Experimental performance comparison and optical characteristics of aperture mechanisms for solar cavity receivers

Dynamic control of solar thermal processes is a critical bottleneck demanding to be addressed for the development of viable alternatives to traditional industrial processes that rely on high temperature process heat from fossil fuel combustion. Current technologies possess various concentrated solar power (CSP) control strategies to compensate for natural fluctuations of solar irradiation while some of the dynamic control problems are addressed by creative reactor design to capture solar energy more efficiently. In this paper, a solar receiver with two light entry controlling aperture mechanisms is proposed as a promising control method for manipulated radiation entry into the cavity. Design and motion characteristics of these aperture mechanisms govern the amount of flux intercepted by the receiver. Performance of these apertures in maintaining semi-constant temperature inside the receiver was tested numerically and experimentally for comprehensive explanation of the physics behind the results. An optical analysis was done by implementing two different Monte Carlo Ray Tracing (MCRT) approaches for comparison and verification of complex aperture designs. Total radiative losses were determined by considering the energy balance on the quartz window by taking the spectral-dependent optical properties of the window into account. A thermal analysis was done in conjunction with the optical analysis to demonstrate thermal-optic performance of both mechanisms. A 7 kW high flux Xenon bulb was optically modeled according to experimentally obtained radiation maps. The optical model was coupled as an input to a thermal computation with lumped parameter assumption. Numerical results were validated by comparison with experimental results for both steady and dynamic responses. Estimation of a high flux scenario at steady state as well as for a given DNI pattern were numerically predicted and comparisons with conventional control methods were made. Results demonstrated the potential of the variable aperture in terms of temperature control and receiver efficiency.