US20060277933A1

US20060277933A1 - Sorption cooling systems, their use in personal cooling applications and methods relating to the same

Info

Publication number: US20060277933A1
Application number: US11/422,836
Authority: US
Inventors: Douglas Smith; Kevin Roderick; Peter Campbell
Original assignee: NanoPore Inc
Current assignee: NanoPore Inc
Priority date: 2005-06-08
Filing date: 2006-06-07
Publication date: 2006-12-14

Abstract

A personal cooling device using a sorption cooling system to provide cooled fluid to a garment and methods relating to the same are provided. The sorption cooling system generally includes a lightweight, compact evaporator and adsorbent bed for providing the cooled fluid to the garment.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 60/688,599, filed on Jun. 8, 2005, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to sorption cooling systems incorporating a compact, lightweight evaporator and adsorbent bed structure and the use of such sorption cooling systems in conjunction with a personal cooling device.
2. Description of Related Art
Garments designed to cool users while they are wearing the garment are known in the art. Typically, cooling devices and/or materials are incorporated into a portion of the garment to achieve such cooling. For example, U.S. Pat. No. 6,298,907 by Colvin et al. discloses a vest incorporating micropouches that contain a phase-change material (“PCM”) to cool a person wearing the vest. The micropouches are in thermal communication with the person wearing the vest and the PCM is selected such that as the body temperature of the person increases, the phase-change materials will change from a solid to a liquid phase at a selected temperature, thereby extracting heat from the individual at the selected phase-change temperature. Colvin et al. also discloses the use of air convection to cool the micropouches and further increase the efficiency of the vest.
However, phase-change materials have a relatively low cooling density for direct cooling of a person and typically have a phase change temperature significantly below body temperature (37° C.). Moreover, PCMs are not effective where there is a possibility of vapor build up within the garment. For example, firemen, military and other public service personnel frequently use hazardous materials (Hazmat) suits when fighting fires and/or when undergoing biological and chemical warfare or corresponding training. Similar suits are also often used in an uncomfortably hot atmosphere, such as steel mills and deep mines and, as a result, the individual will often perspire significantly. The resulting perspiration can cause a rapid buildup of water vapor within the suit, which lowers the user's rate of heat rejection. In the case of a sealed suit, the build-up of water vapor can cause visibility problems from condensation of the vapor on the user's visor. Physical and physiological discomfort may also result from the increased temperature of the suit and the build-up of perspiration on the user's skin and clothing.
U.S. Pat. No. 6,125,645 by Horn discloses the use of ice as a phase-change material within a garment to cool the individual. Aside from the impracticality of maintaining temperatures suitable to maintain the ice within the garment, ice has a low cooling density and has logistical disadvantages in that the ice must be available to the user at or near the hostile environment. Ice also cannot remove water vapor from the garment. In fact, as the ice melts, it will likely contribute to the humidification of the garment. The use of ice also suffers in that the user cannot control the rate of cooling within the garment.
In contrast to ice and phase-change materials, adsorption cooling can overcome many of these problems by providing lighter or longer lasting cooling, simplified logistics, sustained visibility and controllability. The use of an adsorption cooler to cool air within a garment is disclosed in commonly owned U.S. Pat. No. 6,601,404 by Roderick. Cooling loads from about 50 W to about 500 W for garment cooling applications and cooling temperatures of from about 10° C. to about 25° C. are disclosed.
U.S. Pat. No. 5,289,695 by Parrish et al. discloses a garment incorporating a desiccant. The desiccant adsorbs water generated during the wearing of the garment (e.g., perspiration) and is located adjacent to the outer surface of the garment or in a separate case. An open-cell thermal insulating layer is located opposite the exterior of the garment and adjacent to the desiccant to prevent heat from dissipating back toward the skin of the person wearing the garment. A phase-change material can be located between the users skin and the adsorbent bed to enable thermal control within the garment by controlling the heat transfer to the adsorbent vest.
Commonly owned U.S. Pat. No. 6,858,068 discloses a personal cooling device employing an adsorber to dehumidify air provided to the wearer. A desiccant material is included within the adsorber to adsorb water vapor and a phase-change material is located in thermal communication with the desiccant to increase the loading capacity of the desiccant and maintain a cool gas stream by extracting the heat of adsorption from the desiccant.
There remains a need for a lightweight sorption adsorption device for providing a cooled fluid to an individual wearing a garment and/or controlling the micro-climate in an enclosure. It would be advantageous if the device could provide a high degree of water evaporation and corresponding adsorption, thereby providing a high rate of cooling.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention to provide a lightweight, compact closed-loop sorption cooling device capable of providing a cooled liquid to a garment.
It is also an object of the present invention to provide a lightweight, compact adsorbent bed capable of enabling the sorption cooling device of achieving the desired cooling rates.
It is also an object of the present invention to provide a lightweight, compact multiple-chamber evaporator capable of cooling a liquid flowed therethrough so that the cooled liquid can be provided to a garment.
These and other objects of the present invention are achieved using the sorption cooling device and sorption cooling method of the present invention.
In one embodiment of the present invention, a method for cooling a person using a personal cooling system is provided. The method generally includes the steps of flowing dry air from an adsorber to an evaporator, such as at a flow rate of between about 0.33 liters per second and about 0.83 liters per second, evaporating a first portion of liquid water contained in the evaporator into the dry air, thereby creating wet air and cooling a second portion of the liquid water, and flowing the cooled second portion of the liquid water through a garment worn by the person, such as at a flow rate of about 10 liters per second. The method may further include the steps of returning the wet air to the adsorber, and adsorbing water contained in the wet air to create the dry air. The method may also include the step of returning the second portion of water to the evaporator.
In another embodiment of the present invention, a personal cooling system is provided. The personal cooling system generally includes a garment, adapted to be worn by a person, and including at least one cooling channel. The personal cooling system further includes an evaporator, a circulating means for circulating refrigerant through the cooling channels of the garment and the evaporator, and an air flow means for flowing air through the evaporator. Generally, the adsorber is adapted to gaseously communicate with the evaporator, and is further adapted to adsorb vaporous refrigerant contained in an air stream. The personal cooling system may further include a refrigerant source for supplying a supplemental portion of refrigerant to the evaporator.
The evaporator generally includes a plurality of vapor permeable membrane material layers, each having a first side and a second side. The evaporator further includes airflow channels disposed proximal to each of the first membrane material sides and refrigerant flow channels disposed proximal to each of the second membrane material sides. The refrigerant flow channels generally each also include liquid water. Generally, the evaporator has a volumetric cooling density of at least about 80 W per liter and a mass cooling density of at least about 100 W per kilogram.
The membrane material layers separate the airflow channels from the refrigerant channels, but enable vaporous refrigerant to flow from a refrigerant channel to an adjacent air flow channel. The membrane material used in the evaporator is generally a porous vapor permeable material, such as a dense polyurethane or expanded PTFE material. In one embodiment of the present invention, the vapor permeable membrane material is selected from the group consisting of dense polyurethanes, expanded PTFE materials and combinations thereof. Generally, the membrane material is vapor permeable, but liquid impermeable at liquid pressures of from between about 1.5 psi to 15 psi.
The adsorber is preferably in the form of an adsorbent bed having a fluid impermeable casing that includes a refrigerant inlet, a refrigerant outlet, and a coolant inlet and coolant outlet. The adsorbent bed also includes a plurality of desiccant sheets, such as first and second desiccant sheets. The desiccant sheets generally have an adsorbent first side and are covered by a fluid impermeable barrier on a second side. The desiccant sheets each include at least one aperture, such as at least two apertures, extending through the desiccant sheets. The apertures of the desiccant sheets are a portion of one of a refrigerant flow path and a coolant flow path. In a particular embodiment, at least one of the refrigerant flow path and coolant flow path is non-linear. In one embodiment, both the refrigerant flow path and the coolant flow path are non-linear.
A refrigerant flow path for flowing a refrigerant fluid between the refrigerant inlet and refrigerant outlet extends through each adsorbent bed. The refrigerant flow path is at least partially defined by the first adsorbent sides of the desiccant sheets. Preferably, the adsorbent sides of opposing desiccant sheets face each other to help define at least a portion of the refrigerant flow path. In one embodiment, the desiccant sheets are substantially parallel to one another, and are separated by not more than 5 mm. In one embodiment, the apertures are a portion of the refrigerant flow path and the refrigerant flow path is non-linear. In one embodiment, the refrigerant flow path is in gaseous communication with an airflow channel of an evaporator.
A coolant flow path for flowing a coolant fluid between the coolant inlet and coolant outlet also extends through each adsorbent bed. The coolant flow path is fluidly isolated from the refrigerant flow path, but is adjacent to at least one of the desiccant sheets. In one embodiment, the apertures are a portion of the coolant flow path and coolant flow path is non-linear. In one embodiment, the coolant fluid is air.
In one embodiment, the adsorbent bed includes at least three desiccant sheets, each of the desiccant sheets having at least one aperture extending therethrough, where the apertures define at least a portion of one of the refrigerant flow path and the coolant flow path. Preferably, a coolant flow path is adjacent to both a first and second desiccant sheet.
In one embodiment, at least one of the desiccant sheets includes a carbon material impregnated with a metal salt. In a particular embodiment, the metal salt is selected from the group consisting of lithium chloride, calcium chloride and mixtures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a typical sorption cooling device.
FIG. 2 is a flow-diagram of one embodiment of a sorption cooling system of the present invention.
FIG. 3 is a cross-sectional, perspective view of one embodiment of an adsorbent bed of the present invention.
FIG. 4 is a cross-sectional, perspective view of one embodiment of an adsorbent bed of the present invention.
FIG. 5A is a cross-sectional, perspective view of one embodiment of an adsorbent bed of the present invention.
FIG. 5B is a top view of one embodiment of a portion of a fluid casing of an adsorbent bed of the present invention.
FIG. 5C is a top view of one embodiment of a desiccant sheet of an adsorbent bed of the present invention.
FIG. 5D is a top view of one embodiment of a spacing material of an adsorbent bed of the present invention.
FIG. 5E is a top view of one embodiment of a portion of a fluid casing of an adsorbent bed of the present invention.
FIG. 6 is a graph depicting the adsorption capacities of various materials.
FIG. 7 is a graph depicting the heat of adsorption as compared to water heat of vaporization of a high-capacity desiccant.
FIG. 8 is a graph depicting the adsorption capacity of a high-capacity desiccant as a function of temperature.
FIG. 9 is a cross-sectional perspective view of one embodiment of an adsorbent bed of the present invention.
FIG. 10A is a side view of one embodiment of an adsorbent bed of the present invention.
FIG. 10B is a side view of one embodiment of an adsorbent bed of the present invention.
FIG. 11 is a cross-sectional perspective view of one embodiment of an evaporator of the present invention.
FIG. 12 is a cross-sectional view of one embodiment of an evaporator of the present invention.
FIG. 13 is a perspective view of a garment employing a sorption cooling system of the present invention.
FIG. 14 is a perspective view of one embodiment of an adsorbent bed and an evaporator of the present invention.
FIG. 15 is a graph depicting water adsorption isotherms for CaCl₂and LiCl based desiccants.
FIG. 16 is a graph depicting cooling rate and capacity as a function of time for one embodiment of a sorption cooling device of the present invention.
FIG. 17 is a graph depicting relative humidity and temperature measurements versus time for one embodiment of a sorption cooling device of the present invention.
FIG. 18 is a graph depicting cooling rate and capacity as a function of time for one embodiment of a sorption cooling device of the present invention.
FIG. 19 is a graph depicting cooling rate and capacity as a function of time for one embodiment of a sorption cooling device of the present invention.
FIG. 20 is a graph depicting cooling rate and capacity as a function of time for one embodiment of a sorption cooling device of the present invention.
FIG. 21 is a graph depicting cooling rate and capacity as a function of time for one embodiment of a sorption cooling device of the present invention.
FIG. 22 is a graph depicting heat transfer effectiveness and pressure drop as a function of gap size for one embodiment of an adsorbent bed of the present invention.
FIG. 23 is a graph depicting relative humidity and temperature measurements versus time for one embodiment of a sorption cooling device of the present invention.
FIG. 24 is a graph depicting cooling rate and capacity as a function of time for one embodiment of a sorption cooling device of the present invention.
FIG. 25 is a perspective view of a single membrane material test cell apparatus.
FIG. 26 is a graph depicting relative humidity and temperature measurements versus time for one embodiment of a sorption cooling device of the present invention.
FIG. 27 is a graph depicting water cooling rate versus air flow rate through an evaporator according to one embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides an improved sorption cooling system that may include an improved, lightweight adsorbent bed structure capable of adsorbing a relatively high amount of refrigerant. The sorption cooling system may also include an improved lightweight, compact evaporator capable of evaporating refrigerant at rates sufficient to provide a sufficiently cooled fluid. The sorption cooling system may be used to provided the cooled fluid, such as water, to a garment to cool the wearer of the garment.
The operation of a sorption cooling system is well known in the art. Generally, a sorption cooling system 100 comprises an evaporator 102, an adsorber 106 and a refrigerant source 104, as depicted in FIG. 1. Refrigerant supplied to the evaporator 102 from the refrigerant source 104 evaporates in the evaporator 102, thereby cooling a portion of the evaporator 102. The evaporated refrigerant is then adsorbed in the adsorber 106 to remove the vaporous refrigerant from the evaporator 102, thereby increasing its cooling capacity. A vapor passageway 105 enables the evaporated fluid to flow from the evaporator 102 to the adsorber 106.
The present invention enables the use of adsorption techniques to provide cooling using a solid-phase, high-capacity desiccant capable of adsorbing a significant amount of vapor phase refrigerant. The mechanism by which the high-capacity desiccant functions can be adsorption, absorption or a combination of adsorption and absorption, and as used herein, the terms adsorb, adsorptive, adsorption and the like refer to the retention of fluid, regardless of the actual mechanism by which the fluid is retained. Likewise, the mechanism by which the adsorbent refrigerant is released from the high-capacity desiccant can be desorption, desorbtion, or a combination of desorption and desorbtion, and as used herein, the terms desorb, desorbtive, desorbtion and the like, as well as the terms regenerate, regenerable, regenerated and the like when used in reference to the desiccant material, refer to the release of fluid from the adsorbent, regardless of the actual mechanism by which the fluid is released.
Application of In Situ Regenerable and Ex-Situ Regenerable Sorption Cooling Systems
Sorption cooling systems can generally be categorized into two types: in situ regenerable and ex situ regenerable. In situ regenerable sorption cooling systems are generally capable of regenerating the adsorbed refrigerant from the adsorbent bed during operation of the sorption cooling system, and therefore generally include a means for regenerating the adsorbed refrigerant and subsequently liquefying such refrigerant. Generally these means include a heat source for heating the adsorbent bed material and a cooling source for cooling the regenerated refrigerant into a liquid. Often, the cooling source is a condenser.
Ex situ regenerable sorption cooling systems are generally not capable of regenerating the adsorbed refrigerant from the adsorbent bed during operation of the sorption cooling system. According to one aspect of the present invention, the sorption cooling system is an ex situ sorption cooling system utilized in providing a cooled fluid to a garment. A flow diagram depicting one embodiment of the present invention is depicted in FIG. 2. According to this embodiment, the sorption cooling system 200 includes an evaporator 202, a refrigerant source 204 in fluid communication with the evaporator 202, and an adsorbent bed 206 adapted to fluidly communicate with the evaporator 202 and a coolant fluid 216. During operation, liquid refrigerant flows from the refrigerant source 204 to the evaporator 202, where the refrigerant is evaporated. As is well known in the art, this evaporation cools the evaporator 202. Fluid within the evaporator 202 is cooled by such evaporation and is supplied to the garment 208, such as by a pumping means 218 for cooling the wearer of the garment. The evaporated refrigerant is supplied to the adsorbent bed 206 where it is adsorbed by a desiccant material located therein. Coolant fluid 216 is supplied to the adsorbent bed to cool the desiccant material located therein, thereby increasing its adsorption capacity. A valve 220 may be used to control the flow of refrigerant to the evaporator.
For convenience, the term “desiccant”, “desiccant material” and the like are used herein to described the adsorbent material utilized in the adsorber, but such terms are not intended to limit the adsorbent material to just desiccants. The adsorbent can be any material capable of adsorbing vaporous refrigerant and that is also capable of regenerating (i.e., desorbing) refrigerant at temperatures below 120° C.
Preferably, the sorption cooling system is contained in a hermetically sealed system to maintain refrigerant to adsorbent ratios and to effect the desired cooling, as well as to prevent contamination of the high-capacity desiccant. Since the high-capacity desiccant does have a high affinity for water, it cannot be exposed to atmospheric conditions where water vapor and contaminants are present. Exposure to the atmosphere will likely poison and/or sterilize the high-capacity desiccant.
As noted above, a coolant fluid can be supplied to the adsorbent bed to increase the adsorption capacity of the desiccant contained therein. In one preferred embodiment, the coolant fluid is a gas-phase coolant. In a particularly preferred embodiment, the gas-phase coolant is air.
Generally, the flow rate of refrigerant to the evaporator is a function of the cooling (evaporation) rate and adsorption/desorption rate. Generally, to achieve sufficient cooling using a sorption cooling system in a personal cooling device, the refrigerant flow rate to the evaporator is in the range of from 0.5 to 8 liters per second, such as about 4 liters per second.
Generally, the flow rate of coolant through the adsorbent bed is a function of the desired desiccant cooling rate. Generally, temperature changes are exponentially related to difference in temperature between two objects. Therefore, flow rate in relation to temperature of the coolant and desired temperature and mass of desiccant to be cooled/heated must be evaluated to determine the appropriate coolant flow rate through the adsorbent beds. In one embodiment of the present invention, the mass of the desiccant sheets to cooling capacity ratio is <0.01 pounds per Watt-hour.
The pressure within the evaporator, adsorber and refrigerant source of sorption cooing system is also much less than generally required for a traditional compression-based air conditioner. Typically, the pressure within such components is not greater than 14.7 psig, preferably not greater than 10 psig, more preferably 5 psig, and even more preferably not greater than 2.5 psig.
The Refrigerant Source of the Sorption Cooling System
As noted above, the refrigerant source supplies the evaporator with the necessary refrigerant to provide the cooling. Generally, ex situ regenerable sorption cooling systems can use any type of refrigerant source, but a simple liquid reservoir, such as a lightweight, flexible plastic housing, will generally suffice so as to minimize the size, weight and complexity of the system. Other types of liquid reservoirs useful in accordance with the present invention are disclosed in commonly-owned U.S. Pat. No. 6,701,724 to Smith et al. and U.S. Pat. No. 6,858,068 Smith et al., each of which is incorporated herein by reference in its entirety.
The Refrigerant of the Sorption Cooling System
Any suitable refrigerant may be employed in the sorption cooling system of the present invention. In one embodiment, the refrigerant has a high vapor pressure at ambient temperature so that a reduction of pressure will produce a high vapor production rate. Suitable liquids include ammonia, various alcohols such as methyl alcohol or ethyl alcohol, ketones (e.g., acetone) or aldehydes (e.g., acetaldehyde). Other useful liquids can include chlorofluorocarbons (CFC) or hydrochlorofluorocarbons (HCFC) such as FREON (E.I. Dupont de Nemours, Wilmington, Del.), a series of fluorocarbon products such as FREON C318, FREON 114, FREON 21, FREON 11, FREON 114B2, FREON 113 and FREON 112.
In one preferred embodiment, the refrigerant is an aqueous-based liquid and in a particularly preferred embodiment the liquid consists essentially of water. Water is preferred for its very high mass heat of vaporization in addition to its environmental, costs and safety advantages. Approximately 2,500 J of cooling per gram of water evaporated at ambient temperature can be achieved.
The Adsorbent Bed of the Sorption Cooling System
The adsorber of the sorption cooling system of the present invention may be any type of adsorber adapted to adsorb refrigerant from the evaporator. For example, the adsorber may be in the form of a packed bed. In a preferred embodiment, the adsorber is an adsorbent bed that is lightweight, compact and has a relatively high adsorption capacity. Generally, the adsorbent bed is structured to contain distinct refrigerant and coolant flow paths. Generally, the refrigerant and coolant flow paths are fluidly isolated. The refrigerant flow path generally allows a fluid (e.g., a gas) to flow through the adsorbent bed and contact the high-capacity desiccant material. The coolant flow path is fluidly isolated from the desiccant material of the adsorbent beds and refrigerant flow path, but is adjacent to such desiccant material for providing thermal communication thereto, as discussed in further detail below.
One embodiment of an adsorbent bed useful in accordance with the present invention is depicted in FIG. 3. The adsorbent bed 300 includes a fluid impermeable outer casing 301 including a refrigerant inlet 304, a refrigerant outlet 306, coolant inlets 308 and coolant outlets 310. A refrigerant flow path 318 lies between the refrigerant inlet 304 and refrigerant outlet 306, and a coolant flow path 320 lies between the coolant inlet 308 and coolant outlet 310. A plurality of desiccant sheets 312 are disposed within the casing, each of the desiccant sheets having at least one aperture 314 extending therethrough. A first side of the desiccant sheets 312 preferably includes a high-capacity desiccant (not shown). A second side of the desiccant sheets preferably is covered by a fluid impermeable barrier 316. Spacing materials 322 separate the desiccant sheets 312 from one another and help to define the refrigerant and coolant flow paths. The spacing materials 322 and apertures 314 of the desiccant sheets are arranged such that the refrigerant flow path 318 and coolant flow path 320 are fluidly isolated from one another. Generally, the apertures 314 are a portion of and help define either the refrigerant or coolant flow path, although a first set of apertures can be used to define the refrigerant flow path, while a second set of apertures can be used to define the coolant flow path. It will be appreciated that the adsorbent bed can be designed such that the flow paths depicted in FIG. 3 are switched by switching the coolant inlets and outlets with the refrigerant inlets and outlets and flipping the desiccant sheets over, as depicted in FIG. 4.
During adsorption operations, vaporous refrigerant from the evaporator 302 flows through the refrigerant flow path 318 contacting the adsorbent sections of the desiccant sheets 312, where the refrigerant is adsorbed. As discussed above, the coolant may be flowed through the coolant flow paths 320 to cool the desiccant sheets 312 and increase their adsorption capacity. The fluid impermeable barrier 316 on the second side of the desiccant sheets 312 prevents unwanted interaction (e.g., chemical interaction) between the coolant and the desiccant.
While a single refrigerant and/or coolant flow path may be used in accordance within the present invention, generally the adsorbent bed will include a plurality of apertures per desiccant sheet, and a plurality of spacers, coolant inlets and outlets and/or refrigerant inlets and outlets to define a plurality of refrigerant and coolant flow paths. One embodiment of such an adsorbent bed is provided in FIGS. 5A-5E. A fluid impermeable casing 501 includes a plurality of refrigerant inlets 504, refrigerant outlets 506, coolant inlets 508 and coolant outlets 510. A plurality of desiccant sheets 512 are disposed within the casing, each of the desiccant sheets having at least one aperture 514 extending therethrough. A first side of the desiccant sheets preferably includes a high-capacity desiccant. A second side of the desiccant sheets preferably is covered by a fluid impermeable barrier 516. A plurality of refrigerant flow paths 518 lie between the refrigerant inlets 504 and refrigerant outlets 506, and a plurality of coolant flow paths 520 lie between the coolant inlets 508 and coolant outlets 510. Spacing materials 522 separate the desiccant sheets 512 from one another and help to define the plurality of refrigerant flow paths 518 and plurality of coolant flow paths 520. The spacing materials 522 and apertures 514 of the desiccant sheets 512 are arranged such that the refrigerant flow paths 518 and coolant flow paths 520 are fluidly isolated from one another. Generally, the apertures 514 are a portion of and help define the flow path of either the refrigerant or coolant flow paths, although a first set of apertures can be used to define the refrigerant flow paths and a second set of apertures can be used to define the coolant flow paths. It will be appreciated that the adsorbent bed can be designed such that the flow paths depicted in FIG. 5A are switched by switching the coolant inlets and outlets with the refrigerant inlets and outlets and flipping the desiccant sheets over.
The refrigerant flow paths should generally be designed to maximize mass transfer rates of refrigerant during adsorption and regeneration operations. As is discussed in further detail below, mass transfer rates can be maximized by optimizing, inter alia, the surface area of the adsorbent sections of the desiccant sheets (i.e., the surface area of the first, adsorbent sides of the desiccant sheets minus the area occupied by apertures) and the spacing between the adsorbent sides of the desiccant sheets.
The coolant flow paths should generally be designed to maximize heat transfer rates between the desiccant sheets and the coolant fluid. As is discussed in further detail below, coolant rates can be maximized by optimizing, inter alia, the surface area of available for heat transfer and the spacing between the fluid impermeable sides of the desiccant sheets.
The casing of the adsorbent bed is preferably made of a rigid, lightweight and fluid impermeable material to help minimize the mass of the bed, prevent atmospheric interaction with the bed and structurally protect the bed. Non-rigid, fluid-impermeable materials can also be used in some instances. Suitable casing materials include simple plastic films such as polyethylene, nylon, PVC, metal foils with plastic heat seal layers such as sold those by Toyo Aluminum (Japan), metallized plastic barriers, such as those available from E. I. Du Pont de Nemours and Co. (Wilmington, Del., United States of America), molded polyethylene or polypropylene, such as those available from Rexam (Evansville, Ind., United States of America), COVEXX from Wipak (Finland), multilayer plastic and combinations thereof.
The spacing materials used in the adsorbent bed are preferably lightweight, rigid and include an fluid-impermeable perimeter. As the spacing materials are used to help define at least one of the refrigerant and coolant flow paths, and often both of such flow paths, the spacing materials are also preferably inert to both the refrigerant and coolant used in the sorption cooling system. Preferred spacer materials include polyethylenes, polycarbonates, polypropylenes, acrylonitrile-butadiene-styrene, polyoxymethylenes (e.g., DELRIN available from E. I. Du Pont de Nemours and Co.), polyvinyl chlorides, chlorinated polyvinyl chlorides, epoxies, thermoset polyester elastomers (e.g., HYTREL available from E. I. Du Pont de Nemours and Co.), polyphenylene ethers (e.g., NORYL available from General Electric Corp., Fairfield, Conn., United States of America), polyamides (e.g., NYLON available from E. I. Du Pont de Nemours and Co.), polyphenylene sulfides (e.g., RYTON available from Chevron Phillips Chemical Company, The Woodlands, Tex., United States of America), polytetrafluoroethylenes (e.g, TEFLON available from E. I. Du Pont de Nemours and Co.), polyvinylidine difluorides and combinations thereof. Of these, polyethylenes and polypropylenes are particularly preferred spacer materials. In some instances, the desiccant sheet material can also be cut to the size of the desired spacing material and utilized as the spacing material.
The desiccant sheets of the adsorbent bed are preferably lightweight, thin, have a high adsorption capacity and are adapted to be regenerated at a relatively low temperature. One preferred desiccant sheet according to the present invention includes a low-cost, high-capacity desiccant material, such as composite desiccant comprising a porous-support material having a high pore volume, controlled pore size and an adsorbent dispersed onto the porous support material, such as those desiccants described in commonly-owned U.S. Pat. No. 6,559,096 to Smith et al., which is incorporated by reference herein in its entirety.
The high-capacity desiccant is preferably capable of adsorbing at least about 0.2 grams of liquid refrigerant per gram of desiccant, preferably at least about 0.5 grams of liquid refrigerant per gram of desiccant, more preferably at least about 1.0 grams of liquid refrigerant per gram of desiccant, and even more preferably at least about 2.0 grams of liquid refrigerant per gram of desiccant. FIG. 6 illustrates typical adsorbent capacities of the high-capacity desiccant in comparison with other adsorbents. The high-capacity desiccant also preferably has relatively low ratio of heat of vaporization to heat of adsorption, such as less than 1:1.5, preferably less than 1:1.4, and more preferably less than 1:1.3. By way of comparison, a typical zeolite has a heat of vaporization to heat of adsorption ratio of about 1:1.8. FIG. 7 illustrates the heat of adsorption as compared to water heat of vaporization of a high-capacity desiccant comprising modified carbon.
According to one embodiment, when the refrigerant consists essentially of water, the preferred high-capacity desiccant can adsorb at least about 20 percent of its weight in water at 10 percent relative humidity at ambient temperature (e.g., 25° C.), and at least 40 percent of its weight in water at 50 percent relative humidity at ambient temperature. More preferably, the high-capacity desiccant will adsorb at least 40 percent of its weight at 10 percent humidity at ambient temperature and 60 percent of its weight at 50 percent relative humidity at ambient temperature. Even more preferably, the high-capacity desiccant will adsorb at least about 60 percent of its weight at 10 percent humidity at ambient temperature and at least about 80 percent of its weight at 50 percent humidity at ambient temperature.
Suitable desiccants include zeolites, barium oxide, magnesium perchlorate, calcium sulfate, calcium oxide, activated carbon, modified carbon, calcium chloride, glycerin, silica gel, alumina gel, calcium hydride, phosphoric anhydride, phosphoric acid, potassium hydroxide, sodium sulfate, and combinations thereof. A particularly preferred high-capacity desiccant is a surface modified porous support material. The porous support material can be a material such as activated carbon, carbon black or silica. Preferably, the porous support material has a pore volume of at least about 0.8 cm³/g and average pore size of from about 1 nm to about 20 μm. The surface modification can include impregnating the porous support material with one or more metal salts, such as any one of calcium chloride, lithium chloride, lithium bromide, magnesium chloride, calcium nitrate, potassium fluoride and combinations thereof. The porous support material is preferably loaded with from about 20 to about 80 weight percent of the metal salt and more preferably from about 40 to about 60 weight percent of the metal salt.
The high-capacity desiccant is also preferably of such a nature and quantity as to desorb most adsorbed liquid during an optional regeneration phase. In one embodiment, the high-capacity desiccant is capable of desorbing at least about 90% of the refrigerant at temperatures not greater than 120° C., preferably at least about 95% of adsorbed refrigerant at temperatures not greater than 120° C. and more preferably at least about 98% of adsorbed refrigerant at temperatures not greater than 120° C. FIG. 8 illustrates the adsorption capacity of the high-capacity desiccant material as a function of temperature. As noted above, ex situ regenerable sorption cooling system may employ adsorbent beds that may be regenerated after use, such as by placing in an oven or contacting with a hot carrier gas.
One particularly preferred desiccant sheet includes a porous carbon sheet having a metal salt impregnated thereon. In a preferred embodiment, the desiccant sheets are made of, inter alia, a carbon sheet that includes at least about 50% weight carbon impregnated with a metal salt selected from the group consisting of lithium chloride and calcium chloride. Preferably, the desiccant sheets include at least about 0.01 grams of carbon/cm²of adsorbent side surface area, more preferably at least 0.015 grams of carbon/cm²and more preferably at least 0.02 grams of carbon/cm². Preferably the desiccant sheets include at least 0.01 grams of metal salt/cm²of adsorbent side surface area, more preferably at least 0.02 grams of metal salt/cm², and more preferably at least 0.04 grams of metal salt/cm².
The desiccant sheets are preferably made by impregnating a carbon paper with the metal salt. Generally, the processing includes pre-treating the carbon paper with a surfactant, such as a mixture of 50% deionized water and 50% methanol, uniformly spraying the mixture onto the paper surface until the surface is just moistened, and contacting the moistened carbon sheet with a solution including the metal salt. When the pore volume of the carbon sheet has been filled, the salt/water/alcohol mixture will no longer wick into the surface. If a higher loading of salt is desired than was obtained with one wicking step, the paper sheet may be placed in an oven at 50° C. for several hours to dry the bulk of the liquid out of the pores and the pretreatment and wicking steps are repeated. Once the desired loading is achieved, the sheet may be placed in an oven at 70° C. under vacuum overnight to complete drying. After the sheet has been dried under vacuum, it is kept in a sealed container or handled in a nitrogen environment to prevent it from adsorbing atmospheric water. The above-described carbon sheets can be any sheets comprising a high surface area carbon material (e.g., activated carbon or carbon black), such as those produced by Mead-Westvaco, South Lee, Mass.
The desiccant sheets included in the adsorbent bed may be in any suitable form so long as the desiccant sheets are in fluid communication with the refrigerant flow path and thermal communication with the coolant flow path. The cross-sectional area of the desiccant sheets can be anything, but is generally slightly less than the corresponding cross-section area of the adsorbent bed casing. As noted above, the cross-sectional area of the desiccant sheets should be maximized to maximize the mass transfer and heat transfer capabilities of the adsorbent bed.
The desiccant sheets generally have a relatively low thickness to enable efficient heat transfer between the coolant and the desiccant sheets. Preferably, the desiccant sheets have a thickness of not greater than about 5 mm, preferably not greater than about 2 mm, and more preferably not greater than about 1 mm. To maintain structural integrity, the desiccant sheets should have a thickness of at least about 0.1 mm and more preferably at least about 0.5 mm.
The desiccant sheets should also be as light as possible to minimize the weight of the adsorbent bed. The desiccant sheet should also be structurally sound to withstand forces from the refrigerant and coolant fluid flows. The above-described carbon sheets are particularly preferred due to their low weight and sturdy nature.
As noted above, a fluid impermeable layer should cover one side of the desiccant sheets to prevent unwanted interaction between the desiccant sheets and the coolant. The fluid impermeable layer should also be relatively thermally conductive to enable efficient heating and/or cooling rates between the coolant and the desiccant sheets. The fluid impermeable layer should also be lightweight to help minimize the weight of the adsorbent bed. In this regard, preferred fluid impermeable layer materials include polyethylene, polyurethane, polyesters, COVEXX (available from Wipak, Finland) and metal foils. The fluid impermeable layer may be adhered to the desiccant sheet by any known method, such as lamination, ultrasonic welding, solvent welding and heat sealing.
Each of the desiccant sheets generally has at least one aperture. As noted above, the apertures define a portion of either the refrigerant or coolant paths. In one embodiment, the apertures define a portion of only the coolant flow path. In another embodiment, the apertures define a portion of only the refrigerant flow path. In yet another embodiment, a first set of apertures define a portion of the coolant flow path and a second set of apertures define a portion of the refrigerant flow path.
The desiccant sheets may have one aperture per sheet, but generally have a plurality of apertures per sheet. As the number of apertures increases, the number of fluid paths through the adsorbent bed correspondingly increases and the total frictional surface area available to the fluid decreases. This correspondingly decreases the friction factor through the adsorbent bed, which helps decrease the pressure drop through the adsorbent bed. Moreover, when the apertures are a portion of the refrigerant flow path, an increased number of refrigerant flow paths is witnessed, which can lead to greater utilization of the desiccant material.
According to one embodiment of the present invention, at least one of the desiccant sheets includes a plurality of apertures, such as at least 2, at least 4, at least 6, at least 8 and even at least 16 apertures per 40 in²of cross-sectional area of each sheet. However, it will be appreciated that the number of apertures also increases the complexity of the plumbing that must be accomplished to fluidly isolate flow paths through the adsorbent bed. Thus, according to one embodiment of the present invention the desiccant sheets preferably contain not greater than 256 apertures, such as not greater than 128 apertures, and in some instances not greater than 64 apertures per 40 in²of cross-sectional area of each sheet.
The desiccant sheets of the adsorbent bed can have the same number of apertures per sheet or the desiccant sheets can have different number of apertures per sheet, depending on the desired fluid flow paths and pressure drop within the system. In one embodiment of the present invention, a majority or even all of the desiccant sheets have the same number of apertures per sheet.
The orientation of the apertures through a desiccant sheet can be any orientation that enables efficient fluid flow through the device. In a preferred embodiment, the apertures are substantially orthogonal to the sheet. The apertures in the desiccant sheets can be created by any known means, including laser cutting and die cutting.
The apertures can be any shape that enables efficient fluid flow through the adsorbent bed. For example, the apertures can be substantially cylindrical, conical or a rectangular solid. In a preferred embodiment, the apertures are cylindrical to maximize the surface area available for adsorption while maximizing the amount of fluid flow paths through the adsorbent bed.
The apertures can be any width that enables efficient fluid flow through the bed while helping to maximize mass and heat transfer between the desiccant sheets and the refrigerant and coolant, respectively. The width/diameter of the apertures can be substantially the same or the size of the apertures can vary. In one embodiment of the present invention, all the apertures have substantially the same width. In another embodiment, the width of the apertures is varied through the adsorbent bed to facilitate minimal pressure drop while maximizing fluid flow paths through the system. In a particularly preferred embodiment, the width of the apertures increases in a first direction across the plane of at least one of the desiccant sheets, and in a preferred embodiment across a majority or even all of the desiccant sheets.
In a particularly preferred embodiment, as depicted in FIG. 9, a first desiccant sheet 912 and a second desiccant sheet 912′ each include at least first, second and third apertures, 901, 901′, 902, 902′, 903, and 903′. The first apertures 901, 901′ have a first diameter, the second apertures 902, 902′ have a second diameter, and the third apertures 903, 903′ have a third diameter, where the first diameter is less than the second diameter, and the second diameter is less than the third diameter. The first apertures 901, 901′ are also aligned within a first flow plane 901-FP of the adsorbent bed. Correspondingly, the second apertures 902, 902′ and third apertures 903, 903′ are also aligned in a second and third flow plane, 902-FP and 903-FP, respectively. Preferably, these flow planes are substantially parallel to one another. This arrangement provides multiple fluid flow paths through the adsorbent bed while increasing fluid contact with the desiccant sheets with decreased pressure drop through the adsorbent bed.
The desiccant sheets can be arranged in any suitable manner within the adsorbent casing so long as efficient refrigerant and coolant flow paths are provided. In a preferred embodiment, the desiccant sheets are substantially parallel to one another. In one embodiment, the apertures of a majority of the desiccant sheets are aligned within one another, as depicted in FIG. 9. In one embodiment, the substantially parallel sheets are substantially orthogonal to a refrigerant inlet, a refrigerant outlet or both. Correspondingly, the substantially parallel sheets may also be substantially parallel to a coolant inlet, a coolant outlet or both. In an alternative embodiment, the substantially parallel sheets are orthogonal to a coolant inlet, a coolant outlet, or both. Correspondingly, the substantially parallel sheets may also be substantially parallel to a refrigerant inlet, a refrigerant outlet or both.
The length and/or width of neighboring desiccant sheets may vary to help decrease/minimize flow imbalances within the system. In one embodiment, depicted in FIG. 10A, the width of the desiccant sheets 1012A decreases from the top of the bed to the bottom to help decrease flow imbalances of a fluid path (e.g., the coolant path) through the adsorbent bed. As coolant enters the casing 1001A via the coolant inlets 1008A, in this instance from the bottom, the amount of force required for the coolant to enter the top of the adsorbent bed will be more than the amount of force required for the coolant to enter the middle or lower portions of the adsorbent bed as the coolant will want to follow the path of least resistance. However, as the motive force for flowing the coolant (e.g., a fan or pump—not depicted) is above the coolant outlet 1010A of the adsorbent bed, the greatest motive force acting on the coolant will be at the top of the adsorbent bed. Therefore, this tapered bed structure helps to balance the forces acting on the fluid at the various levels of the bed.
Another embodiment of a structure designed to help decrease flow imbalances is provided in FIG. 10B. Coolant flows through coolant inlets 1008B and exits via coolant outlet 1010B. Again, the motive force (not depicted) for flowing the coolant is disposed above the coolant outlet 1010B, thereby providing the greatest motive force at the top of the adsorbent bed. However, in this embodiment, the adsorbent bed casing 1001B is tapered in relation to the sides of the desiccant sheets 1012B, and the desiccant sheets 1012B are all relatively the same size. As with the previous embodiment, this tapered bed structure helps to balance the forces acting at the various levels of the bed. It will be appreciated that while the foregoing tapered bed embodiments have been described in relation to a coolant fluid, a refrigerant fluid could also be used in such embodiments. Additionally, both the desiccant sheet length/width and casing size could be varied to achieve the desired flow.
The space between the adsorbent sides of the desiccant sheets has been found to be an important factor in adsorbent bed performance, and relates to the mass transfer efficiency of and pressure drop of the refrigerant through the adsorbent bed. Preferably, the space between the first (adsorbent) sides of two desiccant sheets (“the first gap size”) is such that the refrigerant contacts a large amount of desiccant with a small pressure drop. In one embodiment, the first gap size is not greater than 5 mm, preferably not greater than 2 mm, and more preferably not greater than 1 mm. However, preferably the first gap size is at least 0.05 mm, preferably at least 0.1 mm, more preferably at least 0.25 mm, and even more preferably at least 0.5 mm to decrease the amount of refrigerant pressure drop through the bed and help minimize complexity of manufacture. As noted above, the spaces between the desiccant sheets are provided by the spacing materials, described above.
With respect to the spaces between the second sides of the desiccant sheets (i.e., the fluid impermeable sides), these spaces relate to the volume of coolant and pressure required to flow coolant through the bed. Preferably, the space (“the second gap size”) between the second side of a desiccant sheet and a second material (e.g., another second side of a desiccant sheet or the casing) is such that the coolant contacts a large amount of fluid impermeable layer surface area with a small coolant volume and small coolant pressure drop. In this regard, it will be appreciated that the volume of coolant is directly related to the second gap size and the phase of the coolant used (i.e., gas or liquid phase). It will be appreciated that the type of coolant, surface area to be heated/cooled, coolant inlet and outlet temperature, regeneration temperature, cool-off temperature and thermal conductivity of the fluid impermeable barrier and desiccant material are all considerations in evaluating the appropriate second gap size.
Preferably the pressure drop through the adsorbent bed is as small as possible to help minimize the size of the fans, pumps or other motive force required to circulate refrigerant and/or coolant therethrough. In one embodiment of the present invention, the pressure drop of refrigerant flow through adsorbent bed is not greater than 4 inches of H₂O, preferably not greater than 2 inches of inches of H₂O, more preferably not greater than 1 inches of H₂O, and even more preferably not greater than 0.5 inches of H₂O.
The adsorbent bed generally comprises a plurality of desiccant sheets. The number of desiccant sheets required is a function of several factors, including the desired cooling rate, which relates to the amount the evaporation rate of the evaporator, the type of refrigerant, the adsorption capacity of the desiccant and the amount and size of adsorbent beds utilized in the system. With this information the number of sheets can be calculated.
As noted above, the refrigerant flow paths of the adsorbent bed provide fluid communication between the adsorbent beds and the evaporator. Generally, the refrigerant flow paths should be defined to enable a high mass transfer rate within the bed at the lowest pressure drop. As noted above, the number of refrigerant flow paths should be maximized to help adsorbent utilization.
The refrigerant flow paths are defined by the refrigerant inlet(s) and outlet(s) in the casing, and at least one of: (a) the aperatures within the desiccant sheets, and (b) the spacing materials. In one embodiment, a plurality of refrigerant flow paths are defined by a plurality of refrigerant inlets, outlets and apertures. Referring back to FIG. 5A, a plurality of refrigerant inlets 504 are provided for the refrigerant to enter the adsorbent bed casing 501. Spacing materials 522 and apertures 514 define the path through which the refrigerant fluid may flow. The fluid exits the bed through refrigerant outlets 506.
The refrigerant flow path may be substantially linear or non-linear through the adsorbent bed. A substantially linear flow path generally results in a low pressure drop, but may also witness an underutilization of adsorbent. It may also be more difficult to plumb coolant to the desiccant sheets. A non-linear flow path generally has a higher pressure drop, but witnesses an increased adsorbent utilization. Ease of plumbing coolant may also be witnessed. One embodiment of an adsorbent bed having multiple non-linear (i.e., tortuous) flow paths is provided in FIG. 5A, described above. Wet air flows through the refrigerant inlets 504 and into the adsorbent casing 501. Apertures 514 and spacing materials 522 in the desiccant sheets 512 help define non-linear flow paths for the refrigerant fluid to communicate with the first (adsorbent) side of the desiccant sheets. Dry air exits the casing via refrigerant outlets 506. A fan (not shown) is generally used to circulate the air through the refrigerant flow paths.
As used herein, the term “dry air” refers to an air or other gaseous stream containing a relatively small amount or no refrigerant. As used herein, the term “wet air” refers to an air or other gaseous stream containing a relatively high or even saturated amount of refrigerant. As noted below, a variety of refrigerants can be used within the sorption cooling system of the present invention, including water. However, neither the term “dry air” nor the term “wet air” is meant to imply the absence or presence of water within such air unless the refrigerant comprises water.
The coolant flow paths are designed to provide high thermal exchange between the coolant and desiccant sheets with minimal volume, pressure drop and plumbing complexity. Like the refrigerant paths, the coolant paths are defined by the coolant inlet(s) and outlet(s), and at least one of: (a) the apertures within the desiccant sheets and (b) the spacing materials. Like the refrigerant flow paths, the coolant flow paths may also be linear or non-linear, and similar issues exist with respect to the coolant flow paths (e.g., pressure drop, heat transfer, plumbing complexity, etc.). The coolant flow paths are fluidly isolated from the refrigerant flow path to prevent interaction between the two streams. The fluid isolation is generally accomplished using the spacing materials and the fluid impermeable barrier on the second side of the desiccant sheet. The coolant flow paths are generally adjacent to the adsorption sections of the desiccant sheet to thermally communicate therewith. In personal cooling applications, the coolant is generally air, although a liquid coolant, such as water, could be utilized.
The coolant flow paths can flow through the plurality of apertures or the spaces in the adsorbent bed created by the spacing materials, and the coolant generally flows through whichever path the refrigerant flow paths do not flow. Therefore, if the apertures are a portion of the refrigerant flow paths, the coolant flow paths would not be defined by the apertures and vice-versa.
Although certain embodiments of the adsorbent bed have been described with reference to certain orientations, shapes, sizes, numbers, etc., it will be appreciated that any number, size, shape and orientation of apertures, desiccant sheets, spacing materials and fluid inlets and outlets can be used in the adsorbent bed to create an infinite number of refrigerant flow and coolant flow paths having various orientations and capabilities.
The Evaporator of the Sorption Cooling System
The evaporator of the sorption cooling system of the present invention may be any evaporative cooling type of evaporator adapted to cool a fluid stream in contact with at least a portion thereof. In one embodiment of the present invention, the evaporator is a multiple-chamber evaporator that is lightweight, compact and includes multiple air and liquid refrigerant flow channels, embodiments of which are depicted in FIGS. 11 and 12. The evaporator 1102 comprises a casing 1101, air flow inlets 1104 and outlets 1106 in the casing, membrane layers 1108 and spacing materials 1110 for defining the air flow channels 1112 and refrigerant flow channels 1114. Dry air enters the air flow inlets 1104 and travels proximal to (e.g., adjacent to, in contact with) the membrane layers 1108 where, due to pressure and concentration gradients, refrigerant evaporates therein, thereby cooling the refrigerant in the refrigerant flow channels. Wet air exits via air flow outlets 1106 and is recycled back to an adsorbent bed (not shown) to adsorb said refrigerant. Cooled refrigerant exits the evaporator where it is circulated through a garment to cool the garment and correspondingly the user wearing it. Optionally, the evaporator includes a refrigerant surplus inlet for receiving surplus refrigerant, such as from a back-up liquid reservoir, a condenser or other refrigerant source.
The evaporator casing can be any rigid, fluid impermeable casing, such as any of those materials described above for use as the fluid casing of the adsorbent bed.
The air flow channels and refrigerant flow channels are created using a spacing material and the membrane material. The spacing material can be any inert, fluid impermeable material, such as any of those described above for use as the spacing material in the adsorbent bed.
The air flow channels and refrigerant flow channels are preferably in a cross-flow arrangement to achieve a high degree of evaporation with a small volume.
Preferably, the height of each air channel is designed to achieve the desired flow rate though the channel while occupying a small volume. In one embodiment, the height of the air flow channels (i.e., the distance between opposing membrane materials) is not greater than 5 mm, such as not greater than 3 mm, preferably not greater than 2 mm.
Preferably, the width and length of the refrigerant flow channels (equivalent to the length and width of the air flow channel in a cross-flow arrangement) is such that the desired amount of cooling can occur to the refrigerant at the desired air flow and refrigerant flow rates. An evaporator having a 45 mm refrigerant flow path width and 101 mm refrigerant flow path length is capable of achieving a water evaporation rate equivalent to at least about 6.0 W of cooling at a refrigerant (water) flow rate of about 10 liters per second (LPS) and an air flow rate of from about 0.33 to about 0.83 liters per second. The available evaporative surface area for one side of this refrigerant flow channel is about 45.6 cm².
The evaporator of the present invention is generally capably of achieving a high cooling rate with little membrane material. Generally, the ratio of achieved cooling to available evaporative surface area is at least about 250 W/m², preferably at least about 500 W/m², more preferably at least about 750 W/m², and even more preferably, at least about 1000 W/m². In this instance, cooling density is defined as the ratio of the achieved cooling rate to the available evaporative surface area of the membrane material.
The evaporator can include any number of air flow and refrigerant flow channels to achieve the desired amount of cooling. An evaporator with 32 air flow channels and 24 refrigerant flow channels can achieve at least about 150 Watts of cooling with at least about 150 Watt-Hr cooling capacity using air flow rates of from about 0.33 to about 0.83 liters per second through the air flow channels and a refrigerant flow rate of about 10 liters per second through the refrigerant flow channels, using water as the refrigerant. The fabricated evaporator included 4 separate columns, each column having 6 refrigerant flow channels stacked on top of one another (e.g., vertically stacked), such as depicted in FIG. 12. It will be appreciated that a horizontal arrangement of refrigerant flow channels could also be employed.
As illustrated in FIG. 12, the evaporator can include a plurality of membrane sheets, refrigerant flow channels and air flow channels to achieve the desired cooling rate within the personal cooling device. As illustrated in FIG. 13, the sorption cooling system/device can be used to circulate a cooled refrigerant liquid through a garment to cool the user of such garment.
The membrane material is used to tailor the evaporation rates to achieve the desired amount of cooling within the evaporator. Generally, the membrane material has a high vapor permeance and no or little liquid permeance. In one embodiment, the membrane material is a vapor permeable material/fluid impermeable material. There are two general classes of vapor permeable membranes. The first are porous and hydrophobic, such as made from expanded PTFE, so that liquid refrigerant will not penetrate through the pores below some pressure, which depends upon, pore size and contact angle. The second class comprises dense membranes typically produced from substituted polyurethanes. These have the advantage of higher operating pressure but also have lower vapor fluxes.
Particularly preferred membrane materials in accordance with the present invention include such dense substituted polyurethanes, such as those produced by Porvair PLC (Norflolk, United Kingdom), including PORVAIR P3 SB75. Expanded PTFE membranes may also be used, such as TETRATEX produced by the Donaldson Company (Minneapolis, Minn., United States of America).
Preferably, the membrane material is tailored to achieve efficient mass transport between the refrigerant and air. With typically required evaporation rates in personal cooling applications, the membrane material generally has a thickness sufficient to withstand from about 1.5 psi to about 15 psi of liquid pressure without liquid cross-over, such as from 3.0 psi to about 5.0 psi of liquid pressure.
It is also preferable in some circumstance to match the size, shape and number of the evaporator wet air outlets to the size, shape and number of wet air inlets in the adsorbent bed for modular application of the sorption cooling system, as depicted in FIG. 14. As will be appreciated, such modular design enables the rapid substitution of elements within the sorption cooling system, realizing efficiencies in operating the sorption cooling system.
The properties of the evaporator should be tailored to ensure maximum cooling capacity with minimal size and weight. It has been found that various relationships exist between the surface area of the membrane material, volume of the evaporator, and weight of the evaporator that aid in providing such an evaporator with a high cooling capacity and minimal size and weight. Preferably, the membrane has an available evaporative surface area within the evaporator in the range of about 0.035 m²to about 0.45 m²per total liter volume of evaporator, and preferably from about 0.08 m²to about 0.25 m²per total liter volume of evaporator. As used in this application, available evaporative surface area means the total surface area available for evaporation. Preferably, the evaporator weighs no more than 1000 grams, more preferably no more than 750 grams, and even more preferably no more than 600 grams per 1.2 liters of evaporator volume. Preferably, the ratio of evaporator volume to evaporator size is no greater than 1000 grams per liter, more preferably not greater than 750 grams per liter, and even more preferably not greater than 600 grams per liter. Preferably, the evaporator is able to achieve a cooling rate of at least about 100 W, preferably at least about 150 W per 1.2 liters of evaporator volume or a volumetric cooling rate of at least about 80 W/liter. Preferably, the evaporator is able to achieve a cooling rate of 100 W, preferably at least about 150 W per 1000 grams, preferably per 750 grams, and even more preferably per 600 grams of evaporator mass or a mass cooling density of at least about 100 W/kg.
The cooled fluid may be circulated through the garment and evaporator using any well known mechanical means, such as a fan for a gas or a pump for a liquid. One preferred liquid pump is a positive displacement, self-priming diaphragm pump, such as those produced by KNF Neuberger (Trenton, N.J., United States of America) or a gear pump, such as those produced by Greylor (Cape Coral, Florida, United States of America) In one approach, the evaporator of the present invention is a lightweight, compact personal cooling evaporator capable of providing 150 Watts of cooling with a 150 Watt-Hr cooling capacity and may be adapted for modularly integrating with a adsorbent bed of the present invention.

EXAMPLES

Example 1

Water Adsorption Isotherms For Desiccant Materials

Water adsorption isotherms were measured at 25° C. for a CaCl₂-based and LiCl-based desiccant materials. The results are depicted in FIG. 15. Although both desiccant materials perform similarly at less than about 10% RH, at higher humidity the LiCl-based material was more adsorbent.

Example 2

Performance of A First Adsorbent Bed Having 64 Adsorption Sections

An adsorbent bed having 64 adsorption sections was fabricated (an adsorption section is defined by the number of apertures and spacing materials per sheet and their physical relation to one another). An air stream at 35° C. and 95% RH was flowed through the adsorbent bed, and the outlet temperature and humidity was measured over time. The measured cooling rate and cooling capacity are illustrated in FIG. 16.

Example 3

Performance of A Second Illustrated Adsorbent Bed Having 64 Adsorption Sections

An adsorbent bed having 64 adsorption sections was fabricated (an adsorption section is defined by the number of apertures and spacing materials per sheet and their physical relation to one another). An air stream at 35° C. and 95% RH was flowed through the adsorbent bed, which included a CaCl₂desiccant material, and the outlet temperature and humidity were measured over time. A total of six runs were conducted. Table 1 summarizes the results. Measured inlet and outlet temperatures and measured inlet and outlet relative humidity for one test are depicted in FIG. 17. The measured cooling rate and cooling capacity for this test are depicted in FIG. 18. The sorption cooling system averaged about 320 watts of cooling over 30 minutes, with a cooling capacity of 157 W-Hr.

TABLE 1


	Surface						Average	Cooling
	Modification	Air flow	Time	Water	Inlet	Outlet	Cooling	Capacity
Test	Material	(LPS)	(min)	Gained (g)	RH	RH	(W)	(W-Hr)

1	CaCl₂	10.2	60	267	90*	75	340	340
2	LiCl	10.2	60	214	80*	50	183	183
3	Non-modified	8.0	30	101	80*	60	160	81**
4	CaCl₂	9.0	30	98	100	50	320	157**
5	LiCl	11.1	30	157	100	60	420	208**
6	CaCl₂	3.8	30	80	100	70	138	69**

*Inlet humidity was lower than target because the adsorbent bed was removing water vapor faster than the humidifier could produce it.
**Only tested for 30 minutes.

Example 4

Measured Effect of Spacing Between Desiccant Sheets

In the above Example 3, the adsorption beds used a 0.5 mm space between the adsorbent sides of the desiccant sheets and a 1 mm space between the cooling sides of the adsorbent sheets. An adsorbent bed having a 1.0 mm gap space between the adsorbent sides and a 1.25 mm space between the coolant (i.e. fluid impermeable) sides of the adsorbent sheets was fabricated. Humid air at 4 LPS was flowed through the refrigerant paths and air at 23 LPS was flowed through the coolant paths. The measured pressure drop through the refrigerant paths was 0.35 in. H₂O as compared to a 1 in. H₂O for the previous bed. Cooling rates and capacity using this 1.0 mm adsorbent side gap spacing at 4 LPS of refrigerant are illustrated in FIG. 19. The test was stopped after 30 minutes. Cooling rates and capacity using this 1.0 mm adsorbent side gap spacing at 8 LPS of refrigerant are illustrated in FIG. 20, and evidenced about a 180 W-Hr cooling capacity.
Another adsorbent bed was produced using a 1.5 mm adsorbent gap size instead of the above 1.25 mm. The pressure drop through this adsorbent bed was 0.30 in. H₂O. The performance of this adsorbent bed was nearly as good as the adsorbent bed having a 1.25 mm adsorbent side gap spacing, as depicted in FIG. 21.
As noted above, the gap spacing between the coolant sides of the sheets is also important. FIG. 22 illustrates the calculated heat transfer efficiency between the desiccant sheets and an air coolant fluid and the corresponding pressure drop within an adsorbent bed as a function of coolant side gap spacing.

Example 5

Adsorbent Bed Using Desiccant Sheet As Spacer Material

An adsorbent bed was fabricated using CaCl₂impregnated sheets and laser cut desiccant sheets having a thickness of 0.5 mm as the spacing material instead of using a 0.5 mm thick polycarbonate spacing material. The desiccant sheets weigh less than the polycarbonate spacing materials, have a higher adsorption capacity and more readily adhere to the adjoining desiccant sheets. For this example, the total weight reduction was 35 grams. An air stream at 35° C. and 100% RH was fed to the adsorbent bed, and the output temperature and humidity measured were measured over time, the results of which are illustrated in FIG. 23. The measured cooling rate and capacity are illustrated in FIG. 24. The sorption cooling system averaged 260 watts of cooling, peaking at 440 watts, and a cooling capacity of 130 watt-hours.

Example 6

Testing of Membrane Permeability For Thin-Filmed Evaporator

A single membrane test cell was constructed that allowed the testing of a 1″×2″ membrane material having flowing water on one side and flowing air on the other side. The test cell is depicted in FIG. 25. The inlet and outlet temperature of both air and water streams were measured, as well as the inlet and outlet humidity of the air streams. The water inlet temperature was 20° C. and the flow rate was maintained such that the water temperature was essentially constant. Compressed air at ˜28° C. and 20% RH was fed into the test cell and the flow rate was controlled over the range of 1 to 30 liters per minute. Results are summarized in Table 2.

TABLE 2


Membrane	g/m²day at lpm	g/m²day at lpm	g/m²day at lpm

Expanded	9,400 at 1 lpm	20,670 at 9.2 lpm	19,620 at 30 lpm
PTFE
(1 μm thick)
Porvair	25,840 at 10 lpm	34,070 at 20 lpm	34,810 at 30 lpm
P3-SB-75

Example 7

Evaporator Testing

An evaporator module was constructed having an available evaporative surface area of 0.05 m². A polyurethane-coated expanded PTFE membrane was used as the membrane material. The inlet air measured 23.7° C. and 11% relative humidity (RH). The outlet air measured 16.8° C. and 40% RH. The air flow rate was 30 LPM. The water inlet temperature was 19.3° C. The water outlet temperature was 16.3° C. The water flow rate was 2.62 g/s (9.4 LPH). A cooling rate of 32.9 W was achieved.

Example 8

Evaporator Performance Using Dense Polyurethane Membrane

An evaporator module was assembled using a dense polyurethane membrane. Water at about 26° C. was flowed through the water flow channels of the evaporator at 3 grams/sec (˜2.8 gal per hour). Compressed air at about 7-9% RH was flowed through the air flow channels at various flow rates (0.33, 0.50, 0.67 and 0.83 LPS). The inlet and outlet air temperature and relative humidity were measured over time, the results of which are depicted in FIG. 26 for the 0.33 LPS air flow rate. Even though the inlet air stream had only about 7% RH, the outlet air stream had an about 70% RH, which remained constant for a 1 hour test. An increased cooling performance was also witnessed at higher air flow rates, as depicted in FIG. 27.
While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, is to be expressly understood that such modifications and adaptations are within the spirit and scope of present invention.

Claims

1. A method of cooling a person using a personal cooling system, the method comprising:

(a) flowing dry air from an adsorber to an evaporator, said evaporator comprising:

i. a plurality of vapor-permeable membrane material layers, each of said plurality of vapor-permeable membrane materials having a first side and a second side;

ii. air-flow channels disposed adjacent to each of said first sides; and

iii. refrigerant flow channels disposed adjacent to each of said second sides, each of said refrigerant flow channels comprising liquid water;

(b) evaporating a first portion of said liquid water into said dry air thereby cooling a second portion of said liquid water; and

(c) flowing said cooled second portion of said liquid water through a garment worn by said person.

2. The method of claim 1, wherein wet air is created from said step of evaporating a first portion of said liquid water into said dry air, the method further comprising the steps of:

(d) returning said wet air to said adsorber; and

(e) adsorbing water contained in said wet air.

3. The method of claim 2, wherein said dry air is created from said adsorbing said water step.

4. The method of claim 1, further comprising the step of:

(d) returning said second portion of water to said evaporator.

5. The method of claim 1, wherein said flowing dry air step comprising flowing said dry air into said evaporator at a flow rate of between 0.33 liters per second and 0.83 liters per second.

6. The method of claim 1, wherein said flowing said cooled second portion of said liquid water step comprises flowing said cooled second portion of said liquid water at a flow rate of about 10 liters per second.

7. The method of claim 1, wherein said evaporator has a volumetric cooling density of at least about 80 W/liter.

8. The method of claim 1, wherein said evaporator has a mass cooling density of at least about 100 W/kg.

9. The method of claim 1, wherein said membrane material is selected from the group consisting of dense polyurethanes, expanded PTFE and combinations thereof.

10. The method of claim 1, wherein said membrane material is substantially vapor permeable and liquid impermeable at between about 1.5 psi and 15 psi of liquid pressure.

11. A personal cooling system comprising:

(a) a garment adapted to be worn by a person, said garment comprising at least one cooling channel;

(b) an evaporator comprising:

i. a first refrigerant flow channel containing a refrigerant, said first refrigerant flow channel adapted to fluidly communicate with said at least one cooling channel;

ii. a first air flow channel;

iii. a membrane material liquidly separating said first refrigerant flow channel from said first air flow channel;

(c) a circulating means for circulating said refrigerant through said cooling channels of said garment and said first refrigerant channel; and

(d) an air flow means for flowing air through said first air flow channel.

12. The personal cooling system of claim 11, further comprising:

(e) an adsorber adapted for gaseous communication with said air flow channel of said evaporator, said adsorber being further adapted to adsorb vaporous refrigerant contained in an air stream.

13. The personal cooling system of claim 12, wherein said adsorber comprises a plurality of desiccant sheets, each of said desiccant sheets comprising at least one aperture, said at least one aperture being a portion of an air flow path in gaseous communication with said air flow channel of said evaporator.

14. The personal cooling system of claim 11, further comprising a refrigerant source for supplying a supplemental portion of refrigerant to said evaporator.

15. The personal cooling system of claim 11, wherein said membrane material is from the group consisting of dense substituted polyurethanes, expanded PTFE and combinations thereof.

16. The personal cooling system of claim 11, wherein said refrigerant flow channel and said cooling channel operate in a closed-loop.