CO2 refrigeration system with supercritical subcooling control

A refrigeration system includes a gas cooler/condenser configured to remove heat from a refrigerant, a temperature sensor configured to measure a temperature of the refrigerant leaving the gas cooler/condenser, a pressure sensor located along the high pressure conduit and configured to measure a pressure of the refrigerant leaving the gas cooler/condenser, a pressure control valve operable to regulate the pressure of the refrigerant leaving the gas cooler/condenser, and a controller. The controller is configured to determine whether the refrigerant leaving the gas cooler/condenser is in a subcritical region based on at least one of the measured temperature of the refrigerant or the measured pressure of the refrigerant. If the refrigerant leaving the gas cooler/condenser is not in the subcritical region, the controller is configured to add a pseudo-subcooling temperature value to the measured temperature of the refrigerant to calculate a summed temperature, calculate a supercritical pseudo-saturated pressure as a function of the summed temperature, and operate the pressure control valve to drive the pressure of the refrigerant leaving the gas cooler/condenser to the supercritical pseudo-saturated pressure corresponding to the summed temperature.

BACKGROUND

The present disclosure relates generally to a refrigeration system and more particularly to a refrigeration system that uses carbon dioxide (i.e., CO2) as a refrigerant. The present disclosure relates more particularly still to a CO2refrigeration system that controls an amount of subcooling of a CO2refrigerant.

Refrigeration systems are often used to provide cooling to temperature controlled display devices (e.g. cases, merchandisers, etc.) in supermarkets and other similar facilities. Vapor compression refrigeration systems are a type of refrigeration system which provides such cooling by circulating a fluid refrigerant (e.g., a liquid and/or vapor) through a thermodynamic vapor compression cycle. In a vapor compression cycle, the refrigerant is typically compressed to a high temperature high pressure state (e.g., by a compressor of the refrigeration system), cooled/condensed to a lower temperature state (e.g., by rejecting heat to ambient air or another fluid in a gas cooler or condenser), expanded to a lower pressure (e.g., through an expansion valve), and evaporated to provide cooling by absorbing heat into the refrigerant. CO2refrigeration systems are a type of vapor compression refrigeration system that use CO2as a refrigerant.

Heat absorption and heat rejection are two of the four thermodynamic paths that make up the vapor compression cycle. Both heat absorption and heat rejection take advantage of latent heat transfer, causing a refrigerant to change state from a saturated liquid to saturated vapor (i.e., evaporation) or from a saturated vapor to a saturated liquid (i.e., condensation). As heat is absorbed or rejected during evaporation and condensation, the pressure and the temperature may remain constant (this may not be the case if the refrigerant is a blend of refrigerants that exhibit different saturation characteristics). Any heat transfer that occurs outside of this phase changing process is known as sensible heat transfer and results in a change in temperature of the refrigerant. Sensible heat transfer can be defined as either a subcooling of liquid or a superheating of gas. When pressure is constant and the temperature of a refrigerant decreases below its saturated temperature at that pressure, its subcooling value increases. Likewise, when pressure is constant and the temperature of the refrigerant increases above its saturation temperature at that pressure, its superheating value increases. Alternatively, if the temperature remains constant, subcooling and superheating can be achieved by either increasing the pressure of the refrigerant above its saturation pressure at that temperature or decreasing the pressure of the refrigerant below its saturation pressure at that temperature, respectively. Some refrigeration systems seek to achieve a subcooling setpoint by increasing the pressure of a refrigerant to be greater than its saturation pressure. However, a refrigerant not in a subcritical region (i.e., having a temperature above the critical temperature of the refrigerant) does not have the capability of latent heat transfer (condensing or evaporating) and thus cannot be condensed isothermally by increasing its pressure. Therefore a refrigerant having a temperature greater than its critical temperature has no corresponding saturation pressure. For this reason, it is common for non-subcooling control schemes (such as methods to maximize system COP) to be implemented to control the high side of supercritical vapor compression cycle systems.

SUMMARY

One implementation of the present disclosure is a refrigeration system. The refrigeration system includes a gas cooler/condenser configured to remove heat from a refrigerant and discharge the refrigerant into a high pressure conduit, a temperature sensor located along the high pressure conduit and configured to measure a temperature of the refrigerant leaving the gas cooler/condenser, a pressure sensor located along the high pressure conduit and configured to measure a pressure of the refrigerant leaving the gas cooler/condenser, a pressure control valve located along the high pressure conduit and operable to regulate the pressure of the refrigerant leaving the gas cooler/condenser, and a controller. The controller is configured to determine whether the refrigerant leaving the gas cooler/condenser is in a subcritical region based on at least one of the measured temperature or the measured pressure of the refrigerant. In response to determining that the refrigerant leaving the gas cooler/condenser is not in the subcritical region, the controller is configured to add a pseudo-subcooling temperature value to the measured temperature of the refrigerant to calculate a summed temperature, calculate a supercritical pseudo-saturated pressure as a function of the summed temperature, and operate the pressure control valve to drive the pressure of the refrigerant leaving the gas cooler/condenser to the supercritical pseudo-saturated pressure corresponding to the summed temperature.

In some embodiments, the controller is configured to generate a supercritical pseudo-saturation function for the refrigerant using supercritical pressure (P), enthalpy (H), and temperature (T) data for the refrigerant and calculate the supercritical pseudo-saturated pressure using the supercritical pseudo-saturation function.

In some embodiments, wherein the controller is configured to generate the supercritical pseudo-saturation function by identifying inflection points of supercritical P-H isotherms for the refrigerant using the supercritical P-H-T data for the refrigerant and deriving the supercritical pseudo-saturation function from the inflection points of the supercritical isotherms.

In some embodiments, deriving the supercritical pseudo-saturation function from the inflection points of the supercritical isotherms includes fitting a supercritical pseudo-saturated line to the inflection points of the supercritical isotherms. In some embodiments, deriving the supercritical pseudo-saturation function from the inflection points of the supercritical isotherms includes deriving an equation that defines the supercritical pseudo-saturated line. In some embodiments, the supercritical pseudo-saturated line is continuous with a subcritical saturation curve (on a pressure-temperature plot), as shown inFIG. 5B.

In some embodiments, the pseudo-subcooling temperature value is a dynamic value and the controller is configured to calculate the dynamic pseudo-subcooling temperature value as a function of the measured temperature of the refrigerant leaving the gas cooler/condenser. In some embodiments, the dynamic pseudo-subcooling temperature values and/or the slope of the function that yields such values are modifiable to allow for different dynamic pseudo-subcooling temperature values, as may be desired in various implementations.

In some embodiments, in response to determining that the refrigerant leaving the gas cooler/condenser is in the subcritical region, the controller is configured to add a fixed temperature value to the measured temperature of the refrigerant to calculate a second summed temperature, calculate a subcritical saturated pressure as a function of the second summed temperature, and operate the pressure control valve to drive the pressure of the refrigerant leaving the gas cooler/condenser to the subcritical saturated pressure corresponding to the second summed temperature.

DETAILED DESCRIPTION

Referring generally to the FIGURES, a CO2refrigeration system is shown, according to various exemplary embodiments. The CO2refrigeration system may be a vapor compression refrigeration system which uses primarily carbon dioxide (i.e., CO2) as a refrigerant. In some implementations, the CO2refrigeration system is used to provide cooling for temperature controlled display devices in a supermarket or other similar facility.

Referring now toFIG. 1, a CO2refrigeration system100is shown, according to an exemplary embodiment. CO2refrigeration system100may be a vapor compression refrigeration system which uses primarily carbon dioxide (CO2) as a refrigerant. However, it is contemplated that other refrigerants can be substituted for CO2without departing from the teachings of the present disclosure. CO2refrigeration system100and is shown to include a system of pipes, conduits, or other fluid channels (e.g., fluid conduits1,3,5,7,9,13,23,25,27, and42) for transporting the CO2refrigerant between various components of CO2refrigeration system100. The components of CO2refrigeration system100are shown to include a gas cooler/condenser2, a high pressure valve4, a receiver6, a gas bypass valve8, a medium-temperature (“MT”) subsystem10, and a low-temperature (“LT”) subsystem20. In some embodiments, CO2refrigeration system100includes a parallel compressor26which may replace high pressure valve4or work in parallel with high pressure valve4. Parallel compressor26may be implemented with an ejector or without the ejector. Both parallel compressor26and the ejector are described in greater detail below.

Gas cooler/condenser2may be a heat exchanger or other similar device for removing heat from the CO2refrigerant. Gas cooler/condenser2is shown receiving CO2gas from fluid conduit1. In some embodiments, the CO2gas in fluid conduit1may have a pressure within a range from approximately 45 bar to approximately 100 bar (i.e., about 650 psig to about 1450 psig), depending on ambient temperature and other operating conditions. In some embodiments, gas cooler/condenser2may partially or fully condense CO2gas into liquid CO2(e.g., if system operation is in a subcritical region). The condensation process may result in fully saturated CO2liquid or a two-phase liquid-vapor mixture (e.g., having a thermodynamic vapor quality between 0 and 1). In other embodiments, gas cooler/condenser2may cool the CO2gas (e.g., by removing only sensible heat) without condensing the CO2gas into CO2liquid (e.g., if system operation is in a supercritical region). In some embodiments, the cooling/condensation process may be assumed to be an isobaric process. Gas cooler/condenser2is shown outputting the cooled and/or condensed CO2refrigerant into fluid conduit3.

In some embodiments, CO2refrigeration system100includes a temperature sensor33and a pressure sensor34configured to measure the temperature and pressure of the CO2refrigerant exiting gas cooler/condenser2. Sensors33and34can be installed along fluid conduit3(as shown inFIG. 1), within gas cooler/condenser2, or otherwise positioned to measure the temperature and pressure of the CO2refrigerant exiting gas cooler/condenser2. In some embodiments, CO2refrigeration system100includes a condenser fan35configured to provide airflow across gas cooler/condenser2. The speed of condenser fan35can be controlled to increase or decrease the airflow across gas cooler/condenser2to modulate the amount of cooling applied to the CO2refrigerant within gas cooler/condenser2. In some embodiments, CO2refrigeration system100also includes a temperature sensor37and/or a pressure sensor38configured to measure the temperature and/or pressure of the ambient air that flows across gas cooler/condenser2to provide cooling for the CO2refrigerant contained therein.

High pressure valve4receives the cooled and/or condensed CO2refrigerant from fluid conduit3and outputs the CO2refrigerant to fluid conduit5. High pressure valve4can be operated to control the high side pressure of the CO2refrigerant (i.e., the pressure of the CO2refrigerant in fluid conduit1, gas cooler/condenser2, and/or fluid conduit3) by adjusting an amount of CO2refrigerant permitted to pass through high pressure valve4. High pressure valve4can be operated automatically (e.g., by a controller50) to control the high side pressure of the CO2refrigerant. In some embodiments, CO2refrigeration system100includes an ejector in place of high pressure valve4or in parallel with high pressure valve4. Like high pressure valve4, the ejector can be operated automatically (e.g., by controller50) to control the high side pressure of the CO2refrigerant. In some embodiments, controller50receives measurements of the temperature and/or pressure of the CO2refrigerant exiting gas cooler/condenser2from sensors33-34. Controller50can calculate an appropriate high side pressure setpoint for the CO2refrigerant and can operate high pressure valve4to achieve the high side pressure setpoint within fluid conduit1, gas cooler/condenser2, and/or fluid conduit3. The high side pressure control performed by controller50is described in greater detail with reference toFIGS. 2-7.

In some embodiments, high pressure valve4is a high pressure thermal expansion valve (e.g., if the pressure in fluid conduit3is greater than the pressure in fluid conduit5). In such embodiments, high pressure valve4may allow the CO2refrigerant to expand to a lower pressure state. The expansion process may be an isenthalpic and/or adiabatic expansion process, resulting in a two-phase flash of the high pressure CO2refrigerant to a lower pressure, lower temperature state. The expansion process may produce a liquid/vapor mixture (e.g., having a thermodynamic vapor quality between 0 and 1). In some embodiments, the CO2refrigerant expands to a pressure of approximately 38 bar (e.g., about 550 psig), which corresponds to a temperature of approximately 40° F. The CO2refrigerant then flows from fluid conduit5into receiver6.

Receiver6collects the CO2refrigerant from fluid conduit5. In some embodiments, receiver6may be a flash tank or other fluid reservoir. Receiver6includes a CO2liquid portion16and a CO2vapor portion15and may contain a partially saturated mixture of CO2liquid and CO2vapor. In some embodiments, receiver6separates the CO2liquid from the CO2vapor. The CO2liquid may exit receiver6through fluid conduits9. Fluid conduits9may be liquid headers leading to MT subsystem10and/or LT subsystem20. The CO2vapor may exit receiver6through fluid conduit7. Fluid conduit7is shown leading the CO2vapor to a gas bypass valve8and a parallel compressor26(described in greater detail below). In some embodiments, CO2refrigeration system100includes a temperature sensor31and a pressure sensor32configured to measure the temperature and pressure within receiver6. Sensors31and32can be installed in or on receiver6(as shown inFIG. 1) or along any of the fluid conduits that contain CO2refrigerant at the same temperature and/or pressure as receiver6(i.e., fluid conduits5,7,9, or27).

Still referring toFIG. 1, MT subsystem10is shown to include one or more expansion valves11, one or more MT evaporators12, and one or more MT compressors14. In various embodiments, any number of expansion valves11, MT evaporators12, and MT compressors14may be present. Expansion valves11may be electronic expansion valves or other similar expansion valves. Expansion valves11are shown receiving liquid CO2refrigerant from fluid conduit9and outputting the CO2refrigerant to MT evaporators12. Expansion valves11may cause the CO2refrigerant to undergo a rapid drop in pressure, thereby expanding the CO2refrigerant to a lower pressure, lower temperature two-phase state. In some embodiments, expansion valves11may expand the CO2refrigerant to a pressure of approximately 25 bar-33 bar and a temperature of approximately 13° F.-30° F. The expansion process may be an isenthalpic and/or adiabatic expansion process.

MT evaporators12are shown receiving the cooled and expanded CO2refrigerant from expansion valves11. In some embodiments, MT evaporators may be associated with display cases/devices (e.g., if CO2refrigeration system100is implemented in a supermarket setting). MT evaporators12may be configured to facilitate the transfer of heat from the display cases/devices into the CO2refrigerant. The added heat may cause the CO2refrigerant to evaporate partially or completely. According to one embodiment, the CO2refrigerant is fully evaporated in MT evaporators12. In some embodiments, the evaporation process may be an isobaric process. MT evaporators12are shown outputting the CO2refrigerant via suction line13, leading to MT compressors14.

MT compressors14compress the CO2refrigerant into a superheated gas having a pressure within a range of approximately 45 bar to approximately 100 bar. The output pressure from MT compressors14may vary depending on ambient temperature and other operating conditions. In some embodiments, MT compressors14operate in a transcritical mode. In operation, the CO2discharge gas exits MT compressors14and flows through fluid conduit1into gas cooler/condenser2.

Still referring toFIG. 1, LT subsystem20is shown to include one or more expansion valves21, one or more LT evaporators22, and one or more LT compressors24. In various embodiments, any number of expansion valves21, LT evaporators22, and LT compressors24may be present. In some embodiments, LT subsystem20may be omitted and the CO2refrigeration system100may operate with an AC module or parallel compressor26interfacing with only MT subsystem10.

Expansion valves21may be electronic expansion valves or other similar expansion valves. Expansion valves21are shown receiving liquid CO2refrigerant from fluid conduit9and outputting the CO2refrigerant to LT evaporators22. Expansion valves21may cause the CO2refrigerant to undergo a rapid drop in pressure, thereby expanding the CO2refrigerant to a lower pressure, lower temperature two-phase state. The expansion process may be an isenthalpic and/or adiabatic expansion process. In some embodiments, expansion valves21may expand the CO2refrigerant to a lower pressure than expansion valves11, thereby resulting in a lower temperature CO2refrigerant. Accordingly, LT subsystem20may be used in conjunction with a freezer system or other lower temperature display cases.

LT evaporators22are shown receiving the cooled and expanded CO2refrigerant from expansion valves21. In some embodiments, LT evaporators may be associated with display cases/devices (e.g., if CO2refrigeration system100is implemented in a supermarket setting). LT evaporators22may be configured to facilitate the transfer of heat from the display cases/devices into the CO2refrigerant. The added heat may cause the CO2refrigerant to evaporate partially or completely. In some embodiments, the evaporation process may be an isobaric process. LT evaporators22are shown outputting the CO2refrigerant via suction line23, leading to LT compressors24.

LT compressors24compress the CO2refrigerant. In some embodiments, LT compressors24may compress the CO2refrigerant to a pressure of approximately 30 bar (e.g., about 450 psig) having a saturation temperature of approximately 23° F. In some embodiments, LT compressors24operate in a subcritical mode. LT compressors24are shown outputting the CO2refrigerant through discharge line25. Discharge line25may be fluidly connected with the suction (e.g., upstream) side of MT compressors14.

Still referring toFIG. 1, CO2refrigeration system100is shown to include a gas bypass valve8. Gas bypass valve8may receive the CO2vapor from fluid conduit7and output the CO2refrigerant to MT subsystem10. In some embodiments, gas bypass valve8is arranged in series with MT compressors14. In other words, CO2vapor from receiver6may pass through both gas bypass valve8and MT compressors14. MT compressors14may compress the CO2vapor passing through gas bypass valve8from a low pressure state (e.g., approximately 30 bar or lower) to a high pressure state (e.g., 45-100 bar).

Gas bypass valve8may be operated to regulate or control the pressure within receiver6(e.g., by adjusting an amount of CO2refrigerant permitted to pass through gas bypass valve8). For example, gas bypass valve8may be adjusted (e.g., variably opened or closed) to adjust the mass flow rate, volume flow rate, or other flow rates of the CO2refrigerant through gas bypass valve8. Gas bypass valve8may be opened and closed (e.g., manually, automatically, by a controller, etc.) as needed to regulate the pressure within receiver6.

In some embodiments, gas bypass valve8includes a sensor for measuring a flow rate (e.g., mass flow, volume flow, etc.) of the CO2refrigerant through gas bypass valve8. In other embodiments, gas bypass valve8includes an indicator (e.g., a gauge, a dial, etc.) from which the position of gas bypass valve8may be determined. This position may be used to determine the flow rate of CO2refrigerant through gas bypass valve8, as such quantities may be proportional or otherwise related.

In some embodiments, gas bypass valve8may be a thermal expansion valve (e.g., if the pressure on the downstream side of gas bypass valve8is lower than the pressure in fluid conduit7). According to one embodiment, the pressure within receiver6is regulated by gas bypass valve8to a pressure of approximately 38 bar, which corresponds to about 37° F. Advantageously, this pressure/temperature state may facilitate the use of copper tubing/piping for the downstream CO2lines of the system. Additionally, this pressure/temperature state may allow such copper tubing to operate in a substantially frost-free manner.

In some embodiments, the CO2vapor that is bypassed through gas bypass valve8is mixed with the CO2refrigerant gas exiting MT evaporators12(e.g., via suction line13). The bypassed CO2vapor may also mix with the discharge CO2refrigerant gas exiting LT compressors24(e.g., via discharge line25). The combined CO2refrigerant gas may be provided to the suction side of MT compressors14.

In some embodiments, the pressure immediately downstream of gas bypass valve8(i.e., in suction line13) is lower than the pressure immediately upstream of gas bypass valve8(i.e., in fluid conduit7). Therefore, the CO2vapor passing through gas bypass valve8and MT compressors14may be expanded (e.g., when passing through gas bypass valve8) and subsequently recompressed (e.g., by MT compressors14). This expansion and recompression may occur without any intermediate transfers of heat to or from the CO2refrigerant, which can be characterized as an inefficient energy usage.

Still referring toFIG. 1, CO2refrigeration system100is shown to include a parallel compressor26. Parallel compressor26may be arranged in parallel with MT compressors14and arranged in series with LT compressors24. Although only one parallel compressor26is shown, any number of parallel compressors may be present. Parallel compressor26may be fluidly connected with receiver6and/or fluid conduit7via a connecting line27. Parallel compressor26may be used to draw non-condensed CO2vapor from receiver6as a means for pressure control and regulation. Advantageously, using parallel compressor26to effectuate pressure control and regulation may provide a more efficient alternative to traditional pressure regulation techniques such as bypassing CO2vapor through bypass valve8to the lower pressure suction side of MT compressors14.

In some embodiments, parallel compressor26may be operated (e.g., by a controller50) to achieve a desired pressure within receiver6. For example, controller50may activate or deactivate parallel compressor26when the flow rate of the CO2refrigerant through gas bypass valve8exceeds a threshold value to assist with regulating the pressure within receiver6. Parallel compressor26may have a minimum flow rate requirement and may activate and remain on as long as the flow rate of the CO2refrigerant through parallel compressor26is at least its minimum required flow rate. When active, parallel compressor26compresses the CO2vapor received via connecting line27and discharges the compressed gas into discharge line42. Discharge line42may be fluidly connected with fluid conduit1. Accordingly, parallel compressor26may operate in parallel with MT compressors14by discharging the compressed CO2gas into a shared fluid conduit (e.g., fluid conduit1).

Parallel compressor26may be arranged in parallel with both gas bypass valve8and with MT compressors14. CO2vapor exiting receiver6may pass through either parallel compressor26or the series combination of gas bypass valve8and MT compressors14. Parallel compressor26may receive the CO2vapor at a relatively higher pressure (e.g., from fluid conduit7) than the CO2vapor received by MT compressors14(e.g., from suction line13). This differential in pressure may correspond to the pressure differential across gas bypass valve8. In some embodiments, parallel compressor26may require less energy to compress an equivalent amount of CO2vapor to the high pressure state (e.g., in fluid conduit1) as a result of the higher pressure of CO2vapor entering parallel compressor26. Therefore, the parallel route including parallel compressor26may be a more efficient alternative to the route including gas bypass valve8and MT compressors14.

In some embodiments, gas bypass valve8is omitted and the pressure within receiver6is regulated using parallel compressor26. In other embodiments, parallel compressor26is omitted and the pressure within receiver6is regulated using gas bypass valve8. In other embodiments, both gas bypass valve8and parallel compressor26are used to regulate the pressure within receiver6. All such variations are within the scope of the present disclosure.

Controller

Referring now toFIG. 2, a block diagram illustrating controller50in greater detail is shown, according to an exemplary embodiment. Controller50may receive signals from one or more measurement devices (e.g., pressure sensors, temperature sensors, flow sensors, etc.) located within CO2refrigeration system100. For example, controller50is shown receiving temperature and pressure measurements from sensors33-34and a valve position signal from high pressure valve4. Although not explicitly shown inFIG. 2, controller50may also receive measurements from any of sensors31-34and37-38, a fan speed signal from condenser fan35, a valve position signal from gas bypass valve8, and/or any other measurements or inputs from the various devices of CO2refrigeration system100.

Controller50may use the input signals to determine appropriate control actions for controllable devices of CO2refrigeration system100(e.g., compressors14and24, parallel compressor26, condenser fan35, valves4,8,11, and21, flow diverters, power supplies, etc.). For example, controller50is shown providing control signals to high pressure valve4. Although not explicitly shown inFIG. 2, controller50may also provide control signals to MT compressors14, LT compressors24, parallel compressor26, gas bypass valve8, condenser fan35, and/or any other controllable device of CO2refrigeration system100.

In some embodiments, controller50is configured to operate high pressure valve4to maintain the high side pressure Phighof the CO2refrigerant (i.e., the pressure measured by pressure sensor34) at a high side pressure setpoint Psp. Controller50can generate the high side pressure setpoint Pspto ensure that the CO2refrigerant exiting gas cooler/condenser2has a desired amount of subcooling. The desired amount of subcooling may vary depending on whether the CO2refrigerant exiting gas cooler/condenser2is in a subcritical region or a supercritical region. In some embodiments, controller50may compare the high side pressure Phighof the CO2refrigerant exiting gas cooler/condenser2(i.e., the pressure measured by pressure sensor34) to the critical pressure Pcritof the CO2refrigerant to determine whether the CO2refrigerant is in a supercritical region or subcritical region. In other embodiments, controller50may compare the temperature TGCof the CO2refrigerant exiting gas cooler/condenser2(i.e., the temperature measured by temperature sensor33) to the critical temperature Tcritof the CO2refrigerant to determine whether the CO2refrigerant is in a supercritical region or subcritical region.

In some embodiments, if the high side pressure Phighof the CO2refrigerant exiting gas cooler/condenser2is less than the critical pressure Pcrit(i.e., Phigh<Pcrit), controller50may determine that the CO2refrigerant is in a subcritical region. In other embodiments, if the temperature TGCof the CO2refrigerant exiting gas cooler/condenser2is less than the critical temperature Tcrit(i.e., TGC<Tcrit), controller50may determine that the CO2refrigerant is in a subcritical region. In response to determining that the CO2refrigerant is in a subcritical region, controller50may identify a predetermined or fixed subcooling value TSC,fixed. Controller50may then add the fixed subcooling value TSC,fixedto the measured temperature TGCand identify a corresponding saturation pressure Psat(TGC+TSC,fixed), where the function Psat( ) calculates the saturation pressure Psatof the CO2refrigerant at a given subcritical temperature (i.e., the summed temperature TGC+TSC,fixed). Controller50may then set the high side pressure setpoint Pspequal to the calculated saturation pressure Psatand operate high pressure valve4to drive the high side pressure Phighto the pressure setpoint Psp.

In some embodiments, if the high side pressure Phighof the CO2refrigerant exiting gas cooler/condenser2is greater than the critical pressure Pcrit(i.e., Phigh>Pcrit), controller50may determine that the CO2refrigerant is in a supercritical region. In other embodiments, if the temperature TGCof the CO2refrigerant exiting gas cooler/condenser2is greater than the critical temperature Tcrit(i.e., TGC>Tcrit), controller50may determine that the CO2refrigerant is in a supercritical region. In response to determining that the CO2refrigerant is in a supercritical region, controller50may identify a dynamic pseudo-subcooling value TSC,dynamicthat corresponds the measured temperature TGC. The dynamic pseudo-subcooling value TSC,dynamicmay vary as a function of the measured temperature TGCas well as a user controlled modification to the dynamic pseudo-subcooling values, yielding either a lower or higher controlled pressure at any given supercritical temperature. Controller50may then add the dynamic pseudo-subcooling value TSC,dynamicto the measured temperature TGCand identify a corresponding pseudo-saturation pressure Psat*(TGC+TSC,dynamic), where the function Psat*( ) defines the pseudo-saturation pressure Psat* of the CO2refrigerant at a given supercritical temperature (i.e., the summed temperature TGC+TSC,dynamic). The pseudo-saturation pressure Psat* for supercritical temperatures and the function Psat*( ) are described in greater detail below. Controller50may then set the high side pressure setpoint Pspequal to the calculated pseudo-saturation pressure Psat* and operate high pressure valve4to drive the high side pressure Phighto the pressure setpoint Psp.

In some embodiments, controller50is configured to operate gas bypass valve8and/or parallel compressor26to maintain the CO2pressure within receiving tank6at a desired setpoint or within a desired range. In some embodiments, controller50operates gas bypass valve8and parallel compressor26based a flow rate (e.g., mass flow, volume flow, etc.) of CO2refrigerant through gas bypass valve8. Controller50may use a valve position of gas bypass valve8as a proxy for CO2refrigerant flow rate. In some embodiments, controller50operates high pressure valve4and expansion valves11and21to regulate the flow of refrigerant in system100.

Controller50may include feedback control functionality for adaptively operating the various components of CO2refrigeration system100. For example, controller50may receive or generate a setpoint (e.g., a temperature setpoint, a pressure setpoint, a flow rate setpoint, a power usage setpoint, etc.) and operate one or more components of system100to achieve the setpoint. The setpoint may be specified by a user (e.g., via a user input device, a graphical user interface, a local interface, a remote interface, etc.) or automatically determined by controller50based on one or more measurements collected by the sensors of CO2refrigeration system100. In some embodiments, controller50includes some or all of the features of the controller described in P.C.T. Patent Application No. PCT/US2016/044164 filed Jul. 27, 2016, the entire disclosure of which is incorporated by reference herein.

Controller50may be a proportional-integral (PI) controller, a proportional-integral-derivative (PID) controller, a pattern recognition adaptive controller (PRAC), a model recognition adaptive controller (MRAC), a model predictive controller (MPC), or any other type of controller employing any type of control functionality. In some embodiments, controller50is a local controller for CO2refrigeration system100. In other embodiments, controller50is a supervisory controller for a plurality of controlled subsystems (e.g., a refrigeration system, an AC system, a lighting system, a security system, etc.). For example, controller50may be a controller for a comprehensive building management system incorporating CO2refrigeration system100. Controller50may be implemented locally, remotely, or as part of a cloud-hosted suite of building management applications.

Still referring toFIG. 2, controller50is shown to include a communications interface54and a processing circuit51. Communications interface54can be or include wired or wireless interfaces (e.g., jacks, antennas, transmitters, receivers, transceivers, wire terminals, etc.) for conducting electronic data communications. For example, communications interface54may be used to conduct communications with gas bypass valve8, parallel compressor26, compressors14and24, high pressure valve4, various data acquisition devices within CO2refrigeration system100(e.g., temperature sensors, pressure sensors, flow sensors, etc.) and/or other external devices or data sources. Data communications may be conducted via a direct connection (e.g., a wired connection, an ad-hoc wireless connection, etc.) or a network connection (e.g., an Internet connection, a LAN, WAN, or WLAN connection, etc.). For example, communications interface54can include an Ethernet card and port for sending and receiving data via an Ethernet-based communications link or network. In another example, communications interface54can include a Wi-Fi transceiver or a cellular or mobile phone transceiver for communicating via a wireless communications network.

In some embodiments, communications interface54receives a measurement of a gas cooler/condenser exit temperature TGCfrom temperature sensor33and a measurement of the high side pressure Phighfrom pressure sensor34. The gas cooler/condenser exit temperature TGCmay indicate the temperature of the CO2refrigerant at the outlet of gas cooler/condenser2, whereas the high side pressure Phighmay indicate the pressure of the CO2refrigerant at the outlet of gas cooler/condenser2. If the cooling/condensation of the CO2refrigerant within gas cooler/condenser2is isobaric, the high side pressure Phighmay also be the pressure of the CO2refrigerant within the high side components of CO2refrigeration system100(i.e., fluid conduit1, gas cooler/condenser2, and/or fluid conduit3). Communications interface54may also receive a valve position signal from high pressure valve4. Communications interface54may provide control signals to high pressure valve4(e.g., to an electromechanical actuator that operates high pressure valve4) to drive the high side pressure Phighof the CO2refrigerant to a high side pressure setpoint.

Processing circuit51is shown to include a processor52and memory53. Processor52can be implemented as a general purpose processor, an application specific integrated circuit (ASIC), one or more field programmable gate arrays (FPGAs), a group of processing components, a microcontroller, or other suitable electronic processing components. Memory53(e.g., memory device, memory unit, storage device, etc.) may be one or more devices (e.g., RAM, ROM, solid state memory, hard disk storage, etc.) for storing data and/or computer code for completing or facilitating the various processes, layers and modules described in the present application. Memory53may be or include volatile memory or non-volatile memory. Memory53may include database components, object code components, script components, or any other type of information structure for supporting the various activities and information structures described in the present application. According to an exemplary embodiment, memory53is communicably connected to processor52via processing circuit51and includes computer code for executing (e.g., by processing circuit51and/or processor52) one or more processes or control features described herein.

Still referring toFIG. 2, controller50is shown to include a criticality detector55. Criticality detector55can be configured to determine whether the CO2refrigerant leaving gas cooler/condenser2is in a subcritical region or supercritical region. In some embodiments, criticality detector55may receive the measured pressure Phighof the CO2refrigerant exiting gas cooler/condenser2and may compare the measured pressure Phighwith the critical pressure Pcritof the CO2refrigerant. The critical pressure Pcritis a known constant and may be stored as a fixed value within memory53. If the pressure Phighof the CO2refrigerant exiting gas cooler/condenser2is less than the critical pressure Pcrit(i.e., Phigh<Pcrit) criticality detector55may determine that the CO2refrigerant is in a subcritical region. However, if the pressure Phighof the CO2refrigerant exiting gas cooler/condenser2is greater than the critical pressure Pcrit(i.e., Phigh>Pcrit), criticality detector55may determine that the CO2refrigerant is in a supercritical region.

In other embodiments, criticality detector55may receive the measured temperature TGCof the CO2refrigerant exiting gas cooler/condenser2and may compare the measured temperature TGCwith the critical temperature Tcritof the CO2refrigerant. The critical temperature Tcritis a known constant and may be stored as a fixed value within memory53. If the temperature TGCof the CO2refrigerant exiting gas cooler/condenser2is less than the critical temperature Tcrit(i.e., TGC<Tcrit), criticality detector55may determine that the CO2refrigerant is in a subcritical region. However, if the temperature TGCof the CO2refrigerant exiting gas cooler/condenser2is greater than the critical temperature Tcrit(i.e., TGC>Tcrit), criticality detector55may determine that the CO2refrigerant is in a supercritical region.

In response to determining that the CO2refrigerant is in a subcritical region, criticality detector55may trigger subcooling value generator56and saturation pressure calculator58to generate the pressure setpoint Psp. Subcooling value generator56may identify a predetermined or fixed subcooling value TSC,fixedand may provide the fixed subcooling value TSC,fixedto saturation pressure calculator58. The fixed subcooling value TSC,fixedcan be stored in memory53, specified by a user, and/or received from an external data source.

Saturation pressure calculator58may add the fixed subcooling value TSC,fixedto the measured temperature TGCand may identify a saturation pressure Psatthat corresponds to the summed temperature TGC+TSC,fixed. In some embodiments, saturation pressure calculator58uses a function Psat(TGC+TSC,fixed) to calculate the saturation pressure Psatas a function of the summed temperature TGC+TSC,fixed. The function Psat( ) may define the saturation pressure Psatof the CO2refrigerant as a function of temperature. In other embodiments, saturation pressure calculator58uses a lookup table that defines pairs of saturation pressures and corresponding saturation temperatures of the CO2refrigerant and interpolates within the lookup table to calculate the saturation pressure Psatas a function of the summed temperature TGC+TSC,fixed.

Saturation pressure calculator58may then set the high side pressure setpoint Pspequal to the calculated saturation pressure Psat. By setting the high side pressure setpoint Pspequal to the calculated saturation pressure Psat, saturation pressure calculator58ensures that the CO2refrigerant has the desired amount of subcooling (i.e., TSC,fixed) at the exit of gas cooler/condenser2. For example, the saturation temperature of the CO2refrigerant at the calculated saturation pressure Psatis equal to the summed temperature TGC+TSC,fixed. Because the actual temperature of the CO2refrigerant at the exit of gas cooler/condenser2is TGC, the difference between the actual temperature and the saturation temperature is TSC,fixed. In other words, the CO2refrigerant is subcooled by the desired amount TSC,fixed.

In response to determining that the CO2refrigerant is in a supercritical region, criticality detector55may trigger pseudo-subcooling value generator57and pseudo-saturation pressure calculator59to generate the pressure setpoint Psp. Pseudo-subcooling value generator57may identify a dynamic pseudo-subcooling value TSC,dynamicthat corresponds the measured temperature TGC. The dynamic pseudo-subcooling value TSC,dynamicmay vary as a function of the measured temperature TGC. In some embodiments, the dynamic pseudo-subcooling value TSC,dynamicis relatively larger at higher values of the measured temperature TGCand relatively smaller at lower values of the measured temperature TGC. In other embodiments, the dynamic pseudo-subcooling value TSC,dynamicis relatively smaller at higher values of the measured temperature TGCand relatively larger at lower values of the measured temperature TGC. The function that defines the dynamic pseudo-subcooling values TSC,dynamicas a function of the measured temperature TGCcan be adjusted (e.g., by a user) to control the relationship between the dynamic pseudo-subcooling values TSC,dynamicand the temperature TGC. For example, a user can provide input to controller50(as shown inFIG. 8) and can adjust a slope of a supercritical control line to adjust whether the dynamic pseudo-subcooling values TSC,dynamicincrease, decrease, or remain constant as the measured temperature TGCincreases. Pseudo-subcooling value generator57may calculate TSC,dynamicas a function of TGCand provide the calculated value of TSC,dynamicto pseudo-saturation pressure calculator59.

Pseudo-saturation pressure calculator59may add the dynamic pseudo-subcooling value TSC,dynamicto the measured temperature TGCand may identify a pseudo-saturation pressure Psat* that corresponds to the summed temperature TGC+TSC,dynamic. In some embodiments, pseudo-saturation pressure calculator59uses a function Psat*(TGC+TSC,dynamic) to calculate the pseudo-saturation pressure Psat* as a function of the summed temperature TGC+TSC,dynamic. The function Psat*( ) may define the pseudo-saturation pressure Psat* of the CO2refrigerant as a function of temperature. In other embodiments, pseudo-saturation pressure calculator59uses a lookup table that defines pairs of pseudo-saturation pressures and corresponding supercritical temperatures of the CO2refrigerant and interpolates within the lookup table to calculate the pseudo-saturation pressure Psat* as a function of the summed temperature TGC+TSC,dynamic. The pseudo-saturation pressure Psat* for supercritical temperatures and the function Psat*( ) are described in greater detail with reference toFIG. 3.

Pseudo-saturation pressure calculator59may then set the high side pressure setpoint Pspequal to the calculated pseudo-saturation pressure Psat*. By setting the high side pressure setpoint Pspequal to the calculated pseudo-saturation pressure Psat*, pseudo-saturation pressure calculator59ensures that the CO2refrigerant has the desired amount of pseudo-subcooling (i.e., TSC,dynamic) at the exit of gas cooler/condenser2. For example, the pseudo-saturation temperature of the CO2refrigerant at the calculated pseudo-saturation pressure Psat* is equal to the summed temperature TccTSC,dynamic. Because the actual temperature of the CO2refrigerant at the exit of gas cooler/condenser2is TGC, the difference between the actual temperature and the pseudo-saturation temperature is TSC,dynamic. In other words, the CO2refrigerant is subcooled by the desired amount TSC,dynamic.

Still referring toFIG. 2, controller50is shown to include a valve controller60. Valve controller60may receive the pressure setpoint Pspfrom saturation pressure calculator58or pseudo-saturation pressure calculator59, depending on whether the CO2refrigerant is in a subcritical region or supercritical region exiting gas cooler/condenser2. Valve controller60may also receive the high side pressure Phighmeasured by pressure sensor34. Valve controller60can operate high pressure valve4using any of a variety of feedback control techniques (e.g., PID, PI, MPC, etc.) to drive the measured high side pressure Phighto the pressure setpoint Psp. For example, valve controller60is shown providing control signals to high pressure valve4. The control signals may cause high pressure valve4to variably open or close to adjust the flowrate of the CO2refrigerant through high pressure valve4, thereby affecting the high side pressure Phighupstream of high pressure valve4. In some embodiments, valve controller60causes high pressure valve4to open more to decrease the high side pressure Phigh(i.e., if the high side pressure Phighis greater than the pressure setpoint Psp) and causes high pressure valve4to close more to increase the high side pressure Phigh(i.e., if the high side pressure Phighis less than the pressure setpoint Psp).

Pseudo-Saturation Pressure Calculator

Referring now toFIG. 3, a block diagram illustrating pseudo-saturation pressure calculator59in greater detail is shown, according to an exemplary embodiment. As described above, pseudo-saturation pressure calculator59can be configured to calculate a pseudo-saturation pressure Psat* of the CO2refrigerant at various supercritical temperatures. When the measured pressure of the CO2refrigerant Phighexceeds the supercritical pressure Pcritand/or the measured temperature of the CO2refrigerant TGCexceeds the supercritical temperature Tcrit, pseudo-saturation pressure calculator59can add the desired amount of pseudo-subcooling TSC,dynamicto the measured temperature TGCand calculate the corresponding pseudo-saturation pressure Psat*(TGC+TSC,dynamic). Pseudo-saturation pressure calculator59can then set the pressure setpoint Pspto the calculated pseudo-saturation pressure Psat* to ensure that the CO2refrigerant has the desired amount of pseudo-subcooling TSC,dynamic. The components of pseudo-saturation pressure calculator59and steps performed by pseudo-saturation pressure calculator59to calculate the pseudo-saturation pressure Psat* are described in detail below.

Pseudo-saturation pressure calculator59is shown to include a refrigerant pressure (P), enthalpy (H), and temperature (T) database61. P-H-T database61may store data defining various potential states of the CO2refrigerant. Each potential state of the CO2refrigerant may have a corresponding pressure value, a corresponding enthalpy value, and a corresponding temperature value. In other words, P-H-T database61may store various P-H-T data points for the CO2refrigerant. The P-H-T data points may be based on known properties and chemical characteristics of the CO2refrigerant and may be received from an external data source.

The data stored in P-H-T database61can be represented graphically as shown inFIG. 4.FIG. 4is a pressure-enthalpy (P-H) diagram400of the CO2refrigerant. In P-H diagram400, pressure is shown along the vertical axis, whereas enthalpy is shown along the horizontal axis. Isotherms402are curves of constant temperature that increase in value from left to right. Isotherms402having a temperature and pressure above the critical temperature Tcritand critical pressure Pcritdefined by critical point406(i.e., supercritical isotherms) represent the supercritical region of the CO2refrigerant, whereas isotherms402having a temperature below the critical temperature Tcritand/or a pressure below the critical pressure Pcrit(i.e., subcritical isotherms) represent the subcritical region of the CO2refrigerant. Subcritical isotherms402are somewhat parallel to the vertical axis in the liquid region to the left of saturated liquid line408, exactly parallel to the horizontal axis in the two-phase region within vapor dome404(meaning there is no change in pressure as enthalpy increases within vapor dome404), and have a semi-steep negative slope in the gas region to the right of saturated vapor line410.

In the subcritical region of P-H diagram400, the CO2refrigerant may exhibit well-defined and widely-accepted saturated temperatures and pressures where evaporation and condensation processes can occur. The saturated temperatures and pressures are shown as horizontal lines within vapor dome404between saturated liquid line408and saturated vapor line410. However, the CO2refrigerant may not always be confined to saturated states; subcooled liquid or superheated gas states are common in vapor compression cycles. The subcooling of a liquid occurs when the refrigerant's pressure is greater than its saturation pressure at a given temperature. Conversely, a gas exists in a superheated state when the pressure of the refrigerant is less than its saturation pressure at a given subcritical temperature (i.e., when the temperature of the gas is below Tcrit) or less than its critical pressure Pcritat a given supercritical temperature (i.e., when the temperature of the gas is above Tcrit). The portions of subcritical isotherms402to the left of saturated liquid line408represent the CO2refrigerant in a subcooled state, whereas the portions of subcritical isotherms402to the right of saturated vapor line410represent the CO2refrigerant in a superheated state. This notion of manipulating a refrigerant's pressure at a given temperature to achieve subcooling or superheat can be used in commercial refrigeration controls.

To move forward in this discussion, an understanding of the difference between a vapor and a gas is important. Vapor is characterized by a gas state which, during an isothermal process (maintaining constant temperature), can condense by increasing its pressure. For the CO2refrigerant, this can only occur if the temperature of the CO2refrigerant gas is less than the critical temperature Tcritof the CO2refrigerant. If the temperature of the CO2refrigerant gas is greater than the critical temperature Tcritof the CO2refrigerant and the pressure of the CO2refrigerant is increased isothermally, the CO2refrigerant gas will never condense into the liquid state. Therefore, a superheated gas that cannot condense is not a vapor and has no corresponding saturation pressure. However, careful observation of the CO2refrigerant's isotherms402in P-H diagram400can reveal that there is in fact a way to naturally associate pressures with non-vapor superheated gas temperatures.

Referring again toFIG. 3, pseudo-saturation pressure calculator59is shown to include an isotherm extractor62and an inflection point identifier63. Isotherm extractor62may identify the supercritical isotherms of the CO2refrigerant using the P-H-T data stored in P-H-T database61. Each supercritical isotherm may be defined by a set of P-H-T data points that have the same supercritical temperature. In some embodiments, isotherm extractor62generates pressure-enthalpy functions that represent the supercritical isotherms. For example, each supercritical isotherm402may be represented by a function that defines pressure as a function of enthalpy (i.e., P=ƒ(H)). In some embodiments, isotherm extractor62uses a regression process to fit a curve (e.g., a cubic polynomial) to the set of P-H-T points that define each supercritical isotherm.

Inflection point identifier63may receive the supercritical isotherms from isotherm extractor62. Inflection point identifier63can be configured to identify the inflection point of each supercritical isotherm. The inflection point of a supercritical isotherm can be defined as the point at which the change in pressure per unit enthalpy along the supercritical isotherm reaches a minimum. In other words, the inflection point of a supercritical isotherm is the point at which the slope of the supercritical isotherm is closest to zero. These inflection points are shown graphically inFIG. 5Aas isotherm inflection points508.

Referring now toFIG. 5A, another pressure-enthalpy (P-H) diagram500representing the P-H-T data for the CO2refrigerant is shown, according to an exemplary embodiment. P-H diagram500is shown to include several isotherms502. Some of isotherms502have a temperature less than the critical temperature Tcrit(defined by critical point506), pass through vapor dome504, and are therefore subcritical isotherms502. Other isotherms502have a temperature above Tcrit, do not pass through vapor dome504, and are therefore supercritical isotherms502. Isotherm inflection points represent the points along each supercritical isotherm502at which the slope of the supercritical isotherm502is closest to zero.

Inflection point identifier63can identify the inflection point of each supercritical isotherm508using an analytical or numerical technique. For example, inflection point identifier63can use a pressure-enthalpy function that defines a supercritical isotherm502(i.e., P=ƒ(H)) to calculate the slope of the supercritical isotherm502as a function of enthalpy value, as shown in the following equation:

dPdH=f⁡(H)
Inflection point identifier63can then identify the enthalpy value H at which the slope

dPdH
is closest to zero and can select the corresponding P-H-T data point as the inflection point508of the supercritical isotherm502.

As another example, inflection point identifier63can use the set of P-H-T data that defines a supercritical isotherm502to calculate changes in pressure ΔP and changes in enthalpy ΔH between each pair of adjacent P-H-T data points. Inflection point identifier63can then identify the pair of P-H-T data points at which the value of ΔP/ΔH is closest to zero. Inflection point identifier63can select either P-H-T data point in the identified pair as the inflection point508or can interpolate between the identified pair of P-H-T data points to calculate the inflection point508(e.g., an average of the P-H-T data points in the pair). Inflection point identifier63can repeat this process for each supercritical isotherm502to identify the corresponding inflection point508.

Referring again toFIG. 3, pseudo-saturation pressure calculator59is shown to include a pseudo-saturation function generator64. Pseudo-saturation function generator64may receive the inflection points508from inflection point identifier63. Each inflection point508may include a pressure value, an enthalpy value, and a temperature value. Pseudo-saturation function generator64can use the temperature values and pressure values of the inflection points508to fit a two-dimensional line to the inflection points508, shown inFIG. 5Aas pseudo-saturated line510. Pseudo-saturated line510can be generated by fitting a line to inflection points508using any of a variety of curve fitting techniques (e.g., polynomial regression). In some embodiments, pseudo-saturated line510a linear (i.e., straight) line that best fits inflection points508. In other embodiments, pseudo-saturated line510may be quadratic, cubic, or may have any other polynomial order. Pseudo-saturated line510may pass through all of inflection points508or may be a line that best fits inflection points508.

Pseudo-saturated line510may define a relationship between temperature and pseudo-saturation pressure Psat* for various supercritical states of the CO2refrigerant. In some embodiments, pseudo-saturation function generator64generates a function that defines pseudo-saturated line510(i.e., a pseudo-saturation function). The pseudo-saturation function may define a pseudo-saturation pressure Psat* as a function of a supercritical temperature of the CO2refrigerant (i.e., Psat*=ƒ(T)). The pseudo-saturation function may be an equation that represents pseudo-saturated line510and may define the points along pseudo-saturated line510. In other words, each point along pseudo-saturated line510may be a solution to the pseudo-saturation function Psat*=ƒ(T). The pseudo-saturation function Psat*=ƒ(T) can be generated by fitting a polynomial function (e.g., a linear function, a quadratic function, a cubic function, etc.) to inflection points508, as previously described.

Referring now toFIG. 5B, a pressure-temperature graph550representing the pressure and temperature data (a subset of the P-H-T data) for the CO2refrigerant is shown, according to an exemplary embodiment. In graph550, line514represents the critical pressure Pcritof the CO2refrigerant (i.e., 1070 pisa) whereas line516represents the critical temperature Tcritof the CO2refrigerant (i.e., 87.8° F.). The intersection of lines514and516defines the critical point506.

When both the temperature of the CO2refrigerant is below the critical temperature Tcritand the pressure of the CO2refrigerant is below the critical pressure Pcrit, the CO2refrigerant can exist as a liquid (within liquid region518), a vapor (within vapor region520), or as a liquid-vapor mixture (along saturation line512). When the temperature of the CO2refrigerant is above the critical temperature Tcritand the pressure of the CO2refrigerant is below the critical pressure Pcrit, the CO2refrigerant is a gas (within gas region522). When the temperature of the CO2refrigerant is below the critical temperature Tcritand the pressure of the CO2refrigerant is above the critical pressure Pcrit, the CO2refrigerant is a high density (liquid-like) compressed fluid (within compressed fluid region524). When both the temperature of the CO2refrigerant is above the critical temperature Tcritand the pressure of the CO2refrigerant is above the critical pressure Pcrit, the CO2refrigerant is a low density (gas-like) supercritical fluid (within supercritical fluid region526).

In graph550, vapor dome504(shown inFIG. 5A) is collapsed into a single saturation line512. Saturation line512defines the boundary of vapor dome504for subcritical states of the CO2refrigerant and terminates at critical point506(at the top of vapor dome504). In some embodiments, pseudo-saturated line510is continuous with saturation line512and extends from critical point506into supercritical fluid region526. Accordingly, pseudo-saturated line510can be thought of as the extension of saturation line512into supercritical fluid region526.

Referring again toFIG. 3, pseudo-saturation pressure calculator59is shown to include a dynamic subcooling temperature adjuster66and a pressure setpoint calculator65. Dynamic subcooling temperature adjuster66may receive the temperature TGCof the CO2refrigerant measured by temperature sensor33and the dynamic pseudo-subcooling value TSC,dynamicgenerated by pseudo-subcooling value generator57. Dynamic subcooling temperature adjuster66may add the temperature TGCto the dynamic pseudo-subcooling value TSC,dynamicto calculate a temperature T (i.e., T=TGC+TSC,dynamic) and can provide the calculated temperature T to pressure setpoint calculator65.

Pressure setpoint calculator65can receive the pseudo-saturation function Psat*=ƒ(T) from pseudo-saturation function generator64and the calculated temperature T from dynamic subcooling temperature adjuster66. Pressure setpoint calculator65may apply the calculated temperature T as an input to the pseudo-saturation function Psat*=ƒ(T) to calculate a corresponding pseudo-saturation pressure Psat*. The calculated pseudo-saturation pressure Psat* may be a point along pseudo-saturated line510that has the calculated temperature T. Pressure setpoint calculator65may then set the pressure setpoint Pspequal to the calculated pseudo-saturation pressure Psat* and provide the pressure setpoint Pspto valve controller60.

Pressure Control Process

Referring now toFIGS. 6 and 7, a pressure-enthalpy (P-H) diagram600and flowchart illustrating a pressure control process700performed by controller50are shown, according to exemplary embodiments.

Process700is shown to include measuring the pressure Phighand/or the temperature TGCof the CO2refrigerant leaving gas cooler/condenser2(step702). The pressure Phighcan be measured by a pressure sensor34positioned at the exit of gas cooler/condenser2or along a fluid conduit3coupled to the exit of gas cooler/condenser2, whereas the temperature TGCcan be measured by a temperature sensor33positioned at the exit of gas cooler/condenser2or along a fluid conduit3coupled to the exit of gas cooler/condenser2, as shown inFIG. 2. The pressure Phighand/or temperature TGCmeasured in step702may indicate whether the CO2refrigerant is in a subcritical region or a supercritical region.

Process700is shown to include comparing the measured pressure Phighand/or temperature TGCwith the critical pressure Pcritand/or temperature Tcritof the CO2refrigerant defined by critical point616(step704). If the measured pressure Phighis not greater than the critical pressure Pcritand/or the measured temperature TGCis not greater than the critical temperature Tcrit(i.e., the result of step704is “no”), the pressure of the CO2refrigerant is controlled using a subcritical control method (step706). Accordingly, a point defining the state of the CO2refrigerant may be located within vapor dome604or along an isotherm that passes through vapor dome604. However, if the measured pressure Phighis greater than the critical pressure Pcritand/or the measured temperature TGCis greater than the critical temperature Tcrit(i.e., the result of step704is “yes”), the pressure of the CO2refrigerant is controlled using a supercritical control method (step718). Accordingly, a point defining the state of the CO2refrigerant may be along an isotherm that does not pass through vapor dome604. In some embodiments, the supercritical control method is used when the measured Phighis greater than the critical pressure Pcrit, regardless of whether the measured temperature TGCis greater than the critical temperature Tcrit.

In response to selecting the subcritical control method (step706), process700may proceed with comparing the measured temperature TGCwith a minimum temperature setpoint Tsp,minof the CO2refrigerant (step708). If the measured temperature TGCis less than the minimum temperature setpoint Tsp,min(i.e., the result of step708is “yes”), process700may include overwriting the measured temperature TGCwith the minimum temperature setpoint Tsp,min(step710) and proceeding to step712. However, if the measured temperature TGCis greater than or equal to than the minimum temperature setpoint Tsp,min(i.e., the result of step708is “no”), process700may proceed directly to step712without adjusting the measured temperature TGC.

Process700is shown to include adding a fixed subcooling temperature value TSC,fixed606to the measured temperature TGC(step712) and calculating a saturated pressure Psatas a function of the summed temperature TGC+TSC,fixed(step714). The fixed subcooling value TSC,fixed606added in step712results in a subcritical subcooled control line612that is substantially parallel to the saturated liquid line602defining the left edge of vapor dome604. The saturated pressure Psatcalculated in step714is the saturated pressure corresponding to the summed temperature TGC+TSC,fixed. However, because the actual temperature of the CO2refrigerant is TGCand not TGC+TSC,fixed, the state of the CO2refrigerant will be along subcritical subcooled control line612and not saturated liquid line602when the pressure of the CO2refrigerant is controlled to Psat.

Process700is shown to include setting the pressure setpoint Pspequal to the saturated pressure Psatcalculated in step714and operating high pressure valve4to achieve the pressure setpoint (step716). Accordingly, the temperature of the CO2refrigerant at the exit of gas cooler/condenser2will be TGCand the pressure of the CO2refrigerant at the exit of gas cooler/condenser2will be Psat(TGC+TSC,fixed), which places the state of the CO2refrigerant along subcritical subcooled control line612.

Returning to step704, in response to selecting the supercritical control method (step718), process700may proceed to adding a dynamic pseudo-subcooling temperature value TSC,dynamic608to the measured temperature TGC(step720). The dynamic pseudo-subcooling temperature value TSC,dynamic608may vary as a function of the measured temperature TGCand results in a supercritical subcooled control line614. Supercritical subcooled control line614may be sloped relative to pseudo-saturated line610due to different values of TSC,dynamicbeing added in step720for different measured temperatures TGC. The values of TSC,dynamiccan also be manipulated by a user to control the pressure either higher or lower than the default TSC,dynamicvalue at any value of the measured temperature TGCin the supercritical control method. A graphical illustration for manipulating the values of TSC,dynamicis shown inFIG. 8. As described above, pseudo-saturated line610may be the line that passes through or best fits the inflection points of the supercritical isotherms in P-H diagram600.

Process700may then proceed to calculating a pseudo-saturated pressure Psat* as a function of the summed temperature TGC+TSC,dynamic(step722). The pseudo-saturated pressure Psat* calculated in step722is the pressure defined by pseudo-saturated line610at the summed temperature TGC+TSC,dynamic. However, because the actual temperature of the CO2refrigerant is TGCand not TGC+TSC,dynamic, the state of the CO2refrigerant will be along supercritical subcooled control line614and not pseudo-saturated line610when the pressure of the CO2refrigerant is controlled to Psat*.

Process700is shown to include setting the pressure setpoint Pspequal to the pseudo-saturated pressure Psat* calculated in step722and operating high pressure valve4to achieve the pressure setpoint (step724). Accordingly, the temperature of the CO2refrigerant at the exit of gas cooler/condenser2will be TGCand the pressure of the CO2refrigerant at the exit of gas cooler/condenser2will be Psat*(TGC+TSC,dynamic), which places the state of the CO2refrigerant along supercritical subcooled control line614.

Graphical Illustration of Control

Referring now toFIG. 8, a graphical illustration800for adjusting the dynamic pseudo-subcooling temperature values TSC,dynamicis shown, according to an exemplary embodiment. Graphical illustration800is shown to include two graphs810and820and a transcritical control pressure modifier822. Graph810is a pressure-temperature graph, whereas graph820is a pressure-enthalpy graph. Both graphs810and820illustrate several subcooled control lines812,814,816, and818for the CO2refrigerant.

Line818is a subcritical subcooled control line (similar to line612in diagram600) and defines the subcooled control region for a range of subcritical temperatures of the CO2refrigerant (i.e., 83° F. or less). In some embodiments, control line818is formed by adding a fixed subcooling value to the saturated liquid line along the left edge of the vapor dome. Line816is another subcritical subcooled line (similar to line612in diagram600) and defines the subcooled control region for a range of subcritical temperatures of the CO2refrigerant (i.e., between 84° F. and 87° F.). In some embodiments, control line816is formed by adding dynamic pseudo-subcooling temperature values TSC,dynamicto the saturated liquid line along the left edge of the vapor dome.

Line812is a baseline supercritical subcooled control line (similar to line614in diagram600) and defines a baseline subcooled control region for supercritical temperatures of the CO2refrigerant (i.e., above 87° F.). In some embodiments, control line812is formed by adding baseline dynamic pseudo-subcooling temperature values TSC,dynamicto pseudo-saturated line610. Line814is a modified supercritical subcooled control line (similar to line614in diagram600) and defines a modified subcooled control region for supercritical temperatures of the CO2refrigerant (i.e., above 87° F.). In some embodiments, control line814is formed by adding adjustable dynamic pseudo-subcooling temperature values TSC,dynamicto pseudo-saturated line610.

A user can adjust the slope of modified supercritical subcooled control line814(relative to baseline supercritical subcooled control line812) by adjusting transcritical control pressure modifier822. Setting transcritical control pressure modifier822to a negative value may cause the slope of modified supercritical subcooled control line814to be lesser (i.e., less positive or more negative) than the slope of baseline supercritical control line812. Increasingly negative values of transcritical control pressure modifier822may cause the slope of modified supercritical subcooled control line814to be more negative (or less positive). When the value of transcritical control pressure modifier822is negative, the dynamic pseudo-subcooling temperature values TSC,dynamicmay decrease as the measured temperature TGCof the CO2refrigerant increases. For example, the dynamic pseudo-subcooling temperature values TSC,dynamicmay be close to 5° F. at relatively lower values of the measured temperature TGCand decrease to approximately 3.6° F. at relatively higher values of the measured temperature TGC.

Conversely, setting transcritical control pressure modifier822to a positive value may cause the slope of modified supercritical subcooled control line814to be greater than the slope of baseline supercritical control line812. Increasingly positive values of transcritical control pressure modifier822may cause the slope of modified supercritical subcooled control line814to be more positive. When the value of transcritical control pressure modifier822is positive, the dynamic pseudo-subcooling temperature value TSC,dynamicmay increase as the measured temperature TGCof the CO2refrigerant increases. For example, the dynamic pseudo-subcooling temperature values TSC,dynamicmay be close to 5° F. at relatively lower values of the measured temperature TGCand increase to approximately 6.4° F. at relatively higher values of the measured temperature TGC.

Configuration of Exemplary Embodiments