It is  very  difficult to obtain fluid temperatures in excess 
  of  150° C  using  even  the  most  sophisticated  flat-plate 
  collectors,  because  their  surface  thermal  losses  become 
  appreciable.  A  possible  and  practical  means of achieving 
  higher  temperatures is by  using concentrating type of solar 
  collectors.  This necessarily implies that practically all of 
  the  diffuse  component  of  solar  radiation  is lost to the 
  system. Therefore,  attempts  must be made to utilize as much 
  of the direct solar beams as possible. Hence,the concentrator 
  is normally  equipped with a tracking device which, in effect, 
  ensures that it "follows the sun" continuously.  The absorber 
  in this  case is located  close to the geometric focus of the 
  concentrator,  to  ensure  that  it  intercepts  most  of the 
  incident  direct radiation.  There are, in general, two types 
  of  concentrators: the  linear-focusing concentrators and the 
  point-focusing ones.  The former is generally equipped with a 
  single-axis  tracking  system, and the latter with a two-axis 
  system.  In the  former case, the absorber is a tube on which 
  the  solar  image shifts as a function  of the sun's position, 
  while in the  latter, the absorber covers the area around the 
  focal point. In practice, linear concentrations are generally 
  limited to a factor of   30 to 40, while point  concentrators 
  range from 500 to 2,000. For the two classes of concentrators, 
  the  efficiencies of direct  solar beam conversion is usually 
  in the order of  60-70 per cent.  Point-focusing  systems are 
  far  more  sophisticated  and  complex  than  linear-focusing 
  systems,  and  can  only be justified  when high temperatures 
  (3000C to 1,0000C) are to be reached.