Two-dimensional transient heat transfer and optical analysis of a solar receiver

Transient nature of solar radiation creates challenges in operating solar reactors requiring semi-constant cavity temperatures. A promising approach to overcome this problem is to implement a variable aperture. This paper presents a heat transfer model of a solar reactor featuring a variable aperture. The model is an in-house developed code comprising transient two-dimensional heat transfer analysis. The model is coupled with an optical analysis based on the Monte Carlo Ray Tracing (MCRT) or the Radiosity Net Exchange (RNE) method to simulate radiation within the receiver. The model is also coupled with a fluid dynamics analysis based on implementing a staggered grid system and the SIMPLE algorithm to obtain the velocity field of the fluid flow. Validation of the model was made through experimental results obtained using a 7 kW high flux solar simulator with an input current of 155 A and a fully opened aperture at 60 mm radius. Results showed satisfactory accuracies given an experimental maximum uncertainty of ± 8.6°C. However, the MCRT method provided more accurate results. The difference between the experimental and the model's steady state temperature values had an average of 6.3°C and 11.8°C for the MCRT and RNE methods, respectively. Based on the temperature distribution results, it was noted that the RNE method fails to predict accurate radiation distribution where two surfaces meet at perpendicular angles. Finally, the in-house code was used to determine an optimum aperture radius per highest possible exhaust temperature value which was 52.5 mm for the operating conditions outlined above.