Towards a Carbon-Free Future of Solar Thermal with Nonimaging Concentrators

ABSTRACTTowards a Carbon-Free Future of Solar Thermal with Nonimaging ConcentratorsBy Yogesh Bhusal Doctor of Philosophy in Mechanical Engineering University of California, Merced Professor James PalkoDue to the pi-effect of concentration in tubular absorbers, the existing commercial single-stage concentrating parabolic trough collectors are limited to the concentration ratio of 20-30x and working temperature of 400 oC with thermal oils and 550 oC with molten salts. This work presents the design and development of a two-stage concentrated parabolic trough collector that will enhance the performance of existing commercial parabolic trough systems, by employing nonimaging secondary optics. The novel receiver developed in this work includes a nonimaging secondary concentrator designed to accept and further concentrate the wide-angle edge rays coming from the high rim-angle parabolic primary to the reduced size absorber. An optical program was developed to generate nonimaging secondary optics for a chosen absorber size and primary mirror parameters and is used to develop an optimized combination of absorber size and secondary based on a primary parabolic mirror that is used in this work. The optimized receiver tube with a 42 mm diameter absorber and secondary concentrator would boost the concentration ratio of the existing system (80 mm absorber) from 28X to 53X, surpasses the working temperatures from ~550 oC to >650 oC, and thus improve the solar-thermal-electric conversion efficiency. The novel receivers with secondary optics were manufactured in commercially produced size to demonstrate that they can replace current commercial receivers and use the existing primary mirrors. The optical and thermal design of the two-stage system with secondary optics was conducted, prototype receivers were assembled, and a 100-hours long heating experiment at 650 oC was conducted to measure the heat loss from the prototype receiver and its emittance throughout the test. The heat loss of ~1026 Watts/m and an emittance of <0.2 was measured from the prototype receiver at 650 oC for more than 100 hours and was compared with the measurements done by different institutions on commercial single-stage systems. The two-stage system designed in this work can achieve 70% optical efficiency provided the commercially available mirror (ρ=0.94), AR coated glass tube (τ =0.96), and absorber coating with (α=0.95). The employment of secondary optics also delivered almost uniform illumination around the absorber thereby requiring a much lower HTF flow rate to homogenize the circumferential temperatures at the absorber and eliminating the thermal stress induced deformation of the absorber experienced by single-stage concentrated absorbers. The thermal models predicted that the two-stage system could operate at appreciable solar to thermal efficiencies of >45% at a temperature of 700 oC with HTF like molten Chloride salts, Refractory particles, and Supercritical CO2. Multiple prototype receivers were mounted on a commercial primary mirror in series and On-Sun optical efficiency was measured