Researchers at Rice University have found a new way to improve a key element of thermophotovoltaic (TPV) systems, which convert heat into electricity via light. Using an unconventional approach inspired by quantum physics, Rice engineer Gururaj Naik and his team designed a thermal emitter that can deliver high efficiencies within practical design parameters.

The research could inform the development of thermal-energy electrical storage, which holds promise as an affordable, grid-scale alternative to batteries. More broadly, efficient TPV technologies could facilitate renewable energy growth—an essential component of the transition to a net-zero world. Another major benefit of better TPV systems is recouping waste heat from industrial processes, making them more sustainable. To put this in context, up to 20-50% of the heat used to transform raw materials into consumer goods ends up being wasted, costing the United States economy more than $200 billion annually.

TPV systems involve two main components: photovoltaic (PV) cells that convert light into electricity and thermal emitters that turn heat into light. Both of these components have to work well in order for the system to be efficient, but efforts to optimize them have focused more on the PV cell.

Using conventional design approaches limits thermal emitters’ design space, and what you end up with is one of two scenarios: practical, low-performance devices or high-performance emitters that are hard to integrate in real-world applications.

—Gururaj Naik

In a new open-access study published in npj Nanophotonics, Naik and his former Ph.D. student Ciril Samuel Prasad—who has since earned a doctorate in electrical and computer engineering from Rice and has taken on a role as a postdoctoral research associate at Oak Ridge National Laboratory—demonstrated a new thermal emitter that promises efficiencies of over 60% despite being application-ready.

A new thermal emitter developed by Rice University engineers composed of a tungsten metal sheet, a thin layer of a spacer material and a network of silicon nanocylinders promises efficiencies of over 60%. (Photos by Gustavo Raskosky/Rice University)

The emitter is composed of a tungsten metal sheet, a thin layer of a spacer material and a network of silicon nanocylinders. When heated, the base layers accumulate thermal radiation, which can be thought of as a bath of photons. The tiny resonators sitting on top “talk” to each other in a way that allows them to “pluck photon by photon” from this bath, controlling the brightness and bandwidth of the light sent to the PV cell.

Instead of focusing on the performance of single-resonator systems, we instead took into account the way these resonators interact, which opened up new possibilities. This gave us control over how the photons are stored and released.

—Gururaj Naik

This selective emission, achieved through insights from quantum physics, maximizes energy conversion and allows for higher efficiencies than previously possible, operating at the limit of the materials’ properties. To improve on the newly achieved 60% efficiency, new materials with better properties would need to be developed or discovered.

These gains could make TPV a competitive alternative to other energy storage and conversion technologies such as lithium-ion batteries, particularly in scenarios where long-term energy storage is needed. Naik noted that this innovation has significant implications for industries that generate large amounts of waste heat such as nuclear power plants and manufacturing facilities.

The team’s technology could also be used in space applications such as powering rovers on Mars.

The research was supported by the National Science Foundation (1935446) and the US Army Research Office.

Resources

• Samuel Prasad, C., Naik, G.V. Non-Hermitian selective thermal emitter for thermophotovoltaics. npj Nanophoton. 1, 44 (2024). doi: 10.1038/s44310-024-00044-3