Life time for solar collectors for solar heating plants

Two HT solar collectors, which have been in operation at high temperature levels in Ottrupgaard solar heating plant for 15 years and in Marstal solar heating plant for 13 years, were in the spring of 2009 tested with regard to efficiency. The collectors were also inspected with the aim to evaluate the life time of the collectors. An old version of the HT solar collector, which has been in operation in a Swedish test facility since 1982, was tested with regard to the thermal performance. The measurements showed that the efficiencies of the solar collectors from the two Danish solar heating plants have been decreased since the installation. The reductions of the yearly thermal performance of the solar collectors are at a temperature level of 40 degrees C, 1% and 4%, respectively, for the Marstal collector and the Ottrupgaard collector. At a temperature level of 60 degrees C the reduction of the yearly thermal performance is 10% and 11 %, respectively, for the Marstal collector and the Ottrupgaard collector. At a temperature level of 80 degrees C the reduction is 27% and 23%, respectively, for the Marstal collector and the Ottrugaard collector. Based on the inspection, it is estimated that the reason for the reduction of thermal performance is the condition of the teflon foil and the installation of the teflon foil. The teflon foil is wrinkled and folded and expanded in such a way that the distance between the absorber and the teflon foil is far too small. Further, cracks in the teflon foil have been observed. The thermal performance of the Swedish solar collector in the test facility is after 26 years of operation reduced compared to the thermal performance of the collector when it was first installed. For this collector the reduction in thermal performance is only 2-5%. The collectors from Ottrupgaard solar heating plant and from Marstal solar heating plant were in a very good condition with exception of the above mentioned problems with the teflon foil. However, some corrosion was observed in the horizontal absorbers trips. The progress of the corrosion is limited and it is evaluated that the life time of the collectors will be about 30 years. Based on the investigations it is concluded that the largest problems for the collectors are related to the teflon foil and the installation of the teflon foil