Heat efficiency of a solar trough receiver with a hot mirror compared to a selective coating

  • Marie C. Cyulinyana Centre for Theoretical Physics, School of Physics, University of the Witwatersrand
  • Phil Ferrer Centre for Theoretical Physics, School of Physics, University of the Witwatersrand
Keywords: solar trough, selective coating, hot mirror, efficiency, receiver coating


In this article, we present a solar trough system in which the receiver pipe is enclosed in a glass cover under vacuum. The dominant radiation losses from the receiver are reduced by the use of a ‘hot mirror’ on the glass cover instead of a selective coating on the receiver pipe. We present the results for a general heat transfer model and compare the performance of a selective coating with that of a hot mirror, using simulations. We determined that a hot mirror is a viable alternative to a selective coating, and certainly allows higher temperatures of the working fluid. We recommend the use of a hybrid system, in which a selective coating is used in the part of the receiver pipe in the low temperature region, and a hot mirror is used in the high temperature region to reduce radiation losses.


1. Twidell J, Weir T. Renewable energy resources. New York: Taylor & Francis, 2006; p. 115–145.

2. Solar Cookers International. Solar cookers: How to make, use and enjoy. 10th ed. Sacramento: Solar Cookers International; 2004. Available from: http://solarcooking.org/plans/Plans.pdf

3. Mckay JC. Sustainable energy – without the hot air [book on the Internet]. c2009 [cited 2010 Jul xx]. Available from: www.withouthotair.com

4. Kaltschmitt M, Streicher W, Wiese A. Renewable energy technology – economics and environment. Berlin: Springer; 2007.

5. Sen Z. Solar energy fundamentals and modeling techniques. London: Springer Verlag; 2008.

6. Sargent & Lundy LLC Consulting Group. Assessment of parabolic trough and power tower solar technology cost and performance forecasts: Prepared for the US Department of Energy and National Renewable Energy Laboratory. Subcontractor Report, SL-5641 [document on the Internet]. c2003 [cited 2007 May]. Available from: http://blum.home.cern.ch/blum/Studie/Dok-A4/CSP-DoE-Assessment.pdf

7. SCHOTT DURAN®. Tubing, capillary and rod of borosilicate glass 3.3 [document on the Internet]. No date [cited 2010 Sep]. Available from: http://www.schott.com/tubing/english/download/schott-tubing_brochure_duran_english.pdf

8. Kennedy CE, Price H. Progress in development of high-temperature solar-selective coatings. NREL/CP-520-36997. Paper presented at: ISEC2005. Proceedings of the 2005 International Solar Energy Conference; 2005 Aug 06–12; Orlando, FL, USA.

9. Kennedy CE. Review of mid- to high-temperature solar selective absorber materials. Golden, CO: National Renewable Energy Laboratory; 2002. http://dx.doi.org/10.2172/15000706

10. Sørensen B, Breeze P, Storvick T, et al. Renewable energy focus handbook. Oxford: Elsevier; 2009.

11. Kreith F, Kreider JK. Principles of solar engineering. Washington DC: Hemisphere Publishing; 1978.

12. You Y, Hu EJ. A medium-temperature solar thermal power system and its efficiency optimization. Appl Thermal Eng. 2002;22:357–364. http://dx.doi.org/10.1016/S1359-4311(01)00104-1

13. Ferrer P. Enhanced efficiency of a parabolic solar trough system through use of a secondary radiation concentrator. S Afr J Sci. 2008;104:383–388.

14. Cyulinyana MC. Investigation into applicability of existing renewable energy technologies and possible efficiency increase. Case study: Rwanda. MSc thesis, Johannesburg, University of the Witwatersrand, 2011.

15. Microsoft Office Excel. 2007. Redmond, WA: Microsoft Corporation; 2007.

16. Mathcad Plus. Version 6.0 Professional edition. Cambridge, MA: Mathsoft Inc.; 1995.

17. Kumashiro Y. Electric refractory materials. New York: Marcel Dekker; 2000. http://dx.doi.org/10.1201/9780203908181

18. Xu Y, Gao J, Zheng X, Wang X, Wang T, Chen H. Deposited indium-tin-oxide (ITO) transparent conductive films by reactive low-voltage ion plating (RLVIP) technique. J Lumin. 2007;122–123:908–910.