FIELD OF THE INVENTION

The present invention relates generally to a water purification plant and relates particularly, though not exclusively, to a desalination plant. The present invention further relates generally to a process for producing distilled or potable water.

BACKGROUND TO THE INVENTION

Fresh water and particularly potable water are limited resources. Potable water is commonly obtained underground as groundwater from an aquifer. The aquifer forms a level referred to as the water-table below which the ground is saturated with water. The water-table is susceptible to contamination with other liquids which may leak underground. For example, tanks containing liquid hydrocarbons may leak and contaminate groundwater. This is an increasing problem particularly around industrial sites.

Alternative potable water supplies have been sought for some years due to at least the above problems. Distillation of salt or contaminated water is a known process for producing desalinated or decontaminated water, respectively. The salt or contaminated water is heated to a temperature such that a water fraction thereof evaporates and is then cooled producing a desalinated or decontaminated water condensate. This distillation process may, for example, be energised by solar energy in suitable climates and is known as solar desalination or decontamination. Alternatively, steam may be used within a shell and tube type heat exchanger to evaporate the water fraction of the salt or contaminated water.

An inherent problem with solar desalination or decontamination is that heat from sunlight is generally seasonal, and limited to daylight hours. Furthermore,

known solar desalination plants are relatively inefficient and thus unable to produce sufficient quantities of desalinated or decontaminated water. The plant may, therefore, need to be electrically boosted or alternatively large supplies of desalinated or decontaminated water produced and then stored. Storage can be expensive and the water may then require further treatment if it is to be used as potable water. Electrical heating may not be possible in remote areas and a solar desalination or decontamination plant which is electrically heated may be expensive to run.

Steam driven desalination plants require steam which is generally produced by evaporating water by burning a combustible fuel, such as natural gas or coal. Combustion of such fuels in turn produces combustion gases such as CO, NOX, and SOX gases which are known to be harmful to the atmosphere, in particular by destroying the ozone layer. Furthermore, the generation of steam can be relatively expensive requiring the running and maintenance of a steam utility plant.

STJM ARY OF THE INVENTION

An intention of the present invention is to provide a water purification plant that can efficiently produce a relatively large supply of distilled or potable water.

According to one aspect of the present invention there is provided a water purification plant for producing distilled or potable water said plant comprising: a collector adapted to absorb heat from a solar heat source, the collector being in heat conductive communication with salt water or contaminated water so as to preheat the salt water or contaminated water; an evaporator designed to receive preheated salt or contaminated water from the collector and evaporate as

water vapour a water fraction of the preheated salt or contaminated water; a condensor having a condensor surface disposed above the evaporator so that the water vapour can readily condense on the condensor surface, the condensor surface designed to be force cooled to a relatively low temperature via a heat exchange fluid or solid; and catchment means disposed relative to the condensor for collecting a condensed fraction of the water vapour so that, in use, the salt or contaminated water can absorb heat from the collector and thereafter the preheated salt or contaminated water can flow across a surface of the evaporator where a water fraction of said preheated salt or contaminated water can evaporate as water vapour, the heat exchange fluid or solid then promoting cooling and thus condensation of a fraction of the water vapour on the condensor surface, the condensed fraction then being collected by the catchment means as distilled or potable water.

Preferably, the condensor comprises at least one fluid passage adapted to contain or carry the heat exchange fluid or solid so that, in use, the heat exchange fluid absorbs heat from the condensor surface thus cooling said surface and promoting condensation of the water vapour on said surface.

Typically, the condensate catchment means consists of a catchment tray disposed below the condensor surface so that, in use, condensed water vapour which forms on said condensor surface falls or flows under the influence of gravity into the tray.

In another embodiment the water purification plant further comprises an accumulator containing a phase change substance having a relatively high latent heat of fusion, the accumulator being in liquid communication with both the

collector and the evaporator so that, in use, the salt or contaminated water preheated in the collector can flow to the accumulator and fuse at least a portion of the phase change substance contained therein, and thereafter salt or contaminated water at a temperature substantially less than the solidification temperature of the phase change substance can cool and solidify said fused portion of the phase change substance thereby releasing its latent heat and preheating said salt or contaminated water which then flows to the evaporator.

Advantageously, the accumulator contains more than one phase change substance each substance having a different predetermined melting point wherein latent heat can be accumulated at a range of predetermined temperatures .

Typically, the water purification plant further comprises a buffer tank in liquid communication with both the evaporator and the collector so that, in use, salt or contaminated water flowing across the evaporator, that is not evaporated, passes to the buffer tank and thereafter is pumped to the collector for recirculation through the plant. More typically, the buffer tank is also in liquid communication with the condensor so that, in use, the volume of salt or contaminated water recirculated through the plant is maintained substantially constant by adding a portion of the heat exchange fluid from the condensor to the buffer tank to compensate for the distilled or potable water removed from said salt or contaminated water during evaporation of the water fraction thereof.

According to another aspect of the present invention there is provided a process for producing distilled or potable water comprising the steps of: heating a collector by absorbing solar heat on the collector;

preheating salt or contaminated water by placing said salt or contaminated water in heat conductive communication with the collector thus absorbing heat from the collector; flowing the preheated salt or contaminated water across a surface of an evaporator so that a water fraction of said salt or contaminated water evaporates as water vapour,- and condensing at least a fraction of the water vapour on a condensor surface of a condensor which is disposed above the evaporator, the condensor surface being force cooled to a relatively low temperature via a heat exchange fluid or solid; and collecting said condensed fraction in catchment means, said condensed fraction being collected as distilled or potable water.

Preferably, the step of flowing the preheated salt or contaminated water across a surface of an evaporator involves having the evaporator surface inclined at a first predetermined angle so that the flow is effected by gravity. Similarly, the step of collecting a condensed fraction of the water vapour involves having the condensor surface inclined at a second predetermined angle to effect flow of the condensed fraction along said surface and into the catchment means.

Typically, the first and/or second predetermined angles at which the evaporator surface and/or condensor surface are inclined is between 20° to 40° relative to a horizontal plane. More typically, the first and/or second predetermined angles are approximately 30°.

In another embodiment, the process further comprises the steps of: passing preheated salt or contaminated water from the collector to an accumulator, the accumulator containing

a phase change substance having a relatively high latent heat of fusion, thereby fusing at least a portion of the phase change substance contained therein; thereafter cooling and solidifying said fused portion of the phase change substance using relatively cool salt or contaminated water from the collector, said cool water being at a temperature substantially less than the solidification temperature of the phase change substance, thereby releasing latent heat and preheating the relatively cool salt or contaminated water; and flowing the preheated relatively cool salt or contaminated water to the evaporator.

Typically, the heat exchange fluid or solid comprises force cooled or chilled water or air. Alternatively, the heat exchange fluid comprises a force cooled or chilled portion of the salt or contaminated water.

Preferably the phase change substance comprises a hydrate salt having a relatively high latent heat of fusion. Typically, the phase change substance has a melting point of between 80°C to 100°C. In one example, the hydrate salt comprises hexahydrate manganese nitrate or a derivative thereof .

Typically, the solar heat source is sunlight.

BRIEF DESCRIPTION OF DRAWINGS In order to achieve a better understanding of the nature of the present invention a preferred embodiment of a water purification plant will now be described in some detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic of a water purification plant;

Figure 2 is a cross-sectional view of a collector taken from the water purification plant shown in Figure 1; and,

Figure 3 is a perspective view of a condensor taken from the water purification plant shown in Figure 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in Figure 1 there is a water purification plant, in this example a desalination plant shown generally as 10, comprising a collector 12, a condensor 14, an evaporator 16, and a buffer tank 18.

The desalination plant 10 comprises two (2) flow circuits, namely a collector/evaporator flow circuit 20, and a condensor flow circuit 22. In this example, a dual pump 24A, 24B, is used to circulate salt water through the collector/evaporator circuit 20 and the condensor circuit 22, respectively. The condensor pump 24B circulates a portion of the salt water through the condensor 14, whereas the collector/evaporator pump 24A recirculates salt water through the collector 12 and evaporator 16 via the buffer tank 18.

As shown in Figure 2 the collector 12 comprises a base layer 26, a heat conductive layer 28, a heat absorbent layer 30 and an upper layer 32. A series of tubes 34 adapted to carry salt or contaminated water through the collector 12 are laid directly on the heat conductive layer 28.

The base layer 26 is constructed of a foamed polystyrene or polyurethane material having a relatively high density. The heat conductive layer 28 is formed from an aluminium alloy sheet having a thickness of between approximately

0.05 to 0.5 mm. The heat absorbent layer 30 consists of a composite bitumen/latex base material and is sprayed or otherwise coated on an exposed surface of the tubes 34 and

the heat conductive layer 28. The construction of the collector 12 is otherwise disclosed in Australian provisional patent application No. P03568 which is included by way of reference. The collector 12 can be mounted as an integral part of the desalination plant 10 or located remote from the plant 10.

As illustrated in Figure 3 the condensor 14 comprises a condensation panel 36. The condensation panel 36 has a pair of substantially parallel and opposed transparent sheets 38A, 38B, interconnected by a series of parallel ribs 40 designed to stiffen the panel 36 and increase its surface area. A series of fluid passages 42 having a substantially rectangular cross-section are defined in the space between the opposed transparent sheets 38A, 38B and adjacent ribs 40.

In use, a heat exchange fluid, in this example a chilled or force cooled portion of the salt or contaminated water, can flow through the fluid passages 42 so as to cool a lower surface of the transparent sheet 38A. Alternatively or additionally, the condensation panel 36 may be force cooled by a chilled solid or fluid, such as chilled water, or any suitable refrigerant. Cooling of the condensation panel 36 may also be effected by chilled air produced by an air conditioning plant. The air conditioning plant may be powered by solar, electrical, renewable energy sources, or any combination thereof .

Generally, the lower surface of the condensation panel 36 is cooled to a temperature of less than approximately 10°C with a temperature of approximately 5°C being preferred. Forced cooling may be effected naturally where ambient conditions naturally provide adequate cooling of the condensation panel 36.

In this example the evaporator 16 has an upper surface constructed of a substantially flat polycarbonate material. The polycarbonate material is insulated on an underside by a suitable insulating material such as high density polystyrene.

The collector 12, condensor 14, and evaporator 16 are located one above the other supported in a framework (not shown) . The framework includes suitable heat insulation. A lower surface of the condensor 14 and an upper surface of the evaporator 16 are inclined at a first and second predetermined angle, respectively. In this example, the first and second predetermined angles are approximately 30° to a horizontal plane.

Catchment means, in this example a catchment tray (not shown), is located beneath a lower end of the condensor 14. Thus, in use, condensed water vapour that collects on the lower condensor surface flows or passes along said lower surface and is collected in the catchment tray.

Preferably, the buffer tank 18 and transfer pipes which are used for circulating salt or contaminated water through the collector/evaporator circuit 20 are heat insulated.

Operation of the desalination plant 10 described above will now be explained in some detail.

When the desalination plant 10 is first started up salt or contaminated water is pumped, using the condensor pump 24B through the condensor 14 and into the buffer tank 18. Once a predetermined maximum level of salt water is received in the buffer tank 18 the flow of salt water to the buffer tank 18 is stopped by the combination of a level switch and valve (not shown) operatively coupled to the tank 18.

The collector/evaporator circuit pump 24A then begins to recirculate salt water through the collector/evaporator circuit 20. The salt water is preheated to a temperature of between 80 to 100°C as it passes through the collector. The preheated salt water then cascades down an upper surface of the evaporator 16. A water fraction of the preheated salt water is evaporated as water vapour.

Salt water is pumped by the condensor pump 24B through the condensor circuit 22 so as to cool the lower surface of the condensor 14. The salt water is force cooled or chilled to a relatively low temperature so as to promote condensation of water vapour on said lower surface. The lower surface of the condensor 14 is usually at a temperature of less than 10°C although approximately 5°C is preferred. Water vapour thus readily condenses on the lower surface of the condensor 14. The condensed water vapour is retained on the lower surface of the condensor 14 due to the surface tension therebetween. The condensed water vapour flows down the inclined lower surface of the condensor 14 into the catchment tray (not shown) . This fraction of the condensed water vapour is then pumped from the desalination plant 10 as distilled or potable water.

As a water fraction of the salt water is removed from the salt water, as distilled or potable water, the volume of salt water contained in the buffer tank 18 gradually reduces. Once the level in the buffer tank 18 falls below a predetermined minimum level the float switch actuates the solenoid valve to effect a flow of salt water into the buffer tank 18 from the condensor circuit 22. The buffer tank 18 is thus filled with make-up water and the solenoid valve shut down once the predetermined maximum level of salt water is once again received in the buffer tank 18.

The desalination plant 10 may also comprise an accumulator (not shown) containing a phase change substance having a

relatively high latent heat of fusion, such as hexahydrate manganese nitrate or a derivative thereof. Hexahydrate manganese nitrate is a hydrate salt having a melting point of approximately 90°C. The accumulator is located in the collector/evaporator circuit 20 downstream of the collector 12 and upstream of the evaporator 16. In use, salt water circulated through the collector 12 heats and fuses at least a portion of the hydrate salt contained in the accumulator (generally during sunlight hours) . Once the salt water flowing through the collector 12 is at a temperature substantially less than the solidification temperature of the hydrate salt, the hydrate salt is cooled and solidified by the salt water thereby releasing latent heat. The latent heat released preheats the salt water circulating through the accumulator before it flows to the evaporator 16.

The accumulator can, therefore, be charged during the day by fusing the hydrate salt, and the latent heat of the hydrate salt used during the evening to heat salt water flowing to the evaporator 16. Thus, the desalination plant 10 can be designed to supply distilled or potable water during periods where the collector 12 is not sufficiently charged by sunlight.

Now that a preferred embodiment of the water purification plant has been described in some detail it will be apparent to those skilled in the relevant art that the invention has at least the following advantages over the admitted prior art:

(1) the water purification plant including forced cooling of the condensor can efficiently provide relatively large volumes of distilled or potable water;

(2) the water purification plant can be designed to provide distilled or potable water for extended

periods by incorporating an accumulator into the system; and (3) the water purification plant provides an environmentally safe method for producing distilled or potable water.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. For example, the collector, condensor, evaporator, and/or catchment means described may be configured in a variety of shapes. Furthermore, the collector and/or catchment means may be located remote from the evaporator and condensor. The water purification plant is not restricted to including a buffer tank as described herein. Heat may be absorbed by the collector from any solar heat source and the invention is not restricted to the absorption of sunlight by the collector. Furthermore, the construction of the collector described herein is not to restrict the scope of the invention in any way. The invention is not limited to the purification of only salt water and may involve the purification of any form of contaminated water. All such variations and modifications are to be considered within the scope of the present invention the nature of which is to be determined from the foregoing description.