An important possibility for the development of medium temperature collectors is to reduce heat losses by concentration. For example INETI in Portugal is investigating a stationary CPC type collector without vacuum for medium temperature applications /4/. The concentration factor is in the range of 2.

Also the developments of MaReCo’s (=maximum reflector collectors) of Vattenfall and Finsun Energy AB in Sweden is based on using reflectors for improved performance at higher temperatures /5/. Figure 3 shows an application of an early MaReCo collector in a district heating system in Sweden.

Small parabolic trough collectors

Especially for the temperature range of 150°C to 250°C it is extremely interesting to consider the parabolic trough collector technology. A lot of experience is available from the high temperature applications where parabolic trough collectors are used at 400°C to 600°C for electric power production. But adjustments have to be made for the medium temperature range. As examples for current investigations with this aim developments from Spain, Austria, and Germany are mentioned:

CIEMAT in Spain plans to develop a parabolic trough collector (‘FASOL’) with an aperture width of 2.62 m and a focal distance of 0.7m. The length of every parabolic trough module will be 6 m and about 8 troughs will form a collector with tracking device. The concentration factor will be in the range of 20 to 25 /6/.

AEE INTEC in Austria works on the development and optimisation of a small parabolic trough collector with glass cover for operating temperatures from 100 to 200°C. The prototype dimensions of a module are 0.5 m x 4 m and the focal length is 10 cm. The first prototype has a receiver with a diameter of 8 mm, a non — evacuated glass cover tube
and was coated with a non-selective varnish /7/, see Figure 4. This Austrian national project is funded by the Austrian Ministry of Transportation, Innovation and Technology as part of the research program ‘Fabrik der Zukunft’. Co-operation partners are the manufacturer of the parabolic trough (the company Knopf Design) and a number of other Austrian companies.

In Germany, a parabolic trough collector is developed by DLR and SOLITEM /8/. A first collector field was recently installed in an hotel in Turkey, see Figure 5.

In Israel, the company SOLEL is marketing a parabolic trough collector. It is for example installed in an application in which solar thermal energy is used for the production of heat, cooling and electricity.

The collector field consists out of a set of 4 modules of 6 rows of 48 m long parabolic troughs. The collecting area totals to 860 m2. The collector field is installed 10 meters above the company’s parking lot and working area, see Figure 6.

A water sprinkling system is mounted on each row of collectors to wash off dirt and prevent deterioration in performance. The water is filtered and recycled. The heat carrier of the solar loop is thermal oil. The collector output is used to heat the water in the steam generator at a temperature up to 180° C. This then powers a condensed steam turbine and produces 50 kW of electricity. The waste heat is stored in a 20 m3 water tank supplying enough hot water for space heating in winter as well as 30 tons of air conditioning (by absorption) in summer for the offices and the corporate manufacturing facilities. The system is installed in Beit Shemesh, Israel.

01

Conclusion

For applications where temperatures up to 250°C are needed the experiences are rather limited and therefore also suitable collectors, components and systems are missing. The aim of IEA Task 33/4 is to make use of the huge potential for solar heat in the industry and to open new market sectors for the solar thermal industry. The aim is to integrate solar thermal systems into industrial processes in the best and most suitable way. To achieve this, new ‘medium temperature collectors’ have to be developed for the temperature range of 80°C to 250°C. Improved flat plate collectors, collectors with reflectors and low concentration factors as well as appropriate parabolic trough collectors are under development. Solar heat for industrial processes is needed for applications such as air conditioning and cooling, solar ice production, heat for food industry, textile industry, washing, processes in diary farms, pasteurisation, sterilisation, water purification, disinfection and sea water desalination. In these sectors, the new medium temperature collectors will open new markets.

Acknowledgement

Many thanks for their input to this paper to the colleagues from Subtask C, especially to: Maria Joao Carvalho, Esther Rojas Bravo, Bjorn Karlsson, Klaus Hennecke, Dagmar Jahnig and to Werner Weiss.

References

/1/ Matthias Rommel, Andreas Gombert, Joachim Koschikowski, Arim Schafer, Yan Schmitt, Which Improvements can be achieved using single and double AR — glass covers in flat-plate collectors? European Solar Thermal Energy Conference estec 2003, 26 — 27 June 2003, Freiburg, Germany

/2/ Matthias ROMMEL, Joachim KOSCHIKOWSKI, Marcel WIEGHAUS, Thermally driven desalination plants based on membrane distillation, International Conference ‘RES for island — Tourism & Water, 26-28 May 2003, Crete, Greece

/3/ M. Hermann, J. Koschikowski, M. Rommel, Corrosion-free solar collectors for thermally driven seawater desalination, SOLAR ENERGY, Vol 72, No.5, pp. 415-426, 2002

/4/ Maria Joao Carvalho, Stationary CPC type collector without vacuum for medium temperature applications (air-conditioning, industrial applications) (2004) mjoao. carvalho@ineti. pt

/5/ Bjorn Karlsson and Gunnar Wilson, MaReCo design for horizontal, vertical or tilted installations, Eurosun 2000, Copenhagen (bjorn. karlsson@vattenfall. com)

/6/ Esther Rojas, FASOL collector, CIEMAT-PSA, ES, 2004 (esther. rojas@ciemat. es)

/7/ Dagmar Jahnig, Development and Optimisation of a smal-scale parabolic trough collector for production of process heat, AEE INTEC, 2004 (d. jaehnig@aee. at)

/8/ Klaus Hennecke, Solitem PTC 1800, 2004 (Klaus. Hennecke@dlr. de) and http:www/solitem. de

/9/ http://www. solel. com