MIT researcher on solid-state heat engine: ‘This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid.’

Researchers at the Massachusetts Institute of Technology (MIT) and the National Renewable Energy Laboratory (NREL) have developed a solid-state heat engine that they say is more energy-efficient than a steam turbine.

Called the thermophotovoltaic (TPV) cell, it can convert heat to electricity with over 40 percent efficiency, surpassing the 35% efficiency of traditional steam turbines. The TPV cell is designed to passively capture high-energy photons from a heat source of between 1,900 and 2,400 degrees Celsius and convert them into electricity.

Their research has been published in the journal Nature, with co-authors at MIT including Alina LaPotin, Kyle Buznitsky, Colin Kelsall, Andrew Rohskopf, and Evelyn Wang, along with Kevin Schulte and collaborators at the National Renewable Energy Laboratory (NREL) in Golden, Co.

“This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid," said Asegun Henry, the Robert N. Noyce Career Development Professor in MIT’s Department of Mechanical Engineering. 

Henry added that thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept and that research is one step closer to the expansion of renewable energy and establishing a fully decarbonized grid. A challenge for solar- and wind-powered electric generation is the storage of energy and its recapture as electricity when the sun isn't shining or the wind isn't blowing. Thermal batteries aim to address that challenge.

"More than 90 percent of the world’s electricity comes from sources of heat such as coal, natural gas, nuclear energy, and concentrated solar energy," an MIT press release says.  (Statistica and Our World In Data say the total is around 75%.) "For a century, steam turbines have been the industrial standard for converting such heat sources into electricity," but steam turbines can't operate at the high thermal battery temperatures supported by the research, the press release says. 

The researchers said they have shown TPV cells' effectiveness in small-scale experiments. They plan to incorporate the TPV cell into a grid-scale thermal battery that would store excess energy from renewable sources, such as solar energy, in heavily insulated banks of hot graphite. When needed, TPV cells would convert the heat into electricity and dispatch the energy to a power grid.  

TPV cells have the potential to be more efficient than steam turbines because they do not have moving parts.

The TPV cell is made up of three regions: a high-bandgap alloy, which sits over a slightly lower-bandgap alloy, and a mirror-like layer of gold. The first layer captures a heat source's highest-energy photons and converts them into electricity while the second layer captures lower-energy photons that are then added to the voltage.