Medium Temperature Collector Developments

Reports and discussions during the two experts meetings in Rome and Lisbon in 2006 showed that many development projects are now underway. And still more projects are starting to study new concepts for concentrating collectors in the 150 to 250°C temperature range. This very positive situation is considered to also be a result of the successful work of Task 33/IV.

For example, AEE INTEC reported on further development of Button Energy’s Parasol One collector in Austria. This is a parabolic trough collector designed for a temperature range up to 200°C. Successful experiments in direct steam generation were carried out during the summer of 2006. Suitable heat transfer media for a first application in a solar cooling system were also investigated.

PSE GmbH in Germany is developing a Fresnel process heat collector for the 150 to 250°C temperature range. The first experimental collector was erected in December 2005 in Freiburg, Germany. The total reflector area of this collector is 88 m². It is being operated in a test loop for experimental research. And the second Fresnel process heat collector with a reflector area of 132 m² was already setup in a solar cooling system in Bergamo, Italy in summer 2006. The heat of the collector is used to operate an absorption chiller made by the Italian firm, Robur. First experience in operating the collector and absorption chiller and their interaction was acquired in summer 2006. The data measured while monitoring the collectors and cooling system have now been evaluated and development work will be continued.

The CCStaR collector developed by the Universitat de les Illes Balears, Spain

Development of a new collector was also started in Spain: The CCStaR collector applies the concept of fixed Fresnel reflectors and a moving receiver. Work was started at the Universitat de les Illes Balears in Mallorca. In June 2006, Tecnologia Solar Concentradora S.L. was founded for the purpose of industrial development of the CCStaR concept.

Finally, it should be mentioned here that a new parabolic trough collector has been developed in Australia. The NEP SOLAR Polymer Carrier PTC has about a 1‑m aperture width and a new reflector design. It was set up for testing during summer 2006 and is scheduled to go on the market in 2007.

Medium Temperature Collector Testing

New medium temperature collectors require new testing facilities for collector performance measurements in the temperature range up to about 200°C. An important step was taken in summer 2006 concerning testing of process heat collectors. A "Round Robin" or intercomparison test by test labs was initiated. The testing institutions, Fraunhofer ISE in Freiburg, Germany, ITW in Stuttgart, Germany and INETI in Lisbon, Portugal, will carry out thermal-performance tests on a vacuum-tube collector with a CPC reflector. Three collectors were acquired for the Round Robin test and the thermal performance of all three of them was measured at Fraunhofer ISE with a recently designed Medium Temperature Test Stand (MTTS) with a solar simulator in the indoor test facility. The highest collector inlet temperature in these tests was 185°C. The laboratories at ITW and INETI will now perform their tests so that the results can be compared in the meetings in 2007. Recommendations for medium temperature collectors will be made based on conclusions drawn from the experience gained in these tests.

The stagnation behavior of large (medium temperature) collector fields is still a very crucial point which needs much more attention and further research in order to avoid stagnation problems in industrial applications of solar collectors.

In this context, Aiguasol Engineering, Spain, reports on their operating experience and stagnation behavior of the CONTANK system. Stagnation problems were due to the system load being considerably less than design point. The safety valve had to be opened several times due to excessive steam produced in the collector. Nighttime trough cooling in the collector field reduces the problem a little, but not sufficiently. The membrane expansion vessel has been redimensioned and research will continue.

Results of the ongoing German stagnation-proof projects indicate that degradation of the water-glycol fluid is caused mainly by high temperatures. Blocking by degraded glycol residue is especially risky in small collector absorber tubes with low heat losses that cannot be vacated well. The discussions showed that a lot of experience is still needed and there are no standard procedures for solving stagnation problems in existing plants. More experience is especially needed in large systems with collector areas of hundreds to thousands of m² (for process heat systems) based on solid experimental experience and knowledge.