|Essential parts of a TPV system. Source: MIT LEES|
MIT researchers are working to improve an old concept—the thermophotovoltaic (TPV) conversion of heat into electricity using photovoltaic (PV) diodes—to power vehicle electric subsystems.
The new TPV system would use a small amount of fuel to heat an emitter to approximately 1,500 Kelvin (1,227° C), shine the resulting intense light on new types of photo diode cells to generate electricity, and bounce any excess light back to the light source to help keep it glowing-hot.
Such a light-based system would not replace the car’s engine, but it would consume far less fuel than currently required to keep a heavy, multi-cylinder engine running, even at low speed. In theory, TPV system efficiency could be as high as 40% or 50%.
The system consists of a vertical-cavity enhanced resonant thermal emitter, a spectral control component and PV cells. The PV cells, made of a new material such as gallium-antimonide, would surround the glowing emitter picking up the radiated light.
A highly specialized filter—the spectral control component—set between the two, would let the most useful light wavelengths pass through to hit the photo diodes, while reflecting light of less useful wavelengths back to the heating element, pumping up the temperature.
What’s new here is the opportunity for a much more effective energy system to be created using new semiconductor materials and the science of photonics.—Professor John Kassakian, director of the Laboratory for Electromagnetic and Electronic Systems (LEES)
The relatively high efficiency compared to photovoltaic systems in use today is expected to come from the ability to fine-tune all three main parts of this system: the light emitter, the photo diode cells, and a way to scavenge light at wavelengths that might otherwise be wasted.
The concept of TPV systems is not new. In the late 1960s and early 1970s, much research was done on TPV and light-harvesting technology, first to create solar energy systems for spacecraft, and then in response to energy shortages that spurred an intense burst of research into various alternative energy technologies. It is the advances in the various constituent technologies that now makes this more viable.
This new technology is what makes it a very attractive system. There are the new materials that let us build more appropriate photo diodes. There’s our new understanding of photonics that lets us build the selective emitters. And there’s the photonic band-gap filter, made of thin silicon and silicon-dioxide layers that act as selective mirrors, letting the desired wavelengths through and reflecting back the rest.—John Kassakian
Cooling remains an issue to be solved, and the team is working with different materials to see which work best in terms of light emissions, light harvesting and light reflection.
The researchers are focusing on developing an automotive system that will take excess heat from the TPV system and use it to drive the car’s heating and air conditioning systems. This would replace both the alternator and air conditioner, both of which are now run by the engine.
TPV systems could mesh with hybrid vehicle technology, and be applied to providing auxiliary power for long-haul trucks.
Initial funding for the research was from the MIT/Industry Consortium on Advanced Automotive Electrical/Electronic Components and Systems. The work is presently funded in part by Toyota, but Toyota has made no decision to develop this technology for automobiles.
MIT LEES Thermophotovoltaics project site
Photonic Crystals for Thermophotovoltaic Applications (poster)
“Resonant-cavity enhanced thermal emission”; Ivan Celanovic, David Perreault, and John Kassakian; Phys. Rev. B 72, 075127 (2005)
“Optical characteristics of one-dimensional Si/SiO2 photonic crystals for thermophotovoltaic applications”; Francis O’Sullivan, Ivan Celanovic, Natalija Jovanovic, John Kassakian, Shoji Akiyama, and Kazumi Wada; J. Appl. Phys. 97, 033529 (2005)