Radiant Energy Spectrum Converters for Solar Energy Harvesting

Solar energy is a particularly attractive form of renewable energy because it is widely available and the amount of solar energy received on Earth each year is ~3.5 x 10^6 EJ, which is more than 7000 times greater than the annual global energy consumption. However, solar energy remains largely untapped because it is a broadband, intermittent and sparse resource, making it difficult to harness. Herein, the implementation of newly designed optical cavities, referred to as Radiant Energy Spectrum Convertors (RESC), in the form of ellipsoids, spheroids and/or paraboloids is presented for broadband solar energy harvesting and conversion applications. In this thesis RESC structures are designed and their application in four broadband solar energy harvesting applications is numerically analyzed: 1) photobioreactors, 2) agri-voltaics, 3) hybrid solar lighting, and 4) Solar Thermophotovoltaics (STPV). 1) The RESC structure in the photobioreactor is a luminescent solar spectrum splitter that partitions the solar irradiance into photosynthetically active radiation (PAR) and photosynthetically inactive radiation (non-PAR) to simultaneously power algae cultivation systems and PV cells, respectively. Results show that a RESC structure enables 0.25 MJ of electric power generation in a photobioreactor with a projected area of and volume of 0.2 m^2 and 0.2 m^3, respectively. 2) The RESC structure implemented in agri-voltaics is an elliptic array luminescent solar concentrator for combined power generation and microalgae growth with the similar concept of partitioning solar irradiance into its PAR and non-PAR components. Considering the combined effects of emission, transmission and surface scattering losses, numerical results show the optical efficiency of the elliptic array luminescent solar concentrator (LSC) is 63%, whereas in comparison the optical efficiency for a conventional planar LSC of the same size is 47.2%. 3) The RESC structures used in hybrid solar lighting applications are based on luminescent solar spectrum splitters that partition the incoming solar irradiance into its visible and non-visible components to simultaneously power fibre optic lighting system and PV cells, respectively. Numerical analysis shows that the non-visible portion of the solar irradiance can be converted to electricity with an efficiency of 13.4% using a double-junction PV cell within a RESC structure. 4) RESC structures implemented into STPV systems are based on highly specular IR reflective optical cavities in the form of oblate and prolate spheroid structures to enhance the power density and photoconversion efficiency of the STPV systems. The optical cavity partially encloses a solar receiver that is located at the focal point of an ellipsoid/paraboloid. Concentrated solar radiation is absorbed by the receiver, which functions as both an absorber and an emitter. Radiation is emitted from the emitter, and the internal surface of the cavity is able to reflect a large portion of this emitted radiation either back to the emitter or to a PV cell. Emitted radiation that is returned to the emitter is referred to as "photon recycling". A high degree of photon recycling can be used to achieve high emitter temperatures, which enhances the performance of STPV systems. The results presented in this thesis show that optical structures in the form of spheroids and paraboloids can be used to partition, control and harness broadband solar energy to simultaneously provide for multiple applications, which ultimately increases overall solar energy conversion efficiencies.