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 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
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.
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
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Costs and Rates One dish costs around
$250,000 averagely, depending on the
capacity of it. Once production rates rise,
The Stirling engine produces electricity using
the heat gathered by the receiver directly.
they could cost less than $150,000.
Southern California Edison Electric Company
Click here for an animation on how it works:
http://www.keveney.com/Vstirling.html
cannot give away the actual price per kWh, but they say it is well below the 11.33 cents seen
currently.
More Designs
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
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 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 Southwest
In 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
the end of a Luz-2 collector
collector thus weights less than steel design and
credit: Henry Price
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 same as the current
market price in terms of grid base load in the
U.S.24
In
Ausra’s 5-MW plant in Calf. Source:
Ausra.com
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
Heat collection element (HCE) used in Luz system
(Source: Flabeg Solar International)
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
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)
Schematic presentation of a solar chimney
tower
“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.
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 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
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
exchanger. The now high-temperature ST flows back to a high-temperature storage tank 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 Thermocline
To 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 energy than molten salt), easy to
Figure
11
The
German
Aerospace
Center
constructed a facility at the University of Stuttgart
for testing a concrete, thermal energy storage
system.
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 researches propose
Proposal of a cascaded latent heat storage tank with 5 PCMs according to Dinter et al. (1991)
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 technologies
National Renewable Energy Lab in Golden, Colorado:
http://www.nrel.gov/csp/
U.S. Department of Energy, Energy Efficiency and Renewable Energy
http://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
research
http://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 living
http://www.daviddarling.info/encyclopedia/AEalphindex/AE_categories.html
Solar Thermal Power Plants, Technology Fundamentals
http://www.volker-quaschning.de/articles/fundamentals2/index_e.html
Renewable Energy World, latest news on renewable energies where you can post your
resumes as well
http://www.renewableenergyworld.com/rea/home
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.26-33
2 “CST Research-Technology Basics” National Renewable Energy Lab http://www.nrel.gov/csp/technology_basics.html
1
3
Angelino,G., Invernizzi, C. Binary Conversion Cycles For Concentrating Solar Power Technology Solar Energy Volume 82,
Issue 7, Jul 2008, Pages 637-647
4 C. H. Forsberg et al, High-Temperature Liquid-Fluoride-Salt Closed-Brayton-Cycle Solar Power Towers Journal of Solar
Energy Engineering May 2007, Vol. 129 141-146
5
Müller-Steinhagen H, Trieb F. Concentrating Solar Power—A Review Of The Technology. Ingenia 18, 2004
List of solar thermal power stations, http://en.wikipedia.org/wiki/List_of_solar_thermal_power_stations
6
7Woody,
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.html
9
Kennedy; C.E. Advances in Concentrating Solar Power Collectors: Mirrors and Solar Selective Coatings National
Renewable Energy Laboratory NREL/PR-550-43695, Oct 2007
10Bowles
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.html
11 Solarpaces.org http://www.solarpaces.org/CSP_Technology/docs/solar_trough.pdf
12 Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts, Oct 2003
NREL/SR-550-34440
13 “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.pdf
16 “Learning About Renewable Energy: Concentrating Solar Power” NREL http://www.nrel.gov/learning/re_csp.html
17 Farret, F.A.; Simoes, M.G. Integration of Alternative Sources of Energy IEEE Press 2006 pp.112-127
18 Zhang, X.R., Yamaguchi, H. An experimental study on evacuated tube solar collector using supercritical CO Applied
2
Thermal Engineering, 28 (2008) 1225–1233
19 Gee, R.C. and Hale, M.J. Solargenix Energy Advanced Parabolic Trough Development Solargenix Energy Conference
Paper NREL/CP-550-39206 No. 2005
20 Ford, G., CSP: Bright Future For Linear Fresnel Technology? Renewable Energy Focus Volume 9, Issue 5,Sep-Oct 2008,
Pages 48-49, 51
21
“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 2008.08.004
23 Inslee, Jay; Hendricks, Bracken, Apollo's fire : igniting America's clean-energy economy Island Press for economic and
social association, 2008 pp 84-87
24 “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.112-127
26
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.htm
28 “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.htm
30 Jones, J. Concentrating Solar Thermal Power, Renewable Energy World Magazine Sep 2, 2008
31
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. 127
32
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. 127
33 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.htm
35 Fluri, T.F. et al Cost analysis of solar chimney power plants J. Solar Energy July 2008
36“Parabolic Trough Thermal Energy Storage Technology”NREL
http://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.html
38 Stine, W.B., Harrigan, R.W. an online update version of the book "Power From The Sun"
http://www.powerfromthesun.net/Chapter11/Chapter11.htm
39 Solar Power; Sunny Future For Parabolics In Granada And Nevada Modern Power System February 14, 2007
40 “National solar thermal testing facilities” Sandia National Laboratories
http://www.sandia.gov/Renewable_Energy/solarthermal/NSTTF/salt.htm
41 Taggard, S., Parabolic troughs: CSP’s quiet achiever Renewable Energy Focus Volume 9, Issue 2, March-April 2008,
Pages 46-48, 50
42 “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_systems
43 Michels, H., Pitz-Paal, R., Cascaded Latent Heat Storage For Parabolic Trough Solar Power Plants Solar Energy 81 (2007)
829–837
44 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 919-927
45 Nava P, Herrmann, U. Trough Thermal Storage Status Spring 2007 NREL/DLR Trough workshop -Denver Mar 2007
46
Leopold, G., Solar thermal technology heats up Electronic Engineering Times August 2008 4 Pg 38
“ 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.html
48 Kennedy; C.E. Advances in Concentrating Solar Power Collectors: Mirrors and Solar Selective Coatings National
47
Renewable Energy Laboratory NREL/PR-550-43695, Oct 2007