Heat, whether from the sun or from engines and furnaces, is often wasted. Thermophotovoltaic devices, which convert heat into electricity, promise a super efficient way to scavenge it. And they could enable compact, simple systems for grid energy storage that are cheaper than batteries. But devices made so far have yet to catch up to this promise because they are too expensive and not efficient enough. A new thermophotovoltaic design with a record-high 32 percent efficiency could turn that around (Nature 2020, DOI: 10.1038/s41586-020-2717-7).
Thermophotovoltaic systems use an energy source like concentrated sunlight or a stream of hot gas to heat a thermal material, which in turn emits low-energy infrared radiation. A specially engineered photovoltaic cell captures that radiation and converts it to electricity.
To get high efficiencies, thermophotovoltaic systems made so far have had to incorporate expensive materials. Most infrared radiation from thermal emitters is low energy. So today’s devices use specially engineered emitters that can radiate at least some high-energy light and expensive indium gallium arsenide photovoltaic cells that can absorb low-energy light. Still, much of the emitter’s radiation goes to waste, so researchers have recently made photovoltaic cells with a mirrorlike metal layer on their back surface to reflect the low-energy light back to the emitter for reuse. Even in the best cells, chemical engineer Andrej Lenert says, the metal mirror absorbs at least 5% of the light, so a significant amount of energy is wasted.
To weed out this inefficiency, Lenert’s team at the University of Michigan and electrical engineer Stephen Forrest came up with a cell design that reflects 99% of the low-energy light back to the emitter. They did this by adding an air layer between the semiconductor and the metal. Too thick a layer adds electrical resistance, so they had to get it just right: 600 nm.
The team made thermophotovoltaic devices with and without the air layer. Adding the air layer increased the heat-to-electricity conversion efficiency from 24% to 32%. This simple scheme improves efficiency.
“The biggest bottleneck for thermophotovoltaic efficiency has been the wasted low-energy light,” Lenert says. “Now that we have 99% reflectance, this really paves the way for very high efficiency.”
Alejandro Datas of the Polytechnic University of Madrid’s Institute of Solar Energy calls the work a breakthrough and says it could enable conversion efficiencies of more than 40 percent in the near future. This increase would bring the efficiency of thermophotovoltaics close to that of today’s most efficient heat engines, “but with the important difference that thermophotovoltaics can be made simple and small.”
Such devices could lead to compact energy-storage systems that use surplus renewable power to produce heat that is stored in materials such as molten salt. That heat could then be used to produce thermophotovoltaic electricity on demand. “Because heat can be stored, thermophotovoltaics have a remarkable role to play in solving the energy storage challenge,” Datas says.