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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¡ªharnessing 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). 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¡±.

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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 ¡ãF/300 ¡ãC 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%. Another method proposed for Solar Tower uses a liquid-fluoride-salt coolant system to achieve to 1100 ¡ãC and operates through a multi-stage turbine system to obtain an efficiency of 60 percent. 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. That is generally within the SunBelt¡ªbetween 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

list of CSP plants
(from wiki, for a more complete list of plants under construction, announced in the U.S. and elsewhere around the world click on here

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. , 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. Yet this large-scale acquisition of land has brought concerns about the desert environment and fragile ecosystems there.

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.

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.

levilized cost of PTPP and tower.bmp

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


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, 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.



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

¡°CST Research-Technology Basics¡± National Renewable Energy Lab

Angelino,G., Invernizzi, C. Binary Conversion Cycles For Concentrating Solar Power Technology Solar Energy Volume 82, Issue 7, Jul 2008, Pages 637-647

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

M¨¹ller-Steinhagen H, Trieb F. Concentrating Solar Power¡ªA Review Of The Technology. Ingenia 18, 2004

Woody, T. The Southwest desert's real estate boom July 11 2008

Bureau of Land Management Initiates Environmental Analysis of Solar Energy Development

Kennedy; C.E. Advances in Concentrating Solar Power Collectors: Mirrors and Solar Selective Coatings National Renewable Energy Laboratory NREL/PR-550-43695, Oct 2007

Bowles J., Hearings to debate impact of solar farms on threatened species The Press-Enterprise

Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts, Oct 2003 NREL/SR-550-34440


Copyright Yiting Wang 2008