review of renewable solar energy a project
TRANSCRIPT
REVIEW OF RENEWABLE SOLAR ENERGY
A Project
Presented to the faculty of the Department of Mechanical Engineering
California State University, Sacramento
Submitted in partial satisfaction of
the requirements for the degree of
MASTER OF SCIENCE
in
Mechanical Engineering
by
Usha Kiranmayee Bhamidipati
SUMMER
2012
ii
REVIEW OF RENEWABLE SOLAR ENERGY
A Project
by
Usha Kiranmayee Bhamidipati
Approved by:
__________________________________, Committee Chair
Dr. Dongmei Zhou
___________________________
Date
iii
Student: Usha Kiranmayee Bhamidipati
I certify that this student has met the requirements for format contained in the University format
manual, and that this project is suitable for shelving in the Library and credit is to be awarded
for the thesis.
__________________________, Graduate Coordinator ___________________
Dr. Akihiko Kumagai Date
Department of Mechanical Engineering
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Abstract
of
REVIEW OF RENEWABLE SOLAR ENERGY
by
Usha Bhamidipati
The major challenge that our planet is facing today is the anthropogenic driven climate changes
and its link to our global society’s present and future energy needs. Renewable energy sources are
now widely regarded as an important energy source. This technology contributes to the reduction
of environmental impact, improved energy security and creating new energy industries.
Traditional Fossil fuels such as oil, natural gas, coals are in great demand and are highly effective
but at the same time they are damaging human health and environment. In terms of environment
the traditional fossil fuels are facing a lot of pressure. The most serious challenge would perhaps
be confronting the use of coal and natural gas while keeping in mind the greenhouse gas
reduction target. It is now clear that in order to keep the levels of CO2 below 550 ppm, it cannot
be achieved fundamentally on oil or coal based global economy.
Renewable energies can provide sustainable energy services, based on the use of routinely
available, indigenous resources. A transition to renewable-based energy systems is looking
increasingly likely as their costs decline while the price of oil and gas continue to fluctuate.
Renewable Energies are considered as a clean source of energy and optimal use of these
resources minimizes environmental impacts, produces minimum secondary waste and is sustained
v
based on current and future economic and social needs. Sun is a major source of all energies. The
primary forms of Sun’s energy are heat and light. Sun energy is transformed and absorbed in
different way these transformations result in renewable energies like biomass and wind energy.
Renewable energy technologies provide an excellent opportunity for mitigation of greenhouse gas
emission and reducing global warming through substituting conventional energy sources. Solar
energy is the most abundant renewable energy source available and in most regions its
availability is far in excess when compared to the current primary energy supply. Therefore solar
energy acts as a key tool to reduce carbon emissions and providing a clean environment.
The transition from a carbon –based energy system to renewable solar energy system involves
some technological, scientific and socioeconomic barriers to the implementation of solar energy
as a clean technology for the future. . This research serves a review on “Renewable Energy
Sources” mainly focused on Solar Energy. It summarizes how the resources are used in energy
production in terms of resource potential, existing capacity. It discusses the historical trends and
future growth prospects of solar energy in terms of what has been done till now in that field of
development and what is the research which is going on for the future. The project also provides
comments on the importance, benefits, and the key scientific and technical challenges of Solar
Energy Technology as well as the detailed description on the different technologies that are being
used worldwide and a review on the future prospects of solar energy supply under various
scenarios by 2020, 2030 and 2050.
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The key concepts to this review are to develop a solid understanding of the multi facet
technological, geopolitical, sociological, and the economic impacts of energy use and to provide
insights into the solar energy development and usage
_______________________, Committee Chair
Dr. Dongmei Zhou
_______________________
Date
vii
ACKNOWLEDGMENTS
I would like to express my deep gratitude to my advisor Dr. Dongmei Zhou for her
encouragement and belief in me, for the uncountable number of hours spent sharing her
knowledge and discussing various ideas, and for many useful comments and suggestions while
examining my work. I would also like to express my thanks to faculty members of Mechanical
Engineering Department California State University, Sacramento and its management who
contributed in completing and helping me to finish this work in time and in comprehensive
manner suitable to California State University formulated guidelines.
To my husband Raghu, for being my sounding board several times: Thanks for letting me vent
my frustrations and occasionally for solving my problems in doing so. I would also like to thank
my Parents and all my family members who believed in me and for their continual support and
encouragement in attaining my academic achievements.
Usha Kiranmayee Bhamidipati
M.S Mechanical Engineering
Summer 2012
viii
TABLE OF CONTENTS
Acknowledgments....................................................................................................................... vii
List of Tables ................................................................................................................................ x
List of Figure ............................................................................................................................... xi
Chapter
1. INTRODUCTION .................................................................................................................... 1
1.1. Background .......................................................................................................................... 1
1.2 World’s Energy Supply ......................................................................................................... 3
1.3. What is Renewable Energy? ................................................................................................ 3
1.4. Kinds of Renewable Energy ................................................................................................ 4
1.5. Renewable Energy Sources .................................................................................................. 4
1.6. Climate Change Scenario ..................................................................................................... 6
1.7. Objective .............................................................................................................................. 7
2. SOLAR ENERGY .................................................................................................................... 8
2.1. Introduction .......................................................................................................................... 8
2.2 Significance of Resource: Historical, Present and Future ..................................................... 9
2.3. Advantages and Disadvantages of Solar Energy ............................................................... 13
2.4. The Economics of Solar Energy ........................................................................................ 15
2.5. Summary ............................................................................................................................ 16
3. SOLAR ENERGY TECHNOLOGIES ................................................................................... 17
ix
3.1. Introduction ........................................................................................................................ 17
3.2. Passive and Active Systems ............................................................................................... 18
3.3 Thermal And Photovoltaic .................................................................................................. 21
3.3.1. Photovoltaic Cells ..................................................................................................... 21
3.3.2 Solar Thermal Energy ................................................................................................ 29
3.4. Concentrating and Non-Concentrating Technologies ........................................................ 32
3.4.1. Stationary or Non-Concentrating Technology .......................................................... 33
3.5. Summary ............................................................................................................................ 38
4. CONCENTRATED SOLAR THERMAL POWER ............................................................... 40
4.1 Introduction ......................................................................................................................... 40
4.2 Solar Radiation.................................................................................................................... 44
4.3 Direct Normal Insolation .................................................................................................... 46
4.4. Concentrated Solar Technologies ...................................................................................... 47
4.4.1 Parabolic Trough Collector Technology .................................................................... 49
4.4.2. Linear Fresnel Collector Technology ....................................................................... 62
4.4.3. Parabolic Dish Collector Technology ....................................................................... 67
4.4.4. Heliostat Field Collector Or Power Tower ............................................................... 71
4.5. Comparision of Concentrated Solar Power (CSP) Technologies ...................................... 75
4.6. Current Market Status CSP ................................................................................................ 77
x
4.7. Applications ....................................................................................................................... 78
4.8 Summary ............................................................................................................................. 80
5. DIRECT STEAM GENERATION TECHNOLOGY ............................................................. 82
5.1. Introduction ........................................................................................................................ 82
5.2. Direct Solar Steam Generation [DISS] in Parabolic Trough Collectors ............................ 84
5.3. Comparison of DSG and Synthetic Oil Based Parabolic Trough Plant ............................ 87
5.4. Power Tower With Direct Steam Generation .................................................................... 89
5.5. Current Status of Direct Steam Generation ....................................................................... 90
5.6. Energy Storage Technology for Direct Steam Generation ................................................ 91
5.7. Innovations and Improvement in DSG .............................................................................. 93
5.8. Summary ............................................................................................................................ 95
6. CONCLUSION AND FUTURE WORK ............................................................................... 97
6.1 Conclusion .......................................................................................................................... 97
6.2 Future Work ...................................................................................................................... 100
References ................................................................................................................................. 102
LIST OF TABLES
Tables Page
xi
Table2.1. Yearly solar fluxes and Human energy Consumption.................................................10
Table3.1. Classifications of Solar Energy Technologies.............................................................18
Table3.2. Overall Comparison of Solar Energy Technologies…………………..…….…..…...37
Table4.1. Overall Comparison of Concentrated Solar Power Technologies…………………..76
LIST OF FIGURES
Figures Page
xii
Figure 1.1: Types of Renewable Energy Sources...........................................................................05
Figure 2.1: Technical Potential of Renewable Energy Technologies.............................................11
Figure 2.2: Sun’s Position Vector...................................................................................................13
Figure 3.1: Solar Energy heating building with Trombe Wall.......................................................20
Figure 3.2: The Photovoltaic Cell...................................................................................................22
Figure 3.3: Types of PV Systems …..............................................................................................24
Figure 3.4: Total Installed Capacity of PV at Global Level...........................................................28
Figure 3.5: Box Type Solar Cooker................................................................................................31
Figure 3.6: Schematic Diagram of Solar water heater....................................................................32
Figure 3.7: Flat plate collectors typically mounted on the roof…..................................................34
Figure 3.8: Schematic Diagram of CPC collector.........................................................................34
Figure 3.9: Schematic Diagram of an Evacuated Tube Collector..................................................36
Figure 4.1: Main Components of a CSP System............................................................................41
Figure 4.2: CSP System Efficiency Variation with Operating Temperature.................................42
Figure 4.3: Levelized Electricity Cost (cents/kWh) Projections of CSP........................................44
Figure 4.4: Solar Irradiance variation within a day measured on a flat plate positioned
horizontal and tracking the sun and direct normal irradiance (DNI)..........................45
Figure 4.5: The solar insolation (KWh/m2/day) on an optimally tilted surface during the
worst month of the year...............................................................................................46
Figure 4.6: Line-Focusing Systems......................................................................…......................48
Figure 4.7: Point-Focusing Systems ..........................................................................…................48
Figure 4.8: Types of CSP Technology..................................................................…......................49
Figure 4.9: A Typical Parabolic Trough System. .......................................................…...............50
xiii
Figure 4.10: Schematic Diagram of Parabolic Trough Collector...................................................51
Figure 4.11: Typical Schematic Diagram of SEGS Plants............................................................52
Figure 4.12: Parabolic Trough at 30 MWe (net) SEGS Plant in Kramer Junction, CA................53
Figure 4.13: Typical Cost breakdown of a Parabolic Trough SEGS plant.....................................55
Figure 4.14: Parallel Sun Rays being concentrated onto the focal line of the collector.................56
Figure 4.15: Tracking of Sun rays by Parabolic Trough Collectors with a Collector axis
oriented north south.................................................................................................57
Figure 4.16: Schematic Representation of Linear Fresnel Solar Collectors...................................62
Figure 4.17: The effect of storage on utility load during a typical day..........................................63
Figure 4.18: Schematic Diagram showing interleaving of mirrors in a CLFR with
reduced shading between mirrors….………………………....…………….….…...64
Figure 4.19: Schematic diagram of inverted air cavity receiver.....................................................65
Figure 4.20: Wave platform structure for a CLFR system allows maximization of solar
radiation collected from a given area……………………………...........................66
Figure 4.21: Schematic Diagram of Parabolic Dish Collector.......................................................68
Figure 4.22: Stirling Dish Systems at Sandia National Labs.........................................................69
Figure 4.23: Schematic Diagram of Heliostat Field Collector.......................................................72
Figure 5.1: Basic Concepts for the DISS in Parabolic Trough Collectors………...……..……....84
Figure 5.2: View of the PSA DISS solar field in Operation..........................................................86
Figure 5.3: Arrangement of the Solar Power Plant with DSG......................................................86
Figure 5.4: DSG in Power Tower with Saturated Steam Receiver................................................89
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Figure 5.5: Selected DSG Storage Options……………………....................................................92
1
CHAPTER 1
INTRODUCTION
1.1. BACKGROUND:
Energy plays a vital role in today's life. Our work, leisure, our social, economic and physical
welfare depends on sufficient supply of energy. Yet we take it for granted and now we have a
worldwide demand for energy at an alarming rate. The traditionally used fossil fuels such as oil
are ultimately limited and the gap between the energy needed to the energy available is will in not
too distinct future, have to be met in increasingly from alternative primary energy sources. We
must strive to make these more sustainable to avoid the negative impact of the global climate
change, the growing risk of supply disruptions, price volatility and air pollution that are
associated with today’s energy system. This calls for immediate actions to promote green house
gas emissions free energy sources such as renewable energy source, alternative fuels for transport
and to increase energy efficiency.
The Simplest definition of renewable energy would be energy that comes from naturally
replenished energy sources such as sunlight and wind. The main reason why the renewable
energy is so closely connected to environment and ecology in eyes of many people is because it is
environmental friendly and reduces the climate changes and Global warming unlike the
traditional sources do.
People usually refer to renewable energy as the antithesis to fossil fuels. The Fossil Fuels are
being used since a long time while the renewable energies has just started developing and this is
the main reason why renewable energy is still finding it hard to compete with fossil fuels.
Renewable energy still needs to improve its cost-competitiveness, because most renewable
energy sources still require subsidies to remain competitive with fossil fuels in term of costs
(though it also has to be said that the prices of renewable energy technologies are constantly
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dropping so it's only really a matter of time before renewable energy will become cost
competitive with traditional fuels without subsidies.)
Together with costs renewable energy will also need to improve its efficiency. For instance,
average solar panels have efficiency of around 15% which means that lot of energy gets wasted
and transferred into heat, instead of some other form of usable energy. However, there are many
ongoing researches with the goal to improve efficiency of renewable energy technologies, some
of which have been really promising, though we are yet to see some highly efficient and
commercially viable renewable energy solution.
Renewable energy sector could decide to choose a "sit and wait strategy" because fossil fuels will
eventually become depleted and renewable energy would then remain as the best alternative to
satisfy world's hunger for energy. But this would be a bad strategy for two reasons: energy
security and climate change.
Once fossil fuels become depleted renewable energy sector will have to be developed enough to
replace coal, oil, and natural gas and this can be only done if renewable energy technologies
continue with progress in years to come. By failing to further develop renewable energy
technologies we would endanger our future energy security, and this is something world mustn't
allow. Renewable energy is often considered as the best way to tackle global warming and
climate change. The more renewable energy we use the less fossil fuel we burn, and less burning
of fossil fuels means less carbon dioxide emissions and lesser impact to climate change.
There are really plenty of reasons to choose renewable energy over fossil fuels but we must not
forget that renewable energy is still not ready to completely replace fossil fuels. Some day it will
be but not just yet. The most important thing to do right now is to further develop different
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renewable energy technologies in order to ensure that once this day comes world wouldn't have to
worry whether renewable energy will be able to deliver the goods or not.
1.2 WORLD’S ENERGY SUPPLY:
The increase in greenhouse gasses in the atmosphere and the potential global warming and
climatic change associated with it, represent one of the greatest environmental dangers of our
time. The anthropogenic reasons of this impending change in the climate can for the greater part
be put down to the use of energy and the combustion of fossil primary sources of energy and the
emission of CO2 associated with this. Today, the world’s energy supply is based on the non-
renewable sources of energy: oil, coal, natural gas and uranium, which together cover about 82%
of the global primary energy requirements. The remaining 18% divide approximately 2/3 into
biomass and 1/3 into hydropower.
The effective protection of the climate for future generations will demand at least a 50%
reduction in the world-wide anthropogenic emission of greenhouse gases in the next 50 to 100
years. With due consideration to common population growth scenarios and assuming a
simultaneity criterion for CO2 emissions from fossil fuels, one arrives at the demand for an
average per-capita reduction in the yield in industrial countries of approximately 90%. This
means 1/10 of the current per-capita yield of CO2.A reduction of CO2 emissions on the scale will
require the conversion to a sustained supply of energy that is based on the use of renewable
energy with a high share of direct solar energy use.
1.3. WHAT IS RENEWABLE ENERGY? [36]
Renewable energies are the energy sources that come from natural resources such as sun, wind,
rain which can naturally replenish. Most of these energies are directly derived from the sun such
as the thermal, photochemical and photoelectric and some of them are derived indirectly like the
4
wind, hydropower and the photosynthesis energy stored in biomass and some from the natural
movement and mechanism from the environment. This energy is environmental benign, they do
not emit any toxic gases while being used and they do not try to deplete any natural resources.
Renewable energy does not include energy resources derived from fossil fuels, waste products
from fossil sources, or waste products from inorganic sources. Renewable energy flows involve
natural phenomena such as sunlight, wind, tides, plant growth, and geothermal heat. Renewable
energy is derived from natural processes that are replenished constantly. In its various forms, it
derives directly from the sun, or from heat generated deep within the earth.
1.4. KINDS OF RENEWABLE ENERGY:
Renewable energy is the energy which comes from the natural resources such as sunlight, wind,
rain tides and geothermal heat which are renewable. Classifying an energy form as “renewable”
encompasses a range of assumptions regarding the time scale. The implication is that the
renewable energy is available continuously without depleting and degrading. For example solar
energy is available for some time period every day virtually everywhere on the surface of the
earth. There is a natural 24 hour diurnal cycle, as well as seasonal vibration due to the changes in
the relative angle of our rotating earth tilted on its axis as it makes its yearly orbit around the sun.
Due to these effects are the daily fluctuations that result because of the cloud cover. Other
renewable types such as Biomass, Hydro Power and Wind Energy have analogous variations over
different time scale.
1.5. RENEWABLE ENERGY SOURCES:
The renewable energy sources play an important role in the sufficing the need for growing
demand for energy resources of the world in the near future and they all are very cost efficient.
The energy sources are divided into three categories: 1.Fossil fuels, 2. Renewable energy
5
resources, and 3. nuclear resources. Renewable energy sources are those which can be produced
again and again, like solar energy, wind energy, geothermal energy, biomass, hydro power energy
these are also often known as alternative sources of energy.
Figure 1.1: Types of Renewable Energy Sources [32]
As the renewable energy sources meet the energy requirements they have the potential to provide
services with zero or no emissions any harmful gases. The development of these resources
provide solutions for crucial tasks like energy reliability and solving problems of energy and
water supply, increases the standard of living for a common man and also increase the level of
employment and also as these energies are available everywhere it provides service in remote
regions like the desserts and mountain areas. Harvesting the renewable energy in decentralized
6
manner is one of the options to meet the rural and small scale energy needs in a reliable,
affordable and environmentally sustainable way.
1.6. CLIMATE CHANGE SCENARIO:
Climate change is one of the primary concerns for humanity in the 21st century. It may affect
health through a range of pathways, for example as a result of increased frequency and intensity
of heat waves, reduction in cold related deaths, increased floods and droughts, changes in the
distribution of vector-borne diseases and effects on the risk of disasters and malnutrition. The
overall balance of effect on health is likely to be negative and populations in low income
countries are likely to be particularly vulnerable to the adverse effects. The potentially most
important environmental problem relating to energy is global climate change (global warming or
the greenhouse effect). The increasing concentration of greenhouse gases such as CO2, CH4 ,
CFCs, halons, N2O, ozone, and peroxyacetylnitrate in the atmosphere is acting to trap heat
radiated from Earth’s surface and is raising the surface temperature of Earth. Humankind is
contributing with a great many economic activities to the increase atmospheric concentration of
various greenhouse gases.
Many scientific studies reveal that overall CO2 levels have increased 31% in the past 200 years,
20 Gt of Carbon added to environment since 1800 only due to deforestation and the concentration
of methane gas which is responsible for ozone layer depletion has more than doubled since then.
The global mean surface temperature has increased by 0.4–0.8 ◦C in the last century above the
baseline of 14 ◦C. Increasing global temperature ultimately increases global mean sea levels at an
average annual rate of 1–2mm over the last century. Arctic sea ice thinned by 40% and decreased
in extent by 10–15% in summer since the 1950s.
7
Industry contributes directly and indirectly (through electricity consumption) about 37% of the
global greenhouse gas emissions, of which over 80% is from energy use. Total energy-related
emissions, which were 9.9 Gt CO2 in 2004, have grown by 65% since 1971 [26].There is ample
scope to minimize emission of greenhouse gases if efficient utilization of renewable energy
sources in actual energy meeting route is promoted
1.7. OBJECTIVE:
The main objective of this project is to provide information on the role of Renewable energy
sources in this growing demand for energy mainly focused on Solar Energy mainly summarizing
how are the resources used in energy production in terms of resource potential, existing capacity,
along with historical trends and future growth prospects. The outline of this project is to write a
review about the importance, benefits, and the key scientific and technical challenges of Solar
Energy Technology. The project mainly summarizes how the resources are used in energy
production, So far what has been achieved in this field of development and what research is
required in that field of development for future. A review on Direct Steam Generation (DSG) in
parabolic trough collectors is a promising option for the improvement of the reliable CSP
technology. This review mainly summarizes about what is DSG, The research review discusses
all the latest research in the field along with feasibility features design and control concepts being
used. To achieve the above mentioned objective a detailed review on solar energy and different
technologies being used to produce electricity using the Sun’s energy and the advances taking
place in this Field is being discussed and what future developments are required to make solar
energy as the main energy source to produce electricity.
8
CHAPTER 2
SOLAR ENERGY
[46] [33] [34] [17] [5]
2.1. INTRODUCTION:
Sun is the vital source of life. It is vast, environmentally friendly and generally synchronous with
daily energy demands. Meeting all future energy demands with solar energy is technically
possible, but further technology development and cost reductions are required before this
immense resource will be able to provide a significant portion of world’s energy needs reliably
and at an acceptable cost.
Throughout the human history, Sun energy has been used for household’s domestic purposes like
cooking and heating. As Sun’s energy is everywhere and the ability to use it effectively over a
range of scale makes solar energy the popular choice among other energies. The Sun’s energy
incident on the earth is the intrinsic source for many forms of renewable energy (including wind,
ocean thermal, and bio energy) and over a long time scales all of the fossil energy.
Solar energy as well as secondary solar powered energy such as wind, biomass, wave power,
hydropower account for major part of renewable energy but only a part of solar energy is being
used. The Solar industry began developing during the years 1980 but the fossil fuels slowed the
solar industry growth in 1990 as the cost of fossil fuel is still low. But in 2000 the world market
for solar energy had a rapid growth due to the increasing cost and concerns for global climate
changes of fossil fuels and also the improving technologies and lowering costs of solar energy
itself.
Solar energy is the most commonly used energy when compared to other energies, the resource is
well understood, and conversion technologies have long and positive operational track records
9
still there are three barriers that prevent the widespread utilization of solar energy. First, while the
solar energy is vast it requires significant surface area as it is not highly concentrated, second,
In order to establish a large scale power plant the cost of producing energy in large scale is still
costly compared to other energies. Third, the solar resource’s intermittency and cyclical nature
pose challenges for integrating solar at a large scale into the existing energy infrastructure.
Although there is nothing we can do about the nature of sun still the cost for utilization of sun’s
energy can be achieved by using advanced technologies and improved manufacturing technique
these will help in making solar energy be the major contributor in meeting the world’s energy
needs.
2.2 SIGNIFICANCE OF RESOURCE: HISTORICAL, PRESENT AND FUTURE [33]
The sun’s energy is being used by us right from moment the humans inhabit the earth for the
purpose of lighting, drying food and heat water. Overall, the sun emits about 7,000 times more
energy than is required for human consumption. Presently the total amount of solar energy
consumed for human use is less than 1% of our entire energy requirements with a little more
knowledge of the basic characteristics of Sun and on the utilization of solar radiation in different
fields we could increase the contribution of solar power to world energy consumption.
Technologies in these areas with a few under development and few that are available give an
opportunity to contribute to the world’s energy needs.
The most commonly used application of solar energy is light, heat and electricity. Today’s solar
industry supplies reliable products to provide heat and electricity for residential, commercial, and
industrial applications using simple equipment such as flat-plate collectors. Natural sunlight is
increasingly utilized in modern building design; day-lighting can be successfully incorporated
into almost any structure, even underground buildings.
10
Several solar electric power plants has been built in US and outside US using concentrating
collector’s optics to achieve high temperature required to produce electricity. Currently
photovoltaic cells are capable to do the job at an efficiency of 15to 20% but there are researches
going on for cells that are work at an efficiency of 40-45%. Most of the applications are cost
effective while cost for other application is drastically decreasing.
A key issue about solar energy is its variability along with time it is not the same at all seasons
and at all times therefore in order to accommodate the nation’s energy needs we need a solar
energy system with adequate storage capacity or some other form of energy that gives a backup
support. Although the market cost for these technologies are relatively high the desirable
characteristics of solar energy like its synchronous with its demand, zero to low emission and
water requirements and sun resources available makes looks promising for the energy needs in
the future.
Table 2.1 Yearly solar fluxes and Human energy Consumption [33]
Yearly Solar fluxes & Human Energy Consumption
Solar 3,850,000 EJ
Wind 2,250 EJ
Biomass 3,000 EJ
Primary energy use (2005) 487 EJ
Electricity (2005) 56.7 EJ
11
According to table 2.1 the total solar energy absorbed by Earth's atmosphere, oceans and land
masses is approximately 3,850,000 (EJ) per year. In 2002, this was more energy in one hour than
the world used in one year. Photosynthesis captures approximately 3,000 EJ per year in biomass.
The amount of solar energy reaching the surface of the planet is so vast that in one year it is about
twice as much as will ever be obtained from all of the Earth's non-renewable resources of coal,
oil, natural gas, and mined uranium combined.
Figure 2.1: Technical Potential of Renewable Energy Technologies. [46]
In mathematical terms the capture efficiency (η solar) of the solar collector can be represented as
[33]
η solar = useful energy recovered/ total solar flux incident on the collector x 100%
12
Recovering efficiency can be in two form- thermal energy (heat) and electrical energy (current x
voltage). In thermal energy application the efficiency is ranging from 30-60% and in electrical
energy application the efficiency is only about 8-15%.
An operating variable that can influence the capture efficiency of the solar collector is the
pointing error ψ which can be represented as
Ψ = pointing error = € - β in degrees
And α = collector tilt relative to the latitude = β – φ
Where β = tilt angle of the collector in degrees from the horizontal
Φ = latitude in degrees, € = pointing angle of the sun, θ = altitude angle in degrees
ω = azimuth in degrees.
The altitude angle θ and azimuth ω are defined as the angle of the sun above the horizon and the
angle from true south, respectively. The hourly variation of the sun’s position are usually
represented by the azimuth or hour angle, ω that varies about 15 degrees per hour and ranges
from 0 to a maximum value that changes depending on the time of the year. The value of ω is
zero at solar noon when the sun reaches its highest position in the sky for its specific location and
reaches its maximum value when the sun sets below the horizon. The maximum value is less than
90 degrees in fall and winter months and greater than 90 during spring and summer months.
Seasonal variations are usually given as a function of declination angle δ, which provides a
qualitative measure of tilted earth’s position relative to the sun as the earth moves around the sun
annually. Value of δ is zero at autumn and vernal equinoxes September 21 and March 21
13
respectively and in northern latitude +23.5 degrees and at summer solstice on june21 and -23.5
degrees at winter solstice on December 21.
Figure 2.2: Sun's Position Vector [33]
2.3. ADVANTAGES AND DISADVANTAGES OF SOLAR ENERGY:
Advantages: Solar energy is a clean energy, environmentally friendly.it does not require any
depletion of any natural resources, it does not emit any harmful gases,and it does not leave any
liquid or solid waste.keeping in mind sustainable development the direct and indirect advantages
of Solar energy are as follows
1. No emissions of greenhouse (mainly CO2, NOx) or toxic gasses (SO2,particulates);
2. Reclamation of degraded land;
3. Reduction of transmission lines from electricity grids;
4. Improvement of quality of water resources;
The sun’s position vector relative to the earth-center frame, in the earth-
center frame, CM, CE and CP represent three orthogonal axes from the
center of the earth pointing towards meridian, east and Polaris,
respectively.
14
5. Increase of regional/national energy independence;
6. Diversification and security of energy supply;
7. Acceleration of rural electrification in developing countries.
Disadvantages: Although solar energy is considered to be a clean source and also considered as a
infinite energy source when compared to fossil fuels, like any other energy solar energy has a few
disadvantages too.
1. Though the solar energy systems do not emit any harmful gases during their operation
their modules may contain chemical that might be exposed to environment during a fire.
2. Solar systems do not produce sound during their operation but during the construction
time there might be some noise
3. There might be a little visual impact depending on the type of scheme and surroundings
being used by the solar energy system.
4. One of the main disadvantages of solar energy is its initial cost of the equipment used to
harness the sun energy. Solar energy is still a costly alternative when compared to readily
available fossil fuels.
5. The solar energy technologies installation requires a large area for the system to be
sufficient.
6. Pollution can be a disadvantage to solar panel; pollution can degrade the efficiency of
photovoltaic Clouds also provide the same effect, as they can reduce the energy of the
sun’s rays. This certain disadvantage is more of an issue with older solar components, as
newer designs integrate technologies to overcome the worst of these effects.
7. Since not all the light from the sun is utilized by the solar panels therefore the solar panel
have 40% efficiency rate which means 60% of sun’s energy is going waste. New
15
technologies are trying to increase the rate of efficiency of the solar panels from 40-80%
and on the downside have increased the cost solar panels
2.4. THE ECONOMICS OF SOLAR ENERGY: [46]
A new era for solar energy is rising which was once thought uneconomical with the new
technologies rising and the price for traditional fuels are increasing. These wide variety of solar
energy technologies try to compete with technologies in different energy markets, like the
centralized power supply, distributed power generation, or off grid or stand alone applications.
On the other hand, small-scale solar energy systems, which are part of distributed energy
resources (DER) systems, compete with a number of other technologies. The traditional approach
for comparing the cost of generating electricity from different technologies depends on levelized
cost method. The levelized cost of the power plant is calculated as follows. (1)
The LCOE methodology is an abstraction from reality and is used as a benchmarking or ranking
tool to assess the cost-effectiveness of different energy generation technologies. Where OC is the
overnight construction cost (or investment without accounting for interest payments during
construction); OMC is the series of annualized operation and maintenance (O&M) costs; FC is
the series of annualized fuel costs; CRF is the capital recovery factor; CF is the capacity factor; r
is the discount rate and T is the economic life of the plant.
16
2.5. SUMMARY:
It is clear from the above discussion that solar energy constitutes the most abundant renewable
energy resources available in most of the regions throughout the world. Its potential is far in
excess when compared to the total primary energy supply in those regions. Sun is the main source
of solar energy. Most commonly used application of solar energy is light, heat and electricity.
Today’s solar industry supplies reliable products to provide heat and electricity for residential,
commercial and industrial applications using simple equipment. Natural sunlight is increasingly
utilized in modern building design. Solar energy is environmentally friendly. It does not emit any
harmful gases into the environment. It reduces transmission lines from electricity grids, improves
quality of water resources and increases energy independence. However it also has some
disadvantages. Even though solar energy does not emit harmful gases during their application
their modules may emit chemicals which can be exposed to environment during fire. Although
Solar energy is an expensive option when compared to other renewable energies as it requires
huge investment cost when used in large scale and it requires huge maintenance cost when
compared to conventional energy sources .Solar technologies like Concentrated solar power
might require more land, still with more intense research and new developments in this field solar
energy will be a better option compared to other technologies. Using solar energy to generate
electricity is one of the greatest achievements by mankind, and is set for even greater things in the
future.
17
CHAPTER 3
SOLAR ENERGY TECHNOLOGIES
[11], [14], [16-18] [22] [28-29] [35] [51]
3.1. INTRODUCTION:
Solar energy is being used by mankind since a long time, for example 2000 years back solar
energy was used to extract salt from sea water. The Ancient greek used solar energy technology
which is now widely used as paasive systems for heating and cooling the building. A number of
technologies are being used to harness the sun energy for different purposes including heating,
lighting drying, generating electricity using solar cells and solar collectors.Solar energy systems
can be classified into two types:- solar thermal‖ applications that convert solar radiation to
thermal energy, which can be directly used (e.g., solar hot water systems) or converted further
into electricity (e.g., CSP); and applications that directly generate electricity from sunlight using
the photovoltaic effect.
In other sense any phenomenon that traces its origin to energy from the sun and harness it in a
useful way directly or indirectly, this may include phenomenon such as wind and photosynthesis.
However, for our purposes, we limit use of the term ―solar energy to sources of energy that can
be directly attributed to the light of the sun or the heat that sunlight generates.
As such, solar energy technologies can be arranged along the following continuum:
1) Passive and active;
2) Thermal and photovoltaic and
3) Concentrating and non-concentrating.
18
3.2. PASSIVE AND ACTIVE SYSTEMS: [35,51]
The most simple and direct application of solar energy is directly converting solar energy into
low temperature heat,temperature about 212 0F.In general two classes of technologies can be
distinguished as Passive and Active Solar energies system. The table below shows classification
of solar energy into active and passive systems and where they are used.
Table 3.1. Classifications of Solar Energy Technologies
Active Solar
Passive solar Photovoltaic Solar thermal
Electric Non-electric
Centralized(>200kW) Concentrating
PV arrays
(CPV) Utility-
scale PV
Concentrating
solar thermal
(CSP)
District
water
heating
Large-scale
distributed (>20kW)
Commercial
building
PV
Commercial
hot
water
systems
Small-scale
distributed (<20kW)
Small
commercial &
Residential
building PV
Residential
water
heating
systems
Heating
&
cooling
Off-grid
Applications
Stand alone
systems for
remote
applications,
solar-home
systems
Day - lighting
3.2.1. Passive Energy: is more of a day to day usage purposes like heating, lighting,cooling. That
is the building itself or some parts of it take advantageof the natural energy charecteristics and air
created by the exposure to the sun. Passive systems are simple and have very less number of
moving parts, no mechanical parts therefore requires very minimal maintenance. Passive solar
19
designs tries to optimise the amount of energy that can be derived directly from the sun, Similarly
by taking good care in the consideration of building material and fabric can help to reduce the
need for secondary heating ,ventilation and artificial lighting.
There are three types of passive solar heating systems:
1. Direct Gain: this is the simplest and the basic form of passive solar. This can be achieved by
facing all the window in the house facing the equator. The sunlight enters the room through the
windows and hits the thermal surface (walls, floors) gets absorbs and stores the thermal heat. At
night time the heat stored in the thermal mass convects and radiates into the room.
2. Indirect Gain: these systems have a thermal storage such as the trombe wall between the
sunlight and the room. This wall store and releases the thermal heat to the room over a period of
several hours. This way energy supplied to the room is more controllable when compared to the
direct gain system. A Trombe wall is made up of heat absorbing material and painted dark.
During the day this absorbing maaterial absorbs the sunlight and heat is radiated when the sun
goes down.
20
Figure 3.1: Solar Energy heating building with Trombe Wall. [23]
3. Isolated Gain: the simplest type of isolated gain system is sunrooms. Heat is distributed
into the house through ceiling and floor level fans, windows and vents.
3.2.2. Active Systems: Active solar energy technology refers to the harnessing of solar energy to
store it or convert it for other applications. The only difference between active system and passive
systems is that the active systems employ collectors to capture the sun’s energy and to transfer
this thermal energy to a working fluid circulating which can be used immediately or store for
later.
21
Active solar energy technologies reduce the use of fossil fuels for the sake of energy requirements
and associated fuel costs. The basic benefit of active systems is that controls (usually electrical)
can be used to maximize their effectiveness. The downside to active solar systems is that the
external power sources can fail (probably rendering them useless), and the controls need
maintenance. The two main applications of active solar sources are for homes/ buildings i.e.,
electricity, and heat.
3.3 THERMAL AND PHOTOVOLTAIC: [42, 46]
Solar thermal and photovoltaic electricity generation are two promising technologies for climate
compatible power with such enormous potential that, theoretically, they could cover much more
than just the present worldwide demand for electricity consumption. Together both technologies
can provide an important contribution to climate protection. Photovoltaic systems have
advantages for low-power demand, stand-alone systems and building-integrated grid-connected
systems. Solar thermal power plants are best operated in large grid-connected systems.
3.3.1. PHOTOVOLTAIC CELLS: [52]
(a). Introduction:
Photovoltaic conversion is the direct conversion of sunlight to electricity this is called as
photovoltaic effect this effect was first discovered in 1954 by Bell Telephone when he discovered
that the Silicon an element found in sand when exposed to sunlight produces electricity. Since
then the photovoltaic cells are being used to power up space satellites and also used in smaller
devices like calculators, watches. Today they are being used to power up homes, building and
businesses with individual PV systems. Utility companies are also using PV technology for large
power stations.
22
Figure 3.2: The Photovoltaic Cell. [5]
Traditional photovoltaic systems are typically made up of silicon and are proved to be very
efficient and are flat plate. The Second generation PV systems also known as thin-film solar cells
because they are made up of amorphous silicon, non-silicon material called cadmium telluride.
Companies are also using PV technology for large power stations.
The third generation of PV systems is using materials other than silicon, including solar in using
conventional press technologies, solar dyes, and conductive plastics. Some new cells use
conductive plastic lenses or mirrors to concentrate the sunlight on a small piece of PV material.
Although the PV material is costly as we need only a small piece of this PV material which are
highly efficient these are turning to be a cost effective system for utility and industry. The use of
23
concentrating collectors is limited to sunniest part of the country because the lenses must be
pointed at the sun.
Solar panels are used to power home and businesses typically hold up to 40 solar cells into a
module. In general we need 20 to 30 solar panels to produce power for a house or business. These
panels are fixed in an angle facing the south and are connected to a tracking device which follows
the sun, and allow them to capture sunlight. A number of solar panels combined to create a solar
array. For a large electric utility or industrial application a number of solar arrays are
interconnected to form a large utility PV system.
Photovoltaic systems are broadly classified into two types: stand alone and grid connected
system. Stand alone systems are systems which are not connected to grid; In general the energy
produced by this is matched by the energy required by the load. They come with an energy
storage system usually the rechargeable battery support when there is no sunlight. Stand alone
systems are generally used in areas that are not easily accessible and have no access to electricity.
Stand alone system usually have a PV module or a module with batteries and charge controllers.
An inverter can be used to change the direct current produced by the PV module into alternating
current form which is required for normal appliances.
On the other had grid systems are the systems which are connected to the public grid i.e., the PV
module is connected to the local electric network. This means the electricity produced during the
day can be immediately used or can be sold to the electricity supply companies in the evenings
when solar energy is not available this system acts as an energy storage module and provides
electricity. This kind of connection removes the dilemma by stand alone systems. They demand
energy from grid when there is not enough power generation on the panels and feed in the power
24
to the grid when there is more than required power by the system. This trend is a concept called
“net metering”. It is expected that grid connected systems are growing in the developed countries
while the priority is given for the stand alone systems in developing and non-developed countries.
Small PV power systems are wildly used in building industries where they can generate
electricity for lights, water pumps, TVs, refrigerators and water heaters. There are also some
villages called “solar village” that all the houses are operated by solar energy system. Although
20 years ago PVs were considered as a very expensive solar system the present cost is around
5000$ per kWe and there are good prospects for further reduction in the coming years.
Figure 3.3: Types of PV Systems
Photovoltaic technology is the highest of all the active solar technologies. It started in 1950 as an
energy source for satellites but this application is considered to be highly insensitive in terms of
cost wise, but this did help to create a solar photovoltaic industry in United States. This
technology expanded since 1970 and for 15 years it maintained a steady growth of 15%
shipments of PV’s. In the early 1990’s the off grid PV system like the home or village power
systems accounted for 20% of the market while the grid connected systems accounted for 11% of
25
the market and the rest is for the stand alone applications like communication, leisure and so
forth.
3.3.1. (b) ADVANTAGES AND DISADVANTAGES OF PV SYSTEMS [52]
ADVANTAGES:-
1. It is clean and emission free technology as all they need is sunlight, they do not harm the
air or water recourses as there are no harmful gases comes out of these systems, they do
not deplete any natural resources.
2. It is quiet and unobtrusive system
3. For small scale PV plant the unwanted space on the rooftop is sufficient so that it won’t
occupy any space
4. PV systems are originally developed for operation in space where repairs are nearly
impossible or extremely expensive PV nearly powers every satellite circling the earth as
they operate continuously with no maintenance.
5. A PV system can be constructed to any size based on energy requirements. Furthermore,
the owner of a PV system can enlarge or move it if his or her energy needs change. For
instance, homeowners can add modules every few years as their energy usage and
financial resources grow. Ranchers can use mobile trailer-mounted pumping systems to
water cattle as the cattle are rotated to different fields.
DISADVANTAGES:
1. Some toxic chemicals, like cadmium and arsenic, are used in the PV production process.
These environmental impacts are minor and can be easily controlled through recycling
and proper disposal.
26
2. Solar energy is somewhat more expensive to produce than conventional sources of
energy due in part to the cost of manufacturing PV devices and in part to the conversion
efficiencies of the equipment. As the conversion efficiencies continue to increase and the
manufacturing costs continue to come down, PV will become increasingly cost
competitive with conventional fuels.
3. Solar power is a variable energy source, with energy production dependent on the sun.
Solar facilities may produce no power at all some of the time, which could lead to an
energy shortage if too much of a region's power come from solar power.
(c). ENVIRONMENTAL IMPACTS OF SOLAR PHOTOVOLTAIC TECHNOLOGY: - [52]
The main concern about the occupational and health risk factor in the life cycle of a PV system is
the toxic substances used to manufacture the PV. The risk can occur during manufacturing
process. The list of chemicals in the final PV cell is different from the chemicals used to
manufacture them, as solvents and acids for cleaning the semiconductors part or gases for
depositing the thin film layers are not present in the final product.
The properties of the material, some of which are toxic, flammable, concentration, frequency and
duration of human exposure, it is also important to note that not-known interactions paths
between components (for instance while operation of the PV system) exist and not all the
interactions have been tested in the laboratory.
From a life cycle approach, the impacts can occur during manufacturing, an accidental release
may result in risk for the worker or communities in the nearby as a number of gases involved.
The toxicity and explosive nature may create both physical and biological damages and also long
periods of exposure to toxic gases could affect both workers and the general public. During use
27
and operation there is danger of potential human danger can occur from leaching of materials
from a broken PV which is made up heavy material like cadmium and selenium are of main
concern. During decommissioning Disposal of large quantities of modules to a single landfill
presents potential risks for humans, communities and the environment as the leaching of
chemicals can contaminate local ground and surface water. . Many of the chemicals found in
electronic waste (e-waste) are also found in solar PV, including lead, flame retardants, cadmium,
and chromium. The disposal of e-waste is becoming an escalating environmental and health
problem in countries in West Africa, Asia and Latin America. This should be prevented in the
case of PV systems.
(d). CURRENT MARKET STATUS OF PV: [46]
Over the last few decades the installation of solar energy is grown exponentially in a global level.
As illustrated in the example given below in fig 7, the capacity of globally installed PV both off-
grid and grid increased from 1.4GW in 2000 to approximately 40 GW in 2010 with an annual
average growth rate around 49%, of which 85% grid connected and remaining 15% off grid. In
the current phase the market is dominated by crystalline silicon based PV cells. And the
remainder of the market is almost entirely consists of thin film technologies.
28
Figure 3. 4: Total Installed Capacity of PV at Global Level. [46]
As shown in fig (b) many countries dominate the market of PV. Right now there are two types in
the market grid connected and off grid connected. The recent trend is strong growth in the grid
connected PV development of installation over 200kW, operating as centralized power plant. In
the present market condition the off grid application has been overtaken by grid connected PV but
still in a few countries like India and China are still in favor of off grid system. This trend could
be a reflection of their large rural populations, with developing countries adopting an approach to
solar PV that emphasizes PV to fulfill basic demands for electricity that are unmet by the
conventional grid.
(e). COMPARISON OF PV WITH OTHER ENERGY SOURCES: [14]
If we compare present day PV technology with other energy options we see that PV has
considerable low green house gas emissions than all the fossil fuel options, but in comparison
with wind and nuclear energy the solar cells have relatively high green house gas emission
especially when we install PV systems at lower irradiation regions. On the other hand we have
29
shown that there are good prospect if we reduce the green house gas emissions to a low value of
15g/kWh. Sustainability comprises of more than only greenhouse gas emission, using PV reduces
the burdens for the future generation provided we cover the material loops by developing
effective recycling processes. Also one should not forget that PV has a very large potential for
application, larger than wind energy and probably also larger than nuclear and carbon storage.
3.3.2 SOLAR THERMAL ENERGY: [8], [41- 42]
(a). Introduction: Solar thermal energy is an innovative technology to use sunlight to heat water
and other heat transfer fluids to do variety of applications. The simple and most commonly used
applications of solar thermal energy include solar water heating, swimming pool heating and
agricultural drying. In the U.S solar pool, water and space heating are the major applications of
thermal energy.
Solar thermal energy is broadly classified into three types low, medium, and high temperature
collectors. Low temperature collectors are generally flat plate used for heating swimming pool,
the medium temperature collectors are also flat plate and are used more for residential and
commercial places for heating water and air. The high temperature collectors are generally used
for electricity production by harnessing the sunlight with the help of mirrors or lens. When
compared to photovoltaic cells solar thermal energy is different and more efficient.
Solar collectors are the key component of active solar-heating systems. Solar collectors gather the
sun's energy, transform its radiation into heat, and then transfer that heat to water, solar fluid, or
air. The solar thermal energy can be used in solar water-heating systems, solar pool heaters, and
solar space-heating systems. There are several types of solar collectors: Flat-plate collectors,
30
Evacuated-tube collectors, Integral collector-storage systems. Residential and commercial
building applications that require temperatures below 200°F typically use flat-plate collectors,
whereas those requiring temperatures higher than 200°F use evacuated-tube collectors.
3.3.2. (b) Low Temperature Solar Applications: [36]
As far as renewable energy sources is concerned solar thermal energy is the most abundant one
and is available in both direct as well as indirect forms. The Sun emits energy at a rate of
3.8×1023 kW, of which, approximately 1.8×1014 kW is intercepted by the earth. There is vast
scope to utilize available solar energy for thermal applications such as cooking, water heating,
crop drying, etc. Solar cooking is the most direct and convenient application of solar energy.
Solar energy is a promising option capable of being one of the leading energy sources for
cooking. Various types of solar cookers are available, out of them box type solar cooker is widely
used all over the world. A study was conducted in Costa Rica and in the world as a whole, and
then compared the advantages and limitations of solar ovens with conventional firewood and
electric stoves. The payback period of a common hot box type solar oven, even if used 6–8
months a year, is around 12–14 months, roughly 16.8 million tons of firewood can be saved and
the emission of 38.4 million tons of carbon dioxide per year can also be prevented.
31
Figure 3.5: Box Type Solar Cooker. [36]
Solar water heater of domestic size, suitable to satisfy most of the hot water needs of a family of
four persons, offers significant protection to the environment and should be employed whenever
possible in order to achieve a sustainable future. It is estimated that a domestic solar water
heating system of 100 l per day capacity can mitigate around 1237 kg of CO2 emissions in a year
at 50% capacity utilization and in hot and sunny region it is about 1410.5 kg. A schematic of solar
water heater is illustrated in Figure. Solar-drying technology offers an alternative which can
process the vegetables and fruits in clean, hygienic and sanitary conditions to national and
international standards with zero energy costs. It saves energy, time, occupies less area, improves
product quality, makes the process more efficient and protects the environment.
32
Figure3.6: Schematic Diagram of Solar water heater.[36]
3.4. CONCENTRATING AND NON-CONCENTRATING TECHNOLOGIES [22, 15]
The final category in our continuum of solar energy technologies is concentrating vs. non-
concentrating technologies. The CSP technologies just discussed are a family of concentrating
solar energy technologies that use mirrors or lenses to focus sunlight and thus increase the
intensity of light in the focus area. In addition to CSP the principle of concentrating solar energy
is applied to PV as well by using a dish collector to concentrate sunlight on a smaller cell area.
A non-concentrating collector has the same area for intercepting and for absorbing solar radiation
whereas a sun-tracking concentrating solar collector usually has concave reflecting surfaces to
intercept and focus the sun’s beam radiation to a smaller receiving area, thereby increasing the
radiation flux. Solar energy collectors are basically distinguished by their motion, i.e., stationary,
single axis tracking and two-exes tracking, and by their operating temperature. A large number of
solar collectors are available in the market.
33
3.4.1. STATIONARY OR NON-CONCENTRATING TECHNOLOGY: [20]
(MEDIUM TEMPERATURE APPLICATIONS)
(a). Introduction:
These collectors are fixed they do not track the sun. There are three types of collectors that fall in
this category.
Flat plate collectors
Stationary compound parabolic collector
Evacuated tube collector
Flat Plate Collector: When radiation passes through the transparent cover and impinges on the
blackened absorber surface of high absorptivity, a large portion of this energy is absorbed by the
plate and then the rest is transferred to the storage unit for later use. The inside of the absorber
plate and the side of the casing are properly insulated to reduce conduction losses. Flat plate
collectors (FPC) are by far the most used type of collector. Flat-plate collectors are usually
employed for low temperature applications up to 80°C. Flat plate collectors are permanently fixed
in position and require no tracking of the sun. The collectors should be oriented directly towards
the equator, facing south in the northern hemisphere and north in the southern. Flat-plate
collectors have been built in a wide variety of designs and from many different materials. They
have been used to heat fluids such as water, water plus antifreeze additive, or air. The collector
should also have a long effective life, despite the adverse effects of the sun’s ultraviolet radiation,
corrosion and clogging because of acidity, alkalinity or hardness of the heat transfer fluid,
freezing of water, or deposition of dust or moisture on the glazing.
34
Figure 3.7: Flat plate collectors typically mounted on the roof
Compound Parabolic Collectors (CPC):
Compound parabolic collectors have the capability of reflecting to the absorber all of incident
radiation within wide limits. The necessity of moving the concentrator to accommodate the
changing of solar orientation can be eliminated by using the trough technology with two sections
of a parabola facing each other; this can accept radiations coming from a wide range of angles.
By using multiple internal reflections, any radiation that is entering the aperture, within the
collector acceptance angle, finds its way to the absorber surface located at the bottom of the
collector. The absorbers can be cylindrical or flat as shown in the figure 3.8.
Figure 3.8: Schematic Diagram of CPC collector. [20]
35
These collectors are normally used as linear or trough type collectors. The orientation of the
collector is related to the acceptance angle. Depending on this acceptance angle the collector can
be stationary or tracking.
Evacuated Tube Collectors: Evacuated heat pipe solar collectors (tubes) consist of a heat pipe
inside a vacuum-sealed tube, Evacuated tube collectors have demonstrated that the combination
of a selective surface and an effective convection suppressor can result in good performance at
high temperatures. The vacuum envelope reduces convection and conduction losses, so the
collectors can operate at higher temperatures (~150°C). Both direct and diffuse radiation can be
collected A type of solar collector that can achieve high temperatures, in the range 170°F (77°C)
to 350°F (177°C) and can, under the right set of circumstances, work very efficiently. Evacuated-
tube collectors are, however, quite expensive, with unit area costs typically about twice that
of flat-plate collectors. They are well-suited to commercial and industrial heating applications and
also for cooling applications (by regenerating refrigeration cycles). They can also be an effective
alternative to flat-plate collectors for domestic space heating, especially in regions where it is
often cloudy. For domestic hot water heating, flat-plate collectors tend to offer a cheaper and a
more reliable option. An evacuated-tube collector consists of parallel rows of glass tubes
connected to a header pipe. Each tube has the air removed from it to eliminate heat loss through
convection and radiation.
36
Figure 3.9: Schematic Diagram of an Evacuated Tube Collector [20]
Another type of collector developed recently is the integrated compound parabolic collector
(ICPC). This is an evacuated tube collector in which at the bottom part of the glass tube a
reflective material is fixed. The collector combines the vacuum insulation and non-imaging
stationary concentration into a single unit. For high temperature applications, a tracking ICPC
may be used
(b). Current Market Status Of Non-Electric/Non Concentrating Solar Thermal Technology: [54]
The total area of installed solar collector is about 185GW by early 2010. Three types of solar
collector are presently in market glazed, unglazed and evacuated. By the end of 2009, of the total
installed capacity of 172.4 GW, 32% was glazed flat-plate collectors; 56% was evacuated tube
collectors; 11% was unglazed collectors; and the remaining 1% was glazed and unglazed air
collectors. The use of solar thermal non-electric technologies varies greatly in scale as well as
type of technology preferred. For instance, the market in China, Taiwan, Japan and Europe is
dominated by glazed flat-plate and evacuated tube water collectors. On the other hand, the North
37
American market is dominated by unglazed water collectors employed for applications such as
heating swimming pools
3.2. Overall Comparision of Solar Energy Technologies
Advantages of
Solar Energy
Technologies
Disadvantages of
Solar Energy
Technologies
Current Market
Status
Future
Prospective
Photovoltai
c Systems
1. It is quiet and
clean energy
Source.
2.occupies less
space,
3.has a long life
span of 20-25
years so can be
used in space
where repairs
can be nearly
impossible
1. It consist of toxic
materials like
cadmium which
might have a minor
impact on the
environment.2.they
are expensive due in
part of
manufacturing cost
and due the
conversion
efficiencies of the
engines.3. it is a
variable energy
source as its energy
production depends
on Sun
There are two
types in the
market grid
connected and
off grid
connected. In
the present
market
condition the
off grid
application has
been overtaken
by grid
connected PV
but still in a few
countries like
India and China
are still in favor
of off grid
system.
emphasis should
be on
developing cost-
effective
Manufacturing
technologies.
work being
done to improve
cell efficiencies
by
concentrating
sunlight and
using multi-
junctions using
nano-
technology to
increase the
range of places
where solar PV
can be used
Low
Temperatur
e Solar
Thermal
Application
These
applications
saves energy,
time, occupies
less area,
improves
product quality,
makes the
process more
efficient and
protects the
environment
These can operate
only in the morning
and might need a
back up support to
provide energy in the
night time.
These tend to be
used
traditionally in
developing
countries. Many
technological
advances have
been made in
design of ‘solar
buildings’ in
developed
countries during
the last two
decades but
again the level
of technology is
often high and
Development in
terms of solving
some of the key
issues includes
cost reduction,
higher quality,
aesthetics and
building
integration.
38
expensive and
out of reach for
rural
communities in
developing
countries
Medium
Temperatur
e Solar
Thermal
Application
s
Built in a wide
variety of
designs and
from many
different
materials and a
long effective
life.
These are stationary
and cannot track the
sun therefore they
need a backup
support to provide
energy in night
times.
Three types of
solar collector
are presently in
market glazed,
unglazed and
evacuated, of
which 32% was
glazed flat-plate
collectors, 56%
was evacuated
tube collectors;
11% was
unglazed
collectors and
the 1% was
glazed and
unglazed air
collectors
Initial RD&D
efforts will be
directed towards
the: control of
heat loss; and
maximization of
energy
collection.
3.5. SUMMARY:
As discussed previously solar energy is the most abundant renewable energy resource available
and in most of the regions its potential is far in excess compared to the current total primary
energy supply. Solar energy technologies could help address energy access to rural and remote
communities help improve long-term energy security and help greenhouse gas mitigation. Table
3.2 shows the overall comparison of these solar technologies discussing their advantages and
disadvantages and their current and future market structure. The market for these technologies has
been dramatically increased over the past few decades. The fundamental barrier to increase in the
utilization of these solar technologies continues to be their cost. While the cost of energy from
many solar energy technologies remains high compared to conventional energy technologies, the
39
cost trend of solar energy technologies demonstrates rapid declines in the recent past and the
potential for significant declines in the near future. For instance the cost of PV declined over 80%
during the last two decades. The emerging technology known as concentrating solar power, or
CSP which has been discussed in next chapter, holds much promise for countries with plenty of
sunshine and clear skies. Its electrical output matches well the shifting daily demand for
electricity in places where air-conditioning systems are spreading. When backed up by thermal
storage facilities and combustible fuel, it offers utilities electricity that can be dispatched when
required, enabling it to be used for base, shoulder and peak loads. Continuation and expansion of
existing supports would be necessary for several decades to enhance the further deployment of
solar energy in both developed and developing countries, given current technologies and
projections of their further improvements over the near few decades. The future projections for
solar energy technologies are broadly optimistic. The market for solar energy technology is
expected to grow significantly in the long-term as well as short-term. Further, despite its technical
and economic limitations at present, it is expected that solar energy will play an important role in
the transportation sector in the future.
40
CHAPTER 4
CONCENTRATED SOLAR THERMAL POWER
(HIGH TEMPERATURE APPLICATIONS)
[1, 3, 4, 6, 7, 9, 10, 12, 13, 15, 20-21, 24-27, 34, 37-43, 49, 50, 54]
4.1 INTRODUCTION:
Concentrated solar thermal energy is a power generation technology which harnesses sunlight
using mirrors and lens, these mirrors collect sunlight and use this energy to heat the fluid and
generate steam this steam is used to drive the turbine and generate power just like in a
conventional power plant. For example a turbine fed from parabolic trough might require steam at
750K and reject heat into the atmosphere at 300K thus having an ideal thermal efficiency of about
60% with an intelligent management of heat waste the overall system efficiency of about 35% is
feasible. The solar heat can be collected with different types of concentrating solar power
technologies to produce high temperatures and generate steam to drive conventional power cycles
like Rankine, Brayton and Stirling. During the day time while generating power the sun’s energy
can be used and a part of it can be stored generally in a phase change medium such as molten salt.
The stored heat can be used to generate power during the night time. A simple schematic diagram
is shown below (Figure 4.1) which describes the main elements of a CSP
41
Figure 4.1: Main Components of CSP System [15]
In general CSP are made up of different collectors, power cycles and different power storage
systems. The market and application of CSP dictate the category of the system and its
components, in general the CSP are categories according to the size of the system-low(<100 kW),
medium(<10MW), and high(<10MW).CSP processes heat like any other conventional power
plant as such that the plant efficiency depends on the operating temperatures of the system.
Therefore, the useful energy produced will depend on the solar field collection and the power
cycle efficiencies, as illustrated in Figure 4.2. The efficiency of a solar collector field is defined
as the quotient of usable thermal energy versus received solar energy. The power generation
subsystem efficiency is the ratio of net power out to the heat input
42
Figure4.2: CSP System Efficiency Variation with Operating Temperature. [15]
CSP has been under investigation for several decades despite being a simple scheme of using
mirrors and collecting heat which in turn is being used as a power to drive a turbine generating
electricity this method involves several steps that can each be implemented in a number of
different ways. The chosen execution method of every stage in solar thermal power production
must be optimally matched to various technical, economic and environmental factors that may
favor one approach over another. Extensive research is being carried out on the solar collector
type, material and structures. The progress made in every aspect of CSP directs towards
increasing the efficiency of power production, and also being affordable when compared to near
future fossil fuel derived power.
Unlike traditional power plants CSP provide an environmental friendly source of energy produces
no emissions, and requires no fuel other than sunlight. About the only impact CSP plants have on
43
the environment is land use. Although the land used by CSP is greater than the fossil fuel, still
both the plants use almost same amount of land because fossil fuels plants use additional land for
mining and exploration as well as road building to reach the mines.
Other benefits include low operating cost and the ability to produce power during high demand
energy periods thus increasing our energy security. As they can store energy they can operate
even in the night times and in the cloudy days too. When CSP is combined with a fossil plant
forming a hybrid system they can operate round the clock regardless of any weather conditions.
CSP technologies require a very large Direct Normal Incidence unlike PV systems that can use
diffuse, scattered irradiance. The Solar Electricity Generation Plant in the southwest dessert of
California has considerably high Direct Normal Incidence which makes impressive cost
reduction. These parabolic trough power plants have been operating over three decades providing
valuable data, in order to further reduce the Levelized electricity cost by more advanced concepts,
improved plant operation and proper maintenance.
44
Figure 4.3: Levelized Electricity Cost (cents/kWh) Projections of CSP.[15]
Because of rapid developments occurring both in technology and electricity market strategies.
Because of rapid developments occur both in technology and electricity market strategies; CSP
has the greatest potential of any single renewable energy area. It also has significant potential for
further development and achieving low cost because of its guaranteed fuel supply (the sun).
4.2 SOLAR RADIATION:
The potential to have a CSP plant in any geographical location is determined by its solar radiation
characteristics. The power of electromagnetic radiation per unit area incident on a surface is
called irradiance. When integrating irradiance over a certain time period it becomes solar
irradiation. The solar radiation energy received on a given surface area over a course of a day is
called solar insolation. Solar radiation consists of direct beam and diffuses scattered components.
The term “global” solar radiation simply refers to the sum of these two components. The daily
45
variation of the different components depends upon meteorological and environmental factors
(e.g. cloud cover, air pollution and Humidity) and the relative earth-sun geometry.
Figure 4.4: Solar Irradiance variation within a day measured on a flat plate positioned
horizontal and tracking the sun and direct normal irradiance (DNI). [15]
The direct normal irradiance (DNI) is synonymous with the direct beam radiation and it is
measured by tracking the sun throughout the sky. Figure 4.4 shows an example of the global solar
radiation that is measured on a stationary two flat plate and a plate that is tracking the sun. The
measured DNI is also included and its lower value can be attributed to the fact that it does not
account for the diffuse radiation component.
In CSP applications, the DNI is important in determining the available solar energy. It is also for
this reason that the collectors are designed to track the sun throughout the day. Figure 4.5 shows
the daily solar insolation on an optimally tilted surface during the worst month of the year around
46
the world. Regions represented by light and dark red colors are most suitable for CSP
implementation.
Figure 4.5: The solar insolation (kWh/m2/day) on an optimally tilted surface during the
worst month of the year. [15]
4.3 DIRECT NORMAL INSOLATION:
Extraterrestrial solar radiation follows a direct line from the sun to the Earth. Upon entering the
earth’s atmosphere, some solar radiation is diffused by air, water molecules, and dust within the
atmosphere. The direct normal insolation represents that portion of solar radiation reaching the
surface of the Earth that has not been scattered or absorbed by the atmosphere. The adjective
“normal” refers to the direct radiation as measured on a plane normal to its direction. For more
practical purpose a time average of the direct insolation is considered over the course of the year.
47
This takes into account the absence of sunlight during the night, increased scattering in the
morning and evening hours as well as the seasonal variations that take place.
4.4. CONCENTRATED SOLAR TECHNOLOGIES: [15]
In a nutshell concentrating solar technologies are environmental friendly technology and has
emerged as a promising technology for electricity generation.CSP plant produces electricity in a
similar way to conventional power station the only difference is that CSP obtain their energy
input by concentrating solar radiation and converting it to the high temperature steam or gas to
drive a turbine, unlike PV cells or Flat plate solar thermal plant uses the diffuse part of solar
irradiation which results from scattering of the direct sunlight by clouds, particles, or molecules in
the air.
The process of energy conversion consists of two parts:
• The concentration of solar energy and converting it into usable thermal energy
• The conversion of heat into electricity this is generally realized by a conventional steam
turbine (Rankine cycle).
Because of the apparent movement of the sun across the sky, conventional concentrating
collectors must follow the sun's daily motion. There are two methods by which the sun's motion
can be readily tracked
a. Line-focusing systems or One Axis Tracking Mechanism: such as the parabolic trough
collector (PTC) and linear Fresnel collector. These systems track the sun position in one
dimension (one-axis-tracking), Line focus is less expensive, technically less difficult, but not as
efficient as point focus. The basis for this technology is a parabola-shaped mirror, which rotates
on a single axis throughout the day tracking the sun.
48
Figure 4.6: Line Focusing Systems: Left: Parabolic trough collector; Right: Linear Fresnel
Collector [34]
b. Point-focusing systems or Two Axis Tracking Mechanism: such as solar towers or solar dishes.
These systems realize higher concentration ratios than line-focusing systems. Their mirrors track
the sun position in two dimensions (two axis-tracking). Point focus technique requires a series of
mirrors surrounding a central tower, also known as a power tower. The mirrors focus the sun’s
rays onto a point on the tower, which then transfers the heat into more usable energy.
Figure 4.7: Point Focusing Systems: Left: Solar Tower Plant PS10, 11 MW in Seville, Spain;
624 so-called heliostats, 120 m2 each, focus the sunlight onto a receiver on top of a 100 m
high tower (Abgengoa, 2010); Right: Dish Stirling Prototype Plant of 10 kW each in
Almeria, Spain; diameter 8.5m (DLR, 2010). [34]
49
The four main types of concentrating solar collectors are
(1) Parabolic trough collectors;
(2) Heliostat field collectors;
(3) Linear Fresnel reflectors; and
(4) Parabolic dish collectors.
Figure 4.8: Types of CSP Technology. [15]
4.4.1 PARABOLIC TROUGH COLLECTOR TECHNOLOGY: [39]
4.4.1 (a) Introduction:
Parabolic trough is the most proven and lowest cost solar plant available today; primarily because
of the nine large commercial scale solar power plants that are being operated in California desert.
Concentrating solar power (CSP) plants which produce electricity using the thermal energy
collected from a series of concentrating solar collectors. This thermal energy drives a
50
conventional Rankine steam power cycle to produce electricity. Parabolic trough power plant
consists of large fields of parabolic trough collectors, a heat transfer fluid/ steam generation
system and optional thermal storage system and/ or a fossil fuel backup system. The parabolic
shaped mirrors are constructed by forming a sheet of reflective material into parabola shape that
concentrates incoming sunlight onto a central receiver tube at the focal line of the collector. The
arrays of mirrors can be 100 meters (m) long or more, with the curved aperture of 5 m to 6 m.
The collector field is made up of a large field of single-axis-tracking parabolic trough solar
collectors.
Parabolic trough collector technology design is modular in nature and comprises of many
parallels rows of collectors generally set in north south horizontal axis. As shown in Figure 4.9
Figure 4.9: A Typical Parabolic Trough System. [55]
Each solar collector has a parabolic shaped reflector that focuses the sun radiation directly on to
the heat collection element that runs through the focal line of each trough. The collectors track the
sun from east to west just to make sure that the sun is continuously focused on the linear receiver.
51
The receiver comprises the absorber tube 70 mm diameters which is coated with black chrome or
a selective ceramic material/metal (cermets) surface coating inside an evacuated glass envelope as
shown in Figure 4.10. The absorber tube is generally a coated stainless steel tube, with a
spectrally selective coating that absorbs the solar (short wave) irradiation well, but emits very
little infrared (long wave) radiation. This helps to reduce heat loss. Evacuated glass tubes are used
because they help to reduce heat losses.
The focused radiant energy is absorbed through the heat collecting element and transferred to heat
transfer fluid which is either water or liquids like synthetic oil such as a mixture of biphenyl and
diphenyl oxide that is pumped through each HCE tube. The heated HTF is pumped back to the
power plant, where it becomes the thermal resource for steam generation in the power cycle
Figure 4.10: Schematic Diagram of Parabolic Trough Collector. [20]
Within the power cycle portion of the plant, the hot HTF is piped through a series of counter flow
heat exchangers that transfer the thermal energy from the HTF to a feed water stream to produce
superheated steam. This steam serves as the working fluid in a conventional Rankine power
cycle. Steam is condensed at the bottom of the cycle through a water cooled condenser and
pumped back through a series of feed water heaters to the cycle’s steam generator. The heat
52
absorbed by the condenser water is rejected to the environment through an induced draft cooling
tower.
Figure 4.11: Typical Schematic Diagram of SEGS Plant. [39]
Nine Solar Electric Generating Systems (SEGS) were built in the Mojave Desert in southern
California between 1984 and 1990. The first two SEGS plants (SEGS I and SEGS II) were built
in Daggett, CA, between 1984 and 1985, and are rated at 14 [MWe] and 30 [MWe], respectively.
A power park of five
SEGS plants (SEGS III through VII), rated at 30 [MWe] each, was then assembled in Kramer
Junction, CA, between 1986 and 1988. The final two SEGS plants (SEGS VIII and IX) are each
rated at 80 [MWe] and were built in Harper Lake, CA, between 1989 and 1990. All nine SEGS
plants were designed, built, and sold by Luz International. All of the SEGS plants are still in
operation today and, collectively, they generate a combined peak power of 354 [MW]. A portion
53
of the solar field for one 30 MWe SEGS plant is shown in f igure b elow . The SEGS plants
also include an ancillary natural gas fired boiler, which may be used to supplement solar steam
production (up to 25%). The levelized cost of electricity from the SEGS plants was estimated at
$0.14/kWh in 2002.
Figure 4.12: Parabolic Trough at 30MWe (net) SEGS Plant in Kramer Junction, CA. [55]
The existing parabolic trough plant has been designed to use solar energy as a primary source of
energy source to produce electricity. Given sufficient solar input, the plants can operate at full
rated power using solar energy alone. During the summer times the plant operates on 10-12
hr/day on soar energy in order to achieve the full rated electric output during the cloudy days or
nighttimes the plant have been designed as hybrid solar/fossil plants i.e., a backup fossil fired
capability can be used to supplement the solar output during the days with low solar radiation. In
54
addition, thermal storage can be integrated into the plant design to allow solar energy to be stored
and dispatched when power is required.
The parabolic trough produces power based on conventional Rankine cycle which is widely used
for steam power cycle. The cycle collects the superheated steam from the parabolic trough field
this superheated vapor expands to lower pressure values in the steam turbine that drives the
generator to produce electricity. The turbine exhaust steam is then condensed and recycled as the
feed water for the superheated steam generation to begin the cycle again. The SEGS system
experiences show that the plant’s conversion from solar compared to the efficiency of fossil fuels
but when we compare both the technologies the operating and maintenance cost for SEGS plant
are negligible due to the absence of any fuel costs, thus making the LEC largely electric
efficiency ranges from 10-14% which is less when depend on capitol costs.
The figure 4.13 shows the major breakdown of investment on a typical parabolic trough plant that
operates on Rankine Cycle. We can see that the majority of the initial investments are associated
to the solar field. But in recent time much progress has been made recently with the introduction
of lightweight space frame structure designs and the development of efficient highly reflective
film13, such as ReflecTechTM
and 3M’s new solar mirror film.
55
Figure 4.13: Typical Cost breakdown of a Parabolic Trough SEGS plant. [15]
The heat transfer fluid (HTF) system moves the heat from the solar field to the power block and
it requires an HTF with the following properties: high temperature operation with high thermal
stability, good heat transfer properties, low energy transportation losses, low vapor pressure, low
freeze point, low hazard properties, good material compatibility, low hydrogen permeability of
the steel pipe and economical product and maintenance costs. As a result, synthetic organic HTFs
are most suitable for the parabolic trough plants. The last of the major components is the power
block, which consists of a conventional steam turbine based system, the costs of which are well
established and a number of new players from China and India have made the prices quite
competitive. Any significant reduction in the cost of any of these three major components will
result in a lower LEC for CSP systems
One important advantage of solar thermal power plants is that they can operate with
other means of water heating and thus a hybrid system can ensure security of supply. During
periods of insufficient irradiance, a parallel burner can be used to produce steam. Climate-
compatible fuels such as biomass or h y d r o g e n p r o d u c e d b y r e n e w a b l e e n e r g y
c a n a l s o f i r e t h i s p a r a l l e l b u r n e r .
56
4 . 4 . 1 . (b) Working Principle of Parabolic Trough Technology: [ 5 4 ]
The reflecting surface of parabolic trough collectors, also called linear imaging concentrators,
has a parabolic cross section. The curve of a parabola is such that light travelling parallel to the
axis of a parabolic mirror will be reflected to a single focal point from any place along the curve.
Because the sun is so far away, as shown in the figure 4.14 all direct solar beams (i.e., excluding
diffuse) are essentially parallel so if the parabola is facing the sun, the sunlight is concentrated at
the focal point. A parabolic trough extends the parabolic shape to three dimensions along a single
direction, creating a focal line along which the absorber tube is run.
Figure 4.14: Parallel Sun Rays being concentrated onto the focal line of the collector. [54]
Parabolic trough collectors like other solar concentrating systems have to track the sun. The
troughs are normally designed to track the sun along one axis oriented in the north-south or east-
west direction as shown in figure 4.15. As parabolic troughs use only direct radiation, cloudy
skies become a more critical factor than when using flat-plate collectors, which can also use
57
diffuse sunlight. Periodic cleaning of mirrors also is essential to assure an adequate parabolic
trough field performance.
Figure 4.15: Tracking of Sun rays by Parabolic Trough Collectors with a Collector axis
oriented north south. [54]
4.4.1. (c) Solar Collector Technology:
a.) Luz Collectors: The Luz International LTD., established in 1979, designed three generations
of parabolic trough collectors LS-1, LS-2, LS-3 installed in SEGS plant. The first two generations
LS-1 and LS2 consist of similar assemblies mounted on a structure of similar length the structure
is based on a rigid structural support tube, called the torque tube which supports the steel tubes to
which the parabolic troughs are connected. In the LS-3, the torque tube is replaced by a metal
lattice framework, the aperture width is 14% wider than the LS-2 and collector length is doubled.
Changes were made in the pedestal and reflector supports, and the collectors are positioned by a
hydraulic control system instead of the mechanical gear and cable system used in the LS-2. LS-3
collector design makes use not only of previous Luz power plant experience (SEGS-I to SEGS-
VI), but also mass production, cost and performance requirements. However, SEGS plant
operating experience shows that any benefit to cost has been clearly offset by associated
performance and maintenance issues.
58
The Heat Transfer Element is made up of stainless steel with a special coating sealed in a vacuum
tight glass tube. The outer tube is low-iron glass (max. 0.015%) and has an anti-reflective coating
on both sides to maximize solar transmission. Hydrogen traps, often referred to as passive
vacuum pumps, are installed in the vacuum cavity to absorb the hydrogen which migrates slowly
across the steel tube. The selective coating material used in LS1 and LS2 is made up of black
chrome where as for LS3 a new ceramic metal layer is used. The glass is given its parabolic shape
by heating it on accurate parabolic moulds in special ovens. Ceramic pads are cemented with a
special adhesive to the back of the reflectors for mounting to the support structure.
In 1991 Luz filed for bankruptcy and in 1992, Solel Belgium (nowadays Solel Solar Systems
Ltd.) purchased Luz manufacturing assets, providing a reserve for the Luz collector technology
and key collector components. Before the demise of Luz, the company had designed a fourth
generation collector, the LS-4, with the intention of studying DSG inside the absorber tubes. The
LS-4 collector had a 10.5-m aperture width (almost double the LS-3), 49 m total length and
absorber outer diameter of 0.114 m. Working fluid temperature and pressure foreseen were
400 °C and 10 MPa, respectively. Two special features were that the absorber tube was tilted 8°
and did not move, because at that time, no ball joints were able to work at such pressures
(d). Option Of Thermal Energy Storage Systems For Parabolic Trough Collectors:
One main advantage of solar thermal power plants over other renewable power technologies, such
as photovoltaic and wind energy converters, is the option of energy storage. Unlike the storage of
electric energy, thermal energy storage is practically and economically feasible already today,
even in large-scale applications. Solar thermal power plants can be equipped with thermal energy
storage with a full-load storage capacity in the range of several hours.
59
Usually, the storage is filled during the day, and emptied again after sunset, so that electricity is
still produced even after sunset. This allows for plant operation in concordance with load
requirements from the grid, because in many countries there is an electricity demand peak after
sunset. During such demand peaks, electricity prices are usually far higher than base-load prices,
creating a very important added value of CSP and storage. Various thermal storage technologies
are in principle feasible for solar thermal power plants, based on different physical mechanisms
(such as sensible heat storage, latent heat storage, and chemical energy storage), and by applying
different types of storage materials (such as molten salt, oil, sand, and concrete).
The storage material needs to be cheap, because large quantities are required. It should also be
noted that different heat transfer fluids (HTFs) used in the solar field require and allow different
storage options. Thermal storage is in principle applicable not only to parabolic trough power
plants, but also to the other CSP technologies. However, the only power plants that are in
operation today using thermal storage are the Andasol power plants shown in Figure. The
Andasol plants use a two-tank molten salt storage; the working principle is it stores heat by
heating up a medium (sensible heat storage).When loading the storage, the hot heat-transfer fluid,
coming from the solar field, passes through a heat exchanger and heats up the molten salt. In turn,
the storage is unloaded by transferring the heat from the salt back to the heat-transfer fluid. Many
operation strategies are feasible for the operation of the plant and the storage. The most common
one is to feed primarily the turbine directly with the heat from the solar field. Whenever excess
solar heat is available, it is stored. Other options may also aim at storing the solar energy from the
morning hours instead of directly converting it into electricity, and thereby using the storage for
shifting rather than for maximizing the plant’s operational hours.
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(e). System Applications, Benefits and Impacts of Parabolic Trough Technology: [39]
1. The primary application for parabolic trough power plants is large scale grid connected power
application in the 30 to 300 MW range. Because the technology can be easily hybridized with
fossil fuels, the plant can be designed to provide film peaking to intermediate load power. The
plants are a good match for applications where the solar radiation resources correlate closely with
the peak electric power demands in the region
2. The domestic market opportunity for parabolic trough plants is in the south western deserts
where the best DNI solar resource exists. These regions also have peak power demands that could
benefit from parabolic trough technologies. All of the existing Luz-developed SEGS projects
were developed as independent power projects and were enabled through special tax incentives
and power purchase agreements such as the California SO-2 and SO-4 contracts. However, with
utility restructuring, and an increased focus on global warming and other environmental issues,
many new opportunities such as renewable portfolio standards and the development of solar
enterprise zones may encourage the development of new trough plants.
3. With the high demand for new power generation in many developing countries, the next
deployment of parabolic troughs could be abroad. Many arid regions in developing countries are
ideally suited for parabolic trough technologies. India, Egypt, Morocco, Mexico, Brazil, Crete
(Greece), and Tibet (China) have expressed interest in trough technology power plants.
Benefits of PTR:
1. The parabolic trough technology is the only one which generates electricity at the lowest
cost sources available. They are backed by considerable valuable operating experience.
This will be the least cost solar option for another few more years until other technologies
are developed.
61
2. Trough plants generate their peak output during sunny periods when air conditioning
loads are at their peak. Integrated natural gas hybridization and thermal storage have
allowed the plants to provide firm power even during non-solar and cloudy periods.
3. Trough plants reduce operation of higher-cost, cycling fossil generation that would be
needed to meet peak power demands during sunny afternoons at times when the most
photochemical smog, which is aggravated by NOx emissions from power plants, is
produced.
4. Construction of trough technology has a huge positive impact on today’s economy most
of the part are local made. Also the trough plant requires very less labor during
construction and operation.
Impacts:
1. The HTF used is Monsanto Therminol VP-1 although it is considered to be non
hazardous in US but it is considered to hazardous in California. In additions to the liquid
spills, few vapor emissions during normal operations.
2. Water availability can be significant issue in arid regions which is best suited for trough
plants. Another issue is the waste water coming out of the plant. This water coming out of
the plants as it must be sent to the evaporation pond.
3. Parabolic trough plants require a significant amount of land that typically cannot be used
concurrently for other uses. Parabolic troughs require the land to be graded level. One
opportunity to minimize the development of undisturbed lands is to use parcels of
marginal and fallow agricultural land instead.
4. Solar fossil hybrid plants do operate with fossil fuels during some periods. During this
time the plant will generate emissions consistent with fuel consumption.
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4.4.2. LINEAR FRESNEL COLLECTOR TECHNOLOGY:
This is a single axis tracking technology but differs from a parabolic trough in that the absorber is
fixed in space above the mirror field and the reflector is composed of many long row segments
which focus collectively on an elevated long tower receiver running parallel to the reflector
rotational axis. The first such array was a small one built by Francia (1961), but since then little
was done until the last few years when development began on two LFR designs in Australia and
Belgium.
Fresnel lenses are used as solar collectors where the reflector is composed of many long rows of
flat mirrors, which concentrate beam radiation directly on the receiver located at meters height
running parallel to the axis of rotation. Linear Fresnel collectors follow the same principles of
parabolic trough technology, but replace the curved mirrors with long parallel lines of flat or
slightly curved shape mirrors. A representation of an element of an LFR collector field is shown
in Figure 4.16 shown below.
Figure 4.16: Schematic Representation of Linear Fresnel Solar Collectors. [20]
63
In the field of CSP applications Fresnel collectors are one of the best choices because of
its advantages such as small volume, light weight, mass production with low cost. These
were first discovered in 1822 by Augustin Jean Fresnel, the early Fresnel lens is first made up of
glass. Glass is an attractive option when lenses are to be used at high temperatures or when they
are used for glazing.
Figure 4.17: The effect of storage on utility load during a typical day. [15]
However polymethylmethacrylate (PMMA) which is a light-weight, clear, and stable polymer
with optical characteristics nearly same as that of glass, serves as a suitable material for the
manufacturing of Fresnel lenses. Modern plastics, new molding techniques, and computer-
controlled diamond turning machines have improved the quality of Fresnel lenses and have
opened new horizons for the design of Fresnel lenses for solar energy concentration applications.
Fresnel lenses can be pressure-molded, injection-molded, cut, or extruded from a variety of
plastics and the production costs for large outputs are considerably low [50].
The main advantage of Linear Fresnel is its lower investment and operational costs. The flat plate
mirrors are cheaper and easier to produce when compared to parabolic trough technology (which
will be discussed later) and are readily available. The structure has a low profile with mirrors over
64
one or two meters above the ground this means that the plant can operate in strong winds. It can
be a viable solution because of it light weight and simple collector structure. Although the
technology is much simpler and a cost effective option it is not been tested enough to determine
its viability as an alternative to parabolic trough [40].
The main difficulty of Linear Fresnel reflector technology is avoidance of shading of incoming
solar radiation and blocking of reflected solar radiation by adjacent reflector. Blocking can be
removed by increasing the height of the absorber towers, but this increases cost. Compact linear
Fresnel reflector (CLFR) technology has been recently developed at Sydney University in
Australia. This is in effect a second type of solution for the Fresnel reflector field problem which
has been overlooked until recently.
Figure 4.18: Schematic Diagram showing interleaving of mirrors in a CLFR with reduced
shading between mirrors. [20]
The classical LFR model has only one receiver and therefore there is no choice given for the
direction or orientation of the reflector. Therefore if the linear absorbers are close enough, each
reflector will have an option of directing reflectors solar radiation to at least two absorbers. This
65
provides the means for much more densely packed arrays, because patterns of alternating
reflector orientation can be such that closely packed reflectors can be positioned without shading
and blocking. The arrangement minimizes beam blocking by adjacent reflectors and allows high
reflector densities and low tower heights to be used. Close spacing of reflectors also reduces land
usage. The avoidance of large reflector spacing and tower heights is an important cost issue when
the cost of ground preparation, array substructure cost, tower structure cost, steam line thermal
losses and steam line cost are considered [27]. For commercial power production (greater than 1
MW scale), it is very reasonable to have multiple receivers, and thus the CLFR design is very
useful without incurring extra costs, especially in areas where land is limited.
A very useful addition to the CLFR design is the inverted cavity receiver attached to the planner
array of boiling tubes as shown in the figure [4.19]. This structure allows the plant operation
using direct steam generation process (which will be discussed later), this design consideration of
inverted cavity receiver indicates that this design bypasses the receiver thermal uniformity
challenges faced by the parabolic trough direct steam generation technology.
Figure 4.19: Schematic diagram of inverted air cavity receiver. [14]
66
Linear Fresnel collectors seem to be more open for redesign and adaptation to local conditions.
Local content is probably higher than for the parabolic trough due to the simpler components. All
commercial Fresnel collectors use pressurized water / steam as an environmentally friendly heat-
transfer fluid. A power plant with direct steam generation thus requires fewer heat exchangers
than the one using HTF thermal oil. Another innovative design to further limit wasted solar
radiation in a CLFR with a purpose of blocking or shading while maximizing the field layout
density this design proposes the reformation of platform on which reflectors are resting into a
wave form as shown in figure 4.20
To summarize LFR technology offers many advantages of parabolic trough collectors systems
while incurring smaller reflector costs. It too can be easily coupled to direct steam generation as
well as molten salts for thermal energy transport. The costs is further reduced due to central
receiver regime it incorporates , but tags on the challenge of maximizing the amount solar
radiation that can be collected. Innovation in receiver design and reflector organization has made
LFR relatively inexpensive in comparison with other CSP technologies. It readily couples to
thermal storage methods and numerous applications
Figure4.20: Wave platform structure for a CLFR system allows maximization of
solar radiation collected from a given area. [4]
67
4.4.3. PARABOLIC DISH COLLECTOR TECHNOLOGY:
(a) Introduction:
This is a point focus collector that tracks the sun in two axes, concentrating solar energy onto a
receiver located at a focal point of the dish. The dish structure fully tracks the sun and sends it to
the thermal receiver. The receiver absorbs solar radiation and converts it into thermal energy in a
circulating fluid. This thermal energy can either be directly converted into electricity using a
generator that is directly converted to receiver or it can be transported to power conversion
system in order to increase the efficiency of this design they must have dual tracking mechanisms
so that the dish aperture is always normal to the incoming solar radiation.
Parabolic dish technology can achieve very high temperatures up to 15000c because the receivers
are distributed throughout the collector field just like the parabolic trough collector. The greatest
challenge faced by distributed dish systems is developing a power conversion system which is
low capital and maintenance cost, long life, high efficiency and automatic operations. Several
different engine have been used such as Gas turbine, reciprocating steam engine, organic Rankine
engine have been used. But in recent year Stirling engine is often being used for these
applications as its performance is better at temperatures below 9500c. At high temperatures
combined cycle gas turbines must be used to achieve higher efficiency.
68
Figure4.21: Schematic Diagram of Parabolic Dish Collector. [20]
Due to its high concentration ratio the dish technology can also be used for concentrated
photovoltaic’s the large solar flux concentrated on a small area can produce enough power to
reduce the high capital investment required. These PV cells are very costly yet they are very heat
resistant and perform better under high concentration ratio. Adding such PV cells to the dish
collector technology is fairly simple, and can give results that are comparable to or better
compared to heat engine systems, with much longer lifetime.
The Dish Stirling systems have highest efficiency when compared to other solar power generation
systems by converting nearly 31.25% of direct normal incident solar radiation into electricity
after taking into account of parasitic power losses. Therefore Dish Stirling is better compared to
parabolic trough in producing electricity at much cheaper rates and higher efficiencies. As the
Dish Stirling systems are modular they can be assembled into plants ranging for a kilowatt to 10
MW plant. Dish Stirling systems are applicable in areas where there is high direct normal
irradiance the so called sun belt areas like Mexico, Australia, Africa etc.,
69
Figure4.22: Stirling Dish Systems at Sandia National Labs. (Optics .org website)
Parabolic dish collectors are high cost devices; they are large mirrors with concavity to
effectively concentrate solar radiation. They are also very heavy and their tracking systems must
be very sensitive and finely tuned. In order to reduce the high collector cost an approximate
Parabolic dish with small mirrors can be manufactured. These small mirror arrangements
approximate a parabolic collector in a relatively inexpensive way.
Presently there are two types of Dish Stirling systems in market: the Euro Dish from Schlaich-
Bergermann und Partner (SBP) and the Sun Catcher Dish Stirling system developed by Stirling
Energy System. A refined design of Sun Catcher Dish Stirling system that is being used in
commercial scale deployments started in 2010. Innovation in parabolic dish reflector technology
has promoted this highly efficient yet expensive technology towards the goal of being reasonably
affordable. Novel improvements in reflector structure and collector design continue to boost the
thermal efficiency of this concentrated solar power scheme. The use of a Stirling engine at a
PDC’s focus helps alleviate the losses and costs associated with heat transport. However, this
regime does not comply with thermal storage in a simple manner, a significant issue in the scope
of year-round power production. A possible solution for this problem can be using a
electrochemical battery that can provide power after sunset.
70
(b) System Application, Benefits and Impacts:
Dish Stirling has high efficiency, low cost, versatility and hybrid operations. Due to its versatility
and hybrid operations it is used in wide range of applications with high power ranges from
megawatts to gigawatt. Their ability to be quickly installed, their inherent modularity, and their
minimal environmental impact make them a good candidate for new peaking power installations.
Although these systems do not have cost effective storage systems, their ability to work with
fossil fuels and bio derived fuels makes them more dispatchable.
The Dish Stirling systems can be used individually to stand alone applications, although there
power ranges and modularity is ideal for standalone applications there are a few challenges in
maintenance and operation in remote environments. In addition, to enable operation until the
system can become self sustaining, energy storage (e.g., a battery like those used in a diesel
generator set) with its associated cost and reliability issues is needed. Therefore, it is likely that
significant entry in stand-alone markets will occur after the technology has had an opportunity to
mature in utility and village-power markets. The Dish Stirling is well suited for intermediate
applications as well.
Because the Dish systems use a heat engine the ability to use fossil fuels is possible. The use of
the same power conversion equipment, including the engine, generator, wiring, and switch gear,
etc., means that only the addition of a fossil fuel combustor is required to enable a hybrid
capability. For Dish/Brayton systems addition of a hybrid capability is straightforward. For
dish/Stirling systems, on the other hand, addition of a hybrid capability is a challenge. The
external, high temperature, isothermal heat addition required for Stirling engines is in many ways
easier to integrate with solar heat than it is with the heat of combustion.
The Dish Stirling has very minimal impact on environments. It has been known for being quiet,
relative to internal combustion gasoline and diesel engines, and even the highly recuperated
71
Brayton engines are reported to be relatively quiet. The biggest source of noise is the cooling fan
of the radiator. Emissions from dish/engine systems are also quite low. Other than the potential
for spilling small amounts of engine
Oil or coolant or gearbox grease, these systems produce no effluent when operating with solar
energy. Even when operating with a fossil fuel, the steady flow combustion systems used in both
Stirling and Brayton systems result in extremely low emission levels.
4.4.4. HELIOSTAT FIELD COLLECTOR OR POWER TOWER:
(a). Introduction: The most recent CSP technology to emerge into commercial utility was the
heliostat field collector design. This design is incorporated in a very few locations around the
world because of its expensive, powerful design. The 10MW solar one and solar two are the first
HFC plants to be built. The heliostat field collector design features a large array of flat mirrors
distributed around the central receiver mounted on a solar tower. The major components of this
design are heliostat field, the heliostat controls, the receiver, the storage system and the heat
engine system which drives the generator. This design must ensure that the radiation is delivered
to the receiver at the desired flux density at the minimum cost. Several receivers have been
considered and the cylindrical receiver has advantage when used with Rankine cycle engines.
Cavity receivers with large tower are height to heliostat field area ratios are used for higher
temperatures required for the operation of Brayton cycle turbines.
Each heliostat is on a two-axis tracking mount, and has a surface area ranging from 50 to 150m2.
Using slightly concave mirror segments on heliostats can increase the solar flux they reflect,
though this elevates manufacturing costs. Every heliostat is individually oriented to reflect
incident light directly on to the central receiving unit. Mounting the receiver on a tall tower
decreases the distance mirrors must be placed from one another to avoid shading. A fluid
72
circulating in a closed-loop system passes through the central receiver, absorbing thermal energy
for power production and storage. An advantage of HFC’s is the large amount of radiation
focused on a single receiver which minimizes heat loses and simplifies heat transport and storage
requirements. Power production is often implemented by steam and turbine generators. The
single-receiver scheme provides for uncomplicated integration with fossil-fuel power generators.
As the collectors represent the largest cost in the system an efficient heat engine is required to
obtain maximum conversion of the collected energy. Several thermodynamics cycles can be
considered like, Brayton or sterling gas cycle engines operated at the inlet temperature of 800-
10000c which provides high engine efficiencies. But are limited for low gas heat transfer
coefficient and by practical constrains on collector design imposed by the requirements of very
high temperatures.
Figure4.23: Schematic Diagram of Heliostat Field Collector. [20]
73
The integration of a solar reformer with a heliostat field array was proposed in2002.Solar
reforming of methane with steam or CO2 is an efficient chemical heat storage method. The
Syngas produced can be converted into electricity using a gas turbine or combined cycle. The
suggested reformer rests on the ground, and has a collector mounted above it. A solar reflector
tower is used to concentrate solar flux from heliostats on to the ground reformer. In this fashion,
the power producing unit can be separated from the concentrator field entirely.
Power tower systems currently under development use either nitrate salt or air as the heat transfer
medium. In the USA, the Solar One plant in Barstow, CA was originally a water/steam plant and
is now converted to Solar Two, a nitrate salt system. The use of nitrate salt for storage allows the
plant to avoid tripping off line during cloudy periods and also allow the delivery of power after
sunset. The heliostat system consists of 1818 individually oriented reflectors, each consisting of
12 concave panels with a total area of 39.13 m2, for a total array of 71 100 m
2. The reflective
material is back-silvered glass. The receiver is a single pass superheated boiler, generally
cylindrical in shape, 13.7 m high, 7 m in diameter, with the top 90 m above the ground. It is an
assembly of 24 panels, each 0.9 m wide and 13.7 m long. Six of the panels on the south side,
which receives the least radiation, are used as feed-water pre-heaters and the balance are used as
boilers. The panels are coated with a non-selective flat black paint which was heat cured in place
with solar radiation. The receiver was designed to produce 50 900 kg/h of steam at 516 8C with
absorbing surface operating at a maximum temperature of 620 0C.
Heliostat field collector technology has greatly improved over the last few decades, and continues
to draw much attention as a suitable scheme for large solar thermal plants. The exceedingly high
temperatures at which they operates it grant HFC plants excellent efficiencies, while allowing
them to be coupled to a variety of applications. The high capital investment necessary for the
74
construction of HFC systems is an obstacle, however, and further technological advancements in
efficiency must be accompanied by low cost materials and storage schemes for this CSP method
to become more economical
Recent research and development efforts have focused on polymer reflectors and stretched-
membrane heliostats. A stretched-membrane heliostat consists of a metal ring, across which two
thin metal membranes are stretched. A focus control system adjusts the curvature of the front
membrane, which is laminated with a silvered-polymer reflector, usually by adjusting the
pressure (a very slight vacuum) in the plenum between the two membranes. Stretched-membrane
heliostats are potentially much cheaper than glass/metal heliostats because they weigh less and
have fewer parts.
(B). System Application, Benefits and Impacts: [52]
As non-polluting energy sources become more favored, molten-salt power towers will have a
high value because the thermal energy storage allows the plant to be dispatchable. Consequently,
the value of power is worth more because a power tower plant can deliver energy during peak
load times when it is more valuable. Energy storage also allows power tower plants to be
designed and built with a range of annual capacity factors (20 to 65%). Combining high capacity
factors and the fact that energy storage will allow power to be brought onto the grid in a
controlled manner (i.e., by reducing electrical transients thus increasing the stability of the overall
utility grid), total market penetration should be much higher than an intermittent solar technology
without storage.
The availability of an inexpensive and efficient energy storage system may give power towers a
competitive advantage. Thermal-energy storage in the power tower allows electricity to be
dispatched to the grid when demand for power is the highest, thus increasing the monetary value
75
of the electricity. Much like hydro plants, power towers with salt storage are considered to be a
dispatchable rather than an intermittent renewable energy power plant.
One possible concern with the technology is the relatively high amount of land and water usage.
This may become an important issue from a practical and environmental viewpoint since these
plants are typically deployed within desert areas that often lack water and have fragile landscapes.
No hazardous gaseous or liquid emissions are released during operation of the solar power tower
plant. If a salt spill occurs, the salt will freeze before significant contamination of the soil occurs.
Salt is picked up with a shovel and can be recycled if necessary. If the power tower is hybridized
with a conventional fossil plant, emissions will be released from the non-solar portion of the
plant.
4.5. COMPARISION OF CONCENTRATED SOLAR POWER (CSP) TECHNOLOGIES: [49]
CSP technologies differ in a significant way from each other. Not only in terms of technical and
economical but also in terms of reliability, maturity and operational experience. Parabolic troughs
are the most widely commercially deployed CSP plant, but are not so matured and improvement
in performance and cost reduction are expected. Most of the parabolic trough collectors currently
used do not have thermal energy storage and only generates electricity during daylights hours.
Most CSP projects currently under construction or development are based on parabolic trough
technology, as it is the most mature technology and shows the lowest development risk.
Solar tower and linear Fresnel systems are only beginning to develop and there is a significant
potential to reduce prices and improve efficiency especially solar tower. However parabolic
trough systems are more promising with longer operational experience of utility-size plants;
represent a less flexible, but low-risk option today. There is increased interest in solar towers
using high temperature molten salt or other alternatives for synthetic oil as the HTF and storage
76
medium due to the potential for cost reduction, higher efficiency and extended energy storage
opportunities. This appears to be the most promising CSP technology for the future.
While the levelized cost of electricity (LCOE) of parabolic trough systems does not tend to
decline with higher capacity factors, the LCOE of solar towers tends to decrease as the capacity
factor increases. This is mainly due to the significantly lower specific cost (up to three times
lower) of the molten-salt energy storage in Solar Tower plants. CSP technologies offer a great
opportunity for local manufacturing, which can stimulate local economic development, including
job creation. It is estimated that solar towers can offer more local opportunities than trough
systems.
In the longer term, the ability to achieve higher operating temperatures may give tower
technologies efficiency and cost advantage versus parabolic troughs if new thermal cycle
technologies can be integrated with the power tower heat source. While utility-scale photovoltaic
systems are on a trajectory to achieve lower energy costs, the CSP cases presented here have
capacity factors 2-3 times greater than utility-scale PV systems. The advantage thermal energy
storage offers for reliability and dispatch flexibility is expected to allow these CSP technologies
to maintain a competitive edge with respect to PV systems. While these factors are minor at low
penetration, they become essential for renewable energy systems to achieve higher grid
penetration.
Table 4.1: Overall Comparision of Concentrated Solar Power Technologies:
Advantages Disadvantages Current
Status
Future Work Applicati
ons
Parabolic
Trough
commercially
available,
with 4500 G-
W-h
operational
experience,
lower
temperatures
restrict
output to
moderate steam
qualities
commercial size 80 MWe
units, total 354 MWe
operating; designs for
Cost
reduction and
greater
efficiencies,
expansion of
location
grid
connected
plants,
process
heat
77
hybrid
concept
proven, storage
capability
through
temperature
limits of oil
integration
with
combined
cycles
possibilities
and uses,
electricity
generation
throughout
the clock
Linear
Fresnel
It is simpler
and less
expensive.
Fluid
temperatures
are relatively
low compared
to other
technologies
A 177 MW
Linear
Fresnel solar
power plant is
scheduled to
begin
operations
soon in
California.
Are still in the
demonstration
phase and not
yet reached
the
commercial
market
steam
powered
electricity
generatio
n
Dish
Stirling
very high
conversion
efficiencies,
modularity,
hybrid
operation in
development
fossil back-up
not yet
proven, storage
a problem,
high cost an
issue,
development
has reached
prototype
stage
Test and
demo units:
stand-alone
systems 50
kWe and
farms 5
MWe;
commercial
status about
1998.
Study towards
system
hybridization,
system
amortization ,
Thermal
storage
system
stand
alone
applicatio
ns or
small
power
systems
Power
Tower
good long-term
perspective
for high
efficiencies
and storage
through high
temperatures,
hybrid
operation
possible.
capital cost
projections not
yet proven;
heliostats
require
very high
tracking
accuracy; air
receiver has
reached
prototype stage;
promising salt
receiver
system not yet
proven
Test and
demo units;
maximum 10
MWe;
commercial
status about
1999; designs
for
integration
with
combined
cycles.
Development
towards
advanced
receivers for
heating air
efficiently.
Efforts should
be put in
making this
option a more
cheaper
option than
conventional
sources
grid
connected
plants,
high
temperatu
re process
heat
4.6. CURRENT MARKET STATUS CSP: [46]
The CSP market first emerged in the early 1980s but lost pace in the absence of government
support in the United States. However, a recent strong revival of this market is evident with 14.5
78
GW in various stages of development across 20 countries and 740 MW of added CSP capacity
between 2007 and 2010 While many regions of the world, for instance, Southwestern United
States, Spain, Algeria, Morocco, South Africa, Israel, India and China, provide suitable
conditions for the deployment of CSP, market activity is mainly concentrated in Southwestern
United States and Spain, both of which are supported with favorable policies, investment tax
credits and feed-in tariffs. Currently, several projects around the world are either under
construction, in the planning stages, or undergoing feasibility studies6 and the market is expected
to keep growing at a significant pace.
4.7. APPLICATIONS: [4]
Concentrated solar power provides large variety of application in addition to the main objective
of electricity generations for which solar thermal energy can be harnessed. Industrial heat
processes, chemical production, salt-water desalination, heating and cooling are just a few
examples of the plethora of available applications that can be implemented using CSP
technologies. Some technologies require specific CSP design while some others can be coupled to
theses designs. In regions of the world where clean drinking water is scarce also have an
abundance of solar radiation, makes this CSP application worthwhile. Processes like Desalination
are generally done by evaporating salt-water to leave salt behind, then condensing salt free vapor
back into its liquid state. The process of heating large amounts of water for drinking and
agricultural purposes requires immense amount of energy. Concentrating solar radiation and
converting it to heat is an efficient method by which this process can be achieved using emission-
free, renewable energy.
The large amount of thermal energy that can be harvested using solar concentrators makes them a
lucrative option for integration with industrial heat processes. A substantial fraction of these
processes run below 300 0C, an operational temperature achievable by most solar concentrator
79
regimes. Solar power can be utilized for temperature control of buildings, providing both heating
and cooling mechanisms. A cascade of mini-dish collector and gas micro-turbine produces
electricity that drives a mechanical chiller, with turbine heat rejection running absorption chiller.
A special feature of this system is that energy can be stored compactly as ice. The compactness of
the solar mini-dish system is conducive for small-scale ultra-high-performance solar cooling
systems.
The utilization of Fresnel lenses was also suggested for lighting and temperature control of
buildings. A collection system using a Fresnel lens concentrator and a solar receiver generally
absorbs between 60% and 80% of incoming radiation. The remaining solar flux can be distributed
in the interior space for illumination and heating needs. On days when solar radiation is high, this
provides cooling of interior spaces as well as brightness control. During low solar intensity
periods, the absorber can be shifted off-focus to permit 100% of light to be distributed around the
interior. The receiver can be of PV type, thermal type or a hybrid of the two, and will collect solar
energy for heat and/or electricity generation. A parabolic trough collector system was constructed
to study the potential of this CSP regime in solar heating and cooling.
A great deal of work has also been done to develop small-scale, solar powered food (fruit,
vegetables and nuts) dryers that can be built with local materials. However, the existing dryer
designs are suited to cloudless, dry environments and they dry too slowly in hazy situations,
typical of many tropical developing countries. Excessively slow drying allows product
degradation caused by microbial decay, insects and naturally occurring enzymes. Some existing
designs are also expensive and relatively inefficient, and have low capacity. Adding a solar
concentrating surface increases the heat output of solar devices operating in cloudy or hazy
conditions. With indirect solar dryers this can be accomplished by adding glazed concentrated
solar panels to the system.
80
Concentrating solar panels can be used to inexpensively increase the heat output for indirect
dryers. Additionally, they can be used to focus a greater light flux onto the drying zone in direct
dryers, allowing them to operate in low-insolation environments. The reflective surfaces can be as
sophisticated as precision-machined, polished surfaces or as simple as cardboard covered in
aluminum foil. The development of a multitude of CSP applications is beneficial in many
regards; such applications help turn many carbon emitting industrial processes into ‘clean’ ones,
conserve large amounts of electricity that would otherwise be used up and promote a general
environmentally friendly approach to energy consumption for both industries and individuals.
Furthermore, the growing number of these applications aids CSP technologies in taking root,
increasing the demand for solar thermal power and advancing it into world markets.
4.8 SUMMARY:
Concentrating solar thermal power (CSP) is a proven technology, which has significant potential
for further development and achieving low cost. Concentrating solar power (CSP) is thermal solar
power that uses a means of magnifying or concentrating the effective radiation from the sun onto
a receiving device that collects the power so that it can be used directly as thermal energy or used
to generate electricity. CSP is the most developed of the solar technologies and is on the verge of
being competitive with conventional power plants. CSP technologies relatively low cost and
ability to deliver power during periods of peak demand mean that it has the potential to be a
major contributor to our electrical power needs. The solar resource for generating power from
concentrating solar power systems is plentiful.
It is clear from the above discussion that a large variety of collectors have been developed over
the period of time, which can be used in variety of applications depending from the temperature
variation. Some areas in the field of solar energy are fully developed and needs less attention like
81
the flat plate collectors and parabolic collectors but still a lot of research is required in this field to
make it one of the major source of energy production at the lowest cost available.
Although photovoltaic is projected to have a lower cost than concentrated solar power in the
medium to long term, concentrated solar power will play a vital role in utility scale installations
due to its storage capabilities and other benefits to utilities and societies. Trough technologies are
being implemented first and will have technical improvements and cost reductions while other
CSP technologies are still under development. These technologies will suffer from slower
ramping and scale, but should become competitive at the utility scale within the next decade.
In the 21st century the concentrating solar power around the world has seen a rapid increase of
interest from governments and industry as well as from other groups than the environmental
organizations. New areas of expertise are developing and work opportunities are increasing,
promising a sustainable labor market ahead. The commercializing of the CSP technology will
give incitements to solutions and improvements of the challenges that are faced by the market
today including solar collectors and control systems, storage solutions and its media to more
flexible and durable power block integration.
In chapter 5 we discuss about direct steam generation technology which is still under
development stage helps to drive down the investment costs and also the size of the plant by
eliminating the need for expensive heat transfer fluids and heat exchangers respectively. This can
be promising technology as DSG allows the solar field to operate at higher temperatures,
resulting in higher power cycle efficiencies. Furthermore, since hazardous HTFs are removed
from the balance, DSG presents a lower environmental risk. Additionally, because heat can be
stored without HTFs and heat exchangers, steam accumulators can be employed when they
become commercially viable, enabling the storage capacity of parabolic trough plants to be
expanded.
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CHAPTER 5
DIRECT STEAM GENERATION TECHNOLOGY
[55-61]
5.1. INTRODUCTION:
Parabolic trough power plants are currently the most commercially applied systems for CSP
power generation. To improve their cost effectiveness, one focus of research is the development
of processes with other heat transfer fluids than the currently used synthetic oil. One option is the
utilization of water/steam in the solar field, the so-called direct steam generation (DSG). Several
previous studies show the economic potential of DSG technology. Analyses’ results showed that
live steam parameters of up to 500 0C and 120 bars are most promising and could lead to a
reduction of the levelized electricity cost (LEC) of about 11%.
Current parabolic trough solar thermal power plants connected to the electricity grid are using
synthetic oil as heat transfer fluid in the collectors. The main disadvantage of this technology is
the maximum power block inlet temperature, which is limited to the oil upper working
temperature in order to guarantee this fluid thermal stability. Although there are some
alternatives, like the use of molten salts in the parabolic trough collector, none of them have been
scaled to a commercial size. Besides that, all these options, called heat transfer fluid (HTF)
technologies, require a heat recovery steam generator (HRSG) between the solar field and the
power block, which introduces additional heat losses and pressure drops in the global efficiency.
Direct steam generation is considered a very promising option to increase the efficiency of
parabolic trough systems, not only because there is no need for heat exchanger between the solar
field and power block.
83
Direct steam generation in the absorber pipes of parabolic trough collectors, the so called DSG
process can significantly reduce the cost of electricity produced by solar thermal power plants
using this type of solar collector. Nevertheless, implementation of the DSG technology is subject
to the successful investigation of those technical constraints and possible problems that could
exist in a commercial DSG plant. Replacement of the oil by direct steam generation results in
lower investment and operating costs, as well as reduced environmental risk and fire hazard in
case of leaks. Simultaneously, the performance can be improved by avoiding the thermodynamic
losses associated with the oil–water/steam heat exchanger of the SEGS plants. In combination
with further improvements of the collector field and overall system integration, a 26% reduction
in the electricity cost seems to be achievable. The direct steam generation in the parabolic trough
collectors is a feasible improvement of this reliable technology.
Direct steam generation is considered a very promising option to increase the efficiency of
parabolic trough systems, not only because there is no need of a heat exchanger between the solar
field and the power block , but also owing to the higher temperatures that can be attained in the
collector receivers. This last reason is especially important at present, when new commercial
absorber tubes, for working at higher temperatures, have been developed.
At present, there are two projects to develop pre-commercial demonstration plants based on DSG
technology, they all to be implemented in the southern of Spain. Net electrical power of these
plants will be 3 MWe (Zarza et al., 2008) and 5 MWe (Eck et al., 2008), respectively. The
analysis presented in this paper is referred to a 50 MWe net DSG power plant. It has been
selected this power because it is a relevant size for commercial projects.
84
5.2. DIRECT SOLAR STEAM GENERATION [DISS] IN PARABOLIC TROUGH
COLLECTORS: [56]
The feasibility of the direct steam generation technology was demonstrated within the DISS
project funded by European Union. The DISS parabolic-trough solar plant is the leading DSG
test facility in the world. Three different operating concepts have been studied, namely the once-
trough mode, the recirculation mode and the injection mode of operation, as shown in figure
given below.
Figure 5.1: Basic Concepts for the DISS in Parabolic Trough Collectors. [56]
In the once-through process, high temperature gradients can be avoided by tilting the collectors.
Feed water is preheated, evaporated, and converted into superheated steam as it circulates from
the inlet to the outlet of the long rows of solar collectors. The main advantage is its simplicity,
while the main technical problem is to control the superheated steam parameters at the solar field
outlet under solar radiation transients.
85
In the injection process, the collectors are horizontal and small amounts of water are injected
along the row of collectors. High temperature gradients may be avoided by keeping the mass flow
in the absorber pipes above a threshold level. The main advantage of this process is that the
parameters of the superheated steam at the solar field outlet are easy to control. On the other
hand, the injection system is more complex and costs more.
The third option, recirculation, is the most conservative. In this case, there is a water-steam
separator at the end of the evaporating section. The inlet feed-water flow rate is much higher than
that of the steam to be produced by the system. Only a fraction of this water is converted into
steam as it circulates through the collectors of the evaporating section. The steam is separated
from the water by the separator, and the remaining water is sent back into the solar field inlet by a
recirculation pump. The excess water in the evaporating section makes stratification impossible.
This type of system can be controlled well, but the excess water that has to be recirculated and the
pump necessary for it increases system parasitic loads and costs. So, each process has advantages
and disadvantages when compared to each other, and they have to be experimentally evaluated to
find out which process is the best for a commercial DSG power plant.
86
Figure 5.2: View of the PSA DISS solar field in Operation. [58]
Figure 5.2 is a picture of the DISS facility in operation. The absorber pipes are at the focal line
illuminated by the concentrated solar radiation reflected by the mirrors
Figure5.3: Arrangement of the Solar Power Plant with DSG.. 1. Parabolic trough field; 2.
Pump and flow meter; 3. Steam trap and Separator; 4. Steam motor; 5. Electric Generator;
6. Valve for the recirculation Process
87
Figure 5.3 shows a typical SEGS plant with DSG the figure describes the arrangement of the
power block for electricity generation using the recirculation process. In this figure, the parabolic
trough field had an orientation east west. A flow control valve was used at the input of the first
parabolic trough concentrator in order to control the quantity of water in the inlet. The increase in
quality of steam was carried out mainly in the last two modules. In the early hours of morning
water steam flow recirculates in the first three modules until it reaches almost the condition of
saturated steam. When this condition is reached the valve which goes to the steam motor is open.
When the system is working under this condition the water is extracted in the steam trap is
recirculated by the opening the appropriate valve.
Additionally a water steam separator and traps are used before the injection of steam at the engine
in order to supply only saturated steam or superheated steam to reduce the probability of damage
when clouds were present or low insolation was available. A thermocouple and pressure
transducer is also used to measure the type of steam injected to the engine as the steam motor
needs only 93kg h-1
of steam.
5.3. COMPARISON OF DSG AND SYNTHETIC OIL BASED PARABOLIC TROUGH
PLANT: [61]
Solar energy can play a fundamental role in the near future to replace fossil-fuel plants for
stationary applications. Reducing fossil-fuel consumption is necessary because of reservoir
depletion and environmental concerns related to rising CO2 concentration in the atmosphere.
Renewable energy and—for regions with high solar radiation solar energy can play a fundamental
role to move from a carbon economy to a green economy. Among solar energy conversion
systems, concentrating solar energy is a very promising technology because it can decouple the
solar energy source from electricity production due to storage systems. Today, this feature is not
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possible for photovoltaic plants because systems for storing electrical energy are not
economically competitive.
For solar thermal power plants, parabolic trough technology the heat-transfer fluid (HTF), which
collects and transfers the solar thermal energy to the power block, is synthetic oil (Therminol VP-
1). With this system the two primary issues hindering the diffusion of this technology is the cost
of the solar field and relative performance of steam cycle due to the temperature limits with the
synthetic oil. Previously the receiver’s thermal stability was limited to 4000c but today receivers
can handle up to 5800c. Based on these limitations different technologies have been investigated
in which DSG is one.
The main advantage of DSG technology is in the direct cycle configuration steam is directly
generated in the solar field thus avoiding the use of boilers. Besides this advantages there are
other advantages for this technology are (i) solar field constant temperature in the evaporation
section with benefits for solar thermal efficiency, (ii) receiver maximum temperature coincident
with the steam-cycle temperature, and (iii) reduced solar field recirculation pump consumption.
There are a few more advantages that make DSG more preferable like DSG concept is
environment friendly and it avoids usage of flammable & environmental hazard materials, The
DSG allows significant reductions of the total investment costs and levelized electricity cost.
With DSG a higher operating temperature will be achievable.
There are a few drawbacks to this technology which are (i) high volumetric flow in the solar field,
(ii) superheating section composed of several short loops placed in parallel, and (iii) storage
issues (i.e. no commercially available storage for steam), with negative effects during transient
conditions. The last aspect is important from an operations perspective.
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5.4. POWER TOWER WITH DIRECT STEAM GENERATION [62]
Examples of saturated steam Power Towers are PS10 and PS20 in Spain. PS10 is an 11 MW
DSG Power Tower plant that started operating in 2007 at Solúcar Solar Park, at Sancùlar la
Mayor in Seville. The steel drum has a pressure above the system steam pressure to ensure
boiling of the water into steam when opening the valves. The higher pressure the thicker the steel
drums have to be, therefore to optimize the drum material is important.
Figure 5.4: DSG in Power Tower with a Saturated Steam Receiver. [62]
The PS10 plant presented by Solúcar Solar S.A (2006) has 624 Heliostats; each heliostat of 121
m2 is tracking the sun in two axis. The heliostats are curve shaped arranged in 35 circular rows
around the tower .The receiver has a slant range where the focal point is at a distance equal to the
slant range. The solar receiver is of cavity type, formed by four vertical panels of 5.4 m width x
12 m height. Each panel has the heat exchange surface of about 260m2. These panels are arranged
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into a semi-cylinder of 7 m of radius. The receiver is basically a forced circulation radiant boiler
with low ratio of steam at the outlet, in order to ensure wet inner walls in the tubes. Feed water
around 50 ˚C vaporizes at 250 ˚C and 40 bars. During operation at full load, absorber panels will
receive about 55 MWT of concentrated solar radiation with peaks of 650 kW/m2. Thermal storage
comprises four pressurized steel tanks with a thermal capacity of 20 MW·h, equivalent to an
effective operational capacity of 50 minutes at 50% turbine workload, and also provides
controlled temperature conditions during the steam turbine start/stop-cycles. Natural gas back-up
is used to power production of 12-15 % of PS10’s capacity. PS20 has the same design as PS10
but has the twice the capacity. The number of heliostats are 1 255 and the height of the tower is
165 m.
5.5. CURRENT STATUS OF DIRECT STEAM GENERATION: [60]
The research activities in the field of direct steam generation in parabolic troughs have been
started since the middle of 90’s. After theoretical analysis and testing various operation strategies
a 700m demonstration loop was installed on the Plataforma Solar Almeria (Spain). Here the
different operation strategies were first tested and evaluated. Also the concept functionality is also
proven here. Today the test loop is used for component development, like high temperature
absorbers tubes and steam separators as well as the development of operation and control
strategies to optimize the dynamic behavior of the DSG in parabolic troughs.
A first step towards a demonstration plant was taken with the pre-engineering of a 5MWel DSG
plant in the project INDITEP. This pre-design is now used as base for a 3 MW demonstration
plant, which was built by a Spanish Consortium at the Plataforma Solar de Almeria on March
2011. Another consortium aiming at the development of commercial parabolic trough plant with
DSG has already started with the development of components for 500 °C application (e.g.
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absorber tubes, storage system, flexible tube connections). The next steps will be the
demonstration and qualification of the components at the Litoral steam power plant in Carboneras
and afterwards the erection of a stand-alone demonstration plant with 5 MW. Along with this
project, basic designs and operation strategies for 50 MW DSG power plants are developed.
To enlarge the operation time range and the flexibility of the power plant another essential
component for commercial DSG power plants is an integrated thermal storage system. In several
projects the development and demonstration of such a storage system is pursued. Most promising
is a modular storage system based on a high temperature concrete for sensible heat storage and on
a phase changing material for latent heat storage. This modular concept is now tested in pilot
storage systems
5.6. ENERGY STORAGE TECHNOLOGY FOR DIRECT STEAM GENERATION: [61]
A big advantage of solar thermal power plants is that they can store energy in the form of heat.
This means that the facilities can produce power to meet demand during cloudy weather or at
night. Direct solar steam generation requires storage technologies that are adapted to suit this new
technique. An important requirement for these technologies is that they efficiently store the large
amount of energy released during the condensation of steam, a process that occurs at constant
temperature. In the pilot facility now in operation, this challenge is met using a combined storage
system with storage units for both sensible and latent heat.
The main cost driver of the reference DSG plant is the storage system. Due to the characteristics
of the DSG concept, Phase change Material [PCM} storage seems the only reasonable way for
evaporation/condensation storage, i.e. charging and discharging with two-phase flow media. The
main configuration changes are therefore limited to the sensible part or to the different usage of
the PCM storage system.
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Figure 5.5 shows three options of storage systems for direct steam generation in which figure 5.5
(a) is an option of concrete storage for sensible parts the main advantages for this is it does not
require any additional parts like pump and the disadvantage is that the achievable outlet
temperature during discharge depends on the remaining ‘fill level’ or the energy still useable in
the storage, respectively. In figure 5.5 [b] it is the reference storage system which uses three
sensible tanks with a buffer tank the main advantage of this is it has good controllability over the
process and constant outlet temperatures are available throughout discharge. The disadvantage of
this system is it requires lot of additional items and three tanks might increase the total investment
cost of the system.
Option c (figure 5.5c) is the currently favored design for a high temperature system. PCM storage
is not only used for condensation/evaporation, but also for sub-cooling/preheating. The de-
/superheating of the steam are performed by a two-tank molten salt system. This system has not
been demonstrated so far, but its operation is not supposed to cause any problems in a modular
Figure 5.5: Selected DSG Storage Option. [61]
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PCM storage design. Compared to the oil system’s storage, the specific investment of this two-
tank variant differs due to two main effects. First, it is reduced due to the higher temperature
difference. Second, it is increased by the use of more expensive material for the hot storage tank
and heat exchangers. Compared to the three-tank option, option c offers high reduction potential
as the specific investment for the PCM system also decreases, while the absolute size of the
system is expected to be the same.
5.7. INNOVATIONS AND IMPROVEMENT IN DSG: [59]:
Typically, the technical innovations considered show are made to improve the performance of the
system and helps in reducing the total investment cost; they have an impact on more than one of
the input parameters of the model. Specific innovations and associated parameter sets for the
parabolic trough DSG system are as follows:
1. Up scaling of Power Block to 47 MW: by using more sophisticated steam cycle and
steam turbine increases the system efficiency from 26%-38.5%, As a result of the
increased cycle efficiency, the total reflective area is reduced significantly.
2. Thin Glass Mirrors: The usage of thin glass mirrors instead of the sagged float glass
mirrors leaves the mean reflectivity unchanged at 0.88 or gives a slightly increased value
of 0.89 pessimistic and optimistic values. The specific investment costs for the solar field
are reduced to 90/95%.
3. Multilayer Plastics and innovative Structure: for the usage of these parabolic trough
components it is assumed that the specific investment costs for the solar field may be
reduced to 70/90% of the reference value _133 € /m2 /171 € /m2_ while the technical
parameters are not modified compared to the reference plant.
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4. Dust-Repellent Mirrors: Dust-repellent mirrors will leave the mean reflectivity
unchanged at 0.88 or increase it to 0.91 to account for lower soiling. Additionally, these
mirrors need less manpower for field cleaning. Thus, the specific number of persons is
reduced from 0.03 to 0.02/0.025 persons per 1000 m2 of aperture.
5. Advanced Storage: Although no storage was considered for the aforementioned types of
DSG parabolic trough plants, there are recent research projects dealing with storage
options for steam. This storage could probably be a combination of phase change material
for the latent heat and a concrete storage for the sensible heat. Assuming that this storage
type would be available in a commercial scale, 3 h full-load storage is added. The solar
field size is increased to deliver excess thermal energy for storage charging, and specific
costs of 20/30 € /kWh for the storage are assumed.
6. Increased Solar Field Outlet Temperature: Higher live steam temperatures are desirable
for better Rankine cycle performance. For direct steam generation, the temperature is not
limited by thermal degradation of the fluid, but the selective absorber coatings may be the
limiting factor for this technology. Assuming that a durable absorber is available without
additional costs, the steam temperature at field outlet is increased from 411°C to
450/480°C. At the same time, the design efficiency of the power block is increased to
39.5/41.5%. Since the mathematical model calculates the thermal losses of the solar field
depending on the mean fluid temperature, these losses are increasing.
7. Reduced Parasitic: Parasitic energy consumption, especially for pumping, is an important
item and could be reduced through the usage of improved tube joints and water/steam
separators. Therefore, the factor for solar field parasitic is decreased from 0.009 kW/m2
to 0.004/0.006 kW/m2 of aperture.
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Not only the impact of the individual measures, but also the combination of them, is an
important figure. Therefore, all above-mentioned measures were combined and the
resulting cost reduction was calculated. In particular, combination means that the high
average reflectivity of the dust repellent mirrors has been used together with the low costs
of multilayer plastics and innovative structures.
5.8. SUMMARY:
Direct steam generation drives down the investment costs and also the size of the plant by
eliminating the need for expensive heat transfer fluids, reducing efficiency losses and heat
exchanges. The cost and thermodynamic disadvantages of using the synthetic oils and heat
exchangers impact negatively on the Plant and, with accumulated Parabolic Trough capacity set
to rise to around 2,250 MW by 2020 (Global CSP Industry Report 2010–2011), several
companies are examining whether Direct Steam Generation could boost Parabolic Trough Plant
competitiveness. The technical feasibility of DSG in parabolic trough plants has been
demonstrated in the DISS loop. The overall plant configuration is simple and environment
friendly. Alongside molten salts; DSG is the most promising solution for driving down costs in
the near future. DSG allows the solar field to operate at higher temperatures, resulting in higher
power cycle efficiencies and lower fluid pumping parasitic. Furthermore, since hazardous HTFs
are removed from the balance, DSG presents a lower environmental risk. Additionally, because
heat can be stored without HTFs and heat exchangers, steam accumulators can be employed when
they become commercially viable, enabling the storage capacity of parabolic trough plants to be
expanded.
Considering these advantages, it is highly probable that this technology will be fully ready for
commercialization in the near term. But Research suggests that a number of technical issues still
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need to be addressed. The two of the main problems for this technology are the controllability of
the two-phase water/steam flow and the temperature gradients inside the absorber tubes. Strong
transients in solar radiation can make it difficult to maintain steam temperature because a
minimum feed flow rate must be guaranteed in the solar field to avoid high temperature gradients
when radiation levels recover. Furthermore, absorber tubes are subject to higher pressures when
DSG is employed. As R&D brings down the cost and as experience of operating and maintaining
these systems accumulates, it will gradually become established due to the higher temperatures,
improved efficiencies and greater outputs that can be achieved.
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CHAPTER 6
CONCLUSION AND FUTURE WORK
6.1 CONCLUSION:
From the above discussion it is clear sun is the most abundant, clean energy resource that can
replace fossil fuels and meet world’s energy demands. Presently only a part of solar energy is
being harnessed but more research and much effort can be put in order to harness more energy
from sun. Though a lot is being done still more effort has to be put in to harness solar energy in
the right way. Both political and economical players involvement are required in order to produce
energy in large scale using solar energy technologies but these technologies still require further
development in conversion efficiency and reducing the cost for manufacturing. More research
effort is required in order to overcome these barriers and to find more innovative technologies.
A wide variety of solar technologies have the potential to become a large component of the
future energy portfolio. Direct production of chemicals fuels, and particularly hydrogen, from
solar energy is a promising alternative to using fossil fuels for the development of a sustainable
carbon-free fuel economy. Thermo-chemical and biological conversion processes are promising
technologies with potential for high efficiency. However, only a few thermo-chemical processes
have been investigated to date and biological systems require more understating of genetics and
biological conversion to become efficient and stable. Solar energy has a large potential to be a
major fraction of a future carbon-free energy portfolio, but technological advances and
breakthroughs are necessary to overcome low conversion efficiency and high cost of presently
available systems.
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Solar energy is broadly classified into two types Passive and Active Energy. Passive technologies
are used for indoor lighting and heating of buildings and water for domestic use. Also, various
active technologies are used to convert solar energy into various energy carriers for further
utilization. Active solar energy is further divided into Photovoltaic and Solar Thermal energy.
Photovoltaic directly convert energy into electricity. These devices use inorganic or organic
semiconductor materials that absorb photons with energy greater than their band gap to promote
energy carriers into their conduction band. In the last few years, photovoltaic technologies have
experienced an astonishing evolution that led to the increase of the efficiency of crystal-silicon
solar cells up to 25% and of thin-film devices up to 19%. Recently, nano-technology, innovative
deposition and growth techniques, and novel materials opened routes for reaching higher
performances and for developing very low-cost devices such as organic-based PVs. Although
these technologies face comparable fundamental issues related to the steps involved in the
conversion of energy into electricity. Both fundamental research and technical development are
critical requirements for these technologies to become more efficient, stable, and reliable.
Solar thermal technologies convert the energy of direct light into thermal energy using
concentrator devices. The simple and most commonly used applications of solar thermal energy
include solar water heating, swimming pool heating and agricultural drying. Solar thermal energy
is broadly classified into three types, namely low, medium, and high temperature collectors. Low
temperature collectors plate used for heating swimming pool, the medium temperature collectors
are used more for residential and commercial places for heating water and air. The high
temperature collectors are generally used for electricity production by harnessing the sunlight
99
with the help of mirrors or lens. When compared to photovoltaic cells solar thermal energy is
different and more efficient.
Concentrated Solar Power or the high temperature solar thermal technology is the most
developed of the solar technologies and is on the verge of being competitive with conventional
power plants. CSP technologies have relatively low cost and ability to deliver power during
periods of peak demand meaning that it has the potential to be a major contributor to our
electrical power needs. The solar resource for generating power from concentrating solar power
systems is plentiful. Their ability to overcome the intermittency problem using hybridization and
thermal storage renders these technologies particularly suitable for large-scale electricity
production. The concept is technically simple and sustainable and the potential lies both in the
electricity generation sector and in the industrial processing sector. A large variety of collectors
have been developed over the period of time, which can be used in variety of applications
depending from the temperature variation. Trough technologies are being implemented first and
will have technical improvements and cost reductions while other CSP technologies are still
under development.
It is evident that direct steam generation is the most promising solution for driving down costs in
the near future. Direct steam generation drives down the investment costs and also the size of the
plant by eliminating the need for expensive heat transfer fluids, reducing efficiency losses and
heat exchanges. DSG allows the solar field to operate at higher temperatures, resulting in higher
power cycle efficiencies and lower fluid pumping parasitic. These advantages makes direct steam
generation is highly probable that this technology will be fully ready for commercialization in the
near term. But there are few technical difficulties that can be overcome with further research this
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technology can become established due to the higher temperatures, improved efficiencies and
greater outputs that can be achieved.
It is believed that solar energy has a large potential to be a major fraction of a future carbon-free
energy portfolio, but technological advances and breakthroughs are necessary to overcome low
conversion efficiency and high cost of presently available systems. However, there is still much
to do and even though the technology has been largely demonstrated. This challenge can only be
met by deepening collaborative work and by strengthening the Research Area in this field.
6.2 FUTURE WORK:
In the near future, we will be forced to lay more emphasis on the research and development of
alternative energy sources. Our current rate of fossil fuel usage will lead to an energy crisis this
century. In order to survive the energy crisis many energy industry are inventing new ways to
extract energy from renewable sources. While the rate of development is slow, mainstream
awareness and government pressures are growing.
In the 21st century, solar energy has become a small part of our daily life. From solar heated
swimming pools to solar powered home. Yet many wonder if small applications will be all solar
power is capable of handling. Certainly, the difficulties of large solar plants are many, although
many experts continue to insist that the future of solar energy is quite bright.
The key issue that we are facing regarding the future of solar energy is the space requirement for
solar power plants. A solar plant is comprised of thousands of solar panels and requires large
place to accommodate these panels along with other equipment. Because of this, solar plants
require a consistently sunny area and a considerable amount of space. Currently, the one of the
largest solar power stations in the world covers more than 10 square miles (16.9 km squared) and
creates enough power to run about 200,000 homes. In addition to this the other problems related
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to solar energy is to make its technologies. Therefore on resolving these two issues solar energy
can be very beneficial for our near future.
One encouraging factor about the future of solar energy is that many of the world's greatest
innovators are choosing to focus their considerable talent and funds on improving alternative
energy technology. One of the challenges is the task of making solar energy an economical
solution to the growing power needs of the world.
Although there are many reasons to believe that the future of solar energy is bright and coming
soon, the answer really lies in the hands of the world's citizens. In a world largely governed by
economics and politics, what ordinary citizens choose to buy and support will dictate the trends of
the future. By installing solar panels, donating to research organizations involved in alternative
energies, majoring in science or engineering, and voting for measures that give money to
alternative energy development, anyone can influence the future of solar energy.
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