Secondary mirrors for homogenization of parabolic trough radiative flux distributions on cylindrical receivers

Abstract

For high-pressure applications, like direct steam generation or hydrothermal liquefaction, homogenizing the radiative flux on the tubular receiver of the parabolic trough concentrators can help avoid thermal stresses and potential material degradation. In the present study Monte Carlo ray tracing is used to analyze different secondary mirrors to enhance the flux uniformity level over the receiver tube of four parabolic troughs with different rim angles. Elliptical, V-shaped, Lambda-shaped (inverted V), and Compound Elliptical Concentrator (CEC) mirrors are considered. The design of these mirrors is optimized for each case using a Differential Evolution Algorithm. Solutions are sought with the maximum possible uniformity while keeping high optical efficiency. Results of the optical modeling show reductions in peak flux over the tube wall by approximately 60% when secondary mirrors are implemented with PTC systems. Furthermore, optical efficiencies of 90% and flux uniformity levels of 86.6% can be achieved by integrating the Elliptical and Lambda-shaped geometries as secondary mirrors. The effect of the secondary mirrors on the PTC efficiency for off-normal incidence was also studied. CECs are very good at increasing off-normal rays’ acceptance, followed by V-shaped mirrors, but at the cost of a worsened overall performance.