Wave-powered seawater desalination.

Wave-powered seawater desalination.
I wondered about using wave-action to power desalination at our cottage, because it's a readily available source of the huge power (pressures) needed to drive reverse osmosis to extract water from seawater. Here is a very scholarly examination of the power available from wave action versus the amount of evaporation that occurs in that climate. The numbers are staggering, and the souks fixation of the calculations is intriguing.
...Along arid, sunny coastlines, an efficient wave-powered desalination plant could provide water to irrigate a strip of land 0.8 km wide if the waves are 1 m high, increasing to 5 km with waves 2 m high.
...[Potable water] availabilities below about 1700 m3/capita/y are generally considered to indicate water scarcity.
...Waves will generally be available where sea- water is desalinated. But the harnessing of wave energy is, as with other forms of renewable energy, expensive in terms of capital plant and the effort needed to develop the technology.
...Of the 173,000 TW of solar power arriving at the earth's atmosphere, 114,000 TW is absorbed in the atmosphere, oceans and the earth's surface. About 1200 TW of this thermal energy is then converted into the kinetic energy of the wind [10]. The shearing action of the wind on the surface of the ocean generates currents and waves, involving energy transfer at a rate of around 3 TW.
...It is evident that wave energy is generally out-of- phase with water demand for irrigation...Between 100 and 200 days of storage are needed to eliminate the need for overcapacity due to seasonal mismatch.
...In Morocco, for example, wave-powered desalina- tion could supply 16% of the shortfall, increasing to 64% in the case of Oman. Somalia is the only mainland nation of those studied where potential supply clearly exceeds the shortfall, by a factor of about 6...there are many arid ocean-facing regions belonging to countries that do not figure as being short of water at the level of national statistics.
...Canaries and the Maldives, where lack of rainfall tends to coincide with abundant wave resource
...suggest that the energy cost of desalinating water is equivalent to horizontal transport over 100's of km. ...vertical transport of water requires about 1000 times more energy than horizontal transport.
...Wave energy stands out from other types of renewable energy resource, not only in terms of the intensity of the primary resource, but also in terms of the conversion efficiencies actually and theoretically obtainable. For comparison, the effi- ciency of solar energy conversion is commonly held to be limited to 86.7%...Real devices can only approximate such ideal devices crudely and the record efficiency of photovoltaic conversion actually attained is 35% [22].
Similarly, wind energy converters are normally interpreted as being subject to the Betz momentum theory that places a limit of 59% on achievable efficiency, with real wind turbines achieving effi- ciencies up to about 50% [23].
In contrast, there appears to be no theoretical reason why wave energy converters cannot reach 100% efficiency in theory and wave tank devices yielding over 80% have been demon- strated in practice

http://www.desline.com/articoli/6390.pdf



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