Thermoelectric Energy Harvesting for Efficient Aircraft
For jet engine manufacturers, hotter is better. The higher the temperature in the combustion chamber, the more efficient the engine and the less fuel the aircraft consumes. Today’s commercial jet engines can reach temperatures as high as 1,700 degrees Celsius (that’s 3,092 degrees Fahrenheit) because of the highly effective thermal barrier coatings that line the inside of the chamber. Without them, the temperature would be limited to about 1,150 degrees, the point at which heat-resistant nickel superalloys used for jet engines lose their strength as they approach their melting point.
For Professor Patrick Hopkins, a mechanical engineer and specialist in microscale heat transfer and high-temperature materials science, that dramatic drop in temperature between the side of the coating exposed to the heat of combustion and the side protecting the steel of the chamber was suggestive. If he could harness the thermoelectric effect — the generation of voltage from a temperature gradient within the coating material — he would have a way to make jet engines incrementally more efficient.
“If you could make a coating that could not only survive in this hot environment but also produce current, you could harvest electricity that you could plug back into the aircraft,” Hopkins says.
This work is published in Advanced Materials https://doi.org/10.1002/adma.201805004.
Joining Forces with Rolls-Royce
Thanks to UVA Engineering’s close relationship with Rolls-Royce, one of the leading manufacturers of jet engines, Hopkins found a receptive audience for his idea. UVA Engineering is one of approximately two dozen institutions world-wide that are part of the company’s University Technology Center network. With Dr. Ann Bolcavage, a Rolls-Royce associate fellow in materials and coatings, Hopkins secured a nearly $300,000 grant from the National Science Foundation’s Grant Opportunities for Academic Liaison with Industry (GOALI) program to explore his idea.
“Although the funding from a GOALI award goes to the academic institution, companies like it because it is a way for them to help shape long-term basic research in areas of interest to them,” Hopkins says. “This is not the kind of research that companies normally do.”
Another benefit of the GOALI project is that it has expanded Hopkins’ network with Rolls-Royce, which has led to additional research. “The advantage of our relationship with Rolls-Royce is that it is enabling me to bring in new graduate students and to expand the menu of activities we are pursuing in my lab,” he says. “We are now their thermal transport people.”
Searching for a Two-for-One Material
Although all materials can produce the thermoelectric effect, some are better at it than others. The challenge that Hopkins and Bolcavage are taking on is to design a material with poor thermal conductivity, making it a good thermal barrier and good electrical conductivity.
To begin with, they are investigating the nano- and microscale thermal properties of thermal barrier coatings used in jet engines, with the hope that this knowledge might allow them to design coatings from oxides, which have pronounced thermoelectric properties at elevated temperatures. “We are trying to determine ahead of time what the characteristics of this new material should be,” Hopkins says. This approach might also reduce the cost of thermal barrier coatings, which currently rely on expensive rare earth elements. “The challenge is learning how to take the thermal barrier effect of a certain class of coatings and bridge it with the thermoelectric characteristics of a different class of materials,” Hopkins says.
Fortunately for the team, they do not need to set their sights on an ideal thermoelectric material. “Even using a material with a poor thermoelectric efficiency, we will be able to produce a significant amount of current because the temperature differential in a jet engine is so high,” Hopkins says. “But any waste energy we can scavenge is a net gain. For the airlines, even small gains in efficiency can produce millions of dollars in fuel savings."
Rolls-Royce is donating a helicopter engine to UVA Engineering, which will be on display in the Mechanical and Aerospace Engineering building. “We really appreciate their commitment and generosity,” Hopkins says.