After correct assembly of the collectors and positive testing of the flux distributions in the focal line, thermal tests should complete the acceptance tests. In order to reduce cost for sensor mounting and reduce the risk of leak­age of heat transfer fluid, clamp-on sensors might be preferred for this testing, in spite of the lower precision. For temperature meas­urements, thermally well-insulated and cali­brated surface resistance thermo probes (PT100) in four-wire technique, designed for temperatures up to 400 °C are used. An ultra­sonic flow meter can be used for working ranges up to 300°C on a wide range of pipe diameters. One ultrasonic sensor attached to a pipe is shown in Figure 7. The instrument also determines the wall thickness of the pipe in use, which is necessary because this value can vary significantly. Knowing the heat capacity, the specific density, the sound velocity and the viscosity of the fluid for the measured temperature range, mass flow rate can be measured with an accuracy of 1 to 3 percent.

Performance Impact of Geometric Precision

The optical performance of a parabolic trough collector is determined by the optical proper­ties of its key components, the mirrors and the receiver tubes. But of course their proper­ties have to match, and the concentrating collector as a whole has to be manufactured on the appropriate precision level to reach the design performance. The methods for geomet­
ric evaluation of concentrating collectors presented in the previous sections provide infor­mation about the actual geometry of the product. However the classification in pass/fail categories has been very difficult at some stages. Apart from the common criteria to fit components together, there is a need for appropriate criteria and tolerances that have to be fulfilled in order to reach the design energetic performance of the final product.

Ray tracing has being used to model the capture fraction of the reflected sunrays on the absorber tube. A detailed approach uses finite mirror facet elements and Monte-Carlo methods with millions of rays to find out the intercept factor of the solar radiation. If well modelled it reveals the optical efficiency and also the flux distribution of a part of a large collector under certain geometric conditions. This method is not practical for the analysis of large collector fields over longer time periods (e. g. one year).

So different ray-tracing techniques, as proposed by Rabl [6], have been used for the more extended annual analysis of solar collector fields. Certain simplifications reduce drastically the computational effort required. As usual for studies with a large number of independent, stochastically varying inputs the individual input will be replaced by the statistical model of a Gaussian distribution characterized by the standard deviation. So the beam spread oc­curring to the sunrays when interacting with the imperfect concentrator is represented by its standard deviation. The same can be applied, within a certain range of validity, for the sunbeam spread due to the size of the solar disc. Basing on this model the effect of irregu­larities can be respected in dependence of their frequency distribution. The individual ef­fects sum up with their weighted squares:

2 2 ^total = ai Wi

This equation also suggests that the standard deviation for each component (e. g. struc­ture, mirror) is the quality measure, which can be assessed easily from large quantities of measurement results. As given by this theory, the intercept factors for line focusing collec­tors have a dependence of the total beam spreads. The result for the EuroTrough geome­try (and because of identical concentrator and receiver geometry also for the LS 3- collector) is shown in Figure 8.

Conclusions

The systematic analysis and specific measurement systems used until now in solar ther­mal concentrating technology used to serve for the evaluation and qualification of proto­types in test or demonstration installations. Numerous techniques have been developed and used for measuring and optimizing the performance of prototypes. At the moment of the continuous transition from research and development work to market introduction in large series fabrication, the role of measurement techniques change. Their former applica­tion experience however is the basis for its further deployment in concepts for the quality control in large-scale projects.

The experience from tedious manual work in geometric measurements, leveling, photo — grammetry and flux density measurements has contributed to the collection of very de­tailed knowledge about the EuroTrough collector. The fastest and most reliable techniques from R&D experience are now transferred to quality control tools in order to assist the manufacturing and assembly of thousands of trough collector modules for the large solar power plant projects in Spain. Close-range photogrammetry is among the favorites. The contact-less measurement with digital camera equipment has been identified to fulfill the precision requirements of trough collector structure assembly. Further effort is underway with the objective to automate the caption and evaluation processes. The work on flux measurement and intercept factor analysis has identified the significant potential of im­provement in collector quality, which can be exploited basing on the detailed knowledge gained of the complexity of a concentrating solar collector.

The application of the proposed quality control concepts will reduce the effort on meas­urements and reworking. But even more: The potential in solar field performance gain amounts to several percent, and savings are reflected in cost reduction for less solar field area needed. In addition the knowledge that has been gathered on how to check and ver­ify in efficient manner the good performance of large parabolic trough collector fields will help to reduce the risk for construction companies and thus cut down solar field cost sig­nificantly.

The authors gratefully acknowledge financial support by the German Federal Ministry for the Environment (BMU) within the scope of “PARASOL/OPAL", the contributions by G. Johnston, S. Ulmer, and K.-J. Riffelmann, and the collaboration with the SKAL-ET project partners.