DE102020123960B4 - Method for operating a heat pump and heat pump - Google Patents

Abstract

Method for operating a heat pump (1) with an evaporator (2) for evaporating a refrigerant, the evaporator (2) having a fan (8), a compressor (4) for compressing the refrigerant, a condenser (5) for liquefying the refrigerant, a controllable expansion valve (7) for expanding the refrigerant and a control device (10), the control device (10) controlling a degree of opening of the expansion valve (7) and/or a speed of the fan (8), and the method comprising the following method steps comprises:measuring an actual overheating value and an evaporation temperature of the refrigerant in the evaporator (2) and comparing the measured evaporation temperature with an upper first limit value and a lower second limit value; andif the evaporation temperature of the refrigerant is greater than the first limit value, operating the heat pump (1) in a first operating state; orif the evaporation temperature of the refrigerant is equal to or lower than the first limit value and equal to or higher than a second limit value, operating the heat pump (1) in a second operating state; orif the evaporation temperature of the refrigerant is lower than the second limit value, the heat pump (1) is operated in a third operating state,wherein the first limit value is greater than the second limit value.wherein the control device (10) regulates the degree of opening of the expansion valve (7) in the second operating state controls depending on a deviation between a specified overheating setpoint and the actual overheating value, and wherein the control device (10) in the second operating state increases the overheating setpoint depending on a deviation of the measured evaporation temperature from the second limit value by a variable offset.

Description

The present invention relates to a method for operating a heat pump and a heat pump. in particular, the proposed method prevents the formation of a layer of ice on an evaporator of an air-to-water heat pump.

Modern air-to-water heat pumps are characterized by their high level of efficiency and can therefore be used to heat a building in a particularly attractive way from an ecological and economic point of view. On the one hand, heating with environmental heat is climate-friendly. On the other hand, many energy suppliers have been offering special heat pump tariffs for a number of years, which are financially more attractive than a normal electricity tariff.

A generic air-water heat pump has a circuit for a refrigerant. At an evaporator, the refrigerant absorbs heat from the ambient air. A possible cause of faults in heat pumps is that a layer of ice forms on the evaporator, which impedes the heat transfer on the evaporator. The layer of ice then usually has to be removed by a defrosting process. The need for defrosting leads to shorter heating cycles and thus reduced operational readiness of the heat pump.

A method and a device for defrosting a heat pump is, for example, in the European patent application EP 0 142 663 A2 described. Measuring the temperature difference between the ambient air and the evaporator is intended to prevent unnecessarily frequent defrosting. If the evaporator temperature exceeds the freezing point (0°C), the defrosting process is blocked.

The Japanese publication JP 2004 - 61 061 A discloses a cooling device using CO2 as a refrigerant. From US Patent No. U.S. 5,253,483 A discloses a refrigeration cycle having a variable speed compressor, a condenser, an evaporator, and a variable speed evaporator fan for moving air across the evaporator, a compressor speed controller, a superheat monitor for monitoring the condition at the inlet of the compressor, and a primary superheat controller.

US patent application no. U.S. 2012/0 260 688 A1 discloses a heat pump system with a variable-capacity compressor, the pumping capacity of a circulating pump being controlled as a function of the evaporation temperature.

1 schematically illustrates the structure of an air-to-water heat pump 1 for a building G. The heat pump 1 uses the ambient air as a heat source to supply the building with heat. during operation, a fan 8 sucks in ambient air and directs it via a heat exchanger or evaporator 2, which can be arranged outside or inside the building G in order to absorb heat from the ambient air. A refrigerant circulates in the evaporator 2, which due to its thermal properties has a relatively low physical state

changes temperature from liquid to gas, i.e. vaporizes. Such a heat pump is not limited to use in a building and could also be used in a vehicle, for example.

In the evaporator 2, heat is extracted from the supplied (warm) outside air W, so that the refrigerant heats up and evaporates. The temperature of the resulting steam is still relatively low for typical heating or hot water applications. One of the reasons for this is that the heat absorbed is used as evaporation heat and does not lead to heating of the refrigerant. From the evaporator 2, the vapor flows further via a four-way valve 3 to an (electrically driven) compressor 4. The compressor 4 increases the pressure so that the temperature of the refrigerant rises to a desired temperature level. As a result, the heated refrigerant flows on to another heat exchanger, the condenser 5. In the condenser 5, the refrigerant transfers its heat to a heat transfer medium of a heating system H, not shown, of the building G and liquefies again.

The heat W' obtained in this way can be used, for example, for heating or hot water preparation. The liquefied refrigerant first collects in a collector 6. The refrigerant then flows through an expansion valve 7, with the pressure and temperature of the refrigerant dropping back to the initial level so that the cycle can begin again.

In or on the evaporator 2, for example at the outlet of the evaporator 2, a temperature sensor 9 is arranged, which measures the temperature, in particular the evaporation temperature of the refrigerant. A control device 10 of the heat pump 1 monitors the evaporation temperature and can regulate a speed of the fan 8 and/or a degree of opening of the expansion valve 7 depending on the measured evaporation temperature. Furthermore, in or on the evaporator 2, for example at the outlet of the evaporator 2, a pressure sensor for measuring the pressure, in particular the evaporation pressure of the refrigerant may be arranged. By measuring the temperature and pressure of the refrigerant in the evaporator, the evaporating temperature and superheat of the refrigerant can be determined.

The object of the present invention is to overcome the problems known in the prior art and to specify a method for operating a heat pump that is improved compared to the prior art. in particular, the method should avoid the formation of glacial ice on the evaporator.

The object is achieved by a method for operating a heat pump according to claim 1 and by a heat pump according to claim 7.

character list

Further advantageous configurations are described in more detail below with reference to an exemplary embodiment which is illustrated in the drawings and to which the invention is not restricted, however.

• 1 illustriert eine Wärmepumpe gemäß einem Ausführungsbeispiel der Erfindung.
• 2 zeigt ein Ablaufdiagramm eines erfindungsgemäßen Verfahrens.
• 3 zeigt ein Diagramm, das einen zeitlichen Verlauf bei Regelung des Überhitzungssollwerts illustriert.
• 4 zeigt ein Diagramm, das einen zeitlichen Verlauf bei Regelung der Lüfterdrehzahl illustriert.
• 5 zeigt ein erstes Regelschema bei Regelung des Überhitzungssollwerts.
• 6 zeigt ein zweites Regelschema bei Regelung der Lüfterdrehzahl.

They show schematically:

• 1 illustrates a heat pump according to an embodiment of the invention.
• 2 shows a flowchart of a method according to the invention.
• 3 shows a diagram that illustrates a time course when controlling the overheating setpoint.
• 4 shows a diagram that illustrates a time course when controlling the fan speed.
• 5 shows a first control scheme for controlling the overheating setpoint.
• 6 shows a second control scheme for controlling the fan speed.

DETAILED DESCRIPTION OF THE INVENTION ON THE BASIS OF EXEMPLARY EMBODIMENTS

In the following description of a preferred embodiment of the present invention, the same reference symbols designate the same or comparable components.

1 1 illustrates an exemplary embodiment of a heat pump 1 according to the invention, in particular for a building G. The heat pump 1 comprises an evaporator 2, a four-way switching valve 3, a compressor 4, a condenser 5, a collector 6, a controllable expansion valve 7, a fan 8 and a control device 10.

The evaporator 2 serves to transfer heat from the ambient air to a heat transfer medium circulating in the circuit of the heat pump, which is referred to below as refrigerant. The refrigerant absorbs the thermal energy from the heat source (air, ground or water) during evaporation and delivers it to a consumer (here condenser 5) by condensing again. there is always energy in these phase transitions, so theoretically any substance is conceivable as a refrigerant. However, a refrigerant that is suitable for a heat pump must have a number of special properties: It should have the lowest possible boiling point, a small vapor volume and a high volume-related cooling capacity. In addition, it must not attack the components and lubricants used, it should be as non-toxic, non-explosive and non-flammable as possible. The impact on the ozone layer and on the greenhouse effect must be as small as possible. Known refrigerants for air-to-water heat pumps include, for example, pentafluoroethane, CO ₂ , propane or butane.

The evaporator 2 can be arranged, for example, outside a building G, or can be supplied with ambient air via an air supply. The amount of air supplied can be controlled via the fan 8 (fan). For efficient heat transfer from the ambient air to the refrigerant, the evaporator 2 can have a large number of lamellae, which increase the surface area of the evaporator. The coolant is caused to evaporate by the heat transfer from the ambient air to the evaporator 2 .

The refrigerant first flows from the evaporator 2 to the four-way switching valve 3 and from there on to the compressor 4. The valve 3 ensures that only superheated vapor reaches the compressor 4. The compressor 4 is, for example, an electrically driven scroll compressor and is used to compress the refrigerant. Compression increases the temperature of the gaseous refrigerant so that it can be used for applications such as heating or water heating.

The compressed refrigerant flows on to the condenser 5, in which a heat transfer from the refrigerant to a heat transfer medium (for example water), for example a heating system H of the building G, takes place. During this heat transfer, the refrigerant liquefies, so that the the heat of condensation released in the process is transferred as usable heat for the heating system H. The condenser 5 can be a plate heat exchanger, for example.

The liquefied refrigerant first collects in the collector 6. From here it flows through the controllable expansion valve 7. In the heat pump circuit, the expansion valve 7 has the task of expanding the liquid refrigerant, which is still under high pressure, after the heat has been transferred to the heating system H. The refrigerant is thus brought into a state that enables it to absorb ambient heat again. In order to prevent liquid from getting into the compressor 2, the expansion valve 7 regulates the amount of refrigerant (refrigerant mass flow) so that only as much refrigerant gets into the evaporator 2 as can be completely evaporated there. The regulated expansion valve 7 can in particular compensate for fluctuations in the source temperature and the power.

The control device 10 regulates the degree of opening of the expansion valve 7 and/or the speed of the fan 8. The controlled variable is the evaporation temperature of the refrigerant in the evaporator 2. A temperature sensor 9 is arranged in the evaporator 2 to measure the evaporation temperature and outputs a measured value to the control device 10. The manipulated variables in the control circuit are thus the degree of opening of the expansion valve 7 and/or the speed of the fan 8. Furthermore, an actual value of the overheating (actual overheating value) of the refrigerant in the evaporator is measured.

An overheating setpoint is specified as the setpoint. in particular, the evaporation temperature in the evaporator 2 can be lowered by lowering the speed of the fan 8 . Alternatively or additionally, the evaporation temperature in the evaporator 2 can be regulated via the degree of opening of the expansion valve 7 , since the evaporation temperature depends, among other things, on the pressure of the refrigerant in the evaporator 2 .

The evaporation temperature can also be measured by measuring the pressure in the evaporator 1 . Furthermore, a temperature of the ambient air can be measured and used by the control device 10 to determine the evaporation temperature of the refrigerant in the evaporator. For this purpose, the heat pump 1 can have suitable sensors in addition to or as an alternative to the temperature sensor 9 shown.

In order for heat to be transferred from the ambient air to the refrigerant, the refrigerant must always be colder than the ambient air. This can lead to moisture from the ambient air condensing on the surfaces of the evaporator 2 . If the evaporator 2 has a temperature of 0°C or lower, this condensate may freeze, so that a layer of frost or ice may form on the surfaces of the evaporator 2 . Since a layer of frost or ice can have negative effects on the efficiency of the heat pump 1, the ice is thawed regularly. This produces liquid water, which flows from top to bottom over the fins of the evaporator 2 due to gravity.

In the ideal case, the evaporator 2 would frost evenly over its entire surface. in reality, however, this is usually not the case. One reason for this can be manufacturing tolerances and control methods. The evaporator 2 can therefore have an uneven temperature distribution on its surface. As a rule, that part of the evaporator circuit through which flow occurs first in the flow direction is colder than the part through which flow lasts. For example, with a corresponding arrangement, the lower part of the evaporator 2 can be colder than the upper part. It can therefore happen that moisture in the ambient air condenses in the upper (warmer) part of the evaporator 2 and then flows along the fins of the evaporator 2 to the lower area of the evaporator 2 . The lower (colder) area of the evaporator 2, on the other hand, frosts up more quickly. In addition, the condensate running down from the upper area freezes in the lower area, so that a solid layer of ice can gradually form. This layer of ice formed from condensate is called glacial ice. The formation of glacial ice can be favored in particular by conventional defrosting methods for removing a layer of frost on the evaporator 2 .

A major problem with the formation of glacial ice is that it cannot be removed by conventional thawing processes and can even continue to build up. If the defrosting is incomplete, the liquid water that forms during the defrosting process can run over the remaining glacier ice and freeze when the heat pump 1 starts up again. As a result, very large layers of ice can form, so that the heat pump 1 can no longer be operated efficiently, or goes into malfunction mode, for example low-pressure shutdowns.

Experience has shown that glacial ice is mainly formed at evaporation temperatures between -4.5 °C and -2 °C. These evaporation temperatures usually occur at an outside temperature (ambient air temperature) of 5 to 7 °C. Under these conditions, there is still a relatively large amount of moisture in the air, which encourages heavy condensation to form.

The present invention aims to avoid the formation of glacial ice. One aspect of the invention is therefore referred to as a function of glacier ice avoidance. The aim of the method is to achieve a surface temperature of the evaporator 2 that is everywhere outside of the critical range mentioned above, which should lead to uniform frosting of the evaporator 2 so that no more condensation water runs down the evaporator. The uniform frosting of the evaporator 2 can thus prevent the formation of glacier ice.

In the following, the mode of operation of the control device 10 is explained using the 2 illustrated flowchart described.

The method begins in step S1 with the normal operation of the heat pump 1. This normal operation is also referred to below as the first operating state B1. In the first operating state, the control device 10 regulates the degree of opening of the expansion valve 7 to a predetermined first superheat setpoint ÜS.

The evaporation temperature of the refrigerant in the evaporator 2 and the actual value of the overheating of the refrigerant are measured continuously. In addition, the measured evaporation temperature is compared with an upper first limit value T1 and a lower second limit value T2.

In step S2 it is checked whether the evaporation temperature falls below the first limit value T1 and whether it is above the second limit value T2. The upper first limit value T1 is -2° C., for example. The lower second limit value T2 is -4.5° C., for example. The first and second limit values thus define an area in which the glacier ice avoidance should be active.

If the evaporation temperature of the refrigerant is greater than the first limit value, the heat pump 1 continues to be operated in the first operating state B1 (normal operation). The glacier ice avoidance function remains inactive (step S8).

If the comparison of the evaporation temperature with the limit values T1 and T2 in step S2 shows that the evaporation temperature of the refrigerant is equal to or lower than the first limit value T1 and equal to or higher than the second limit value T2 - i.e. that the evaporation temperature is within the critical range, the heat pump 1 is operated in a second operating state B2 and the function of preventing glacier ice is activated (step S3).

In step S4, the control device 10 controls the opening degree of the expansion valve 7 in the second operating state B2 as a function of a deviation between the specified overheating setpoint ÜS and the measured overheating actual value. Furthermore, a deviation between the measured evaporation temperature and the second limit value T2 is determined and the overheating setpoint ÜS is increased by a variable offset Δ, the offset Δ being determined as a function of the deviation between the measured evaporation temperature and the second limit value T2. The control device 10 can increase the variable offset Δ as a function of the change in the evaporation temperature up to a maximum offset Δ _max .

In step S5, the control device 10 checks whether the evaporation temperature of the refrigerant is lower than the second limit value T2. If the evaporation temperature of the refrigerant is lower than the second limit value T2, then the heat pump 1 is operated in a third operating state B3. in the third operating state B3, the function of avoiding glacier ice is inactive (S8).

If it is determined in step S5 that the evaporation temperature is greater than (or the same as) the second limit value T2 and less than (or the same as) the first limit value T1), the control device 10 can further increase the variable offset Δ in step S6 depending on the change in the evaporation temperature (up to the maximum offset Δ _max ). In step S7, the control device 10 continuously checks whether the evaporation temperature of the refrigerant is lower or higher than the second limit value T2. Steps S6 and S7 are repeated as long as the evaporation temperature of the refrigerant remains greater than the second limit value T2.

If it is found in step S5 or S7 that the evaporation temperature is lower than the second limit value T2, then the function for avoiding glacier ice becomes inactive (S8). In the third operating state B3, the control device 10 can reduce the variable offset Δ to zero as a function of the change in the evaporation temperature. Sooner or later this will lead to the evaporation temperature rising again, so that after a certain time the second operating state B2 is reached again.

3 12 illustrates a time course of the evaporation temperature VT in the evaporator 1. The horizontal axis is a time axis t. The vertical axis denotes the temperature T. T1 is the first limit of the evaporating temperature, for example -2°C, and T2 is the second limit of the evaporating temperature, for example -4.5°C.

First, the heat pump 1 is operated in the first operating state B1. Here the control device 10 regulates the overheating of the refrigerant in the evaporator 2 via the degree of opening of the expansion valve 7 to the overheating setpoint ÜS. For example, due to the falling outside temperature, the evaporation temperature VT can fall and at time t1 fall below the first limit value T1, so that the heat pump 1 is now operated in the second operating state B2, as described above. The consequence of this is that the overheating setpoint ÜS is increased by an offset Δ depending on the deviation of the evaporation temperature from the second limit value T2. In the second operating state, the control device 10 regulates the degree of opening of the expansion valve 7 as a function of the deviation between the overheating setpoint ÜS and the actual overheating value.

Despite raising the superheat setpoint, the evaporation temperature can continue to fall and at time t2 fall below the second limit value T2, so that the heat pump 1 is operated in the third operating state B3. In the third operating state B3, the control device 10 controls the degree of opening of the expansion valve 7, the control device 10 reducing the variable offset Δ as a function of the change in the evaporation temperature down to zero.

Between time t2 and t3, the evaporation temperature begins to rise again and at time t3 again exceeds the second limit value, so that the heat pump 1 is again operated in the second operating state B2. The control device 10 now increases the variable offset Δ up to the maximum offset Δ _max .

As soon as the evaporation temperature rises above the first limit value T1 again (time t4), the heat pump is again operated in the first operating state B1, with the control device 10 controlling the degree of opening of the expansion valve 7 back to the original overheating setpoint ÜS.

4 shows an alternative embodiment of the method, in which the control device 10 controls the speed of the fan 8 instead of the degree of opening of the expansion valve 7 . 4 essentially corresponds to the 3 , where the fan speed is plotted against time instead of the superheat setpoint. In order to lower the evaporation temperature in the evaporator 2, the control device 10 regulates the fan speed down from a predetermined desired value (max) to a minimum value (fan off, speed equal to zero).

An upper (warmer) area of the evaporator 2 can be cooled by a lower evaporation temperature. As a result, the upper area of the evaporator 2 also has frost. Overall, therefore, a more uniform temperature distribution of the surface of the evaporator 2 and thus uniform frosting can be achieved. The even tires prevent condensate from running down from the top area.

The control method according to the invention can be summarized as follows: The heat pump is initially operated normally (first operating state B1). As soon as the evaporation temperature falls below the first limit value T1 (e.g. -2 °C), the overheating setpoint is increased (second operating state B2). The overheating setpoint is increased until the evaporation temperature falls below the second limit value T2 (e.g. -4.5 °C) (third operating state B3).

The first operating state can be reached, for example, by an increasing outside temperature. If the outside temperature drops, the overheating setpoint can be lowered again. Here, too, normal operation can be established in the first operating state.

5 illustrates the control scheme when controlling the degree of opening of the expansion valve 7 depending on the overheating or the evaporation temperature of the refrigerant in the evaporator 2. The expansion valve 7 thus serves as an actuator of the control loop, which has two controllers, which are either present as separate control devices or together in the control device 10.

The first control loop has the evaporation temperature of the refrigerant in the evaporator 2 as a reference variable. The measured evaporating temperature is compared with the limit values T1 and T2 as described above. If the evaporation temperature is in the critical range between T1 and T2, the second control loop is activated to avoid the formation of glacier ice.

The second control loop has the superheat setpoint with the variable offset Δ as a command variable. The degree of opening of the expansion valve 7 is also used as the manipulated variable. Here, the actual overheating value is compared with the variable setpoint overheating value and the degree of opening of the expansion valve 7 is adjusted as a function of the deviation between the actual overheating value and the setpoint overheating value.

6 illustrates the control scheme when controlling the fan speed. Here again the evaporation temperature of the refrigerant in the evaporator 2 is used as a reference variable. The fan 8 serves as an actuator. The evaporation temperature as a rule variable is measured and depending on a deviation between the measured evaporation temperature and the second limit value T2, the speed of the fan 8 is regulated.

The features disclosed in the above description, the claims and the drawings can be important both individually and in any combination for the implementation of the invention in its various configurations.

Reference List

1

heat pump

2

Evaporator

3

Four-way valve

4

compressor

5

condenser

6

collector

7

expansion valve

8th

Fan

9

temperature sensor

10

control device

Claims (6)

Method for operating a heat pump (1) with an evaporator (2) for evaporating a refrigerant, the evaporator (2) having a fan (8), a compressor (4) for compressing the refrigerant, a condenser (5) for liquefying the refrigerant, an adjustable expansion valve (7) for expanding the refrigerant and a control device (10), wherein the control device (10) controls a degree of opening of the expansion valve (7) and/or a speed of the fan (8), and the method comprising the following method steps: Measuring an actual overheating value and an evaporation temperature of the refrigerant in the evaporator (2) and comparing the measured evaporation temperature with an upper first limit value and a lower second limit value; and if the evaporation temperature of the refrigerant is greater than the first limit value, operating the heat pump (1) in a first operating state; or if the evaporation temperature of the refrigerant is equal to or lower than the first limit value and equal to or higher than a second limit value, operating the heat pump (1) in a second operating state; or if the evaporation temperature of the refrigerant is lower than the second limit value, operating the heat pump (1) in a third operating state, wherein the first limit is greater than the second limit. wherein the control device (10) in the second operating state controls the degree of opening of the expansion valve (7) as a function of a deviation between a predetermined overheating setpoint and the actual overheating value, and wherein the control device (10) in the second operating state increases the overheating setpoint by a variable offset as a function of a deviation of the measured evaporation temperature from the second limit value. procedure after claim 1 , wherein the control device (10) in the second operating state reduces a speed of the fan (8) depending on a deviation of the measured evaporation temperature from the second limit value. procedure after claim 1 , wherein the control device (10) reduces the variable offset to zero in the third operating state as a function of a change in the evaporation temperature. Method according to one of the preceding claims, in which the first limit value corresponds to a temperature of -2°C and the second limit value corresponds to a temperature of -4.5°C. A heat pump (1) comprising: an evaporator (2) for evaporating a refrigerant, the evaporator (2) having a fan (8); a compressor (4) for compressing the refrigerant; a condenser (5) for condensing the refrigerant; a controllable expansion valve (7) for expanding the refrigerant; and a control device (10) for controlling a degree of opening of the expansion valve (7) and/or a speed of the fan (8), the control device (10) being configured: to measure an actual overheating value and an evaporation temperature of the refrigerant in the evaporator (2) and to compare the measured evaporation temperature with an upper first limit value and a lower second limit value; and if the evaporation temperature of the refrigerant is greater than a first limit value, to operate the heat pump (1) in a first operating state; or if the evaporation temperature of the refrigerant is equal to or lower than the first limit value and equal to or higher than a second limit value, to operate the heat pump (1) in a second operating state; or if the evaporating temperature of the refrigerant is smaller than the second limit value, to operate the heat pump (1) in a third operating state, the first limit value being greater than the second limit value, the control device (10) being configured to control the degree of opening of the expansion valve (7) in the second operating state as a function of a deviation between a predefined overheating setpoint and the actual overheating value, and the control device (10) being configured to control the overheating setpoint in the second operating state as a function of a deviation of the measured evaporation temperature from the second limit value to increase a variable offset. Heat pump (1) after claim 5 , wherein the control device (10) is configured to increase the variable offset depending on a change in the evaporation temperature up to a maximum offset in the second operating state.

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