Machine Design

Electricity from light gets hot

Researchers from the Laboratory for Electromagnetic and Electronic Systems (LEES) at the Massachusetts Institute of Technology are converting light from superheated materials into electricity more efficiently than ever.

Professor John Kassakian and several of his doctoral students examine a thermophotovoltaic device. Thermophotovoltaic systems generate electric current from light emitted by superheated materials. Photo by Donna Coveney, MIT

To do so, they burn fuel to heat a cylindrical tungsten element till it's white hot. The element's surface is covered with microscopic pits called nanoholes that emit intense light at specific wavelengths when heated to temperatures around 2,200°F. Photodiodes tuned to the frequency of the light turn escaping photons into electricity through a process called thermophotovoltaic (TPV) conversion. Special filters let only the most usable wavelengths of light through to the photodiodes, reflecting less useful frequencies back to the heating element. The reflected energy helps pump up the temperature. In theory, the conversion efficiencies could reach 50%.

TPV technology isn't new. Back in the late 1960s and early 70s, TPV and light-harvesting technologies were first explored to create solar-energy systems for spacecraft. Energy shortages in the mid-70s spurred research into various alternative-energy technologies, including TPV. Energy-conversion efficiencies were low, however, and the return of cheap gas made many researchers abandon the technology.

Today, new materials and manufacturing techniques, along with greater understanding of photonic principles, have boosted efficiency levels. But there are still many engineering problems to solve. For one, the emitter must run as hot as possible, but not melt everything. And it's essential to keep the light-collecting photodiodes cool but still as close as possible to the emitter for efficiency. Different materials are being tested to see which works best in terms of light emission, harvesting, and reflection.

First commercial use of TPV power is targeted for the automotive area. A TPV system could replace present-day alternators and air conditioners, removing their demands from the engine to boost fuel efficiencies. The technique would also let hybrid car engines shut down completely at traffic stops while maintaining the air conditioning for passengers. Trucks would no longer need to run their diesel engines overnight to power lighting, air conditioning, and electronic systems.

Massachusetts Institute of Technology,

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