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Photonic crystals to help drive auxiliary power in cars

26 May 2006

Researchers in the US are studying the potential of thermophotovoltaic systems as auxiliary power generation units (APU) in cars.

Researchers at MIT are working on a thermophotovoltaic (TPV) system which could one day power a car's air-conditioning system - even when the engine is switched off. And it's not only the automotive industry that could benefit from TPVs, the team lists portable power supplies and powering space vehicles as other potential applications.

The TPV system would not replace a car's engine. It would simply supply enough electricity to run auxiliary systems, consuming far less fuel than is required to keep the engine ticking over.

The MIT system consists of a burner, thermal emitter, photonic-crystal based spectral control components and a gallium-antimonide (GaSb) photovoltaic diode.

The heat extracted from burning fuel is converted into thermal radiation. Using two photonic crystal components, this thermal radiation is reshaped to better match the GaSb diode response characteristics and then converted into electricity.

"Our main focus has been the exploration of photonic crystal metamaterials as a means to shape, guide and process thermal radiation with the goal of optimizing the efficiency of TPV systems," Dave Perreault, a researcher at MIT's Laboratory for Electromagnetic and Electronic Systems, told optics.org. "It is clear that photonic crystals offer remarkable opportunities in this particular application."

The first photonic crystal component - a 2D hexagonal photonic crystal - ensures that the thermal emitter produces only selected wavelengths. The second element is a 1D photonic crystal that essentially filters the light before it hits the GaSb diode.

The 1D photonic crystal consists of an alternating Si and SiO2 layer dielectric stack, manufactured by low-pressure chemical vapour deposition. By combining both photonic crystals, the researchers predict "significant performance improvements over conventional TPV system architectures".

The TPV system operates in the temperature range 1300-1500K. "The thermal emission is cut-off by the 1D photonic crystal filter at 1.8 micron wavelength, which is matched to the GaSb diode," explained Perreault. "We use commercially available GaSb PV cells since they are relatively inexpensive. Other semiconductors with band-gaps as low as 0.5 eV and quantum efficiencies approaching one can be used as well."

According to Perreault, the idea is to burn gasoline in the steady state fashion to produce heat. "The benefit of doing this is that one can generate power independent of running the engine, giving better support stop-start operation of the engine," he said. "In addition to electrical power generation, a complete automotive system would also provide heat and cooling (via heat-driven air conditioning) from the combustion energy."

To date, the team has completed some initial system design studies to identify the key technical challenges. "Our goal is to reach efficiencies of 20%," added Perreault. "Future steps include further system integration and component validation and demonstration of a full system."

Darius Nikbin is Science/Technology Reporter on Optics.org and Opto & Laser Europe magazine.

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