Design, optimization and thermodynamic analysis of a parabolic trough collector with varying cross-section for utilization in a solar-thermal system

One of the methods to utilize solar energy, which is the primary source of energy is concentrated solar power systems. Parabolic trough collectors are one of the most common systems. They consist a parabolic mirror which focuses sun rays to a focal line and an absorber placed along this line converts solar energy to thermal energy. These collectors are employed both for power and process heat generation. Different studies are being conducted to achieve higher thermal efficiencies which enable these collectors to become more popular. Most of the studies are focusing on either different absorber configurations or different heat transfer fluids to achieve higher heat transfer rates. On the other hand, the number of studies which are about reflector's structure is relatively low. In this study, a novel parabolic surface is defined. This surface contains a fixed focal line while the focal distance varies throughout the length of the absorber. A numerical method is developed to investigate the effect of this surface on thermal performance. Through a Matlab code, with the input of geometrical parameters, the surface is divided into smaller parabolic segments, and each elements' data of focal length and location is obtained. Then this data is applied to the SolTrace, an open source ray tracing software, to obtain the heat flux profile around the absorber. Then achieved heat flux applied to the ANSYS CFX, a computational fluid dynamics program, and flow and heat transfer simulations are conducted. Model's results are compared with the other results given in the literature, for validation. Also, grid independence studies are performed. Unlike the conventional parabolic reflectors, defined geometry is asymmetrical trough the length of the absorber thus flow direction becomes another design parameter. These flow configurations are named as higher flux at the inlet (HFI) and higher flux at the outlet (HFO). The defined geometry also holds an additional geometric factor, the ratio of focal lengths (k) at the ends. The effects of this parameter and flow direction on the flow properties were investigated. With increasing k, effects become more visible, and the flow direction determines if the impact is positive or negative on that characteristic.