Conductive composites for seawater desalination
Seawater is plentiful, but drinking water is scarce. Desalination plants can convert seawater, but these plants use tubes made of special steel or titanium. Thermally conductive plastic composites can alternatively be used for heat exchangers in seawater desalination plants to replace expensive and hard-to-find titanium alloys.
Drinking water is a scarce commodity - and this is no longer true for desert regions alone. In Mediterranean countries like Spain and Portugal, too, fresh water is scarce in the hot summer months. Plants that desalinate seawater and convert it into drinking water are therefore on the rise.
The principle of desalination: seawater is sprayed onto pipes through which hot gas or water flows and which are thus heated. Pure water evaporates from the seawater, leaving behind salty waste liquor. This process places a wide range of demands on the material and its properties: The tube material should transfer heat as well as possible and be particularly robust against corrosion and the formation of deposits - over a long period of time. For optimum evaporation, the tube material must also be easily wetted by the seawater. Manufacturers therefore use titanium and high-alloy steels, but these are very expensive and only available in limited quantities, as demand for titanium is constantly increasing in the course of lightweight construction (e.g. in the aviation industry).
Polymer composite tubes as an alternative to titanium tubes
Fraunhofer IFAM has developed an alternative to titanium tubes for this application: Tubes made of polymer composites. Up to 50 percent by volume of copper microfibers have been incorporated into the material. This does not change the processing properties of the composite, which can still be processed like a polymer. The special feature here is that these polymer composites have plastic properties but are also good at transferring heat. Another advantage is that they can be produced by extrusion as continuous goods and are correspondingly less expensive than metal pipes.
In a pilot seawater desalination plant, it is possible both to test the thermal conductivity and to check how much coating is formed on the pipes by microorganisms and how strongly the material corrodes in the salty environment. Based on these results, the composite properties and thermal conductivity can be optimized at Fraunhofer IFAM. On the other hand, the researchers have adjusted the evaporation process so that it takes place at a temperature of only 70 degrees Celsius – thus, 70 degree hot gas flows through the tubes. This offers several advantages: Less deposit forms on the tubes, the material does not corrode as quickly and the pressure difference between the inside and outside of the tube does not become as great.
Thermally conductive polymer composite with versatile applications
The field of application for the material is not limited to heat exchangers for seawater desalination, but it can also be used in process plants in the food and pharmaceutical industries.
Shaping and functionalization are core competencies of Fraunhofer IFAM. In the "Functional Composites" group, Arne Haberkorn develops thermally conductive composites for seawater desalination plants. In the "Smart Systems" department, the group has extensive know-how and the latest technologies in conductive composites. The fields of application range from environmental technology to the food and pharmaceutical industries.
The development of polymer-based heat exchangers for use in seawater desalination plants was funded by the German Federal Ministry for Economic Affairs and Energy (IGF project 16959 N).