WO2013168129A1

WO2013168129A1 - A passive radiative condenser

Info

Publication number: WO2013168129A1
Application number: PCT/IB2013/053790
Authority: WO
Inventors: Craig Michael SHERIDAN
Original assignee: University Of The Witwatersrand
Priority date: 2012-05-11
Filing date: 2013-05-10
Publication date: 2013-11-14

Abstract

A passive radiative condenser device (10) that is suitable for collecting water from atmospheric water vapour. The device (10) has a water collection arrangement (20) with a substantially convex surface (22) for collecting water from atmospheric water vapour through passive radiative cooling, and a water storage container (66) in fluid communication with the water collection arrangement. Passive radiative cooling of the water collection arrangement lowers the surface temperature thereof so that atmospheric water vapour condenses and accumulates on the water collection arrangement (20) and flows under gravity to the water storage container (66).

Description

A PASSIVE RADIATIVE CONDENSER

FIELD OF THE INVENTION

THIS invention relates to a passive radiative condenser device, in particular to a radiative condenser device suitable for use in collecting water from atmospheric moisture, in particular from atmospheric water vapour.

BACKGROUND TO THE INVENTION

Water availability and water security is an ever increasing concern in a world with a rapidly growing population. There are several known examples of methods or devices that have been developed that have as its aim the harvesting of atmospheric moisture for use as potable water. As early as 1865 reference was made to "dew ponds" in the Journal of the Royal Agricultural Society. Dew ponds were first constructed upon the Downs in South East England many centuries ago. In 1900, the Lancet (The Dew- Ponds on the Downs, 1900) published an item describing the formation of Dew Ponds on the Downs and their ability to capture water, especially in times of water stress when shallow pans in the valleys were dry. There has been some research on dew ponds and the general consensus is that they are recharged through a combination of rain, groundwater recharge, dew and fog, with the precise recharge source and quantity varying by location.

An air well or aerial well is another example of a structure or a device that collects water, arguably by promoting the condensation of moisture from air. Designs for air wells are many and varied, but the simplest designs are completely passive, require no external energy source and have few, if any, moving parts. At the beginning of the 20^th century F.I. Zibold constructed an air well that consisted of a heap of stones, placed to form a truncated cone with a bowl shaped depression at the top. It was claimed that up to 360 litres of water per day could be produced. However, subsequent attempts using the same approach did not produce any substantial quantities of water except on rare occasions, which are believed to have been related to fog interception, as opposed to atmospheric moisture collection.

Beysens et al. (D. Beysens et al. Energy 31 (2006) 2303 - 2315) describes, and reports on the effectiveness, of passive foil-based radiative dew condensers which have been established in regions where dew is frequent and where the potential for moisture collection is high. The passive condensers described in Beysens are large flat surfaces, usually positioned at an angle relative to the ground.

Another method of producing potable water is through the use of fog harvesting nets. The fog harvesting net is usually a simple, flat, rectangular net of nylon supported by a post at both ends, and arranged perpendicular to the direction of the prevailing wind. The surface of fog collectors is usually made of fine-mesh nylon or polypropylene netting. As water collects on the net, the droplets join to form larger drops that fall under the influence of gravity into a trough or gutter at the bottom of the panel, from which it is conveyed to a storage tank. The collector itself is completely passive, and the water is conveyed to the storage system by gravity. Fog harvesting nets are relatively maintenance intensive, and are therefore not considered ideal. Also, it collects atmospheric water from only one source, i.e. fog.

Another method of producing potable water is through the process of desalination. Desalination is a rainfall independent method of producing potable water through the removal of salts and minerals from saline water. Desalination processes typically uses large amounts of energy and specialized technologies, making it more expensive than potable water from other sources.

According to a recently published work, in an unrelated technical field, by Vogela and Muller-Doblies (Vogela S., Muller-Doblies U., 2011 , Desert geophytes under dew and fog: The "curly-whirlies" of Namaqualand (South Africa). Flora. 206: 3-31) the geophytes of the Namaqualand, South Africa, have been observed to have an increased capacity to extract dew and fog from the atmosphere. These plants have evolved leaves or appendices which seem to increase the ability of the plant to extract moisture from the atmosphere.

Based on the above considerations, there remains a need for an effective, relatively low cost, light-weight device that is portable and that can harvest and collect atmospheric moisture including atmospheric water vapour, as a source of potable water.

It is therefore an object of the present invention to at least partly address some of the shortcomings experienced with the prior art. SUMMARY OF THE INVENTION

According to the present invention there is provided a passive radiative condenser device, suitable for collecting water from atmospheric water vapour, the device comprising a water collection arrangement having a substantially convex surface for collecting water from atmospheric water vapour through passive radiative cooling, and a water storage container in fluid communication with the water collection arrangement, wherein in use, passive radiative cooling of the water collection arrangement lowers the surface temperature thereof to below the dew point temperature, such that atmospheric water vapour condenses and accumulates on the water collection arrangement and flows under gravity to the water storage container.

In a preferred embodiment, the water collection arrangement further includes a plurality of biomimetic appendages extending from the substantially convex surface thereof.

The shapes of the biomimetic appendages may be selected from the group consisting of serpentine, helical, circinate, tortuous, ciliate, undulate, and/or combinations thereof.

Preferably, the biomimetic appendages are serpentine or helical in shape.

In a preferred embodiment, the device further comprises a shield member adjacent the water collection arrangement.

Preferably, the shield member comprises a convective shield portion and a radiative shield portion.

The convective shield portion may be formed from fiberglass or polystyrene, and the radiative shield portion may be formed from aluminium foil. In one embodiment, at least a portion of the periphery of the substantially convex surface of the water collection arrangement terminates in a gutter member.

The substantially convex surface of the water collection arrangement may have a surface area of equal to or greater than 0.8 m².

Preferably, the substantially convex surface of the water collection arrangement has a surface area of greater than 1.5 m².

More preferably, the substantially convex surface of the water collection arrangement has a surface area of greater than 10.0 m².

Preferably, the water collection arrangement is formed from a material selected from the group consisting of a metal, a metal alloy, a composite material, a plastic, or any combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Without thereby limiting the scope, the invention will now be described in more detail with reference to the following Figures in which:

Figure 1 shows a perspective view of one embodiment of the passive radiative condenser according to the present invention;

Figure 2 shows a perspective view of a second embodiment of the water collection arrangement of a passive radiative condenser according to the present invention; and

Figure 3 shows a front view of possible shapes of the biomimetic appendages of the water collection arrangement of the passive radiative condenser according to the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 shows one embodiment of a passive radiative condenser device 10 according to the present invention. The condenser device 10 has a water collection arrangement 20, a radiation shield member 40, and a gutter member 60 for channelling the water that flows under gravity from the surface of the water collection arrangement 20 via an opening 62 in the gutter member 60, through a conduit 64 into a water storage container 66.

In the embodiment illustrated in Figure 1 , the condenser device 10 is mounted on a stand 80 which is collapsible.

As is shown in Figures 1 and 2, the water collection arrangement 20 of the condenser device 10 has a substantially convex surface 22. In the embodiment shown in Figure 1 , the substantially convex surface 22 is formed by a combination of a number of polygonal, substantially planar members 26.

In an alternative embodiment shown in Figure 2, the water collection arrangement 20 has a substantially convex surface 22 that is not modular, but that is instead formed from a single element. In the embodiment shown in Figure 2 the outer surface of the water collection arrangement 20 is predominantly smooth.

As used in this specification, the term "substantially convex" should be understood to mean a surface that is essentially, or for the most part, of a convex nature. That is to say, a surface that is essentially, or for the most part, in the shape of a truncated sphere, or a shape that is curved like the exterior of a circle or sphere, or a surface that is shaped such that all interior angles of the surface measure less than 180 degrees.

In the embodiment shown in Figure 2 the water collection arrangement 20 has a number of randomly distributed biomimetic appendages 24 extending radially from the substantially convex surface 22. The biomimetic appendages 24 shown in Figure 2 are all helical in shape. As used in this specification, the term "biomimetic" should be understood to mean a device or an apparatus that has been based on an examination of nature, its models, systems, processes, and elements. It is a device or an apparatus that is based on inspiration from natural systems in order to solve human problems. It may also be described as innovation created from biologically inspired engineering. A number of significant and highly innovative inventions have arisen as a result of biomimicry including creating highly efficient wind turbines by using the shape of whale fins, designing low noise fans by mimicking the shape of aquatic mollusc shells, designing bacteria- resistant surfaces by mimicking cells on the basking shark's skin (amongst others).

In alternative embodiments, the biomimetic appendages 24 attached to or extending from the substantially convex surface 22 may also be any combination of the shapes shown in Figure 3 (a) to (f).

Figure 3 (a, b) shows serpentine shaped biomimetic appendages, (c, d) shows helically shaped biomimetic appendages, (e, f) shows circinate shaped biomimetic appendages, (g, h) shows ciliate shaped biomimetic appendages, (i) shows a tortuous shaped biomimetic appendage, and (j) shows a undulate shaped biomimetic appendage.

As used in this specification, the term "serpentine" (Figure 3 (a) and (b)) should be understood to mean a shape resembling a repeating pattern of "s"- shapes in a plane, i.e. a sinus wave, or alternatively a shape having repeating left and right-hand windings in a plane, i.e. snake-like.

As used in this specification, the term "helical", "helix", or "helically-shaped" (Figure 3 (c) and (d)) should be understood to mean a type of smooth space curve, i.e. a curve in three-dimensional space. It has the property that the tangent line at any point makes a constant angle with a fixed line called the axis. An example of a helix is a coil spring. The shape can also be described as spirally twisted, like a corkscrew. These terms should also be understood to include a "helicoid".

As used in this specification, the term "circinate" (Figure 3 (e) and (f)) should be understood to mean spirally bent, like the shape of a watch spring, or a crosier, in a plane.

As used in this specification, the term "ciliate" (Figure 3 (g) and (h)) should be understood to mean a shape comprising a central spine having a plurality of hair-like members extending therefrom at any angle relative the central spine.

As used in this specification, the term "tortuous" (Figure 3 (i)) should be understood to mean a shape resembling an elongate body twisted about its own substantially central longitudinal axis.

As used in this specification, the term "undulate" (Figure 3 (j)) should be understood to mean a shape resembling an elongate body having a wavy surface, or a wavy edge.

The condenser device 10 includes a shield member 40 that is adjacent and operatively below the water collection arrangement 20. The radiation shield member comprises a convective shield portion 42 and a radiative shield portion 44.

The convective shield portion 42 is made from a material that resists the transfer of heat through convection. This material may be fibreglass, polystyrene foam, other thermally insulating foams, or natural insulators such as coir, wool, cotton, bamboo, or any combination of these materials. In the condenser device embodiments shown in Figures 1 and 2 the convective shield material of the shield member 40 is envisaged to be fibreglass.

The radiative shield portion 44 is made from a material that resists the transfer of heat through radiation. The material may be aluminium foil, tin, cadmium or other material with an emissivity less than 0.1. In the condenser device embodiments shown in Figures 1 and 2 the radiative shield material of the shield member 40 is envisaged to be aluminium foil.

As is shown in Figure 1 , the water collection arrangement 20 of the water condenser 10 has a periphery that terminates in a gutter member 60. The gutter member 60 includes an opening 62. A conduit 64 is connected to the opening 62 in the gutter member 60, and leads to a water storage container 66.

The water condenser device 10 shown in Figure 1 is mounted above the ground on a collapsible stand 80.

One source of water that is collected by the water collection arrangement 20 of the water condenser 10 is water from atmospheric water vapour. Without thereby wishing to be bound by any particular existing theory, it is believed that the water condenser device 10 primarily collects water in the form of dew (or frost) through the capture of atmospheric water vapour on the substantially convex surface 22 through the mechanism of passive radiative cooling.

Passive radiative cooling is said to occur when heat is lost from a surface to the surrounding atmosphere via infrared radiation, such that the temperature of the radiative surface is thereby lowered. Objects such as the water collection arrangement 20 will passively lose heat to the atmosphere by radiative transfer.

Water droplets in the form of dew or frost will condense on the surface 22 of the water collection arrangement 20 when the surface temperature is lowered to a temperature that is below the dew point temperature, or the frost point temperature, of a surrounding pocket of air.

The dew point is the temperature below which the water vapour in a volume of humid air at a given constant barometric pressure will condense into liquid water at the same rate at which it evaporates. In other words, water which has condensed onto a surface from atmospheric water vapour is referred to as dew. An example of this occurrence is the formation of dew by the release of infrared emissions from the earth's surface to the sky. Frost formation will occur when the temperature of the surface 22 is below the freezing point of water and also below the frost point.

It is believed that the substantially convex surface 22 of the water collection arrangement 20 provides an advantage over conventional, substantially planar surfaces, in terms of the processes involved with passive radiative cooling.

One advantage of the substantially convex surface 22 is that such a surface will accumulate comparatively less heat from the process of auto-radiation, when compared to conventional planar surface and will therefore have a relatively increased radiative flux. The term "auto-radiation" refers to a mechanism of heating, whereby the surface re-heats itself through radiation. Radiation beams exit a radiating surface at all angles from a certain point and, as would be appreciated by a person skilled in the art that, while certain beams may exit the surface at substantially right angles, certain other beams may exit the surface at angles substantially along the radiating surface, thereby reheating the surface itself.

The substantially convex surface 22 shown in Figures 1 and 2 also has the further advantage that it facilitates the movement of the atmospheric moisture to the gutter member 60 under gravitational force.

The water collection arrangement 20 not only collects water (in the form of dew or frost) from atmospheric water vapour (through radiative cooling processes). The water collection arrangement 20 also collects other sources of water in air including fog, rain, and snow. These sources of water are not collected on the surface of the condenser through radiative cooling processes as the moisture is already suspended in the air, and no condensation therefore takes place. Again, without thereby wishing to be bound by any particular existing theory, it is believed that the biomimetic appendages 24 will, in certain atmospheric conditions, assist the water collection arrangement 20 in trapping atmospheric moisture from suspended moisture such as fog, rain and snow, and also from atmospheric water vapour in the form of dew and frost.

The shield member 40 further assists in keep the water collection arrangement 20 of the condenser 10 cool during the process of radiative cooling by blocking the radiation energy emitted by the surface of the earth, or any other surface over which the condenser is installed, during the radiative cooling of the earth's surface (ground heat flux).

The shield member 40 is a composite shield that comprises both a convective shield portion 42 and a radiative shield portion 44. The term "convective shield" or "convective shielding", refers to the portion which insulates the device from convective heating. The term "radiative shield" or "radiative shielding", on the other hand, refers to the layer of foil or low emissivity material which prevents ground or other radiation from reaching the condenser and heating it thereby reducing its effectiveness.

It is known that condensation is influenced by a number of factors including the radiative power of a substrate, its thermal properties, as well as its heat exchange capacity. It would be understood that all of these factors are also inextricably linked to the surface area of the passive radiative condenser. Therefore, as a minimum, the substantially convex surface 22 of the water collection arrangement 20 has a surface area of 0.8m² in order to collect a sufficient amount of atmospheric water vapour through radiative cooling. In another embodiment the convex surface 22 has a surface area of greater than 1.5m², and in a preferred embodiment the surface area of the convex surface 22 is greater than 10m².

The water collection arrangement 20, appendages 24, and the gutter 60 of the preferred embodiment are made from corrosion resistant steel. In alternative embodiments, the material used for each of the water collection arrangement 20, appendages 24, and the gutter 60 may be a metal, a metal alloy, a composite material, a plastic, or any combination thereof. It is also envisaged that the water collection arrangement 20, appendages 24, and the gutter 60 may be coated with a coating material that promote condensation to enhance the water collection potential of the passive radiative condenser.

Claims

1. A passive radiative condenser device, suitable for collecting water from atmospheric water vapour, the device comprising:

- a water collection arrangement having a substantially convex surface for collecting water from atmospheric water vapour through passive radiative cooling, and

- a water storage container in fluid communication with the water collection arrangement,

wherein in use, passive radiative cooling of the water collection arrangement lowers the surface temperature thereof to below the dew point temperature, such that atmospheric water vapour condenses and accumulates on the water collection arrangement and flows under gravity to the water storage container.

2. A device according to claim 1 , wherein the water collection arrangement further includes a plurality of biomimetic appendages extending from the substantially convex surface thereof.

3. A device according to claim 2, wherein the shapes of the biomimetic appendages are selected from the group consisting of serpentine, helical, circinate, tortuous, ciliate, undulate, and/or combinations thereof.

4. A device according to any one of the preceding claims, further comprising a shield member adjacent the water collection arrangement.

5. A device according to claim 4, wherein the shield member comprises a convective shield portion and a radiative shield portion.

6. A device according to claim 5, wherein the convective shield portion is formed from fiberglass or polystyrene, and the radiative shield portion is formed from aluminium foil.

7. A device according to any one of the preceding claims, wherein at least a portion of the periphery of the substantially convex surface of the water collection arrangement terminates in a gutter member.

8. A device according to any one of the preceding claims, wherein the substantially convex surface of the water collection arrangement has a surface area of equal to or greater than 0.8 m².

9. A device according to any one of the preceding claims, wherein the wherein the substantially convex surface of the water collection arrangement has a surface area of greater than 1.5 m².

10. A device according to any one of the preceding claims, wherein the wherein the substantially convex surface of the water collection arrangement has a surface area of greater than 10.0 m².

11. A device according to any one of the preceding claims, wherein the water collection arrangement is formed from a material selected from the group consisting of a metal, a metal alloy, a composite material, a plastic, or any combination thereof.

12. A passive radiative condenser device according to claim 1 , substantially as herein illustrated and described.