A thermophotovoltaic (TPV) cell (size 1 cm x 1 cm) mounted on a heat sink designed to measure the TPV cell efficiency. (Source: Felice Frankel, MIT)
MIT and the National Renewable Energy Laboratory (NREL) announced the design of a heat engine with no moving parts that can convert heat to electricity with a greater than 40% efficiency. Researchers say the solution outperforms traditional steam turbines and could someday enable a fully decarbonized power grid.
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. However, the system’s moving parts are not able to withstand temperatures higher than about 3600 degrees Fahrenheit.
That’s why scientists have studied heat engines with no moving parts, that could potentially work efficiently at higher temperatures.
The MIT and NREL heat engine is a thermophotovoltaic (TPV) cell, similar to a solar panel’s photovoltaic cells. The TPV cell passively captures high-energy photons from a white-hot heat source and converts them into electricity. The partners’ design can generate electricity from a heat source between roughly 3,400 to 4,300 degrees Fahrenheit.
“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,” said Asegun Henry, with MIT’s Department of Mechanical Engineering. “They just sit there and reliably generate electricity.”
The researchers plan to incorporate the TPV cell into a grid-scale thermal battery system. The system would absorb excess energy from 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 a power grid.
The new TPV cell represents what the researchers called a successful demonstration in separate, small-scale experiments. They are now working to integrate the parts to demonstrate a fully operational system. From there, they eventually hope to scale up the system to replace plants powered by fossil fuels and allow a decarbonized power grid supplied solely by renewable energy.
TPV cells can be made from semiconducting materials with a particular bandgap — the gap between a material’s valence band and its conduction band. If a photon with a high enough energy is absorbed by the material, it can kick an electron across the bandgap. The electron can then conduct, and thereby generate electricity, all doing so without moving parts or motors.
Until this point, MIT researchers said most TPV cells have reached efficiencies of around 20%, with the record at around 32%. That performance largely is a because they have been made of relatively low-bandgap materials that convert lower-temperature, low-energy photons, and therefore convert energy less efficiently.
The new TPV 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 a heat source’s highest-energy photons and converts them into electricity, while lower-energy photons that pass through the first layer are captured by the second and converted to add to the generated voltage.
Any photons that pass through this second layer are then reflected by the mirror, back to the heat source, rather than being absorbed as wasted heat.
The team tested the cell’s efficiency by placing it over a heat flux sensor — a device that directly measures the heat absorbed from the cell. They exposed the cell to a high-temperature lamp and concentrated the light onto the cell. They then varied the bulb’s intensity, or temperature, and observed how the amount of power the cell produced compared with the heat it absorbed — changed with temperature. Over a range of 1,900 to 2,400 degrees Celsius, the new TPV cell maintained an efficiency of around 40%.
The cell in the experiments is about a square centimeter. For a grid-scale thermal battery system, researchers envisioned the TPV cells would have to scale up to about 10,000 square and would operate in climate-controlled warehouses to draw power from huge banks of stored solar energy. MIT's Asegun Henry pointed out that an infrastructure exists for making large-scale photovoltaic cells, which could also be adapted to manufacture TPVs.
