In the development of flat-plate collectors and vacuum tube collectors for domestic hot water and for room heating applications it was sufficient to carry out collector efficiency measurements up to collector inlet temperatures of about 100°C. But the situation is different for the development of collectors which will have their main operating temperature in the range of 80 to 250°C. It is essential to carry out efficiency measurements directly at these high temperatures and not to rely on extrapolations from measured efficiency points at lower temperatures.

Therefore, at Fraunhofer ISE a new test facility was developed with which we can determine collector efficiencies at measuring temperatures up to 200 °C. It can be used in indoor measurements (with our solar simulator, see figure 6) and outdoor (with the tracker) [7].

Figure 7 shows examples of measured efficiency curves of three different collectors. All curves were measured in the solar simulator laboratory (indoor measurements) and with the new testing facility.

The measurement points at the highest temperatures were taken at mean fluid temperatures of about 185° — 190°C for all three collectors. The actually measured efficiency points are indicated in the diagram for the evacuated tubular collector 1. It is a collector without a CPC reflector and with relatively narrow gaps between the single evacuated tubes. These measurements were taken at an irradiation of 931.9 W/m2. Therefore also the two other curves are given for this irradiation which is necessary in order to plot all three curves in one diagram. In all measurements the ambient temperature was in the range of about 30°C. The second evacuated tubular collector uses a CPC reflector. The heat losses based on the aperture area are therefore smaller and the efficiency curve is higher than for collector number 1 at higher operating temperatures. The efficiency curves show that both evacuated tubular collectors are suitable for system applications in which the collector operating temperature is in the range above 100°C and may be up to 150°C. The flat-plate collector has a highly selective absorber coating and is glazed with a single, anti-reflectively coated glass.

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Figure 7: Measured efficiency curves of three different collectors (indoor measurements with solar simulator, highest mean collector temperatures about 185° — 190°C for all three collectors. The dots show the actually measured efficiency points for collector 1 and the mean collector fluid temperature in the measurement.

3. Conclusions

For a summary on the achievements of the work of IEA-SHC Task 33 SHIP considering the development of process heat collectors I want to make the following statements:

• A good start has been made. Different new collectors for the operating temperature range of 80 to 250°C are under development. The Task had a positive triggering and integrating impact with a high degree of information exchange.

• With regard to realized demonstration plants and built application systems, it has to be stated that almost all of them are working at temperatures below 100°C. Of course, there is also a big potential for solar heat in industrial processes below 100°C, but the full potential for heat up to about 250°C can only be used if new and appropriate collectors are developed. As mentioned, a good start is made, but a lot of development work and research activity is still needed.

• New requirements have to be fulfilled with respect to collector components, materials and system components of the solar loop:

— appropriate heat transfer fluids sufficiently temperature stable, anti-freeze properties, efficient thermodynamic and hydraulic performance

— cost effective reflectors with high performance and long service time

— tracking systems which are reliably operating for the whole service time

— appropriate receivers (with and without selective coating, applicable in vacuum of in atmospheric conditions)

— appropriate and temperature stable pumps, piping and connection systems, heat exchangers and valves and other system components.

• New collector testing standards and testing facilities are required, especially for CPC and concentrating collectors. The aim must be to achieve a full technical and economical comparison for the full range of the collector technologies.

Acknowledgement

I want to thank all colleagues who contributed to Subtask C and to the booklet Process Heat

Collectors. The work contributed by Fraunhofer ISE has been carried out with financial support by the

German Ministry for the Environment, Nature Conservation and Nuclear Safety.

References

[1] Werner Weiss, Irene Bergmann, Gerhard Faninger, (2008), Solar Heat Worldwide — Markets and Contribution to the Energy Supply 2006, Edition 2008, http://www. iea-shc. org/publications/statistics/IEA- SHC_Solar_Heat_Worldwide-2008.pdf

[2] Werner Weiss (2008), Solare Prozesswarme — Potenziale, Einsatzbereiche und Herausforderungen fur die Solarindustrie, 18. Symposium Thermische Solarenergie, Bad Staffelstein 23.-25. April 2008, pp159-164.

[3] Henning, Hans-Martin (Ed.) (2004), Solar-assisted air-conditioning in buildings — A handbook for planners, Springer Verlag Wien New York, ISBN 3-211-00647-8

[4] M. Wieghaus, J. Koschikowski, M. Rommel, (2008) Solar desalination for an autonomous water supply, Desalination and Water Reuse; for more information visit www. solarspring. de

[5] Chr. Thoma, Th. Weick, J. Richter, Th. Siems, M. Rommel (2008) Testing fo solar air collectors, Proceedings of Eurosun 2008, Lisbon

[6] Werner Weiss and Matthias Rommel (Ed.), (2008) Process Heat Collectors, downloadable from http://www. iea-shc. org/publications/downloads/task33-Process_Heat_Collectors. pdf.

[7] M. Rommel, K. Kramer, S. Mehnert, A. Schafer, T. Siems, C. Thoma, W. Striewe, (2007) Testing Unit for the Development of Process Heat Collectors up to 250°C, estec 2007, Proceedings of the 3rd European Solar Thermal Energy Conference, June 19-20, 2007, Freiburg, Germany, pp 414-418