The ancient concept seems ingenious, but why doesn’t everyone use the free heat energy from the sun to desalinate seawater into drinking water anymore?
By: Ringo Bones
Believe it or not, the knowledge that salty seawater can be made into safe fresh drinkable water is more that 2,000 years old. Ancient Mediterranean sailors embarking on long seafaring voyages have supplemented their stores of shipboard drinkable fresh water by placing pots of seawater under the sun and trapping the condensed vapor. This very same technique – in an updated scaled-up form – had been tried in some large-scale experimental desalination plants back in the 1960s.
Surprisingly, the concept of using the sun’s free thermal energy to convert salty seawater to potable fresh water can easily work when scaled up to a several thousand-gallon-a-day capacity. Back in the 1960s, the 4,083 inhabitants of Symi, an island near Greece, used to get all of their potable fresh water from a newly constructed experimental solar-distillation unit which can supply about 4,000 gallons a day. It works by tapping the sun’s free thermal energy – i.e. heat – to turn seawater into fresh water by first piping seawater into a flat shallow trough enclosed under a transparent plastic dome. The sun’s heat causes the water to evaporate that re-condenses into chemically pure fresh water on the cooler underside of the dome. This pure salt-free water then trickles down the dome, drips into collecting trough at the edges of the unit and is then collected. The briny residue that’s left behind – which is several times saltier than seawater – is then flushed away back to the sea.
The method is very inexpensive given that the energy source used to desalinate the seawater is virtually free, unlike the more popular reverse osmosis method used today which uses electricity to pressurize seawater up to several thousand pounds per square inch to be squeezed though banks of salt-filtering polymer membranes. But using the sun’s free thermal or heat energy to convert seawater into drinkable fresh water is for all intents and purposes an inefficient and impractical process in most cases because the yield is quite low: at best only 0.13 gallons per square foot of basin area per day. This makes a typical solar thermal desalination plants that can be able to compete the output of a typical modern reverse osmosis desalination plant occupy a prohibitively large real estate for every gallon of fresh water produced.