by Katherine Derbyshire, Contributing Editor, Solid State Technology
April 17, 2008 – The vast majority of photovoltaic (PV) installations capture photons emitted by the sun, but many other sources of electromagnetic radiation exist. Since combustion of any kind emits photons in the infrared portion of the spectrum, the waste heat produced by diesel generators, internal combustion engines, and industrial processes offers a potential source of radiation for PV cells with the appropriate bandgap.
Though some systems generate electricity as a byproduct of combustion, they generally do so by mechanical means. Industrial waste heat powers steam turbines. In engines, some of the energy from the drive train may be used to drive an alternator, in which the rotation of a magnet generates electricity for use by the vehicle.
In both these cases, the conversion from thermal to mechanical to electrical energy increases system complexity and introduces mechanical and other losses. Such systems are also difficult to miniaturize for portable applications. While microturbines can generate a high power density, they require precise, high speed, moving parts, notes W. M. Yang of the U. of Singapore. Creating and assembling millimeter-scale microturbines is quite challenging. 1. Moreover, such mechanical systems make noise and are subject to friction and wear.
Military applications in particular would benefit from a lightweight, near-silent source of electricity. For example, unmanned reconnaissance vehicles use significant amounts of electricity to operate cameras and sensors and transmit information back to the human controller. Batteries are heavy, while their limited charge constrains the vehicle’s range. Internal combustion engines are noisy and also add substantial weight.
Conventional diesel generators also convert thermal energy to electricity by way of a mechanical system. At the same time, generators are often used in parallel with heat sources for cooking and climate control. A system that could generate electricity as a byproduct of heating might offer substantial weight savings.
Applications like these are driving interest in thermophotovoltaic (TPV) devices. TPV devices place photovoltaic cells in close proximity to a combustion heat source (usually 1000-1600K, or 1340°-2420°F). They have no moving parts, substantially reducing weight and complexity compared to conventional generators. Because of their simplicity, TPV generators can be very small. Yang’s group, for instance, demonstrated a 0.92W unit with a micro-combustor diameter of 3.0mm. In fact, such small units can deliver more output power/unit volume than larger units because of their high surface-to-volume ratio.
The most significant difference between TPV and conventional solar PV cells is the use of low-bandgap materials to exploit low energy, long wavelength IR photons. Yang’s group used GaSb cells, which have a bandgap of about 0.8eV and absorb wavelengths up to 1.8μm. While silicon cells are much less expensive, K. Qiu of Canada’s CANMET Energy Technology Centre explained that most thermal radiation falls short of their bandgap (silicon Eg = 1.12 eV) and cannot be used. Qiu’s group used a selective Yb2O3 radiator to shift the spectrum of their natural gas burner to the desired range 2.
Control of excess heat is extremely important for successful TPV cells. They encounter far higher temperatures and energy densities than solar PV cells, yet can only absorb and convert a small fraction of that energy. To protect themselves from destructive heat loads, most designs reflect excess heat back to the combustion chamber, where it serves to preheat the air-gas mixture and increase the combustion efficiency.
As J. van der Heide and coworkers at IMEC explained, TPV cells generate a higher current density than conventional solar cells; it’s important to minimize the resistance of the front contact structure. For the back surface, a highly reflective contact is desirable. Without some form of optical confinement, many long wavelength photons would simply pass through the thin film cells without being absorbed. The IMEC group, working with germanium TPV cells (Eg = 0.66 eV), used an amorphous silicon/SiO2/aluminum stack to form this reflector, realizing the contact structures by laser firing. For the front surface, they diffused palladium from a thin coating through the silicon passivation to form the contact, with a thick silver layer on top to reduce series resistance. 3
In many applications, TPV cells will compete with batteries or fuel cells, not with conventional generators or with solar PV. Energy density (power per unit weight or volume) will be as important as cost or net efficiency. Yet parameters like contact resistance, optical confinement, and cell efficiency contribute to energy density as well. TPV and solar PV designs are likely to learn from each other, enriching the range of photovoltaic applications in the process. — K.D.
References:
[1] W. M. Yang, et. al., “Experimental study of micro-thermophotovoltaic systems with different combustor configurations,” Energy Conv. And Mgmt., vol 48, pp. 1238-1244 (2007).
[2] K. Qiu and A.C.S. Hayden, “Development of a silicon concentrator solar cell based TPV power system,” Energy Conv. And Mgmt., vol. 47, pp 365-376 (2006).
[3] J. van der Heide, et. al., “Optimisation and characterisation of contact structures for germanium thermophotovoltaic cells,” presented at 22nd EU PVSEC, 3-7 September, 2007, Milan.