Patent Publication Number: US-2022214079-A1

Title: Carbon dioxide refrigeration system with low temperature mode

Description:
FOREIGN PRIORITY 
     This application claims priority to European Patent Application No. 21150423.8, filed Jan. 6, 2021, and all the benefits accruing therefrom under 35 U.S.C. § 119, the contents of which in its entirety are herein incorporated by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to refrigeration systems and more particularly to carbon dioxide based refrigeration systems operable in a low ambient temperature mode. 
     BACKGROUND 
     The advantages of carbon dioxide as a refrigerant fluid for various refrigeration applications include being non-flammable and non-toxic, as well as offering favourable environmental properties, i.e. negligible Global Warming Potential (GWP) and zero Ozone Depletion Potential (ODP), and advantageous thermo-physical properties. Carbon dioxide refrigerant fluid (e.g. R744) is also inexpensive in comparison with man-made refrigerants. 
     However, the performance of simple “CO2 only” vapor-compression systems is significantly more sensitive to ambient temperature than other refrigerant type systems. Specifically, as a result of the critical temperature of CO2 being 31° C., unwanted subcritical or transcritical running conditions arise from fluctuations in the ambient temperature around this value. 
     Low pressure lift ejector systems are simpler systems than high pressure lift ejector systems. At ambient temperatures of around 17-18° C., the CO2 refrigerant fluid leaves the gascooler at around 23-25° C. At these ‘high’ temperatures and pressures the low pressure lift ejector is able to operate to provide a pressure lift, entraining and mixing the low pressure fluid from the suction inlet (from the evaporator) with the high pressure fluid from the motive inlet. 
     However, in especially low ambient temperature conditions, such as in winter, the CO2 refrigerant fluid leaves the gascooler at ‘low’ temperatures and pressures such that the low pressure lift ejector is unable to provide sufficient pressure lift to entrain the fluid from the suction inlet. In these conditions, the ejector is working as a high-pressure valve but providing no benefits to the system. 
     Conventionally, in these low-pressure lift ejector systems, a refrigerant pump is used to overcome the insufficient pressure lift across the ejector. However, it will be appreciated that an additional refrigerant pump requires the consumption of additional energy. It will always remain the case that reductions in part usage and energy consumption are desirable. 
     SUMMARY 
     According to a first aspect, the invention provides a refrigeration system for a carbon dioxide based refrigerant fluid, wherein the refrigeration system comprises a refrigerant circuit, the refrigerant circuit comprising a compression device, a heat rejecting heat exchanger, an ejector, a receiver, an expansion device, and a heat absorbing heat exchanger; wherein the ejector includes a primary inlet, a secondary inlet and an outlet; wherein the receiver includes an inlet, a liquid outlet and a gas outlet; wherein the ejector primary inlet is arranged to receive fluid from an outlet of the heat rejecting heat exchanger, the ejector secondary inlet is arranged to receive fluid from an outlet of the heat absorbing heat exchanger, and the ejector outlet is arranged to direct flow to the receiver inlet; wherein a suction inlet of the compression device is arranged to receive refrigerant fluid from the gas outlet of the receiver; and wherein the liquid outlet of the receiver is connected via the expansion device to an inlet of the heat absorbing heat exchanger; characterised in that the refrigerant circuit comprises a bypass line and a bypass control valve, with the bypass line providing a fluid connection between the outlet of the heat rejecting heat exchanger and the expansion device, wherein, in an ejector mode of the refrigeration system, the bypass control valve prevents fluid flow through the bypass line such that all fluid exiting the heat rejecting heat exchanger enters the ejector primary inlet; and wherein, in a bypass mode of the refrigeration system, the bypass control valve permits fluid exiting the heat rejecting heat exchanger to flow through the bypass line to the expansion device and then to the heat absorbing heat exchanger without first passing through the ejector. 
     The use of a bypass line to avoid operating the ejector at a low pressure lift has the advantage, in comparison to using an additional refrigerant pump to secure an effective pressure lift over the ejector, that it reduces the cost and complexity of the refrigeration system. Furthermore, the bypass line consumes no energy, and thus provides a refrigeration system with a lower overall energy consumption. Effectively, the bypass control line enables the refrigeration system to act as two differing types of refrigerant circuit depending on the state of the bypass control valve, which can be changed based on external conditions. For example, in periods with a lower ambient temperature and hence reduced cooling requirements then the bypass valve can be opened and the bypass line used for an efficient low power/low cooling load mode of operation. Alternatively, when there is a higher ambient temperature the bypass line can be closed and the ejector and receiver components are utilised to provide enhanced performance of the refrigeration system and provide an efficient high power/high cooling load mode of operation. 
     The bypass control valve may be implemented with any suitable valve arrangement, such as one or more valves in the bypass line and/or at the junction of the bypass line with a line between the heat rejecting heat exchanger and the expansion device. The bypass control valve may comprise an on/off valve. The bypass control valve may be operated manually (e.g. ball or plug valve), or the refrigeration system may comprise a controller for automatic control of the bypass control valve (e.g. solenoid valve), in order to achieve switching between the bypass mode and the ejector mode. 
     The bypass line may be arranged to provide a direct fluid connection between the outlet of the heat rejecting heat exchanger and an inlet of the expansion device. 
     The bypass line may provide a fluid flow path (e.g. a conduit, a pipe) between the outlet of the heat rejecting heat exchanger and an inlet of the expansion device that is only interrupted by the bypass control valve. In other words, the bypass line may comprise no further components. 
     The bypass line may be arranged such that fluid does not undergo heat exchange with another portion of the refrigeration system i.e. lose and/or gain heat to and/or from another portion of the refrigeration system, when flowing from the outlet of the heat rejecting heat exchanger to the inlet of the expansion device through the bypass line. 
     By providing a direct connection between the outlet of the heat rejecting heat exchanger and an inlet of the expansion device, the number of components (and complexity) of the refrigeration system is minimised. Thus, when operated in the bypass mode, the refrigeration system provides a simple single-stage vapor-compression refrigeration system. Being able to switch to a simple refrigerant circuit with minimal components provides the option of reliable and robust refrigeration of a temperature controlled environment. 
     The refrigeration system may include a check valve between the liquid outlet of the receiver and the expansion device. 
     The bypass control valve may be a three-port valve, wherein a first port of the valve may be connected to the expansion device, a second port of the valve may be connected to the bypass line, and a third port of the valve may be connected to the liquid outlet of the receiver. In the ejector mode of the refrigeration system, the bypass control valve may be controlled to allow fluid communication between the first port and the third port; and in the bypass mode of the refrigeration system, the bypass control valve may be controlled to allow fluid communication between the first port and the second port. 
     A three-port valve may provide the function of the bypass control valve and the check valve in a single valve, thus reducing the number of components of the refrigeration system and accordingly providing improved reliability and reduced cost. 
     The refrigeration system may comprise a sensor for monitoring an ambient air temperature, and the controller may be configured to control the bypass control valve to switch to the bypass mode of the refrigeration system in response to determining that the ambient air temperature is below a predetermined threshold. 
     The refrigeration system may comprise a sensor for monitoring an ambient air temperature, and the controller may be configured to control the bypass control valve to switch to the ejector mode of the refrigeration system in response to determining that the ambient air temperature is above a predetermined threshold. 
     Thus the refrigeration system is provided with the ability to switch between the ejector mode and the bypass mode automatically in response to the ambient air temperature in order to optimise performance based on external conditions. 
     The refrigeration system may comprise a refrigerant fluid temperature sensor located between the outlet of the heat rejecting heat exchanger and the ejector primary inlet, and the controller may be configured to control the bypass control valve to switch to the bypass mode in response to determining that a sensed pressure at the outlet of the heat rejecting heat exchanger is below a predetermined threshold. 
     Thus the refrigeration system is provided with the ability to switch between the ejector mode and the bypass mode automatically in response to the temperature of the refrigerant fluid at the outlet of the heat rejecting heat exchanger in order to optimise performance based on internal conditions of the refrigeration system. 
     The refrigerant circuit may not generally include any further components, i.e. it may consist of a compression device, a heat rejecting heat exchanger, an ejector, a receiver, an expansion device, a heat absorbing heat exchanger, a bypass line, a bypass control valve and a check valve. 
     The refrigerant circuit may not include any other components between the compression device and the heat rejecting heat exchanger. 
     The refrigerant circuit may not include any other components between the heat rejecting heat exchanger and the primary inlet of the ejector. 
     The refrigerant circuit may not include any other components between the ejector outlet and the inlet of the receiver. 
     The refrigerant circuit may not include any other components between the gas outlet of the receiver and the compression device. 
     The refrigerant circuit may not include any other components between the heat rejecting heat exchanger and the bypass control valve. 
     The refrigerant circuit may not include any other components between the bypass control valve and the expansion valve. 
     The refrigerant circuit may not include any other components between the expansion device and the heat absorbing heat exchanger. 
     The refrigerant circuit may not include any other components between the heat absorbing heat exchanger and the secondary inlet of the ejector. 
     According to another aspect, the invention provides a method of controlling a refrigeration system for a carbon dioxide based refrigerant fluid, wherein the refrigeration system comprises: a refrigerant circuit comprising a compression device, a heat rejecting heat exchanger, an ejector, a receiver, an expansion device, and a heat absorbing heat exchanger; wherein the ejector includes a primary inlet, a secondary inlet and an outlet; wherein the receiver includes an inlet, a liquid outlet and a gas outlet; wherein the ejector primary inlet is arranged to receive fluid from an outlet of the heat rejecting heat exchanger, the ejector secondary inlet is arranged to receive fluid from an outlet of the heat absorbing heat exchanger, and the ejector outlet is arranged to direct flow to the receiver inlet; wherein a suction inlet of the compression device is arranged to receive refrigerant fluid from the gas outlet of the receiver; and wherein the liquid outlet of the receiver is connected via the expansion device to an inlet of the heat absorbing heat exchanger; characterised in that the refrigerant circuit comprises a bypass line and a bypass control valve, with the bypass line providing a fluid connection between the outlet of the heat rejecting heat exchanger and the expansion device; the method comprising: running the refrigeration system in either an ejector mode in which all refrigerant fluid exiting the heat rejecting heat exchanger enters the ejector primary inlet, or a bypass mode of the refrigeration system in which refrigerant fluid exiting the heat rejecting heat exchanger is permitted to flow through the bypass line to the expansion device and then to the heat absorbing heat exchanger without first passing through the ejector; and controlling the bypass control valve to switch to running the refrigeration system in the other of the ejector mode or the bypass mode. 
     The use of a bypass line to avoid operating the ejector at a low pressure lift has the advantage, in comparison to using an additional refrigerant pump to secure an effective pressure lift over the ejector, reduces the cost and complexity of the refrigeration system. Furthermore, the bypass line consumes no energy, and thus provides a refrigeration system with a lower overall energy consumption. 
     The bypass control valve may be an on/off valve. The valve may be operated manually (e.g. ball or plug valve), or the refrigeration system may comprise a controller for automatic control of the bypass control valve (e.g. solenoid valve), in order to achieve switching between the bypass mode and the ejector mode. 
     The bypass line may be arranged to provide a direct connection between the outlet of the heat rejecting heat exchanger and an inlet of the expansion device. 
     The bypass line may provide a fluid flow path (e.g. a conduit, a pipe) between the outlet of the heat rejecting heat exchanger and an inlet of the expansion device that is only interrupted by the bypass control valve. In other words, the bypass line may comprise no further components. 
     The bypass line may be arranged such that fluid does not undergo heat exchange with another portion of the refrigeration system i.e. lose and/or gain heat to and/or from another portion of the refrigeration system, when flowing from the outlet of the heat rejecting heat exchanger to the inlet of the expansion device through the bypass line. 
     By providing a direct connection between the outlet of the heat rejecting heat exchanger and an inlet of the expansion device, the number of components (and complexity) of the refrigeration system is minimised. Thus, when operated in the bypass mode, the refrigeration system provides a simple single-stage vapor-compression refrigeration system. Being able to switch to a simple refrigerant circuit with minimal components provides the option of reliable and robust refrigeration of a temperature controlled environment. 
     The refrigeration system may include a check valve between the liquid outlet of the receiver and the expansion device. 
     The bypass control valve may be a three-port valve, wherein a first port of the valve may be connected to the expansion device, a second port of the valve may be connected to the bypass line, and a third port of the valve may be connected to the liquid outlet of the receiver. In the ejector mode of the refrigeration system, the bypass control valve may be controlled to allow fluid communication between the first port and the third port; and in the bypass mode of the refrigeration system, the bypass control valve may be controlled to allow fluid communication between the first port and the second port. 
     A three-port valve may provide the function of the bypass control valve and the check valve in a single valve, thus reducing the number of components of the refrigeration system and accordingly providing improved reliability and reduced cost. 
     The method may comprise monitoring an ambient air temperature outside of the refrigeration system; and controlling the bypass control valve to switch to from the ejector mode to the bypass mode in response to determining that the ambient air temperature is below a predetermined threshold. 
     The method may comprise monitoring an ambient air temperature outside of the refrigeration system; and controlling the bypass control valve to switch from the bypass mode to the ejector mode in response to determining that the ambient air temperature is above a predetermined threshold. 
     The method may comprise monitoring a refrigerant fluid temperature at an outlet of the heat rejecting heat exchanger; and controlling the bypass control valve to switch from the ejector mode to the bypass mode in response to determining that the refrigerant fluid temperature is below a predetermined threshold. 
     Thus automatic switching between the ejector mode and the bypass mode in response to external and/or internal conditions is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which: 
         FIG. 1  is a schematic diagram of an ejector refrigeration system including a bypass line, the ejector refrigeration system being run in an ejector mode of operation; and 
         FIG. 2  is a schematic diagram of the ejector refrigeration system of  FIG. 1 , the ejector refrigeration system being run in a bypass mode of operation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As seen in  FIG. 1 , a refrigeration system includes a compression device  12 , a heat rejecting heat exchanger  14 , an ejector  20 , a receiver  22 , an expansion device  18  and a heat absorbing heat exchanger  16  that together form an ejector refrigerant circuit. The ejector refrigerant circuit contains a carbon dioxide based refrigerant fluid (e.g. R744) and circulation of the refrigerant fluid via the compression device  12  enables the ejector refrigeration system to utilise a refrigeration cycle to satisfy a cooling load. In this example the compression device  12  is at least one compressor  12  for compression of the carbon dioxide based refrigerant fluid, the heat rejecting heat exchanger  14  is a gascooler for at least partially condensing the refrigerant fluid, and the heat absorbing heat exchanger  16  is an evaporator for at least partially evaporating the refrigerant fluid. The refrigeration system may advantageously be arranged so that the fluid is fully condensed at the gascooler  14 , and fully evaporated at the evaporator  16 . 
     The refrigeration system is configured to provide control and maintenance of temperature conditions of an environment, such as the inside of a freezer cabinet. The ejector refrigerant circuit may be situated outside of the temperature controlled environment. Air passages may be provided to allow air to circulate between the ejector refrigerant circuit and the temperature controlled environment, and the refrigeration system may include fans (not shown) configured to direct ambient air across the gascooler  14  and air across the evaporator  16  to or from the temperature controlled environment. 
     The ejector  20  comprises a primary inlet  201  (e.g. a high-pressure motive inlet), a secondary inlet  202  (e.g. a low-pressure suction inlet) and an outlet  203 . The ejector  20  includes a high-pressure fluid passage extending from the primary inlet  201  to a high-pressure fluid nozzle; a suction fluid passage extending from the secondary inlet  202  to a suction chamber surrounding the high-pressure fluid nozzle; a mixing chamber  204  in fluid communication with the high-pressure fluid passage and the suction fluid passage respectively; and a diffusion chamber  205  downstream of the mixing chamber  204 . The working principle of the ejector  20  is generally described as follows: a high-pressure fluid is converted into a high-momentum fluid when passing through the high-pressure fluid nozzle, the suction fluid is suctioned into the mixing chamber with the high-momentum fluid and mixed with the high-momentum fluid in the mixing chamber, and then diffuses in the diffusion chamber to recover the pressure of the fluid, the fluid then passing through the outlet  203 . 
     The receiver  22  (e.g. an accumulator) comprises an inlet  221 , a liquid outlet  222  and a gas outlet  223 . Inlet  221  is connected to the outlet  203  of the ejector  20  and receives refrigerant fluid therefrom, the liquid outlet  222  is connected to the inlet of the expansion device  18 , and the gas outlet  223  is connected to the inlet of the compressor  12 . In many cases it is beneficial to avoid the presence of liquid at the inlet to the compressor  12 . 
     The refrigeration system includes a bypass line  24  for use in a bypass mode of operation. The inlet of the bypass line  24  is attached to the ejector refrigerant circuit via the line between the outlet of the gascooler  14  and the primary inlet  201  of the ejector. The outlet of the bypass line  24  is attached to the ejector refrigerant circuit via the line between the liquid outlet  222  of the receiver  22  and the inlet of the expansion valve  18 . 
     The bypass line  24  includes a bypass control valve  26 , which is shown as a solenoid valve with an open state and a closed state. Optionally, the ejector refrigerant circuit may include a check valve  28  on the line between the outlet of the receiver  22  and the inlet of the expansion valve  18 . In this embodiment, as shown in  FIG. 1 , the outlet of the bypass line  24  is attached to the ejector refrigerant circuit via the line between the outlet of the check valve  28  and the inlet of the expansion valve  18 . 
     In an alternative embodiment, the bypass control valve  26  is a three-port valve. The first port of the valve  26  is connected to the inlet of the expansion device  18 , a second port of the valve  26  is connected to the outlet of the bypass line  24 , and a third port of the valve  26  is connected to the liquid outlet  222  of the receiver  22 . The use of a three-port valve prevents fluid communication between the liquid outlet  222  of the receiver  22  and the outlet of the bypass line  24 , such that the need for the check valve  28  is eliminated. 
     Optionally, the ejector refrigerant circuit may comprise a plurality of gascoolers, e.g. first gascooler  14  and second gascooler (heat rejecting heat exchanger)  14   a . The first gascooler  14  and second gascooler  14   a  may advantageously be arranged so that the fluid is fully condensed at the outlet of the second gas cooler  14   a.    
     Optionally, the ejector refrigerant circuit may comprise a plurality of evaporators (not shown). 
     Optionally, the compression device  12  may comprise a plurality of compressors in parallel. 
     Optionally, the ejector  20  may comprise a plurality of ejectors in parallel. 
     The refrigeration system may include a controller (not shown) for automatic control of the bypass control valve  26 . The refrigeration system may include various temperature and pressure sensors (not shown) in wired or wireless communication with the controller. 
     The operation of the refrigeration system is now described with reference to  FIGS. 1 and 2 . 
     With reference to  FIG. 1 , in an ejector mode of operation the carbon dioxide based refrigerant fluid flows through the ejector refrigerant circuit, and does not flow through the bypass line  24 . 
     With reference to  FIG. 2 , in a bypass mode of operation the carbon dioxide based refrigerant fluid flows through the bypass line  24 , and does not flow through the primary inlet  201  of the ejector  20 . When running in a bypass mode of operation the carbon dioxide based refrigerant fluid flows through the compressor  12 , the gascooler  14 , the expansion valve  18  and the evaporator  16  in that order, and accordingly the refrigeration system can be considered to be operating as a typical single-stage vapor-compression refrigeration system. During the bypass mode of operation the ejector  20  acts as conduit between the secondary inlet  202  and the outlet  203  for the refrigerant fluid flow. Similarly, the receiver  22  acts as a conduit for between the inlet  221  and the gas outlet  223 . 
     The ejector mode of operation may be initiated automatically, for example upon start-up. Alternatively, on start-up, the controller may be configured to determine whether the refrigeration system should be initiated in the ejector mode of operation or the bypass mode of operation. 
     During the operation of the refrigeration system (in either the ejector mode or the bypass mode) the controller may be configured to switch to a different mode of operation in response to received information (e.g. measurements). The controller may receive temperature measurements from sensors, such as a sensor for ambient air temperature (outside air temperature), a sensor for temperature of the temperature controlled environment, and/or sensors within the ejector refrigerant circuit or the bypass line such as for measuring temperatures and/or pressures. The sensors may be comprised as a part of the refrigeration system. 
     Alternatively the switching may be performed manually, by a user (e.g. engineer or operator) or performed automatically, for example at certain times of the day. 
     Advantageously, in situations when the ambient air temperature (outside air temperature) is high (e.g. during the day and/or during summer), the refrigeration system can switch to be ran in the ejector mode. When the ambient air temperature is high, the fluid leaving the outlet of the gascooler is correspondingly also at a high temperature (and a high pressure). Accordingly, because the pressure of the motive fluid (i.e. the fluid entering the primary inlet  201  of the ejector  20 ) is high enough to provide a sufficient pressure lift to the suction fluid (i.e. the fluid entering the secondary inlet  202 ), the performance advantages of the ejector  20  (such as improved efficiency and/or productivity of the refrigeration system) can be realised. 
     However, in situations when the ambient air temperature (outside air temperature) is low (e.g. during the night and/or during winter), the fluid leaving the outlet of the gascooler is correspondingly at a low temperature (and a low pressure). As such, because the pressure of the motive fluid is low, the pressure lift provided by the ejector  20  is low. The ejector  20  thus operates poorly and the performance of the refrigeration system suffers. 
     Advantageously, the refrigeration system can switch to be ran in the bypass mode if it is determined that the ambient air temperature, or the temperature and/or pressure of the motive fluid (i.e. the fluid leaving the outlet of the gascooler  14 ) is low, e.g. below a predetermined threshold. 
     As discussed above, when operated in the bypass mode, the bypass valve  26  is opened. Essentially all the fluid flowing from the output of the gascooler  14  thus flows through the bypass line  24 , as the high-pressure nozzle of the ejector  20  presents a significantly higher pressure barrier for the refrigerant fluid to overcome (as opposed to the expansion valve  18 ). Thus the ejector  20  does not act as an ejector but acts instead as a fluid conduit (e.g. pipe), providing fluid communication between the outlet of the evaporator  16  and the inlet  221  of the receiver. Thus, in accordance with an embodiment of the invention, the refrigeration system is operated as a typical single-stage vapor-compression refrigeration system in conditions where the operation of the ejector  20  would be detrimental to the performance of the refrigeration system. 
     The refrigeration system may not include any components or elements other than those shown in  FIG. 1  and  FIG. 2 , i.e. the refrigeration system may consist of a compression device  12 , a heat rejecting heat exchanger  14 , an ejector  20 , a receiver  22 , an expansion device  18 , a heat absorbing heat exchanger  16 , a bypass line  24 , a bypass control valve  26  and a check valve  28 . Alternatively, the refrigeration system may consist of a compression device  12 , a heat rejecting heat exchanger  14 , an ejector  20 , a receiver  22 , an expansion device  18 , a heat absorbing heat exchanger  16 , a bypass line  24  and a three-port bypass control valve  26 . 
     The refrigeration system may also include other more complex additions to the ejector refrigerant circuit or bypass line  24  such as to adapt the refrigeration system for particular requirements.