Integration of phase changing materials in parabolic solar collectors and their effects

In this thesis, a new solar receiver with cavity, phase change material and insulation that can be used for parabolic trough collectors has been designed. In the first stage of the study, optical efficiency was studied, which primarily affects the overall efficiency of a collector system. The most important parameter affecting optical efficiency is the net amount of heat absorbed by the receiver where solar radiation is concentrated. In this context, three different geometries (triangle, rectangle and polygon), three different aperture width and height, and three different positions of the cavity receiver relative to the parabolic reflecting surface were taken as optimization parameters. The Response Surface Method was used within a design-of-experiment approach to evaluate the effects of these parameters and also to examine their impact on the radiative heat rate absorbed by the receiver. For optical analysis open-source numerical ray-tracing software SolTrace, accompanied with the Monte Carlo Ray Tracing method, was used to investigate the effects of these parameters. The results showed that the optimum cavity geometry was polygonal and the cavity depth and span are both equal to 0.05m. Moreover, it is found that the most effective parameter is the position of the cavity receiver and the optimum position was at the focal line of the parabolic concentrator. For the optimum cavity receiver design, the optical efficiency of the system was found to be 81.05%. In the new receiver design, verification studies of the new model were carried out with Computational Fluid Dynamics (CFD). In this context, two-stage validation was performed from the literature. Firstly, the non-uniform radiation map was integrated in a tubular solar receiver and thermal analyses were verified with an experimental study. Then, the Phase Change Material (PCM) melting analyses were verified with an experimental study in the literature. After the validation studies, the PCM integrated, self-insulating cavity receiver was modeled and the results were evaluated by performing CFD analyses. High density polyethylene (HDPE) with melting and freezing temperatures of 130 and 120°C, respectively, was used as PCM. According to the CFD results, PCM started to melt in the 13th minute. The complete melting time was 31st minute. This can also be called PCM storage time. The exit temperature of the water was 437.52K during the period until PCM was completely melted. After this stage, the radiation map on the cavity walls was removed for the case where solar radiation was cut off and the analyses continued again depending on time. In these analyses, it was seen that FDM started to solidify from the 5th minute and was completely solid at the 21st minute. FDM integrated self-insulating cavity solar receiver was compared with FDM integrated non-insulating cavity receiver under the same boundary conditions. While the fluctuation in water outlet temperature was only 17.5°C during the solidification period of FDM (16 minutes), this value was 58°C in the receiver without FDM integration. FDM integration to the solar receiver allowed the system to work more stably, especially during cloud transitions during the day, with low water outlet temperature fluctuations for minutes. It was also shown that hot water or steam could continue to be obtained from the FDM integrated receiver for a while longer in the evening hours when the sun completely stopped.