The next set of parameters refers to control settings and auxiliary boiler data (Figure 9). Most significant influences the boiler outlet temperature fsax, ext. The higher this temperature, the lower is the possibility of the solar collector to heat this volume to higher temperatures. Of nearly no importance within the varied values seems to be the collector controller settings for start up and the boiler flow rate.

Figure 9 Dependency of fsav, ext on specific parameter change of control settings and auxiliary boiler data

Figure 10 Dependency of fsav, ext on specific parameter change of temperature sensor positions

In the same way as above the position of the auxiliary boiler sensor is very important (Figure 10). The higher this sensor is placed the less volume is heated by the auxiliary boiler. Less important, but still of influence, is the position of the collector sensor in the store. As lower this sensor, as earlier is the collector pump switched on, when the store is discharged. On the other hand the running time for the collector pump increases. As long as the temperature sensor is positioned around the collector heat exchanger the change of fsax, ext is very small.

Conclusion

Nine different solar combisystems were simulated and deeply analyzed in IEA SHC Task 26 using the same reference conditions and the same simulation tool. For each system between 12 and 30 parameters were varied starting from a base case. These parameters covered climate, collector type, size, orientation, mass flow, store size, store geometries, size of heat exchangers, heights of inlets and outlets, insulation, control settings of thermostats and control strategy of the whole system.

The influence of the parameters was analyzed for each system and general conclusion of the dependency on combisystems were drawn using a statistical approach.

Besides the well known influence of climate, collector size, orientation, and store volume on fractional energy savings, it was found that the store insulation on top and bottom should be around 15 cm (already taking into account additional heat losses at inlets and outlets, sensors etc. by a correction factor). The bottom insulation is not that important, because the store should be cold at the lower part anyway. The store volume that is heated by the auxiliary heater should be as small as possible but still big enough that the heat load can be covered. Otherwise the fractional savings indicator drops significantly.

Literature

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Bales, C., 2003a: Generic System #12: Space Heating Store with DHW Load Side Heat Exchanger(s) and External Auxiliary Boiler (Advanced Version), Technical Report for Subtask C, IEA SHC-Task 26, Appendix 7 to Streicher, W, 2003 Bony, J., Pittet, J., 2003: Generic System #8: Space Heating Store with Double Load — Side Heat Exchanger for DHW, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 4 to Streicher, W, 2003

Cheze, D, Papillon, P., 2003: Generic System #3a: Advanced Direct Solar Floor, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 2 to Streicher, W, 2003 Ellehauge, K., 2003: Generic System #2: A Solar Combisystem based on a Heat Exchanger between the Collector Loop and Space-Heating Loop, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 1 to Streicher, W, 2003

Heimrath, R., 2003: Generic System #19: Centralized Heat Production, Distributed Heat Load, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 9 to Streicher, W, 2003

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Peter, M., 2003: Generic System #9b: Space Heating Store with Immersed DHW tank and External DHW store with Auxiliary, Technical Report for Subtask C, IEA SHC-Task 26, Appendix 5 to Streicher, W, 2003

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Streicher, W., Heimrath, R., 2004: Analysis of System Reports of Task 26 for Sensitivity of Parameters, Technical Report for Subtask C, IEA SHC-Task 26 Weiss, W. (ed.), 2003: Solar Heated Houses — A Design Handbook for Solar Combisystems, James&James Science Publisher