Advancements in thermophotovoltaic cells edge closer to grid-scale applications

Researchers at the University of Michigan have made significant strides in the efficiency of that convert heat into , pushing these technologies closer to practical use on the electrical grid. These developments, which include reaching near-theoretical maximum efficiencies, could revolutionize how we store and utilize .

Harnessing Heat for Storage

The innovation centers on the concept of heat batteries, which can store energy produced during peak times of renewable energy generation. These devices utilize a thermal version of solar , known as thermophotovoltaic (TPV) cells, to convert stored heat into electricity.

“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,” explained Andrej Lenert, an associate professor of at U-M and corresponding author of the study published in Joule.

How Thermophotovoltaic Cells Work

Similar to traditional (solar cells), TPV cells convert into electricity. However, instead of using the higher-energy photons of visible light, TPV cells harness lower-energy infrared photons. This makes them particularly suitable for converting heat stored in high-temperature materials.

The research team has achieved a power conversion efficiency of 44% at an operating temperature of 1,435°C. This efficiency is within the target range for high-temperature energy storage systems, typically operating between 1,200°C and 1,600°C, and represents a significant improvement over the 37% efficiency of previous designs.

Advantages Over Other Storage Technologies

Stephen Forrest, the Peter A. Franken Distinguished University Professor of at U-M and contributing author of the study, highlighted several advantages of TPV-based heat batteries. Unlike electrochemical batteries, which require mined materials like lithium, TPV cells are passive and do not compete with other industries for resources. Moreover, unlike hydroelectric storage, TPV systems do not require a nearby water source and can be deployed in a variety of locations.

In a typical heat battery setup, TPV cells would surround a block of heated material maintained at a minimum temperature of 1,000°C. This heat could be generated using excess electricity from renewable sources or by capturing waste heat from industrial processes. The heated material would then emit thermal photons, a portion of which the TPV cells convert into electricity.

Optimizing the Semiconductor Material

A key challenge addressed by the research team was optimizing the semiconductor material within the TPV cells to efficiently capture the broad range of photon energies emitted by the heat source. At 1,435°C, about 20-30% of the emitted photons have sufficient energy to generate electricity.

To maximize efficiency, the team designed an “air bridge” structure within the TPV cells, which includes a thin air layer and a gold reflector. This design helps trap photons with the right energies, redirecting them back into the heat storage material for another chance at conversion, thus recuperating energy that would otherwise be lost.

“Unlike solar cells, thermophotovoltaic cells can recuperate or recycle photons that are not useful,” noted Bosun Roy-Layinde, a doctoral student of chemical at U-M and first author of the study.

Future Prospects

The research indicates potential for further improvements in efficiency. “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,” Forrest stated.

The team has applied for patent protection with the assistance of U-M Innovation Partnerships and is currently seeking collaborators to help bring this promising technology to market. As advancements continue, thermophotovoltaic cells could play a crucial role in the transition to a more sustainable and resilient energy grid.

Source: University of Michigan