Waste Exhaust Heat Generates Electricity to Make Cars Efficient

Only about a quarter of the energy in a gallon of gas goes into moving a car down the road. Much of the remaining energy exits the exhaust pipe as heat. Thus, automotive engineers are working on ways to capture some of this energy and put it to use as a way to conserve fuel.

One promising technique is to use a thermoelectric generator, or TEG, to capture heat contained in the engine’s exhaust stream. TEGs can use virtually any heat source, such as waste exhaust heat, to produce electricity using the thermoelectric effect. In this case a difference in temperature produces an electric voltage. The result is that TEGs directly convert exhaust heat to electricity while reducing the load placed on an engine’s alternator.

BMW plans to use TEGs on some of its models as early as 2014. The TEG would power electric accessories like the climate control system without placing the additional drag on the engine caused by the need for belt-driven compressors, thereby increasing fuel efficiency. Heat could also be stored and used to pre-heat an engine to reduce fuel usage and decrease cold start emissions.

According to BMW, TEGs could potentially reduce fuel consumption by about 5 percent. Surprising, this is a bigger savings than provided by such technologies as engine stop-start and regenerative braking. BMW recently won an award with a TEG system that can generate about 200 watts of power.

Volkswagen has also demonstrated a TEG that generates electricity through the reaction of two different metals as heat passes through, with electrical generation in the range of up to 600 watts. This is about 30 percent of the electrical requirement of a Golf Plus, an amount that could reduce fuel consumption by over 5 percent.

Furukawa in Japan is developing a skutterudite (a cobalt arsenide mineral thermoelectric material) for use in a TEG. Furukawa’s module, measuring 5cm x 5cm x 8mm, produces up to 33 watts. For automotive applications, about 20 modules could be attached around the exhaust system with about 7 percent of the exhaust heat potentially converted to electricity, easing engine load and reducing fuel consumption by about 2 percent.

Since 2004, the U.S. Department of Energy has funded TEG research being conducted by three teams comprised of participants from the automobile industry, universities, NASA, and DOE national laboratories. The goal is a TEG for vehicles that can deliver a 10 percent improvement in fuel economy.