The conventional dimensioning of a solar heating system for one — or multi-family houses is commonly based on a trade-off between the solar fraction and the level of utilisation [2], whereas the customer and user might rather be interested in elements like additional cost and primary energy savings. To meet these requirements a different dimensioning directive was introduced [1][3]. The simulation program TRNSYS was coupled with the optimisation program GenOpt for the dimensioning in response to this directive. A short description and the resulting optimal dimensions of the MaxLean system concept are given in the following and serve as basis for the subsequent sensitivity analysis.

3.1 Dimensioning directive

The objective function minimised by the optimisation algorithm is the cost/benefit ratio described in Eq. (1):

a annuity factor

I0 total investment costs of the solar thermal system

BMaxLean annual operation costs of the MaxLean system concept (including the heating circuit)

Bref annual operation costs of the conventional reference system

Eprim, sav primary energy savings

As long as a solar heating system is not economically rewarding, the primary energy saving is the preferential merit on which value is laid. The additional cost for these primary energy savings consists of the additional investment cost minus the difference in operation costs between the conventional and the solar assisted heating system. The investment costs of a solar heating system are calculated by Eq. (2), (cf. [2]):

A ( V 6536

I0 = 2559€ + *368€ + 3983€I I Eq 2

m2 t m3 I 4′

Acoll flat-plate collector area

Vstor storage device capacity

The annual primary energy savings are calculated from the difference between the energy consumption of the solar and non-solar heating system, as well as the embodied energy of the solar system. (Thus, the embodied energy of the non-solar heating system —i. e. a water heater store— is not accounted for). The gas and electricity consumption is converted to primary energy consumptions. To obtain annual values of the embodied energy the total amount is divided by the service lifetime.