A new heat-to-energy converter has reached a record efficiency of 44% – the average steam turbine manages about 35%, for comparison. This thermophotovoltaic cell is a major step on the way to sustainable, grid-scale renewable energy storage.

With renewable energy prices dropping fast, the barrier now is their intermittency – the first point any renewable energy skeptic will throw at you is “but what happens at night or when the wind isn’t blowing?” A little thing called “batteries” can help there, and there’s no shortage of grid-scale storage systems that can save energy for (literally) rainy days. That includes scaling up classics like lithium-ion batteries, or more experimental designs like iron-air, water-in-salt, flow batteries, or a variety of gravity-based systems.

One of the most promising avenues is storing energy as heat. The medium itself can be broad – sand, molten salt, volcanic ash, carbon blocks, clay bricks and many others are in the works – but unfortunately getting the energy back out of heat and turning it into electricity can be the tricky part.

That’s where the new system comes in. Developed by researchers at the University of Michigan, this device works on the principle of thermophotovoltaics. It’s similar to solar cells, which are photovoltaic, generating electricity (volts) from light (photons). Thermophotovoltaics obviously add heat (thermo) to the mix. In practice, that means they absorb photons from the infrared part of the spectrum, rather than the higher-energy, visible-light photons that solar cells capture.

The new thermophotovoltaic cell was tested with silicon carbide as the heat-storage material, although it could be swapped out for whatever else works. This is surrounded by a semiconductor material made of indium, gallium and arsenic, carefully designed to capture the broadest range of photons, and specifically those produced by the heated material.

When the team heated the material to 1,435 °C (2,615 °F), it starts radiating thermal photons at a range of energy levels, with 20 to 30% of them able to be captured by the semiconductor. To make use of some of those with higher or lower energies, the cell contains a thin layer of air after the semiconductor, followed by a gold reflector layer. This bounces some photons back to the semiconductor to be converted into electricity, while others bounce back to the heat storage material, giving them another chance of being emitted as the right kind of photon.