Closing in on the theoretical maximum efficiency, devices for turning heat into electricity are edging closer to being practical for use on the grid, according to University of Michigan research.
Heat batteries could store intermittent renewable energy during peak production hours, relying on a thermal version of solar cells to convert it into electricity later.
“As we include higher fractions of renewables on the grid to reach decarbonization goals, we need lower costs and longer durations of energy storage as the energy generated by solar and wind does not match when the energy is used,” Andrej Lenert, U-M associate professor of chemical engineering.
Thermophotovoltaic cells operate much like traditional photovoltaic cells, converting electromagnetic radiation into electricity. However, they utilize lower energy infrared photons instead of the higher energy photons of visible light.
The research team has announced that their new device achieves a power conversion efficiency of 44% at 1435°C, falling within the target range for existing high-temperature energy storage (1200°C-1600°C). This represents a significant improvement over the previous 37% efficiency achieved by designs operating within this temperature range.
“It’s a form of battery, but one that’s very passive. You don’t have to mine lithium as you do with electrochemical cells, which means you don’t have to compete with the electric vehicle market. Unlike pumped water for hydroelectric energy storage, you can put it anywhere and don’t need a water source nearby,” said Stephen Forrest, the Peter A. Franken Distinguished University Professor of Electrical Engineering at U-M.
In a heat battery, thermophotovoltaics would surround a block of heated material at a temperature of at least 1000°C. It might reach that temperature by passing electricity from a wind or solar farm through a resistor or by absorbing excess heat from solar thermal energy or steel, glass, or concrete production.
“Essentially, using electricity to heat something up is a very simple and inexpensive method to store energy relative to lithium-ion batteries. It gives you access to many different materials to use as a storage medium for thermal batteries,” Lenert said.
The heated storage material emits a wide range of thermal photons, with 20-30% of them having the potential to generate electricity in our thermophotovoltaic cells at 1435°C. Our study’s success lies in optimizing the semiconductor material to capture a broader range of photon energies while aligning with the dominant energies from the heat source. However, without careful engineering, the excess photons produced by the heat source could go to waste.
To solve this problem, the researchers built a thin layer of air into the thermophotovoltaic cell just beyond the semiconductor and added a gold reflector beyond the air gap – a structure they call an air bridge. This cavity helped trap photons with the right energies so that they entered the semiconductor and sent the rest back into the heat storage material, where the energy had another chance to be re-emitted as a photon the semiconductor could capture.
“Unlike solar cells, thermophotovoltaic cells can recuperate or recycle photons that are not useful,” said Bosun Roy-Layinde, U-M doctoral student of chemical engineering.
This innovative design achieves an impressive total power conversion efficiency of 44%, surpassing other designs operating at the same temperature, which top out at 37%. While some designs have exceeded 40% efficiency, they operate at much higher temperatures that may not be practical in many scenarios.
The concept involves heating the storage material using electricity generated by wind or solar farms or by directly absorbing excess heat from industrial processes or solar thermal energy systems.
Although it may only have half the efficiency of lithium-ion batteries, its enhanced safety and lower production and operational costs make it a cost-effective solution for utilizing excess electricity, especially considering the abundance of this resource. Furthermore, the research team believes there is still potential for further improvement.
“We’re not yet at the efficiency limit of this technology. I am confident that we will get higher than 44% and be pushing 50% in the not-too-distant future,” said Forrest.
