TECHNICAL FIELD

The present invention relates to a refrigeration system. One particular application of such a refrigeration system is in a refrigerator for storing foodstuffs.

BACKGROUND

One kind of refrigerator for storing foodstuffs comprises a refrigeration system comprising a compressor, a condenser, an expansion arrangement, an evaporator, and a system of conduits interconnecting these components. A refrigerant is circulated in the refrigeration system by means of the compressor compressing the refrigerant in gaseous form. The refrigerant condenses to liquid form in the condenser and is subjected to a pressure reduction as it passes through the expansion arrangement. The liquid refrigerant evaporates to gaseous form in the evaporator. The evaporator cools a compartment of the refrigerator as the refrigerant evaporates in the evaporator. In the following, this kind of refrigeration system will be referred to as a compressor operated refrigeration system, and its operation will be referred to as compressor operation or compressor operated, and a refrigeration system may be referred to as being compressor driven when the refrigerant is circulated in the refrigeration system by means of the compressor.

A different kind of refrigeration system, sometimes referred to as a thermosiphon, comprises an evaporator and a condenser interconnected by two conduits. The evaporator and the condenser of the thermosiphon are subjected to different temperatures. Thus, a refrigerant is circulated in the refrigeration system by means of buoyancy and gravity, gaseous refrigerant flowing from the evaporator to an upper end of the condenser, condensing in the condenser and flowing from a lower end of the condenser to the evaporator and rising through the evaporator as it evaporates. The operation of a thermosiphon refrigeration system herein will be referred to as thermosiphon operation or thermosiphon operated, and a refrigeration system may be referred to as being run as a thermosiphon when the refrigerant is circulated in the refrigeration system by means of buoyancy and gravity.

In some refrigeration systems a thermal storage is utilized for providing a source of cooling during certain periods. The thermal storage may comprise a phase change material (PCM), e.g. a solution of salt and water which changes between solid and liquid phase depending on its temperature. The thermal storage may for instance be cooled by the refrigeration system during hours when energy costs are low and the thermal storage may be used for cooling during hours when energy costs are high.

EP 641978 discloses a refrigeration apparatus capable of operating in i) vapour compression mode or ii) thermosiphon mode. The refrigeration apparatus defines a refrigerant path including a compressor, a condenser, an expansion valve and a chiller/evaporator where the refrigerant absorbs heat from a fluid to be cooled. The refrigerant circuit includes valves configurable for selectively directing refrigerant i) to pass through the compressor and the expansion valve or ii) to bypass the compressor and the expansion valve. A fluid circuit comprises a region to be cooled by the fluid cooled in the chiller/evaporator. A thermal store is provided in the fluid circuit, and is i) cooled while the apparatus operates in the vapour compression mode and ii) provides a chilling effect during changeover to thermosiphon mode. US 5251455 discloses a refrigeration appliance having at least two refrigeration

compartments, each compartment having its own access door. There is a first evaporator for the first compartment, the first evaporator operating at a first pressure level and a second evaporator for the second compartment, the second evaporator operating at a pressure level higher than the first pressure level. There is a control device for directing refrigerant to a selected one of the evaporators from a condenser and for preventing a flow of refrigerant into the first evaporator when refrigerant is being directed into the second evaporator to cool the second compartment. A phase change material may be used in association with one or both of the evaporators to store thermal energy from excess capacity of an oversized compressor. GB 180342 discloses a compression system in which a high pressure evaporator enclosed in a brine tank is connected to a liquid separator, communicating with a compressor and with a low pressure evaporator. There is provided between the two evaporators an additional communication with a one-way valve. The low pressure evaporator communicates with the compressor through. The high pressure evaporator thus functions as a cold-accumulator, such that when the compressor is stopped, a circulation is set up between the two evaporators owing to the temperature surrounding the low pressure evaporator being higher than that surrounding the high pressure evaporator. The pressure of the vapour in the low pressure evaporator increases and the vapour flows through the one-way valve to the high pressure evaporator where it is condensed and the liquid passes through the separator to the low pressure evaporator. The high pressure evaporator therefore functions as a condenser when the compressor is stopped, i.e. the high pressure and low pressure evaporators form a thermosiphon. SUMMARY

It is an object of the present invention to provide a refrigeration system comprising a thermal storage unit, and being adapted for thermosiphon operation, in which refrigeration system energy consumption may be better controlled than in some of the prior art systems mentioned above.

According to an aspect of the invention, the object is achieved by a refrigeration system comprising a compressor, a condenser, an expansion arrangement, an evaporator, a thermal storage unit comprising a thermal energy transfer conduit, and a refrigerant arranged to circulate in at least part of the refrigeration system. A first refrigerant circuit comprising the evaporator and the thermal energy transfer conduit is adapted for gravity and buoyancy driven circulation of the refrigerant. The refrigeration system comprises a second refrigerant circuit and a third refrigerant circuit adapted for compressor driven circulation of the refrigerant, wherein the second refrigerant circuit comprises the compressor, the condenser, the expansion arrangement, and the thermal energy transfer conduit, and wherein the third refrigerant circuit comprises the compressor, the condenser, the expansion arrangement, and the evaporator.

Since the thermal energy transfer conduit and the evaporator are arranged in separate compressor driven refrigerant circuits, the time period over which the refrigerant circulates through, and evaporates in, the thermal energy transfer conduit to charge the thermal storage unit may be controlled independently of the refrigerant evaporating in the evaporator, and vice versa. As a result, the above mentioned object is achieved. Moreover, the energy consumption may be reduced compared to systems wherein the thermal storage unit always is incorporated in the refrigerant circuit during compressor operation since the refrigerant may be circulated through the thermal energy transfer conduit only until the thermal storage unit is fully charged.

It has been realized by the inventors that arranging the thermal energy transfer conduit in series with the evaporator, as disclosed in some of the above-mentioned prior art documents will not provide sufficient control of energy consumption for a modern refrigeration system. The inventors further realized that a parallel arrangement of the thermal energy transfer conduit and the evaporator according to the above-mentioned aspect will provide basis for a precise control of the time periods during which the compressor circulates refrigerant through either the thermal energy transfer conduit or the evaporator. Due to the thermal energy storage unit, the compressor of the refrigeration system may be shut off during peak hours when energy costs are high. During such peak hours the refrigeration system may instead be run as a thermosiphon with the refrigerant circulating in the first refrigerant circuit. Also during power outages the refrigeration system may be run as a thermosiphon with the refrigerant circulating in the first refrigerant circuit. A phase change material (PCM) may be used in the thermal storage unit. When the compressor is shut off, the energy stored in the thermal storage unit may be utilized for condensing the refrigerant in the thermal energy transfer conduit during thermosiphon operation of the refrigeration system.

The second refrigerant circuit excludes the evaporator and the third refrigerant circuit excludes the thermal energy transfer conduit.

When the refrigeration system operates as a thermosiphon a temperature difference between the evaporator and the thermal storage unit causes the refrigerant to circulate through the first refrigerant circuit. The pressure in the evaporator and the thermal energy transfer conduit is substantially the same during thermosiphon operation. The thermal energy transfer conduit and the evaporator are subjected to different temperatures. Thus, the refrigerant may be circulated in the refrigeration system by means of buoyancy and gravity, gaseous refrigerant flowing from the evaporator to an upper end of the thermal energy transfer conduit, condensing in the thermal energy transfer conduit and flowing from a lower end of the refrigerant channel to the evaporator and rising through the evaporator as it evaporates. According to embodiments, the refrigeration system may comprise a first conduit forming at least a portion of a length of conduit extending from the condenser to the thermal energy transfer conduit and a second conduit forming at least a portion of a length of conduit extending from the condenser to the evaporator. In this manner conduits portions of the second and third refrigerant circuits may be provided.

According to embodiments, portions of the first conduit and the second conduit may be connected via a bypass valve to form a first thermosiphon conduit of the first refrigerant circuit, the first thermosiphon conduit may be adapted to conduct the refrigerant from the evaporator to the thermal energy transfer conduit when the bypass valve is open. In this manner a conduit may be provided in the first refrigerant circuit. Moreover, the bypass valve may prevent flow of refrigerant between the thermal energy transfer conduit and the evaporator during compressor operation of the refrigeration system. The bypass valve may be controlled by a control system of the refrigeration system.

According to embodiments, the expansion arrangement may comprise a first expansion device arranged in the first conduit and a second expansion device arranged in the second conduit. In this manner expansion devices adapted for a relevant evaporation temperature in the thermal energy transfer conduit and the evaporator, respectively, may be provided.

According to embodiments, the expansion arrangement may comprise a third expansion device arranged downstream of the condenser and upstream of the first and second conduits. In this manner a first pressure drop may be provided in the third expansion device prior to the first and second expansion devices.

Any one of the first, second, and/or third devices may be for instance a capillary tube or an expansion valve.

According to embodiments, the expansion arrangement may comprise a controllable expansion valve arranged downstream of the condenser and upstream of the first and second conduits. In this manner one expansion valve which may be set for a respective pressure drop suited for the thermal energy transfer conduit or the evaporator, respectively, may be provided. The expansion valve may be controlled by a control system of the refrigeration system.

According to embodiments, the refrigeration system may comprise a valve arrangement for controlling flow of the refrigerant through the first and second conduits. In this manner the flow of refrigerant may be directed through the second or third refrigerant circuit during compressor driven circulation of the refrigerant. The valve arrangement may be controlled by a control system of the refrigeration system. According to embodiments, the valve arrangement may comprise a first valve arranged in the first conduit and a second valve arranged in the second conduit.

According to embodiments, the valve arrangement may comprise a three-way valve connecting the condenser and the first and second conduits.

According to embodiments, the refrigeration system may comprise a check valve arranged in a second thermosiphon conduit of the first refrigerant circuit. The second thermosiphon conduit may be adapted to conduct refrigerant from the thermal energy transfer conduit to the evaporator. In this manner circulation of the refrigerant through the first refrigerant circuit may be provided. Moreover, refrigerant entering the thermal energy transfer conduit after exiting the evaporator during circulation of the refrigerant through the third refrigerant circuit may be avoided.

According to embodiments, in use of the refrigeration system, an outlet end of the thermal energy transfer conduit may be arranged above an inlet end of the evaporator, and an inlet end of the refrigerant channel may be arranged above an outlet end of the evaporator. In this manner the refrigerant is circulated in the refrigeration system during thermosiphon operation by means of buoyancy and gravity. The gaseous refrigerant flows via the first thermosiphon conduit from the evaporator to the thermal energy transfer conduit in the thermal storage unit, the thermal energy transfer conduit forming a condenser of the first refrigerant circuit during thermosiphon operation. The liquid refrigerant flows from the thermal energy transfer conduit to the evaporator via the second thermosiphon conduit and rises through the evaporator as it evaporates therein.

According to embodiments, the refrigeration system may comprise a compartment for storing foodstuffs, wherein the evaporator may be arranged in thermal communication with the compartment.

The refrigeration system may comprise one or more doors for providing access to the compartment. The refrigeration system may be arranged for above and/or below 0 degree Celsius storage of the foodstuffs. The refrigeration system may be adapted for domestic or commercial use.

The refrigerant may evaporate in the evaporator and cool the compartment both when the refrigeration system is compressor operated with the refrigerant circulating through the third refrigerant circuit, and when thermosiphon operated with the refrigerant circulating through the first refrigerant circuit. During compressor operation with the refrigerant circulating through the second refrigerant circuit, the thermal storage unit may be cooled by the refrigerant evaporating in the thermal energy transfer conduit. In thermosiphon operation the thermal storage unit, previously cooled during compressor operation, is utilized as a condenser. Accordingly, heat may be extracted from the thermal storage unit, or put differently the thermal storage unit may be charged, during compressor driven circulation of refrigerant through the second refrigerant circuit during hours when energy costs are low. During hours when energy costs are high, the thermal storage unit is utilized as condenser for condensing liquid refrigerant in thermosiphon operation of the refrigeration system.

Besides charging the thermal storage unit when energy costs are low, this has the advantage that in thermosiphon operation of the refrigeration system, during high energy cost hours, the compressor does not consume any energy.

According to embodiments, the thermal storage unit may comprise a thermal energy storage material and the thermal energy transfer conduit may be arranged in thermal communication with the thermal energy storage material. The thermal energy storage material may comprise a phase change material (PCM), e.g. a solution of salt and water. Other phase change materials which may be used are alcohol and water mixtures, paraffin, or fatty acids. The phase change material is cooled by liquid refrigerant flowing through the thermal energy transfer conduit during compressor operation of the refrigeration system with the refrigerant circulating through the second refrigerant circuit. Thus, the phase change material changes phase from liquid phase to solid phase. During thermosiphon operation of the refrigeration system the phase change material condenses gaseous refrigerant in the thermal energy transfer conduit to liquid refrigerant. Thus, the phase change material changes phase from solid phase to liquid phase. Other materials than phase change materials may be used as thermal energy storage materials, e.g. metals.

According to embodiments, in a first mode of operation of the refrigeration system, the refrigerant may be arranged to be circulated through the first refrigerant circuit by means of buoyancy and gravity, wherein in a second mode of operation of the refrigeration system the refrigerant may be arranged to be circulated through the second refrigerant circuit by means of the compressor, and wherein in a third mode of operation of the refrigeration system the refrigerant may be arranged to be circulated through the third refrigerant circuit by means of the compressor. In this manner the refrigeration system may be compressor operated in the second and third modes of operation and thermosiphon operated in the first mode of operation. In the refrigeration system, thermal energy may be stored in an efficient way in the thermal storage unit during the second mode of operation, and during the first mode of operation the thermal energy may be utilized for cooling a space being arranged in thermal communication with the evaporator. During the third mode of operation, the space is cooled as in any ordinary compressor driven refrigeration system.

In the first mode of operation, refrigerant circulation may be excluded through the compressor, the condenser, and the expansion arrangement. According to embodiments, the refrigeration system may comprise an accumulator for refrigerant arranged in a conduit extending from the thermal energy transfer conduit and the evaporator to the compressor. In this manner liquid refrigerant may be stored in the accumulator if during any of the three different modes of operation of the refrigeration system, less refrigerant is circulated through the refrigeration system than during other modes of operation.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the invention, including its particular features and advantages, will be readily understood from the example embodiments discussed in the following detailed description and the accompanying drawings, in which:

Fig. 1 illustrates a refrigeration system according to embodiments, and

Fig. 2 illustrates a portion of a refrigeration system according to embodiments.

DETAILED DESCRIPTION

Aspects of the present invention will now be described more fully. Like numbers refer to like elements throughout. Well-known functions or constructions will not necessarily be described in detail for brevity and/or clarity.

Fig. 1 illustrates a refrigeration system 2 according to embodiments. The refrigeration system 2 comprises a compressor 4, a condenser 6, an expansion arrangement 8, an evaporator 10, a thermal storage unit 12 comprising a thermal energy transfer conduit 14, and a refrigerant arranged to circulate in at least part of the refrigeration system 2.

A first refrigerant circuit 16 of the refrigeration system 2 comprises the evaporator 10 and the thermal energy transfer conduit 14, and excludes the compressor 4, the condenser 6, and the expansion arrangement 8. The first refrigerant circuit 16 is adapted for gravity and buoyancy driven circulation of the refrigerant therein. The refrigeration system 2 further comprises a second refrigerant circuit 18 and a third refrigerant circuit 20 adapted for compressor driven circulation of the refrigerant therein. The second refrigerant circuit comprises the compressor 4, the condenser 6, the expansion arrangement 8, and the thermal energy transfer conduit 12, and excludes the evaporator 10. The third refrigerant circuit 20 comprises the compressor 4, the condenser 6, the expansion arrangement 8, and the evaporator 10, and excludes the thermal energy transfer conduit 14. The refrigeration system 2 comprises a control system 22 adapted to control the operation of the compressor 4, which may be a constant speed or variable speed compressor, and the position of valves of the refrigeration system 2. Thus, in a first mode of operation of the refrigeration system 2, the refrigerant may be circulated through the first refrigerant circuit 16 by means of buoyancy and gravity, in a second mode of operation of the refrigeration system 2 the refrigerant may be circulated through the second refrigerant circuit 18 by means of the compressor 4, and in a third mode of operation of the refrigeration system 2 the refrigerant may be circulated through the third refrigerant circuit 20 by means of the compressor 4. In this manner the refrigeration system 2 is compressor operated in the second and third modes of operation and thermosiphon operated in the first mode of operation.

According to embodiments, the thermal storage unit 12 comprises a thermal energy storage material. The thermal energy transfer conduit 14 is arranged in thermal communication with the thermal energy storage material in the thermal storage unit 12. Thus, during the second mode of operation of the refrigeration system 2, the thermal energy storage material may be cooled by the refrigerant evaporating in the thermal energy transfer conduit 14. Moreover, during the first mode of operation, gaseous refrigerant is condensed to liquid refrigerant in the thermal energy transfer conduit 14, the gaseous refrigerant being cooled by the thermal energy storage material previously cooled during the second mode of operation.

The refrigeration system 2 comprises a compartment 23 for storing foodstuffs. The evaporator 10 is arranged in thermal communication with the compartment 23. A fan 25 for circulating air inside at least a portion of the compartment 23 may be provided in the compartment 23 to even out temperature differences in at least a portion of the compartment 23. A door 27 is arranged for closing, and providing access to, the compartment 23.

The refrigerant evaporates in the evaporator 10 and cools the compartment 23 both when the refrigeration system 2 is compressor operated with the refrigerant circulating through the third refrigerant circuit 20, and when thermosiphon operated with the refrigerant circulating through the first refrigerant circuit 16. During compressor operation with the refrigerant circulating through the second refrigerant circuit 18, the thermal storage unit 12 is cooled by the refrigerant evaporating in the thermal energy transfer conduit 14. In thermosiphon operation the thermal storage unit 12, previously cooled during compressor operation, is utilized as a condenser.

The refrigeration system 2 comprise a first conduit 24 forming at least a portion of a length of conduit extending from the condenser 6 to the thermal energy transfer conduit 14, and a second conduit 26 forming at least a portion of a length of conduit extending from the condenser 6 to the evaporator 10. Portions of the first conduit 24 and the second conduit 26 are connected via a bypass valve 28 to form a first thermosiphon conduit 30 of the first refrigerant circuit 16. The first thermosiphon conduit 30 extends between the evaporator 10 and the thermal energy transfer conduit 14 and thus, is adapted to conduct the refrigerant from the evaporator 10 to the thermal energy transfer conduit 14 when the compressor 4 is shut off and the bypass valve 28 is open, i.e. during thermosiphon operation when the refrigerant circulates through the first refrigerant circuit 16. The bypass valve 28 is controlled by the control system 22 of the refrigeration system 2.

The refrigeration system 2 comprises a check valve 32 arranged in a second thermosiphon conduit 34 of the first refrigerant circuit 16. The second thermosiphon conduit 34 extends between the thermal energy transfer conduit 14 and the evaporator 10. The check valve 32 permits flow of refrigerant through the second thermosiphon conduit 34 from the thermal energy transfer conduit 14 to the evaporator 10. Moreover, the check valve 32 prevents refrigerant from entering the thermal energy transfer conduit 14 after exiting the evaporator 10 during circulation of the refrigerant through the third refrigerant circuit 20.

The expansion arrangement 8 comprises a first expansion device 36 arranged in the first conduit 24 and a second expansion device 38 arranged in the second conduit 26. The first expansion device 36 is adapted to provide a pressure drop such that the refrigerant evaporates at a temperature suitable for storing thermal energy in the thermal energy storage material of the thermal storage unit 12 during circulation of the refrigerant through the second refrigerant circuit 16. The second expansion device 38 is adapted to provide a pressure drop such that the refrigerant evaporates in the evaporator 10 at a temperature suitable for cooling foodstuffs in the compartment 23, either to a temperature above 0 degrees Celsius or to a temperature below 0 degrees Celsius, depending on the kind of refrigeration system 2. The first and second expansion devices 36, 38 may comprise capillary tubes.

The refrigeration system 2 comprises a valve arrangement 40 for controlling flow of the refrigerant through the first and second conduits 24, 26 and accordingly, also through the first and second refrigerant circuits 18, 20. That is, via the valve arrangement 40 the control system 22 controls, through which of the second and third refrigerant circuits 18, 20 the refrigerant is circulated by the compressor 4. In these embodiments, the valve arrangement 40 comprises a first valve arranged 42 in the first conduit 24 and a second valve 44 arranged in the second conduit 26. Alternatively, a three-way valve may be used, as discussed below in connection with Fig. 2.

The expansion arrangement 8 may comprise a third expansion device 46 arranged downstream of the condenser 6 and upstream of the first and second conduits 24, 26. The third expansion device 46 may provide an initial pressure drop prior to the first or second expansion device 36, 38. The third expansion device 46 is optional and may be omitted in alternative embodiments.

The refrigeration system 2 comprises an accumulator 48 for refrigerant arranged in a conduit 50 extending from the thermal energy transfer conduit 14 and the evaporator 10 to the compressor 4. A heat exchanger 51 for heat exchange between refrigerant flowing to the compressor 4 and refrigerant flowing from the condenser 6 may be provided in a customary manner.

Fig. 2 illustrates a portion of a refrigeration system 2 according to embodiments. These embodiments are similar to the embodiments discussed in connection with Fig. 1 .

Accordingly, only the main differences with the Fig. 1 embodiments will be discussed below. The expansion arrangement 8 comprise a controllable expansion valve 52 arranged downstream of the condenser 16 and upstream of the first and second conduits 24, 26. The control system 22 may set the expansion valve 52 either for a pressure drop suited for the thermal energy transfer conduit or the evaporator.

Again, the refrigeration system 2 comprises a valve arrangement 40 for controlling flow of the refrigerant through the first and second conduits 24, 26. Via the valve arrangement 40 the control system 22 controls, through which of the second and third refrigerant circuits the refrigerant is circulated by the compressor 4. In these embodiments the valve arrangement 40 comprises a three-way valve 54 connecting the condenser 6 and the first and second conduits 24, 26. Alternatively, the valve arrangement 40 may comprise a first valve arranged in the first conduit 24 and a second valve arranged in the second conduit 26, as discussed above in connection with Fig. 1 . The control system 22 sets the three-way valve 54 to direct the refrigerant either into the first or the second conduit 24, 26.

This invention should not be construed as limited to the embodiments set forth herein. A person skilled in the art will realize that different features of the embodiments disclosed herein may be combined to create embodiments other than those described herein, without departing from the scope of the present invention, as defined by the appended claims. The control system 22 may be arranged for switching between compressor operation and thermosiphon operation of the refrigeration system 2. During compressor operation, the control system 22 may switch on and off the compressor 8. The control system 22 may comprise a thermo sensor, which may be arranged in direct or indirect thermal

communication with the compartment 23. In a refrigeration system 2 according to

embodiments comprising a compartment 23 with below 0 degree Celsius temperature, the air mean temperature may be between - 22 to - 10 degrees Celsius. In a refrigeration system 2 according to embodiments comprising a compartment 23 with above 0 degree Celsius temperature, the air mean temperature may be between 0 to 8 degrees Celsius. According to embodiments, the evaporator 10 mean temperature may be between - 35 to -15 degrees Celsius. An example of a phase change material used as a thermal energy storage material in the thermal storage unit 12 is the PluslCE ® Phase Change Material Eutectic (E) Range, E-26 by PCM Phase Change Material Products Limited, United Kingdom. The refrigerant in circulating in the refrigeration system may be e.g. R134a or R600.

Although the invention has been described with reference to example embodiments, many different alterations, modifications and the like will become apparent for those skilled in the art. One or more controllable expansion valves may be utilized in the embodiments of the refrigeration system 2 discussed in connection with Fig. 1 and two or more expansion devices in the form of capillary tubes may be utilized in the embodiments of the refrigeration system 2 discussed in connection with Fig. 2. Therefore, it is to be understood that the foregoing is illustrative of various example embodiments and that the invention is defined only by the appended claims.

As used herein, the term "comprising" or "comprises" is open-ended, and includes one 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.