Spatially Varying Extinction Coefficient for Direct Absorption Solar Thermal Collector Optimization

Direct absorption solar thermal collectors have recently been shown to be a promising technology for photothermal energy conversion but many parameters affecting the overall performance of such systems have not been studied in depth, yet alone optimized. Earlier work has shown that the overall magnitude of the extinction coefficient can play a drastic role, with too high of an extinction coefficient actually reducing the efficiency. This study investigates how the extinction coefficient impacts the collector efficiency and how it can be tuned spatially to optimize the efficiency, and why this presents a unique design over conventional solar thermal collection systems. Three specific extinction profiles are investigated: uniform, linearly increasing, and exponentially increasing with the exponentially increasing profile demonstrating the largest efficiency improvement. [DOI: 10.1115/1.4003679]