The area of the collector is varied between 2 and 16 m2, and for each, the volume of the storage vessel is varied between 0.15 and 10 m3. Although the latter value is perhaps a little high for a single family house, the simulations are carried out to show the potential and limitations of the system. The results are compared to those of the base case, where a 3 m2 vacuum collector with a 150 l storage vessel is used solely for DHW production. The added savings on primary energy demand for space heating, DHW and hot fill are shown in fig 3.

Fig. 3. Savings on primary energy — compared to base case — as a result of the solar collector contributing to space heating, DHW and hot fill as a function of collector area and at different storage vessel sizes

The solar contribution to the total primary energy savings for space heating is calculated assuming that in the absence of a solar collector, the heat has to be generated with a boiler with an efficiency of 95% (on lower heating value). Similarly, for DHW the efficiency of the boiler is assumed to be 75%. For the hot fill the savings are calculated assuming that in the absence of the solar collector, the electricity has to be generated in a power plant with a primary energy efficiency of 50%. When the solar collector cannot supply enough heat for the hot fill, there is still a gain in primary energy because the heat is then generated by the boiler with an efficiency of 75% rather than in the electricity plant at 50%.

From Figure 3 it appears that even with rather modest collector sizes in the order of 8 m2, there is a substantial saving on primary energy of 15-20 kWh/m2a over the base case. This corresponds to a solar fraction of about 45% of the total heating demand for space heating, DHW and hot fill. It also appears that at this collector area, the size of the vessel is of secondary importance, as long as it is at least 0.6 m3.