A team at MIT has discovered a way to harvest solar energy more efficiently and potentially on demand. The new solar thermophotovoltaic (STPV) device combines photovoltaic (PV) systems, which turn sunlight directly into electricity, and solar thermal systems, which allow delayed use of energy since heat can be stored more easily than electricity. A nanophotonic solar thermophotovoltaic device is composed of an array of multiwalled carbon nanotubes as the absorber, a one-dimensional silicon/silicon dioxide photonic crystal as the emitter, and a 0.55-eV photovoltaic cell. Courtesy of John Freidah. In contrast to a conventional silicon-based solar cell, whose bandgap misses many wavelengths of light, the STPV offers more broadband absorption of sunlight. The device also offers scalability and compactness because it is based on existing chip-manufacturing technology, and its reliance on heat offers ease of energy storage. The team inserted a two-layer absorber-emitter device between sunlight and a conventional PV cell. One layer consists of multi-walled carbon nanotubes, which absorb the light's energy and convert it to heat; the other layer is made of photonic crystals. When the two layers come together, the nanotubes heat the photonic crystal until it glows and emits light of a particular wavelength, which in this case is tuned to match the bandgap of the PV cell mounted nearby. Optical image of the vacuum-enclosed device illustrating the energy conversion processes in a nanophotonic solar thermophotovoltaic device. Sunlight is converted to useful thermal emission, and ultimately electrical power, via a hot absorber-emitter. Courtesy of MIT researchers. Simulated sunlight was used, and the team found that its peak efficiency came when its intensity was equivalent to a focusing system that concentrates sunlight by a factor of 750. This light heated the absorber-emitter to a temperature of 962o C. While previous experiments have been unable to produce an STPV device with an efficiency of greater than 1 percent, this new device shows a measured efficiency of 3.2 percent. With further work, the team says, it could reach 20 percent efficiency. Ongoing improvements will be made to the device, researchers say, including the size of the test objects. Since the absorber-emitter relies on high temperatures, its size is crucial: The larger an object, the less surface area it has in relation to its volume, so heat losses decline rapidly with increasing size. Subsequent tests will be performed on larger surface areas. The article was published last week in Nature Nanotechnology. For more information, visit www.mit.edu.