The co-production of large quantities of fresh water is one of the main advantages of the OTEC process. Up to 0.7 to 0.8 MGD (Million Gallons per Day) of fresh water can be produced per MW (Megawatt) of installed gross electric power capacity. The fresh water results from the evaporated warm seawater used as the working fluid in the Open-Cycle OTEC power production process or as an additional parallel system component in a Kalina Cycle^® power system configuration. The evaporation occurs because the warm seawater is exposed to a partial vacuum which turns 0.5% of it into low temperature – low pressure steam. In an Open-Cycle OTEC power configuration, the generated steam is passed through a turbine (which powers a generator to produce electricity) and then is condensed into liquid fresh water by transferring heat to the cold seawater through a heat exchanger. The fresh water is then pumped out for storage and distribution.

     The efficiency of the Open-Cycle OTEC process (as well as the efficiency of any seawater distillation process) is increased by the removal of non-condensable gases from the seawater. Research done at the University of Hawaii by Drs. Hans Krock (OCEES), Stephen Oney (OCEES), and Manfred Zapka resulted in two patents related to the enhanced exchange of non-polar gases into and out of seawater. These patented processes allow the design of a pre-deaeration and a gas reinjection system which make the OC-OTEC system (or the separate fresh water production system in conjunction with a Kalina Cycle^® system) inherently more efficient, less costly, and environmentally benign.

     The production of fresh water from the Open-Cycle OTEC process was demonstrated at a functioning plant at the Natural Energy Laboratory of Hawaii (NELHA) on the island of Hawaii. The fresh water produced was of high quality with approximately 80 mg/l of TDS (Total Dissolved Solids). The TDS of water from a full production integrated Kalina Cycle^® OTEC plant or an OC-OTEC facility is expected to be even lower because the "steam" used for fresh water production in the NELHA plant was diverted before the mist screen allowing for more salt carry-over via water droplets than would be experienced in current OCEES fresh water production plant designs. As with any desalination product water (which is essentially distilled water), it will necessarily require minor adjustments in pH and dissolved oxygen in order to improve the taste and control corrosivity.

     A third method to produce fresh water in the OTEC systems approach is to exploit the natural humidity inherent in the tropical regions of the earth by passing the cold seawater through a heat exchanger which is in contact with this humid air, effectively condensing fresh water on the surface of the irrigation tube. This method is primarily utilized in the Cold Water Agriculture application discussed in the corresponding section of this website as developed by Dr. John Craven of the Common Heritage Corporation. This method not only provides a natural irrigation technique further utilizing the cold seawater resource inherent in an OTEC system but also works well at conditioning temporal plants for suitable cultivation in an "unnatural" environment like the tropics.