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Parabolic Trough Power Plants (PTPP)are thus far mostly developed CSP thermal plants that are operating commercially. They consist 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:

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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¡ãC. 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.



Schematic of a PTPP with a thermal storage system


In the solar field, cold heat transfer fluid comes in,picks up the heat collected by the trough and exits at a high temperature

The Shape and Material of the collector 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.

 

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, ?

The end of a LUZ-2 collector
credit: Henry Price

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. It has a much lower concentrating temperature, at 285 ¡ãC (545¡ãF) , , 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. In October 2008, Ausra just launched a 5-MW solar thermal plant in Bakersfield, California, with a 177-MW plant in planning.

Ausra¡¯s 5-MW plant in Calif. Source: Ausra.com

 

 

 

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 (black chrome , 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). , .


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. .


Parabolic Trough Power Plants in the U.S.

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.

 

 

 

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

A list of the 11 CSP plants in the United States
US pptp.bmp
(credit: NREL for specific information of each of the plant click on http://www.nrel.gov/csp/troughnet/power_plant_data.html )

Bibliography


¡°300-MW Array and More Planned for Florida, California¡± Engineering News October 8, 2007 Pg. 14 Vol. 259 No. 13

¡°CSP project developments in Spain¡± SolarPaces http://www.solarpaces.org/News/Projects/Spain.htm

Jones, J. Concentrating Solar Thermal Power, Renewable Energy World Magazine Sep 2, 2008


Farret, F.A.; Simoes, M.G. Integration of Alternative Sources of Energy IEEE Press 2006 pp.112-127

Wengenmayr, Roland; Buhrke, Thomas Renewable Energy: sustainable energy concepts for the future Wiley-VCH,2008? pp.26-33


Gee, R.C. and Hale, M.J. Solargenix Energy Advanced Parabolic Trough Development Solargenix Energy Conference Paper NREL/CP-550-39206 No. 2005

Ford, G., CSP: Bright Future For Linear Fresnel Technology? Renewable Energy Focus Volume 9, Issue 5,Sep-Oct 2008, Pages 48-49, 51

¡°How Ausra¡¯s technology works¡±, Ausra Inc., http://ausra.com/technology/

Garc¨ªa-Valladares, O.; Vel¨¢zquez, 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

Inslee, Jay; Hendricks, Bracken, Apollo's fire : igniting America's clean-energy economy Island Press for economic and social association, 2008 pp 84-87

¡°Corporate Overview¡± Ausra Inc,. http://ausra.com/about/


Zhang, X.R., Yamaguchi, H. An experimental study on evacuated tube solar collector using supercritical CO2 Applied Thermal Engineering, 28 (2008) 1225¨C1233

 

Copyright Yiting Wang 2008