Abstract:
Disinfecting a sample of water includes generating a current using an array of photovoltaic cells, using the current to power an array of light emitting diodes, wherein the array of light emitting diodes emits a germicidal wavelength of radiation, and exposing the sample of water to the radiation. Another method for disinfecting a sample of water includes placing the sample of water within a container, wherein the container includes an array of photovoltaic cells encircling an exterior wall of the container and an array of light emitting diodes encircling an interior wall of the container, placing the container in a location exposed to solar radiation, converting the solar radiation to a current using the array of photovoltaic cells, and powering the array of light emitting diodes using the current, wherein the array of light emitting diodes emits a germicidal wavelength of radiation sufficient to disinfect the sample of water.

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to water disinfection and relates more specifically to electricity-less water disinfection systems. 
     BACKGROUND OF THE DISCLOSURE 
     Recent studies by the World Health Organization indicate that as many as one billion people lack access to a source of improved drinking water. Consequently, more than two million people die per year of waterborne disease, and more still are afflicted with non-fatal waterborne diseases. Most of these people live in developing countries, refugee camps, or disaster relief shelters, where conventional water treatment systems may be cost-prohibitive (or the resources required to power such systems—e.g., electricity, fuel, etc.—may not be readily available). 
     Conventional approaches to electricity-less water disinfection include of ultraviolet (UV) germicidal irradiation, which typically uses a mercury vapor lamp to deliver germicidal UV radiation. Although such systems compare favorably with other water disinfection systems, they also introduce environmental hazards that other systems do not. For instance, a full-spectrum mercury vapor lamp will produce ozone at certain wavelengths. Moreover, exposure to germicidal wavelengths of UV radiation can be harmful to humans (e.g., resulting in sunburn, skin cancer, or vision impairment). 
     SUMMARY OF THE DISCLOSURE 
     Disinfecting a sample of water includes generating a current using an array of photovoltaic cells, using the current to power an array of light emitting diodes, wherein the array of light emitting diodes emits a germicidal wavelength of radiation, and exposing the sample of water to the radiation. 
     Another method for disinfecting a sample of water includes placing the sample of water within a container, wherein the container includes an array of photovoltaic cells encircling an exterior wall of the container and an array of light emitting diodes encircling an interior wall of the container, placing the container in a location exposed to solar radiation, converting the solar radiation to a current using the array of photovoltaic cells, and powering the array of light emitting diodes using the current, wherein the array, of light emitting diodes emits a germicidal wavelength of radiation sufficient to disinfect the sample of water. 
     Yet another method for disinfecting a sample of water includes converting solar radiation into an electrical current, using the electrical current to power a source of a germicidal wavelength of radiation, and exposing the sample of water to the germicidal wavelength of radiation until a desired percentage of microorganisms in the sample of water is sterilized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
         FIG. 1A  is a plan view illustrating one embodiment of a water disinfection system, according to the present invention; 
         FIG. 1B  is a cross-sectional view of the water disinfection system illustrated in  FIG. 1A , taken along line A-A′ of  FIG. 1A ; 
         FIG. 2  is a flow diagram illustrating one embodiment of a method for disinfecting water, according to the present invention; and 
         FIG. 3  is a flow diagram illustrating one embodiment of a method for manufacturing the water disinfection system illustrated in  FIGS. 1A-1B . 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the Figures. 
     DETAILED DESCRIPTION 
     In one embodiment, the present invention is a method and apparatus for electricity-less water disinfection. Within the context of the present invention, “electricity-less” is understood to refer to the absence of a conventional infrastructure for delivering electricity (e.g., a power distribution grid). However, as will become apparent, embodiments of the present invention employ mechanisms for converting renewable sources of energy into direct current electricity. In particular, embodiments of the present invention disinfect water using an array of light emitting diodes (LEDs) powered by photovoltaic cells, thereby obviating the need for a conventional source of electricity. The water is efficiently and effectively disinfected using a system that is more compact, consumes less power, and is safer environmentally than conventional disinfection systems. 
       FIG. 1A  is a plan view illustrating one embodiment of a water disinfection system  100 , according to the present invention.  FIG. 1B  is a cross-sectional view of the water disinfection system  100  illustrated in  FIG. 1A , taken along line A-A′ of  FIG. 1A . The water disinfection system  100  employs a chemical-free process that directly attacks the vital deoxyribonucleic acid (DNA) of microorganisms (e.g., bacteria, mold, yeast, viruses, protozoa, etc.) in a water sample, thereby sterilizing the microorganisms and rendering the water sample suitable for human consumption. 
     Referring simultaneously to  FIGS. 1A-1B , the system  100  generally comprises a rigid container  102 , such as a jug or a bottle. The container  102  includes a neck  112  or other opening that allows water to be poured into the container  102  and a lid or cap  114  that seals the neck  112  (and thus the container  102 ). The container  102  thus defines a volume within which a quantity of water can be contained and disinfected according to the embodiments described below. In one embodiment, the container  102  holds up to approximately five gallons of liquid, although the container  102  can be manufactured in any size. In one embodiment, the container  102  is formed from a material that is known to be environmentally and health-safe (i.e., does not cause any significant negative environmental or health-related side effects), such as a Bisphenol A (BPA)-free polymer or plastic. 
     The system  100  further comprises an array  104  of photovoltaic cells (i.e., semiconductors that convert solar radiation to direct current electricity) coupled to the exterior wall  106  of the container  102 . In one embodiment, the array  104  of photovoltaic cells encircles an entire perimeter of the exterior wall  106 . In one embodiment, the array  104  comprises a plurality of micro-photovoltaic cells (e.g., photovoltaic cells having a size between approximately ten and one hundred micron). In a further embodiment, the photovoltaic cells are spalled (i.e., thin-film), flexible photovoltaic cells. In one embodiment, one or more of the photovoltaic cells is formed from at least one of: amorphous silicon, crystalline silicon, silicon germanium (SiGe), germanium (Ge), indium gallium arsenide (InGaAs), or indium arsenide (InAs). 
     In addition, an array  108  of LEDs is coupled to the interior wall  110  of the container  102 . In one embodiment, the array  108  of LEDs encircles an entire perimeter of the interior wall  110 . The array  108  of LEDs is also connected (e.g., by a system of interconnects) to the array  104  of photovoltaic cells such that current can pass from the photovoltaic cells to the LEDs. In one embodiment, the array  108  comprises a plurality of micro-LEDs (e.g., LEDs having dimensions less than or equal to one hundred micrometers×one hundred micrometers). In a further embodiment, the LEDs are spalled, flexible micro-LEDs arranged on a substrate (e.g., a silicon substrate) and coupled via a system of interconnects. In one embodiment, the micro LEDs are formed from aluminum gallium nitride (AlGaN) and/or gallium nitride (GaN). In one embodiment, each of the LEDs has a power output of approximately one milliwatt. The system  100  has been demonstrated to be capable of sterilizing up to at least ninety-nine percent of many different types of microorganisms in water. Water that has been sterilized to this degree would generally be considered potable. 
       FIG. 2  is a flow diagram illustrating one embodiment of a method  200  for disinfecting water, according to the present invention. In particular,  FIG. 2  illustrates how water may be disinfected using the water disinfection system  100  illustrated in  FIGS. 1A-1B . As such, reference is made in the discussion of the method  200  to various items illustrated in  FIGS. 1A-1B . 
     The method begins in step  202 . In step  204 , the container  102  is filled with a quantity of water to be treated. The container  102 , including the water, is then placed in a location where it will be exposed to radiation (e.g., sunlight) in step  206 . 
     In step  208 , the array  104  of photovoltaic cells generates a current in response to the radiation. In one embodiment, the current generated by the array  104  of photovoltaic cells is in the milliwatt range. 
     In step  210 , the array  108  of LEDs is activated and emits germicidal radiation in response to the current provided by the array  104  of photovoltaic cells. In one embodiment, the germicidal radiation is UV radiation (e.g., having a wavelength in the range of approximately 265 to 280 nanometers). Prolonged exposure to this germicidal radiation results in the sterilization of microorganisms in the water that is held within the container  102 . As a result, the water is disinfected and rendered suitable for human consumption. In one embodiment, the length of time for which the water must be exposed to the germicidal radiation depends at least on the amount of water to be treated, the desired percentage and type of microorganisms to be sterilized, and the intensity of the germicidal radiation emitted by the array  108  of LEDs. For instance, in one embodiment, the water is exposed to the germicidal radiation for at least one minute; in further embodiments, the water is exposed to the germicidal radiation for up to an hour. Disinfection of the water is thus a product of the intensity of the germicidal radiation emitted by the array  108  of LEDs over the time of exposure and within the given area (i.e., the volume of the container  102 ). This exposure may be expressed in microwatt seconds per square centimeter. 
     The method  200  ends in step  212 . 
     The method  200  thus employs a physical, chemical-free process that effectively and efficiently disinfects water without consuming electricity or causing any significant environmental side effects. Because the system  100  is compact and does not require electricity or fuel other than sunlight, it can be used in substantially any environment. 
     Moreover, the system  100  is cost effective to manufacture and to use. In particular, certain techniques, such as spalling, may be used to manufacture the system  100  in a manner that minimizes waste of materials or energy. 
       FIG. 3  is a flow diagram illustrating one embodiment of a method  300  for manufacturing the water disinfection system  100  illustrated in  FIGS. 1A-1B . In particular, the method  300  is one embodiment of a method for producing the array  108  of LEDs on the interior wall  110  of the container  102 . The particular method  300  illustrated in  FIG. 3  relies on a spalling technique to produce the LED array  108 . 
     The method  300  begins in step  302 . In step  304 , an array of LED structures is produced on a wafer (e.g., a silicon substrate). The array of LED structures may be produced using any one or more known manufacturing techniques. For instance, a stack of layers comprising a silicon substrate, an aluminum nitride layer formed on the silicon substrate, and a gallium nitride layer formed on the aluminum nitride layer can be fabricated. The stack may additionally comprise a plurality of contacts (e.g., p- and n-type contacts). Dry etching of the aluminum nitride and gallium nitride layers can expose the silicon substrate, which may then be anisotropically etched using potassium hydroxide (KOH), leaving an array of anchored gallium nitride/aluminum nitride structures. 
     In step  306 , the array of LED structures is transferred from the wafer to a stamp. For instance, a patterned polydimethylsiloxane (PDMS) stamp may be brought into contact with the wafer and then quickly removed, causing chips of gallium nitride/aluminum nitride to be released from the wafer and adhered to the stamp as a plurality of discrete thin film devices. This technique may also be referred to as “spalling.” 
     In step  308 , the array of LED structures is transferred from the stamp to a substrate. For instance, the stamp may be brought into contact with the substrate and then slowly removed, causing the array of LED structures to adhere to the substrate as a plurality of discrete thin film devices (i.e., the array of LEDs). This may be accomplished using a transfer printing technique. In one embodiment, the substrate already includes a layer of interconnects (and adhesive) onto which the thin film devices are deposited. An additional layer of interconnects may then be deposited on the thin film devices (e.g., after planarization of the thin film devices). As a result, a printed array of micro LEDs is fabricated upon the substrate. In one embodiment, the substrate is or will become the inner surface  110  of the container  102 . Thus, in one embodiment, the substrate is a BPA-free polymer. 
     The method  300  ends in step  310 . 
     The method  300  thus results in the application of an array  108  of thin-film LEDs to the inner surface  110  of the container  102 . As discussed above, spalling can also be used to apply the array  104  of photovoltaic cells to the outer surface  106  of the container  102 . This technique allows a dense array to be distributed on a sparse array, thereby making economical use of materials by reducing the cost and area of material used. 
     Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.