What is concentrated solar power plants?

When it comes to solar power, many people would have the picture of big

photovoltaic panels on someone’s roof popping out in their head. What is less

obvious on the stage and more like a hidden dragon (some calls it “sleeping

solar giant”) is the technology called concentrated solar thermal power—

harnessing the sun for heat at high temperature, which today generates the

same amount of the electricity sent to the grid as photovoltaic systems

worldwide (about 500 GWh per year).1 In places like Spain and the west of

United States, there are already large power plants in place, feeding households

and industries in a steady flow. With its potentially large capacity, cheap energy

storage system, reliable dispatchability and market price, National Renewable

Energy Lab (NREL) has claimed it “can be a major contributor to our nation's

future need for new, clean sources of energy”.2

In this page:

Concentrated Solar Power Power Conversion Other Applications Different System Designs Solar Thermal Power Plants In The World Land Rush In The Southwest Cost Reduction Of Csp Plants Some Challenges And Environmental Concerns

Concentrated Solar Power (CSP) is a major utility-scale application of solar

thermal energy. Instead of being used directly to heat up houses or swimming

pools, sunlight is focused by mirrors or lenses to reach a high temperature (at

least 570 °F/300 °C to be effective and economically applicable) to either

generate steam to propel a turbine to produce an electric current or convert

heat to electricity directly using a Stirling engine. The former is the same

concept as in a conventional power plant, but rather than burn fossil fuels it

collects the sun’s radiation and sends off no pollution or greenhouse gases.

Power Conversion Different temperature levels mean different conversion

methods; generally, the higher the temperature the more efficient the

conversion. Different materials and technologies add up to different cost. Most

commonly adopted steam turbines (i.e Rankine cycle) have an efficiency of up

to 41.7% while a combined cycle of Rankine and Brayton (has gas turbine using

pressured air) can achieve a reasonable target of 50 % at a turbine inlet

temperature of 1200 C. Moreover, a binary cycle using alkali-metal (i.e. most

experiments use potassium) as a second working fluid in the topping cycle has

demonstrated an efficiency of 57%.3 Another method proposed for Solar Tower

uses a liquid-fluoride-salt coolant system to achieve to 1100 °C and operates

through a multi-stage turbine system to obtain an efficiency of 60 percent.4 Its

high working temperature requires the plants to be built in locations with direct

normal insolation (DNI) above 1800KWh/ (m2day) (circa 5KWh/ (m2day)) to be

economical. 5 That is generally within the SunBelt—between the 35th northern

and 35th southern latitudes. But via an efficient electric transmission system, it

theoretically has the capacity to meet the world with its electricity demand.7

Other Applications CSP could also be integrated into other industries to

provide power. Desalination using waste heat from power generation pumps

out freshwater to the desert regions where the mirrors are most ideally suited.

The cold water can also be used to provide air conditioning. Solar electricity

could also be used in the production of hydrogen, an increasingly important

clean fuel. Solar furnace made of parabolic dish or heliostat mirror can

process fullerenes and large carbon molecules with major potential commercial

applications in semiconductors and superconductors.

Different System Designs There are currently three major types of CSP

systems with respect to how the sunlight is concentrated and different

conversion processes. They are Parabolic Dishes, Solar Towers and Parabolic

Trough Power Plants (PTPP).

A list of operational solar thermal power plants in the world 6

(credit:wiki for a more complete list of plants under construction, announced in the U.S. and

elsewhere click on here http://en.wikipedia.org/wiki/List_of_solar_thermal_power_stations)

Land Rush in the Southwest

Until earlier this year, U.S. Bureau of Land Management had already received

125 applications for solar energy development on federal land totaling around

4000km2 (1544 mile2) or enough land for 70GW.7,8 While according to a 2003

NREL report on Southwest Solar Energy Potential, it estimates an total area of

53,727 mile2 of land that has no primary use today, excluding land with slope >

1%, <5 contiguous km2, and sensitive lands. Assuming 5 acres per MW, this

size of land have the potential of 6,877,055 MW from solar power.9 Yet this

large-scale acquisition of land has brought concerns about the desert

environment and fragile ecosystems there.10

Cost reduction of CSP plants

During the 1980’s, the early parabolic trough power plants in Europe generated

electricity at a cost equivalent to 70-140 U.S. cents per KWh. It quickly went

down to 30 cents when the SEGS 1 came into place in the U.S. Now it has

reached a range of 8-16 cents. These cost reductions primarily come from

larger plants being built, increased collector production volumes, building

projects in solar power park developments, and savings through competitive

bidding. A general rule is that the larger the size of the plant the lower the per

kW capital cost of power plants. Today in Southern California for example, peak

power costs anywhere between US cent 10-18/kWh, almost no difference with

CSP.11,45

These fast cost reduction is also a result of CSP’s fundamentally simple

technology. It is the same principle as you burn a piece of paper using a

magnifying glass. With CSP, you just need to have a good many of them and a

traditional thermal power plants. There is no complex material selection as in

PV production, no holes to drill as geothermal has to.30

A cost reduction study of PTPP and Solar Tower credit: NREL12

Some Challenges And Environmental Concerns

A big challenge for CSP to power greater area is transmission as the highest

resource potential does not match with populous regions. High capacity power

lines are needed for CSP’s long-term development. Competition with

agricultural, industrial and residential use of water would also be a spiny issue,

water being sucked up from Colorado River. Some scientists have brought up

the concerns over the fragile ecosystem in the desert area. The only emission

from solar thermal power plants running on steam turbines, water vapor, clean

as it is, yet contributes to global warming. Some underlying safety concerns

include the incidental leakage and explosion of some toxic oil heat transfer

fluid.

After years of worldwide campaigns on global climate change, we finally do not

have to dedicate much energy in arguing for it. Now is the time for us to take

our steps to actually shift of our energy use. Taking advantage of the non-

sensitive deserts, no pollution and the lowest carbon emission among other

renewable energy technologies,6 and with the sun pouring more than 7 KWh/m2

day of its energy onto the golden landscape of the southwest,43 concentrated

solar power has been quietly chasing around the sun for some 20 years, just

like the sunflowers. CSP will and should exert a bigger play in the grand picture

of America’s future renewable energy mix with duly confidence.

Parabolic Dish systems use satellite-like mirror dish(es) to focus the light onto

a single central receiver in front of the

mirror. They so far have the highest

heat-electricity conversion efficiencies

among all CSP designs (up to 30 %).

The size of the concentrator is

determined by its engine. A

dish/Stirling system’s concentrator with

Dish/engine system schematic. The dish that follows the sun on two axes focuses the sunlight onto one single point on a receiver posed right in front of the mirror.

a nominal maximum direct normal solar insolation of 1000 W/m2 and a 25-kW

capacity has a diameter of approximately 10 meters. It could also run on a

single Brayton cycle, where air, helium or other gas is compressed, heated and

expanded into a turbine. Parabolic dish could be applied individually in remote

locations, or grouped together for small-grid (village power, 10 KW) or end-of-

line utility (100 MW) applications. The electricity has to be used immediately or

transmitted to the gird as the system has no storage device.1,13 Intermittent

cloud cover can cause weakening of highly concentrated receiver source flux.

Sensible energy storage in single-phase materials was proposed to allow a

cylindrical absorber element not only absorb the energy but also store it in its

mass, thus reducing the amplitude of cloud cover transients.14Although this

design only allows short period energy storage, potential longer time storage

technology would make parabolic dish more appealing.

Stirling Energy System Inc.’s 300 M first commercial one in California <object width="425" height="344"><param name="movie"

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Costs and Rates One dish costs

around $250,000 averagely,

depending on the capacity of it.

Once production rates rise, they

could cost less than $150,000. Southern California Edison Electric

Company cannot give away the actual price per kWh, but they say it is well

below the 11.33 cents seen currently.

More Designs

Infinia’s Modular Solar Thermal Dish

$50 million investment 20-30% cheaper energy production

than PV cells 334 dishes per 1MW of power designed to be assembled with

mass produced parts that an auto

Dish/engine system with stretched-membrane mirrors: this design allows wind to pass through to minimize the destructive force of wind. Picture from Sunlab, Department of Energy

The Stirling engine produces electricity using the heat gathered by the receiver directly.Click here for an animation on how it works:http://www.keveney.com/Vstirling.html

parts supplier could manufacture each dish costs approximately $20,000

The History Of Solar Dishes

-Solar dishes have been in use since ancient Mesopotamian times -Polished gold dishes were used to concentrate the sun and light altar fires -In the 17th century glass lenses were used to smelt iron, copper, and

mercury -In the 18th century, concentrated solar power was used to heat ovens and

furnaces -Supposedly the Greek scientist Archimedes used reflective bronze shields

to focus sunlight at wooden Roman ships to set fire to them

Solar Tower, sometimes called Central Receiver System, has rings of small

individual flat mirrors (heliostats) surrounding a central power tower (up to 100-

200 m), on top of which sits a receiver that gathers the reflected radiation. The

receiver contains a kind of fluid medium, be it water, air, mineral oil, liquid

metal, molten salt or diluted salt. The heated fluid goes to a hot fluid storage

tank (where excessive heat is stored) and then to a steam generator to

engender electricity. The medium is then reused, returning to a cold fluid

storage tank and being pumped up to the tower again. Solar tower can reach

the highest temperature of all concentrator designs. The scheme of a solar

tower plant is shown in Figure 3.

Scheme of Solar Two, a molten-salt power tower system15

Solar tower possesses a higher efficiency than parabolic trough power plants

(approximately 20% vs. 15%) resulting from its higher concentrating ratio and

higher temperature. Therefore they are expected to be more cost efficient than

parabolic trough power plants when producing at a large scale (100-200 MW) in

a longer run. Pilot projects, Solar One (later converted into Solar Two) in the

Mojave deserts in the U.S have demonstrated well-maintained functionality.

They use molten/diluted salt which could maintain the heat energy for several

days. A big challenge for solar tower now is the high cost of the overall

construction and operation, with the heliostat and the rest of the system each

accounting for half of the total cost.16,17 Several more solar tower plants are

scheduled for installation in the Mojave Desert, California

America’s pilot solar tower project that has been proven to operate functually--Solar Two, In

Daggett, CA, 10 MWe, HTF/Storage Molten Nitrate Salt, 30 Acres in size

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A fully operation PS10 solar tower plant near Seville, Spain that can generate 10MW of electricity.

Expansion into 20 MW will be completed January 2009, enough to power 11,000 homes.

(guardian.co.uk)

Parabolic Trough Power Plant (PTPP) consists of a solar field filled with

hundreds or thousands of solar collector assemblies (SCA). Each SCA is an

independently tracking parabolic trough solar collector consisting of four major

subsystems: parabolic reflectors (mirrors)

receiver tube metal support structure tracking system that includes the drive, sensors, and controls. Also in this page: PTPPs in the U.S. PTPP around the world Land Rush in the SouthwestIn parabolic trough collector, long, U-curved mirrors focus the rays of the sun

into an absorber pipe. The mirrors track the sun on one linear axis from north to

south during the day. The pipe is seated above the mirror in the center along

the focal line and has a heat-absorbent medium (mineral oil, synthetic oil,

molten salt etc.) running in it. The sun’s energy heats up the oil, which carries

the energy to the water in a boiler heat exchanger, reaching a temperature of

about 400°C. The heat is transferred into the water, producing steam to drive

turbine. A study supported by Japanese government found an annually-

averaged collector efficiency using supercritical CO2 as the working fluid, higher

than water/vapor.18

Schematic of a PTPP with a thermal storage system

The Shape and Material of the parabolic troughs differ from different designs

as well. The collector is generally composed of one bent glass mirror, with

either silver or aluminum coated on the backside of the glass. The glass is

about four-millimeter thick and low in iron, maximizing the reflectance of

incoming sunlight (about 93.5% with silver coating protected by multilayer

paint). Although National Renewable Energy Lab (NREL) uses silver for its

collector and it has a higher reflectance, aluminum is also adopted by others for

its cheaper cost and stronger resistance to erodent environment.12

Most current solar thermal power plants uses a parabolic trough design called

Luz system (LS-1, 2 and 3) collectors.

Made from galvanized steel to support

its torque-tube structure, Luz collector

represents the standard design.

Solargenix Energy and NREL

collaborated to have developed a new

collector structure that uses extruded

aluminum. Solargenix SGX-1 collector

thus weights less than steel design and is easier to assemble and be aligned.13,19

A simpler design called compact linear fresnel reflector (CLFR) solar

collector reduces the cost significantly. It uses simple flat (or slightly curved)

mirrors, an optical system originally developed by French engineer Augustin-

Jean Fresnel. It weighs 3 kg/m2 , only one third

of parabolic trough mirror.20 It has a much

lower concentrating temperature, at 285 °C

(545°F) 21,22,23 Ausra Inc.’s Fresnel Principle

technology, originally developed by founder

David Mills at Sydney University, currently can

operate in a $10-cent-per-KW range, about the Ausra’s 5-MW plant in Calf. Source: Ausra.com

the end of a Luz-2 collector credit: Henry Price

same as the current market price in terms of grid base load in the U.S.24 In

October 2008, Ausra just launched a 5-MW solar thermal plant in Bakersfield,

California, with a 177-MW plant in planning.

The Absorber Pipe, also

called heat collection

element (HCE), is made up

of a several-meter-long

metal tube and mostly a

glass envelope covering it.

In between these two

usually resides either air or

a vacuum to reduce

convective heat losses and

allow for thermal expansion. A glass-to-metal seal is crucial in reducing heat

losses as well. The metal tube is coated with a selective material (chrome black,

cermet etc.) that has high solar radiation absorbance (filters out infrared rays)

and low thermal remittance (attracts more visible light). The HCE is the core

part that enables PTPP to acquire high efficiency (with only a 10% heat losses). 25,26.

Other supporting structures of an SCA include pylons, drive, controls,

collector interconnect. Pylons are the foundations that hoist the mirrors; drive

enables the collector to track the sun. The local controller for each SCA,

connected to a central computer, keeps track of the drive and also watches out

for any abnormal conditions. Collector interconnect are the insulated hoses that

link up the whole power cycle.27.

U.S. Parabolic Trough Power Plants

Heat collection element (HCE) used in Luz system (Source: Flabeg Solar International)

11 Parabolic Trough Power Plants have been operating in the southwestern

U.S. (9 of them in California) since 1980s, producing roughly 420 megawatts of

annual net output. The recently completed Nevada Solar One PTPP has a

capacity of 64 MW.

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Florida Power & Light is investing a 300 MW CSP plant, bigger than any existing

ones.28 It will adopt Ausra Inc.'s compact linear fresnel reflector solar collector

and steam generation system. Spain has a layout of 1000 MW capacity for solar

thermal power plants, the first 200 MW already in place.29 Despite the just

launched Kimberlina concentrating solar thermal power plant in Bakersfield,

Calif.by Ausra Inc., Governor Schwarzenegger mandated a Solar Task Force of

implement 3,000 MW of new solar power by 2015. New Mexico has even

outlined a CSP specific task force.30

(credit: NREL for specific information of each of the plant click on

http://www.nrel.gov/csp/troughnet/power_plant_data.html )

Solar Chimney Tower Plant

Prototype of the solar tower prototype plant at Manzanares, Spain(Schlaich, J. et al 31)

“Hot air rises.” This is the most basic fact employed in the design of the

gigantic solar chimney tower plant. The spread-out solar collectors receive the

sunlight and act like a greenhouse together with the ground. Air in the

“greenhouse” is heated and pushed toward the turbines at the bottom of the

chimney at speeds of up to 70km/h (43.5 mi/h). The buoyancy effect created by

the pressure difference from the air under the collectors and ambient

(surrounding/outside) air produces a driving force to make sure the air moves

fast.

Schematic presentation of a solar chimney tower

The size of the collector and area and the height of the chimney decide the

capacity of the electricity production. The larger the collecting area, the more

air flow and heat it traps; the higher the height of the chimney, the greater the

pressure difference. This is called the stack effect in physics.

Heat Can Be Stored by the Ground. The ground beneath the collector roof

absorbs the heat and re-radiates it during the night, therefore able to provide

energy 24 hours a day. Other uses for the space in between the roof and

ground have been proposed, such as dehydration of fruits or vegetables.

Principle of thermal energy storage with water-filled black tubes for additional

thermal storage capacity. This works better than soil alone as water as water’s

heat capacity is five times larger than that of soil. Also heat transfer between

water tubes and water is much higher than that between ground surface and

the soil layers underneath. (Schlaich, J. et al 32)

The First Prototype Plant was established in Manzanares, Spain in 1981,

jointly invested by German government and a Spanish Utility .133 The chimney is

194.8 meter (639.1 ft) in height and 10 meter(32.8 ft) in diameter ; collector

zone(greenhouse) of 244m(800.5ft) in diameter. It produced an upwind velocity

of 15 m/s(33.5mi/h), reaching a total output of 50 KW. It was set up mainly for

experimental use to test different materials and other parameters. One sections

1

of the collector zone is actually used as a greenhouse to grow plants. Here is

video clips of the plant:

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A Future Plant In 2002, an Australian company EnviroMission acquired the

permission from the government to build a 1000 m high by 7 km diameter solar

chimney plant. A power output of 200 MW34 is expected. The greenhouse will

use heat enhancing properties materials including glass, polycarbonate and

polymer while the chimney will just be forged with reinforced concrete. It will

prevent over 900,000 tons of greenhouse gases otherwise to be created by

fossil fuel plants.

In Terms Of Conversion Efficiency, the Australian SCPP project estimated

that they can utilize about 0.5 percent, or 5 W/m² of 1 kW/m², of the solar

radiation the sun pours onto the whole collecting area. It is a rather low

conversion rate considering the 15%-30% of other concentrated solar power

technologies (PTPP and Parabolic dish respectively). But the reliability of these

calculations remains to be further investigated because of insufficient testing

data.

Click here for an animation of what it looks like!

http://www.enviromission.com.au/SolarTower%20Animation

%20Metric.wmv

The Bulk of the Cost of a SCPP falls on the initial construction of the plants.

It involves relatively less sophisticated technologies and therefore very ideal for

less developed countries with optimal solar insolation and large area of unused

inferior flat land. Countries like Botswana and Namibia have been looking into

the possibility of investing such a plant. Carbon credits will also help reduce the

overall leveled cost of the plant.35

Solar Thermal Storage System

One big shining point of Parabolic Trough Power Plant (PTPP), the so-called dispatchability, is

its potential to provide power 24 hours a day, by storing the heat energy in a thermal storage

unit for later use during peak hours, in the evening or on a cloudy day. It enhances the annual

capacity of a plant by 50 % over one without a thermal energy storage system (TES). Within

current technology, heat is much cheaper to store than electricity. Nearly all current existing

solar thermal plants that have back-up systems are supported by fossil fuels, but a TES

completely hoisted by the power the plant generates itself is within reach. Several storage

mechanisms have been put in place while other proposals are still in lab-scale. Progress is

being achieved by improvements on old systems and alternative designs.36,37

Two-tank direct storage system

Two-tank indirect storage system

Single-Tank Thermocline

Phase-Change Materials

Two-tank direct storage system

The early two-tank direct system was used in the first Luz mirror plant, the “Solar Energy

Generating System I (SEGS I)”in California. It has two tanks, one of low and one of high

temperature. Only one heat transfer fluid (HTF), in this case mineral oil (Caloria), circulates

from the low-temperature tank through the solar collectors picking up the heat. Part of the

heat goes to generate the steam to run the turbine and the excessive heat goes back to the

high-temperature tank for storage. After passing through a heat exchanger, the cooled fluid

flows back to the low-temperature tank to be reused. The Solar Two power tower in California

also uses this system, only with molten salt as the HTF.1,3

Two-tank sensible heat storage38

But as later SEGSs moved to synthetic oil (a eutectic mixture of biphenyl-diphenyl oxide) to

achieve a higher operating temperature and hence a higher efficiency, the two-tank direct was

no longer suitable. The old mineral oil has a high vapor pressure so it cannot be used in the

large unpressurized storage tank system as the one adopted for SEGS I. Pressurized storage

tanks are very expensive. In addition, the HTF in some places is too expensive or not suitable

to also serve as a storage fluid. It takes the freezing point and local temperature (day and

night) into consideration in terms of choosing the transfer medium.1,2

Two-tank indirect storage system

The subsequently developed two-tank indirect

storage system has not only a HTF but also a

storage fluid (ST) and an extra heat exchanger.

The storage fluid coming out of the low-

temperature tank absorbs the heat energy of

the high-temperature HTF in the extra heat

exchanger. The now high-temperature ST

flows back to a high-temperature storage tank

Two-tank indirect thermal energy storage system for Andasol 1 and 2. The storage tank is 10m in height and 37m in diameter. The storage fluid is a mixture of 60%NaNO3

40% of KNO3 Credit: Flagsol

and the now low-temperature HTF moves on to the solar collector to start the power cycle

again. Despite the extra cost resulting from a second heat exchanger and smaller temperature

difference between the two tanks, the two-tank indirect system with molten salt as the ST is

still dominant in most of the PTPPs around the world. The technology originated from the

experiment of Solar Two power tower in California. Two PTPP in plan, the 50MW AndaSol

project in Granada, Spain and the 280MW Solana, in Gila Bend, Arizona, will both adopt the

molten salt thermal storage system.1,2,39 Andasol, for example, aims at a capacity of 1,010 MWh,

equivalent to 7.5 hours of full load operation.

For high temperature thermal storage, above 400°C, organic HTFs tend to thermally

decompose, while molten-salt or liquid metal is still generally stable. It is also non-flammable

and nontoxic and has been used in other industries40. But problem with molten salt is its

relatively high freezing temperature 120 to 220°C (250-430°F). Special operating maintenance

needs to be done to make sure it doesn’t freeze during cold night, especially in deserts.41

Single-Tank ThermoclineTo further reduce the cost of the storage fluid and the storage tanks, researchers moved

forward to a single tank called thermocline. Energy is stored in a tank made of solid storage

medium--commonly concrete or silica sand—instead of a storage fluid. High-temperature fluid

flows into in the tank from the top, all the way down through to the bottom and cools. It creates

two different temperature regions from high to low, between which there is a space called

temperature gradient or thermocline. When the stored-up thermal energy is needed, the flow

reverses taking up the heat on its way up. Buoyancy effects make sure that hot, less dense

materials stay on top of cool, dense materials at the bottom, creating thermal stratification of

the fluid.

Sandia National Laboratories in New Mexico has tested a 2.5 MWhr, backed-bed thermocline

storage system with binary molten-salt fluid, and quartzite rock and sand for the filler material.

The cost for a TSE system is reduced substantially by replacing most of the storage fluid and

cheap filling material for the tank.2,3

Thermocline test at Sandia National Laboratories. Credit: Sandia National Laboratories

The research goals now directing current R&D in solar thermal storage encompass finding

heat-transfer fluid that can operate at higher temperature with low freezing point, hence a

higher overall heat transfer efficiency. Another goal is to develop a storage fluid that has high

heat capacity so that less amount of fluid is needed in the system.42

Although these above-mentioned systems are very reliable technically, they still pose a high

overall cost. Other concepts for a cheaper cost are being explored and investigated too. Some

research is under way to find more efficient and less costly filler materials for the one-tank

system which possesses high potentiality for cost reduction.

Phase-Change Materials

Although using concrete as the filler

materials is very cost efficient(it is much

cheaper to hold the same amount of

Figure 11 The German Aerospace Center constructed a facility at the University of Stuttgart for testing a concrete, thermal energy storage system.

energy than molten salt), easy to handle and has higher strength, it faces

problems such as maintaining good contact between the concrete and pipelines

and low efficiency of heat transfer from the concrete to the HTF.

Another rather promising solution is phase-change materials (PCMs), use d in

high temperature latent heat thermal energy storage system (HTLTTES) for

direct steam generation (DSG).Its primary advantage resides in its ability to

hold up large amounts of energy in relatively small volumes, at one of lowest

costs among other storage materials. It utilized different PCM’s different latent

heat of fusion (melting), which should be matched to the temperature of the

incoming sensible HTF. The PCMs are cascaded from low melting temperature

at the bottom of the tank to high temperature at the top (maximum operating

temperature around 390°C).

The HTF flows downward when charging (melting the PCMs) and upward when

discharging providing heat to generate steam (solidifying the PCMs). Current

Proposal of a cascaded latent heat storage tank with 5 PCMs according to Dinter et al. (1991)

researches propose nitrate/nitrite salts and eutectic mixtures of these salts,

such as lithium nitrate and potassium nitrate as the PCMs for HTLHTES, for their

enthalpy and economic feasibility.

Despite its encouraging prospect, however, PCMs is challenged by the

complexity of the system itself, unstable lifespan of the PCMs and low heat

conductivity. Researchers are looking for other material sources that possess

more sufficient heat of fusion, corrosiveness and high heat conductivity (at least

2 W/(m K)). Or it can also be improved by developing proper heat transfer

techniques to offset the low conductivity of PCMs.1,2, 43,44

A cost comparison of the three storage concepts in different parts including 2-tank

direct liquid salt, thermocline (concrete, solid salt and liquid salt), and PCM. The

latter two are only in testing phrase.45

"Thermal energy storage is the killer app of concentrating solar power

technology," said Andrew McMahan, vice president of SkyFuel, New Mexico, told

a packed solar technology conference last month held in conjunction with

Semicon West.46 This month, the U.S. Department of Energy (DOE) just

announced a funding of $35 million to facilitate developing lower-cost energy

storage for CSP technology.47An increasing number of major venture capital also

flows into researches that focus on more cost efficient solar thermal storage

technologies.

Solar radiation is a general term for the electromagnetic radiation emitted by

the sun. Solar thermal energy captures the radiation and converts it into heat to

produce energy. Concentrated Solar Power utilizes the high temperature heat to

generate electricity.

Photovoltaic systems in contrast convert the radiation into electricity directly.

The sun’s waves hit a photovoltaic cell and excite the electrons within layers of

the cell. The excited electrons jump back and forth, creating electricity. This

electricity is captured by wires running through the PV cells and sends the

electricity into your home.

Unlimited Solar Resources

In one hour, enough sunlight(1000 Wh per m² = 1 kWh/m²) falls on the

earth to power the world for an entire year If 1% of the Sahara Desert were covered in solar thermal systems, enough

energy would be produced to power the entire world Solar radiation along with secondary solar resources such as wind and wave

power, hydroelectricity and biomass account for 99.97% of the available

renewable energy on Earth. 53,727 mile2 of land in the American southeast that has no primary use

today has the potential of 6,877,055 MW from solar power., (excluding land

with slope > 1%, <5 contiguous km2, and sensitive lands) and assuming 5

acres per MW.48

Types of Solar Radiation

Diffuse Solar Radiation is the sunlight that is absorbed, scattered and

reflected by all kinds of particles in the air(such as water vapor and clouds). Direct Solar Radiation is the solar radiation that reaches the Earth's surface

without being diffused. Direct-Normal Radiation refers to the portion of sunlight that comes directly

from the Sun and strikes a surface at a 90-degree angle. The sum of the diffuse and direct solar radiation is called global solar

radiation.

Measurement

Insolation is a measure of solar radiation energy received on a given

surface area in a given time. It is commonly expressed as average

irradiance in watts per square meter (W/m²) or kilowatt-hours per square

meter per day (kW·h/(m²·day)) (or hours/day). Direct estimates of solar

energy may also be expressed as watts per square meter (W/m2). In photovoltaics it is commonly measured as kWh/kWp•y (kilowatt hours per

year per kilowatt peak rating). Radiation data for solar water heating and space heating systems are

usually represented in British thermal units per square foot (Btu/ft2).

Some History of the Concentration Use of Solar Energy—Reclaim the Sun!

Ancient Greeks and Romans

saw great benefit in what

we now refer to as passive

solar design—the use of

architecture to make use of

the sun’s capacity to light

and heat indoor spaces. Romans advanced the art by covering south facing

building openings with glass or mica to hold in the heat of the winter sun.

Through calculated use of the sun’s energy, Greeks and Romans offset the need

to burn wood that was often in short supply.

*this page is contributed by Molly 11’ Hampshire College and Ally 09’ Hampshire College

MORE LINKS

Here are more links about concentrated solar power technologiesNational Renewable Energy Lab in Golden, Colorado:http://www.nrel.gov/csp/

U.S. Department of Energy, Energy Efficiency and Renewable Energyhttp://www1.eere.energy.gov/solar/csp.html

SolarPACES, an international cooperative organization, one of a number of collaborative programs managed under the umbrella of the International Energy Agency:http://www.solarpaces.org/

Solar Energy Industrial Association, works to expand the use of solar energy and promote researchhttp://www.seia.org/

The official website for the book “Profit from clean energy”http://www.profitfromcleanenergy.com/index.asp

An encyclopedia of alternative energy and sustainable livinghttp://www.daviddarling.info/encyclopedia/AEalphindex/

AE_categories.html

Solar Thermal Power Plants, Technology Fundamentalshttp://www.volker-quaschning.de/articles/fundamentals2/index_e.html

Renewable Energy World, latest news on renewable energies where you can post your resumes as wellhttp://www.renewableenergyworld.com/rea/home

1 Pitz-paal, R. “How The Sun Gets Into The Power Plant”, Renewable Energy: Sustainable Energy Concepts For The Future Wengenmayr, R.; Buhrke, T. Eds. Wiley-VCH,2008 pp.26-332 “CST Research-Technology Basics” National Renewable Energy Lab http://www.nrel.gov/csp/technology_basics.html 3 Angelino,G., Invernizzi, C. Binary Conversion Cycles For Concentrating Solar Power Technology Solar Energy Volume 82, Issue 7, Jul 2008, Pages 637-6474 C. H. Forsberg et al, High-Temperature Liquid-Fluoride-Salt Closed-Brayton-Cycle Solar Power Towers Journal of Solar Energy Engineering May 2007, Vol. 129 141-1465 Müller-Steinhagen H, Trieb F. Concentrating Solar Power—A Review Of The Technology. Ingenia 18, 20046 List of solar thermal power stations, http://en.wikipedia.org/wiki/List_of_solar_thermal_power_stations7 Woody, T. The Southwest desert's real estate boom CNN.com July 11 2008 http://money.cnn.com/2008/07/07/technology/woody_solar.fortune/index.htm

8Bureau of Land Management Initiates Environmental Analysis of Solar Energy Development http://www.blm.gov/wo/st/en/info/newsroom/2008/may_08/NR_053008.html9 Kennedy; C.E. Advances in Concentrating Solar Power Collectors: Mirrors and Solar Selective Coatings National Renewable Energy Laboratory NREL/PR-550-43695, Oct 200710Bowles J., Hearings to debate impact of solar farms on threatened species The Press-Enterprise http://www.pe.com/localnews/inland/stories/PE_News_Local_S_solar15.48dbdb9.html11 Solarpaces.org http://www.solarpaces.org/CSP_Technology/docs/solar_trough.pdf12 Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts, Oct 2003 NREL/SR-550-3444013 “Solar dish engine” SolarPaces http://www.solarpaces.org/CSP_Technology/docs/solar_dish.pdf 14Lund, K. O.A., Direct-Heating Energy-Storage Receiver for Dish-Stirling Solar Energy Systems, J. Sol. Energy Eng. Feb1996 Volume 118, Issue 1, 15 15 SolarPaces.org http://www.solarpaces.org/CSP_Technology/docs/solar_tower.pdf16 “Learning About Renewable Energy: Concentrating Solar Power” NREL http://www.nrel.gov/learning/re_csp.html17 Farret, F.A.; Simoes, M.G. Integration of Alternative Sources of Energy IEEE Press 2006 pp.112-12718 Zhang, X.R., Yamaguchi, H. An experimental study on evacuated tube solar collector using supercritical CO2 Applied Thermal Engineering, 28 (2008) 1225–123319 Gee, R.C. and Hale, M.J. Solargenix Energy Advanced Parabolic Trough Development Solargenix Energy Conference Paper NREL/CP-550-39206 No. 200520 Ford, G., CSP: Bright Future For Linear Fresnel Technology? Renewable Energy Focus Volume 9, Issue 5,Sep-Oct 2008, Pages 48-49, 5121 “How Ausra’s technology works”, Ausra Inc., http://ausra.com/technology/22 García-Valladares, O.; Velázquez, N., Numerical Simulation Of Parabolic Trough Solar Collector: Improvement Using Counter Flow Concentric Circular Heat Exchangers International Journal of Heat and Mass Transfer 2008.08.00423 Inslee, Jay; Hendricks, Bracken, Apollo's fire : igniting America's clean-energy economy Island Press for economic and social association, 2008 pp 84-8724 “Corporate Overview” Ausra Inc,. http://ausra.com/about/25 Farret, F.A.; Simoes, M.G. Integration of Alternative Sources of Energy IEEE Press 2006 pp.112-12726 Wengenmayr, Roland; Buhrke, Thomas Renewable Energy: sustainable energy concepts for the future Wiley-VCH,2008 pp.26-33 27 “CST-how it works” SolarPACES http://www.solarpaces.org/CSP_Technology/csp_technology.htm28 “300-MW Array and More Planned for Florida, California” Engineering News October 8, 2007 Pg. 14 Vol. 259 No. 13 29 “CSP project developments in Spain” SolarPaces http://www.solarpaces.org/News/Projects/Spain.htm30 Jones, J. Concentrating Solar Thermal Power, Renewable Energy World Magazine Sep 2, 200831 Schlaich, J. et al Design of Commercial Solar Updraft Tower Systems—Utilization of Solar Induced Convective Flows for Power Generation J. Solar Energy Engineering Feb 2005, Vol. 12732 Schlaich, J. et al Design of Commercial Solar Updraft Tower Systems—Utilization of Solar Induced Convective Flows for Power Generation J. Solar Energy Engineering Feb 2005, Vol. 12733 Pasumarthi, N. and Sherif, S.A. Experimental and theoretical performance of a demostration solar chimney model–Part 1: mathematical model development, J Energy Res 22 (1998), pp. 277–288.34 http://www.enviromission.com.au/faqs/faqs.htm35 Fluri, T.F. et al Cost analysis of solar chimney power plants J. Solar Energy July 2008 36 “Parabolic Trough Thermal Energy Storage Technology”NRELhttp://www.nrel.gov/csp/troughnet/thermal_energy_storage.html#direct 37 “Thermal Storage” U.S Department of Energy Efficiency and Renewable Energy http://www1.eere.energy.gov/solar/thermal_storage.html38 Stine, W.B., Harrigan, R.W. an online update version of the book "Power From The Sun" http://www.powerfromthesun.net/Chapter11/Chapter11.htm39 Solar Power; Sunny Future For Parabolics In Granada And Nevada Modern Power System February 14, 200740 “National solar thermal testing facilities” Sandia National Laboratories http://www.sandia.gov/Renewable_Energy/solarthermal/NSTTF/salt.htm41 Taggard, S., Parabolic troughs: CSP’s quiet achiever Renewable Energy Focus Volume 9, Issue 2, March-April 2008, Pages 46-48, 5042 “Solar Storage And Research Development”, U.S Department of Energy Efficiency and Renewable Energy

http://www1.eere.energy.gov/solar/thermal_storage_rnd.html#storage_systems43 Michels, H., Pitz-Paal, R., Cascaded Latent Heat Storage For Parabolic Trough Solar Power Plants Solar Energy 81 (2007) 829–83744 Guo, C., Zhang, W. Numerical simulation and parametric study on new type of high temperature latent heat thermal energy storage system Energy Conversion and Management Volume 49, Issue 5, May 2008, Pg 919-92745 Nava P, Herrmann, U. Trough Thermal Storage Status Spring 2007 NREL/DLR Trough workshop -Denver Mar 200746 Leopold, G., Solar thermal technology heats up Electronic Engineering Times August 2008 4 Pg 3847 “ DOE to invest $35 million in concentrating solar plant projects” National Renewable Energy Lab, Sep 19, 2008 http://www.nrel.gov/csp/news/2008/634.html48 Kennedy; C.E. Advances in Concentrating Solar Power Collectors: Mirrors and Solar Selective Coatings National Renewable Energy Laboratory NREL/PR-550-43695, Oct 2007