August 21, 2014 - One of the biggest complaints about solar energy is that the setups aren’t very attractive. A team of material engineers at Michigan State University have developed solar concentrators that are completely transparent, which means they can be used on windows without disturbing the view or even on smartphone screens. The research was led by Richard Lunt and the paper was featured on the cover of the journal Advanced Optical Materials.
Solar concentrators are used to lens sunlight, focusing it on a small area to maximize the amount of light, maximizing the amount of electricity generated. This is similar to using a magnifying glass to set leaves on fire. These can be quite large and a bit of an eyesore, as they are very much function over form. While many people have been attempting to create a see-through lens for years, the balance between efficiency and transparency has been hard to come by, with most prior attempts resulting in a colored product.
"No one wants to sit behind colored glass," Lunt said in a press release. "It makes for a very colorful environment, like working in a disco. We take an approach where we actually make the luminescent active layer itself transparent.”
The secret to their success was developing a system that could operate at wavelengths outside of the visible spectrum. This was accomplished by using organic luminophores, which are compounds that are responsible for luminescence. The concentrator absorbs light at certain wavelengths, and then is able to transmit them in another. Because the visible spectrum isn’t involved, the researchers were able to have a finished product as clear as glass.
"We can tune these materials to pick up just the ultraviolet and the near infrared wavelengths that then 'glow' at another wavelength in the infrared," he explained.
Rather than concentrate the light directly over the solar cell (like holding a magnifying glass over a leaf), the infrared glow is transferred to the solar cells at the perimeter of the device, where it is converted into electricity. With all of the action happening along the edge, there are incredible opportunities to integrate the technology nearly anywhere.
This technology is still new, so there are efficiency concerns that need to be addressed before it can be scaled up. Some concentrators that aren’t transparent are operating at an efficiency of about 7%, while this currently is nearing 1%. The researchers stated that they hope to surpass 5% in time. Though the goal is technically less than colored concentrators, a transparent device would have more possible applications, making it a much more practical option.
"It opens a lot of area to deploy solar energy in a non-intrusive way," Lunt continued. "It can be used on tall buildings with lots of windows or any kind of mobile device that demands high aesthetic quality like a phone or e-reader. Ultimately we want to make solar harvesting surfaces that you do not even know are there."