By far most of the stationary solar collectors are flat-plate 
  type. Simple  flat-plate collectors have been able to achieve 
  temperatures of  400 - 800 C with  conversion efficiencies of 
  between  40 and  80 per cent.  To achieve higher temperatures, 
  the  absorber  is  often  covered  with a  special  selective 
  coating.  For temperatures in excess of about 1000 C, special 
  designs of stationary  collectors, such as the evacuated tube 
  collectors, have been designed and tested.  Advantages of the 
  flat-plate collector are its simplicity and the absorption of 
  beam, as well as, diffuse  radiation.  Disadvantages  are the 
  low  maximum temperatures that can be reached (40-1200 C) and 
  the rather low efficiencies at "elevated" temperatures.
  Constructed in a  modular form (units 1-4 m2), the flat-plate 
  collectors can be coupled to make collector arrays of various 
  sizes,  depending  on  the  application.  However, there is a 
  maximum  size  for  collector  arrays, owing to their thermal 
  losses. It is therefore possible to design a heating facility 
  for a block of houses, but it would  be  necessary to install 
  several units for a whole agglomeration.