The general methodology for the determination of the energy payback time of solar combisystems is the same as explained above. The particularities that arise from various different system concepts for the realization of solar space heating will be explained in the following. This will be demonstrated by calculating the energy payback time for four solar combisystems — two combisystems without integrated burner and two combisystems with integrated burner.

4.1 System Boundaries

In order to be able to compare the energy payback time of solar combisystems with different system concepts, system boundaries have to be defined.

The system boundary for solar combisystems without integrated burner, i. e. where the boiler is a separate component, is directly at the store. The auxiliary heating loop with boiler and hydraulic station is not taken into consideration as these components do not represent specific solar components. They are also necessary for a “conventional” heating system without using solar energy. The same applies to the hot water loop. Components that are situated beyond the system boundary are not considered.

Solar combisystems with integrated burner are thermal solar systems where a gas or oil burner is integrated in the store. As for combisystems without integrated burner, the system boundary is directly at the store. But in this case the balance comprises the integrated burner and the hydraulic station. As flue pipes are situated outside of the system boundary, they are not taken into account.

1.4 Credits

In order to be able to compare solar combisystems with integrated burner with solar combisystems without integrated burner, standardized components are defined that can later be credited to solar combisystems with integrated burner. This will be done by special credits, as already described above with the store credit for the hot water preparation. Depending on the system concept these credits comprise burner, hydraulic station and controller of the heating loop. The single components are balanced with the average values indicated below.

The reference hydraulic station consists of a 3-way valve, a mixer, a pump, 3 m copper tubing with insulation and an expansion vessel of 35 l for the heating loop. The cumulative energy demand for the production of this reference hydraulic station amounts to 247 kWh.

For the reference controller of the heating loop a controller with a weight of 1.25 kg (including temperature sensors) is defined. The cumulative energy demand for the production of this reference controller amounts to 89 kWh. The nominal power of the reference controller is specified with 3 W. This results in an annual energy consumption of 26 kWh. With the primary energy equivalent for electric power (3.8 kWhprim/kWh) the cumulative energy demand for the operation of this reference controller amounts to 100 kWh per year.

As reference burner a standard burner for oil or gas with a weight of 45 kg is specified. It is composed of different materials like mild steel, stainless steel, aluminium, copper and polypropylene. The cumulative energy demand for the production of this burner amounts to 972 kWh.

The application of the credits will be demonstrated with the help of the calculation of the cumulative energy demand for the production of four different solar combisystems.

As shown in Table 8, a solar combi-system with integrated burner can be credited with a maximum cumulative energy demand of 1308 kWh (system 5). In the case that the controller of the combisystem also includes the control algorithm for the heating loop, credits for the controller are also applicable for solar combisystems without integrated burner, as can be seen in system 3.

As the operation of the controller consumes electrical power, the amount of power consumption has to be credited too. Therefore the cumulative energy demand of the operation will be reduced by a credit of 100 kWh that represents the cumulative energy demand of the reference controller of the heating loop.

1.5 Example

Table 9 contains the calculated results for the four solar combisystems. The determination of the primary energy that will be saved by the thermal solar system during its lifetime will be explained below.

The yearly primary energy demand of the conventional system includes the primary energy demand for hot water preparation as well as the primary energy demand for space heating. According to the European draft standard prEN 12977-2 an amount of 2945 kWh is assumed for the hot water consumption and an amount of 644 kWh is considered for heat losses of the store. The space heating demand of a single family house with low energy consumption standard and approximately 130 m2 heated area is 9090 kWh. This results in a total energy consumption of 12679 kWh per year. Taking into account the efficiency of the boiler of p = 85% and the primary energy equivalent of oil or gas with 1.11 kWhprimar/kWh yields to the primary energy demand of the reference system Qconv, tot. Finally the primary energy saved is calculated by subtraction of the auxiliary heat demand from the primary energy demand of the reference system.

5. Conclusion

The energy payback time is a suitable method for the integral assessment of thermal solar systems. Solar domestic hot water systems have energy payback times between 1.3 to 2.3 years. For solar combisystems typical energy payback times are slightly higher, from 2.0 to

4.3 years. Taking into consideration that thermal solar systems have a minimum lifetime of 20 years or more, the substantial potential for saving of primary energy has hence been demonstrated.