Thermoelectric devices could revolutionize HVAC

  • 22-May-2009 03:24 EDT
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Zoned thermoelectric devices permit targeted clima­te control, particularly motorist-only, for most driving conditions.

As the automotive world looks to change to a low-global-warming refrigerant, teams of engineers are looking beyond those systems to solid-state air-conditioning—with no refrigerant and no compressor. The principle is familiar—Peltier effect thermoelectric cooling and heating, with its history in picnic coolers and more recently in temperature-controlled car seats.

The mention of such systems raises eyebrows, as conventional HVAC has at least a 4:1 coefficient of performance advantage over thermoelectric devices (TEDs) and is half the cost. Yes, the R-134a global-warming issue seems to be solved at modest cost by R-1234yf, but other factors improve TED prospects, primarily the work to electrify the car.

Research is closing the cost and efficiency gaps faced by TEDs, according to Steve Ayres, a Senior Engineer at BSST, a research company wholly owned by Amerigon, the manufacturer of 2 million of those TED passenger-seat systems.

Speaking at the 2009 SAE World Congress, Ayres pointed out that the U.S. Department of Energy (DOE) is backing industry initiatives in this area and that his company is in a research venture with Ford. Newly engineered TEDs use 1/6th to 1/25th of the thermoelectric materials, the amount of which is a key cost factor in TEDs, he said. Further, the real competition is not the belt-driven compressor but the more expensive electrically driven unit used to provide A/C with an engine start/stop feature.

Although electric-drive compressors do the job, they are not as efficient as belt-driven units, and there also are energy losses from the battery discharge cycle itself.

Plug-in hybrids initially will have electric-drive compressors, but the potential for TEDs will increase, as "occupant comfort demands under hot/cold startup will make engine-off preconditioning highly desirable," Ayres said.

TEDs are inherently solid-state heat pumps, providing heating or cooling, and because multiple modules are used, they offer much flexibility. Although an A/C system can be made as a heat pump, it is still a single system per vehicle. Electromechanical requirements to go from cooling to heating add complexity and may raise issues with windshield defogging during cabin heating.

As heat pumps in heat mode, TEDs would provide BTU multiples of over two compared with the resistance-type PTC heaters used in some cars, particularly small diesels, where PTCs provide supplemental heating during a cold start.

Substantial TED improvements in efficiency may come from new materials, Ayres said. Although bismuth telluride (BiTe) is the current choice for most TEDs, other man-made and natural materials with inherent thermoelectric characteristics are being tested.

Car-seat TEDs operate purely in air, but BSST research on TED-HVAC includes installing them in liquid-to-air and liquid-to-liquid system modules (using oil and water/antifreeze mixture), Ayres told the Congress meeting.

A zoning approach, for which TEDs are well suited, could provide highly effective efficiency. The typical car holds only the driver 80% of the time, Ayres said, so operating only a driver's TED zone could overcome the efficiency advantage of conventionally heating and A/C cooling the entire passenger compartment. BSST estimates that zoned TED climate control, with only a driver's area zone operating much of the time, could reduce HVAC energy consumption by 50% compared with conventional air conditioning and PTC-assisted heating. All-electric cars offer an additional opportunity.

Initially, Ayres said that BSST sees TED-cooled seating with the addition of TED zones as a supplem­ental system to a belt-driven compressor, reducing overall energy use for air-conditioning by about 18% and providing engine-off cooling. As the TED-zoned system is improved, it eventually would permit elimination of the conventional compressor, he said.

Another dual-approach, shown last year by Delphi and Lear (see AEI's Sept. 2008 print edition), is to use TED seats in conjunction with conventional A/C, adding a duct from the HVAC case to the seat. At first, the seats reduce compressor work; later conventional HVAC reduces the electrical load of the seat TEDs.

Also in DOE’s playbook is an aerospace technology: the thermoelectric cycle that uses waste heat from the exhaust to generate electricity—an application of the Seebeck effect, which actually predates Peltier—1821 vs. 1834. BSST participates in a research project with BMW, Ford, and Visteon. The DOE objective is to improve fuel economy and perhaps lead to eliminating the engine-driven alternator.

Ayres said analyses of waste-heat-to-electricity indicate a potential improvement of up to about 4%, based on the combined U.S. drive cycle. The heat exchangers would be built into the exhaust system, likely at the catalytic converter outlet. Exhaust heat not converted to electricity might be used to speed engine warm-up, even assist cabin heating. A 500-750 W high-temperature TED generator is being developed for dynamometer testing at DOE's National Renewable Energy Laboratory.

A Honda Research & Development study, presented at SAE World Congress by research engineer Masayoshi Mori, indicates that current TED generator fuel economy benefits are too small, 0.08% on the US06 drive cycle to a high of 0.36% on the highway cycle, but that's with today's technology. However, he added that Honda has great interest in the future of this technology and has active research programs in place.

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