Patent Application
20080034966
Before providing a more detailed description of the properties and characteristics of the semi-permeable osmotic wall and osmotic fluid, and the interrelationships between them, it is helpful to first have a general understanding of the overall operation of the air conditioning system 100 within which they are incorporated.
The system 100 typically operates as follows: A fan 128 or other suitable airflow generator pulls humid outside air represented by the arrow 130, into an inlet duct 132 and blows it into and through the airflow compartment 116. A valve 134 within the duct 132 may direct all, some or none of that air around the dehumidifier through a bypass duct 136, depending upon dehumidification requirements. In this schematic representation, the valve 134 is controlled based upon a signal from a sensor 135 which measures the humidity of the air stream as it leaves the dehumidifier 108.
As the air passes through the compartment 116, water vapor in the air condenses into liquid form within the pores of the osmotic membrane 126. The water subsequently travels through the osmotic membrane 126 into the osmotic fluid within the compartment 118. The air, now lower in humidity, leaves the air flow compartment 116 through a duct 139 and is directed into the cooling apparatus 106, which may be of any desired type. The cooling apparatus 106 cools the air, if necessary or desired, and exhausts it via a duct 141 into the enclosed space 102 being conditioned, as depicted by the arrow 137. Part of that cooled and dehumidified air may be recirculated through the dehumidifier and cooling apparatus, such as by returning a portion of it, via a duct 138, to the inlet duct 132, to be mixed with incoming outside air 130. A valve 140, or the like, controls the amount of air recirculated, and may be responsive to any number of parameters, such as the humidity and/or temperature of either or both the outside air 130 and air within the space 102. Although not shown in
Further regarding operation of the system 100, the osmotic solution leaves the compartment 118 via a duct 142, passes through the evaporator 110, and is returned to the compartment 118 via a duct 144. As stated above, an evaporator 110 is an example of a means for regenerating the osmotic fluid, and the present invention is not limited to use with an evaporator 110. In the evaporator 110, water within the osmotic fluid evaporates into the atmosphere at a rate selected to maintain a desired water concentration within the osmotic fluid in the compartment 118. As shown in
An alternate method for maintaining the proper water concentration in the osmotic fluid is to direct the osmotic fluid from the compartment 118 into a large surface area overflow pan exposed to the outside air. Fresh osmotic fluid with an appropriate water concentration would be pumped into the compartment, as needed.
The semi-permeable osmotic wall 120 typically includes a macroporous structure 149 that provides support to the osmotic membrane 126. The macroporous structure 149 may comprise the same material as the osmotic membrane 126, a different material, or some combination thereof. The macroporous structure 149 may be disposed on one or both sides of the osmotic membrane 126, or be integral with the osmotic membrane 126. The macroporous structure 149 is typically porous, having cells, pores, or the like that permits water vapor to access the first side of the osmotic membrane 126 and/or osmotic fluid to access the second side of the osmotic membrane 126, depending on the particular macroporous structure 149 used and its position relative to the osmotic membrane 126.
The osmotic membrane 126 is a hydrophilic membrane having a thickness typically in the range of between about five (5) nanometers and one hundred (100) nanometers. The thinner the membrane 126, the greater the water flux through the membrane 126 and into the osmotic fluid, since flux across the osmotic membrane 126 is inversely proportional to the thickness of the osmotic membrane 126. Due to the large Gibbs Free Energy drive across the osmotic membrane 126 exerted by the osmotic fluid, the pore morphology of the osmotic membrane 126 allows water transfer through the osmotic membrane 126 and into the osmotic fluid compartment 118. That is because the water condensed in the pores of the osmotic membrane 126 is water in the pure liquid state, and the osmotic fluid is chosen to have a high concentration of solute. The osmotic fluid exerts a type of osmotic “pressure” on the condensed pure water. The magnitude of the osmotic pressure is described by the Van't Hoff equation, while the osmotic pressure gradient is in direct proportion to this magnitude and indirectly proportional to the thickness of the osmotic membrane 126.
The preferred pore size for the osmotic membrane 126 will depend upon the nature of the solute used in the osmotic fluid. As mentioned above, the pores should not be so large that the hydrated solute molecules can pass through them or enter and block the pores. Pore diameters (i.e., separation distances between membrane internal surfaces) on the order of about ten to twenty Angstroms (10-20 Å) would be acceptable for use with most osmotic fluids. If a salt solution is used as the osmotic fluid, pore diameters of between about five to ten Angstrom (5-10 Å) are preferred. The water flux across the osmotic membrane 126 is a function of the membrane's permeability and the water concentration difference across the osmotic membrane 126. Flux equals the product of permeability, cross sectional area, and concentration difference across the membrane 126. The permeability is inversely proportional to the membrane 126 thickness.
The osmotic membrane 126 of the present invention is preferably made from synthetic materials, including, but not limited to synthetic polymers. Cellulose acetate and polyamide are examples of acceptable synthetic polymers.
The dehumidifier 108, and more specifically the osmotic wall 120, the airflow compartment 116, and the osmotic fluid compartment 118, can assume a variety of different configurations. As shown schematically in
An osmotic fluid having solute molecules that: 1) do not permeate the osmotic membrane 126 in a dehydrated and/or a hydrated state; 2) have high solubility in water; and 3) do not degrade the osmotic membrane 126, is an example of an acceptable osmotic fluid. Osmotic fluids may be either ionic solutions or nonionic solutions. Nonaqueous solutions may also be used. Examples of osmotic fluids are lithium and magnesium salt solutions and phosphate salt solutions, although other salts may be used. Examples of two non-aqueous osmotic fluid solutions are glycerol and ethylene glycol.
In the foregoing description, methods are described for maintaining a high water concentration gradient across the osmotic membrane 126 that involve either evaporating excess water or to adding fresh osmotic fluid to the osmotic fluid compartment 118. In a further embodiment the osmotic fluid intentionally includes solute in excess of the saturation limit. Thus, initially, solute crystals will be present in the osmotic fluid. As water passes through the osmotic membrane 126 and into the osmotic fluid, more solute will dissolve, due to the presence of the additional water; and thus the concentration of solute in solution will remain at the highest level, i.e., saturation. Eventually, when all the undissolved solute crystals dissolve and even more water enters the osmotic fluid, the concentration of solute will gradually decrease and the osmotic driving force will decrease, thereby reducing the amount of water transported through the osmotic membrane 126. At this point, the water must be removed and the osmotic solution reconcentrated.
Examples of commercially available materials that can be used to form the osmotic membrane 126 include “Polyamide RO AK Membrane” and “Thin Film NF HL Membrane” both of which are manufactured by GE Osmonics, and marketed by Sterlitech Corporation of Kent, Wash., U.S.A. for purification of brackish water by reverse osmosis. Other commercially available materials that can be used to form the osmotic membrane 126 include “X-Pack” and “Expedition” marketed by Hydration Technologies, Inc. of Albany, Oreg., U.S.A. for purification of water by forward osmosis.
Now referring to
From these experiments it was apparent that capillary condensation occurs in regions of small pore size in each semi-permeable osmotic wall 120, and that water traveled across the thickness of the osmotic wall 120 likely via water bridges formed across the thickness of the wall 120. Each osmotic wall 120 maintained a high water concentration gradient by not allowing significant permeation of the osmotic fluid in the direction of the humid air, thereby resulting in water being driven through the osmotic wall 120 and into the osmotic fluid. It is believed that the various osmotic walls 120 exhibited different water fluxes because of their different thicknesses and permeabilities. These characteristics would influence the water bridges linking the liquid traveling across the walls 120 and into the osmotic fluid, and therefore the water flux rate.
Although the invention has been described and illustrated with respect to the exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made without departing from the spirit and scope of the invention.
