GEA is supplying plate heat exchangers for seawater desalination plant
Clean drinking water is a valuable asset. More than a billion people have no access to this primary staple commodity. And global water consumption will continue to increase. Estimates predict by up to 40 percent by 2025. Solutions are called for.
This is also what Dr. Ulrich Plantikow thought 10 years ago, when he built a regenerative seawater desalination plant on the German island of Rügen. The mobile pilot plant uses the power of the wind to produce fresh water from seawater. Plantikow's approach was met with great interest throughout the world. Particularly in tourist regions and in countries where water is scarce, fresh water obtained by desalinating seawater can solve acute shortage problems. And this is why in previous years there has been a certain expectancy from the small research plant run by WME GmbH on the Baltic island of Rügen.
The pilot phase was completed some months ago. WME GmbH erected the first commercial-scale plant on the Greek island of Symi, 23 kilometres to the north of Rhodes. Up until now the island was supplied with drinking water by ships. GEA PHE Systems supplied the plate heat exchangers for this pioneering plant. A 350 kilowatt wind turbine produces electricity for a vacuum evaporator that can supply up to 20 cubic metres of desalinated water per hour at maximum wind speeds. This is sufficient to supply around 3000 people per day. The seawater desalination plant works on the principle of mechanical vapour compression (MVC) using a falling film evaporator. The seawater flows through two plate heat exchangers arranged in parallel and is preheated.
GEA installed two NT 150 plate heat exchangers fitted with titanium plates. The gasketed heat exchangers are distinguished by optimised plate profiling that requires less heat transfer surface to achieve the same performance. This means that the units can be kept small in size, which is an essential prerequisite considering the restricted space conditions available in the containers. These plate heat exchangers were developed using the latest hydro- and thermodynamic know-how in cooperation with the University of Hannover.
A degassing unit and a vacuum pump extract all non-condensable gases from the water so that a temperature of between 40 and 85 degrees Celsius and a steam pressure of between 100 and 550 millibars are sufficient to evaporate the water. A heat recovery system ensures that the efficiency of the plant remains attractive. The power consumption is between 6 and 15 kilowatt hours per cubic metre of drinking water. There are certain special aspects to be considered when operating plants using regenerative energy and in particular with wind power. As the wind does not blow at a constant rate the whole time, the electrical power generated also fluctuates severely. The desalination plants cannot be operated at a constant performance as with conventional systems and have to be adapted to suit the severe fluctuations in the power available. The WME GmbH system operates using modified steam compression - a modified heat pump process.
This method makes it possible to take up the complete generator output even under blustery wind conditions and to use this at high efficiency for desalination. Instead of surplus electrical energy the surplus drinking water produced during strong wind periods is stored in water reservoirs. During light wind periods the stored water can be used to offset any deficits. A restart after a period of wind calm is possible in seconds. Of particular interest in this development is that besides the exclusive use of regenerative wind power no eco-unfriendly chemicals have to be used in the desalination process. These chemicals could cause widespread ocean pollution where the concentrated seawater from the desalination plant is discharged into the sea.
