Method for operating a supercritical refrigerant circuit

Abstract

The present invention relates to a method for operating a supercritical refrigerant circuit (10), in particular for a motor vehicle. The refrigerant circuit (10) comprises a main circuit (12) with a compressor (14), an electronically controlled and lockable first expansion valve (18), a gas cooler (16) for heating a medium to be heated, and at least one pressure sensor (26B), which are arranged downstream of the compressor (14) and upstream of the first expansion valve (18), a chiller (20) and / or an evaporator (22) for cooling a medium to be cooled, which are arranged downstream of the first expansion valve (18) and upstream of the compressor (14), and a refrigerant receiver (24) downstream of the chiller (20) and / or evaporator (22) and upstream of the compressor (14).The refrigerant circuit (10) further comprises at least a first branch (30) with a lockable second expansion valve (32), which branches off downstream from the compressor (14) and upstream from the gas cooler (16) and is connected to the refrigerant receiver (24), wherein the pressure between compressor (14) and first expansion valve (18), preferably between compressor (14) and gas cooler (16), is determined.The refrigerant circuit (10) is characterized in that, when it is detected that the pressure downstream of the compressor (14) and upstream of the first expansion valve (18) is below a specified first pressure value (P1), the main circuit (12) is shut down, in particular by closing the first expansion valve (18) and opening the second expansion valve (32), so that the refrigerant flow downstream of the compressor (14) is essentially completely routed through the first branch (30) to be drawn back into the compressor (14) via the refrigerant receiver (24).

Description

[0001] The present invention relates to a method for operating a supercritical refrigerant circuit, in particular for a motor vehicle.

[0002] Supercritical refrigerants are known as such. They are fluids that, during operation, partially pass through a pressure range above a so-called critical point, where no noticeable phase transition occurs during an isentropic or isothermal change.

[0003] Carbon dioxide, for example, has long been known as a supercritical refrigerant under the code R744. It is a non-flammable, odorless, non-corrosive, non-water polluting, chemically stable gas with a low global warming potential. It also has the advantage of being naturally available, non-toxic to humans in low concentrations, chemically inert, and therefore compatible with all common materials. Its use as a refrigerant presents some challenges because the critical point is typically exceeded on the high-pressure side of the refrigerant circuit, preventing condensation. Therefore, the pressure on the high-pressure side must be actively monitored and regulated by the expansion valve.

[0004] Battery-electric vehicles generate an insufficient amount of waste heat to meet the temperature requirements of the vehicle cabin, battery, or other components. Therefore, heat pump systems are commonly used for heating. At low ambient temperatures, the heating capacity is inadequate or may even become technically impossible or impractical.

[0005] To avoid costs for auxiliary electric heaters as much as possible, or at least keep them to a minimum, operating modes are known in which the additional electrical energy is provided by the compressor. Such operating modes are known as hot gas modes, in which the compressor is intentionally operated inefficiently to consume more electrical energy, which then enters the system as heat, as disclosed, for example, in US2019 / 0070924 A1. In this case, the refrigerant flow between the compressor and the gas cooler can be split into two separate branches, with the secondary branch being routed back to the compressor as a bypass.

[0006] For supercritical refrigerants such as R744, a special design and a special control strategy are required in the high-pressure range due to the above-mentioned properties in order to operate the system safely.

[0007] It is the object of the present invention to provide a method for operating a supercritical refrigerant circuit which solves the above-mentioned problems, particularly under low temperature conditions, and enables safe operation.

[0008] This problem is solved by a method with the features of claim 1. Advantageous further developments and refinements result from the dependent claims.

[0009] The supercritical refrigerant circuit according to the present disclosure comprises a main circuit with a compressor and an electronically controlled and closable first expansion valve, between which a high-pressure section is formed downstream of the compressor and upstream of the first expansion valve. At least one gas cooler for heating a medium to be heated and at least one pressure sensor are arranged in the high-pressure section. The main circuit further comprises a chiller and / or an evaporator for cooling a medium to be cooled, which are arranged downstream of the first expansion valve and upstream of the compressor in a low-pressure section. An external heat exchanger can also be connected downstream of the first expansion valve and downstream of the compressor to operate as an evaporator in heat pump mode, as is familiar to those skilled in the art.Furthermore, a refrigerant collector is provided downstream of the chiller and / or evaporator and upstream of the compressor. Furthermore, the refrigerant circuit has at least one first branch with a closable second expansion valve, which branches off downstream of the compressor and upstream of the gas cooler and is connected to the refrigerant collector. This provides a hot gas bypass.

[0010] The medium to be heated by the gas cooler is arbitrary. The gas cooler can be a refrigerant-to-air heat exchanger that regulates the temperature of an air stream. It can also be a refrigerant-to-coolant heat exchanger, in which case the coolant then either interacts with the area or object to be cooled (e.g., electrical and electronic components in the vehicle) or, in turn, regulates the temperature of an air stream via a coolant-to-air heat exchanger. The target area to be cooled can, in particular, be a passenger cabin, but is not limited to this.

[0011] Typically, several heat exchangers can be arranged in the low-pressure section. If these are connected in parallel branches, a separate (third, fourth, and / or fifth) electronically controlled, lockable expansion valve can be provided upstream for each of the heat exchangers, in particular instead of the first expansion valve. In particular, when shutting down the main circuit, all of these parallel branches must be capable of being shut down via the respective expansion valves (or separately provided switching valves).

[0012] Optionally, a second lockable branch can be provided, branching off downstream from the gas cooler and containing the outdoor heat exchanger. The outdoor heat exchanger is thus not connected to the first branch (hot gas bypass). Depending on the operating mode, the outdoor heat exchanger can be configured as a gas cooler for heat dissipation in the high-pressure section (AC operation) or as an evaporator for heat recovery in the low-pressure section (heat pump operation).

[0013] In the method according to the invention for operating the refrigerant circuit, the pressure between the compressor and the first expansion valve, preferably between the compressor and the gas cooler, is determined. This pressure in the high-pressure section generally drops only slightly downstream, so that calculations and / or simulations can determine the pressure at a suitable point in the high-pressure section, if necessary, allowing conclusions to be drawn about the pressure at another point. In particular, the pressure is determined immediately downstream of the compressor outlet. The method according to the invention is characterized in that, if it is detected that the pressure downstream of the compressor and upstream of the first expansion valve is below a predetermined first pressure value, the main circuit is shut down, wherein in particular the first expansion valve is closed and the second expansion valve is opened.In this phase 1 of hot gas operation, the refrigerant flow downstream of the compressor is essentially completely directed through the first branch to be sucked back into the compressor via the refrigerant receiver.

[0014] The inventors recognized that, unlike subcritical systems, such operation is safely possible if the pressure, particularly in the high-pressure section, is appropriately controlled and a refrigerant receiver is located directly on the suction side of the compressor. In subcritical systems, the compressor's suction pressure can drop below ambient pressure for extended periods during start-up at low ambient temperatures, resulting in sealing problems and making such a process unsafe to operate. Such a problem does not exist, or is far less so, with supercritical refrigerants (particularly R744). The refrigerant receiver directly on the suction side of the compressor ensures that refrigerant is quickly available for hot-gas operation during start-up and / or at low temperatures and that it flows through the system in all circumstances.

[0015] The first pressure value (and the additional pressure values ​​introduced below) are determined depending on the situation. They are not fixed values, but rather lie within a predetermined corridor or band. A pressure value determined under specific conditions can also consist of a pair of pressure values, e.g., to account for system hysteresis and measurement error tolerances. This is well known to those skilled in the art as a dead band and amounts to, for example, 1-3 bar around the target value.

[0016] The first pressure value P1 is typically in the critical pressure range. Below the critical pressure, it can be assumed that the gas cooler cannot dissipate sufficient heat to achieve the heating capacity, as the compressor's waste heat is too low. Shutting down the main circuit helps the system quickly start up via hot gas operation.

[0017] The necessity or expediency of switching to hot gas operation or starting the system with it can already be recognized from other operating conditions. For example, a performance gap may have been detected before the main circuit was shut down (or the first expansion valve was closed) and / or the second expansion valve was opened. This allows a proactive switch to hot gas operation to be recognized as a further indication. The performance gap is known to the person skilled in the art and results from the difference between the power requirement for providing the heating function via the gas cooler and the heat output available via the chiller and / or evaporator (whereby any evaporator such as a cabin evaporator in the air conditioning unit or the outdoor heat exchanger may be considered as an evaporator), plus the waste heat from the compressor released into the refrigerant circuit.This can be obtained in a conventional manner from tests, simulations or other calculations.

[0018] After shutting down the main circuit (or closing the first expansion valve) and / or opening the second expansion valve, the compressor speed is then ramped up according to the present disclosure. Ramping up can occur according to any predetermined pattern, e.g., linearly, gradually, or asymptotically, up to a maximum or maximum specified speed.

[0019] According to an advantageous embodiment of the present disclosure, the first expansion valve is opened when the pressure in the high-pressure section (i.e., between the compressor and the first expansion valve, or better, between the compressor and the gas cooler) exceeds the first pressure value. If the first pressure value or pressure value corridor is selected such that, for example, the critical point of the supercritical refrigerant lies within this value range, then, starting from this pressure value ("phase 2"), an attempt can be made step by step to transfer heat to the medium to be heated via the gas cooler. This pressure value is advantageously also determined, among other things, by a pressure difference at the compressor (i.e., between the high-pressure level and the low-pressure level).

[0020] Advantageously, the first expansion valve is opened only cautiously and gradually. Electrically switchable expansion valves typically have a defined minimum opening. When the first pressure value is exceeded, the first expansion valve is then only partially opened, preferably this minimum opening. This prevents the pressure in the high-pressure section, particularly at the gas cooler, from suddenly dropping again, which would cause the control to oscillate back and forth.

[0021] In this phase 2 of hot gas operation, additional control mechanisms can advantageously be provided to support a controlled ramp-up to continuous operation. General control mechanisms (actual value acquisition, deviation from the target value, and readjustment of the manipulated variable) are known per se and can be readily applied here.

[0022] After the first expansion valve opens, the compressor speed is increased further, according to one embodiment, up to a maximum or maximum defined speed D-max, if not already reached. The degree of opening of the first expansion valve remains unchanged as long as the pressure downstream of the compressor and upstream of the first expansion valve (or upstream of the gas cooler) is below a specified second pressure value P2. The second pressure value P2 is particularly related to a technical limit value of the system pressure P2', which is determined by the system components and must not be exceeded for safety reasons. The second pressure value P2 is, for example, slightly below this limit value P2'.

[0023] The opening degree of the first expansion valve is increased when the pressure downstream of the compressor and upstream of the first expansion valve (or upstream of the gas cooler) exceeds the specified second pressure value P2. Further opening of the first expansion valve initially causes the pressure in the high-pressure section to drop. In other words, this control mechanism consists in regulating the pressure in the high-pressure region via the opening degree of the first expansion valve. With an electronically controlled first expansion valve, typically only the next larger opening degree is set step by step, although with a dynamic pressure increase, an arbitrarily larger opening degree can also be set.If necessary, an iteration loop can be used to ensure that counteraction is taken by slightly closing the first expansion valve if the pressure drops by more than one tolerance from the first pressure limit.

[0024] Furthermore, according to one embodiment, a third pressure value P3 is defined for the pressure on the suction side of the compressor, and the pressure on the suction side of the compressor is recorded. The opening degree of the second expansion valve is then reduced if the pressure on the suction side of the compressor exceeds the third pressure value P3. The third pressure value P3 is advantageously in a range slightly above the vapor pressure of the supercritical refrigerant used. In the case of R744, the pressure value is typically between 20 and 45 bar (the vapor pressure of R744 is 19.7 bar at -20°C and 45 bar at +10°C). In other words, the opening degree of the second expansion valve regulates the pressure in the low-pressure section.

[0025] Furthermore, according to a further embodiment, a first temperature value is defined for the temperature at the outlet of the gas cooler, and the temperature at the outlet of the gas cooler is measured. The compressor speed is reduced if the temperature at the outlet of the gas cooler exceeds the first temperature value. This allows the heating output to be adjusted. If sufficient thermal energy is available, the input of thermal energy by the compressor can be gradually reduced by lowering the compressor speed. In other words, the compressor speed regulates the temperature at the outlet of the gas cooler.

[0026] The temperature value can typically be defined as a target value based on the air temperature flowing through the gas cooler. However, it can also be determined from measured values ​​obtained in the refrigerant circuit, at the air outlet of the area to be conditioned (e.g., the vehicle air conditioning system), and / or possibly in intermediate secondary coolant circuits, for example, by means of estimates or calculations taking into account the specific system design.

[0027] The compressor is conveniently operated at a speed within a target range, which in common systems is typically 4000–6000 rpm. The pressure in the high-pressure range does not need to be further adjusted as long as the compressor operates within this target range. If the compressor speed has left the desired target range for a certain period of time, the high-pressure target value must be adjusted. For this purpose, an upper threshold D1 and a lower threshold D2 of the compressor speed, as well as an optimal compressor speed D3, can be defined, which is lower than the upper threshold D1 and higher than the lower threshold D2.

[0028] Depending on the variant, the compressor speed D may only be ramped up to this upper threshold D1, without exploiting the compressor's full performance potential. Furthermore, the optimal compressor speed D3 may coincide with or be very close to one of the upper or lower thresholds D1, D2.

[0029] The second pressure value P2 is reduced if the pressure falls below the lower threshold D2 for a predetermined period of time, whereby the opening degree of the second expansion valve is increased. The second pressure value P2 is increased if the upper threshold D1 has been exceeded for a predetermined period of time, whereby the opening degree of the second expansion valve is reduced. Changing the opening degree of the second expansion valve indirectly causes the pressure in the high-pressure section to change. When the second expansion valve opens, the pressure difference between the high-pressure section and the low-pressure section decreases, which is why the compressor tends to rotate faster (and conversely, rotates slower if the pressure difference increases with the closing of the second expansion valve).

[0030] The high-pressure target value can now be adjusted depending on the compressor speed, as described above, by changing the opening angle of the first expansion valve. Furthermore, additional heat exchangers can be gradually added, such as the second low-pressure heat exchanger (chiller or evaporator, if not already connected) and / or an outdoor heat exchanger. When the evaporator is connected, dehumidification operation can also be implemented, in which the air flow for the area to be air-conditioned is first cooled (and thus dehumidified) and then reheated downstream as it passes through the gas cooler.

[0031] According to a further development, the outdoor heat exchanger can be connected to a fifth expansion valve arranged upstream between the gas cooler and the chiller and / or evaporator in order to dissipate excess heat to the environment. This allows the thermal energy required to provide the heating function to be balanced in the short term so that neither too much nor too little thermal energy is available. Depending on the operating point and ambient conditions (e.g., at an outside temperature of 10 - 15°C), a point may arise at which normal dehumidification would generate too little thermal energy, but the heating function disclosed here would generate too much for reheating. In this case, optimal comfort can be achieved by dissipating the excess thermal energy.

[0032] The invention will now be explained in more detail using exemplary embodiments and with reference to the figures. The Fig. 1 and Fig. 2 schematically show refrigerant circuits which can be operated with the method according to the invention, the Fig. 3 to 5 show schematic flow diagrams of a first, second and third phase of the hot gas operation according to an embodiment of the method according to the invention and the Fig. 6 and Fig. Figure 7 schematically shows various measurement curves of the pressure conditions in the high-pressure section and low-pressure section, the temperature conditions at the gas cooler, and the speed development of the compressor, as exemplified over the three phases according to Fig. 3 to 5 were measured according to an embodiment of the invention.

[0033] In the Fig. 1 and Fig. Figure 2 schematically depicts refrigerant circuits 10, which are associated with the method according to the invention and can be operated by such a method. The refrigerant circuit 10 consists of Fig. 1 is a simplified form of the refrigerant cycle 10 of the Fig. 2 with the components essential for the method according to the invention. The use of the refrigerant circuit 10 is intended in particular for a motor vehicle air conditioning system, but is not limited thereto.

[0034] The refrigerant circuit 10 is designed for operation with a supercritical refrigerant, in particular carbon dioxide (R744), and comprises a main circuit 12 and a secondary circuit 30. The main circuit 12 contains, in series, a compressor 14, a gas cooler 16, a first electronically controllable, fully shut-off first expansion valve 18, and at least one low-pressure heat exchanger as a chiller 20, evaporator 22, or external heat exchanger 34. Downstream of this is a low-pressure refrigerant collector 24, via which the main circuit 12 of the refrigerant circuit 10 closes to the suction side of the compressor 14. The area downstream of the compressor 14 and upstream of the first expansion valve 18 is the high-pressure section, while the area downstream of the first expansion valve 18 and upstream of the compressor 14 is the low-pressure section.If at least two of the chiller 20, evaporator 22 and / or external heat exchanger 34 are connected in the low-pressure section, these are each connected to their own expansion valves 38A - 38C, as described in connection with the . Fig. 2 is shown.

[0035] Upstream and downstream of the compressor 14, e.g., directly at the suction-side inlet and at the high-pressure-side outlet of the compressor 14, there are one pressure sensor each 26A, 26B, as well as a further pressure sensor 26C (optionally a combined pressure-temperature sensor) near the gas cooler 16. The pressure in the high-pressure section downstream of the compressor 14 can be continuously determined and monitored via the pressure sensors 26B and 26C. The gas cooler 16 is a refrigerant-air heat exchanger for heating an air stream for a passenger cabin of a motor vehicle (not shown), with a temperature sensor 28 arranged downstream in the air stream in order to be able to record the temperature conditions at the gas cooler 16 and in the air stream flowing through it.

[0036] The sensors 26A - 26C, 28 and possibly further sensors (not shown) as well as the electrically controlled components (compressor 14, first and second expansion valve 18, 32 and possibly further components) are connected in a control circuit with a control unit (not shown) and can be operated according to the method according to the invention, as will be described below with reference to the Fig. 3 to 7 will be explained in more detail.

[0037] The secondary circuit 30 branches off from the main circuit 12 between the compressor 14 and the gas cooler 16 and has a second electronically controlled expansion valve 32. The secondary circuit 30 is then fed back into the main circuit 12 downstream of the low-pressure heat exchanger (chiller 20 and / or evaporator 22) and thus flows into the refrigerant receiver 24.

[0038] In the Fig. 2 is an advantageous configuration to that shown in the Fig. The basic configuration shown in 1 is illustrated according to an exemplary embodiment. In addition to the features related to the Fig. 1, the refrigerant circuit 10 in the low-pressure section has the chillers 20 and evaporator 22 arranged in parallel to one another, each with an upstream, separately controllable third and fourth expansion valve 38A, 38B. The chiller 20 is in heat-exchanging contact with a coolant circuit 50. Downstream of the gas cooler 16, an external heat exchanger 34 can be connected via a fifth expansion valve 38C, which is located in a separate secondary branch that is not connected to the first secondary circuit 30. Depending on requirements, during normal heat pump operation (i.e. with heat absorption from the ambient air), the external heat exchanger 34 and possibly also the chiller 20 and / or the evaporator 22 in the low-pressure section flow through one another in parallel.

[0039] Downstream of the external heat exchanger 34, the high-pressure part of an internal heat exchanger 36 can optionally be flowed through, which then opens into an upstream area shortly before the first expansion valve 18. The low-pressure part of the internal heat exchanger 36 is flowed through between the refrigerant collector 24 and the suction side of the compressor 14. Additional shut-off valves 40A - 40D as well as check valves 40E, 40F are available to enable the various connections of the refrigerant circuit 10 for different operating modes (e.g., heat pump operation, cooling operation, and dehumidification operation), as is known per se or can be derived from the prior art. Among other things, valve 40D must be open and valve 40B closed if, in pure ambient heat pump operation, energy is only to be absorbed via the external heat exchanger 34, whereby the fifth expansion valve 38C then sets the pressure stage between the high-pressure and low-pressure sections.

[0040] The method according to the invention will now be described using an exemplary embodiment and with reference to the Fig. 3 to 7 are explained in more detail. Fig. Figures 3 to 5 are schematic flowcharts of a first, second and third phase of hot gas operation with the refrigerant R744 (carbon dioxide), while with reference to the Fig. 6 and Fig. 7 schematically and by way of example, various associated measurement curves of the pressure conditions in the high-pressure section and low-pressure section, the temperature conditions at the gas cooler 16, the speed development D of the compressor 14 and the opening degrees of the first expansion valve 18 and second expansion valve 32 (if necessary, by way of example, one of the expansion valves 38A, 38B, if both the chiller 20 and the evaporator 22 are provided) are shown.

[0041] The system predetermines several pressure values ​​P1 - P3, a temperature value T1 (target temperature at gas cooler 16), and several speed values ​​D1 - D3 and D-max. These are set with a variable component depending on external conditions, so that they can be constant for given ambient conditions, but also variable within a range of values, depending on the ambient temperature and other parameters. The value ranges in the example are as follows: P1: 70 - 75 bar (in the range of the critical pressure ~73 bar of R744) P2: 100 - 120 bar (above P1, below max. permissible system pressure) P3: 20 - 45 bar (in the range of the saturation pressure of R744) T1: 40 - 60 °C (to achieve a heating effect in the vehicle cabin) D-max: 7000 rpm (maximum permissible continuous speed) D1: 6000 rpm (upper desired speed limit) D2: 4000 rpm (lower desired speed limit) D3: 5000 rpm (optimal speed)

[0042] At system startup, 100 in the Fig. 3, it is checked whether the refrigerant circuit 10 can be started in the usual heat pump mode with thermal energy absorption via the external heat exchanger 34 and, if applicable, via the chiller 20. For this purpose, the power requirement for providing the heating function, which is to be provided via the gas cooler 16, is recorded and the extractable thermal energy from the potential heat sources (chiller 20, evaporator 22, external heat exchanger 34, in evaporator mode) as well as the heat output of the compressor 14 delivered into the refrigerant circuit 10 is estimated. The calculation of the required heating output and the available heat output can be carried out in any desired manner, in particular based on pressure conditions in the high and low pressure sections, the outside temperature and the target temperature of the air in the room to be air-conditioned. The power balance in heat pump mode is the difference between the sum of the thermal outputs of the heat sources (i.e.the heat exchanger in the low-pressure range, in particular chiller 20, evaporator 22, external heat exchanger 34 in low-pressure operation) and the additional waste heat via the compressor 14 on the one hand and the power requirement via the gas cooler 16 on the other hand, as is familiar to the person skilled in the art.

[0043] A power gap exists when the power balance is negative, i.e. more heat output is requested via the gas cooler than can be provided via the other heat exchangers and the compressor 14. If this is the case, or if the pressure detected via the pressure sensor 26B in the high-pressure section is below a specified pressure value P1, a special operating mode ("hot gas mode / loss mode") is started in phase 1 in step 110. For this purpose, the first expansion valve 18 is completely closed and the second expansion valve 32 is opened. As a result, the refrigerant flow downstream of the compressor 14 is essentially completely directed through the first secondary branch 30 in order to be sucked in again by the compressor 14 via the refrigerant receiver 24. Alternatively, the power gap alone can also be used as a decision criterion as to whether the special operating mode needs to be started.

[0044] If there is no performance gap or the pressure at the pressure sensor 26B is high enough, the heat pump mode can be started normally (not shown).

[0045] In step 120, the compressor speed D is ramped up linearly according to a defined rate (typically, for example, an increase in the range of 200-400 rpm per second). If the compressor speed D already reaches the maximum permissible continuous speed D-max during this phase, a flag is set and the speed is not ramped up further linearly, but is kept constant during this step.

[0046] In step 130, it is checked whether the first pressure value P1 has been reached. If not, a timer is set in step 140, e.g., in the range of 0.5 to 2 seconds, and then the condition is checked again in step 130. If the condition is met after the timer expires, step 200 transitions to phase 2. As long as the speed flag has not yet been set, the compressor speed continues to increase as described above up to the permissible maximum speed D-max (not shown).

[0047] Phase 2 will now be carried out with reference to the Fig. 4. In step 200, the first expansion valve 18 is opened to its minimum opening. This temporarily dampens the pressure increase at pressure sensor 28B because the refrigerant flow can partially circulate in the main circuit 12.

[0048] The temperature at the outlet of the gas cooler 16 is recorded in a step 210. The temperature value T1 is variably set for the outlet of the gas cooler 16. The temperature value is a target value and is typically defined by the air temperature of the air flowing through the gas cooler 16, but can also be determined using measured values ​​obtained in the refrigerant circuit and / or at the air outlet of an air conditioning system, etc., and corresponding estimates / calculations. Based on the system components, a model or simulation can be used to estimate at which temperature value T1 a target temperature is reached at the air outlet to the cabin. These values ​​are typically stored in a control curve in the control unit.

[0049] If the temperature at the outlet of the gas cooler 16 has not yet exceeded the first temperature value T1, a check is performed in step 220 to determine whether the pressures measured by the pressure sensors 26A in the low-pressure section and 26B in the high-pressure section have exceeded certain preset pressure values. If this is not yet the case, a timer is set in step 230, e.g., in the range between 1 and 5 seconds.

[0050] Exceeding the specified pressure values ​​is specified as follows: For the high-pressure section, the specified pressure value is the second pressure value P2 (here specifically 110 bar), for the low-pressure section, the specified pressure value is the third pressure value P3 (here specifically 35 bar).

[0051] As long as the pressure measured at pressure sensor 26B in the high-pressure section is below the specified second pressure value P2, the opening degree of the first expansion valve 18 remains unchanged. If the pressure at pressure sensor 26A exceeds the specified second pressure value P2, the opening degree of the first expansion valve 18 is increased. If the low-pressure region exceeds the third pressure value P3, the opening degree of the second expansion valve (32) is reduced. The first expansion valve 18 is closed slightly again if the pressure drops below the pressure value P2 again. The process is then repeated at step 210.

[0052] If the temperature value T1 is reached in step 210 and the pressures no longer rise in step 220, the process transitions to phase 3 in step 300.

[0053] The compressor is conveniently operated at a speed within a target range, which in common systems is typically 4000–6000 rpm. The pressure in the high-pressure range does not need to be further adjusted as long as the compressor operates within this target range. If the compressor speed D leaves the desired target range for a certain period (alternatively or additionally for a certain deviation), the high-pressure target value must be adjusted. For the compressor speed D, a lower threshold value D2 = 4000 rpm and an upper threshold value D1 = 6000 rpm, as well as an optimal compressor speed D3 = 5000 rpm, are defined.

[0054] In phase 3 with reference to the Fig. 5, in a first control loop in step 310 it is checked whether the compressor speed D continues to be within this range. If this is the case (step 320), the compressor speed D is checked again after a timer has expired in step 370, e.g. after 10 to 30 seconds in step 310. If it is determined in step 330 that the upper threshold value D1 has been exceeded for the time period specified by the timer, the degree of opening of the second expansion valve 32 is reduced in step 340. The second pressure value P2 then increases, which counteracts the increase in the compressor speed D. If, on the other hand, it was determined in step 350 that the lower threshold value D2 has been exceeded for the time period specified by the timer, the degree of opening of the second expansion valve 18 is increased in step 360. The second pressure value P2 then decreases, which counteracts a further drop in the compressor speed D.After the timer expires in step 370, the iteration loop is run through again at 310.

[0055] In general, changing the opening degree of the second expansion valve 32 causes the pressure difference between the high-pressure and low-pressure sections to change, thus correcting the compressor speed D accordingly in the desired direction. At a lower pressure difference, the compressor must rotate faster, and conversely, at a higher pressure difference, it must rotate slower. The speed values ​​and other control values ​​can be averaged over the period of the corresponding timer.

[0056] The different effects of the control according to the specified procedure on the different system values ​​can be seen from the Fig. 6 and Fig. 7 is evident. In the Fig. 6 shows by way of example how a permanently high compressor speed D at or slightly above the threshold value D1 is adjusted to the optimum compressor speed D3 by slightly increasing the pressure in the high-pressure section within the permissible range by reducing the degree of opening of the second expansion valve 32.

[0057] Out of Fig.7 shows in particular how, after the compressor 14 has initially started up, the compressor speed initially rises to D-max, while in parallel the pressure in the high-pressure section rises via P1 to P2, before then being stabilized in this range. When the pressure value P1 is exceeded, the first expansion valve 18 (designated EXV1) is then opened minimally for the first time and then further, causing the temperature at the gas cooler 16 to rise abruptly at first and then more slowly until it settles at the target temperature T1 (here 40°C). The compressor speed D can then be reduced in order to operate essentially in the desired corridor between D1 and D2. After the gradual increase in pressure in the low-pressure section, it later reaches the pressure value P3, whereupon this is also stabilized by successive closing of the second expansion valve 32 (designated EXV2).

[0058] In phase 3, the system has now reached a stable state. Gradually, it can be checked whether it is possible to switch back to normal heat pump operation or whether additional heat exchangers can be used to balance the thermal management system. During this phase, it can also be provided that the external heat exchanger 34 between the gas cooler 16 and the chiller 20 and / or evaporator 22 can be switched on, e.g. in a separate branch between the fifth expansion valve 38C and the high-pressure section of the internal heat exchanger 36, in order to dissipate excess heat to the environment. Using the fifth expansion valve 38C, an intermediate pressure can be set between the gas cooler 16 and the low-pressure section so that the heat released via the external heat exchanger 34 can be regulated. For this purpose, the valves 40B and 40D are closed. LIST OF REFERENCE SYMBOLS 10 Refrigerant circuit 12 Main circuit 14 Compressor 16 gas coolers 18 first expansion valve 20 Chiller 22 evaporators 24 refrigerant collectors 26A - 26C Pressure sensors 28A temperature sensor 30 secondary circulation 32 second expansion valve 34 outdoor heat exchangers 36 Internal heat exchanger 38A - 38C third to fifth expansion valve 40A - 40D shut-off valves 40E, 40F check valves 50 Coolant circuit 100 - 400 process steps P1 - P4 pressure ranges T1 temperature value D Compressor speed D1 - D3 Speed ​​ranges QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 2019 / 0070924 A1

[0005]

Claims

[1] Method for operating a supercritical refrigerant circuit (10), in particular for a motor vehicle, wherein the refrigerant circuit (10) comprises - a main circuit (12) with - a compressor (14), - an electronically controlled and lockable first expansion valve (18), - a gas cooler (16) for heating a medium to be heated and at least one pressure sensor (26B), which are arranged downstream of the compressor (14) and upstream of the first expansion valve (18), - a chiller (20) and / or an evaporator (22) for cooling a medium to be cooled and / or an external heat exchanger (34), which are arranged downstream of the first expansion valve (18) and upstream of the compressor (14), and - a refrigerant collector (24) downstream of the chiller (20) and / or evaporator (22) and upstream of the compressor (14), - at least one first branch (30) with a lockable second expansion valve (32), which branches off downstream of the compressor (14) and upstream of the gas cooler (16) and is connected to the refrigerant collector (24), - wherein the pressure between the compressor (14) and the first expansion valve (18), preferably between the compressor (14) and the gas cooler (16), is determined, characterized by that, when it is detected that the pressure downstream of the compressor (14) and upstream of the first expansion valve (18) is below a predetermined first pressure value (P1), - the main circuit (12) is shut down, in particular by closing the first expansion valve (18) and - the second expansion valve (32) is opened so that the refrigerant flow downstream of the compressor (14) is passed substantially completely through the first secondary branch (30) in order to be sucked back into the compressor (14) via the refrigerant collector (24). [2] Method for operating a refrigerant circuit (10) according to claim 1, characterized by that a performance gap has been detected before the main circuit (18) is shut down and / or the second expansion valve (32) is opened. [3] Method for operating a refrigerant circuit (10) according to claim 1, characterized by that after shutting down the main circuit (18) and / or opening the second expansion valve (32), the compressor speed is increased [4] Method for operating a refrigerant circuit (10) according to one of claims 1 to 3, characterized bythat when the pressure between the compressor (14) and the first expansion valve (18) exceeds the first pressure value (P1), the first expansion valve (18) is opened. [5] Method for operating a refrigerant circuit (10) according to claim 4, characterized by , that - a minimum opening is defined for the first expansion valve (18) and - when the first pressure value (P1) is exceeded, only a partial opening, preferably the minimum opening, is set at the first expansion valve (18). [6] Method for operating a refrigerant circuit (10) according to one of claims 4 or 5, characterized by , that - the compressor speed is increased further to a maximum defined speed (D-max) after the first expansion valve (18) has opened, - the degree of opening of the first expansion valve (18) remains unchanged as long as the pressure downstream of the compressor (14) and upstream of the first expansion valve (18) is below a predetermined second pressure value (P2), and - the degree of opening of the first expansion valve (18) is increased when the pressure downstream of the compressor (14) and upstream of the first expansion valve (18) exceeds the predetermined second pressure value (P2). [7] Method for operating a refrigerant circuit (10) according to one of claims 4 to 6, characterized by , that - a third pressure value (P3) is set for the pressure on the suction side of the compressor (14) and the pressure on the suction side of the compressor (14) is recorded and - the degree of opening of the second expansion valve (32) is reduced if the pressure on the suction side of the compressor (14) exceeds the third pressure value (P3). [8] Method for operating a refrigerant circuit (10) according to one of claims 4 to 7, characterized by , that - a first temperature value (T1) is set for the temperature at the outlet of the gas cooler (16) and the temperature at the outlet of the gas cooler (16) is recorded and - the compressor speed (D) is reduced when the temperature at the outlet of the gas cooler (16) exceeds the first temperature value (T1). [9] Method for operating a refrigerant circuit (10) according to one of claims 4 to 8, characterized by , that - an upper threshold (D1) and a lower threshold (D2) of the compressor speed as well as an optimal compressor speed (D3) are defined, which is greater than the upper threshold (D1) and greater than the lower threshold (D2), - the second pressure value (P2) is reduced if the lower threshold value (D2) has been undershot for a predetermined period of time, whereby the degree of opening of the second expansion valve is increased, and - the second pressure value (P2) is increased if the upper threshold value (D1) has been exceeded for a predetermined period of time, the degree of opening of the second expansion valve (32) being reduced. [10] Method for operating a refrigerant circuit (10) according to one of claims 4 to 9, characterized by that the external heat exchanger (34) between the gas cooler (16) and chiller (20) and / or evaporator (22) can be switched on in order to dissipate excess heat to the environment.

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