The advantage of the chosen solar collector (figure 8) for small systems is its resistance against corrosive liquids. Its absorber consists of selectively coated glass tubes and silicone header tubes. Therefore the absorber is resistant to hot sea water and the collector can directly be implemented into the feed circuit of the MD-module. No additional collector loop and heat exchanger is necessary. An aluminium zigzag-reflector is mounted behind the glass tubes to utilise the whole aperture area for the collection of solar radiation. This kind of collector was developed in our institute for the SODESA-project and investigated during a one year field test at a desalination plant in Gran Canaria from summer 1999 to summer 2000 (Hermann 2002). The tests with respect to performance and long term resistance were successful, identified week points were improved for future collectors. The graph in figure 9 shows the collector efficiency curve. The range of operation for MD is between AT/I of 0.06 and 0.09 (K m2)/W. The collector efficiency in this range is between 53% and 40%.

Small scale test system

A compact experimental desalination system as sketched in figure 10 consisting of the MD module, a corrosion free solar collector, a pump and a temperature hysteresis controller was installed on the outdoor test site of our institute. Sensors for temperatures, volume flow and solar insolation were integrated for the monitoring of the operational parameters.

A PV-power supply was not integrated but all electrical parts were supplied by the grid.

Since the energy for the distillation process is almost independent from the salt concentration, the system was operated with drinking water to avoid trouble with corrosion at auxiliary components.

The results from the experimental investigations (figure 11) showed that the handling of the system is quite easy and long term operation periods without maintenance are possible. The performance of the system is shown as an example for one day in June 2002 in the diagram of figure 7. The system starts operation at 10:15 h when the solar insolation was in the range of 700 W/m2. The feed flow is manually adjusted at about 225 l/h. The maximum evaporator inlet temperature rises up to 90°C. At the same time the maximum of distillate production reached 15 l / h. The total amount of distillate gained on that day was about 81 litres. The maximum of distillate gain during the test period of summer 2002 was about 130 l/d under the meteorological conditions at Freiburg in central Europe.

System simulations

For system simulation calculations an empirical simulation model of the MD-module was developed which is based on its measured performance data. The model was implemented into the simulation program for thermal systems ColSim (Wittwer 1999). The system design consists of one MD-module with 7m2 membrane area connected to a 6 m2 SODESA-collector, a pump and a distillate storage.

One-day and annual simulation calculations for three different locations, (Eilat in Israel, Muscat in Oman and Palma de Majorca in Spain) were carried out using weather data sets of these locations. It can be seen from the graph in figure 12 that for example in Eilat a maximum distillate output of 28 l per day and m2 collector area (equal to total amount of 161 l/d) can be gained on a day with good weather conditions during summer. The minimum production rate is in the range of 11 l/d and square meter collector area (equal to total amount of 63 l/d) in December. Two different control strategies were investigated (Wieghaus 2002).

Since the most common small scale solar desalination systems in the third world countries are solar stills a briefly comparison between the simulated performance of a MD-system and the performance of a simple solar still (V. Janisch, 1995) is drawn in figure 13. The used insolation data for the performance calculations were averaged from the weather data sets of Eilat. Since the MD-system is modular and each module has a maximum distillate capacity in the range of 150l/d, the number of modules rise step by step (the graph represents these steps since the system performance rises non-linear when a new module is attached). The comparison shows that the simulated MD-system has a 4.5 times higher distillate output.

The development of small stand-alone operating desalination systems is an important task to provide people in rural remote areas with clean portable water. The fact that the lack of drinkable water often corresponds with a high solar insolation speaks for the use of solar energy as the driving force for a water treatment system. Membrane distillation is a process with several advantages regarding the integration into a solar thermally driven desalination system. Simulation calculations for such systems with module characteristics derived from several experimental investigations were carried out for different potential installation locations. The simulation results show that a very simple compact system with a collector area less than 6m2 and without heat storage can distill 120 to 160 l of water during a day in summer in a southern country. Experimental investigations on a testing system are currently carried out at Fraunhofer ISE. New MD-modules will be developed aiming at a higher GOR value and a lower pressure drop.