Optical efficiency of parabolic troughs with a secondary flat reflector; effects of non-ideal primary mirrors

Secondary flat receivers enhance concentration of parabolic trough collectors with cylindrical receivers. An increase of 70 % was possible for commercial primary mirrors while allowing the optical efficiency to be less sensitive to misalignments. Including the secondary flat reflector close to a parabolic-trough receiver may, however, reduce the optical efficiency of the trough due to the shadow projected. In this work ray-tracing simulations were conducted to compare the optical efficiency and the misalignment sensitivity of the standard absorber and its secondary flat reflector variation. In some cases, the required dimensions for the secondary flat reflector would make it a non-realistic solution due to the impossibility of encapsulating it within a glass cover. To overcome this issue, a truncated version of the secondary mirror is proposed and evaluated. A truncated secondary flat reflector will decrease the deleterious effects of the shading, enhancing concentration further but changing the tolerance to misalignments of the troughs. Analysis of two benchmark primary mirrors shows a worst-case scenario of optical efficiency reductions less than 3.5 and 2 %, with concentration increases of 20 and 80 % respectively. The reduction on absorber size offsets the lower optical efficiency by the reduction of radiation losses, especially for high-temperature applications.