ECE 7800: Renewable Energy SystemsTopic 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 SystemsA 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 H2 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 TroughsIn 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)

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)

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 1982-1988 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