Photos: Catching rays with a Stirling engine

Stirling Energy Systems is one of a number of start-ups focused on solar power. But there's one key difference: It's using an engine technology first developed in 1816.

The Stirling system's power conversion unit, seen here on a maintenance stand. The power comes from a cycle of heating and cooling--a gas in the system expands with exposure to an external heat source, such as the sun's rays.

The power conversion unit with its heater head removed. Stirling Energy is working with Boeing and the U.S. Department of Energy on performance tests.

The original design of the Stirling engine, with a picture of its inventor, Church of Scotland minister Robert Stirling.