The models used for the parabolic trough collector fields are based on the model of Lippke [5] which has been developed for solar thermal power plants. The implementation of these models in Greenius has been described in [6]. The model parameters of the specific collector type have been measured at DLR and published in [7].

3. Comparison with another simulation tool

In order to validate the model for parabolic trough process heat plants, the simulation results shall be compared with measurements of a pilot plant which is just under construction in Ennepetal,

Germany. Therefore the validation of this model will be starting soon when sufficient data is available.

For the validation of the flat plate and vacuum tube collector models comparisons with the simulation program T*SOL have been made. A simple solar system for drinking water heating was used for this purpose. The system has a heat demand of 2700 kWh/a. In Greenius the operation mode “constant load” was used for the auxiliary boiler with a demand of 0.308 kW. A hot water storage of 0.3m3 was used as thermal storage. This storage has a thermal capacity of 17 kWh and a time constant of 250 hours. The flat plate collector Viessmann Vitosol 100 2.5s was used as solar collector. This collector has an aperture area of 2.5 m2 and a conversion factor of 0.8.

The length of the pipes was 8 meters, 7 meters inside the building. The collectors were tilted 30° towards south. The location was Freiburg (Germany). A design temperature of 50 °C was chosen. The number of collectors was varied.

In general, since different models are used for the simulation a total conformity of the simulation results cannot be expected. In the current version Greenius does not have a stratification model for the thermal storage. The missing stratification leads to different temperature behaviour of T*SOL and Greenius. An adaptation can be made using different mean design temperatures. Therefore, the two design temperatures 45°C and 50°C have been considered in Greenius for the comparison. Figure 4 shows the system defined in T*SOL. Figure 3 shows the simulation results of T*SOL and Greenius with varying collector field size.

Besides the missing temperature stratification in Greenius the different definitions of the solar share produce further differences. T*SOL additionally considers the storage losses when calculating the solar share, Greenius not. Therefore, the solar share calculated with T*SOL is a little higher than with Greenius. When considering the model differences between T*SOL and Greenius, the matching of the results is good.