Extracting much more energy from thermal batteries

According to a study conducted at the University of Michigan, led by scientists from the University of Michigan and published in: JouleDevices that convert heat into electricity are getting closer to being able to use them in the network.

Thermal batteries can store renewable energy periodically during peak times and then convert it into electricity using a thermal version of solar cells.

As we integrate more renewables into the grid to meet decarbonization goals, we need lower costs and longer energy storage periods because the energy produced by solar and wind does not match the energy consumed.

Andrej Lenert, corresponding author of the study and associate professor at the University of Michigan

Thermophotovoltaic cells work similarly to photovoltaic cells (also known as solar cells). Both convert electromagnetic radiation into electrical energy. However, thermophotovoltaics uses lower energy infrared photons instead of higher energy photons in visible light.

The team claims that their novel device has a power conversion efficiency of 44% at 1,435°C, which is within the target range for existing high-temperature energy storage (1,200-1,600°C). It outperforms previous designs in this temperature range, which averaged 37%.

It is a form of battery, but very passive. There is no need to mine lithium as with electrochemical cells, which means there is no need to compete with the electric vehicle market. Unlike pumped water for hydropower storage, it can be placed anywhere and no nearby water source is needed.

Stephen Forrest, study co-author, and Peter A. Franken, Distinguished University Professor, Electrical Engineering, University of Michigan

The thermal battery would contain a block of heated material with a minimum temperature of 1000 °C surrounded by thermophotovoltaics. This temperature can be achieved by absorbing excess heat from solar energy, producing steel, glass or concrete, or transmitting electricity from a wind or solar farm via a resistor.

Lenert added: “Basically, using electricity to heat something 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 carrier for thermal batteries.“

The heated storage material emits thermal photons of many energies. At 1,435 °C, about 20–30% of them had enough energy to power the team’s thermophotovoltaic cells. The key to this study was adapting the semiconductor material that collects photons to expand the preferred photon frequencies while remaining consistent with the dominant energy provided by the heat source.

However, the heat source generates photons with energy higher and lower than that which the semiconductor can convert into electricity. Without careful engineering they would have been lost.

To solve this problem, the researchers introduced a small layer of air into the thermophotovoltaic cell just behind the semiconductor. They added a gold reflector, creating an air bridge behind the air gap.

This cavity helped trap photons at the appropriate energies, allowing them to enter the semiconductor and return the remainder to the heat storage material, where the energy could be re-emitted as a photon that the semiconductor could collect.

Unlike solar cells, thermophotovoltaic cells can recover or convert photons that are not useful.

Petty Officer Roy-Layinde, first author of the study and a Ph.D. student at the University of Michigan

Recent research has shown that stacking two air bridges improves the design, improving the range of photons converted to electricity and the range of temperatures usable for thermal batteries.

Forrest added: “We have not yet reached the limits of this technology’s performance. I am confident that in the near future we will exceed 44% and then exceed 50%.“

Forrest is also the Paul G. Goebel Professor of Engineering and professor of electrical engineering, computer science, materials science and engineering, and physics.

The team has applied for patent protection with the help of UM Innovation Partnerships and is looking for partners who could bring the technology to market.

This research was funded by the National Science Foundation (grant numbers 2018572 and 2144662) and the Army Office of Research (grant number W911-NF-17-0312).

References in magazines:

Roy-Layinde, B., it cops. (2024) High-efficiency air-bridge thermophotovoltaic cells. Joule. doi:10.1016/j.joule.2024.05.002.

Roy-Layinde, B., it cops. (2024) Integrated tandem airbridge thermophotovoltaics with high performance over a wide range of heat source temperatures. ACS energy lists. doi:10.1021/acsenergylett.4c00774.

Source: http://umich.edu/