Systems and methods for controlling a heat pump system

Abstract

Methods and systems for controlling a heat pump system are disclosed. The heat pump system (1) comprises a compressor unit (2) comprising one or more compressors for compressing a refrigerant, a heat rejecting heat exchanger (4) for performing a heat exchange between the refrigerant and a secondary fluid (3), an expansion device (7), and an evaporator (8). The method comprises the step of deriving an optimal temperature difference dTopt between an inlet temperature, Tin,w, of the secondary fluid (3) and an outlet temperature, Tout,ref, of the refrigerant, at the heat rejecting heat exchanger (4). The method further comprises the step of deriving a measured temperature difference dTm and the step of controlling a pressure of the refrigerant leaving the heat rejecting heat exchanger (4) based on a comparison between dTm and dTopt.

Description

[0001] SYSTEMS AND METHODS FOR CONTROLLING A HEAT PUMP SYSTEM

[0002] TECHNICAL FIELD

[0003] The disclosed technology generally relates to a method and a system for controlling a heat pump system comprising a compressor unit, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant path.

[0004] BACKGROUND

[0005] Heat pumps are widely used in residential, commercial, and industrial applications for heating and cooling. Heat pumps typically comprise a compressor unit comprising one or more compressors, a heat rejecting heat exchanger, an expansion device and an evaporator arranged in a refrigerant path. Refrigerant flowing in the refrigerant path is thereby compressed by the compressor(s) of the compressor unit before being supplied to the heat rejecting heat exchanger. When passing through the heat rejecting heat exchanger, heat exchange takes place between the refrigerant and the ambient or a secondary fluid flowing across the heat rejecting heat exchanger, in such a manner that heat is rejected from the refrigerant. The refrigerant then passes through the expansion device, where it undergoes expansion, before being supplied to the evaporator.

[0006] One of the challenges in operating heat pump systems is optimizing the performance of a heat pump. Different ways to optimize the performance may include regular maintenance and adjusting temperature. Additionally, suitable design parameters of the heat pump allow meeting the specific heating and cooling demands of a space, maximizing its effectiveness.

[0007] Moreover, optimizing pressure levels in a heat pump system allows for maintaining its efficiency and performance. For example, the pressure difference between an evaporator and a heat rejecting heat exchanger, such as a condenser or a gas cooler, directly affects the overall efficiency of the heat pump, as it influences the thermodynamic cycle and the compressor's workload. Improper pressure control can therefore lead to suboptimal heat transfer, increased energy consumption, and premature component wear, such as compressor failure.

[0008] A further challenge is optimizing the performance of heat pumps across their full range of operating conditions. Heat pumps are typically designed for specific temperature and load ranges, and their efficiency can diminish significantly when operating outside these parameters. For instance, maintaining high performance during rapid changes in heat demand further complicates the overall system efficiency. There is therefore a need in the art for addressing above-described challenges and for further improvements in methods and systems. Developing such technologies can allow that heat pump systems operate efficiently, reliably, and with reduced environmental impact.

[0009] SUMMARY

[0010] The herein disclosed technology seeks to mitigate, alleviate or eliminate one or more deficiencies and disadvantages in the prior art to address various problems relating to heat pump systems.

[0011] It is therefore an object of the herein disclosed technology to provide a system and a method that alleviate all or at least some of the drawbacks of presently known systems and methods.

[0012] Further, it is an object of the herein disclosed technology to provide a system, and a method that provide a means for ensuring that a heat pump system operates efficiently.

[0013] Various aspects and embodiments of the herein disclosed technology are defined below and in the accompanying independent and dependent claims.

[0014] A first aspect of the present disclosure relates to a method for controlling a heat pump system, the heat pump system comprising a compressor unit comprising one or more compressors for compressing a refrigerant, a heat rejecting heat exchanger for performing a heat exchange between the refrigerant and a secondary fluid, an expansion device, and an evaporator, wherein the compressor unit, the heat rejecting heat exchanger, the expansion device and the evaporator are being arranged in a refrigerant path, where the refrigerant passing through the heat rejecting heat exchanger has an inlet temperature, Tin,ref, when entering the heat rejecting heat exchanger and an outlet temperature, Tout,ref, when leaving the heat rejecting heat exchanger, and the secondary fluid has an inlet temperature, Tin,w, when entering the heat rejecting heat exchanger and an outlet temperature, Tout, , when leaving the heat rejecting heat exchanger, the method comprising the steps of: deriving an optimal temperature difference dToptbetween the inlet temperature, Tin,w, of the secondary fluid and the outlet temperature, Tout,ref, of the refrigerant by o obtaining design values of

[0015] ■ the inlet temperature, Tin,wo, of the secondary fluid

[0016] ■ the outlet temperature, Tout,wo, of the secondary fluid and ■ a temperature difference, dT0, between the outlet temperature, Tout,refo, of the refrigerant and the inlet temperature, Tin,wo, of the secondary fluid, o measuring the inlet temperature, Tin,w, o obtaining a target outlet temperature, Tout, _target, of the secondary fluid, o deriving an optimal temperature difference, dTopt, from the design values, the measured inlet temperature, Tin,w, and the obtained target temperature, Tout, _target, of the secondary fluid; deriving a measured temperature difference dTmby o measuring the outlet temperature, Tout,ref, of the refrigerant o obtaining the measured temperature difference, dTm, as the difference between the measured outlet temperature between the, Tout,ref, of the refrigerant and the measured inlet temperature, Tin,w, of the secondary fluid; controlling a pressure of the refrigerant leaving the heat rejecting heat exchanger based on a comparison between dTmand dTopt.

[0017] Thus, the herein disclosed method comprises the steps of: deriving an optimal temperature difference dTopt, deriving a measured temperature difference dTm, and controlling a pressure of the refrigerant leaving the heat rejecting heat exchanger based on a comparison between dTm and dToPt

[0018] The herein disclosed method further comprises the steps of: obtaining design values, measuring the inlet temperature, obtaining a target outlet temperature of the secondary fluid, and deriving an optimal temperature difference, dTopt, from the design values, the measured inlet temperature and the obtained target temperature of the secondary fluid.

[0019] The herein disclosed method further comprises the steps of: measuring the outlet temperature of the refrigerant and obtaining the measured temperature difference.

[0020] The herein disclosed method is adapted for controlling a heat pump system, which comprises a compressor unit, a heat rejecting heat exchanger, an expansion device, and an evaporator, and in which the compressor unit, the heat rejecting heat exchanger, the expansion device and the evaporator are being arranged in a refrigerant path.

[0021] The refrigerant passing through the heat rejecting heat exchanger has an inlet temperature, Tin, ref. The inlet temperature of the refrigerant corresponds to the refrigerant temperature when the refrigerant enters the heat rejecting heat exchanger. The refrigerant passing through the heat rejection heat exchanger has an outlet temperature, Tout,ref. The outlet temperature of the refrigerant corresponds to the refrigerant temperature when the refrigerant leaves / exits the heat rejecting heat exchanger. Similarly, the secondary fluid has an inlet temperature, Tin,w, and an outlet temperature, Tout,w. The inlet temperature of the secondary fluid is the temperature when the secondary fluid enters the heat rejecting heat exchanger and the outlet temperature of the secondary fluid is the temperature when the secondary fluid leaves / exits the heat rejecting heat exchanger.

[0022] The target outlet temperature, Tout, _target, represents a temperature level which the secondary fluid is required to be at when leaving the heat rejecting heat exchanger, in order to ensure that heating requirements of the system are met. Thus, the target outlet temperature, Tout, _target, reflects the amount of heat that needs to be extracted from the refrigerant in the heat rejecting heat exchanger, and it will typically be dictated by the system.

[0023] A temperature difference, dT, between the temperature of refrigerant leaving the heat rejecting heat exchanger, i.e. Tout,ref, and the temperature of the secondary fluid entering the heat rejecting heat exchanger, Tin,w, is a suitable indication of the efficiency of the heat transfer taking place in the heat rejecting heat exchanger, between the refrigerant and the secondary fluid. For instance, if this temperature difference is large, this is an indication that the refrigerant passes through the heat rejecting heat exchanger without fully utilizing the potential heat transfer to the secondary fluid. It is therefore desirable to operate the heat pump system in such a manner that an optimal temperature difference is obtained.

[0024] Identifying an optimal temperature difference for a given heat pump system is not trivial, and may require customized measurements to be performed on the individual systems.

[0025] Thus, the method according to the first aspect of the invention comprises the step of deriving an optimal temperature difference dToptbetween the inlet temperature, Tin,w, of the secondary fluid and the outlet temperature, Tout,ref, of the refrigerant. This includes obtaining design values of the heat pump system, measuring a set of relevant temperatures and obtaining a target value for outlet temperature of the secondary fluid, and based on these obtained design values and measured values deriving an optimal temperature difference, dTopt. The design values are the set values and can be obtained for a specific heat pump system. The obtained design values are the inlet temperature, Tin,wo, of the secondary fluid; the outlet temperature, Tout,wo, of the secondary fluid; and, a temperature difference, dT0, between the outlet temperature, Tout,refo, of the refrigerant and the inlet temperature, Tin,wo, of the secondary fluid.

[0026] This implies that the optimal temperature difference, dTopt, is derived from the design values, the measured inlet temperature, Tin,w, and the obtained target outlet temperature, Tout, _target, of the secondary fluid. The optimal temperature difference, such as a target temperature difference, is driven from design values and measurements.

[0027] The step of deriving an optimal temperature difference dToptalso comprises measuring the inlet temperature, Tin,w, and obtaining target value, such as a desired value, Tout, _target, of the outlet temperature, Tout, , of the secondary fluid. The optimal temperature difference, dToptis then obtained based on the design values, the measured temperature value of the secondary fluid and the target outlet temperature, Tout, _target, of the secondary fluid. This implies that the optimal temperature is calculated at a desired pinch point, i.e., at the outlet (seen from the refrigerant flow direction) of the heat rejecting heat exchanger. The optimal temperature difference may be used as a reference or set value, for example against a measured temperature difference. Since the optimal temperature difference, dTopt, is derived solely based on readily available design values and simple measurements of the inlet and outlet temperatures of the secondary fluid, this is a semi-generic and easy manner of determining an optimal operation parameter of the heat pump system.

[0028] Accordingly, the method comprises deriving a measured temperature difference dTm, as the difference between the measured, and actually occurring, outlet temperature, Tout,ref, of the refrigerant and the measured, and actually occurring, inlet temperature, Tin,w, of the secondary fluid. This implies that the measured temperature difference represents the actually occurring temperature difference, at the outlet (seen from the refrigerant flow direction) of the heat rejecting heat exchanger.

[0029] After obtaining the optimum temperature difference dToptand the measured temperature difference dTm, the method comprises controlling a pressure of the refrigerant leaving the heat rejecting heat exchanger based on a comparison between dTmand dTopt. In other words, the optimum temperature difference dToptmay be seen as a setpoint or reference value for the measured temperature difference, dTm, and by comparing the measured temperature difference dTmwith the set reference, for example by assessing or obtaining how much dTmdiffers from dTopt, the heat pump system, more specifically the pressure of the refrigerant at the outlet (seen from the refrigerant flow direction) of the heat rejecting heat exchanger is controlled. Accordingly, the pressure of the refrigerant passing through the heat rejecting heat exchanger is controlled in order to achieve an actual temperature difference, dTm, which is equal to the optimum temperature difference, dTopt, thus obtaining optimal operation of the heat pump system.

[0030] It has been found by the present inventors that the heat transfer in the heat rejecting heat exchanger may be characterized by the pinch point, i.e., the location in the heat rejecting heat exchanger where the temperature difference between the refrigerant and the secondary fluid is minimal. The pinch point in the heat rejecting heat exchanger has an impact on the system performance of the heat pump. Specifically, the heat flow in a heat rejecting heat exchanger is limited by the pinch point and a location of the pinch point. If the pinch point is located internally in the heat exchanger, this may indicate that the heat flow is constrained, and that the heat exchanging surface of the heat rejecting heat exchanger is not optimally used. It is therefore important to have the pinch point at the outlet (seen from the refrigerant flow direction) of the heat rejecting heat exchanger.

[0031] In particular, the presently disclosed method comprises controlling the pressure of the refrigerant at the outlet, i.e., the pressure of the refrigerant leaving the heat rejecting heat exchanger. Controlling the pressure is based on a comparison between dTmand dTopt. Specifically, the disclosed method comprises calculating a reference for the optimal temperature difference, dTopt, at the heat rejecting heat exchanger outlet. dToptis calculated based on a priori known design values of the heat rejecting heat exchanger, and measured temperatures of the secondary fluid at the inlet and at the outlet of the heat rejecting heat exchanger. The disclosed method further comprises measuring the outlet (seen from the refrigerant flow direction) temperature of the refrigerant and obtaining the measured temperature difference, i.e., dTm, that is the temperature difference between the refrigerant outlet and the secondary fluid inlet temperatures. Then the method comprises controlling the pressure of the refrigerant, based on a comparison between dTmand dTopt, for example to obtain desired optimal temperature difference dTopt.

[0032] Accordingly, an advantage of the herein disclosed method is improved efficiency and performance of the heat pump system. Because the disclosed method takes the effect of pinch point into account, the performance of the heat pump system can be improved.

[0033] A further advantage of the herein disclosed technology is providing a generic method for optimizing the heat flow for various heat pump systems. Generally, finding the optimal pressure for a generic heat pump depends on various conditions. Thus, assessing optimal pressure for a heat pump typically requires a specific evaluation that considers several factors such as operating conditions, system design. With the herein disclosed technology, based on priori known design values for the heat exchange and measured temperatures, optimal operations, also in off-design condition, can be obtained. Accordingly, significant energy savings may be achieved.

[0034] In a second aspect the present disclosure relates to a heat pump system comprising, a compressor unit comprising one or more compressors for compressing a refrigerant, a heat rejecting heat exchanger for performing a heat exchange between the refrigerant and a secondary fluid, an expansion device, and an evaporator. The compressor unit, the heat rejecting heat exchanger, the expansion device and the evaporator are arranged in a refrigerant path, where the refrigerant passing through the heat rejecting heat exchanger has an inlet temperature, Tin,ref, when entering the heat rejecting heat exchanger and an outlet temperature, Tout,ref, when leaving the heat rejecting heat exchanger, and the secondary fluid has an inlet temperature, Tin,w, when entering the heat rejecting heat exchanger and an outlet temperature, Tout, , when leaving the heat rejecting heat exchanger. The heat pump system further comprises a control unit configured to perform the method according to any one of the methods of the present disclosure. With this aspect of the disclosed technology, similar advantages and preferred features are present as in the previously discussed aspects.

[0035] Further embodiments are defined in the dependent claims. These and other features and advantages of the disclosed technology will in the following be further clarified with reference to the embodiments described hereinafter.

[0036] DETAILED DESCRIPTION

[0037] It is to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may refer to more than one unit in some contexts, and the like. Furthermore, the words "comprising", "including", "containing" do not exclude other elements or steps. The term "and / or" is to be interpreted as meaning "both" as well and each as an alternative.

[0038] It should be noted that, the term "if" may be construed to mean "when" or "upon" or "in response to determining" or "in response to detecting" depending on the context. Similarly, the phrase "if it is determined" or "when it is determined" or "in an instance of" may be construed to mean "upon determining" or "in response to determining" or "upon detecting and identifying occurrence of an event" or "in response to detecting occurrence of an event" depending on the context.

[0039] Furthermore, the term "obtaining" is herein to be interpreted broadly and encompasses receiving, retrieving, collecting, acquiring, and so forth directly and / or indirectly between two entities configured to be in communication with each other or further with other external entities. However, in some embodiments, the term "obtaining" is to be construed as determining, deriving, forming, computing, etc. Thus, as used herein, "obtaining" may indicate that a parameter is received, or that the parameter is determined based on received data.

[0040] As disclosed above, a first aspect of the present disclosure relates to a method for controlling a heat pump system. Specifically, the presently disclosed method suggests calculating a reference for an optimal temperature difference dToptat the outlet of the heat rejecting heat exchanger, based on design conditions for the heat rejecting heat exchanger and measured deviations from the design conditions.

[0041] The design values are obtained based on the priori known design conditions. The obtained design values are the inlet temperature, Tin,wo, of the secondary fluid, the outlet temperature, Tout.wo, of the secondary fluid, and a temperature difference, dT0, between the outlet temperature, Tout,refo, of the refrigerant and the inlet temperature, Tin,wo, of the secondary fluid. The design values may be provided by data sheet. The design values may be parameters of the heat pump system. Generally, the heat pump may be designed to deliver a certain power capacity (kW) at a given design point / location, such as at the secondary fluid inlet point and / or outlet point. This means the heat rejecting heat exchanger may be designed and optimized for the design point during a component selection process. The component selection process may be based on a simulation run in a dedicated simulation program using several inputs on the geometry, material, surface roughness etc. of the heat rejecting heat exchanger. Accordingly, the heat rejecting heat exchanger may be optimized for the design point and the corresponding design values may be obtained. Alternatively, and / or additionally, for developing a heat rejecting heat exchanger, several tests may be performed at varying operating pressures with an aim to optimize a coefficient of performance (COP) of a heat pump system. Advantageously, the herein disclosed technology may allow taking into account the operating conditions and further altering the operating conditions when / if the heat pump system is not operating at a design point.

[0042] For deriving dTopt, the method comprises measuring the inlet temperature, Tin,w, of the secondary fluid and obtaining a target outlet temperature, Tout, _target, of the secondary fluid, e.g. a system dictated target outlet temperature.

[0043] In some embodiments, the step of deriving the optimal temperature difference value dToptcomprises applying an eguation of dToPt=dTo T Ci(Tin / wo - Tinzw) T C2(Tout,wo—Tout,w_ target) , where Ci and C2are constants. Ci and C2may be empirically obtained for a given refrigerant. For example, Ci and C2may be obtained in response to performing computer implemented simulation and / or experiments in off-design point operations and finding an optimal operation point under these conditions. Ci and C2may generally be independent of a specific heat rejecting heat exchanger layout and hence may be considered to be generic for a given refrigerant.

[0044] The method may further comprise comparing dToptwith dTm. dTmis the measured temperature difference between the refrigerant outlet temperature, i.e. the temperature value of the refrigerant when leaving the heat rejecting heat exchanger, and the secondary fluid inlet temperature, i.e. the temperature value of the secondary fluid when entering the heat rejecting heat exchanger. Based on the comparison between dTmand dTopt, the method comprises controlling a pressure of the refrigerant leaving the heat rejecting heat exchanger. More particularly, the pressure of the refrigerant leaving the heat rejecting heat exchanger may advantageously be controlled in such a manner that the measured temperature difference dTmapproaches the optimal temperature difference dTopt, thus ensuring that the heat pump system operates at an optimal pressure.

[0045] In some embodiments, the step of controlling a pressure comprises controlling the pressure such that the difference between dTmand dToptis minimized. Optimal temperature difference calculation corresponds to having the pinch point at the outlet (seen from the refrigerant flow direction) of the heat rejecting heat exchanger. By minimizing the difference between dTmand dToPt, the system may be led to operate at the pinch point. Because the pressure control may be to minimize the difference of dTmfrom dTopt.

[0046] In some embodiments, the step of controlling a pressure comprises controlling the pressure of the refrigerant leaving the heat rejecting heat exchanger in such a manner that the pressure remains above a pre-defined lower limit pressure value. The pre-defined lower limit may represent a pressure level below which the operation of the heat pump system becomes inefficient or inappropriate for other reasons than the heat transfer taking place in the heat rejecting heat exchanger.

[0047] In some further embodiments, the pre-defined lower limit pressure value represents an energy optimal pressure value of the heat pump system. The energy optimal pressure value may be calculated based on a measured temperature of the refrigerant leaving the heat rejecting heat exchanger. For example, when there is not any constraint for reaching a specific outlet temperature of the secondary fluid for heating purposes, the pressure of the heat rejecting heat exchanger may be optimized to improve overall energy efficiency of the system. The pressure may be reduced to a point where the energy saved in the compression process balances out, such that the balance considers that reducing pressure also reduces the system's overall cooling capacity. Thus, the pre-defined lower limit pressure value or a curve comprising lower limit pressure values may allow maximising the system efficiency and effectiveness. Since heat recovery is the object of a heat pump, it may be considered more efficient and optimal to operate the system at a pressure level which is above the optimal curve, as long as this results in a desired increase in heat recovery, and as long as the energy consumed by the compressors is not excessive compared to the obtained increase in heat recovery. However, operating the system at a pressure level below the optimal curve will result in sub- optimal operation of the system, and this should be avoided.

[0048] In some embodiments, the step of controlling a pressure based on the comparison between dTmand dTopt, comprises increasing the pressure of the refrigerant leaving the heat rejecting heat exchanger when dTmis greater than dTopt. In some other embodiments, the step of controlling a pressure based on the comparison between dTmand dTopt, comprises decreasing the pressure of the refrigerant leaving the heat rejecting heat exchanger when dTmis smaller than dToPt. By increasing or decreasing the pressure, while maintaining the pressure above the pre-defined lower limit, the difference between dTmand dToptmay be minimized.

[0049] For instance, when dTmis greater than dTopt, this is an indication that the actual temperature difference at the outlet of the heat rejecting heat exchanger is higher than the optimal temperature difference. Therefore, when this is the case, the pressure of refrigerant leaving the heat rejecting heat exchanger needs to be adjusted in such a manner that the actual temperature difference decreases. Increasing the pressure may result in a decrease in the temperature. This may be the case for example when CO2is the refrigerant. CO2may have a lower heat capacity at high temperatures, i.e. at the inlet of the heat rejecting heat exchanger. When the temperature reduces with constant pressure, the heat capacity may have a peak value, and when the temperature reduces even further the heat capacity may fall again. Hence in some embodiments, by adjusting the pressure, the heat capacity versus temperature profile through the heat rejecting heat exchanger may be adjusted.

[0050] Similarly, when dTmis smaller than dTopt, the pressure of refrigerant leaving the heat rejecting heat exchanger needs to be decreased, in order to cause the actual temperature difference dTmto increase, thus approaching the optimal temperature difference dTopt.

[0051] In some embodiments of the method for controlling a heat pump system, the heat pump system comprises a receiver arranged in the refrigerant path between the outlet of the heat rejecting heat exchanger and the expansion device, and a high-pressure valve arranged between the outlet of the heat rejecting heat exchanger and the receiver, wherein the step of controlling a pressure comprises adjusting an opening degree of the high-pressure valve.

[0052] Since the high-pressure valve is connected directly to the outlet of the heat rejecting heat exchanger, increasing the opening degree of the high-pressure valve results in a decrease in the pressure of refrigerant leaving the heat rejecting, and decreasing the opening degree of the high-pressure valve results in an increase in the pressure of refrigerant leaving the heat rejecting heat exchanger. Thus, appropriately adjusting the opening degree of the high- pressure valve is a very easy and simple manner of controlling the pressure of refrigerant leaving the heat rejecting heat exchanger.

[0053] In some embodiments, a working cycle of the heat pump system is a transcritical refrigeration cycle. This implies that the pressure and the temperature of the refrigerant at the outlet of the heat rejecting heat exchanger may be controlled independently, however the pressure and the temperature may influence each other.

[0054] In some embodiments, the refrigerant is CO2. This is an example of a refrigerant which is often applied in a transcritical refrigeration cycle. As an alternative, other suitable transcritical refrigerants may be used. As another alternative, the working cycle of the heat pump system may be one that is not transcritical.

[0055] In some embodiments, the secondary fluid is water. In this case, the heated water may, e.g., be applied for water-based room heating, such as floor heating and / or for heating domestic water. As an alternative, the secondary fluid may be another suitable kind of liquid or a gaseous fluid, e.g. atmospheric air.

[0056] Moreover, it is to be noted that the refrigerant flow and the secondary fluid flow through the heat rejecting heat exchanger may preferably be in a counterflow arrangement. An advantage of such an arrangement is that the conditions for the heat transfer may be improved. Alternatively, a co-flow arrangement may also be provided. In the co-flow arrangement, the refrigerant and the secondary fluid may flow through the heat rejecting heat exchanger towards the same direction. In an embodiment with the co-flow arrangement, the temperature differences between the two outlet temperatures of the heat rejecting heat exchanger would be relevant to look at. Specifically, the method for the coflow arrangement may comprise deriving an optimal temperature difference between the outlet temperature of the secondary fluid and the outlet temperature of the refrigerant and obtaining the measured temperature difference between the measured outlet temperature of the refrigerant and the measured outlet temperature of the secondary fluid.

[0057] It is to be understood that the order of the steps of a method may differ from what is depicted. In addition, two or more steps may be performed concurrently or with partial concurrence. Such variation will depend on the software and hardware systems chosen and on designer choice. All such variations are within the scope of the disclosure. Moving on, the presently disclosed technology further relates to a heat pump system comprising a control unit configured to perform any one of the methods as described above.

[0058] The heat pump system may further comprise one or more sensors, such as at least one temperature sensor. The temperature sensors may be configured to measure one or more of the inlet temperature, Tin,w, of the secondary fluid and / or the outlet temperature, Tout,ref, of the refrigerant.

[0059] It will be appreciated that when the present disclosure is described in terms of a method, it may also be embodied in an apparatus or a system comprising one or more processors, one or more memories coupled to the one or more processors, where computer code is loaded to implement the method. For example, the one or more memories may store one or more computer programs that causes the apparatus to perform the steps, services and functions disclosed herein when executed by the one or more processors in some embodiments.

[0060] The heat pump system may comprise a receiver arranged in the refrigerant path between the outlet of the heat rejecting heat exchanger and the expansion device.

[0061] The heat pump system may further comprise a high-pressure valve provided such that the refrigerant leaving the heat rejecting heat exchanger passes through the high-pressure valve before entering the receiver.

[0062] The disclosed aspects and preferred embodiments may be suitably combined with each other in any manner apparent to anyone of ordinary skill in the art, such that one or more features or embodiments disclosed in relation to one aspect may also be considered to be disclosed in relation to another aspect or embodiment of another aspect.

[0063] BRIEF DESCRIPTION OF THE DRAWINGS

[0064] The above aspects, features and advantages of the disclosed technology, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of example embodiments of the present disclosure, when taken in conjunction with the accompanying drawings, in which:

[0065] Fig. 1 is a diagrammatic view of a heat pump system in accordance with an embodiment of the present disclosure, Fig. 2 is a log(P)-h diagram of a refrigerant of a heat pump system in accordance with an embodiment of the present disclosure,

[0066] Fig. 3 is a graph illustrating a coefficient of performance of a heat pump system in accordance with an embodiment of the present disclosure,

[0067] Fig. 4 is a graph illustrating a relationship between a measured temperature difference and a refrigerant outlet pressure in accordance with the embodiment of the present disclosure, and

[0068] Figs. 5a-5c are graphs illustrating temperature profiles along a heat rejecting heat exchanger in accordance with an embodiment of the present disclosure.

[0069] DETAILED DESCRIPTION OF THE DRAWINGS

[0070] The present disclosure will now be described in detail with reference to the accompanying drawings, in which some example embodiments of the disclosed technology are shown. The disclosed technology may, however, be embodied in other forms and should not be construed as limited to the disclosed example embodiments. The disclosed example embodiments are provided to fully convey the scope of the disclosed technology to the skilled person.

[0071] Fig. 1 is a diagrammatic view of a heat pump system 1 in accordance with an embodiment of the present disclosure. This implies that, the heat pump system 1 of Fig. 1 is controlled in accordance with a method according to some embodiments of the present disclosure.

[0072] The heat pump system 1 comprises a compressor unit 2 comprising one or more compressors, a heat rejecting heat exchanger 4, an expansion device 7, in the form of an expansion valve, and an evaporator 8 arranged in a refrigerant path. The heat pump system 1 further comprises a receiver 6 arranged in the refrigerant path between the heat rejecting heat exchanger 4 and the expansion device 7. The heat pump system 1 further comprises a high-pressure valve 5 arranged between the outlet of the heat rejecting heat exchanger 4 and the receiver 6.

[0073] The compressor 2 is fluidly connected to an outlet of the evaporator 8. Refrigerant flowing in the refrigerant path is compressed by the compressor unit 2, before being supplied to the heat rejecting heat exchanger 4. In the heat rejecting heat exchanger 4, heat exchange takes place between the refrigerant flowing through the heat rejecting heat exchanger 4 and a secondary fluid flow 3 across the heat rejecting heat exchanger 4, in such a manner that heat is rejected from the refrigerant and absorbed by the secondary fluid flow 3. The flow direction of the secondary fluid is indicated in Fig. 1 with arrows and dashed lines.

[0074] In the case that the heat rejecting heat exchanger 4 is in the form of a condenser, the refrigerant is thereby at least partly condensed. In the case that the heat rejecting heat exchanger 4 is in the form of a gas cooler, the refrigerant flowing through the heat rejecting heat exchanger 4 is cooled, but it remains in a gaseous or transcritical state.

[0075] In some embodiments, the working cycle of the heat pump system 1 is a transcritical refrigeration cycle, in which case the heat rejecting heat exchanger 4 may advantageously be a gas cooler. In this case the refrigerant flowing in the refrigerant path may be CO2or another suitable transcritical refrigerant.

[0076] The heat pump system comprises a receiver 6, and the refrigerant leaving the heat rejecting heat exchanger 4 passes through the high-pressure valve 5, where it undergoes expansion before being supplied to the receiver 6. In the receiver 6, the refrigerant is separated into a liquid part and a gaseous part. The liquid part of the refrigerant leaves the receiver 6 via a liquid outlet 9, and is supplied to the expansion device 7, where it undergoes expansion before being supplied to the evaporator 8. The refrigerant being supplied to the evaporator 8 is thereby in a mixed gaseous and liquid state. The gaseous part of the refrigerant in the receiver 6 may be supplied directly to the compressor unit 2, via a gaseous outlet 10 and a bypass valve 11.

[0077] In the evaporator 8, heat exchange takes place between the refrigerant flowing through the evaporator 8 and the ambient or the secondary fluid flow across the evaporator 8, in such a manner that heat is absorbed by the refrigerant, while the liquid part of the refrigerant is at least partly evaporated. Direction of the secondary fluid is shown with arrows and dashed lines. Finally, the refrigerant leaving the evaporator 8 is supplied to the compressor unit 2.

[0078] The heat pump system 1 is controlled by deriving an optimal temperature difference dToptbetween an inlet temperature, Tin,w, of the secondary fluid and an outlet temperature, Tout,ref, of the refrigerant; deriving a measured temperature difference dTmas the difference between the measured outlet temperature between the, Tout,ref, of the refrigerant and the measured inlet temperature, Tin,w, of the secondary fluid; and controlling a pressure of the refrigerant leaving the heat rejecting heat exchanger based on a comparison between dTmand dTopt.

[0079] Fig. 2 is a log(P)-h diagram of a heat pump system in accordance with some embodiments of the present disclosure. The heat pump system may, e.g., be the heat pump system 1 illustrated in Fig. 1. From point 14 to point 15 the refrigerant is compressed by the compressor unit 2. Thereby the pressure as well as the enthalpy increases. From point 15 to point 16 the refrigerant passes through the heat rejecting heat exchanger 4, where heat exchange takes place between the refrigerant and the secondary fluid flow 3 across the heat rejecting heat exchanger 4, in such a manner that heat is rejected from the refrigerant. Thereby the enthalpy decreases, while the pressure remains constant. From point 16 to point 17 the refrigerant passes through the high-pressure valve 5, where the refrigerant undergoes expansion, and is received in the receiver 6. Thereby the pressure decreases, while the enthalpy remains substantially constant.

[0080] In the receiver 6, the refrigerant is separated into a liquid part and a gaseous part. Point 18 represents the liquid part of the refrigerant in the receiver 6, and point 19 represents the gaseous part of the refrigerant in the receiver 6. From point 18 to point 20 the liquid part of the refrigerant in the receiver 6 is passed through the expansion device 7, where it undergoes expansion. Thereby the pressure is reduced while the enthalpy remains constant. From point 20 to point 14 the refrigerant passes through the evaporator 8, where heat exchange takes place between the refrigerant and the ambient or the secondary fluid flow 3 across the evaporator 8, in such a manner that heat is absorbed by the refrigerant. Thereby the enthalpy increases, while the pressure remains constant.

[0081] The position of the point 17 corresponds to the enthalpy of the refrigerant which leaves the heat rejecting heat exchanger 4 and is supplied to the receiver 6. This enthalpy determines the liquid-vapour ratio of the refrigerant entering the receiver 6. Thus, when the enthalpy of the refrigerant entering the receiver is low, corresponding to the point 17 being arranged far to the left, a large portion of the refrigerant entering the receiver 6 is liquid. Similarly, when the enthalpy of the refrigerant entering the receiver 6 is high, corresponding to the point 17 being arranged far to the right, a large portion of the refrigerant entering the receiver is gaseous, i.e. in the form of vapour.

[0082] For a transcritical refrigeration cycle, pressure and temperature at the exit of the heat rejecting heat exchanger, such as a gas cooler, may be controlled independently but they may influence each other. The curve 30 represents an optimal curve for a refrigerant at the outlet of the heat rejecting heat exchanger 4. For a given outlet temperature of the refrigerant, the optimal curve 30 provides a lower limit for a pressure value of the refrigerant. Generally, the pressure may be lowered until the level where the reduction in compression work (driven by the pressure increase) is outweighed by an even larger reduction in delivered cooling or heating capacity. The lower limit pressure value may be predefined based on the refrigerant properties. In the transcritical refrigeration cycle, the area towards the right of the optimal curve 30, may comprise a gas loop area. In the gas loop area, towards the low-pressure high temperature, a small drop in the temperature results in a large drop in the enthalpy. A gas fraction of the refrigeration cycle of the heat pump system increases and this may result in a situation where the system cycles gas with limited, if any, cooling or heating.

[0083] Thus, the heat pump system 1 may be configured to control the pressure of the refrigerant leaving the heat rejecting heat exchanger 4 in such a manner that the pressure remains above a pre-defined lower limit pressure value. Advantageously, improvements in the safety and efficiency of the heat pump systems and methods may be achieved.

[0084] Moving on, the heat pump system is configured such that a pressure of the heat rejecting heat exchanger, thereby the pressure of the refrigerant, is controlled based on a comparison between the optimal temperature difference dToptand the measure temperature difference dTm. By controlling the pressure, the location of the pinch point may be changed. dTopt may indicate an optimal state when the pinch point is at the outlet (seen from the refrigerant flow direction) of the heat rejecting heat exchanger. dTmmay indicate an actual temperature difference. By comparing these temperature differences, such as by receiving, obtaining and / or calculating how much dTmdiffers from dTopt, the pressure may be controlled such that the actual pinch point shifts towards the outlet. More specifically, controlling the pressure may comprise increasing and / or decreasing the pressure such that the difference between dTmand dToptis minimized. As a result, the heat pump system may operate while a desired position of the pinch point, i.e., at the outlet (seen from the refrigerant flow direction) of the heat rejecting heat exchanger, is reached.

[0085] Controlling the pressure of the refrigerant may comprise controlling an opening degree of the high-pressure valve. Alternatively, or additionally controlling the pressure of the refrigerant may comprise controlling the compressor unit. Controlling the compressor unit may comprise adjusting the compressor to operate at a higher or a lower pressure. For example, when the capacity of the compressor unit increases, the pressure of the refrigerant at the outlet of the heat rejecting heat exchanger may increase. This may improve the efficiency of the heat pump system. However, increasing the pressure further may result in an inefficient process, since the energy consumption of the compressor unit increases.

[0086] Fig. 3 is a graph illustrating a relationship between a coefficient of performance (COP) of a heat pump system and a refrigerant outlet pressure in accordance with an embodiment of the present disclosure. Fig. 4 shows a graph illustrating a relationship between a measured temperature difference dTmand a refrigerant outlet pressure in accordance with the embodiment of Fig. 3. The measured temperature difference dTmis derived by measuring the outlet temperature, Tout,ref, of the refrigerant, CO2, measuring the inlet temperature, Tin,w, of the secondary fluid, water; and calculating the temperature difference between said measured values.

[0087] As shown in Fig. 4, dTmdecreases with increased pressure. However, increasing the pressure further above a certain value, such as above 114 bar, has less influence on the decrease in dTm. dTmconverges to a certain value, such as 2. A further increase in the pressure does not have a significant effect on dTm. This may be due to the constrained heat transfer. Because the temperatures of the refrigerant and the secondary fluid are close to each other, there would not be further significant heat transfer despite an increase in the pressure. In contrast, increasing the pressure further above a specific value may result in an inefficient process. This phenomenon is illustrated in Fig. 3. As shown in Fig. 3, COP increases with increased pressure up to a pressure value of around 114 bar. However, COP decreases if the pressure is above 114 bar.

[0088] Thus, by appropriately controlling the pressure of refrigerant leaving the heat rejecting heat exchanger, the maximum possible performance of the system may be achieved. Based on measured deviations from the optimal conditions of the temperature difference, pressure may be adjusted. This implies that, controlling the pressure of the refrigerant leaving the heat rejecting heat exchanger may comprise performing an act, such as adjusting a high pressure valve or controlling a compressor, configured for obtaining a higher COP while minimizing the deviations between the measured temperature and the optimal temperature.

[0089] Fig. 5a-5c are graphs illustrating a temperature profile along the heat rejecting heat exchanger in accordance with an embodiment of the present disclosure. Each graph is based on a different refrigerant pressure at the outlet of the heat rejecting heat exchanger. Each graph shows temperature profiles for the refrigerant and for the secondary fluid, water. Dotted lines represent water, and the solid lines represent the refrigerant, e.g., CO2.

[0090] Fig. 5a shows that when the pressure is 97.3492, a temperature difference at the outlet of the heat rejecting heat exchanger (relative length 100%) is larger than a temperature difference within the heat rejecting heat exchanger. Thus, the pinch point, i.e., the location in the heat rejecting heat exchanger where the temperature difference between the refrigerant and the secondary fluid is minimal, is located internally in the heat exchanger. This indicates that the heat flow is constrained, and that the surface of the heat rejecting heat exchanger is not optimally used. When the pressure increases to 109.1995 bar, the pinch point shifts towards the outlet of the heat rejecting heat exchanger as shown in Fig. 5b. Accordingly, the performance in the heat pump system is optimized, by maximizing the heat flow in the heat rejecting heat exchanger, e.g. in the gas cooler in CO2systems. However, increasing the pressure further may not provide any further advantage. Fig. 5c illustrates the temperature profiles of the refrigerant and water when the pressure is 113.1497 bar. Although the pinch point is at the outlet of the heat rejecting heat exchanger, operating with such a pressure may consequences and affect the efficiency of the system adversely because increasing the pressure may cause additional and unnecessary work in the compressor and results in lower COP as discussed above. Reaching to the pinch point internally within the heat rejecting heat exchanger limits the amount of heat that can be effectively transferred, which in return makes the system inefficient in terms of energy usage. In such cases, the heat transfer is constrained, and the heat exchanging surface of the heat rejecting heat exchanger is not optimally used. Accordingly, with the herein disclosed methods and systems, actual, measured, temperatures as well as the pinch point may be taken into consideration while offsetting the pressure. Overall objective of the present disclosure is to improve the efficiency of the heat pump systems. By controlling a pressure of the refrigerant at a location when the refrigerant leaves the heat rejecting heat exchanger, and by basing this control on a comparison between dTmand dTopt, the heat exchanging surface of the heat rejecting heat exchanger may be optimally used, for example as shown in Fig.5b.

Claims

1. CLAIMS1. A method for controlling a heat pump system (1), the heat pump system (1) comprising a compressor unit (2) comprising one or more compressors for compressing a refrigerant, a heat rejecting heat exchanger (4) for performing a heat exchange between the refrigerant and a secondary fluid (3), an expansion device (7), and an evaporator (8), wherein the compressor unit (2), the heat rejecting heat exchanger (4), the expansion device (7) and the evaporator (8) are being arranged in a refrigerant path, where the refrigerant passing through the heat rejecting heat exchanger (4) has an inlet temperature, Tin,ref, when entering the heat rejecting heat exchanger (4) and an outlet temperature, Tout,ref, when leaving the heat rejecting heat exchanger (4), and the secondary fluid has an inlet temperature, Tin,w, when entering the heat rejecting heat exchanger (4) and an outlet temperature, Tout, , when leaving the heat rejecting heat exchanger (4), the method comprising the steps of: deriving an optimal temperature difference dToptbetween the inlet temperature, Tin,w, of the secondary fluid and the outlet temperature, Tout,ref, of the refrigerant by o obtaining design values of■ the inlet temperature, Tin,wo, of the secondary fluid■ the outlet temperature, Tout,wo, of the secondary fluid and■ a temperature difference, dT0, between the outlet temperature, Tout,refo, of the refrigerant and the inlet temperature, Tin,wo, of the secondary fluid, o measuring the inlet temperature, Tin,w, o obtaining a target outlet temperature, Tout, _target, of the secondary fluid (3), o deriving an optimal temperature difference, dTopt, from the design values, the measured inlet temperature, Tin, „ and the obtained target outlet temperature, Tout, _target, of the secondary fluid (3); deriving a measured temperature difference dTmby o measuring the outlet temperature, Tout,ref, of the refrigerant o obtaining the measured temperature difference, dTm, as the difference between the measured outlet temperature between the, Tout,ref, of therefrigerant and the measured inlet temperature, Tin,w, of the secondary fluid (3); controlling a pressure of the refrigerant leaving the heat rejecting heat exchanger (4) based on a comparison between dTmand dTopt.

2. The method according to claim 1, wherein the step of controlling a pressure comprises controlling the pressure such that the difference between dTmand dToptis minimized.

3. The method according to any one of the preceding claims, wherein the step of deriving the optimal temperature difference value dToptcomprises applying an equation of dTopt=dTo + Ci (Tjn,wO ” Tjn,w) + C2(Tout,wO—Tout,w_target) , where Ci and C2are constants.

4. The method according to any one of the preceding claims, where in the step of controlling a pressure comprises controlling the pressure of the refrigerant leaving the heat rejecting heat exchanger (4) in such a manner that the pressure remains above a pre-defined lower limit pressure value (30).

5. The method according to claim 4, wherein the pre-defined lower limit pressure value represents an energy optimal pressure value of the heat pump system (1).

6. The method according to any one of the preceding claims, wherein the step of controlling a pressure based on the comparison between dTmand dTopt, comprises increasing the pressure of the refrigerant leaving the heat rejecting heat exchanger (4) when dTmis greater than dToptor decreasing the pressure of the refrigerant leaving the heat rejecting heat exchanger (4) when dTmis smaller than dTopt.

7. The method according to any one of the preceding claims, wherein the heat pump system (1) further comprises a receiver (6) arranged in the refrigerant path between the outlet of the heat rejecting heat exchanger (4) and the expansion device (7), and a high pressure valve (5) arranged between the outlet of the heat rejecting heat exchanger (4) and the receiver (6), wherein the step of controlling a pressure comprises adjusting an opening degree of the high pressure valve (5).

8. The method according to any one of the preceding claims, wherein a working cycle of the heat pump system (1) is a transcritical refrigeration cycle.

9. The method according to any one of the preceding claims, wherein the refrigerant is CO2.

10. The method according to any one of the preceding claims, wherein the secondary fluid is water.

11. A heat pump system (1) comprising, a compressor unit (2) comprising one or more compressors for compressing a refrigerant, a heat rejecting heat exchanger (4) for performing a heat exchange between the refrigerant and a secondary fluid (3), an expansion device (7), and an evaporator (8), wherein the compressor unit (2), the heat rejecting heat exchanger (4), the expansion device (7) and the evaporator (8) are being arranged in a refrigerant path, where the refrigerant passing through the heat rejecting heat exchanger (4) has an inlet temperature, Tin,ref, when entering the heat rejecting heat exchanger (4) and an outlet temperature, Tout,ref, when leaving the heat rejecting heat exchanger (4), and the secondary fluid has an inlet temperature, Tin,w, when entering the heat rejecting heat exchanger (4) and an outlet temperature, Tout, , when leaving the heat rejecting heat exchanger (4), the heat pump system (1) further comprising a control unit configured to perform the method according to any of the preceding claims.

12. The heat pump system according to claim 11, further comprising a receiver (6) arranged in the refrigerant path between the outlet of the heat rejecting heat exchanger (4) and the expansion device (7).

13. The heat pump system (1) according to claim 12, further comprising a high pressure valve (5) provided such that the refrigerant leaving the heat rejecting heat exchanger (4) passes through the high pressure valve (5) before entering the receiver (6).

14. The heat pump system (1) according to anyone of claims 11-13, further comprising one or more sensors.

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