Solar for Dark Climates

Solar technology that generates both heat and electricity could make solar energy practical in places that aren't sunny.

Cool Energy, a startup based in Boulder, CO, is developing a system that produces heat and electricity from the sun. It could help make solar energy competitive with conventional sources of energy in relatively dark and cold climates, such as the northern half of the United States and countries such as Canada and Germany.

Solar generator: A prototype of a Stirling engine that's powered by a solar water heater. Credit: Cool Energy

The company's system combines a conventional solar water heater with a new Stirling-engine-based generator that it is developing. In cool months, the solar heater provides hot water and space heating. In warmer months, excess heat is used to drive the Stirling engine and generate electricity.

Samuel Weaver, the company's president and CEO, says that the system is more economical than solar water heaters alone because it makes use of heat that would otherwise be wasted during summer months. The system will also pay for itself about twice as quickly as conventional solar photovoltaics will, he says. That's in part because it can efficiently offset heating bills in the winter--something that photovoltaics can't do--and in part because the evacuated tubes used to collect heat from the sun make better use of diffuse light than conventional solar panels do.

The system is designed to provide almost all of a house's heating needs. But the generator, which will produce only 1.5 kilowatts of power, won't be enough to power a house on its own. The system is designed to work with power from the grid, although the power is enough to run a refrigerator and a few lights in the event of a power failure.

The company's key innovation is the Stirling engine, which is designed to work at temperatures much lower than ordinary Stirling engines. In these engines, a piston is driven by heating up one side of the engine while keeping the opposite side cool. Ordinarily, the engines require temperatures of above 500 °C, but Cool Energy's engine is designed to run at the 200 degrees that solar water heaters provide.

Story continues below

The success of the technology, however, hinges on achieving the efficiency targets, says Dean Kamen, the inventor of the Segway, who is developing high-temperature Stirling engines for other applications, including transportation. "We need data," he says. The company's second prototype was only 10 percent efficient at converting heat into electricity. Its engineers hope to reach 20 percent with a new prototype.