For system simulation calculations an empirical simulation model of the MD-module must be developed which is based on its measured performance data. Exact performance measurements must be feasible to determine improvements concerning new module constructions. The dynamical behaviour of the MD-module is a very important property for the system design with respect to an intermittent operation of a solar thermal collector field as heat source.

A test facility allows the determination of the specific energy consumption of the modules for different inlet temperatures and different feed volume flows. Also the dynamic start up and cool down behaviour can be investigated.

The specific energy consumption and the GOR value were determined depending on the feed volume flow and the evaporator inlet temperature. The measured parameters are the distillate volume flow, the feed volume flow, the condenser in — and outlet temperature and the evaporator in — and outlet temperature. The additional heat supplied into the system from outside can be calculated from the temperature difference between the condenser outlet and the evaporator inlet, the feed volume flow and the specific heat capacity cp of the feed. The heat demands for different feed flow rates between 200 and 400 l/h depending on the evaporator inlet temperature are shown in the right hand diagram of figure 6. The diagram to the left shows the corresponding distillate volume flow. The GOR value can be calculated by dividing the product of distillate output and the specific enthalpy of evaporation (m-distiiiate * r) by the heat input (Q^n). For example the calculated GOR for a volume flow of 350 l/h at an evaporator inlet temperature of 75°C (r70°C=2321.5 kJ/kg) is 5.5. The specific energy consumption per cubic meter distillate for these operation conditions is in the range of 117kWh / m3.

Figure 7: Investigations on the dynamic performance of the MD-module.

For larger systems a pressurised heat storage is used if the investment costs for the MD — modules are much higher than the additional system costs for the storage system. In that case the MD-modules can be operated 24 hours a day.

The advantage of a storage system concerning the energy efficiency is that the system can be operated at the optimal working point (temperature and volume flow) for a long part of the daily operation period. On the other hand storage heat losses and lower collector efficiencies caused by higher collector temperatures which are necessary for a reasonable storage, decrease the energy efficiency.