High-Efficiency Solar Thermophotovoltaic Systems Based on Spectrally Selective Emitters

Recent advances in low-bandgap semiconductor materials for solar cell technology has generated renewed interest in the development of solar thermophotovoltaic (STPV) devices, given their potential for high-efficiency solar energy harvesting by utilizing the full solar spectrum. STPV systems aim to achieve efficiencies higher than the Shockley-Queisser (SQ) limit for photovoltaic conversion through the use of an intermediate element that absorbs the broadband sunlight and re-emits the absorbed energy as narrow-band thermal radiation tuned to the characteristic spectral response of the solar cell. Spectrally selective absorbers and emitters can greatly enhance the STPV system efficiency by maximizing the absorption and suppressing the emission of sub-bandgap and excessive energy photons. STPV is a promising technology for building scalable, reliable, and maintenance-free high-energy-density electrical power generator systems. These features along with their versatility of utilizing numerous input sources of heat, such as concentrated solar energy, industrial waste heat, radioisotope heaters, combustible materials, etc., make thermophotovoltaic (TPV) systems very appealing for many terrestrial and space applications.
This dissertation study focused on the design, optimization, and fabrication of a fully operational high-efficiency planar STPV system comprising GaSb cells and spectrally selective emitters. Designing an efficient STPV system is a balancing act and requires a comprehensive understanding of all the loss mechanisms at various stages of the energy transport. A combination of thermodynamic modeling and transfer matrix method (TMM) simulation was used to formulate a detailed-balance analysis required for the design and optimization of high-performance selective surfaces that are essential components of efficient STPV systems. Significance of determining the optimal values of the emitter temperature, spectral cut-off wavelengths, absorber-to-emitter area ratio, and emitter bandwidth for global system optimization was discussed. The relevance of photon recycling on both the absorbing and emitting sides for achieving high thermal extraction and overall system conversion efficiency was investigated.
Utilizing the knowledge gained from the simulation study, a high-efficiency planar STPV system was designed, fabricated and evaluated. A micro-textured selective absorber and a Si3N4/W/Si3N4 coated selective emitter were fabricated on a W substrate. The absorptivity of 0.92 was measured for the textured absorber for wavelengths below 1 m. The selective emitter exhibited a high surface emissivity in spectral regimes matching the quantum efficiency of the GaSb solar cells. Photon recycling was incorporated to suppress the thermal emission loss within the system. The performance of the STPV system was evaluated using a 300 W continuous-wave laser as a simulated source for incident radiation. An output power density of 1.75 W/cm2 and a system efficiency of 8.6% were recorded at the operating system temperature of 1670K. This experimental efficiency is higher than those of previously reported STPV systems. Various optical and thermal losses occurred at multiple stages of the energy conversion process were quantified.
This dissertation also studied the dependence of the surface spectral absorptivity upon temperature and quantified its impact on the performance of the selective emitter. The long-term thermal stability of the selective surfaces was also assessed. Combining the simulation and experimental results, essential guidelines to further improve the system efficiency were also provided.