Researchers at the University of Michigan in Ann Arbor have created new heat harnessing solar cells that store energy that doesn’t convert to electricity to help bring down the price of storing renewable energy as heat, as well as harvesting waste heat from exhaust pipes and chimneys.
The energy storage application, known as a sun in a box, stores extra wind and solar power generation in a heat bank.
“This approach to grid-scale energy storage is receiving widespread interest because it is estimated to be 10-fold cheaper than using batteries,” says Andrej Lenert, an assistant professor of chemical engineering.
The “sun” in the solution is inexpensive, such as a tank of molten silicon. The relatively expensive parts are the photovoltaic panels that turn the stored heat back into electricity.
Traditional solar panels turn light, rather than heat, into electricity. Thermal photovoltaics must accept lower energy photons of heat or light because the heat source is at a lower temperature than the sun.
To maximize efficiency, engineers have been looking to reflect the photons that are too low-energy back into the heat bank so the energy will be reabsorbed and have another chance to turn into an electricity-producing higher-energy proton.
“It’s a recycling job,” says Steve Forrest, the Peter A. Franken distinguished university professor of engineering and the Paul G. Goebel professor of engineering. “The energy emitted by the heat bank has over 100 chances to be absorbed by the solar cell before it gets lost.”
The conventional gold-backed thermophotovoltaic reflects 95 percent of light it can’t absorb. While this isn’t bad, if 5 percent of the light is lost with each bounce, it has on average 20 chances to be re-emitted in a photon with enough energy to be turned into electricity.
Increasing the number of opportunities to be re-emitted in a photon means one could use cheaper solar cell materials that are choosier about what proton energies they’ll accept. This has additional benefits: higher energy photons make higher energy electrons, which means higher voltages and less energy lost while getting the electricity out.
To improve the reflectivity, the team added a layer of air between the semiconductor – the material that converts the photons into electricity – and the gold backing. The gold is a better reflector if the light hits it after traveling in air, rather than coming straight from the semiconductor. 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.
“It was not clear at the beginning if this ‘air bridge’ structure, with such a long span and without any mechanical support in the middle, could be built with high precision and survive multiple harsh fabrication processes,” says Dejiu Fan, electrical engineering and computer science doctoral student.
The researchers are looking to raise the efficiency further. Raising the reflectivity to 99.9 percent would give heat 1,000 chances to turn into electricity.
U-M has applied for patent protection and is seeking commercial partners to bring the technology to market. The research was funded by the Army Research Office and the National Science Foundation.