January 20, 2014
Nanotubes Let Solar Cells Capture Heat And Light
[ Watch the Video: Tapping The Sun's Energy In New Ways ]
Like a beautiful piece of music being played on an out-of-tune instrument, conventional photovoltaic material (PV) used to capture solar energy is slightly out of tune with natural sunlight. A new system being developed by researchers at MIT looks to correct this dissonance by using sunlight to heat a high-temperature material that then radiates an optimal wavelength of energy into a conventional photovoltaic cell.
According to the team’s report in the journal Nature Nanotechnology, the intermediate step improves on existing technology because it captures wavelengths of sunlight that ordinarily go to waste.
For the energy of a photon to be converted into electricity, the photon’s energy level must match that of a characteristic of PV material called a bandgap. To address that limitation in silicon and other PV materials, the MIT team placed a two-level absorber-emitter apparatus — made of carbon nanotubes and photonic crystals — between the incoming sunlight and the PV cell. This intermediary material gathers energy from a wide spectrum of sunlight, which causes it to heat up. Once the material begins to glow red hot, it gives off infrared light calibrated to match the bandgap of its PV cell.
“This is the first time that we’ve been able to incorporate nanophotonic surfaces into the absorber-emitter,” said Andrej Lenert, a PhD candidate in MIT’s Department of Mechanical Engineering. “In our experiment we start with a light source that simulates the solar spectrum and we focus it down to high solar intensities. This light is absorbed and turned into heat. It results in thermal re-radiation towards the PV cell.”
This so-called solar thermophotovoltaic (STPV) system could offer a way to skirt a theoretical limit of 33.7 percent on the energy-conversion efficiency of semiconductor-based solar energy devices, the researchers said.
“We’ve been able to demonstrate about three times better efficiencies than what’s been previously demonstrated experimentally and we think near-term that we can reach close to 20 percent efficiencies, which is just relying on existing components that we have used, but just scaling up and introducing some additional filters,” said study author Evelyn Wang, associate professor of mechanical engineering at MIT. “Once you reach about 20 percent conversion efficiencies… you become competitive with photovoltaics.”
The researchers said the two-layer absorber-emitter material is a major factor in their improvement on previous STPV systems. The outer layer of multi-walled carbon nanotubes is bonded firmly to a layer of a photonic crystal, which is specifically designed to glow with a peak-intensity light that is mostly above the bandgap of the adjacent PV cell, meaning nearly all of the energy gathered by the absorber is then converted into electricity.
The team said their system has another advantage over conventional PV technology: delayed use of captured energy because heat can be more easily stored than electricity.
“This work is a breakthrough in solar thermophotovoltaics, which in principle may achieve higher efficiency than conventional solar cells because STPV can take advantage of the whole solar spectrum,” noted Zhuomin Zhang, a professor of mechanical engineering at the Georgia Institute of Technology who was not involved in this research. “This achievement paves the way for rapidly boosting the STPV efficiency.”