The new thermophotovoltaics recovers 44% of the energy stored in the heat accumulator

Research from the University of Michigan (UoM) shows that devices that convert heat into electricity are getting closer to being used in the network and are approaching their theoretical maximum efficiency.

The team at (UoM) reports that their new device achieves a power conversion efficiency of 44 percent at 2,615 degrees Fahrenheit (1,435°C), which puts it well within the target range for existing high-temperature energy storage systems (1,200-1,600°C) . °C).

According to the researchers, this represents a significant improvement compared to previous designs, which achieved an efficiency of 37 percent in the same temperature range.

Details of the study, conducted under the supervision of Andrej Lenert, professor of chemical engineering at the Medical University of Warsaw and corresponding author, were published in the journal Joule.

Thermophotovoltaic energy solutions

During periods of high capacity, intermittent renewable energy can be stored in thermal batteries and then converted into electricity using a thermal variant of solar cells.

Scientists emphasize that as larger fractions of renewable energy sources are integrated into the grid to achieve decarbonization goals, there is a need for cheaper and more durable energy storage. This is because the energy produced by the sun and wind does not always coincide with the moment of its consumption.

Thermophotovoltaic (TPV) cells work similarly to photovoltaic cells, commonly known as solar cells. Although both types convert electromagnetic radiation into electrical energy, thermophotovoltaic cells specifically use lower energy infrared photons instead of the higher energy photons found in visible light.

In a heat accumulator, TPVs would be placed around a block of material heated to at least 1000°C. This high temperature can be achieved by passing electricity from a wind or solar farm through a resistor, or by capturing excess heat from solar energy or industrial processes such as steel, glass or concrete production.

“Essentially, using electricity to heat something is a very simple and inexpensive method of storing energy compared to lithium-ion batteries. It provides access to many different materials that can be used as a carrier for thermal batteries,” Lenert said in a statement.

Improving energy conversion with air bridges

The heated storage medium emits a variety of energetic thermal photons. About 20 to 30 percent have enough energy at 1,435°C to power the assembly’s thermophotovoltaic cells with electricity.

According to the researchers, optimizing the semiconductor material that absorbs photons to broaden the preferred photon energies while matching the dominant energies generated by the heat source was crucial to this study.

However, photons above and below the energy that the semiconductor can convert into electricity are also produced by the heat source. These would be lost with careless engineering.

To solve this problem, the researchers integrated a thin layer of air with a thermophotovoltaic cell placed just behind the semiconductor and placed a gold reflector outside the air gap.

This structure, called an air bridge, helps capture photons with appropriate energies and enter the semiconductor, while redirecting the rest back to the heat-storing material. This provides another opportunity to re-emit energy in the form of photons, which the semiconductor can 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.

The researchers point out that a recent study examined improving the design by stacking two air bridges. This increases the temperature range at which thermal batteries can be used and the range of photons that can be converted into energy.

This work demonstrates the potential of airbridge cells for large-scale TPV implementation using single-junction cells at lower emitter temperatures and highlights the adaptability of the architecture to a variety of semiconductor materials.

The performance limits of this technology have not yet been reached. Scientists believe efficiency will exceed 44 percent and soon approach 50 percent.