FIELD OF THE INVENTION

The present invention relates to a tilted multi effect distillation module, which may be used for freshwater production and more particularly for the production of freshwater from fluids such as sea- water, brackish water or water containing dissolved impurities using heat energy, such as solar energy sources supplied at the bottom of the module. The in¬ vention is a modularized component that when stacked with other similar components form a multi effect distillation module.

BACKGROUND OF THE INVENTION

Several solar powered multi effect distillation solutions for freshwater production have earlier been described. US-A-4.475.988, US-A-4.402.793, US-A- 4.329.205 and JP 11.156341 all describe - as this system - proposed implementations of a solar distil¬ lation system. However, none of the above mentioned applica¬ tions has ever been brought to market due to the low efficiency, high manufacturing cost and high need of maintenance of the systems. To mention some of the fundamental challenges of solar desalination, and re- late them to the above patents: 1) The need of very much energy/high tempera¬ ture in order to have water evaporate at a rate where the efficiency of the system is high enough. The start-up speed at sunrise of the system is important in order to have reasonable production in the time window of sun during daytime. Too little collector area, too much mass (basin type multi effect) and un- controlled use of energy will in the above systems result in very poor efficiency. This is a problem in any tray type single or multiple effect distillation system such as those described in US-A-4.475.988 , US- A-4.402.793, US-A-4.329.205. 2) Corrosion. Salt water is very aggressive to any metal at high temperature and due to the need of use of high quality metal foils (e.g. titanium al¬ loys) it is nearly impossible to have durability and cost match (No solution offered in US-A-4.475.988 , US-A-4.402.793, US-A-4.329.205) . 3) Dosing of the salt water is highly critical to control the energy consumption within any of the above implementations. The use of wicks have been proposed, but no wick based on capillary suction will work over time or be precise enough (No solution of¬ fered in US-A-4.475.988, US-A-4.402.793 , US-A- 4.329.205) . 4) Scaling and cleaning. Salt water will at high temperature very quickly both crystallize and create scaling problems within the system. Any known system will fail shortly after installation if it is not cleaned frequently. A system should therefore have an integrated/automatic cleaning system for the salt crystallization and biological growth. The scal- ing that cannot be chemically removed, should be easy to remove mechanically from the system. None of the existing patents brings forward a solution for clean¬ ing and elimination of salt crystallisation. 5) Bacteria. In order to have good water qual¬ ity during operation, the operating temperature should be as high as possible. Most bacteria/vira will not give problems at temperatures above 600C and in a non-pressurized distillation process. Any multi effect system using an external heat source (solar collector or other) that will not bring the operating temperature above 6O0C will have problems with the quality of the distillate. None of the existing pat- ents brings forward a solution for any issues related to bacteria or other biological infection. Due to the above neither the system described in US-A-4.475.988, which have some structural simi¬ larities with the prior invention and may therefore be considered the closest prior art, nor any other solar multi effect distillation solution is currently in the market. Research and implementations over the latest 30 years has shown the same result: The prin¬ ciple is too expensive and too complex. The best and latest example for a product is "AquaKids" launched by TOP ECOLOGY CO. (Japan) in 1998/99 claimed to pro¬ duce 48 litres pr day with a basin area of 4,5 m2. The solution is very expensive and is not known to be in the market for the same reason. Other examples of re- search and implementations can be found in scientific databases or at any of the world's desalination com¬ munities such as International Desalination Associa¬ tion, European Desalination Society, Indian Desalina¬ tion Association and others. Thus, there is a need for a new design of a multi effect distillation module for mass production and easy assembly, operation and maintenance. In short, it is the object of the invention to provide a multi effect distillation module and a layer there¬ fore, which yields a relatively high efficiency, a relatively low level of maintenance and costs and which is of a simple construction.

TERMINOLOGY

In the present application the below listed terms are used: Foil: A foil where the upper surface of the foil serves as a surface for evaporation and the lower surface of the foil as a surface for condensa¬ tion. Frame: The spacer between foils either encapsu- lating the rim of foil or being placed between foils. The frame contains necessary functions such as inlet (s) and outlets. Layer: The combination of a foil and a frame, where the foil is surrounded by and embedded in the frame. In the present application the term "sur¬ rounded" is used to indicate that opposing edges or peripheral areas of the foil are in contact with the frame and the term "embedded" to indicate a physical joint between the foil and the frame. Chamber: The space between a lower foil for evaporation and an upper foil for condensation. Module: A number of layers stacked to thereby form a module of multiple chambers. System: One or more modules combined with oper- ating equipment such as solar collectors, reservoirs, pumps, filters, piping, valves and other peripheral equipment. SUMMARY OF THE INVENTION

There is provided a stackable layer to form a chamber comprising a foil, where the edge of said foil is embedded in a frame, said frame comprising one or more inlet opening(s) in one end of the layer and two outlet openings in the opposite end of the layer and said frame preferably insulating at least 10 times better than the foil. The foil can obtain the better heat transfer by having a higher thermal conductivity and/or being very thin compared to the thickness of the frame. The material of the foil is made of a material, which is non-toxic, resistant to salt water and possible chemicals in the fluid as well as any water vapour in the temperature range from O0C - 1000C. The foil may be made of materials such as metals, ceramics, glass, enamelled metals or composites, as well as lamination of one or more of the above materials . The frame of the layer may be made of a suit¬ able material to provide non-corrosion, water and air tightness, flexibility in a large temperature range, insulation, UV-stability, opaqueness and should also be non-toxic. According to embodiments of the inven- tion, the frame of the layer is made of a plastic or composite material. The frame comprises an integrated collecting part for collecting distillate condensed at the con¬ densation surface and a collecting part for collect- ing residue fluid at the evaporation surface. This ensures guaranteed safe collection of the distillate and has not previously been implemented in an inte¬ grated design of a frame. It is preferred that the evaporation surface of a layer is capable of holding back fluid or reducing the velocity of the flow of fluid and allow evapora¬ tion. This may be achieved by increasing the surface tension, chemically or by granularity, of the foil surface or by at least partly covering the surface with a layer of a hygroscopic or water restraining material. Such a material is e.g. a plastic material. The condensation surface of a layer must allow the drops of distilled fluid to run off the surface into the corresponding collecting part(s) without loosing the grip of the surface. The grip is provided through the natural surface tension of water combined with a preferred higher surface tension of the foil . The surface tension of the material used for the foil should be with a contact angle less than 60° at tem¬ peratures between 20°C and 95°C and as close to 0° as possible. The dimensions of the different parts of a layer may vary according to the materials used. How¬ ever, it is within the invention that the thickness of the foil is in the range of 0.2-2 mm, preferably about 1 mm. The frame may have a thickness in the range of 2 - 20 mm, preferably about 8 mm, and a width in the range of 10-50 mm. The tolerances of the frame and foil in this invention are highly important to exploit the available low quality low temperature thermal energy for multi effect distillation and the emphasis on this has not been pointed out earlier. The width of a layer from one side to the other is limited by the stiffness of the foil used, and may thus be designed within a wide range, depending on the materials used and where the module is to be in- stalled. Preferably, the layer should be as wide as possible, but according to an embodiment of the in¬ vention, the width of a layer may be in the range of 0.5-2 m, preferably about 1 m. For an embodiment of a layer the length from top to bottom may be limited by the ability to retain distillate on the condensation surface without loss before collecting it in the collection part(s) . This is determined by many factors such as inclination an- gle, temperature, and surface tension of the foil. Preferably the layer should be as long as possible, but for handling purposes the length of a layer may be in the range of 0.5-5 m, typically 1 - 3 m. At least two layers are to be stacked to form a distillation chamber. The chamber hereby has a bottom part and a top part being parallel to each other. The bottom part is an evaporation surface and the top part is a condensation surface in said chamber. When said layers are stacked on top of each other to form chambers, the foil of one layer becomes condensation surface in one chamber and evaporation surface in the overlying chamber. This design allows industrialisa¬ tion of the design for mass-production. The layers should provide a thermal contact for the latent heat provided through natural evaporation and natural condensation taking place in each cham¬ ber. The thermal contact should preferably be through the foil and not through the frame that keeps the layers apart and thereby forming the chamber. The number of chambers in a module is at least 2 and up to 15. The maximum number of chambers de¬ pends of the energy level supplied to the system and the energy transition through the chambers. The lower chambers will produce the most and if not cooled on the upper outside, the highest chamber will produce least. A preferred embodiment is between 3 and 10 chambers. It is within the present invention that the stacked chambers of a module share at least one com¬ mon fluid supply and two fluid outlets. It is given that the frame and surfaces of the chamber are watertight and airtight. To ensure water and air tightness to the surrounding atmosphere the layers are pressed together so that the frame in it¬ self provides the wanted spacing and sealing. Inter¬ nally between the chambers a controlled vapour dis¬ tribution may be provided through a number of holes in the frame connecting each chamber. Size and place¬ ment of these vapour passage openings depend on en¬ ergy level and materials used. The inside of a chamber is preferably shielded from light and at least some of the outer parts of a module are made of an opaque material to obtain this. Opaqueness is to prevent bacteria, algae and UV- breakdown of materials. This is highly important to obtain good water quality and minimise maintenance. In order to obtain an effective evaporation, the distance between the lower evaporation surface and the upper condensation surface in a chamber should be rather small. Thus, it is preferred that the distance is in the range of 3-20 mm. The stackable design of the present invention allows very easy assembly and disassembly of the mod¬ ule for cleaning or other maintenance. A chamber as described above may be sub-divided into two or more sub-chambers lengthwise and the thickness of a shared sidewall of such sub-chambers may be in the range of 1-20 mm, preferably about 3 mm. The internal sidewalls provide stiffness and ri¬ gidity to the foil . Thereby it is possible to produce bigger layers without excessive deflection of the foil(s) . In order to prevent build up of bacteria or sedimentation of any kind inside the module, the sys¬ tem may comprise a flushing system allowing a me- chanical and/or chemical flushing. This may be ob¬ tained in separate inlets in the layer. By temporar¬ ily closing the outlets any dissolvable build-ups can be dissolved and washed out by filling up the chamber with fluid for 5-30 minutes and then reopen the out- lets. For flushing purposes the common inlet may also be used, just adding more pressure to the inlet de¬ vice. In order to control the right volume of inlet fluid to the chamber, a device integrated in the frame and connected to the common fluid supply is provided to ensure correct flow according to avail¬ ability of thermal energy. This device for controlled inlet of fluids may e.g. be achieved through fixed geometry, through individually adjustable valves or dosing units as described in the applicants own Dan¬ ish patent application PA 2004 01694, not published. The controlled flow of inlet fluid may be individual for each chamber according to the present energy level and demand for flow to avoid build-up of sol- ids. The common fluid supply for all inlet devices in a module may be integrated in the frame by a hole perpendicular to the frame plane in each frame creat- ing an internal tube when being stacked. In another implementation the common fluid supply may be kept outside the module only having the inlet devices in¬ side the frames. In order to achieve as large a surface and thereby energy consumption by evaporation as possible and to avoid build up of solids, a fluid spread de¬ vice may enhance the fluid to be spread over the en¬ tire evaporation surface in an even flow. The device to spread the inlet fluid is preferably an integrated part of the layer's frame or an add-on to the layer. In the preferred embodiment the spread device is a flat beam stretching over the width and at the upper end of the foil . This beam is not connected to the foil and will through capillary tension pull the inlet fluid sideways under the beam. The outlet of the chamber serves two purposes : collection of distillate and collection of the resi¬ due fluid - e.g. salt water. It is preferred that the integrated collecting part in the frame for collecting fluid is arranged between the bottom part and the top part at the fluid outlet end of the chamber and stretching from one side of the chamber to the other side of the chamber. The collecting part for collecting distillate should be arranged at a distance above the evaporation sur¬ face of the bottom part of a chamber so that residue fluid will not enter this collecting part. Here, it is preferred that said distance is at least 1 mm, preferably at least 2 mm. The frame integrated collecting parts of a layer may be formed so that each chamber has two gut¬ ters, preferably one on each side of the foil. The collection of either fluid may thus be done in a gut¬ ter leading the fluid (either distillate or residue fluid) out of the chamber in separate outlet pipes at the lower end of the chamber. The gutters in the chamber may be angled to allow both fluids to run out by gravity. A divider between the two gutters may en¬ sure a safe collection of distillate and residue fluid by choosing correct angle and positioning. The outlet of a chamber may allow vapour to spread to other layers, but not liquids. It is sub¬ stantial for the performance of the invention that excess vapour in one chamber in a controlled way (through the size/number of the outlet openings) may condense in other chambers. Other objects, features and advantages of the present invention will be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accom¬ panying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic view of a layer according to the present invention, Fig. 2 presents sectional views of a layer ac¬ cording to the present invention Fig. 3 is a schematic view of a water restrain¬ ing material on the evaporation surface of a layer Fig. 4 is a schematic view of a chamber made by two layers Fig. 5 is a schematic view of a layer having external fluid supply Fig. 6 is a conceptual design of a distillation system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 illustrates an embodiment of a layer. The layer 101 may be used as a building block for a freshwater-producing module according to a further aspect of the present invention. In Fig. 1 the layer 101 has a lower surface 102 of a foil 112, an upper surface 103 of a foil 112, a frame 104 with a first sidewall 105 and a second sidewall 106, a water inlet end wall 107 and a water outlet end wall 108. The layer 101 is pierced at the inlet end wall by one or more inlet hole(s) 109 and at the outlet end wall 108 by a distillate fluid outlet 110 and a residue fluid outlet 111. The lower surface 102 has a smooth surface let¬ ting condensed distillate run off down to the outlet end wall 108. On the lower surface 102 of the layer 101 the outlet end wall 108 is sloping towards the distillate outlet 110 to ease the collection of distillate drops running off the lower surface 102. On the upper sur¬ face 103 of the layer 101 the outlet end wall 108 is sloping towards the residue fluid outlet 111 to ease the collection of fluid running off the upper surface 103. The distillate outlet 110 and the residue fluid outlet 111 at the outlet end wall 108 are substan- tially closed and airtight towards ambient atmosphere with the exception of fresh water drained through distillate outlet 110 and residue fluid drained through outlet 111. When layers 101 are stacked into one or more chambers, inlet (s) and outlets (109, 110, 111) form conduits connecting the chambers. The resulting out¬ let conduits may connect the chambers that will thereby share the same air/vapour pressure. Fluid will be dosed in the inlet (s) in layer specific vol¬ umes by adjusting the hole dimensions, e.g. diameter or length (by adding a stub) , or by partially filling the hole with longitudinally oriented fibres and in combination with a constant gravitational push from the inlet fluid reservoir. Pig. 2 illustrates sectional views of the layer (101) from Fig. 1 and introduces the sheet/plate 202 separating distillate from residue fluid on top of layer 201. When the plate 202 is placed between stacked layers 201, the plate 202 becomes the lower part of a distillate collection trough keeping dis¬ tillate separate from residue fluid. The plate 202 is overlapping with the frame of layer 201 ensuring proper sealing of the distillate collection trough. In another embodiment (not shown) , the separator plate may be moulded integrally with the frame. The plate 202 has two holes 203, 204 similar to the outlet holes 111, 110 in the layer 201 and when the plate is placed correctly between stacked layers these holes becomes an integrated part of the drains for distillate 204, 110 and residue fluid 203, 111. A sectional view 2B of a layer with a separating plate on top is presented for residue fluid outlet 203 hav- ing a residue collection part 205. A sectional view 2C of a layer with a separating plate on top is pre¬ sented for distillate outlet 204 having a distillate collection part 206. Sectional view 2D (207) depicts the foil 208 of the layer 101 being completely encapsulated by the frame material 209. To ensure proper sealing of a chamber when stacking two or more layers the frame 209 is made of a flexible material and furthermore has a bead 210 throughout the length of the frame on all four sides 105, 106, 107, 108 of the layer. Fig. 2E shows a notch 211 in each side of the frame 209 designed to insert a bar across the upper surface of the layer 201. The bar holds a wick (not shown) lying on the upper surface of 201. As indi¬ cated on the sectional view 2E, the notch in the frame is only on the upper side of the layer 201, not on the lower side. The bar fitting into the notches 211 is illustrated between two layers at 212 showing that it will let fluid pass underneath the bar to the wick on the upper surface of the layer 201, but no fluid can pass over it to the lower surface of the overlying layer. Sectional view 2F depicts the access 213 from the inlet hole (s) 109 to the upper surface of the layer 103. Fig. 3 presents an implementation 301 introduc¬ ing a wick 302 on the upper surface 103 of a layer 101. The wick absorbs the supplied fluid and distrib¬ utes the fluid evenly across the width of the upper surface 103 of layer 101. The wick is made in a chemically resistant material with regards to salt and substances typically found in seawater. The wick is based on a net with a mesh size big enough to let any air/vapour bubbles pass through. Fig. 4 is an illustration of a chamber formed by two layers. The uppermost layer 401 in a distilla- tion module is illustrated in Fig. 4A with the under¬ lying layers not being visible. A sectional view 4B of the chamber presents the upper layer 402 and Fig. 4C a lower layer 403. The lower layer has a wick 407 on top attached to a bar 406 across the width of the layer. When put together 402 and 403 form a chamber 404 also presented without the frame as 405 in Fig. 4E. Fig. 5 illustrates a layer 501 having an inlet hole 502 through the frame wall for the purpose of supplying and dosing fluid externally to the frame . Fluid can then be supplied to the chamber e.g. by a hose or a tube connected to the inlet hole 502. Fig. 6 illustrates a distillation system 601 including a number of distillation modules 604. In a preferred embodiment of the invention the fluid or feed water supplied from the reservoir 602 is sea- water or polluted water to be distilled into freshwa¬ ter. The feed water is supplied to the distillation modules 604 and the thermal reservoir 607 via a feed water hose 603. The distillation modules 604 are placed next to each other and across from each other forming a roof and an insulating gable 606 (partly cut away in fig. 6) closes both ends of the roof. The space within the roof is substantially air and vapour tight towards ambient atmosphere and thermally insu¬ lated towards the ground and ambient atmosphere to reduce energy loss from lost vapour and heat radia¬ tion. In this air and vapour tight space a thermal reservoir 607 holding feed water is placed and water evaporating from this reservoir will condensate on the lower surfaces of the distillation modules 604 and thereby produce distilled water and warm up the distillation modules starting the multi-effect dis¬ tillation process. The distillation modules 604 on one side of the roof shares one drainage system col¬ lecting distilled fluid 610 and residue fluid 609. The thermal reservoir 607 receives heat from a solar collector 611 through a heat exchange loop connected by the fluid hoses 612 and 613. Thermal energy may alternatively be supplied to the basin 607 through convection by using the concept of a heat exchanger transferring thermal energy from natural underground thermal energy, or it may be waste heat from any kind of combustion based process. To maintain an acceptable salt concentration in the thermal reservoir 607 excess water is drained through 608. Each distillation module 604 is covered by a cover 605 in order to maintain a low temperature on the upper cooling surface of the distillation modules 604. While the invention has been particularly shown and described with reference to particular embodi¬ ments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention, and it is intended that such changes come within the scope of the following claims.