One of the major hurdles in developing high-efficiency, long-lasting solar cells is keeping the cells cool, even in the scorching heat of the noontime sun, and scientists may have overcome this hurdle.
Researchers led by Shanui Fan, an electrical engineering professor at Stanford University in California, have found a way to let solar cells cool themselves by buffering away unwanted thermal radiation by adding a specialty patterned layer of silica glass to the surface of ordinary solar cells.
Today, solar cells are among the most promising and commonly used renewable energy technologies on the market. Even though they are readily available and easily manufactured, even the best designs convert can only convert a fraction of the energy they receive from the sun into usable energy. Part of this loss is the inevitable consequence of converting sunlight into electricity. A surprisingly troublesome amount, though, is due to solar cells overheating.
Under normal operating conditions, solar cells may easily reach temperatures of 130 degrees Fahrenheit (55 degrees Celsius) or more. These kinds of harsh conditions quickly diminish efficiency and can drastically shorten the lifespan of a solar cell. However, actively cooling solar cells – either by ventilation or coolants – would be very expensive and in opposition with the need to optimize exposure to the sun.
The design that is being proposed avoids these problems by taking a more elegant, passive approach to cooling. The researchers have found a way of redirecting unwanted heat – in the form of infrared radiation – from the surface of the cells, through the atmosphere and back into space, by embedding tiny pyramid and cone-shaped structures on an incredibly thin layer of silica glass.
Solar cells work by converting the sun’s rays into electrical energy. Photons of light pass into the semiconductor regions of the solar cells, and they knock off electrons from the atoms, thus allowing electricity to flow freely, creating a current. Silicon semiconductors are the most successful and widely used design; they convert less than 30 percent of the energy they receive from the sun into electricity. The energy that is not converted into electricity generates waste heat, which unavoidably lessens a solar cell’s performance. For every one-degree Celsius (1.8 degree F) increase in temperature, solar cell efficiency declines by about half of a percent.
“That decline is very significant,” said Aaswath Raman, a postdoctoral scholar at Stanford. “The solar cell industry invests significant amounts of capital to generate improvements in efficiency. Our method of carefully altering the layers that cover and enclose the solar cell can improve the efficiency of any underlying solar cell. This makes the design particularly relevant and important.”
Also, solar cells “age” more rapidly when their temperatures increase, and the rate of aging doubles for every increase of 18 degrees Fahrenheit. To passively cool the solar cells, allowing them to emit excess heat without expelling energy to do so, requires exploiting the basic properties of light as well as a special infrared “window” through Earth’s atmosphere. Different wavelengths of light interact with solar cells in completely different ways – with visible light being the most efficient at generating electricity while infrared carries heat more efficiently. Also, different wavelengths bend and refract differently, depending on the type and shape of the material that they are passing through.
These basic principles are harnessed in the newly proposed design to allow visible light to pass through the added silica layer unimpeded while improving the amount of energy that is able to be carried away from the solar cells at thermal wavelengths.
To test the new design, two different silica coverings were compared: one a flat surface approximately 5 millimeters thick and the other a thinner layer covered with pyramids and micro-cones just a few microns (one-thousandth of a millimeter) thick in any dimension. The size of these features was crucial. By precisely controlling the width and height of the micro-cones and pyramids, they could be tuned to refract and redirect only the unwanted infrared wavelengths away from the solar cell and back into space.
The next step is to demonstrate radioactive cooling of solar cells in an outdoor environment.
Source: Raman, Aaswath. “Self-cooling Solar Cells.” Self-cooling Solar Cells. Phys.org, 22 July 2014. Web. 22 July 2014.