The “acceptance curve” shown in Figure 16.16 enables an evaluation of service life of an absorber made in the actual PPS material. Note that the thermal impact only has been considered. Other factors that influence the life time can also have significant effects, although previous studies [16, 17] have shown that the thermal load is by far the most critical.
A solar collector oriented towards the south with a tilt angle of 30° has been evaluated. The latitude is 40 °N. The thermal load has been simulated day by day, based on assumptions about the ambient temperature given in Table 16.5 and by
Figure 16.16 Measured critical failure points (Table 16.4, O) and calculated relation for failure according to Eq. (16.5) (—).
experimental data for stagnation temperature versus solar irradiation for the actual solar collector.
The acceptance curve shows that degradation of the material due to thermal load is negligible during normal operation when the absorber temperature is typically in the range from 40 to 90 °C. The significant thermal load appears when the solar system is out of operation, and the absorber temperature reaches the stagnation value with a balance between absorbed heat and heat loss from the collector to the ambient. We assume that the total load due to stagnation can be represented by three perfect sunny days per month, with the extreme ambient temperature (Table 16.5). Figure 16.17 shows the actual absorber temperature during these days.
These continuous temperature functions are inputs in the integral given in Eq. (16.6), resulting in a thermal dose per day. The dose estimate is normalized in such a way that the full acceptance dose is represented by the value 1. The results obtained for one year are shown in Figure 16.18, where both the monthly load and the
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Figure 16.18 Thermal load per year on an absorber under the conditions described in the text. The total annual load indicates a service life for the collector of more than 18 years.
accumulated load are presented. The total yearly load, under these conditions, is 0.054. Hence, the thermal loads indicate a service life of 18.5 years.
The described method provides estimates of the expected values for operation without failures for a solar absorber under given climatic conditions. Other factors than temperature can influence the material properties. Furthermore, the temperature variations during real operation can cause effects other than those revealed in the present experiments.
The example shown indicates that a material like the PPS provided by Chevron Phillips Chemicals, and extruded into twin wall sheets for circulation of the heat carrier, can sustain absorber temperatures up to 160 ° C during stagnation in the warmest season. Less demanding climates will of course give a much longer service life.
This project is partly financed by The Norwegian Research Council, and this support is acknowledged. Parts of the work have been carried out within the framework of the IEA-SHC, Task 39 project and we acknowledge good and inspiring discussions, ideas, and comments. Aventa AS is acknowledged for providing the materials and for financial support.