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SOLAR CELLS New heat-harnessing solar cells get more electricity from heat

From Luke James

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New heat-harnessing “mirror-like” thermophotovoltaic solar cells, which reflect almost all of the light that they can’t turn into electricity, could help cut the costs associated with storing renewable energy as heat, as well as harvesting waste heat.

A research team from the University of Michigan presents a so-called "air-bridge thermophotovoltaic cell" with almost perfect photon utilization.
A research team from the University of Michigan presents a so-called "air-bridge thermophotovoltaic cell" with almost perfect photon utilization.
(Source: gemeinfrei / Pixabay)

This is an energy storage application that has been referred to as “sun in a box,” and it works by storing extra solar and wind power generation in a heat bank. It’s a novel approach to grid-scale energy storage which is receiving a lot of interest from researchers because “it is estimated to be ten-fold cheaper than using batteries,” says Andrej Lenert, an assistant professor of chemical engineering at the University of Michigan’s College of Engineering.

That’s because the sun itself is low cost. It’s sitting there up in the sky and it costs nothing to harness its energy save for the parts and materials that do it, and that’s where things get expensive, for instance the photovoltaic panels that turn stored heat back into electricity. Compared to regular solar panels that turn light into electricity, thermal photovoltaics need to be able to accept lower energy photons because the heat source is at a lower temperature.

To improve and maximize efficiency, engineers have been looking at ways to reflect low-energy photons back into the heat bank. This way, the energy isn’t wasted and is instead reabsorbed into a higher-energy photon which can be accepted by the cell. “The energy emitted by the heat bank has over 100 chances to be absorbed by the solar cell before it gets lost,” says Peter A. Franken, Distinguished University Professor of Engineering and the Paul G. Goebel Professor of Engineering.

Thermophotovoltaic solar cells

This is where the University of Michigan research team’s work comes in. In a study published in the journal Nature on September 21, the team presents what it calls an “air-bridge thermophotovoltaic cell” with near-perfect photon utilization.

Thermophotovoltaic cells (TPVCs) are similar to solar cells. However, they are designed to utilize locally radiated heat rather than convert solar radiation into electricity. If researchers are able to develop highly efficient TPVCs, widespread applications in grid-scale energy storage could be unlocked.

To achieve high efficiencies, TPVCs must use the broad spectrum of a radiative thermal source. This is problematic, however, because most thermal radiation is in a low-energy range which cannot be used for electron excitation to generate electricity.

One potential solution is to reflect low-energy photons so that they can be re-absorbed by the thermal emitter. This essentially gives the photon’s energy another chance at contributing towards the TPVC’s photogeneration. However, current methods are limited by insufficient bandwidth or parasitic absorption which leads to efficiency losses, and this is what the Michigan team set out to address in their research.

Near-perfect reflection of low-energy photons

In this study, the Michigan researchers describe “near-perfect reflection of low-energy photons,” achieved by using air.

An electron microscope image showing the air gap (darkest gray) between the gold backing (bottom) and the semiconductor (top), supported with gold beams.
An electron microscope image showing the air gap (darkest gray) between the gold backing (bottom) and the semiconductor (top), supported with gold beams.
(Source: Dejiu Fan, University of Michigan)

In a conventional gold-backed thermophotovoltaic, 95% of light is reflected and 5% is lost with each bounce. While this is a good result, it wasn’t enough for the researchers who wanted to find a way to improve on this and increase the opportunity for a low-energy photon to be turned into electricity—improving efficiency and driving down material costs.

In order to improve the reflectivity, the researchers added a layer of air between the semiconductor and the gold backing. To reduce instances of the light waves cancelling one another out, the air gap’s thickness was made similar to the photons’ wavelengths. The air gap’s thickness has to be so precise, in fact, that PhD student Dejiu Fan was initially sceptical of the project’s potential. “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,” he said.

Working with Tobias Burger, another PhD student, and other colleagues, Fan managed to achieve this. The team laid the gold beams on top of the semiconductor and then coated the silicon back plate with gold to make the mirror. They then cold-welded the gold beams to the gold backing. This way, the gold beams’ thickness could accurately control the air bridge’s accuracy, enabling near-perfect mirroring.

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Overall, this increases efficiency to 99%, and the researchers are already looking at raising this to 99.9%. The University of Michigan has applied for patent protection and is currently looking to collaborate with commercial partners to bring the technology to market.