Abstract:
A carbon dioxide absorption heat pump cycle is disclosed using a high pressure stage and a super-critical cooling stage to provide a non-toxic system. Using carbon dioxide gas as the working fluid in the system, the present invention desorbs the CO 2  from an absorbent and cools the gas in the super-critical state to deliver heat thereby. The cooled CO 2  gas is then expanded thereby providing cooling and is returned to an absorber for further cycling. Strategic use of heat exchangers can increase the efficiency and performance of the system.

Description:
ORGIN OF INVENTION 
     The invention described hereunder was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the Contractor has elected not to retain title. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed generally to heat pumps, and more specifically to a high pressure heat pump using carbon dioxide as the circulating fluid. 
     2. Description of Related Art 
     Heat pumps are well known in the art. A heat pump is simply a device for delivering heat or cooling to a system, whereas a refrigerator is a device for removing heat from a system. Thus, a refrigerator may be considered a type of heat pump. Throughout the application, the invention will be referred to as a heat pump with the understanding that the designation of refrigerator could be substituted without changing the operation of the device. 
     In absorption heat pumps, an absorbent such as water absorbs the refrigerant, typically ammonia, thus generating heat. When the combined solution is pressurized and heated further, the refrigerant is expelled. When the refrigerant is pre-cooled and expanded to a low pressure, it provides cooling. The low pressure refrigerant is then combined with the low pressure depleted solution to complete the cycle. 
     Many current absorption heat pump/refrigerators make use of either a water-ammonia couple, or a water-lithium bromide. These two absorption couples suffer from certain drawbacks. The water-ammonia couple raises security problems in view of the toxicity and flammability of ammonia, and LiBr is corrosive and very failure prone due to low pressure operation, i.e., small leaks create contamination. Moreover, the tendency to crystallize can be a clogging problem. Operating at very low pressures is often impossible due to the freezing of water. Other absorption processes have been proposed, but all involve working fluids that are toxic, flammable, ozone-depleting, or have high atmospheric green house effects. The art lacks an environmentally friendly and efficient cycle that uses a non-toxic, non-corrosive working fluid with a positive working pressure. 
     SUMMARY OF THE INVENTION 
     The present invention is a safe, environmentally friendly absorptive cooling/heating process. The process uses a carbon dioxide absorption cycle that utilizes a liquid, non-toxic absorbent such as alcohol, from which the carbon dioxide gas is absorbed. Only the carbon dioxide refrigerant is circulated to the evaporator and condenser heat exchangers, the components directly in contact with breathable air, thus avoiding the problem of alcohol&#39;s flammability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The exact nature of this invention, as well as its objects and advantages, will become readily apparent upon reference to the following detailed description when considered in conjunction with the accompanying drawing, wherein the figure illustrates a schematic of the process for the heat pump of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor of carrying out his invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the general principles of the present invention have been defined herein specifically to provide a heat pump method and apparatus using carbon dioxide as the working fluid. 
     The preferred embodiment of the present invention described below comprises a generator, two heat exchangers, an absorber, and an evaporator. The generator heats the alcohol/carbon dioxide mixture to approximately 250° F., at which point carbon dioxide is vented. The carbon dioxide then passes through the heat exchanger, is pre-cooled and expanded, and passes through an evaporator to provide cooling at about 40° F. The CO 2  then passes back through a heat exchanger to an absorber where it is absorbed back into the solution and heat is released. A more detailed description is provided below. 
     FIG. 1 is a schematic diagram of a preferred embodiment of the present invention. A first reservoir, or “absorber”  100  stores a strong solution  195  of an absorbent such as alcohol, and carbon dioxide at low temperature such as 100° F. and a low pressure of 560 PSI. Numerical values are provided for illustrative purposes only and the invention is not limited a particular temperature or pressure discussed herein. The solution  195  is “strong” in that there is a higher concentration of the carbon dioxide dissolved in the alcohol as compared with a second reservoir of a “weak” solution to be discussed below. 
     The strong solution  195  of alcohol and carbon dioxide is directed through conduit  105  to a pump  110  that elevates the pressure of the solution to approximately 1400 PSI. The now high pressure solution is passed via conduit  115  through a heat exchanger  120  to increase the temperature in the solution to approximately 275° F. This high temperature solution is directed by conduit  125  to a second reservoir, or “generator”  130  in order to extract the carbon dioxide from the solution. From the high temperature high pressure solution  135 , carbon dioxide gas  140  is boiled off or desorbed at the high temperature and pressure condition (250° F. and 1400 PSI) and directed through conduit  145 . The weak alcohol solution  135  remaining after the quantity of carbon dioxide gas has been bled off is passed through an expansion valve  190  to reduce the pressure of the solution back to approximately 560 PSI, and then is directed to the heat exchanger  120  to recoup some of the heat used to heat the strong solution. After leaving the heat exchanger  120 , the weak solution returns to the reservoir  100  where it is mixed with carbon dioxide gas returning from the cycle to regenerate the strong solution  195 . 
     The quantity of high temperature, high pressure carbon dioxide gas  140  removed from the strong solution returns back through heat exchanger  120  to recoup some of the heat which in turn is used to heat more strong solution passing through the heat exchanger  120 . Upon exiting the heat exchanger, the temperature of the CO 2  gas is approximately 175° and the pressure remains approximately 1400 PSI. 
     The CO 2  is delivered by conduit  150  to a super critical pre-cooler  160  to cool the gas to approximately 100° F. while providing heat represented by arrow  165  to the system. The decrease in the temperature of the carbon dioxide gas from approximately 175° to 100° occurs while the gas is in a super-critical state, enabling heat to be removed across a significant temperature range instead of at a single condenser temperature. The cooled super critical CO 2  gas next passes through a second heat exchanger  170  further reducing the temperature of the gas to approximately 60° F. At this point, the high pressure, low temperature carbon dioxide gas (partially liquefied) flows through conduit  175  to an expansion valve  180  where the carbon dioxide&#39;s pressure is reduced to approximately 560 PSI while the temperature is reduced still further to approximately 40° F. The now low temperature (40° F.), low pressure (560 PSI) gas is returned through the heat exchanger  170  to elevate the temperature of the gas to close to the original 100° F., although some loss is expected. The CO 2  gas then travels through conduit  185  back to reservoir  100  to regenerate the strong solution  195  and begin the cycle over again. 
     An advantage of the carbon dioxide cycle just discussed is the capacity to pre-cool the gas over a much larger super-critical temperature range compared to a common refrigerant such as R134a, which must be condensed at one specific temperature for a given pressure. Here, the carbon dioxide gas can be cooled in the super-critical regime from between about 200° F. and 100° F. depending upon the state of the gas exiting the heat exchanger  120 , thus enabling heat to be removed from the system continuously within this range rather than limiting the heat extraction only at a specific temperature. This in turn significantly reduces the required heat exchanger mass. 
     The present invention has many potential practical applications, especially in the automobile air conditioning. Other applications include industrial heating and air conditioning systems. The present invention has a number of distinct advantages over existing systems of water/ammonia and LiBr in that the refrigerant is high pressure, thus requiring smaller heat exchangers, and utilizes a non-toxic working fluid. Furthermore, there is no potential for crystallization as is the case with LiBr. The application is also well suited for indoor applications because of the absence of toxic working fluids which may leak into human occupied spaces. 
     While alcohol is described as the absorbent for the present invention, other absorbents are also possible. Table 1 lists some additional absorbents for use with the present invention, Important parameters to consider are normal boiling point (NBP) higher normal boiling points are desired to minimize distillation heat losses, and toxicity. Generally, compounds that have high NBPs and are above 50 ppm toxicity should be considered safe, in that the amount of solvent in contact with the CO 2  is small and will become smaller if mixed with air should a leak occur. From Table 1, isobutyl acetate and amyl acetate are also good candidates for CO 2  absorption, as CO 2  absorbs well with acetates compared to alcohols, and these fluids have a relatively high normal boiling point (NBP) and low toxicity. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 CO 2  Solvents 
               
             
          
           
               
                   
                 cc CO 2   
                   
                   
                   
               
               
                   
                 dissolved/cc Solvent 
                 (° C.) 
                 TLV, ppm 
               
               
                 Compound 
                 Ostwald Coefficient* 
                 NBP 
                 Toxicity 
                 Comment 
               
               
                   
               
             
          
           
               
                 Water 
                 0.817 
                 100 
                 — 
                 low Ost. 
               
               
                   
                   
                   
                   
                 Coef. 
               
               
                 Acetone 
                 6.38 
                 56 
                 750 
                 low NBP 
               
               
                 Pyridine 
                 3.58 
                 115 
                 5 
                 toxic 
               
               
                 Methyl Alcohol 
                 3.40 
                 64 
                 200 
                 low NBP 
               
               
                 Ethyl Alcohol 
                 2.70 
                 78 
                 1000 
                 low Ost. 
               
               
                   
                   
                   
                   
                 coef. 
               
               
                 Amyl Acetate 
                 4.40 
                 148 
                 100 
                 good 
               
               
                 Acetate Acid 
                 4.82 
                 118 
                 10 
                 toxic 
               
               
                 Heptane 
                 2.63 
                 98 
                 400 
                 low Ost. 
               
               
                   
                   
                   
                   
                 coef. 
               
               
                 Isobutyl 
                 4.69 
                 117 
                 150 
                 good 
               
               
                 Acetate 
               
               
                 Acetic 
                 5.21 
                 140 
                 5 
                 toxic 
               
               
                 Anhydride 
               
               
                   
               
               
                 *1 atm., 25° C.  
               
             
          
         
       
     
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein, and particularly with other solvents for CO 2 .