The typical thickness of a PUR storage insulation is in the range of 10 to 12 cm [5]. Within this study the thickness has been varied between 7.5 cm and 17.5 cm (base case 15 cm). The results of the optimisation process for these variations are shown in Figure 3 (d) and Table 1.

The optimisation with 7.5 cm of insulation leads to a system with a storage device capacity of 0.77 m3 and a solar collector area of 12 m2. Thicker storage insulation leads to an improvement of the thermal behaviour of the system. Therefore with the same dimensioning and investment cost but thicker insulation a dot in Figure 3 would shift towards higher primary energy savings and simultaneously towards smaller additional costs (due to a reduction of running costs). Thus, the optimisation curves in Figure 3 (d) shift down and flatten with an enlargement in insulation. A solely parallel movement of the curves to smaller additional costs would mean that the point of intersection of the tangent would move left, whereas a flattening of the curves would lead to a point of intersection with higher primary energy savings. Figure 3 (d) shows that the optimised

system configurations shift towards smaller primary energy savings and Table 1 indicates that the resulting dimensions are getting smaller. This means that the reduction of additional cost dominates the increasing primary energy savings. The bigger the thickness of insulation gets the smaller the distance between the resulting curves become. With an insulation thickness of 17.5 cm nearly no difference is apparent to the base case. A cross check taking into account increasing investment costs with thicker insulation showed that the results summarised in Table 1 change insignificantly.