Harvesting Vehicles’ Waste Heat

An innovative car-exhaust mechanism could raise cars’ energy efficiency by 20%.

Most of gasoline’s stored energy never actually powers a single car, according to General Motors (GM) researchers. Half to three-fourths of gas energy is lost as waste heat spilling out of the cars’ tailpipes. But GM and competitors BMW and Ford are all separately working on ways to capture that heat energy before it leaves the tailpipe and convert it back into mechanical energy that the cars can use.

“You’ve got a lot of this waste heat. Let’s try to turn it into a mechanical heat and put it to work,” says Jeffrey Brown, vice president of Dynalloy Inc.

Dynalloy is helping GM design a thermal recovery system that would be installed near a car’s exhaust system and use the escaping heat to generate enough electricity to fully power the car’s radio or air-conditioning. The system consists of a thin belt of nickel-titanium alloy that loops around three pulleys to form a triangle. One corner of the triangle lies close to the thermal exhaust system, where it is very hot; another corner is farther away, where it is cooler.

The belt automatically expands and contracts in response to changes in temperature: Heat makes it tighten up, while cold causes it to loosen. So as the different areas of the belt are exposed alternately to blasts of hot and cool air, the belt moves along and turns the three pulleys, which in turn move a shaft that drives a generator. The more heat that strikes the belt, the more electricity the generator creates.

“It uses low-grade waste heat that can’t be used in a conventional motor,” says Alan L. Browne, a GM Technical Fellow and one of the project’s leading team members. “We’re just harvesting this stuff that is otherwise being dumped into the environment.”

The U.S. Department of Energy awarded GM an $8 million contract for waste-heat recovery R&D this year. Ford and BMW are working separately with partner firm BSST.

“This is one of many [waste-heat recovery concepts] that are being explored, but it’s also the newest boy on the block. And right now, we are now producing some outputs that are looking very competitive,” says Browne.

So far, a 10-gram strand yields 2 watts, enough to power a small nightlight. That would amount to harvesting 4% or 5% more energy. Since the typical combustion engine’s energy yield is now just 25%, that would constitute a 20% overall energy-efficiency increase.

“It’s not tremendous, but the impact is huge, because it’s all for free, because it’s heat that’s currently lost,” Browne notes, adding that further refinements could bring up the energy yield even more.

Diesel trucks are also prime candidates for waste-heat recovery systems, according to Browne. He also foresees the systems going into use in farm vehicles such as tractors.

“In farm areas or other rural areas where fuel is hard to bring out there, you could potentially make a pump out of it,” he says. “It’s hard to bring power to anyplace out in the bush.”

Browne sees even bigger opportunities in public mass transit. Subway trains get much more use than cars, after all, with the miles of rail line that they pass back and forth every day. Every mile of rail could go to generating heat that could be turned into mechanical energy.

Cars are driven sporadically—perhaps an hour or two a day—but it would be better to have the heat engine continuously; trains would get more output. “Your cost factor for the waste-heat recovery system goes down if it’s on a train,” says Browne. “You’d be getting much more life cycle energy out of them than out of a car.”

Other mechanical structures besides vehicles might eventually deploy waste-heat recovery mechanisms, too, according to Jan Aase, director of GM’s Vehicle Development Research Lab. He speculates that oil pipelines or fuel stations, for instance, could use them to collect some of their machines’ waste heat.

“That’s more of an aspirational concept,” says Aase, who estimates that any application of the technology will be at least another five to 10 years in the making. The energy output will have to significantly increase, and the production costs lowered, before GM and other companies will want to use it. They are working toward a near-term goal of 200 milliwatts per gram of material.

“We’re hopeful but cautious at this point. The economics has to work, and the packaging has to work,” says Aase.—Rick Docksai

Sources: Interviews with Jeffrey Brown, Dynalloy Inc., www.dynalloy.com; Jan Aase, Vehicle Development Research Laboratory, GM, www.gm.com; Alan L. Browne, Technical Fellow, GM R&D, www.gm.com/ design-technology.