FIELD OF THE INVENTION

The present invention relates to a transcritical vapour compression system comprising a chilling section and a freezing section, where the refrigerant circuits of the chilling section and the freezing section are fluidly interconnected. Such a vapour compression system is sometimes referred to as a booster refrigeration system.

The transcritical vapour compression system of the present invention is suitable for use in hot climate, e.g. at locations where the ambient temperature increases above 31°C, which is the triple point of carbon dioxide (C0 ₂ ). BACKGROUND OF THE INVENTION

Vapour compression systems having a chilling section as well as a freezing section may be in the form of so-called cascading systems, where heat exchange takes place between refrigerant flowing in the chilling section, or medium temperature section, and refrigerant flowing in the freezing section, or low temperature section. As an alternative, the vapour compression system may be in the form of a so-called booster system. In this case, the refrigerant circuits of the chilling section, or medium temperature section, and the freezing section, or low temperature section, are fluidly interconnected in such a manner that refrigerant leaving the compressor of the low temperature refrigerant circuit is fed to the inlet of the compressor of the medium temperature refrigerant circuit. Booster systems are generally more energy efficient than cascade system, and therefore booster systems are often preferred over cascade systems. However, in order for a booster system to operate, the ambient temperature must be below the triple point of the refrigerant flowing in the system. This is due to the fact that, at higher ambient temperatures, the air cooling is not sufficient to bring the refrigerant from the transcritical state to a subcritical state. The triple point of carbon dioxide (C0 ₂ ), which is a suitable refrigerant for transcritical vapour compression systems, is at 31°C. In many regions it is not uncommon that the ambient temperature increases above 31°C, at least during the day time in the summer period. In these regions booster systems will not operate optimally, and it may therefore be necessary to use cascade systems instead. Since cascade systems are generally less energy efficient than booster system, this has the consequence that the energy consumption is increased. US 7,644,593 discloses a C0 ₂ refrigeration circuit for circulating a refrigerant in a

predetermined flow direction. The refrigeration circuit comprises a medium temperature loop and a low temperature loop. The refrigeration circuit further comprises a liquid line connecting the liquid portion of a receiver with at least one of the medium and low temperature loops and having an internal heat exchanger, and a flash gas line connecting a flash gas portion of the receiver via the internal heat exchanger with the inlet of the low temperature compressor. Thus, in use the internal heat exchanger transfers heat from the liquid flowing through the liquid line to the flash gas flowing through the flash gas line.

Accordingly, the heat remains in the refrigeration circuit. WO 2008/019689 discloses a transcritical refrigeration system with a booster and a bypass valve. The transcritical refrigeration system may comprise a high temperature heat exchanger for improvement of the efficiency of the system. The high temperature heat exchanger has a first flow circuit with an input connected to the output of a gas cooler and an output connected to an input of a high temperature expansion valve. The high temperature heat exchanger further has a second flow circuit in thermal communication with the first flow circuit and having an input connected with the output of a medium temperature evaporator and an output connected with the input of a high pressure compressor. Thus, heat exchange takes place in the refrigeration system, but the heat remains in the refrigeration system.

DESCRIPTION OF THE INVENTION It is an object of embodiments of the invention to provide a transcritical vapour compression system which is energy efficient, while being operable at ambient temperatures above 31°C.

It is a further object of embodiments of the invention to provide a transcritical vapour compression system of the booster type which is operable at ambient temperatures above the triple point of the refrigerant used in the vapour compression system. It is an even further object of embodiments of the invention to provide a method for operating a transcritical vapour compression system in an energy efficient manner at temperatures above the triple point of the refrigerant used in the vapour compression system.

According to a first aspect the invention provides a transcritical vapour compression system comprising: - a medium temperature compressor, a gas cooler, a medium temperature expansion device and a medium temperature evaporator being fluidly interconnected in a medium temperature refrigerant circuit,

- a low temperature compressor, a low temperature expansion device and a low

temperature evaporator being fluidly interconnected in a low temperature refrigerant circuit, a refrigerant inlet of the low temperature expansion device being fluidly connected to a refrigerant outlet of the gas cooler,

- the medium temperature refrigerant circuit and the low temperature refrigerant

circuit further being fluidly interconnected by means of a flow path interconnecting a refrigerant outlet of the low temperature compressor to a refrigerant inlet of the medium temperature compressor, wherein the transcritical vapour compression system further comprises an external heat exchanger arranged downstream relative to a refrigerant outlet of the gas cooler, said external heat exchanger being arranged to cool refrigerant leaving the gas cooler by means of heat exchange with an external heat sink.

In the present context the term Vapour compression system' should be interpreted to mean any system in which a flow of fluid medium, such as refrigerant, circulates and is

aiternatingly compressed and expanded, thereby providing either refrigeration or heating of a volume. Thus, the vapour compression system may be a refrigeration system, an air condition system, a heat pump, etc.

The vapour compression system according to the first aspect of the invention is a transcritical vapour compression system. This should be interpreted to mean that the refrigerant flowing in the vapour compression system is sometimes in a subcritical state, and sometimes in a transcritical state. The vapour compression system comprises a medium temperature refrigerant circuit and a low temperature refrigerant circuit. In the medium temperature refrigerant circuit a medium temperature compressor, a gas cooler, a medium temperature expansion device, e.g. in the form of an expansion valve, and a medium temperature evaporator are fluidly

interconnected. Thus, refrigerant is aiternatingly compressed in the medium temperature compressor and expanded in the medium temperature expansion device. The refrigerant is further at least partly evaporated in the medium temperature evaporator, thereby providing medium temperature cooling at the location of the medium temperature evaporator. The medium temperature cooling may, e.g., be in the form of chilling, such as providing a target temperature of approximately 5°C. The medium temperature evaporator may, e.g., be arranged at a chilling compartment in a supermarket.

The medium temperature compressor may be in the form of a single compressor. The compressor may be a fixed speed compressor or a variable speed compressor. As an alternative, the medium temperature compressor may be in the form of a compressor rack comprising two or more compressors. In this case each of the compressors of the compressor rack may be a fixed speed compressor or a variable speed compressor.

The medium temperature evaporator may be in the form of a single evaporator. The evaporator may comprise a single evaporator coil, or it may comprise two or more evaporator coils arrange fluidly in series or in parallel. As an alternative, the medium temperature evaporator may comprise two or more evaporators arranged fluidly in series or in parallel. In this case each of the evaporators may comprise a single evaporator coil or two or more evaporator coils arranged fluidly in series or in parallel.

Similarly, in the low temperature refrigerant circuit a low temperature compressor, a low temperature expansion device, e.g. in the form of an expansion valve, and a low temperature evaporator are fluidly interconnected. The low temperature compressor may be a single compressor or a compressor rack, as described above, and the low temperature evaporator may be a single evaporator or two or more evaporators, as described above. Thus, refrigerant is alternatingly compressed in the low temperature compressor and expanded in the low temperature expansion device. The refrigerant is further at least partly evaporated in the low temperature evaporator, thereby providing low temperature cooling at the location of the low temperature evaporator. The low temperature cooling may, e.g., be in the form of freezing, such as providing a target temperature of approximately -18°C. The low

temperature evaporator may, e.g., be arranged at a freezing compartment in a supermarket. A refrigerant inlet of the low temperature expansion device is fluidly connected to a refrigerant outlet of the gas cooler. Thus, refrigerant leaving the gas cooler is supplied to the medium temperature expansion device as well as to the low temperature expansion device, possibly via one or more further components arranged in the refrigerant path. This will be described in further detail below. Furthermore, the medium temperature refrigerant circuit and the low temperature refrigerant circuit are fluidly interconnected by means of a flow path interconnecting a refrigerant outlet of the low temperature compressor to a refrigerant inlet of the medium temperature compressor. Thus, refrigerant leaving the low temperature compressor is supplied to the medium temperature compressor, where it undergoes further compression. This may advantageously be obtained by mixing refrigerant leaving the low temperature compressor with refrigerant leaving the medium temperature evaporator, before supplying the mixed refrigerant to the medium temperature compressor. In this case refrigerant leaving the low temperature evaporator is initially compressed to a temperature and pressure which are comparable to the temperature and pressure of the refrigerant leaving the medium temperature evaporator, in order to allow the two refrigerant flows to mix before supplying the mixed refrigerant to the medium temperature compressor.

Accordingly, the refrigerant circuits are fluidly interconnected in such a manner that the same refrigerant flows in the two refrigerant circuits. Furthermore, since the refrigerant leaving the low temperature compressor is supplied to the medium temperature compressor, the vapour compression system is a so-called booster system. As described above, booster systems are known to be more energy efficient than cascade systems, and it is therefore preferred to use booster systems wherever it is possible.

The vapour compression system according to the first aspect of the invention further comprises an external heat exchanger arranged downstream relative to a refrigerant outlet of the gas cooler. Thus, refrigerant leaving the gas cooler is subsequently passed through the external heat exchanger. The external heat exchanger is arranged to cool refrigerant leaving the gas cooler by means of heat exchange with an external heat sink. In the present context the term 'external heat exchanger' should be interpreted to mean a heat exchanger which thermally interacts with an entity, in this case in the form of a an external heat sink, which does not form part of the vapour compression system. Accordingly, the external heat exchanger removes heat from the refrigerant and transfers the removed heat out of the vapour compression system. This is in contrast to an internal heat recovery arrangement where the heat being removed from the refrigerant is used for heating something which forms part of the vapour compression system, e.g. refrigerant flowing in other parts of the refrigerant circuits. The refrigerant is cooled in the gas cooler. However, as described above, since the gas cooler exchanges heat with an ambient air flow, it is not possible to cool the refrigerant to a temperature below the ambient temperature by means of the gas cooler. In the case that the ambient temperature is higher than the temperature at the triple point of the refrigerant, this means that the gas cooler is not capable of bringing the refrigerant into a subcritical state, and the vapour compression system is not operating properly. According to the present invention, the external heat exchanger further cools the refrigerant leaving the gas cooler. Thus, if the refrigerant leaving the gas cooler is in a transcritical state, due to a high ambient temperature, the external heat exchanger cools the refrigerant to a temperature below the triple point, thereby bringing the refrigerant into a subcritical state, and ensuring that the vapour compression system can be operated properly. This allows use of a booster system in regions where high ambient temperature are expected. In particular, it allows the use of booster systems applying carbon dioxide (C0 ₂ ) as refrigerant, in regions where the ambient temperature is expected to reach 31°C or higher temperatures. This is very advantageous, since such booster systems are very energy efficient and cost effective to manufacture.

Thus, the external heat exchanger preferably cools the refrigerant sufficiently to ensure that the refrigerant is brought into a subcritical state.

The external heat sink may comprise a flow system having a heat sink cooling fluid flowing therein. According to this embodiment, the heat sink cooling fluid also flows through the external heat exchanger in such a manner that heat exchange takes place between the refrigerant and the heat sink cooling fluid in the external heat exchanger. The flow of heat sink cooling fluid provides efficient heat exchange with the refrigerant, and the heat exchange may even be controlled by controlling the flow of the heat sink cooling fluid in the flow system. This will be described in further detail below. The heat sink cooling fluid may, e.g., be water.

As an alternative, the external heat exchanger may provide a direct thermal contact between the refrigerant and a heat sink or heat reservoir. For instance, the external heat exchanger may be directly submerged in a water reservoir, a lake, the sea, under ground, etc.

The external heat sink may comprise one or more energy piles arranged under ground. According to this embodiment, the heat sink cooling fluid transfers the heat received from the refrigerant to the energy piles. The energy piles, in turn, deliver the heat to the ground. Accordingly, in this case the ground absorbs the heat removed from the refrigerant.

The flow system may comprise a controllable pump arranged to control the flow of heat sink cooling fluid in the flow system of the external heat sink. According to this embodiment, the heat sink cooling fluid is driven along the flow system by means of the controllable pump. Thus, stopping operation of the pump stops the flow of heat sink cooling fluid, and starting operation of the pump starts the flow of heat sink cooling fluid through the external heat exchanger. Furthermore, in the case that the pump is a variable speed pump, the flow velocity of the heat sink cooling fluid can be controlled by controlling the speed of the pump. Thereby the heat exchange between the refrigerant and the heat sink cooling fluid can also be controlled by controlling the speed of the pump. Thus, the flow system can be operated in such a manner that heat exchange is only taking place when required, and to the extent required. Thereby it is ensured that the refrigerant is always brought into a subcritical state, while minimising the energy consumption of the system. The external heat sink may comprise a water reservoir. The water reservoir may, e.g., be in the form of a sea, a lake, a river, a water tank, or any other suitable kind of water reservoir. In this case the water of the water reservoir absorbs the heat which is removed from the refrigerant. The external heat exchanger may be arranged in direct contact with the water reservoir, in which case the heat exchange in the external heat exchanger takes place directly between the refrigerant and the water of the water reservoir. For instance, the external heat exchanger may be directly submerged in the water.

As an alternative, the external heat exchanger may be thermally connected to the water of the water reservoir via a heat sink cooling fluid and an additional heat exchanger, The vapour compression system may have carbon dioxide (C0 ₂ ) flowing in the refrigerant circuits thereof. C0 ₂ is known as a suitable refrigerant, since it is cheap, readily available and environmental friendly. Furthermore, the piping required for a C0 ₂ vapour compression system has significantly smaller dimensions than piping required for vapour compression systems using other types of refrigerant, e.g. R404a. This reduces the manufacturing costs of the vapour compression system.

The vapour compression may further comprise a receiver being fluidly interconnected between the external heat exchanger, and the medium temperature and low temperature expansion devices, said receiver being adapted to separate liquid refrigerant from gaseous refrigerant. The vapour compression system may further comprise a flow path fluidly connecting a part of the receiver which contains gaseous refrigerant to a refrigerant input of the medium temperature compressor. According to this embodiment, the gaseous refrigerant is supplied directly to the medium temperature compressor, while the liquid refrigerant is supplied to the medium and low temperature expansion devices. The vapour compression system may further comprise a bypass valve arranged to allow refrigerant leaving the gas cooler to bypass the external heat exchanger. The bypass valve is switchable between a position in which the flow of refrigerant passes through the external heat exchanger, and a position in which the flow of refrigerant bypasses the external heat exchanger. Thereby it is possible to control whether or not the refrigerant passes through the external heat exchanger. For instance, the refrigerant may be passed through the external heat exchanger when the ambient temperature is high and/or if it is established that the refrigerant leaving the gas cooler is in a transcritical state. Similarly, when the ambient temperature is low and/or if it is established that the refrigerant leaving the gas cooler is in a subcritical state, the refrigerant flow bypasses the external heat exchanger because, in this case, it is not necessary to cool the refrigerant further. Thereby it is ensured that the refrigerant is brought into a subcritical state, regardless of the ambient temperature, while minimising the energy consumption of the system.

Alternatively or additionally, the vapour compression system may further comprise a bypass valve arranged to allow refrigerant to bypass the gas cooler. This bypass valve is switchable between a position in which the refrigerant passes through the gas cooler and a position in which the refrigerant bypasses the gas cooler. It may, e.g., be desirable to temporarily bypass the gas cooler if the vapour compression system comprises a heat recovery system arranged to recover heat from refrigerant leaving the medium temperature compressor, before the refrigerant reaches the gas cooler. In this case the heat recovery system may cool the refrigerant to a temperature below or close to the ambient temperature. In this case the gas cooler is not able to cool the refrigerant further, and passing the refrigerant through the gas cooler is therefore superfluous, and may in the worst case result in a temperature increase of the refrigerant. It may therefore be desirable to bypass the gas cooler under these circumstances.

The vapour compression system may further comprise at least one temperature sensor and/or at least one pressure sensor arranged at or near a refrigerant outlet of the gas cooler. Thereby the temperature and/or the pressure of the refrigerant leaving the gas cooler can be measured. Based on such measurements it is possible to establish whether or not the refrigerant leaving the gas cooler is in a transcritical state. Based on this, the heat exchange taking place in the external heat exchanger can be controlled in such a manner that heat exchange only takes place when required, and to the extent required. As described above, this may be obtained by controlling a bypass valve allowing the flow of refrigerant to bypass the external heat exchanger, and/or by controlling a controllable pump arranged to drive a flow of heat sink cooling fluid flowing through the external heat exchanger.

According to a second aspect the invention provides a method for controlling a transcritical vapour compression system, the vapour compression system comprising :

- a medium temperature compressor, a gas cooler, a medium temperature expansion device and a medium temperature evaporator being fluidly interconnected in a medium temperature refrigerant circuit,

- a low temperature compressor, a low temperature expansion device and a low

temperature evaporator being fluidly interconnected in a low temperature refrigerant circuit, a refrigerant inlet of the low temperature expansion device being fluidly connected to a refrigerant outlet of the gas cooler, - the medium temperature refrigerant circuit and the low temperature refrigerant circuit further being fluidly interconnected by means of a flow path interconnecting a refrigerant outlet of the low temperature compressor to a refrigerant inlet of the medium temperature compressor, the method comprising the steps of:

- alternatingly compressing and expanding refrigerant flowing in the transcritical

vapour compression system by means of the medium and low temperature compressors and expansion devices, thereby providing medium temperature cooling at the medium temperature evaporator and low temperature cooling at the low temperature evaporator, and

- allowing refrigerant leaving the gas cooler to pass through an external heat

exchanger arranged downstream relative to a refrigerant outlet of the gas cooler, thereby cooling the refrigerant leaving the gas cooler by means of heat exchange with an external heat sink. The transcritical vapour compression system being controlled by means of the method according to the second aspect of the invention may advantageously be a transcritical vapour compression system according to the first aspect of the invention. Thus, a person skilled in the art would readily recognise that any feature described in combination with the first aspect of the invention could also be combined with the second aspect of the invention, and vice versa. Accordingly, the remarks set forth above are equally applicable here.

According to the method of the second aspect of the invention, the refrigerant flowing in the transcritical vapour compression system is alternatingly compressed and expanded by means of the medium and low temperature compressors and expansion devices. Thereby medium temperature cooling is provided at the medium temperature evaporator, and low temperature cooling is provided at the low temperature evaporator. This has already been described above.

Furthermore, refrigerant leaving the gas cooler is allowed to pass through an external heat exchanger arranged downstream relative to a refrigerant outlet of the gas cooler. Thereby the refrigerant leaving the gas cooler is cooled by means of heat exchange with an external heat sink. As described above, this step ensures that the refrigerant which leaves the gas cooler is brought into a subcritical state, even if the ambient temperature is so high that the gas cooler is not capable of cooling the refrigerant to a temperature below the triple point of the refrigerant, thereby bringing the refrigerant into a subcritical state. The method may further comprise the step of causing a flow of heat sink cooling fluid to flow through the external heat exchanger, thereby causing heat exchange between the refrigerant leaving the gas cooler and the heat sink cooling fluid. According to this embodiment, heat exchange between the refrigerant and the heat sink cooling fluid takes place in the external heat exchanger. This step may be performed by controlling a pump. This has already been described in detail above.

The method may further comprise the step of determining whether the refrigerant leaving the gas cooler is in a transcritical state. This step may, e.g., comprise measuring the temperature and/or the pressure of the refrigerant leaving the gas cooler. Alternatively, other parameters may be measured, and used for determining whether the refrigerant leaving the gas cooler is in a transcritical state. For instance, the ambient temperature may be measured, and in the case that the ambient temperature is above or close to the

temperature of the triple point of the refrigerant, then the refrigerant leaving the gas cooler is likely to be in a transcritical state. The method may further comprise the step of controlling heat exchange in the external heat exchanger, based on the step of determining whether the refrigerant leaving the gas cooler is in a transcritical state. According to this embodiment, it can be ensured that heat exchange in the external heat exchanger only takes place when required, and to the extent required. For instance, if it is established that the refrigerant leaving the gas cooler is not in a transcritical state, i.e. that the refrigerant is in a subcritical state, then further cooling of the refrigerant is not required. Accordingly, the external heat exchanger can be controlled in such a manner that no heat exchange takes place, and energy may thereby be conserved, and/or unnecessary use of the heat sink can be avoided. On the other hand, in the case that it is determined that the refrigerant is in a transcritical state, then further cooling of the refrigerant is required in order to ensure proper operation of the vapour compression system. Accordingly, the external heat exchanger can be controlled in such a manner that heat exchange takes place in the external heat exchanger. Furthermore, it may be ensured that heat exchange only takes place to the required extent. Thereby the energy consumption is minimised, and the heat sink is only utilised to the required extent. Thus, the method may further comprise the steps of:

- in the case that it is determined that the refrigerant leaving the gas cooler is in a transcritical state, leading the refrigerant through the external heat exchanger, and - in the case that it is determined that the refrigerant leaving the gas cooler is not in a transcritical state, leading the refrigerant through a bypass flow path, bypassing the external heat exchanger.

According to this embodiment, the heat exchange is controlled by controlling the refrigerant flow. The refrigerant flow is controlled, e.g. by controlling a bypass valve, in such a manner that the refrigerant passes through the external heat exchanger when it is required, and bypasses the external heat exchanger when heat exchange is not required.

As an alternative, the method may further comprise the steps of:

- in the case that it is determined that the refrigerant leaving the gas cooler is in a transcritical state, causing a heat sink cooling fluid to flow through the external heat exchanger, and

- in the case that it is determined that the refrigerant leaving the gas cooler is not in a transcritical state, preventing the heat sink cooling fluid from flowing through the external heat exchanger. According to this embodiment, the heat exchange is controlled by controlling the flow of heat sink cooling fluid through the external heat exchanger, e.g. by controlling a controllable pump. In this case, the refrigerant may always be passed through the external heat exchanger, but whether or not, and to what extent, heat exchange takes place in the external heat exchanger is determined by the flow of heat sink cooling fluid through the external heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 1 is a diagrammatic view of a transcritical vapour compression system according to an embodiment of the invention,

Fig. 2 is a flow diagram illustrating a method according to a first embodiment of the invention, and Fig. 3 is a flow diagram illustrating a method according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagrammatic view of a transcritical vapour compression system 1 according to an embodiment of the invention. The vapour compression system 1 comprises a medium temperature compressor 2, in the form of a compressor rack, a gas cooler 3, a receiver 4, a medium temperature expansion device 5, in the form of an expansion valve, and a medium temperature evaporator 6, arranged along a refrigerant path forming a medium temperature refrigerant circuit. The vapour compression system 1 further comprises a low temperature compressor 7, in the form of a compressor rack, a low temperature expansion device 8, in the form of an expansion valve, and a low temperature evaporator 9, arranged along a refrigerant path forming a low temperature refrigerant circuit.

The medium temperature evaporator 6 is arranged at or near a location which requires chilling, e.g. a cooling compartment in a supermarket. Similarly, the low temperature evaporator 9 is arranged at or near a location which requires freezing, e.g. a freezing compartment in the supermarket. Thus, the temperature provided at the location of the low temperature evaporator 9 is significantly lower than the temperature provided at the location of the medium temperature evaporator 6. For instance, the temperature at the location of the medium temperature evaporator 6 may be approximately 5°C, and the temperature at the location of the low temperature evaporator 9 may be approximately -18°C.

The medium temperature refrigerant circuit and the low temperature refrigerant circuit are fluidly interconnected in the sense that the same refrigerant flows in both refrigerant circuits. This will be described further below. The vapour compression system 1 of Fig. 1 operates in the following manner. Refrigerant is compressed at the medium temperature compressor 2. The compressed refrigerant then flows towards valve 10 which can be switched to a position which directs the refrigerant flow towards a heat exchanger 11, which extracts heat from the refrigerant and supplies the heat to a tap water system 12. After having passed through the heat exchanger 11, the refrigerant flows towards the gas cooler 3. Alternatively, the valve 10 may be switched to a position which directs the refrigerant directly towards the gas cooler 3, without passing through the heat exchanger 11. In the gas cooler 3, the gaseous refrigerant is cooled. However, since the vapour

compression system 1 is operated transcritically, the refrigerant is not condensed, i.e. the refrigerant remains in a gaseous state while passing through the gas cooler 3.

From the gas cooler 3 the refrigerant flows towards the receiver 4, where liquid refrigerant is separated from gaseous refrigerant. The gaseous part of the refrigerant is then passed via flow path 13 to an inlet of the medium temperature compressor 2. The liquid part of the refrigerant flows along flow path 14 towards the expansion devices 5, 8. Thus, the refrigerant flow is divided into a part which flows towards the medium temperature expansion device 5, via flow path 15, and a part which flows towards the low temperature expansion device 8, via flow path 16.

In the medium temperature expansion device 5 the refrigerant is expanded before entering the medium temperature evaporator 6, where it is evaporated, thereby providing medium temperature cooling, or chilling, as described above. Subsequently, the refrigerant is supplied to the medium temperature compressor 2. Similarly, in the low temperature expansion device 8 the refrigerant is expanded before entering the low temperature evaporator 9, where it is evaporated, thereby providing low temperature cooling, or freezing, as described above. Subsequently, the refrigerant is supplied to the low temperature compressor 7, where it is compressed. The refrigerant leaving the low temperature compressor 7 is mixed with the refrigerant leaving the medium temperature evaporator 6, via flow path 17, before being supplied to the medium

temperature compressor 2. Thus, the refrigerant leaving the low temperature evaporator 9 is first compressed in the low temperature compressor 7, and then in the medium temperature compressor 2. This ensures that a desired pressure of the refrigerant can be reached without loading the compressors 2, 7 excessively. A vapour compression circuit with this feature is sometimes referred to as a booster system.

When the refrigerant leaves the gas cooler 3, and before entering the receiver 4, it passes through valve 18. The valve 18 may be in a position in which the refrigerant is simply passed directly towards the receiver 4. Alternatively, the valve 18 may be in a position in which the refrigerant is passed towards an external heat exchanger 19. In the external heat exchanger 19, heat exchange takes place between the refrigerant and a heat sink cooling fluid flowing in a heat sink flow system 20. The heat sink flow system 20 comprises a number of energy piles 21, three of which are shown, which are arranged under ground, and through which the heat sink cooling fluid passes. Accordingly, the refrigerant leaving the gas cooler 3 is cooled before entering the receiver 4, and the heat is removed from the vapour compression system 1 by means of the external heat exchanger 19, and supplied to the ground, via the heat sink cooling fluid and the energy piles 21. The heat sink cooling fluid may simply be water which is driven by a simple water pump.

In the gas cooler 3 it is not possible to cool the refrigerant to a temperature which is lower than the ambient temperature, since the gaseous refrigerant is cooled by means of heat exchange with ambient air. The triple point of carbon dioxide (C0 ₂ ) is approximately at 31°C. Thus, in the case that C0 ₂ is used as a refrigerant and the ambient temperature is at or above 31°C, it is not possible to cool the refrigerant to a temperature below the triple point in the gas cooler 3, i.e. the gas cooler 3 is not capable of bringing the refrigerant into a subcritical state. Thereby the vapour compression system 1 is not operated properly. By cooling the refrigerant in the manner described above, it is ensured that the refrigerant leaving the gas cooler 3 is cooled to a temperature below the triple point, i.e. the refrigerant is brought into a subcritical state, before entering the receiver 4. Thereby the vapour compression system 1 can be operated properly, even though the ambient temperature is above the triple point of the refrigerant. Accordingly, it is possible to provide C0 ₂ booster systems in regions where it was previously necessary to either provide cascade systems or use another refrigerant. This is an advantage, because booster systems are more energy efficient than cascade systems, and because C0 ₂ is environmental friendly compared to alternative refrigerants. Furthermore, the initial costs involved with manufacturing and installing the system are lower than corresponding costs for systems using another refrigerant, such as 404a, because the required size of the pipes of the system is significantly smaller.

The valve 18 may be controlled in such a manner that it directs the refrigerant towards the external heat exchanger 19 if it is established that the refrigerant leaving the gas cooler 3 is in a transcritical state and/or the ambient temperature is close to or above the triple point of the refrigerant, and directs the refrigerant directly towards the receiver 4 if it is established that the refrigerant leaving the gas cooler 3 is in a subcritical state and/or the ambient temperature is well below the triple point of the refrigerant. In this case the refrigerant is only passed through the external heat exchanger 19 when it is necessary. In an alternative embodiment, the valve 18 may be omitted, i.e. the refrigerant is always passed through the external heat exchanger 19. Instead a pump driving the heat sink cooling fluid may be controlled in such a manner that it is turned on if it is established that the refrigerant leaving the gas cooler 3 is in a transcritical state and/or the ambient temperature is close to or above the triple point of the refrigerant, and turned off if it is established that the refrigerant leaving the gas cooler 3 is in a subcritical state and/or the ambient temperature is well below the triple point of the refrigerant. In order to establish whether the refrigerant leaving the gas cooler 3 is in a transcritical or a subcritical state, the temperature and/or the pressure of the refrigerant leaving the gas cooler 3 may be measured by means of one or more sensors arranged in the refrigerant path. It should be noted that even though Fig. 1 shows a heat sink flow system 20 comprising energy piles 21 arranged under ground and a circulating heat sink cooling fluid, alternative arrangements for removing the heat from the refrigerant are also within the scope of the present invention. For instance, instead of energy piles 21 arranged under ground, the heat sink cooling fluid could exchange heat with other heat sinks, such as sea water or lake water. Furthermore, the heat sink cooling fluid may be omitted, and the external heat exchanger 19 may exchange heat directly with a heat sink, such as sea water or lake water, in which case the external heat exchanger 19 may be directly submerged in water.

The vapour compression system 1 of Fig. 1 further comprises a bypass valve 22 arranged to allow the refrigerant flow to bypass the gas cooler 3. In the case that the tap water system 12, via heat exchanger 11, cools the refrigerant to a temperature which is below or near the ambient temperature, then the gas cooler 3 is not able to cool the refrigerant further, and the temperature of the refrigerant may even increase if the refrigerant is passed through the gas cooler 3. In this case it may be desirable to bypass the gas cooler 3.

Fig. 2 is a flow diagram illustrating a method of operating a vapour compression system according to a first embodiment of the invention. The vapour compression system being operated may, e.g., be the vapour compression system of Fig. 1. The process is started at step 23. At step 24 the vapour compression system is operated in the normal manner, i.e. the refrigerant is alternatingly compressed and expanded in order to obtain medium temperature cooling at the medium temperature evaporator, and low temperature cooling at the low temperature evaporator.

At step 25 the temperature and/or pressure of refrigerant leaving the gas cooler is measured. This may, e.g., be done by means of one or more sensors arranged in the refrigerant path at or near the refrigerant outlet of the gas cooler. Based on the measurement it is investigated whether or not the refrigerant leaving the gas cooler is in a transcritical state, at step 26. Thus, it is investigated whether or not the temperature and pressure of the refrigerant is above the triple point.

If the refrigerant is not in a transcritical state, there is no need to cool the refrigerant further. Accordingly, the external heat exchanger can be bypassed. Therefore it is investigated, at step 27, whether or not the bypass valve is in heat exchanger state, i.e. whether or not the bypass valve is in the position which leads the refrigerant through the external heat exchanger. If this is not the case, the bypass valve is in the correct position, and the process is returned to step 25. If step 27 reveals that the bypass valve is in the heat exchanger state, then the bypass valve is switched at step 28, and the process is returned to step 25. Thus, the bypass valve is moved into the position in which the refrigerant flow bypasses the external heat exchanger.

If step 26 reveals that the refrigerant leaving the gas cooler is in a transcritical state, then it is necessary to further cool the refrigerant. Accordingly, the refrigerant must be passed through the external heat exchanger in order to obtain this additional cooling. Therefore it is investigated, at step 29, whether or not the bypass valve is in the heat exchanger state. If this is the case, the state of the bypass valve is correct, and the process is returned to step 25. If step 29 reveals that the bypass valve is not in the heat exchanger state, then the bypass valve is switched, at step 30, in order to move the bypass valve to a position in which the refrigerant is passed through the external heat exchanger, in order to obtain further cooling of the refrigerant.

Thus, in the method illustrated in Fig. 2, the refrigerant is passed through the external heat exchanger, thereby obtaining additional cooling of the refrigerant, if the refrigerant leaving the gas cooler is in a transcritical state. If the refrigerant leaving the gas cooler is not in a transcritical state, the external heat exchanger is bypassed, and additional cooling of the refrigerant is not provided. Accordingly, additional cooling of the refrigerant is provided only when it is required.

Fig. 3 is a flow diagram illustrating a method of operating a vapour compression system according to a second embodiment of the invention. The vapour compression system being operated may, e.g., be the vapour compression system of Fig. 1. The process is started at step 31. At step 32 the vapour compression system is operated in the normal manner, i.e. the refrigerant is alternatingly compressed and expanded in order to obtain medium temperature cooling at the medium temperature evaporator, and low temperature cooling at the low temperature evaporator.

At step 33 the temperature and/or pressure of refrigerant leaving the gas cooler is measured. This may, e.g., be done by means of one or more sensors arranged in the refrigerant path at or near the refrigerant outlet of the gas cooler. Based on the measurement it is investigated whether or not the refrigerant leaving the gas cooler is in a transcritical state, at step 34. Thus, it is investigated whether or not the temperature and pressure of the refrigerant is above the triple point. If the refrigerant is not in a transcritical state, there is no need to cool the refrigerant further. Accordingly, there is no need to circulate a heat sink cooling fluid passing through the external heat exchanger. Therefore it is investigated, at step 35, whether or not the heat sink pump is operating. If this is not the case, the heat sink fluid is not circulated, and the process is returned to step 33. If step 35 reveals that the heat sink pump is operating, then the pump is deactivated at step 36, and the process is returned to step 33. Thus the heat sink fluid is no longer circulated.

If step 34 reveals that the refrigerant leaving the gas cooler is in a transcritical state, then it is necessary to further cool the refrigerant. Accordingly, the heat sink fluid must be circulated through the external heat exchanger in order to obtain this additional cooling. Therefore it is investigated, at step 37, whether or not the heat sink pump is operating. If this is the case, the heat sink fluid is circulated through the external heat exchanger, and the process is returned to step 33. If step 37 reveals that the heat sink pump is not operating, then the pump is activated, at step 38, in order to circulate the heat sink cooling fluid through the external heat exchanger, in order to obtain further cooling of the refrigerant.

Thus, in the method illustrated in Fig. 3, the heat sink pump is operated, thereby obtaining additional cooling of the refrigerant, if the refrigerant leaving the gas cooler is in a transcritical state. If the refrigerant leaving the gas cooler is not in a transcritical state, the pump is not operated, and additional cooling of the refrigerant is not provided. Accordingly, additional cooling of the refrigerant is provided only when it is required.