Aglow Like Tungsten Wire, Thermal Batteries Efficiently Store Solar Energy

A team at the Massachusetts Institute of Technology (MIT) and the National Renewable Energy Laboratory recently developed thermal batteries that emits light like a tungsten wire, effectively storing wind and solar energy. The technology achieves a nearly 30 percent jump in the efficiency of thermophotovoltaic (TPV), a semiconductor structure that converts photons from a heat source into electricity, just as solar cells convert sunlight into electricity.

"This is a very exciting thing," said Andrej Lenert, a materials engineer at the University of Michigan, Ann Arbor. "It's the first time TPVs have moved into the higher efficiency range, which is critical for many applications." This new work gives a significant boost to progress in thermal batteries.

The idea of this research is to feed wind or solar energy to a heating element that raises the temperature of a liquid metal bath or graphite block to several thousand degrees. The heat can be converted to electricity by creating the steam that drives the turbine, and the high temperature increases the conversion efficiency, but the turbine material begins to decompose at about 1500 C. TPVs offer an alternative. The stored heat is delivered to a metal film or filament that emits light like a tungsten wire in an incandescent lamp, and then thermoplastic sulfides are used to absorb the emitted light and convert it to electrical energy.

When the first thermoplastic sulfides were invented in the 1960s, they converted only a few percent of their heat energy into electricity. That efficiency jumped to about 30 percent in 1980 and has remained essentially constant since then. One reason for this is that tungsten and other metals tend to radiate photons across a wide spectrum, from high energy ultraviolet to low energy far infrared. But all photovoltaic devices, including photovoltaics, are optimized to absorb photons in a narrow range, meaning that higher and lower frequencies of light tend to be wasted.

For this new device, Asegun Henry, a mechanical engineer at MIT, tinkered with both the emitter and the TPV itself. The previous TPV setup heated the emitter to about 1400°C, which maximized its brightness over the wavelength range for which the TPV was optimized. Henry's goal was to push the temperature up by 1000°C, where the tungsten would emit more photons at higher energies, which could improve energy conversion. But that means also reworking the TPV.

The team laid down more than two dozen different thin layers of semiconductors to create two separate cells, one stacked on top of the other. The top cell absorbs primarily visible and ultraviolet photons, while the bottom cell absorbs primarily infrared light. The thin gold sheet under the bottom cell reflects the low energy photons that the TPVs cannot harvest. The tungsten reabsorbs these energies and prevents their loss. This TPV tandem converts 41.1% of the energy emitted by the 2400°C tungsten wire into electricity.

Henry's team saw a better way, and in the Oct. 8, 2020, issue of Nature, Lenert and his colleagues report a reflection capable of reflecting nearly 99 percent of the unabsorbed infrared photons back to the heat source. Combining that reflection with the MIT group's improved TPV could yield another huge boost. Henry says, " We think we have a clear path to 50% efficiency."

This progress has triggered commercial interest. Antora Energy in California launched a thermal energy company in 2016. Lenert and others are eyeing their own startups. And Henry recently launched a venture—Thermal Battery Corp.—to commercialize his group’s technology, which he estimates could store electricity for $10 per kilowatt-hour of capacity, less than one-tenth the cost of grid-scale lithium-ion batteries. “Storing energy as heat can be very cheap,” even for many days at a time, says Alina LaPotin, an MIT graduate student and first author of the current Nature paper.

Henry and others add that thermal batteries systems are modular, unlike fossil fuel plants, which are most efficient at a massive, gigawatt scale. “That makes them equally good at providing power for a small village or a large power plant,” says Alejandro Datas, an electrical engineer at the Polytechnic University of Madrid—and for storing power from solar energy and wind farms of any size. “This is the beauty.

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