Greek desalination plant powered by floating wind turbine

Any successful strategy for meeting society's current and future energy needs while simultaneously mitigating climate change must contain a prominent role for wind energy. Electricity generated from many land-based wind projects already compete favorably with coal and natural gas.

The potential of offshore wind energy is enormous. One key to realizing this potential will be the development of floating platforms that will allow the winds over deep waters to be harvested without the need for bottom-mounted foundations which would be prohibitively expensive.

Of course, floating wind turbine platforms present a number of challenges in their own right and Researchers around the world are working to overcome them. One such effort that has somehow operated "under the radar" is a combined floating wind turbine-desalination plant that has been developed and deployed by a group of Greek researcher centers, companies and universities.

This article appears in the April/May 2008 issue of WindTech International -- an outstanding publication of the most up-to-date information on wind energy developments around the world. (GW)

Floating Wind Turbine

By Theodore Lilas and Nikatas Nikitakos
WindTech International
April/May 2008

Transportation of drinkable water from mainland or bigger islands is an incon­venient solution to the water shortage problems, of small islands in southern Europe because with such transportation the cost of the water goes up to 10 per cubic metre. Desalination of seawater can be one of the solutions technology can provide. The cost of production is significantly lower. However, there are problems in this process related to the high energy demand, the use of chemi­cals in sensitive environmental areas, and the disposal of brine. But by combin­ing those resources that are to be found in abundance in these islands, such as wind, sun and sea water, it should be possible to produce one resource that they are poor in: potable water.

Based on this concept, a floating, autonomous desalination unit has been designed and developed by a group of Greek companies, universities and research centres. A pilot unit has been created that uses sea water as feedwater, and the wind and sun as energy sources for producing 70,000 litres of potable water daily, a quantity covering the daily needs of three hundred persons.

The pilot unit was anchored, and has now been working for 9 months, off Iraklia, a small island in the Aegean Sea. Many other small islands in this region experience permanent water shortages during the dry period, which coincides with the high tourist season of the Greek archipelago.

The Wind Generation Component

For a number of reasons the current trend in wind energy production is to build offshore wind farms. These reasons include higher wind speed, improved wind profile, and limited environmental impact.

In northern Europe, offshore wind farms are gaining in popularity all the time helped by the relatively shallow seas for some distance away from the coast (e.g. in Baltic areas), which allows the wind generators to be fixed to the seabed.

In the Mediterranean region, as well as in Japan and the USA, where the sea depth is much greater, big floating wind farms can be a suitable solution, and therefore a lot of research effort is cur­rently invested in this topic.

The Problem of Water Scarcity in the Mediterranean Islands

The problem of water scarcity and short­ages during a dry period is quite com­mon in many parts of the world. This is the rule for most of the hundreds of islands in the Mediterranean region that experience seasonal population increase and water scarcity during the summer period every year.

Desalination, which is the process of removing soluble salts from water in order to render it suitable for drinking, irrigation and industrial uses, is typi­cally used in coastal and island regions for addressing water scarcity problems. Desalination of underground (normally brackish) water risks ending with the intrusion of seawater into the under­ground water table. The above facts favour the use of desalination units in the islands for the production or potable water from sea water. However, desalination units are energy-demand­ing installations and therefore the price of the water produced remains relatively high. Moreover, the cost of producing electricity in the islands is already sig­nificantly higher than on the mainland because they are not connected to a national power grid. As an alternative wind turbines have been installed on many islands, but the creation of wind parks on small islands faces' a lot of difficulties, and in most cases becomes inconvenient, due to the lack of infra­structure such as access roads, construc­tion companies and equipment (cranes, bulldozers, loaders, etc). In addition, the lack of connection to a national power grid tends to make over-reliance on wind power inadvisable. Thus the creation and maintenance of wind parks on isolated, small islands is a task fraught with dif­ficulties that has often proved unviable from the economic point of view.

Other major problems for desalina­tion units include the environmental impact of conventional onshore units. The water intakes, the brine composi­tion (including the chemicals that are commonly used in traditional units), and the discharge of brine (with the chemicals) in an enclosed area, are all problems that can have a serious impact addressed and solved by the floating unit described in this article, which is powered by environmentally friendly energy sources and has the fur­ther advantages of:

• reduced cost of connection between the units, since long network trans­mission is not required;
• the possibility of placement of the unit far from villages, so that it does not bother the residents;
• the possibility of transporting the unit, so that it is possible to achieve better utilisation in different regions depending on the conditions.

Technical Solution

The wind powered floating desalination unit developed and described here is an integrated solution that combines all the previously mentioned advantages.

A 30KW wind turbine (Pitch Wind Sweden) produces and supplies, through advanced electrical and electronic ener­gy conversion components, energy to a desalination unit that uses a reverse osmosis technique to produce 70,000 litres of potable water from the sea on a daily basis. No chemical additives are used in the desalination process. The whole plant works on a special floating platform. The platform is designed in such a way that the wind turbine and the whole system will work even in adverse weather conditions. The result is an eco­logically safe sea water treatment plant for producing potable water for isolated, small islands. The system is composed of several subsystems 'wind turbine, desali­nation unit, pumping unit, etc' that can be exploited independently.

The system described represents the first ever fully-functional floating wind turbine anywhere in the world, since the unit can­not be considered to be just a prototype because it has been using the power that it produces in an embedded fully opera­tional desalination plant for the last 15 months. For the last nine of those months the unit has been anchored near the island of Iraklia in the Aegean Sea.

During this time the desalination unit's production and energy efficiency has been improved. Major advances in comparison to conventional desalination units have involved minimising the scaling and foul­ing effects on the membranes, increasing the energy efficiency of the cycle and avoiding any chemical treatment.

The floating water production unit is autonomous, which means that no con­nection to a national power grid or diesel generator is required. Because it is a floating structure, it is possible that larger units can be built in ship­yards and towed to wherever they are to be installed. It is obvious that such installation need not be permanent but can be periodical or occasional for serv­ing specific requirements. The structure complies with the marine safety regula­tions and the requirements of the clas­sification societies.

An important innovation of the current system is that it is adapted to oper­ate with a varying power input, which enables it to use all the available wind power. Thus, when the wind speed is higher more power is generated and consumed, consequently increasing me quantity of the potable water produced. A further advantage of the system is that it is fully automated, remotely control­led and can be tele-operated.

The Platform

The first version of the supporting plat­form measures 20 x 20 metres. As can be seen in the figures, the platform includes five metallic floatation cylinders. The central cylinder is linked by a tubular truss to four cylinders placed around the circumference of the platform. The cen­tral cylinder is divided into three com­partments. At the bottom is the potable water compartment, in the middle is the desalination plant compartment, and at the top is the control room compart­ment. A 22-metre high tower, the base of which rests on the central cylinder, hosts the wind turbine.

The system's design and development uses approaches and research results drawn from several relevant scientific domains. The construction of the plat­form took advantage of the significant expertise available in the shipbuilding sector and this contributed to the devel­opment of a particularly cost-effective system. All the main components are provided by European manufacturers.

The current version provides an aver­age water production up to 70,000 litres daily, which corresponds to the needs of approximately three hundred persons. Larger units are technically attainable and economically more efficient. The next version is expected to produce more than 1 million litres per day, with only a fourfold increase in equipment costs.

The system has so far faced winds of up to 120km/hour, without any prob­lem. At the same time the platform is actually a research laboratory, carrying several sensors, data measurers and log­ging units that collect information on wind and water-related parameters. Thus it will provide the research commu­nity with data that will help to support the increasing use of renewable energy sources by developing economically effi­cient floating wind parks.

Conclusions

The development of the floating, envi­ronmentally friendly desalination unit for producing potable water is consid­ered to be an important technological solution for addressing water shortage problems in isolated and island commu­nities worldwide.

The envisaged system incorporates a number of innovative features beyond those originally planned. There are innovative improvements to the energy management and desalination process­es, as well as in the unit's ability to work without any chemical pre-treatment of the sea water; these are in addition to the originality of the design of the float­ing platform with its need to support an operating wind turbine.

In summary, the system design provides the following advantages:

• it can be constructed in places where all the facilities and materials are available;
• the finished plant can be easily towed to the place of operation;
• it has the potential to address the seasonal water scarcity problems of isolated, small islands;
• there is a reduction in installation expenses;
• it overcomes any constraints of land availability;
• it avoids permanent disturbance;
• it also reduces the ecological impact and stresses to the environment.

The proposed solution encourages the use of renewable resources and avoids the environmental impact that is associ­ated with the opening of new roads that are usually required when constructing new land-based wind parks together with the transmission infrastructure needed for an onshore desalination unit. The system makes use of sea space in a sustainable way using an environmen­tally friendly approach for addressing the critical issue of scarce water resources; a problem likely to become more acute in the light of the dispiriting scenarios of climate change worldwide.

Experience gained so far proves that the local population is much more posi­tive about floating desalination units in comparison to land-based ones since they avoid permanent installations and reduce environmental impact.

An occasional alternative use for the sys­tem would be in post-disaster situations, particularly in coastal areas, because it could be towed to the area and put into production (of electricity as well as water) relatively quickly.