Researchers at the University of Michigan, Ann Arbor, Mich., have found a way to get more energy out of thermophotovoltaic cells, which could have a big impact on energy storage down the line.
Thermophotovoltaic cells are similar to solar cells, but instead of converting sunlight into electricity, they convert locally radiated heat into electricity. The cells offer the opportunity for storing extra power generated by renewables as heat, as well as harvesting waste heat from sources like exhaust pipes and chimneys. To make the cells more efficient, a mirrorlike metal layer is used to reflect low-energy photons back onto the thermal emitter to be reabsorbed for more photogeneration in the cell.
“However, current methods for photon recuperation are limited by insufficient bandwidth [of electricity-producing wavelengths] or parasitic absorption [of photons by components that are not designed to do so], resulting in large efficiency losses relative to theoretical limits,” the researchers wrote in their study published in Nature.
To minimize such waste, the team embedded a layer of air between the semiconductor and gold backing within a thin-film cell to “demonstrate near-perfect reflection of low-energy photons.”
“The gold is a better reflector if the light hits it after traveling in air, rather than coming straight from the semiconductor,” according to a subsequent university blog post. “To minimize the degree to which the light waves cancel each other out, the thickness of the air layer must be similar to the wavelengths of the photons.”
The air layer was able to reduce parasitic absorption fourfold, increasing conversion efficiency from 24% to 32%. The researchers are now exploring how to increase that even further by adding extra “nines” to the percentage of photons reflected. If reflectivity is raised to 99.9%, heat would have “1,000 chances to turn into electricity.”
The implications are substantial, the researchers asserted in their study.
“The development of high-efficiency thermophotovoltaic cells has the potential to enable widespread applications in grid-scale thermal energy storage, direct solar energy conversion, distributed co-generation and waste heat scavenging,” the researchers wrote.
Alejandro Datas of the Polytechnic University of Madrid’s Institute of Solar Energy, Madrid, Spain, told Chemical & Engineering News that the research is “a breakthrough” that could lead to even greater conversion efficiencies akin to efficient heat engines, “but with the important difference that thermophotovoltaics can be made simple and small.”
One application of such cells could be in compact energy-storage systems that store heat produced from surplus renewable power in molten salt or other materials, according to Datas.
“Because heat can be stored, thermophotovoltaics have a remarkable role to play in solving the energy storage challenge,” he said.