Sudarshan P, a blogger, has an interesting idea for green power-generation: solar-heat / gas hybrid steam turbines. He theorizes a power plant where some gas-burning is necessary to reach the high temperatures required to push a turbine with steam, but 60% of the energy comes from solar heat gain, only 40% from gas (which, for extra green points, he specifies should be biofuel, not petroleum), and may be as good as 50% efficient.
How would it work? Basically solar heat (concentrated by reflectors) can boil water and superheat it to 200C, then gas is burned to heat the steam to 400C - 500C (where a turbine is very efficient). Solar heat ends up being ~60% of your heat input because it only takes 1 calorie to raise the temperature of a gram of water (or steam) 1 degree C, but it takes 80 calories for a gram of water to go from liquid to gas, so most of the energy input is used just to boil the water.
Sudarshan starts out the article with what may be an unfairly harsh critique of Stirling engines (he doesn't even spell Stirling right, which makes me think he hasn't done adequate homework), but his solar/gas turbine idea seems a very clever one. I'd like to see someone build one, and see how it performs.
