Effects of reflector designs and material properties on the efficiency of solar vacuum-tube collectors: a ray-tracing study

The optical performance of evacuated tube-in-tube solar collectors is inves tigated. A ray-tracing method is applied to calculate the local and the tot al transmittance-absorptance product (tau alpha) in-dependence of the incid ence angle Theta. Using the commercial Sydney collector as a starting point the reflector design is optimized. The suggested reflectors improve (tau a lpha) significantly upon the original one for Theta<10 degrees and Theta=40 ...70 degrees. Further, the losses due to non-ideal optical mate rial param eters and the effectiveness of additional plane booster mirrors are determi ned.