Turning sunlight into heat before generating electrical power can more than double efficiency of solar cells and break through an efficiency limit established more than 50 years ago.
In 1961, American physicists William Shockley and Hans-Joachim Queisser calculated the maximum theoretical efficiency of a solar cell. For a conventional single-layer cell made of silicon, this so-called Shockley–Queisser limit was set to about 32 per cent. That means that the maximum the technology can achieve is to convert 32 per cent of the oncoming sunlight into electricity. The rest is turned into unusable heat that only heats up the panel.
However, a team of engineers from the Massachusetts Institute of Technology has now invented a technique that could more than double this efficiency by first turning the light into heat and then into radiation again in a controlled manner. They developed a high-tech thermophotovoltaic layer that could be used to cover the conventional solar panel to improve its efficiency.
"We believe that this new work is an exciting advancement in the field," said professor Evelyn Wang, one of the authors of the invention, in an article in the latest issue of the journal Nature Energy.
The thermophotovoltaic layer is based on carefully engineered nanophotonic crystals that emit exactly-defined wavelengths of light when heated. The layer absorbs all the oncoming light and turns it into thermal radiation of exactly the right wavelengths that could be efficiently processed by the underlying silicon layer and turned into electricity.
"A lot of the work thus far in this field has been proof-of-concept demonstrations," said David Bierman, PhD student at MIT and lead author of the paper.
"This is the first time we've actually put something between the sun and the PV cell to prove the efficiency of the thermal system.”
The nanophotonic crystals of the thermophotovoltaic layer consist of carbon nanotubes that operate at temperatures of up to 1,000 °C.
"The carbon nanotubes are virtually a perfect absorber over the entire colour spectrum," Bierman said. "All of the energy of the photons gets converted to heat."
The system could use a conventional solar-concentrating system with lenses or mirrors that focus the sunlight to maintain the high temperature. An additional component, an advanced optical filter, would let through all the desired wavelengths of light to the PV cell, while reflecting back any unwanted wavelengths. These unwanted wavelengths would get re-absorbed, helping to maintain the heat of the photonic crystal.
The researchers said that in addition to the improved efficiency, the solar thermophotovoltaic solar panel would deliver electricity consistently regardless of brief weather changes.
In the experiment, the team used a PV cell with a very low efficiency of only 6.8 per cent. When paired with the solar thermophotovoltaic layer, the results were more than convincing.
The team first tested their device under direct sunlight and then with the sun completely blocked so that only the secondary light emissions from the photonic crystal were illuminating the cell. The results showed that the actual performance matched the predicted improvements.