The design of a new type of two-mirror solar concentrator for a tubular 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 thermodynamic limit with high collection efficiencies. This characteristic makes the XX unique and, contrary to current two-stage designs, allows for the 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 efficiency of 96.8% (i.e., 3.2% of the rays incident on the primary mirror within ±0.73 deg are rejected). Another XX design is 92.5% efficient and receives 95.1% of the maximum concentration. These values are the highest reported for practical concentrators, to our knowledge. The gap between the absorber and the secondary mirror is 6.8 and 10.5 times the absorber radius for each concentrator. Moreover the rim angle of the primary mirror is 98.8 and 104.4 deg in each case, which is of interest for the collector's good mechanical stability.
