Catch the light, trap the heat

A new type of device that uses both heat and light from the sun should be more efficient than conventional solar cells, which convert only the light into electricity.

The device relies on a physical principle discovered and demonstrated by researchers at Stanford University. In their prototype, the energy in sunlight excites electrons in an electrode, and heat from the sun coaxes the excited electrons to jump across a vacuum into another electrode, generating an electrical current. The device could be designed to send waste heat to a steam engine and convert 50 percent of the energy in sunlight into electricity--a huge improvement over conventional solar cells.

The most common silicon solar cells convert about 15 percent of the energy in sunlight into electricity. More than half of the incoming solar energy is lost as heat. That's because the active materials in solar cells can interact with only a particular band of the solar spectrum; photons below a certain energy level simply heat up the cell.

Stanford's Nicholas Melosh was inspired by highly efficient cogeneration systems that use the expansion of burning gas to drive a turbine and the heat from the combustion to power a steam engine. But thermal energy converters don't pair well with conventional solar devices. Solar cells, by contrast, get less efficient as they heat up.

The breakthrough came when the Stanford researchers realized that the light in solar radiation could enhance energy conversion in a different type of device, called a thermionic energy converter, that's conventionally driven solely by heat.