WASTE heat can be turned into electricity using a type of solar cell that鈥檚 powered by heat rather than light. The system鈥檚 secret is an egg-shaped 鈥渆nergy concentrator鈥� inspired by the exquisite creations of the nineteenth century jeweller Carl Faberg茅.
Developed by engineers at Dalarna University鈥檚 Solar Energy Research Center in Borl盲nge, Sweden, the system is based on a technology called thermo-photovoltaics. In TPV, thermal radiation from a hot source powers a photovoltaic (solar) cell geared to work best at the wavelength of the thermal radiation.
However, although TPV was conceived by the US military some 30 years ago, the idea has made little headway due to its low energy-conversion efficiency. But the Swedish researchers believe they have hit upon a way to make the technology efficient enough, thanks to their novel 鈥淔aberg茅 optics鈥�.
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TPV generators use a heat source 鈥� which can be a dedicated combustion chamber or waste heat from, say, a furnace 鈥� to warm a metal element chosen for its ability to emit a great deal of infrared radiation. This radiation strikes a conventional solar cell and is converted into electricity.
But sources like this emit infrared over a wide range of wavelengths, whereas solar cells work best across a narrow band of wavelengths. Bombarding them with a range of wavelengths reduces their efficiency. One way to overcome this is to place a filter between the emitter and the solar cell. While this restores efficiency, such filters only work well when the radiation hits them almost straight on. 鈥淏ut in practice the emitter radiates in all directions,鈥� says Eva Lindberg, one of the Dalarna team.
Enter Faberg茅. Lindberg and her colleague Lars Broman have placed their whole TPV system inside an egg-shaped optical chamber with a highly reflective interior coating, making it look just like one of the goldsmith鈥檚 famous bejewelled eggs. It turns out the three-dimensional, ellipsoidal shape ensures that most of the radiation emitted reflects off the inside of the egg and hits the wavelength filter almost head on (see Graphic). It鈥檚 similar to the way the hemispherical, shiny interior surface of car headlights straightens out the light beam.
Reporting their invention at a TPV conference in Rome this week, Lindberg and Broman say this approach decreases the range of incidence angles into the filter from around 45掳 to 20掳, allowing about 96 per cent of the emitted rays to reach their goal 鈥� the solar cell 鈥� with other wavelengths being reflected back towards the emitter.
TPVs are certainly an attractive option, says Tim Coutts, a researcher for the National Renewable Energy Laboratory in Golden, Colorado. Since they have no moving parts, there are efficiency savings to be made and maintenance costs are lower. And they work at both large and small scales, with applications ranging from small domestic generators, producing heat and electricity, to megawatt power plants or back-up stations.
But Coutts believes their best application will be in recycling waste energy. 鈥淵ou can imagine TPV being used in conjunction with high-temperature industries, such as glass manufacturing,鈥� he says.