AU2011378695A1

AU2011378695A1 - Refrigerator and method of operating refrigeration system

Info

Publication number: AU2011378695A1
Application number: AU2011378695A
Authority: AU
Inventors: Andreas Aschan; Richard Furberg
Original assignee: Electrolux Home Products Corp NV
Current assignee: Electrolux Home Products Corp NV
Priority date: 2011-10-03
Filing date: 2011-10-03
Publication date: 2014-04-17
Anticipated expiration: 2031-10-03
Also published as: KR20140071411A; US20170159983A1; WO2013050055A1; RU2562834C1; US20140318161A1; CN103874896A; BR112014007624A2; EP2764304A1; AU2011378695B2

Abstract

A refrigeration system 4 controlled by a control system 26, wherein the refrigeration system 4 comprises: an evaporator 8, a compressor 10, a condenser 12, and an expansion arrangement 15.The control system 26 is adapted: to establish a momentary cooling requirement based on the differential between a set-point temperature and an actual temperature averaged over time, to forma requirement variable relating to the momentary cooling requirement, to fix a first duration during which the compressor is switched on and a second duration during which the compressor is switched off, and to switch on and to switch off the compressor according to the first and second durations. The refrigeration system 4 comprises a first shut-off valve 20. The control system 4 is adapted: to close the first shut-off valve 20 in connection with switching off the compressor 10, and to open the first shut-off valve 20 in connection with switching on the compressor 10.

Description

WO 2013/050055 PCT/EP2011/067213 1 REFRIGERATOR AND METHOD OF OPERATING REFRIGERATION SYSTEM TECHNICAL FIELD The present invention relates to a method of operating a refrigeration system. The 5 invention further relates to a refrigerator comprising a compartment cooled by a refrigeration system. BACKGROUND A refrigeration system, as used in connection with cooled domestic and commercial 10 foodstuff storage compartments comprises: an evaporator, a compressor, a condenser, and an expansion arrangement, such as a capillary tube, interconnected by conduits. Such cooled domestic and commercial foodstuff storage compartments may form part of above freezing point refrigerators or below freezing point refrigerators, the latter are sometimes referred to as freezers. Hereinafter, both types will be referred to as 15 refrigerators. A refrigerant circulates in the refrigeration system while the compressor runs, compressing the gaseous refrigerant coming from the evaporator. As the refrigerant circulates, the gaseous refrigerant is cooled and condenses to a liquid state in the 20 condenser, which is arranged outside of the compartment. Thereafter, the liquid refrigerant is subjected to a pressure drop in the expansion arrangement and evaporates from the liquid state in the evaporator. The evaporator is arranged in thermal communication with the compartment. Thus, the compartment is cooled by the refrigerant. 25 A refrigeration system is designed to provide a specific cooling capacity. The refrigeration system provides the specific cooling capacity when it reaches a steady "on" state after the compressor has been running for a while. The specific cooling capacity is higher than the long term cooling requirement of the compartment. Thus, the compressor 30 is running during a first duration, an on-period, and not running during a second duration, an off-period. One compressor cycle during the operation of the refrigeration system is constituted by one on-period and one off-period. Accordingly, during operation the WO 2013/050055 PCT/EP2011/067213 2 refrigeration system runs through many compressor cycles. The operation of the compressor and its cycle length is dependent of the temperature inside the compartment. Commonly used compressor control parameters are a maximum allowed temperature in the compartment, at which the compressor is started, and a minimum 5 temperature, at which the compressor is stopped. EP 727628 discloses a control device for controlling temperature in a refrigerator and a method of controlling the temperature in refrigerators. The purpose is to provide a control device and a method of controlling the temperature in refrigerators by means of which 10 the cooling source may be controlled to minimize the energy consumption of the refrigerator. This is achieved by means of a control algorithm in which a requirement variable is formed. The requirement variable formed indicates a ratio of the switch-on duration (the above-mentioned on-period) of the cooling source to the switch-off duration (the above-mentioned off-period) of the cooling source. Furthermore, an actual 15 temperature averaged over time is used as the actual temperature for forming a differential between a set-point temperature and an actual temperature. During each compressor cycle, a refrigeration system is subjected to cyclic losses. The losses are due to: 1. Liquid refrigerant, instead of gaseous refrigerant, being extracted 20 from the evaporator at the beginning of an on-period. 2. Gaseous refrigerant being pumped through the expansion arrangement in the form of a capillary tube before a liquid seal is formed at the beginning of the on-period. 3. Improperly charged condenser and evaporator before equilibrium has been established during the on-period. 4. Gaseous and liquid refrigerant entering the evaporator through the expansion 25 arrangement during the off-period. Furthermore, a high start up current may be required depending on the type of compressor operation, which also may contribute to losses in systems comprising a refrigeration system. In refrigeration systems running with long compressor cycle times, cyclic losses are 30 negligible in comparison with the total energy consumption of the refrigeration system. However, in refrigeration systems running with short cycle times the cyclic losses may be responsible for a considerable portion of the total energy consumption of the refrigeration system. In refrigerators, lowering energy consumption has been a goal for years. For instance the EU Directive 92/75/EC establishes an energy consumption WO 2013/050055 PCT/EP2011/067213 3 labeling scheme applicable inter alia for refrigerators. A good energy consumption rating is thus aspired by refrigerator manufacturers. The control device and method of EP 727628 does not take account of cyclic losses in a 5 relevant refrigeration system. WO 2006/044787 is concerned with a pressure equalization system which solves a problem specific to HVAC (Heating Ventilation Air Conditioning) systems. In connection with the background it is stated that pressure tends to equalize between low pressure 10 and high pressure sides when the compressor stops operating, and that in a refrigeration cycle energy is required at start up to create a high pressure in the compressor. In HVAC systems however, pressure does not equalize between the condenser and the evaporator (high pressure side and low pressure side of the refrigeration system) during the off-periods of the compressor. This creates a need for additional components, which 15 enable a high pressure difference start-up of the compressor. The pressure equalization system of WO 2006/044787 comprises a bleed port interconnecting the high pressure side of the compressor with the low pressure side of the compressor (outlet of compressor with inlet of compressor). Via the bleed port the pressure level in the compressor may be equalized to the low pressure level. Thus, the compressor may be 20 started under low pressure difference conditions. Accordingly, the electrical system driving the compressor does not require any costly start capacitor or start relay. There exists a need to decrease energy consumption in some types of refrigeration systems. 25 SUMMARY Accordingly, an object of the present invention is to further decrease energy consumption in refrigeration systems. 30 According to an aspect, the object is achieved by a method of operating a refrigeration system, wherein the refrigeration system comprises: - an evaporator adapted to be arranged in thermal communication with a compartment to be cooled, WO 2013/050055 PCT/EP2011/067213 4 - a compressor, -a condenser, - an expansion arrangement, and - conduits interconnecting the evaporator, the compressor, the condenser and the 5 expansion arrangement. The method comprises: - establishing a momentary cooling requirement of the compartment based on the differential between a set-point temperature and an actual temperature averaged over time, - forming a requirement variable relating to the momentary cooling requirement, the 10 requirement variable being a ratio between a switch-on duration of the compressor and a switch-off duration of the compressor, - fixing a first duration during which the compressor is switched on and a second duration during which the compressor is switched off on the basis of the requirement variable, and 15 - switching on and switching off the compressor according to the first and second durations. The refrigeration system further comprises a first shut-off valve arranged in a conduit extending between the condenser and the evaporator. The method further comprises: - closing the first shut-off valve in connection with switching off the compressor, and 20 - opening the first shut-off valve in connection with switching on the compressor. Since in a method, which provides an optimal refrigeration cycle length from an energy consumption point of view, further a first shut-off valve is closed when the compressor is switched off and not opened again until the compressor is switched on again to maintain 25 a pressure difference between the condenser and the evaporator of the refrigeration system when the compressor is not running, cyclic losses in the refrigeration system as laid out above under points 1 - 4 are avoided, at least to a large extent. As a result, the above mentioned object is achieved. 30 The refrigeration system and compartment may form part of a refrigerator, such as a domestic or commercial refrigerator for foodstuffs. The averaged temperature may for instance be calculated over a fixed period of time or over a number of compressor cycles. The feature "ratio between a switch-on duration of the compressor and a switch off duration of the compressor" encompasses both alternative calculations of the ratio, WO 2013/050055 PCT/EP2011/067213 5 i.e. switch-on duration in relation to switch-off duration, as well as switch-off duration in relation to switch-on duration of the compressor.The requirement variable in the form of a ratio between a switch-on duration and a switch-off duration of the compressor may be applied to one compressor cycle length to establish the first duration and the second 5 duration. The compressor cycle length may be fixed or may be set iteratively, e.g. to maintain a number of compressor cycles per time unit within a specified interval, such as e.g. 1 - 8 cycles/hour. More specifically, the conduit in which the first shut-off valve is arranged is the conduit in which also the expansion arrangement is arranged. 10 According to embodiments the refrigeration system may comprise the compartment and the compartment may be adapted for domestic foodstuff storing. In this manner the refrigeration system and the compartment may form part of a domestic refrigerator. According to embodiments, the compressor may be a single speed compressor 15 controlled by a control system to run at a constant speed during the first duration. Thanks to the method, a comparatively simple compressor may be operated in an economic manner. The compressor, and more specifically the electric motor driving the compressor, may be provided with a start capacitor and/or a start relay and/or other means to start under high pressure difference conditions prevailing between an inlet side 20 and an outlet side of the compressor due to the first shut-off valve being closed during the second duration. According to embodiments, the refrigeration system further may comprise a valve arrangement in a conduit between the compressor and the condenser, and a by-pass 25 conduit extending between an outlet side of the compressor and an inlet side of the compressor. The method further may comprise: - equalizing a pressure difference between the outlet side and the inlet side of the compressor during the second duration. In this manner for instance a single speed compressor may be started at low pressure difference conditions and a start capacitor or 30 other means may be omitted since the pressure difference between the inlet side and the outlet side of the compressor is equalized via the by-pass conduit. According to embodiments, the valve arrangement may comprise a check valve and the refrigeration system further may comprise a second shut-off valve arranged in the by- WO 2013/050055 PCT/EP2011/067213 6 pass conduit. The method further may comprise: - maintaining the second shut-off valve closed during the first duration, and - opening the second shut-off valve during the second duration to achieve the said equalizing the pressure difference. 5 According to embodiments, the valve arrangement may comprise a 3-way valve, the 3 way valve being connected to the by-pass conduit. The said equalizing the pressure difference may include: - opening a connection between the outlet side of the compressor and the by-pass 10 conduit and closing a connection between the compressor and the condenser by means of the 3-way valve. The method further may comprise: - closing the connection between the outlet side of the compressor and the by-pass conduit and opening the connection between the compressor and the condenser by means of the 3-way valve during the first duration. 15 According to embodiments, the expansion arrangement may comprise a capillary tube. According to embodiments, the first shut-off valve may form part of the expansion arrangement. 20 According to embodiments, the first duration and the second duration may collectively have a length of between 1 - 100 minutes. That is, the compressor cycle may have said length. According to further embodiments, the first duration and the second duration may collectively have a length of between 3 - 30 minutes. Generally, short total first and 25 second durations, i.e. a short compressor cycle length, is advantageous from an energy consumption point of view. Further, a short compressor cycle length allows for a more even air temperature inside a compartment cooled by the evaporator. Furthermore, a higher humidity of air inside the compartment may be achieved. 30 According to embodiments, the compressor may be adapted to provide a cooling capacity of between 10 - 500 W according to ASHRAE LBP or HMBP standard. According to further embodiments, the compressor may be adapted to provide a cooling capacity of between 20 - 300 W according to ASHRAE LBP or HMBP standard. Such a WO 2013/050055 PCT/EP2011/067213 7 compressor may provide sufficient cooling capacity for most domestic refrigerator applications. According to a further aspect, the above-mentioned object is further achieved by a 5 refrigerator comprising a compartment cooled by a refrigeration system controlled by a control system, wherein the refrigeration system comprises: an evaporator arranged in thermal communication with the compartment, a compressor, a condenser, an expansion arrangement, and conduits interconnecting the evaporator, the compressor, the condenser and the expansion arrangement. The control system is 10 adapted: - to establish a momentary cooling requirement of the compartment based on the differential between a set-point temperature and an actual temperature averaged over time, - to form a requirement variable relating to the momentary cooling requirement, the 15 requirement variable being a ratio between a switch-on duration of the compressor and a switch-off duration of the compressor, - to fix a first duration during which the compressor is switched on and a second duration during which the compressor is switched off on the basis of the requirement variable, and 20 - to switch on and to switch off the compressor according to the first and second durations. The refrigeration system comprises a first shut-off valve arranged in a conduit extending between the condenser and the evaporator, and the control system is further adapted: - to close the first shut-off valve in connection with switching off the compressor, and 25 - to open the first shut-off valve in connection with switching on the compressor. Embodiments mentioned above in relation to the method and the refrigeration system of the method, may be embodied in a refrigerator. 30 According to embodiments, the compressor may be a single speed compressor controlled by the control system to run at a constant speed during the first duration. According to embodiments, the refrigeration system further may comprise a valve arrangement in a conduit between the compressor and the condenser, and a by-pass WO 2013/050055 PCT/EP2011/067213 8 conduit extending between an outlet side of the compressor and an inlet side of the compressor. According to embodiments, the valve arrangement may comprise a check valve and the 5 refrigeration system further may comprise a second shut-off valve arranged in the by pass conduit, and wherein the control system may be adapted: - to maintain the second shut-off valve closed during the first duration, and - to open the second shut-off valve during the second duration to equalize a pressure difference between the outlet side and the inlet side of the compressor during the 10 second duration. According to embodiments, the valve arrangement may comprise a 3-way valve, the 3 way valve being connected to the by-pass conduit. The control system may be adapted: - to open a connection between the outlet side of the compressor and the by-pass conduit and to close a connection between the compressor and the condenser, by 15 means of the 3-way valve during the second duration, and - to close the connection between the outlet side of the compressor and the by-pass conduit and to open the connection between the compressor and the condenser, by means of the 3-way valve during the first duration. 20 According to embodiments, the compressor alternatively may be a variable speed compressor controlled by the control system to run at variable speed during the first duration. In this manner a compressor may be provided which may be started at high pressure difference conditions. 25 According to embodiments, the refrigeration system may comprise a filter arranged in a conduit between the condenser and the expansion arrangement. According to embodiments, the first shut-off valve may be arranged in a conduit between the filter and the expansion arrangement. In this manner it may be ensured that when 30 the first shut-off valve is opened in connection with switching on the compressor, liquid refrigerant will enter the expansion arrangement. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following detailed description. Those skilled WO 2013/050055 PCT/EP2011/067213 9 in the art will realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention, as defined by the appended claims. 5 BRIEF DESCRIPTION OF THE DRAWINGS The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which: Fig. 1 illustrates schematically a refrigerator according to embodiments and a 10 refrigeration system according to embodiments, Fig. 2 illustrates a method of operating a refrigeration system according to embodiments, Figs. 3 and 4 illustrate portions of refrigeration systems according to embodiments, and Fig. 5 illustrates a refrigeration system according to embodiments. 15 DETAILED DESCRIPTION The present invention will now be described more fully with reference to the accompanying drawings, in which example embodiments are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Disclosed features of example embodiments may be combined as readily understood by 20 one of ordinary skill in the art to which this invention belongs. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for brevity and/or clarity. 25 Fig. 1 illustrates schematically a refrigerator 2 according to embodiments and a refrigeration system 4 according to embodiments. The refrigerator 2 comprises the refrigeration system 4 and a compartment 6, e.g. for storing foodstuff. The refrigeration system 4 comprises an evaporator 8, a compressor 10, a condenser 12, a filter 14 and 30 an expansion arrangement 15 comprising a capillary tube 16. Conduits are arranged to interconnect the mentioned components of the refrigeration system 4. Accordingly, the filter 14 is arranged in a conduit ext ending between the condenser 12 and the expansion arrangement 15.

WO 2013/050055 PCT/EP2011/067213 10 A refrigerant circulates in the refrigeration system 4. Circulation of the refrigerant is driven by a pressure difference between the condenser 12 and the evaporator 8. The pressure difference is created by the compressor 10, compressing gaseous refrigerant 5 which has evaporated from liquid refrigerant in the evaporator 8. The gaseous refrigerant is cooled and condenses to a liquid state in the condenser 12. The liquid refrigerant passes through the filter 14, which may collect debris and water in the refrigerant. In the expansion arrangement 15 the liquid refrigerant is subjected to a pressure drop to thereafter evaporate from the liquid state in the evaporator 8. The evaporator 8 is 10 arranged in thermal communication with the compartment 6 and thus, cools the compartment 6 when refrigerant evaporates in the evaporator 8. The refrigeration system 4 further comprises a first shut-off valve 20 arranged in a first conduit 22 extending between the condenser 12 and the evaporator 8, i.e. a conduit 22 15 also comprising the expansion arrangement 15. The first shut-off valve 20 is arranged in the first conduit 22 between the filter 14 and the expansion arrangement 15. The first conduit 22 may be closed by means of the first shut-off valve 20. The first shut-off valve 20 has two discrete positions, one fully closed position and one fully open position. 20 An electric motor 24 drives the compressor 10. A control system 26 is arranged to control the operation of the refrigeration system 4. The control system 26 may comprise a microprocessor programmed to control the operation of the refrigeration system 4. Alternatively, the control system 26 may comprise discrete electric components connected to control the refrigeration system 4. The control system 26 further comprises 25 a temperature sensor 28 arranged in the compartment 6 and is connected to the electric motor 24 for controlling the compressor 10, and to the first shut-off valve 20. Control parameters of the refrigeration system 4 may be preset in the control system 26. Alternatively, at least some control parameters may be set via a control panel 30. 30 Fig. 2 illustrates a method of operating a refrigeration system according to embodiments. The refrigeration system may be a refrigeration system 4 as described in connection with Fig. 1. The method comprises: - Establishing 100 a momentary cooling requirement of the compartment 6 based on a differential between a set-point temperature and an actual temperature averaged over WO 2013/050055 PCT/EP2011/067213 11 time. The set-point temperature may be set in the control system 26, e.g. by means of the control panel 30. The averaged temperature over time may be calculated by the control system 26 utilizing temperature measurements of the temperature sensor 28 in the compartment 6. The said establishing 100 the momentary cooling requirement may 5 comprise simply establishing whether the refrigeration system 4 is to be switched on or off, i.e. whether the compressor 10 should be running or not. - Forming 102 a requirement variable relating to the momentary cooling requirement. The requirement variable may be a ratio between a switch-on duration of the compressor 10 and a switch-off duration of the compressor 10. 10 - Fixing 104 a first duration during which the compressor 10 is switched on and a second duration during which the compressor 10 is switched off on the basis of the requirement variable. For example, if a compressor cycle length is 20 minutes and the requirement variable is 40% switch-on duration and 60 % switch-off duration, the first duration is 8 minutes and the second duration is 12 minutes. 15 - Switching on 106 and switching off 108 of the compressor 10 according to the first and second durations. The control system 26 is adapted to switch on and off the compressor 10 by means of switching on and off the electric motor 24. - Closing 110 the first shut-off valve 20 in connection with switching off the compressor 10. 20 - Opening 112 the first shut-off valve 20 in connection with switching on the compressor 10. It may be clarified that in a refrigeration system without a first shut-off valve operated according to this method, the pressure between the condenser 12 and the evaporator 8 25 is gradually equalized via the expansion arrangement 15 during switch-off durations of the compressor, which causes cyclic losses as initially discussed under points 1 - 4. A refrigeration system 4 operated according to this method is advantageous in that it firstly, thanks to the requirement variable and the average temperature over time is used 30 allows the compressor 10 to be run for optimal durations from a cooling requirement and energy consumption point of view. Secondly, thanks to the closing and opening of the first shut-off valve 20, cyclic losses in the refrigeration system 4 may be avoided, at least to a large extent. Thus, the method allows a refrigeration system 4 to be operated with WO 2013/050055 PCT/EP2011/067213 12 low energy consumption by optimizing running characteristics and by eliminating cyclic losses. Method steps 100 - 104 are suitably performed at regular intervals by the control system 5 16 of the refrigeration system 4. Method steps 106 - 112 may be performed at the same regular intervals or at different intervals. With reference to the method disclosed in connection with Fig. 2, it is accordingly concluded that the control system 26 illustrated in Fig. 1 is adapted: 10 - to establish a momentary cooling requirement of the compartment 6 based on the differential between a set-point temperature and an actual temperature averaged over time, - to form a requirement variable relating to the momentary cooling requirement, the requirement variable being a ratio between a switch-on duration of the compressor 10 15 and a switch-off duration of the compressor 10, - to fix a first duration during which the compressor 10 is switched on and a second duration during which the compressor 10 is switched off on the basis of the requirement variable, - to switch on and to switch off the compressor 10 according to the first and second 20 durations, - to close the first shut-off valve 20 in connection with switching off the compressor 10, and - to open the first shut-off valve 20 in connection with switching on the compressor 10. 25 The compressor 10 may be a single speed compress or controlled by the control system 26 to run at a constant speed during the first duration. That is, the electric motor 24 has only one operational speed and the control system 26 is adapted to switch on and off the electric motor 24. The control system 26 may include a start capacitor and/or a start relay in order to be able to start the compressor 10 with the high pressure maintained in 30 the condenser 12, and at the outlet side of the compressor, due to the first shut-off valve 20 being closed during the second duration when the compressor 10 is switched off. Alternatively, the compressor 10 may be a variable speed compressor controlled by the control system 26 to run at variable speed during the first duration. That is, the control WO 2013/050055 PCT/EP2011/067213 13 system may either run the electric motor 24 at, at least, two different speeds during one first duration or alternatively, at a constant speed during one first duration and at a different constant speed during a following first duration. In the latter alternative, the requirement variable of the method may be specified to be maintained within a specified 5 interval. If the requirement variable is outside the specified interval, the constant speed is increased or decreased, depending on whether the requirement variable is above or below the specified interval, in a following first duration. A variable speed compressor and its electric motor are adapted to be started against a high pressure difference between the condenser 12, i.e. the outlet side of the compressor 10 and the evaporator 10 8, ie. inlet side of the compressor 10. The compressor 10, single speed or variable speed, may be adapted to provide a cooling capacity of between 10 - 500 W according to ASHRAE LBP or HMBP standard (at 55 degrees Celsius condensing temperature and - 23,3 degrees Celsius evaporating 15 temperature). More specifically, in some applications the compressor 10 may be adapted to provide a cooling capacity of between 20 - 300 W according to ASHRAE LBP or HMBP standard. The first duration and the second duration may collectively have a length of between 1 - 100 minutes, i.e. the compressor cycle may have a length of 1 100 minutes. More specifically, the first duration and the second duration may 20 collectively have a length of between 3 - 30 minutes. Fig. 3 illustrates a portion of a refrigeration system 4 according to embodiments. According to these embodiments there is provided a solution which allows a compressor 10, such as a single speed compressor, to be started under low pressure difference 25 conditions despite a high pressure prevailing in a condenser 12 arranged downstream of the compressor 10. The refrigeration system 4 comprises a valve arrangement 32 in a conduit 34 between the compressor 10 and the condenser 12. A by-pass conduit 36 extends between an outlet side of the compressor 10 and an inlet side of the compressor 10. The valve arrangement 32 comprises a check valve 38. A second shut 30 off valve 40 is arranged in the by-pass conduit 36. When the second shut-off valve 40 is open, any pressure difference between the inlet side and the outlet side of the compressor 10 is equalized to substantially the pressure prevailing at the inlet side of the compressor 10. The check valve 38 ensures that a high pressure in the condenser 10 cannot be equalized in an upstream direction from the condenser 10.

WO 2013/050055 PCT/EP2011/067213 14 The second shut-off valve 40 is connected to a control system 26 of the refrigeration system 4. The control system 26 is adapted: - to maintain the second shut-off valve 40 closed during a first duration when the 5 compressor 10 is running, and - to open the second shut-off valve 40 during a second duration to equalize a pressure difference between the outlet side and the inlet side of the compressor 10 during a second duration when the compressor 10 is not running. 10 Accordingly the method illustrated in Fig. 2 may further comprise: - Equalizing 114 a pressure difference between the outlet side and the inlet side of the compressor 10 during the second duration. - Opening 116 the second shut-off valve 40 during the second duration to achieve the said equalizing 114 the pressure difference between the outlet side and the inlet side of 15 the compressor 10 during the second duration. - Maintaining 118 the second shut-off valve 40 closed during the first duration. Fig. 4 illustrates a portion of a refrigeration system 4 according to embodiments. Also these embodiments provide a solution which allows a compressor 10 to be started under 20 low pressure difference conditions despite a high pressure prevailing in a condenser 12. The refrigeration system 4 comprises a valve arrangement 32 in a conduit 34 between the compressor 10 and the condenser 12. A by-pass conduit 36 extends between an outlet side of the compressor 10 and an inlet side of the compressor 10. Via the by-pass conduit 36, any pressure difference between the inlet side and the outlet side of the 25 compressor 10 may be equalized to substantially the pressure prevailing at the inlet side of the compressor 10. The valve arrangement 32 comprises a 3-way valve 42. The 3 way valve is connected to the by-pass conduit 36. The 3-way valve is connected to a control system 26 of the refrigeration system 4.The 30 control system 26 is adapted: - to open a connection between the outlet side of the compressor 10 and the by-pass conduit 36 and to close a connection between the compressor 10 and the condenser 12, by means of the 3-way valve 42 during the second duration when the compressor 10 is not running to equalize pressure via the by-pass conduit 36, and WO 2013/050055 PCT/EP2011/067213 15 - to close the connection between the outlet side of the compressor 10 and the by-pass conduit 26 and to open the connection between the compressor 10 and the condenser 12, by means of the 3-way valve during the first duration when the compressor 10 is running. 5 Accordingly the method illustrated in Fig. 2 may comprise: - Equalizing 114 a pressure difference between the outlet side and the inlet side of the compressor 10 during the second duration, the said equalizing 114 the pressure difference including: - Opening 120 a connection between the outlet side of the compressor 10 and the by 10 pass conduit 36 and closing 122 a connection between the compressor 10 and the condenser 12 by means of the 3-way valve 42. The method may further comprise: - Closing 124 the connection between the outlet side of the compressor 10 and the by pass conduit 36 and opening 126 the connection between the compressor 10 and the 15 condenser 12 by means of the 3-way valve 42 during the first duration. Thanks to the embodiments according to Figs. 3 and 4, a constant speed compressor does not require any start capacitor or other means to permit starting despite a high pressure prevailing in the condenser 12. The embodiments of Figs. 3 and 4 may be used 20 in refrigeration systems 4 according to Figs. 1 and 5. Fig. 5 illustrates a refrigeration system 4 according to embodiments. The refrigeration system 4 comprises an evaporator 8, a compressor 10, a condenser 12, and an expansion arrangement 15. The expansion arrangement 15 comprises a first shut-off 25 valve 20 or put differently, the first shut-off valve 20 forms part of the expansion arrangement 15. Accordingly, when the compressor 10 is running and liquid refrigerant evaporates in the evaporator 8, the first shut-off valve 20 acts as an expansion valve for liquid refrigerant. When the compressor 10 is not running the first shut-off valve 20 is closed to prevent cyclic losses in the refrigeration system 4. A control system 26 of the 30 refrigeration system 4 controls inter alia the first shut-off valve 20. A refrigeration system 4 according to embodiments may be operated according to a method according to embodiments for a period of time. Whereas during other periods of time the refrigeration system 4 may be operated according to a different method, e.g. if a WO 2013/050055 PCT/EP2011/067213 16 difference between a set-point temperature and an actual temperature exceeds a threshold value. Example embodiments and components described above may be combined as 5 understood by a person skilled in the art. Accordingly, when herein reference is made to some components relating to the compressor, such as a start capacitor, the electric motor of the compressor is encompassed in the expression "compressor". Although the invention has been described with reference to example embodiments, 10 many different alterations, modifications and the like will become apparent for those skilled in the art. For instance, the check valve 38 of the Fig. 3 embodiments may be arranged in the conduit extending between the evaporator 8 and the compressor 10 upstream of the by-pass conduit 36. Similarly, the 3-way valve 42 in the Fig. 4 embodiments may be arranged in the conduit extending between the evaporator 8 and 15 the compressor 10 and be connected to the compressor inlet side of the by-pass conduit 36. Therefore, it is to be understood that the foregoing is illustrative of various example embodiments and the invention is not to be limited to the specific embodiments 20 disclosed and that modifications to the disclosed embodiments, combinations of features of disclosed embodiments as well as other embodiments are intended to be included within the scope of the appended claims. As used herein, the term "comprising" or "comprises" is open-ended, and includes one 25 or more stated features, elements, steps, components or functions but does not preclude the presence or addition of one or more other features, elements, steps, components, functions or groups thereof. As used herein, the term "and/or" includes any and all combinations of one or more of 30 the associated listed items. As used herein, the common abbreviation "e.g.", which derives from the Latin phrase "exempli gratia," may be used to introduce or specify a general example or examples of a previously mentioned item, and is not intended to be limiting of such item. If used WO 2013/050055 PCT/EP2011/067213 17 herein, the common abbreviation "i.e.", which derives from the Latin phrase "id est," may be used to specify a particular item from a more general recitation. The terminology used herein is for the purpose of describing particular embodiments 5 only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Unless otherwise defined, all terms (including technical and scientific terms) used herein 10 have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 15 It will be understood that although the terms first, second, third etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used top distinguish one element, component, region, layer 20 or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed herein could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 25

Claims

1. A method of operating a refrigeration system (4), wherein the refrigeration system (4) comprises: an evaporator (8) adapted to be arranged in thermal communication with a 5 compartment (6) to be cooled, a compressor (10), a condenser (12), an expansion arrangement (15), and conduits interconnecting the evaporator (8), the compressor (10), the condenser (12) and the expansion arrangement (15), wherein the method comprises: - establishing (100) a momentary cooling requirement of the compartment (6) based 10 on the differential between a set-point temperature and an actual temperature averaged over time - forming (102) a requirement variable relating to the momentary cooling requirement, the requirement variable being a ratio between a switch-on duration of the compressor (10) and a switch-off duration of the compressor (10), 15 - fixing (104) a first duration during which the compressor (10) is switched on and a second duration during which the compressor (10) is switched off on the basis of the requirement variable, and - switching on (106) and switching off (108) the compressor (10) according to the first and second durations, 20 characterized in that the refrigeration system (4) comprises a first shut-off valve (20) arranged in a conduit extending between the condenser (12) and the evaporator (8), and in that the method comprises: - closing (110) the first shut-off valve (20) in connection with switching off (108) the 25 compressor (10), and - opening (112) the first shut-off valve (20) in connection with switching on (106) the compressor (10).

2. The method according to claim 1, wherein the refrigeration system (4) comprises the 30 compartment (6) and the compartment (6) is adapted for domestic foodstuff storing. WO 2013/050055 PCT/EP2011/067213 19

3. The method according to any one of claims 1 or 2, wherein the compressor (10) is a single speed compressor controlled by a control system to run at a constant speed during the first duration. 5

4. The method according to claim 3, wherein the refrigeration system (4) further comprises a valve arrangement (32) in a conduit between the compressor (10) and the condenser (12), and a by-pass conduit (36) extending between an outlet side of the compressor (10) and an inlet side of the compressor (10), and wherein the method further comprises: 10 - equalizing (114) a pressure difference between the outlet side and the inlet side of the compressor (10) during the second duration.

5. The method according to claim 4, wherein the valve arrangement (32) comprises a check valve (38) and wherein the refrigeration system (4) further comprises a second 15 shut-off valve (40) arranged in the by-pass conduit (36), and wherein the method further comprises: - maintaining (118) the second shut-off valve (40) closed during the first duration, and - opening (116) the second shut-off valve (40) during the second duration to achieve 20 said equalizing (114) the pressure difference.

6. The method according to claim 4, wherein the valve arrangement (32) comprises a 3-way valve (42), the 3-way valve (42) being connected to the by-pass conduit (36), and wherein said equalizing (114) the pressure difference includes: 25 - opening (120) a connection between the outlet side of the compressor (10) and the by-pass conduit (36) and closing (122) a connection between the compressor (10) and the condenser (12) by means of the 3-way valve (42), and wherein the method further comprises: - closing (124) the connection between the outlet side of the compressor (10) and the 30 by-pass conduit (36) and opening (126) the connection between the compressor (10) and the condenser (12) by means of the 3-way valve (42) during the first duration.

7. The method according to any one of the preceding claims, wherein the expansion arrangement (15) comprises a capillary tube (16). WO 2013/050055 PCT/EP2011/067213 20

8. The method according to any one of claims 1 - 6, wherein the first shut-off valve (20) forms part of the expansion arrangement (15). 5

9. The method according to any one of the preceding claims, wherein the first duration and the second duration collectively have a length of between 1 - 100 minutes.

10. The method according to any one of the preceding claims, wherein the compressor (10) is adapted to provide a cooling capacity of between 10 - 500 W according to 10 ASHRAE LBP or HMBP standard.

11. A refrigerator comprising a compartment (6) cooled by a refrigeration system (4) controlled by a control system (26), wherein the refrigeration system (4) comprises: an evaporator (8) arranged in thermal communication with the compartment (6), a 15 compressor (10), a condenser (12), an expansion arrangement (15), and conduits interconnecting the evaporator (8), the compressor (10), the condenser (12) and the expansion arrangement (15), and wherein the control system (26) is adapted: - to establish a momentary cooling requirement of the compartment (6) based on the differential between a set-point temperature and an actual temperature averaged 20 over time, - to form a requirement variable relating to the momentary cooling requirement, the requirement variable being a ratio between a switch-on duration of the compressor (10) and a switch-off duration of the compressor (10), - to fix a first duration during which the compressor (10) is switched on and a second 25 duration during which the compressor (10) is switched off on the basis of the requirement variable, and - to switch on and to switch off the compressor (10) according to the first and second durations, characterized in that 30 the refrigeration system (4) comprises a first shut-off valve (20) arranged in a conduit extending between the condenser (12) and the evaporator (8), and in that the control system (26) is further adapted: - to close the first shut-off valve (20) in connection with switching off the compressor (10), and WO 2013/050055 PCT/EP2011/067213 21 - to open the first shut-off valve (20) in connection with switching on the compressor (10).

12. The refrigerator according to claim 11, wherein the compartment (6) is adapted for 5 domestic foodstuff storing.

13. The refrigerator according to any one of claims 11 or 12 wherein the compressor (10) is a single speed compressor controlled by the control system (26) to run at a constant speed during the first duration. 10

14. The refrigerator according to claim 13, wherein the refrigeration system (4) further comprises a valve arrangement (32) in a conduit between the compressor (10) and the condenser (12), and a by-pass conduit (36) extending between an outlet side of the compressor (10) and an inlet side of the compressor (10). 15

15. The refrigerator according to claim 14, wherein the valve arrangement (32) comprises a check valve (38) and the refrigeration system (4) further comprises a second shut-off valve (40) arranged in the by-pass conduit (36), and wherein the control system (26) is adapted: 20 - to maintain the second shut-off valve (40) closed during the first duration, and - to open the second shut-off valve (40) during the second duration to equalize a pressure difference between the outlet side and the inlet side of the compressor (10) during the second duration. 25

16. The refrigerator according to claim 14, wherein the valve arrangement (32) comprises a 3-way valve (42), the 3-way valve (42) being connected to the by-pass conduit (36), and wherein the control system (26) is adapted: - to open a connection between the outlet side of the compressor (10) and the by pass conduit (36) and to close a connection between the compressor (10) and the 30 condenser (12), by means of the 3-way valve (42) during the second duration, and - to close the connection between the outlet side of the compressor (10) and the by pass conduit (36) and to open the connection between the compressor (10) and the condenser (12), by means of the 3-way valve (42) during the first duration. WO 2013/050055 PCT/EP2011/067213 22

17. The refrigerator according to any one of claims 11 or 12 wherein the compressor (10) is a variable speed compressor controlled by the control system (26) to run at variable speed during the first duration. 5

18. The refrigerator according to any one of claims 11 - 17, wherein the expansion arrangement (15) comprises a capillary tube (16).

19. The refrigerator according to any one of claims 11 - 17, wherein the first shut-off valve (20) forms part of the expansion arrangement (15). 10

20. The refrigerator according to any one of claims 11 - 19, where the refrigeration system (4) comprises a filter (14) arranged in a conduit between the condenser (12) and the expansion arrangement (15). 15

21. The refrigerator according to claim 20, wherein the first shut-off valve (20) is arranged in a conduit between the filter (14) and the expansion arrangement (15).