Potential improvements in the optical and thermal efficiencies of parabolic trough concentrators

Abstract

A detailed 3D heat transfer model is used to analyze the improvement potential of a typical parabolic trough concentrator (PTC) system with step-wise idealizations of system components. Sigmoid functions are used to idealize and optimize the optical behavior of the absorber tube selective coating. Reflectance, absorptance, and transmittance behavior is evaluated for the glass envelope, assessing the potential performance of a system with an ideal selective glass. Optical properties of the primary concentrator mirror, such as reflectivity as well as tracking and surface errors, are successively idealized to reveal the differences between ideal and real concentrators. In addition, the effect of the supporting structure is analyzed by reducing the number of heat collection elements (HCE) per mirror module, lowering the shaded area between HCEs, and removing the structure altogether. Applying these component idealizations individually yielded increases in thermal efficiency between 4.3% and 7.3% compared to a selected benchmark PTC design, while a combination of all idealizations resulted in an increase of 23%. The analysis of an alternative PTC design has shown that the idealization approach is robust in that it predicts similar increases in thermal efficiency. In a second step, several secondary mirror designs are evaluated and optimized, including a partially reflective glass surface, insulation in the vacuum annulus with reflective surfaces, as well as aplanatic mirror and tailored secondary designs. The analysis of systems with secondary optics revealed potential increases in thermal efficiencies between 0.8% and 1.6% compared to the benchmark design.