New heat engine is as efficient as steam turbines and could one day enable a fully decarbonized power grid

China Energy Storage Network: According to a study recently published in the British journal Nature, engineers from the National Renewable Energy Laboratory and the Massachusetts Institute of Technology designed a heat engine with no moving parts. Demonstrations show that it converts thermal energy into electrical energy with an efficiency of more than 40%, which is better than traditional steam turbines.

The heat engine is a thermophotovoltaic (TPV) cell, similar to the photovoltaic cells in solar panels. The researchers plan to integrate the TPV cell into a grid-scale thermal battery. The system would draw excess energy from renewable energy sources such as solar power and store that energy in a highly insulating thermal graphite reservoir. When the energy is needed, such as on a cloudy day, the TPV cell would convert the heat into electricity and distribute the energy to the grid.

"TPV batteries are the last critical step in proving that thermal batteries can become a viable concept," says Asegan Henry, a professor in MIT's Department of Mechanical Engineering. "This is a critical step on the path to scaling renewable energy and achieving a fully decarbonized grid."

In recent years, scientists have been investigating solid-state energy converters, one advantage of which they believe is that they can operate at higher temperatures and with lower maintenance costs because they have no moving parts.

TPV cells offer a path to explore for solid-state heat engines. Like solar cells, TPV cells can be made from semiconductor materials with specific band gaps. But to date, most TPV cells have an efficiency of only about 20%, with the highest being only 32%.

In the new TPV design, Henry and his colleagues hope to capture higher energy photons from higher temperature heat sources, thereby converting energy more efficiently. Compared with existing TPV designs, the team's new cell uses higher bandgap materials and multiple junctions, or layers of materials.

The cell is made of three main regions: a high-bandgap alloy sits on top of a slightly lower-bandgap alloy, with a mirror-like layer of gold underneath. The first layer captures the highest-energy photons from the heat source and converts them into electricity, while lower-energy photons that pass through the first layer are captured and converted by the second layer to increase the voltage generated. Any photons that pass through the second layer are reflected by the mirror back to the heat source rather than being absorbed as waste heat.

The research team exposed the cells to a high-temperature lamp and focused the light on the cells. They then changed the intensity, or temperature, of the bulb and observed how the power efficiency of the cells changed with temperature. The efficiency of the new TPV cells remained around 40% over a temperature range of 1900°C to 2400°C.

The experimental cell was about one square centimeter. For a grid-scale thermal battery system, the researchers envision that the TPV cell would have to be scaled up to about 900 square meters.

Henry said the technology is safe and environmentally friendly over its life cycle and could have a huge impact on reducing carbon dioxide emissions from electricity production.

【Editor-in-Chief’s Comments】

Is electricity clean and green? This depends on what energy it is converted from. Because electricity is a secondary energy source, it needs to be obtained through primary energy conversion. For example, when we drive a pure electric car, although the car does not produce exhaust gas, if the electricity used to charge the car comes from fossil energy, it cannot be said to be completely green and clean. The same is true for promoting the green and low-carbon transformation of the power grid. Using more and more advanced technology to connect cleaner energy to the power grid and enter thousands of households is an important way for people to achieve a green and low-carbon life. (Zhang Mengran)