There is a tantalizing hope that someday someone will come up with a real breakthrough in direct heat-to-electricity conversion. No moving parts, “solid-state”, self-contained, scalable, and so on. Such miracles do exist, but they are costly and inefficient, and can find use only in specialized niche applications like satellite power, IC chip cooling, novelty items like picnic coolers, and most recently as comfort conditioning in automobiles.
The sought-after breakthrough would be in performance and cost, for example, to “make the internal combustion engine obsolete” and do many other marvelous things. As one example, cold climate utilities have attempted unsuccessfully to use thermoelectic generation to develop self-powered home heating systems which could continue to operate during power outages.
The fundamental underlying processes have been known for a long time, e.g., Thermoelectric (TE) (Seebeck, Peltier), Thermionic, ThermoPhotoVoltaic, etc. NASA, for one, has spent decades fine tuning these for use in space, and a hardy band of scientific, engineering and business people continue the quest. Some companies actually earn a decent living at making and selling such devices, but it is strictly a matter of small niches. Note that TE can be used reversibly to either provide cooling (heat pump) or generate electricity (heat engine).
There are some interesting stirrings of late. For a number of years, researchers at MIT and elsewhere have focused on nanostructures which create one and two dimensional worlds for electrons (known as “quantum wells”), which theoretically should yield higher efficiencies. Experimental results are slow in coming. Last October, the Research Triangle Institute published a major paper in Nature claiming dramatic improvements (in the lab) in TE performance, based on nanolayers of traditional TE materials. Most research in the field has focused on trying to find new bulk materials that have better properties, so this layering approach caught people by surprise. Prior claims to boost “ZT” (the figure of merit for TE) much above 0.7 – 1 haven’t held up, but RTI seems really to have a ZT of 2.4. Such a doubling or tripling of “ZT” could hugely expand the range of applications for both cooling and power — assuming of course that the cost is low enough.
RTI is putting on a conference Oct 28-30 in Dallas:
“Next Generation Thermal Management Materials and Systems – for Cooling and Power Conversion”
Full agenda at: http://techventure.rti.org/fall2002/
* The latest advances in thermal management materials and systems, and how recent developments can spur commercialization.
* Market trends and opportunities for new thermal management technologies in cooling and power conversion – in wide ranging applications – from micro electronics to refrigeration.
* The status of commercial applications – impact on enabling new markets and displacing current markets.
One of the speakers has recently given a paper at a recent TE conference*. (I have the papers if anyone is interested.) A clever way** of arranging an array of TE modules more than doubles the overall system efficiency for cooling. A commercial product using this technique already is in use, cooling seats of luxury cars. (http://www.amerigon.com/)
(The TE conference* was the ICT2002, held August 26-29, Long Beach, CA. This is an annual meeting of the worldwide thermoelectric R&D community. For a brief account of the conference, see the Sept 30 “ZTSpam” at Cronin Vining’s website:
Cronin is a renowned expert in TE, and a good friend and colleague of UFTO.)
Besides TE, thermionic and TPV continue to get attention. (In thermionic conversion, electrons boil off a heated surface and are collected on another electrode. In TPV, the heated surface sends out photons of a particular variety which go to a specialized PV cell. It’s PV with its own built-in custom light source, which is heat-driven.) Some of the most promising new developments use nanoscale approaches to overcome traditional obstacles to cost and performance. The “Nano-TPV” work is being done at Draper Laboratory, and involves reducing the spacing between the heated emitter and PV receiver to nanoscale dimensions. Experiments confirm a dramatic increase in the photo current. In another development, Eneco in Salt Lake City continues to make progress on its nanoscale method which they say combines thermionic and TE effects. (See UFTO Note 28 Nov 2001.)
** As explained in the papers, the configuration involves (as I describe it) a counterflow heat exchanger where a number of parallel heat pumps push heat from the cold side to the hot side. Each heat pump sees a temperature difference that is only half of the “delta-T” that the overall system provides, leading to higher overall efficiency. Whether this would be practical in a larger system using compressors is hard to say.