EP2212629A1 - Non-vacuum absorption refrigeration - Google Patents

Non-vacuum absorption refrigeration

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Publication number
EP2212629A1
EP2212629A1 EP07852808A EP07852808A EP2212629A1 EP 2212629 A1 EP2212629 A1 EP 2212629A1 EP 07852808 A EP07852808 A EP 07852808A EP 07852808 A EP07852808 A EP 07852808A EP 2212629 A1 EP2212629 A1 EP 2212629A1
Authority
EP
European Patent Office
Prior art keywords
refrigerant
solution
membrane
membrane contactor
absorption refrigeration
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP07852808A
Other languages
German (de)
French (fr)
Other versions
EP2212629A4 (en
Inventor
Zidu Ma
Joseph J. Sangiovanni
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Carrier Global Corp
Original Assignee
Carrier Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Carrier Corp filed Critical Carrier Corp
Priority to PCT/US2007/022121 priority Critical patent/WO2009051583A1/en
Publication of EP2212629A1 publication Critical patent/EP2212629A1/en
Publication of EP2212629A4 publication Critical patent/EP2212629A4/en
Withdrawn legal-status Critical Current

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Classifications

    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25BREFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT-PUMP SYSTEMS
    • F25B15/00Sorption machines, plant, or systems, operating continuously, e.g. absorption type
    • F25B15/14Sorption machines, plant, or systems, operating continuously, e.g. absorption type using osmosis
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25BREFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT-PUMP SYSTEMS
    • F25B15/00Sorption machines, plant, or systems, operating continuously, e.g. absorption type
    • F25B15/02Sorption machines, plant, or systems, operating continuously, e.g. absorption type without inert gas
    • F25B15/06Sorption machines, plant, or systems, operating continuously, e.g. absorption type without inert gas the refrigerant being water vapour evaporated from a salt solution, e.g. lithium bromide
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02ATECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A30/00Adapting or protecting infrastructure or their operation
    • Y02A30/27Relating to heating, ventilation or air conditioning [HVAC] technologies
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02BCLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B30/00Energy efficient heating, ventilation or air conditioning [HVAC]
    • Y02B30/62Absorption based systems

Abstract

A non-vacuum absorption refrigeration system is disclosed that operates at ambient pressure using membrane distillation to perform evaporator, absorber and boiler and condenser functions, The absence of a vacuum enables the use of low-pressure, non-corrosive piping and vessels that obviate the maintenance requirements associated with vessels and piping that are susceptible to corrosion and also reduces total system weight.

Description

NON-VACUUM ABSORPTION REFRIGERATION
BACKGROUND OF THE INVENTION
[0001] The invention relates generally to the field of absorption refrigeration. More specifically, the invention relates to an absorption refrigeration system that does not require a vacuum and uses membrane distillation to perform evaporator, absorber and concentrator functions.
[0002] The basic absorption cycle employs a refrigerant and an absorbent. Typically, water is used as the refrigerant and lithium bromide/water solution (LiBr/H2O) is used as the absorbent. These fluids are separated and recombined during the absorption cycle.
[0003] In the absorption cycle, refrigerant vapor is absorbed into the absorbent releasing a large amount of heat. The concentration of the diluted (weak) absorbent solution may be 55 percent weight or higher. The weak absorbent solution is pumped to a high temperature boiler. The added heat causes the refrigerant in the weak absorbent to desorb from the absorbent and vaporize. The vapors flow to a condenser, where heat is rejected, and condenses to a liquid. The liquid is metered to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling. The remaining liquid absorbent in the boiler is recombined with the refrigerant vapors returning from the evaporator so the cycle can be repeated.
[0004] The absorption process operates under a high vacuum. For example, a 6.35 mmHg (0.85 kPa) vacuum is used in an evaporator/absorber section which corresponds to the saturated vapor pressure of water at 41 0F (5 0C) . A 76.2 mmHg (10.2 kPa) vacuum is used in the boiler and condenser section which corresponds to the saturated condensing pressure of water at 115 0F (46 0C) . These two sub-ambient pressures must be used to maintain the absorption cycle with boiler/condenser and evaporator/absorber type of designs.
[0005] Because the evaporator/absorber, and boiler/condenser sections operate in a vacuum, they require high-pressure, air-tight designs. Thick metal walls are necessary to withstand the external pressure on the vessel sections. However, the absorbent solution is highly corrosive to metals. Inhibitor chemicals are used to control corrosion. Periodic chemical analysis on alkalinity and inhibitor chemical concentration in the absorbent solution is required to maintain the normal operation of the absorption chillers.
SUMMARY OF THE INVENTION
[0006] The inventors have discovered that it would be desirable to have an absorption refrigeration system that operates without vacuum using membrane distillation to perform evaporator, absorber and boiler and condenser functions. The absence of a vacuum enables the use of low-pressure, inexpensive corrosion resistive piping and vessels that obviate the maintenance requirements associated with vessels and piping that are susceptible to corrosion. The use of corrosion resistive materials such as plastics also reduces total system weight.
[0007] Non-vacuum absorption refrigeration systems according to this aspect of the invention include a membrane contactor (evaporator/absorber) for cooling a refrigerant fluid and generating a refrigerant vapor, and carrying an absorption solution for absorbing the refrigerant vapor to produce a refrigerant-absorption (weak) solution, and a membrane contactor (concentrator) for removing the refrigerant from the weak solution to provide a concentrated absorption solution for the membrane contactor (evaporator/absorber) .
[0008] Another aspect of the non-vacuum absorption refrigeration system is where non-corrosive material is used for system piping and vessel construction.
[0009] Another aspect of the non-vacuum absorption refrigeration system is where the membrane contactor (evaporator/absorber) and membrane contactor (concentrator) are designed and operated for membrane distillation, including direct contact membrane distillation, air gap membrane distillation, sweep gas membrane distillation, and vacuum membrane distillation.
[0010] Another aspect of the non-vacuum absorption refrigeration system is where the membrane contactors comprise microporous membranes having hydrophobic inner and outer surfaces .
[0011] Another aspect of the non-vacuum absorption refrigeration system is where the pore size and the hydrophobicity are such that the absorbent solution and the refrigerant do not penetrate the membrane pores . [0012] Another aspect of the non-vacuum absorption refrigeration system is where the thermal efficiency of the membrane contactor (evaporator/absorber) ηA is greater than 50 percent.
[0013] Another aspect of the invention provides a method for non-vacuum absorption refrigeration. Methods according to this aspect of the invention start with adding a refrigerant to a refrigerant fluid flow, circulating a concentrated absorbent solution and the refrigerant fluid flow through a membrane contactor (evaporator/absorber) , generating a refrigerant vapor and cooling the refrigerant fluid in the membrane contactor (evaporator/absorber) , absorbing the refrigerant vapor producing a refrigerant-absorbent (weak) solution, circulating the weak solution and the refrigerant through a membrane contactor (concentrator) , generating a refrigerant vapor in the membrane contactor (concentrator) , absorbing the refrigerant vapor into the refrigerant and providing a concentrated absorbent solution and cooling the solution for the membrane contactor (evaporator/absorber) .
[0014] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims .
BRIEF DESCRIPTION OF THE DRAWINGS [0015] FIG. 1 is an exemplary absorption refrigeration machine. [0016] FIG. 2 is an exemplary non-vacuum absorption refrigeration system using membrane distillation.
[0017] FIG. 3 is an exemplary microporous membrane contactor.
[0018] FIG. 4 is a section view of vapor exchange taking place in the porous hydrophobic polymeric membrane wall.
[0019] FIG. 5 is a photomicrograph of a single microporous membrane cross section in a partial tube-and-shell evaporator/absorber arrangement with other fibers.
[0020] FIG. 6 is a photomicrograph of section of the wall of similar microporous membranes shown in FIG. 5.
DETAILED DESCRIPTION
[0021] Embodiments of the invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of the examples set forth in the following description or illustrated in the figures. The invention is capable of other embodiments and of being practiced or carried out in a variety of applications and in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having," and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms "mounted, " "connected, " and "coupled, " are used broadly and encompass both direct and indirect mounting, connecting, and coupling. Further, "connected," and "coupled" are not restricted to physical or mechanical connections or couplings .
[0022] By way of background, absorption refrigeration is a process that is different from compression refrigeration. The absorption process uses heat as a driving force instead of electrical or shaft power.
[0023] FIG. 1 shows a simplified absorption chiller machine 101. The machine 101 includes an evaporator 103 and an absorption section 105. [0024] The refrigerant 107 in this example is water which is metered into the evaporator section 103. A refrigerant circulating pump 109 circulates the water through spray heads 111 to be sprayed over a chilled water tube bundle 113. This wets the tube bundle 113 through which circulating water from a cooling water system passes. The heat from the system water 113 evaporates the refrigerant 107 to create water vapor schematically illustrated at 115. Water is constantly being evaporated and must be made up.
[0025] In the absorption section 105, the absorbent (LiBr) solution 117 has a lower vapor pressure than that of the evaporated water from section 103, and readily absorbs the water vapor 115 into the solution 117. The LiBr solution 117 is recirculated via a LiBr solution circulating pump 119 through spray heads 121 to give the solution more surface area to attract the water vapor 115. As the solution 117 absorbs water, it becomes diluted. If the water is not removed, the solution 117 will become so diluted that it will no longer have any attraction potential and the absorption process will stop. Another pump 123 constantly removes some of the solution 117 and pumps it to a concentrator 125. The solution that is pumped to the concentrator 125 is referred to as the weak solution because it contains water absorbed from the evaporator 105.
[0026] The concentrator (generator) 125 includes a boiler 127 and a condenser 129. The boiler 127 requires a heat source which may be either steam or hot water 131. The condenser 129 requires a stream of cool water usually from a cooling tower system 133. The weak solution is pumped into the concentrator 125 where it is boiled. The boiling action changes the water to a vapor which leaves the absorbent solution and water vapor is attracted to the condenser coils 129. The water is condensed to a liquid where it gathers and is metered back to the evaporator section 103 through an orifice 135. The absorbent solution becomes concentrated 137 and is drained back through line 139 to the absorption section 105 for circulation by the absorbent pump 119.
[0027] The absorption process 101 is simple considering that the only moving parts are the pump motors and pump impellers. The absorption chiller may include more than one stage which results in an absorption machine that is more efficient than a single-stage design.
[0028] FIG. 2 shows a non-vacuum absorption refrigeration system 201 that does not operate under a vacuum and may therefore use non-corrosive materials for piping and vessels. The system 201 uses membrane distillation to replace the boiler 127 and condenser 129 used in the concentrator 125, and the evaporator 103 and absorber 105. Membrane distillation employs low temperature heat to vaporize water from one side of a membrane contactor, and condenses the water vapor on the other side of the membrane contactor. Due to evaporation, membrane distillation can chill water. The system 201 uses a membrane contactor (concentrator) 203 and a membrane contactor (evaporator/absorber) 205.
[0029] The membrane contactors are devices that allow refrigerant vapor transport between the two sides of the membrane that are in contact with two different liquid phases without the liquid phases penetrating through the membrane. [0030] A refrigerant 207 circulating loop is defined by a refrigerant circulating pump 209, a refrigerant heat exchanger (refrigerant cooler) 211, a metering orifice 213 and the membrane contactor (concentrator) 203 tube side. An absorbent solution 215 circulating loop is defined by an absorbent solution circulating pump 217, primary side of an absorbent solution heat exchanger (recuperator) 219, a weak absorbent solution heater 221, the membrane contactor (concentrator) 203 shell side, secondary side of the absorbent solution heat exchanger (recuperator) 219, primary side of another absorbent solution heat exchanger (solution cooler) 223 and the membrane contactor (evaporator/absorber) 205 shell side. The shell or tube sides may be switched with no loss of function. The recuperator is used to recover the heat from hot absorbent solution 241 with diluted cool absorbent solution 231 to increase system efficency. The absorbent solution used in the exemplary embodiment is LiBr/H20, but other absorbents may be used. The refrigerant used in the exemplary embodiment is water, but other refrigerants may be used.
[0031] A another refrigerant flow 225 (to be used for cooling purposes) is coupled with a metered refrigerant flow 227 to produce refrigerant fluid 229 to be chilled by membrane contactor (evaporator/absorber) 205. The refrigerant fluid 229 is coupled to the membrane contactor (evaporator/absorber) 205 tube side. The invention uses membrane distillation to cool the refrigerant fluid 229 in the membrane contactor (evaporator/absorber) 205 and to concentrate the heated weak absorbent solution 237 by the membrane contactor (concentrator) 203. [0032] Heat is removed from the refrigerant fluid 229 flowing through the membrane contactor (evaporator/absorber) 205 through vaporization of the refrigerant 207 from the fluid 229 flowing through the tube side of membrane contactor 205. In the membrane contactor (evaporator/absorber) 205 shell side, the absorbent solution 215 has a vapor pressure less than that of the refrigerant fluid 229 on the tube side of the membrane. The absorbent solution 215 absorbs the vapor of refrigerant fluid 229 transported through the membrane pores. The absorption induces more vaporization of refrigerant fluid 229 inside the membrane contactor.
[0033] Similarly, when the diluted absorbent solution is heated to a temperature such that the refrigerant vapor pressure from the shell side in the concentrator is higher than that of refrigerant on the tube side in the concentrator, refrigerant vapor will be transported from the absorbent solution side to the refrigerant side. The absorbent solution looses refrigerant and becomes concentrated.
[0034] FIG. 3 shows a cut-away view of a typical membrane contactor 301 configuration that may be used for the membrane concentrator (concentrator) 203 and membrane contactor (evaporator/absorber) 205. The membrane contactor 301 has a construction analogous to that of a tube-and-shell exchanger where tubes constructed of hydrophobic microporous membranes are arranged, coupling input 303 and output 305 tube side ends where one fluid flows, and another fluid flows over the tubes through a shell input 307 and output 309. Since the surfaces of the microporous membrane are hydrophobic, the membrane will not allow liquid water or LiBrZH2O solution to pass through the pores to the opposite sides of the membrane. The microporous membranes in both contactors (concentrator) and (evaporator/absorber) may¬ be made of polypropylene, polyvinylidene difluoride (PVDF) , polytetrafluoroethylene (PTFE) or other materials that have surface energy less than the surface tension of pure water. Or, the membrane surface, made of a material or materials, has surface energy less than the surface tension of pure water or the refrigerant .
[0035] FIG. 4 shows a section view of a membrane wall 401 in the membrane contactors (concentrator) 203 and (evaporator/absorber) 205. The membrane wall 401 includes a membrane 403 and a microporous skin 405. The membrane surfaces 407 are hydrophobic. The pores 409 become gas filled and form two interfaces, a liquid refrigerant and refrigerant vapor interface 411 and a liquid absorbent solution and refrigerant vapor interface 413. In the membrane contactor (evaporator/absorber) 205, the absorbent solution 245 flows over the tubes (shell side) and the refrigerant fluid 229 flows within the tubes (tube side) . In the membrane contactor (concentrator) 203, the hot absorbent solution 237 flows over the tubes (shell side) and the cool refrigerant 207 flows within the tubes (tube side) .
[0036] FIG. 5 shows an exemplary cross section of one hollow fiber tube in the membrane contactor (evaporator/absorber) 205. In the membrane contactor (evaporator/absorber) 205, the cold absorbent solution 245 is shown flowing across the hollow fiber tubes in the shell side. The refrigerant fluid 229 is shown flowing within each hollow fiber. FIG. 6 shows an enlarged view of the hollow fiber wall shown in FIG. 5 where the transition from vaporization to condensation for a solution having a water vapor pressure greater than the other side.
[0037] The microporous membrane has a pore size in the range of from about 0.1 to 0.6 micrometer and a porosity of greater than 50 percent. The membrane acts as a barrier between the two phases of the absorbent solution and the refrigerant fluid. The membrane's surface energy is sufficiently less than the lesser of the absorbent solution's surface tension or the refrigerant fluid's surface tension. The membrane in conjunction with process parameters evaporates the refrigerant from the refrigerant fluid 229 in the membrane contactor (evaporator/absorber) 205 and from the weak absorbent solution 237 in the membrane contactor (concentrator) 203. The membrane allows the refrigerant vapor to transfer through. The driving force is the vapor pressure difference across the membrane.
[0038] On the membrane contactor (evaporator/absorber) 205 tube side, refrigerant from refrigerant fluid 229 will be vaporized from a meniscus formed near the mouths of the membrane pores (see FIG. 4) . The vapor will be transported through the membrane pores to the shell side. The water vapor is absorbed by the absorbent solution meniscus near the membrane pore mouth on the shell side.
[0039] Because refrigerant vapor pressure difference is the driving force in membrane distillation, vaporization and absorption can be sustained without vacuum. This surface vaporization in combination with a large contact area is achieved using hollow fiber membrane contactors. Refrigerant absorption may be sustained by the concentrated absorbent in the membrane contactor (evaporator/absorber) 205 shell side to produce chilled refrigerant fluid 229 through evaporation, and by the cooler water flowing on the tube side in the membrane contactor (concentrator) 203 to concentrate a weak absorbent solution 237.
[0040] The cool, concentrated absorbent solution 245 may have a percent weight concentration of 56.8 percent and a corresponding refrigerant vapor pressure vpevap/absorbenl of, for example, 4.3 mmHg
(0.57 kPa) at 31.44 0F (0.31 0C) at the inlet of the membrane contactor 205 on absorbent solution side. This vapor pressure is lower than 7.1 mmHg (0.95 kPa) , which is the vapor pressure of refrigerant fluid 229 at 45 0F (7.22 0C) at the refrigerant outlet of the membrane contactor 205. After passing through the membrane contactor 205, the concentrated absorbent solution becomes diluted to 56 percent and the temperature increases to 46.9 0F (8.27 0C) . At the outlet of absorbent solution of the membrane contactor 205, the refrigerant vapor pressure is 11.1 mmHg (1.48 kPa) still less than the refrigerant vapor pressure from the refrigerant fluid 229, 11.3 mmHg (1.51 kPa) , at 55 0F (12.77 0C) at the inlet of the refrigerant side of the membrane contactor 205. This vapor pressure differential ensures absorption of refrigerant vapor from the refrigerant fluid 229 by the absorbent solution 245 inside the membrane contactor 205.
[0041] The refrigerant f luid 229 temperature te°vap/refrigeranl_βuid in the membrane contactor 205 determines the refrigerant vapor pressure vPevap/refrigeran,- flu* • τ° cool the refrigerant f luid 229 , the vapor pressure of the refrigerant f luid 229 must be greater than the vapor pressure of the absorbent solution 245 in the membrane contactor (evaporator/absorber) 205.
[ 0042 ] VPevap/ refrigerant -fluid > VP evap / absorbent ' ^ '
[0043] The percent weight concentration of the absorbent solution is known, and using the percent weight concentration and solution temperature C-p/αtoortenr i-n the membrane contactor (evaporator/absorber) 205, the absorbent solution vapor pressure VPevap/absorbent mave found. The conversions from temperature and concentration, to vapor pressure may be found either using an equation or a memory look-up table.
[0044] The absorbent solution cooler 223 is configured to output the absorbent solution 245 at a predefined temperature te'vaplabsorbent that corresponds to a predefined vapor pressure vpevap/absorbenl for a given capacity absorption refrigeration system, ensuring that the membrane contactor (evaporator/absorber) 205 functions to that capacity. If a system perturbation occurs and the vapor pressure relationship (1) is not met, the absorbent solution cooler 223, for example, may be thermostatically controlled such that the absorbent solution temperature tl°implabsorbatt will be decreased, in turn decreasing the absorbent solution vapor pressure vp^aplabsorbent . In this manner, the relationship (1) will be maintained throughout any system perturbation.
[0045] After the absorbent solution 231 passes through the membrane contactor (evaporator/absorber) 205 shell side, the absorbent solution 231 concentration is lower than the absorbent solution 245 due to the absorbent solution 245 absorbing refrigerant from refrigerant fluid 229, decreasing its absorption capacity. The absorbent solution 245 becomes absorbent solution 231, which is diluted and called weak absorbent solution.
[0046] The weak absorbent solution 231 is circulated to the absorbent recuperator 219 which preheats the weak solution 231 by the hotter concentrated absorbent solution 241. The preheated weak solution 233 output by the recuperator 219 is heated in the weak solution heater 221. The weak solution heater 221 is heated to about 203 0F (95 0C) using a hot water or steam source 235. The heated weak absorbent solution 237 is input to the membrane contactor (concentrator) 203 shell side.
[0047] The hot, weak absorbent solution 237 may have a 56 percent weight concentration and a corresponding vapor pressure Ψconcentrator,waksoiuuon °f, for example, 125 ItImHg (16.67 kPa) at 203 °F (95 0C) . The vapor pressure of the weak absorbent solution 237 must be higher than the refrigerant 239 vapor pressure VPconcentratorirefrigerant which may be, for example, 99 mmHg (13.20 kPa) at 96.8 0F (36 0C) after passing through the refrigerant cooler 211.
[0048] The differential vapor pressure drives the vapor transport through the membrane pores . The weak absorbent solution side of the membrane is at a temperature high enough to generate vapor pressure that is higher than that of the refrigerant on the refrigerant side of the membrane. [0049] The refrigerant temperature tc°onceπlmtor/refngeranl in the membrane contactor 203 determines the refrigerant vapor pressure Wconcentratorirefngeran, ■ To concentrate the weak absorbent solution, the vapor pressure of the weak absorbent solution must be greater than the vapor pressure of the refrigerant in the membrane contactor 203.
I J Jr concentrator I weak solution r concentrator I refrigerant * '
[0051] The percent concentration weight of the weak absorbent solution 237 is known, and using the percent concentration weight and weak solution temperature tc°oncentratorlweaksoluUon in the membrane contactor 203, the weak solution vapor pressure VPconcentratorineaksoiunon maY be foundTne conversions from temperature and concentration, to vapor pressure may be found either using an equation or a memory look-up table .
[0052] The weak solution heater 221 and the refrigerant cooler 211 are configured to output the weak solution 237 and cold refrigerant 239 at predef ined temperatures tc°oncentratorl weaksolutlon , Concentrator i refngeram that correspond to predef ined vapor pressures
VP concentrator/ weak soluLon • VP concentrator I refngerant f Or a gin Capaci ty absorption refrigeration system, ensuring that the membrane contactor (concentrator) 203 functions to that capacity. If a system perturbation occurs and the vapor pressure relationship (1) is not met, the weak solution heater 221, for example, may be thermostatically controlled such that the weak absorbent solution temperature tc°oncentratorlweaksolutωn will be increased, in turn increasing the weak absorbent solution vapor pressure VPconcentiweaksol*™ ■ Conversely, if the relationship (1) is not met, the refrigerant cooler 211, for example, may be thermostatically controlled such that the refrigerant temperature tc°oncenlralor/refrigeranl will be decreased, thereby decreasing the refrigerant vapor pressure Ψconcentrator/refrigeraπl ■ In this manner, the relationship (1) will be maintained throughout any system perturbation. Control arrangements controlling both the weak solution heater 221 and the refrigerant cooler 211 may be provided.
[0053] The weak absorbent solution after passing through the membrane contactor (concentrator) 203 becomes concentrated 241, recovering its absorption capacity. The absorbent solution 241 is circulated by the absorbent solution circulating pump 217 through the absorbent solution recuperator 219 where it is precooled 243 and then passed through the absorbent solution cooler 223 completing the absorbent solution cycle.
[0054] The refrigerant pump 209 circulates the refrigerant 239 through the cooler 211 which is cooled by cooling tower water 243. A portion of the cooled refrigerant 238 is passed to the membrane contactor 203 while the rest of the refrigerant is sent to the evaporator/absorber 205.
[0055] As can be seen, the refrigerant in refrigerant fluid 229 that is absorbed by the absorption solution 245 into the absorption solution 231 circulating loop (by the membrane contactor 205) is returned back to the refrigerant 207 circulating loop (by the membrane contactor 203) . Refrigerant 207 losses that may occur may be made up using head or storage tanks (not shown) .
[0056] The membrane contactor 203 can employ low temperature heat to vaporize water from the weak absorbent solution 237 in the shell side and condenses the vapor in the tube side where the refrigerant 207 is circulated. The heated refrigerant 239 is cooled by the refrigerant cooling heat exchanger 211 using cooling tower water 243.
[0057] An non-vacuum absorption refrigeration system using the membrane contactors of the invention 201 may achieve a high coefficient of performance (COP) . The coefficient of performance of a system is the ratio of the amount of cooling to the amount of heat supplied to drive the absorption cycle.
[0058] The non-vacuum absorption refrigeration system 201 uses a membrane contactor in the evaporator/absorber 205 and in the concentrator 203. A thermal efficiency η for each membrane contactor 203, 205 is calculated.
[0059] The thermal efficiency for the membrane contactor (concentrator) 203 is defined as
[0061] where ηG is the thermal efficiency of the membrane contactor (concentrator) 203, QHV is the vaporization heat for water from the absorbent solution 237 and QCG is the heat lost through conduction from the absorbent solution side of the membrane in concentrator 203 to that of the refrigerant side of the membrane in the concentrator 203.
[0062] The thermal efficiency for the membrane contactor (evaporator/absorber) 205 is defined as
[0064] where ηA is the thermal efficiency of the membrane contractor (evaporator/absorber) 205, QHV is the vaporization heat of water from the refrigerant fluid 229 and QCA is heat lost from the absorbent solution side of the membrane in the membrane contactor 205 to that of the refrigerant side of the membrane in the membrane contactor 205.
[0065] The heat of conduction, QCA , needs to be as small as possible to have a high thermal efficiency. So does QCG .
[0066] In the evaporator/absorber 205, the evaporation of the refrigerant 207 from the refrigerant fluid 229 provides a cooling action. However, the condensing of the refrigerant 207 on the cold absorbent solution 245 side of the membrane heats the absorbent solution as it weakens it. The evaporator/absorber 205 membrane material must be chosen for its insulating ability. [0067] The coefficient of performance (COP) for the non-vacuum absorption refrigeration system 201 can be derived as
[0068] COP=ηQ 21A I (5)
[0069] where ηA is the thermal efficiency of the membrane contactor (evaporator/absorber) 205, ηG is the thermal efficiency of the membrane contactor (concentrator) 203 and F is the percent of thermal energy recuperation through the absorbent solution recuperator 219 from absorbent solution 241 to absorbent solution 233.
[0070] As can be seen from (5) , in order for the non-vacuum absorption refrigeration system 201 to chill refrigerant fluid 229, the thermal efficiency of the membrane contactor (evaporator/absorber) 205 ηA must be greater than 50 percent,
[0071] ηA>5$%, (6)
[0072] If the thermal efficiency of the membrane contactor (evaporator/absorber) 205 ηA is less than 50 percent, the membrane contactor (evaporator/absorber) 205 will heat the refrigerant fluid 229 instead of cool it. The membrane materials in the membrane contactor (evaporator/absorber) 205 must satisfy (6) . [0073] The benefits of the non-vacuum absorption refrigeration machine include eliminating all metal components, and therefore corrosion due to the corrosive nature of the absorbent solution by using non-corrosive materials such as plastics that also reduce weight and size significantly. The use of membrane distillation eliminates carry-over issues by isolating the absorbent solution from the refrigerant .
[0074] One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

Claims

CLAIMS What is claimed is:
1. A non-vacuum absorption refrigeration system comprising: a membrane contactor (evaporator/absorber) for cooling a refrigerant fluid and generating a refrigerant vapor, and carrying an absorption solution for absorbing the refrigerant vapor to produce a refrigerant-absorption (weak) solution; and a membrane contactor (concentrator) for removing the refrigerant from the weak solution to provide a concentrated absorption solution for the membrane contactor (evaporator/absorber) .
2. The non-vacuum absorption refrigeration system according to claim 1 wherein non-corrosive material is used for system piping and vessel construction.
3. The non-vacuum absorption refrigeration system according to claim 2 wherein the non-corrosive material is plastics.
4. The non-vacuum absorption refrigeration system according to claim 1 wherein the membrane contactor (evaporator/absorber) and membrane contactor (concentrator) are designed and operated for membrane distillation, including direct contact membrane distillation, air gap membrane distillation, sweep gas membrane distillation, and vacuum membrane distillation.
5. The non-vacuum absorption refrigeration system according to claim 4 wherein the membrane contactors comprise microporous membranes having hydrophobic inner and outer surfaces.
6. The non-vacuum absorption refrigeration system according to claim 5 wherein the microporous membrane material on the inner and outer surfaces or throughout is selected from the group consisting of polypropylene, polyvinylidene difluoride (PVDF) , polytetrafluoroethylene (PTFE) or any other materials that have a lower surface energy than the surface tension of the refrigerant .
7. The non-vacuum absorption refrigeration system according to claim 5 wherein the microporous membranes have a wall porosity greater than 50 percent.
8. The non-vacuum absorption refrigeration system according to claim 7 wherein the microporous membranes have a pore size in the range of from about 0.1 to 0.6 micrometers .
9. The non-vacuum absorption refrigeration system according to claim 8 wherein the pore size and the hydrophobicity are such that the absorbent solution and the refrigerant do not penetrate the membrane pores .
10. The non-vacuum absorption refrigeration system according to claim 9 wherein the membrane's surface energy in the membrane contactor (evaporator/absorber) and membrane contactor (concentrator) is less than the lesser of the weak absorbent solution's surface tension or the refrigerant's surface tension.
11. The non-vacuum absorption refrigeration system according to claim 5 further comprising: an absorption solution circulating loop wherein the absorption solution circulating loop and refrigerant fluid flow through the membrane contactor (evaporator/absorber) ; a refrigerant circulating loop wherein the refrigerant circulating and absorption solution circulating loops flow through the membrane contactor (concentrator) ; and a metered refrigerant flow coupled to a refrigerant fluid flow.
12. The non-vacuum absorption refrigeration system according to claim 11 further comprising: a heater configured to heat the weak absorbent solution to a predefined temperature before entering the membrane contactor (concentrator) ; a cooler configured to cool the refrigerant to a predefined temperature before entering the membrane contactor (concentrator) ; and a cooler configured to cool the concentrated absorbent solution to a predefined temperature before entering the membrane contactor (evaporator/absorber) .
13. The non-vacuum absorption refrigeration system according to claim 12 wherein the weak absorbent solution predefined temperature determines a weak absorbent solution vapor pressure and the refrigerant predefined temperature determines a refrigerant vapor pressure.
14. The non-vacuum absorption refrigeration system according to claim 13 wherein the weak absorbent solution vapor pressure is greater than the refrigerant vapor pressure and refrigerant is vaporized from the weak absorbent solution on one side of the membrane contactor where the weak absorbent solution is circulated and the vaporized refrigerant is condensed on the other side of the membrane contactor where the refrigerant is circulated.
15. The non-vacuum absorption refrigeration system according to claim 12 wherein if the weak absorbent solution vapor pressure is less than or equal to the refrigerant vapor pressure, the weak absorbent solution heater output temperature is increased.
16. The non-vacuum absorption refrigeration system according to claim 12 wherein if the weak absorbent solution vapor pressure is less than or equal to the refrigerant vapor pressure, the refrigerant cooler cooling temperature is decreased.
17. The non-vacuum absorption refrigeration system according to claim 12 wherein the concentrated absorbent solution predefined temperature determines a concentrated absorbent solution vapor pressure and the refrigerant fluid predefined temperature determines a refrigerant fluid vapor pressure.
18. The non-vacuum absorption refrigeration system according to claim 17 wherein the concentrated absorbent solution vapor pressure is less than the refrigerant fluid vapor pressure and refrigerant is vaporized from the refrigerant fluid solution on one side of the membrane contactor where the refrigerant fluid is circulated and the vaporized refrigerant is condensed on the other side of the membrane contactor where the concentrated absorbent solution is circulated.
19. The non-vacuum absorption refrigeration system according to claim 12 wherein if the concentrated absorbent solution vapor pressure is greater than or equal to the refrigerant fluid vapor pressure, the concentrated absorbent solution cooler cooling output temperature is decreased.
20. The non-vacuum absorption refrigeration system according to claim 1 wherein the refrigerant is water.
21. The non-vacuum absorption refrigeration system according to claim 1 wherein the absorbent solution is a LiBr solution.
22. The non-vacuum absorption refrigeration system according to claim 11 wherein the refrigerant removed from the weak solution originates from the metered refrigerant flow.
23. The non-vacuum absorption refrigeration system according to claim 22 wherein the refrigerant removed from the weak solution is returned to the refrigerant circulating loop by the membrane contactor (concentrator) .
24. The non-vacuum absorption refrigeration system according to claim 1 wherein the thermal efficiency of the membrane contactor (evaporator/absorber) ηA is greater than 50 percent.
25. A method for non-vacuum absorption refrigeration comprising: adding a refrigerant to a refrigerant fluid flow; circulating a concentrated absorbent solution and the refrigerant fluid flow through a membrane contactor (evaporator/absorber) ; generating a refrigerant vapor and cooling the refrigerant fluid flow in the membrane contactor (evaporator/absorber) ; absorbing the refrigerant vapor producing a refrigerant-absorption (weak) solution; circulating the weak solution and the refrigerant through a membrane contactor (concentrator) ; generating a refrigerant vapor from the weak solution in the membrane contactor (concentrator) ; absorbing the refrigerant vapor into the refrigerant; and providing a concentrated absorbent solution and cooling the solution for the membrane contactor (evaporator/absorber) .
26. The method according to claim 25 wherein the thermal efficiency of the membrane contactor (evaporator/absorber) ηA is greater than 50 percent.
EP07852808A 2007-10-16 2007-10-16 Non-vacuum absorption refrigeration Withdrawn EP2212629A4 (en)

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KR20100080552A (en) 2010-07-08
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WO2009051583A1 (en) 2009-04-23
CN101849147A (en) 2010-09-29

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