• It absorbs sunlight from a wide range of angles and from the appropriate spectrum
• It withstands temperatures of up to 1,000 degrees centigrade
• It can be produced on a large scale and at low cost using current technology
For the optimum conversion of solar energy into heat, an “ideal” material is needed; one which absorbs practically all the wavelengths of light which reaches the Earth’s surface from the Sun, but which does not absorb too much of the rest of the spectrum so that the energy reradiated by the material does not increase and it is not lost at the moment of conversion. Researchers at the Massachusetts Institute of Technology (MIT) have recently created a material which comes very close to this ideal.
The Challenges They Faced
If we want to prevent the global temperature on Earth from rising more than 2 degrees centigrade in the coming decades and avoid suffering the dire consequences of the effects this temperature rise would mean, we must increase the generation of energy from clean sources, with low carbon emissions, like solar energy.
One of the greatest challenges for those working in the MIT Photovoltaic Research Laboratory over the past few years has been the manufacture of more efficient photocells than those currently on the market; in other words, cells which increase the percentage of sunlight which is transformed into energy. To do this, it was essential to find more suitable materials for its production on a large scale (less scarce and less expensive) and more efficient in the absorption of sunlight wavelengths.
The Research and the Results
Now, a team of researchers from the MIT Mechanical Engineering Department have succeeded in developing a material which unites these conditions: a two-dimensional dielectric photonic crystal which is almost ideal for solar absorption. Notable among the characteristics which make it nearly perfect – apart from absorbing the appropriate light spectrum – are its capacity to withstand temperatures of up to 1,000 degrees centigrade (necessary to take the utmost advantage of the systems which concentrate sunlight using mirrors), the fact that it can be manufactured at low cost and on a large scale using current technology, and that its absorption capacity can be controlled very precisely. The material is made from a collection of nanocavities, and “you can tune the absorption just by changing the size of the nanocavities,” explains the main author of the article on the work, Jeffrey Chou. (The article has been published in the magazine Advanced Materials.)
The new material functions as part of a solar-thermophotovoltaic device (STPV): it converts sunlight into heat, which makes the material glow and, consequently, emit light which is then converted into an electrical current.
The team of researchers had previously been working on an STPV device made of hollow cavities, with only air inside. According to the MIT website, until then nobody had tried placing a dielectric material in such an environment. When they did the test, they noticed that it had some interesting properties to utilise solar energy, related to the correct absorption of light and its emission, which increased the STPV performance.
The research has received support from the Solid-State Solar Thermal Energy Conversion Center and the United States Department of Energy. Jeffrey Chou predicts that the system could be commercially viable within five years.