Rensselaer Researchers Nano-Engineer Solar to 'Near Perfect' Efficiency

November 15th, 2008 by Susan Kraemer

Nano-engineering students at Rensselaer have created a solar power game-changer: more than 96% absorption of sunlight from all angles, from sunrise to sunset.

The two biggest efficiency hurdles for solar efficiency have been:

1. Solar cells absorb only part of the light spectrum.

2. The sun always moves in relation to the panel.

To solve problem number one, researchers nano-invented an anti-reflective coating to make the solar cell capture the full light spectrum. Currently, solar cells reflect almost 1/3 of the sunlight that hits them. That reflected light is not harvested, which has reduced solar cell efficiency. Problem one solved.

To solve problem two, they stopped the sun in its tracks.

Well, no, actually, that would be a roundabout way to solve that problem.

Instead, they designed a nano-coating to ‘follow’ the sun’s movements and absorb every last photon of light, regardless of the suns moving position in the sky.

The problem:

Most surfaces and coatings absorb or transmit light through them from only a specific range of angles. Your glasses, for instance, absorb-transmit all the light in front of you. But much less from the periphery.

That’s why some solar panels are mechanized to slowly move so they always face the moving sun. But that uses energy, too. So the energy it takes reduces the efficiency of the panel.

‘At the beginning of the project, we asked ‘would it be possible to create a single anti-reflective structure that can work from all angles?’ said Shawn-Yu Lin, professor of physics at Rensselaer and a member of the university’s Future Chips Constellation who led the research project.

‘Then we attacked the problem from a fundamental perspective, tested and fine-tuned our theory, and created a working device,’ Lin said. Rensselaer physics grad student Mei-Ling Kuo played a key role in the investigations.

How their solution works:

Unlike typical antireflective coatings that are engineered to transmit light of only one particular wavelength, this coating stacks seven of these layers, one on top of the other, in such a way that each layer enhances the anti-reflective properties of the layer below it.

These additional layers also help to ‘bend’ the flow of sunlight to an angle that augments the coating’s anti-reflective properties. This means that each layer transmits sunlight and also helps to capture any light that may have otherwise been reflected off of the layers below it.

The seven layers, each with a height of 50 nanometers to 100 nanometers, are made up of silicon dioxide and titanium dioxide nanorods positioned at an oblique angle.

Each layer looks like (and functions like) a dense forest where sunlight is ‘captured’ between the trees. The nanorods were attached to a silicon substrate via chemical vapor disposition.

The silicon surface absorbed 96.21 percent of sunlight, after treating the material with the reflective coating.

Only 3.79 percent of the sunlight was reflected and unharvested.

The entire spectrum of sunlight from UV to visible light to infrared was absorbed, for the first time.

‘To get maximum efficiency when converting solar power into electricity, you want a solar panel that can absorb nearly every single photon of light, regardless of the sun’s position in the sky,’ said Lin. ‘Our new antireflective coating makes this possible.’

The bottom line:

This is a game changer. This nano-engineered coating could be applied to nearly any photovoltaic material for use in solar cells, says Lin. These two huge gains move solar power forward to being cost-effective for mass production.

Photo Credit: Flikr user ZebSnaps via Creative Commons License, Rensselaer press release

Source: Rensselaer Polytechnical Institute









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