PHOTOVOLTAICS: Heat-transfer simulation aids in TPV prototyping

April 1, 2005

A prototype TPV system is built around a tube-shaped methane burner within a ytterbium oxide mantle emitter that selectively radiates electromagnetic energy matched to the bandgap of the silicon solar cells attached to water-cooled aluminum plates in the outer diameter. The quartz cylinder surrounding the burner protects the solar cells from hot exhaust gas [5, 6].

Unlike photovoltaic (PV) cells, which generate electricity from electromagnetic radiation in the visible spectrum, thermophotovoltaic (TPV) cells generate electricity from IR radiation, for example as produced through the combustion of fuel (see Laser Focus World, July 2004, p. 15). The idea in TPVs is to actually bring the PV cell into close proximity with the radiation source (see Fig. 1). This presents a unique heat-transfer problem that researchers in Germany have analyzed using a computer simulation.¹

The thermal radiation for TPV conversion is generated by combustion of conventional fuels, such as petroleum, gas, biomass, or nuclear fuel, that burn in a temperature range from 1000 K to 2500 K and heat a thermal emitter. Design approaches among various research groups have included blackbody-emitter materials such as silicon carbide, rare-earth materials that emit over relatively narrow wavelength bands to match a chosen PV cell, combined burners and emitters, and emitters that are textured with tungsten and ceramic or other materials. As an alternative to a selective emitter, a filter stage can tune the bandwidth of radiation reaching the PV cell and also improve system efficiency, by transmitting a relatively narrow wavelength spectrum to the PV cell while reflecting radiant energy that was not transmitted back to the emitter.²

In terms of the PV cell, many of the various groups researching TPV energy generation have tried alternatives to the silicon used in traditional PV materials because of silicon’s high bandgap. Compound semiconductors that absorb IR radiation, such as gallium antimonide, indium gallium antimonide, gallium indium arsenic antimonide and indium gallium arsenide/indium phosphide, have been widely investigated; work has also been done with silicon, germanium and silicon germanium PV materials and selective emitters. Thermophotovoltaic photocells have also been developed with multiple junctions, each converting different portions of the incident energy spectrum. Quantum-well cells made of indium gallium arsenide phosphide have also been developed to enable tuning of the cell bandgap to a desired energy spectrum without changing the materials.³

Potential applications include solar power for spacecraft, generating electricity from home heating systems, improving efficiency of fossil-fuel-driven power plants, and powering cleanly combusting, electrically driven automobiles.4 While the technology has yet to have a significant impact on the energy market, it has been the subject of international research initiatives. Efficiencies ranging from 1% to 20% have been achieved, with high efficiency being much more important for electrical power source applications, such as automobiles, than for cogeneration of electricity from waste heat, as in home furnaces.

How hot should it be?

Thermophotovoltaic systems present design challenges in terms of heat transfer because unlike a typical electronic system, efficiency improves when radiative heat transfer is maximized, but radiated heat that is not converted to electricity and heat that is transmitted conductively increase the temperature of the PV cells, which have a limited operating-temperature range. So the heat-transfer problem tends to be one of maximizing surface-to-surface radiative heat fluxes while minimizing conductive heat flux, which involves optimizing factors such as the emitter temperature and the geometry of focusing mirrors to maximize PV efficiency.

As a potential alternative to assessing actual device prototypes in various system configurations, Wilhelm Durisch, Bernd Bitnar, and colleagues at the Paul Scherrer Institute (Villigen, Switzerland) performed computer simulations of device performance for a range of emitter temperatures from 1000 K to 2000 K using the heat-transfer module of Femlab 3 modeling software (Comsol; Stockholm, Sweden).

At a 2000 K emitter temperature, the temperature of the PV cells (whose efficiency drops to zero above their 1600 K maximum) climbed to 1800 K. At a 1600 K emitter temperature, however, the PV-cell temperature dropped to 1200 K, where electrical power output (PV efficiency multiplied by irradiated energy) was maximized. The researchers also noted a temperature of 800 K at the outer edge of the device insulation, indicating a significant amount of heat transfer to the air surrounding the TPV device.