CAM ThermoPhotoVoltaics

Center for Advanced Materials

	High Effieciency
	Thermophotovoltaics

	Technology Thermophotovoltaic (TPV) energy conversion is a potentially environmentally friendly approach to achieve high efficiency, compact and reliable sources of electrical energy. In TPV conversion, a source of energy such as concentrated sunlight, nuclear power, fossil fuel, or a radioisotope heat source is used to heat an intermediate thermal emitter. The emitter then radiates photons which impinge on a photovoltaic cell.Common to all TPV systems operating at moderate emitter temperatures is the desire for low-bandgap photovoltaic devices that can efficiently convert the infrared-rich spectrum emanating from the emitter. In conventional photovoltaic cells the electron and hole result from absorption of a photon with energy above the bandgap. These carriers rapidly thermalize to their respective band edges. The fundamental efficiency limitation in a conventional cell results from the trade-off between a low bandgap, which maximizes light absorption and hence the output current, and a high bandgap, which maximizes output voltage. Power conversion in thermo-photovoltaics, or any other photovoltaic device, can be increased by implementing monolithically series connected multi-bandgap structure in the device. The main concern for multi-band gap material is the availability of different band gaps for the optimal operation of the device. Based on the recent at the Center for Advanced Materials, GaAsN/InAsN superlattice lattice- matched to InP has shown the potential of achieving band gaps in the range of 0.7-0.4eV, which is a technologically important range for the TPV structure due to the availability of the photon energies in this range from the heat source. CAM is developing patented materials and processes based on multi-quantum well and multi-junction technologies (US 6,150,604; US 6,147,296; US 6,372,980 B1) focused on maximizing efficiencies for applications and systems integration.

	Market Gas, coal, nuclear fuel, petrol, and also biomass all burn at temperatures in the range of 1000 - 2500 degrees K, radiating energy over a broad spectrum like the sun but at longer wavelengths. This can be converted into electricity by low-band-gap cells. There is the possibility of surrounding the source by an “emitter” which re-radiates in a narrower spectrum just like an old fashioned gas mantle. Traditional fuels have the added advantage of an established distribution infrastructure. There is also now renewed interest in TPV energy conversion using nuclear energy sources or combustion driven systems operating at low temperatures (<1500 degrees K). Possible uses include power generation for deep space exploration, silent portable gas or natural gas operated generators, non-polluting and silent energy generation for natural gas operated vehicles, power co-generation (using conversion of waste heat into electricity), and peak-time electricity for power utilities or household use.


	CLICK on image to enlarge

	Available Energy The spectral energy peak of a 1500 deg. K black body falls at a wavelength of 2 microns. As a result, for silicon photovoltaic devices only a very small portion (<2%) of the emitted energy is above the bandgap and is available for PV conversion (above). Therefore the use of narrower bandgap semiconductors has been identified as a necessary condition to achieve higher efficiencies. Most of the existing development work is concentrated around two semiconductor systems: the ternary InGaAs cells fabricated on InP substrates and GaSb cells and the GaInAsSb quaternary alloys fabricated on GaSb substrates. Nanostructure Modeling The Photovolatics and Nanostructures Laboratories at the Center for Advanced Materials employs computer modeling to optimize the design parameters of nano-engineered TPV material. Optimized band gaps for well-behaved p-i-n junction subcells were estimated by finding the optimal current to provide the maximum power through the series-connected double, triple and quadruple junction cells for given blackbody radiation as an incident flux. Once the optimal current for the given set of tandem cells is found, modeling can be used to determine the output power of the tandem. For a fixed first subcell band gap, the second subcell band gap can be varied to find the maximum power output of the tandem, giving the corresponding second subcell band gap as the optimal band gap of the double junction tandem. This variation is shown in the blue curve (below) for first fixed cell at the band gap of 0.74eV.



	The optimal power can be reached for the second subcell band gap of 0.615eV. Similarly, we can find the band gaps of third (0.525eV) and fourth (0.46eV) subcell to achieve maximum power output in triple and quadruple junction cells. If the first band gap is not fixed, optimal band gaps of the first two subcells also can be found by searching all the possible combinations of the band gaps of the first subcell and the second subcell, giving the iso-power surfaces as shown (below). It can be seen that the optimal band gaps for first and second subcells come out to be at 0.64eV and 0.46eV respectively.


	For more information contact:

	Dr. Alex Freundlich - Project Leader
	Nanostructures and Photovoltaic Devices
	afreundl@central.uh.edu

Center for Advaced Materials

724 Science & Research Building One

Houston, Texas 77204-5004

713-743-3621