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.