Developments in absorber technology have generated selective coatings with high values of absorbance over the solar spectrum in parallel with low levels of emissivity in the infrared region. Using the method described by Bhowmik6 it was possible to show that absorber plate using black chrome coatings exhibit selectivity’s of ~10 whereas the newer so called ‘blue’ coatings exhibit improved selectivities of ~25 due to lower emissivity values. As a result radiation losses have been minimised for evacuated solar tubes in comparison to earlier absorber plate technologies. Also Chow et al7 showed that
conduction and convention loss mechanisms shutdown within evacuated tubes at pressures less than 1×10-3 mbar. The dominant mechanisms of heat loss for the evacuated collector systems are therefore due to convection and conduction in the area of the manifold and corresponding tube connections. The use of Infrared-imaging techniques can reveal this to be the case; Figure 6 shows a thermal map for a vertically mounted direct-flow collector system installed outdoors. The glass temperature of the evacuated tubes was observed to be 12.5 ± 2.5 °C whereas the temperature of the manifold cover indicated by the dashed box was observed to be 32.5 ± 2.5 °C. Modern insulations with low k-values in the order of 2×10-2 Wm-2K-1 can minimise these heat losses from the manifold, which is especially important in low flow systems.

2 Conclusion

It was reported that the optical efficiency of an evacuated tube collector of direct-flow design was stable with incident irradiance power densities. The effect of mass flow-rate on the optical efficiency was found to be significant. The low flow-rate penalty was found to decrease efficiency by up to 25% of the quasi-stable value at higher mass flows. Using a simple empirical technique thermal losses from the collector were calculated under these conditions where Tc was held at 3 K above the ambient temperature. These losses were shown to depend heavily on mass flow and incident irradiance. The influence of increasing collector slope on collector performance was found to be beneficial. Linear losses within the collector were found to decrease by 9% over the range of 0° to 60°, however square dependency losses were found to increase by 750%. However, square dependency losses have a minimal effect on the collector performance and therefore the overall result on increasing the slope was found to be favourable. Losses from the collector system were found to be concentrated around the manifold and the connections to the solar tubes. Losses within the evacuated tube were found to be minimal, radiation losses were dominant for pressures less than 1×10-3 mbar.

3 Further work

Plans to repeat this work for comparison with an evacuated heat-pipe collector system are in the pipeline. Also mathematical modelling of the collector and solar simulator using TRNSYS based set-up may be employed at a later stage for theoretical comparison with experimental results.

Acknowledgements

I would like to thank my colleagues at Thermomax R&D, David McClenaghan, Gareth McWha and Richard Pelan who helped me with the construction of Thermomax’s first solar simulator, thanks guys I couldn’t have done it without you.

Nomenclature

References

[1] Solar Energy — The State of the Art: Edited Jeffrey Gordon — Chapter 5 Solar Water Heating by G. L. Morrison: James and James (Science Publishers) Ltd, London, 2001

[2] EN 12975-2:2001 — Thermal solar systems and components — Solar collectors

[3] SRCC STANDARD 100 — Test methods and minimum standards for certifying solar collectors, 1995

[4] C. Muller-Scholl, S. Brunold, U. Frei / Proceedings EUROSUN 2002 Conference in Bologna Italy

[5] K. A.R Ismail, M. M. Abogderah: ASME Journal of Solar Energy Engineer Engineering, 120, 51-58, 1998

[6] N. C. Bhowmik, J. Rahman, M. A. Alam Khan, Z. H. Mazumder: Renewable Energy, 24, 663, 2001

[7] S. P. Chow, D. R. Mills and G. L. Harding: Solar Energy 31 (4), 433, 1983