by Emil Morhardt
Low-level heat—temperatures 100–200°C above ambient, the temperature range of a kitchen oven more-or-less—are abundant in the exhausts of all sorts of industrial processes from drying biomass to operating internal combustion engines. They are also much more common in geothermal fields than the higher temperatures needed for traditional geothermal steam power generation, although low-level heat can be used to vaporize high-volatility organic compounds such as propane, which can then power a turbine much as steam would. For the most part, though, this heat is wasted, just released into the environment; but it needn’t be. Researchers at the China University of Geosciences in Beijing and at Stanford University experimented with an array of commercially available thermoelectric power generators (TEGs) with the intent of producing a 500-Watt power source generated solely from low-level heat. The least expensive and evidently best TEGs for low level heat conversion are solid-state devices made of bismuth telluride, costing less that $3.50 per unit, and generating more that 4.5 W with a temperature gradient of only 140°C.
By combining 96 of these the engineers figured they could generate 500 W if they got the temperature up to 200°C. This is a little more expensive than an equivalent output from photovoltaic panels in full sun, but has the distinct advantage that if the heat source is constant, as it would be in a geothermal situation or in an ongoing industrial process, the electrical output would be equally continuous. They may also be preferable to the propane-driven turbines now used in low-level geothermal electrical generation because they have no moving parts. It seems to me that they would be particularly valuable in northern climes where low ambient temperatures would allow generation from much lower-temperature geothermal or other sources, since it is the temperature gradient across the devices that determines the amount of electrical output.
Liu, C., Chen, P., Li, K., 2014. A 500 W low-temperature thermoelectric generator: Design and experimental study. International Journal of Hydrogen Energy. Abstract at: http://bit.ly/1sfVmtX