In order to demonstrate uncertainty in modelling solar systems a large solar hot water system, is employed. A schematic diagram of the system is shown in Fig. 1. The system consists of a collector array, a storage tank, solar pump and auxiliary heater. A differential thermostat is used which compares the temperature at the exit of the collectors and the storage tank and gives a signal to switch on the pump. The collectors employed in this application are flat plate collectors. Their characteristics are shown in Table 1.

The characteristics of the solar system are shown in Table 2. These are obtained from an optimization exercise of the same solar system presented in [2].

The solar system examined can satisfy the hot water needs of 10 houses or flats or any other similar application of same hot water requirement. For this application, a hot water consumption (load) profile is required. This load is subject to a high degree of variation from day to day and from consumer to consumer, however, it is impractical to use anything but a repetitive load profile. This is not quite correct for the summer period, where the consumption pattern is somewhat higher due to frequent bathing. However, during this period, the temperature requirement for hot water is not as high as during winter. Consequently, the total thermal energy requirement is reasonably constant throughout the year. For the present study, the hot water consumption profile, illustrated in Fig. 2 is used. The same daily hot water consumption profile, of 120 liters at 50°C, is assumed for each house (families of four persons, 30 liters/person).

Fig. 2 Hot water daily consumption profile for one family