Solar for Dark Climates

Technology Review Wednesday, July 08, 2009

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

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.

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.

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.

A Stirling engine's efficiency is limited by the difference in temperature between the cool and hot side. Typically, reaching the necessary high temperatures using sunlight requires mirrors and lenses for concentrating the light and tracking systems for keeping the concentrators pointed at the sun. The concentrators require direct sunlight, so they don't work on overcast days, and they're too bulky to be mounted on the roof of a house.

To make a practical Stirling engine that runs at low temperatures and doesn't require concentrators, the engineers at Cool Energy addressed a problem with conventional engines that leads to wasted energy: heat leaks from the hot side of the system to the cool side, lowering the temperature difference between them. This happens because the materials required for high temperatures and pressures--typically metals--conduct heat. Working at lower temperatures, the engineers concluded, allows them to use materials such as plastics and certain ceramics that don't conduct heat, reducing these losses. These materials also help lower costs: they're cheaper than some of the metals typically used, and they don't require lubrication, improving the reliability of the engines and reducing maintenance costs.

Cool Energy's engineers are currently assembling the company's third prototype, which they say will allow them to reach their efficiency targets by the end of this summer, after which they plan to test pilot systems outside the lab. Within two years, they plan to manufacture enough systems to drive costs down and achieve their payback targets.

Copyright Technology Review 2009.