The company had toyed with the idea of using only thermoelectric modules hooked up to PV panels. In a thermoelectric module, voltage applied across a thermoelectric material sandwiched between two ceramic plates makes one side hot and the other cold. However, existing thermoelectrics (which are used in temperature-controlled car seats, lasers, and portable picnic coolers), typically bismuth or lead telluride, are not efficient enough for large refrigerators.

Gang Chen, a professor of mechanical engineering at MIT, says that the efficiency of a cooling unit depends on its size. “As you shrink the size to a hotel refrigerator, the compressor itself becomes less efficient,” he says. “In those cases, thermoelectric becomes increasingly more attractive.” Promethean’s approach to combining thermoelectrics with compressors sounds like a logical argument to increase cooling efficiency in commercial-scale systems, Chen says.

The company’s 60-liter prototype used bismuth-telluride modules from Dallas-based Marlow Industries. That is the most efficient cooling material known so far, says Boston College physics professor Zhifeng Ren. But there is still room for improvement, and Grama says that the company is on the lookout for new, possibly more-efficient materials.

The startup company might be in luck. Many advances in thermoelectric materials have come out of laboratories recently. MIT’s Chen, for one, has increased the efficiency of bismuth antimony telluride by 40 percent by using nanocrystalline materials. Researchers are also tinkering with lead telluride and are starting to use silicon nanowires and silicon-germanium composites. Chen and Ren have founded a company called GMZ Energy, headquartered in Newton, MA, to commercialize their nanocomposite material, and they’re expecting commercial thermoelectric modules within one year.