Aerodynamics of new solar parametric troughs: Two dimensional and three dimensional single module numerical analysis

Abstract

Two new symmetric Non Imaging Parametric Trough Collectors (PmTC) with circular and flat evacuated receivers have been recently proposed with a potential improvement in the net concentration ratio relative to the thermodynamic ideal limit beyond 65% compared to commercial Parabolic Trough Collectors (PTC) while maintaining or increasing the rim angle. Both collectors are composed of a symmetrical parametric primary discontinuous reflector geometry and a secondary concentrator with potential to reduce the wind loads and effectively reduce the cost of the solar field. Computational Fluid Dynamics (CFD) has been used as “virtual” wind tunnel to compare the flow around a single model-scale module of the two PmTCs and the two commercial LS2 and LS3 PTCs, in a range of pitch angles. Two case studies - 2D and 3D simulations - have been analyzed. Wind turbulence intensity was not taken into account, as the aim is to compare qualitatively the aerodynamic behavior of the different collectors. Velocity vector fields, mean values of aerodynamic drag, lift and moment coefficients and flow patterns were computed. Results confirm that the PmTC with circular receiver behaves very similarly to the LS2 and LS3 geometries for the drag, lift and moment coefficients. The PmTC with flat absorber shows the worst performance showing more than 25% penalization in terms of maximum drag and moment values in comparison with the other three collectors. The 2D case study shows worse coefficients when compared to the 3D case by a factor of 2. Analyses of averaged velocity vector fields at mid-section show a smaller wake in the 3D case study for the four collectors and all pitch angles and a lower influence of the gap in the primary reflector. An additional comparison of the LS2 3D results with and without turbulence intensities - the latter results taken from an earlier work - shows very good agreement both for the mean lift and moment values and a 20% difference for the mean drag due to the fact that turbulence fluctuations over the mean velocity profile add more energy to the system. Further 3D CFD simulations with turbulence intensities for a complete solar field design are needed in order to evaluate the potential of both PmTC to effectively reduce the cost of the solar field. This work has permitted to advance in the understanding of the potential of non-imaging optics to generate new geometries able to improve thermosolar technology from an aerodynamic point of view.