The small FracTherm® absorber had been mounted in a collector casing and tested as a stand-alone device, which means that interconnection between several collectors arranged in parallel had not been relevant. Therefore one important future task is to develop concepts for header channels and appropriate adaptors. It is important that the cross section of the headers is large enough in order to keep the pressure drop as low as possible and to ensure a uniform flow distribution in all collectors.

There are basically the following different possibilities to build header channels:

• Additional header tubes connected to the roll-bond absorber

• Wide, flat header channels integrated into roll-bond absorber (same height as absorber channels)

• Narrower, higher header channels integrated into roll-bond absorber (much higher than absorber channels)

• Other concepts generating channels by forming parts of the roll-bond absorber

These different concepts have to be investigated in detail especially with respect to feasibility, production tolerances, costs, pressure resistance, corrosion, coating, flexibility.

The FracTherm® algorithm allows generating channel structures on given areas which need not to be rectangular. One of the industry partners of the BIONICOL project already now offers custom-made non-rectangular collectors which fit the shape of the roof (Fig. 4, left). However, these are individually made meander absorbers, resulting in high production costs and high pressure drops in operation. With FracTherm® it is possible to generate channel structures which fit the given boundary form and lead to a uniform flow distribution as well as a low pressure drop (Fig. 4, right). Since roll-bonding is a series production process, it will not be economical to produce individual absorbers, but it would be possible to offer some standard non-rectangular absorbers which might be adapted to customers’ specifications to a certain degree.

The collectors developed in the BIONICOL project will be tested at Fraunhofer ISE. The collector performance tests will be carried out with aluminium roll-bond prototype collectors with different absorber designs (meander absorber, harp absorber, FracTherm® absorber) as well as with state-of-the — art collectors for comparison. The tests will be carried out according to the European standard EN 12975 (collector efficiency curve, exposition test, rain penetration test, internal and external thermal shock test, pressure drop). Moreover, the emptying behaviour of the collectors will be tested

and optimized which is important with respect to stagnation behaviour and a potential application in drain-back systems. Stagnation tests can be carried out using the indoor solar simulator of Fraunhofer ISE (determination of steam penetration depth, “steam producing power”, volume of fluid remaining in the absorber).

Field test collectors will be produced in a small-scale series production and installed in different test sites in Europe (maritime, urban, rural and industrial atmosphere). The field test collectors will be operated for one year. It is planned to publish the results of the final evaluation in several workshops.