Solar cells add another function to an already long list of beneficial uses – they can transform heat into usable energy, a study has revealed.

Humans are on the lookout for more sources of renewable energy. In the past, scientists have been able to develop a solar panel that converts raindrops into electricity. Now, researchers from Massachusetts Institute of Technology (MIT) have come up with a device inspired by the solar cells' ability to surpass a theoretical limit on the amount of sunlight they can transform into electricity.

Solar cells were predicted to have an absolute theoretical ceiling, also known as Shockley-Queisser Limit, that defined the capacity of solar cells to convert energy. For example, a one-layer silicon cell has an upper limit of approximately 32 percent. To increase this capacity, several layers of cells would be added. An alternative method is to convert the energy first before it is allowed to generate electricity. The latter technique, also called solar thermophotovoltaics (STPVs), is what the researchers developed.

While the known theoretical energy cap still exists in conventional photovoltaics (PV), researchers explained that there is the possibility that that particular limit can be breached by the STVPs.

In explaining the theory, lead study author David Bierman, who is a doctoral student at MIT, calculated that the STPVs, which use traditional solar cells coupled with sheets of high-tech materials, can even exceed the efficiency limit by twofold.

For their study, the researchers demonstrated that the STPV device indeed has the ability to convert solar energy into electricity at a greater degree versus a device that uses only a low-efficiency PV cell.

How Does It Work?

The principle of STPVs works by having a transitional component that first absorbs all the heat and energy to a temperature that would initiate heat radiation. By altering the configuration and materials of the added layers, the radiation can be emitted via accurate wavelengths of light that the solar cell can gather. This method makes the solar cell perform efficiently while reducing the solar cell heat generation.

The important part of the device is the use of nanophotonic crystals that have the ability to discharge determined wavelengths precisely when the cells are heated.

During demonstration, the researchers integrated the nanophotonic crystals within the system via vertically aligned carbon nanotubes and subjected to 1,000 degrees Celsius (1,832 degrees Fahrenheit) of heat. During the application of heat, the crystals emitted a narrow band of wavelength that perfectly matched the corresponding PV cell that captured and converted it to electricity.

Bierman explained that the carbon nanotubes absorb full solar spectrum that all of the captured photons convert into heat, complementing the PV cell's efficiency limit.

The method also uses a traditional solar-concentrating system equipped with mirrors that direct the sunlight to keep the high temperature focused. An advanced optical filter then allows all the preferred wavelengths of light to the PV cell, while unnecessary wavelengths were reflected back and re-absorbed. The reabsorption of these unwanted wavelengths maintains the photonic crystal's temperature.

Through this operation, the photonic crystal creates emissions via heat instead of light, which means that environmental changes would not affect its efficiency. Solar power can then be used continuously 24 hours, 7 days a week - no energy goes to waste.

"This is the first time we've actually put something between the sun and the PV cell to prove the efficiency," said Bierman. "We showed that just with our own unoptimized geometry, we in fact could break the Shockley-Queisser limit."

The team is now looking at expanding their research to look for methods to scale up their experimental unit.

Purdue University's Electrical and Computer Engineering assistant professor Peter Bermel acknowledged that the device indeed has significant value to the scientific community. It is the only device that used solar TPV, which can be successfully replicated outdoors.

"It also shows that solar TPV can exceed PV output with a direct comparison of the same cells, for a sufficiently high input power density, lending this approach to applications using concentrated sunlight," said Bermel.

The study was published online in Nature Energy on May 23.

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