MIT’s sun funnel could slit solar power’s efficiency bottleneck

November 28, 2012 by  
Filed under Green Energy News

MIT's sun funnel

Every day, our Sun beats more energy into the world’s deserts than is used by the entire human race in a year. Every joule of energy in the world’s oil reserves was put there by the sun, millions of years ago. Even uranium was created in the death-rattle explosion of some ancient star. We have found or developed hundreds of middle-men, from coal turbines to AAA batteries, energy carriers who power up on star energy and release it only grudgingly to human beings. This week, a team of researchers at MIT hope to start our great global austerity measure, the elimination of the middle-men, and the rise of true, sustainable solar power.

The flaw in solar power has always been efficiency. For decades, scientists struggled to capture even a sizable minority of the solar energy falling on a particular panel — the recent development of a system with 32% efficiency was heralded as a major breakthrough. At that rate, solar farms would need to be huge, truly gigantic, to collect useful amounts of energy, and their price per square foot has always been worrying. The reason for this is that solar energy is not one homogeneous force, like wind; any one collector will be able to absorb some of the energy falling on it, but will miss most of the rest as a result.

Ergon Energy Station in Windorah, Australia

Ergon Energy Station in Windorah, Australia

Until now. A recent MIT study [PDF] has proposed that an “atomically thin” sheet of semiconducting material could be stretched by pushing a pin down onto the center. The resulting funnel shape would have a gradient of internal strain that tapers off further from the center. Prior research has shown that stretching silicon semiconductors by just 1% can increase electron flow by more than 50%, and so the researchers propose using strained semiconducting materials to “tune” a solar collector to a particular wavelength of light. Within the limits of the material, changing the pin pressure will adjust the shape of the funnel, and thus the edges of the absorption spectrum.

This neatly sidesteps the major problem with collection of solar energy: there’s so much of it. Spread across a spectrum far larger than that visible to the human eye, energy from the sun comes in a variety of sizes and power levels. MIT’s new model says that by creating a gradient of strain between the semiconducting molecules, they will also create a gradient of absorptive ability, a physical property called the bandgap. Plants use a variety of different pigments to trap a wide variety of light, but are ultimately best with red; even using multiple absorbers can only get you so far. High-bandgap materials can accommodate higher-energy light but exclude the weaker wavelengths that make up most of our potential photons, while low-bandgap materials let us collect many photons, each with little energy. What we need is a cell that can absorb all along that spectrum.

A solar funnel, as proposed by MIT, could catch many different wavelengths of light all along its surface — and unlike modern organic cells, which rely on diffusion to bring excited units to the collector, this one would direct every captured photon quickly down toward the collector. That’s ultimately half the point of their work, to make not just the absorption but the collection phases efficient enough for mass use.

Combined with existing techniques to increase solar efficiency, like farms and panel stacks, this study hopes to lay the foundation for a viable solar future. In the past, solar power has been about clean energy production, about saving the environment and ourselves. As our energy needs continue to increase, however, we may find that we’re turning to solar simply so we can keep the lights on. Sun funnels are a lot more approachable than star-encompassing Dyson spheres, after all.

Now read: Solar panel made with ion cannon is cheap enough to challenge fossil fuels

Research paper: doi:10.1038/nphoton.2012.285 – “Strain-engineered artificial atom as a broad-spectrum solar energy funnel”

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