Figure 1 summarises the situation until recently in a simplified way: There existed well developed collector technologies (flat-plate and vacuum tube collectors) for systems that operate at collector temperatures up to 80°C. And since many years large parabolic trough collectors operating at about 400°C are available. But what is an appropriate collector technology in the temperature range between these two established technologies? The development of suitable process heat collectors in this temperature range (sometimes called ‘medium temperature collectors’ for this reason) is important for new developments and further diversification of applications of the solar thermal technology. The practical experience up to now is only limited to and concerns mainly the lower temperature range of up to 80°C. Solar systems which deliver heat for washing and cleaning processes in the food and beverage industry may be mentioned, see [2]. Important and very interesting fields for collector applications at higher operating temperatures are solar air conditioning and cooling [3], solar sea water desalination [4] and water purification, besides industrial process heat.

It has to be mentioned here that well developed evacuated tubular collectors exist which can be regarded as process heat collectors for applications up to about 160°C with respect to their thermal performance. But also those are not often used up to now for the application in process heat systems.

Partly this is due to the fact that other additional conditions which are often or generally met in industrial processes are of concern, such as the large size of the systems, costs, stagnation times and

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Figure 1: What is the appropriate collector technology for the medium temperature range from 80°C to 250°C? Experience from concentrating and non-concentrating collector technology will merge in the development of new process heat collectors for operating system temperatures above 80°C.

stagnation behaviour of large collector fields, appropriate solar loop fluid with sufficient temperature resistance etc. The temperature limit for water-glycol mixtures as solar fluid lies in the range of 150°C. With respect to this topic, some system providers work with water in the solar loop. But then other solutions for freeze protection are required. Not only, but also with respect to these solar fluid related difficulties Fraunhofer ISE has started to work again on solar air collectors [5].