Matthias Rommel, Thorsten Siems, Rainer Becker, Kurt Schule
Fraunhofer Institute for Solar Energy Systems ISE
Heidenhofstr. 2, D-79110 Freiburg
email: matthias. rommel@ise. fraunhofer. de

The operation of collector fields under stagnation conditions has to be regarded as an important operation mode. A failure-free behaviour of a thermal system under stagnation conditions is important in order to achieve a long system life time. The stagnation behaviour has to be well understood and dealt with for the successful further development of small and large solar thermal systems.

The basic processes that take place when solar collector fields with pressurised closed solar loops are subjected to stagnation conditions were investigated in the past years in different projects. Fraunhofer ISE has worked in this field and elaborated important contributions to it /1, 2/. For information on the basic investigations see for example the contributions from ISE and AEE to Eurosun 2000 and from AEE to Eurosun 2002 /3/.

The proper control and containment of stagnation conditions is especially important for the further development of large solar thermal systems for multi-family houses that are designed to contribute not only to the domestic hot water demand but also to the room heating demand. On the one hand, these applications are extremely important to increase the share of solar energy in the European thermal energy market and to reduce CO2 emission. On the other hand, in these applications large collector fields will regularly be under stagnation conditions during summer time so that it is extremely important to achieve a non-problematic stagnation behaviour.

Fraunhofer ISE now carries out new experimental and simulation investigations in the frame of a larger joint project of several German partners and companies. The aims of this joint project are described in the paper by A. Schenke et al. ‘Outline of a joint research project of SWT, ZfS, ISFH and Fraunhofer-ISE: Analysis and evaluation of large scale thermal solar „combi-plants“ ‘ to this Eurosun 2004 conference. The joint project is supported by the German Ministry of Environment, Environmental Protection and Reactor Safety (BMU).

Method and approach

One of the main aims of the investigations carried out by Fraunhofer ISE is to study in detail the stagnation behaviour of single collector modules. For these experimental investigations the solar simulator and indoor test facility of Fraunhofer ISE is used /4, 5/. A complete pressurised closed solar loop was set up under the solar simulator. The emptying behaviour and process that occurs due to stagnation conditions will be analysed for different collectors
and different hydraulic absorber designs. One of the aims is to determine the maximum steam production rate from the measurements. The further aim is to conclude from the observations and measurements of a single collector module on the behaviour of a complete collector field in which a certain number of modules are connected in series and some of these rows are connected in parallel.

Experimental set-up

Figure 1: Indoor-collector test facility of Fraunhofer ISE with solar simulator.

The pipes of the solar loop connected to the collector are equipped with temperature sensors. The distance between two adjacent sensors is one meter. The sensors are attached to the outer wall of the copper pipe. The pipe is insulated with temperature resistant insulation material (EPDM foam, 165°C). The total length of the pipes prepared like that is 15 m on the left connection of the collector field and 15 m on the right.

In the experimental set-up it is taken care that the pipes are installed such that they continuously lead downwards to the pump, membrane expansion vessel and filling valves of the solar loop, see Figure 2. Under these conditions, the steam reaches into the pipes and consecutively passes the temperature sensors when stagnation conditions start and the collector fluid evaporates further in the course of the stagnation situation.

Figure 3 and Figure 4 show first measurement results which were measured with one collector module of an evacuated tubular collector. The fluid is a water/glycol mixture in this measurement. Figure 3 shows the course of the temperatures. Tin and Tout are measured at the collector inlet and outlet. These two sensors are immersed in the fluid. All other temperature sensors are attached to the outer wall of the pipes of the solar loop. Figure 4 shows the pressure (above ambient pressure) in the solar loop measured near to the membrane expansion vessel.

1 Collector, pe = 2 bar, pMAG = 1,5 bar, Fluid = Glycol, VMAG = 33I, 03/2004

Figure 3: Measured temperatures at collector inlet and outlet and along one half of the pipe of the solar loop.

The lamps of the solar simulator were switched on at 18:15h. The different phases of the

stagnation process can be seen in the measurement:

1. phase 18:30h to 19:15h: Expansion of the fluid. Temperature at the collector inlet and outlet is increasing rapidly. The temperature in the pipes increases very little and only due to the fact that the ambient temperature in the room of the test facility with the solar simulator increases from 20 to 25°C.

2. phase 19:15 to 19:30h: The first evaporation occurs and the steam presses the fluid out of the collector. The over pressure at the expansion vessel is 2.15 bar, Tin=130°C.

3. phase 19:30h to 20:00h: phase with saturated steam — emptying of the collector by boiling. The temperatures at the different sensors on the connecting tube increase very rapidly whenever the steam reaches them. The maximum pressure of 2.7 bar is reached. The steam reaches somewhere between the temperature sensors that are 9 and 10 m away from the collector inlet.

4. phase 20:00h to 9:15h: phase with overheated steam. The steam producing power of the collector is reduced because an increasing part of the absorber is not filled with fluid any more. The depth to which the fluid is penetrating the connecting pipe is decreasing. Whenever the fluid/steam front passes a temperature sensor the temperature falls rapidly. Finally at 22:30h the fluid is only 1 m apart from the collector inlet. Later in the course it reaches almost back to the collector inlet. Then there is no motion of the fluid in the connecting pipe any more and the fluid in the loop decreases all along the tube due to
heat losses of the pipe.

5. phase 9:15h to end of measurement: At 9:15h the simulator is switched out and the irradiation drops suddenly from 1070 W/m2 to zero. The collector is refilled by the fluid from the connecting tubes. When the fluid enters the absorber the increase again shortly due to steam produced from heat that was stored in the absorber material.

2 bar, Pmag = 1,5 bar, Fluid = Glycol, VMAG = 33I, 03/2004

Figure 4: Pressure (above ambient pressure) measured in the solar loop at the height of the expansion vessel. At the point in time when the pressure reaches its maximum the collector reaches its maximum steam producing power.

Conclusions

As mentioned before, these are just first measurements. The experiments and their evaluation are ongoing. More results and conclusions to be drawn with respect to the stagnation behaviour of large collector fields will be presented at the conference.

Information on the status and the results of the joint project can be found on the website http://www. Solarkombianlagen-XL. info.

Literature

/1/ Konrad Lustig, Experimentelle Untersuchungen zum Stillstandsverhalten thermischer Solaranlagen, Dissertation, University Karlsruhe, elaborated at Fraunhofer ISE, 2002.

/2/ Konrad Lustig, Matthias Rommel and Dirk Stankowski, EXPERIMENTAL RESEARCH OF STAGNATION IN SOLAR THERMAL SYSTEMS, Eurosun 2000 Copenhagen.

/3/ Hausner, Fink, Stagnation Behaviour of thermal solar systems, Eurosun 2002 Bologna

/4/ Joachim Koschikowski, Neuer Solarsimulator zur Indoor-Vermessung thermischer Solarkollektoren am Fraunhofer ISE, OTTI 2002 Bad Staffelstein

/5/ Joachim Koschikowski, Charakterisierung des neuen Solarsimulators am Fraunhofer ISE, OTTI 2003 Bad Staffelstein

The scientific work is financed by the German Federal Ministry for Environment, Environmental Protection and Reactor Safety (BMU). The authors gratefully acknowledge this support. The authors are responsible for the content of this publication.