WO2003010093A1

WO2003010093A1 - Apparatus for preparing fresh water from (non-potable) water

Info

Publication number: WO2003010093A1
Application number: PCT/NL2002/000478
Authority: WO
Inventors: Dick Nicolaas De Ridder
Original assignee: Coöperatie Sun Water Systems U.A.
Priority date: 2001-07-16
Filing date: 2002-07-16
Publication date: 2003-02-06
Also published as: JO2186B1; NL1018558C2

Abstract

An apparatus for preparing fresh water from non-potable water, comprising an evaporator operating on solar energy with a water basin (14) for moistening air in an enclosed space (10) above the water basin and at least one fan (v-1) for supplying moistened air from above the water basin to a cooling apparatus (k). The cooling apparatus is provided with means for drawing off water and with a return line for directly or indirectly recirculating air leaving the cooling apparatus to the space.

Description

Title: Apparatus for preparing fresh water from (non-potable) water

The present invention relates to an apparatus for preparing fresh water from non-potable water, comprising an evaporator operating on solar energy having a water basin, for moistening aix in an enclosed space above the water basin and at least one fan for supplying moistened air from above the water basin to a cooling apparatus comprising a cooling unit.

An apparatus of the above-described type is known from Swiss patent CH 672 227. This known apparatus comprises a number of adjacent channels formed by a corrugated bottom plate, which are covered by a transparent foil. Seawater is passed through the channels, while in the space above the water flowing through the channels, in operation, an air flow is created by means of a fan. The air flow absorbs water vapor and is discharged at one end of the channel system and then passed to an underground system of porous tubes. The water vapor condenses at least partly against the relatively cool walls of the porous tubes, and the thus obtained fresh water can then penetrate into the earth via the porous tube walls to irrigate crops. The air saturated with water vapor and passed through the porous tubes is blown into the outside air at the free end of the porous tubes that extends above the ground.

The known apparatus is not suitable for forming a supply of fresh water that can be collected and stored for later use. Further, the known apparatus has a relatively low efficiency and can be easily contaminated internally by dust, air pollution and organisms. Also, the capacity of this apparatus is limited because of the limited width of the channels covered with foil. Moreover, it is a problem that the condensation occurs in particular at the beginning of the porous tubes and that the degree of irrigation of the crops is therefore position-dependent.

Further, WO 98/16474 discloses an apparatus operating on solar energy for producing fresh water, which utilizes an elongated cylindrical chamber having, for instance, a diameter of 1 and a length of 10 or a mattress- shaped apparatus having a number of juxtaposed elongated chambers. The chambers are divided by longitudinal walls into an evaporation space and one or more condensation spaces. Seawater is fed to the evaporation space via a perforated tube provided at the top in that space, which seawater is used to moisten a layer of hydrophilic material applied to the inner wall of the evaporation space. This prevents the formation of drops against the inner wall of the evaporation space. Air flowing through the evaporation space is returned at the end of the chambers via the condensation space(s). The evaporated water condenses in the condensation space(s) and is discharged via suitable conduits. For the transport of air through the evaporation space and the condensation space(s) an electric fan is provided, which also provides for recirculation of the air leaving the condensation s ace(s). This known apparatus is rather complicated by the use of internal hydrophilic layers. Furthermore, the known apparatus comprises no water basin.

For preparing drinking water, further, evaporator apparatuses are known from practice, in which condensate is formed against a glass, foil or plastic cover and is collected in a gutter and then drawn off.

Also, primitive constructions are known in which a piece of transparent foil is stretched, for instance, over a pit.

Other techniques for producing fresh water are distillation, which has the drawback of a high fuel consumption, and techniques such as membrane filtration. These techniques are less simple and are less suitable for primitive conditions. Although they have long been known, such techniques have never offered the solution towards coming to a lasting fresh water production in areas with a shortage of fresh water.

It is an object of the present invention to obviate the drawbacks inherent to the known techniques and to provide an effective, wind- and storm-resistant, simple, and cheap-to-construct apparatus that further requires no fossil fuels for the intended evaporation in tho system.

According to the invention, this object is achieved in that in an apparatus for preparing fresh water from non-potable water, the cooling apparatus is provided with means for drawing off water and with a return line for returning the air leaving the cooling apparatus to the space mentioned.

In the evaporator, the air and the water to be processed are heated by the solar heat, so that the amount of water vapor in the air increases, after which the air is blown into the cooling unit by means of the fan. Applied here is the physical principle that heated air, with increasing temperature, can hold exponentially more moisture than cold air.

The cooling unit may be placed, for instance, in the ground, under water (for instance in the sea) or in the water basin. The construction of evaporator and cooling unit enables, with low investments and the utilization of simple materials, an easily maintainable system which produces reliable water. By virtue of the recirculation of air and the excess pressure prevailing in the apparatus, internal contamination of the apparatus and penetration of organisms is prevented and the efficiency of the apparatus is improved.

It is noted that the return pipe does not to lead back directly to the evaporator, but may form a circuit loop with one or more evaporators and cooling units communicating in series with each other.

By recirculating air which has flowed through the cooling unit, an excess pressure with which the covering, like an air hall, is held in erected condition can easily be maintained in the apparatus.

An evaporator with air hall covering or a covering kept upright by inflated bodies can have dimensions of many hundreds or even many thousands of ². In view of the feasible scale size, this is an important breakthrough in the use of solar energy for the purpose of fresh-water production. Condensation may occur both in the evaporator itself and in the cooling unit and therefore, if desired, fresh water can be produced in both places. If desired, however, condensation in the evaporator can be largely avoided, for instance by using anti-condensation foil. In order to use as much as possible of the irradiated solar energy for heating the air/water vapor mixture, it is an option to use foil types with infrared block, which reduce the emission of heat from the air hall. Converting an amount of water of 1 kg into an amount of vapor of 1 kg at 0 degrees Centigrade requires about 2500 K evaporation energy. Accordingly, the less heat is lost, the better the efficiency.

In operation, solar energy is collected in the air hall, so that the temperature in the inner space of the air hall rises. A property of air is that according as the air is warmer, it can contain more water vapor. Because in the inner space of the evaporator the temperatures can run up high, the air can heat up strongly. According as the air is passed further through the air hall, which may advantageously be tunnel-shaped, both the temperature and the air humidity increase. The thus formed hot air is passed, through the propulsion caused by one or more fans, to a cooling unit, where the air is cooled. Condensation occurs both against the foil of the air hall and in the cooling unit, so that in both places fresh water can be drawn off. The materials for making the installation are cheap and simple and easy to handle, resulting in a cost price per m^s of water produced that is low by prevailing standards, while the water quality is high grade. The usability of the system in sunny/warm countries having a water problem is therefore very great,

For an example of a possible implementation of the evaporator, reference is made to Dutch patent application 1007941. That application describes an air hall which, after adaptations, could serve as an evaporator for an apparatus according to the invention. Also known are implementations of air halls in which the longitudinal edges of the foil are anchored to concrete, or halls that are anchored to the ground by means of ground anchors and nets.

Particular embodiments of the invention are laid down in the dependent claims.

In the following, the invention will be described in more detail with reference to the appended drawing.

Fig. 1 diagrammatically shows an example of a known apparatus for preparing drinking water; Fig. 2 diagrammaticaEy shows, in longitudinal section, an example of an apparatus according to the invention;

Fig. 3 shows, in top plan view, an example of an apparatus according to the invention;

Fig. 4 shows a graph of the possible water content of air at different temperatures;

Figs. 5 and 6 show examples of constructional details of an apparatus according to the invention;

Fig. 7 shows a sid,e view of an apparatus according to a second example of an apparatus according to the invention; Fig. 8 shows a cutaway view of a part of a cooling apparatus of a third example of an apparatus according to the invention;

Fig. 9 shows a side view, in longitudinal section, of an apparatus according to a fourth example of an apparatus according to the invention;

Fig. 10 shows a detail X in Fig. 9; Fig. 11 diagrammatically shows a detail XI in Fig. 12;

Fig. 12 diagrammatically shows a top plan view of an apparatus according to a fifth example of an apparatus according to the invention as well as a part designed according to a sixth exemplary embodiment of an apparatus according to the invention; Fig. 13 shows a side view, in cross-section, of an evaporator of an apparatus according to a fifth example of an apparatus according to the invention; and

Fig. 14 shows a cut-of side view on the line XIV-XTV in Fig. 13. Fig. 1 diagrammatically shows a known construction for producing potable water. Positioned at the bottom of the construction is a receptacle with (non-potable) water 1, the sun shines through a glass cover 2, which results in a temperature rise in the enclosed space 3. Water will condense against the colder glass cover 2, being in contact with the outside air, and the condensate will run off to a collecting gutter 4. The condensate is passed to a collecting reservoir, not shown. Instead of a glass cover, a cover of plastic may also be used.

The exemplary embodiment diagrammatically shown in Figs, 2 and 3 comprises an elongated tunnel or air hall 10 of a strong foil 12, which covers a basin 14 containing (non-potable) water. Located at the beginning of the tunnel are fan units V-1 and V-2. Unit V-1 provides for air circulation through, the assembly of evaporator 10 and a cooling apparatus having one or more cooling units K, and unit - provides for excess pressure in the tunnel (or air hall) with respect to the ambient pressure so as to keep the evaporator in the erected condition of use. The sun shining, the air in the inner space 13 of the tunnel or hall is heated. The air advances in the direction of the arrow P from point T-l to point T-2 whilst becoming increasingly hot and increasingly moist. During the transport of the air through the air tunnel 10, condensation occurs against the inner wall of the tunnel, as the inner wall is relatively cold with respect to the rest of the inner space 13. If desired, however, this condensation can be largely avoided with anti-condensation foils.

At point T-2 at the end of the air tunnel, the air is passed on to a cooling- unit K. As is diagrammatically shown in Fig. 2, the cooling apparatus can be built up in a^' simple manner from pipes 20 laid in the ground, which are coupled with suitable distributing and collecting pieces 21, 22. The cooling apparatus absorbs the heat f om the air. Ground, specifically deeper in the soil, can absorb much heat. As the day progresses, the temperature of the ground itself will also rise slightly; at night, the ground will release/emit this heat substantially to the surroundings again. Depending inter alia on climate, soil, and evaporator design, the dimensioning of the cooling apparatus may vary. The air leaving the cooling apparatus is returned via one or more return pipes 15 to the beginning of the tunnel. An equilibrium will be established with respect to the day and night rhythm, with the evaporation unit each time adding heat to the soil by means of the circulating air. After the installation has run for some time, a pendulum of temperature will occur: the hot air from the tunnel is cooled down by the soil and will be heated up again in the evaporator. During the day, the temperature of the soil is increased, but the temperature in the evaporator will also run up further. In areas where at night strong cooling occurs through radiation, optionally, drinking water may also be produced at night by allowing water to condense against the cold inner wall of the air hall.

At the lowermost point in the cooling apparatus, a water draw-off point WT is situated, via which the condensed water can be discharged from the cooling apparatus to a storage reservoir and/or consumers. It is not necessary that the cooling apparatus is level, but it is necessary that it has at least one relatively low point.

The ground cooling system comprises a pipe system 20 to 22, which is preferably of such design that each ground cooling pipe gets to process a substantially equal amount of air flowing through, and hence the heat absorption by the ground cooling system will proceed uniformly. The cooling apparatus can be made of large-scale design. In a practical embodiment, the pipes can have a substantial length, e.g, 80 m or more, yielding a long path of transport for the air flowing through. This promotes the precipitation of the water vapor in that air, As already observed, condensation also takes place in the air hall or air tunnel itself against the inner side of the foil covering. The water condensed in the air hall or air tunnel flows down along the wall and, if desired, can be collected in suitable gutters. A few constructional exemplary embodiments are diagrammatically shown in Figs. 5 and 6.

Fig. 5 shows an exemplary embodiment in which a gutter SO is used, which may be made of plastic or metal, and which is located in the inner space 13 of the air hall or air tunnel. The lower edge of the covering foil 12 hangs in the gutter and is attached to the outer edge of the gutter. To that end, in the example shown, an auxiliary strip 31 is used, which prevents tearing of the edge of the cover foil. The auxiliary strip and the cover foil edge are attached to the gutter 30 with bolts or blind rivets. The gutter itself in turn is attached, with a number of anchor strips 33, to ground anchors 32, which may comprise e.g. a threaded end 34 and an anchor plate 35. The anchor strips 32 may be attached to the gutter in any suitable manner, e.g. by welding, soldering, with rivets, bolts etc. Optionally, the attachment of the anchor strips may be combined with the attachment of the cover foil. Also, the anchor strip could have a bent end embracing the edge of the gutter 30. Fig. 6 diagrammatically shows a construction different from that of

Fig. 5 in that the gutter 30 lies on a gutter-enveloping edge of a plastic bottom foil S6, which also forms the bottom of the water basin in the air hall. In the example shown, along the inner side of the gutter, an earthen wall 37 has been formed, constituting a limit of the water basin 14. At the level of the water basin, insulating material 38 may be located under the bottom foil 36 to prevent cooling of the water 39.

A drawback of collecting water of condensation having condensed against the foil may be that the tunnel must be rather high, because otherwise the droplets do not flow along the foil. Due to its height the tunnel is then relatively fragile during storms. If anti-condensation foil (such as acrid) is used, the tunnel can remain relatively flat. The foil can then have an angle of inclination of about 10°, so that the wind/storm cannot get a grip on it. Another advantage of not producing water via condensation against the foil of the air hall is that at night the air hall tunnel can lie flat. For if the foil is contaminated with salt, this does not affect the eventual water quality since condensation, or at least collection, of condensed water takes place exclusively in the cooling apparatus. The apparatus described here has major plus points:

• cheap per m²; • high energetic efficiency per m²;

• exclusively solar energy is the engine behind the evaporation from the basin;

• there are no minimum requirements with respect to the starting water (as long as this water will evaporate and is not covered by veils of dirt);

• the installation can also be built and maintained by do-it-yourselfers;

• the price of the installation is low, and so is the cost price of the water produced; • there are many possibilities of adaptation to the local conditions, double or triple foil cover on the evaporator, dimensioning of tunnel length, and dimensioning of cooling unit; « the apparatus is highly storm-resistant in that the air hall can easily be lowered by switching off the fan(s).

It is observed that within the framework of the proposed invention, many modifications are possible. Thus, for instance, the number of fans may vary, or the fans for air circulation and for increased air pressure, respectively, may be combined in whole or in part. Furthermore, for instance, the cooling apparatus may be arranged in whole or in part under the evaporator. Also, the length/width/height ratio may be altered or the arrangement of the installation may be modified from e.g. land to sea or inland waters. When used on surface waters, the evaporator may be designed, for instance, to float on, for instance, insulating material and/or air cushions, and the cooling unit may use (sea)water present under the evaporator. The ground cooling unit may, if desired, be combined or replaced by placing a condensation apparatus, for instance a unit with a very large number of tubes of e.g. glass or any other suitable material, through which cool seawater flows, and on which condensation takes place. If an apparatus according to the invention is to be placed on a hard e.g. rocky or coral substrate, it is difficult to install an in-the-ground cooling apparatus. By way of alternative, the cooling apparatus can then comprise e.g. a number of pipes placed in the water of, for instance, the sea.

An example of such an apparatus is shown in Fig. 7. According to this example, the elongated tunnel or air hall 60 is mounted on a frame 74 with floats 75, and the apparatus floats in a surface water 95. Located at the beginning of the tunnel 60 is a fan unit V-51, which communicates with the return line 65 and with a supply line 76 for the supply of outside air. Included in the return line 65 and the supply line 76 are valves 77, 78, which are each provided with a control communicating with a sensor 79 which indicates whether sufficient pressure prevails in the inner space 63 of the air hall 60. If this is not the case, the valve 77 closes temporarily and the valve 78 opens temporarily, so that a temporary switch is made from rccirculation to supply of outside air, so as to bring the pressure in the air hall 60 back to the intended levol again. If the sensor 79 signals that the predetermined maximum pressure has been reached, the valve 78 closes again and the valve 77 is opened again. The water in the air hall is indicated by reference numeral 51.

The cooling apparatus is formed by a set of parallel-connected lines 70 passing under the frame and in operation extending under water. Unit V-51 provides for air circulation through the assembly of evaporator 60 and cooling lines 70.

From the water draw-off point WT, the water condensed in the lines 70 can be discharged from the cooling apparatus to a storage reservoir and/or the consumers, who will generally be on shore.

A system of lines supported by a floating structure, in which evaporated water condenses, can also be combined with an evaporator supported on shore or supported towards the ground. This has the advantage that the construction is less sensitive to swell. The sensitivity to swell may be further limited by also supporting the lines in which evaporated water condenses towards the ground or by placing them on shore and passing the seawater along the lines.

Also, the cooling apparatus could consist of a number of juxtaposed tubes on which a layer of water is present. Cooling of the tubes may optionally be promoted by evaporation of the water on the tubes. It is also possible to cool the tubes with water, which, after having been heated by heat exchange with the condensation tubes, is supplied to the evaporator, for instance by including channels 80 in which the water condenses, in a heat exchanger with lines 81 through which flows the supplied water from which drinking water is to be produced (see Fig. 8). Thus, a further efficiency improvement can be achieved.

Figs. 9 and 10 show an example of a particularly efficient and effective construction of an apparatus according to the invention. According to this example, the air hall 110 is placed on a soil paving 131. The paving 131 is covered with a layer of insulating material, e.g. rock wool. Over the rock wool a watertight lower foil 114 is laid. This foil is attached along edges thereof to a framework composed of inter alia two tubes 104, 133 at the ends of the elongated evaporator and two tubes 146 along the longitudinal sides of the evaporator. The tubes may be e.g. PVC pipes having a diameter of 10-20 cm. The lower foil 114 may be simply attached to the tubes 104, 133 with self-tapping screws 134. The foil 102 forming the covering of the water basin is likewise attached along the edges thereof to the framework by means of self-tapping screws 135. There are of course many other attachment possibilities available for the attachment of the foils to the tubes. The foils may also be attached to each other with line or point connections, e.g. by gluing, welding and/or stitching. Thus, the sealing of the evaporator can be better ensured.

The tubes 104, 133, 146 are anchored in the ground by means of ground anchors. According to this example, the ground anchors consist of tie rods 136 and anchor plates 137. The ground anchors may reach through openings in the tubes. By coupling the ground anchors to the tubes by means of hooks, straps or sleeves around the tubes, the necessity of sealing holes in the tubes for the attachment of the ground anchors is avoided. The tube 104 is provided with passages 138, via which moistened air can flow from the inner space 103 of the air hall 110 into the tube. The tube 133 is also provided with such passages. These, however, serve to admit recirculated air from the tube 133 to the inner space 103. Finally, the tubes 146, too, may be provided with such passages 138, for supplying water to be processed and for discharging drain water. The passages 138 in the tubes 104, 133 via which air is passed are located in a section of the tube at a distance above the lower foil 114, so that water 101 cannot flow away through the openings 138. The lower sides of the passages 138 in the tube 146 via which water is discharged are preferably located at the intended level of the water surface in the basin. The ground anchors reliably keep the tubes 104, 133 in the right orientation about the longitudinal axis, so that the openings 138 are reliably kept at the intended level.

For collecting rainwater and possibly water condensing against the evaporators at night, gutters 147 are provided along the edges of the evaporators. The gutters communicate with the tube 104 via passages 148 through the gutter 147 and the tube 104. Thus, optionally, rainwater may also be collected and contribute to the amount of drinking water made available. Because the foil 102 bulges slightly, the rainwater and water of condensation will reliably drain to the gutters 147.

Connected to the tubes 104, 133 are connecting lines 121 and the return line 115. Connected to the connecting lines 121, in turn, are the lines 120 of the cooling apparatus K, in which water condenses from the air moistened in the inner space 103. The condensation tubes 120 can have a diameter of, for instance, 5-15 cm and can be prepared, for instance, from plastic such as PVC or from metal. A suitable position of the tubes 120 under the soil surface 156 is 40-80 cm in most situations.

A comparison of Figs. 7 and 9 shows that the length of the return line 65, 115 can be considerably reduced by positioning the cooling apparatus under the evaporator. However, placing the cooling apparatus in the soil involves the drawback that the condensation tubes, once installed, are difficult to reach, for instance to remedy a defect. The length of the return line can also be limited by placing the cooling apparatus along a long side of an elongated evaporator. However, this implies that the direction of flow of the air must be reversed at the two ends of the evaporators. Figs. 11 and 12 diagrammatically show a configuration allowing very short return lines to be used, but in which it is less often necessary to reverse the direction of flow of the air.

According to the example shown in Figs. 11 and 12, in each case two elongated evaporators 160, 160' are placed behind each other in the longitudinal direction and staggered with respect to each other in the direction of width. To limit the occupation of space, the evaporators

160, 160' are further placed so as to overlap each other in the direction of width.

At the ends thereof, the evaporators 160, 160' are provided with lines 154, 154', to which connect the condensation lines 170, 170' of the cooling apparatuses K, K'. The cooling apparatuses K, K' are thus located in each case in line with the associated evaporator 160, 160'. The return lines 165, 165' via which air is returned from the condensation lines 170, 170' extend from an end area of the condensation lines 170, 170' which is remote from the evaporator 160, 160' arranged upstream. In contrast with the apparatus shown in Fig. 9, however, the return line 165, 65' does not pass back to the evaporator 160, 160' from where air was received, but passes to the other evaporator 160', 160, respectively, of the two evaporators located behind each other and staggered with respect to each other. Fig. 11 further shows: supply channels 190, 190' for supplying water to be treated, and fans V-151 and V-152 for, respectively, recirculating air and keeping the system under pressure, as well as discharge lines 192 for discharging produced drinking water from the draw-off points WT, WT'. The discharge lines 192 open into a collecting line 193. It is noted that one set of fans is sufficient to operate both evaporators 160, 160' and both cooling apparatuses K, K In the configuration represented in Fig. 12, it is also possible that the evaporators 160 and cooling apparatuses K located in line behind each other in the longitudinal direction are series-connected to each other to form a circuit and the last cooling apparatus is connected with a last evaporator of an adjacent circuit and the last evaporator 160 is connected with the last cooling apparatus of an adjacent circuit. Thus, in an efficient manner, a circuit 194 with a large number of evaporators and cooling apparatuses is obtained, in which the air displacements can be driven with a single fan or a single fan assembly. In that case, along the circuit extend distribution lines for supplying water to be processed and collecting lines 193 for collecting drinking water produced.

As is illustrated by the exemplary embodiment shown in Figs. 13 and 14, the excess pressure which keeps the covering 202 over the water 201 in, the basin erect, need not prevail in the inner space 203 above the water 201, but may also prevail in chambers 249 of the covering. In that case, in the space 203, for instance, the same air pressure as in the surroundings may prevail.

The covering 202 is moreover of double-walled design, so that heat losses via the covering and condensation of water against the inner side of the covering 202 are prevented.

Under the water 201 in the basin, a structure with air channels 250 may be located. Furthermore, the water basin is divided in the longitudinal direction into channels 252. Under the air channels 250 there is located a thermal insulation 231 corresponding to the insulation 131 in the example shown in Figs. 9 and 10.

In operation, air is guided through the inner space 203 in a direction indicated by an arrow 252, thereby entraining moisture from the water 201, which is heated under the influence of solar radiation and flows at a low speed in the same direction as indicated by an arrow 253, Via the air channels, air originating from the space 203 is passed under the water 201. When the air then flows under a part of the water 201 that has not yet, or at least hardly, been heated, moisture which has evaporated from the more intensely heated part of the water will condense against the walls of the air channels 250 which bound the water or at least are in thermally conductive contact with it. The partitions 251 separating the channels from each other may then also serve as heat conductors and condensation surfaces. The heat released during condensation is then utilized for heating the water taken in and to be processed.

In order to promote condensation in the air channel 250, a part of the covering 210, on the side of the evaporator where water to be processed is taken in, is made at least largely impervious to light, so that heating of the water through solar radiation in that part of the evaporator is limited. The water 251 thereby remains relatively cool in that part of the evaporator, which promotes condensation in the air channel 250. The at least largely light-impervious part of the covering may be designed in a dark color. This l β

provides the advantage that air passing under this part of the covering is effectively heated, but the water in the basin is relatively little heated.

It is noted that the construction shown in Figs. 13 and 14 may also be designed for exclusive use as cooling apparatus. In that case, the covering 210 is omitted, so that water can evaporate from the basin. The air channels 250 may optionally be designed as channels in a double-walled foil in which an excess pressure is maintained.

As a further alternative for the cooling apparatus, air/air heat exchangers, known per se, may bo used in an embodiment suitable for collecting water condensing from the cooled air. When using such heat exchangers, too, heat released during the condensation can be utilized for heating water and/or air to promote evaporation.

Claims

1. An apparatus for preparing fresh water from non-potable water, comprising an evaporator operating on solar energy having a water basin for moistening air in an enclosed space above the water basin and at least one fan for supplying moistened air from above the water basin to a cooling apparatus comprising a cooling unit, characterized in that the cooling apparatus is provided with means for drawing off water and with a return line for returning the air leaving the cooling apparatus to said space.

2. An apparatus according to claim 1, characterized in that the water basin is covered by an air hall of foil transmitting solar radiation, which foil is brought into and maintained in the erected condition of use through an air pressure increased with respect to the ambient pressure.

3. An apparatus according to claim 1 or 2, characterized in that the air hall is provided with a number of collecting gutters for collecting and discharging condensate formed against the inner side of the wall of the air hall.

4. An apparatus according to any one of claims 1 to 3, characterized in that the. air hall has an anti-condensation surface on the inner side.

5. An apparatus according to any one of claims 1 to 4, characterized in that the air hall is of tunnel-shaped design with at least one fan unit at one end and a connection with the cooling apparatus at the other end.

6. An apparatus according to any one of claims 1 to 5, characterized in that the water basin is formed from plastic foil.

7. An apparatus according to any one of claims 1 to 6, characterized in that the edges of the water basin are formed by elevations which are covered with plastic foil.

8. An apparatus according to claim 7, characterized in that the elevations are earthen walls.

9. An apparatus according to any one of claims 1 to 8, characterized in that insulating material is arranged under the water basin.

10. An apparatus according to claim 3 or any one of the preceding claims 4 to 9 as far as referring to claim 3, characterized in that the collecting gutters have an outer longitudinal edge to which the lower edge of the foil forming the air hall is attached.

11. An apparatus according to any one of claims 6 to 10, characterized in that the outer edge of the foil forming the water basin is attached to the collecting gutters at a distance above the lower side of the collecting gutters.

12. An apparatus according to claim 3 or any one of the claims 4 to 11 as far as referring to claim 3, characterized by ground anchors for anchoring the collecting gutters to the soil.

13. An apparatus according to any one of claims 1 to 12, characterised in that the cooling apparatus is situated in the water.

14. An apparatus according to any one of the preceding claims, characterized in that the cooling apparatus is positioned at least partly under the water basin.

15. An apparatus according to any one of the preceding claims, characterized in that the cooling apparatus comprises a number of tubes through which, in operation, relatively cool water is passed.

16. An apparatus according to any one of claims 1-12, characterized in that at least a part of the return hue is in heat-exchanging contact with the water basin to cause water vapor in the return line to condense.

17. An apparatus according to claim 16, wherein said at least one part of the return line is in heat-exchanging contact with a part of the water basin near an inlet for water to be processed.

18. An apparatus according to claim 16 or 17, wherein a part of the water basin near an inlet for water to be processed is designed so as to be substantially impervious to light and heat radiation.