Control of a heat pump under variable operating conditions

Abstract

Method for operating a heat pump (10), in particular in a motor vehicle (100), with an ambient heat exchanger (20) and an interior heat exchanger (40), which are connected to each other via a refrigerant circuit (50), wherein an icing degree (V) is calculated as an operating parameter of the heat pump (10), wherein a defrosting process (A) of the ambient heat exchanger (20) is initiated when a limit value (G) of the determined icing degree (V) is exceeded or reached, characterized in that the icing degree (V) of the ambient heat exchanger (20) is calculated to be increased from an initial value (V0) of the icing degree (V) until the limit value (G) is exceeded or reached during the operation of the heat pump (10), and in that the icing degree (V) is determined proportionally to a pressure loss coefficient (ζ) of the ambient heat exchanger (20).

Description

[0001] The invention relates to a method for operating a heat pump, particularly in a motor vehicle, with an ambient heat exchanger and an interior heat exchanger, which are connected to each other via a refrigerant circuit. The invention further relates to a motor vehicle equipped with a heat pump.

[0002] Electrically powered vehicles often use heat pumps to heat the passenger compartment. In this process, the heat required to heat the passenger compartment is extracted from the vehicle's surroundings and transferred to the passenger compartment. During this process, a heat exchanger thermally coupled to the vehicle's surroundings cools down to such an extent that the dew point of the surrounding air is reached, and frost forms on the surface of the heat exchanger.

[0003] Frost buildup increasingly ices the surface of the heat exchanger, thus preventing the heat pump from operating efficiently. To allow air to flow through the heat exchanger again, defrosting cycles are necessary, during which the iced-up heat exchanger is heated. Consequently, the ice melts, and air can flow through the heat exchanger once more. However, such defrosting cycles impair the efficiency and performance of the heat pump.

[0004] Methods for performing defrost cycles are already known, which initiate the defrosting process of an evaporator after a predefined operating period. In heat pumps with changing operating conditions, such time-controlled defrost cycles can further reduce the efficiency of the heat pump, since the duration is usually optimized for a specific operating condition or represents a compromise between several operating conditions.

[0005] To determine when defrosting is necessary, stationary refrigeration units use sensors that measure the degree of icing on the evaporator. For mobile applications, such as vehicle-mounted heat pumps, no sensors for determining the degree of icing are currently known. Furthermore, such sensors can increase the installation effort of the heat pump and require evaluation electronics, thus increasing the overall cost.

[0006] US Patent 5,186,016 A describes a method for defrosting a heat exchanger in an air conditioning system. In one step, the time required to heat an interior space to a desired temperature is calculated. Based on this time, the resulting amount of ice buildup after that time is calculated, assuming the air conditioning compressor is running continuously.

[0007] In US 2009 / 0241561A1 and JP 2000-274916A, measurement data from a humidity sensor inside a refrigerator are evaluated to determine absolute humidity values ​​and the rate of change of humidity. When the rate of change of humidity decreases, the amount of icing is determined, and if the amount of icing exceeds a certain threshold, defrosting is initiated.

[0008] US patent 2013 / 0291577A1 discloses an air conditioning system for a vehicle with reduced energy consumption. A control unit detects whether an ice layer is forming on the ambient heat exchanger at low outside temperatures and then initiates a defrosting process. This ice layer check is performed independently of the operation of the air conditioning compressor.

[0009] DE 31 10 042 A1 describes a method for defrosting an ambient heat exchanger.

[0010] In DE 10 2012 203 564 A1, an air conditioning system with a frost condition calculator for a vehicle is described, which informs the vehicle occupants of a defrosting process.

[0011] The invention is based on the objective of providing a method for determining the degree of icing of evaporators without the use of additional sensors. This objective is achieved by the features specified in claim 1. Further advantageous embodiments of the invention are described in the dependent claims.

[0012] According to one aspect of the invention, a method for operating a heat pump, particularly in a motor vehicle, is provided. The heat pump comprises an ambient heat exchanger and an interior heat exchanger, which are connected to each other via a refrigerant circuit. A refrigerant can be circulated through the refrigerant circuit by means of a compressor or a pump. For example, the ambient heat exchanger and the interior heat exchanger can perform the function of a heat pump via the refrigerant circuit.

[0013] The degree of icing is calculated as an operating parameter of the heat pump. If a limit value of the calculated degree of icing is exceeded or reached, a defrosting process of the ambient heat exchanger is initiated.

[0014] The calculation of the degree of icing and the control of defrosting processes can be implemented by a control unit that can be connected to the heat pump via data transmission. Optionally, the control unit can be connected to at least one sensor via data transmission to receive operating parameters such as the temperature of the ambient heat exchanger, air temperature, operating status of the heat pump, speed of the refrigerant compressor, and the like.

[0015] According to the invention, the degree of icing of the ambient heat exchanger is computationally increased from an initial value until a limit is exceeded or reached during operation of the heat pump. During operation, the icing of the ambient heat exchanger increases steadily depending on the operating state of the heat pump and the environmental conditions. In particular, the icing, and thus the degree of icing of the ambient heat exchanger, increases with the duration of operation. By calculating the degree of icing, the actual icing or frost layer can be computationally replicated or simulated. The initial value of the degree of icing can, for example, be 0, since no icing of the ambient heat exchanger is present after a completed defrosting process or when the vehicle starts driving.

[0016] This method allows the operating parameter "degree of icing" to be defined and used in the control of operating states and defrosting processes of the heat pump. This operating parameter characterizes the degree of icing of the ambient heat exchanger or evaporator. The degree of icing can be used across different operating states.

[0017] The degree of icing can be calculated without the use of sensors. Preferably, the degree of icing can be determined mathematically based on operating time and environmental conditions such as air temperature, humidity, and weather data. Additional factors, such as the refrigerant flow rate and the surface area of ​​the ambient heat exchanger, can also be included in the calculation.

[0018] According to the invention, the degree of icing is determined with particular precision by calculating it proportionally to a pressure loss coefficient of the ambient heat exchanger, and advantageously also proportionally to the mass of a frost layer on the ambient heat exchanger and / or to the thermal insulation effect of the frost layer on the ambient heat exchanger. This allows further factors influencing the thermodynamic interaction between the ambient heat exchanger, a frost or ice layer, and the ambient air to be taken into account when calculating the degree of icing.

[0019] The operating mode and required output of the heat pump can be precisely determined by the degree of icing of the ambient heat exchanger if it is increased depending on at least one operating condition and / or operating time of the heat pump. The degree of icing can be increased iteratively or continuously with operating time. As the heat output of the heat pump at the indoor heat exchanger increases, the ambient heat exchanger cools down more, causing it to ice up more quickly and the degree of icing to rise more rapidly.

[0020] The operating states of the heat pump can be adjusted depending on parameters such as the outside air temperature, ambient humidity, and the heat pump's heating output. These parameters can change while the vehicle is in motion. Therefore, the heat pump's control system, including defrosting, must be adapted to these changes. As the heat pump's operating state changes, so does the rate at which the degree of icing increases.

[0021] In another embodiment, the degree of icing of the ambient heat exchanger is reset to its initial value after a defrosting process is complete and / or when the vehicle starts driving. This measure allows the degree of icing to be reliably determined from a constant initial value. A corresponding calculation of the degree of icing can, for example, be performed using an integral and is therefore particularly easy to implement technically.

[0022] By changing the operating states of the heat pump in response to changing environmental conditions or performance requirements, the degree of icing can be easily transferred between operating states if the degree of icing of the ambient heat exchanger is stored, at least temporarily, and continuously increased during each operating state change from one operating state to at least one second operating state. Preferably, the degree of icing in the first operating state increases at different rates than the degree of icing in the second operating state over the operating time. This allows the degree of icing to be used as an operating parameter even with changing operating states of the heat pump and to represent a reliable measure of the icing of the ambient heat exchanger.

[0023] According to one embodiment, the degree of icing of the ambient heat exchanger is calculated using an integral over the operating time. V=r0+∫0tmde˙subdt

[0024] In this process, a time integral of the mass increase due to the formation of the frost layer on a surface of the ambient heat exchanger is formed to determine the degree of icing, with the unit being mass.

[0025] The mass increase due to the formation of the frost layer on the surface of the ambient heat exchanger depends on various parameters, especially environmental factors. mde˙sub=β⋅F⋅ρAir⋅(XAir(T)−X'Air(T))

[0026] In particular, the surface area of ​​the ambient heat exchanger, the water content in the air, the saturation level of the air, the mass transfer coefficient and the maximum water storage capacity of the ambient heat exchanger can be used to calculate the mass increase.

[0027] Such a computational determination of the degree of icing using integral calculus is robust and technically simple to implement. This allows the icing to be determined based on the operating conditions since the last defrosting and based on the current degree of icing. This corresponds to a relative calculation of an increase or decrease in the degree of icing starting from the initial value or a previous value.

[0028] Alternatively or additionally, a situation-dependent or absolute calculation of the degree of icing can also be performed based on the current operating state.

[0029] According to a further aspect of the invention, a motor vehicle equipped with a heat pump for carrying out a method according to the invention is provided. An interior heat exchanger of the heat pump can be used to cool or heat the interior of the motor vehicle. When the interior is being heated, an ambient heat exchanger, which is coupled to the interior heat exchanger via a refrigerant circuit, is cooled. Over time, frost can form on the surface of the ambient heat exchanger. This frost layer on the surface of the ambient heat exchanger reduces the efficiency of the heat pump and impairs the heating performance of the interior or passenger compartment.

[0030] This method allows the degree of icing of the ambient heat exchanger to be estimated or simulated and used for efficient defrosting processes. A defrosting process can be initiated when the degree of icing reaches or exceeds a certain threshold.

[0031] Depending on the operating state of the heat pump, the limit value for the degree of icing can be constant or variable.

[0032] Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. The drawings show: Fig. 1 a schematic representation of a motor vehicle according to the invention with a heat pump according to one embodiment, and Fig. 2a and Fig. 2b Schematic diagrams of an air-side pressure loss coefficient of an ambient heat exchanger and a determined degree of icing to illustrate a method according to the invention.

[0033] In the figures, the same constructive elements each have the same reference numerals.

[0034] The Fig. Figure 1 shows a schematic side view of a motor vehicle 100 with a heat pump 10 for air treatment according to one embodiment. The heat pump 10 is exemplified as a flat-roof air conditioning unit mounted on a motor vehicle 100 configured as a bus. The heat pump 10 can be used analogously in any motor vehicle 100, such as passenger cars, trucks, agricultural vehicles, and the like.

[0035] The heat pump 10 has an ambient heat exchanger 20, which is supplied with ambient air (U) via an optional fan 30. This allows the ambient heat exchanger 20 to extract heat from the ambient air (U) during heating operation of the heat pump 10. The ambient heat exchanger 20 can, for example, be configured as an evaporator during heating operation of the heat pump 10.

[0036] Furthermore, the heat pump 10 has an interior heat exchanger 40. The interior heat exchanger 40 is arranged in a passenger compartment or vehicle interior 101 to heat or cool the passenger compartment 101.

[0037] The ambient heat exchanger 20 and the interior heat exchanger 40 are connected to each other via a fluid-carrying refrigerant circuit 50. A refrigerant can be circulated through the refrigerant circuit 50 by means of a refrigerant pump 51 or a refrigerant compressor to enable the extraction of heat from the environment U and the supply of heat to the passenger compartment 101.

[0038] By extracting heat from the surroundings U, the ambient heat exchanger 20 can ice up and thus lose its efficiency. In the illustrated embodiment, the ambient heat exchanger 20 can be defrosted. For this purpose, the heat pump 10 can, for example, be operated in the opposite direction to its heating operation to briefly heat the ambient heat exchanger 20.

[0039] A control unit 60 can be connected to the refrigerant pump 51 and configured to control the heat pump 10. In particular, the control unit 60 can implement requirements for the heating or cooling capacity of the heat pump 10 by adjusting its operating state. Adjusting the operating state of the heat pump 10 can, for example, change the speed of the refrigerant pump 51, the refrigerant flow rate, the control of valves, and the like.

[0040] In the Fig. 2a and Fig. Figure 2b shows schematic diagrams illustrating a method according to the invention. Fig. Figure 2a shows an example of an air-side pressure loss coefficient ζ of the ambient heat exchanger 20, which changes with time or operating time t. In the Fig. Figure 2b shows a determined degree of icing V, which also depends on the operating time t of the heat pump 10.

[0041] The air-side pressure drop coefficient ζ of the ambient heat exchanger 20 is a dimensionless measure of the pressure drop across the ambient heat exchanger 20 and serves to illustrate the consequences of icing. With increasing icing, the ambient heat exchanger 20 can extract less heat from the environment U, thus reducing the efficiency of the heat pump 10. To remove the icing from the ambient heat exchanger 20, defrosting processes A are initiated. A defrosting process A removes the icing from the ambient heat exchanger 20, causing the air-side pressure drop coefficient ζ to increase again.

[0042] In the Fig. Figure 2a shows the course of the air-side pressure loss coefficient ζ with defrosting cycles A performed in two different operating states or operating modes 70, 80. The heat pump 10 is operated in a first operating state 70 by the control unit 60 and switched to a second operating state 80 by the control unit 60 at an operating state change 90.

[0043] In the second operating state 80, the heat pump 10 is operated at a lower power output, so that the icing of the ambient heat exchanger 20 proceeds more slowly.

[0044] In the Fig.Figure 2b illustrates the degree of icing V calculated during the operation of the heat pump 10, for example by the control unit 60. The degree of icing V is calculated as an operating parameter of the heat pump 10. If a limit value G of the calculated degree of icing V is exceeded or reached, a defrosting process A of the ambient heat exchanger 20 is initiated. This can also be implemented technically by the control unit 60.

[0045] By switching between operating states 70 and 80 of the heat pump 10 in response to changing environmental conditions or performance requirements, the degree of icing V can be transferred between these states. For this purpose, the control unit 60 can at least temporarily store the degree of icing V of the ambient heat exchanger 20 and continuously calculate it during an operating state change 90 of the heat pump 10 from the first operating state 70 to the second operating state 80. The degree of icing V increases at different rates over the operating time t compared to the degree of icing V in the second operating state 80. Due to the slower increase in the degree of icing V in the second operating state 80, the defrosting process A is activated at a later operating time, as the limit value G of the degree of icing V is reached more slowly.

[0046] The degree of icing V of the ambient heat exchanger 20 is calculated using an integral over the operating time t. V=r0+∫0tmde˙subdt

[0047] This involves calculating a time integral of a mass increase m desub formed by the formation of the frost layer on a surface of the ambient heat exchanger 20 to determine the degree of icing V with the unit of mass.

[0048] The operating time t is defined as the time during which heat is extracted from the vehicle's surroundings by the ambient heat exchanger 20. The degree of icing increases depending on the mass increase m. desub during operating hours t.

[0049] The mass increase m desub The formation of the frost layer on the surface of the ambient heat exchanger 20 is calculated according to the following formula. mde˙sub=β⋅F⋅ρAir⋅(XAir(T)−X'Air(T)) with F a surface area of ​​the ambient heat exchanger 20, X a water content of the air at an ambient temperature T, X' a saturation level of the air, β a mass transfer coefficient, r0 a maximum water storage capacity of the ambient heat exchanger 20 and ρ Luft air density.

[0050] The degree of icing of the ambient heat exchanger 20 is therefore dependent on the ambient conditions, which promote the formation of a frost layer on the ambient heat exchanger 20.

[0051] After a completed defrosting process A of the ambient heat exchanger 20 and / or when the motor vehicle 100 starts driving, the degree of icing V is reset to an initial value V0 of the degree of icing V. Reference symbol list 100 motor vehicles 101 Passenger compartment 10 Heat pump 20 ambient heat exchangers 30 fan 40 interior heat exchangers 50 Refrigerant circuit 51 Refrigerant pump / Refrigerant compressor 60 Control unit 70 first operating state 80 second operating state 90 operating state changes ζ air-side pressure loss coefficient ρ Luft airtightness A defrosting process β mass transfer coefficient F Surface area of ​​the ambient heat exchanger G Limit value of the degree of icing r0 maximum water storage capacity of the ambient heat exchanger t operating time Ambient temperature U surroundings Degree of icing V0 Initial value of the degree of icing X Water content of the air X' Saturation level of the air

Claims

[1] Method for operating a heat pump (10), in particular in a motor vehicle (100), with an ambient heat exchanger (20) and an interior heat exchanger (40) which are connected to each other via a refrigerant circuit (50), wherein an icing degree (V) is calculated as an operating parameter of the heat pump (10), wherein a defrosting process (A) of the ambient heat exchanger (20) is initiated when a limit value (G) of the determined icing degree (V) is exceeded or reached, characterized by , that the degree of icing (V) of the ambient heat exchanger (20) is calculated to increase from an initial value (V0) of the degree of icing (V) until the limit value (G) is exceeded or reached during the operation of the heat pump (10), and by determining the degree of icing (V) proportionally to a pressure loss coefficient (ζ) of the ambient heat exchanger (20). [2] Method according to claim 1, wherein the degree of icing (V) is further determined proportionally to a thermal insulation effect of the frost layer on the ambient heat exchanger (20). [3] Method according to claim 1, wherein the degree of icing (V) of the ambient heat exchanger (20) is increased depending on at least one operating condition (70, 80) and / or an operating time (t) of the operation of the heat pump (10). [4] Method according to claim 1 or 3, wherein the degree of icing (V) of the ambient heat exchanger (20) is reset to an initial value (V0) after a completed defrosting process (A) of the ambient heat exchanger (20) and / or when the motor vehicle (100) starts driving. [5] Method according to any one of claims 1 to 4, wherein the degree of icing (V) of the ambient heat exchanger (20) is stored at least temporarily and is continuously increased during an operating state change (90) of the heat pump (10) from a first operating state (70) to at least a second operating state (80) of the heat pump (10), wherein a value of the degree of icing (V) in the first operating state (70) increases at different rates over the operating time (t) compared to a value of the degree of icing (V) in the second operating state (80). [6] Method according to any one of claims 1 to 5, wherein the degree of icing (V) of the ambient heat exchanger (20) is calculated using an integral over the operating time (t): V=r0+∫0tmde˙subdt with mde˙sub=β⋅F⋅ρAir⋅(XAir(T)−X'Air(T)) and with F, a surface of the ambient heat exchanger X, the water content of the air at an ambient temperature T X', the saturation content of the air β, a mass transfer coefficient r0, a maximum water storage capacity of the ambient heat exchanger [7] Motor vehicle (100) which has a heat pump (10) for carrying out a method according to one of the preceding claims.

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