Solar Energy, Vol.159, 20-36, 2018
Aperture size adjustment using model based adaptive control strategy to regulate temperature in a solar receiver
One of the main challenges of solar thermal technology is the intermittency of solar radiation which adversely affect temperature stability of the solar receiver. A promising technique to tackle this problem is the use of a variable aperture mechanism to regulate the light entry into solar receiver. Efficiency analysis confirms the advantage of this control technique over shutter adjustment method, which is also based on regulation of solar radiation entry. In order to regulate the temperature in a closed loop circuit based on aperture size adjustment, a model based control strategy was developed. To show the robustness and comprehensiveness of this control strategy, it was applied to a cavity receiver heated by two different radiative heat sources demonstrating the applicability of this control strategy consistently in most commonly practiced solar thermal systems. The first heat source studied is a solar furnace housing a parabolic dish, whereas the second one is a high flux solar simulator. For each radiative heat source, flux entering the receiver was determined using Monte Carlo ray tracing (MCRT) method. MCRT model was then coupled with energy balance equations to derive numerical model describing dynamic temperature variation in solar receiver. Comparison of simulated and experimentally measured temperatures showed appreciable accuracy of the dynamic model. Simulation results of the numerical model were then used to identify a nonlinear adaptive model for use in designing a model predictive controller (MPC). Parameters of the adaptive model were updated continuously to make the controller more robust against model mismatches and external disturbances. Simulation results for both radiative heat sources showed that the proposed controller yields faster response with less overshoot compare to proportional integral derivative (PID) controller. Results showed that this controller exhibits robust performance during sunrise and sunset times as well as passing clouds conditions where significant fluctuations in solar radiation is experienced.