The new term ‘medium temperature collectors’ is used to address collectors with operating temperatures in the range of 80°C to 250°C. The aim is to develop collectors that are suitable for applications in this temperature range in which up to now only very limited experience exists. What are the activities that are investigated in the Task 33/4? To give a short overview, three categories may be introduced:

• improved flat plate collectors

• low concentration collectors (flat plate or vacuum tube collectors)

• small parabolic trough collectors.

Improved flat plate collectors

Especially for applications in the temperature range of 80 to 120°C there exists a number of possibilities to improve flat plate collectors such that they become suitable for those applications. In order to achieve this, it is necessary to reduce the collector heat losses without sacrificing too much of the optical performance at the same time. To mention some possibilities: hermetically sealed collectors with inert gas fillings, double glazed flat plate collectors, low concentration collectors using reflectors, or even vacuum flat plate collectors.

Figure 1 shows estimated efficiency curves of single, double and triple glazed flat plate collectors when newly developed anti-reflection glazings (‘AR-glass’) are used.

First of all, it is important to point out that there is a big potential for improvement of flat plate collectors if the normal solar glass is replaced by AR-glass. This improvement is independent of the operating temperature (comparison standard flat — plate collector to 1AR). Secondly, it is interesting to note with respect to double glazings that the 2AR collector reaches the same no-value as the standard flat-plate collector. Therefore, the complete efficiency curve of the 2AR-collector is above the standard flat-plate collector. Especially for higher operating temperatures the advantage of 2AR collectors is promisingly large. At AT/G=0.1 (K m2)/W the efficiency of the 2AR collector (in Fig. 1) is better than the standard flat-plate collector by more than 33% (relative)! These results from estimated efficiency curves have been verified by experiments and measured efficiency curves, by Fraunhofer ISE /1/. This temperature range of 80° to 150° is important for new fields of applications such as solar cooling and air conditioning, sea water desalination and other process heat applications like cleaning, washing, dying etc. Figure 2 shows the collector field of a solar driven desalination system. Fraunhofer ISE presently develops a desalination technology based on membrane distillation which is especially suitable the operating conditions for solar thermal driven desalination application /2/. Pilot plants will be
installed in Jordan, Egypt, Morocco and Spain within the frame of projects funded by the EC. It is also considered to use 2AR collectors in some of these plants.