Previous experiences show that service and maintenance mainly consist of general revision tasks like checking the concentration of the heat transfer fluid, control of the impermeability of the system, primary pressure of the expansion vessel, control of the system operation pressure etc. In general replacement of certain components is not necessary. These revision tasks mainly cause labor costs. Therefore for the calculation of the cumulative energy demand for maintenance only the driving distance of 30 km (one­way) with a passenger car is considered. Furthermore it is assumed that the inspection is done once a year.

1.3 Primary Energy Saved

The amount of primary energy saved by the thermal solar system PEAsub is determined by the difference between Qconv, tot and the auxiliary primary energy demand required by the thermal solar system Qaux, tot. Here, Qconv, tot represents the total primary energy requirement of a conventional reference system that is necessary to meet the hot water and in case of a combisystem also the space heating demand.

The energy demand of a conventional domestic hot water system as well as the heat losses of the domestic hot water store are based on the European draft standard prEN 12977-2 which specifies a unique European reference system. The yearly heat demand for domestic hot water preparation (including heat losses of the store) is 3589 kWh.

For the determination of the yearly energy demand of the reference system (in the form of oil or gas) Qconv, the efficiency of the boiler (p = 85 %) of the conventional (non-solar) reference system has to be considered. Taking into account the primary energy value of gas of 1,11 kWhprimar/kWh, the yearly primary energy demand of the conventional reference system Qconv, tot amounts to 4687 kWh/a.

The auxiliary primary energy demand required by the thermal solar system can be calculated based on the fractional energy savings. For the calculation of the fractional energy savings the energy saved by the thermal solar system is compared with the energy demand of a conventional (none solar) system. System 1 and system 2 both have fractional energy savings of 55%, which lead to an auxiliary primary energy demand Qaux, tot of 2109 kWh per year.