Patent Publication Number: US-10775086-B2

Title: Method for controlling a vapour compression system in ejector mode for a prolonged time

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage application of International Patent Application No. PCT/EP2016/074765, filed on Oct. 14, 2016, which claims priority to Danish Patent Application No. PA 2015 00645, filed on Oct. 20, 2015, each of which is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a method for controlling a vapour compression system comprising an ejector. The method of the invention allows the ejector to be operating in a wider range of operating conditions than prior art methods, thereby improving the energy efficiency of the vapour compression system. 
     BACKGROUND 
     In some vapour compression systems an ejector is arranged in a refrigerant path, at a position downstream relative to a heat rejecting heat exchanger. Thereby refrigerant leaving the heat rejecting heat exchanger is supplied to a primary inlet of the ejector. Refrigerant leaving an evaporator of the vapour compression system may be supplied to a secondary inlet of the ejector. 
     An ejector is a type of pump which uses the Venturi effect to increase the pressure energy of fluid at a suction inlet (or secondary inlet) of the ejector by means of a motive fluid supplied to a motive inlet (or primary inlet) of the ejector. Thereby, arranging an ejector in the refrigerant path as described above will cause the refrigerant to perform work, and thereby the power consumption of the vapour compression system is reduced as compared to the situation where no ejector is provided. 
     An outlet of the ejector is normally connected to a receiver, in which liquid refrigerant is separated from gaseous refrigerant. The liquid part of the refrigerant is supplied to the evaporator, via an expansion device, and the gaseous part of the refrigerant may be supplied to a compressor unit. It is desirable to operate the vapour compression system in such a manner that as large a portion as possible of the refrigerant leaving the evaporator is supplied to the secondary inlet of the ejector, and the refrigerant supply to the compressor unit is primarily provided from the gaseous outlet of the receiver, because this is the most energy efficient way of operating the vapour compression system. 
     At high ambient temperatures, such as during the summer period, the temperature as well as the pressure of the refrigerant leaving the heat rejecting heat exchanger is relatively high. In this case the ejector performs well, and it is advantageous to supply all of the refrigerant leaving the evaporator to the secondary inlet of the ejector, and to supply gaseous refrigerant to the compressor unit from the receiver only, as described above. When the vapour compression system is operated in this manner, it is sometimes referred to as ‘summer mode’. 
     On the other hand, at low ambient temperatures, such as during the winter period, the temperature as well as the pressure of the refrigerant leaving the heat rejecting heat exchanger is relatively low. In this case the ejector is not performing well, and refrigerant leaving the evaporator is therefore often supplied to the compressor unit instead of to the secondary inlet of the ejector. When the vapour compression system is operated in this manner, it is sometimes referred to as ‘winter mode’. As described above, this is a less energy efficient way of operating the vapour compression system, and it is therefore desirable to operate the vapour compression system in the ‘summer mode’, i.e. with the ejector operating, at as low ambient temperatures as possible. 
     US 2012/0167601 A1 discloses an ejector cycle. A heat rejecting heat exchanger is coupled to a compressor to receive compressed refrigerant. An ejector has a primary inlet coupled to the heat rejecting heat exchanger, a secondary inlet and an outlet. A separator has an inlet coupled to the outlet of the ejector, a gas outlet and a liquid outlet. The system can be switched between first and second modes. In the first mode refrigerant leaving the heat absorbing heat exchanger is supplied to the secondary inlet of the ejector. In the second mode refrigerant leaving the heat absorbing heat exchanger is supplied to the compressor. 
     SUMMARY 
     It is an object of embodiments of the invention to provide a method for controlling a vapour compression system comprising an ejector, in an energy efficient manner, even at low ambient temperatures. 
     It is a further object of embodiments of the invention to provide a method for controlling a vapour compression system comprising an ejector, in which the method enables the ejector to operate at lower ambient temperatures than prior art methods. 
     The invention provides a method for controlling a vapour compression system, the vapour compression system comprising a compressor unit, a heat rejecting heat exchanger, an ejector comprising a primary inlet, a secondary inlet and an outlet, a receiver, at least one expansion device and at least one evaporator, arranged in a refrigerant path, the method comprising the steps of:
         obtaining a temperature of refrigerant leaving the heat rejecting heat exchanger,   deriving a reference pressure value of refrigerant leaving the heat rejecting heat exchanger, based on the obtained temperature of refrigerant leaving the heat rejecting heat exchanger,   obtaining a pressure difference between a pressure prevailing in the receiver and a pressure of refrigerant leaving the evaporator,   comparing the pressure difference to a predefined first lower threshold value,   in the case that the pressure difference is higher than the first lower threshold value, controlling the vapour compression system on the basis of the derived reference pressure value, and in order to obtain a pressure of refrigerant leaving the heat rejecting heat exchanger which is equal to the derived reference pressure value, and   in the case that the pressure difference is lower than the first lower threshold value, selecting a fixed reference pressure value corresponding to a derived reference pressure value when the pressure difference is at a predefined level which is essentially equal to the first lower threshold value, and controlling the vapour compression system on the basis of the selected fixed reference pressure value, and in order to obtain a pressure of refrigerant leaving the heat rejecting heat exchanger which is equal to the selected fixed reference pressure value.       

     The method according to the invention is for controlling a vapour compression system. 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 alternatingly 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 comprises a compressor unit, comprising one or more compressors, a heat rejecting heat exchanger, an ejector, a receiver, at least one expansion device and at least one evaporator arranged in a refrigerant path. The ejector has a primary inlet connected to an outlet of the heat rejecting heat exchanger, an outlet connected to the receiver and a secondary inlet connected to outlet(s) of the evaporator(s). Each expansion device is arranged to control a supply of refrigerant to an evaporator. The heat rejecting heat exchanger could, e.g., be in the form of a condenser, in which refrigerant is at least partly condensed, or in the form of a gas cooler, in which refrigerant is cooled, but remains in a gaseous state. The expansion device(s) could, e.g., be in the form of expansion valve(s). 
     Thus, refrigerant flowing in the refrigerant path is compressed by the compressor(s) of the compressor unit. The compressed refrigerant is supplied to the heat rejecting heat exchanger, where heat exchange takes place with the ambient, or with a secondary fluid flow across the heat rejecting heat exchanger, in such a manner that heat is rejected from the refrigerant flowing through the heat rejecting heat exchanger. In the case that the heat rejecting heat exchanger is in the form of a condenser, the refrigerant is at least partly condensed when passing through the heat rejecting heat exchanger. In the case that the heat rejecting heat exchanger is in the form of a gas cooler, the refrigerant flowing through the heat rejecting heat exchanger is cooled, but remains in a gaseous state. 
     From the heat rejecting heat exchanger, the refrigerant is supplied to the primary inlet of the ejector. As the refrigerant passes through the ejector, the pressure of the refrigerant is reduced, and the refrigerant leaving the ejector will normally be in the form of a mixture of liquid and gaseous refrigerant, due to the expansion taking place in the ejector. 
     The refrigerant is then supplied to the receiver, where the refrigerant is separated into a liquid part and a gaseous part. The liquid part of the refrigerant is supplied to the expansion device(s), where the pressure of the refrigerant is reduced, before the refrigerant is supplied to the evaporator(s). Each expansion device supplies refrigerant to a specific evaporator, and therefore the refrigerant supply to each evaporator can be controlled individually by controlling the corresponding expansion device. The refrigerant being supplied to the evaporator(s) is thereby in a mixed gaseous and liquid state. In the evaporator(s), the liquid part of the refrigerant is at least partly evaporated, while heat exchange takes place with the ambient, or with a secondary fluid flow across the evaporator(s), in such a manner that heat is absorbed by the refrigerant flowing through the evaporator(s). Finally, the refrigerant is supplied to the compressor unit. 
     The gaseous part of the refrigerant in the receiver may be supplied to the compressor unit. Thereby the gaseous refrigerant is not subjected to the pressure drop introduced by the expansion device(s), and energy is conserved, as described above. 
     Thus, at least part of the refrigerant flowing in the refrigerant path is alternatingly compressed by the compressor(s) of the compressor unit and expanded by the expansion device(s), while heat exchange takes place at the heat rejecting heat exchanger and at the evaporator(s). Thereby cooling or heating of one or more volumes can be obtained. 
     According to the method of the invention, a temperature of refrigerant leaving the heat rejecting heat exchanger is initially obtained. This may include measuring the temperature of refrigerant leaving the heat rejecting heat exchanger directly by means of a temperature sensor arranged in the refrigerant path downstream relative to the heat rejecting heat exchanger. As an alternative, the temperature of refrigerant leaving the heat rejecting heat exchanger may be obtained on the basis of temperature measurements performed on an exterior part of a pipe interconnecting the heat rejecting heat exchanger and the ejector. As another alternative, the temperature of refrigerant leaving the heat rejecting heat exchanger may be derived on the basis of other suitable measured parameters, such as an ambient temperature. 
     Next, a reference pressure value of refrigerant leaving the heat rejecting heat exchanger is derived, based on the obtained temperature of refrigerant leaving the heat rejecting heat exchanger. For a given temperature of refrigerant leaving the heat rejecting heat exchanger there is a pressure level of refrigerant leaving the heat rejecting heat exchanger, which results in the vapour compression system operating at optimal coefficient of performance (COP). This pressure value may advantageously be selected as the reference pressure value. The higher the temperature of refrigerant leaving the heat rejecting heat exchanger, the higher the pressure level providing the optimal coefficient of performance (COP) will be. 
     Next, a pressure difference between a pressure prevailing in the receiver and a pressure of refrigerant leaving the evaporator is obtained, and this pressure difference is compared to a first lower threshold value. 
     The pressure difference between the pressure prevailing in the receiver and the pressure of refrigerant leaving the evaporator is decisive for whether or not the ejector is able to operate efficiently, i.e. whether or not the ejector is able to suck refrigerant leaving the evaporator(s) into the secondary inlet of the ejector. The first lower threshold value may advantageously be selected in such a manner that it corresponds to a pressure difference below which the ejector is expected to operate inefficiently. 
     In the case that the pressure difference is higher than the first lower threshold value, it can therefore be assumed that the ejector is able to operate efficiently. Therefore, in this case the vapour compression system can be operated in order to obtain optimal coefficient of performance (COP), and the ejector will still operate efficiently. Therefore, the vapour compression system is, in this case, operated in a normal manner, i.e. on the basis of the derived reference pressure value, and in order to obtain a pressure of refrigerant leaving the heat rejecting heat exchanger which is equal to the derived reference pressure value. This situation will often occur when the ambient temperature is relatively high. 
     On the other hand, in the case that the pressure difference is lower than the first lower threshold value, then it can be assumed that the ejector is unable to operate efficiently. Therefore, if the vapour compression system is operated in a normal manner in this case, the ejector will not be operating, and the energy efficiency of the vapour compression system is therefore decreased. This situation will often occur when the ambient temperature is relatively low. 
     If the vapour compression system is operated in such a manner that the pressure of refrigerant leaving the heat rejecting heat exchanger is slightly higher than the pressure level which provides optimal coefficient of performance (COP), then the coefficient of performance (COP) will only be slightly decreased. A slightly higher pressure of refrigerant leaving the heat rejecting heat exchanger results in a slightly higher pressure difference across the ejector. This increases the ability of the ejector to suck refrigerant from the outlet of the evaporator towards the secondary inlet of the ejector. Accordingly, operating the vapour compression system to obtain a slightly higher pressure of refrigerant leaving the heat rejecting heat exchanger will result in the ejector being capable of operating at lower ambient temperatures, thereby improving the energy efficiency of the vapour compression system, even though the increased pressure of refrigerant leaving the heat rejecting heat exchanger causes a slight decrease in the coefficient of performance (COP). 
     Therefore, in the case that the pressure difference between the pressure prevailing in the receiver and the pressure of refrigerant leaving the evaporator is lower than the first lower threshold value, a fixed reference pressure value, for the refrigerant leaving the heat rejecting heat exchanger, is selected instead of the derived reference pressure value. The fixed reference pressure value corresponds to a derived reference pressure value when the pressure difference is at a predefined level which is essentially equal to the first lower threshold value. Essentially, when the pressure difference decreases, the reference pressure value is simply maintained at the current level, when the first lower threshold value is reached. Subsequently, the vapour compression system is controlled on the basis of the fixed reference pressure value, and in order to obtain a pressure of refrigerant leaving the heat rejecting heat exchanger which is equal to the selected fixed reference pressure value. This allows the ejector of the vapour compression system to operate at lower ambient temperatures, thereby improving the energy efficiency of the vapour compression system. 
     The method may further comprise the steps of, in the case that the pressure difference is lower than the first lower threshold value:
         obtaining a difference between the derived reference pressure value and the selected fixed reference pressure value,   comparing the obtained difference to a second upper threshold value, and   in the case that the obtained difference is higher than the second upper threshold value, selecting the derived reference pressure value, and controlling the vapour compression system according to the derived reference pressure value, and in order to obtain a pressure of refrigerant leaving the heat rejecting heat exchanger which is equal to the derived reference pressure value.       

     According to this embodiment, if the pressure difference is lower than the first lower threshold value, and the fixed reference pressure value has therefore been selected, the temperature of refrigerant leaving the heat rejecting heat exchanger is still monitored, and the corresponding reference pressure value is derived. Thereby, the reference pressure value, which would normally be applied, is still derived, even though the fixed reference pressure value has been selected and the vapour compression system is controlled in accordance therewith. 
     Furthermore, a difference between the derived reference pressure value and the selected fixed reference pressure value is obtained and compared to a second upper threshold value. 
     In the case that the obtained difference is higher than the second upper threshold value, the derived reference pressure value is selected, and the vapour compression system is subsequently controlled on the basis thereof, as described above. Thus, if the difference between the derived reference pressure value and the fixed reference pressure value becomes too large, it is no longer considered appropriate to maintain the increased pressure of refrigerant leaving the heat rejecting heat exchanger, and therefore the ‘normal’ derived reference pressure value is selected instead of the increased, fixed reference pressure value, i.e. the vapour compression system is operated without the energy efficiency benefit of the ejector. 
     It should be noted that the second upper threshold value could be a fixed value. As an alternative, the second upper threshold value could be a variable value, such as a suitable percentage of the derived reference pressure value. 
     The step of obtaining a pressure difference between a pressure prevailing in the receiver and a pressure of refrigerant leaving the evaporator may comprise the step of measuring the pressure in the receiver and/or the pressure of refrigerant leaving the evaporator. As an alternative, the pressures may be obtained in other ways, e.g. by deriving the pressures from other measured parameters. As another alternative the pressure difference may be obtained without obtaining the absolute pressures of refrigerant inside the receiver and refrigerant leaving the evaporator, respectively. 
     The step of deriving a reference pressure may comprise using a look-up table providing corresponding values of temperature of refrigerant leaving the heat rejecting heat exchanger, pressure of refrigerant leaving the heat rejecting heat exchanger, and optimal coefficient of performance (COP) for the vapour compression system. The look-up table may, e.g., be in the form of curves representing the relationship between temperature, pressure and COP. According to this embodiment, a pressure providing optimal COP for a given temperature of refrigerant leaving the evaporator can readily be obtained. 
     Alternatively or additionally, the step of deriving a reference pressure value may comprise calculating the reference pressure value based on the temperature of refrigerant leaving the heat rejecting heat exchanger. This may, e.g., be done by using a predefined formula. 
     The steps of controlling the vapour compression system on the basis of the derived reference pressure value or on the basis of the selected fixed reference pressure value may comprise adjusting a secondary fluid flow across the heat rejecting heat exchanger. Adjusting the secondary fluid flow across the heat rejecting heat exchanger affects the heat exchange taking place in the heat rejecting heat exchanger, thereby affecting the pressure of refrigerant leaving the heat rejecting heat exchanger. In the case that the secondary fluid flow across the heat rejecting heat exchanger is an air flow, the fluid flow may be adjusted by adjusting a speed of a fan arranged to cause the air flow, or by switching one or more fans on or off. Similarly, in the case that the secondary fluid flow is a liquid flow, the fluid flow may be adjusted by adjusting a pump arranged to cause the liquid flow. 
     Alternatively or additionally, the steps of controlling the vapour compression system on the basis of the derived reference pressure value or on the basis of the selected fixed reference pressure value may comprise adjusting a compressor capacity of the compressor unit. This causes the pressure of refrigerant entering the heat rejecting heat exchanger to be adjusted, thereby resulting in the pressure of refrigerant leaving the heat rejecting heat exchanger being adjusted. 
     Alternatively or additionally, the steps of controlling the vapour compression system on the basis of the derived reference pressure value or on the basis of the selected fixed reference pressure value may comprise adjusting an opening degree of the primary inlet of the ejector. The opening degree of the primary inlet of the ejector determines a refrigerant flow from the heat rejecting heat exchanger towards the receiver. If the opening degree of the primary inlet of the ejector is increased, the flow rate of refrigerant from the heat rejecting heat exchanger is increased, thereby resulting in a decrease in the pressure of refrigerant leaving the heat rejecting heat exchanger. Similarly, a decrease in the opening degree of the primary inlet of the ejector results in an increase in the pressure of refrigerant leaving the heat rejecting heat exchanger. Furthermore, in the case that the vapour compression system comprises a high pressure valve arranged in parallel with the ejector, the pressure of refrigerant leaving the heat rejecting heat exchanger may be adjusted by opening or closing the high pressure valve, or by adjusting an opening degree of the high pressure valve. 
    
    
     
       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 vapour compression system being controlled in accordance with a method according to a first embodiment of the invention, 
         FIG. 2  is a diagrammatic view of a vapour compression system being controlled in accordance with a method according to a second embodiment of the invention, 
         FIG. 3  is a log P-h diagram for a vapour compression system being controlled in accordance with a method according to an embodiment of the invention, 
         FIG. 4  is a graph illustrating coefficient of performance as a function of ambient temperature for a vapour compression system being controlled in accordance with a method according to the invention and a vapour compression system being controlled in accordance with a prior art method, respectively, 
         FIG. 5  illustrates control of pressure of refrigerant leaving the heat rejecting heat exchanger of a vapour compression system, 
         FIG. 6  is a block diagram illustrating operation of the high pressure control unit of  FIG. 5 , and 
         FIG. 7  is a block diagram illustrating operation of the fan control unit of  FIG. 5 . 
     
    
    
     DEATAILED DESCRIPTION 
       FIG. 1  is a diagrammatic view of a vapour compression system  1  being controlled in accordance with a method according to a first embodiment of the invention. The vapour compression system  1  comprises a compressor unit  2  comprising a number of compressors  3 ,  4 , three of which are shown, a heat rejecting heat exchanger  5 , an ejector  6 , a receiver  7 , an expansion device  8 , in the form of an expansion valve, and an evaporator  9 , arranged in a refrigerant path. 
     Two of the shown compressors  3  are connected to an outlet of the evaporator  9 . Accordingly, refrigerant leaving the evaporator  9  can be supplied to these compressors  3 . The third compressor  4  is connected to a gaseous outlet  10  of the receiver  7 . Accordingly, gaseous refrigerant can be supplied directly from the receiver  7  to this compressor  4 . 
     Refrigerant flowing in the refrigerant path is compressed by the compressors  3 ,  4  of the compressor unit  2 . The compressed refrigerant is supplied to the heat rejecting heat exchanger  5 , where heat exchange takes place in such a manner that heat is rejected from the refrigerant. 
     The refrigerant leaving the heat rejecting heat exchanger  5  is supplied to a primary inlet  11  of the ejector  6 , before being supplied to the receiver  7 . When passing through the ejector  6  the refrigerant undergoes expansion. Thereby the pressure of the refrigerant is reduced, and the refrigerant being supplied to the receiver  7  is in a mixed liquid and gaseous state. 
     In the receiver  7  the refrigerant is separated into a liquid part and a gaseous part. The liquid part of the refrigerant is supplied to the evaporator  9 , via a liquid outlet  12  of the receiver  7  and the expansion device  8 . In the evaporator  9 , the liquid part of the refrigerant is at least partly evaporated, while heat exchange takes place in such a manner that heat is absorbed by the refrigerant. 
     The refrigerant leaving the evaporator  9  is either supplied to the compressors  3  of the compressor unit  2  or to a secondary inlet  13  of the ejector  6 . 
     The vapour compression system  1  of  FIG. 1  is operated in the most energy efficient manner when all of the refrigerant leaving the evaporator  9  is supplied to the secondary inlet  13  of the ejector  6 , and the compressor unit  2  only receives refrigerant from the gaseous outlet  10  of the receiver  7 . In this case only compressor  4  of the compressor unit  2  is operating, while compressors  3  are switched off. It is therefore desirable to operate the vapour compression system  1  in this manner for as large a part of the total operating time as possible. However, at low ambient temperatures, where the pressure of refrigerant leaving the heat rejecting heat exchanger  5  is normally relatively low, the ejector  6  is not performing well, and therefore the refrigerant leaving the evaporator  9  will normally be supplied to the compressors  3 , thereby resulting in a less energy efficient operation of the vapour compression system  1 . 
     According to the method of the invention, the temperature of refrigerant leaving the heat rejecting heat exchanger  5  is obtained, e.g. by simply measuring the temperature of the refrigerant directly or by measuring the ambient temperature. 
     Based on the obtained temperature of refrigerant leaving the heat rejecting heat exchanger  5 , a reference pressure value of refrigerant leaving the heat rejecting heat exchanger  5  is derived. This may, e.g., be done by consulting a look-up table or a series of curves providing corresponding values of temperature, pressure and optimal coefficient of performance. Alternatively, the reference pressure value may be derived by means of calculation. The derived reference pressure value may advantageously be the pressure of refrigerant leaving the heat rejecting heat exchanger  5 , which causes the vapour compression system  1  to be operated at optimal coefficient of performance (COP), at the given temperature of refrigerant leaving the heat rejecting heat exchanger  5 . 
     Furthermore, a pressure difference between a pressure prevailing in the receiver  7  and a pressure of refrigerant leaving the evaporator  9  is obtained and compared to a first lower threshold value. When this pressure difference becomes small, it is an indication that the operation of the vapour compression system  1  is approaching a region where the ejector  6  is not performing well. However, when the pressure difference is large, the ejector  6  can be expected to perform well. 
     Therefore, in the case that the pressure difference is higher than the first lower threshold value, the derived reference pressure value is selected, and the vapour compression system  1  is operated based on this reference pressure value. Accordingly, the vapour compression system  1  is simply operated as it would normally be, in order to obtain a pressure of refrigerant leaving the heat rejecting heat exchanger  5  which results in optimal coefficient of performance (COP), and the ejector  6  will automatically be operating. 
     On the other hand, in the case that the pressure difference is lower than the first lower threshold value, it must be expected that a region in which the ejector  6  no longer performs well is approached. Therefore, instead of the derived reference pressure value, a fixed reference pressure value is selected. The fixed reference pressure value is slightly higher than the derived reference pressure value, and it corresponds to a derived reference pressure value when the pressure difference is at a predefined level which is essentially equal to the first lower threshold value. Accordingly, in this case the vapour compression system  1  is not operated in accordance with a pressure of refrigerant leaving the heat rejecting heat exchanger  5 , which provides optimal coefficient of performance (COP). Instead the ejector  6  is kept running for a prolonged time, and this provides an increase in COP which exceeds the impact of operating the vapour compression system  1  being operated at the slightly increased pressure of refrigerant leaving the heat rejecting heat exchanger  5 . Thereby the overall energy efficiency of the vapour compression system  1  is improved. 
     The pressure of refrigerant leaving the heat rejecting heat exchanger  5  could, e.g., be adjusted by adjusting an opening degree of the primary inlet  11  of the ejector  6 . Alternatively, it could be adjusted by adjusting the pressure prevailing inside the receiver  7 , e.g. by adjusting the compressor capacity of the compressor  4  being connected to the gaseous outlet  10  of the receiver  7 , or by adjusting a bypass valve  14  arranged in a refrigerant path interconnecting the gaseous outlet  10  of the receiver  7  and the compressors  3 . 
       FIG. 2  is a diagrammatic view of a vapour compression system  1  being controlled in accordance with a method according to a second embodiment of the invention. The vapour compression system  1  of  FIG. 2  is very similar to the vapour compression system  1  of  FIG. 1 , and it will therefore not be described in detail here. 
     In the compressor unit  2  of the vapour compression system  1  of  FIG. 2 , one compressor  3  is shown as being connected to the outlet of the evaporator  9  and one compressor  4  is shown as being connected to the gaseous outlet  10  of the receiver  7 . A third compressor  15  is shown as being provided with a three way valve  16  which allows the compressor  15  to be selectively connected to the outlet of the evaporator  9  or to the gaseous outlet  10  of the receiver  7 . Thereby some of the compressor capacity of the compressor unit  2  can be shifted between ‘main compressor capacity’, i.e. when the compressor  15  is connected to the outlet of the evaporator  9 , and ‘receiver compressor capacity’, i.e. when the compressor  15  is connected to the gaseous outlet  10  of the receiver  7 . Thereby it is further possible to adjust the pressure prevailing inside the receiver  7 , and thereby the pressure of refrigerant leaving the heat rejecting heat exchanger  5 , by operating the three way valve  16 , thereby increasing or decreasing the amount of compressor capacity being available for compressing refrigerant received from the gaseous outlet  10  of the receiver  7 . 
       FIG. 3  is a log P-h diagram, i.e. a graph illustrating pressure as a function of enthalpy, for a vapour compression system being controlled in accordance with a method according to an embodiment of the invention. The vapour compression system could, e.g., be the vapour compression system illustrated in  FIG. 1  or the vapour compression system illustrated in  FIG. 2 . 
     During normal operation of the vapour compression system, at point  17  refrigerant enters one or more compressors of the compressor unit being connected to the outlet of the evaporator. From point  17  to point  18  the refrigerant is compressed by this compressor or these compressors. Similarly, at point  19  refrigerant enters one or more compressors of the compressor unit being connected to the gaseous outlet of the receiver. From point  19  to point  20  the refrigerant is compressed by this compressor or these compressors. It can be seen that the compression results in an increase in pressure as well as in enthalpy for the refrigerant. It can further be seen, that the refrigerant received from the gaseous outlet of the receiver, at point  19 , is at a higher pressure level than the refrigerant received from the outlet of the evaporator, at point  17 . 
     From points  18  and  20 , respectively, to point  21  the refrigerant passes through the heat rejecting heat exchanger, where heat exchange takes place in such a manner that heat is rejected by the refrigerant. This results in a decrease in enthalpy, while the pressure remains constant. 
     From point  21  to point  22  the refrigerant passes through the ejector, and is supplied to the receiver. Thereby the refrigerant undergoes expansion, resulting in a decrease in the pressure of the refrigerant and a slight decrease in enthalpy. 
     Point  23  represents the liquid part of the refrigerant in the receiver, and from point  23  to point  24  the refrigerant passes through the expansion device, thereby decreasing the pressure of the refrigerant. Similarly, point  19  represents the gaseous part of the refrigerant in the receiver, being supplied directly to the compressors which are connected to the gaseous outlet of the receiver. 
     From point  24  to point  17  the refrigerant passes through the evaporator, where heat exchanger takes place in such a manner that heat is absorbed by the refrigerant. Thereby the enthalpy of the refrigerant is increased, while the pressure remains constant. 
     From point  19  to point  17  the refrigerant passes from the gaseous outlet of the receiver to the suction line, i.e. the part of the refrigerant path which interconnects the outlet of the evaporator and the inlet of the compressor unit, via a bypass valve. 
     In the case that the control of the vapour compression system approaches a region where the ejector no longer performs well, e.g. due to low ambient temperatures, the vapour compression system is instead controlled in such a manner that the pressure of refrigerant leaving the heat rejecting heat exchanger is slightly increased, as illustrated by the dashed line of the log P-h diagram. This has the consequence that the decrease in pressure when the refrigerant passes through the ejector from point  21   a  to point  22  is larger than the decrease in pressure during normal operation, i.e. from point  21  to point  22 . This improves the capability of the ejector to drive a secondary fluid flow, i.e. to suck refrigerant from the outlet of the evaporator to the secondary inlet of the ejector. Accordingly, the increased pressure of the refrigerant leaving the heat rejecting heat exchanger allows the ejector to operate at lower ambient temperatures. 
       FIG. 4  is a graph illustrating coefficient of performance as a function of ambient temperature for a vapour compression system being controlled in accordance with a method according to the invention and a vapour compression system being controlled in accordance with a prior art method, respectively. The dotted line represents operation of the vapour compression system according to a prior art method, and the solid line represent operation of the vapour compression system in accordance with a method according to the invention. 
     At high ambient temperatures, the ejector is performing well, resulting in the vapour compression system being operated at a higher coefficient of performance (COP) than is the case when the vapour compression system is operated without the ejector. 
     When the ambient temperature reaches approximately 25° C., the vapour compression system approaches a region where the ejector no longer performs well. This corresponds to a pressure difference between a pressure prevailing in the receiver and a pressure of refrigerant leaving the evaporator decreasing below a first lower threshold value. Under normal circumstances, the ejector would simply stop operating at this point, resulting in the vapour compression system being operated as indicated by the dotted line. Thereby the coefficient of performance (COP) of the vapour compression system is abruptly decreased at this point. 
     Instead, according to the present invention, the pressure of refrigerant leaving the heat rejecting heat exchanger is maintained at a slightly increased level, resulting in the ejector being capable of operating at the lower ambient temperatures, as described above, i.e. the solid line is followed instead of the dotted line. This is illustrated by the ‘kink’  25  in the graph. The increased pressure level of refrigerant leaving the heat rejecting heat exchanger is maintained until the ambient temperature reaches a level where it is no longer an advantage to keep the ejector operating, because it no longer improves the COP of the vapour compression system. This corresponds to a difference between the derived reference pressure value and the selected fixed reference pressure value increasing above a second upper threshold value. This occurs at point  26 , corresponding to an ambient temperature of approximately 21° C. At lower ambient temperatures, the vapour compression system is simply operated without the ejector. 
     It is clear from the graph of  FIG. 4  that the method according to the invention provides a transitional region between a region where the ejector performs well and a region where the ejector is not operating, thereby allowing the ejector to operate at lower ambient temperatures, i.e. approximately between 21° C. and 25° C. 
       FIG. 5  illustrates control of pressure of refrigerant leaving the heat rejecting heat exchanger  5  of a vapour compression system. The vapour compression system could, e.g., be the vapour compression system of  FIG. 1  or the vapour compression system of  FIG. 2 . 
     The temperature of refrigerant leaving the heat rejecting heat exchanger  5  is measured by means of temperature sensor  27 , and the pressure of refrigerant leaving the heat rejecting heat exchanger  5  is measured by means of pressure sensor  28 . Furthermore, the ambient temperature is measured by means of temperature sensor  29 . 
     The measured temperature and pressure of the refrigerant leaving the heat rejecting heat exchanger  5  are supplied to a high pressure control unit  30 . Based on the measured temperature of refrigerant leaving the heat rejecting heat exchanger  5 , the high pressure control unit  30  selects a reference pressure value for the refrigerant leaving the heat rejecting heat exchanger, being either a derived reference pressure value or a fixed reference pressure value, as described above. The high pressure control unit  30  further ensures that the vapour compression system is controlled in order to obtain a pressure of refrigerant leaving the heat rejecting heat exchanger  5  which is equal to the selected reference pressure value. The high pressure control unit  30  does this on the basis of the measured pressure of refrigerant leaving the heat rejecting heat exchanger  5 . 
     In order to control the pressure of refrigerant leaving the heat rejecting heat exchanger  5 , the high pressure control unit  30  generates a control signal for the ejector  6 . The control signal for the ejector  6  causes an opening degree of the primary inlet  11  of the ejector  6  to be adjusted. A decrease in the opening degree of the primary inlet  11  of the ejector  6  will cause the pressure of refrigerant leaving the heat rejecting heat exchanger  5  to be increased, and an increase in the opening degree of the primary inlet  11  of the ejector  6  will cause the pressure of refrigerant leaving the heat rejecting heat exchanger  5  to be decreased. 
     A fan control unit  31  receives the temperature of refrigerant leaving the heat rejecting heat exchanger  5 , measured by the temperature sensor  27 , and a temperature signal from the temperature sensor  29  measuring the ambient temperature. Based on the received signals, the fan control unit  31  generates a control signal for a motor  32  of a fan driving a secondary air flow across the heat rejecting heat exchanger  5 . In response to the control signal, the motor  32  adjusts the speed of the fan, thereby adjusting the secondary air flow across the heat rejecting heat exchanger  5 . A decrease in the secondary air flow across the heat rejecting heat exchanger  5  will result in an increase in the temperature of refrigerant leaving the heat rejecting heat exchanger  5 . This will cause the high pressure control unit  30  to increase the pressure of refrigerant leaving the heat rejecting heat exchanger  5 . Similarly, an increase in the secondary air flow across the heat rejecting heat exchanger  5  will result in a decrease in the pressure of refrigerant leaving the heat rejecting heat exchanger  5 . 
     Alternatively, a secondary liquid flow may flow across the heat rejecting heat exchanger  5 . In this case the fan control unit  31  may instead generate a control signal for a pump driving the secondary liquid flow across the heat rejecting heat exchanger  5 . 
       FIG. 6  is a block diagram illustrating operation of the high pressure control unit  30  of  FIG. 5 . The temperature (Tgc) of refrigerant leaving the heat rejecting heat exchanger is measured and supplied to a reference pressure deriving block  33 , where a reference pressure value for the pressure of refrigerant leaving the heat rejecting heat exchanger is derived, based on the measured temperature of refrigerant leaving the heat rejecting heat exchanger. The reference pressure value may be derived from a look-up table or a series of curves providing corresponding values of temperature of refrigerant leaving the heat rejecting heat exchanger, pressure of refrigerant leaving the heat rejecting heat exchanger, and coefficient of performance (COP). Thereby the derived reference pressure value is preferably the pressure value which causes the vapour compression system to be operated at optimal coefficient of performance (COP). 
     The derived reference pressure value is supplied to an evaluator  34 , where a pressure difference between a pressure prevailing in the receiver and a pressure of refrigerant leaving the evaporator (Ej offset) is compared to a first lower threshold value. Based thereon, the evaluator  34  determines whether the derived reference pressure value or a fixed reference pressure value should be selected as a reference value for the pressure of refrigerant leaving the heat rejecting heat exchanger. 
     The selected reference pressure value is supplied to a comparator  35 , where the reference pressure value is compared to a measured value of the pressure of refrigerant leaving the heat rejecting heat exchanger. The result of the comparison is supplied to a PI controller  36 , and based thereon the PI controller  36  generates a control signal for the ejector, causing the opening degree of the primary inlet of the ejector to be adjusted in such a manner that the pressure of refrigerant leaving the heat rejecting heat exchanger reaches the reference pressure value. 
       FIG. 7  is a block diagram illustrating operation of the fan control unit  31  of  FIG. 5 . The ambient temperature (T amb) is measured and supplied to a first summation point  37 , where an offset (dT) is added to the measured ambient temperature. The result of the addition is supplied to another summation point  38 , where an offset (Ej offset), originating from the method according to the present invention, is added to thereto. Thereby a final temperature setpoint (Setpoint) is obtained. 
     The final temperature setpoint is supplied to a comparator  39 , where the temperature setpoint is compared to the measured temperature of refrigerant leaving the heat rejecting heat exchanger. The result of the comparison is supplied to a PI controller  40 , and based thereon the PI controller  40  generates a control signal for the motor of the fan driving the secondary air flow across the heat rejecting heat exchanger. The control signal causes the speed of the fan to be controlled in such a manner that the temperature of refrigerant leaving the heat rejecting heat exchanger reaches the reference temperature value. 
     While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.