Which solar thermal technology works best? The debate will soon begin in earnest.

Developer Tessera Solar has created a 1.5 megawatt power plant out of 60 SunCatcher solar thermal devices from Stirling Energy Systems. Later this year, Stirling will go into volume production which will enable Tessera to break ground on commercial scale solar plants in California (one 750 megawatt plant and one 850 megawatt plant) and Texas (a 27 megawatt plant).

The prototype 1.5 megawatt power plant comes after a few delays and a financial glitch for Stirling. Still, now that the plant is up, Stirling will be able to compare the results its gets from its Stirling engines from heliostat prototype power plants erected by eSolar in Southern California and BrightSource Energy in Israel as well as parabolic trough systems that have already been commercially deployed. Parabolic companies, BrightSource and eSolar collect solar heat on mirrors and use it to heat fluid. The warmth causes the fluid to expand, which creates pressure that gets exploited to crank a turbine.

The SunCatcher is made up of a giant parabolic dish of mirrors (40 feet across) to concentrate the sun onto a receiver called a "power conversion unit (PCU)." Sunlight heats up the hydrogen gas in tubes in the PCU, and the gas goes through a heat exchanger to run a four-cylinder Stirling engine. The engine then drives a generator to produce electricity.

The history of Stirling engines goes back to 1816, when Robert Stirling in Scotland designed the first machine and built it two years later to pump water from a quarry.

Each 25-kilowatt SunCatcher is its own mini electricity-generating unit. Thus, it represents a different way of looking at the problem. Stirling has claimed that its system is the most efficient: a prototype converted a record 31 percent of the energy striking it into electricity. Stirling engines, however, transfer heat through the air. The others transfer heat through liquid. As a result, there is no inherent mechanism for storing heat, Stirling execs have said. Which one of these works best in which sort of environments will be one of the big issues for the solar thermal world.

And there are other solar thermal ideas emerging as well: HelioFocus (high temperature Stirling engines linked directly to turbines) and so-called beam down concepts from the Masdar Institute and Solar Fusion.