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MIT students seek to harness waste heat

Device could make MIT's cogeneration plant even more efficient
MIT graduate students are working on a thermoelectric system that could make MIT's cogeneration plant even more efficient.
MIT graduate students are working on a thermoelectric system that could make MIT's cogeneration plant even more efficient.
Photo / Donna Coveney

MIT's cogeneration plant, which provides most of the electricity, heat and air conditioning for the campus, could get even more efficient if a team of students' project to harness surplus heat works as expected.

Andy Muto and Daniel Kraemer, graduate students in mechanical engineering, and Bryan Ho, a graduate student in materials science and engineering, have been working together on a thermoelectric system that could be installed in a hot-water pipe, or in exhaust flues at the plant, to get some extra electric power from heat currently going to waste.

Thermoelectric devices are solid-state heat engines that produce electricity from a difference in temperatures, without using any moving parts, so they can be extremely simple and durable. To test the feasibility of the plan, the team installed a small test module on one of the plant's rooftop flues this winter and will be checking it periodically to collect data on its performance and durability after exposure to the weather.

But the more promising possibility, Kraemer says, is to install thermoelectric devices in a hot-water line, where it can pick up even more of the waste heat.

The cogeneration plant was installed on campus in 1995, and it provides the electricity, heat and air conditioning needs of 80 percent of campus buildings. Already a model of energy efficiency, the student project aims to eke out a small additional percentage of energy output from the natural gas that powers the plant.

Since the plant produces 22 megawatts of electricity in addition to heating and cooling the buildings, even a small increase in efficiency could have a substantial impact--and could provide a model for other cogeneration plants in operation or under construction around the world, the students say.

The increase in efficiency that could be harnessed by such a system "looks like small numbers," Muto says, "But if you look at the scale of the plant, it's big." And once installed, the operating costs would be negligible, he says.

Currently, high-temperature steam at 300 to 400 degrees Celsius generated by the gas-fired boiler is used to drive a steam turbine to provide electricity, and then is used directly for heating and cooling. But even the steam returning to the plant after being piped through the system still contains enough heat to be useful, but is not hot enough to drive the turbines. That's where the potential of the thermoelectric system comes in.

In addition, hot air from the system is exhausted through flues connected to the smokestack, at temperatures of 150 to 170 degrees Celsius--hot enough to provide some additional power through the thermoelectric modules. "Everyone just exhausts the gases at that temperature, they don't try to use them for anything else," Muto says.

Muto, along with Kraemer and others, is working on a heat-exchanger system that resembles a car radiator, to be placed in the hot-air or hot-water systems to convey the heat to the module. They hope to have a prototype system ready to install at the plant by the end of the summer.

The system would not only squeeze some extra efficiency out of the plant, but could also have environmental benefits by further reducing the heat in the exhaust air that is vented to the outside.

Ultimately, Kraemer says, "this could work anywhere--even on a home furnace. Anywhere where there's excess heat."

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