Device for desalting sea or brackish water by using solar energy

A device for desalting sea or brackish water using solar energy, characterized by the fact that it includes two adjacent canals or equivalent structures fed with the sea or brackish water, a green house type structure over one of the canals to vaporize water from it and a structure for condensation of water and for collecting softwater, the structure being in communication with the greenhouse structure and largely immersed in the other canal, which acts as the cold source for the condensating unit.

The invention aims at providing a solution to the production of softwater 
using solar energy in desertic regions, where the sea or brackish water is 
available, by simple means requiring little and in any case unskilled 
labor for their maintenance, and only simple typically road building 
workmanship for their implementation, this in such a way that any 
automated or electronic control system is avoided. 
With this aim in mind, the device for desalting sea or brackish water using 
solar energy according to the invention is characterized essentially by 
two adjacent canals or equivalent systems fed with sea or brackish water, 
a greenhouse type structure placed over one of the said canals for 
vaporizing the water and a condensating and softwater collecting structure 
in communication with the said greenhouse structure and for a large 
measure placed under water in the other canal, which acts as the cold 
source for the condensating unit. 
Preferably one couples the greenhouse structure with a structure for the 
optical concentration of the sun rays, which can take different simple and 
efficient embodiments, as seen further on. 
It is necessary to be able to trap and also to renew the sea or brackish 
water in at least the canal used for vaporizing water. Thus a device for 
desalting water according to the invention will also include preferably 
another canal or equivalent structure running parallel to the canals 
mentioned above and used for feeding the said canals with sea or brackish 
water. The means for trapping and renewing the sea or brackish water will, 
then, consist of sluices placed between the feeder-canal and each of the 
said canals. 
One knows, of course, that sea water contains approximately 4.5% by weight 
of salt (density 1.03), while it could contain up to 26% before being 
saturated. It is, thus, possible to obtain softwater by vaporization of 
sea water without the formation of salt-deposits as long as one renews the 
seawater one uses for that purpose before its salinity exceeds the 
critical salinity. One must, nevertheless, be also able to maintain in the 
canal used for the vaporization a temperature corresponding to a useful 
partial vapor-pressure. Because the solar energy available is small, one 
should not use more sea or brackish water than required. This is why the 
periodical trapping and renewing of the water is necessary in order to 
obtain an acceptable efficiency. 
The cold source provided by the said second canal is simply the consequence 
both of the natural evaporation of the sea water which acts as a regulator 
and also of the reradiation at night.

The layout for the canals or equivalent structures of a desalting device 
shown on FIG. 1 shows two horizontal canals 1 and 2 linked by ordinary 
manually activated sluices 1a, 1b, 2a, 2b to a canal 3 or to a pipe fed 
with sea or brackish water. The pressure drop along canal 3 should be 
sufficient to get sea water flowing inside canals 1 and 2 by simply 
opening the head sluices 1a and 1b and the foot sluices 2a and 2b. The 
closing of these sluices traps the water inside both canals 1 and 2. 
Canal 1 is meant to be the hot temperature heat source for vaporizing water 
while canal 2 is meant to be the cold temperature heat source for a 
structure on which softwater vaporized from canal 1 will be condensating. 
As seen on FIG. 2, bottom and sides of canal 1 are provided with a 
radiation absorbant coating (for instance a black paint). The canal is 
embedded in a refractory layer made for instance of sand or bricks. 
The canal is enclosed and topped by a transparent pane 6 for instance made 
of glass, placed at a slant to either assure that the surface of said pane 
is orthogonal on the average to the sun rays at noon (the slant of the 
pane varies with latitude), or satisfy other conditions mentioned later. 
A wall 7 which supports the topside of pane 6 is made of refracting 
material (such as bricks). 
The device described so far acts as a greenhouse over canal 1. 
Wall 7 is pierced with holes allowing for pipes 8 which link canal 1 with a 
pipe 9 placed in canal 2 (for instance at the bottom). This latter 
constitutes the structure for the condensation of water. Through gravity 
alone pipe 9 can lead to a storage tank (not shown) where the condensed 
softwater accumulates, or can lead to a series of smaller storage tanks 
placed from time to time along its length which can for instance act as 
constant level tanks placed at the head of underground irrigation ducts, 
in the fashion required for example by the irrigation process named BIP of 
the French Corporation BERTIN. 
Canal 2 is open to the ambient air so that the sea water it contains stays 
at a temperature defined by the ambient temperature and the vaporization 
it generates. The capacity of this canal can be chosen for keeping proper 
its temperature according to the ambient conditions at hand. If desired, 
wall 7 can be outfitted with any additional screen to protect canal 2 from 
direct sun light. 
With such a plant, one can obtain, from a canal 1 m large, from 4 to 8 
m.sup.3 of fresh water per day per km of canal. A benchmark for 
agriculture is the need for 1 liter/second/hectare per sunshine hour for a 
non-decerning watering system, or about 2 liters/day/m.sup.2. A more 
advanced watering scheme (such as BIP mentioned earlier for example) 
reduces the quantity of water necessary by 30%, thus requires only 0.6 
liter/day/m.sup.2. Thus, the device described so far allows one to 
irrigate from 7 to 14 m.sup.2 per m of canal. One can also foresee that 
the device could satisfy the softwater needs of 1 to 2 people per m of 
canal. 
To obtain better results, it is necessary to concentrate the solar energy. 
FIG. 3 shows the device shown on FIG. 2 to which one has added a structure 
for the concentration of solar energy made of transparent panels (10) 
molded on their exterior face (it could also be on both faces) into a 
Fresnel lens which focuses the sun energy inside the greenhouse 4. 
Such panels can be made from plastics, for instance from methacrylic 
resins, and formed into a cylindrical Fresnel lens by molding. The grooves 
providing the lens effect will stretch horizontally parallely to canal 1. 
The slope given to panels 10 is the same as the one given to panes 6. 
Thus, the annual average direction of the sun rays at noon will be chosen 
normal to panels 10 and during the day the sun rays will remain more or 
less within a plane both orthogonal to panels 10 and to the cross-section 
of canals 1 and 2. In this manner, the focusing of the rays will continue 
to occur within the greenhouse 4. 
Because the distance between 10 and 4 must be of the same order as the 
width of 10 (which is the distance AB) the height of the structure 
supporting 10 may become important, even though that structure may require 
in principle only posts 11a, 11b, linked by L-shaped bars 11c holding the 
panels. 
The wall 7a between the two canals can however be made higher, so that also 
its shadow will protect canal 2 from the sun. 
At night, it is advisable to unfurl or place screens or any kind of blanket 
above the panes 6, to minimize the losses by radiation in the greenhouse. 
This could also be done in the case of FIG. 2. 
A device such as the one shown on FIG. 3 permits one to irrigate from 70 to 
140 m.sup.2 per m of canal (assuming canal 1 to be 1 m wide) or to satisfy 
the water needs of from 10 to 20 people per meter of canal. 
One has shown on FIG. 4 another device for the concentration of solar 
energy which is of greater capacity. This device is also made of panels. 
Each panel is made of elongated cells 12, seen in cross-section on FIG. 4 
(such that their vertical sides 13 are metallized--rendered reflecting). 
The sun rays at noon enter the transparent material of each cell normally 
to the face BA. Face AC is orthogonal to face BA so that it cannot 
interfere with the sun rays striking the next cell. Angle .alpha. made by 
face BA with the horizontal BC is chosen so that the sun rays 14 are 
refracted for the most part at face DE, along that face. Thus angle r must 
correspond substantially to the Brewster conditions, this means that, if n 
is the refractive index of the cell's material, .alpha.=arc sin (1/n). As 
refractive indices do not vary much (between 1.4 and 1.6) .alpha. is of 
the order of 45.degree. (correct value when n=1.414). 
Thus, such panels bend part of the solar energy in a given direction 15 
along their bottom surface 16. 
FIG. 5 corresponds to the implementation of a desalting device using the 
means for solar concentration shown on FIG. 4. 
The solar rays such as 14 find themselves after crossing the cells 12 bent 
in a direction parallel to the bottom face of panels 10a. They then strike 
a reflecting roof (mirror) 17 and enter canal 1 through pane 6. 
The solar concentrator is kept above ground by a very light structure which 
is made of poles 18 interconnected by beams 19, these beams remaining in 
the plane of the Figure in order to avoid the presence of beams parallel 
to the canals, which would intercept the rays bent as indicated. 
The slope .beta. is so chosen that the solar rays, when the sun is at the 
zenith, are orthogonal to the incident faces of the cells. The slope would 
be zero for rays making a 45.degree. angle with the horizontal, it is 
15.degree. for rays making a 60.degree. angle with the horizontal, it is 
25.degree. for rays making a 70.degree. angle with the horizontal. 
The refractory material 20 is placed behind mirror 17 to also screen the 
greenhouse against radiation losses during the night. 
Another solution for the concentration of solar energy shown on FIG. 6 
consists in compensating the annual variations in solar attitude by means 
of a screen 20 made of prisms 21. Following a precise calendar schedule, 
one can either change such screen as 20, it means use screens made of 
prisms of different apex angles, or adjust the slope of the screen. To do 
the latter, circular mounts 22 can be affixed to wall 7 and provided with 
rungs in which the screen can be held according to the slope desired, if 
said screens are also mounted in 20a in such a way that they can rotate. 
The number of times one changes screens or one changes the slope of the 
screen within a year can be computed, taking into account the losses 
through the screen and the losses through pane 6 when the solar rays do 
not strike the screen normally. Such screens can be molded and be made of 
plastic material just as well as the panels 10 which act as Fresnel lens. 
The screens will be kept in place by a frame imparting to them the 
necessary mechanical qualities. 
As shown on FIG. 7, such a screen 20 can also be used in conjunction with a 
panel 10 acting as a Fresnel lens (mounted as shown on FIG. 3) so that the 
sun rays will keep striking normally the panel 10 during all year. 
In both the cases of FIG. 6 and FIG. 7, the slope of the panel 6 or of the 
panel 10 acting as a Fresnel lens will be so chosen that the sun rays, at 
noon on the longest day of the year, will be normal to 6 and or 10, so 
that screen 20 will be taken off at that time. During the rest of the 
year, when the sun at noon will be lower in the sky, the screen will be 
used and will be placed at a slant which will always be greater than the 
one given to 6 or 10. 
FIG. 8 shows a solution of the same type as the one shown on FIG. 7, but it 
differs from the latter by the size, and principally because the width of 
panel 10 acting as a Fresnel lens is only, for instance, twice the one of 
pane 6. One can propose that in this case instead of erecting a wood frame 
or even walls, the overall optical structure (by which we mean the panel 
10 and its support) can constitute a sort of casing 23, which can be 
transported either as a kit or premounted. Thus, in the case of a canal 50 
cm wide for instance, the dimensions of the casing will be of the order of 
a meter. 
A reduction of the size of panel 10 acting as a Fresnel lens can also lead 
to another device for the concentration of solar energy, as shown on FIG. 
9. It is a casing such that its dimensions can be for example of the order 
of a meter which includes a floor 24 on which is placed an elliptic mirror 
25 used for refocusing the solar energy focused in F2, by the panel 10 
acting as a Fresnel lens and the screen 20, into a further focal point F1. 
While the faces of the casing parallel to the plane of the figure can be 
filled, the faces orthogonal to the plane of the figure (at least the ones 
facing F1) must be hollow so that, in general, the mounting pieces named 
26a, 26b are posts. 
With such a device a certain precision is necessary and screen 20 becomes 
an absolute necessity in order to use the device all year long. In the 
case when F1 is meant to occur under pane 6 in canal 1, its position can 
vary by dimensions of the order of 50 cm which over a span of 20 m leads 
to a possible angular tolerance of less than 2.degree.. 
As shown on FIG. 10, one can then use a number of casings of the type shown 
on FIG. 9 to concentrate the solar energy captured over a very large 
surface at a point F1 which falls in canal 1 under pane 6. The shape of 
the roof of the greenhouse over canal 1 is somewhat modified to allow for 
the entrance of the sun rays captured by the casings, it means that the 
poles 27, supporting pane 6 and interconnected on top, support also 
another more or less vertical transparent pane 28. Also the casings for 
concentrating the sun energy are placed in such a way that neither 
obstructs the rays coming from the ones that are further away. 
The necessary precision for the device can be obtained at the factory 
before transportation to the location where it will be used. For that 
purpose the vertical edges 26a and 26b of adjacent casings will be 
provided with notches or holes for screws or with any other means such 
that it will be impossible to place the different casings of a set 
differently from the intended way. 
The angle .alpha. for the rays entering canal 1 can be chosen close to 
20.degree. and the number of casings one can place along a direction 
orthogonal to the canal, while limited, can easily reach 20. 
Naturally other variations can be imagined without straying from the domain 
of this invention.