Solar Thermal to Electric Energy Conversion
Once the sunlight is concentrated by concentrators, several methods can be used to convert the
heat into electrical energy. The conventional approach is to use a heat engine.
Several power conversion methods have been developed for solar central receivers. The power
conversion unit of large central receivers (20–200 MWe) is likely to be a steam Rankine turbine,
while smaller central receivers can have intrinsically better optics and thus accommodate
Brayton and combined cycles operating at higher temperatures. A 10-MWe system using molten
salt as heat transfer fluid and storage medium, combined with a steam Rankine turbine at up to
about 850K, has been demonstrated in the U.S. Department of Energy’s Solar II project. Other
methods are (a) steam generation and superheating in the receiver, (b) heating atmospheric air to
about 950K in the receiver and then using it to superheat steam, and (c) heating compressed air
in the receiver to over 1100K and using it in a solar/fuel hybrid gas-turbine.
An important area that has not been sufficiently explored is the development of heat engines
specifically designed for integration in a solar thermal system, as opposed to the customary
approach of modifying existing engines. Such approach could take advantage of recent
turbomachinery component developments (e.g., combustors, recuperators, alternators, bearings,
ceramic rotors). The use of such engines instead of a modified existing fuel-driven engine should
substantially simplify the power conversion unit, increase system efficiency, and lead to a
significant cost reduction.
In general, new developments and innovations in the various methods described above can
reduce the cost of solar thermal electricity production to about $2/W, or even less (e.g., in
dish/engines). Such installed costs are low enough to provide cost-competitive electrical energy
if fuel costs remain at their present value and carbon emission limitations are implemented.
Solar Thermoelectric Power Generators. Direct thermal-to-electric energy conversion
engines based on thermoelectric devices and thermophotovoltaic (TPV) energy converters
provide new opportunities for medium power ranges that may rival direct photovoltaic (PV)
power conversion and involve no moving parts.
Thermoelectric energy conversion technology, based on the Peltier effect and the Seebeck effect,
exploits the thermal energy of electrons (and holes) for the energy conversion between heat and
electricity, including power generation, refrigeration, and heat pumping.
A thermoelectric power generator has a maximum efficiency given by
η=Th−Tc
Th1 +ZTm− 1
1 +ZTm+Tc/Thwhere
Th,Tc = temperatures at the hot and cold sides