Frequency distribution of maximum absorber temperatures

Figure 3 illustrates the absolute frequency in hours per year of maximum absorber temperatures for collectors without and with thermotropic overheating protection (solar transmittance of thermotropic layer in clear state: 0.85; residual solar transmittance of thermotropic layer in opaque state of 0.20,

0. 30, 0.40, 0.50 or 0.60). The collector was assumed to face stagnation only. For the collector without overheating protection absorber materials are required which display a long term service temperature of 125°C associated by adequate durability when exposed to temperatures between 125 and 160°C for 400 hours per year. By the use of thermotropic layers with a residual solar transmittance of 0.20 in the opaque state the absorber material is exposed to a maximum temperature of ~90°C and to temperatures between 80 and 90°C for 1000 hours per year. If thermotropic layers with a solar transmittance of 0.40 above the switching threshold are applied, absorber materials are required that can withstand temperatures between 80 and 100°C for periods of ~1000 hours per year. For thermotropic layers with a moderate reduction of solar transmittance to values of 0.60 absorber materials are exposed to temperatures between 110 and 130°C for less than 200 hours per year and for 1000 hours between 80 and 110°C. Special polyolefin grades are available with show sufficient durability under such stagnation conditions [6]. This indicates that the development and design of thermotropic layers exhibiting a solar transmittance below 0.60 in the opaque state, would allow for the application of cost — efficient plastics as absorber materials.

image109

absorber temperature [°C]

 

Fig. 3. Frequency of maximum absorber temperatures per year for a collector with twin-wall sheet glazing and
black absorber (a=0.95, є=0.90), being installed in Graz (south oriented, slope 45°) and facing stagnation only;
dotted line: collector without overheating protection; other lines: collectors with thermotropic glazing (solar
transmittance of thermotropic layer: 0.85 in clear and 0.20, 0.30, 0.40, 0.50 or 0.60 in opaque state).

2. Summary and Conclusions

Within this paper theoretical modeling was applied to characterize the potential of thermotropic layers to prevent overheating of an all polymeric flat-plate collector. The investigations showed that for a collector with twin-wall sheet glazing and black absorber stagnation temperatures can be reduced by using thermotropic layers in the glazing. For thermotropic glazing the switching temperature should range between 55 and 60°C. In general, the impact of a thermotropic layer on overall collector efficiency was detected to be low as long as the solar transmittance exceeds 0.85 in the clear state. To provide an excellent limitation of the stagnation temperature to a maximum operating temperature of ~85°C a residual solar transmittance of the functional material about 0.25 was found to be effectual. If thermotropic materials are applied exhibiting an excellent reduction of solar transmittance to values of 0.20 the absorber reaches temperatures of 80°C to maximum 90°C for 1000 hours per year when installed in Graz (AUT) and facing stagnation only. Even with functional materials that show moderate switching ranges to a solar transmittance of 0.60 in opaque state stagnation temperatures between 110 and 130°C as well as between 80 and 110°C are reached for ~200 and ~1000 hours per year, respectively. Thus, the development and design of thermotropic layers with a residual transmittance below 0.60 in opaque state would allow for the application of cost-efficient plastics as absorber materials.

References

[1] A. Khan, A. Brunger (1997). In Proceedings of the 23rd Annual Conference of the Solar Energy Society of Canada Inc, Vancouver, Canada, pp. 119-124.

[2] G. M. Wallner, K. Resch, R. Hausner, Solar Energy Materials and Solar Cells, 92 (2008) 614-620.

[3] P. Nitz, H. Hartwig, Solar Energy, 79 (2005) 573-582.

[4] SOLAR, 1997-2006, “Warme — und stromungstechnische Berechnungen, Unterprogramm: Theoretischer Kollektor“, AEE INTEC, Gleisdorf, AUT.

[5] M. Meir, J. Rekstad, (2003). In Proceedings of 1st Leobner Symposium Polymeric Solar Materials, Leoben, Austria, pp II-1-II-8.

[6] S. Kahlen, G. M. Wallner, M. Meir, J. Rekstad, (2006). In Proceedings of EuroSun 2006, Glasgow, UK, pp. 7ff.

Acknowledgements

The research work of this paper was performed at the Polymer Competence Center Leoben GmbH within the framework of the Non Kpius Program.

Подпись: Steiermark

This work is funded by the State Government of Styria, Department Zukunftsfonds Steiermark.