Recovering exhaust heat to generate electricity and boost efficiency

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GM engineers are testing a prototype thermoelectric exhaust-heat energy-recovery device on a Chevy Suburban.

Despite the unprecedented technical sophistication of today’s cars, automotive engineers can never really overlook the fact that those same vehicles waste more than two-thirds of the energy that is in the gasoline they burn. Most of that—around 40%—is squandered out the exhaust system as hot gases exit at a temperature from 300 to 700°C (570 to 1300°F).

Facing the prospect of high fuel prices, automakers have long considered how their vehicles might squeeze out even a few percent more useful energy from fuel, but nothing practical has yet emerged.

That may be changing, however. Recent improvements in the performance of thermoelectric (TE) semiconductors—perhaps most familiar as the solid-state devices that heat and chill luxury car seats—has prompted the U.S. Department of Energy (DOE) to help fund researchers at General Motors, Ford, and BMW find out if the materials could recapture energy that is lost in engine exhaust by turning waste heat into electricity. The low conversion efficiencies and high costs of existing TE materials have so far kept such an application from becoming reality.

Significant fuel savings

But if ongoing work on TE-based exhaust energy-recovery systems is successful, the technology could enhance a car’s fuel economy by as much as 5 or maybe even 10%, according to Greg Meisner, a staff researcher at GM Global R&D in Warren, MI. Such TE generators could also help supply electricity to run the many power-hungry electrical devices that are proliferating on cars.

Perhaps even more significant is the notion that TE energy-harvesting systems “would be especially useful on hybrid vehicles,” said Clay Maranville, a senior research scientist at Ford Research and Advanced Engineering in Dearborn, MI. The technology, he explained, “complements hybrid powertrains, which work best in city driving, whereas TE generators work best on the freeway at high loads,” when engines tend to be engaged and produce hot exhaust.

Ideally, several TE specialists agreed, a successful exhaust heat recuperation product would weigh less than 10 kg (22 lb) and cost about $500 while generating 1 kW of electrical power.

Notably, the DOE has also awarded contracts to GM and Ford to develop TE-based air-conditioning and heating devices. These units, which use zonal concepts, would warm and chill the passenger compartments more efficiently than conventional, full-cabin HVAC systems.

Thermoelectricity basics

Unlike the car-seat heater/chiller application, which feeds electricity to TE materials to make them hot or cold, the concept behind a TE generator is to run the temperature-sensitive substance in reverse, subjecting it to waste heat, which induces it to produce electricity. “When you apply a temperature gradient across a thermoelectric semiconductor, you get a voltage,” said Jim Salvadore, a senior researcher (chemistry) at GM. “You can use that open circuit voltage to drive an external load."

In a car exhaust application, one side of the TE material would be exposed to the hot gases while the other sees engine coolants at 100°C (212°F). “The higher the temperature difference,” Salvadore noted, “the greater the voltage.”

TE applications lag

Applied TE technology got its start in the early 1960s, when manufacturers began producing cooling and power-supply devices for the defense and aerospace industries, said Dan Coker, Chief Executive Officer of Amerigon, a supplier of TE-based climate-control seat devices in Northville, MI. These products offer benefits such as small size, no moving parts, and no greenhouse gas emissions or environmentally harmful coolant fluids.

“Unfortunately, thermoelectric materials are not terribly efficient,” he said. “Typically, the conversion efficiency of heat to electricity is from 4 to 5 percent, and that’s just the materials alone, not including system losses.”

The trick is to find thermoelectric materials that can maintain a large temperature difference between opposite sides. An ideal TE needs to be both a good electrical conductor and a poor thermal conductor, which involves a difficult compromise in physical properties. If the material conducts heat too well, the temperature difference is lost and the thermoelectric effect halts.

Most of the relatively few TE products that have been marketed to date largely rely on the TE properties of bismuth telluride, one of the first materials found to strike the necessary compromise between high electrical and low thermal conductivities, Coker said. Bismuth telluride’s so-called thermoelectric figure of merit, ZT—a dimensionless metric that indicates the intensity of the TE effect—is about 1.0 at room temperature. On the downside, bismuth telluride contains expensive tellurium and only works at temperatures below 250°C (480°F).

For practical use, manufacturers dope the materials to make them function as p-type and n-type semiconductors (positive and negative charge-carriers, respectively) and then place them in a circuit. Typically, many of these p-type and n-type pairs are connected in series, yielding arrays that can supply large voltages and handle substantial heat flows.

Improved TE materials

Driven by recent fundamental materials advances in performance, conversion efficiency, as well as higher-temperature operating ranges, researchers are now working to identify new TE materials and new ways to use them.

Scientists at Amerigon, Coker reported, “are looking to deliver materials with ZTs of 1.4 to 1.5 using new, rather sophisticated doping, compounding, and crystal-growth methods.” The company is reportedly experimenting with blends of hafnium and zirconium that work well at high temperatures.

A GM-led team is in the meantime working on yet another group of materials, skudderudites, which are cobalt-antimony-based compounds containing rare-earth elements, Salvadore said. GM researchers did work on skudderudites starting around 2004 or 2005, and owns basic patents in the area, he noted. Skudderudites have high ZTs at the elevated temperatures typical of auto exhaust systems.

Skudderudite has large voids, Salvador explained. “When you fill them with rare-earth or alkaline-earth element cations, you can modulate the charge-carrier population to optimize the electrical transport properties while lowering the thermal conductivity to slow heat transfer.”

But many roadblocks to success in exhaust systems still remain to be overcome, GM’s Meisner warned. High temperatures tend to degrade the performance of TE materials over time, for instance. But beyond that lie the engineering challenges. The devices must be highly robust and heat-resistant while minimizing weight, volume, and cost.

Vehicle testing

As part of a multiyear, multimillion-dollar project partly sponsored by the DOE, GM is testing “strap-on” exhaust heat-recovery prototypes on a Chevrolet Suburban SUV. The R&D work, which is being pursued in collaboration with partners including Dallas-based Marlow Industries and various universities, has shown that such devices can improve fuel efficiency by at least 3 percent, Meisner said.

Meanwhile, teams funded by parallel R&D contracts involving Ford, BMW, and Amerigon have begun testing TE generators on a Ford Fusion hatchback and a BMW X6 crossover vehicle.

Although “a cost-effective TE generator is still somewhere between the applied research stage and the engineering development stage,” working systems could be available in five years if engineers can overcome the remaining challenges, Ford’s Maranville concluded.

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