The recovery of electricity from heat storage reaches an efficiency of 44%.

Research from the University of Michigan shows that as they approach their theoretical maximum efficiency, devices that convert heat into electricity are getting closer to practical use on the grid.

Thermal batteries could store renewable energy periodically during peak times, relying on a thermal version of solar cells to later convert it into electricity.

“As we integrate a greater share of renewable energy sources into the grid to achieve decarbonization goals, we need lower costs and longer energy storage periods, because the energy produced by solar and wind energy does not match energy consumption,” – Andrej Lenert, UM associate professor of chemical engineering and co-author of a study recently published in Joule.

Thermophotovoltaic cells work similarly to photovoltaic cells, commonly known as solar cells. Both convert electromagnetic radiation into electrical energy, but thermophotovoltaics uses lower energy infrared photons rather than higher energy visible light photons.

The research team reports that their new device has a power conversion efficiency of 44% at 1435°C, which is within the target range of existing high-temperature energy storage devices (1200-1600°C). It exceeds the 37% result achieved by previous designs in this temperature range.

“It’s a kind of battery, but very passive. There is no need to extract lithium, as is the case with electrochemical cells, which means there is no need to compete with the electric vehicle market. Unlike pumped water, hydropower storage can be placed anywhere and doesn’t need to be near a water source, said Stephen Forrest, professor of electrical engineering at UM’s Peter A. Franken University and co-author of the study.

In a heat battery, thermophotovoltaics would surround a block of heated material with a temperature of at least 1000°C. It can reach this temperature by passing electricity from a wind or solar farm through a resistor, or by absorbing excess heat from solar energy or steel, glass or concrete production.

“Essentially, using electricity for heating is a very simple and inexpensive method of storing energy compared to lithium-ion batteries. It gives access to many different materials that can be used as a storage medium for thermal batteries,” Lenert said.

The heated storage material emits thermal photons of various energies. At 1435°C, approximately 20-30% of them have enough energy to generate electricity in the assembly’s thermophotovoltaic cells. The key to this study was optimizing the photon-capturing semiconductor material to broaden the preferred photon energies while adapting to the dominant energies produced by the heat source.

However, the heat source also produces photons above and below energy, which the semiconductor can convert into electricity. Without careful engineering, these elements would be lost.

To solve this problem, the researchers incorporated a thin layer of air into the thermophotovoltaic cell just behind the semiconductor and added a gold reflector behind the air gap – a structure they call an air bridge. This cavity helped trap photons at the appropriate energies so that they would enter the semiconductor and send the rest back into the heat storage material, where the energy had another chance to be re-emitted in the form of a photon that the semiconductor could capture.

“Unlike solar cells, thermophotovoltaic cells can recover or recycle photons that are not useful,” said Bosun Roy-Layinde, a doctoral student in chemical engineering at UM and first author of the study.

A recent study found that stacking two air bridges improves the design by increasing both the range of photons converted to electricity and the range of temperatures usable for thermal batteries.

“We have not yet reached the performance limits of this technology. “I am confident that we will exceed 44% and in the not too distant future we will exceed 50%,” said Forrest, who is also the Paul G. Goebel Professor of Engineering and Professor of Electrical Engineering and Computer Science, Materials Science and Engineering, and Physics.

The team has applied for patent protection with the support of UM’s innovation partnerships and is looking for partners to bring the technology to market.