The system design follows two different approaches. The first one is the “Compact System” for very low fresh water capacities between 100 and 500 l/day. The second one is a “Two-Loop System” for higher capacities.
The low thermal capacity of MD-modules enables a quick change of feed volume flow and operation temperature without instabilities in the desalination process. Thus the MD-modules can directly be connected to a corrosion free solar thermal collector without any heat storage. Intermittent operation is possible without reasonable efficiency losses.
Fig. 4: Compact System - sketch of the principle set up (left), demo-system installed in December 2006 in
Tenerife, Spain (right)
The main components of the Compact System are a 500 l feed storage (not represented in the sketch of Fig.4, left side), 1 MD-module, a 6- 7m2 sized solar thermal collector, a pump and a PV module for the electrical power supply of the pump and control system. While the feed storage is mounted above the collectors, most of the hydraulic components are installed in a closed housing covered by the back site of the collectors. However, the cold feed water is pumped into the condenser channel. Afterward the pre-heated feed water leaves the condenser outlet and enters the solar thermal collector which is directly connected to the condenser outlet. The feed temperature is increased by about 5 to 10 K and is re-circulated to the evaporator inlet. The advantage of this configuration is its simplicity and the good efficiency because there are no additional heat losses by a heat exchanger. The disadvantage is that no common flat plate collectors can be used, but the collectors must have sea water resistant riser and header tubes.
The distillate flows to a fresh water tank. The brine rejected from the evaporator outlet of the MD module is recirculated to the feed storage. So the salt concentration as well as the temperature of feed water in the feed storage increases over the day while the content of water decreases due to distillate production. The feed storage is refilled automatically when a certain level or temperature is reached.
Six Compact Systems were constructed, installed and operated since December 2004 in Pozo Izquierdo (Gran Canaria), Alexandria (Egypt), Irbid (Jordan), Kelaa (Morocco) and Freiburg (Germany). In December 2007 an improved compact system with a maximum daily capacity of 120 litres was installed in Tenerife, Spain (Fig. 4, right hand side).