In this paragraph the main characteristics of the studied solar heating system and the elaborated physically-based model applied to the system are described in details.

1.1. Main characteristics of the solar heating system

The monitored combined solar heating system installed at the campus of Szent Istvan University (SIU), Godollo, Hungary is sketched in Fig. 1.

The term combined means actually that the installation has more than one consumers. It preheats water for an outdoor swimming pool and, in the idle period of this operation, provides domestic hot water for a kindergarten nearby. The main system components are the flat plate solar collector field, with a total area of 33.3 m2, oriented to the south and its inclination angle is 45°, plate heat exchangers, a 700 m3 outdoor swimming pool with a surface of 350 m2 and a 2000 litre solar storage tank. Auxiliary gas heated boilers are also included which operate in the same time with the solar heating if necessary. According to the notations above the following parameters are monitored on the system: temperatures (T, °C), specific solar irradiance (I, W/m2), volumetric flows (v, m3/s). Measured data are available from the year 2001, apart from minor interruptions.