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TECHNOLOGYREVIEW.COM MIT TECHNOLOGY REVIEW VOL. 120 | NO. 2 BREAKTHROUGH TECHNOLOGIES
By converting heat to focused beams of light, a new solar device could create cheap and continuous power.
S olar panels cover a growing num- ber of rooftops, but even decades after they were first developed, the slabs of silicon remain bulky, expensive, and inefficient. Funda-
mental limitations prevent these conven- tional photovoltaics from absorbing more than a fraction of the energy in sunlight.
But a team of MIT scientists has built a different sort of solar energy device that uses inventive engineering and advances in materials science to capture far more of the sun’s energy. The trick is to first turn sunlight into heat and then convert it back into light, but now focused within the spec- trum that solar cells can use. While various researchers have been working for years on
Hot Solar Cells
Breakthrough A solar power device that could theoretically double the efficiency of conventional solar cells.
Why It Matters The new design could lead to inexpensive solar power that keeps working after the sun sets.
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Key Players - David Bierman, Marin
Soljacic, and Evelyn Wang, MIT
- Vladimir Shalaev, Purdue University
Availability 10 to 15 years
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Hot Solar Cells
A view of the solar device seen by looking through the equipment used to focus simulated sunlight on it.
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so-called solar thermophotovoltaics, the MIT device is the first one to absorb more energy than its photovoltaic cell alone, demonstrating that the approach could dramatically increase efficiency.
Standard silicon solar cells mainly capture the visual light from violet to red. That and other factors mean that they can never turn more than around 32 percent of the energy in sunlight into electricity. The MIT device is still a crude prototype, operating at just 6.8 percent efficiency— but with various enhancements it could be roughly twice as efficient as conven- tional photovoltaics.
The key step in creating the device was the development of something called an absorber-emitter. It essentially acts as a light funnel above the solar cells. The absorbing layer is built from solid black carbon nanotubes that capture all the energy in sunlight and convert most of it into heat. As temperatures reach around 1,000 °C, the adjacent emitting layer radiates that energy back out as light, now mostly narrowed to bands that the photovoltaic cells can absorb. The emitter is made from a photonic crystal, a structure that can be designed at the nanoscale to control which wavelengths
of light flow through it. Another critical advance was the addition of a highly spe- cialized optical filter that transmits the tailored light while reflecting nearly all the unusable photons back. This “pho- ton recycling” produces more heat, which generates more of the light that the solar cell can absorb, improving the efficiency of the system.
There are some downsides to the MIT team’s approach, including the relatively high cost of certain components. It also currently works only in a vacuum. But the economics should improve as efficiency levels climb, and the researchers now have
Above: Black carbon nanotubes sit on top of the absorber-emitter layer, collecting energy across the solar spectrum and converting it to heat.
Facing page: The absorber-emitter layer is situated above an optical filter and photovoltaic cell, which is visible underneath.
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a clear path to achieving that. “We can fur- ther tailor the components now that we’ve improved our understanding of what we need to get to higher efficiencies,” says Evelyn Wang, an associate professor who helped lead the effort.
The researchers are also exploring ways to take advantage of another strength of solar thermophotovoltaics. Because heat
is easier to store than electricity, it should be possible to divert excess amounts gener- ated by the device to a thermal storage sys- tem, which could then be used to produce electricity even when the sun isn’t shining. If the researchers can incorporate a stor- age device and ratchet up efficiency levels, the system could one day deliver clean, cheap—and continuous—solar power.
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Concentrated light from a solar simulator shines through the window of a vacuum chamber, where it reaches the solar thermophotovoltaic device and generates electricity.
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