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'Greenhouse effect' used to generate electricity

MIT

Researchers are working on a device that traps the sun's energy using a greenhouse-like effect and converts it into electricity.

A device that gets scorching hot as it captures and traps much of the sun's energy using a greenhouse-like approach could usher in an era of inexpensive electricity from the sun.

The breakthrough comes from a sunlight-absorbing material made of photonic crystals that are arranged to prevent the escape of most of the energy it captures from direct sunlight.


The concept is similar to the way carbon dioxide molecules in the atmosphere trap the sun's energy, which keep the planet warmer than it would be if all the energy escaped to space.

In this case, infrared radiation from the sun enters the device through holes in the surface, but the reflected rays are blocked when they try to escape, explains Peter Bermel, an electronics researcher working on the device at the Massachusetts Institute of Technology. 

This blockage is achieved by a geometry that limits re-radiation of the sun's rays to a narrow range of angles — the solar disk and region right around the sun. The rest of the rays stay in the device and heat it up.

All this concentrated heat is focused on the production of high-energy photons, which are used to generate electricity via a thermophotovoltaic device.

Conventional photovoltaic cells are limited in their ability to convert sunlight into electricity due to the inefficient conversion of the broad spectrum of sunlight that hits the cells. 

This limit, known as Shockley-Queisser, is 31 percent. 

"What we're doing is a way around that limit … we are taking a very broad spectrum and then we are squeezing it, in some sense," Bermel told me.

Peter Bermel / MIT

This is a diagram of the angle-selective thermophotovoltaic system. In theory, such devices could produce electricity more efficiently than conventional photovoltaic cells.

That's because heat is absorbed across a broad range of wavelengths and then tailored to generate the high-energy photons needed to generate electricity. The approach, Bermel said, could reach efficiencies of 35 to 36 percent, which is higher than the Shockley-Queisser limit.

Thermophotovoltaic devices have existed since the 1950s, but the concentration of sunlight is traditionally done with giant and expensive arrays of mirrors. Bermel's approach, by contrast, can be made with inexpensive chip-manufacturing technology, he said.

A major expense, though, will come in the equipment needed to track the sun so that the device is always getting direct sunlight to take advantage of the selective-angle approach.

Other solar concentrators, such as the luminescent solar concentrators we reported on in November, get around the outlay for tracking technology by absorbing diffuse sunlight and pumping it to conventional solar cells.

However, some sunlight is still reabsorbed in the LSC technology and control of the wavelengths is difficult, Bermel noted.

"The nice thing about our angle-selective approach is that it can keep losses to extremely low levels, relatively speaking," he said.

What's more, the higher efficiencies of the thermophotovoltaics, in theory, could make up for the added costs of the tracking, he added.

To get there, though, will require more work on optimizing the angular selectivity of their material to reach the theoretical efficiencies.

"I don't want to oversell the research and say we've already figured it all out and it is going to be in your home in the next year or two," Bermel said. "That's not realistic."

Nevertheless, finding new ways to concentrate sunlight to generate electricity is welcome news as global concentrations of carbon dioxide reach new highs, raising worries about that other greenhouse effect.

More on solar energy breakthroughs:


Bermel and colleagues describe their work in the journal Nanoscale Research Letters. 

John Roach is a contributing writer for msnbc.com. To learn more about him, check out his website. For more of our Future of Technology series, watch the featured video below.

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