As mentioned above, the properties of the small test absorber are not directly scalable to larger absorbers. In [6] Treikauskas compares the simulated pressure drop of an optimized roll-bond absorber (harp absorber, 985 mm x 1925 mm) with state-of-the-art absorbers and with the measurement results of the small FracTherm® test absorber (590 mm x 1000 mm). However, it is difficult to compare the pressure drop of absorbers with different sizes. The pressure drop of a large FracTherm® absorber cannot yet be anticipated, and for a given volume flow it will probably be different from the small one. Therefore it will be very interesting to compare the large FracTherm® absorber to be developed with another large, optimized roll-bond absorber with a different channel design.

The development of the header channel is an important task with respect to connectivity and pressure drop. In order to have a first estimation of the pressure drop in a flat, wide header channel, simple analytical calculations of 1 m long channels with different rectangular cross sections were carried out. The channel width b was varied from 50 mm to 150 mm, the height h from 2.95 mm (height used in small test absorber) to 4 mm. The results for different volume flows Q are shown in Fig. 5. The change from laminar to turbulent flow is obvious: the pressure drop rises with Q according to a linear function in the laminar region and approximately according to a power function in the turbulent region. Fig. 6 shows both the pressure drop for different rectangular cross sections for a volume flow of Q=0.36 m3/h and the cross sectional areas. It can be seen that same cross sectional areas (which means same fluid volume and thus thermal capacity) lead to different pressure drops. The example in the diagram reveals a pressure drop difference of about 24 % for A=300 mm2 (see dashed arrows). It is evident that for a given cross sectional area the pressure drop becomes lower with increasing h/b ratio. But it is a question of technical feasibility whether the height of the channels can really be increased.

2. Conclusion

The results of the FracTherm® test absorber investigations carried out earlier are not directly scalable to standard size collectors. Therefore the main challenges of the European project BIONICOL are the further development of the FracTherm® program, the development of appropriate heat transfer fluids in order to prevent corrosion, the adaptation of a glass batch coating plant for coating solar absorbers, the development of concepts for header channels and the interconnection of collectors and finally the field tests to be carried out for one year in different sites in Europe. Some first rough calculations of the pressure drop in rectangular channels were already carried out. They can serve as a basis of dimensioning roll-bond header channels with a given maximum channel height.

3. Acknowledgement

The contract for the BIONICOL project was not yet signed when this paper was written. This is the reason why the project partners are not mentioned by name. The consortium applied for the project in the call FP7-ENERGY-2007-2-TREN within the Seventh Research Framework Programme (FP7) of the European Commission.

References

[1] V. Weitbrecht, D. Lehmann, A. Richter, Flow distribution in solar collectors with laminar flow conditions. Solar Energy 73(6), pp. 433-441, 2002

[2] M. Hermann, FracTherm — Fractal hydraulic structures for energy efficient solar absorbers and other heat exchangers. Proceedings, EuroSun 2004, Freiburg, Germany, 20-23 June 2004, Volume 1, pp. 332-338

[3] M. Hermann, Entwicklung des FracTherm-Absorbers — Simulationen und Experimente. Proceedings, 15. Symposium Thermische Solarenergie OTTI, Bad Staffelstein, Germany, 27-29 April 2005, pp. 94-99

[4] M. Hermann, (2005). Bionische Ansatze zur Entwicklung energieeffizienter Fluidsysteme fur den Warmetransport. Dissertation, Faculty of Mechanical Engineering, Universitat Karlsruhe (TH)

[5] C. Mattheck, Teacher tree: The evolution of notch shape optimization from complex to simple. Engineering Fracture Mechanics 73 (2006), pp. 1732-1742

[6] F.-D. Treikauskas, W. Zorner, V. Hanby, Volumetrische Absorber: Die neue Generation von Solarabsorbern in Theorie und Praxis. Proceedings, 18. Symposium Thermische Solarenergie OTTI, Bad Staffelstein, Germany, 23-25 April 2008, pp. 182-187