Method for operating a refrigerant circuit and motor vehicle

Abstract

Method for operating a refrigerant circuit (10) for a motor vehicle (12), in which at least a first compressor (14) and a second compressor (16) jointly or each individually pump a refrigerant through the refrigerant circuit (10), and in which the first compressor (14) has a lower maximum delivery capacity than the second compressor (16), wherein at least during a first period of operation of the refrigerant circuit (10) the second compressor (16) is operated alone when a mass flow rate of the refrigerant to be pumped through the refrigerant circuit (10) is greater than a mass flow rate of the refrigerant that can be achieved by operating the first compressor (14) alone at its maximum delivery capacity, characterized in thatthat when the first compressor (14) is operated alone at its maximum delivery capacity, the mass flow rate of the refrigerant conveyed through the refrigerant circuit (10) is essentially equal to the mass flow rate of the refrigerant conveyed through the refrigerant circuit (10) when the second compressor (16) is operated alone at its minimum delivery capacity.

Description

The invention relates to a method for operating a refrigerant circuit for a motor vehicle, in which at least a first compressor and a second compressor jointly or individually circulate a refrigerant through the refrigerant circuit. The first compressor has a lower maximum delivery rate than the second compressor. Furthermore, the invention relates to a motor vehicle with a refrigerant circuit. JP 2020 083 111 A describes a vehicle air conditioning system with a main compressor and a parallel auxiliary compressor, which has a lower delivery capacity than the main compressor. In a low-capacity operating mode, refrigerant is supplied only by the auxiliary compressor. In a medium-capacity operating mode, refrigerant is supplied only by the main compressor. In a high-capacity operating mode, refrigerant is supplied by both the main and auxiliary compressors. The operating modes are switched under thermal load. JP 2002 130 890 A describes a refrigerated vehicle with a first compressor driven by a transport engine and a second compressor driven by an electric motor. When the refrigerated vehicle is moving, the first compressor is driven by the transport engine. When the refrigerated vehicle is parked, the second compressor is operated. US 2020 / 0298670 A1 describes an air conditioning system for a vehicle with a first compressor driven by the vehicle's engine to compress a refrigerant, and a second compressor driven by an electric motor. US Patent 4,757,694 A describes a cooling system in which an auxiliary compressor with a lower capacity is connected in parallel to a main compressor. EP 0 444 332 A1 describes a refrigeration system with multiple motor-driven compressors, each with a different delivery capacity. During operation, the refrigeration system initially starts a first compressor with a low delivery capacity. Subsequently, as the demand increases, this first compressor is switched off and a second compressor with a higher delivery capacity is simultaneously started. A disadvantage of this operating mode is that it results in undesirable, abrupt changes in operating conditions. This is because the first compressor is switched off and thus completely and suddenly taken out of service, while at the same time the second compressor is suddenly switched on and put into operation. DE 10 2005 009 173 A1 describes a refrigeration system with at least two refrigerant compressors, each of which has an additional compressor stage. The additional compressor stages can have different stroke volumes. Furthermore, DE 10 2005 016 094 A1 describes a refrigeration system with several screw compressors. The object of the present invention is to provide a method of the type mentioned at the outset which results in a particularly uniform conveyance of refrigerant, and to specify a motor vehicle designed for carrying out the method. This problem is solved by a method with the features of claim 1 and by a motor vehicle with the features of claim 9. Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims and in the following description. In the inventive method for operating a refrigerant circuit for a motor vehicle, at least a first compressor and a second compressor jointly or individually circulate a refrigerant through the refrigerant circuit. The first compressor has a lower maximum delivery capacity than the second compressor. At least during an initial period of operation of the refrigerant circuit, the second compressor operates alone when the mass flow rate of the refrigerant to be circulated through the refrigerant circuit is greater than the mass flow rate achievable by operating the first compressor alone at its maximum delivery capacity. This is based on the understanding that if a comparatively high demand for cooling capacity is anticipated from the refrigerant circuit, it makes sense to operate the second, more powerful compressor alone from the very start of commissioning. If, instead, the first compressor were initially operated alone, and its capacity is insufficient to provide the mass flow required by the refrigerant circuit, the second compressor would have to be added or operated jointly with the first compressor anyway to provide the required refrigerant mass flow. The associated effort, which would be required by first commissioning the first compressor and then the second, can be advantageously avoided in this case. This is because the second refrigerant compressor is activated or put into operation from the very beginning of the refrigerant circuit's commissioning to provide the mass flow that needs to be conveyed through the refrigerant circuit. This results in a particularly consistent refrigerant flow, as it avoids the operational fluctuations that can occur when switching the compressors on and off. The use of at least two compressors or refrigerant compressors, which are dimensioned differently with regard to the delivery volume or delivery capacity, also comes with a number of other advantages. Depending on the cooling or heating capacity required by the refrigerant circuit during operation, at least one of the compressors can be operated alone, or at least two compressors can be operated together. And since the maximum delivery rate of the first compressor is lower than that of the second compressor, it is possible to operate the refrigerant circuit with a particularly low mass flow rate of refrigerant. This is because, when only the first compressor is operating due to a very low cooling or heating demand, only a very small mass flow rate of refrigerant flows through the pipes and other components of the refrigerant circuit. This prevents, in particular, one of the compressors from being operated in a pulsed manner, i.e., being switched on and off alternately, in order to avoid icing up at least one evaporator in the refrigerant circuit when a minimal mass flow of refrigerant is continuously delivered. Avoiding such pulsed or two-point control operation of the first and / or second compressor in the refrigerant circuit is advantageous with regard to the long-term functionality of the compressor. Even if cycling of the compressor is unavoidable in certain operating situations despite the use of the small, first compressor, this cycling operation is at least significantly limited in terms of time compared to a situation in which only the larger, second compressor is in use. Compressors with different maximum delivery capacities are typically dimensionally different. However, due to limitations regarding available installation space, a single, large compressor is often more difficult to install in a vehicle than two compressors or refrigerant compressors with different maximum delivery capacities. Providing at least two refrigerant compressors with different maximum delivery capacities is therefore also advantageous with regard to utilizing the available installation space in the vehicle. This is particularly true if the first and second compressors are located at different points within the refrigerant circuit. When only the first compressor is operating at its maximum capacity, the mass flow rate of the refrigerant through the refrigerant circuit is essentially the same as the mass flow rate of the refrigerant through the circuit when only the second compressor is operating at its minimum capacity. This is because, when the maximum capacity of the first compressor is essentially equal to the minimum capacity of the second compressor, particularly smooth, gradual transitions from operating one compressor alone to operating the other can be achieved. The first compressor, and thus its displacement volume, can be designed such that, taking into account the rotational speed and efficiency of the first compressor, this displacement volume covers the lower operating range of the second compressor and only partially overlaps with the operating range of the second compressor. Both compressors then exhibit an overlap between the maximum volume flow of the first compressor and the minimum volume flow of the second compressor. As an example, with regard to maximum volume flow or displacement, the first compressor can have approximately 25% of the maximum delivery capacity of the second compressor and consequently be smaller in size. This clearly assigns the first compressor to low-load operating conditions, while the second compressor is suitable for medium and high-load operating conditions. Particularly in cases of pronounced high load, both compressors can operate in parallel. The mass flows at the maximum delivery capacity of the first compressor and the minimum delivery capacity of the second compressor can be described as essentially equal, in particular if these mass flows differ from each other by no more than about 10 percent. Preferably, following the initial operating period, the first compressor is started up and operated in addition to the second compressor. This is advantageous, for example, if a mass flow rate is required through the refrigerant circuit after the initial operating period that is greater than the mass flow rate achievable by operating the second compressor alone at its maximum capacity. Furthermore, connecting the first compressor to the second compressor also ensures a very uniform flow of refrigerant through the circuit. This is because, even during initial operation, the first compressor preferably has a lower minimum capacity than the second compressor. Preferably, following joint operation of the first compressor and the second compressor, the second compressor is operated alone again if the mass flow rate of the refrigerant to be conveyed through the refrigerant circuit is lower than a mass flow rate of the refrigerant achievable by operating the second compressor alone at its maximum delivery capacity. This takes into account the fact that, following the joint operation of the first compressor in addition to the previously commissioned second compressor, it may be more efficient to operate only one of the two compressors, namely the second compressor. Advantageously, switching the first compressor on and off results in smaller fluctuations in the operation of the refrigerant circuit than would be the case when switching the second refrigerant compressor on or off. It may be planned that during the initial phase, the first compressor operates alone if the refrigerant mass flow rate to be circulated through the refrigerant circuit is lower than the mass flow rate achievable by operating only the first compressor at its maximum capacity. For example, if it is determined that a comparatively low refrigerant mass flow rate is to be circulated through the refrigerant circuit anyway, it is sufficient to operate only the first compressor at the start of commissioning. This also promotes a particularly uniform flow of refrigerant through the lines and other components of the refrigerant circuit. Furthermore, operating only the first compressor ensures that, particularly at its minimum output, a correspondingly low mass flow of refrigerant passes through the refrigerant circuit. This is advantageous because it prevents the provision of an undesirably high cooling capacity, which could otherwise lead to icing of at least one evaporator in the refrigerant circuit. In particular, when a low cooling capacity is required, only the first refrigerant compressor can be operated directly and alone, thus excluding the other, in this case second, refrigerant compressor. As a measure to prevent evaporator icing due to excessive cooling capacity, the compressor can be operated intermittently in a manner known per se. In this case, any necessary implementation of such intermittent operation can occur later and / or less frequently if the first compressor, which is smaller in terms of displacement volume or delivery capacity, is operated alone. If, during operation of the first compressor alone, a larger mass flow of refrigerant needs to be conveyed through the refrigerant circuit than is possible when operating the first compressor at its maximum delivery capacity, the second compressor can also be put into operation and essentially support the first compressor in conveying the refrigerant. Should it transpire that operating only the second compressor is sufficient to provide the required cooling capacity, and / or even more efficient than operating both the first and second compressors simultaneously, then the second compressor can be operated alone and the first compressor can be taken out of service. This also contributes to particularly efficient and trouble-free operation of the refrigerant circuit. Preferably, when the first and second compressors jointly circulate the refrigerant through the refrigerant circuit, the compressors are operated at different speeds. Such a speed offset between the compressors minimizes acoustic disturbances caused by their operation. This applies both to the audibility of the respective compressor's operation by a person inside the passenger compartment of the vehicle and by a person outside the vehicle. In particular, this method avoids beat frequencies, i.e., additive superposition of vibrations during compressor operation that differ only slightly in frequency. This is advantageous for ensuring acoustically unobtrusive operation of the refrigerant circuit. Preferably, to determine the mass flow rate of refrigerant to be conveyed through the refrigerant circuit, an ambient temperature and / or a cooling capacity to be provided by at least one evaporator of the refrigerant circuit are taken into account. Using such parameters, it is possible to estimate very accurately which mass flow rate of refrigerant should be conveyed through the refrigerant circuit. Depending on these parameters, a decision can be made as to whether one of the compressors alone or both compressors together should provide or convey the desired mass flow rate of refrigerant. A heat exchanger can be used as at least one evaporator in the refrigerant circuit, directly cooling an airflow during operation. This is advantageous, for example, when the temperature of an airflow entering a passenger compartment of a motor vehicle needs to be reduced and / or dehumidified. Additionally or alternatively, a chiller can be used as at least one evaporator in the refrigerant circuit, which absorbs heat from a coolant stream during operation of the refrigerant circuit. By using the evaporator designed as a chiller, a particularly large amount of heat can be extracted from the coolant stream. This is especially advantageous when components such as an electrical energy storage device of the vehicle and / or an electric drive motor of the vehicle, particularly one designed for propelling the vehicle, and / or power electronics or the like are to be cooled indirectly by means of the coolant stream. Additionally or alternatively, it may be possible to indirectly cool and / or dehumidify an airflow via a cooling fluid-air heat exchanger, which is connected downstream of the chiller-designed cooling fluid heat exchanger. Particularly when the refrigerant circuit includes both the evaporator, which absorbs heat from the airflow during operation, and the chiller, which absorbs heat from the refrigerant flow during operation, the inclusion of at least two compressors in the refrigerant circuit is advantageous. This is because when the at least two compressors are operated together, the at least two heat-absorbing evaporators of the refrigerant circuit can be supplied with a comparatively large mass flow of refrigerant. This allows a comparatively high cooling capacity to be provided at each evaporator. It has proven further advantageous to use heat released by the refrigerant compressed by at least one of the compressors to raise the temperature of an airflow introduced into the passenger compartment of the vehicle. Accordingly, the refrigerant circuit can preferably be operated as a heat pump. This is advantageous with regard to the diverse applications of the refrigerant circuit. Furthermore, a particularly high heating capacity can be provided, especially by operating both refrigerant compressors in the refrigerant circuit with heat pump functionality. Particularly when only a comparatively small amount of heat energy is available from heat sources such as the coolant or cooling water or the ambient air, it can prove advantageous to initially activate the first compressor alone when starting the system or refrigerant circuit and only switch to the second compressor during further operation of the refrigeration system or refrigerant circuit, or to operate both compressors in parallel mode. The motor vehicle according to the invention has a refrigerant circuit. A first compressor and a second compressor of the refrigerant circuit are configured to pump a refrigerant through the circuit, either jointly or individually. The first compressor has a lower maximum delivery capacity than the second compressor. The motor vehicle further comprises a control device configured to operate the second compressor alone, at least during an initial period of operation of the refrigerant circuit. The control device is configured to do this when the mass flow rate of the refrigerant to be pumped through the circuit is greater than the mass flow rate achievable by operating the first compressor alone at its maximum delivery capacity.When operating only the first compressor at its maximum delivery capacity, the mass flow rate of the refrigerant that can be conveyed through the refrigerant circuit is essentially the same as the mass flow rate of the refrigerant that can be conveyed through the refrigerant circuit when operating only the second compressor at its minimum delivery capacity. The vehicle's control unit thus enables a particularly uniform flow of refrigerant through the refrigerant circuit. Accordingly, the vehicle is designed to carry out the method according to the invention or an advantageous embodiment thereof. The invention therefore also includes the vehicle's control unit. The control unit can comprise a data processing device or a processor unit configured to perform an embodiment of the method according to the invention. For this purpose, the processor unit can comprise at least one microprocessor and / or at least one microcontroller and / or at least one FPGA (Field Programmable Gate Array) and / or at least one DSP (Digital Signal Processor). Furthermore, the processor unit can comprise program code configured to perform the embodiment of the method according to the invention when executed by the processor unit. The program code can be stored in a data memory of the processor unit. The control unit can be equipped with, for example, material data information for the refrigerant used, calculation methods for energy balances, characteristic curves and / or maps relevant to the operation of the refrigerant circuit, or similar data. Furthermore, the control unit can be equipped with interfaces for an input and an output of request signals and / or measurement signals and / or control signals. The advantages and preferred embodiments described for the method according to the invention also apply to the motor vehicle according to the invention and vice versa. The invention therefore also includes further developments of the motor vehicle according to the invention, which have features as already described in connection with the further developments of the method according to the invention. For this reason, the corresponding further developments of the motor vehicle according to the invention are not described again here. The motor vehicle according to the invention is preferably designed as a motor vehicle, in particular as a passenger car or truck, or as a passenger bus. The invention also includes combinations of the features of the described embodiments. The invention therefore also includes realizations that each exhibit a combination of the features of several of the described embodiments, provided that the embodiments have not been described as mutually exclusive. The following are exemplary embodiments of the invention. Figure 1 shows a highly schematic representation of a refrigerant circuit of a motor vehicle in which two compressors or refrigerant compressors with different maximum delivery capacities are arranged in parallel; and Figure 2 schematically shows the motor vehicle comprising the refrigerant circuit according to Figure 1. The exemplary embodiments described below are preferred embodiments of the invention. In these exemplary embodiments, the described components each represent individual features of the invention, which can be considered independently of one another and each further develops the invention independently. Therefore, the disclosure is intended to include combinations of features of the embodiments other than those shown. Furthermore, the described embodiments can also be supplemented by further features of the invention already described. In the figures, identical reference symbols denote functionally equivalent elements. Figure 1 schematically shows a refrigerant circuit 10 of a motor vehicle 12, which is schematically depicted in Figure 2. The refrigerant circuit 10 comprises a first compressor 14 and a second compressor 16. The maximum delivery rate of the first compressor 14 is lower than the maximum delivery rate of the second compressor 16. Consequently, particularly with electrically driven compressors 14, 16 with a scroll compressor unit, the minimum delivery rate of the first compressor 14 is lower than the minimum delivery rate of the second compressor 16. The two compressors 14 and 16, or refrigerant compressors, are arranged in parallel in the refrigerant circuit 10. Accordingly, refrigerant to be compressed can be supplied to both the first compressor 14 and the second compressor 16 from a branch point 18 of the refrigerant circuit 10. At a junction point 20 of the refrigerant circuit 10, the mass flows of refrigerant delivered by the two compressors 14 and 16 can be recombined. The two compressors 14, 16 can pump the refrigerant through the refrigerant circuit 10 together or individually. Downstream of the inlet 20, a condenser 22 or gas cooler is arranged in the refrigerant circuit 10 in a manner known per se, by means of which the refrigerant compressed by at least one of the compressors 14, 16 can be cooled during operation of the refrigerant circuit 10. In the exemplary configuration of the refrigerant circuit 10 shown here, the cooled refrigerant, which depending on the refrigerant used is condensed, coming from the condenser 22, can be fed to an evaporator 24 and / or a chiller 26 during operation. Alternative configurations of the refrigerant circuit 10 may include only one such evaporator or more than the two evaporators shown here as examples. A first expansion element 28 is connected upstream of the evaporator 24 in a manner known per se, which serves to expand the refrigerant. Similarly, a further expansion element 30 is connected upstream of the chiller 26, by means of which the refrigerant supplied to the chiller 26 can be expanded. The evaporator 24 and the chiller 26 are arranged in parallel in the refrigerant circuit 10. Consequently, the evaporator 24 and the chiller 26 can be supplied with expanded refrigerant simultaneously or individually. The evaporator 24 is designed in this case as a heat exchanger which cools an airflow during the operation of the refrigerant circuit 10, for example an airflow which can be introduced into a passenger compartment 32 of the motor vehicle 12 (compare Fig. 2 ). In contrast, the chiller 26 absorbs heat from a coolant flow 34 during operation of the refrigerant circuit 10, which is shown only in a highly schematic form in Fig. 1. This coolant flow 34 can be used, for example, to cool an electrical energy storage device 36 of the motor vehicle 12 and / or at least an electric motor 38, which can be designed to propel the motor vehicle 12 if the motor vehicle 12 is designed as an electric vehicle or hybrid vehicle. Preferably, the two compressors 14, 16 are designed as electrically driven refrigerant compressors. This allows the speed and thus the delivery rate of each compressor 14, 16 to be set very precisely, for example by means of a control unit 40 of the motor vehicle 12, which is shown schematically in Fig. 1. As already mentioned, the maximum delivery capacity of the first compressor 14 is lower than the maximum delivery capacity of the second compressor 16. Accordingly, the first compressor 14 has a low delivery volume, whereas the second compressor 16 has a medium or large delivery volume. Particularly when only a comparatively low cooling capacity is to be provided by means of the at least one evaporator, in this case by means of the evaporator 24 and the chiller 26, it may be provided that only the first compressor 14 is operated. In other words, the first compressor 14 is preferably used alone, especially at low loads. If the first compressor 14 is operated at a minimum speed, then only a minimum mass flow of refrigerant is conveyed through the refrigerant circuit 10, or rather, a minimum delivery quantity of refrigerant is provided by the first compressor 14. This makes it easy to prevent icing of the evaporator 24. The first compressor 14 can be used, in particular, up to medium loads of the refrigerant circuit 10 and preferably operate up to its efficiency limit. This efficiency limit can be reached, in particular, when the first compressor 14 exhibits an excessively steep drop in its delivery rate characteristic or, alternatively, when it provides its maximum delivery capacity. If the control unit 40 then detects that the cooling capacity requirement is increasing, the second compressor 16 can also be put into operation. Accordingly, both the first compressor 14 and the second compressor 16 can then be operated together or in parallel. In addition to the sketched and dashed-marked communication lines between the control unit 40 and the first compressor 14 and the second compressor 16, the control unit or control device 40 preferably has further communication interfaces, for example in the form of signal inputs, which are not shown for the sake of clarity. It may also be possible to operate the second compressor 16 alone after both compressors 14, 16 have been operated together, and to deactivate or take out of service the first compressor 14. This can be effected by the control unit 40, in particular, if such an operating mode of only the second compressor 16 is more efficient than operating the first compressor 14 and the second compressor 16 together. Furthermore, this operating mode takes advantage of the fact that a refrigerant compressor operating at a comparatively low speed is less audibly noticeable than one operating at a high speed. Accordingly, the second compressor 16 can be operated independently, particularly if this results in a lower speed for the second compressor 16 than the speed required by the first compressor 14 to supply or circulate the same mass flow of refrigerant. This is advantageous in terms of minimizing the audible noise of the respective compressors 14 and 16. If the control unit 40 determines that there is a medium to high power demand, i.e., that the refrigerant circuit 10 is to be operated at a medium to high load, then the second compressor 16 is directly put into operation. Accordingly, it is provided that, at least during the initial operating period of the refrigerant circuit 10, the second compressor 16 is operated alone, provided that the mass flow rate of the refrigerant to be conveyed through the refrigerant circuit 10 is greater than the mass flow rate of the refrigerant that can be achieved by operating only the first compressor 14 at its maximum delivery capacity. If the control unit 40 determines, for example, that a mass flow of refrigerant is to be conveyed through the refrigerant circuit 10, as occurs with a medium to high cooling or heating demand, the second compressor 16 is activated and operated alone from the outset. If the power demand, i.e., the cooling or heating capacity to be provided in the operation of the refrigerant circuit 10, subsequently increases further, the speed of the second compressor 16 can first be increased. Once the second compressor 16 is operating at its optimal or maximum speed with regard to efficiency, the first compressor 14 can also be put into operation. If the power demand decreases again, or if only one of the compressors 14 or 16 operates more efficiently, the first compressor 14 can be taken out of service. The second compressor 16 will then operate on its own again. Particularly good, smooth transitions in the operation of each of the compressors 14, 16 alone or of both compressors 14, 16 together can be achieved if a maximum delivery capacity of the first compressor 14 essentially corresponds to a minimum delivery capacity of the second compressor 16. An operating strategy implemented by the control unit 40 can be estimated or implemented, in particular with regard to the respective integration or exclusion of one of the two compressors 14, 16 and the operation of both compressors 14, 16 simultaneously, depending on the prevailing boundary conditions. In this context, the control device 40 can in particular take into account an ambient temperature, i.e. a temperature present in the vicinity of the motor vehicle 12, and / or a cooling or heating requirement of the passenger compartment 32 and / or the electrical energy storage device 36 and / or the at least one electric motor 38. In addition to or as an alternative to an efficiency-optimized operating mode of the first compressor 14 or the second compressor 16 alone, or the simultaneous operation of both compressors 14, 16, the operation of the refrigerant circuit 10 can also be optimized with regard to the acoustics both inside the vehicle compartment 32 and outside the vehicle 12. In this context, it can be taken into account that operating the respective compressor 14, 16 at a low speed is acoustically advantageous because the operation of the respective compressor 14, 16 is then less audibly perceptible. It has also proven advantageous if the two compressors 14, 16, which jointly or simultaneously pump refrigerant, are not operated in the same speed ranges or at speeds that are critical to each other with regard to acoustics. Rather, it is advantageous if, when the first compressor 14 and the second compressor 16 jointly pump the refrigerant through the refrigerant circuit 10, the compressors 14, 16 are operated in different speed ranges. This helps to avoid, in particular, acoustically noticeable beat frequencies. In the case of minimal to low power requirements, the first compressor 14 can be operated alone. Conversely, in the case of medium to high power requirements, the second compressor 16 can be operated alone. In the case of maximum power requirements, however, the first compressor 14 and the second compressor 16 can jointly circulate the refrigerant through the refrigerant circuit 10. In particular, transitions from the combined operation of compressors 14, 16 to an operating state of the refrigerant circuit in which only one of the two compressors 14, 16 circulates the refrigerant can be determined by the control unit 40 depending on the load case. The operating mode appropriate to the respective operating state can be optimized with regard to efficiency and / or acoustics. As can be seen from Fig. 1, the refrigerant circuit 10 may have an internal heat exchanger 42 to further increase the efficiency of the refrigerant circuit 10. Preferably, the refrigerant circuit 10 can also be operated as a heat pump. In this case, by providing appropriate valves and lines not shown in Fig. 1, at least one further heat exchanger of the refrigerant circuit 10 can be operated as a heating coil and supplied with the compressed refrigerant. After passing through the heating coil, the refrigerant can be expanded by means of an expansion device and then fed to another heat exchanger, where the expanded refrigerant absorbs heat from a heat source before the refrigerant is fed back to the compressors 14, 16. For example, the chiller 26 can be used as such a heat exchanger, in which the refrigerant, which is expanded by means of the further expansion element 30, absorbs heat from the coolant stream 34 which serves as a heat source. Additionally or alternatively, the condenser 22 can, for example, be used as a heat exchanger, in which, during heat pump operation of the refrigerant circuit 10, the refrigerant (expanded by means of a further expansion device, not shown here) absorbs heat from an airflow. This airflow then serves as the heat source. Therefore, in this heat pump operation of the refrigerant circuit 10, the condenser 22 can be referred to as an air-source heat pump evaporator. In such operating modes of the refrigerant circuit 10 as a heat pump, it is advantageous that by operating both compressors 14, 16 together or jointly at the (not shown) heating register or alternative heat exchangers of the refrigerant circuit 10, a particularly high heating output can be provided. Overall, the examples show how a refrigeration system or a refrigerant circuit 10 can be provided with at least two compressors 14, 16 or refrigerant compressors.

Claims

Method for operating a refrigerant circuit (10) for a motor vehicle (12), in which at least a first compressor (14) and a second compressor (16) jointly or each individually pump a refrigerant through the refrigerant circuit (10), and in which the first compressor (14) has a lower maximum delivery capacity than the second compressor (16), wherein at least during a first period of operation of the refrigerant circuit (10) the second compressor (16) is operated alone when a mass flow rate of the refrigerant to be pumped through the refrigerant circuit (10) is greater than a mass flow rate of the refrigerant that can be achieved by operating the first compressor (14) alone at its maximum delivery capacity, characterized in thatthat when the first compressor (14) is operated alone at its maximum delivery capacity, the mass flow rate of the refrigerant conveyed through the refrigerant circuit (10) is essentially equal to the mass flow rate of the refrigerant conveyed through the refrigerant circuit (10) when the second compressor (16) is operated alone at its minimum delivery capacity. Method according to claim 1, characterized in that following the first time period the first compressor (14) is put into operation and operated in addition to the second compressor (16). Method according to claim 2, characterized in that, following joint operation of the first compressor (14) and the second compressor (16), the second compressor (16) is operated alone again if the mass flow rate of the refrigerant to be conveyed through the refrigerant circuit (10) is less than a mass flow rate of the refrigerant achievable by operating the second compressor (16) alone at its maximum delivery capacity. Method according to one of the preceding claims, characterized in that during the first period the first compressor (14) is operated alone if the mass flow rate of the refrigerant to be conveyed through the refrigerant circuit (10) is less than the mass flow rate of the refrigerant achievable by operating the first compressor (14) alone at its maximum delivery capacity. Method according to one of the preceding claims, characterized in that when the first compressor (14) and the second compressor (16) jointly convey the refrigerant through the refrigerant circuit (10), the compressors (14, 16) are operated in different speed ranges. Method according to one of the preceding claims, characterized in that, for determining the mass flow of refrigerant to be conveyed through the refrigerant circuit (10), an ambient temperature and / or a cooling capacity to be provided by at least one evaporator (24, 26) of the refrigerant circuit (10) are taken into account. Method according to claim 6, characterized in that a heat exchanger (24) is used as the at least one evaporator of the refrigerant circuit (10), which cools an air stream during operation of the refrigerant circuit (10), and / or a chiller (26) is used, which absorbs heat from a coolant stream (34) during operation of the refrigerant circuit (10). Method according to one of the preceding claims, characterized in that heat which is released by the refrigerant compressed by means of at least one of the compressors (14, 16) is used to increase the temperature of an airflow which is introduced into a passenger compartment (32) of the motor vehicle (12). Motor vehicle with a refrigerant circuit (10), wherein a first compressor (14) and a second compressor (16) of the refrigerant circuit (10) are configured to pump a refrigerant through the refrigerant circuit (10) jointly or individually, and wherein the first compressor (14) has a lower maximum delivery capacity than the second compressor (16), and with a control device (40) configured to operate the second compressor (16) alone, at least during a first period of operation of the refrigerant circuit (10), when a mass flow rate of the refrigerant to be pumped through the refrigerant circuit (10) is greater than a mass flow rate of the refrigerant that can be achieved by operating the first compressor (14) alone at its maximum delivery capacity, characterized in thatthat when operating only the first compressor (14) at its maximum delivery capacity, the mass flow rate of the refrigerant that can be conveyed through the refrigerant circuit (10) is essentially equal to the mass flow rate of the refrigerant that can be conveyed through the refrigerant circuit (10) when operating only the second compressor (16) at its minimum delivery capacity.

Images