By Sean Sullivan, The University of Texas at Austin, Nanomaterials and Thermo-Fluids Laboratory and MRS Student Chapter
The Big Picture – Solar Thermophotovoltaic Devices
Traditional methodologies for extracting energy from the sun typically focus on two seemingly disparate aspects. The first is light—using solid state devices to convert photons into an exploitable electric potential. The second is heat, which is used to drive mechanical engines with the help of a working fluid. Each of these technologies has its limitations: a single-junction photovoltaic cell, for example, may only access a small portion of spectral sunlight, thereby bounding its maximum efficiency with the Shockley-Queisser limit. Past efforts have attempted to combine both the thermal and the photonic characteristics of solar radiation to enhance energy extraction. These solar thermophotovoltaic (STPV) devices typically consist of a light concentrator to focus and intensify sunlight thousands of times, a thermal absorber-emitter material, and a photovoltaic cell. Using a similar design with updated materials, a team from MIT has recently developed an STPV that bests previous generations of devices while reducing the required amount of light concentration1.
More in Depth – Improving STPV Efficiency with Nanophotonic Materials
In the 1 cm2 device, sunlight concentrated 750x heats the absorber layer, consisting of an array of multi-walled carbon nanotubes, to around 1000°C. This stored thermal energy is then released with the aid of a silicon-based emitter. By creating a photonic crystal consisting of alternating Si/SiO2 layers, the peak blackbody emittance energy can be tuned to a level just above the bandgap of the InGaAsSb solar cell (0.55 eV). This effectively takes advantage of full spectral sunlight and “funnels” it into a more usable energy distribution for the chosen photovoltaic. With such a device, the MIT team has demonstrated an energy conversion efficiency of 3.2%, a three-fold improvement over previous STPV devices. However, the authors believe there is plenty of room for improvement: device efficiency scales with size, so by increasing the active area to 10 cm2, they estimate an improvement to 20% efficiency. Other challenges must be overcome, such as the cost of manufacturing IR-active photovoltaic cells, ambient heat loss, and the need for light concentration. Regardless of the feasibility of STPV devices, this work represents a major step forward in the field. With further optimization, STPV devices could present another approach to surpass the Shockley-Queisser limit.
The paper detailing the novel STPV device
1. Lenert, A. et al. "A nanophotonic solar thermophotovoltaic device," Nat. Nanotechnol. 9, 126-130(2014)
Previous generation STPV with conversion efficieny of 0.8%
2. Datas, A. & Algora, C., "Development and experimental evaluation of a complete solar thermophotovoltaic system," Prog. Photovolt. Res. Appl. 21, 1025–1039 (2012).
Theoretical studies on the optimization of STPV devices
3. Bermel, P. et al. "Design and global optimization of high-efficiency thermophotovoltaic systems," Opt. Express 18 Suppl 3, A314–34 (2010).
4. Rephaeli, E. & Fan, S., "Absorber and emitter for solar thermo-photovoltaic systems to achieve efficiency exceeding the Shockley-Queisser limit," Opt. Express 17, 15145 (2009).
Efficient radioisotope thermophotovoltaic for spacecraft
5. Teofilo, V. L., Choong, P., Chang, J., Tseng, Y.-L. & Ermer, S., "Thermophotovoltaic Energy Conversion for Space," J. Phys. Chem. C 112, 7841–7845 (2008)