Patent Publication Number: US-2012036854-A1

Title: Transcritical thermally activated cooling, heating and refrigerating system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This disclosure relates to pending U.S. application Ser. No. 07/18958, assigned to the assignee of the present disclosure. 
     Reference is made to and this application claims priority from and the benefit of U.S. Provisional Application Ser. No. 61/173,776, filed Apr. 29, 2009, entitled “TRANSCRITICAL THERMALLY ACTIVATED COOLING, HEATING AND REFRIGERATING SYSTEM”, which application is incorporated herein in their entirety by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to vapor compression systems and, more particularly, to a combined vapor compression and vapor expansion system. 
     BACKGROUND OF THE DISCLOSURE 
     It is known to combine a vapor compression system with a vapor expansion, i.e. Rankine cycle, system. See, for example, U.S Pat. No. 6,962,056, assigned to the assignee of the present invention, and U.S Pat. No. 5,761,921. 
     U.S. Pat. No. 5,761,921 generates power in the Rankine cycle which is then applied to drive the compressor of the vapor compression cycle, and the combined systems operate on three pressure levels, i.e. the boiler, condenser and evaporator pressure levels. A common refrigerant R-134 is used in both the vapor compression and the Rankine cycle systems. Such combined systems have generally not allowed use of transcritical refrigerants, since transcritical systems have generally not had a condenser (but only a gas cooler), and therefore no liquid refrigerant available downstream of the gas cooler for pumping through the Rankine circuit. The expander requires a high entering pressure, but the high inlet pressure elevates the boiling temperature and the leaving temperature of the heating fluid carrying the thermal power. The elevated leaving temperature reduces the extent of the waste heat utilization. For those reasons the systems do not sufficiently utilize available thermal energy and, therefore have a low level of thermodynamic efficiency. Further, they do not provide an adequate performance when the available hot source is below 180° F. 
     U.S. patent application Ser. No. 07/18958 provides for a combined flow of refrigerant from the two systems at the discharge of the compressor and the expander, respectively. Further, a suction accumulator is provided such that liquid refrigerant is always available to the pump in the Rankine cycle system such that transcritical operation is made possible. However, such use of a suction accumulator may be undesirable because of the need for a larger pump with greater power requirements. The pump power is defined by a product of pressure differential across the pump and the specific volume of the refrigerant stream at the pump inlet. Although the liquid in the suction accumulator has a low specific volume, the pump may be required to work against high pressure differentials. When the disadvantage of the pressure differential increase exceeds the advantage of the liquid specific volume reduction, feeding of the pump with liquid refrigerant from the condenser is considered to be an advantage over the use of a suction accumulator. 
     DISCLOSURE 
     Briefly, in accordance with one aspect of the disclosure, a combined vapor compression circuit and vapor expansion circuit includes a common refrigerant which enables a supercritical high pressure portion and a sub-critical low pressure portion of the vapor expansion circuit, and combines the refrigerant from the expander discharge and the compressor discharge at the entrance to the outdoor heat exchanger. The outdoor heat exchanger is so sized and designed that the refrigerant discharge therefrom is always in a liquid form so that it can flow directly to the vapor expansion circuit pump. The pump and expander are so sized and designed that the high pressure portion of the vapor expansion circuit is always super-critical. 
     In accordance with another aspect of the disclosure, the outdoor heat exchanger includes a cooling tower to ensure that the refrigerant is converted to a liquid in the heat exchanger. 
     In accordance with another aspect of the disclosure, a liquid to suction heat exchanger is provided between the outdoor heat exchanger and the pump in order to increase subcooling and refrigerant density prior to the refrigerant liquid&#39;s passing to the pump. 
     In accordance with yet another aspect of the invention, a topping heat exchanger is provided downstream of the expander outlet for the purpose of regenerating enthalpy of the hot stream. 
     In accordance with yet another aspect of the invention, a power generation vapor expansion circuit is used as a stand alone system and generates electrical power, which may be used as an electrical power supply for different purposes, including driving a refrigeration system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a further understanding of these and objects of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, where: 
         FIG. 1  is a schematic illustration of a thermally activated refrigerant system for cooling or heating only. 
         FIG. 2  is a schematic illustration of a temperature-entropy, (T-S) diagram of processes for the thermally activated refrigerant system for cooling or heating only. 
         FIGS. 3A-3C  are schematic illustrations comparing glides in supercritical and subcritical applications, respectively. 
         FIG. 4  is a schematic illustration of a thermally activated vapor expansion system with multi-stage expansion. 
         FIG. 5  is a schematic illustration of a T-S diagram of processes for the thermally activated vapor expansion system with multi-stage expansion. 
         FIG. 6  is a schematic illustration of a thermally activated refrigerant system providing both air conditioning and refrigeration. 
         FIG. 7  is a schematic illustration of a thermally activated heat pump with two expansion devices. 
         FIG. 8  is a schematic illustration of a thermally activated heat pump with one bidirectional expansion device. 
         FIG. 9A and 9B  are schematic illustrations of reversing and check valve arrangements, respectively. 
         FIG. 10  is a schematic illustration of a thermally activated heat pump with two different hot sources. 
         FIG. 11  is a schematic illustration of a thermally activated heat pump with multi-stage compression. 
         FIG. 12  is a schematic illustration of a thermally activated heat pump with a vapor-to-vapor ejector. 
         FIG. 13  is a schematic illustration of a thermally activated heat pump with a two-phase ejector. 
         FIG. 14  is a schematic illustration of a thermally activated heat pump with an economized cycle. 
         FIG. 15  is a schematic illustration of a thermally activated heat pump with a two-phase expander. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     While the present disclosure has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the disclosure as defined by the claims. 
     In accordance with  FIG. 1  a thermally activated refrigerant system incorporates a vapor compression circuit  21  shown as solid lines and a vapor expansion circuit  22  shown as dashed lines. The vapor compression circuit  21  includes a compressor  23 , a condenser  24 , a liquid-to-suction heat exchanger  26 , an expansion device  27 , and an evaporator  28 . The vapor expansion circuit  22  consists of a pump  29 , a topping heat exchanger  31 , a heater  32 , an expander  33 , and the condenser  24 . A refrigerant vapor stream at the outlet from the compressor and a vapor refrigerant stream at the outlet from the expander are connected at the condenser inlet to provide a combined flow through the condenser  24 . A refrigerant liquid stream at the condenser outlet, or at the outlet of the liquid to suction heat exchanger  26  as shown, splits into two streams: one feeds the pump, and another circulates through the components of the vapor compression circuit. 
     The thermally activated refrigeration system has three pressure levels: a heating pressure, a heat rejection pressure level, and evaporating pressure. The heating pressure is the pump discharge pressure, the heat rejection pressure is compressor or expander discharge, and the evaporating pressure is the compressor suction pressure. The heating and heat rejection pressures are high and low pressures of the vapor expansion circuit. The heat rejection and evaporating pressures are high and low pressures of the vapor compression circuit 
     One common working fluid is used for both the vapor compression and the vapor expansion circuits. The working fluid has the following feature: it provides super-critical operation for a high pressure portion of the vapor expansion circuit and a sub-critical operation for the low-pressure portion of the vapor expansion circuit. Thus, the working fluid in the vapor expansion circuit at the high pressure remains gaseous, but the working fluid in the condenser appears in the region to the left of the vapor dome and is liquefied. Examples of such working fluid are CO 2  or CO 2  based mixture, such as CO 2  and propane, or the like. 
     The heater  32  provides a thermal contact between a heating medium and the pumped refrigerant stream. Usually the heat source is a waste heat such as may be available from a fuel cell, a solar device, a micro-turbine, a reciprocating engine, or the like. Pressure in the heater is supercritical, that is, above the critical pressure of the refrigerant. This provides a favorable temperature glide compatible with a temperature glide of the heating medium shown on  FIG. 2 . The heater  32  should be designed to provide equality of heat capacity rates of both streams and enable the highest temperature differentials across each stream. The glides and equality of the heat capacity rates provide a higher extent of waste heat utilization and a high entering expander temperature, resulting in improved expander performance. If the hot source is not a waste heat, the equality of heat capacity rates may not be required; the temperature glide provides a higher refrigerant temperature at the expander inlet, which improves the performance characteristics of the expander. 
     The condenser  24  provides a thermal contact between a cooling medium and the combined refrigerant stream outgoing from the compressor  23  and expander  33 . The temperature of the cooling medium in the condenser  24  is always maintained below the refrigerant critical point to enable refrigerant condensation at the heat rejection pressure, with the liquid refrigerant feeding the pump  29 . 
     During periods of operation at higher ambient temperatures, the condenser  24  may be fed by a cooling tower  34  to ensure condensation of the refrigerant vapor. Another option is to use CO 2  and propane or the like in order to elevate the critical point of the fluid sufficiently above the level of ambient temperature to enable the condensation process at the heat rejection pressure. 
     The heating pressure in the heater  32  is controlled by an expander-to-pump capacity ratio, which is defined by an expander-to-pump rotating speed ratio, a liquid refrigerant temperature at the pump inlet, and a vapor refrigerant state at the expander inlet. 
     The liquid-to-suction heat exchanger  26  is optional. It slightly sub-cools a liquid stream outgoing from the condenser  24  and substantially superheats a vapor stream flowing from the evaporator  28 . The subcooling reduces the pump power due to reduction of the refrigerant density at the pump inlet. Also, it increases the enthalpy difference across the evaporator  28  and increases the evaporator effect. The superheat decreases the refrigerant density at the compressor inlet and reduces the compressor mass flow rate and the evaporator capacity. The superheat effect is usually stronger and the overall effect is usually detrimental. Therefore, the liquid-to-suction heat exchanger  26  is only used if a certain superheat at the compressor inlet is required. 
     The topping heat exchanger  31  substantially improves thermodynamic efficiency of the system when the hot source temperature is high. When the hot source temperature is low, the topping heat exchanger is not needed. 
     Power generated in the expander  33  may drive the compressor  23  and the pump  29 . All three machines may be placed on the same shaft. There is an option to couple the shaft with a power generator  36  to provide not only cooling or heating duty, but also electrical power. The expander  33  may be coupled with a power generator only, in which case the power generator  36  powers the compressor  23  and pump  29 . In addition, optionally, it may generate supplemental electrical power. 
     The vapor expansion circuit may be implemented as a separate power generation system. Power generated in the power generation system may be used to power a heat pump, air conditioner, refrigerator, or any other electrical device. 
     All components sitting on the same shaft may be covered by a semi-hermetic or hermetic casing to reduce risk of leakage. 
     The pump  29  may be a variable or multiple speed device or a constant speed device. Speed variation helps to satisfy the variable demands of refrigeration, air conditioning or heating. 
     Referring now to  FIG. 2 , the T-S diagram is shown for both the vapor compression circuit  21  and the vapor expansion circuit  22  of  FIG. 1 , with the various points of interest in the two figures being shown by the numerals  1 - 12 . As will be seen, the line  9 - 10  is representative of the temperature and enthalpy increases that occur as the working fluid passes through the heater  32 . Also, it should be appreciated that the alternate dash-dot line  37  is indicative of the T-S diagram for the cooled heating fluid passing through the heater  32 . In this regard, it is desirable to not only use hot source fluids at temperatures of 180° F. and above, as are used in conventional systems, but also enable the use of hot source fluids at temperatures below that level. This is made possible by the “glide” or slope of the line  37  that results from the use of CO 2  as the working fluid. This will be more clearly understood by reference to  FIGS. 3A-3C . 
     Shown in  FIG. 3A  is a vapor expansion circuit which includes, in serial flow relationship a pump  38 , a topping heat exchanger  39 , a heater  41 , an expander  42  and a condenser  43 . 
     Shown in  FIG. 3B  is a T-S diagram for the  FIG. 3A  circuit when operating in a supercritical mode such as with CO 2  as the refrigerant. The numbers  1 - 8  in  FIG. 3B  correspond to the positions  1 - 8  in the  FIG. 3A  drawing. As will be seen, the line  3 - 4  in  FIG. 3B  represents the increases in temperature and enthalpy as the CO 2  passes through the heater  41 , and the alternate dash and dot line  44  represents the T-S diagram for the cooled heating fluid. It will recognized that the “glide”, or the slope of this line is substantial. 
     In contrast, the  FIG. 3C  illustration is a T-S diagram of the  FIG. 3A  circuit when operating in a subcritical mode, i.e. with a refrigerant other than CO 2 . Here, it will be recognized that the glide/slope of the line  46  is substantially less than that of the line  44  in  FIG. 3B . The vertical component of the two lines  44  and  46 , as shown by the arrowed lines  47  and  48 , respectively, show the degree of waste heat utilization of the two alternatives of  FIGS. 3B and 3C . As will be seen, the line  47  extends downwardly further then the line  48  which, in turn, indicates that heat sources (state  7 ) at lower temperatures may be employed as long as the temperature in state  8  is below the temperature in state  7 . Thus, temperatures below 180° F. may be suitable, such as, for example, temperatures of 150° F. 
     Referring now to  FIG. 4 , there is shown another embodiment wherein, rather than a single stage expander  33  as shown in  FIG. 1 , a two stage expander  49  is provided, as well as a second heater  51 . The second heater  51  receives the heating fluid along line  52  and returns it to a point of the heater  32  by way of line  53 . The temperature of the heating fluid in the heater  51  should be equal to the temperature of the point in the heater  32 , where the line  53  is attached to. In operation, the refrigerant passes from the heater  32  to the first stage of the two stage expander  49  and then passes through the second heater  51 , after which it passes through the second stage of the two stage expander  49 , and then to the topping heat exchanger  31 . The remainder of the circuit is as described above. The effect of using the two stage expander  49  and the second heater  51  is shown by the T-S diagram of  FIG. 5  wherein the numbers ( 1 - 14 ) are indicative of the locations indicated in  FIG. 4 . It is known that the method of multi-stage expansion with reheat improves the expander efficiency, and reduces required pump power to thereby enable the use of smaller pumps and to reduce use of pump power to thereby improve the overall efficiency of the system. 
     Another embodiment is shown in  FIG. 6  wherein a second vapor compression circuit  54  is provided in parallel with the vapor compression circuit  21 . This enables the system to provide for both air conditioning, i.e. by way of the second vapor compression circuit  54  and refrigeration, i.e. by way of the vapor compression circuit  21 . 
     The second vapor compression circuit  54  includes a second expansion device  56 , a second evaporator or indoor unit  57  and a second compressor  58 . The flow of refrigerant for that circuit originates upstream of the expansion device  27 , and the discharge flow from the second compressor  58  is combined with the refrigerant flow from the topping heat exchanger  31  prior to the combination being combined with the flow from the discharge of the compressor  23 . Thus, each of the vapor compression circuits  21  and  54  has its own compressor and evaporator unit, and all other components are shared between the two circuits. As will be seen both of the compressors are powered by the expander  33 . 
     If the condenser  24  is an outdoor unit and the evaporator  28  is an indoor unit then the thermally activated refrigerant system generates cooling. If the condenser is an indoor unit and the evaporator is an outdoor unit then the thermally activated refrigerant system generates heating. To switch between the two modes of operation, one or more reversing or check valves may be provided as shown in  FIGS. 7-15 . 
     In order to allow the system to operate as a heat pump, a pair of reversing valves  59  and  61  are provided as shown in  FIG. 7 . Further, in addition to the expansion device  27  that is operable for use in the cooling mode, a second expansion device  62  is provided for use in the heating mode. Each of the expansion devices  27  and  62  include a bypass valve, i.e. valves  63  and  64 , respectively, to permit operation in the respective cooling and heating modes. The expansion devices  27  and  62  are single directional expansion devices. In order to switch between the cooling and heating modes, the reversing valves  59  and  61 , and the bypass valves  63  and  64 , are all operated simultaneously. 
     A suction accumulator  66  maybe provided in order to satisfy the refrigerant charge demands for cooling and heating operation. Also, the suction accumulator  66  provides charge management and capacity control accumulating redundant amount of liquid refrigerant. 
     Further, a liquid-to-suction heat exchanger  67  may be provided as indicated. 
     A variation of the  FIG. 7  system is shown in  FIG. 8  wherein the two expansion devices are replaced by a single expansion device  68  which is designed for bi-directional use. Thus when switching between the cooling and heating modes, the single expansion device and the reversing valves  59  and  61  are all switched simultaneously. 
     In  FIG. 9A , the respective positions of the reversing valve  59  are shown to provide either cooling or heating operation. Thus, in cooling, the refrigerant passes from the reversing valve  59  through the heat exchanger  67 , the expansion device  27 , and then to the indoor unit. In heating, refrigerant passes from the reversing valve  59 , through the heat exchanger  67 , the expansion  27 , and then to the outdoor unit. 
     As will be seen in  FIG. 9B , rather than using reversing valves as described hereinabove, check valves maybe substituted to accomplish the same function. Thus, rather than reversing valves, four check vales  71 ,  72 ,  73  and  74  are provided. In the cooling mode, the refrigerant passes through the check valve  71 , the heat exchanger  67 , the expansion device  27 , and the check valve  73  to go to the indoor unit, with check valves  72  and  74  being closed. During operation in the heating mode, the check valves  71  and  73  are closed, and refrigerant passes through the check valve  74 , the heat exchanger  67 , the expansion device  27 , and the check valve  72  to pass then to the outdoor unit. 
       FIG. 10  represents a case when two hot sources, high temperature and low temperature sources, are available. A second heater  74  utilizes the high temperature source. The heater  32  utilizes the low temperature source. 
     A further embodiment is shown in  FIG. 11  wherein a multi-stage compressor  76  is provided. After passing through the first stage, the refrigerant passes through a gas cooler  77 , and then through the second stage of the two stage compressor  76  before passing to the reversing valve  61  and the condenser  24 . In this way, the total compressor power is reduced to thereby improve the thermodynamic efficiency of the compression circuit and therefore that of the total system. 
     The embodiment of  FIG. 12  provides an ejector  78  for boosting the flow of refrigerant vapor to the suction accumulator  66  to thereby improve the thermodynamic efficiencies of the vapor compression circuit and of the total system. The ejector  78  is driven by a high pressure stream along line  79  or, alternatively, from lines  81  or  82 . In this particular embodiment the liquid-to-suction heat exchanger  67  is a mandatory component. The heat exchanger  67  provides completion of evaporation of liquid portion of the refrigerant stream outgoing from the ejector  78 . 
       FIG. 13  embodiment shows a heat pump with an ejector  83  being driven by high pressure refrigerant from line  84  or, alternatively, from line  86 . The bi-directional expansion device  87  could be replaced by two one directional expansion devices, i.e. one for the indoor unit and another for the outdoor unit as it was shown above on  FIG. 7 . 
     It is known that ejectors improve performance characteristics of vapor compression cycles. The combined vapor compression and vapor expansion cycle improves with a better vapor compression cycle. 
     Shown in  FIG. 14  is an alternative embodiment that includes an economizer cycle which includes an economizing heat exchanger  88 , an economizer expansion device  89 , and a economizer port  91  leading into a mid-stage of the compressor  23 . A further alternative may be that of a multi-stage compressor with intermediate vapor cooling. It is known that economized cycles improve performance characteristics of vapor compression cycles. The combined vapor compression and vapor expansion cycles improves with a better vapor compression cycle. 
     The  FIG. 15  embodiment provides a two-phase expander  92  fluidly interconnected between an inlet to the pump  29  and the reversing valve  59  as shown. Its use tends to increase the cooling effect while recovering additional power to drive the cycle. This, in turn, reduces required pump size and pump power. 
     Although the present disclosure has been particularly shown and described with reference to a preferred embodiment as illustrated by the drawings, it will be understood by one skilled in the art that various changes in detail made be made thereto without departing from the scope of the disclosure as defined by the claims.