After finishing the code, the storage model was validated in two steps. In a first step, the simulation behaviour was compared to measured data from previous system tests using measured hot water draw off curves and storage heat loss curves from several different testing days (Figure 3).

In a second step, the thermal behaviour of the double mantle heat exchanger store was compared with CARNOT’s existing simple water store model — insulated cylindrical hot water storage without any additional heat exchangers — validated by Hafner et al [4]. In order to compare both storage types, the gap between the inner and outer mantle of the double mantle storage model was assumed to be zero.

Both types of validation tests showed a deviation between simulation and measurements of less then 5%.

3. System Simulation

In the next step a complete thermosiphon system composed of solar collector, the necessary piping, the so-called thermosiphon pump (a block setting the thermosiphonic flow rate according to pressure differences in the system) and the newly developed double mantle heat exchanger storage was built up (Figure 4). The implemented technical data is identical to a system which was tested according to ISO 9459-2 at the CENTRE OF EXCELLENCE FOR SOLAR ENGINEERING in Ingolstadt (Table 1).

All blocks in the system are connected via data vectors according to their position in the system. The most important vector is the so called thermo hydraulic vector (THV), in which all relevant values are bundled. It includes the fluid identity, e. g. water or water-glycol mixture, fluid pressure, pressure drop (calculated in the block before), fluid temperature and density. Due to this THV, a realistic system behaviour can be achieved.

In the following, two different thermosiphon systems were simulated according to data measured at Ingolstadt University, using irradiation on the collector plane, the thermosiphonic flow rate and temperatures at collector / storage in — and outlet. The corresponding simulation runs show a good correlation between measured and simulated data, as can be seen in Figure 5 showing the flow rate and the accumulated flow through collector and storage over a whole day.

Fig. 5. Measured and simulated flow rate in a thermosiphon solar hot water heater