In this July 31, 2008, photo released by General Motors, a module for a thermoelectric generator is shown in Warren, Mich. The generator, which converts exhaust heat to electricity, could improve fuel economy. (General Motors, Lee Short/Associated Press)

The hot air that escapes from a car's tailpipe could help us use less gas.

Researchers are competing to meet a challenge from the U.S. Department of Energy to improve fuel economy 10 per cent by using wasted exhaust heat as energy to help power the vehicle.

General Motors Corp. is close to reaching the goal, as is a BMW AG supplier working with Ohio State University. Their research into thermoelectrics — the science of using temperature differences to create electricity — couldn't come at a better time, as high gas prices accelerate efforts to make vehicles as efficient as possible.

GM researcher Jihui Yang said a metal-plated device that surrounds an exhaust pipe could increase fuel economy in a Chevrolet Suburban by about five per cent, an improvement that would be even greater in a smaller vehicle.

Reaching the goal of a 10 per cent improvement would save more than 455 million litres of fuel per year in GM vehicles in the U.S. alone.

"The take-home message here is: It's a big deal," Yang said.

Searching for an alternator alternative

The U.S. DOE, which is partially funding the auto industry research, helped develop a thermoelectric generator for a heavy-duty diesel truck and tested it for the equivalent of more than 885,000 kilometres about 12 years ago.

John Fairbanks, the department's thermoelectrics technology development manager, said the success of that generator justified the competitive search in 2004 for a device that could augment or replace a vehicle's alternator. Three teams were selected to participate in the program, with GM and thermoelectrics manufacturer BSST separately working on cars and a team from Michigan State University focusing on heavy-duty trucks.

Fairbanks said thermoelectric generators should be on the verge of production in about three years.

"It's probably the biggest impact in the shortest time that I can think of," he said.

The technology is similar to what NASA uses to power deep space probes, a perk being it doesn't seem to be susceptible to wear. Probes have used a thermoelectric setup for about 30 years.

How it works

Thermoelectric devices can work in two ways — using electricity to provide heating or cooling, or using temperature differences to create electricity.

The second method is Yang's focus, and for good reason. In an internal combustion engine, only about a quarter of the total energy from gasoline is used to actually turn the wheels, while 40 per cent is lost in exhaust heat and 30 per cent is lost through cooling the engine. That means about 70 per cent of the available energy is wasted, according to GM.

In an internal combustion engine, only about a quarter of the total energy from gasoline is used to actually turn the wheels, while 40 per cent is lost in exhaust heat and 30 per cent is lost through cooling the engine. — General Motors

"If I can use some of that heat energy and convert it to electricity, you can improve the overall efficiency," Yang said.

The thermoelectric generator works when one side of its metallic material is heated, and excited electrons move to the cold side. The movement creates a current, which electrodes collect and convert to electricity.

A Suburban produces 15 kilowatts of exhaust heat energy during city driving, which is enough to power three or four air conditioners simultaneously.

But it's not possible to harness all the exhaust heat a vehicle produces, so when the Suburban is cruising at highway speeds, the generator can produce about 800 watts of power, Yang said. That electricity could go to accessories such as a GPS device, DVD player, radio and possibly the vehicle's water pumps.

Yang's prototype device is to be tested in a Suburban next year. A similar prototype created by Ohio State scientists and BSST should be tested in a BMW in 2009.

While it's not clear how much the device would add to the price of a vehicle, the whole point of the research is to make it cost-effective, Yang said.

"There are several other steps that are required to commercialize the material, but we're cautiously optimistic that these steps can be carried out successfully," said Lon Bell, president of BSST, a subsidiary of Northville-based thermoelectrics supplier Amerigon Inc.

BSST also is working with Ford Motor Co. to develop climate control systems based on thermoelectrics.

Ford wants a system that would target a person's extremities when it's cold or the back of the neck in summer heat, rather than blow out a lot of air to change the temperature of the entire vehicle.

"We think we can make people feel cooler more quickly, feel comfortable more quickly, and that will translate into less power in the central AC system," said Clay Maranville, a Ford senior research scientist.