EP4685415A1

EP4685415A1 - Method for controlling a refrigeration system and refrigeration system

Info

Publication number: EP4685415A1
Application number: EP24191190.8A
Authority: EP
Inventors: Damian FORNALSKI; Tobias JANITSCHKE; Thomas KRETZSCHMAR; Matthias GEUKE; Franziska CLAUS; Thomas Exner
Original assignee: Viessmann Holding International GmbH
Current assignee: Viessmann Holding International GmbH
Priority date: 2024-07-26
Filing date: 2024-07-26
Publication date: 2026-01-28

Abstract

Provided is a method for controlling a refrigeration system, comprising a primary circuit in which a refrigerant circulates, a secondary circuit in which a liquid cooling medium circulates, and a heat exchanger which couples the primary circuit to the secondary circuit. The method comprises operating a compressor in the primary circuit, wherein the compressor receives the refrigerant at a suction pressure from the heat exchanger and provides the refrigerant to a condenser at a discharge pressure. Following steps are carried out periodically: detecting a characteristic pressure of the refrigerant; comparing the characteristic pressure of the refrigerant with a predetermined first threshold; calculating a cooling capacity of the compressor as a function of the characteristic pressure of the refrigerant and a speed of the compressor; and calculating an amount of heat transferred from the cooling medium to the refrigerant as a function of the cooling capacity. If the characteristic pressure of the refrigerant is lower than the first threshold, an accumulated amount of heat is calculated by adding the amount of heat of a current time interval to the accumulated amount of heat of a previous time interval. The speed of the compressor may be reduced in dependence on the accumulated amount of heat or by a predetermined increment. Heat exchange between the cooling medium and the refrigerant may be maximized by performing flow control and/or temperature control on the secondary circuit.

Description

The present invention relates to a method for controlling a refrigeration system. Furthermore, the present invention relates to a refrigeration system. In particular, the refrigeration system may be embodied by a heat pump operated in a reversed cycle for providing cooling to a liquid medium.
Heat pumps are distinguished by a high degree of efficiency and are particularly attractive from ecological and economic points of view for providing heating and/or cooling for a building or for providing a hot or cold medium used in industrial processes. Heating and/or cooling using environmental heat is climate-friendly on the one hand. On the other hand, many energy suppliers have been offering special heat pump tariffs for some years which are more attractive financially than a normal electricity tariff.
Known heat pumps, such as for example air-water heat pumps or water/brine-water heat pumps, have a primary circuit in which a refrigerant circulates. As an example, R290 (propane) is widely used as the refrigerant. Since propane is combustible, an inadvertent leakage must be prevented in order to fulfil safety regulations. In a secondary circuit, a liquid medium, such as water or a mixture of water and additives which can lower the freezing temperature, is used as a carrier for heating or cooling, for example of the building or in an industrial process.
The primary circuit comprises at least two heat exchangers. A first exchanger couples the primary circuit to the secondary circuit. Depending on the operation mode of the heat pump, heat is either transferred from the refrigerant to the liquid medium (heating operation) or from the liquid medium to the refrigerant (cooling operation). Depending on the operation mode, the first heat exchanger may be referred to as an evaporator (cooling operation) or a condenser (heating operation). The first heat exchanger is often provided as a plate heat exchanger. In the following description, the "first heat exchanger" is also simply referred to as the "heat exchanger".
Furthermore, the heat pump comprises a second heat exchanger. At the second heat exchanger, the refrigerant absorbs heat from the environment (for example from ambient air or brine) when the heat pump is operated in heating mode. When operated in cooling mode, the refrigerant dissipates heat to the environment at the second heat exchanger. Accordingly, the second heat exchanger may be operated as a condenser (cooling mode) or as an evaporator (heating mode).
During operation in a cooling mode of the heat pump or during operation in a defrosting mode for removing ice from the second heat exchanger, the heat pump is operated in a reversed mode with respect to operation in the heating mode. Since heat is transferred from the liquid carrier medium to the refrigerant, it is possible to cool the liquid carrier medium below its freezing point. Since the liquid carrier medium at least partially consists of water, this can lead to damage of first heat exchanger due to density increase of solid water.
DE 10 2007 052 532 A1 discloses a safety system for refrigeration compressors in refrigeration systems. The safety system detects states in the refrigeration system under computer control. In this case, a safety high-pressure limiter and a high-pressure switch are simultaneously detected and evaluated. Each safety device of the refrigeration system has an influence on the function of the refrigeration compressor. In the event of a fault, the safety high-pressure limiter immediately switches off the compressor and an automatic reset is not possible. In the event of a fault, the high-pressure switch immediately switches off the compressor and a repeated automatic reset is possible. The safety system can be used in all refrigeration systems with at least one refrigeration compressor, i.e. also in heat pumps, drying systems, ice machines and dehumidification systems with one or more compressors.
DE 10 2020 112 376 A1 discloses a heat pump system with a plate heat exchanger and a low-pressure switch which is arranged on the plate heat exchanger and is intended to switch off the compressor of the heat pump system when a predefined switch-off low pressure is reached or undershot.
Furthermore, German patent application DE 10 2024 119 230 relates to a safety device for a reversible heat pump with a pressure limiter and a temperature limiter. The pressure limiter comprises a first switch that opens when a pressure of the refrigerant is lower than a predefined pressure threshold. The temperature limiter comprises a second switch that opens when a temperature of the refrigerant is higher than a predefined temperature threshold. First and second switch are arranged in parallel such that a power supply to the heat pump is disrupted only if both switches are open.
Operating a safety device according to the prior art may lead to unwanted switching operations of the heat pump under certain conditions. Starting out from the prior art, the present invention aims at providing an improved method for operating a refrigeration system under above mentioned challenging conditions which reduces the amount of switching operations of a compressor of the refrigeration system. In particular, the method determines an amount of heat transferred in a heat exchanger between a liquid medium and a refrigerant in order to determine a potential risk of freezing of the liquid medium inside the heat exchanger.
According to a first aspect, the present invention provides a method for controlling a refrigeration system in accordance with claim 1. According to a second aspect, the present invention provides a refrigeration system in accordance with claim 15. Further aspects of the invention are set forth in the dependent claims, the drawings, and the following description of embodiments.
A refrigeration system according to the invention comprises a primary circuit in which a refrigerant circulates and a secondary circuit in which a liquid medium circulates. A (first) heat exchanger couples the primary circuit to the secondary circuit. For example, the refrigeration system can be embodied as a heat pump which is primarily used for heating during winter and is operated in cooling mode during summer. In particular, the refrigeration system may provide cooling for a building, a vehicle, and/or for industrial processes.
The primary circuit comprises at least one compressor, a condenser, and an expansion device. The secondary circuit comprises a pump and a mixing valve.
Preferably, a first pressure sensor may be provided in the primary circuit for detecting a suction pressure of the refrigerant between the heat exchanger and the compressor. The suction pressure may be used to determine a characteristic pressure of the refrigerant. In particular, the characteristic pressure determined from the suction pressure is a measure for the temperature of the refrigerant during evaporation inside the first heat exchanger. An advantage of using a pressure sensor for measuring the characteristic pressure is, for example, that a pressure sensor usually has a shorter reaction time compared to a temperature sensor. Besides this, measuring a temperature of a system or part of a system is usually slower than measuring a pressure, as the intrinsic processes required for measuring a meaningful temperature usually take longer than for measuring a pressure. Thus, even when fast sensors are available, it usually takes longer to detect a change in temperature than detecting a pressure change.
According to preferred embodiments, the characteristic pressure of the refrigerant may be determined using other methods, other sensors and/or additional sensors. For example, at least one temperature sensor may be provided inside the first heat exchanger or at least very close to the first heat exchanger. In particular, a thermocouple or a thermistor may be used to directly measure the temperature.
A second pressure sensor may be provided in the primary circuit for detecting a discharge pressure of the refrigerant between the compressor and the expansion device or between the compressor and the condenser. The discharge pressure of refrigerant after the compressor may be used as a measure for the refrigerant's temperature. From the discharge pressure and the suction pressure, a pressure difference caused by the compressor may be determined. In particular, a cooling capacity of the compressor may be calculated.
The refrigeration system further comprises a control device configured to execute a method according to the present invention. At least some functions of the control device may be carried out by a geographically distant server and/or cloud system connected to a local control device by means of a suitable network interface. An external control device may have the benefit of a higher computational power. Using a local control device may have the benefit of not needing a network connection.
The compressor is operated in the primary circuit for compressing the refrigerant received at a suction pressure from the heat exchanger. The refrigerant is compressed to a discharge pressure by the compressor and subsequently provided to a condenser. In a preferred embodiment of the invention, two or more, three or more, four or more, or five or more compressors may be operated in the primary circuit to compress the refrigerant. For example, the compressors may be provided in series or parallel. By using more than one compressor, a higher cooling capacity may be achieved.
Each step or some steps of the method are carried out periodically at predetermined discrete time intervals. The time intervals may be denoted using an index or counter n which is incremented at each interval and which runs from 0 to N, where N is a positive integer. Accordingly, values denoted with a subscript n may refer to a value of the corresponding time interval.
In a first step, a characteristic pressure of the refrigerant is detected. Throughout this application, the characteristic pressure is denoted as P_s . In particular, the characteristic pressure of the refrigerant is detected at a portion of the primary circuit where the refrigerant is in a gaseous state or at least predominantly gaseous. For example, the characteristic pressure of the refrigerant may be detected by measuring a pressure of the refrigerant. Alternatively, as described above, the characteristic pressure may be detected directly using a temperature sensor.
In a second step, the detected characteristic pressure of the refrigerant is compared to a predetermined first threshold. In particular, it is evaluated whether the characteristic pressure of the refrigerant is lower than, equal to, or higher than the first threshold.
Preferably, the first threshold p_f may be determined based on a freezing temperature of the liquid medium. In case of water, the first threshold may thus be set at or at least close to 0°C. For example, the first threshold may be set to -1°C, -0.5°C, 0.5°C, 1°C, 1.5°C, or 2°C.
In a third step, a cooling capacity of the compressor is calculated as a function of the characteristic pressure of the refrigerant and a speed of the compressor. The speed of the compressor is a measure for the compressor's performance. In particular, the speed may relate to a rotational speed of the compressor. In alternative embodiments, a power of the compressor may be used instead of speed.
According to a preferred embodiment, the following equation (1) expresses the functional dependence of the cooling capacity of the compressor from the detected pressure values and the compressor's speed: $\dot{Q} = f (P_{s}, P_{d}, s)$
In equation (1), the cooling capacity of the system is represented as a flow of heat Q which can be calculated using a mathematical model of the compressor with the input variables suction pressure P_s , discharge pressure P_d , and speed s of the compressor. For example, the mathematical model describing the performance of the compressor may be provided by the manufacturer of the compressor or may be determined experimentally. Further alternatively, a lookup table with empirically or experimentally determined values may be used.
The cooling capacity of time interval n is denoted as Q̇_n . Preferably, the cooling capacity Q̇_n .of the compressor may be calculated as a function of the suction pressure P_s of the refrigerant measured at a low-pressure portion of the primary circuit, a discharge pressure P_d of the refrigerant measured at a high-pressure portion of the primary circuit, and the speed s of the compressor, see equation (1).
In a fourth step, an amount of heat transferred from the liquid medium to the refrigerant is calculated as a function of the cooling capacity. According to the preferred embodiment, the following equation (2) represents the amount of heat Q_i transferred from the liquid medium to the refrigerant at the heat exchanger: $Q_{i} = \sum_{n = 0}^{N} \int_{A_{n}}^{B_{n}} \dot{Q} \cdot {dt}_{f}$
In equation (2), a time integral is calculated from starting time A_n to end time B_n of each time interval n. Then a summation over index n is calculated in order to determine an accumulated amount of heat transferred from the liquid medium to the refrigerant. According to a preferred embodiment, the summation is carried out for every consecutive time interval n as long as the characteristic pressure is lower than the first threshold.
If the characteristic pressure P_s of the refrigerant is lower than the first threshold p_f , an accumulated amount of heat is calculated by adding the amount of heat of a current time interval to the accumulated amount of heat of a previous time interval. According to the preferred embodiment, this calculation can be performed using above equation (2).
Furthermore, the speed s of the compressor is reduced in dependence on the accumulated amount of heat or by a predetermined increment. The speed of the compressor is reduced in order to reduce the cooling capacity and drive the characteristic pressure above the first threshold. By simply reducing the speed of the compressor, it may be possible to avoid an operation of switching off the compressor. Reducing switching operations may improve the compressor's lifetime, increase the length of maintenance intervals and generally lead to a more efficient operation of the refrigeration system.
Additionally, flow control on the secondary circuit can be performed by controlling a pump performance, in particular a pump speed, of a pump provided in the secondary circuit. Furthermore, temperature control on the secondary circuit can be performed by controlling a mixing valve provided in the secondary circuit. For example, the flow of the liquid medium through the first heat exchanger may be maximized in order to prevent freezing of the liquid medium. Furthermore, the temperature of the liquid medium may be increased by switching on or off different portions of the secondary circuit. This operation may also avoid freezing of the liquid medium without switching off the compressor.
A preferred embodiment of the method further comprises a step of comparing the characteristic pressure of the refrigerant with a predetermined second threshold which is higher than the first threshold. As a result, it can be determined that a reduction of the compressor speed in not necessary. In particular, it may be determined that the refrigeration system is in an optimal operating range and the compressor speed may be maintained.
If the characteristic pressure of the refrigerant is higher than the first threshold and lower than the second threshold, the accumulated amount of heat of the current time interval is set equal to the accumulated amount of heat of the previous time interval. In other words, if the characteristic pressure is between the first threshold and the second threshold, the control algorithm is paused and the compressor speed is kept at the current value. Furthermore, the heat exchange between the liquid medium and the refrigerant may be adjusted by performing flow control and/or temperature control on the secondary circuit as described above. If the temperature drops back below the first threshold, the accumulation of the heat integral continues. Only after a certain predefined duration above the second threshold, the integral can be reset to zero and the risk of freezing is fully reset.
If the characteristic pressure of the refrigerant is higher than the second threshold for a predetermined number of consecutive intervals, i.e. during a preset time period, the accumulated amount of heat of the current time interval is set to zero. Furthermore, the speed of the compressor is increased by a predetermined increment or by an increment depending on an actual demand of cooling.
According to a preferred embodiment, the accumulated amount of heat of the current time interval may be compared to a predetermined upper limit for the accumulated amount of heat. If the accumulated amount of heat of the current time interval is equal to or greater than the upper limit, the operation of the compressor is stopped. In other words, the compressor speed is set to zero.
Preferably, the upper limit for the accumulated amount of heat is determined based on a probability for freezing of the liquid medium inside the heat exchanger. For example, the upper limit may be tuned to the system and can be estimated by looking at the amount of heat it takes to freeze a certain percentage of the volume of the first heat exchanger.
In a preferred embodiment the speed s of the compressor may be further controlled based on a specified cooling demand d. A higher demand means that more cooling is requested. A lower demand means that less cooling is requested. Accordingly, higher demand may be met by increasing the speed s of the compressor, whereas a lower demand may lead to a reduced speed s of the compressor. In general, the compressor may require a minimum speed to be operated. Thus, when operating at or near the minimum speed of the compressor, a further reduction in demand or control to avoid freezing of the liquid medium may result in switching off the compressor. However, it is a goal of the present invention to avoid unnecessary switching operations of the compressor.
In a preferred embodiment, the characteristic pressure of the refrigerant can be measured at a low-pressure portion of the primary circuit. In particular, the characteristic pressure of the refrigerant is determined from the suction pressure of the refrigerant measured between the heat exchanger and the compressor.
According to a preferred embodiment, the cooling capacity Qn.of the compressor may be calculated based on a mathematical model describing the performance of the refrigeration system with a particular compressor and refrigerant. Such a mathematical model may be provided by the manufacturer of the compressor. In general, the mathematical model depends on various parameters of the compressor including dimensions and power of the compressor as well as physical properties of the used refrigerant. Alternatively, a lookup table may be used to determine the cooling capacity Q̇_n .of the compressor.
The amount of heat Q_i transferred from the liquid medium to the refrigerant during the time interval may be calculated by integrating the compressor's cooling capacity over the time interval, see equation (2).
The second threshold is set higher than the first threshold by a predetermined offset. In particular, the second threshold may be chosen such, that at the second threshold it is unlikely that the liquid medium is frozen. In other words, the liquid medium is in a liquid state at the second threshold p_m. The offset may preferably correspond to a temperature difference of at least 2 K.
In a preferred embodiment, the refrigeration system is a heat pump which is operated in a reversed cycle. In order to achieve this, the heat pump may comprise a 4-2-way valve which allows inverting the primary cycle from am operation in heating mode to an operation in cooling mode.
The invention is illustrated in greater detail with the aid of a schematic drawing.

Figure 1: Fig 1 illustrates an exemplary embodiment of a refrigeration system according to the invention.
Figure 2: Fig. 2 illustrates an exemplary embodiment of a method for controlling a refrigeration system according to the invention.
Figure 3: Fig. 3 illustrates results of a simulation of the method depicted in Fig. 2.

Detailed description of the embodiments of the invention

Fig. 1 illustrates a portion of a refrigeration system 10 according to an embodiment of the present invention comprising a primary circuit A in which a refrigerant 3 circulates and a secondary circuit B in which a liquid medium 1 circulates. A heat exchanger 2 couples the primary circuit A to the secondary circuit B. This heat exchanger 2 corresponds the "first heat exchanger" mentioned in the above description.
The secondary circuit B comprises a mixing valve 9 which can be operated to control the temperature of the liquid medium provided to the heat exchanger 2. Furthermore, the secondary circuit B comprises a pump 8 for providing a flow of the liquid medium. The volume flow of the liquid medium in the secondary circuit B can be controlled by controlling a performance of the pump 8, in particular a speed it is operating at.
Not illustrated in Fig. 1 is a load disposed in the secondary circuit B. For example, one or more heat exchangers, forward and return lines of an underfloor heating, radiators, means for industrial process cooling, an air condition, etc. may be provided upstream of the mixing valve 9 and downstream of the heat exchanger 2.
The primary circuit A comprises a compressor 5, a condenser (not illustrated) disposed downstream of the compressor, and at least one expansion device (not illustrated) disposed downstream of the condenser. Furthermore, a first pressure sensor 6 is arranged for detecting a suction pressure Ps of the refrigerant at an inlet of the compressor 5. In particular, the first pressure sensor 6 is provided between the heat exchanger 2 and the compressor 5. A second pressure sensor 4 for detecting a discharge pressure Pd of the refrigerant is arranged at an outlet of the compressor 5. In particular, the second pressure sensor 6 is provided downstream of the compressor 6 and upstream of the condenser (not illustrated).
A control device 7 is provided for controlling operation of the switching valve 9, the pump 8, and the compressor 5. Furthermore, the control device 7 receives measurement values from the first and second pressure sensors, respectively. The control device 7 is configured to carry out a method according to the present invention.
According to preferred embodiments, the control device 7 may be embodied as a single local apparatus executing all functions of the method according to the invention. Alternatively, at least some or all functions of control device 7 may be carried out by a geographical distant data processing system, server, or cloud computing system, which may be connected to a local portion of the control device 7 by a network and a suitable interface (not illustrated).
Fig. 2 shows a flow chart illustrating the method according to an embodiment of the present invention. Arrows in Fig. 2 illustrate interactions between function blocks of the flow chart and relate the respective functions to elements of the refrigeration system 10 of Fig. 1.
As depicted in Fig. 2, the first pressure sensor 6 is configured to detect the suctions pressure P_s of the refrigerant upstream of the compressor 5. The second pressure sensor 4 is configured to detect the discharge pressure P_d of the refrigerant downstream of the compressor 5.
Formula (1) describes the dependence of the cooling capacity on the speed s of the compressor 5 and the suctions pressure P_s and discharge pressure P_d of the refrigerant: $\dot{Q} = f (P_{s}, P_{d}, s)$
The cooling capacity of a discrete time interval n is denoted as Q̇_n .
Using equation (2), an accumulated amount of heat Q_i transferred from the liquid medium to the refrigerant at the heat exchanger 2 can be calculated as: $Q_{i} = \sum_{n = 0}^{N} \int_{A_{n}}^{B_{n}} \dot{Q} \cdot {dt}_{f}$
The function block "update Q_i " illustrates how Q_i is updated depending on a comparison between the detected suction pressure P_s , which is a measure of a characteristic pressure of the refrigerant, with a first threshold p_f and a second threshold p_m.
In a first case, the detected suction pressure P_s is smaller than the first threshold p_f , i.e. P_s < p_f. This means that the refrigerant has a characteristic pressure which could potentially freeze the liquid medium inside the heat exchanger 2. In the case of water, the first threshold is thus chosen at or close to 0°C, wherein the exact value of the first threshold (and the second threshold) may vary with the accuracy of the pressure or temperature measurement and external parameters such as geodetic altitude. As a result of the low characteristic pressure of the refrigerant, the accumulated amount of heat at discrete time interval n which has been transferred from the liquid medium to the refrigerant is larger than the total accumulated amount of heat transferred at previous discrete time interval n-1, i.e.: $Q_{i_{n}} > Q_{i_{n - 1}}$
As a result of this condition, the speed of the compressor 5 of the present discrete time interval n is reduced compared to the previous discrete time interval n-1, i.e.: $s_{n} < s_{n - 1}$
Reducing the speed of the compressor 5 reduces the cooling capacity. As a result, the characteristic pressure of the refrigerant should start to rise above the first threshold, in order to avoid freezing of the liquid medium in the heat exchanger.
If the detected suction pressure P_s is larger than the first threshold p_f , but smaller than a second threshold p_m , i.e. p_f < P_s < p_m , then the amount of heat transferred at discrete time interval n from the liquid medium to the refrigerant is equal to the amount of heat transferred at the previous discrete time interval n-1, i.e.: $Q_{i_{n}} = Q_{i_{n - 1}}$
As a result of this condition, the speed of the compressor 5 of the present discrete time interval n is kept at the same value of the previous discrete time interval n-1, i.e.: $s_{n} = s_{n - 1}$
If the detected suction pressure P_s is larger than the second threshold p_m for a predefined amount of time, i.e. P_s > p_m , then the amount of heat transferred at discrete time interval n from the liquid medium to the refrigerant is set to 0, i.e.: $Q_{i_{n}} = 0$
As a result of this condition, the speed of the compressor 5 of the present discrete time interval n is increased compared to the previous discrete time interval n-1, i.e.: $s_{n} > s_{n - 1}$
In the last case where the compressor speed is increased, the amount of increase may depend on a demand d received from, for example, a user input or the like. Preferably, for providing user inputs and/or for providing outputs to the user, the system may comprise a mobile/handheld terminal device (not illustrated) which can be connected to the control device 7. In particular, this terminal device may be used as a human-machine interface between the control device 7 and the user.
As indicated by the arrow going from the function block "update s" to the compressor 5, the control device 7 performs control to adjust the speed of the compressor according to the outcome of the comparison described above.
Additionally to controlling the speed of the compressor 5, the method according to the invention may perform control on the secondary circuit B. For example, in the above-described cases of the detected suction pressure P_s being smaller than the first threshold p_f or at least smaller than the second threshold p_m , the pump 8 in the secondary circuit may be controlled to maximize a flow of the liquid medium through the heat exchanger. This way, freezing of the liquid medium inside the heat exchanger 2 may be avoided.
Furthermore, temperature control may be performed on the secondary circuit B by controlling the mixing valve 9. In particular, the mixing valve 9 may be controlled to increase temperature of the liquid medium in the heat exchanger 2, for example by avoiding a bypass loop and/or by returning liquid medium to the heat exchanger 2 which has an increased temperature after providing cooling to a load disposed in the secondary circuit. This way, a potential freezing of the liquid medium inside the heat exchanger 2 may further be avoided.
Moreover, as shown on the right side of Fig. 2, the accumulated amount of transferred heat Q_i at time interval n is compared to a maximally allowed accumulated amount of transferred heat Q _{i max}. If this upper limit is exceeded, the compressor 5 is switched off, in other words, s_n is set to zero. At the upper limit, the possibility of liquid medium freezing in the heat exchanger 2 is high. To avoid freezing, the compressor 5 is shut off, such that no further cooling takes place and the cooling capacity is zero.
Once the characteristic pressure P_s is higher than the second threshold pm for the predetermined duration, the value of Q_i of the corresponding time interval n is set to zero again and the compressor speed s_n can be increased again.
The upper graph of Fig. 3 shows a simulated timeline of a characteristic pressure of the refrigerant measured using the first pressure sensor 6 of Fig. 1. The upper graph plots temperature in units of °C over time in arbitrary units. The first temperature threshold p_f and the second temperature threshold p_m are indicated as a broken line at 0°C and a dot-dashed line at 1°C, respectively. The dotted curve indicates the measured characteristic pressure derived from the detected suction pressure P_s .
The lower graph of Fig. 3 illustrates cooling capacity of the compressor in units of kW as a dotted line. Furthermore, the vertical bars indicate the accumulated amount of heat transferred from the liquid medium to the refrigerant in units of kJ. The horizontal dashed line indicates the maximally allowed value for the accumulated amount of heat. Vertical dashed lines a to e indicate events where a threshold is reached and are explained in more detail below.
The dotted curve of the characteristic pressure starts at about 1.5°C and quickly drops below the second threshold. As a result, the speed of the compressor is first increased and then maintained. Subsequently, the characteristic pressure drops below the first threshold for the first time at the vertical dashed line a for just a short period. As a result, the accumulated heat is increased, see first bars in the lower graph, and the compressor speed is reduced.
At vertical dashed lines b and c the characteristic pressure again drops below the first threshold for the second and third time. Each time the accumulated heat is increased, see first bars in the lower graph, and the compressor speed is further reduced. Each time the characteristic pressure rises again above the first threshold but stays below the second threshold, the compressor speed is maintained. The reduction of the compressor speed is reflected in the reduced cooling capacity.
At line d, the characteristic pressure again drops below the first threshold. As a result, the accumulated heat, which had been increased three times at lines a, b, and c, would now exceed the maximum accumulated heat (dashed horizontal line). Therefore, the compressor is switched off, which is reflected by a zero cooling capacity near vertical dashed line e. Furthermore, the characteristic pressure starts to rise above the second threshold. After a predefined delay, the accumulated heat is set to zero at line e, the compressor speed is increased and the characteristic pressure stars falling again.
Similar events as previously described occur in the duration after line e. Not shown in Fig. 3 is the control performed on the pump and mixing valve of the secondary circuit which also have influence on the characteristic pressure.
In summary, the described method can efficiently prevent freezing of liquid medium in the heat exchanger coupling the primary circuit with the secondary circuit while at the same time reducing switching operations of the compressor. As a result, a more reliable operation of the refrigeration system can be achieved. In particular, the method performs control of the compressor depending on an accumulated amount of heat transferred between the secondary circuit and the primary circuit. Additionally, flow control and temperature control can be performed on the secondary circuit, for example by controlling a pump and/or a mixing valve.
The features described in the above description, claims and figures can be relevant to the invention in any combination. Their reference numerals in the claims have merely been introduced to facilitate reading of the claims. They are by no means meant to be limiting.

Claims

Method for controlling a refrigeration system comprising:
a primary circuit in which a refrigerant circulates;

a secondary circuit in which a liquid medium circulates; and

a heat exchanger which couples the primary circuit to the secondary circuit, the method comprising:
operating a compressor in the primary circuit, wherein the compressor receives the refrigerant at a suction pressure from the heat exchanger and provides the refrigerant to a condenser at a discharge pressure, wherein the following steps are carried out periodically at predetermined time intervals:
detecting a characteristic pressure of the refrigerant;

comparing the characteristic pressure of the refrigerant with a predetermined first threshold;

calculating a cooling capacity of the compressor as a function of the characteristic pressure of the refrigerant and a speed of the compressor; and

calculating an amount of heat transferred from the liquid medium to the refrigerant as a function of the cooling capacity, wherein:

if the characteristic pressure of the refrigerant is lower than the first threshold:
calculating an accumulated amount of heat by adding the amount of heat of a current time interval to the accumulated amount of heat of a previous time interval; and

reducing the speed of the compressor in dependence on the accumulated amount of heat or by a predetermined increment; and/or

maximizing heat exchange between the liquid medium and the refrigerant by performing flow control and/or temperature control on the secondary circuit.
Method according to claim 1, further comprising the following steps, carried out periodically at the predetermined time intervals:
comparing the characteristic pressure of the refrigerant with a predetermined second threshold which is higher than the first threshold, wherein:
if the characteristic pressure of the refrigerant is higher than the first threshold and lower than the second threshold:
setting the accumulated amount of heat of the current time interval equal to the accumulated amount of heat of the previous time interval; and

maintaining the speed of the compressor; and/or

increasing heat exchange between the liquid medium and the refrigerant by performing flow control and/or temperature control on the secondary circuit.
Method according to claim 2, further comprising the following steps, carried out periodically at the predetermined time intervals:
if the characteristic pressure of the refrigerant is higher than the second threshold for a predetermined number of consecutive intervals:
setting the accumulated amount of heat of the current time interval to zero; and

increasing the speed of the compressor by a predetermined increment.
Method according to one of the previous claims, wherein:
flow control on the secondary circuit is performed by controlling a speed of a pump provided in the secondary circuit; and/or

temperature control on the secondary circuit is performed by controlling a mixing valve provided in the secondary circuit.
Method according to one of the previous claims, further comprising the following steps, carried out periodically at the predetermined time intervals:
comparing the accumulated amount of heat of the current time interval with a predetermined upper limit for the accumulated amount of heat; and

if the accumulated amount of heat of the current time interval is equal to or greater than the upper limit, stopping operation of the compressor.
Method according to claim 5, wherein the upper limit for the accumulated amount of heat is determined based on a probability for freezing of the liquid medium inside the heat exchanger.
Method according to one of the previous claims, wherein the speed of the compressor is further controlled based on a specified cooling demand.
Method according to one of the previous claims, wherein the characteristic pressure of the refrigerant is measured at a low-pressure portion of the primary circuit.
Method according to claim 8, wherein the characteristic pressure of the refrigerant is determined from the suction pressure of the refrigerant measured between the heat exchanger and the compressor.
Method according to one of the previous claims, wherein the cooling capacity of the compressor is calculated as a function of:
the suction pressure of the refrigerant measured at a low-pressure portion of the primary circuit;

a discharge pressure of the refrigerant measured at a high-pressure portion of the primary circuit; and

the speed of the compressor.
Method according to claim 10, wherein the cooling capacity of the compressor is calculated based on a mathematical model describing the performance of the compressor with the refrigerant.
Method according to one of the previous claims, wherein the amount of heat transferred from the liquid medium to the refrigerant during the time interval is calculated by integrating the compressor's cooling capacity over the time interval.
Method according to one of the previous claims, wherein:
the first threshold is determined based on a freezing temperature of the liquid medium; and/or

the second threshold is higher than the first threshold by a predetermined offset.
Method according to one of the previous claims, wherein the refrigeration system is a heat pump operated in a reversed cycle by switching a 4-2-way valve.
Refrigeration system comprising:
a primary circuit in which a refrigerant (3) circulates and having a compressor (5), a condenser, and an expansion device;

a secondary circuit in which a liquid medium (1) circulates and having a pump (8) and a mixing valve (9);

a heat exchanger (2) which couples the primary circuit to the secondary circuit;

a first pressure sensor (6) for detecting a suction pressure of the refrigerant between the heat exchanger (2) and the compressor (5);

a second pressure sensor (4) for detecting a discharge pressure of the refrigerant between the compressor (5) and the expansion device or between the compressor (5) and the condenser; and

a control device (7) configured to execute a method according to one of claims 1 to 14.