Fresh Water produced by desalination processes

Renewable Energy Systems (RES) generate electricity but production is often limited by the intermittent character of solar radiation or wind speed. As described in electrical transmission, it may not be economically feasible to transmit the electricity onshore from an offshore installation due to the high costs of cabling and transmission losses therefore it may prove beneficial to convert the electricity into a suitable medium which can then be stored or distributed onshore using cheaper alternatives (pipelines).

Due to the location of offshore installations and the existing infrastructure of pipelines (particularly gas, being cleaner) it is our aim to establish whether it could be feasible to re-use them as desalination plants to produce potable water from seawater.

Water is an ideal medium for the storage of the variable amount of electricity generated by RES.

The production of drinking water from seawater has been carried out for many years, leading to the development of various desalination processes. Due to the intermittent nature of the resources, only a few methods exist which can operate under undefined process parameters.

Water is one of the earth's most important resources and in particular countries which do not have the benefit of continual fresh water supplies, competition for fresh water continues to grow. Increasing population and industrialisation is taxing the supply leading to shortages of brackish water. Drinking and industrial facilities that use purified river water in their operations will be forced to find an alternative to brackish water supplies. This inevitable leads to the use of seawater as a viable alternative to shrinking brackish water supplies (1).
Dr Irvine Moch, president of I Moch & Associates, a water consulting firm based in Wilmington, Delaware believes that many areas in the United States will begin to face brackish water shortages and has quoted 'Very simply we are having a more difficult time preserving our nations aquifiers. The bottom line is this, soon sea water will be used as the raw material for a significant portion of the industrial water produced in this country. It's a question of when, not if'

• Reverse Osmosis (RO)
• SEP - Mechanical Vapour Compression Plant (MVC)

1. Reverse Osmosis

   On-shore tests on a prototype wind-powered RO process are currently being carried out on the Canary Islands (North Africa) by the NEL (East Kilbride, Scotland).
   

   Osmosis is the natural phenomenon where dissimilar liquids when mixed will try to reach equilibrium. The only way for this to happen in our example is for pure water to pass through the membrane. This process involves pressure being applied by an electric pump to the seawater which forces water molecules through a semi-permeable membrane. The salt particles do not pass therefore creating potable water on the other side of the membrane. The water may have to be passed through many membranes depending on the salinity of the water. A heat exchanger is required to decrease the temperature of the cylinder containing the membranes due to the fact that the process runs at pressures of 60 bar.

   Further to a meeting with Graham Skivington, a Strathclyde University PhD student who is directly involved in monitoring the process, it was established that the system was successfully producing potable water.
   
   We also arranged a meeting with Calum Mc Gill. a Mechanical Engineer from Weir Westgarth (Glasgow, Scotland) to determine the feasibility of using an existing RO system on an offshore platform using renewable energy systems. It was established that desalination plants are currently manufactured for use on offshore installations (modular in design and enclosed in steel containers for protection) which provide fresh water for drilling and drinking purposes. This process does however require defined process parameters however we anticipate that this problem could be addressed by implementing a Logic Control System to manage the electricity produced and batteries to store excess electricity for use at times of low power output from the RES.
   
   
   • Merits:
     • Modular design
     • Units are currently manufactured for use on offshore installations.
   • Drawbacks:
     • Membranes are prone to fouling and splitting which can be expensive to replace (dependant on salinity of water and operation of plant).
     • Membranes require cleaning or changing every 2-3 months.
     • Extensive Pre and post treatment of the water is required.
     • Needs a special pretreatment for the particules of oil contained in the water
     • Pre-treatment process needs to be changed regularly (2 months).
     • The success of RES systems still to be proven.

   Cost:

   t.b.d
   
   

2. Mechanical Vapour Compression

   An on-shore prototype using wind-power has been operating successfully on the Island of Rugen (North Sea) since March 1995. This process was developed by SEP Gesellschaft fur Technische (Munich, Germany) is producing a maximum of 15 m3/hr of potable water from a 300kW wind turbine.

   Plant Start Up

   A vacuum pump is activated to evacuates the evaporation / condenser unit to a pressure corresponding to the actual evaporation temperature. The circulating seawater is heated by the electric resistance heating within the evaporator/condenser unit to an adjustable minimum temperature.

   Operation

   The preheated seawater form the bottom of the evaporator/condenser unit is diffused uniformly into the vertical tubes by a trickling installation and evaporators inside the tubes. The compressor transports the vapour from inside the tubes . The compressor transports vapour from inside the tubes to the condensation section outside the tubes. The concentrated seawater and the distillate emerging from the evaporator/condenser unit are fed into heat exchangers in order to transfer heat to the incoming feed-water. The de-gasifier is required to remove in-condensible gases.
   
   The prototype wind powered SEP MVC plant has to operate under completely different conditions than conventional plants that operate stationery at defined parameters. A fixed speed and power input of the compressor results in a constant evaporation temperature and a steady distillate production.
   
   Depending on the actual wind conditions, the power output of the wind turbine can vary in ratios 1:10 or more. Despite this high dynamic power input, the desalination plant has to operate without any tendency to oscillate or generate any un-necessary feedback to the wind power plant.
   
   
   • Merits:
     • Unit:
       • Proven distillation system compatible with variable power input.
       • Low energy demand
       • Almost complete utilisation of wind power for desalination.
       • Low process temperatures
       • Simple process
       • System requires cleaning once per year.
     • Turbine:
       • Variable rotor speed for higher energy gains
   • Drawbacks: There are few known drawbacks to this system (contact project team).

   Cost:

   Unfortunately we could not obtain cost data for a large 2MW plant due to problems contacting the project team.

We have established that a 2MW wind turbine located in the following areas could produce in the region of 30 m³per hour: Gulf of Mexico, West Coast America, West Coast Africa.

• Proven to be compatible with RES.
• Minimal maintenance and operator attendance requirement.
• Low energy requirements.
• Minimal pre-treatment required.
• No need for expensive membranes.