MidTempColl

In order to increase the market of solar thermal systems the high potential of new applications like integration in district heating networks, industrial processes and other craft applications shall be used. In such cases typically large collector fields with operating temperatures above 70 deg C are installed. Therefore large dimensioned collector modules with high efficiency in the temperature range of 80 to 120 deg C are required. Additionally high flexibility for the hydraulic concept and mounting technology are needed in combination with high cost efficiency.

Up to now typically standard flat plate or vacuum tube collectors are used which are not specifically designed for applications described before. Large scale flat plate collectors are available on the market, but already at the lower end of the design temperature range (80 to 120 deg C) the efficiency typically is rather low. Three European producers are known who offer modified double covered (glass/glass or glass/plastic foil) flat plate collectors in order to reduce convection heat losses. But the energetic advantages of these collectors are more than compensated by other disadvantages. Collectors with double glazing are significantly heavier and more expensive. Collectors with plastic foil (typically Fluor-polymer is used) as second cover in practice show significant problems in terms of long-term durability of the foil material itself and in context with the clamping of the foil. Due to these problems a remarkable percentage of collectors is in bad condition direct after production or after some time of operation resulting in reduced efficiency and bad appearance (cracks of the foil or strong sagging and contact to the absorber or crinkles of the foil).

Market available vacuum tube collectors in principle suit perfect in terms of thermal performance at the mentioned operating conditions with increased temperature. Since these collectors are not specifically designed for large scale collector fields (module size, hydraulics, assembly technique, etc.) this leads to high effort for assembling, increased heat losses in the collector field and investment cost out of scale. Additionally practical examples show that hydraulic problems occur (bad distribution of collector flow, partial stagnation) due to the high number of single components which need to be connected. This can lead to significant reduction of solar gain of the entire solar heating system.

Based on above mentioned reasons from this project consortium point of view an increased demand in research and development of especially designed and long-term durable large scale solar thermal collectors for operating conditions in the temperature range of 80 to 120 deg C is given. Therefore within this project a completely new concept of cost efficient vacuum tube collectors based on the direct flow through concept and special designed for large scale collector areas will be developed. Main focus of the work shall be the adapted hydraulics, (serial connection of several vacuum tubes for long thermal lengths, new concept of hydraulic junctions and good emptying behaviour during stagnation periods) and a simple and cheap assembling concept in order to reach the goal of a significantly decrease of production cost per m2 collector area.

As conclusion, the transfer of the project results takes place to the main target groups and to the IEA SHC working groups as well. After this successful project a major step is done towards a broad increase of the market of large scale solar thermal heating systems in combination with district heating networks and process heat applications.