In the following, the methodology of determining the energy payback time is explained by an example of two thermal solar systems. Both systems are domestic hot water systems (SDHW-systems) with the same design parameters: 5 m2 collector area, 300 l total store volume including an auxiliary volume of 150 l. The two systems differ only in materials used for the collector and the supporting frame. The fractional energy savings are equal for both systems investigated.

2.1 Cumulative Energy Demand for Production

The cumulative energy demand (KEAp) comprises the energy required for the production of the goods at all phases, including extraction, mining of raw materials, semi­manufactured products and the production process itself. For all following calculations the values are taken from an extensive database called "Okoinventare fur Energiesysteme” from Switzerland.

Table 1 shows the impact of the use of different materials for the collector on the cumulative energy demand. The basis is system 1 with a collector that has a low cumulative energy demand. System 2 varies only in some collector materials used so that the impact on the energy payback time can be shown. The two systems only differ in absorber coating technique, the casing and insulation of the collector and in the material of the supporting frame. It can be seen that the absorber coating technique has only a minor influence on the cumulative energy demand of the whole system. Concerning the cumulative energy demand the relevant components of the collector are the absorber and the casing material.

The cumulative energy demand has to be determined for each component of the thermal solar system as shown in Table 2. It has to be considered that the store volume of the conventional heating system is reduced by using a thermal solar system. Therefore both systems are credited with the cumulative energy demand of a conventional hot water store of 135 litres (store credit in Table 2). The collector of system 1 is integrated in the roof, saving a large number of rooftiles. The cumulative energy demand for the saved rooftiles is therefore also credited to the thermal solar system.

In addition the cumulative energy demand of the transport of the thermal solar system from the manufacturer to the place of installation has to be considered. It was assumed that a distance of 300 km from the manufacturer to the wholesale dealer is covered with a truck and that a distance of 100 km from the wholesale dealer to the place of installation is covered by a delivery van. The cumulative energy demand for transportation is directly coupled with the total weight of the thermal solar system (including package).

With respect to the integrated mounting mode a credit for the rooftile transport has to be granted. With a general approach that the average transport distance is 400 km and that the transport is carried out by truck, the transport credit amounts to 205 kWh.

Concerning assembly and installation of the thermal solar system no general data base is available. Since the effort of installation varies depending on the kind of thermal solar system, it is calculated with a general approach of 10% of the cumulative energy demand for production of the materials and for the transport.

Table 3 shows the impact on the energy payback time. It can be seen that both systems only differ in the cumulative energy demand for the production that comprises materials used, transport, assembly and installation of the system. All other influences on the energy payback time such as cumulative energy demand for operation and for maintenance and the primary energy saved by the solar system are equal for both systems investigated.

System 1 with a minor cumulative energy demand for production has an energy payback time of 1.4 years. The cumulative energy demand for production of system 2 is 43 % above the value of system 1. This results in an increase of the energy payback time to 2.1 years.

1.2 Cumulative Energy Demand for Operation

The cumulative energy demand for operation includes the electrical power consumption of the solar loop pump and the electrical power consumption of the controller. The power consumption in [W] is multiplied by the respective operating hours of the pump and the controller. For the determination of the cumulative energy demand the resulting electrical power consumption has to be multiplied by the primary energy equivalent for electrical power.

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