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1. WO2003022746 - APPAREIL DE DESSALEMENT D'EAU DE MER

Note: Texte fondé sur des processus automatiques de reconnaissance optique de caractères. Seule la version PDF a une valeur juridique

[ EN ]

SEAWATER DESALINATION SYSTEM

TECHNICAL FIELD
The present invention relates to a seawater desalination system.
BACKGROUND ART
Various types of seawater desalination systems are known, each with its own desalination method. Though most systems are highly effective and reliable, actual implementation is limited by the high- construction and running costs involved, by the systems occupying large tracts of land which could otherwise be farmed or put to other uses, and by the systems at times creating serious problems in terms of environmental impact.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide a seawater desalination system designed to provide a straightforward, low-cost solution to the above problems, and which in particular is cheap and easy to construct and extremely cheap to run.
According to the present invention, there is provided a seawater desalination system comprising desalination means for receiving seawater and supplying desalinated water, and desalinated-water storage means; characterized in that both the desalination means and desalinated-water storage means float on the sea.
BRIEF DESCRIPTION OF THE DRAWINGS
A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which:
Figure 1 shows, schematically and substantially in block form, a preferred, non-limiting embodiment of the seawater desalination system according to the teachings of the present invention;
Figure 2 is similar to, and shows a variation of a detail in, Figure 1.
BEST MODE FOR CARRYING OUT THE INVENTION
Number 1 in Figure 1 indicates as a whole a system for desalinating seawater and. supplying drinking water.
System 1, which has no on-land components and floats entirely on the sea, comprises a desalination unit 2 supported on a number of known buoyancy elements 3 moored to the seabed,- and a floating desalinated-water storage device 4 conveniently, though not necessarily, located close to unit 2 and moored iri known manner to the seabed.

In the preferred embodiment described, unit 2 is a distillation system, and comprises an evaporating unit 6 in turn comprising one or more conveniently black-bottomed tanks 7 - only one shown in the accompanying drawings - each for containing a mass 8 of saltwater conveniently pumped into tank 7 by a motor-driven pump 9. Evaporating unit 6 also comprises, for each tank 7, a respective cover shell 10, which is permeable /to light, extends over and closes tank 7, and is preferably made of transparent plastic material. Conveniently, each shell 10 is defined by a plastic film - preferably polyethylene -to which substances are added to maximize retention of infrared rays inside the shell. On the opposite side to the incident light, shell 10 may conveniently be metallized on the inside, from the base to a height depending on the latitude involved, to assist reflection, inside the water in the tank, of any reflected light which might issue partly from shell 10. Each tank 7 and respective shell 10 define a light chamber 12, part of which is occupied by the mass 8 of saltwater, and the rest of which, in use, contains water vapour produced by evaporation of the water in tank 7. The water vapour is withdrawn from light chamber 12 by a fan 13 housed at one end of chamber 12, and is fed by a conduit (not shown) to the inlet of a dark airtight chamber 15 underneath, the outlet of which, opposite the inlet, is also connected to light chamber 12 by a further conduit (not shown) .
Dark chamber 15 forms part of a condensing unit 16 also comprising a tube bundle 18, which is located between the inlet and outlet of chamber 15, and is conveniently a mat type made of plastic material - i.e. comprises a number of adjacent, coplanar tubes 19 connected parallel - e.g. of the type known commercially as "Heliocol™" manufactured by Magen Plastic. To maximize system performance, the internal dimensions of dark chamber 15 must be approximately equal to, but no smaller than, the external dimensions of tube bundle 18. In other words, dark chamber 15 must be large enough to house tube bundle 18, and to define, with tube bundle 18, a passage 20 large enough to circulate the hot, vapour-saturated air contained in chamber 12, and which is blown by fan 13 over the outside of tube bundle 18, to the outlet of chamber 15, and then back into light chamber 12.
Coolant is circulated inside tube bundle 18, and is conveniently defined by seawater drawn, by a submersible pump (not shown) or by pump 9, from such a depth that the water circulating in tube bundle 18 is cooler than, and so condenses, the vapour.
Dark chamber 15 is defined at the bottom by a smooth, inclined catch tank 21 designed to feed the condensate simply by force of gravity to the inlet of a drain pipe 22 connecting condensing unit 16 to storage device 4.
In the embodiment shown, storage device 4 is lower than catch tank 21 of dark chamber 15, so as to fill simply by force of gravity, and comprises a bag 23 made of sheet material, conveniently a sheet of plastic or other elastomeric material sealed along the lateral edges and laid directly on the surface of the sea. Bag 23 is preferably housed in a retaining net 24 made of the same material as the bag or in other materials, e.g. vegetable or synthetic fibres, and having a number of mooring portions 25 by which to moor bag 23 to the seabed. Alternatively, net 24 is replaced by a number of retaining sashes surrounding bag 23 and fitted with said mooring portions . If the sheet material from which the bag is made is such as to permit direct attachment of the mooring portions, both the net and sashes may be dispensed with.
Bag 23 has an outlet opening 26 connected to a pipe 27 for piping the distilled water in bag 23 to land, but also for feeding rainwater into bag 23 in rainy seasons. In the latter case, bag 23 is black to prevent algae growth.
Operation of system 1 will now be described as of the condition in which tank 7 contains a mass 8 of seawater, and seawater drawn from underneath (so that it is much cooler than the water in tank 7) circulates in tube bundle 18. As of the above condition, as the sun increases the temperature inside light chamber 12, vapour is formed inside chamber 12, and is blown continuously by fan 13 onto the outside of tube bundle 18 and then back into chamber 12 along an endless circuit in which the same air circulates at all times; which air is hot and moisture-saturated before blowing over tube bundle 18, and is cooler at the outlet of dark chamber 15, and therefore at the inlet of light chamber 12.
As a result of the difference in the temperature of tube bundle 18 and the air blown by fan 13 , condensate forms on the outside of tube bundle 18 and drips into tank 21; and, tank 21 being tilted and higher than bag 23, the distilled water flows by force of gravity into bag 23. Both as and after it is filled, bag 23 continues to float on the surface of the sea, by the specific weight of the distilled water being less than that of seawater; and, at the outlet of dark chamber 15, the air, by now cooled and dehumidified, is fed back into light chamber 12 to be heated again.
As will be clear from the foregoing description, system 1, as compared with existing solutions, has the following advantages :
First of all, it is extremely cheap to construct, by featuring commonly marketed, low-cost materials. More specifically, system 1 comprises an extremely straightforward evaporating unit 2; and an equally straightforward condensing unit 16 comprising a normal dark chamber and a normal side by side tube bundle, but which at the same time is highly effective and, above all, resistant to corrosion by seawater.
System 1 described is also extremely cheap to run by featuring only one fan and one pump for pumping seawater into the tank and through the tube bundle; both of which components consume very little energy, and this only during sunlight hours, since the vapour condensation process does not operate at night. The electrical power required may therefore be supplied by an ordinary known photovoltaic device indicated 28.
No pumping devices at all are required for feeding the water to the storage device, which is done simply by force of gravity.
Using floating storage devices provides, on the one hand, for locating the storage device right next to the distillation unit, and, on the other, for solving all the problems typically associated with on-land storage devices. More specifically, using floating as opposed to on-land storage devices, large tracts of land formerly used for reservoirs can be put to other uses; and the land itself is unaffected, thus eliminating any problems in terms of environmental impact. Moreover, floating storage devices involve no pollution of the stored water, and no distilled water losses by percolation or evaporation, which are inevitable using either natural or artificial reservoirs, and which, despite the considerable cost involved, are only partly solved by waterproofing the reservoirs .
By featuring only a small number of component parts which are extremely straightforward functionally, system 1 is not only effective but also highly reliable. Housing tube bundle 18 in a dark chamber increases both the working life and reliability of the system , by the dark chamber preventing decay caused by light on the material of the condenser, which is normally made of plastic material to prevent corrosion.
The Figure 2 variation relates to a seawater desalination system 30 differing from system 1 by comprising a condensing unit 31 which, unlike condensing unit 16, has no dark chamber 15, and comprises a tube bundle 32 similar to tube bundle 18 and immersed directly in the seawater beneath tank 7. The moisture-saturated air from light chamber 12 is blown by fan 13 along the inside of tube bundle 32, which, unlike tube bundle 18, comprises a number of tubes with a larger inside diameter than those of tube bundle 18 to assist passage of the moisture-saturated air from chamber 12, and is positioned substantially vertically or at any rate tilted to receive the moisture-saturated air downwards along a pipe 34, and to feed the water gradually condensing along the tubes to an outlet header 35 of tube bundle 32. The air flowing along the tubes also flows along outlet header 35, from where it is sucked back by fan 13 into light chamber 12 connected to header 35 by a pipe 36. The condensed water in header 35 flows down simply by force of gravity into a water trap 37 secured at a lower level than header 35 by a mooring shown schematically. Trap 37 houses an immersion pump 38 which, controlled by a floating sensor 39, pumps the water into bag 23. The tilt of tube bundle 32 and the depth of trap 37 are so selected as to minimize the head required of pump 38.
System 30, as compared with system l, further reduces desalinated water production costs in terms of system energy by totally eliminating the energy required to recirculate the coolant. That is, tube bundle 32 is immersed directly in the seawater, which flows over the outside of tube bundle 32, and the heat produced by the system is dispersed free of charge simply by convection, assisted by the movement of the waves mixing the seawater.
Clearly, changes may be made to systems 1 and 30 as described herein without, however, departing from the scope of the present invention. In particular, condensing units 16 and 31 may be formed otherwise than as described by way of example, and may comprise a number of superimposed tube mats or different tube arrangements. Also, the tubes may be replaced with a number of hollow elements differing in shape and/or geometry from an ordinary tube.
Condensing unit 16 need not necessarily be higher than the storage device, and a desalinated water pumping unit may be provided- between the two.
Finally, the floating storage device may be formed otherwise than as described by way of example, while still floating on the surface of the sea.