The simulation code has allowed to estimate, in transient condition, the performances of the system including solar collectors — tank — building to be heated. Table V shows for each localities and for each system configuration the quantities concerning the period of heating: incident solar energy, energy supplied to the building and for domestic hot water, solar fraction of the system (ratio of the energy supplied by the tank to seasonal requirement) and eventually the system efficiency (ratio of the energy supplied by the system to solar incident energy on the tilted plane); DHW annual energy and annual solar fraction are also reported in the tables. In the period in which building heating is not expected, domestic hot water requirement is completely satisfied, while in the ramaining period of the year an DHW fraction depending on collectors’ surface and tank’s storage volume takes place. Annual solar fractions calculated vary between 30 % and 72% for Cosenza, between 25% and 68% for Rome and between 14% and 41% for Milan.

Figures 4, 5 and 6, each refered to one locality, show the solar fraction and system efficiency values, in ralation to the variation of the collector’s surface and at the different storage volumes. These data only concern the period of heating. Solar fraction grows at increasing of storage volume. The four curves, each referring to a different tank storage volume, have the same solar fraction for collectors’ surface of 4 m2. The bigger the surface, the more the curves are different from each other. A value of solar fraction
variable between 12% and 62% for Cosenza and Rome and between 7% and 36% for Milan has been found. The benefit produced by the increment of storage volume increases with the growing of the collectors’ surface. Figures also show a diminuition of the efficiency of the whole system with the growth of collectors’ surface; for S=4 m2 it evaluates 55% for Cosenza, 42% for Rome, 45 % for Milan. While for, S=28 m2, it assumes a value between 28% and 36% for Cosenza, 23 and 29% for Rome and 30 and 35% for Milan. The more the storage volume increases, the more the system is efficient, both because of the better collection efficiency, as a consequence of a lower mean temperature of the tanking water; and for the smaller frequency with which the maximum temperature in the tank is touched. Even though among the three localities there is a difference in value of solar fraction and efficiency of the system, seasonal energy supplied only for building heating is not so different instead, because of the different period of heating. As example Figure 7 shows the energy supplied to the building for a storage volume 2m3 ; the best performance takes place in Rome. With a 28 m2 collector’s surface it can supply 16.7 GJ, followed by Cosenza with 15.5 GJ and Milan with 13.8 GJ.