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Heating Plug-in Hybrids

Heating and air-conditioning systems that use thermoelectrics could make plug-in hybrids more practical.
April 14, 2008

The potential of plug-in hybrids and electric vehicles to curb petroleum use has grabbed a lot of attention lately. But there is still a big obstacle to clear before such cars can become the dominant vehicles on the road: automakers will need to find an efficient way to supply them with heat and air conditioning. That’s because conventional heating and cooling systems either don’t work or are inefficient in such vehicles, significantly lowering their range in hot and cold weather.

One of the leading candidates for an alternative system is based on thermoelectrics, semiconductor devices that can provide either heat or cooling, depending on the direction the electric current is flowing. Major automakers, such as GM and Ford, are now developing systems based on existing thermoelectric semiconductors, and experimental materials that use nanotechnology promise to make such systems even more appealing.

The first plug-in hybrids–cars that can be recharged by plugging them into an electrical socket, but have small gasoline engines to extend their range–will make use of electric heaters. When they start appearing from major automakers near the end of 2010, they’ll cost thousands more than conventional cars, so automakers are looking for ways to make them less expensive to broaden their appeal. One way to do so is to find more-efficient systems of heating and cooling, which make it possible to use smaller, less expensive batteries. As a result, thermoelectric systems could start appearing in cars in 2012.

The heating systems in today’s cars rely on the fact that internal combustion engines are terribly inefficient–about two-thirds of the energy gained from burning gasoline does nothing to propel the car. But this inefficiency does generate massive amounts of heat. Some of that heat is used to warm passengers. Plug-in hybrids, which run mostly or entirely on electricity for local driving, don’t generate such quantities of waste heat. So, heat has to be generated using power from the battery, draining thousands of watts that could otherwise have been used to propel the vehicles. While plug-in hybrids consume dramatically less gas thanconventional cars in mild weather, in cold weather the benefit will be much less, according to Clay Maranville, a senior researcher at Ford Motor Company.

A similar problem holds true with air conditioning–the electric range of plug-in hybrids will drop in hot weather, either because the gasoline engine needs to kick in to spin a conventional compressor, or because an electric compressor will drain the battery.

Thermoelectrics are attractive for heating because they’re more efficient than ordinary electric heaters, which generate heat by running a current through a material that has high electrical resistance. What’s more, thermoelectrics make it possible to completely redesign the cooling and heating system. In most of today’s vehicles, the heating and cooling systems are designed to heat the entire passenger compartment. Thermoelectrics are compact devices that can be installed at points throughout the car–in the headrest, the seat, at a person’s feet, and so on. So, instead of heating and cooling the whole cabin, they can heat or cool individual passengers.

The savings of electrical energy such a system could provide to a plug-in hybrid car could be enormous. Where a conventional electric heating and cooling system requires up to 4,500 watts to maintain the desired temperature inside a car, a thermoelectric system would use 3,000 watts if the car is full of passengers or less than 700 watts if the driver is alone, according to estimates from the Department of Energy. Future generations of thermoelectric materials that rely on nanotechnology now being demonstrated in academic labs could improve these savings even more. (See “Cheap, Efficient Thermoelectrics” and “Turning Waste Heat into Power.”)

Both Maranville and Jihui Yang, a staff scientist at GM, say thermoelectrics are likely to be incorporated gradually as different components become available. One of the first ways it can help is in downsizing conventional air conditioning to make it more efficient. Yang says GM’s cars use oversized air conditioners to meet a requirement that they cool the passenger compartment in two minutes, even on very hot days. Well-placed thermoelectrics could make passengers feel cooler immediately by cooling their necks or face while a small, conventional air conditioner gradually lowers the temperature everywhere else.

In general, automakers have worried that most consumers won’t be willing to put up with inconveniences to drive more-efficient cars. Thermoelectric heating and cooling could help. “Our big challenge in research is to make the technology transparent to the customer, so that when they get into a vehicle, it doesn’t matter whether they’re in a conventional vehicle or in an advanced technology vehicle,” Maranville says.

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