fullbook.pdf

7/30/2019 fullbook.pdf

1/168

1

Chapter-1

INTRODUCTION

The energy consumption in the world, particularly in the industrialized countries, hasbeen growing at an alarming rate. Fossil fuels, which today meet major part of the energy

demand, are being depleted quickly. World has started running out of oil and it is estimated that80% -of the worlds supply will be consumed in our life times. Coal supplies may appear to be

large but even this stock may not last longer than a few decades. Moreover the pollution hazard

arising out of fossil fuel burning has become quite significant in recent years. Nuclear power hasposed a number of problems and nuclear fusion is still a speculative technology.

Thus we are forced to look for unconventional energy sources such as geothermal, ocean

tides, wind and sun. It is also hoped that these alternative energy sources will be able to meetconsiderable part of the energy demand in coming future. Among all these, solar energy seems to

hold out the greatest promise for the mankind. It is free, inexhaustible, non-polluting and devoidof political control. Solar water beaters, space heaters and cookers are already on the market andseem to be economically viable. Solar photovoltaic cells, solar refrigerators and solar thermal

power plants will be technically and economically viable in a short time. It is optimistically

estimated that 50% of the world power requirements in the middle of 21st century will comeonly from solar energy.

Enough strides have been made during last two decades to develop the direct energy

conversion systems to increase the plant efficiency 60 to 70% by avoiding the conversion ofthermal energy into mechanical energy. Still this technology is on the threshold or the success

and it is hoped that this will also play vital role in power generation in coming future.


2/168

2

1.1 Renewable and Non-conventional

Recently considered energy resources are solar, wind, geothermal, ocean thermal, oceanwave, ocean tide, mini-hydro, bio-mass, chemicals, waste fuels etc. These are available from

nature in renewable but periodic/intermittent form. Energy technologies for use of renewab1es

have been rapidly developed. Present contribution of renewable in the world is less than 20%(excluding hydro). This is likely to increase to about 10% by 2015 AD and to about 15% by

2025 AD. Renewable are cheap, clean energy resources. However, solar and wind sources are

intermittent, diffused and their conversion technologies are presently costly and suitable only ofsmaller plant capacities. 100% storage facility is necessary on output side.

Comparison between Conventional and Renewable

Feature Conventional

Non Renewable

Renewable

- Technologies Established Commercially weak

- Plant size Large (MW range) Small (KW range)

- Main Power Plants Suitable Non Sufficient- Energy density of source High Low

- Pollution problems More Less

- Energy reserves Limited Renewable

- Storage Easy Uneconomical

- Cost of Generation Low High

1.2 Energy Chains or Energy Routes

The concept of energy chain (energy route) is very useful in energy studies, 'chain' or

'route' signify a sequential path.

Energy is present in several sequential activities. Individual 'link' or 'individual step'

represent 'particular energy transformation'. A sequence of such energy transformations forms anenergy chain (energy route).

A required type of usable energy can be obtained from alternate energy routes, e.g. heatcan be obtained by burning of wood or from electrical heater or from solar energy.

Energy chain has three or more energy links. These are associated with

- Primary energy source.- Intermediate energy forms.

- Final usable energy form.

Energy resources are primary energy forms available from nature in raw form. These are

explored, located, extracted, processed and converted to intermediate form / forms.

Intermediate forms of energy are 'vehicles of energy' for transportation and processing.


3/168

3

Final usable forms of energy are for consumption e.g. heat, mechanical motion, fuels, electricity,

chemicals, lubricating oils, steam, hot water, natural gas etc.

Energy Technology deals with various 'energy chains' and 'energy links'.

1.3 Energy Resources and Forms of Energy

Primary energy resources are those available in nature in raw form. (Coal, Petroleum-Oil,

Natural Gas, Fire-wood, wind, water at high level, Uranium-ore, solar irradiation, Geothermalfluid, Ocean-waves, Ocean-thermal fluid, Ocean tides, biomass fuels etc.)

Search for new, renewable primary energy resources are in progress. The technology for

conversion, processing etc. should also be available. The primary resources should also berecoverable. Some resources are partly recoverable and some are not recoverable e.g. coal at

very great depth is non-recoverable.

Primary resources include conventional, Alternate (Non- conventional), Renewable andNon-renewable, futuristic.

Intermediate energy forms. The primary resources are generally not suitable for ultimate

use. They are transformed to intermediate form by one or more processes (e.g. coal may be

gasified, supplied to gas power plant.)

Secondary energy sources (Usable energy) are those supplied to the user for consumption

(electrical energy; steam, hot water, liquid petroleum gas in cylinders or pipe-lines, petroleum

oils, fire-wood etc.). .

Secondary energy sources are either commercial or non-commercial. For example

electricity is an expensive commercially supplied secondary (usable) energy. Solar energy isnon-commercial source of renewable energy.

Renewable are those, which are renewed by the nature again and again, and their supply

is not affected by rate of consumption. (Wind energy; Solar-energy, Geothermal energy, Oceanwave; Hydro-energy etc.) Renewable are renewed by nature periodically (e.g. Sun light) or

intermittently (e.g. wind).

Alternative energy source are those, which are non-traditional. They are alternatives to

the conventional energy resources.

The demarcation between conventional and non-conventional is not rigid. Todays non-

conventional become conventional after a few decades. For our reference, we consider the

following:

-Energy resources, which are in use during 1950-1975, are called conventional.


4/168

4

-Energy resources, which are considered for large-scale use after 1973 oil crisisare, called Non-

conventional or Alternate.

-Non-conventional energy technologies are presently under development/ commercialization.

- Non-conventional energy resources are likely to have more and more share of energy market incoming decades (1990s, after 2000 AD).

Conventional and Renewable Recourses For Electrical Generation

Conventional Alternative, Renewable

Coal Wind Power

Petroleum Oils Solar Power

Natural Gas Geo Thermal

Hydro Ocean Waves

Nuclear Fission Fuels Ocean Tide

Fire Wood Bio- mass fuelsWaste fuelsBio-Gas

Synthetic Gasses

Nuclear Fusion fuelsFuels or fuel cells

Fire wood

Ocean algae fuel

Ocean Salinity gradient

Non-renewable energy resources are those which do not get replenished after their

consumption e.g. coal once burnt is consumed without replacement of the same (Fossil fuels,Nuclear fission fuels).

The energy resources which are formed very slowly in nature and which are likely to be

exhausted in a few more decades or centuries are called Non-renewable. World is presently

dependant on such resources (90% supplies of world primary resources are by Nonrenewables-1990)

Advantages of renewable energy. Even though renewable options are not likely to supply asubstantial amount of energy to developing countries over the short term, they do have these

advantages:

(1) Renewable energy is an indigenous resource available in considerable quantities to alldeveloping nations and capable, in principle, of having a significant local, regional or national

economic impact. The use of renewable energy could help to conserve foreign exchange and

generate local employment if conservation technologies are designed, manufactured, assembledand installed locally.


5/168

5

(2) Several renewable options are financially and economically competitive for certain

applications, such as in remote locations, where the costs of transmitting electrical power or

transporting conventional fuels are high, or in those well endowed with biomass, hydro orgeothermal resources.

(3) Because conversion technology tends to be flexible and modular, it can usually be rapiddeployed. Other advantages of modular over very large individual units include easy in adding

new capacity, less risk in comparison with 'lumpy' investments, lower interest on borrowed

capital because of shorter lead times and reduced transmission and distribution costs fordispersed rural locations.

(4) Rapid scientific and technological advantages are expected to expand the economic range of

renewable energy applications over the next 8-10 years, making it imperative for internationaldecision makers and planners to keep abreast of these developments.

Obstacles to the Implementation of renewable energy systems

Experience with renewable energy projects in the developing countries indicates that

there are a number of barriers to the effective development and widespread diffusion of thesesystems. Among these are:

(1) Inadequate documentation and evaluation of past experience, a paucity of validated field

performance data and a lack of clear priorities for future work.

(2) Weak or non-existent institutions and policies to finance and commercialize renewable

energy systems. With regard to energy planning, separate and completely uncoordinatedorganizations are often responsible for petroleum, electricity, coal, forestry, fuel wood,

renewable resources and conservation.

(3) Technical and economic uncertainties in many renewable energy systems, high economic and

financial coat. for some systems in comparison with conventional supply options and energy

efficiency measures.

(4) Skeptical attitudes towards renewable energy systems on the part of the energy planner. And

a lack of qualified personnel to design, manufactures, market, operate and maintain such

systems.

(5) Inadequate donor coordination in renewable energy assistance activities, with little or no

information exchange on successful and unsuccessful projects.


6/168

6

Chapter-2 (A)

SOLAR POWER PLANTS

2.1 Principle of Solar Thermal Power Generation

In a solar thermal power production using a solar pond, a flat plate collector or afocussing collector first collects system the energy. This energy is used to increase the internal

energy or temperature of a fluid. This fluid may be directly used in any of the common or known

cycles such as Rankine, Brayton or Stirling or passes through a heat exchanger to heat a

secondary fluid working fluid) which is being used in the cycle to " produce mechanical powerfrom which electrical power can be produced easily.

The following three systems are discussed in the following, section:

[1] Low temperature cycles using flat plate collector or solar pond.

(2) Concentrating collectors for medium and high temperature cycle.

(3) Power tower concept.

2.2 Low temperature systems

The flat plate collector and solar pond are classified as low temperature collectors,because temperature achieved is of the order of 60 to 100C, with collection efficiency of 30 to

50%. If Rankine Cycle solar thermal power production system is employed, since the

temperature of the fluid (water) is usually below l00C (with solar pond the maximumtemperature is limited approximately 80C) and it is not possible to generate steam with flat plate

collector or solar pond, so this can not be used directly to run the prime mover. Therefore, some

other organic fluid is used (Freon group etc.) which evaporates at low temperature and high

pressure by absorbing the heat from the heated water. The vapour formed can be used to run aturbine or engine which may generate power, which will be sufficient to light the group of

houses for rural areas and for irrigation purposes.

A schematic view of basic Rankine cycle power production system is shown in Fig. 2.2.1

the T-S diagram of the cycle is also shown.

Rankine cycle, a version of the theoretical Carnot Cycle happens to be me most

frequently employed not only for solar thermal power production system but also for the

conventional power plants. The various processes that comprise the cycle are as follows:


7/168

7

Fig. 2.2.1 Schematic of a basic solar Rankine Cycle.

1.2. Reversible adiabatic pumping process in the pump.

2.3. Constant pressure transfer of heat in the vapour/steam generator.

3.4. Reversible adiabatic expansion in the expander (turbine, 'reciprocating

engine etc.)4.1. Constant pressure transfer of heat in the condenser.

The Rankine cycle also has the possibility of superheating the vapour, which is notshown in the figure.

2.3 A number of system configurations are currently under development under the basiccategory of Rankine cycle solar thermal power production system. However the major choices

involved are the following:

(a) Choice of the collection system and cycle operating pressure andtemperature.

[b] Choice of the working fluid & Choice of the expander.

(c) Choice of the storage system.

(a) Choice of the collection system (solar collectors). A flat plate collector or solar pond

(concentrator for higher temperature cycle) may be employed. The flat plate collector may besimple one or with selective coating. The collector cost, increasing in the order presented above

along with its conversion efficiency and maximum operating temperature play important role in

the choice of the collector only. A temperature up to 100C can be achieved with such collectors.With solar pond the maximum temperature is limited approximately 80C. Though solar pond is

very cheap collecting device and works as storage also but because of some other limitations itcannot be conclude the only choice at low temperature. With concentrating collectors

temperature up to C can be achieved but additional cost of tracking with a increment in thesystem's overall efficiency has to be taken into consideration. The Rankine cycle efficiency

increases with boiler temperature whereas solar collectors efficiency decreases with rise in

temperatures, therefore one should find an optimum operating boiler temperature by consideringseveral collectors and Rankine cycle with their costs and overall cost should be minimized. The

choice of the collector partly depends upon the desired fluid or it can be said vice-versa. Suppose


8/168

8

a working fluid of higher boiling point is to be used then the use of solar pond or flat-plate solar

collector does not arises. Similarly, if it is desired to use a solar pond as solar energy collection

device a working fluid of low boiling point has to be used.

(b) Choice of the working fluid:- As mentioned earlier that the primary fluid going through the

collector and the secondary fluid going through the power plant may be the same or differentfrom each other. Using the same fluid throughout the system has the advantage of eliminating

the heat exchanger. The main advantage when used with flat plate collectors are that a large

quantity of working fluid of low boiling point has to be used, a greater chance of leakage in thecollector and loss of fluid exist, and the collector has to operate at higher pressure. Therefore, it

is more advantageous to use different fluids for a plant of higher capacity 10-100 kW. The

choice of the working fluid depends on the operating temperature in the boiler and condenser and

the type of expander to be used. Generally either water or organic fluid (Freon group etc.) is usedas working fluid. The desirable properties of the working fluid can be briefly listed as follows:

(i) Nature of the saturation curve - this will be largely determined how close the practical cycle

can come to the theoretical efficiency limits.

(ii) Working pressure of the cycle - the important consideration on the low-pressure side isweather it is sub atmospheric or atmospheric. The maximum working fluid pressure on the other

hand, decide the system complexity to a certain extent. Specific volume and latent heat these two

properties will determine the volume flow rate for a given power out and partly govern the

choice of the prime movers.

(iii) Other considerations like chemical stability and non-corrosiveness within the operating

temperature; good heat transfer and flow properties etc.

(c) Choice of the storage system.

The storage system is required in all the solar thermal power generation system whether a

Rankine cycle powers generation system, a Stirling cycle power generation system or Brayton

cycle power generation system. Owing to the intermittent and variable nature of solar energy,

every solar thermal power system must have an energy storage system, which could stabilize thehalf sinusoidal incident energy, the constant temperature thermal energy and could provide

energy for generation of the system during non-sunny hours. Energy storage can be

accomplished through routes, i.e. sensible heat storage, storage of energy in phase changematerials, chemical reaction storage and electro-chemical storage. If the power generation is

considered by using solar pond then storage is not required.

2.4 Rankine Cycle Solar Thermal Power Generation System.

To convert solar energy into electricity through thermal conversion, researches in theworld have done a considerable work in varying capacity power systems. Organic fluid Rankine

cycle has been extensively used in these studies. The development work in the generation of

solar thermal power in higher range, i.e. 10 kW and above up to megawatts, is in line in various

parts of the world. The systems have been developed by using a solar pond, flat -plate collector, a


9/168

9

focussing collector (distribution type) or a heliostat. The heliostat systems are used normally in a

very high range of solar thermal power production (around megawatts). It will be described later.

A low temperature solar engine, using heated water from flat plate solar collector and

butane as the working fluid is shown in Fig.2.4.2,

Which is developed in France for lift irrigation. The system has array of flat-plate

collectors to heat water up to nearly 70C and in the heat exchanger, the heat of water is used for

boiling butane. The high-pressure butane vapour runs a butane turbine, which operates ahydraulic pump, which pumps the water from the well and used for irrigation. The exhaust

butane vapour from butane turbine is condensed in a condenser with the help of water, which is

pumped by the pump. This condensate is fed to the heat exchanger or butane boiler.

Fig.2.4.2. Schematic of a low temperature solar power plant

The system is applied for small power plants of about 10 kW capacities. It has the advantage of

simplicity.

2.5 Medium Temperature Systems with Concentrating Collectors

Cylindrical parabolic concentrating collectors (line Focus system) give a temperature

range of 250 to 700C with efficiency of 50- 70%. High temperature collectors such as parabolictype concentrators consists of many flat mirrors give a temp.range of 600-2000C with an

efficiency of 60-75%.

A simple parabolic cylindrical concentrator for medium temperature system is shown in

Fig. 2.5.3. It consists of a parabolic cylinder reflector to concentrate sun light on to a collecting

pipe within a Pyrex or glass envelope. A selecting coating of suitable material is applied to pipeto minimize infrared emission. The space between the transparent tubes surrounding the pipe can


10/168

10

be evacuated to reduce convection heat loss. Proper sun-tracking arrangement is made so that

maximum sunlight is focussed on the reflector. The line focus system, also called the trough

system, uses concentrators in the form of long troughs of cylindrical or parabolic cross-sections,which are lined with mirrors to collect and concentrate the sun's radiation onto a focal linear

conduit through which the primary coolant flows. Because of their geometry such troughs are

usually made to track the sun in only one plane, by being rotated about their focal line, Thus,other than solar noon, they receive sun's rays that get more inclined with respect to their

projected surface as the sun deviates from solar noon.

Fig. 2.5.3. Basic geometry of a parabolic cylindrical concentrator.

They, therefore, usually operate in the lower temperature ranges of about 90 to 315C. Line-

focus systems are thus believed suitable only for small-sized electric-generating systems for

which thermal efficiency is not of prime importance and for other applications such as driving

irrigation pumps, providing industrial process heat, space heating and cooling, and otherindustrial applications, but not for large scale electric generation. [Refer Fig.2.5.4 (see next

page).

Concentrating collectors use reflective surfaces to focus the sun's rays onto a receiver or

absorber where the solar energy heats a circulating fluid. The hot fluid can then be used directly

for an industrial process, to power a turbine for mechanical work, or to generate electricity.

Thermal storage systems accumulate thermal energy to be used during cloudy weather or

at night. Currently, storage system capacities range from "buffer" storage for short intervals such

as during cloud passage, to as long as six continuous hours.

Modularity is an advantage of a parabolic trough system. The basic collector module is a

row of troughs coupled to a drive motor that rotates the trough module about a single axis totrack the sun's position. A control system connects as many modules as required to raise the fluid

in the troughs to a specified outlet temperature. The sub-system are called delta T loops and

array of such loops is called a field. A field can consists of one loop or many, depending uponthe energy required for a particular application.


11/168

11

Fig.2

.5.4Schematicofasolarpowergenerationmethodusingcylindr ical

parabolic(line

focussing)concentrators.


12/168

12

The single-axis tracking trough concept exhibits favourable performance and cost-

effectiveness in the mid-temperature range, i.e.100C to 350C. Energy requirements within this

range are significant, including industrial process heat, production of mechanical or electricalenergy such as for irrigation pumping, steam generation for enhanced oil recovery, and "total

energy" production or cogeneration (i.e. providing for both electrical and direct heating

processes).

The five components of a complete parabolic trough system are :

(i) The concentrator with its support and drive system,

(ii) The receiver,

(iii) Thermal transport,

(iv) Controls andiv) Thermal storage (optional)

A variation of the line-focus system, called the line-focus bowl or the solar bowl concept,

it involves a fixed mirror collector, shaped in the form of a hemisphere and tilted upward fromthe horizon along a north-south line depending on the latitude of location. The slender cylindrical

receiver, helical coiled tubing on pipe, is supported by a cantilevered beam pivoted on a dual-axis mount to track the sun. The system can convert pressurized water to superheated steam that

can drive a turbine for the production of electricity. It can also use other heat transfer fluids at a

lower pressure and temperature than water for industrial process heat or for heating and cooling

buildings.

Bowls can be larger than other single-unit concentrating devices since the reflecting

surface does not have to be moved to track the sun. A 65-ft (20 m) diameter solar bowl wasdesigned and constructed in the United States to determine the technical feasibility of the

concept.

2.6 Stirling Cycle Solar Thermal Power Generation

The stirling cycle is an old concept, first proposed by Stirling in 1816. In its original form

the engine was slow, heavy and inefficient and with the advancement in I.C. engine this wasoverlooked. Again because of oil crisis researches started working on this engine as it can work

almost on any fuel including solar. Engine based on stirling cycle using solar energy have high

promise. Like any heat engine, the stirling engine goes through the four basic processes ofcompression, heating, expansion and cooling. This is illustrated in Fig. 2.6.5. There are three

basic type of stirling engine one, the ALPHA type, use two pistons. These pistons mutually

compress the working fluid in the cold space, move it to the hot space where it is expanded andthen move it back. The second and third type of engine uses a piston and a displacer. The piston

does the compressing and expanding, and the displacer does the gas transfer from hot to cold

space. The displacer arrangement with the power piston in line is called the BETA


13/168

13

Fig. 2.6.5. The Stirling cycle.

type of engine and if the piston off-set from the displacer, to allow a simple mechanicalarrangement, is called the GAMA type of stirling engine. As for as the main components of the

stirling cycle solar thermal power generation systems are concerned, basically they are solarenergy collection system. Working fluid, stirling engine and storage. Use of flat-plate solar

collector and solar pond cannot be made for this system, point focussing collector {paraboloidal)

is the only choice. Various working fluids like air, hydrogen and helium etc. have been tried.

Limitation of stirling cycle solar thermal power generation systems is that it generates a

fraction of horsepower as there is practical limit to the size of three dimensional concentrator dueto wind loading and their tracking.

2.7 Solar Thermal Power Generation Using Brayton Cycle

Brayton cycle is the basic gas power cycle for gas turbine power plants. Very less

research work has been done on this cycle for solar


14/168

14

Fig.2.7.6. Closed cycle gas turbine with on Brayton cycle

energy application due to capital cost. Heliostat type of solar collection system is supposed to beused in this system.

The schematic diagram of the closed cycle gas turbine Brayton cycle is shown in Fig.

2.7.6 along with TS and PV diagram. The closed Bray ton cycle using rotary compressors andgas turbines have been fully developed in the megawatt range for space application. But this

cycle has not been tried, for solar energy application due to capital cost. Its efficiency depends

strongly on the pressure ratio in the system. In solar energy system it is very costly to get high-pressure ratio. Some research institutions and research manufacturing companies in the world are

developing a solar power system using this cycle.

2.8 Tower Concept for Power Generation (Central Receiver Power Plants) :

High Temperature Systems

Two main approaches to solar power generation are (A) the solar furnace in which sun

light reflects from many different locations is concentrated on a single heat exchanger, and (B)

the solar farms where a large numbers of linear reflectors focus solar radiation on long pipeswhich collect heat.

The above two basic arrangements for converting solar radiation into electrical energyare also called, the central receiver system and the distributed collector system respectively.

In the central receiver system, known colloquially as the "Power tower" design, an arrayof sun-tracking mirrors (heliostats) reflects solar radiation onto a receiver mounted on top of a

central tower.

Solar energy absorbed in the central receiver is removed as heat by means of a heattransport fluid and converted into electrical energy in a turbine-generator. The distributed

collector system may consist of a number of parabolic trough-type (line focusing) collectors or


15/168

15

of parabolic dish-type (point focusing) collectors. The absorber pipes (or receivers) of the

individual collectors are connected so that all the heated fluid is carried to a single location

where the electricity is generated. The basic difference between the central receiver anddistributed collector systems is that in the former the solar energy falling on a large area is

transmitted to a central point as radiation, but in the latter, the energy is carried as heat in a fluid.

Analysis of the two systems indicates that they may have different preferred applications.

The energy losses in transmission by radiation are less than in transport as a hot fluid. Hence,

higher temperature should be attainable at the turbine inlet with the central receiver design thanif the same amount of fluid were transported from distributed collectors. Furthermore, in a large

distributed collector system, the costs of the long pipelines and of the energy required to pump

the heat-transport fluid through them would be considerable. However, these costs are offset

somewhat by the high cost of heliostats and the central receiver tower.

It appears, therefore, that the central receiver system is preferable for the large-scale

generation of power for an electric utility.

On the other hand, the distributed collector design may be more suitable for power plants ofsmaller electrical capacity, perhaps less than 2 megawatts (MW). The so-called total energy (or

cogeneration) systems, intended to supply both electric power and heat or process steam to aninstitution, small community, or industry, may fall into the latter category.

(A) Tower Power Plants (Central Receiver Systems)

(I) Principle and working: - In this system as stated the incoming solar radiation is focused to a

central receiver or a Boiler mounted on tall tower using thousands of plane reflectors, which are

steer able about two axes and are called heliostats.The tower concept illustrated in Fig. 2.8.7 is an example of the solar furnace approach.

Fig.2.8.7 Tower concept for power generator.

As shown in it an assembly of separate flat mirrors is oriented in such away that all theincident light beams are reflected towards the same point, the concentration factor achieved is

roughly equal to the number of mirrors.


16/168

16

A schematic view of an electric power plant making use of this idea is shown in Fig.

2.8.8. The mirrors are installed on the ground and are oriented so as to reflect the direct beam

radiation into an absorber or receiver (boiler) which is mounted on the top of a tower locatednear the centre of the field of mirrors to produce high temperature

2.9 Schematic of a solar-thermal central-receiver system power plant.

This factor makes it possible to position the boiler in the field of view of all mirrors at all

hours of the day. Beam radiations incident on boiler absorbed by black pipes in which working

fluid circulates and is heated. The hot working fluid is allowed to drive a turbine and producemechanical energy. The turbine, which is coupled to an alternator, produces electrical energy. As

in any thermodynamic conversion, the heat sink is provided. Suitable heat storage is also

provided to supply the heat energy during the periods of cloudiness.

The characteristics of the system are summarized below:

i. All parts of it use known technologies.

ii. The heat conversion sub-system comprising a turbine and an alternator may be of

conventional type, thus avoiding the need of future development work.(iii) Boiler or absorber is a light absorber, and low volume unit, resulting in low heat

losses from it.

Fig.2.8.8 Schematic of a Solar-thermal central-receiver system power plant


17/168

17

(iv) The heat need not be transported over long distances as compared to parabolic

troughs, thus avoiding excessive plumbing and heat losses, all the power is

transferred to the central point by optical transmission.

Fig. 2.9.9 shows the schematic view of central receiver solar thermal power plant using a field of

flat mirrors and a gas turbine.

Fig.2.9.9 Schematic of a central tower receiver associated with a field of flat mirrors and a gas turbine.

The system should incorporate storage for nighttime and cloudy periods, as shown in Fig.

2.8.8. The receiver output is made greater than that required by the steam cycle, and the excess

output during periods of greatest solar incidence is bypassed to a thermal storage system. During

periods of low or no solar incidence, the feed water is shunted to the storage system, instead of tothe receiver, where it vaporizes for use in the turbine. Proper valving in the system allows

operation in either mode.

Because solar-thermal electric plants are most likely to be located in hot arid areas where

land is plentiful (for the large heliostat field) and where the sun's energy is plentiful and

dependable, but where cooling water is scarce, the condenser water is most probably cooled by adry-cooling tower. Such towers are less effective and cause a reduction in Rankine cycle

efficiency but require practically no make up water. )


18/168

18

Chapter-2 (B)

SOLAR ELECTRIC POWER GENERATION:

SOLAR PHOTO- VOLTAIC

2.10 Introduction:

The direct conversion of solar energy into electrical energy by means of the photovoltaic

effect, that is, the conversion of light (or other electromagnetic radiation) into electricity. The

photovoltaic effect is defined as the generation of an electromotive force as a result of theabsorption of ionizing radiation. Energy conversion devices, which are used to convert sunlight

to electricity by the use of the photovoltaic effect, are called solar cell. A single converter cell is

called a solar cell or, more generally, a photovoltaic cell, and combination of such cells;designed to increase the electric power output is called a solar module or solar array.

Photovoltaic cells are made of semiconductors that generate electricity when they absorblight. As photons are received, free electrical charges are generated that can be collected on

contacts applied to the surfaces of the semiconductors. Because solar cells are not heat engines,

and therefore do not need to operate at high temperatures, they are adapted to the weak energy

flux of solar radiation, operating at room temperature. These devices have theoretical efficienciesof the order of 25 per cent. . Actual operating efficiencies are less than half this value, and

decrease fairly rapidly with increasing temperature.

The best-known application of photovoltaic cells for electrical power generation has been

in spacecraft, for which the silicon solar cell is the most highly developed type. The silicon cell

consists of a single crystal of silicon into which a doping material is diffused to form a

semiconductor. Since the early days of solar cell development, many improvements have beenmade in crystal growing and doping, electrical contact and cell assembly and production

methods. Large number of cells has been manufactured with areas 2 x 2 cm, efficiencies

approaching 10 per cent, and operating at 28C. The efficiency is the power developed per unitarea of array divided by the solar energy flux in the free space (1.353 kW/m2).

For terrestrial applications, silicon solar cells have shown operating efficiencies of about12 to 15 per cent. Though silicon is one of the earth's most abundant materials, it is expensive to

extract (from sand, where it occurs mostly in the form SiO2 and refine to the purity required for

solar cells. The greater barrier to solar cell application lies in the costs of the cells themselves.Reducing the cost of silicon cells is difficult because of the cost of making single crystal. One

very promising method is being developed to produce continuous thin ribbons of single crystalsilicon to reduce fabrication costs. Cells made from the ribbon have so far shown efficiencies ofaround 8 per cent. Several other kinds of photocells are in the laboratory stage of development.

Cadmium sulfide and CdS/Cu2S cells are other possibilities. So far, efficiencies have been in the

range of 3 to 8 per cent, and these cells have been less durable than silicon cells owing to

degradation with exposure to oxygen, water vapour and sunlight, especially at elevatedtemperatures. The active part of the CdS cell is a thin polycrystalline layer of Cds, about 10um

thick, on which a layer of CU2S compound perhaps 0.1um thick is grown. These cells can be


19/168

19

made by deposition on long sheets of substrates a process that might be adaptable to expensive

mass production,

Photovoltaic cells could be applicable to either small or large power plants, since they

function well on a small scale, and may be adaptable to local energy generation on building

rooftops. The cost of energy storage and power conditioning equipment might, however, makegeneration in large stations the most economical method. Solar cells have also been used to

operate irrigation pumps, navigational signals, highway emergency call systems, rail road

crossing warnings, automatic meteorological stations, etc., in location where access to utilitypower lines is difficult.

A PV (photo-voltaic) system consists of:

(i) Solar cell array (ii) Load leveler (iii) Storage system (iv) Tracking system (where necessary).

In actual usage, the solar cells are interconnected in certain series/parallel combinations

to form modules. These modules are hermetically sealed for protection against corrosion,

moisture, pollution and weathering. A combination of suitable modules constitutes an array. Onesquare meter of fixed array kept facing south yields nearly 0.5 kWh of electrical energy on a

normal sunny day if the orientation of the array is adjusted to face the sun's rays at ally tune, theoutput can increase by 30 per cent. Solar PV system call produces an output only if sunlight is

present. If it is required to be used during no sunshine hours, a suitable system of storage

batteries will be required.

2.11 Solar Cell Principles. The photo-voltaic effect can be observed in nature in a variety of

materials, but the materials that have shown the best performance in sunlight are the semi-

conductors as stated above. When photons from the sun are absorbed in a semiconductor, theycreate free electrons with higher energies than the electrons, which provide the bonding in the

base crystal. Once these electrons are created, there must be an electric field to induce these

higher energy electrons to flow out of the semi-conductor to do useful work. A junction ofmaterials, which have different electrical properties, provides the electric field in most solar

cells.

To obtain a useful power output from photon interaction in a semi-conductor three

processes are required.

1. The photons have to be absorbed in the active part of the material and result in

electrons being excited to a higher energy potential.2. The electron-hole charge carrier created by the absorption must be physically

separated and moved to the edge of the cell.

3. The charge carriers must be removed from the cell and delivered to a useful loadbefore they loose their extra potential.

For completing the above processes, a solar cell consists of :

(a) Semi-conductor in which electron hole pairs are created by absorption of

incident solar radiation.

(b) Region containing a drift field for charge separation, and


20/168

20

(c) Charge collecting front and back electrodes.

The photovoltaic effect can be described easily for p-n junction in a semi-conductor. Inan intrinsic semi-conductor such as silicon, each one of the four valence electrons of the material

atom is tied in a chemical bond, and there are no free electrons at absolute zero. If apiece of such

a material is doped on one side by a five valence electron material, such as arsenic orphosphorus, there will be an excess electrons in that side, becomes an n-type semiconductor. The

excess electrons will be practically free to move in the semiconductor lattice. When a three

valence electron material, such as boron, drops the other side of the same piece there will bedeficiency of electrons leading to a p-type semiconductor. This deficiency is expressed in terms

of excess of holes free to move in the lattice. Such a piece of semi-conductor with one side of the

p-type and the other of the n-type is called a p-n junction. In this junction after the photons are

absorbed, the free electrons of the n-side will tend to flow to the p-side, and the holes of the n-side will tend to flow to the n region to compensate for their respective deficiencies. This

diffusion will create an electric field Eli' from the n region to the p-region. This field will

increase until it reaches equilibrium for V ", the sum of the diffusion potentials for holes and

electrons. If electrical contacts are made with the two semiconductor materials and the contactsare connected through an external electrical conductor, the free electrons will flow from the n-

type material through the conductor to the p-type material (Fig, 2.11.10). Here the free electronswill enter the holes and become bound electrons; thus, both free electrons and holes will be

removed. The flow of electrons through the external conductor constitutes an electric current,

which will continue as long as more free electrons and holes are being formed by the solar

radiation. This is the basis of photovoltaic conversion, that is, the conversion of solar energy intoelectrical energy. The combination of n-type and p-type semiconductors thus constitutes a

photovoltaic (PV) cell or solar cell. All such cells generate direct current, which can be

converted into alternating current if desired.

The most normal configuration for a solar cell to make a p-n junction semiconductor is as

shown schematically in Fig. (2.11.10). The junction of the 7) type' and 'n type' materials providesan inherent electric field which separates the charge created by the absorption of sunlight. This

p-n junction is usually obtained by putting a p-type base material into a diffusion furnace

containing a gaseous n-type dopant.

Fig. 2.11.10


21/168

21

such as phosphorus and allowing the n-dopant to diffuse into the surface about 0.2 !1m. The

junction is thus formed slightly below the planar surface of the cell and the light impingesperpendicular to the junction.

The positive and negative charges created by the absorption of photons are thusencouraged to drift to the front and back of the solar cell. The back is completely covered by a

metallic contact to remove the charges to the electric load. A fine grid of narrow metallic fingers

aids the collection of charges from the front of the cell. The surface coverage of the conductingcollectors is typically about 5 per cent in order to allow as much light as possible to reach active

junction area. An antireflective coating is applied on the top of the cell.

Fig. 2.11.11 p-n. Junction electric fields.

Fig. 2.11.11 demonstrates how this p-n junction provides an electrical field that sweeps

the electrons in one direction and the positive holes in the other. If the junction is in

thermodynamic equilibrium, then theFermi energy must be uniform throughout. Since the Fermilevel is near the top of the gap of an n-doped material and near the bottom of the p-doped side,

an electric field must exist at the junction providing the charge separation function of the cell.Important characteristic of the Fermi level is that, in thermodynamic equilibrium, it is always

continuous across the contact between the two materials.

Each of the individual solar cells will produce power at about 0.5 V with the current

directly proportional to the cell's area. The individual cells are connected in series-parallel

combination to meet the voltage, power and reliability requirements of the particular application.

Space cells are covered with transparent 'cover slips' to absorb the high-energy particles in spacethat could cause damage in the cell and result in a degradation of output. For terrestrial

applications, the solar cell panels have to be encapsulated to protect them from atmosphericdegradation due to oxidation of the metal contacts, which would cause peeling and open circuitsmaterials such as glass, acrylics or silicon epoxies are used to provide a clear, weather fight front

covering for the panels.


22/168

22

2.12 A Basic Photovoltaic System for Power Generation

A basic photovoltaic system integrated with the utility grid is shown in Fig. 2.12.12. Itpermits solarly generated electrical power to be delivered to a local load. It consists of:

(i) Solar -Array, large or small, which converts the insolation to useful DC electrical power.

Fig. 2.12.12. Basic photovoltaic system integrated with power grid.

(ii) A Blocking Diode, which lets the array-generated power flow only toward the battery or

grid. Without a blocking diode the battery would discharge back through the solar array during

times of no insolation (recall from Fig. 5.6.3 that the cell equivalent circuit has a forward biaseddiode in it).

(iii) Battery Storage, in which the solarly generated electric energy may be stored.

(iv) Inverter/converter, usually solid state which converts the battery bus voltage to AC of

frequency and phase to match that needed to integrate with the utility grid. Thus it is typically aDC, AC inverter. It may also contain a suitable output step up transformer, perhaps some

filtering and power factor correction circuits and perhaps some power conditioning, i.e. circuitry

to initiate battery charging and to prevent over charging. Power conditioning may be shown as a

separate system functional block. This block may also be used in figure shown to function as arectifier to charge the battery from the utility feeder when needed and when no insolation was

present.

(v) Appropriate Switches and Circuit Breakers, to permit isolating parts of the system, asthe battery. One would also want to include breakers and fusing protection (not

shown) between the inverter output and the utility grid to protect both thephotovoltaic system and the grid.


23/168

23

2.13 Solar Cell Modules (Solar Photovoltaic Arrays)

There may be tracking arrays or modules or fixed arrays. A tracking array is defined asone, which is always kept mechanically perpendicular to the sun-array line so that all times it

intercepts the maximum insolation. Such arrays must be physically movable by a suitable prime

mover and are generally considerably more complex than fixed arrays. A fixed array is usuallyoriented east west and tilted up at an angle approximately equal to the latitude of the site. Fixed

arrays are mechanically simpler than tracking arrays. Thus the array designs fall into two broad

classes:

(1) Flat-plate Arrays. Wherein solar cells are attached with a suitable adhesive to some kind of

substrate structure usually semi-rigid to prevent cells being cracked.

This technology springs from the space-related photovoltaic technology, and many such

arrays have been built in various power sizes.

(2) Concentrating Arrays. Wherein suitable optics, e.g. Fresnel lenses, parabolic mirrors,compound parabolic concentrators (CPC), and others, are combined with photovoltaic cells in an

array fashion. This technology is relatively new to photovoltaic in terms of hardwaredevelopment, and comparatively fewer such arrays have actually been built.

Solar cell connecting arrangements. Cells may be connected in parallel to achieve the

desired current and then stacked in series to achieve the desired voltage. The optimum operatingvoltage of a photovoltaic cell is generally about 0.45 volt at normal temperatures, and the current

in full sunlight may be taken to be 270amperes/sq. m. If the exposed area of a cell is 40 sq. cm or

40 x 10-4 sq. m. the current would be 1.08 amperes and the electric power output 0.45 x 1.08 =0.49 watts, in full sunlight. A decrease (or increase) in the solar radiation has little effect on the

voltage, but the current and power are decreased (or increased) proportionately.

By combining a number of solar cells in series (i.e. in a string) the voltage is increased

but the current is unchanged. For example, 110 volts, for operating commercial tools, motors, or

domestic appliances,

Would require = 244 cells in series. To increase the current output at the same time,

several strings of 244 cells would be connected in parallel, as depicted in Fig. (2.13.13).

Fig. 2.13.13 Solar cell arrangements in series and parallel.


24/168

24

Suppose there were ten such strings in parallel, the current under optimum conditionswould then be 10 x 1.08 = 10.8 amperes, and the power output would be 10.8 x 110 = 1190 watts

or 1.19 kW. The so-called solar panels on spacecraft consist of modules (or arrays) of cells

connected in series and parallel to produce the required voltage and power.

If even a single cell in a string should fail, the whole string would become inoperative. The cells

in the remaining strings would maintain the voltage, but the current (and power) output of thesystem would be decreased by the loss of one string of cells. A short circuit in a cell would not

disable the string, although there would be a slight drop in voltage. There is a possibility that the

other cells might cause current to flow in the wrong direction through any string having a

reduced voltage. This danger is eliminated by including a diode, which permits current flow inone direction only, at end of each string. Instead of a number of strings of cells in parallel, the

current (and power) could be increased by locating a single string of cells at the focal line of a

sun tracking, parabolic trough, concentrating collector. There would be some decrease in voltage

because of the inevitable higher temperature of the solar cell material, but the current wouldincrease approximately in proportion to the concentration factor of the collector. Thus, with

concentration factors of 100, the current from a single string would be increased to about 100amperes. If the number of cells in the string were sufficient to produce 110 volts, the total

electric power output would be approximately 12 kW.


25/168

25

Chapter-3

WIND POWER PLANTS

3.1 The power from the wind

The power in the wind can be computed by using the concept of kinetics. The windmillworks on the principle of converting kinetic energy of the wind to mechanical energy. We knowthat power is equal to energy per unit time. The energy available is the kinetic energy of thewind. The kinetic energy of any particle is equal to one-half its mass times the square of itsvelocity, or 1/2 MV2. The volume of air passing in unit time, through an area A, with velocity V,is A . V and its mass M is equal to its volume, multiplied by its density p of air, or

M=p.AV

(M is the mass of air traversing the area A swept by the rotating blades of a wind mill typegenerator).

Substituting this value of the mass in the expression for the kinetic energy, we obtain,

Kinetic energy =. pAVV2

=. pAV3 watts

It is not possible to convert all the wind energy into another form of energy because theload would reduce the wind speed through the generator to zero, thus stopping the machine. It is

concluded from theoretical considerations that the maximum conversion rate is 16/27 (0.593) ofthe energy of the wind, (for horizontal axis wind mill).

Wind power P is given by the following expression

P =.pAV3CpCp = power coefficient

= Energy availableEnergy input

i.e. fraction of the available energy that is converted is called the power coefficient.

P can be written =KAV3 Where K is a constant.

There are losses to be encountered before the energy is delivered, due to bearings, gearsand other transmission system, represented through nm, i.e. mechanical efficiency.

Here P = nm KA V3


26/168

26

Further conversion factors like nE and nT, electrical and transmission efficiency beincluded in the above equation.

In practice, the wind power is measured by wind mill type devices or anemometers. Thewind power P is related to the wind speed by the empirical formula :

P = 0.37 ( )3

where P is in kilowatts per square metre of area normal to direction of the wind, and V is inmetres per second.

It is important to note that the convertible power or energy is proportional to the cube ofthe wind speed. Thus if the wind speed decreases by 20%, the power output is reduced by almost50%. The wind speed may vary considerably from day to day and from season to season.Standard wind maps are available, in which the mean annual wind velocity zones are marked.One can see that only some parts of the country have reasonably good velocity from the point of

view of wind mill operation.There are various ways the data on wind behaviour is collected depending on the use it is

intended to be put into. The hourly mean wind velocity as collected by the meteorologicalobservations is the basic data used in wind mill designs. The hourly mean is the averaged over aparticular hour of the day, over the day, month, year and years. The factors, which affect thenature of the wind close to the surface of the earth, they are:

(i) Latitude of the place,(ii) Altitude of the place,(iii) Topography of the place,(iv) Scale of the hours, month or year.

Winds being an unsteady phenomenon, the scale of the periods considered is animportant set of data required in the design. The hourly mean velocity (for many years) providesthe data for establishing the potential of the place for tapping the wind energy. The scale of themonth is useful to indicate whether it is going to be useful during particular periods of the yearand what storage if necessary is to be provided for. The data based on scale of the hour is usefulfor mechanical aspects of design.

In addition to the data on the hourly mean velocity, two other informations required arespells of low wind speeds and gusts. A number of criteria can be applied in estimating theimportance of wind potential as a function of height and location. First of all, careful siting isimportant because wind speed near the ground is greatly affected by houses, trees and similarfeatures. Wind speed increases with height above ground, the rate of increase being about thesame at all locations. Therefore, if the wind speed at a given height is known, the speed at anyother height may be calculated.

V10


27/168

27

The wind flow in the atmosphere is also influenced by some other parameters. Thefollowing guidelines may be useful (all figures are for a height of 20 m, which seems to be areasonable minimum):

1. The best sites for wind energy are found off shore and on the sea coast. An average

value on the coast is 2400 kWh/m2 per year.

2. The second best sites are in mountains. Atypical average value is 1600 kWh/m2 per year.

3. The lowest level of the wind energy is found in plains where values are generally threeor four times lower than that at the coast. A typical average is 750 kWh/m2 per year.

As regards climates, other criteria interact with those discussed thus for:

1. In the humid equatorial region, there is virtually no wind energy, whether at sea,along the coast or in the mountains.

2. The amount of convertible wind energy is fair or good in dry or hot climates, as wellas in temperature and cold climates.

3. In some warm, windly countries, wind energy may not be usual because of thefrequency of cyclones. (eq. Japan, Caribbean area).

3.2 Advantages and disadvantages of wind energy:-

Advantages of wind energy are:

(i) It is a renewable source of energy.

(ii) Like all forms of solar energy, wind power systems are nonpolluting, so it has noadverse influence on the environment.

(iii) Wind energy systems avoid fuel provision and transport.

(iv) On a small scale, upto a few kilowatt system, is less costly. On a large scale costscan be competitive with conventional electricity and lower costs could be achievedby mass production.

Disadvantages of wind energy are:

(i) Wind energy available is dilute and fluctuating in nature.

(ii) Unlike water energy wind energy needs storage capacity because of its irregularity.

(iii) Wind energy systems are noisy in operation; a large unit can be heard manykilometers away.


28/168

28

(iv) Wind power systems have a relatively high overall weight, because they involve theconstruction of a high tower and include also a gearbox, a hub and pitch changer, agenerator, coupling shaft etc. For large systems a weight of 110 kg/kW (rated) hasbeen estimated.

(v) Large areas are needed; typically, propellers 1 to 3 m in diameter deliver power in the30 to 300 W ranges.

(vi) Present systems are neither maintenance free nor practically reliable. However, thefact that highly reliable-propeller engines are built for aircraft suggests that thepresent troubles could be overcome by industrial development work.

3.3 Windmills Types and Performance

A wind mill is a machine for wind energy conversion. A wind turbine converts the

kinetic energy of the wind's motion to mechanical energy transmitted by the shaft. A generatorfurther converts it to electrical energy, thereby generating electricity.

Wind mills are generally classified as

- Horizontal axis type, and- Vertical axis type,

Depending on their axis of rotation.

Some authors refer to them also as wind axis rotors and crosswind axis rotors

respectively. In the former types, the rotors are oriented normal to the direction of wind, while inthe latter types, the effective surface of the rotor moves in the same direction as the wind.Horizontal axis wind mills further sub-classified as single bladed, double bladed, multiblade andbicycle multibladed type, sail, wing, multibladed are example of horizontal axis wind mills.Savonius and Darrius rotors are example of vertical axis rotors.

The vertical axis wind mill is again sub-divided into two major types:

(i) Savonius or'S' type rotor mill (low velocity wind),

(ii) Darrieus type rotor mill (high velocity wind), based on the working speed of the

machine and the velocity ranges required by the machine for operation.

Vertical axis machines are of simple design as compared to the horizontal axis.

3.4 Vertical Axis Type Wind Mills

(a) The Savonius Rotor. Perhaps the simplest of the modern types of wind energyconversion systems is the Savonius rotor, which works like a cup anemometer. S.J. Savonius


29/168

29

invented this type in the year 1920. This machine has become popular since it requires relativelylow velocity winds for operation.

Constructional details and principle of operation: -. It consists of two half-cylindersfacing opposite directions in such a way as to have almost an S-shaped cross-section (refer Fig.

3.4.1).

Fig. 3.4.1. The Savonius rotor and its stream flow.

Their two-semi-circular drums are mounted on a vertical axis perpendicular to the winddirection with a gap at the axis between the two drums. Irrespective of the wind direction therotor rotates such as to make the convex sides of the buckets head into the wind. From the rotorshaft we can tap power for our use like water pumping, battery charging, grain winnowing etc.However, instead of having two edges together to make an S-shape, they overlap to leave a widespace between the two inner edges, so that each of these edges is near the central axis of theopposite half cylinder, as shown in the figure. The main action of the wind is very simple; theforce of the wind is greater on the cupped face that on the rounded face. In detail it is a bit more

complicated. The wind curving around the backside of the cupped face exerts a reduced pressuremuch as the wind does over the top of an airfoil and this helps to drive the rotation. The wide slotbetween the two inner edges of the half cylinders, lets the air whip around inside the forward-moving cupped face and then around the inside of the backward moving face, thus pushing bothin the direction of the rotation.

Advantages and Disadvantages of Savonius Rotor. A Savonius wind energy conversionsystem has a vertical axis, which eliminates the expensive power transmission system from therotor to the axis. Since it is a vertical axis machine it does not matters much about the winddirection. The machine performs even at lower wind-velocity ranges.

Another advantage of the Savonius rotor is its low cut in speed (the wind speed requiredfor switching electric power into the line); it produces power effectively in winds as slow as 8km/hr, whereas most-propeller type wind mills require about 16 km/hour, for effective operationand large wind mill still more. This means that it is useful for most of the time and is thus lessdependent on storage or supplementary power.

In common with all vertical-axis machines, the Savonius rotor has the advantage that theweight of the electric generator may be carried at ground level without the use of bevel gears.This is, however, not a very important advantage.


30/168

30

The main disadvantage of the type of machine is that it is too solid, having so much metalor other material surface compared with the amount of wind intercepted. This not only leads toexcessive weight for a large installation but also leaves the machine at the mercy of severe storm,since there is no way to reduce the effective area.

It is not useful for a very tall installation because long drive shaft problems and also thebracing of the top most bearing above the rotor of a very tall vertical-axis machine is awkward,requiring very long guy wires. In a conventional horizontal-axis wind electric machine with thegenerator a loft, the strength of the structure required to carry the added weight of the generatoris small compared with that needed to servive a severe storm and the generator housing addslittle to the area presented to the storm.

Areas of concern: - The Savonius rotor has moderately good efficiency and satisfactory startingcharacteristics, the latter being particularly important for use with a positive displacementpumps. The rotor area requirement for getting the required amount of power is higher than anyother systems. It is commonly used for pumping, and to operate small agricultural machines like

winnowers, blowers, bird scares, grinders etc. Another use of this type of wind energyconversion system is to use this machine along with Darrieus rotor for starting purposes.

(b) The Darrieus type machines (High velocity wind). This machine was invented originallyand patented in 1925 by G.J.M. Darrieus a French Engineer and this concept has recently beengiven serious consideration once again. This type of windmills is already in use in Canada. Asnoted, a modern rapidly rotating propeller type windmill, by use of an efficient airfoil,effectively intercepts large area of wind with a small blade area. The Darrieus windmill is a typeof vertical axis machine that has the same advantage. An additional advantage is that it supportsits blades in a way that minimizes bending stresses in normal operation.

Constructional details and principle of operation . In this type of machine, the blades are curvedand attached to hubs on the vertical shaft at both ends to form a cage-like structure suggestive ofan ordinary egg beater (refer Fig. 3.4.2). The curved blades has the shape that a rope would takeif subjected to centrifugal force in rapid rotation, some think like the shape of the rope in theexercise of skipping rope. Darrieus rotors have three symmetrical aerofoil blades, both ends ofwhich are attached to a vertical shaft. Thus the force in

Fig.3.4.2.Vertical axis wind mill.


31/168

31

the blade due to rotation is pure tension. This provides a stiffness to help withstand the windforces it experiences. The blades can thus be made lighter than in the propeller type. Whathappens is a severe storm is another question as yet an-answered. When rotating, these air foilblades provide a torque about the central shaft in response to a wind stream. This shaft torque isbeing transmitted to a generator at the base of the central shaft for power generation.

Fig. 3.4.3. Wind mill blade as an airfoil.

As shown in Fig. 3.4.3 we see that the force that propels the blades of a conventionalwind will comes from the chord of the air foil, being tilted away from the direction of motion, sothat the thrust that is almost at right angles to the airfoil is tilted toward the forward direction andhas a component in that direction, labelled "forward thrust". The remarkable thing about theDarrieus rotor is that it cannot have the advantage of this tilt of the airfoil, and yet works

Fig. 3.4.4 Blade orientation in the Darrieus rotor.

without it, with the chord directly along the tangent to the circular path in the equatorial cross-section shown in Fig. 3.4.4. If the chord were tilted away from the tangent so as to tilt the thrustforward where the wind meets the air foil on the windward side of the circle, as indicated by the


32/168

32

broken lines in the figure then the other side of the circle, the wind would meet the other side ofthe aerofoil and the thrust would be tilted backward to retard the motion.

Advantages and Disadvantages. Advantages of such system are:

(1) The major advantage of this design is that the rotor blades can accept the wind fromany point of the compass.

(2) Another added advantage is that the machine can be mounted on the groundeliminating tower structures and lifting of huge weight of the machine assembly,i.e. it can be operated close to the ground level.

(3) Since this machine has vertical axis symmetry it eliminates yaw controlrequirement for its rotor to capture wind energy. A dual purpose and relativelysimple shaft axis support is anticipated as well as ground level power outputdelivery due to presence of vertical shaft. This may in turn, allow easier access and

serviceability.(4) Airfoil rotor fabrication costs are expected to be reduced over conventional rotor

blade costs.

(5) The absence of pitch control requirements for synchronous operation may yieldadditional cost savings.

Disadvantages.

(1) Although a Darrieus has much directional symmetry for wind energy capture, itrequires external mechanical aid for start up.

(2) Rotor power output efficiency of a Darrieus wind energy conversion system is alsosomewhat lower than that of a conventional horizontal rotor.

(3) Because a Darrieus rotor is -generally situated near ground uroximity, it may alsoexperience lower velocity wind compared to a tower mounted conventional windenergy conversion system of comparable projected rotor disc area. This may yieldless energy output.

(4) Because a Darrieus rotor encounters greatly varied local flow conditions perrevolution greater vibratory stresses are encountered which will affect, rotor systemlife. High tension cable tidown of tower-shaft may require large expensive bearing forsupport.

(5) Finally since a Darrieus rotor cannot be yawed out of the wind or its blades feathered,special high torque braking system must be incorporated.


33/168

33

3.5 Horizontal axis type Wind mills. The blade of the windmill may have a thincross-section or the more efficient thick cross section of an aerofoil as suggested in Fig. 3.4.3The motion causing the "wind due to motion" here is the rotation of the blades. At the tip of theblades of a modern wind turbine, the velocity is about six times the wind velocity. This meansthat the blades are set rather flat at a small angle with the plane of the rotation and almost at right

angles to the direction of the wind so that the effective wind properly approach from ahead of theleading edge. At other parts of the blade, between the tip and the axle, the velocity and the idealset of the aerofoil is at a greater angle to the plane of rotation. Ideally the blade should betwisted, but because of construction difficulties this is not always achieved.

Some of the horizontal axis type windmills are briefly described below.

(i) Horizontal axis using two aerodynamic blades. In this type of design, rotor drives agenerator through a step-up gearbox. The blade rotor is usually designed to be orienteddownwind of the tower. The components are mounted on a bedplate, which is attached on apintle at the top of the tower. This arrangement is shown schematically in Fig. 3.5.5. The rotors

blades are continuously flexed by unsteady aerodynamic, gravitational and inertia loads, whenthe machine is in operation. If the blades are made of metal, flexing reduces their fatigue life.

Fig.3.5.5

With rotor the tower is also subjected to above loads, which may cause serious damage. If thevibration modes of the rotor happen to coincide with one of the natural mode of vibration of thetower, the system may shake itself to pieces. Because of the high cost of the blade rotors with

more than two blades are not recommended.Rotors with more than two, say 3 or 4 blades would have slightly higher power

coefficient.

(2) Horizontal axis propeller type using single blade. In this arrangement, a long blade ismounted on a rigid hub (Fig. 3.5.6).


34/168

34

Fig. 3.5.6 Horizontal axis single blade wind mill.

Fig. 3.5.7 Performance of wind mills.


35/168

35

occurs at lower (Utip/V) ratio. This implies that for high performance, the blades rotate athigh r.p.m. (and generally have air-foils for the surface), consequently, the torque is low. Thesetypes are preferred for electric power generation. The multiblade types with high starting torqueon the other hand are more suitable for pumping water. In practice, it is impossible to build winddriven generators capable of operating at the same efficiency at all wind speeds. First, there is a

minimum wind speed below which no power can be generated because of friction losses. Also,above a selected maximum speed, i.e. the rated wind speed, the extracted power is held constantby stabilizing the rotor speed, for instance, when the wind speed exceeds the maximum selectedby the designer, the rotor blades are progressively turned on their axes to reduce the effectivearea facing the wind. As a result, the fraction of power extracted decreases as the wind speedincreases beyond the rated speed. Finally at about 30 m/sec, the rotor is furled to avoid damage.

Only at intermediate wind speeds does the system efficiency reach its optimum' and thepower extracted then follows a y3 law. The range of optimum operation depends on the engine,which was selected so as to give the optimum output over the year. In our example, this could bein the range 10-14 m/sec, 14m/sec being the rated velocity maintained also at higher wind

speeds; only there depending on the degree of sophistication, could between 70 and 85% of theconvertible wind energy by transforming into kinetic energy by the rotor. Upto 20% of thisenergy would be lost in the gear type transmission, which connects the rotor shaft to the electricgenerator. The energy, which is available for conversion over all wind speed, is a function of thewind speed duration spectrum. There is a similarity with solar cells, the efficiency of which isstrongly affected by the wavelength spectrum of sun's radiation. For exploitable spectra, thisspectral efficiency is comprised between 8% and 20%. Hence, altogether, the total systemefficiency of a wind generator amount to 3-7%.

Table 3.1

Percentage of Percentage of total available max. available

Total wind energy 100%,

Maximum theoretical convertible 60% 100%

Steam efficiency (0.60) at rated wind 36% 60%speed. (Rotor efficiency 0.75,transmission gear efficiency 0.80)

Energy integrated over the total speed 3-7% 5-12%spectrum (efficiency 0.08-0.2)

As a general rule, the conversion efficiency at a given location depends on what might betermed the wind quality: a steady wind is the ideal case, never encountered in practice, whereas awind that varies greatly in speed is difficult to convert.


36/168

36

Chapter-4

BIOMASS-POWER PLANTS

4.1 Introduction

Most solar energy applications come under the following three classes:

(i) Trapping of sunlight as heat-photo thermal.

(ii) Direct electric conversion-photovoltaic.

(iii) Photo chemical, i.e. solar energy is stored in the form of chemical energy as in theprocess of photosynthesis. In this mode the energy is not transformed to heat but isutilized in atomic and molecular systems, which undergo chemical changes, and

biomass is produced. This biomass is then used directly by burning or is furtherprocessed to produce more convenient liquid and gaseous fuels. As the word-clearlysignifies, Biomass means organic matter. Then photochemical approach meansharnessing of solar energy by photosynthesis. Hence,

Solar energy Photosynthesis Biomass Energy generation

Out of several sources of renewable energy like solar, wind, ocean thermal energy, tidaland wave energy, energy through biomass are important feature especially in our country.

Biomass resources fall into three categories:

(i) Biomass in its traditional solid mass (wood and agricultural residues) and (ii) biomass in non-traditional form (converted into liquid fuels). The first category is to burn the biomass directlyand get the energy. In the second category, the biomass is converted into ethanol (ethyl alcohol)and methanol (methyl-alcohol) to be used as liquid fuel in engines. The third category is toferment the biomass anaerobically to obtain a gaseous fuel called biogas. It is about this biogastechnology detailed discussion will be given in subsequent article.

The utilization of biogas as a source of energy goes back to the beginning of this century whensewage sludge was anaerobically digested and the resulting gas collected and utilized in thesewage treatment plant itself for heating the sludge during digestion. The idea of production of

biogas from domestic and farm-yard wastes and its utilization in rural areas as a source of energyoriginated in India in the late thirties with the Khadi movement which was concerned with thescale of tree-felling in rural areas for fire wood. It was also argued that burning of cow dungcakes, a wasteful of a valuable resource, which could be better, utilized for fertilizing the fields.A..naerobic digestion of the wastes results not only in valuable biogas production but also in aslurry whose fertilizer value is almost intact or even better since the fertilizing components in thedigested slurry are directly utilizable by plants.


37/168

37

4.2 Photosynthesis

Photosynthesis in the plants is an example of biological conversion of solar energy intosugars and starches, which are energy rich compounds. So if we plant fast growing trees havinghigh photo-synthesis efficiency we can harvest and burn them to produce steam in a similar

manner as in thermal power stations ultimate to produce the electric power. Such an energyplantation would be a renewable resource and an economical means of harnessing solar energy.However, photosynthesis concepts are less attractive as the average efficiency of solar energyconversion in plants is about 10% and the overall efficiency of the conversion sunlight toelectricity would be about O.3% compared to 10% for photovoltaic cells.

The process photosynthesis is extremely complex and not yet completely understood byscientists. (In Greek photo means light and synthesis means combination). It is the mostimportant chemical reaction on the earth, is the reaction of sunlight and green plants. Radiantenergy of sun is absorbed by the green pigment chlorophyll in the plant and is stored within theplant in the form of chemical bond energy. In this reaction, water and CO2 molecules broken

down and a carbohydrate is formed with the release of pure oxygen. The process can beexpressed as follows:

CO2 + H2O + light + chlorophyll (H2CO) 6 + O2 + chlorophyll Sugar

Or 6 CO2 + 12 H2OC6H12O6 + 6 H2O + 6 O2

The absorbed light is in the ultraviolet and infrared range. Visible light having awavelength below 700 Ao is absorbed by the green chlorophyll, which becomes activated andpasses its energy on to the water molecules. A hydrogen atom is then released and reacts with thecarbon dioxide molecule, to produce H2CO and oxygen. H2CO is the basic molecule forming

carbohydrate, stable at low temperature; it breaks at high temperature, releasing an amount ofheat equal to 112,000 cal/mole.

H2CO + O2 CO2 + H2O + 112 kcal/mole.

The absorbed energy of photons should be at least equal to this amount. It is, therefore,possible to produce large amount of carbohydrate by growing say, algae under optimumconditions in plastic tubes or in ponds. The algae could be harvested, dried and burned forproduction of heat that could be converted into electricity by conventional methods.

Thus photosynthesis consists in building up of simple carbohydrates such as sugar etc. in

the green leaf in presence of sunlight. The oxygen liberated is from H2O molecule and not fromCO2. This process is called as carbon fixation or carbon assimilation. Photosynthesis isessentially a reduction and oxidation process.

The process of photosynthesis has two main steps:

(i) Splitting of H2O molecules into H2 and O2 under the influence of chlorophyll andsunlight. This phase of reaction is called the light reaction. In this phase of light absorbed by


38/168

38

chlorophyll causes photolysis of water. O2 escapes and H2 is transformed into some unknowncompound. Thus solar energy is converted into potential chemical energy. In the second phase,hydrogen is transferred from this unknown compound to CO2 to form starch or sugar. Formationof starch or sugar is dark reaction not requiring sunlight.

4.3 The conditions necessary for photo-synthesis are :

1. Light. One of the important inputs for biomass production is the intensity of solarradiation. Only apart of this energy (40-45%) is of the appropriate wavelength (400-700 ) toproduce photosynthesis. The plants use radiations between 400 to 700 . Only a part of thisenergy is actually used in photosynthesis. This range of light is called photo-synthetically activeradiation (PAR). The upper limit of the photosynthesis efficiency is about 5%.

2. CO2 concentration. Carbon dioxide is the primary raw material for photo-synthesis.CO2 constitutes about 0.03% of the atmosphere. However, if CO 2 availability is increasedartificially, linear increase in the yield of several crops, upto a limit, have been observed. Hence

one of the methods of increasing biomass is by supplying additional CO2 to the plants. The mainsources of CO2 are:

(i) Animal respiration,(ii) Combustion of fuel,(iii) The major source is the decay of organic matter by bacteria, and.(iv) Ocean also is an important store of CO2, much of which comes from

photosynthesis by plants. Respiration of marine plants and animal releases CO2 into the water.

3. Temperature. Photosynthesis is restricted to the temperature range which can betolerated by proteins, i.e. 0C-60C. Although photochemical part is not affected by temperature,

but biochemical part, controlled by enzymes, is highly sensitive to temperature.

4.4 Biogas Generation

Biogas, a mixture containing 55-65 per cent methane, 30-40 per cent carbon dioxide andthe rest being the impurities (H2, H2S and some N2), can be produced from the decomposition ofanimal, plant and human waste. It is a clean but slow burning gas and usually has a calorificvalue between 5000 to 5500 kcal/kg. It can be used directly in cooking, reducing the demand forfirewood. Moreover, the material from which the Biogas is produced retains its value as afertilizer and can be returned to the soil. Biogas has been popular on the name 'Gobar Gas'mainly because cow dung has been the material for its production, hitherto. It is not only the

excreta of the cattle, but also the piggery waste as well as poultry droppings are very effectivelyused for biogas generation. A few other materials through which biogas can be generated arealgae, crop residues (agro-wastes), garbage, kitchen wastes, paper wastes, seaweed, humanwaste, waste from sugarcane refinery, water hyacinth etc., apart from the above mentionedanimal wastes. Any cellulosic organic material of animal or plant origin which is easily bio-degradable is a potential raw material suitable for biogas production.


39/168

39

Biogas is produced by digestion, pyrolysis or hydro-gasification. Digestion is a biologicalprocess that occurs in the absence of oxygen and in the presence of anaerobic organisms atambient pressures and temperatures of 35- 70C. The container in which this digestion takesplace is known as the digester.

Anaerobic digestion:- As described the treatment of any slurry or sludge containing alarge amount of organic matter, utilizing bacteria and other micro-organisms under anaerobicconditions is commonly referred to as anaerobic digestion or simply digestion. This anaerobicdigestion consists broadly of three phases :

(i) Enzymatic hydrolysis. Where the fats, starches and proteins contained in cellulosicbiomass are broken down into simple compounds.

(ii) Acid formation. Where the micro organisms of facultative and anaerobic groupcollectively called as acid formers, hydrolyse and ferment, are broken to simplecompounds into acetic acids and volatile solids. As a result complex organic

compounds are broken down to short-chained simple organic acids.(iii) Methane formation. Where these organic acids are then converted into methane

(CH4) and CO2by the bacteria, which are strictly anaerobs. These bacteria are calledmethane fermentors. For efficient digestion these acid formers and methanefermentors, must remain in a state of dynamic equilibrium. This equilibrium is a verycritical factor, which decides the efficiency of generation.

Any remaining indigestible matter that is found in supernatent or sludge, is referred as"slurry". The raw materials are mixed with water and fed into the digester. It is quite importantthat proper attention is paid to the design of the digester; otherwise the gas production not onlyaffected in quality and quantity but also becomes uneconomical. For efficient digestion acidformers and methane fermentation must remain in a state of dynamic equilibrium. It has beendemonstrated that the methane formers are sensitive to pH changes. A pH value 6.5 to 8 is thebest for fermentation and normal gas production. The digester must be such that the temperaturevariation should not be more than 2 to 3C, while anaerobic fermentation is there. The higher thetemperature that is maintained the better is the quantity of gas. Methane bacteria work best at atemperature between 35-38C. The fall in gas production starts at 20C and stops at temperatureof 10C. A specific ratio of carbon to nitrogen (C/N ratio) must be maintained between 25: 1 and30: 1. This ratio will vary for different raw materials. The elements of carbon (in the form ofcarbon hydrates) and nitrogen (as protein, ammonia nitrates etc.) are the main food of anaerobicbacteria. Carbon is used for energy and nitrogen for building the cell structure. The bacteria useup carbon about 30 times faster than they use up nitrogen. There must be suitable water contentand it should be around 90 percent of the weight of the total contents. Both too much and toolittle water are harmful. Agitation of the slurry improves the gas yield. Loading rate, i.e. amountof raw material (usually kg of volatile solid per day per unit volume) should be optimum. If adigester is loaded with too much raw material at a time, acids will accumulate and fermentationwill stop.


40/168

40

4.5 Digesters and their designs

Digestion tanks may be of any convenient shape and provided with a cover to retain thegas. The cover may be a fixed one or floating.

A number of factors are to be taken into account to arrive at optimum size of a biogas

plant. These are:

(1) The volume of waste to be digested daily

(2) The type and amount of waste available for digestion consistently,

(3) Period of digestion,

(4) Method of stirring, the contents, if any,

(5) Method of adding the raw waste and removing digested slurry;

(6) Efficiency of the collection of the raw waste,

(7) The climatic conditions of the region,

(8) The availability of other cellulosic fermentable waste in that area,

(9) Information about sub-soil condition and water table, and

(10) Type of cover.

No separate heating and stirring of the contents are provided for digesting household andfarmyard wastes.

The capacity of the digestion tank may be formulated approximately as

V1 + V2Capacity = . t

2Where V1 = the volume of the

fullbook.pdf

Documents