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Thermoelectric and Thermionic Devices - Energy Conversion Innovation in the Alt Fuels Universe
Submitted by Dan Sweeney on Sun, 2007-04-01 20:15.
Plug-in hybrids are clearly in contention for the position of being the next generation platform for personal transport. I don’t see any wholesale transition occurring any time, and indeed such vehicles may never enjoy more than niche market status, but they’re clearly a lot closer to commercialization than hydrogen fuel cell vehicles. Moreover, a heavy reliance upon grid based electricity rather than liquid fuels certainly makes more sense in terms of carbon abatement in as much as CO2 from power plant can be sequestered while sequestration isn’t practical in individual vehicles. Therefore a plug-in hybrid is arguably greener than a conventional internal combustion engine running on biofuels, and plug-in utilizing biofuels might represent that best of all possible worlds.
Still, there is all that bulk and complexity. Garden variety hybrids are bad enough with their parallel power plants, but with plug-ins the much larger size of the electrical components saddles the manufacturer and ultimately the owner with high materials costs and physically much larger motors, generators, and battery packs. Superior fuel economy and vanishingly low emissions come at a steep price.
But what if you could make a plug-in hybrid whose power plant was little larger than a conventional gasoline engine and one that had superior fuel efficiency to that of a gasoline engine? And, moreover, one that could burn any liquid fuel without adjustment or modification?
No such vehicle is even on the drawing board today, but, if certain innovations in energy conversion prove out, we could eventually see a revolution in motive power and of portable power as well.
Thermal Energy into Electrical Energy
Hybrid vehicles take a number of forms—serial, parallel, and parallel-serial. The most advanced and successful type, exemplified by Toyota’s Hybrid Synergy Drive is the parallel-serial design.
All types of hybrids make use of powerful electrical generators to charge the banks of storage batteries supporting electric drive and powering the electric traction motors. The generators are largest in the serial type and smallest in the pure parallel type, but, in all cases, they are considerably larger in capacity than the alternators used to charge the electrical systems in conventional non-hybrid vehicles.
Charging an electrical system with an ICE (internal combustion engine powered generator) entails multiple inefficiencies. Most gasoline engines are themselves only about 15% efficient (although Toyota’s highly innovative Atkinson Cycle Prius is a bit better than that) and a further five to ten percent are sacrificed in the generator. The battery loses another few percent, and the motor itself subtracts a few percent more.
In view of all these losses one might suppose that a hybrid system would be much less efficient than an ordinary pure ICE power plant, but in fact that is not the case. First of all, a hybrid system, and most especially a serial system, allows the ICE to spend more time operating at that rotational rate that is most conducive to economy. Furthermore, the battery pack can provide momentary surges of power at very good efficiency, and thus the engine can operate at a more nearly constant rate than is the case with pure ICE power plants. For this reason, the vehicle can make use of smaller, lighter, engine with a lower horsepower rating, while still providing good acceleration. The battery pack and the electric motor take up the slack, so to speak.
But suppose you could convert the heat of combustion directly into electrical energy—that is, burn liquid fuel but dispense with an internal combustion engine with its complex mechanical design, noise, bulk, and maintenance problems. You’d still be burning gasoline, or ethanol, or biodiesel or whatever, but the thermal energy, or some portion thereof, would be magically transformed into electricity and could be split between the battery pack and the electric motor.
Modern brushless electric motors themselves are virtually maintenance free and last indefinitely, and they offer beaucoup low end torque and an exhilarating driving experience as a result. They’re also dead silent. So this combination of direct conversion of heat into electricity and a modern high output electric motor would be highly advantageous.
But isn’t this what fuel cells are supposed to do, consume fuel and produce energy directly? This is true, but fuel cells do so by entirely different means. In fuel cells the chemical reactions create ions—single protons and electrons when hydrogen is used—and the free electrons flow through an external circuit producing power. Thermal energy is released in the process but it normally contributes no work, it is simply waste heat. In essence, the operating principle of a fuel cells is the same as that of a battery. The difference is that the electro-chemical reagents are being continually replaced so that the device can operate indefinitely.
Thermoelectric and thermionic systems, our focus here, are something else entirely. Heat is transformed directly into electricity through an interface involving no moving parts, and no expensive catalysts and nanostructures as is the case with fuel cells.
In fact, several types of structures have been shown to have the property of generating electrical flow in the presence of a thermal gradient. Vacuum tubes all have this property. The cathode or negative element of the tube is electrically heated and electrons boil off its surface, cross the internal vacuum of the tube, and congregate at the anode, the positive element. Traditional metallic thermoelectric devices such as Peltier generators also produce current flow in the presence of heat, but take an entirely different form, namely two closely coupled plates of unlike metal. Heating a single metallic surface generates electrical current, and, conversely, sending current through the structure conveys heat away from the negatively charged surface.
Unfortunately, traditional thermionic valves and Peltier devices exhibit very poor efficiencies, however, and thus do not provide any practical means of harnessing thermal energy to perform mechanical work.
A New Energy Conversion Technology
Recently, an intriguing nontraditional thermionic device has been announced by a Salt Lake City based startup named ENECO. If their claims are correct, the field of energy conversion might never be the same.
The ENECO device, according to chief operating officer and technology guru, Lew Brown, is a semiconductor wafer not a vacuum tube. “Our device is made of lead tin telluride, not silicon,” explains Brown. “It is solid state, but it essentially thermionic in nature,” he adds.
What makes it different from previous attempts—and they are many—to make an efficient solid state thermionic device is that the structure of the device is of greater significance in terms of its effectiveness as a trandsucer than is the underlying material technology.
The ENECO wafer is a three layer sandwich like a bipolar transistor with a gap separating the two layers on either side of the thermal gradient. The gap material, a thin film of undisclosed composition, is itself thermoelectrically active and is specially doped as is the base layer of bipolar transistor.
Behind the esoteric construction technique lies a simple objective. “We needed to reduce the work function,” says Brown. “With earlier thermionic solid state devices the work function was about one electron volt which does not conduce to acceptable efficiencies. We got it down to 120 millivolts which gives us about 18% efficiency [slightly better than a typical gasoline engine].”
The operating temperature of the ENECO device is fairly broad, 200 to 600F, and is well matched to typical combustors or to solar concentrators. If the devices were used with combustors, one could convert thermal energy into mechanical energy with roughly the same or better efficiency as an ordinary spark ignition engine while shrinking the size, mass, and complexity of the power plant to considerable degree. It would probably make more sense, however, to use the device to exploit the waste heat of an internal combustion engine, however, particularly in the case of gasoline engine which is normally massively inefficient and squanders most of the chemical energy of gasoline in producing waste heat. Extract 18% of the energy of that waste heat to perform useful work, and you’ve almost doubled the efficiency of the engine, and you end up with higher overall efficiency than if either power plant were used alone.
It is not difficult to see how that much “free” electricity would greatly enhance the performance of a hybrid. In a conventional nonhybrid vehicle it could also serve another purpose, namely, eliminating the need for an alternator with all of its attendant cost and complexity. Instead of the engine generating electricity through electro-mechanical means and sacrificing efficiency in the process, the waste heat could be converted into electricity at absolutely no penalty with respect to fuel efficiency.
A high efficiency thermionic device would have uses far beyond the automotive realm, however. Such devices could replace the engines in gensets, and could also provide for the cogeneration of electricity from waste heat in all sorts of industrial settings. One particularly intriguing application is in geothermal power plants where the low temperature steam available cannot be efficiently utilized in conventional Rankine turbines.
Such devices could also conceivably be used for portable electronic devices, but the presence of a micro-combustor—in essence a tiny furnace—would cause many potential users to hesitate in selecting this sort of power source. We wouldn’t bet on such devices replacing batteries in cell phones, laptops, and portable entertainment systems.
How Soon to Market?
ENECO claims that their technology has been validated by third parties, but published reports are lacking. They also claim that full commercialization is at least a year away.
If company representations are accurate, this new technology could have a major impact on portable and distributed power, provided pricing is in line with that of other mass produced solid state devices.
Still, the relatively modest efficiency rating of the ENECO chips could limit its penetration in a number markets. A combustor shrouded in a solid state jacket would certainly be far cheaper to construct than a diesel engine, but then a diesel has almost double the efficiency of the ENECO device. And the ENECO technology isn’t even close to a fuel cell in efficiency though it promises to be far cheaper. Where it might play is in the markets currently occupied by the Smart Fuel Cells direct methanol fuel cells which currently sell to boaters and owners of recreational vehicles. The Smart Fuel Cell product is only about 25% efficient, puts out a mere 20 watts, and is mainly used to trickle charge battery packs. Plus it requires a special fuel sold only by the manufacturer. And at over four grand, it ain’t cheap. In this context, ENECO’s innovation looks pretty promising.
ENECO’s chief competitor appears to be an English firm calling itself Cool Chips LLC which has received backing from Rolls Royce. The Cool Chips device is also solid state but works on a different principle, namely quantum tunneling of electrons through a constricted vacuum separating two layers of semiconductor material on either side of a temperature gradient. Interestingly, ENECO’s researchers began their quest for a high efficiency thermionic device by experimenting with quantum tunneling before concluding that a commercial embodiment of the principle was infeasible. The U.S.D.O.E. takes a more optimistic view of the potential of quantum tunneling and has published a presentation on the subject recently.
Cool Chips has published little concerning how their devices are constructed and alludes to no third party validation on their Website. Nevertheless, in view of their resources they cannot be dismissed out of hand. Incidentally, Cool Chips has a patent on an automotive propulsion system using a combustor along with their thermoelectric conversion technology and a high output motor. Interestingly, the company also has a patent on a very interesting multi-phase AC motor design.
Low efficiency Peltier devices are currently offered by a number of manufacturers, but these have limited applications and do not constitute a growth category. It should also be mentioned that DARPA has sponsored a lot of research in this area with a view to providing soldiers in the field with a new compact power source. Much of the DARPA efforts have focused on a new class of crystalline substances known as “rattling lattices” by virtue of the relative elasticity of their crystalline structure. These have the rather unique property of poor thermal conductivity and high electrical conductivity—exactly what one wants in a thermoelectric material. So far no one is close to commercialization, however.
Finally, we might make mention of micro ICE’s. Stanford University has produced some prototypes of silicon based Wankel rotary engines running on liquid fuel and occupying a single silicon wafer. Energy density is far higher than any battery and the technology could permit very extended runtimes for portable devices. So far there have been no takers among portable device manufacturers, however. Sometime the notion of an engine running in one’s coat pocket where one’s cellular phone resides is distinctly disquieting.