Refrigeration system

Abstract

This invention relates to a refrigeration system comprising a main refrigeration circuit for maintaining refrigerant fluid, the main refrigeration circuit including a compressor (12), a heat exchanger (14), an expansion device (18), and a heat exchanger (16). Additionally, the refrigeration system includes a buffer tank (20) attached to the main refrigeration circuit, having valves (22, 24) for controlling the flow of refrigerant fluid between the main refrigeration circuit and the buffer tank (20). The refrigeration system is arranged such that the valves (22, 24) are controlled based on a measure of subcooling in the main refrigeration circuit to transfer refrigerant fluid between the main refrigeration circuit and the buffer tank (20). By using the buffer tank (20) and the controllable valves (22, 24), it becomes possible to change the refrigerant charge level of the main refrigeration circuit to allow for increased efficiency during changes in operating conditions, as well as increased refrigerant capacity, potentially expanding the operating range of the refrigerant system, and other enhancements, including greater robustness over time when the refrigerant charge is depleted.

Description

Technical Field

[0001] The present invention relates to a refrigeration system and a method for operating a refrigeration system. Background Art

[0002] As is well known, refrigeration or heating can be provided by a refrigeration system utilizing a refrigeration cycle in which a refrigerant fluid is compressed, cooled, expanded, and then heated. In a common use of a refrigeration system to meet a cooling load, the cooling of the refrigerant fluid is accomplished by discharging heat to the atmosphere via a heat rejection heat exchanger, and the heating of the refrigerant fluid is accomplished by absorbing heat from an object to be cooled (such as a cold storage space for low temperature storage or the interior of a building to be inhabited) via a heat absorption heat exchanger. In this way, the refrigeration system can transfer heat from the interior of the building to the exterior of the building even when the interior temperature is lower than the atmosphere. Alternatively, the refrigeration system can be used as a heat pump to meet the heat demand. In this case, a heat absorption heat exchanger is used to absorb heat from a low temperature source, and then the refrigeration circuit discharges the heat to the high temperature object to be heated. This can once again be the interior of a building. In both cases, all or part of the phase change of the refrigerant fluid can be used to increase the possible temperature difference between the heat rejection phase and the heat absorption phase.

[0003] It will be appreciated that the ability of a refrigeration system to properly handle the refrigerant fluid is important in achieving the most efficient operation of the refrigeration cycle. In particular, it is important to optimize the handling of the refrigerant fluid within the primary refrigeration loop for compression, cooling, expansion, and heating of the refrigerant fluid. In this context, it is known to determine the optimum refrigerant type and refrigerant charge level to improve the efficiency of the refrigerant system. Improved efficiency and practicality are always desired. Summary of the invention

[0004] From a first aspect, the present invention provides a refrigeration system comprising: a main refrigeration circuit for maintaining a refrigerant fluid, the main refrigeration circuit comprising: a compression device, a heat rejection heat exchanger, an expansion device and a heat absorption heat exchanger; wherein the refrigeration system comprises a buffer tank attached to the main refrigeration circuit, having a valve for controlling the flow of the refrigerant fluid between the main refrigeration circuit and the buffer tank; and wherein the refrigeration system is arranged so that the valve is controlled to transfer the refrigerant fluid between the main refrigeration circuit and the buffer tank based on a measure of subcooling in the main refrigeration circuit.

[0005] With the use of a buffer tank and a controllable valve, it becomes possible to vary the refrigerant charge level of the main refrigeration circuit in order to improve efficiency during varying operating conditions, as well as to increase refrigerant capacity, potentially expanding the operating range of the refrigerant system, and other enhancements, including greater robustness over time when the refrigerant charge is depleted. As described above, it is known to determine the optimal refrigerant charge level to allow for improved efficiency. Typically, this is done with reference to an optimal subcooling value associated with the parameters of the compression device and the operating conditions of the refrigeration system (such as the temperatures of heat absorption and / or heat rejection). Conditions vary during operation of the refrigeration system, but in conventional closed refrigeration circuits, the charge level is fixed. The system of the first aspect can advantageously utilize a buffer tank to vary the refrigerant charge level in the main refrigeration circuit in order to achieve improved efficiency for varying operating conditions. Although other systems have been proposed for achieving some of these benefits, these systems are generally more expensive and complex, such as by introducing additional heat exchangers and / or sub-circuits with additional drivers (i.e., additional compressors or pumps). The proposed buffer tank system provides many advantages by making relatively simple and inexpensive modifications to the refrigeration system.

[0006] The buffer tank provides a receiver / reservoir for the refrigerant fluid and can therefore be implemented using components known to function as refrigerant receivers. The buffer tank can be connected to the main refrigeration circuit via a first valve and a second valve, the first valve being used to control flow to or from a higher pressure point on the refrigerant circuit and the second valve being used to control flow to or from a lower pressure point on the refrigerant circuit. When the refrigerant pressure in the buffer tank is lower or higher than the refrigerant pressure in the main refrigeration circuit, then opening the corresponding valve will allow the refrigerant fluid to be transferred to and from the buffer tank. For example, when it is desired to reduce the charge level in the main refrigeration circuit, then the first valve can be opened to fill the buffer tank from a higher pressure point, and when it is desired to increase the charge level in the main refrigeration circuit, then the second valve can be opened to empty the buffer tank to a lower pressure point.

[0007] In one example, the surge tank is connected to the main refrigeration circuit in parallel with the expansion device, with a fluid connection to a higher pressure point before expansion and a lower pressure point after expansion. Thus, with a first valve and a second valve as described above, the first valve can be in the fluid line between the surge tank and a point on the main refrigeration circuit before the expansion device, and the second valve can be in the fluid line between the surge tank and a point on the main refrigeration circuit after the expansion device.

[0008] Any suitable type of subcooling sensor may be used to obtain a measure of subcooling. A refrigeration system may therefore include a subcooling sensor. The subcooling sensor may measure the temperature and pressure of the refrigerant fluid in the main refrigeration circuit. The subcooling sensor is typically located on the main refrigeration circuit after the heat rejection heat exchanger and before the expansion device. The refrigeration system may be arranged to use the output signal of the subcooling sensor, such as via a controller as described below, for example, in relation to the control of a valve.

[0009] The refrigeration system may be arranged so that when there is an excess of subcooling, the refrigerant fluid is directed from the main refrigeration circuit into the buffer tank, and when there is insufficient subcooling, the refrigerant fluid is emptied from the buffer tank into the main refrigeration circuit, thereby refilling the main refrigeration circuit. The refrigeration system may include a controller for controlling valves to achieve this. The controller may also control other elements within the refrigeration circuit, such as a compression device and / or an expansion device. The controller may receive temperature measurements from sensors, such as sensors for ambient air temperature (outside air temperature), sensors for the temperature of the temperature-controlled space, and / or sensors within the refrigeration circuit, such as for measuring temperature and / or pressure. Sensors may be included as part of the refrigeration system.

[0010] In the case of the first valve and the second valve as described above, the controller may be configured to open the first valve when there is excessive subcooling, while the second valve is closed. The controller may also be configured to open the second valve when there is insufficient subcooling, while the first valve is closed. The controller may be arranged so that when it is necessary to maintain the refrigerant charge level of the main circuit unchanged, for example, when the subcooling is at an optimal value or within a range near the optimal value, both the first valve and the second valve will be closed. Hysteresis may be applied to the control loop of the controller. The measurement of subcooling may include a subcooling value, which may be provided by a subcooling sensor.

[0011] Thus, the controller may be arranged to keep both valves closed when the subcooling value remains within a range between an external excessive subcooling threshold and an external insufficient subcooling threshold, wherein the external threshold is above and below the optimum subcooling value. The controller may also be arranged to open the first valve while the second valve is closed if the subcooling value exceeds the external excessive subcooling threshold level, and to open the second valve while the first valve is closed if the subcooling value is below the external insufficient subcooling threshold. The controller may be arranged to close the respective valves again when the subcooling returns to a value within the range. For example, when hysteresis is used in the control of the valves, the value may be a certain amount within the range, rather than just within the threshold.

[0012] Where hysteresis is used, then the controller may be arranged to hold the respective valve open until the subcooling moves back within the outer range by at least a certain amount, such as by 10% or 20% beyond a threshold. Thus, after opening the first valve due to the subcooling value exceeding the outer excessive subcooling threshold level, the controller may be arranged to hold the first valve open until the subcooling value exceeds the inner insufficient subcooling threshold level, which is a value below the outer excessive subcooling threshold value, and then revert to a state where both valves are closed. Similarly, after opening the second valve due to the subcooling value being below the outer insufficient subcooling threshold level, the controller may be arranged to hold the second valve open until the subcooling value exceeds the inner insufficient subcooling threshold level, which is a value above the outer insufficient subcooling threshold value, and then revert to a state where both valves are closed. The inner threshold levels may all be optimal subcooling values. In this case, after opening the first valve to address excessive subcooling, the first valve may be kept open until the subcooling decreases to an optimal value, and then the controller may restore the system to a state where both valves are closed, wherein the second valve is kept open after being opened due to insufficient subcooling until the subcooling increases to an optimal value, after which the controller will restore the system to a state where both valves are closed.

[0013] When the first valve and / or the second valve changes between opening and closing, this can be done gradually to avoid any shock load on the refrigeration system. Moreover, in some cases, depending on the system requirements, the valve may be only partially opened, rather than fully opened. The first valve may be a relatively simple valve having an open state and a closed state, such as a solenoid valve. It has been found that in exemplary embodiments, such as in the case where the first valve controls the flow of fluid from the high pressure side of the expansion device to the buffer tank, the opening of the first valve is not critical, and therefore the opening of the first valve does not need to be subject to any special control. The second valve may be a valve with a controllable opening, such as a PMV, so as to allow the refrigeration system to change the opening when the refrigerant fluid is transferred from the buffer tank to the main refrigeration circuit. This control of the second valve can use a controller, if present. The refrigeration system can be arranged to control the opening of the second valve so as to control the amount of liquid refrigerant fluid at the inlet of the compression device so as to avoid any liquid refrigerant. The control can use a sensor to determine the state of the fluid delivered to the compression device, and thereby avoid the fluid state causing excessive liquid at the inlet of the compression device. This may involve temperature and / or pressure sensors at the heat absorption heat exchanger or at the compression device or in the fluid path between the two.

[0014] The compression device may be any suitable device for increasing the pressure of the refrigerant fluid, and therefore may be any suitable type of compressor, such as a compressor known for a refrigeration circuit. For certain types of refrigeration systems, the compression device may be a pump. The compression device may be arranged to operate with a single-phase refrigerant, i.e., a fully gaseous refrigerant, or with a two-phase refrigerant having a mixture of a liquid phase and a gas phase. The compression device may have an inlet connected to a fluid path from a heat absorption heat exchanger and an outlet connected to a fluid path leading to a heat rejection heat exchanger. In some instances, the fluid path does not provide a direct connection to other refrigeration system components that may change the state of the refrigerant fluid. The compression device may have an intermediate inlet, such as for connecting to an economizer (economiser) line.

[0015] The expansion device may be any suitable device for reducing the pressure of the refrigerant fluid, such as an expansion valve, or a separator having an expansion function. The expansion device may be arranged to provide a controllable degree of expansion, for example by using a valve with a controllable opening. The expansion valve may be an electronic expansion valve. The opening of the expansion valve may be controlled to react to changes in the behavior of the refrigerant circuit according to the adjustment of the charge level by using a buffer tank. The control may be accomplished via a controller as described above, if present. For example, in order to compensate for a pressure drop or a pressure increase, the expansion valve may increase or decrease its opening (for example under the control of a controller) to maintain the same behavior of the refrigerant circuit, for example the opening of the expansion valve may be controlled so as to maintain the same refrigerant mass flow through the circuit.

[0016] The main refrigeration circuit may include an economizer line. The economizer line may be connected to or interact with an expansion device. The economizer line may extend from a branch point in the main refrigeration circuit after the heat rejection heat exchanger and before or at the expansion device to an intermediate inlet of the compressor. There may be an economizer valve in the economizer line for economizing expansion and for controlling economizer flow, and an economizer heat exchanger for exchanging heat between the refrigerant fluid in the economizer line after the economizer valve and the refrigerant fluid in the main refrigeration circuit after the branch point and before the expansion device.

[0017] The heat absorbing heat exchanger may be an evaporator. The heat rejecting heat exchanger may be a condenser.

[0018] Viewed from a second aspect, the invention extends to a method for operating a refrigeration system, wherein the method may include using the refrigeration system of the first aspect. Thus, the method may be a method for operating a refrigeration system, the refrigeration system comprising: a main refrigeration circuit for holding a refrigerant fluid, the main refrigeration circuit comprising: a compression device, a heat rejection heat exchanger, an expansion device and a heat absorption heat exchanger; wherein the refrigeration system comprises a buffer tank attached to the main refrigeration circuit, having a valve for controlling the flow of refrigerant fluid between the main refrigeration circuit and the buffer tank; the method comprises controlling the valve to transfer the refrigerant fluid between the main refrigeration circuit and the buffer tank based on a measure of subcooling in the main refrigeration circuit.

[0019] The method may comprise using a refrigeration system having the other features as described above in combination with the optional features of the first aspect.

[0020] Control of the valve may be performed to vary the refrigerant charge level of the primary refrigeration circuit to allow one or more of improved efficiency during changing operating conditions, expanded operating range of the refrigerant system, and / or adjustment over time as the refrigerant charge is depleted.

[0021] The buffer tank may be connected to the main refrigeration circuit via a first valve for controlling flow to or from a higher pressure point on the refrigerant circuit and a second valve for controlling flow to or from a lower pressure point on the refrigerant circuit, wherein the method includes, when the refrigerant pressure in the buffer tank is lower or higher than the refrigerant pressure in the main refrigeration circuit, opening the corresponding valve to allow transfer of refrigerant fluid to and from the buffer tank. For example, when it is desired to reduce the charge level in the main refrigeration circuit, the first valve may be opened to fill the buffer tank from a higher pressure point, and when it is desired to increase the charge level in the main refrigeration circuit, the second valve may be opened to empty the buffer tank to a lower pressure point. As described above, the buffer tank may be connected to the main refrigeration circuit in parallel with the expansion device.

[0022] The method may include obtaining a measure of subcooling by using a subcooling sensor, which may be any suitable type of sensor and which may measure the temperature and pressure of the refrigerant fluid within the main refrigeration circuit. The subcooling sensor may be located on the main refrigeration circuit after the heat rejection heat exchanger and before the expansion device. The method may include using an output signal of the subcooling sensor in connection with control of the valve via a controller as described above.

[0023] An exemplary embodiment of the method includes controlling a valve for directing refrigerant fluid from a main refrigeration circuit to a buffer tank when excess subcooling exists and directing refrigerant fluid from the buffer tank to the main refrigeration circuit when insufficient subcooling exists.

[0024] In one example, with the first valve and the second valve as described above, the method includes opening the first valve when there is excessive subcooling, while the second valve is closed. The method may also include opening the second valve when there is insufficient subcooling, while the first valve is closed. When it is necessary to maintain the refrigerant charge level of the main circuit unchanged, for example, when the subcooling is at an optimal value or in a range near the optimal value, both the first valve and the second valve may be closed.

[0025] The method may include: keeping both valves closed when the subcooling value remains within a range between an external excessive subcooling threshold and an external insufficient subcooling threshold, wherein the external threshold is above and below the optimal subcooling value; if the subcooling value exceeds the external excessive subcooling threshold level, opening the first valve while the second valve is closed; and if the subcooling value is below the external insufficient subcooling threshold, opening the second valve while the first valve is closed. When the subcooling value returns to a value within the range, the opened valves may be closed again.

[0026] The method may include hysteresis associated with the opening and closing of the valves, and in this case, the method may include keeping the opened valve open until the subcooling moves back within the outer range by at least a certain amount, such as exceeding a threshold value of 10% or 20%. Thus, after the first valve is opened due to the subcooling value exceeding the outer excessive subcooling threshold level, the valve may be kept open until the subcooling value exceeds the inner insufficient subcooling threshold level, which is a value lower than the outer excessive subcooling threshold value, and then both valves may be closed. Similarly, after the second valve is opened due to the subcooling value being lower than the outer insufficient subcooling threshold level, the second valve may be kept open until the subcooling value exceeds the inner insufficient subcooling threshold level, which is a value higher than the outer insufficient subcooling threshold value, and then restored to a state where both valves are closed. The inner threshold levels may all be optimal subcooling values. In this case, after opening the first valve to address excessive subcooling, the first valve may be maintained open until the subcooling decreases to an optimal value, and then the controller may restore the system to a state where both valves are closed, wherein the second valve, after opening due to insufficient subcooling, remains open until the subcooling increases to an optimal value, after which both valves are closed.

[0027] When the first valve and / or the second valve changes between opening and closing, this can be done gradually to avoid any shock load on the refrigeration system. Moreover, in some cases, depending on system requirements, the valve may be only partially opened, rather than fully opened. The first valve may be a relatively simple valve with an open state and a closed state, such as a solenoid valve. The second valve may be a valve with a controllable opening, such as a PMV, and the method may include changing the opening when the refrigerant fluid is transferred from the buffer tank to the main refrigeration circuit. The opening of the second valve may be controlled so as to control the amount of liquid refrigerant fluid at the inlet of the compression device so as to avoid the presence of any liquid refrigerant. The method may include using a sensor to determine the state of the fluid transferred to the compression device, and thereby avoid the fluid state causing excessive liquid at the inlet of the compression device. This may involve a temperature and / or pressure sensor in the fluid path at the endothermic heat exchanger or at the compression device or between the two.

[0028] The compression device may be any suitable device as described above. Optionally, the method includes transferring the refrigerant fluid directly from the heat absorption heat exchanger to the compressor and / or transferring the refrigerant fluid directly from the compressor to the heat rejection heat exchanger. The method may use a fluid path that does not provide direct connection to other refrigeration system components that would change the state of the refrigerant fluid. Optionally, as described above, an economizer may be included and used within the refrigeration system.

[0029] The expansion device may be any suitable device for reducing the pressure of the refrigerant fluid as described above. The method may include controlling the degree of expansion of the refrigerant fluid at the expansion device, for example by using a valve with a controllable opening. The opening of the expansion valve may be controlled to react to changes in the behavior of the refrigerant circuit according to the adjustment of the charge level by using a buffer tank. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Certain preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which: Figure 1 is a schematic diagram of a refrigeration system using a main refrigeration circuit and a buffer tank connected to the main refrigeration circuit; and

[0031] Figure 2 Example graphs of efficiency versus charge level for various operating conditions are shown. DETAILED DESCRIPTION

[0032] like Figure 1As shown, the refrigeration system includes a compression device 12, a heat rejection heat exchanger 14, an expansion device 18, and a heat absorption heat exchanger 16 that together form a main refrigeration circuit. The main refrigeration circuit contains a refrigerant fluid, and the circulation of the refrigerant fluid via the compression device 12 enables the refrigeration system to utilize a refrigeration cycle (or a heat pump cycle) to meet the cooling (or heating) load. In this example, the compression device 12 is a compressor 12 for compressing a gaseous refrigerant fluid, the heat rejection heat exchanger 14 is a condenser for at least partially condensing the refrigerant fluid, the expansion device 18 is an expansion valve for expanding the refrigerant fluid, and the heat absorption heat exchanger 16 is an evaporator for at least partially evaporating the refrigerant fluid. The refrigeration system can be advantageously arranged so that the fluid is completely condensed at the condenser 14 and completely evaporated at the evaporator 16. In many cases, it is beneficial to avoid the presence of liquid at the inlet of the compressor 12.

[0033] The example refrigeration system also includes a surge tank 20 attached to the main refrigerant circuit in parallel with the expansion device 18. The surge tank 20 provides a receiver / reservoir for the refrigerant fluid and is connected to the main refrigeration circuit via a first valve 22 and a second valve 24, the first valve being used to control flow to or from a higher pressure point on the refrigerant circuit and the second valve being used to control flow to or from a lower pressure point on the refrigerant circuit. Figure 1 As shown in , in this example, a surge tank 20 is connected to the main refrigeration circuit in parallel with the expansion device 18 , having a fluid connection to a higher pressure point before expansion via a first valve 22 , and to a lower pressure point after expansion via a second valve 24 .

[0034] The valves 22, 24 are controlled by a controller 26 to control the flow of refrigerant fluid between the main refrigerant circuit and the buffer tank 20. The controller 26 may also be used to control other elements of the refrigeration system, such as the compressor 12. The valves 22, 24 are controlled to divert refrigerant fluid between the main refrigerant circuit and the buffer tank 20 based on a measure of subcooling obtained from a subcooling sensor 28 in the main refrigerant circuit. In this example, the subcooling sensor 28 is placed on the refrigerant fluid path between the heat rejection heat exchanger (condenser) 14 and the expansion device (expansion valve) 18.

[0035] When the refrigerant pressure in the buffer tank 20 is lower or higher than the refrigerant pressure in the main refrigeration circuit, then opening the corresponding valves 22, 24 will allow the refrigerant fluid to be transferred to and from the buffer tank 20. For example, when it is desired to reduce the charge level in the main refrigeration circuit, the first valve 22 can be opened to fill the buffer tank 20 from a higher pressure point, and when it is desired to increase the charge level in the main refrigeration circuit, the second valve 24 can be opened to drain the buffer tank 20 to a lower pressure point. The first valve 22 is a solenoid valve 22, having an open state and a closed state. The second valve 24 is a valve 24 with a controllable opening, such as a PMV24, so as to allow the controller 26 to change the opening when the refrigerant fluid is transferred from the buffer tank 20 to the main refrigeration circuit. The controller 26 can control the opening of the second valve 24 to control the amount of liquid refrigerant fluid at the inlet of the compressor 12 so as to avoid any liquid refrigerant that may damage the compressor 12.

[0036] The refrigerant charge level is varied using valves 22, 24 when evaluated by a subcooling value obtained via a subcooling sensor 28 in order to control the subcooling within the main refrigerant circuit. In the case of excessive subcooling, the controller opens the first valve 22 to reduce the refrigerant charge. Some refrigerant will be stored in the receiver / buffer tank 20. In the case of a low subcooling value, the second valve 24 is opened so that the main refrigerant circuit will be refilled from the buffer tank 20. As described above, the opening of the second valve 24 can be controlled, for example, via the PMV 24 to avoid liquid at the compressor suction inlet. The thresholds for opening and closing the first valve 22 and the second valve 24 are based on a subcooling curve optimization that allows the refrigerant charge to be varied according to conditions and unit loads. As described above, this provides various advantages, including improved efficiency and the potential for a wider operating range. This also increases the available refrigerant charge and allows corrections to be made to maintain efficient operation when the refrigerant charge level decreases over time (i.e., during the use of the refrigerant system). Another advantage of this is a reduced maintenance burden.

[0037] It has been recognized that by allowing the control of varying refrigerant charge levels based on subcooling within the main refrigeration circuit, better control of optimal performance can be achieved. It is well known that there will be an optimal subcooling value for a given refrigeration circuit, depending on various factors, including the size of the compression device and the superheat value (usually a fixed value) selected for the expansion device 18. Operating at the optimal subcooling value will result in maximum efficiency for the refrigeration circuit. It is possible to assess the refrigerant charge level required to achieve this optimal value. Figure 2 Exemplary curves of efficiency versus refrigerant charge level are shown for three different operating conditions (eg, operating conditions associated with varying internal or external temperatures).

[0038] In a typical refrigeration system without the buffer tank 20 proposed herein, the refrigerant charge level is optimized for a specific condition, such as by Figure 2 , a peak is identified on the correlation curve of . However, this generally involves a compromise solution in which the charge level is set based on an estimate of the most likely condition and / or by evaluating a central condition within a range of multiple operating conditions. In actual operation, the refrigeration system will operate under many different conditions, with different external temperatures and different loads during operation. The performance of the refrigeration system is not optimized with respect to the refrigerant charge level for all those conditions that are different from the assumed operating conditions. As a result, existing systems always involve compromises, such as by deciding on a refrigerant charge of 6 kg. This is ideal for condition 3, but the efficiency of the refrigeration system will lose 10% for condition 1 and 5% for condition 2 (compared to the optimal values ​​for those conditions). In the case of using a buffer tank 20 as described above, the controller 26 can adjust the refrigerant charge by using valves 22, 24 so that the refrigerant charge level matches the optimal value for different operating conditions. The controller 26 can use any suitable control system to change the refrigerant charge level with the aim of maintaining the subcooling value at the subcooling sensor at an optimal level, which will therefore subsequently adjust the refrigerant charge level to track the changes in operating conditions. Figure 2 As described above, the controller 26 may allow for some hysteresis relative to the thresholds to be used during control of the valves 22, 24. Other control systems are also possible.

[0039] Refrigeration systems may include Figure 1 Other elements not shown in the drawings, such as economizer lines or other more complex additions to the refrigeration circuit, for example in order to adapt the refrigeration cycle to specific requirements. The refrigeration system may be operated to meet a refrigeration load, or it may be used as a heat pump to provide heating.

Claims

1. A refrigeration system, comprising: a main refrigeration circuit for maintaining a refrigerant fluid, the main refrigeration circuit comprising: a compression device, a heat rejection heat exchanger, an expansion device and a heat absorption heat exchanger; wherein the refrigeration system includes a surge tank attached to the main refrigeration circuit, having a valve for controlling the flow of refrigerant fluid between the main refrigeration circuit and the surge tank; and wherein the refrigeration system is arranged such that the valve is controlled to divert refrigerant fluid between the main refrigeration circuit and the buffer tank based on a measure of subcooling in the main refrigeration circuit; wherein the valve comprises: a first valve for controlling flow to or from a higher pressure point on the main refrigeration circuit, and a second valve for controlling flow to or from a lower pressure point on the main refrigeration circuit; Features: The second valve is a valve having a controllable opening degree, and the refrigeration system is arranged to change the opening degree when the refrigerant fluid is transferred from the buffer tank to the main refrigeration circuit; and The refrigeration system is arranged such that control of the opening of the second valve is used to control the amount of liquid refrigerant fluid at the inlet of the compression device.

2. The refrigeration system according to claim 1, characterized in that: The surge tank is connected to the main refrigeration circuit in parallel with the expansion device, having fluid connections to a higher pressure point before expansion, and a lower pressure point after expansion.

3. The refrigeration system according to claim 1 or 2, characterized in that: The refrigeration system comprises a subcooling sensor for obtaining a measure of the subcooling.

4. The refrigeration system according to claim 3, characterized in that: The subcooling sensor is located on the main refrigeration circuit after the heat rejection heat exchanger and before the expansion device.

5. The refrigeration system according to claim 1 or 2, characterized in that: The refrigeration system includes a controller for controlling the valve, wherein the controller is configured such that when the subcooling measure indicates that there is excessive subcooling, refrigerant fluid is directed from the main refrigeration circuit to the buffer tank, and when there is insufficient subcooling, refrigerant fluid is emptied from the buffer tank into the main refrigeration circuit to refill the main refrigeration circuit.

6. The refrigeration system according to claim 1, characterized in that: The refrigeration system is arranged so that when it is desired to reduce the charge level in the main refrigeration circuit, the first valve is opened to fill the buffer tank from the higher pressure point, and when it is desired to increase the charge level in the main refrigeration circuit, the second valve is opened to empty the buffer tank to the lower pressure point.

7. The refrigeration system according to claim 1 or 2, characterized in that: The expansion device is arranged to provide a controllable degree of expansion, and the refrigeration system is arranged to control the degree of opening of the expansion device in response to changes in the behavior of the main refrigeration circuit in accordance with adjustments to the charge level through use of the buffer tank.

8. A method for operating a refrigeration system according to any preceding claim, the method comprising controlling the valve to divert refrigerant fluid between the main refrigeration circuit and the buffer tank based on a measure of subcooling in the main refrigeration circuit.

9. The method according to claim 8, characterized in that Control of the valve is performed to vary the refrigerant charge level of the primary refrigeration circuit to allow one or more of: improving efficiency during changing operating conditions, expanding the operating range of the refrigerant system, and / or regulating over time when refrigerant is depleted.

10. The method according to claim 8 or 9, characterized in that: The method includes controlling the valve for directing refrigerant fluid from the main refrigeration circuit into the buffer tank when excessive subcooling exists, and for directing refrigerant fluid from the buffer tank into the main refrigeration circuit when insufficient subcooling exists.

11. The method according to claim 8 or 9, characterized in that: The method comprises: When there is excessive subcooling, the first valve is opened and the second valve is closed; When insufficient subcooling exists, opening the second valve while the first valve is closed; and When it is desired to maintain the refrigerant charge level of the main refrigeration circuit unchanged, both the first valve and the second valve are kept closed.

12. The method according to claim 8 or 9, characterized in that: The method includes varying the opening of the corresponding valve while the refrigerant fluid is transferred from the buffer tank to the main refrigeration circuit so as to control the amount of liquid refrigerant fluid at the inlet of the compression device.

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