Patent Publication Number: US-11656005-B2

Title: CO2 cooling system and method for operating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit under 35USC § 119(e) of US provisional patent application 62/154,982 filed Apr. 29, 2015, the specification of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The technical field generally relates to CO 2  cooling systems and to a method for operating a CO 2  cooling system. More particularly, the invention relates to CO 2  refrigeration and air-conditioning systems. 
     BACKGROUND 
     in the last few years, carbon dioxide (CO 2 ) made a come-back in refrigeration applications where it is used as a refrigerant fluid or coolant. This is mainly due to the concerns regarding the effects of refrigerants on ozone layer depletion and global warming. CO 2  is known as a naturally available, safe, environmental friendly refrigerant with good thermo-physical and transport properties. 
     In cooling systems, most of the energy costs come from the motors that drive compressors, fans, and pumps. In the case of an ice-covered surface such as an ice rink, while the use of CO 2  generally allows reducing the energy consumption of the cooling system due to possible higher heat reclaim, some sectors of the surface may require more refrigerant than others in order to maintain a similar ice quality. Similarly, for supermarkets and industrial applications, some sectors may require more refrigerant than others in order to respond to the cooling needs. 
     In view of the above, CO 2  cooling still has a number of challenges. 
     SUMMARY 
     It is therefore an aim of the present invention to address the above mentioned issues. 
     In accordance with an aspect, there is provided a CO 2  cooling system, comprising: a compression stage in which CO 2  refrigerant is compressed; a cooling stage in which the compressed CO 2  refrigerant releases heat; an evaporation stage in which the CO 2  refrigerant, having released heat in the cooling stage, absorbs heat. The evaporation stage comprises: a first evaporation sector and a second evaporation sector; a first CO 2  transfer line feeding a first portion of the CO 2  refrigerant from the cooling stage into the first evaporation sector, the first CO 2  transfer line comprising a first metering device mounted upstream the first evaporation sector; and a second CO 2  transfer line feeding a second portion of the CO 2  refrigerant from the cooling stage into the second evaporation sector, the second CO 2  transfer line comprising a second metering device mounted upstream the second evaporation sector and a CO 2  accumulator mounted upstream the second metering device. The first metering device and the second metering device are operated independently from one another, CO 2  pressure in the first evaporation sector being different than CO 2  pressure in the second evaporation. The CO 2  cooling system also comprises a plurality of CO 2  transfer lines connecting the compression stage, the cooling stage and the evaporation stage, and wherein the CO 2  refrigerant is circulable in a closed-loop circuit. 
     In an embodiment, the CO 2  cooling system further comprises a CO 2  liquid receiver located upstream of the first and the second metering devices and the CO 2  accumulator. The second CO 2  transfer line further comprises a pressure differential unit mounted between the CO 2  liquid receiver and the CO 2  accumulator. 
     In an embodiment, the first metering device comprises an expansion valve and the second metering device comprises a pump. 
     In a further embodiment, the CO 2  cooling system comprises a CO 2  transfer line transferring the CO 2  refrigerant exiting the evaporation stage to the compression stage. 
     In an embodiment, the CO 2  accumulator is mounted in the CO 2  transfer line transferring the CO 2  refrigerant exiting the evaporation stage to the compression stage. 
     In yet a further embodiment, the CO 2  cooling system further comprises a pressure regulating unit mounted to at least one of the first and second transfer line upstream of the CO 2  liquid receiver. 
     In an embodiment, the evaporation stage comprises a circuit of pipes extending under an ice-playing surface. The circuit of pipes includes at least one first pipe line corresponding to the first evaporation sector and at least one second pipe line corresponding to the second evaporation sector. 
     In an embodiment, the at least one first pipe line extends below a central section of the ice-playing surface; and the at least one second pipe line extends below an outer section of the ice-playing surface. 
     In accordance with an aspect, CO 2  pressure in the first evaporation sector is higher than CO 2  pressure in the second evaporation sector. 
     In another embodiment, downstream of a respective one of the first and second metering devices, at least one of the first and second CO 2  transfer lines is divided into a plurality of CO 2  transfer sub-lines. Each one of the CO 2  transfer sub-lines comprises a controllable metering device supplying CO 2  refrigerant to the respective one of the first and the second evaporation sectors. 
     In accordance with another aspect, there is provided a CO 2  cooling system, comprising: a compression stage in which CO 2  refrigerant is compressed; a cooling stage in which the compressed CO 2  refrigerant releases heat; a CO 2  liquid receiver in which the CO 2  refrigerant, exiting the cooling stage, is accumulated in liquid and gaseous states; an evaporation stage in which the CO 2  refrigerant, having released heat in the cooling stage, absorbs heat. The evaporation stage comprises: a first evaporation sector and a second evaporation sector; a first CO 2  transfer line feeding a first portion of the CO 2  refrigerant from the cooling stage into the first evaporation sector, the first CO 2  transfer line comprising a first metering device mounted upstream the first evaporation sector, the first CO 2  transfer line by-passing the CO 2  liquid receiver; and a second CO 2  transfer line feeding a second portion of the CO 2  refrigerant exiting the CO 2  liquid receiver into the second evaporation sector, the second CO 2  transfer line comprising a second metering device mounted upstream the second evaporation sector. The first metering device and the second metering device are operated independently from one another. The CO 2  cooling system also comprises a plurality of CO 2  transfer lines connecting the compression stage, the cooling stage, the CO 2  liquid receiver and the evaporation stage and wherein the CO 2  refrigerant is circulable in a closed-loop circuit. 
     In an embodiment, the first metering device comprises an expansion valve and the second metering device comprises a pump. 
     In another embodiment, the CO 2  cooling system comprises a CO 2  transfer line transferring the CO 2  refrigerant exiting the evaporation stage to the compression stage. 
     In a further embodiment, the CO 2  cooling system comprises a CO 2  accumulator mounted to the CO 2  transfer line extending between the evaporation stage and the compression stage. 
     In an embodiment, the CO 2  cooling system further comprises a pressure regulating unit mounted to at least one of the first and second transfer line upstream of the CO 2  liquid receiver. 
     In yet another embodiment, the CO 2  cooling system comprises a CO 2  transfer line transferring a portion of the CO 2  refrigerant from the CO 2  liquid receiver to the CO 2  accumulator. The CO 2  transfer line includes a pressure differential unit mounted downstream the CO 2  liquid receiver and upstream the CO 2  accumulator. 
     In an embodiment, the evaporation stage comprises a circuit of pipes extending under an ice-playing surface. The circuit of pipes includes at least one first pipe line corresponding to the first evaporation sector and at least one second pipe line corresponding to the second evaporation sector. 
     In another embodiment, the at least one first pipe line extends below a central section of the ice-playing surface; and the at least one second pipe line extends below an outer section of the ice-playing surface. 
     In a further embodiment, CO 2  pressure in the first evaporation sector is higher than CO 2  pressure in the second evaporation sector. 
     In another embodiment, downstream of a respective one of the first and second metering devices, at least one of the first and second CO 2  transfer lines is divided into a plurality of CO 2  transfer sub-lines. Each one of the CO 2  transfer sub-lines comprises a controllable metering device supplying CO 2  refrigerant to the respective one of the first and the second evaporation sectors. 
     In accordance with a further aspect, there is provided a method for operating a CO 2  cooling system. The CO 2  cooling system comprises: a compression stage in which CO 2  refrigerant is compressed; a cooling stage in which the CO 2  refrigerant releases heat; and an evaporation stage. The evaporation stage comprises a first evaporation sector and a second evaporation sector, and the CO 2  refrigerant having released heat in the cooling stage, absorbs heat. The method comprises: circulating the CO 2  refrigerant in a closed-loop circuit between the evaporation stage, the compression stage and the cooling stage. Circulating the CO 2  refrigerant comprises: conveying a first portion of the CO 2  refrigerant exiting the cooling stage into the first evaporation sector; conveying a second portion of the CO 2  refrigerant exiting the cooling stage to a CO 2  accumulator through a pressure differential unit, and conveying the second portion of the CO 2  refrigerant exiting the CO 2  accumulator into the second evaporation sector; and independently controlling a pressure of the CO 2  refrigerant in the first evaporation sector and a pressure of the CO 2  refrigerant in the second evaporation sector. 
     In an embodiment, the CO 2  cooling system further comprises a first metering device downstream of the cooling stage and upstream of the first evaporation sector; and a second metering device downstream of the CO 2  accumulator and upstream of the second evaporation sector. Conveying the first and the second portions of the CO 2  refrigerant comprises conveying a respective one of the first and the second portions through a respective one of the first and the second metering devices. The method further comprises independently controlling the first and the second metering devices so as to feed the first and the second portions of the CO 2  refrigerant to the respective one of the first and the second evaporation sectors, so that CO 2  pressure in the first evaporation sector is higher than CO 2  pressure in the second evaporation sector. 
     In a further embodiment, the CO 2  cooling system further comprises a CO 2  liquid receiver, in which the CO 2  refrigerant is accumulated in liquid and gaseous state, located upstream of the first and the second metering devices and the CO 2  accumulator. Conveying the first and the second portions of the CO 2  refrigerant comprises conveying the first and the second portions of the CO 2  refrigerant exiting the cooling stage to the CO 2  liquid receiver and then conveying the first and second portions of the CO 2  refrigerant to the respective one of the first metering device and the pressure differential unit provided upstream of the CO 2  accumulator. 
     In an embodiment, conveying the CO 2  refrigerant exiting the cooling stage to CO 2  liquid receiver comprises lowering a pressure of the CO 2  refrigerant by conveying at least one of the first and second portions of the CO 2  refrigerant between the cooling stage and the CO 2  liquid receiver through a pressure regulating unit. 
     In a further embodiment, the method comprises conveying the CO 2  refrigerant exiting the evaporation stage to the CO 2  accumulator, and then conveying a portion of the CO 2  refrigerant exiting the CO 2  accumulator to the compression stage. 
     In an embodiment, the method also comprises conveying the CO 2  refrigerant exiting the evaporation stage to the compression stage. 
     In yet another embodiment, the evaporation stage comprises a circuit of pipes extending under an ice-playing surface. The circuit of pipes includes at least one first pipe line corresponding to the first evaporation sector and extending below a central section of the ice-playing surface, and at least one second pipe line corresponding to the second evaporation sector and extending below an outer section of the ice-playing surface. 
     In an embodiment, the method comprises monitoring CO 2  pressure in the CO 2  liquid receiver; and controlling at least one of the pressure regulating unit, the pressure differential unit, the first metering device and the second metering device so that CO 2  pressure in the CO 2  liquid receiver is maintain between 400 and 600 psi. 
     In another embodiment, the method comprises monitoring CO 2  pressure in the CO 2  liquid receiver; and controlling at least one of the pressure regulating unit, the pressure differential unit, the first metering device and the second metering device so that CO 2  pressure in the CO 2  liquid receiver is maintain between 450 and 550 psi. 
     In yet another embodiment, the method comprises monitoring CO 2  pressure in the CO 2  accumulator; and controlling at least one of the pressure differential unit, the first metering device and the second metering device so that CO 2  pressure in the CO 2  accumulator is maintain between 300 and 400 psi. 
     In accordance with another aspect, there is provided a method for operating a CO 2  cooling system. The CO 2  cooling system comprises a compression stage in which CO 2  refrigerant is compressed; a cooling stage in which the CO 2  refrigerant releases heat; a CO 2  liquid receiver in which the CO 2  refrigerant is accumulated in liquid and gaseous states; and an evaporation stage. The evaporation stage comprises first and second evaporation sectors, in which the CO 2  refrigerant having released heat in the cooling stage, absorbs heat. The method comprises: circulating the CO 2  refrigerant in a closed-loop circuit between the compression stage, the cooling stage, and the evaporation stage. Circulating the CO 2  refrigerant comprises: conveying a first portion of the CO 2  refrigerant exiting the cooling stage into the first evaporation sector, the first portion of the CO 2  refrigerant by-passing the CO 2  liquid receiver; conveying a portion of the CO 2  refrigerant exiting the cooling stage to the CO 2  liquid receiver, and conveying a second portion of the CO 2  refrigerant into the second evaporation sector; and independently controlling a pressure of the CO 2  refrigerant in the first evaporation sector and a pressure of the CO 2  refrigerant in the second evaporation sector. 
     In an embodiment, the CO 2  cooling system further comprises a first metering device downstream of the cooling stage and upstream of the first evaporation sector; and a second metering device downstream of the CO 2  liquid receiver and upstream of the second evaporation sector. Conveying the first and the second portions of the CO 2  refrigerant comprises conveying a respective one of the first and the second portions of the CO 2  refrigerant through a respective one of the first and the second metering devices. The method further comprises independently controlling the first and the second metering devices so as to feed the first and the second portions of the CO 2  refrigerant to a respective one of the first and the second evaporation sectors, so that CO 2  pressure in the first evaporation sector is higher than CO 2  pressure in the second evaporation sector. 
     In another embodiment, the CO 2  cooling system further comprises a CO 2  accumulator downstream the evaporation stage and upstream the compression stage. The method comprises conveying a portion of the CO 2  refrigerant exiting the CO 2  liquid receiver to the CO 2  accumulator through a pressure differential unit. 
     In a further embodiment, the method comprises conveying the CO 2  refrigerant exiting the evaporation stage to the CO 2  accumulator, and conveying the CO 2  refrigerant exiting the CO 2  accumulator to the compression stage. 
     In yet another embodiment, the method comprises conveying the CO 2  refrigerant exiting the evaporation stage to the compression stage. 
     In an embodiment, conveying the CO 2  refrigerant exiting the cooling stage to CO 2  liquid receiver comprises lowering a pressure of the CO 2  refrigerant by conveying the first and the second portions of the CO 2  refrigerant from the cooling stage to a respective one of the first metering device and the CO 2  liquid receiver through a pressure regulating unit. 
     In a yet another embodiment, the evaporation stage comprises a circuit of pipes extending under an ice-playing surface. The circuit of pipes includes at least one first pipe line corresponding to the first evaporation sector and extending below a central section of the ice-playing surface, and at least one second pipe line corresponding to the second evaporation sector and extending below an outer section of the ice-playing surface. Each of the at least first and second pipe line comprises a controllable metering device. 
     In an embodiment, the method comprises monitoring CO 2  pressure in the CO 2  liquid receiver; and controlling at least one of the pressure regulating unit, the first metering device and the second metering device so that CO 2  pressure in the CO 2  liquid receiver is maintain between 400 and 600 psi. 
     In another embodiment, the method comprises monitoring CO 2  pressure in the CO 2  liquid receiver; and controlling at least one of the pressure regulating unit, the first metering device and the second metering device so that CO 2  pressure in the CO 2  liquid receiver is maintain between 450 and 550 psi. 
     In yet another embodiment, the method comprises monitoring CO 2  pressure in the CO 2  accumulator; and controlling at least one of the pressure differential unit, the first metering device and the second metering device so that CO 2  pressure in the CO 2  accumulator is maintain between 300 and 400 psi. 
     In accordance with an aspect, there is provided a CO 2  cooling system, comprising: a compression stage in which CO 2  refrigerant is compressed; a cooling stage in which the CO 2  refrigerant releases heat; an evaporation stage in which the CO 2  refrigerant, having released heat in the cooling stage, absorbs heat. The evaporation stage comprises: a first evaporation sector and a second evaporation sector; a first metering device for feeding a first portion of the CO 2  refrigerant into the first evaporation sector at a first pressure; and a second metering device for feeding a second portion of the CO 2  refrigerant into the second evaporation sector at a second pressure; a first CO 2  transfer line for transferring the first portion of the CO 2  refrigerant from the cooling stage to the first metering device; a second CO 2  transfer line for transferring the second portion of the CO 2  refrigerant from the cooling stage to the second metering device, the second transfer line comprising a CO 2  accumulator located upstream of the second metering device, wherein the first metering device and the second metering device are operated independently from one another. The CO 2  cooling system also comprises a plurality of CO 2  transfer lines connecting the compression stage, the cooling stage and the evaporation stage, and wherein the CO 2  refrigerant is circulable in a closed-loop circuit. 
     In an embodiment, the CO 2  cooling system further comprises a CO 2  liquid receiver located upstream of the first metering device and the CO 2  accumulator and the second transfer line extending from the CO 2  liquid receiver to the second metering device further comprises a pressure differential unit mounted between the CO 2  liquid receiver and the CO 2  accumulator. The second CO 2  transfer line can originate from the CO 2  liquid receiver. 
     In accordance with another aspect, there is provided a method for operating a CO 2  cooling system. The CO 2  cooling system comprises: a compression stage in which CO 2  refrigerant is compressed; a cooling stage in which the CO 2  refrigerant releases heat; and an evaporation stage comprising a first evaporation sector with a first metering device and a second evaporation sector with a second metering device and in which the CO 2  refrigerant having released heat in the cooling stage, absorbs heat. The method comprises: circulating a first portion of the CO 2  refrigerant between the cooling stage and the first metering device; operating the first metering device to feed the first portion of the CO 2  refrigerant to the first evaporation sector, at a first pressure; circulating a second portion of the CO 2  refrigerant between the cooling stage and a CO 2  accumulator through a pressure-differential unit and then between the CO 2  accumulator and the second metering device; operating the second metering device independently from the first metering device, so as to feed the second portion of the CO 2  refrigerant to the second evaporation sector, at a second pressure, lower than the first pressure; and circulating the CO 2  refrigerant between the evaporation stage, the compression stage and the cooling stage in a closed-loop circuit. 
     In accordance with a further aspect, there is provided a CO 2  cooling system for an ice-playing surface, comprising: a compression stage in which CO 2  refrigerant is compressed; a cooling stage in which the CO 2  refrigerant releases heat; a CO 2  liquid receiver in which the CO 2  refrigerant is accumulated in liquid and gaseous states; an evaporation stage in which the CO 2  refrigerant, having released heat in the cooling stage, absorbs heat. The evaporation stage comprises: a first evaporation sector and a second evaporation sector; a first metering device for feeding CO 2  refrigerant into the first evaporation sector at a first pressure; and a second metering device for feeding CO 2  refrigerant into the second evaporation sector at a second pressure. The first metering device and the second metering device are operated independently from one another. The CO 2  cooling system further comprises a plurality of CO 2  transfer lines connecting the compression stage, the cooling stage, the CO 2  liquid receiver and the evaporation stage and wherein the CO 2  refrigerant is circulable in a closed-loop circuit. 
     In accordance with still another aspect, there is provided a method for operating a CO 2  cooling system for an ice-playing surface. The CO 2  cooling system comprises: a compression stage in which CO 2  refrigerant is compressed; a cooling stage in which the CO 2  refrigerant releases heat; a CO 2  liquid receiver in which the CO 2  refrigerant is accumulated in liquid and gaseous states; and an evaporation stage comprising first and second evaporation sectors and in which the CO 2  refrigerant having released heat in the cooling stage, absorbs heat. The method comprises: circulating the CO 2  refrigerant in a closed-loop circuit between the compression stage, the cooling stage, the CO 2  liquid receiver and the evaporation stage; and independently controlling a first pressure of the CO 2  refrigerant in the first evaporation sector and a second pressure of the CO 2  refrigerant in the second evaporation sector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of a CO 2  cooling system according to an embodiment, wherein the CO 2  cooling system includes multiple refrigerant metering devices; 
         FIG.  1 A  is a scheme of an evaporation stage of the cooling system of  FIG.  1   . 
         FIG.  2    is a block diagram of a CO 2  cooling system according to another embodiment, wherein the CO 2  cooling system is free of an accumulator; and 
         FIG.  3    is a block diagram of a CO 2  cooling system according to yet another embodiment, wherein the CO 2  cooling system includes a pump and an expansion valve; 
         FIG.  4    includes  FIGS.  4 A,  4 B and  4 C :  FIG.  4 A  is a technical plan of a CO 2  cooling system according to another embodiment, wherein the CO 2  cooling system is designed to cool down an ice-covered surface of an ice rink;  FIGS.  4 B and  4 C  are close-up views of portions of the technical plan of  FIG.  4 A ; 
         FIG.  5    includes  FIGS.  5 A,  5 B and  5 C :  FIG.  5 A  is a technical plan of a CO 2  cooling system according to yet another embodiment, wherein the CO 2  cooling system is designed to cool down an ice-covered surface of an ice rink;  FIGS.  5 B and  5 C  are close-up views of portions of the technical plan of  FIG.  5 A ; and 
         FIG.  6    is the legend of  FIGS.  4  and  5   . 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
     Referring to  FIG.  1   , a CO 2  cooling system  10  according to an embodiment is shown. The CO 2  cooling system  10  can be a CO 2  air-conditioning system of the type used to cool rooms such as computer server rooms. Alternatively, the CO 2  cooling system  10  can be a refrigeration system of the type used to cool ice-playing surfaces including curling, hockey, and skating ice rinks, supermarket refrigerators and freezers, refrigerated rooms, and the like. 
     The CO 2  cooling system  10  is designed to independently control the feeding of CO 2  refrigerant in different sectors of an ice-covered surface or a portion of a building. For example, in the case of an ice-playing surface such as an ice hockey rink, several sectors of the ice-covered surface such as the center ice, and the areas around the goals are subjected to more wear than the other sectors of the ice rink. These over-exposed sectors are therefore typically in need of a greater quantity of refrigerant in order to maintain a similar ice quality. More particularly, more water is added as a thin layer to be frozen to rebuild the thickness of the ice. The CO 2  cooling system  10  is designed to independently control the amount of CO 2  refrigerant which is delivered to each one of the sectors of the ice rink. In other words, a CO 2  pressure in an outer section of the ice-playing surface (i.e. the circumference of the ice rink) and in a central section of the ice-playing surface (i.e. the center or the ice rink) are controlled independently. Throughout this disclosure it is understood that an ice-covered surface is used to exemplify the object to be cooled. However, it is also understood that in what follows, the cooled surface can be substituted with a portion of a building such as a room or a floor, a refrigerator, a freezer, or more generally any refrigerated room, closed space or surface. 
     The CO 2  cooling system  10  includes a compression stage  12  in which CO 2  refrigerant in a gaseous state is compressed. In some embodiments, the compression stage  12  includes one or several suitable compressors. In some embodiments, the compression stage can include a plurality of compressors. In some embodiments, the compressors can be configured in a parallel configuration, wherein the incoming CO 2  refrigerant flow is divided before being supplied to the compressors. The compressor outputs can then be recombined. In some embodiments, the compression stage  12  can include one or more compression units, each including one or more compressors, configured in a parallel configuration. Each one of the compression units can be fed with a different CO 2  refrigerant flow. For instance and without being limitative, a first one of the compression units can be fed with CO 2  refrigerant exiting an evaporation stage  26  through reservoir or accumulator  32 , a second one of the compression units can be fed with CO 2  refrigerant exiting a CO 2  liquid receiver  18 , such as a CO 2  condensation reservoir, and a third one of the compression units can be fed with CO 2  refrigerant exiting a pressure-regulating unit (not shown). In an embodiment, the compression stage  12  is designed to compress CO 2  refrigerant into a sub-critical state or a supercritical state (or transcritical state), as will be described in more details below. However, it is appreciated that the system  10  can be designed to either operate solely in a sub-critical state, solely in a supercritical state, or alternatively in both the sub-critical state and the supercritical state. 
     The CO 2  refrigerant exiting the compression stage  12  is transferred to a cooling stage  14  in CO 2  transfer line  16 . It is understood by the person skilled in the art that a transfer line can be a direct CO 2  connection, such as a conduit or a pipe, between two adjacent components of the CO 2  cooling system or a succession of CO 2  connections between a plurality of components of the CO 2  cooling system. In the cooling stage  14 , CO 2  refrigerant in a compressed state releases heat. In some embodiments, the cooling stage  14  includes a gas cooling stage (or gas cooler). The cooling stage  14  can include one or several cooling units which can be disposed in parallel and/or in series. In some embodiments, in addition to or in replacement of the gas cooling stage, the cooling stage  14  can include a heat reclaim stage wherein heat is reclaimed from CO 2  refrigerant by heating a fluid, such as air, water, or another refrigerant, or by heating equipment. The cooling stage  14  can include one or several heating units. Valve(s) can be provided in relation with the cooling stage units to control the amount of CO 2  refrigerant directed to each of the cooling stage units. 
     In some embodiments, at least a portion of the CO 2  refrigerant exiting the cooling stage  14  is transferred to a CO 2  liquid receiver  18  in CO 2  transfer line  20 . In some embodiments, a pressure regulating unit  22 , such as a valve, is positioned downstream of the cooling stage  14  and upstream of the CO 2  liquid receiver  18 . In the embodiment shown in  FIG.  1   , the pressure regulating unit  22  divides CO 2  transfer line  20  into two sections  20 A and  20 B. However, in alternative embodiments, the pressure regulating unit  22  can be mounted adjacent to one of the cooling stage  14  and the CO 2  liquid receiver  18 . The pressure regulating unit  22  can be any suitable valve or valve assembly that can maintain a pressure differential in line  20 , i.e., that can maintain a higher pressure upstream thereof (the higher pressure side) than downstream thereof (the lower pressure side). In some embodiments, the CO 2  refrigerant is compressed in a supercritical state and the CO 2  refrigerant is returned to the CO 2  liquid receiver  18  in a mixture of liquid and gaseous states. Alternatively, in some embodiments where the CO 2  refrigerant is compressed in a sub-critical state, the CO 2  refrigerant can be directly transferred from the cooling stage  14  to the CO 2  condensation reservoir  18  without going through the pressure regulating unit  22  (i.e., by by-passing the pressure regulating unit  22 ). In other words, in some embodiments, the cooling system  10  can be free of the pressure regulating unit  22  in line  20  when the cooling system is not designed to compress the CO 2  refrigerant in a supercritical state. 
     The CO 2  liquid receiver  18  accumulates CO 2  refrigerant in a combination of liquid and gaseous states. Gaseous refrigerant accumulating in the CO 2  liquid receiver  18  can be circulated back to the compression stage  12  in CO 2  transfer line  23 . More particularly, line  23  can be used to direct flash gas to the compression stage  12 . CO 2  transfer line  24  directs liquid CO 2  refrigerant from the CO 2  liquid receiver  18  to an evaporation stage  26 . 
     In some embodiments, the CO 2  refrigerant exiting the cooling stage  14  is transferred to the evaporation stage  26  without going through the CO 2  liquid receiver  18 . In the embodiment shown in  FIG.  1   , the CO 2  refrigerant can by-pass the CO 2  liquid receiver and be transferred directly to the evaporation stage  26  in CO 2  transfer line  21 . Line  21  by-passes the CO 2  liquid receiver and links lines  20  and  24 . The pressure differential unit  25 , which can be a valve, can be respectively provided in line  21  in order to control the CO 2  refrigerant flowing in both paths (i.e., the CO 2  refrigerant by-passing the CO 2  liquid receiver  18  by going through line  21 , or the CO 2  refrigerant going through the CO 2  liquid receiver  18  in line  20 ). In an embodiment, line  20 , downstream the pressure regulating unit  22  can also be provided with a valve  19  to control the CO 2  refrigerant flow directed to the CO 2  liquid receiver  18 . It is understood that in some embodiments, the CO 2  liquid receiver  18  can be absent from the cooling system  10 . In such case, the CO 2  refrigerant is transferred from the cooling stage  14  to the evaporation stage  26  via CO 2  transfer lines  21  and  24 , and/or to the reservoir or accumulator  32 . 
     In the embodiment shown in  FIGS.  1  and  1 A , the evaporation stage  26  is divided into a plurality of sectors  26 A,  26 B,  26 C,  26 D and  26 E. Each one of the sectors  26 A to  26 E of the evaporation stage  26  can correspond to a sector of the refrigerated surface (or to a sector of the room or zone to refrigerate). The sectors  26 A and  26 E are connected to line  24  via a respective one of CO 2  transfer sub-lines  24 A and  24 E while the sectors  26 B,  26 C, and  26 D are connected to line  21  via a respective one of CO 2  transfer sub-lines  21 B,  21 C, and  21 D. Each one of the sub-lines  24 A to  24 E includes a metering device  28 A,  28 B,  28 C,  28 D and  28 E which can hold CO 2  refrigerant back in a condensed state and can feed the CO 2  refrigerant into the respective one of the sectors  26 A to  26 E. Each one of the metering devices  28 A to  28 E can feed CO 2  refrigerant into the respective one of the sectors  26 A to  26 E at a desired pressure. For example, in the case of an ice-covered surface, some of the metering devices  28 A to  28 E can be configured to release CO 2  refrigerant at a higher pressure in sectors where the ice is damaged, while the remaining metering devices can be configured to release CO 2  refrigerant at a lower pressure in sectors where the ice is of relatively acceptable quality. In some embodiments, each one of the metering devices  28 A to  28 E is one of an expansion valve and a pump. It is understood that a metering device can provide a pressure drop point (i.e., an expansion valve) or a pressure increase point (i.e., a pump). In the embodiments shown in  FIGS.  1  and  1 A , the evaporation stage  26  is divided in five sectors  26 A to  26 E. However, it should be understood that the evaporation stage  26  can be divided into two sectors, three sectors, four sectors, or as many sectors required. Furthermore, the sectors  26 A to  26 E fed by a respective one of lines  21  and  24  can vary from the embodiment shown. 
     In an embodiment, the sectors requiring a higher refrigeration rate are supplied through line  21 . In an embodiment, sub-lines  21 B,  21 C, and  21 D are free of metering devices  28 B,  28 C, and  28 D. The pressure differential unit  25  acts as the metering device for the sectors connected to line  21 . Thus, the pressure differential unit  25  controls the flowrate of CO 2  refrigerant flowing in some sectors of the evaporation stage  26  and, more particularly, the one(s) supplied by line  21 . 
     Now referring to  FIG.  1 A , the evaporation stage  26  can include one or several heat exchanger(s), such as a circuit of pipes  29 , in which the CO 2  refrigerant circulates to absorb heat from ambient air, from another fluid or from a solid. If CO 2  refrigerant absorbs heat from ambient air, air can be propelled on the circuit of pipes through a fan, for instance to increase heat transfer (i.e., forced air convection). In the non-limiting embodiment shown in  FIG.  1 A , the circuit of pipes  29  includes a sub-circuit in each one of the sectors  26 A to  26 E. Each one of the sub-circuits can receive CO 2  refrigerant from a respective one of the metering devices  28 A to  28 E, at a pressure which can be controlled independently in each one of the sectors  26 A to  26 E, by configuring the respective metering device. In each one of the sectors  26 A to  26 E, the CO 2  refrigerant circulates through the sub-circuit so as to absorb heat. In the non-limiting embodiment shown in  FIG.  1 A , the CO 2  refrigerant is then recovered in CO 2  transfer line  30 . In other embodiments (not shown in the Figures), it is understood that separate lines can allow recovering CO 2  refrigerant from each one of the sectors independently. 
     In an embodiment, each one of the metering devices  28 A to  28 E and the pressure differential unit  25 , which can be a metering device, is independently controllable. In an embodiment, the metering devices  28 A to  28 E and pressure differential unit  25  can be operatively connected to a controller (not shown) and their configuration, i.e. opening or speed, can be adjusted in accordance with the required CO 2  flowrate. 
     The CO 2  refrigerant exiting the evaporation stage  26  is directed in CO 2  transfer line  30  to the compression stage  12 . In the embodiment shown in  FIG.  1   , line  30  includes a reservoir or accumulator  32 . For example, the reservoir or accumulator  32  can be a suction line accumulator. In some scenarios, the suction line accumulator can prevent compressor damage from a sudden surge of liquid refrigerant and oil that could enter the compressor stage  12  from line  30 . In some embodiments, CO 2  refrigerant can be directed from the CO 2  liquid receiver  18  to the reservoir or accumulator  32  in CO 2  transfer line  34 . In some embodiments, when the CO 2  liquid receiver  18  is by-passed or not present in the system, CO 2  transfer line (not shown) can transfer CO 2  refrigerant from CO 2  transfer line  21  to CO 2  transfer line  34 . Line  34  can be provided with a pressure regulating unit  36  (such as a valve) which can be configured in a closed position, or in an open position so as to let CO 2  refrigerant through from the CO 2  liquid receiver  18  to the accumulator  32 . In some embodiments, as shown in  FIG.  2   , the accumulator is not present between the evaporation stage  26  and the compression stage  12 , and the CO 2  refrigerant is directly directed to the compression stage  12  from the evaporation stage  26  in line  30  or from the reservoir or accumulator  32  to the compression stage  12 . 
     In some embodiments, the CO 2  refrigerant is transferred from the cooling stage to the evaporation stage by CO 2  transfer lines. The evaporation stage  26  comprises first and second evaporation sectors comprising respectively a first and a second metering devices. A first portion of the CO 2  refrigerant exiting the cooling stage is transferred by a first CO 2  transfer line to the first metering device, and a second portion of the CO 2  refrigerant is transferred by a second CO 2  transfer line to the second metering device. In some embodiment, the first and second transfer CO 2  lines share a conduit section or a pipe section along a portion of their paths, i.e. the CO 2  refrigerant flowing in the first transfer line flows in the same conduit or pipe than the CO 2  refrigerant flowing in the second transfer line along a portion of their respective paths. The second CO 2  transfer line also comprises a CO 2  liquid receiver  18 . Therefore the second portion of the CO 2  refrigerant is circulated between the cooling stage and the CO 2  liquid receiver  18  and then between the CO 2  liquid receiver  18  and the second metering device. The first portion of the CO 2  refrigerant by-passes the CO 2  liquid receiver  18 , and is therefore circulated between the cooling stage and the first metering device through a pressure differential unit  25 . The first and second metering devices can be operated to feed the first and second portions of CO 2  refrigerant into the first and second evaporation sectors respectively. Since the first and second metering devices can be operated independently, a CO 2  pressure in the first evaporation sector can be different from a CO 2  pressure in the second evaporation sector. In some scenario, the CO 2  pressure in the first evaporation sector is higher than the CO 2  pressure in the second evaporation sector. The CO 2  refrigerant is circulated in a closed-loop circuit: between the compression stage to the cooling stage, between the cooling stage and the evaporation stage  26  through the first and second CO 2  transfer lines, the second CO 2  transfer line comprising the CO 2  liquid receiver  18 , and finally between the evaporation stage  26  and the compression stage. 
     Now referring to  FIG.  3   , there is shown an alternative embodiment of a CO 2  cooling system  100 , wherein some features are numbered with reference numerals in the  100  series which correspond to the reference numerals of the embodiment of  FIG.  1   . CO 2  transfer line  124  includes a metering device  128  for feeding the liquid CO 2  refrigerant into the evaporation stage  126 , such that refrigerant can be fed into sector  126 B. In the non-limiting embodiment shown, the metering device  128  is an expansion valve, but it is understood that the expansion valve can be replaced with a pump. The pressure of the CO 2  refrigerant in sector  126 B is controlled by the metering device  128 . 
     Still referring to  FIG.  3   , CO 2  refrigerant is directed to sectors  126 A,  126 C of the evaporation stage  126  from the reservoir  132 , in CO 2  transfer line  137 . Line  137  includes a metering device  138  for feeding the CO 2  refrigerant into the evaporation stage  126 . In the non-limiting embodiment shown, the metering device  138  is a pump, but it is understood that the pump can be replaced with an expansion valve. Line  137 , through metering device  138 , can feed CO 2  refrigerant into sectors  126 A,  126 C of the evaporation stage  126 , and the pressure of the CO 2  refrigerant in sector  126 B is controlled by the metering device  128 . Line  137  is divided into CO 2  transfer sub-lines  137 A and  137 C downstream of the metering device  138  (as opposed to upstream of the metering devices  28 A to  28 E in the embodiment of  FIG.  1   ) such that CO 2  refrigerant can be fed into sectors  126 A and  126 C of the evaporation stage  126 . In an alternative embodiment (not shown), it is appreciated that the flow of CO 2  refrigerant in each one of the sectors  126 A and  126 C can be controlled by its own metering device. 
     In the embodiment shown in  FIG.  3   , the pressure of CO 2  refrigerant in sector  126 B is controlled by metering device  128 , and the pressure of CO 2  refrigerant in sectors  126 A and  126 C is controlled by metering device  138 . However, it should be understood that other configurations are possible. For example, each one of the metering devices  128  and  138  can deliver CO 2  refrigerant to one or more sectors of the evaporation stage  126 . Furthermore, CO 2  transfer line  124  and sub-lines  137 A,  137 C can be provided with flow-limiting devices downstream of the metering device (not shown in the Figures). Such flow-limiting devices can for example include valves such as solenoid valves, motorized valves, one-way flow control devices, pressure-regulating valves, and the like. 
     The pressure of CO 2  refrigerant in the CO 2  liquid receiver  18  is typically higher than the pressure of CO 2  refrigerant in the reservoir or accumulator  32  or  132 . For example, the pressure of CO 2  refrigerant in the CO 2  liquid receiver  18  can be between 400 psi and 600 psi, or between 450 psi and 550 psi. For example, the pressure of CO 2  refrigerant in the reservoir or accumulator  32  or  132  can be between 300 and 400 psi. In some embodiments, the pressure of CO 2  refrigerant in the CO 2  liquid receiver  18  is variable and depends on the amount of CO 2  refrigerant which is condensed and/or the amount of CO 2  refrigerant which is fed into the CO 2  liquid receiver  18 . 
     In some embodiments, the pressure of CO 2  refrigerant in the reservoir or accumulator  32 ,  132  is maintained at a substantially constant value. For example, the pressure in the reservoir or accumulator  32 ,  132  can be set at a given value between 300 and 400 psi (e.g. 350 psi), and CO 2  refrigerant can be allowed into the reservoir or accumulator  32 ,  132  from the evaporation stage  26 ,  126  when the pressure drops below the given value (for example by opening a valve which can be mounted in CO 2  transfer line  30 ,  130  upstream of the reservoir or accumulator  32 ,  132 ). Similarly, when the pressure is higher than the given value, CO 2  refrigerant can be forced out of the reservoir or accumulator  32 ,  132  (for example by opening a valve which can be mounted in line  30 ,  130  downstream of the reservoir or accumulator  32 ,  132 ). 
     In an embodiment, the sectors requiring a higher refrigeration rate are supplied through the high pressure CO 2  liquid receiver  18 , via line  124 , and the metering device  128  is an expansion valve while the metering device  138  is a pump. Higher CO 2  refrigerant flowrates can typically be achieved when supplied from a combination of a higher pressure reservoir and an expansion valve than when supplied from a combination of a lower pressure reservoir and a pump. 
     As for the embodiments described above in reference to  FIGS.  1  and  2   , each one of the metering devices  128  and  138  is independently controllable. In an embodiment, the metering devices  128  and  138  can be operatively connected to a controller (not shown) and their configuration, i.e. opening or speed can be adjusted in accordance with the required CO 2  flowrate. 
     In some embodiments, the CO 2  refrigerant is transferred from the cooling stage to the evaporation stage by CO 2  transfer lines. The evaporation stage comprises a first and a second evaporation sectors, comprising a first and a second metering devices respectively. A first portion of the CO 2  refrigerant exiting the cooling stage is transferred by a first CO 2  transfer line to the first metering device, and a second portion of the CO 2  refrigerant is transferred by a second transfer line to the second metering device. In some embodiment, the first and second transfer lines share a conduit section or a pipe section along a portion of their paths, i.e. the CO 2  refrigerant flowing in the first transfer line flows in the same conduit or pipe than the CO 2  refrigerant flowing in the second transfer line along a portion of their respective paths. The second transfer line also comprises a CO 2  accumulator. Therefore, the second portion of the CO 2  refrigerant is circulated between the cooling stage and the CO 2  accumulator and then between the CO 2  accumulator and the second metering device. The first and second metering devices can be operated to feed the first and second portions of CO 2  refrigerant into the first and second evaporation sectors respectively. The second transfer line also comprises a pressure differential unit between the cooling stage and the CO 2  accumulator. Since the first and second metering devices can be operated independently, a CO 2  pressure in the first evaporation sector can be different from a CO 2  pressure in the second evaporation sector. In some scenario, the CO 2  pressure in the first evaporation sector is higher than the CO 2  pressure in the second evaporation sector. The CO 2  refrigerant is circulated in a closed-loop circuit: between the compression stage to the cooling stage, between the cooling stage and the evaporation stage through the first and second CO 2  transfer lines, the second CO 2  transfer line comprising the CO 2  accumulator and the pressure differential unit. The CO 2  refrigerant is then circulated between the evaporation stage and the CO 2  accumulator, and finally from the CO 2  accumulator to the compression stage. 
     Referring now to  FIGS.  4 A,  4 B and  4 C , there is shown an alternative embodiment of a CO 2  cooling system wherein the features are numbered with reference numerals in the  200  series which correspond to the reference numerals of the previous embodiments. In the embodiment shown in  FIG.  4 A , the CO 2  cooling system  200  includes two CO 2  or accumulators  232  and  218 . The CO 2  liquid receiver  218  is a condensation reservoir while the reservoir or accumulator  232  is a suction accumulator. The CO 2  liquid receiver  218  accumulates CO 2  refrigerant in liquid and gaseous states. The suction accumulator  232  provides storage for the CO 2  refrigerant directed to compression stage  212  from evaporation stage  226  and in which separation of the CO 2  refrigerant in gaseous state from the CO 2  refrigerant in liquid state occurs. 
     The CO 2  cooling system  200  is conceived to cool down an ice-covered surface and, more particularly an ice rink which can be located in an arena. It is understood that other configurations and applications can be foreseen. 
     The CO 2  cooling system  200  includes a compression stage  212  in which CO 2  refrigerant in a gaseous state is compressed by a plurality of compressors  242  mounted in parallel. The compressors  242  are designed to compress CO 2  refrigerant and can compress CO 2  refrigerant into a sub-critical state or a supercritical state (or transcritical state). Oil separators  243  are mounted in the line(s) extending between the output of the compression stage  212  and the cooling stage  214 . Check valves  244  are mounted in the line(s) extending between the outlets of the compressors  242  and the oil separators  243 . Check valves  246  are also mounted between the oil separators  244  and the cooling stage  214 . The purpose of check valves  214  and  216 , as well as other check valves, will be described in more details below. 
     In the embodiment shown, the CO 2  refrigerant exiting the compression stage  212  is transferred to the cooling stage  214  in CO 2  transfer line  216  as compressed CO 2  refrigerant. In the cooling stage  214 , the compressed CO 2  refrigerant releases heat. In the embodiment shown in  FIG.  4 A , the cooling stage  214  includes a gas cooler  248 . The CO 2  refrigerant exiting the cooling stage  214  is transferred to the CO 2  liquid receiver  218  in CO 2  transfer line  220 . A pressure regulating unit  222  is positioned downstream of the cooling stage  214  and upstream of the CO 2  liquid receiver  218 . The pressure regulating unit  222  includes a pressure differential valve  250  (also referred to as an ICMTS valve) in line  220 . The pressure regulating unit  222  also includes CO 2  transfer line  220 A which can be used to bypass the pressure differential valve  250 . The pressure regulating unit  222  includes two isolation valves  251  (one downstream and one upstream) of the pressure differential valve  250  in line  220 , as well as one isolation valve  251 A in line  220 A. The isolation valves  251 ,  251 A allow selecting one flow path or the other (i.e., allow bypassing the pressure differential unit  250  by going through line  220 A, or going through the pressure differential unit  250  in line  220 ). For example, when the CO 2  cooling system  200  is operating in a subcritical state, the pressure differential unit  250  can be by-passed, and when the CO 2  cooling system  200  is operating in a transcritical state, the CO 2  refrigerant can go through the pressure differential valve  250 . It is understood that the purpose of the pressure regulating unit  222  is the same as the purpose of the pressure regulating unit  22  described above. 
     In some embodiments, such as the embodiment shown in  FIG.  4 A , the cooling stage  214  includes optional heat reclaim stages  264  and  266 . For example, heat reclaim stage  264  can allow recovering heat for domestic hot water by actuation of valve  268  and by operating heat exchangers  269 . For example, heat reclaim stage  266  can allow recovering heat for heating the room in which the ice rink  262  is located, by actuation of valve  270  and by operating heat exchangers  271 . In such cases, the CO 2  refrigerant can then be returned to CO 2  transfer line  216  and directed to the gas cooler  248 . 
     Liquid CO 2  refrigerant can be directly sent from the CO 2  liquid receiver  218  to the evaporation stage  226  in CO 2  transfer line  224 , or can first be sent through a dryer  252  in line  224 A, in order to remove traces of moisture content or humidity that may be present in the CO 2  refrigerant. Isolation valves  254  and check valves  256  are provided in lines  224  and  224 A so that one flow path or the other can be selected. 
     Gaseous CO 2  refrigerant, such as flash gas, can be recirculated back from the CO 2  liquid receiver  218  to the compression stage  212  in CO 2  transfer line  223 . A pressure controller  258  is used to regulate the pressure in the CO 2  liquid receiver  218 . The pressure controller  258  is connected to a pressure sensor and a temperature sensor in line  220  upstream of the pressure differential valve  250 , as well as to a pressure sensor and an electronic expansion valve  260  in line  223 . When the CO 2  refrigerant is in a transcritical state, the pressure in the CO 2  liquid receiver  218  is controlled by the ICMTS valve  250 . When the pressure in the CO 2  liquid receiver  218  reaches a certain level, the pressure controller  258  can instruct the electronic expansion valve  260  to release gaseous refrigerant back to the compression stage  212 . 
     CO 2  transfer line  224  directs CO 2  refrigerant, in liquid state, from the CO 2  liquid receiver  218  to the evaporation stage  226 . Line  224  is divided into CO 2  transfer sub-lines  224 A,  224 B,  224 C,  224 D and  224 E, each including an expansion valve  228 A,  228 B,  228 C,  228 D and  228 E. Each of the sub-lines  224 A to  224 E allows CO 2  refrigerant into a respective one of several sectors  262 A,  262 B,  262 C,  262 D,  262 E of an ice rink  262 . In the embodiment shown, the expansion valve  228 E delivers CO 2  refrigerant in pipes located below and around the ice rink  262  (i.e., below and on the exterior of the ice rink  262 ), before being fed into CO 2  transfer line  230  exiting the evaporation stage  226 . Upstream of each one of the expansion valves  228 D,  228 C,  228 B,  228 A, the respective sub-line  224 D,  224 C,  224 B and  224 A is further divided into three paths (which can be circuits of pipes), each path delivering CO 2  refrigerant under a surface of the ice-rink  262  and along the length of the ice rink  262 , and circling back to deliver CO 2  refrigerant into line  230  existing the evaporation stage  226 . The CO 2  refrigerant circulating in the pipes can absorb heat from a heat-transfer fluid or solid surrounding the pipes and located under the ice-covered surface. In some scenarios, the heat-transfer fluid contacting the pipes and located under the ice-covered surface is brine. In some scenarios, the heat-transfer fluid contacting the pipes and located under the ice-covered surface is ambient air. In such case, a plurality of fans can be provided to promote air circulation around the pipes containing CO 2  refrigerant. The air is drawn around the pipes by the action of the fans, promotes heat exchange, and can then exit through an aperture (not shown in the Figure). This configuration can allow for forced convection around the pipes, which can increase heat transfer. In other words, the above-described cooling system  200  can allow a direct heat transfer between CO 2  refrigerant and ambient air, or can be used to cool down gases, liquids, and solids by heat exchange, thereby indirectly transferring heat between the CO 2  refrigerant and ambient air. In some scenarios, the pipes are embedded in concrete, below the ice-covered surface and heat transfer can occur with the ambient air. 
     As for the embodiments described above, each one of the metering devices  228 A to  228 E is independently controllable. In an embodiment, the metering devices  228 A to  228 E can be operatively connected to a controller (not shown) and their configuration, i.e. opening or speed, can be adjusted in accordance with the required CO 2  flowrate. As mentioned above, the sector(s) corresponding to the center of the ice rink and surrounding the goals, if any, has(have) higher cooling needs and thus require(s) a higher CO 2  flowrate. 
     CO 2  refrigerant exiting the evaporation stage  226  is directed to the suction accumulator  232 , in line  230 . It is understood that the suction accumulator  232  has the same purpose as reservoir or accumulator  32  described above. The gaseous CO 2  refrigerant is supplied to the compression stage  212  from the suction accumulator  232  in line  230 . 
     In an alternative embodiment (not shown), one or several sectors of the evaporation stage  226  can be supplied through a line, including a metering device, if connected to the suction accumulator  232 , instead of the CO 2  liquid receiver  218 . For instance, the metering devices  228 A,  228 E can be pumps mounted to CO 2  transfer lines extending between the suction accumulator  232  and the evaporation stage  226 . 
     The CO 2  refrigerant circulates in the CO 2  cooling system  200  mainly through the action of the compression stage  212 . The check-valves which are provided in various CO 2  transfer lines of the CO 2  cooling system  200  (such as check-valves  246 ,  256  among others), prevent CO 2  refrigerant to be directed in an opposite direction. The check-valves are typically one-way valves which allow CO 2  refrigerant circulation in a single direction. For example, check-valves  246  allow CO 2  refrigerant to circulate from the compression stage  212  to the cooling stage  214  and/or other optional heat reclaim stages. 
     A pressure relief valve  272  is provided in a CO 2  transfer line  274  extending from CO 2  transfer line  216  downstream of the compression stage  212  and the optional heat reclaim stages  264  and  266  and upstream the gas cooler  248 . It is appreciated that the location of the pressure relief valve  272 , if any, can vary from the embodiment shown. The CO 2  cooling system  200  also includes other valves to control the fluid flow therein, and a plurality of suitable sensors, such as temperature and pressure sensors, as it is known in the art. For instance, control valves or isolation valves  276  can be provided in the CO 2  transfer lines extending between the CO 2  liquid receiver  218  and the evaporation stage  226 , and/or between the evaporation stage  226  and the suction accumulator  232 , and/or between the suction accumulator  232  and the compression stage  212 , and/or between the compression stage  212  and the cooling stage  214 , and/or between the cooling stage  214  and the CO 2  liquid receiver  218 , and/or at any other suitable location. In some scenarios, the control valves can be configured to control the CO 2  expansion, and therefore the temperature. 
     Referring now to  FIGS.  5 A,  5 B and  5 C , there is shown yet another embodiment of a CO 2  cooling system wherein the features are numbered with reference numerals in the  200  series which correspond to the reference numerals of the previous embodiments. In the embodiment shown in  FIG.  5 A , the CO 2  cooling system  200  includes two CO 2  reservoirs or accumulators  232  and  218 . The reservoir  218  is a CO 2  liquid receiver while the reservoir or accumulator  232  is a suction accumulator. The CO 2  liquid receiver  218  accumulates CO 2  refrigerant in liquid and gaseous states. The suction accumulator  232  provides storage for the CO 2  refrigerant directed to compression stage  212  from evaporation stage  226  and in which separation of the CO 2  refrigerant in gaseous state from the CO 2  refrigerant in liquid state occurs. 
     In the embodiment shown, gaseous CO 2  refrigerant can be directed from the CO 2  liquid receiver  218  to the suction accumulator  232  in CO 2  transfer line  234 . It is understood that isolation valve  236 , located in line  234 , has the same purpose as valve  36  described above. Liquid CO 2  refrigerant is directed from the CO 2  liquid receiver  218  to the evaporation stage  226  in CO 2  transfer line  224 , and liquid CO 2  refrigerant is directed from the suction accumulator  232  to the evaporation stage  226  in CO 2  transfer line  237 . Line  224  is divided into sub-lines  224 B and  224 C, each including a respective expansion valve  228 B and  228 C. Line  237  includes a pump  238  for pumping CO 2  refrigerant in sub-lines  237 A,  237 D and  237 E. Typically, the CO 2  liquid receiver  218  operates at a higher pressure than the suction accumulator  232 . As a non-limiting example, the CO 2  liquid receiver can operate at between 450 and 550 psi (e.g. 500 psi), and the suction accumulator can operate at between 300 psi and 400 psi (e.g. 350 psi). The expansion valves  228 B and  228 C can be configured to deliver a high load of CO 2  refrigerant into the central portion of the ice rink, while the pump  238  can be configured to deliver a lower load of CO 2  refrigerant compared to the expansion valves  228 B and  228 C. Typically, the ice of an ice-covered surface such as an ice rink  262  of an arena is more easily damaged in certain sectors, such as center ice. In the embodiment shown, it is therefore possible to deliver a high flowrate of CO 2  refrigerant to certain sectors (e.g. the center of the ice rink  262 ), while a lower flowrate of CO 2  refrigerant can be delivered to other sectors (e.g. the side sectors of the ice rink  262 ). It is understood that the pump  238  and each one of the expansion valves  228 B,  228 C can be operated and configured independently from one another. Furthermore, each one of the sublines  237 A,  224 B,  224 C,  237 D and  237 E can be provided with flow-limiting devices downstream of the pump and/or each one of the expansion valves (not shown in the Figures). Such flow-limiting devices can for example include valves such as solenoid valves, motorized valves, one-way flow control devices, pressure-regulating valves, and the like. 
     It is understood that combinations of different embodiments of the CO 2  cooling systems  10 ,  100  and  200  described herein can be foreseen. For instance, as a non-limiting example, the several pumps (such as pump  138  of  FIG.  3   ) can be used without using expansion valves in order to control the pressure of CO 2  refrigerant in different sectors of the evaporation stage. As another non-limiting example, one or more pumps (such as pump  138  of  FIG.  3   ) can be used in combination with one or more expansion valves (such as valve  128  of  FIG.  3   ), in the CO 2  cooling system  200 . 
     It is appreciated that the cooling systems  10 ,  100  and  200  can include several CO 2  transfer lines extending in parallel or, in some embodiments, CO 2  transfer lines can combine. For instance and without being limitative, in the evaporation stage  26  shown in  FIG.  1 A , the circuit of pipes can combine into line  30  after exiting the evaporation stage  26 . In alternative embodiments, the sub-lines can exit the evaporation stage  26  without combining in a single line  30 , and can instead extend in parallel to deliver CO 2  refrigerant directly to the reservoir or accumulator  32  and/or the compression stage  12 . 
     In some embodiments, a method for operating a CO 2  cooling system is provided. The CO 2  cooling system includes a compression stage in which CO 2  refrigerant is compressed; a cooling stage in which the CO 2  refrigerant releases heat; a CO 2  liquid receiver in which the CO 2  refrigerant is accumulated in liquid and gaseous states; and an evaporation stage including first and second evaporation sectors and in which the CO 2  refrigerant having released heat in the cooling stage, absorbs heat. For example, the method allows operating CO 2  cooling system including any one of CO 2  cooling systems  10 ,  100  and  200  described above. 
     The method includes circulating the CO 2  refrigerant in a closed-loop circuit between the compression stage, the cooling stage and the evaporation stage. The method also includes independently controlling a first pressure of the CO 2  refrigerant in the first evaporation sector and a second pressure of the CO 2  refrigerant in the second evaporation sector. In some embodiments, the evaporation stage can include more than two evaporation sectors, such as three, four, five or more evaporation sectors. In such cases, it is understood that the method can include independently controlling the pressure of the CO 2  refrigerant in at least two of the evaporation sectors. In some scenarios, the pressure of CO 2  refrigerant can be controlled in all of the sectors. 
     It should be understood that in the expression “independently controlling the pressure of CO 2  refrigerant” in a given sector, it is meant that pressure variations in the given sector do not substantially affect the pressure of CO 2  refrigerant in other sectors, including neighboring sectors. In other words, each one of the independently controlled sectors can be controlled by one or more metering device(s) which is/are not tied to other metering device(s) controlling other independent sectors of the evaporation stage. The independent control can be carried out by operatively connecting the metering devices to a controller. 
     The cooling system described above and the associated method can reduce the total energy requirement of the CO 2  cooling system by allowing independently controlling the amount of CO 2  refrigerant being provided in certain sectors of the evaporation stage. 
     It will be appreciated that the method to operate the CO 2  cooling system described herein may be performed in the described order, or in any suitable order. 
     Several alternative embodiments and examples have been described and illustrated herein. The embodiments of the invention described above are intended to be exemplary only. A person of ordinary skill in the art would appreciate the features of the individual embodiments, and the possible combinations and variations of the components. A person of ordinary skill in the art would further appreciate that any of the embodiments could be provided in any combination with the other embodiments disclosed herein. It is understood that the invention may be embodied in other specific forms without departing from the spirit or central characteristics thereof. The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein. Accordingly, while the specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.