Friday, July 25, 2014

Self-Cooling Solar Cells Yield Better Output And Longer Lifespan

When solar panels operate under full sun they can easily reach temperatures more than 60 degrees Celsius. High temperature will reduce panels' output and shorten their lifespan. For typical silicon solar panel, every degree increases in temperature, about 0.5% drop in output power. For example, if a rated 250 watts of solar panel is operating at Kota Kinabalu, Malaysia, you will only get about 206 watts of output. Using coolant or ventilation to cool the panels is expensive and energy intensive.

Scientists from Stanford University, U.S.A., have developed a passive cooling technology by adding a tiny pyramid-shaped silica glass structures to the surface of ordinary solar cells. The pyramid will help to shepherd away unwanted heat and reduce panel's temperature.

"Silica is transparent to visible light, but it is also possible to fine-tune how it bends and refracts light of very specific wavelengths,” said Prof. Fan, who led the research. “A carefully designed layer of silica would not degrade the performance of the solar cell, but it would enhance radiation at the predetermined thermal wavelengths to send the solar cell’s heat away more effectively.”

The team has tested different shapes and sizes of the structure. They were able to fine tune the shapes to refract and redirect only the unwanted infrared wavelengths (wavelengths that will heat up the panels) away from the solar cell and back out into space.

Their next step is to demonstrate radiative cooling of solar cells in an outdoor environment.

Thursday, July 17, 2014

New Finding Could Increase Solar Efficiency By 30%

Solar industry is a non-stopping battle ground of price and performance. Now, scientists from University of California, Riverside, U.S.A., has found a way to use one photon to generate 2 electrons, which theoretical could boost solar efficiency.

What is photon? Photon is an elementary particle of light. When we see different color of light it is simply photon with different wavelength and energy. When light, or photon, strikes the solar cell, they will penetrate and be absorbed by the solar material. Electron will be generated and electricity will flow when the cell is connected to a load.

Conventionally, one photon will only generate one electron. But newly discovery could double the generation. The technique is called singlet fission, in which an initially excited singlet state spontaneously splits into a pair of triplet excitons, boosting the overall solar efficiency by as much as 30%.

Here’s the Bardeen lab’s diagram of how singlet fission works to spontaneously split into two triplets, effectively dodging the efficiency barrier of the Shockley-Queisser limit.

“The exact mechanism is unknown, but it does happen quickly—at the sub-nanosecond timescale—and with high efficiency,” said Prof. Bardeen, who led the research.

Prof. Bardeen cites recent work at MIT that has already demonstrated an organic photovoltaic cell with more than 100% external quantum efficiency based on this effect. Prof. Bardeen believes similar improvement can be observed in inorganic semiconductors.

The next move of the team is to find new materials that exhibit singlet fission, figuring out how to turn the triplet excitons into electricity.