Abstract
An apparatus and method to harvest moisture from the atmosphere with adequate moisture content without using external energy is disclosed. The apparatus comprises a plate having a non-horizontal surface consisting of majority hydrophilic areas to promote water droplet condensation and minority water hydrophobic water repelling areas to increase water droplet mobility. The non-horizontal surface of the moisture harvesting apparatus may comprise ridges and valleys formed in the approximate direction from about the high end to about the low end of the non-horizontal surface to further increase water droplet mobility.
Claims
1. An apparatus for harvesting moisture in the air with adequate moisture content, comprising: a plate having a non-horizontal surface consisting of majority water condensing areas and minority water repelling areas that are surrounded by the majority water condensing areas.
2. The apparatus of claim 1, wherein: the water condensing areas and water repelling areas of the non-horizontal surface are arranged in alternating patterns, and the water repelling areas form columns following the approximate direction from about the high end to about the low end of the non-horizontal surface.
3. The apparatus of claim 2, wherein: the column width of each water repelling area is about 1 μm to 1 mm, and the spacing between adjacent columns is about 10 μm to 100 mm.
4. The apparatus of claim 1, wherein: the non-horizontal surface has ridges and valleys between adjacent ridges; wherein the ridges and the valleys are formed in the approximate direction from about the high end to about the low end of the non-horizontal surface; and wherein a column of water repelling area is formed in each valley following the valley centerline and is surrounded by water condensing areas.
5. The apparatus of claim 4, wherein: a column of water repelling area is formed on each ridge following the ridge centerline and is surrounded by water condensing areas.
6. The apparatus of claim 4, wherein: each column of water repelling area consists of a plurality of small islands forming a pattern.
7. The apparatus of claim 4, wherein: the column width of each water repelling area is about 1 μm to 1 mm.
8. The apparatus of claim 4, wherein: the distance between adjacent ridges is about 10 μm to 100 mm.
9. The apparatus of claim 1, wherein: the plate includes a rigid substrate material selected from a group consisting essentially of plastic, metal, concrete, cement, wood, and bamboo.
10. The apparatus of claim 1, wherein: the water condensing areas have a water contact angle of less than 20 degrees and the water repelling areas have a contact angle greater than 40 degrees.
11. The apparatus of claim 1, wherein: the non-horizontal surface forms an angle with the horizontal plane from greater than 5° to 90°.
12. A method for harvesting water in the air with adequate moisture content, comprising the steps of: establishing a plate with a non-horizontal surface; forming water condensing areas to cover the majority of the non-horizontal surface; and forming water repelling areas to cover the areas that are not covered by the water condensing areas.
13. The method of claim 12, wherein: the water condensing areas and water repelling areas of foe non-horizontal surface are arranged in alternating patterns, and the water repelling areas form columns following the approximate direction from about the high end to about the low end of the non-horizontal surface.
14. The method of claim 13, wherein: the column width of each water repelling area is about 1 μm to 1 mm, and the spacing between adjacent columns is about 10 μm to 100 mm.
15. The method of claim 12, wherein: the non-horizontal surface has ridges and valleys between adjacent ridges; wherein the ridges and valleys are formed in the approximate direction from about the high end to about foe low end of the non-horizontal surface; and wherein a column of water repelling area is formed in each valley following foe valley centerline and is surrounded by water condensing areas.
16. The method of claim 15, wherein: a column of water repelling area is formed on each ridge following the ridge centerline and is surrounded by water condensing areas.
17. The method of claim 15, wherein: each column of water repelling area consists of a plurality of small islands forming a pattern.
18. The method of claim 15, wherein: the column width of each water repelling area is about 1 μm to 1 mm.
19. The method of claim 15, wherein: the distance between adjacent ridges is about 10 μm to 100 mm.
20. The method of claim 12, wherein: the plate includes a rigid substrate material selected from a group consisting essentially of plastic, metal, concrete, cement, wood, and bamboo.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other objects and features of the invention will become more fully apparent from the following description and appended claims taken in conjunction with the following drawings, where like reference numbers and alphanumeric names indicate identical or functionally similar elements.
[0014] FIG. 1 shows a water droplet on a horizontal surface on the top side of a plate, wherein a contact angle α is formed between the water surface and the plate surface where they meet;
[0015] FIG. 2 depicts a top view of multiple water droplets of different sizes covering a harvesting surface;
[0016] FIG. 3 shows a binary harvesting surface made of majority hydrophilic water condensing areas divided by minority hydrophobic repelling areas, wherein water droplets cover the hydrophilic areas and are repelled by hydrophobic areas;
[0017] FIG. 4 is an inclined plate with the harvesting top surface having contoured ridges and valleys running straight down from the top of the plate surface to the bottom of the plate surface wherein the plate and the horizontal plane thrills an angle β; and
[0018] FIG. 5 is an inclined plate with the harvesting top surface having multifaceted ridges and valleys running straight down from the top of the plate surface to the bottom of the plate surface wherein the plate and the horizontal plane forms an angle β.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The present description will be directed in particular to elements forming part of or cooperating more directly with, apparatus and methods in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
[0020] FIG. 1 shows a water droplet 1 resting on a horizontal surface 2 of a plate 3. Surface tension causes water droplet 1 to bead up from surface 2. At the edge of water droplet, a contact angle α is formed between horizontal surface 2 and the sloped and curved surface of water droplet 1. The magnitude of contact angle a depends or the wettability property of surface 2. A smaller contact angle a means that the water drop has a higher tendency to cling on the surface and spreads to cover a wider area, forming a lower drop height. When a surface has a high tendency to cause a water droplet to spread thin or when the contact angle small, it is considered to be hydrophilic. On the other hand, when a surface has a high tendency to cause a water droplet to bead up or the contact angle is large, it is considered to be hydrophobic.
[0021] Due to its affinity to water, a hydrophilic surface has the tendency to attract moisture in the air to condense on it. However, using a homogeneous hydrophilic surface to condense moisture may not be as effective for harvesting moisture as one might expect. As illustrated in FIG. 2, many water droplets 5 are formed on a horizontal hydrophilic surface 6. Because of the small contact angle due to the hydrophilic surface property, each droplet 5 is clinging to the hydrophilic surface 6, expanding in all directions and farming a thinly covered area. The large contact area that droplets 5 cover make removal of water droplets 5 difficult. Therefore, in the beginning, surface 6 is effective for condensing moisture, But due to the removal difficulty of water droplets 5, soon surface 6 is covered up and condensation is slowed down.
[0022] A solution to the problem is to form hydrophobic areas on the harvesting surface to limit the spreading of the water droplets, thus forming a binary surface composed of hydrophilic areas for water droplet condensation and hydrophobic areas to limit the spread of the droplets formed, Such a binary surface solution is exemplified in FIG. 3, where straight columns of hydrophobic repelling areas 12. are formed on the plate 10, each between larger adjacent hydrophilic condensing surfaces 11. On a two dimensional homogeneous surface, adding periodic hydrophobic repelling areas may serve the purpose of decreasing water droplet size to increase water mobility. Hydrophilic and hydrophobic are relative in terms of contact angle of a water droplet on a specific surface. A surface is considered hydrophobic compared to another surface if its water drop contact angle is 10° or even 20° larger. For example, if surface A has contact angle α<20°, and surface B has α>40°, one may consider that surface A is hydrophilic and surface B is hydrophobic.
[0023] In FIG. 3, when moisture in the air is attracted and condenses on a hydrophilic surface 11 to form droplets 13, these droplets are repelled by the adjacent hydrophobic areas 12, thus limiting the spread of the droplets. Hence, droplets 13 are limited in the horizontal direction and may extend more in the vertical orientation, which helps the droplets overcome the clinging force and drip down more easily if surface 11 is tilted. Given the same water droplet volume, the limited contact area due to the repelling force from repelling areas 12 increases the water droplet height, and die deformed water droplet shapes as depicted in FIG. 3 may expedite water droplet removal from the surface.
[0024] Repelling areas 12 in FIG. 3 may take different shapes. For example, each column of hydrophobic area 12 may be composed of many small islands forming a pattern, such as a straight line of identical shaped and sized small islands or a matrix of identical small islands. Also, instead of periodically spaced, repelling areas 12 may be scattered on the harvesting surface as long as they effectively separate condensing areas 11. In any case, the binary surface of hydrophilic and hydrophobic areas should be mostly composed of hydrophilic condensing areas which are beneficial to water droplet condensation. Then the smaller hydrophobic repelling areas help limit the spreading of the water droplets and enhance mobility. Preferably, hydrophilic condensing areas cover more than 80% of the entire moisture harvesting surface, with hydrophobic repelling areas covering the rest.
[0025] When periodically spaced, the width of each hydrophobic column may be 1 μm to 1 mm, and the distance between adjacent hydrophobic columns may be 10 μm to 100 mm. And the hydrophobic columns may take different shapes, e.g., straight, curved, and etc.
[0026] Preferably, the harvesting surface is set up inclined instead of horizontally. In this way, gravity helps water droplets flow down the slope for collection. For a passive moisture harvesting apparatus, the most efficient way is to use gravity to remove water from the surface. Thus, no external energy is used for the benefit of environmental protection.
[0027] A preferred embodiment is shown in FIG. 4, where an inclined plate 15 forms an angle β with the horizontal plane. The top harvesting surface 16 of the plate 15 has straight ridges 17 and valleys 18 that are contour shaped. Ridges 17 and valleys 18 are aligned in the direction from the top end of the plate 15 to the bottom end of the plate 15. But they can take other arrangements as well as suitable for specific application. As with the previous discussions, the majority of top surface 16 is covered by hydrophilic condensing areas to attract moisture to condensate. The water droplets formed run down from ridges 17 to adjacent valleys 18 by gravity, and then run down the slope of surface 16 in valleys 18 to be collected. Small hydrophobic repelling areas may cover partial areas of valleys 18, preferably along the centerlines of valleys 18, to separate water droplets in order to increase mobility. Optionally, small hydrophobic repelling areas may cover partial areas of ridges 17 in order to cause water droplets to move to valleys 18. Each column of hydrophobic area may be composed of many small islands forming a pattern, such as a straight line of identical shaped and sized small islands or a matrix of identical small islands. Also, instead of periodically spaced, repelling areas may be scattered in the valleys and on ridges as long as they effectively separate condensing areas. The cross-sectional profile of ridge 17 and valley 18 may be sinusoidal, i.e., the width of the ridge is equal to that of the valley. In another embodiment, the width of the ridge is formed smaller than that of the valley. Preferably, each hydrophobic column is about 1 μm to 1 mm wide, and the distance between the adjacent ridges is about 10 μm to 100 mm.
[0028] FIG. 5 shows an alternative embodiment of FIG. 4, where inclined plate 20 forms an angle β with the horizontal plane and the top harvesting surface 21 of plate 20 has straight ridges 22 and valleys 23 running from the top end of plate 20 to the bottom end of plate 20. However, instead of being contoured, top surface 21 is made of multi-faceted straight planes. The same as in FIG. 4, the majority of top surface 20 is covered by hydrophilic areas to attract moisture to condense. The water droplets formed run down from ridges 22 to adjacent valleys 23 by gravity, and then run down the slope of surface 21 in valleys 23 to be collected. Small hydrophobic areas may cover partial areas of valleys 23, preferably along the centerlines of valleys 23, to separate water droplets in order to increase mobility. Optionally, small hydrophobic areas may cover the partial areas of ridges 22 in order to cause water droplets to move to valleys 23.
[0029] In FIGS. 4 and 5, the ridges and valleys may take different orientation arrangements. For example, they may follow curved paths, instead of straight lines from top to bottom of the harvesting surface depending on specific application. And the ridges and valleys may lake a direction that forms an angle with the line of gravity.
[0030] The inclination angle β of plate 15 of FIG. 4 or plate 20 of FIG. 5 may range from near horizontal (>0°) to vertical (90°). Also, plate 15 or plate 20 may be constructed to have both front surface and back surface as moisture harvesting surfaces, to maximize moisture harvesting rate. This type of configuration is especially effective when plate 15 or plate 20 is erected at vertical or near vertical position.
[0031] The binary hydrophilic and hydrophobic surface structure in FIG. 4 or 5 is preferably fabricated on a rigid substrate material that is robust against weather conditioning, including humidity, UV light, and temperature. The rigid substrate material is selected from a group consisting essentially of plastic, metal, concrete, cement, wood, bamboo, and etc. Preferably, the substrate material has high thermal conductivity. When supported by a structure with high thermal conductivity as well, heat conduction ensures that the temperature of the substrate material stays close to that of the supporting structure. When warmer air hits the cooler harvesting surface, moisture condensation occurs more easily. The hydrophilic and hydrophobic areas may be laminated or coated on the substrate material. One method is to coat a layer of hydrophilic layer covering the entire surface and then to coat small areas of a hydrophobic layer on top of the hydrophilic layer. Another method is to coat both hydrophilic and hydrophobic areas in one layer, for example, by digital printing.
[0032] Plate 15 of FIG. 4 or plate 20 of FIG. 5 may be made like a roof tile that can be installed to connect with one another to cover a large area, e.g., to cover part of or the entire roof of a house.
[0033] Clearly, other embodiments and modifications of this invention may occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawing.
