With conventional solar panels, like the silicon panels in most solar installations, the photovoltaic (PV) cells can only convert a certain spectrum of light into electricity. A lot of it, particularly thermal heat, is lost. But now researchers at MIT created a solar thermal photovoltaic (STPV) device that absorbs a broader spectrum of the sun’s energy and converts it into electricity.
Traditional, one-layer PV can’t exceed converting more than about 32 percent of the sun’s energy into electricity. Multi-junction PV cells can exceed that limit, known as the Shockley-Queisser Limit. But even those devices, which use multiple layers of photovoltaic materials to absorb different wavelengths of sunlight have limits. The unique new device from MIT is able to convert the full spectrum of sunlight into heat and then convert that into electricity through a PV cell. s
It’s just the latest new solar technology from the university, which has developed a number of innovative solar materials. For instance, it recently produced solar cells light enough to float on a bubble.
MIT doctoral student David Bierman said that the new device could exceed the Shockley-Queisser Limit and could more than double it. Bierman along with professors Evelyn Wang and Marin Soljačić, MIT alumnus Andrej Lenert PhD ’14, MIT postdocs Walker Chan and Bikram Bhatia, and research scientist Ivan Celanovic published their research in Nature Energy May 23.
“We believe that this new work is an exciting advancement in the field,” Wang said. “We have demonstrated, for the first time, an STPV device that has a higher solar-to-electrical conversion efficiency compared to that of the underlying PV cell.”
The device the researchers created used nanophotonic crystals and carbon nanotubes to absorb all of the energy and heat from the sun. “The carbon nanotubes are virtually a perfect absorber over the entire color spectrum,” said Bierman. “All of the energy of the photons gets converted to heat.” The nanophotonic crystals are designed to convert all of that energy into the exact spectrum of light that match the PV cell’s peak efficiency. Since it’s operating as a thermal device Bierman also posited that it could operate briefly even if the sun is temporarily obscured by clouds.
The demonstration used a relatively low-efficiency PV cell, according to MIT and the resulting system efficiency was only 6.8 percent but it shows the potential of such applications. Ultimately such a device could be incorporated into a concentrating solar PV system where lenses or mirrors focus sunlight on a solar cell to increase its ability to absorb more sunlight. In this case the device would focus heat from the sun.Tweet