You might think that fewer steps in the process of turning sunlight into electricity would be the most efficient way to go. But that’s not the case with a new technique from MIT researchers, who added a layer of complexity to solar power in order to make use of a wider range of the sun’s energy.

The work here is in the solar realm known as thermophotovoltaics, where PV cells are coupled with heat sources, seen as a possible path to bust through the efficiency limit inherent in photovoltaics, generally thought to be 33.7 percent. In a new paper, the MIT researchers haven’t done that, but they say they’ve made a breakthrough that could take thermophotovoltaics far beyond where it’s gone before.

mit thermophotovoltaics
MIT’s nanophotonic solar thermophotovoltaic device, with an array of multi‑walled carbon nanotubes as the absorber, a one‑dimensional silicon/silicon dioxide photonic crystal as the emitter, and a photovoltaic cell. (image via MIT/John Freidah)

Here’s how MIT outlines the development:

(T)he team inserted a two-layer absorber-emitter device — made of novel materials including carbon nanotubes and photonic crystals — between the sunlight and the PV cell. This intermediate material collects energy from a broad spectrum of sunlight, heating up in the process. When it heats up, as with a piece of iron that glows red hot, it emits light of a particular wavelength, which in this case is tuned to match the bandgap of the PV cell mounted nearby.

MIT says that what makes this technique different is the materials in the absorber-emitter.

Its outer layer, facing the sunlight, is an array of multiwalled carbon nanotubes, which very efficiently absorbs the light’s energy and turns it to heat. This layer is bonded tightly to a layer of a photonic crystal, which is precisely engineered so that when it is heated by the attached layer of nanotubes, it ‘glows ‘with light whose peak intensity is mostly above the bandgap of the adjacent PV, ensuring that most of the energy collected by the absorber is then turned into electricity.

Now, the MIT device is only at about 3.2 percent efficiency – a big jump over the less-than 1 percent STPV devices that have been seen before, but still paltry compared to standard PV that usually delivers efficiency levels in the teens. But the researchers think their concept can be refined to reach 20 percent and they point to advantages that thermophotovoltaics could have over PV:

The new solar thermophotovoltaic systems, they say, could provide efficiency because of their broadband absorption of sunlight; scalability and compactness, because they are based on existing chip-manufacturing technology; and ease of energy storage, because of their reliance on heat.

That last point is particularly fascinating, pivoting, it appears, on the idea that storing heat is a less tricky business than storing electricity.

By the way, this isn’t the first foray into thermophotovoltaics at MIT that we’ve reported on. See our earlier story on a technique that one expert said could lead to miniature power supplies and lighter portable electronics.


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