ECE 7800: Renewable Energy Systems Topic 14: Concentrated Solar Power Spring 2010 © Pritpal Singh, 2010

Introduction Concentrating solar power (CSP) technologies convert sunlight to thermal energy to run a heat engine to generate electricity. Three approaches: 1) Parabolic dish systems with Stirling engines; 2) Linear solar trough systems; 3) Power tower concept (with heliostats)

Solar Dish/Stirling Engine Systems A set of mirrors approximate a parabolic dish. The dish tracks the sun and focuses the sunlight onto a thermal receiver. The thermal receiver absorbs the solar energy and using a heat transfer medium, such as a bank of tubes that contain H 2 or He as the working fluid, run a Stirling engine.

Solar Dish/Stirling Engine Systems (cont’d) Another approach is to use heat pipes for boiling and condensing an intermediate fluid between the receiver and the Stirling engine. The cold side is maintained with a water-cooled automotive radiator-type system. This is a closed system and consumes little water. A fuel can be burned when solar energy is not available to provide continuous power output.

Solar Dish/Stirling Engine Systems (cont’d) Average efficiencies of these systems is >20% with record efficiencies of nearly 30%.

Solar Dish/Stirling Engine Systems (cont’d) The SAIC dish comprises 16 stretched-membrane, mirrored facets approx. 3.2 m in diameter around a steel ring (like a drum). The mirror is either a thin glass mirror or a metallized polymer. Evacuating between the stretched membranes allows curving of the mirrors to focus on the receiver.

Solar Dish/Stirling Engine Systems (cont’d) The concentrated sunlight creates a temperature of 725ºC on the Stirling engine. The engine itself has four cylinders, each with a double-acting piston. Connecting rods convert the back and forth motion of the pistons to rotary motion for the generator.

Solar Dish/Stirling Engine Systems (cont’d) The below table shows the efficiencies of the SAIC/STM system from solar to electrical power conversion. Land requirement with these systems is about 1MW/4 acres.

Solar Dish/Stirling Engine Systems (cont’d) This Science Application International Corporation/STM Power Inc. 25 kW Dish-Stirling System is operating at a Salt River Project site in Phoenix, AZ. (Courtesy: Sandia National Labs)

Parabolic Troughs In parabolic trough systems, the power plant comprises rows of parabolic-shaped mirrors that reflect and concentrate sunlight onto linear receivers located at the foci of the reflectors. The receivers are heat collection elements which comprise stainless steel absorber tubes in an evacuated glass envelope (to minimize heat loss).

Parabolic Troughs (cont’d) A heat transfer fluid runs through the stainless steel tubes and delivers the heat to a conventional steam turbine/generator.

Parabolic Troughs (cont’d)

The heat transfer fluid is heated to approximately 400ºC in the receiver tubes. This fluid is passed through a series of heat exchangers to generate high-pressure, superheated steam for the turbine. Thermal storage and/or auxiliary fuel can be used to run the plant when sufficient solar energy is not available. This system uses a considerable amount of cooling water.

Parabolic Troughs (cont’d) One approach to avoiding precious water consumption is based on a modified, organic working fluid, Rankine cycle technology (used in geothermal plants). This fluid can be condensed at above-atmospheric pressures using air-cooled, fan-driven cooling towers. This approach is under evaluation.

Parabolic Troughs (cont’d)

Existing parabolic trough power plants include the SEGS power plants located in the Mojave desert near Bairstow, CA. This is a 354 MW facility. This system has operated reliably and cost-effectively (at a cost of about 12¢/kWh (2001 dollars)).

Parabolic Troughs (cont’d) This solar thermal power plant in Kramer Junction, California, uses parabolic trough collectors. (courtesy NREL)

Solar Central Receiver Systems In this approach, heliostats (computer- controlled mirrors) focus sunlight onto a central tower. This system is also sometimes referred to as a “power tower”. The first system, developed by Sandia National Labs, comprised a 90m tall tower and used water as the working fluid. The first plant, Solar One, was a 10MW power plant located near Barstow, CA. This ran from after which Solar Two was built.

Solar Central Receiver Systems (cont’d) Solar Two used molten nitrate salts (60% sodium nitrate/40% potassium nitrate) as the heat exchange medium. Solar Two was also a 10MW facility and was decommissioned in 1999.

Solar Central Receiver Systems Solar Two Power Plant (Courtesy: Sandia National Labs)

Comparison of CSP Systems Efficiency of systems depends on temperature of working fluid which in turn depends on concentration level. System Solar Conc. Dish Stirling systems 3,000 suns Power Towers 1,000 suns Parabolic troughs 100 suns

Comparison of CSP Systems (cont’d) Efficiency and land requirement comparisons: System Efficiency Dish Stirling Systems 21% Power Towers 16% Parabolic Trough 14% System Land Area Dish Stirling Systems 4 acres/MW Power Towers 8 acres/MW Parabolic Trough 5 acres/MW