(146b) Waste Heat and Thermal Desalination

The largest application of thermal desalination is the use of a desalination plant as the main condenser for a steam power plant. This technique using steam from condensing or back-pressure turbines is widely used in the middle east where single distillers can produce up to 20 million gallons per day (MGD) of fresh water and dozens of such machines may comprise a single facility. The energy consumption to drive a distillation process is high when considered as a stand alone process, but dual purpose generation of power and water yields higher thermodynamic efficiency than making water and electricity separately.

Smaller versions of this middle eastern concept exist in other locations such as the Caribbean where higher energy values mean those systems utilize more efficient, higher capital cost configurations to reach an economic optimum water cost.

Cogeneration of electric power and water may be the most common method of using waste heat for desalination but it is far from the only combination employed. Many merchant marine and cruise vessels produce warm water from engine cooling systems and that warm water is used in distillers to make the ship's water supply. In some cases steam and warm water is used. Naval vessels, particularly nuclear carriers, employ similar technique where much of the water is used for on-deck steam catapults.

There are also industrial applications which may utilize several different types of distiller and feed water, to produce water which can be used by the industry itself, or connected to the local water supply network. In these cases the efficiency of the distiller is usually customized to match the demand for water with the heat available. Newer applications of this type utilize not only traditional heat from stacks and exhausts but also exothermic heat of reactions or are integrated more closely with the industrial processes than ever before.

Oil refineries as well as LNG, Methanol, sulphuric and pteripthalic acid production facilities, have all been coupled with various thermal desalination processes to ensure the water needs of the facility (and its processes) are met. Some examples of these configurations are explained in the text..

The high quality water produced by distillers is usually well matched to the quality required for most industrial processes - including steam generation. Producing this quality of water from the sea, in a single process step, while utilizing otherwise neglected heat sources, is not only environmentally friendly but economically competitive with the ever increasing costs and constantly dwindling volume of traditional water supplies.