The energy payback time is the period, the system has to be in operation in order to save the amount of primary energy that has been spent for production, operation and maintenance of the system. Based on an overall consideration a system only contributes to the saving of our resources if it is operated longer than its energy payback time.

With regard to the determination of the energy payback time arguments are the same valid as discussed for the cost assessment in the previous section: Concerning the systems with hot water stores the listed energy payback times represent only a rough estimation. For systems based on advanced storage technologies it was not possible to determine reliable values of the energy payback time due to the lack of appropriate data.

5 Conclusions

With regard to systems that achieve fractional energy savings of approximately 50 %, the lowest value of the heat price is found for the reference system with 1 m3 store volume and 35 m2 vacuum tube collector with 0.18 EURO/kWh. Partly more expensive are system designs that lead to the same fractional energy savings by using larger store volumes and smaller collector areas. At present, it is more cost effective to enlarge the collector area instead of an enlargement of the store volume. The reasons for this are the following: On one hand the subsidies that were accounted for the determination of the inexpensive value of the heat price are related to the size of the collector area and not to the store volume. On the other hand collectors are already today produced in large quantities and therefore offered relatively cheap. A totally different situation arises if the store volume is enlarged: Large hot water stores are only produced in small quantities or sometimes even as a custom made product. A consequence of this fact is that a high potential for cost reduction can be realised by using large standardised stores.

With regard to the solar heat prices the presented results showed that even today it is already possible to cover approximately 50 % of the heat demand by solar energy at moderate costs. Depending on the chosen configuration of store volume and collector area or collector type respectively, the resulting heat prices are similar to the ones of typical combisystems already offered on the market /2/. However, the fractional energy savings of these systems only amounts to 20 — 30 %.

The step over the “50 % fractional energy saving borderline” from the solar supported conventional heating system to a fossil supported solar heating system is quite small. The

potential of the environmental benefits of such advanced solar combisystems is shown by the fact that the energy payback times for all investigated systems were significantly below the expected lifetime of the systems.

The cost-effective, efficient and environmentally friendly storage of heat is one of the key technologies for the further development of solar technology. Looking on the current building situation and the amount of buildings that have to be retrofitted, it is obvious that a very large potential market exists for relatively small thermal solar systems.

Due to this facts the IEA (International Energy Agency) established within their Solar Heating and Cooling Program an corresponding working group named Task 32 „Advanced Storage Concepts for Solar Buildings". Within this task European manufacturers and research institutes will work together during the next four years in order to bring storage technology, and in this context also solar technology, at least one important step forward.

Literature:

/1/ H. DrQck, H. Kerskes, W. Heidemann, H. MQller-Steinhagen: Solare Kombianlagen der nachsten Generation — Advanced Solar Combisystems, Tagungsband zum zwolften Symposium Thermische Solarenergie, Seiten 59 — 63, Otti, Regensburg, 2002,

ISBN 3-934681-20-4

/2/ H. DrQck, W. Heidemann, H. MQller-Steinhagen: Comparison Test of Thermal Solar Systems for Domestic Hot Water Preparation and Space Heating, proceedings of EuroSun 2004, to be published

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