The Technology Review recently had an article on a somewhat unusual approach to generating power from sunlight. One approach, of course, is to convert the energy in sunlight directly to electricity with photoelectric cells; the cost of the required semiconductor material is a significant issue for this technology. Another approach uses mirrors to concentrate solar energy on a single point, in order to heat water and produce steam to drive a turbine. A drawback of this type of setup is that it requires a supply of water, which is often not easily obtainable in those places which have the most usable sunlight (e.g., deserts). The new approach, developed by Stirling Energy Systems, also uses heat from concentrated sunlight, but the hear is used to power a Stirling engine; since it is a closed-cycle engine, it does not require an external water source.
Stirling Energy Systems technology uses 12-meter-wide mirrors in the shape of a parabolic dish to concentrate sunlight onto a Stirling engine. The difference in temperature between the hot and cool sides of the engine is used to drive pistons and generate 25,000 watts of electricity.
The company is in the process of building a 1.5 megawatt demonstration plant, and has contract to build two full-scale facilities in California.
One potential advantage of this technology is that it is inherently fairly modular, since each reflector dish and engine can work independently. A potential disadvantage is that the system can only work while the sun is shining; unlike a steam-generation plant, there is no obvious way to store energy for use at other times.
This actually points up one of the biggest issue with many “cleaner” forms of energy: how to store it. Hydrocarbon fuels, for all their nasty side effects, do have one clear virtue: they generally have very high energy density. In other words, each kilogram of fuel can generate a lot of power. Any breakthrough in energy storage technology (much better batteries, for example) could make a big difference in the economic attractiveness of alternative energy sources.