Full spectrum utilization for high-efficiency solar energy conversion

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 88-97).Today, two dominant strategies for solar energy harvesting exist: solar thermal and photovoltaic. Solar thermal energy harvesting offers the distinct ability to both utilize the full solar spectrum and provide dispatchable electrical power to the grid. By contrast, the generation of power via the photovoltaic effect can reduce the complexity of a system by eliminating moving parts. Conversion strategies which use both thermal and photovoltaic principles capitalize on the advantages of each. This thesis explores the potential of these technologies through both experimental and theoretical device-level studies. First, we explored solar thermophotovoltaic devices (STPVs) which convert broadband sunlight to narrow-band thermal radiation tuned for a photovoltaic cell. We demonstrated the highest STPV efficiency to date through the suppression of 80% of sub-bandgap blackbody radiation by pairing a one-dimensional photonic-crystal selective emitter with a tandem plasma-interference optical filter. We measured a solar-to-electrical conversion rate of 6.8%, exceeding the performance of the photovoltaic cell alone. Additionally, we show experimentally that STPVs can reduce the heat generation rates in the photovoltaic cell by a factor of two for the same power density. Next, we explored the use of spectral splitting as a different strategy to use both thermal and photovoltaic technologies. A model of an ideal solar spectral-splitting converter was developed to determine the conversion efficiency limit as well as the corresponding optimum spectral bandwidth of sunlight which should illuminate the photovoltaic cell. This bandwidth was also obtained analytically through an entropy minimization scheme and matches well with our model. We show that the maximum efficiency of the system occurs when it minimizes the spectral entropy generation. Beyond solar energy harvesting, we investigated the radiation dynamics of vanadium dioxide (VO2 ), which is of interest because of the abrupt decline of emittance at the insulator-metal transition at ~340 K. Negative differential emission is exploited to demonstrate thermal runaway of this system for the first time. These results are used to validate a radiation heat transfer model which explores the limiting behavior of a VO2 material set.by David M. Bierman.Ph. D