Method and control device for operating a heat pump while stabilizing a mains frequency of an electrical power grid

Abstract

Method for operating a heat pump (10) while stabilizing a grid frequency of an electrical power grid (15), wherein a first heat exchanger (11) is configured to transfer thermal energy to a process medium, wherein a compressor (12) is configured to compress the process medium downstream of the first heat exchanger, wherein a second heat exchanger (13) is configured to transfer thermal energy of the process medium to a consumer, wherein an expander (16) is configured to expand the process medium downstream of the second heat exchanger.If an actual grid frequency deviates from a target grid frequency, the speed of the electric machine (14) and the volume flow of the process medium of the heat pump (10) are adjusted to stabilize the grid frequency in such a way that, with a constant pressure ratio of the compressor (12) and thus a constant temperature of the process medium in the area of ​​the second heat exchanger (13), the actual grid frequency is brought closer to the target grid frequency by changing the electrical power consumption of the electric machine. If the actual grid frequency is higher than the target grid frequency, the speed of the electric machine and the volume flow of the process medium are both increased. If the actual grid frequency is lower than the target grid frequency, the speed of the electric machine and the volume flow of the process medium are both reduced.

Description

The invention relates to a method and a control device for operating a heat pump while stabilizing a grid frequency of an electrical power grid.The construction of a heat pump is sufficiently known from practice. Thus, a heat pump has a first heat exchanger which is configured to transfer thermal energy, in particular from the environment, to a process medium of the heat pump. Downstream of the first heat exchanger, a heat pump comprises a compressor which is configured to compress the process medium of the heat pump, wherein the compressor is driven from an electric machine. Downstream of the compressor, a heat pump has a second heat exchanger, which is configured to transmit thermal energy of the process medium of the heat pump to a consumer or a process medium of a consumer. Downstream of the second heat exchanger, an expander is provided in order to expand the process medium of the heat pump. The expander can be a turbine and / or an expansion valve or else an ejector or a PressureEx.The electric machine of the heat pump, which serves for driving the compressor thereof, is connected to an electric power grid and can be supplied with electric energy from the electric power grid. If the grid frequency of the electrical power grid fluctuates, it is possible in principle to change the electrical power consumption of the electrical machine driving the compressor of a heat pump in order to stabilize the grid frequency of the electrical power grid.If the rotational speed of the electric machine is increased, the electric load pick-up of the heat pump and also the temperature of the process medium of the heat pump in the region of the second heat exchanger increases. If the rotational speed of the electric machine which drives the compressor of the heat pump is reduced, the electrical power consumption of the heat pump and thus also the temperature of the process medium in the region of the second heat exchanger is reduced.If the grid frequency of an electrical power grid is accordingly stabilized via the heat pump, it leads according to practice to a change in the temperature of the process medium of the heat pump in the region of the second heat exchanger and thus also to fluctuations in the thermal energy which can be transmitted to the consumer. This is disadvantageous.There is a need to operate a heat pump in such a way that, when the heat pump is used to stabilize the grid frequency of an electrical power grid, the temperature of the process medium of the heat pump in the region of the second heat exchanger and thus the thermal energy that can be transferred in the direction of the consumer remains unchanged.Proceeding from this, the object of the present invention is to provide a novel method and control device for operating a heat pump while stabilizing a grid frequency of an electrical power grid.US 2021 / 0 116 159 A1 discloses a method according to the preamble of claim 1.JP 2010-84 968 A, EP 3 374 603 B1 and DE 10 2017 205 484 A1 disclose further prior art.This object is achieved by a method according to claim 1 and a control device according to claim 7.With the present invention, it is proposed, for stabilizing the grid frequency of an electrical power grid with the aid of a heat pump, on the one hand to adapt the rotational speed of the electrical machine which drives the compressor of the heat pump and, on the other hand, to adapt the volume flow of the process medium of the heat pump.Then, if an actual grid frequency of the electrical power grid deviates from a setpoint grid frequency, a rotational speed of the electrical machine and the volume flow of the process medium of the heat pump are adapted for stabilizing the grid frequency in such a way that, with a constant pressure ratio of the compressor and thus with a constant temperature of the process medium in the region of the second heat exchanger, the actual grid frequency of the electrical power grid is approximated to the setpoint grid frequency by the change in the electrical power consumption of the electrical machine. Thus, the actual grid frequency of the electrical power grid can be approximated to the setpoint grid frequency for stabilizing the grid frequency without the temperature of the medium of the heat pump changing in the region of the second heat exchanger and thus the thermal energy that can be transferred to the consumer.Then, if the actual grid frequency of the electrical power grid is greater than the setpoint grid frequency, the rotational speed of the electrical machine and the volume flow of the process medium of the heat pump are increased, wherein, if the actual grid frequency of the electrical power grid is less than the setpoint grid frequency, the rotational speed of the electrical machine and the volume flow of the process medium of the heat pump are reduced. This allows a particularly advantageous stabilization of the grid frequency of an electrical power grid while maintaining constant the temperature of the process medium of the heat pump in the region of the second heat exchanger.Preferred developments of the invention are evident from the dependent claims and the following description. Exemplary embodiments of the invention are explained in more detail with reference to the drawings, without being restricted thereto. The following shows: FIG. 1 is a schematic view of a heat pump to illustrate the invention, FIG. 2 is a compressor map.FIG. 1 shows a highly schematic of an exemplary embodiment of a heat pump 10.The heat pump 10 has a first heat exchanger 11 which is configured to transfer thermal energy to a process medium of the heat pump 10.The thermal energy transferred to the process medium of the heat pump with the aid of the first heat exchanger 11 can be thermal energy of ambient air, thermal energy of water of a sea or seawater, water from rivers or municipal / industrial waste / process water or the like.The heat pump 10 has a compressor 12 downstream of the first heat exchanger 11, as seen in the flow direction of the process medium of the heat pump 10. The compressor 12 is configured to compress the process medium of the heat pump downstream of the first heat exchanger 11 and upstream of a second heat exchanger 13 of the heat pump 10.The second heat exchanger 13 is configured to transfer thermal energy of the process medium of the heat pump 10 to a consumer or a process medium of the consumer.The compressor 12 can be driven starting from an electric machine 14 of the heat pump 10. For this purpose, the electric machine 14 is connected to an electric power grid 15. The electric machine 14 can be supplied with electrical energy starting from the electrical power grid 15. Depending on the rotational speed and the torque of the electric machine 14, the electric machine 14 receives or decreases electric load from the electric power grid 15.The heat pump 10 further has an expander 16, which in the exemplary embodiment shown comprises both a turbine 17 and an expansion valve 18. The turbine 17 and the expansion valve 18 are configured to expand the process medium of the heat pump 10 downstream of the second heat exchanger 13 and upstream of the first heat exchanger 11. Energy recovered in turbine 17 may be used to drive electric machine 14.Alternatively to the exemplary embodiment shown, the expander 16 can also comprise exclusively the turbine 17 or exclusively the expansion valve 18. Designs as ejectors or pressure exchangers are likewise included in the invention.FIG. 1 furthermore shows an optional bypass valve 19 to the turbine 17. the quantity of process medium of the heat pump 10, which is guided past the turbine 17 directly in the direction of the expansion valve 18, can be adjusted via the bypass valve 19.If the grid frequency of the electrical power grid 15 changes in such a way that an actual grid frequency of the electrical power grid 15 deviates from a setpoint grid frequency thereof, both the rotational speed of the electrical machine 14 and the volume flow of the process medium of the heat pump 10 are adapted for stabilizing the grid frequency, that is to say for approximating the actual grid frequency to the setpoint grid frequency of the electrical power grid 15.This adaptation is effected in such a way that, with a constant pressure ratio of the compressor and thus with a constant temperature of the process medium downstream of the compressor 12 in the region of the second heat exchanger 13, the change in the electrical load pick-up of the electrical machine 14 caused thereby approximates the actual grid frequency of the electrical power grid to the setpoint grid frequency of the latter.If the actual grid frequency of the electrical power grid 15 is greater than the setpoint grid frequency, the rotational speed of the electrical machine 14 and the volume flow of the process medium of the heat pump 10 are both increased. If, on the other hand, the actual grid frequency of the electric power grid 15 is less than the setpoint grid frequency thereof, the rotational speed of the electric machine 14 and the volume flow of the process medium of the heat pump 10 are both reduced. This brings about the approach of the actual grid frequency to the setpoint grid frequency and thus the stabilization of the grid frequency of the electrical grid 15, specifically at a constant temperature of the process medium downstream of the compressor 12 in the region of the second heat exchanger 13.FIG. 2 shows a characteristic diagram of a compressor 12, wherein FIG. 2 shows the pressure ratio PI of the compressor 12, namely the ratio between the output pressure of the compressor 12 and the input pressure thereof, as a function of the volume flow V of the process medium of the heat pump 10. In FIG. 2, a point 20 visualizes an operating point of the compressor 12 at a defined volume flow V and a defined pressure ratio PI. For the purposes of the invention, this operating point 20 can be shifted in the direction of the double arrow 21 for grid stabilization, i.e. with a constant pressure ratio PI of the compressor 12 between the output pressure and the input pressure thereof. In this case, the volume flow V is adapted, and likewise the rotational speed of the electric machine 14 is adapted in order to maintain the constant pressure ratio PI.Due to the fact that the pressure ratio PI of the compressor 12 is kept constant, the temperature of the process medium downstream of the compressor 12 also remains constant in the region of the second heat exchanger 13, so that the thermal energy that can be transferred to the consumer remains unchanged. A heat pump 10 can thus be used to stabilize a grid frequency of the electrical power grid 15 without this having an effect on the consumer.In the exemplary embodiment of FIG. 1, the volume flow of the process medium of the heat pump 10 can be effected, for example, by changing the open position of the expansion valve 18. The volume flow of the process medium can also be effected by changing the open position of the bypass valve 19. If the turbine 17 and / or the compressor 12 have adjustable guide vanes, so-called adjustable guide vanes, the volume flow of the process medium of the heat pump 10 can also be effected by a corresponding adjustment of the guide vanes at the turbine 17 and / or the compressor 12.In the exemplary embodiment shown, it is accordingly possible for the device for changing the volume flow of the process medium of the heat pump to be a component of the compressor 12, a component of the turbine 17 or a component of the expander 16.A separate device for changing the volume flow of the process medium of the heat pump 10 can also be used, which is designed as a separate module compared to the compressor 12 and the expander 16, for example as a separate valve or as a separate throttle.The invention further relates to a control device which is configured to automatically execute the method according to the invention on the control side.For this purpose, such a control device has data interfaces in order to exchange data with the assemblies involved in carrying out the method according to the invention, for example with the electrical power grid 15, the electrical machine 14 and with the device for changing the volume flow of the process medium of the heat pump 10. The control device is configured to compare the actual grid frequency of the electrical power grid 15 with the setpoint grid frequency thereof and to adapt the rotational speed of the electrical machine 14 and the volume flow of the process medium of the heat pump 10 for stabilizing the grid frequency of the electrical power grid 15, that is to say for approximating the actual grid frequency to the setpoint grid frequency, and specifically in such a way that the pressure ratio PI of the compressor 12 and therefore the temperature of the process medium downstream of the compressor 12 remains constant in the region of the second heat exchanger 13.In order to bring the actual grid frequency closer to the setpoint grid frequency for stabilizing the grid frequency, in the exemplary embodiment shown in FIG. 2, the operating point 20 is shifted in the compressor characteristic diagram of FIG. 2 along the double arrow 21, that is to say with a constant pressure ratio PI of the compressor 12 between the output pressure and the input pressure thereof. In this case, both the rotational speed of the electric machine 14 and the volume flow of the process medium of the heat pump 10 are adapted. This can be effected in a controlled manner as a function of a characteristic diagram or else in a regulated manner.It is also possible, depending on a deviation between the actual grid frequency and the setpoint grid frequency, to determine a load change for the electric machine 14 that is required to bring the actual grid frequency closer to the setpoint grid frequency. Depending on this load change, a required rotational speed and a required torque for the electric machine 14 can be determined depending on an engine characteristic map. Depending on the required torque, an activation variable for the device for changing the volume flow can then be determined.List of reference characters10 Heat pump 11 First heat exchanger 12 Compressor 13 Second heat exchanger 14 Motor 15 Electrical power network 16 Expander 17 Turbine 18 Expansion valve 19 Bypass valve 20 Operating point 21 Double arrow

Claims

Method for operating a heat pump (10) while stabilizing a grid frequency of an electrical power grid (15), wherein the heat pump (10) has a first heat exchanger (11), a compressor (12) that can be driven by an electrical machine (14) connected to the electrical power grid (15), a second heat exchanger (13) and an expander (16), wherein the first heat exchanger (11) of the heat pump (10) is configured to transmit thermal energy to a process medium of the heat pump, wherein the compressor (12) of the heat pump (10) is configured to compress the process medium of the heat pump downstream of the first heat exchanger (11) and upstream of the second heat exchanger (13), wherein the second heat exchanger (13) of the heat pump (10) is configured to transmit thermal energy of the process medium of the heat pump to a consumer, wherein the expander (16) of the heat pump (10) is configured to compress the process medium of the heat pump, the process medium of the heat pump to be expanded downstream of the second heat exchanger (13) and upstream of the first heat exchanger (11), wherein the heat pump (10) has a device with the aid of which a volume flow of the process medium of the heat pump (10) can be adjusted, and wherein, if an actual grid frequency of the electrical power grid (15) deviates from a setpoint grid frequency, the grid frequency of the electrical power grid (15) is stabilized, characterized in that, for stabilizing the grid frequency of the electrical power grid (15), a rotational speed of the electrical machine (14) and the volume flow of the process medium of the heat pump (10) are adjusted in such a way that, that, at a constant pressure ratio of the compressor (12) and thus at a constant temperature of the process medium downstream of the compressor (12) in the region of the second heat exchanger (13), the actual grid frequency of the electrical grid is approximated to the setpoint grid frequency by the change in the electrical power consumption of the electrical machine (14), when the actual grid frequency of the electrical grid (15) is greater than the setpoint grid frequency, the rotational speed of the electrical machine (14) and the volumetric flow rate of the process medium of the heat pump (10) are both increased, whereas when the actual grid frequency of the electrical grid (15) is less than the setpoint grid frequency, the rotational speed of the electrical machine (14) and the volumetric flow rate of the process medium of the heat pump (10) are both reduced.Method according to Claim 1, characterized in that the expander (16) is actuated as a device for changing the volume flow of the process medium of the heat pump, namely a turbine (17) of the expander (16), the guide blades of which are adjusted for changing the volume flow of the process medium, and / or a bypass valve (19) to the turbine (17), the open position of which is adjusted for changing the volume flow of the process medium.Method according to Claim 1 or 2, characterized in that the expander (16) is actuated as a device for changing the volume flow of the process medium of the heat pump, namely an expansion valve (18) of the expander (16), the open position of which is changed for changing the volume flow of the process medium.Method according to one of Claims 1 to 3, characterized in that the device for changing the volume flow of the process medium actuated is a compressor (12), the guide blades of which are adjusted for changing the volume flow of the process medium.Method according to one of Claims 1 to 4, characterized in that a separate valve or a separate throttle, the open position of which can be changed for changing the volume flow of the process medium, is actuated as a device for changing the volume flow.Method according to one of Claims 1 to 5, characterized in that a load change for the electric machine (14) is determined as a function of a deviation between the actual grid frequency and the setpoint grid frequency, said load change being required in order to bring the actual grid frequency closer to the setpoint grid frequency, wherein a required rotational speed and a required torque for the electric machine (14) are determined as a function of said load change and as a function of a motor characteristic diagram, and wherein a control variable for the device for changing the volume flow is then determined as a function of the required torque.Control device for operating a heat pump (10) while stabilizing a grid frequency of an electrical power grid, characterized in that it is configured to automatically execute the method according to one of Claims 1 to 6 on the control side.

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