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Clean Power MIT solar cell efficiency

Published on May 31st, 2016 | by Tina Casey

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New Solar Cell Breaks Efficiency Ceiling, Theory Goes Out Window

May 31st, 2016 by  


If you thought the upper limit of solar cell efficiency was 32 percent in April, think again in May. Last week, MIT News reported on a research team that demonstrated how a silicon solar cell could top the theoretical limit of 32 percent. See you later, Shockley-Queisser Limit, and don’t let the door hit you on the way out.

To be clear, William Shockley and Hans J. Queisser can rest easy. The demonstration involved a silicon solar cell modded out with some other special material, so although MIT is pitching this as a theory-breaker, some people might prefer to mark it with an asterisk. Still, it’s a highly significant finding that could provide the silicon solar cell market with new life even as perovskites and other new solar cell variations emerge on the horizon.

MIT solar cell efficiency

Solar Cell Efficiency Gets The Heat Treatment

The key to the new solar cell efficiency breakthrough is something called thermophotovoltaics, in which carefully tailored materials are deployed to trap heat from the sun before it can reach the solar cell itself.

The extra materials then emit the heat in the form of thermal radiation, which is tuned to wavelengths that can be assimilated by the solar cell.

That’s the short version. If you’re looking for the full rundown you can find it in the journal Nature Energy under the title, “Enhanced photovoltaic energy conversion using thermally based spectral shaping.” Here’s a taste:

Here, we demonstrate enhanced device performance through the suppression of 80% of unconvertible photons by pairing a one-dimensional photonic crystal selective emitter with a tandem plasma–interference optical filter.

I know, right?

So Much For Solar Power Being Unreliable

Clean power foes are still clinging to the idea that solar energy is unreliable, a position that is becoming more and more ridiculous given the rapid progress in energy storage, smart grid technology, and related factors like advanced weather modeling.

The new solar cell breakthrough makes the naysayers look even more ridiculous, by opening up the potential for high efficiency solar conversion even on cloudy days. The extra material — a combination of nanophotonic crystals and vertically aligned carbon nanotubes, to be precise — acts as a mini onboard solar thermal storage unit, smoothing out dips and spikes as clouds pass across the sun.

The thermal element also provides a convenient bridge for pairing the new solar cell with standalone thermal energy storage systems.

The team worked with carbon nanotubes because they are “virtually a perfect absorber” of the color spectrum, enabling the new solar cell to capture all of the solar energy that hits it in the form of heat. As the heat is re-emitted in the form of light, the crystals convert it to colors that enable the photovoltaic material to close in on its peak efficiency.

Breaking the theoretical maximum is quite complicated, as the system requires a concentrating solar system to maintain high heat, and an optical filter to sort the desired wavelengths out from the undesirable ones.

That leads to the question of whether the cost is worth the effort, but the researchers anticipate that the system could be deployed to reduce lifecycle costs of concentrating solar systems, by managing heat generation and reducing the risk of damage.

What Now, Peter Thiel And Donald Trump?

Don’t hold your breath for that new solar cell to hit the market. The new research is a step or two above the proof-of-concept stage, and it describes an early-stage demonstration (see photo at the top of this article) using a low efficiency photovoltaic cell.

The result was an overall efficiency of 6.8 percent, which hardly sounds earth shattering (let alone ceiling-breaking), but the key point is that the figure of 6.8 was a real-life match for the team’s prediction.

Here’s researcher David Bierman describing the significance, as cited by MIT News

“…This is the first time we’ve actually put something between the sun and the PV cell to prove the efficiency” of the thermal system. Even with this relatively simple early-stage demonstration, Bierman says, “we showed that just with our own unoptimized geometry, we in fact could break the Shockley-Queisser limit.

As for what this all has to do with Peter Thiel, the tech tycoon has been all over the news this week for his secretive funding of the Hulk Hogan “sex tape” lawsuit against Gawker Media. Thiel is also well known for encouraging other tech tycoons to invest in software rather than putting up Benjamins for clean tech R&D efforts like this one.

We bring this up because Thiel is an active supporter of presumptive Republican nominee Donald Trump in his quest to become President of the United States. One could assume that if Trump wins the office, Thiel would be in a position to amplify his message to investors and public policy makers.

On the other side, presumptive Democratic nominee Hillary Clinton has carved out a slot for increasing public dollars for solar innovation in her ambitious slate of clean power proposals.

Just saying.

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Photo: via MIT News, courtesy of the researchers.


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About the Author

specializes in military and corporate sustainability, advanced technology, emerging materials, biofuels, and water and wastewater issues. Tina’s articles are reposted frequently on Reuters, Scientific American, and many other sites. Views expressed are her own. Follow her on Twitter @TinaMCasey and Google+.



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