Sun in a box

Sun in a box

Was our ex-president Dilma Rousseff right ? When she created the meme “stock up wind”. Was she having a visionary idea but was misinterpreted? How about “stock up sun”, then? All joking aside, what we have to share with you today, dear reader, is impressive!

Would it be possible to store the extra generation of wind and solar energy? What technologies do we need to do that, and how much does it cost? That is what we’re going to see today, come on!

The energy storage application, known informally as a “sun in a box,” would be able to store the extra wind and solar power generation in a heat bank. We would then have a new form of energy storage, this technology is receiving widespread interest because it is estimated to be tenfold cheaper than using batteries, the “sun’s box” in this approach is already low cost: a tank of molten silicon, for instance. The relatively expensive parts are the photovoltaic panels that turn the stored heat back into electricity.

We know the efficiency of photovoltaic panels  is still low, only part of the photon energy is converted into electricity, but what if we had “solar” cells that use heat that reflect energy that cannot be converted into electricity?

This new technology is a few different compared to ordinary solar panels that turn light, rather than heat, into electricity, thermal photovoltaics need to be able to accept lower energy photons — packets of light or heat — because the heat source is at 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. That way, the energy gets reabsorbed and has another chance to get packaged into an electricity-producing, higher-energy photon.The energy emitted by the heat bank, in this case, has over 100 chances to be absorbed by the solar cell before it gets lost.

The conventional gold-backed thermophotovoltaic reflects 95% of light that it can’t absorb – not bad, but to improve the reflectivity, the team added a layer of air between the semiconductor and the gold backing, that’s  because the gold reflects better when the light hits it after traveling in air, rather than coming straight from the semiconductor. It is important to emphasize that 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.

In order it seemed impossible to create such a cell with such accuracy, but Fan working with Tobias Burger, a doctoral student in chemical engineering, and other collaborators, laid the gold beams onto the semiconductor. Then, they coated a silicon back plate with gold to make the mirror and cold-welded the gold beams to the gold backing. This way, the thickness of the gold beams could accurately control the height of the air-bridge, enabling the near-perfect mirroring.They are already looking ahead to raising the efficiency further, adding extra “nines” to the percentage of photons reflected. For instance, raising the reflectivity to 99.9% would give heat 1,000 chances to turn into electricity.

The study published in Nature is titled “Near-perfect photon utilization in an air-bridge thermophotovoltaic cell.” 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. Forrest is also a professor of electrical engineering and computer science, material science and engineering, and physics. The device was built in the Lurie Nanofabrication Facility.

Source: University of Michigan

Júlia Bittencourt

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