1 Values used in the simulations for reflection, transmission and absorption coefficients for selective coating and hot mirror systems in the infrared and visible solar radiation regions. http://sajs.co.za/index.php/SAJS/article/downloadSuppFile/524/2879
Transmission (tg) and reflection (rg) in the visible region for the glass cover were taken as 0.9 and 0.04, respectively, for both the selective coating and the hot mirror. Transmission of the receiver pipe (tr) was set to zero for both visible and infrared radiation. The selective coating was modelled by setting the reflection of the receiver pipe in the visible region to a low value (rr = 0.04; i.e. it absorbs well) and reflection in the infrared region to a high value (rr = 0.8; i.e. it absorbs poorly and emits poorly). The optical characteristics of the glass cover were within a conventional range.7 The hot mirror system was modelled by making the reflection coefficient in the infrared region high (rg = 0.7). The receiver pipe had thermal characteristics close to a darkened steel pipe without selective coating. 7,11 Results and discussion The temperatures of the glass cover and the receiver pipe for both the selective coating and the hot mirror systems as a function of the length of the solar trough system is shown in Figure 6. The efficiency of the heat transfer into the working fluid (compared to the total incident solar radiation) is shown in Figure 7 for the selective coating and hot mirror systems, as well as for a system with no coating. The simulation was started at a temperature close to where failure of the selective coating is expected (i.e. 600 K). Relative efficiencies of the selective coating and the hot mirror can thus be compared in this region. It can be seen in Figure 6 that, for the chosen parameters of the selective coating and the hot mirror, the selective coating performed better in the sense that the receiver pipe increased to a similar temperature as the hot mirror but over a shorter distance. This result implies a higher heat transfer for the selective coating into the working fluid compared to the hot mirror at the same length, because the temperature gradient between the pipe and the fluid is larger. This effect is also seen in Figure 7. Hence the selective coating should be used at the temperatures where it is stable (below 680 K). After this critical temperature, a substitution to the hot mirror system is advisable. The glass cover temperature is seen to be much lower than the receiver pipe temperature (about 200 K lower), and this allows the glass cover to be coated with a hot mirror, because the hot mirror coating can operate at these temperatures (~500 K). The hot mirror allows the working fluid to be heated to more elevated temperatures, making the overall system more efficient. It should be noted that our choice of values for the hot mirror coating properties was on the conservative side, and the hot mirror can perform better in terms of infrared reflection, elevating the stagnation temperature. This is a detailed engineering concern and will be addressed in a future communication. Figure 7 indicates the efficiency of heat moving into the working fluid [(heat into working fluid, QLC)/(total incident solar radiation, Qv)]. It can be seen that both the selective coating and the hot mirror perform much better in terms of working fluid heat transfer than a system without either, making both suitable for increasing the efficiency of a solar trough plant. Conclusions We investigated the general performance of a hot mirror system compared to that of a selective coating system in terms of efficiency of the heat transferred into the working fluid of a solar trough system. We also investigated the possibility of using a hot mirror system to replace the currently used selective coating system at temperatures beyond which the selective coating system functions. A set of heat transfer equations was derived to model the thermal behaviour of a solar trough receiver unit. Radiative heat transfer within the receiver unit was considered, as well as convective losses to the outside, and heat transfer into the working fluid. A code was written using the equations, and was the main source of our results. Firstly, it was seen that, for our chosen parameters for the hot mirror and the selective coating, the hot mirror system performed slightly more poorly in terms of heat transfer into the working fluid, but much better than a system with no coating. An optimised hot mirror system may be a candidate to replace a selective coating system in a temperature range (above 500 K) for which the selective coating is not useful. Secondly, it was seen that the glass cover temperature was sufficiently low for a hot mirror system to remain operational beyond temperatures currently available to a commercial selective coating system. This finding allows the construction of solar trough systems with longer receiver pipes of two types: a receiver pipe with a selective coating used in a low temperature region and a receiver pipe with a hot mirror system used in the region of high temperatures. This hybrid system will allow the working fluid to reach a higher temperature, and hence to perform better overall. For this purpose, it would be well worth further investigating the hot mirror system. The weakness of the hot mirror system is that it performs slightly more poorly in terms of heat transfer into the working fluid, at least for our choice of parameters. The reflectivity of a hot mirror changes significantly with a change in wavelength, even in the infrared region. Generally, shorter infrared waves are better reflected, making the hot mirror system useful at higher temperatures. Further, it is necessary for a solar trough plant to switch between a selective coating and a hot mirror at some length of the receiver pipe, using both technologies. Also, depending on what type of hot mirror is used, the glass cover may nevertheless reach quite high temperatures. We have not addressed detailed questions such as cost effectiveness or a detailed design for a receiver unit with a hot mirror coating. The main purpose of this study was to highlight the possibility of an alternative to the conventional selective coating. Acknowledgements We wish to thank L. Nayager and P. Taylor for their help and useful comments. Competing interests We declare that we have no financial or personal relationships which may have inappropriately influenced us in writing this article. Authors’ contributions M.C. Cyulinyana completed the study as part of her MSc in Physics at the University of the Witwatersrand. The calculations were performed by her and checked by P. Ferrer. M.C. Cyulinyana worked on the simulations and collated the results. The study was conceived and supervised by P. Ferrer, who formed the outline of how the project should proceed and wrote the code for the simulations. 1.TwidellJWeirTRenewable energy resources20061151452.Solar cookers: How to make, use and enjoy. 10th ed20043.MckayJCSustainable energy – without the hot air [book on the Internet]c2009cited 2010 Jul xx4.KaltschmittMStreicherWWieseARenewable energy technology – economics and environment20075.SenZ Solar energy fundamentals and modeling techniques20086.Assessment of parabolic trough and power tower solar technology cost and performance forecasts: Prepared for the US Department of Energy and National Renewable Energy Laboratoryc2003cited 2007 May7.Tubing, capillary and rod of borosilicate glass 3.3cited 2010 Sep8.KennedyCEPriceHProgress in development of high-temperature solar-selective coatings2005 Aug 06–129.http://dx.doi.org/10.2172/15000706 10.SørensenBBreezePStorvickTetalRenewable energy focus handbook200911.KreithFKreiderJKPrinciples of solar engineering197812. http://dx.doi.org/10.1016/S1359-4311(01)00104-113. FerrerPEnhanced efficiency of a parabolic solar trough system through use of a secondary radiation concentrator200810438338814.CyulinyanaMCInvestigation into applicability of existing renewable energy technologies and possible efficiency increase201115.2007Redmond, WA: Microsoft Corporation200716. Version 6.0 Professional edition199517.http://dx.doi.org/10.1201/978020390818118.XuYGaoJZhengXWangXWangTChenHDeposited indium-tin-oxide (ITO) transparent conductive films by reactive low-voltage ion plating (RLVIP) technique2007122123908910