Abstract:
A solar water purifier is disclosed. The solar water purifier uses waste heat from a solar panel, or direct heat from the sun, to boil, evaporate, and condense water to create a stream of purified water. In one embodiment, a boiling tank is mounted under and in direct thermal contact with a solar panel to absorb waste heat. In another embodiment, a transparent wall of the boiling tank is directly exposed to solar energy. Unpurified water enters the boiling tank from an inlet tank. Once in the boiling tank, a stream of steam and water vapor leaves the tank and is allowed to condense within a condenser. The condenser is located within the inlet tank, such that the heat recovered during condensation is used to preheat the inlet water to the boiling tank.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/832,680, filed Jun. 7, 2013, and also to U.S. Provisional Patent Application No. 61/835,185, filed Jun. 14, 2013, the contents of both of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the use of solar energy to boil and purify drinking water. 
     2. Description of Related Art 
     About two billion people do not have access to clean, fresh water. Most of earth&#39;s water cannot be used for drinking, cooking, or bathing, due to dissolved natural salts, bacterial or parasitic contamination, or chemical contamination. While unpurified water can often be used for some purposes, it must be purified if it is to be used for drinking Water purification can be done by a number of different processes, including reverse osmosis and evaporation-condensation processes. 
     Although effective processes for water purification are well known, these processes are also energy intensive. For example, in evaporation-condensation processes, water is typically heated to convert it to steam and then condensed back into the liquid phase. The heating process kills many organisms found in unpurified water, but requires a great deal of energy. Unfortunately, many of the places that do not have clean, fresh water also do not have access to reliable sources of power or established power grids. A common solution to the lack of reliable power sources, particularly for sunny areas of the globe, is to use photovoltaic panels to convert solar energy to electricity. 
     Photovoltaic cells cannot use all of the wavelengths of light in the solar spectrum to produce electricity. They particularly have trouble using infrared and ultraviolet wavelengths of light, and even for those wavelengths that can be converted into electricity, the conversion is not particularly efficient. An average solar panel is capable of converting only about 20% of the solar energy it absorbs into electricity; the other 80% is lost, typically as heat energy. Solar panels are also relatively expensive. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention relates to a solar water purifier. The solar water purifier uses solar energy to purify water by boiling, evaporation, condensation, and optionally, filtering. In some cases, the energy used may be waste energy from a solar panel. In the solar water purifier, a boiling tank accepts solar energy to boil water. An inlet tank is in fluid communication with the boiling tank and provides unpurified water to it. A stream of steam and water vapor leaving the boiling tank is directed into a condenser, where it is converted to a stream of purified water. The condenser is preferably located within the inlet tank, such that heat withdrawn from stream during condensation is used to preheat the water entering the boiling tank. In some embodiments, the boiling tank may be mounted directly beneath and in thermal communication with the underside of the solar panel. Pumps and other elements used to run the solar water purifier may draw electrical power from the solar panel. 
     Another aspect of the invention also relates to a solar water purifier. The solar water purifier according to this aspect of the invention has a boiling tank with at least one face made of a light-transmissive material, such as glass. The purifier also includes an inlet tank that is in relatively free fluid communication with the boiling tank. The inlet tank may be relatively open, while the boiling tank is sealed with egress of water vapor and steam controlled by a check valve. Once the check valve opens and steam and water vapor do leave the boiling tank, they are routed through a condenser, where they are reconverted to now-purified liquid water. The condenser is disposed in the inlet tank so that waste heat from the condenser can be used to pre-heat the water in the inlet tank. In embodiments according to this aspect of the invention, hydrostatic pressure and the pressure at which the check valve opens can be used to control the rate at which water flows through the purifier and, in some cases, the temperature at which the unpurified water is boiled. 
     Yet another aspect of the invention relates to a solar water purifier as described above with an inlet tank that has higher walls than the boiling tank and can thus maintain higher water levels that create greater hydrostatic pressure. A further aspect of the invention relates to a solar water purifier as described above with an inlet tank that has high, shaped walls that allow greater and varying hydrostatic pressure. 
     Other aspects, features, and advantages will be set forth in the description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURE 
       The invention will be described with respect to the following drawing figures, in which like numerals represent like features throughout the figures, and in which: 
         FIG. 1  is a schematic cross-sectional view of a water purification apparatus according to one embodiment of the invention; 
         FIG. 2  is a schematic cross-sectional view of a water purification apparatus without an integrated solar panel according to another embodiment of the invention; 
         FIG. 3  is a schematic cross-sectional view of a water purification apparatus without an integrated solar panel array, illustrating one type of modification to increase hydrostatic pressure in the apparatus; and 
         FIG. 4  is a schematic cross-sectional view of a water purification apparatus without an integrated solar panel array, illustrating another type of modification to increase hydrostatic pressure in the apparatus. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic cross-sectional view of one embodiment of a water purifier, generally indicated at  10 . In the water purifier  10 , a conventional photovoltaic solar panel  12  is supported on appropriate support structure  13  and is oriented to the sun to absorb its energy. While not shown in  FIG. 1 , the solar panel  12  may have any type of framing and support elements known in the art. 
     As was explained above, in a conventional solar panel, solar energy that cannot be converted to electricity is simply lost, mostly as radiated heat. However, the water purifier  10  makes use of that waste heat and other forms of waste energy to purify water. More specifically, in the water purifier  10 , a boiling tank  14  is mounted under the solar panel  12  and in thermal communication with it, such that waste heat is directed into the boiling tank  14 . In some embodiments, features may be included to increase or optimize thermal transfer between the solar panel  12  and the boiling tank, including conductive plates or adhesives between them, or mechanical features on the rear of the solar panel  12 , like heat sink fin structures, that protrude into the boiling tank  14 . 
     Ideally, the waste heat from the solar panel  12  causes the water in the boiling tank  14  to heat and boil. The boiling of the water in the tank  14  would typically kill most biological contaminants. As water boils off from the boiling tank  14 , it is drawn or directed into a condenser  16  and is allowed to condense there. In  FIG. 1 , the condenser  16  is schematically illustrated as coiled tubing, although it may take any form known in the art. In order to cause the steam and water vapor to condense back into liquid water, the condenser  16  is maintained at a lower temperature. 
     In the illustrated embodiment, the condenser is maintained at a lower temperature by placing the condenser  16  inside an inlet tank  18 , into which unpurified water is pumped or deposited prior to entering the boiling tank  14 . The incoming unpurified water, initially presumed to be at ambient temperature, is significantly cooler than the water vapor or steam in the condenser  16 , and thus, causes the water vapor to condense into liquid water. With the condenser  16  located as it is, the heat drawn off in the condensation process also has the effect of preheating the water in the inlet tank  18  before it enters the boiling tank  14 . However, as those of skill in the art will appreciate, while the condenser  16  may be in thermal contact with the inlet tank  18 , the water within the condenser  16  and the water in the inlet tank  18  are kept isolated from one another. In some embodiments, insulation may be placed between the boiling tank  14  and the inlet tank  18 . 
     The water condensing within the condenser  16  is put through a filter  20 , which may be a standard charcoal filter, before being deposited in a collection tank or entering a water system. 
     The water purifier  10  may be operated in batches or in a continuous feed mode. As was noted briefly above, water may be pumped from a source of polluted water  22  into the inlet tank  18  by a pump  24 . A filter  23  is interposed between the source of polluted water  22  and the pump  24  in order to prevent silt, particulate matter, and other elements that could damage the pump  24  from reaching it, although if the water source is relatively free of such contaminants, the filter  23  may be omitted. Preferably, the pump  24  is a direct current (DC) pump that draws power from the solar panel  12 . Of course, particularly if the water purifier  10  is operated in batch mode, the inlet tank  18  may be the only source of unpurified water, and that water may be placed in the inlet tank  18  manually. 
     As shown in  FIG. 1 , an inlet valve  26  controls the unidirectional flow from the inlet tank  18  into the boiling tank  14 . In some embodiments, the opening of the inlet valve  26  may be timer-controlled. In other embodiments, a simple float  28  at the upper end of the boiling tank  14  would be coupled to the inlet valve  26 , such that when the water level in the boiling tank  14  drops below a pre-set level, the inlet valve  26  is opened and more water is admitted for purification. (A pump coupled to the inlet valve  26  is not shown in  FIG. 1 , but may be included, and, like the pump  24 , would typically be DC-powered and electrically connected to the solar panel  12 .) Both tanks  14 ,  18 , the condenser  16 , and the other elements may have access doors or ports to allow them to be cleaned or flushed. 
     In a continuous or fed-batch mode, the amount of water that the water purifier  10  can purify per unit amount of time will depend on the amount of energy incident on the solar panel  12 , the efficiency of the solar panel  12 , the efficiency of thermal transfer between the solar panel  12  and the boiling tank  14 , the temperature differential between the incoming unpurified water and the boiling point of water, and other factors. 
     Certain other features and variations may be made in other embodiments of the invention. For example, the boiling tank  14  may be thermally insulated, such that it acts as a heat sink for the solar panel  12  but loses less thermal energy to convection or radiation. Additionally, although the inlet tank  18  is shown as being directly underneath the boiling tank  14 , it may be located elsewhere in other embodiments. Moreover, while it is possible for the solar panel  12  to be connected to a solar charge controller and a set of batteries to store electricity or to supply electricity for other purposes, in the interest of simplicity, this may not be done in most embodiments. 
       FIG. 2  is a schematic cross-sectional view of a solar water purifier, generally indicated at  100 , according to another embodiment of the invention. Like the apparatus  10  of  FIG. 1 , the apparatus  100  is supported on support structure  102  at an appropriate angle to catch the sun&#39;s rays. As those of skill in the art will appreciate, the angle may be adjustable. 
     The apparatus  10  of  FIG. 1  includes an integrated solar panel  12 . However, an integrated solar panel  12  is not necessary in all embodiments. Instead, in some embodiments, the sun&#39;s rays may fall directly on a boiling tank, and the sun-facing wall of the boiling tank may be made of glass or another suitable, transparent or energy-transmissive material, so as to expose the water in the boiling tank to the maximum amount of energy possible. 
     In the apparatus  100  of  FIG. 2 , the sun-facing surface of the apparatus is comprised of two sheets  104 ,  106  of glass, separated by a small air gap  108 . In some cases, the sheets  104 ,  106  may be made of another transparent material, such as polymethyl methacrylate (PMMA; PLEXIGLAS®) or polycarbonate, or one sheet  104 ,  106  may be glass and the other a transparent plastic material  104 ,  106 . The air gap  108  provides for insulation, and altogether, the two sheets  104 ,  106  with their interposed air gap  108  serve as the forward wall of a boiling tank  110 . 
     The rear wall  112  of the boiling tank  110  is comprised of insulation, such as polystyrene foam, and may also include thin layers of metal foil or plastic material to isolate the water from the insulation itself. Behind the rear wall  112  of the boiling tank  110  is the inlet tank  114 . Typically, the boiling tank  110  is relatively thin, such that the solar energy can penetrate, reach, and heat all of the water in the tank  110 . For example, in one embodiment, the boiling tank  110  may have a depth in the range of about ½ inch to about 1 inch. The inlet tank  114 , by contrast, may have a depth in the range of about 1 inch to about 2 inches. The thickness of the insulation  112  between the two tanks  110 ,  114  will vary with the nature of the insulating material. In one embodiment, the insulation may be, e.g., polystyrene foam with a thickness of about 4 inches. Of course, any insulative material may be used. 
     An open passageway  116  defined in the rear wall  112  connects the inlet tank  114  and the boiling tank  110  and allows water to flow essentially freely between the two. In contrast to the apparatus  10  of  FIG. 1 , in which an inlet valve  24  controls the flow between the inlet tank  18  and the boiling tank  14 , in the apparatus  100 , there is no such valve. Rather, the inventors have discovered that differences in pressures between the inlet tank  114  and the boiling tank  110  can be used to control the rate of water flow from the inlet tank  114  into the boiling tank  110 . 
     The boiling tank  110  is sealed, except for the passageway  116  connecting it to the inlet tank  114 , while the inlet tank  114  is open to atmosphere. Steam and water vapor from the boiling tank  110  are drawn off into a condenser  120 , and as with the apparatus  10 , the condenser  120  is physically situated within the inlet tank  114 , such that waste heat from the condenser  120  preheats the water in the inlet tank  114 . However, the inlet pipe  122  to the condenser  120  includes a pressure-triggered one-way check valve  124  that allows steam to flow only from the boiling tank  110  into the condenser  120 . That check valve  124  may be selected or configured such that a certain pressure builds up in the boiling tank  110  before the valve  124  opens. 
     Because there is an open passageway  116  between the two tanks  110 ,  114 , the pressure at which the check valve  124  opens and the water level in the inlet tank  114  determine the rate at which water flows from the inlet tank  114  into the boiling tank  110 . Essentially, if the water level in the inlet tank  114  is kept higher than the water level in the boiling tank  110 , the hydrostatic pressure differential will cause water to flow into the boiling tank  110  at a rate that is counterbalanced by the steam pressure within the boiling tank  110 . 
     This type of pressure control has another advantage: it is possible to adjust the pressure of the check valve  124  such that the water will boil at its conventional, sea level temperature. However, in some embodiments, it may be desirable to heat the water to higher temperatures, in order to kill specific organisms, or for other reasons. In that case, the check valve  124  can be selected or configured to open at an appropriate, higher pressure. This technique can also be used to compensate if the apparatus  100  is to be used at higher elevations, where the lower atmospheric pressure would otherwise cause the water to boil at lower temperatures that would not kill all of the pathogens. 
     A pump  126  may be used to pump water into the inlet tank  114  so as to maintain desired water and pressure levels within the apparatus  100 . A float, or another conventional type of water level sensor, may be used to control the water level at which the pump  126  activates. The pump  126  is typically in communication with a supply of polluted water  128  or water that otherwise needs to be purified. While not shown in the figure, the pump  126  may be connected to a solar panel or a battery that draws its energy from a solar panel. A filter  130  may be interposed between the water source  128  and the pump  126  to filter contaminants and prevent the pump  126  from being damaged by particulate matter. Water drawn off from the condenser  120  is passed through another filter  132 . The rear of the inlet tank  114  is defined by a second insulation panel  134 , which is typically thinner than the insulation panel  112 , because the water in the inlet tank  114  is cooler than the water in the boiling tank  110 . 
     Additional changes and improvements may be made to the apparatus  100  to increase the hydrostatic pressure within the apparatus or to increase its controllability. For example,  FIG. 3  is a schematic cross-sectional view of an apparatus  200  according to another embodiment of the invention. The apparatus  200  is similar in most respects to the apparatus  100  of  FIG. 2 , has most of the same components as the apparatus  100 , and the description above will suffice for those components. The primary difference is that apparatus  200  has an inlet tank  214  with walls that extend beyond those of the boiling tank  110 . The higher walls of the inlet tank  214  allow the inlet tank  214  to be filled to a higher water level, which increases the hydrostatic pressure in the apparatus  200  and may contribute to processing water at a higher rate. 
       FIG. 4  is a schematic cross-sectional view of an apparatus, generally indicated at  300 , according to yet another embodiment of the invention. The apparatus  300  is substantially similar to the apparatus  100  of  FIG. 2  and shares many of its features. Therefore, the description above will suffice for those features. 
     Like in the apparatus  200 , the walls of the inlet tank  314  of the apparatus  300  rise above the height of the boiling tank  110 , allowing a higher water level and greater hydrostatic pressure in the inlet tank  314 . However, in addition to the increased wall height of the inlet tank  314 , the upper end  316  of the inlet tank  314  also flares outward. The outward flare of the upper end  316  allows not only increased hydrostatic pressure, but variable, controllable hydrostatic pressure. Although the upper end  316  has a generic, flared shape similar to that of a funnel, the upper end of other embodiments may be made in any shape, in order to establish a desired relationship between water height in the inlet tank  314  and hydrostatic pressure. 
     Although the apparatuses  100 ,  200 ,  300  of  FIGS. 2-4  are not shown in association with a solar panel, in some embodiments, a solar panel may be included and associated with the apparatus  100 ,  200 ,  300 . If a solar panel is used, the solar panel may be smaller than one used in the apparatus  10  of  FIG. 1 , and would generally provide power for any accessory components, such as the pump  126 . 
     Additionally, while it may be desirable in most embodiments to use solar energy as the sole energy source for boiling water in apparatuses  10 ,  100 ,  200 ,  300  according to embodiments of the invention, in some embodiments, it may be necessary or desirable to include electrical heating elements, such as resistive heating elements, in the boiling tank  14 ,  110 . These electrical heating elements may be useful in preheating the water to a desired temperature when the apparatus  10 ,  100 ,  200 ,  300  first begins operation, or in maintaining temperature temporarily if the sun intensity decreases significantly while the apparatus  10 ,  100 ,  200 ,  300  is in use. If present, electrical heating elements may be powered by an accessory solar panel. 
     In other embodiments, the sun&#39;s energy may be focused on the boiling tank  14 ,  110 . In these embodiments, the outer pane of glass  104  may be concave or convex and may serve as a lens to focus the sun&#39;s rays on the boiling tank. For example, a relatively flat Fresnel lens may be particularly suitable in some embodiments. Moreover, while the boiling tank  14 ,  110  and the inlet tank  18 ,  114 ,  214 ,  314  are shown as being roughly the same size in the figures, the boiling tank  14 ,  110  may, in fact, be much smaller, particularly in cases where a lens is used to focus the sun. In that case, the boiling tank  14 ,  110  may be small and centered on the focal point of the lens. As those of skill in the art will appreciate, the spacing of the outer sheet of glass  104  may be altered and the width of the air gap  108  increased, depending on the focal length of the lens. In other embodiments, a separate lens may be mounted above the boiling tank  14 ,  110  on a bracket. 
     Additionally, in some embodiments, reflectors may be used to direct more solar energy onto the boiling tank  14 ,  110 . Generally speaking, a reflector is any kind of reflective panel that can reflect and redirect light. The classic reflector is a mirror, although polished metal surfaces and other suitable materials may also be used. When used with embodiments of the present invention, reflectors may be free-standing and placed in proximity to the apparatus  10 ,  100 ,  200 ,  300 , or one or more reflectors may be directly attached to the apparatus  10 ,  100 ,  200 ,  300 . Preferably, the reflectors are mounted on hinged or jointed mounts that allow the angles of the reflectors to be adjusted. In some cases, both a lens or lenses and reflectors may be used. 
     Although some of the description above may focus on areas of the world where clean water is not readily available, apparatus  10 ,  100 ,  200 ,  300  according to embodiments of the invention may find a wide variety of applications. For example, an apparatus  10 ,  100 ,  200 ,  300  may be connected to a household gutter system and used to purify collected rain water. 
     Additionally, water produced by an apparatus  10 ,  100 ,  200 ,  300  according to embodiments of the invention may be used to produce water for irrigation and other agricultural applications. In that case, desalination may be the primary objective, and it may not be necessary to filter the stream of water from the condenser  16 ,  120 . It may also be possible to omit final filtration of the condensate if the water is to be used for clothes washing and other non-drinking applications. 
     While the invention has been described with respect to certain embodiments, the embodiments are intended to be illuminating, rather than limiting. Modifications and changes may be made within the scope of the invention, which is defined by the appended claims.