Solar power: Building a better suntrap

A novel approach to solar power may help to improve its efficiency

Let’s catch some rays

TO MAKE electricity from sunlight you can convert it directly, using a photovoltaic cell. Or you can use the heat of that sunlight to boil water, and then drive a turbine with the resulting steam. These are both established technologies. But there is, in principle, a third way: use heat directly, without steam or turbines. In this case, unlike a standard solar cell (which is sensitive to some frequencies of light, but not others), almost all of the incident energy is available for conversion. Yet unlike the boiling-water method, no messy mechanical processes are involved. Once set up, such a system could run with the minimum of attention.

Unfortunately, devices that turn sunlight into heat and then into electricity in this way do not get much warmer than boiling water when they are exposed to direct, unconcentrated sunlight. The reason is that at temperatures significantly higher than this the laws of thermodynamics dictate that they shed heat as fast as they absorb it. That has proved problematic, because a direct converter of this sort needs to reach 700°C to become properly efficient, and that is impossible without using special (and expensive) parabolic mirrors to concentrate the incident light.

Peter Bermel of the Massachusetts Institute of Technology and his colleagues, however, think they have found a way round this difficulty. As they describe in Nanoscale Research Letters, they have invented a way of concentrating the energy in the sun’s rays without the need for mirrors. It is, quite literally, a suntrap.

Dr Bermel’s proposed trap is a thin sheet of tungsten (a heat-resistant metal) that has been processed in quite a complicated way. One surface, which faces the sun, is covered in microscopic pits. The other, which faces a specialised type of solar cell made of a material called indium gallium arsenide, is sculpted into a structure called a photonic crystal that causes it to emit infra-red radiation selectively at the frequency best absorbed by the cell. Both of these surfaces would be created by photolithography, the process used to make computer chips.

It is the pits, which are three-quarters of a micron in diameter, three microns deep and arranged in a grid four-fifths of a micron apart, that do the trapping. When the device is aligned so that its pits are pointing straight at the sun, most of the incident radiation goes down them to their bottoms. Here, it is absorbed by the tungsten. As the laws of thermodynamics demand, it is then rapidly reradiated.

Heat radiation coming from inside a pit, however, is more likely than not to encounter the pit’s wall before it escapes into the outside world. If that happens, the whole process of absorption and reradiation starts again. The result is that the pitted tungsten becomes much hotter than a plain sheet of the metal could manage.

To turn that heat into electricity, it is directed towards the solar cell by the photonic crystal. This is a regular geometric pattern etched onto the surface of the tungsten. It acts to amplify infra-red emissions at some frequencies and suppress them at others.

The trick is to tune the crystal, by modifying the details of pattern, so that as much of the emitted energy as possible is at the frequency most efficiently captured by indium gallium arsenide. The process of capture knocks electrons free inside the material and creates a current.

The result, according to Dr Bermel’s calculations, would be a system that converts 37% of sunlight into electricity. This compares with a maximum of 28% by standard silicon-based solar cells that have not had the incident light concentrated by parabolic mirrors, and 31% by those that have—a significant enhancement. The next step, of course, is to try it for real, but Dr Bermel is pretty confident his sums are correct.

Tungsten, as a material, was much used in the filaments of incandescent electric light bulbs. These are going out of fashion because they convert too much of the electricity passing through them into heat, rather than light. A nice irony, then, that running the process backwards may not only give tungsten a new lease of life, but might also help solve the world’s shortage of renewable energy.

The Economist

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