MIT research aims at turning waste heat back into electricity

As if today’s unsustainable levels of energy consumption weren’t already bad enough, consider this: more than half the energy we use worldwide ends up being wasted, mostly in the form of heat. Imagine how much more efficient we could be by finding a way to tap that heat energy.

That’s exactly what researchers at the Massachusetts Institute of Technology (MIT) hope to find.

By developing a technology to convert waste heat into electricity, we could create wildly more efficient devices: mobile phones with double the talk time, for example, or even power plants that put out more electricity for any given amount of fuel.

While thermoelectric devices that turn heat into energy already exist, most are nowhere close to the maximum possible efficiency. In theory, the amount of heat converted into electricity can never exceed a specific value called the “Carnot Limit.” Today’s commercial thermoelectric devices, however, reach only about one-tenth of that limit, according to Peter Hagelstein, associate professor of electrical engineering at MIT. But he and fellow researchers are working to improve that performance level.

Working with Yan Kucherov, a consultant for the Naval Research Laboratory, Hagelstein and his team have managed to achieve an efficiency as high as 40 per cent of the Carnot Limit. Ultimately, they calculate their approach could reach an efficiency as high as 90 per cent.

Hagelstein and his team started from scratch rather than trying to improve the performance of existing devices. They carried out their analysis using a simple system in which power was generated by a single quantum-dot device — a type of semiconductor in which the electrons and holes, which carry the electrical charges in the device, are very tightly confined in all three dimensions. By controlling all aspects of the device, they hoped to better understand how to design the ideal thermal-to-electric converter.

It’s already possible to efficiently convert heat into electricity, but the result is very little power. It’s also possible to get plenty of electrical power — what is known as high-throughput power — from a less efficient and, therefore, larger and more expensive system.

“It”s a tradeoff,” Hagelstein said. “You either get high efficiency or high throughput.”

Using their new system, though, Hagelstein and his team found it would be possible to get both at once. However, the technology remains a few years away from commercial availability.

“There’s a gold mine in waste heat, if you could convert it,” he said.