Patent Publication Number: US-2020284477-A1

Title: Refrigeration system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/AU2018/051262 with a filling date of Nov. 27, 2018, designating the United states, now pending, and further claims the benefit of priority from South Australia Patent Application No. 2017904783, filed on Nov. 27, 2017. The content of the aforementioned application, including any intervening amendments thereto, is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to refrigeration systems and, in particular, refrigeration systems that make use of carbon dioxide (hereinafter referred to as “CO 2 ”) as a refrigerant. 
     BACKGROUND ART 
     The vapour compression cycle has been used extensively in the refrigeration industry for many years. The cycle typically employs the continuous flow of refrigerant between four primary components; a metering device, evaporator, compressor and condenser. 
     The type of refrigerant employed in this cycle varies depending on the application and the refrigeration temperature required. Typically synthetic refrigerants such as CFC&#39;s, HCFC&#39;s and HFC&#39;s have been used. CFC&#39;s and HCFC&#39;s are in the process of being banned in many countries under the Montreal Protocol due to the ozone depleting potential (ODP) of such refrigerants. Similarly, HFC&#39;s are being phased out in those same countries under the Kigali Amendment to the Montreal Protocol due to their high global warming potential (GWP). This phase out in the use of synthetic refrigerants due to their environmental impact has resulted in increased interest in the use of natural refrigerants such as carbon dioxide (CO 2 ), ammonia and hydrocarbons. 
     In low ambient temperature environments, CO 2  refrigeration systems can be more efficient than synthetic refrigerant systems, and for this reason such systems have mostly been used in cooler climates. However, in higher ambient temperatures, the efficiency of CO 2  systems can drop significantly. This reduction in efficiency at higher ambient temperatures is due to the low critical temperature of CO 2  (approximately 31° C.) when compared to other refrigerants. The critical temperature of a refrigerant is the temperature above which that refrigerant exists in a supercritical state. When a refrigerant is in this state, it is unable to be condensed in the condenser and the efficiency of the system drops significantly. 
     In refrigeration systems the condenser condenses the refrigerant by transferring heat from the refrigerant to a cooling medium (e.g. air or water). This heat exchange is induced by a temperature difference between the cooling medium and the refrigerant. Because the temperature of the cooling medium is often dependent on the ambient temperature of the immediate environment, when ambient temperatures are higher (such as in hotter climates), it is increasingly difficult to maintain CO 2  refrigerant in a subcritical state. For example, ambient temperatures of above 25° C. can result in difficulty maintaining the CO 2  refrigerant below its critical temperature. 
     It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art, in Australia or any other country. 
     SUMMARY 
     Disclosed is a CO 2  based refrigeration system comprising a condenser for transferring heat from a CO 2  refrigerant of the refrigeration system to an air stream. The system further comprises an indirect evaporative cooler arranged to cool an ambient air stream and supply the cooled ambient air to the condenser to facilitate the transfer of heat from the CO 2  refrigerant. 
     As should be appreciated by the skilled person, the term “condenser” encompasses gas coolers. The term gas cooler is used to describe a condenser that is operating under conditions where the refrigerant received by the condenser is supercritical rather than subcritical (i.e. such that it is simply cooled rather than condensed). The condenser, or gas cooler, may be an air-cooled condenser. The condenser may alternatively be a water-cooled condenser and heat may be exchanged between the water and the cooled air. 
     The provision of cooled ambient (rather than ambient) air to the condenser may allow the CO 2  refrigerant to be maintained in a subcritical state, even under ambient conditions that would otherwise lead to a CO 2  refrigerant temperature above the critical temperature of CO 2  (31° C.). 
     The use of an evaporative cooler for cooling the ambient air can enable the use of CO 2  refrigerant in a broader range of ambient conditions, and can provide an alternative to systems that make use of non-natural refrigerants (such as CFC&#39;s, HCFC&#39;s and HFC&#39;s) that can be harmful to the environment. The use of an evaporative cooler for cooling the ambient air can also be more efficient than other cooling systems, such that the efficiency benefits gained by the use of CO 2  as a refrigerant are not lost within the process of cooling the air supplied to the condenser. The use of an indirect evaporative cooler can, in some circumstances, be more cost effective than other cooling systems. 
     As should be appreciated, the refrigeration system may also be suitable for (and configured for) various applications, including residential air conditioning, commercial air conditioning (including e.g. cool rooms, chilled cabinets, etc.) and vehicle air conditioning. 
     In one embodiment, the indirect evaporative cooler may comprise a first channel (e.g. a dry channel) for receipt of a first ambient air stream from an air source and a second channel (e.g. a wet channel) separate to the first channel. The second channel may be for receipt of a second air stream and may comprise a wetted surface for supplying water to the second air stream by way of evaporation. The indirect evaporative cooler may further comprise a heat exchanger for exchanging heat between the first and second channels. As is discussed further below, at least a portion of the first ambient and/or second air stream may be supplied to the condenser to facilitate transfer of heat from the CO 2  refrigerant. Preferably, however, at least a portion of the first ambient air stream may be supplied to the condenser to facilitate transfer of heat from the CO 2  refrigerant. 
     In this way, the second air stream may be cooled by the evaporation process (i.e. due to energy being transferred from the air and water due to the phase change). Because of a temperature difference between the cooled second air stream and the first ambient air stream (which is initially at the temperature of the air source), heat is transferred from the first ambient air stream to the cooler second air stream. The evaporation process reduces the dry bulb temperature of the second air stream, but the wet bulb temperature generally remains the same (because of the increase in the moisture entrained in the air). However, in the case of the first ambient air stream, both the wet bulb and dry bulb temperatures are reduced, because the heat loss is a result of heat exchange with the second air stream rather than due to an evaporative process. Hence, the moisture content of the first ambient air stream remains the same. 
     For brevity, only one pair of first and second channels is discussed, but the refrigeration system may comprise a plurality of first channels and a plurality of second channels that are arranged in various configurations. For example, each first channel may be adjacent (and may exchange heat with) a plurality of second channels and vice versa. 
     As should be apparent, the system as disclosed herein requires minimal energy input. The minimal energy input is mostly required by and limited to the energy required to move the air through the channels, but also includes energy for e.g. supply of the water to the second channel. 
     In one embodiment, the indirect evaporative cooler may comprise a diverter to divert at least a portion of the first ambient air stream into the second channel such that the second air stream comprises the diverted portion of the first ambient air stream. The entire first ambient air stream may be diverted, or only a portion of the ambient air stream may be diverted. This may allow the ambient air supplied to the condenser to be cooled to a temperature that is below the wet bulb temperature of the air source (which is not possible with a direct evaporative process). This is because, the cooling (by heat exchange) of the first ambient air stream lowers both the dry bulb and wet bulb temperature. Thus, the diverted portion of the first ambient air stream (which becomes the second air stream) has a lower wet bulb temperature than the air source. This reduces the minimum temperature to which the second air stream can be cooled and, in turn, reduces the temperature to which the first ambient air stream can be cooled (by heat exchange with the second air stream). 
     This arrangement may improve the ability of the system to maintain the temperature of the CO 2  below the critical temperature, so as to maintain the CO 2  refrigerant leaving the condenser in a subcritical state. To an extent, the arrangement may allow the CO 2  refrigerant to be maintained in this state regardless of the ambient air conditions (e.g. temperature and humidity). Hence, the system may be suitable for use in locations that are otherwise unsuitable for CO 2  refrigeration systems. 
     In one embodiment, the first air stream of the indirect evaporative cooler may comprise the cooled ambient air supplied to the condenser for condensing of the CO 2  refrigerant. In another embodiment, the second air stream may comprise the cooled air supplied to the condenser for condensing of the CO 2  refrigerant. In another embodiment the first and second air streams may comprise the cooled air supplied to the condenser for condensing of the CO 2  refrigerant. 
     In one embodiment, the system may further comprise a controller arranged to control the supply of cooled ambient air to the condenser. The controller may be a programmable logic controller (PLC). The system may further comprise a fan to move the ambient air through the indirect evaporative cooler. The fan may be a centrifugal fan. The fan may be a backward curved centrifugal fan. 
     In one embodiment, the controller may be configured to control the fan to control movement of the ambient air through the indirect evaporative cooler. In this way, the power input to the fan may be controlled, and the temperature and pressure of the air supplied by the indirect evaporative cooler may be controlled so as to maximise the efficiency of the refrigeration system. 
     In one embodiment, the controller may be configured to control a condenser fan for moving the ambient air across coils of the condenser. The condenser fan may be a centrifugal fan. The fan may alternatively be a backward curved centrifugal fan. In some circumstances, a centrifugal fan may provide lower operating power requirements than an axial fan, because of the pressure drop induced by the indirect cooler. 
     In one embodiment, the controller may be configured to control the supply of cooler ambient air to the condenser based on the relative humidity and temperature of the air source. Again, this may allow the controller to control the system so as to maximise the efficiency of the system. The relative humidity and temperature may be used to determine a condition of the air (e.g. temperature) being supplied to the condenser. 
     In one embodiment, the controller may be configured to maintain the temperature of the refrigerant in the condenser below a predetermined threshold temperature. The controller may be configured to maintain the temperature of the refrigerant below the critical temperature of the refrigerant. The controller may be configured to maintain the temperature of the refrigerant below a temperature of at least between 30° C. 
     In one embodiment, the refrigeration system may comprise one or more sensors for measuring the temperature and relative humidity of the air source. The sensors may be positioned at an inlet of the indirect evaporative cooler. The sensors may communicate sensed data to the controller in a wired or wireless manner. 
     In one embodiment, the refrigeration system may further comprise a metering device (e.g. a high pressure expansion valve) downstream of the condenser. The metering device may be configured to cause supercritical refrigerant, when received from the condenser, to condense. The metering device may, for example, liquefy the supercritical refrigerant by way of throttling. The metering device may provide a backup solution for when the indirect evaporative cooler is unable to maintain the CO 2  in a subcritical state. 
     In one embodiment, the refrigeration system may further comprise a bypass valve. The bypass valve may be configurable between a first position and a second position. In the first position refrigerant may bypass the metering device. In the second position, the refrigerant may pass through the metering device. The bypass valve may be controlled by the controller to move to the first position when the refrigerant is maintained in a subcritical state. The bypass valve may be controlled by the controller to move to the second position when the refrigerant is not maintained in a subcritical state. Such an arrangement may maximise the efficiency of the system when the refrigerant is able to be maintained in a subcritical state, but also allows the system to continue to operate (in a less efficient but useable manner) when the CO 2  refrigerant is not subcritical (e.g. due to conditions of the air source (e.g. extreme conditions) or a system fault). 
     In one embodiment, the refrigeration system may further comprise a receiver vessel, an expansion valve, an evaporator and a compressor. These components may be disposed in this order (i.e. in the direction of flow of the refrigerant). The refrigeration system may further comprise a bypass line for flash gases formed at the metering device. The bypass line may fluidly connect the receiver vessel to the compressor. Flash gases may form at the metering device due to the pressure drop at the metering device. The flash gases may be separated from the CO 2  refrigerant in the receiver vessel, and can then be directed to the bypass line to bypass the expansion valve and the evaporator. This can avoid a system efficiency loss that could otherwise occur if flash gases pass through the expansion valve. 
     In one embodiment, the bypass line may comprise a bypass line valve for selectively restricting the flow of flash gases through the bypass line. The bypass line valve may be opened when the refrigerant is in a supercritical state, and may be closed when the refrigerant is in a subcritical state. The bypass valve may be operated in conjunction with the bypass valve for bypassing the metering device, such that one is closed when the other is open (i.e. to reflect the state of the refrigerant). 
     In one embodiment, the refrigerant circuit of the refrigeration system may be a closed system. 
     Also disclosed is a method of operating a CO 2  based refrigeration system. The method comprises supplying a first ambient air stream from an air source and cooling a second air stream by moving the second air stream across a wetted surface. The method further comprises transferring heat between the first ambient air stream and the second air stream, and transferring heat between at least a portion of the first ambient air stream and CO 2  refrigerant in a condenser of the refrigeration system to condense the CO 2 . 
     As is provided above, the evaporative cooling of the second air stream, and the transfer of heat from the first ambient air stream to the second air stream can provide an efficient way to maintain the CO 2  below its critical temperature (31° C.). In turn, this may allow the system to operate in an efficient manner (e.g. inefficiency associated with the refrigerant being supercritical may be avoided). 
     In one embodiment, the method may further comprise diverting a portion of the first ambient air stream. The diverted portion may become the second air stream. In this way, the temperature of the air supplied to the condenser (i.e. for the transfer of heat between the CO 2  refrigerant and the air) can be below the wet bulb temperature of the source (e.g. ambient) air. This may otherwise not be the case with direct evaporative cooling. Thus, the present method may not be limited by the wet bulb temperature of the source air and can allow the refrigerant to remain subcritical in locations where this would otherwise not be possible (e.g. due to climatic reasons). 
     In one embodiment, the method may further comprise controlling the rate at which the ambient air is supplied from the air source and/or the rate at which the ambient air is supplied to the condenser based on a condition of the air source. The condition of the air source may be at least one of the relative humidity and the temperature of the air source. As set forth above, this control may allow the efficiency of the refrigeration system to be maximised (for a particular set of conditions). 
     In one embodiment, the method may further comprise controlling the rate at which the ambient air is supplied from the air source and/or the rate at which the ambient air is supplied to the condenser to maintain the refrigerant in a subcritical state. 
     In one embodiment, the method may further comprise determining whether the CO 2  refrigerant is in a subcritical or supercritical state at the step of transferring heat in the condenser. If the CO 2  refrigerant is determined to be supercritical, the method may comprise reducing the pressure of the CO 2  refrigerant by way of a throttling step, to liquefy at least a portion of the CO 2  refrigerant. Thus, even if the CO 2  refrigerant remains supercritical (e.g. due to system fault or extreme ambient conditions), it may still be liquefied such that the method can still be performed. 
     Also disclosed is a method of retrofitting a CO 2  refrigeration system. The method comprises arranging an indirect evaporative cooler so as to be in fluid connection with an air-cooled condenser of the refrigeration system for supplying cooled air to the air-cooled condenser. This may allow the refrigeration system to operate in a more efficient manner than prior to the retrofitting. 
     Also disclosed is a CO 2  based refrigeration system comprising a condenser for transferring heat from a CO 2  refrigerant of the refrigeration system to an air stream, and a metering device downstream of the condenser. The metering device (e.g. a high pressure expansion valve) configured to condense refrigerant from the condenser when received in a supercritical state. The refrigeration system further comprises a bypass arrangement to allow the refrigerant to bypass the metering device. 
     In one embodiment the bypass arrangement may comprise a valve configurable between a first position and a second position. In the first position, the refrigerant may bypass the metering device. In the second position the refrigerant may pass through the metering device. 
     In some circumstances, the process of condensing the CO 2  refrigerant can result in efficiency losses. By bypassing the metering device (e.g. when such condensing is not required), these efficiency losses may be avoided. The refrigeration system may comprise fittings and components that are formed of a copper or steel alloy able to withstand high pressures. Such high pressures may be experienced when CO 2  refrigerant received from the condenser does not pass through the metering device (which may reduce the pressure of the CO 2  refrigerant). 
     In one embodiment the bypass arrangement comprises a bypass line on which the valve disposed. 
     In one embodiment the refrigeration system may be as otherwise defined above 
     Also disclosed is a method of operating a CO 2  based refrigeration system. The method comprises determining whether CO 2  refrigerant being discharged from a condenser of the refrigeration system is in a supercritical state. The method further comprises controlling the system so as to condense the CO 2  refrigerant by way of a throttling process when the CO 2  refrigerant is determined to be in a supercritical state, and so as to bypass the throttling process when the CO 2  is determined to not be in a supercritical state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic showing a refrigeration system as disclosed herein; 
         FIG. 2  schematic illustrates the operation of an indirect evaporative cooler; 
         FIG. 3  is a schematic showing a method of operating the refrigeration system shown in  FIG. 1A ; and 
         FIGS. 4A, 4B and 4C  are top, side and perspective views of a condenser/indirect evaporative cooler assembly. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the drawings and defined in the claims, are not intended to be limiting. Other embodiments may be utilised and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings can be arranged, substituted, combined, separated and designed in a wide variety of different configurations, all of which are contemplated in this disclosure. 
       FIG. 1  shows a CO 2  based refrigeration system (i.e. that uses CO 2  as a refrigerant or working fluid) comprising, among other components, a compressor  102 , condenser  104 , expansion valve  106  and evaporator  108 . In general, these components operate in the same manner as in known refrigeration systems. 
     In operation, CO 2  refrigerant is compressed in the compressor  102 , which increases the pressure and temperature of the refrigerant. The refrigerant subsequently flows from the discharge side of the compressor, via a discharge line  110 , to the condenser  104  for condensing. In the presently described embodiment the condenser  104  is an air cooled condenser (e.g. comprising a coil block and fans  112  that draw air through the coil block to transfer heat from the coil block). 
     Condensers generally operate by way of heat transfer between a cooling medium and the refrigerant (in this case CO 2 ). In an air cooled condenser, such as the condenser  104  illustrated in  FIG. 1 , the cooling medium is an air stream (or a plurality of air streams) that flows across conduits (e.g. tubes or coils) containing flowing refrigerant. The heat exchange is driven by a difference in temperature between the cooling medium (in this case, air) and the refrigerant. As a result, during operation, the temperature of the refrigerant in the condenser  104  is higher (e.g. by  3 - 8 K) than the temperature of the air stream. Thus, even when the ambient temperature is below the critical temperature of CO 2 , the refrigerant temperature can be above the critical temperature (31° C. for CO 2 ), such that the refrigerant exists in a supercritical state. As is set forth above, if the refrigerant is unable to be condensed from a supercritical state to a subcritical state, it can be detrimental to the operation and efficiency of the refrigeration cycle. 
     To avoid this, or to at least reduce the possibility of this occurring, the presently described embodiment further includes an indirect evaporative cooler  114  that supplies an air stream (or a plurality of air streams) to the condenser  104  for the purpose of transferring heat from the refrigerant. As will be described in further detail below, the indirect evaporative cooler  114  is able to receive an air source (i.e. ambient or outside air) and reduce the temperature of the air, before supplying the cooled air to the condenser  104 . In this way, the system  100  is no longer reliant on the ambient temperatures remaining below a particular temperature and, as a result, the refrigerant is able to be maintained in a subcritical state even under high ambient temperatures (e.g. up to 40° C.). Thus, the present system  100  may be operated in locations that would otherwise be unsuitable for CO 2  based refrigeration systems. 
     The indirect evaporative cooler  114  can take various forms, but in general it operates by transferring heat between at least one first air stream that is cooled by an evaporative process, and at least one separate second air stream. 
     Operation of the indirect evaporative cooler  114  is best described by reference to  FIG. 2 . The indirect evaporative cooler  114  of the present embodiment comprises first and second sets of channels that are formed in pads through which the air is drawn. For illustrative purposes, only one first channel  216  and one second channel  218  is shown in  FIG. 2  and described below, but one should be aware that in practice a plurality of first channels and a plurality of second channels may be present. The first channel  216  (or the “dry” channel) is separated from the second channel  218  (or the “wet” channel) by a wall  226  that is water impermeable, but that allows heat transfer between the first  216  and second  218  channels. 
     In operation, the first channel  216  receives a first air stream  220  from an ambient air source (i.e. at ambient temperature). The second channel comprises wetted surfaces  222  and receives a second air stream  224  that causes water on the wetted surfaces  222  to evaporate. The evaporation process results in sensible heat in the air and water becoming latent heat in the vapour, which causes a reduction in temperature of the air and of the water on the wetted surfaces  222 . The difference in temperature between the first  216  and second  218  channels drives heat exchange from the first channel  216  to the associated second channel  218  via a heat exchanger in the form of a channel wall  226  (separating the first  216  and second  218  channels). In this way, the first air stream  220  in the first channel  216  is cooled as it flows along the first channel  216 . This cooled air stream  228  is then supplied to the condenser (such as the condenser  104  shown in  FIG. 1  and described above) for the purpose of transferring heat away from refrigerant in coils, tubes, conduits, etc. of the condenser. 
     The indirect evaporative cooler also comprises a diverter (not shown) that diverts a portion  230  of the first air stream  220  in each first channel  216  into the second channel  218 . In this respect, the diverted portion  230  of the first air stream  220  becomes the second air stream  224  that flows over the wetted surfaces  222  in the second channel  218  (and cools the first air stream  220  via heat exchange across the channel wall  226 ). The cooled air stream  228  (i.e. that is not diverted) is supplied to the condenser, and the remaining diverted portion (the second air stream  224 ) is exhausted to the atmosphere, subsequent to it flowing over the wetted surfaces  222 . 
     Such an arrangement means that, in practice, the cooled air stream  228  supplied from the evaporative cooler  114  (e.g. to the condenser) can be at a temperature that is below the wet bulb temperature of the ambient air received by the evaporative cooler  114  (this is not the case with direct evaporative cooling). This is because, as the first air stream  220  is cooled, both the dry and wet bulb temperatures of the first air stream  220  are lowered. Thus, the wet bulb temperature of the second air stream  224  (which is a redirected portion of the first air stream  220 ) is lower than the ambient wet bulb temperature. 
     Although not shown, the indirect evaporative cooler  114  further comprises a water supply system that supplies water (e.g. via a pump and spray nozzles) to the second channel  218  (or set of second channels). In some cases, second channels of the indirect evaporative cooler  114  may be oriented to facilitate wetting of the wetted surfaces  222 . 
     Returning back to  FIG. 1 , the indirect evaporative condenser  114  comprises fans  132  that move air through the channels ( 216 ,  218 ), exhausts humid air ( 224 ) and supplies cooled air ( 228 ) to the condenser  104 . The condenser  104  also comprises a centrifugal fan  112  that moves the supplied air across the coils to transfer heat from the coils to the air. Each of the evaporative cooler  132  and condenser  112  fans can be controlled (e.g. by a PLC) to maintain the condenser pressure at a desired level. 
     As is set forth above, because the indirect evaporative cooler  114  is capable of supplying air to the condenser  104  at temperatures below the wet bulb temperature of the air, it is possible to maintain the CO 2  refrigerant in a subcritical state (i.e. in environments where this would otherwise not be possible due to ambient air temperatures). In this way, inefficiencies associated with supercritical CO 2  refrigerant can be avoided. 
     Under normal operation, the subcritical CO 2  is condensed to a liquid in the condenser  104 , and flows via a number of components (discussed further below) to the expansion valve  106 , via a receiver vessel  134 . At the expansion valve  106 , the CO 2  refrigerant undergoes a pressure drop and lowers in temperature. The refrigerant subsequently passes through the evaporator  108  and heat is transferred to the refrigerant from the surrounding air or process fluid (i.e. so as to cool the surrounding air or process fluid such as milk, wine, water, etc.). Finally, the refrigerant returns to the compressor  102  via a suction line  136  and the cycle is repeated. 
     The present system  100  also provides means for handling the CO 2  when in a supercritical state (i.e. under non-normal operation). For this purpose, the system  100  further includes a high pressure expansion valve  138  connected between the condenser  104  and the receiver vessel  134 . As is described above, when the CO 2  is supercritical it does not condense into a liquid in the condenser  104 . The high pressure expansion valve  138  is configured to create a pressure drop that liquefies the refrigerant, which then flows to the receiver vessel  134  (and subsequently to the expansion valve  106  and evaporator  108 ). 
     One consequence of the throttling in the high pressure expansion valve  138  is that it forms a flash gas component, which also flows to the receiver vessel  134  (where it separates from the liquid component). To accommodate the flash gas, the present system further comprises a bypass line  140  connecting the receiver vessel  134  (in which the flash gas is separated from the liquid refrigerant) to the compressor  102 . As should be apparent, the flash gas is a portion of the refrigerant that is not useful to the cooling function of the refrigeration system  100  and therefore is representative of an efficiency loss. This loss in efficiency means that the system  102  is more efficient when the CO 2  is maintained in a subcritical state. 
     Nevertheless, even when in a subcritical state, the throttling effect of the high pressure expansion valve  138  results in a reduction in efficiency of the system  100 . To avoid this, the present system  100  further includes a first bypass valve  142  that allows the refrigerant to bypass the high pressure expansion valve  138 . The first bypass valve  142  may avoid (unnecessary) efficiency losses that would otherwise be present due to the CO 2  refrigerant passing through the high pressure expansion valve  138 . The refrigeration system  100  may comprise fittings and components able to withstand the high pressure of the CO 2  bypassing the high pressure expansion valve  138  (e.g. the fitting and components may comprise a copper or steel alloy). 
     The system  100  also includes a second bypass valve  144  positioned on the bypass line  140  (between the receiver vessel  134  and the compressor  102 ) that provides control of refrigerant flow on the bypass line  140 . 
     In this way, when the refrigerant is subcritical (e.g. because the indirect evaporative cooler  114  is operating to maintain it in this state), the first bypass valve  142  can be opened, and the second bypass valve  144  can be closed. This avoids efficiency losses due to throttling in the high pressure expansion valve  138 , and closes the bypass line  140  (which is not required, because no flash gases are produced). Conversely, when the refrigerant is supercritical, the first bypass valve  142  can be closed, and the second bypass valve  144  can be opened such that the supercritical refrigerant flows through the high pressure expansion valve  138 , and the flash gases (created by the throttling effect) are able to flow from the receiver  134  to the compressor  102  via the bypass line  140 . 
     This operation of the valves is depicted in  FIG. 3 , which is a schematic showing exemplary operation of the refrigeration system  100 . The control methodology  346  includes detecting ambient air conditions  348  (i.e. the conditions of the air received by the indirect evaporative cooler  114  for cooling). These conditions may be, for example, the humidity and the temperature of the ambient air and may be detected by suitable sensors (these sensors are discussed in more detail with reference to  FIGS. 4A , B and C). 
     The detected conditions can then be used to determine the conditions at the condenser inlet  350 , which can in turn be used to determine whether the CO 2  refrigerant is in a supercritical state  352 . 
     If the CO 2  refrigerant is supercritical, then the first bypass valve  142  on the high pressure expansion valve bypass (which is normally configured in an open position) is closed  354 , which causes refrigerant to flow through the high pressure expansion valve  138 . This allows the supercritical CO 2  refrigerant to be liquefied by the high pressure expansion valve  138 . At the same time, the second bypass valve  144  on the bypass line  140  is opened  356 , which allows the flash gas component (formed at the high pressure expansion valve  138 ) to bypass the expansion valve  106  and the evaporator  108 . That is, the flash gas flows, via the bypass line  140 , directly to the compressor  102 . 
     An alert may also be raised  358  to notify an operator that the system  100  is operating in a supercritical state. The operator can then correct any issues that might be causing the system  100  to run in this state (i.e. apart from extreme climate conditions). 
     If, on the other hand, the CO 2  refrigerant is determined to be in a subcritical state, the indirect evaporative cooler fan  132  and the condenser fan  112  may be controlled  360 ,  362  (depending on the detected ambient air conditions) to achieve a desired condenser pressure for maximising efficiency of the system  100 . 
       FIG. 4  depicts an exemplary indirect evaporative cooler  414  and condenser  404  assembly  464  that may be used in a refrigeration system, such as the refrigeration system  100  described above. The assembly  464  comprises a plurality of sensors that can communicate data to a controller (not shown) so that the condenser  404  and indirect evaporative cooler  414  can be controlled in a manner that e.g. maximises efficiency of the system. 
     The assembly  464  comprises a humidity and temperature sensor  466  disposed at an inlet of the indirect evaporative cooler  414 . This sensor  466  measures the humidity and temperature of the ambient air that is supplied to the indirect evaporative cooler  414 . The assembly  464  also comprises condenser outlet  468  and condenser inlet  470  temperature sensors that detect the temperature of air entering and leaving the condenser  404 . Also provided is an indirect evaporative cooler inlet pressure sensor  472 , an indirect evaporative cooler exhaust pressure sensor  474 , a condenser pressure sensor  476  and a condenser fan pressure sensor  478 . 
     The data from these sensors is transmitted (e.g. wirelessly or via wired connection) to a controller. The controller makes use of this data to control aspects of the assembly, such as the condenser fans  412  and/or indirect evaporative cooler fans  432  to maximise efficiency. 
     Variations and modifications may be made to the parts previously described without departing from the spirit or ambit of the disclosure. 
     For example, the system may include additional components not discussed above, or may be configured in an alternative manner. 
     The indirect evaporative cooler may be arranged alternatively to that described above. For illustrative purposes,  FIG. 2  shows a single dry (first) channel and a single wet (second) channel, but it should be understood that the indirect evaporative cooler can comprise a plurality of dry channels and a plurality of wet channels. For example, each dry channel may be adjacent to a plurality of wet channels (and vice versa). 
     Similarly, it may not be necessary that a portion of the first air stream be diverted to form the second air stream. Instead, the first (dry) air stream and second (wet) air stream may remain separate so as to be in a cross-flow arrangement. The first and second air streams may not be parallel to one another, and may instead be e.g. perpendicular to one another. 
     Further, and as would be appreciated by the skilled person, the means for providing water to the channels may be other than via spray nozzles. 
     In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the CO 2  based refrigeration system.