CONTACTLESS EFFICIENT 2-STAGE SOLAR CONCENTRATOR FOR TUBULAR ABSORBER

The design of a new type of two-mirror solar concentrator for a tubula r receiver, the XX concentrator, is presented. The main feature of the XX is that it has a sizable gap between the secondary mirror and the absorber and it still achieves concentrations close to the thermodynam ic limit with high collection efficiencies. This characteristic makes the XX unique and, contrary to current two-stage designs, allows for t he location of the secondary outside the evacuated tube. One of the XX concentrators presented achieves an average flux concentration within +/-0.73 deg of 91.1% of the thermodynamic limit with a collection eff iciency of 96.8% (i.e., 3.2% of the rays incident on the primary mirro r within +/-0.73 deg are rejected). Another XX design is 92.5% efficie nt and receives 95.1% of the maximum concentration. These values are t he highest reported for practical concentrators, to our knowledge. The gap between the absorber and the secondary mirror is 6.8 and 10.5 tim es the absorber radius for each concentrator. Moreover the rim angle o f the primary mirror is 98.8 and 104.4 deg in each case, which is of i nterest for the collector's good mechanical stability. (C) 1997 Optica l Society of America.