Engineers at MIT and the National Renewable Energy Laboratory have designed a thermophotovoltaic cell that converts heat into electricity with over 40% efficiency — a performance that exceeds the efficiency of traditional steam turbines. The researchers call their discovery a heat engine, a device with no moving parts that passively captures high energy photons from a white hot heat source and converts them into electricity. The team’s design can generate electricity from a heat source of between 1,900 to 2,400 degrees C (4300 degrees F).
The researchers plan to incorporate the TPV cell into a grid-scale thermal battery — a system that would absorb excess energy from renewable sources such as the sun, and store that energy in heavily insulated banks of hot graphite. When the energy is needed, such as on overcast days, TPV cells would convert the heat into electricity and dispatch the energy to the electrical grid.
So far, the researchers have successfully demonstrated the main parts of the system in small scale experiments and are working to integrate the parts to create a fully operational system. From there, they hope to scale up the system to replace thermal generating plants and enable a fully decarbonized power grid supplied entirely by renewable energy.
“Thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept,” says Asegun Henry, a professor at MIT’s Department of Mechanical Engineering. “This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid.” The efficiency of the new TPV far exceeds that of previous TPV cells. The results of the research have been published in the journal Nature.
Thermophotovoltaic Cells & Electrical Generation
More than 90% of the world’s electricity comes from sources of heat such as coal, natural gas, nuclear energy, and concentrated solar energy, the MIT announcement says. For a century, steam turbines have been the industrial standard for converting such heat sources into electricity.
On average, steam turbines reliably convert about 35% of a heat source into electricity, with about 60% representing the highest efficiency of any heat engine to date. But the machinery depends on moving parts that are temperature-limited. Heat sources higher than 2,000 degrees Celsius are too hot for turbines.
“One of the advantages of solid state energy converters are that they can operate at higher temperatures with lower maintenance costs because they have no moving parts,” Henry says. “They just sit there and reliably generate electricity.”
To date, most TPV cells have only reached efficiencies of around 20%, with the record at 32%, largely because they are designed to operate at lower temperatures, which compromises efficiency. In the design of the new TPV, the researchers wanted to capture high energy photons from a higher temperature heat source to increase efficiency.
The cell is fabricated from three main regions: a high bandgap alloy, which sits over a slightly lower bandgap alloy, underneath which is a mirror-like layer of gold. The first layer captures the highest energy photons from the heat source and converts them into electricity. Lower energy photons that pass through the first layer and are converted to electricity as well.
Any photons that pass through this second layer are reflected by the mirror back to the heat source rather than being absorbed as wasted heat. “We can get a high efficiency over a broad range of temperatures relevant for thermal batteries,” Henry says. [For more technical details, please refer to the MIT blog post or the Nature article.]
The cell in the experiments is about a square centimeter. For a grid-scale thermal battery system, Henry envisions the TPV cells would have to scale up to about 10,000 square feet (about a quarter of a football field), and would operate in climate-controlled warehouses to draw power from banks of stored solar energy. He points out that an infrastructure already exists for making large scale photovoltaic cells and could be adapted to manufacture TPVs.
“There’s definitely a huge net positive here in terms of sustainability,” Henry says. “The technology is safe, environmentally benign in its life cycle, and can have a tremendous impact on abating carbon dioxide emissions from electricity production.”
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