A better heat engine

Engineers at MIT and the National Renewable Energy Laboratory (NREL) have designed a heat engine with no moving parts. It converts heat to electricity with over 40% efficiency—making it more efficient than steam turbines, the industrial standard.

The invention is a thermophotovoltaic (TPV) cell, similar to a solar panel’s photovoltaic cells, that passively captures high-energy photons from a white-hot heat source. It can generate electricity from sources that reach 1,900 to 2,400 °C—too hot for turbines, with their moving parts. The previous record efficiency for a TPV cell was 32%, but the team improved this performance by using materials that are able to convert higher-­temperature, higher-energy photons.

The researchers plan to incorporate the TPV cells into a grid-scale thermal battery. The system would absorb excess energy from renewable sources such as the sun and store that energy in heavily insulated banks of hot graphite. Cells would convert the heat into electricity and dispatch it to a power grid when needed.

The researchers have now successfully demonstrated the main parts of the system in small-scale experiments; the experimental TPV cells are about a centimeter square. They are working to integrate the parts to demonstrate a fully operational system. From there, they hope to scale up the system to replace fossil-fuel plants on the power grid. Coauthor Asegun Henry, a professor of mechanical engineering, envisions TPV cells about 10,000 feet square and operating in climate-controlled warehouses to draw power from huge banks of stored solar energy. 

“Thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept,” 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.”