WO2013070921A2

WO2013070921A2 - Osmotic systems for heating, cooling and refrigeration

Info

Publication number: WO2013070921A2
Application number: PCT/US2012/064159
Authority: WO
Inventors: Aaron Mandell
Original assignee: Oasys Water, Inc.
Priority date: 2011-11-10
Filing date: 2012-11-08
Publication date: 2013-05-16
Also published as: WO2013070921A3

Abstract

The invention relates to improving the efficiency and reliability of heat transfer systems. In one embodiment, the invention is directed to using an osmotic compressor in a vapor compression cycle. In another embodiment, the invention is directed to using forward osmosis to regenerate liquid desiccant in liquid desiccant air-conditioning systems.

Description

OSMOTIC SYSTEMS FOR HEATING, COOLING AND REFRIGERATION

FIELD OF THE INVENTION

[0001] The invention generally relates to using osmotically driven membrane systems to improve the efficiency and reliability of heat transfer systems. In one embodiment the invention generally relates to an osmotic compressor and more particularly to the use of forward osmosis with an engineered draw solution to provide the compression work necessary to pressurize a refrigerant. In another embodiment, the invention generally relates to using forward osmosis in liquid desiccant air conditioning systems.

BACKGROUND

[0002] The vapor-compression cycle is used in most refrigerators and air-conditioning systems, including large commercial and industrial refrigeration and cooling systems. FIG. 1 provides a schematic diagram of the components of a prior art vapor-compression refrigeration system 10. The system 10 includes an evaporator 12, a compressor 16, a condenser 14, and an expansion valve 18 circulating a refrigerant.

[0003] The thermodynamics of the system of FIG. 1 can be analyzed with the diagram as shown in FIG. 2. In this cycle, a circulating refrigerant, such as Freon^® (as available from DuPont), enters the compressor 16 as a vapor. From point 1 to point 2, the vapor is compressed at constant entropy and exits the compressor 16 as a vapor at a higher temperature, but still below the vapor pressure at that temperature. From point 2 to point 3 and on to point 4, the vapor travels through the condenser 14, which cools the vapor until it starts condensing, and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature. Between points 4 and 5, the liquid refrigerant goes through the expansion valve 18 (also called a throttle valve) where its pressure abruptly decreases, causing flash evaporation and auto-refrigeration of, typically, less than half of the liquid. That results in a mixture of liquid and vapor at a lower temperature and pressure as shown at point 5. The cold liquid- vapor mixture then travels through the evaporator coil or tubes 12 and is completely vaporized by cooling the warm air (e.g., from the space being cooled) being blown by a fan across the evaporator 12. The resulting refrigerant vapor returns to the compressor inlet at point 1 to complete the

thermodynamic cycle.

[0004] In the conventional vapor compression process, electricity is converted to mechanical work to compress a refrigerant vapor that serves as a heat pump. The mechanical pump tends to be inefficient and, because there are moving parts, can be unreliable.

[0005] Additional inefficiencies exist in heat transfer systems such as residential, commercial, and industrial air-conditioning systems. The primary energy load in these systems is the humidity control or "conditioning" stage of the cooling process. In this stage, moisture must be removed from the air first in order to both reduce the cooling load (high air-moisture content increases the cooling duty) as well as "condition" the air so as to make it comfortable for the environment (e.g., humans use evaporative cooling and if the surrounding air is humid or contains high amounts of moisture our bodies must work harder to remove heat). As such, in order to make cooling systems more efficient, it is necessary to change the way humidity is controlled.

[0006] The primary means of humidity control in current air-conditioning systems is through condensation. The air is first cooled to a low enough temperature to force the moisture to condense onto a surface and hence be removed from the air. In many systems, the then "dry" air is heated to a comfortable temperature before it is circulated throughout a building. As such, a large portion of the energy load in these systems is the requirement to lower the temperature of the air to the point at which condensation occurs and then raise it back up for comfort. SUMMARY

[0007] In one embodiment, the present invention is directed to a more efficient heat transfer system that uses vapor compression provided by an osmotic compressor. The osmotic compressor uses an osmotically driven membrane system (e.g., forward osmosis) with a recyclable engineered draw solution. While forward osmosis has been around for a long time, not until recently have commercial systems begun to emerge that utilize "engineered" working fluids that can be recovered and regenerated. See, for example, the various patents and publications in the name of McGinnis et al. incorporated herein.

[0008] The use of an osmotic compressor in accordance with the invention as an alternative means to compress a vapor overcomes the primary shortcomings of conventional mechanically- driven vapor compression systems by utilizing a solid state (no moving mechanical parts) osmotic compressor that is inherently more energy efficient and, as a result of having no moving mechanical parts, inherently more reliable. Rather than converting electrical energy to mechanical work and using that work to compress a compressible gas, which is energy inefficient, the osmotic compressor instead uses direct or forward osmosis to generate the same level (or more) of hydraulic pressure and that hydraulic pressure is used to compress a compressible gas or vapor. In this way, the compression stage is done without a mechanical pump and instead by simply passing water through a semi-permeable membrane and transferring the resulting hydraulic pressure to the compression stage of a conventional vapor compression cycle. As such, the vapor compression cycle is nearly identical with the exception of the mechanical compressor, which is now replaced by the osmotic compressor.

[0009] An osmotic compressor in accordance with the invention is similar to conventional compressors or pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. The difference is the mechanism by which the pressure is generated; an osmotic compressor uses forward osmosis to provide the compression work, as opposed to a mechanical pump. Because gases are compressible, an osmotic compressor, similar to a conventional compressor, increases the pressure of a gas by reducing its volume, in this case using forward osmosis. In a particular embodiment, the osmotic compressor pressurizes a liquid, which is relatively incompressible, and this pressure is transferred to a vaporized refrigerant.

[0010] As previously discussed, the osmotic compressor gets its energy through osmosis - the natural tendency of water to flow from a liquid of low solute concentration to a liquid of high solute concentration. The flow rate and resulting hydraulic pressure is then determined by the difference in concentrations of the two fluids - the higher the difference, the higher the flow rate and resulting pressure. The pressures achievable with an osmotic compressor in accordance with the invention are commensurate with the pressure ranges required by a conventional vapor compression cycle. The pressure generated by the direct flow of water across a semi-permeable membrane can be transferred (directly or indirectly) to the vapor stream that needs to be compressed in a cooling or refrigeration system. To regenerate the cycle, the draw solution used in the osmotic compressor is then regenerated using heat, for example, low quality or waste heat. The heat required can be generated using any number of fuels including natural gas, diesel, propane, or any other combustible hydrocarbon. The heat can also be generated electrically and will depend on the application and system scale. There are a variety of other sources of heat that are in the category of low quality heat sources that can be used as well, including industrial heat, geothermal heat or solar radiation.

[0011] In one aspect, the invention relates to a heat transfer system (e.g., an air conditioner) that includes a condenser configured for cooling and condensing a refrigerant, an expansion device (e.g., a temperature-controlled throttling valve) in fluid communication with an outlet of the condenser and configured to throttle the refrigerant introduced from the condenser, an evaporator in fluid communication with an outlet of the expansion device and configured for heating and evaporating the refrigerant, and an osmotic compressor in fluid communication with an outlet of the evaporator and an inlet of the condenser. The osmotic compressor is configured for compressing and discharging the vaporized refrigerant from the evaporator to the condenser.

[0012] In various embodiments of the invention, the osmotic compressor includes an osmotically driven membrane system and a pressure exchanger in fluid communication therewith. In one embodiment, the pressure exchanger includes a first inlet in fluid

communication with the osmotically driven membrane system for receiving a pressurized dilute draw solution therefrom, a second inlet in fluid communication with an outlet of the evaporator for receiving the vaporized refrigerant therefrom at a first temperature, and a first outlet in fluid communication with the inlet of the condenser for providing the vaporized refrigerant thereto at a second temperature. The osmotic compressor can also include a recycling system in fluid communication with a second outlet of the pressure exchanger for receiving the dilute draw solution therefrom. The recycling system can reconcentrate the dilute draw solution and provide same to the osmotically driven membrane system. In one or more embodiments, the recycling system can also provide a source of feed solution to the osmotically driven membrane system. Alternatively, a separate feed source is provided for the osmotically driven membrane system. In one or more embodiments, the osmotically driven membrane system is a forward osmosis membrane system.

[0013] The basic osmotically driven membrane systems for use as an osmotic compressor, along with the engineered draw solutions and recycling systems, can be configured as described in the detailed description below and the incorporated publications. In general, osmotically driven membrane systems involve two solutions separated by a semi-permeable membrane. One solution may be, for example, seawater, while the other solution is a concentrated solution that generates a concentration gradient between the seawater and the concentrated solution. This gradient draws water from the seawater across the membrane, which selectively permits water to pass, but not salts, into the concentrated solution. Gradually, the water entering the concentrated solution dilutes and expands the volume of the concentrated solution.

[0014] Another embodiment of the invention relates to a hybrid heat transfer system that uses a desiccant for humidity control. Desiccants are well known to be efficient absorbers of moisture. In the present invention, humidity control in an air-conditioning system is

accomplished using a liquid desiccant that is regenerated using direct or forward osmosis. This eliminates the need and inefficient energy use required for condensation and instead replaces it with a more energy efficient process.

[0015] Liquid desiccants are solutions that have a high affinity for water vapor. This property is the key to creating cooling systems that dehumidify air without over-cooling. See Andrew Lowenstein, Review of Liquid Desiccant Technology for HVAC Applications, HVAC&R Research, Vol. 14, no. 6, November 2008; the disclosure of which is hereby incorporated by reference herein in its entirety. Since the 1930's, liquid desiccants have been used in industrial dehumidifiers. The liquid desiccants used in these systems commonly are very strong solutions of the ionic salts lithium chloride and calcium chloride. These ionic salts have the attractive characteristic that the salt themselves have essentially zero vapor pressure, and so vapors of the desiccant will not appear in the air supplied by the liquid desiccant air-conditioning (LDAC). However, zero vapor pressure comes with a price: solutions of lithium and calcium chloride are very corrosive. This corrosiveness requires that all wetted parts within the LDAC be protected and that no droplets of desiccant are entrained in the supply air.

[0016] A desiccant has the ability to dry air without cooling, because it forms a relatively strong bond with water molecules (i.e., a stronger bond than that between molecules in pure liquid water). Whereas the heat released when water condenses (i.e., the latent heat of condensation) is approximately 1,000 Btu/lb, more heat— typically an additional 50 to 100 Btu/lb- -will be released when water vapor "condenses" into a liquid desiccant due to the stronger bonds between the molecules. Regenerating the liquid desiccant after it has removed and absorbed moisture is typically energy intensive and inefficient.

[0017] In the present invention, forward osmosis systems using an engineered draw solution provide an efficient way to remove moisture from a liquid desiccant (i.e., regenerate the desiccant). One of the keys to performing this function is the use of a draw solution with a higher osmotic pressure than the desiccant itself. As long as the concentration of solutes in the draw solution exceed the concentration of solutes in the desiccant, dewatering of the desiccant will occur (a semi-permeable membrane must separate both solutions). The engineered draw solution can then be regenerated using low quality or waste heat, as previously discussed.

[0018] In another aspect, the invention relates to a heat transfer system that includes an air- conditioning system, a dehumidification system coupled to the air-conditioning system and including a liquid desiccant, and a desiccant regeneration system in fluid communication with the dehumidification system. The desiccant regeneration system includes a forward osmosis membrane system configured to remove water from the liquid desiccant.

[0019] In various embodiments, the forward osmosis membrane system includes a source of an engineered draw solution. In one or more embodiments, the engineered draw solution includes ammonia and carbon dioxide in a ratio of at least 1: 1; however, other draw solutions are contemplated and considered within the scope of the invention. Additionally, the desiccant regeneration system can include a recycling system in fluid communication with the forward osmosis membrane system for reconcentrating a dilute draw solution output from the forward osmosis membrane system and returning the reconcentrated draw solution to the source of engineered draw solution.

[0020] These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description and the accompanying drawings. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention and are not intended as a definition of the limits of the invention. For purposes of clarity, not every component may be labeled in every drawing. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

[0022] FIG. 1 is a schematic representation of a prior art vapor-compression cycle;

[0023] FIG. 2 is a temperature-entropy diagram for a vapor-compression cycle;

[0024] FIG. 3 is a schematic representation of a vapor-compression cycle utilizing an osmotic compressor in accordance with one or more embodiments of the invention; and [0025] FIG. 4 is a schematic representation of an air-conditioning system using a liquid desiccant dehumidification system and a forward osmosis system for regenerating the liquid desiccant in accordance with one or more embodiments of the invention.

DETAILED DESCRIPTION

[0026] FIG. 3 depicts an exemplary heat transfer system 100 using an osmotic compressor 110. As shown, the system 100 also includes an evaporator 102, a condenser 104, and an expansion valve (or similar throttling device) 106 as found in a conventional vapor-compression refrigeration circuit and which operate in a similar fashion. The osmotic compressor 110 includes an osmotically driven membrane system 120, for example, a forward osmosis module. In one or more embodiments, the membrane system 120 generally includes a first chamber 121 for receiving a feed or working solution, a second chamber 123 for receiving an engineered draw solution, and a semi-permeable membrane 122 separating the two chambers, although other configurations are contemplated and considered within the scope of the invention. Various configurations of membrane systems are described in U.S. Patent Nos. 6,391,205 and 7,560,029; U.S. Patent Publication Nos. 2010/0024423, 2010/0183903, 2011/0203994, 2012/0267306, and 2012/0267307; and PCT Publication No. WO2011/059751; the disclosures of which are hereby incorporated by reference herein in their entireties.

[0027] The membrane system 120 is in fluid communication with a feed/working solution source or stream 112 and an engineered draw solution source or stream 114. The draw solution source 114 includes an osmotic agent to dewater the feed source 112 by osmosis through the forward osmosis membrane 122 within the membrane system 120. In one or more embodiments, the feed solution 112 is deionized water and the engineered draw solution is an ammonia-carbon dioxide solution. Examples of engineered draw solutions are described in the patents and applications incorporated above.

[0028] The osmotic compressor 110 further includes a pressure exchanger or energy recovery device 124 that can take a variety of configurations. In one embodiment, the pressure exchanger 124 is a simple diaphragm type device that directly transfers the hydraulic pressure generated in the second chamber 123 of the membrane system 120 to the vaporized refrigerant within the system 100. In other embodiments, the pressure exchanger can be an isobaric energy recovery device, such as those available from Energy Recovery, Inc. of San Leandro, CA. The specific type of pressure exchanger, and its corresponding operating characteristics, will be selected to suit a particular application (e.g., capacity, material compatibility, etc.).

[0029] The general operation of the osmotic compressor is as follows: the working fluid 112 is introduced to a first side of the semi-permeable membrane 122, the engineered draw solution 114 is introduced to a second side of the semi-permeable membrane 122 causing a portion of the working fluid to flow through the semi-permeable membrane into the draw solution 114 to create a water flux that expands the volume of the draw solution. In one embodiment, the expanded volume of the draw solution results in an increased hydrostatic pressure within the second chamber 123, which can induce the flow of the pressurized draw solution. This increased pressure is used to pressurize the vaporized refrigerant. In an alternative embodiment, the means for transferring hydraulic pressure from the second chamber to the refrigerant can be integral with the second chamber 123 of the membrane system 120.

[0030] In the embodiment depicted in FIG. 3, the pressurized (dilute) draw solution 116 is directed to a first inlet 124a of the pressure exchanger 124. The pressure exchanger 124 includes a second inlet 124b that receives the vaporized refrigerant from the evaporator 102 at a first temperature (TO. The dilute draw solution is depressurized in the pressure exchanger 124 and the vaporized refrigerant is compressed thereby. In the embodiment shown in FIG. 3, the pressure exchanger 124 includes a first outlet 124c at which the compressed refrigerant exits at a second temperature (T₂) and is directed to the condenser 104. The pressure exchanger 124 can further include a second outlet 124d at which the depressurized dilute draw solution exits. One or more pressure exchangers can be used to suit a particular application.

[0031] The osmotic compressor 110 can also include a draw solution recycling or recovery system 126 for receiving the depressurized dilute draw solution and separating the draw solutes from the draw solution, thereby producing new concentrated draw solution 114' for reuse in the osmotic compressor 110. In one or more embodiments, the osmotic compressor 110 is a self- contained unit, where the dilute draw solution is separated into a concentrated draw solution 114' and a nearly deionized working solution 112' via the recovery system 126 for reuse as the draw solution source 114 and the feed solution source 112. Generally, the recovery system 126 will utilize waste or low-grade heat to drive the recovery process. Alternatively, electric power may be utilized to drive the process. Examples of draw solution recovery systems are described in U.S. Patent Publication Nos. 2009/0297431 and 2012/0067819, the disclosures of which are hereby incorporated by reference herein in their entireties. In addition, the membrane system 120 outputs a stream of concentrated solution 118 from the feed stream 112 that can be further processed or recombined with the recovered working solution 112' for reuse in the osmotic compressor 110.

[0032] FIG. 4 depicts a hybrid heat transfer system 200 that includes a liquid desiccant air- conditioning unit 208 and a liquid desiccant regeneration system 210. The LDAC 208 includes an air-conditioning unit 230, such as those known in the art, and a liquid desiccant

dehumidification system 232. As previously discussed, LDAC systems typically require heating the liquid desiccant to a high temperature to regenerate the desiccant, which is energy intensive and inefficient. In addition, there are issues with circulating typically highly corrosive liquid desiccants within the air-conditioning system.

[0033] The desiccant regeneration system 210 of the present invention eliminates the aforementioned problems by utilizing forward osmosis to regenerate the desiccant. The liquid desiccant is circulated to the feed side of the forward osmosis unit, which can be made of compatible materials, and does not require heating to remove moisture. An engineered draw solution, including an osmotic agent, dewaters the liquid desiccant by osmosis through a forward osmosis membrane.

[0034] Specifically, the regeneration system 210 includes an osmotically driven membrane system 220, for example a forward osmosis unit, and a draw solution recovery system 226. In one or more embodiments, the membrane system 220 generally includes a first chamber 221 for receiving a liquid desiccant with a high moisture content 212 from the LDAC 208, a second chamber 223 for receiving an engineered draw solution 214, and a semi-permeable membrane 222 separating the two chambers. As previously discussed, various configurations of membrane systems are described in the patents and applications incorporated herein. Examples of engineered draw solutions are also described in the incorporated patents and applications.

[0035] The draw solution 214 includes an osmotic agent to dewater the liquid desiccant 212 by osmosis through the forward osmosis membrane 222 within the membrane system 220. The dewatered or regenerated liquid desiccant 218 exits the membrane system 220 and is then returned to the dehumidification unit 232 of the LDAC 208. The now dilute draw solution 216 exits the membrane system 220 and is directed to the recovery system 226 to reconcentrate the draw solution for reuse within the regeneration system 210. The recovery system 226 can operate as previously described herein and as described in the incorporated applications. The recovery system 226 also outputs the excess water 228 that was removed from the liquid desiccant. In some embodiments, this water 228 may be directed for further processing, used as potable water, or otherwise disposed of.

[0036] The size and arrangement of the various components of the osmotically driven membrane systems (e.g., osmotic compressor and osmotic regenerator) disclosed herein will be selected to suit a particular application (e.g., material compatibility, flux rates, etc.). In addition, the various systems disclosed herein may include a controller for monitoring, adjusting, or otherwise regulating one or more operating parameter or the overall operation of the various systems.

[0037] Having now described some illustrative embodiments of the invention, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by way of example only. Numerous modifications and other

embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention. In particular, although some of the examples presented herein involve specific combinations of method steps or system elements, it should be understood that those steps and those elements may be combined in other ways to accomplish the same objectives.

[0038] Furthermore, those skilled in the art should appreciate that the parameters and configurations described herein are exemplary and that actual parameters and/or configurations will depend on the specific application in which the systems and techniques of the invention are used. Those skilled in the art should also recognize or be able to ascertain, using no more than routine experimentation, equivalents to the specific embodiments of the invention. It is, therefore, to be understood that the embodiments described herein are presented by way of example only and that, within the scope of any appended claims and equivalents thereto; the invention may be practiced otherwise than as specifically described.

[0039] What is claimed is:

Claims

1. A heat transfer system comprising:

a condenser configured for cooling and condensing a refrigerant;

an expansion device in fluid communication with an outlet of the condenser and configured to throttle the refrigerant received from the condenser;

an evaporator in fluid communication with an outlet of the expansion device and configured for heating and evaporating the refrigerant; and

an osmotic compressor in fluid communication with an outlet of the evaporator and an inlet of the condenser and configured for compressing and discharging the vaporized refrigerant from the evaporator to the condenser.

2. The heat transfer apparatus of claim 1, wherein the osmotic compressor comprises:

an osmotically driven membrane system; and

a pressure exchanger comprising:

a first inlet in fluid communication with the osmotically driven membrane system for receiving a pressurized dilute draw solution therefrom;

a second inlet in fluid communication with an outlet of the evaporator for receiving vaporized refrigerant therefrom at a first temperature; and

a first outlet in fluid communication with the inlet of the condenser for providing vaporized refrigerant thereto at a second temperature.

3. The heat transfer apparatus of claim 2, wherein the osmotic compressor further comprises a recycling system in fluid communication with a second outlet of the pressure exchanger for receiving the dilute draw solution therefrom and configured for reconcentrating the dilute draw solution and providing a concentrated draw solution to the osmotically driven membrane system.

4. The heat transfer apparatus of claim 3, wherein the recycling system is further configured to provide a source of feed solution to the osmotically driven membrane system.

5. The heat transfer system of claim 2, wherein the osmotically driven membrane system is a forward osmosis membrane system.

6. A heat transfer system comprising:

an air-conditioning system;

a dehumidification system coupled to the air-conditioning system and comprising a liquid desiccant; and

a desiccant regeneration system in fluid communication with the dehumidification system and comprising a forward osmosis membrane system configured to remove water from the liquid desiccant.

7. The heat transfer system of claim 6, wherein the forward osmosis membrane system comprises a source of an engineered draw solution.

8. The heat transfer system of claim 7, wherein the engineered draw solution comprises ammonia and carbon dioxide in a ratio of at least 1: 1.

9. The heat transfer system of claim 7, wherein the desiccant regeneration system further comprises a recycling system in fluid communication with the forward osmosis membrane system for reconcentrating a dilute draw solution output from the forward osmosis membrane system and returning the reconcentrated draw solution to the source of engineered draw solution.