Abstract:
The solar-powered desalination system includes a double-walled tank, the inner and outer walls defining a condensation chamber therebetween. The lower portion of the tank provides a thermally insulated storage volume for saltwater, and the upper portion forms an evaporation chamber. Warm saltwater is sprayed upward into the evaporation chamber so that the resulting water vapor passes to the top of the tank and down between the two walls, where it condenses. Condensate is drained off by an outlet at the bottom of the condensation chamber between the two tank walls. A solar collector warms the saltwater before it enters the lower part of the tank. No energy is required, as the water circulates due to a thermosiphon effect. The only energy required is to operate the pump for the internal spray system. Optionally, a heating element may be installed in the bottom of the tank.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to water desalination and/or purification systems, and particularly to a solar-powered desalination system using a humidification and dehumidification process. 
         [0003]    2. Description of the Related Art 
         [0004]    Water that is sufficiently clean for agricultural use and potable for human consumption is an increasingly valuable commodity in many parts of the world, particularly in the Middle East. Increasing population pressures result in an increasing need for water of suitable quality, and natural fresh water supplies are dwindling as they are drawn faster than they are naturally replenished. 
         [0005]    As a result, the desalination of seawater has become an increasingly important source of suitable water in many parts of the world. This is particularly true in the Middle East, where natural fresh water is scarce, population pressures are increasing, and the land masses are surrounded by reasonably accessible bodies of seawater. However, desalination of seawater is also a viable means of obtaining fresh water in many other parts of the world. The practicality of such systems depends primarily upon the energy costs involved. 
         [0006]    A number of different principles are known for the removal of salt and/or other contaminants from water. Filtration and reverse osmosis have been used for such purposes, but these systems are relatively energy intensive due to the high pressure pumps required (and corresponding energy costs) and the maintenance and replacement costs involved for the filters. Accordingly, evaporation or distillation is generally considered to be a more cost effective means for removing salt and/or other contaminants from seawater. However, the efficiency of such evaporative or distillation systems also depends upon the energy input required, and the cost of obtaining or producing that energy. 
         [0007]    Heat is a necessary component of any evaporative desalination system. The warmer the seawater, the greater the rate of evaporation, so heat input is required to produce any reasonably efficient evaporative system. The heat may be produced in a number of different ways, from the burning of combustive fuels to electrical energy from various sources, e.g., fossil fuel plants, nuclear energy, hydroelectric energy, etc. The primary factor that all of these energy sources have in common at the consumer level is their cost. While it may be possible to construct a reasonably inexpensive desalination apparatus, the cost of energy to operate the system may be prohibitive. 
         [0008]    Thus, a solar-powered desalination system solving the aforementioned problems is desired. 
       SUMMARY OF THE INVENTION 
       [0009]    The solar-powered desalination system includes a multifunctional tank that serves as both a storage volume for incoming saltwater and as the evaporation and condensation chambers for the purification of the saltwater contained in the storage volume. The tank is preferably installed aboveground, having an outer surface exposed to the ambient air. The tank has a double wall construction defining a condensation chamber or volume between the inner and outer walls. The lower part of the tank is thermally insulated between the walls in order to retain heat input to the saltwater contained therein. The chamber or volume between the two walls is open in the upper part of the tank, and serves as a condensation chamber for water evaporated from the liquid in the lower portion of the tank. 
         [0010]    A spray system is installed in the tank above the liquid storage portion of the tank. The spray nozzles are oriented upward to maximize the time that spray droplets spend in the air within the tank, thereby maximizing evaporation. The evaporated water passes to the top of the tank and downward between the inner and outer walls in the upper portion of the tank, where relatively cooler temperatures result in condensation of the water on the inner and outer walls of the tank. The condensed pure water runs down the walls of the tank where it is drained off by an outlet immediately above the insulated lower portion of the tank. The saltwater supply is replenished by a saltwater inlet to the lower portion of the tank that is controlled by a float mechanism in the bottom of the tank. 
         [0011]    A solar collector is installed in series with the water supply in the lower portion of the tank. The solar collector circulates saltwater therethrough from the lower portion of the tank. The water circulation is due to a thermosiphon effect, wherein water warmed in the collector rises to the top of the collector to pass to the lower portion of the tank, and cooler water that has collected in the bottom of the tank flows back to the collector. Thus, the only energy required in addition to the solar input is for the operation of the pump for the spray system within the tank, and optionally for an auxiliary heater in the bottom of the tank. 
         [0012]    These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a perspective view of a solar-powered desalination system according to the present invention, illustrating its various features. 
           [0014]      FIG. 2  is an elevation view in section of the solar-powered desalination system according to the present invention, illustrating additional internal features. 
           [0015]      FIG. 3  is a bar graph showing the production of desalinated water using the solar-powered desalination system according to the present invention. 
           [0016]      FIG. 4  is a graph showing desalinated water production output versus incoming water temperature of the solar-powered desalination system according to the present invention. 
       
    
    
       [0017]    Similar reference characters denote corresponding features consistently throughout the attached drawings. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The solar-powered desalination system combines a water storage tank with evaporator and condenser components in a single unit. While the system may include an electrically powered heating element in the water storage tank, the system preferably operates by means of a solar collector disposed external to the tank and water flowing between the solar collector and the tank due to thermosiphon effect. In this manner, the only electrical power required is to drive the pump for the internal spray system for water evaporation in the upper portion of the tank, unless the optional electric heating element is used. 
         [0019]      FIGS. 1 and 2  of the drawings illustrate an external perspective view and an elevation view in section, respectively, of the solar-powered desalination system  10 . The desalination system  10  includes a tank  12  having a closed outer wall  14  and an inner wall  16  within the outer wall  14 , the inner wall  16  and other internal components being shown in the section view of  FIG. 2 . The tank  12  comprises a lower portion  18  that generally serves as the liquid water storage chamber  20  of the system, an intermediate portion  22 , and an upper portion  24 . The intermediate and the upper portions  22  and  24  within the inner wall  16  serve as the evaporation chamber  26  of the system  10 . The outer and inner walls  14  and  16  define an annular space or volume therebetween. The volume  28  in the lower portion  18  of the tank  12  between the outer and inner walls  14  and  16  contains thermal insulation  30 , e.g., glass fiber batting, etc. The annular space or volume of the intermediate and upper portions  22  and  24  between the outer and inner walls  14  and  16  serves as the condensation chamber or volume  32  of the system  10 . The evaporation chamber  26  and the condensation chamber  32  communicate with one another through the open upper end  34  of the inner wall  16 . A waterproof seal  36  is installed between the thermally insulated volume  28  and the condensation chamber  32  between the two walls  14  and  16 . 
         [0020]    A solar collector  38  is disposed external to the tank  12 . The collector  38  communicates with the tank  12  by means of a water inlet line  40  extending from the collector  38  to the water chamber  20  of the tank  12  and a water outlet line  42  extending from the water chamber  20  back to the collector  38 . Saltwater is heated by the sun in the collector  38 . The heated water rises in the collector  38  to flow through the upper water inlet line  40  to the water storage chamber  20  in the lower portion  18  of the tank  12 . The water within the chamber  20  will cool to a temperature cooler than the heated water from the collector  38 . The cooler water settles to the bottom of the chamber  20  due to thermal convection. The cooler water flows through the outlet line  42  and back to the collector  38 , where it is reheated by the sun. This circulation is entirely due to a thermosiphon effect, so that no powered pumping action is required. However, a check valve  44  may be installed in the inlet line from the collector  38  to the chamber  20  to prevent backflow. A filter may also be installed in the collector circuit, e.g., the filter  46  disposed in the outlet line  42  to prevent contaminants from flowing from the chamber  20  into the collector  38 . 
         [0021]    It will be seen that the relatively warm water being supplied to the water storage chamber  20  in the lower portion of the tank  12  will tend to evaporate due to the correspondingly higher vapor pressure. Evaporation is facilitated by a spray system comprising a water pump  48  external to the tank  12  that draws water from the storage chamber  20  by means of a water supply line  50  extending from the water storage chamber  20  to the pump  48  and a water delivery line  52  extending from the pump  48  to the evaporation chamber  26  in the intermediate portion  22  of the tank  12 . A filter  54  is preferably installed in the water supply line  50  between the water storage chamber  20  of the tank  12  and the external water pump  48 . The water delivery line  52  terminates in a plurality of spray nozzles  56  that are directed upward to spray the saltwater up into the evaporation chamber  26  of the upper portion  24  of the tank  12 . While three nozzles  56  are shown distributed along a generally diametric pipe, it will be seen that more or fewer nozzles may be incorporated in any array configuration as desired. In this manner, the saltwater droplets distributed from the nozzles  56  are sent upward in the evaporation chamber  26  before falling. This atomizes the saltwater, increases the surface area and the residence time in the evaporation chamber  26 , thus increasing evaporation of water from the droplets. 
         [0022]    The water vapor resulting from the above system rises to the upper portion of the evaporation chamber  26 , where it begins to condense upon the inner surface of the top of the outer wall and also to descend into the annular condensation chamber or volume  32  between the outer and inner walls  14  and  16  of the intermediate and upper portions  22  and  24  of the tank  12 . The condensed and purified water forms droplets upon the facing surfaces of the two walls  14  and  16  of the condensation chamber  32 , and runs down those wall surfaces to collect in the bottom portion of the condensation chamber  32  above the seal  36 . The purified water is drained from the condensation chamber  32  by an outlet line  58 , which may include a shutoff valve  60 . 
         [0023]    It will be seen that the water supply contained in the water storage chamber  20  will be depleted as water is evaporated from the evaporation chamber  26 , condenses in the condensation chamber  32 , and is drained from the chamber  32  by the outlet line  58 . Accordingly, a water replenishment line  62  is provided from a source of saltwater (not shown) to the upper portion of the water storage chamber  20 . Water is maintained at the proper level in the chamber  20  by an automatic float valve  64  installed at the delivery end  66  of the replenishment line  62  in the water storage chamber  20 . 
         [0024]    The purified water output of the system  10  will depend upon the amount of solar energy received by the collector  38 , among other factors. When relatively little solar energy is being received, the water temperature in the water storage chamber  20  will be correspondingly low and the evaporation rate will also be relatively low. Accordingly, an electric heating element  68  may be installed in the water storage chamber  20 , if desired. The heating element  68  may receive power from an electrical controller and/or junction box  70 , which may also supply and control electrical power to the water pump  48 . An electrically controlled thermostat  72  may be installed in the water storage chamber  20  of the tank  12  to communicate with the electrical heating element  68  through the controller and junction box  70 . A sediment drain line  74  and shutoff valve  76  are also provided, which extends from the bottom of the water storage chamber  20  of the tank  12 . 
         [0025]      FIG. 3  is a bar graph or chart  80  demonstrating the purified water output per energy input, measured in liters of water per kilowatt hours of energy input. The water production (liters) is shown on the vertical scale  82 , and the energy input (either solar or by means of the electric heating element  68 , shown in  FIG. 2 ) is shown along the horizontal scale  84 . It will be seen that generally an increase in energy input results in a greater output of purified water. The purified water output rises generally as an asymptotic curve due to the ever higher vapor pressure of the water as its heat is increased. According to the graph  80  of  FIG. 3 , the system  10  may provide on the order of 1.6 liters of purified water per kilowatt hour when receiving only solar power on the collector  38 . Greater output of purified water is achieved when the electric heating element is activated, but this may result in somewhat less efficiency overall when the cost of the energy required to heat the element  68  is considered. In any event, the chart or graph  80  will be seen as exemplary for only one specific configuration of the system  10 . Variations in the area of the solar collector, the surface area of the tank walls, and other factors of scale result in different efficiencies. 
         [0026]      FIG. 4  is a chart or graph  90  showing the production of desalinated or purified water according to the temperature of the saltwater contained in the water storage chamber  20  ( FIG. 2  of the drawings). The vertical (y) axis  92  represents the production of desalinated water in kilograms (liters) per hour of system operation, and the horizontal (x) axis  94  represents the saltwater temperature in degrees Celsius as measured in the water storage chamber  20  ( FIG. 2 ). Clearly, the higher the water temperature, the greater the production of desalinated water due to the increase in vapor pressure of the water as it is heated to higher temperatures. The line  96  is an approximation of the relationship between desalinated water output and water temperature. The relationship (line slope) is shown empirically by the equation: 
         [0000]        y= 0.0942 x− 2.9348. 
         [0027]    The heat gain (temperature rise) from the solar collector  38  ( FIGS. 1 and 2 ) can be determined by the Hottel-Whillier equation: 
         [0000]        Q   u   =A   c ( F   R (τα) I   t   −F   R   U   L ( T   i   −T   a ))= m   w   C   ρ ( T   o   −T   i )  (1)
 
         [0000]    where Q u =heat gain from collector, A c =collector surface area, F R (τα)=collector efficiency, I t =total radiation incident on the collector, F R U L =heat loss coefficient, T i =inlet temperature at the collector, T a =ambient temperature at the collector, m w =water mass flow rate in the collector, C ρ =water density in the collector, and T o =outlet temperature at collector. 
         [0028]    Energy gain (temperature increase) from the water storage chamber of the tank to the evaporation chamber portion of the tank is: 
         [0000]        Q   e   =m   w   C   ρ ( T   w   −T   o )  (2)
 
         [0000]    where Q e =energy gain from tank to evaporator, m w =water mass flow rate in collector, C ρ =water density in the collector, T w =water temperature in the tank, and T o =outlet temperature at the collector. 
         [0029]    Heat transfer between the condensed desalinated water and the condenser surface is shown as: 
         [0000]        Q   1   =M   d   h   fg   =Ah   x ( T   sat   −T   s )= Q   2   (3)
 
         [0000]        Q   2   =Ah   x ( T   sat   −T   s )  (4)
 
         [0000]    where Q 1 =heat of condensed water in tank, M d =condensate rate, h fg =latent heat of the condensate, A=area of toroidal condensation portion of tank, h x =heat transfer coefficient of condensation at a distance x from the surface, T sat =saturation temperature at atmospheric pressure of 100 kPa, deg. Celsius, T s =tank surface temperature (generally about ambient), and Q 2 =heat balance of the condenser surface. 
         [0030]    In view of the above, h x  (the heat transfer coefficient of condensation at a distance x from the surface) may be defined as: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where h x =heat transfer coefficient of condensation at a distance x from the surface, g=gravitational constant, ρ L =liquid water density, ρ ν =water vapor density, k=thermal conductivity of the condensate, h fg =latent heat of the condensate, μ L =viscosity of liquid water condensate, T sat =saturation temperature at atmospheric pressure of 100 kPa, deg. Celsius, T s =tank surface temperature (generally about ambient), and x=distance from the surface edge of the tank. 
         [0031]    The latent heat of the condensate, h fg , is given by: 
         [0000]        h   fg   =h   fg (1+0.68 J   a )  (6)
 
         [0000]    where: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    and where C ρ =water density in the collector, T sat =saturation temperature at atmospheric pressure of 100 kPa, deg. Celsius, and T s =tank surface temperature (generally about ambient). 
         [0032]    It will be seen that the solar-powered desalination system according to the present invention is not limited to any specific range of sizes or volumes, but may be scaled up or down to handle an amount of water as desired. The various equations noted above are generally valid, regardless of the size or scale of the system. 
         [0033]    It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.