Even though the idea of harvesting exhaust heat is old hat, the real practical application turned out to be little.
Triangular plate-fin heat exchangers, similar to those found in air conditioning units, located inside the exhaust pipe, capture hot exhaust gases.
Researchers design and test a waste-heat recovery system, illustrated here, that attaches to a car tailpipe and converts heat from exhaust into energy.
However, they demonstrate a tangible practical benefit: “the system design of integrated thermoelectric generator (TEG) is lagging behind materials development”, and their work targets precisely this gap.
They describe the study as “an advanced TEG system design bridging the TE device and integrated system transition,” with longer-lasting autonomous aerial platforms as a possible future application.
With every cycle of a gas-powered engine, three-quarters of the energy in the fuel goes out as heat through the exhaust and into the atmosphere. A prototype device has been developed that is hoped to recapture some of this wasted energy and convert it back into electricity.
Now, researchers at Pennsylvania State University have created a compact thermoelectric generator (TEG) system that can be attached to vehicle exhaust pipes in order to convert waste heat into usable electrical energy. The prototype generated up to 40 watts, enough to power a standard lightbulb, in lab tests, while computer simulations suggest that the design could deliver even more when placed on moving cars.
Even though the idea of harvesting exhaust heat is old hat, the real practical application turned out to be little. Current thermoelectric machines are often large and require water cooling to maintain the temperature gradient needed to generate electricity, making them weighty and clunky additions to vehicles. The team took a different approach.
The team used two main components to build this system. Triangular plate-fin heat exchangers, similar to those found in air conditioning units, located inside the exhaust pipe, capture hot exhaust gases. The pipe is enveloped by a cylindrical heatsink, whose grooved surface structure forces convective heat transfer into the surrounding ambient air. The gap between these hot and cold sides drives a bismuth-telluride semiconductor to generate electricity.
Using computer modeling, the team optimized the geometry of a heatsink for the greatest temperature difference. The result: maximizing electrical output.
Researchers design and test a waste-heat recovery system, illustrated here, that attaches to a car tailpipe and converts heat from exhaust into energy. The fanned grooves on the outside of the pipe are the cold side of the device’s heatsink and the triangular components inside the pipe are plate-fin heat exchangers.
What makes the design particularly notable is how it handles fast-moving airflow. The high airflow rate in active exhaust systems isn’t a nemesis; instead, it enhances the device’s operation.
“Thermal energy harvesting for high-speed moving objects is particularly promising in providing an efficient and sustainable energy source to enhance operational capabilities and endurance,” the authors write. That insight shaped the team’s focus on vehicles that are already moving quickly through the air, where the heatsink is better able to shed heat.
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Under laboratory conditions, with a 190 degrees Celsius temperature differential across the device, the prototype produced an output of 40 watts.
The researchers performed computer simulations at different exhaust flow speeds to gauge their performance under conditions approaching the real world. Under car-like conditions, it predicted a maximum of 56 watts, about the power output of five lithium-ion 18650 batteries operating simultaneously. When corrected for the helicopter conditions, the output would have reached 146 watts per kilogram, about 12 times the output of this battery configuration.
While these values are decent, the researchers note that the higher values are still forecasts from simulations rather than measurements, and that there remains a huge divide between laboratory prototypes and aerospace or automotive hardware. However, they demonstrate a tangible practical benefit: “the system design of integrated thermoelectric generator (TEG) is lagging behind materials development”, and their work targets precisely this gap.
The team says the device can be fitted directly into existing exhaust outlets without requiring any additional cooling infrastructure, which could lower the barrier for eventual adoption. They describe the study as “an advanced TEG system design bridging the TE device and integrated system transition,” with longer-lasting autonomous aerial platforms as a possible future application.
The research was funded by the U.S. Army Rapid Innovation Fund Program, the National Science Foundation, and the Office of Naval Research.
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