Abstract:
A carbon dioxide/co-fluid mixture is provided for use in a refrigeration cycle in which the carbon dioxide is alternately absorbed and desorbed from the co-fluid. Suitable co-fluids are selected from the class of alkoxylated carboxylic amides, wherein the amides are cyclic or non-cyclic. It has been discovered that N-2,5,8,11-tetraoxadodecyl-2-pyrrolidinone and its homologs exhibit an advantageous property of a high rate of desorption at lower temperatures.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/329,535, filed on Apr. 29, 2016, and U.S. Provisional Application No. 62/329,586, filed on Apr. 29, 2016. The entire disclosures of the applications referenced above are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to co-fluids for use with carbon dioxide refrigerant in heating, ventilation, air conditioning and refrigeration (HVAC&amp;R) systems. 
       BACKGROUND 
       [0003]    This section provides background information related to the present disclosure which is not necessarily prior art. 
         [0004]    Because carbon dioxide (R744) has a low global warming potential (GWP) of only 1 and no ozone-depleting potential at all (ODP of zero), it makes an excellent environmentally friendly refrigerant as compared to hydrofluorocarbons, hydrofluoroolefins, and other less environmentally sound refrigerants. However, the pressures required to liquefy carbon dioxide prove to be too high for use in conventional heating and cooling systems. To avoid high pressures in a refrigeration cycle, carbon dioxide can be used along with a so-called co-fluid or mixture of co-fluids. 
         [0005]    In operation of an HVAC&amp;R system using carbon dioxide and a co-fluid, carbon dioxide refrigerant is absorbed into and desorbed out of the co-fluid. For example, carbon dioxide is absorbed and the pressure lowered during compression and flow through a condenser or absorber. Subsequent flow through an expansion device and evaporator requires a desirable release (desorption) of a portion of the carbon dioxide refrigerant. 
         [0006]    It has generally been observed that rates of absorption tend to be faster than rates of desorption in co-fluid systems using carbon dioxide as refrigerant. This rate inequality can potentially lead to problems in operating the heating and cooling system. There may not be enough time for proper heat flow to the evaporator needed for cooling. And there could be an accumulation of carbon dioxide in the co-fluid due to the rate difference, causing the system to be inefficient or even inoperable. There is a continuing need for co-fluids that provide a higher rate of desorption to improve operation in cooling systems. 
       SUMMARY 
       [0007]    This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
         [0008]    A carbon dioxide/co-fluid mixture is provided for use in a refrigeration cycle in which the carbon dioxide is alternately absorbed and desorbed from the co-fluid. The mixture includes from 50% to 99% by weight co-fluid and 1% to 50% by weight carbon dioxide. Suitable co-fluids are selected from the class of alkoxylated carboxylic amides, wherein the amides are cyclic or non-cyclic. It has been discovered that N-2,5,8,11-tetraoxadodecyl-2-pyrrolidinone and its homologs exhibit an advantageous property of a high rate of desorption at lower temperatures. 
         [0009]    Pumps or compressors containing the co-fluid as a lubricant are provided for use in a system that includes in sequence a compressor, an absorber (or resorber), an expansion device (or expander), and a desorber. A method of operating a refrigeration system involves circulating the co-fluid and refrigerant around such a system. 
         [0010]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0011]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0012]      FIG. 1  is a schematic representation of a climate-control system according to the principles of the present disclosure; 
           [0013]      FIG. 2  is a schematic representation of an exemplary desorber that can be incorporated into the system of  FIG. 1 ; 
           [0014]      FIG. 3  is a schematic representation of another climate-control system according to the principles of the present disclosure; 
           [0015]      FIG. 4  is a schematic representation of yet another climate-control system according to the principles of the present disclosure; 
           [0016]      FIG. 5  is a schematic representation of a generator that can be incorporated into the system of  FIG. 4 ; 
           [0017]      FIG. 6  compares desorption rates among lubricants; 
           [0018]      FIGS. 7-11  show comparative desorption rates of co-fluids; 
           [0019]      FIG. 12  illustrates carbon dioxide desorption of co-fluids; and 
           [0020]      FIG. 13  compares desorption rates to those of co-fluids. 
       
    
    
       [0021]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0022]    Example embodiments will now be described more fully with reference to the accompanying drawings. 
         [0023]    Co-fluids are provided for use in co-fluid systems where carbon dioxide is used as refrigerant. The co-fluid is an absorbent capable of absorbing and desorbing the carbon dioxide refrigerant. Use of the co-fluids eliminates the need for high system pressures otherwise required to change the phase of the refrigerant carbon dioxide. 
         [0024]    Co-fluids are selected from those with the following generic formulae: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    where m is 1 to 10; R is alkyl, alkenyl, or aryl with 1 to 26 carbon atoms; R′ is H or optionally substituted C 1-6  alkyl; R″ is H, methyl, or ethyl; R′″ is H, methyl, or ethyl; and at least one of R″ and R′″ is H; 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    where m is 1 to 10; p is 1 to 3; R is alkyl, alkenyl, or aryl with 1 to 26 carbon atoms; R′ is H or optionally substituted C 1-6  alkyl; R″ is H, methyl, or ethyl; R′″ is H, methyl, or ethyl; and at least one of R″ and R′″ is H; 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    where x is 1 to 4; n is 1 to 10; R′ is H or optionally substituted C 1-6  alkyl-; R″ is H, methyl, or ethyl; R′″ is H, methyl, or ethyl; and at least one of R″ and R′″ is H; 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    where y is 1 to 4; n is 1 to 10; p is 1 to 3; R′ is H or optionally substituted C 1-6  alkyl; R″ is H, methyl, or ethyl; R′″ is H, methyl, or ethyl; and at least one of R″ and R′″ is H; 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    where z is 1 to 4; n is 1 to 10; R′ is H or optionally substituted C 1-6  alkyl; R″ is H, methyl, or ethyl; R′″ is H, methyl, or ethyl; and at least one of R″ and R′″ is H; 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    where z is 1 to 4; n is 1 to 10; p is 1 to 3; R′ is H or optionally substituted C 1-6  alkyl; R″ is H, methyl, or ethyl; R′″ is H, methyl, or ethyl; and at least one of R″ and R′″ is H. 
         [0025]    Although the invention is not to be limited by any scientific hypothesis or theory of operation, the compounds of formulae (I)-(VI) share chemical features that are believed to contribute to their general usefulness as co-fluids for use with carbon dioxide refrigerant. It is believed that the carboxylic amide (cyclic or open chain) and the polyoxyalkylene moiety combine to provide compositions that desorb carbon dioxide at a high rate, a rate that is higher than homologous compounds without those features, even though the homologous compounds are considered part of the described invention to the extent they have not yet been disclosed as co-fluids. Generally speaking, species with high desorption rates are preferred as co-fluids in carbon dioxide refrigeration systems, because of the operational advantages expected to flow from having high desorption. 
         [0026]    The compounds of formulae (I)-(VI) are characterized by a “side chain” that has a polyoxyalkylene structure denoted by the repeat units of m or n in the formulae. If both of R″ and R′″ are hydrogen (H), the chain is polyoxyethylene; if one of them is methyl (the other being H), the chain is polyoxypropylene; if one of them is ethyl, the chain is polyoxybutylene. Because the repeat units m and n range from 1 to 10, it is also possible to provide so-called heteric polyoxyalkylene chains containing a combination of polyoxyethylene, polyoxypropylene, and polyoxybutylene. That is to say, the formulae should be interpreted as permitting up to 10 repeat units, where each repeat unit is independently based on ethylene-, propylene-, or butylene oxide. 
         [0027]    The non-cyclic amide “alkoxylates” of formulae (I) and (II) are based on carboxylic amides with at least two and up to 27 carbon atoms (since R has 1 to 26 carbon atoms). The nature of the R group (size, level of branching, presence or not of unsaturation) is expected to affect the equivalent weight of and the viscosity of the co-fluid. These are design factors than can be taken into account. 
         [0028]    In all formulae, the terminal hydroxyl of the polyoxyalkylene chain is in the alternative capped with an alkyl group (preferably methyl for ease of synthesis) that is optionally substituted. Although part of the invention, the hydroxyl compounds (R′ ═H) are less preferred in some embodiments because the hydroxyl could contribute to undesirable reactivity, high viscosity, or even corrosion. Capping takes the hydroxyl group out of play. Substitutions on R′ are allowed to the extent they do not spoil the operation of the compound as a co-fluid. In a particular embodiment, the alkyl group R′ is substituted with a carboxylic amide group as shown in the description below and in the examples. Thus, R′ in any of the above can be C 1-6  alkyl substituted with alkylcarbamido or alkenylcarbamido, represented by the following structures where R is alkyl or alkenyl: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0029]    The compounds of formulae (I)-(VI) are formally alkoxylates of the carboxylic amide or -imide shown. The compounds of (I), (III), and (V) can be synthesized by direct alkoxylation of the amide/imide starting group, because the amide/imide group is reactive and can open the oxirane ring of the corresponding alkylene oxides. The compounds of (II), (IV), and (VI) on the other hand, can be made by alkoxylating the free hydroxyl group of a starting material that contains an alkylol moiety attached to the amide or imide. Depending on whether p in the formulae is 1, 2, or 3, the group is a methylol, ethylol, or propylol group. 
         [0030]    Compounds with n (or m) from 1 to 10 can be made by reacting the starting material with n or m equivalents of oxide and reacting to a polydisperse mixture containing an average of n oxide units per amide group. Alternatively, they can be synthesized with a goal of producing a molecular weight distribution where the peak is at a species with n oxide units. Various fractions can be physically separated to provide other distributions of alkoxylation. 
         [0031]    But especially for the lower molecular weight compounds, it can be simpler not to form the compounds by alkoxylation, but instead by reacting a pre-formed monodisperse compound containing n repeat units with the reactive amide nitrogen (or with the hydroxyl of an alkylol group added to the amide, for example by reaction with formaldehyde). This is illustrated by reacting a starting material N-methylolpyrrolidone (N-hydroxymethyl-2-pyrrolidone) with triethylene glycol monomethyl ether in a conventional Williamson ether synthesis. 
         [0032]    In various embodiments, the co-fluids of formulae (I)-(VI) are further characterized by one or any combination of the following: the parameters x and y have a value of either 2 or 4; the parameter z has a value of 2 (meaning the structure is based on a succinimide derivative); the variables n and m are 1 to 4; R′ is methyl; R″ and R′″ are both H; R′ is C 1-3  alkyl substituted with alkylcarbamido. Particular embodiments include the following: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0033]    In operation, the co-fluid acts as lubricant as well a carrier fluid for the refrigerant carbon dioxide. A compressor for use in the cooling circuits described herein contains any of the described co-fluids as a lubricant. 
         [0034]    In operation, the co-fluids absorb (resorb) and desorb refrigerant carbon dioxide as they circulate around a refrigeration or cooling circuit. At various points in the circuit, a cooling composition comprises from 50% to 99% by weight co-fluid and 1% to 50% by weight carbon dioxide. 
         [0035]    Preferred co-fluids have the chemical structures disclosed herein. In various embodiments, performance also relies on a co-fluid having advantageous physical properties as well. Naturally, preferred co-fluids readily absorb and desorb carbon dioxide used as refrigerant. An instantaneous rate (rate essentially at time zero) as well as amount desorbed at 1 minute and at 2 minutes are measured. The results can be used to screen potential candidates. 
         [0036]    The co-fluid also needs to have suitable viscosity. In various embodiments, viscosity is in the range of 1 to 50 centistokes (cSt); 1 to 20 cSt; 3 to 20 cSt; 5 to 20 cSt; 1 to 10 cSt; 3 to 10 cSt. Some good candidates have a viscosity of fairly close to 10 cSt. The viscosity is advantageously in a range of 5 to 15 CSt, 8 to 12 cSt, or 9 to 11 cSt, in various embodiments. Too high a viscosity and fluid flow around the cooling circuit can be impeded. If the viscosity is too low, there could be leakage past seals in the system. A non-limiting illustration of use of the co-fluids follows. 
       Use of the Co Fluids in Refrigeration Methods 
       [0037]    A representative refrigeration cycle based on carbon dioxide as refrigerant (“vapor”) operates as follows. A combination of vapor and liquid (co-fluid) is compressed in a compressor, raising the pressure and forcing some of the vapor into the liquid phase. Heat is rejected in a resorber (absorber) downstream of the compressor. This cools the mixture and causes more of the vapor to be absorbed. The remaining CO 2  vapor and co-fluid are further cooled in an internal heat exchanger. The cool, fully liquefied mixture is then passed through an expansion device, decreasing the pressure, dropping the temperature further, and releasing some of the CO 2  into the vapor phase. Heat is extracted from the refrigerated space into a desorber as the temperature of the mixture rises and further CO 2  escapes from the liquid phase. Finally, the fluids are further warmed in an internal heat exchanger, completing the cycle. 
       Binary-Cycle Climate-Control System 
       [0038]    With reference to  FIG. 1 , a binary-cycle climate-control system  10  is provided that may include a compressor  12 , a liquid-vapor separator  13 , an agitation vessel (e.g., a stirring and/or shaking vessel)  15 , an absorber (or resorber)  14 , an internal heat exchanger  16 , an expansion device  18 , and a desorber  20 . The compressor  12  can be any suitable type of compressor, such as a scroll, rotary or reciprocating compressor, for example. The compressor  12  may include a shell  22 , a compression mechanism  24  disposed within the shell  22 , and a motor  26  (e.g., a fixed-speed or variable-speed motor) that drives the compression mechanism  24  via a crankshaft  28 . The compressor  12  can be a fixed-capacity or variable-capacity compressor. The compressor  12  may compress a mixture of a refrigerant (e.g., carbon dioxide, hydrofluorocarbons, ammonia, bromide, etc.) and a co-fluid (e.g., oil, water, polyalkylene glycol, polyol ester, polyvinyl ether, etc.) and circulate the mixture throughout the system  10 . The co-fluid may be an absorbent capable of absorbing a refrigerant. Compressing the mixture of refrigerant and co-fluid raises the pressure and temperature of the mixture and causes some refrigerant to be absorbed into the co-fluid. 
         [0039]    The liquid-vapor separator  13  may include an inlet  17 , a first outlet (e.g., a gas outlet)  19 , and a second outlet (e.g., a liquid outlet)  21 . The inlet  17  may be fluidly coupled with an outlet  34  of the compressor  12  such that the liquid-vapor separator  13  receives the compressed mixture of refrigerant and co-fluid (e.g., the compressed mixture of refrigerant vapor and liquid co-fluid containing some dissolved refrigerant gas) from the compressor  12 . The liquid co-fluid (which may contain some dissolved refrigerant gas) may settle to the bottom of the liquid-vapor separator  13 , and the undissolved refrigerant vapor may remain at the top (or rise to the top) of the liquid-vapor separator  13  (i.e., above the surface of the liquid co-fluid). The liquid co-fluid may exit the liquid-vapor separator  13  through the second outlet  21  (which may be located below the surface of the liquid in the separator  13 ), and the refrigerant vapor may exit the liquid-vapor separator  13  through the first outlet  19  (which may be located above the surface of the liquid in the separator  13 ). 
         [0040]    The agitation vessel  15  may include a first inlet  23 , a second inlet  25 , a first outlet  27 , a second outlet  29 , and an agitator  31 . The first inlet  23  may be disposed at or generally near a top end of the vessel  15  and may be fluidly coupled with the second outlet  21  of the separator  13  such that liquid co-fluid from the separator  13  enters the vessel  15  through the first inlet  23 . The liquid co-fluid entering the separator  13  through the first inlet  23  may fall to the bottom of the vessel  15 . The second inlet  25  may be below the surface of the liquid co-fluid in the vessel  15  and may be fluidly coupled with the first outlet  19  of the separator  13  such that refrigerant vapor from the separator  13  enters the vessel  15  through the second inlet  25 . In this manner, the refrigerant vapor enters the vessel  15  below the surface of the liquid co-fluid, which causes some of the refrigerant vapor entering the vessel  15  to be absorbed (or dissolved) into the liquid co-fluid. 
         [0041]    The agitator  31  can be or include an impeller (e.g., one or rotating paddles or blades) and/or a shaker, for example, disposed below the surface of the liquid co-fluid in the vessel  15 . The agitator  31  may be driven by a motor  33  and may stir or agitate the liquid co-fluid in the vessel  15  to further promote absorption of the refrigerant vapor into the liquid co-fluid. 
         [0042]    The first outlet  27  of the vessel  15  may be disposed below the surface of the liquid co-fluid such that refrigerant vapor exits the vessel  15  through the first outlet  27 . The second outlet  29  of the vessel  15  may be disposed above the surface of the liquid co-fluid such that liquid co-fluid (with refrigerant vapor dissolved therein) exits the vessel  15  through the second outlet  29 . The first and second outlets  27 ,  29  may both be in communication with a conduit  35  such that the liquid co-fluid from the first outlet  27  and refrigerant vapor from the second outlet  29  are combined and mix with each other (further promoting absorption of the refrigerant vapor into the liquid co-fluid) in the conduit  35 . 
         [0043]    The absorber  14  may be a heat exchanger that may be fluidly coupled with the conduit  35  and may receive the compressed mixture of the refrigerant and co-fluid from the conduit  35 . In configurations of the system  10  that do not include the separator  13  and vessel  15 , the absorber  14  may receive the compressed mixture of the refrigerant and co-fluid directly from the compressor  12 . Within the absorber  14 , heat from the mixture of the refrigerant and co-fluid may be rejected to air or water for example, or some other medium. In the particular configuration shown in  FIG. 1 , a fan  36  may force air across the absorber  14  to cool the mixture of the refrigerant and co-fluid within the absorber  14 . As the mixture of the refrigerant and co-fluid cools within the absorber  14 , more refrigerant is absorbed into the co-fluid. 
         [0044]    The internal heat exchanger  16  may include a first coil  38  and a second coil  40 . The first and second coils  38 ,  40  are in a heat transfer relationship with each other. The first coil  38  may be fluidly coupled with the outlet  32  of the absorber  14  such that the mixture of the refrigerant and co-fluid may flow from the outlet  32  of the absorber  14  to the first coil  38 . Heat from the mixture of the refrigerant and co-fluid flowing through the first coil  38  may be transferred to the mixture of the refrigerant and co-fluid flowing through the second coil  40 . More refrigerant may be absorbed into the co-fluid as the mixture flows through the first coil  38 . 
         [0045]    The expansion device  18  may be an expansion valve (e.g., a thermal expansion valve or an electronic expansion valve) or a capillary tube, for example. The expansion device  18  may be in fluid communication with the first coil  38  and the desorber  20 . That is, the expansion device  18  may receive the mixture of the refrigerant and co-fluid that has exited downstream of the first coil  38  and upstream of the desorber  20 . As the mixture of the refrigerant and co-fluid flows through the expansion device  18 , the temperature and pressure of the mixture decreases. 
         [0046]    The desorber  20  may be a heat exchanger that receives the mixture of the refrigerant and co-fluid from the expansion device  18 . Within the desorber  20 , the mixture of the refrigerant and co-fluid may absorb heat from air or water, for example. In the particular configuration shown in  FIG. 1 , a fan  42  may force air from a space (i.e., a room or space to be cooled by the system  10 ) across the desorber  20  to cool the air. As the mixture of the refrigerant and co-fluid is heated within the desorber  20 , refrigerant is desorbed from the co-fluid. From an outlet  53  of the desorber  20 , the mixture of refrigerant and co-fluid may flow through the second coil  40  and back to the compressor  12  to complete the cycle. 
         [0047]    One or more ultrasonic transducers (i.e., vibration transducers)  44  may be attached to the desorber  20 . As shown in  FIG. 1 , the ultrasonic transducers  44  may be mounted to an exterior surface  46  of the desorber  20 . In some configurations, the ultrasonic transducers  44  are disposed inside of the desorber  20  and in contact with the mixture of refrigerant and co-fluid (as shown in  FIG. 2 ). The ultrasonic transducers  44  can be any suitable type of transducer that produces vibrations (e.g., ultrasonic vibrations) in response to receipt of electrical current. For example, the ultrasonic transducers  44  could be piezoelectric transducers, capacitive transducers, or magnetorestrictive transducers. For example, the ultrasonic transducers  44  may have an output frequency in the range of about 20-150 kHz (kilohertz). The ultrasonic transducers  44  may (directly or indirectly) apply or transmit vibration to the mixture of refrigerant and co-fluid flowing through the desorber  20  to increase a rate of desorption of the refrigerant from the co-fluid. 
         [0048]    The ultrasonic transducers  44  can have any suitable shape or design. For example, the ultrasonic transducers  44  may have a long and narrow shape, a flat disc shape, etc., and can be flexible or rigid. In configurations in which the ultrasonic transducers  44  are mounted to the exterior surface  46  of the desorber  20 , it may be beneficial for the desorber  20  to have a minimal wall thickness at the location at which the ultrasonic transducers  44  are mounted in order to minimize attenuation of the ultrasonic vibration. Furthermore, it may be beneficial to apply the ultrasonic vibration to the mixture of the refrigerant and co-fluid at a location at which the mixture of the refrigerant and co-fluid is static or at a location of reduced or minimal flow rate of the mixture of the refrigerant and co-fluid, because fluids flowing at high rates can be more difficult to excite with ultrasonic energy. 
         [0049]    A control module (or controller)  48  may be in communication (e.g., wired or wireless communication) with the ultrasonic transducers  44  and may control operation of the ultrasonic transducers  44 . The control module  48  can control the frequency and amplitude of electrical current supplied to the ultrasonic transducers  44  (e.g., electrical current supplied to the ultrasonic transducers  44  by a battery and/or other electrical power source) to control the frequency and amplitude of the vibration that the ultrasonic transducers  44  produce. The control module  48  may also be in communication with and control operation of the motor  26  of the compressor  12 , the expansion device  18 , the motor  33  of the agitator  31 , the fans  36 ,  42 , and/or other components or subsystems. 
         [0050]    As described above, applying ultrasonic vibration to the mixture of refrigerant and co-fluid increases the desorption rate. The control module  48  may control operation of the ultrasonic transducers  44  to control the desorption rate. For example, the control module  48  may control the frequency, amplitude, runtime (e.g., pulse-width-modulation cycle time), etc. of the motor  33 , fans  36 ,  42 , and/or the ultrasonic transducers  44  such that the desorption rate matches or nearly matches a rate of absorption of the refrigerant into the co-fluid that occurs upstream of the expansion device  18  (e.g., in the absorber  14  and vessel  15 ). 
         [0051]    Without any excitation of the mixture of refrigeration and co-fluid, the absorption rate may be substantially greater than the desorption rate. The absorption rate may vary depending on a variety of operating parameters of the system  10  (e.g., pressure, compressor capacity, fan speed, thermal load on the system  10 , type of refrigerant, type of co-fluid, etc.). In some configurations, a first sensor  50  and a second sensor  52  may be in communication with the control module  48  and may measure parameters that are indicative of absorption rate and desorption rate. For example, the first sensor  50  can be a pressure or temperature sensor that measures the pressure or temperature of the mixture of refrigerant and co-fluid within the absorber  14 , and the second sensor  52  can be a pressure or temperature sensor that measures the pressure or temperature of the mixture of refrigerant and co-fluid within the desorber  20 . The pressures and/or temperatures measured by the sensors  50 ,  52  may be indicative of absorption rate and desorption rate. 
         [0052]    The sensors  50 ,  52  may communicate the pressure or temperature data to the control module  48 , and the control module  48  may determine a concentration of refrigerant in the co-fluid based on the pressure or temperature data (e.g., using a lookup table or equations). The control module  48  can include an internal clock (or be in communication with an external clock) and can determine the absorption rate and desorption rate based on changes in the concentration of refrigerant in the co-fluid over a period of time. The control module  48  may control operation of the ultrasonic transducers  44  based on the absorption rate and/or the desorption rate. The control module  48  may also control operation of the compressor  12 , the fans  36 ,  42  and/or the expansion device  18  based on the absorption and/or desorption rates and/or to control the absorption and/or desorption rates. In some configurations, the control module  48  may control the ultrasonic transducers  44  based on data from additional or alternative sensors and/or additional or alternative operating parameters. 
         [0053]    Because the absorption rate of many refrigerants into many co-fluids is significantly faster than the desorption rate, the rate of desorption may substantially limit the capacity of the system  10 . Applying ultrasonic energy (e.g., via the ultrasonic transducers  44 ) to the mixture of refrigerant and co-fluid unexpectedly solves the problem of slow desorption rates. It can be shown that desorption rates may increase by about 100%-900% (depending on the refrigerant type and co-fluid type) by exciting the mixture of refrigerant and co-fluid with ultrasonic energy (e.g., using one or more ultrasonic transducers  44 ) as compared to stirring the mixture with a propeller at 400 revolutions per minute. This increase in the desorption rate surpassed reasonable expectations of success. 
         [0054]    Referring now to  FIG. 3 , another binary-cycle climate-control system  100  is provided that may include a compressor  112 , a pump  111 , a liquid-vapor separator  113 , an agitation vessel (e.g., a stirring and/or shaking vessel)  115 , an absorber  114 , an internal heat exchanger  116 , an expansion device  118 , a desorber  120 , a receiver  121 , one or more ultrasonic transducers  144  and a control module  148 . The structure and function of the compressor  112 , liquid-vapor separator  113 , agitation vessel  115 , absorber  114 , internal heat exchanger  116 , expansion device  118 , desorber  120 , ultrasonic transducers  144  and control module  148  may be similar or identical to that of the compressor  12 , liquid-vapor separator  13 , agitation vessel  15 , absorber  14 , internal heat exchanger  16 , expansion device  18 , desorber  20 , ultrasonic transducers  44  and control module  48  described above (apart from any exceptions described below). Therefore, similar features may not be described again in detail. 
         [0055]    The receiver  121  may be fluidly coupled with the internal heat exchanger  116  (e.g., a second coil  140  of the internal heat exchanger  116 ), the compressor  112 , and the pump  111 . The receiver  121  may include an inlet  154 , a refrigerant outlet  156 , and a co-fluid outlet  158 . The inlet  154  may receive the mixture of refrigerant and co-fluid from the second coil  140 . Inside of the receiver  121 , gaseous refrigerant may be separated from liquid co-fluid. That is, the co-fluid accumulates in a lower portion  162  of the receiver  121 , and the refrigerant may accumulate in an upper portion  160  of the receiver  121 . The refrigerant may exit the receiver  121  through the refrigerant outlet  156 , and the co-fluid may exit the receiver  121  through the co-fluid outlet  158 . The refrigerant outlet  156  may be fluidly coupled with a suction fitting  164  of the compressor  112  such that refrigerant is drawn into the compressor  112  for compression therein. The co-fluid outlet  158  may be fluidly coupled with an inlet  166  of the pump  111  so that the co-fluid is drawn into the pump  111 . Outlets  168 ,  170  of the compressor  112  and pump  111 , respectively, are fluidly coupled with an inlet  117  of the separator  113  via a conduit  172  or with an inlet of the absorber  114  such that refrigerant discharged from the compressor  112  and co-fluid discharged from the pump  111  can be recombine in the vessel  115 , in the absorber  114  and/or in the conduit  172  that feeds the separator  113  or the absorber  114 . 
         [0056]    With reference to  FIG. 4 , an absorption-cycle climate-control system  200  is provided that may include a vessel  212  (e.g., a generator), a condenser  214 , a first expansion device  216 , an evaporator  218 , an absorber  220 , an internal heat exchanger  222 , a second expansion device  224 , and a pump  226 . The vessel  212  may include an inlet  228 , a refrigerant outlet  230 , and a co-fluid outlet  232 . The inlet  228  may receive a mixture of refrigerant and co-fluid (i.e., with the refrigerant absorbed into the co-fluid). 
         [0057]    The vessel  212  may be heated by any available heat source (e.g., a burner, boiler or waste heat from another system or machine)(not shown). In some configurations, the vessel  212  may absorb heat from a space to be cooled (e.g., the space to be cooled within a refrigerator, freezer, etc.). As heat is transferred to the mixture of refrigerant and co-fluid within the vessel  212 , the vapor refrigerant desorbs from the co-fluid so that the refrigerant can separate from the co-fluid. The refrigerant may exit the vessel  212  through the refrigerant outlet  230 , and the co-fluid may exit the vessel  212  through the co-fluid outlet  232 . 
         [0058]    One or more ultrasonic transducers  244  may be attached to the vessel  212 . As shown in  FIG. 4 , the ultrasonic transducers  244  may be mounted to an exterior surface  234  of the vessel  212 . In some configurations, the ultrasonic transducers  244  are disposed inside of the vessel  212  and in contact with the mixture of refrigerant and co-fluid (as shown in  FIG. 5 ). The structure and function of the ultrasonic transducers  244  may be similar or identical to that of the ultrasonic transducers  44  described above. As described above, the ultrasonic transducers  244  produce ultrasonic vibration that is transmitted to the mixture of refrigerant and co-fluid to increase the desorption rate of the refrigerant from the co-fluid. In some configurations, ultrasonic vibration may be used to produce a desired amount of desorption without adding heat from another source. In some configurations, ultrasonic vibration and the addition of heat may further accelerate the desorption rate. 
         [0059]    As described above, a control module  248  may be in communication with and control operation of the ultrasonic transducers  244  to increase the desorption rate to a desired level (e.g., to a level matching a rate of absorption). The structure and function of the control module  248  may be similar or identical to that of the control module  48 . The control module  248  may be in communication with sensors  250 ,  252  and may control operation of the ultrasonic transducers  244  based on pressure and/or temperature data received from the sensors  250 ,  252 . The sensor  250  may be disposed within the vessel  212  and may measure a pressure or temperature of the mixture of refrigerant and co-fluid therein. The sensor  252  may be disposed within the absorber  220  and may measure a pressure or temperature of the mixture of refrigerant and co-fluid therein. The control module  248  may also be in communication with and control operation of the pump  226 , the expansion devices  216 ,  224  and/or fans  254 ,  256 ,  257 . 
         [0060]    The condenser  214  is a heat exchanger that receives refrigerant from the refrigerant outlet  230  of the vessel  212 . Within the condenser  214 , heat from the refrigerant may be rejected to air or water for example, or some other medium. In the particular configuration shown in  FIG. 4 , the fan  254  may force air across the condenser  214  to cool the refrigerant within the condenser  214 . 
         [0061]    The expansion devices  216 ,  224  may be expansion valves (e.g., thermal expansion valves or electronic expansion valves) or capillary tubes, for example. The first expansion device  216  may be in fluid communication with the condenser  214  and the evaporator  218 . The evaporator  218  may receive expanded refrigerant from the expansion device  216 . Within the evaporator  218 , the refrigerant may absorb heat from air or water, for example. In the particular configuration shown in  FIG. 4 , the fan  256  may force air from a space (i.e., a room or space to be cooled by the system  200 ) across the evaporator  218  to cool the air. 
         [0062]    The absorber  220  may include a refrigerant inlet  258 , a co-fluid inlet  260 , and an outlet  262 . The refrigerant inlet  258  may receive refrigerant from the evaporator  218 . The co-fluid inlet  260  may receive co-fluid from the second expansion device  224 . Refrigerant may absorb into the co-fluid within the absorber  220 . The fan  257  may force air across the absorber  220  to cool the mixture of refrigerant and co-fluid and facilitate absorption. 
         [0063]    Like the internal heat exchanger  16 , the internal heat exchanger  222  may include a first coil  264  and a second coil  266 . The first coil  264  may receive co-fluid from the co-fluid outlet  232  of the vessel  212 . The co-fluid may flow from the first coil  264  through the second expansion device  224  and then into the absorber  220  through the co-fluid inlet  260 . 
         [0064]    The mixture of refrigerant and co-fluid may exit the absorber  220  through the outlet  262 , and the pump  226  may pump the mixture through the second coil  266 . The mixture of refrigerant and co-fluid flowing through the second coil  266  may absorb heat from the co-fluid flowing through the first coil  264 . From the second coil  266 , the mixture of refrigerant and co-fluid may flow back into the vessel  212  through the inlet  228 . 
         [0065]    It will be appreciated that the climate-control systems  10 ,  100 ,  200  can be used to perform a cooling function (e.g., refrigeration or air conditioning) or a heating function (e.g., heat pump). 
       EXAMPLES 
     Example 1—Comparison to Known Co Fluids 
       [0066]    Several commercial lubricants were compared to a co-fluid of the current teachings. MinOil is mineral oil. POE is polyol ester. PAG is polyalkylene glycol. NMP is N-methylpyrrolidone. PVE is polyvinyl ether. Comparison of their desorption rates at 32° F. under the same initial pressure load, initial desorption pressure and agitation rate is plotted in  FIG. 6  and compared with N-2,5,8,11-tetraoxadodecyl-2-pyrrolidone (abbreviated: Pyrr(EO)3Me) shown here: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    In  FIG. 6 , each value represents the pressure increase in one minute&#39;s time and the values are an average of three trials. 
         [0067]    Pyrr(EO)3Me also shows a rate inversion with temperature. Generally, desorption rates are expected to be faster at higher temperatures and slower at lower temperatures. But the plot in  FIG. 7  shows that desorption is actually faster at the lower temperature. 
       Example 2—Comparison of Analog Compounds 
       [0068]    A) Comparison of Aliphatic Side Chain and Polyoxyalkylene Side Chain 
         [0069]    Pyrr(EO)3Me has a 12 atom chain attached to the nitrogen. A saturated analog, N-dodecyl-2-pyrrolidone (Abbreviated: NDDPy), also has a 12-atom chain. They have similar viscosities (NDDPy at 40° C.=9.29 cSt, Pyrr(EO)3Me at 40° C.=9.06 cSt) and are close in molecular weight. NDDPy had to be evaluated at a higher temperature due to it solidifying at 5° C. Pyrr(EO)3Me has a faster desorption rate, as shown in  FIG. 8 . 
         [0000]    
       
                 
         
             
             
         
       
     
         [0070]    B) Comparison of Carboxylic Amide to Carboxylic Ester 
         [0071]    Pyrr(EO3Me) was compared to a corresponding ester compound IsoV(EO)3Me. Although the comparison ester had a lower viscosity than Pyrr(EO)3Me (which would provide a faster desorption rate, all things equal), the Pyrr(EO)3Me had a faster desorption rate. Data are shown in  FIG. 9 . 
         [0000]    
       
                 
         
             
             
         
       
     
         [0072]    C) Comparison to a Compound without the Cyclic Carboxylic Amide 
         [0073]    Pyrr(EO)3Me was also compared with triethylene glycol dimethyl ether to show that a low viscosity, low molecular weight polyalkylene glycol would not have the same or better desorption rates. Data are shown in  FIG. 10 , demonstrating Pyrr(EO)3Me has a faster desorption rate. The “dimethyl glycol ether” of  FIG. 10  is triethylene glycol dimethyl ether. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0074]    D) N-2,5,8,11-Tetraoxadodecyl-Caprolactam Shows the Same Temperature Rate Inversion as Pyrr(EO)3Me. 
         [0075]    N-2,5,8,11-Tetraoxadodecyl-Caprolactam has a 7-membered lactam ring rather than the five-membered ring on Pyrr(EO)3Me. It shows a temperature rate inversion, with the data shown in  FIG. 11 . The analog&#39;s structure is: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0076]    E) Effect of Ethylene Oxide Chain Length on the Rate of Carbon Dioxide Desorption. 
         [0077]    The graph in  FIG. 12  shows instantaneous rates (rates at time zero) of desorption for a series of compounds with no, 1, 2, or 3 ethylene oxides added to N-hydroxymethyl-2-pyrrolidone, as shown here: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0078]    F) Desorption Rates of Eight Atom Chain Co-Fluids Double Capped with 2-Pyrrolidone Rings. 
         [0079]    The following two compounds were compared at 40° C. This temperature was chosen due to solidification of one of the compounds at 0° C. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    As with the single pyrrolidone capped material (methyl cap at the other end), the compound having both ethylene oxide and pyrrolidone functions desorbs carbon dioxide faster under comparable conditions. This can be seen in the graph in  FIG. 13 . 
       Example 3—Synthesis of Co Fluids 
     Preparation of N-Hydroxymethyl-2-Pyrrolidone 
       [0080]    (This preparation is a slight modification of U.S. Pat. No. 3,073,843) 
         [0081]    To a 250 mL two necked round bottom flask, equipped with a thermometer, magnetic stirrer, and reflux condenser, was added 53.3 g (0.63 moles) 2-pyrrolidone, 19.1 g (0.64 moles) paraformaldehyde, and 0.2 g KOH all at once. The mixture was stirred and heated to 80-90° C. for ˜2.5 hours. Afterwards, 100 mL of hot toluene was added. The solution was then filtered and allowed to cool to room temperature. The resulting crystals were filtered and washed with cold toluene to give 64.5 g (89% yield) of 2-hydroxymethyl-2-pyrrolidone. H NMR and FTIR confirmed the structure. 
       Preparation of N-Hydroxymethyl-2-Caprolactam 
       [0082]    (This preparation is essentially that of U.S. Pat. No. 4,769,454.) 
         [0083]    To a 500 mL round bottom flask equipped with a magnetic stirrer, thermometer, and reflux condenser was added 113.2 g (1.00 mole) of caprolactam, The caprolactam was heated to liquid at which time 31 g (1.0 mole) of paraformaldehyde and 0.7 g of K 2 CO 3  were added at once. A slight exotherm raised the temperature to 97° C., however the reaction mixture was maintained between 70° C. and 95° C. for 2.5 hours. Afterwards, a seed crystal was added at 57° C. and the mixture held at 50° C. for 18 hours. White crystals resulted with a small amount of liquid. The liquid was decanted away from the white crystals to give 138 g (96% yield) of N-hydroxymethyl-2-caprolactam. H NMR and FTIR confirmed the structure. 
       Synthesis of N-2,5,8,11-Tetraoxadodecyl-2-Pyrrolidone 
       [0084]    (See U.S. Pat. No. 3,853,910—hereby incorporated by reference—for hydroxymethyl-2-pyrrolidone ethers of alkyl, aryl, alkenyl, groups etc.) 
         [0085]    To a 500 mL two-necked round bottom flask equipped with a magnetic stirrer, thermometer, and reflux condenser was added 58 g (0.50 moles) of N-hydroxymethyl-2-pyrrolidone and 246 g (1.5 moles) of triethylene glycol monomethyl ether at once. The mixture was cooled to ca. 10° C. and 21 mL of 12N HCl was added in 5 to 10 minutes while maintaining the temperature around 10° C. during the addition. Afterwards, the mixture was warmed to room temperature and was held at this temperature for 2 hours while stirring. Addition of 40 g of 25% NaOH to the mixture between 15° C. and 25° C., followed by stirring for 0.5 hours produced a mixture of NaCl and product. The salt was filtered off and the mixture subjected to vacuum distillation to remove water (27° C. at 0.12 Torr). More salt precipitated out and the distillation stopped and the salt filtered off. The resultant oil was distilled three times through a 6×¾ inch Vigreux Column under vacuum. The final cut distilled at 156° C. at 0.11 Torr to give 71.6 g (55% yield) of N-2,5,8,11-tetraoxadodecyl-2-pyrrolidone. H NMR and FTIR confirmed the structure. 
       Synthesis of N-2,5,8,11-Tetraoxadodecyl-Caprolactam 
       [0086]    To a 500 mL two necked round bottom flask equipped with a magnetic stirrer, thermometer, and reflux condenser was added 71.6 g (0.50 moles) of N-hydroxymethyl-caprolactam and 246 g (1.5 moles) of triethylene glycol monomethyl ether at once. The mixture was cooled to ca. 4° C. and 21 mL of 12N HCl was added in ca. 15 minutes while maintaining the temperature around 10° C. during the addition. Afterwards, the mixture was warmed to room temperature and was held at this temperature for 2.5 hours while stirring. Addition of 40 g of 25% NaOH to the mixture between 15° C. and 25° C., followed by stirring for 0.5 hours produced a mixture of NaCl and product. The mixture was subjected to rotoevaporation to remove water. The resultant salt was filtered off and the mixture subjected to straight take over vacuum distillation collecting a top cut between 82° C. and 84° C. More salt precipitated out and the distillation stopped and the salt filtered off. The resultant oil was distilled through a 6×¾ inch Vigreux column under vacuum. The main cut distilled between 159° C. and 169° C. at 0.2 Torr to give 80.1 g (55% yield) of N-2,5,8,11-tetraoxadodecyl-2-caprolactam. H NMR and FTIR confirmed the structure. 
       Synthesis of 1,8-bis-(Pyrrolidon-1-yl)-3,6-dioxaoctane 
       [0087]    To a single necked 500 mL round bottom flask was added 134 g (1.56 mole) gamma-Butyrolactone and 109.5 g (0.74 mole) 1,8-diamino-3,6-dioxaoctane at once. The flask was fitted with a magnetic stir bar, H-Trap and a condenser. At the top of the condenser a nitrogen source was attached via a Firestone Valve. A vacuum was pulled while heating the mixture to melt any resultant solids. This was followed by a vacuum then nitrogen purge three times. While under nitrogen, the mixture was heated and after 26 mL of water was collected in the H-Trap, the reaction mixture was cooled and subjected to straight take-over vacuum distillation. The material was distilled twice and the final product cut distilled between 196° C. and 214° C. at 0.11 Torr to give 124 g of product. H NMR and FTIR confirmed the structure. 
       Synthesis of 1,8-bis-(pyrrolidon-1-yl)octane 
       [0088]    To a single necked 500 mL round bottom flask was added 134 g (1.56 mole) gamma-butyrolactone and 106 g (0.74 mole) 1,8-diaminooctane at once. The flask was fitted with a magnetic stir bar, Dean-Stark Trap and a condenser. At the top of the condenser a nitrogen source was attached via a Firestone Valve. A vacuum was pulled while heating the mixture to melt any resultant solids. This was followed by a vacuum then nitrogen purge four times. While under nitrogen, the mixture was heated and after 27 mL of water was collected in the Dean-Stark Trap, the reaction mixture was cooled and subjected to vacuum distillation. The material was distilled twice and the final product cut distilled between 200° C. and 208° C. at 0.2 Torr to give 166 g (81% yield) of product. H NMR and FTIR confirmed the structure. 
       Synthesis of 3,6,9-trioxadecyl isovalerate 
       [0089]    To a 500 mL single necked round bottom flask equipped with a magnetic stir bar and a Dean-Stark Trap was added 53.7 g (0.526 mole) isovaleric Acid, 81.6 g (0.50 mole) triethylene glycol monomethyl ether, 0.3 g p-toluenesulfonic acid and 200 mL of toluene at once. The mixture was heated under reflux till 8.5 mL of water was collected. After cooling to room temperature, the toluene solution was washed with 200 mL of 5% aqueous NaOH, 200 mL of saturated salt solution and dried over sodium sulfate. The mixture was filtered and subjected to rotoevaporation. Straight take-over distillation gave 91.7 g (73% yield). The product cut was at 107° C. and 115° C. at 1.0 Torr. FTIR confirmed the structure. 
       Example 4—Measuring Carbon Dioxide Desorption Rate 
       [0090]    A co-fluid (50 g) is added to a 300 mL Parr reactor and the reactor is evacuated to ca. 0.21 Torr while stirring and at the temperature being studied. The stirring is stopped, and the co-fluid allowed to settle for 1 minute. CO 2  is bled in to the reactor at the required pressure, which is 300 psia unless indicated otherwise. The CO 2  is introduced as quietly as possible, with minimal co-fluid agitation. Stirring is then started (400 rpms), time is marked 0 minutes and pressure rate recorded. Equilibrium is recorded generally after 15 minutes of stirring. Note that the equilibrium is reached prior to this. 
         [0091]    Afterwards the stirring is stopped and the co-fluid allowed to settle for 1 minute. The pressure is then rapidly but “quietly” dropped to 50 psi. Stirring is resumed and the pressure rise (indicating release of CO 2  from the co-fluid) is recorded for a period of time. Instantaneous rates are determined by taking measurements for the first 20 seconds and fitting a straight line curve through the data. 
         [0092]    All data points represent at least 3 experimental runs. 
         [0093]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.