Contactless two-stage line focus collectors

Abstract
Two new two-mirror solar collectors for tubular receiver, the Snail concentrator and the Helmet concentrator, are presented. The main feature of these concentrators is that they have a sizeable gap between the secondary mirror and the absorber, and they still achieve concentrations close to the thermodynamic limit with high collection efficiencies. This characteristic makes them unique and, on the contrary to the present two-stage designs, avoids the location of the secondary inside the evacuated tube. One of the differences between the Snail and the Helmet concentrators is that the last is symmetric (as the conventional parabolic trough) but the first is not. For an acceptance angle of α = ±0.73 degs and a collection efficiency of 96.8% (i.e. 3.2% of the rays incident on the primary mirror within the acceptance angle are rejected), the Snail concentrator and the Helmet concentrator achieve an average flux concentration of 91.1% and 72.8% of the thermodynamic limit, respectively. Moreover, both concentrators have also high rim angles of the primary mirror : ±86.2 degs (Helmet) and 98.8 degs (Snail). This is of interest for a good mechanical stability of the collector.