Technical field

The present disclosure relates generally to the field of heat pumps. More particularly, it relates to a heat pump comprising a first heat exchanger and a compressor connected to the first heat exchanger.

Background art

Nearly all large developed cities in the world have at least two types of energy grids incorporated in their infrastructures; one grid for providing electrical energy and one grid for providing space heating and hot tap water preparation. Today a common grid used for providing space heating and hot tap water preparation is a gas grid providing a burnable gas, typically a fossil fuel gas. The gas provided by the gas grid is locally burned for providing space heating and hot tap water. In order to reduce the carbon dioxide emissions there are plans to replace such gas grid with more “green” energy efficient energy systems.

One such energy efficient energy system is cold thermal grids. Cold thermal grids are an evolution of district heating and district cooling systems, where combined district heating and district cooling system with aid of using heat pumps for heating and cooling can provide both cooling, heating and tap water preparation to buildings.

In order to succeed with the replacement of gas grids, where the respective gas burner is replaced by a heat pump, the heat pumps used need to be smaller, less costly and with lower technical complexity, e.g., with fewer and/or less complex sensors for measuring the space heat and tap water energy consumption than presently used heat pumps.

Methods for measuring the heat power transferred from a heat pump are described in WO 1987/005097 A1. However, the methods disclosed have some disadvantages, e.g., the need of a plurality of costly and technically complex pressure sensors. Thus, there may be a need for estimating the heat power transferred from the heat pump with fewer and/or less complex sensors.

Summary

It is an object of some embodiments to solve, mitigate, alleviate, or eliminate at least some of the above or other problems or disadvantages.

According to a first aspect, this is achieved by a heat pump for heating or cooling. The heat pump comprises a first heat exchanger, a compressor connected to the heat exchanger, a second heat exchanger connected to an inlet of the compressor, a valve having an inlet connected to an outlet of the first heat exchanger and an outlet connected to an inlet of the second heat exchanger, a temperature sensor at an inlet of the first heat exchanger, and a temperature sensor at an outlet of the first heat exchanger.

The heat pump further comprises control circuitry configured to determine an inlet temperature of a fluid at the inlet of the first heat exchanger. Furthermore, the control circuitry is configured to determine an outlet temperature of a fluid at the outlet of the first heat exchanger. Moreover, the control circuitry is configured to determine a condensation temperature for a refrigerant based on the determined inlet and outlet temperatures and based on characteristics of the first heat exchanger. The control circuitry is configured to determine a gas pressure at an outlet of the compressor based on the determined condensation temperature and based on characteristics of the refrigerant.

The first aspect may alternatively be expressed as the provision of a heat pump for heating or cooling comprising: a first heat exchanger, and a compressor connected to the first heat exchanger, a second heat exchanger connected to an inlet of the compressor, a valve having an inlet connected to an outlet of the first heat exchanger and an outlet connected to an inlet of the second heat exchanger, a temperature sensor at an inlet of the first heat exchanger, and a temperature sensor at an outlet of the first heat exchanger, the heat pump further comprising control circuitry configured to: determine an inlet temperature of a fluid at an inlet of the first heat exchanger; determine an outlet temperature of a fluid at an outlet of the first heat exchanger; determine a condensation temperature for a refrigerant based on the determined inlet and outlet temperatures and based on characteristics of the first heat exchanger; determine a gas pressure at an outlet of the compressor based on the determined condensation temperature and based on characteristics of the refrigerant.

In some embodiments, the heat pump further comprises one or more of: an electrical power meter connected to the compressor; a temperature sensor at the outlet of the compressor; a second heat exchanger connected to an inlet of the compressor; a valve having an inlet connected to an outlet of the first heat exchanger and an outlet connected to an inlet of the second heat exchanger; a temperature sensor at the inlet of the valve; a pressure sensor at the inlet of the compressor; a temperature sensor at the inlet of the compressor; a temperature sensor at the inlet of the first heat exchanger; a temperature sensor at the outlet of the first heat exchanger; a temperature sensor at the inlet of the second heat exchanger; and a temperature sensor at the outlet of the second heat exchanger.

In some embodiments, a single pressure sensor at the inlet of the compressor is utilized for all pressure measurements.

According to a second aspect, there is provided a method for a heat pump, the heat pump comprising a first heat exchanger, a compressor connected to the first heat exchanger, a second heat exchanger connected to an inlet of the compressor, a valve having an inlet connected to an outlet of the first heat exchanger and an outlet connected to an inlet of the second heat exchanger, a temperature sensor at an inlet of the first heat exchanger and a temperature sensor at an outlet of the first heat exchanger. The method comprises determining an inlet temperature of a fluid at the inlet of the first heat exchanger. Furthermore, the method comprises determining an outlet temperature of a fluid at the outlet of the first heat exchanger. Moreover, the method comprises determining a condensation temperature for a refrigerant based on the determined inlet and outlet temperatures and based on characteristics of the first heat exchanger. The method comprises determining a gas pressure at an outlet of the compressor based on the determined condensation temperature and based on characteristics of the refrigerant.

The second aspect may alternatively be expressed as the provision of a method for a heat pump, the heat pump comprising a heat exchanger and a compressor connected to the heat exchanger, the method comprising: determining an inlet temperature of a fluid at an inlet of the heat exchanger; determining an outlet temperature of a fluid at an outlet of the heat exchanger; determining a condensation temperature for a refrigerant based on the determined inlet and outlet temperatures and based on characteristics of the first heat exchanger; determining a gas pressure at an outlet of the compressor based on the determined condensation temperature and based on characteristics of the refrigerant.

In some embodiments, the method further comprises determining a first heat power transferred at the first heat exchanger based on an electrical power measured by an electrical power meter, based on one or more temperatures measured by one or more temperature sensors, based on a pressure measured by a pressure sensor at the inlet of the compressor, based on the determined gas pressure and based on characteristics of the refrigerant.

In some embodiments, the method further comprises determining a second heat power transferred at the second heat exchanger based on an electrical power measured by an electrical power meter, based on one or more temperatures measured by one or more temperature sensors, based on a pressure measured by a pressure sensor at the inlet of the compressor, based on the determined gas pressure and based on characteristics of the refrigerant.

In some embodiments, the method further comprises determining a third heat power transferred by applying an offset to the first or the second transferred heat power.

In some embodiments, the offset is determined based on one or more temperatures measured by one or more temperature sensors, based on a pressure measured by a pressure sensor at the inlet of the compressor and based on characteristics of the refrigerant.

In some embodiments, the method further comprises acquiring the characteristics of the first heat exchanger from a first lookup table or from a first set of lookup tables; acquiring the characteristics of the refrigerant from a second lookup table or from a second set of lookup tables; and/or acquiring the offset from a third lookup table or from a third set of lookup tables.

According to a third aspect, there is provided a computer program product comprising a non-transitory computer readable medium, having thereon a computer program comprising program instructions, the computer program being loadable into a data processing unit and configured to cause execution of the method of the second aspect or any of the above-mentioned embodiments when the computer program is run by the data processing unit.

Effects and features of the second and third aspects are to a large extent analogous to those described above in connection with the first aspect and vice versa. Embodiments mentioned in relation to the first aspect are largely compatible with the second and third aspects and vice versa.

An advantage of some of the embodiments is that fewer pressure sensors are needed for measuring the heat power transferred from heat pumps. Thus, the technical complexity (and the cost) may be reduced.

Another advantage of some of the embodiments is that the heat power transferred is not overestimated. Thus, the energy grid owner avoids the risk of charging the energy costumer for more than the actual heat power transferred while having a low complexity heat exchanger, thus facilitating and/or speeding up the transition from a gas grid system to a green thermal grid system thereby enabling a faster reduction of carbon dioxide emissions.

Yet another advantage of some of the embodiments is that updating is facilitated, e.g., in case parts of the heat pump, such as heat exchangers or refrigerants, need to be replaced.

A further advantage of some of the embodiments is that the determining of a gas pressure is simplified and/or faster, e.g., by using low complexity temperature sensors and look up tables.

Brief description of the drawings

Further objects, features and advantages will appear from the following detailed description of embodiments, with reference being made to the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the example embodiments.

Figure 1a is a schematic drawing illustrating a heat pump according to some embodiments;

Figure 1b is a flow chart illustrating a prior art heat pump;

Figure 2 is a flow chart illustrating a method for a heat pump;

Figure 3 is a schematic drawing illustrating a heat pump according to some embodiments;

Figure 4 is a schematic drawing illustrating a computer program product;

Figure 5 is a schematic graph illustrating a relation between a temperature difference and a condensation temperature for different inlet temperatures; and

Figure 6 is a refrigerant diagram of a refrigerant, R134a, illustrating a relation between enthalpy and pressure for different condensation temperatures.

Detailed description

It should be emphasized that the term "comprises/comprising" (replaceable by "includes/including") when used in this specification is taken to specify the presence of stated features, integers, steps, or components, but does not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

Generally, when an arrangement is referred to herein, it is to be understood as a physical product, e.g., an apparatus. The physical product may comprise one or more parts, such as controlling circuitry in the form of one or more controllers, one or more processors, or the like.

Embodiments of the present disclosure will be described and exemplified more fully hereinafter with reference to the accompanying drawings. The solutions disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the embodiments set forth herein.

Terminology

The term “Refrigerant” utilized below is a working fluid circulating in the heat pump loop. The Heat pump loop is defined as the loop, in which the working fluid is circulated through the first heat exchanger 110, the compressor 120, and optionally through the second heat exchanger 130, and the valve 140 (all shown in figure 1 ).

The term “heat power transferred” utilized below is a heat usage, such as an amount of heat utilized by a consumer, and is typically measured in kilowatt-hours, kWh.

Figure 1a illustrates a heat pump 100. The heat pump 100 is for heating and/or cooling. The heat pump 100 comprises a first heat exchanger 110 and a compressor 120. The compressor is connected to the first heat exchanger 110. The heat pump 100 further comprises control circuitry 160. The control circuitry 160 is configured to determine 310 an inlet temperature Tin of a fluid at an inlet 112 of the first heat exchanger 110. To this end, the control circuitry 160 may be associated with (e.g., operatively connectable, or connected to) a temperature sensor 116 at the inlet 112 of the first heat exchanger 110. Furthermore, the control circuitry 160 is configured to determine 320 an outlet temperature Tout of a fluid at an outlet 114 of the first heat exchanger 110. To this end, the control circuitry 160 may be associated with (e.g., operatively connectable, or connected to) a temperature sensor 118 at the outlet 114 of the first heat exchanger 110. Optionally, the control circuitry 160 is configured to determine a difference between the fluid temperature at the inlet 112 and the outlet 114 of the first heat exchanger 110, i.e. , a temperature difference AT. To this end, the control circuitry 160 may be associated with (e.g., operatively connectable, or connected to) a temperature difference determining unit (e.g., temperature difference control circuitry or temperature difference control module). Moreover, the control circuitry 160 is configured to determine 330 a condensation temperature Tc for a refrigerant based on the determined inlet and outlet temperatures Tin, Tout and based on characteristics of the first heat exchanger 110. To this end, the control circuitry 160 may be associated with (e.g., operatively connectable, or connected to) a condensation temperature determining unit (e.g., condensation temperature determining circuitry or condensation temperature determining module). If a temperature difference has been determined, the condensation temperature Tc for the refrigerant may instead be based on the determined inlet temperature Tin, the temperature difference AT and characteristics of the first heat exchanger 110. In some embodiments, the condensation temperature Tc for a particular refrigerant is found in a lookup table, LUT or similar describing the characteristics of the heat exchanger by inserting the inlet and outlet temperatures Tin, Tout (or by inserting the inlet temperature Tin and the temperature difference AT). The refrigerant is a known refrigerant or can be determined, e.g., by the control circuitry 160. One example of a refrigerant is R134a. Another example of a refrigerant is R410a. The control circuitry 160 is configured to determine 340 a gas pressure P1 at an outlet 122 of the compressor 120 based on the determined condensation temperature Tc and based on characteristics of the refrigerant. To this end, the control circuitry 160 may be associated with (e.g., operatively connectable, or connected to) a gas pressure determining unit (e.g., gas pressure determining circuitry or gas pressure determining module). In some embodiments, the gas pressure P1 is found in a lookup table, LUT, or similar describing the characteristics of the refrigerant by inserting the determined condensation temperature Tc.

The heat pump 100 may further comprise one or more of: a temperature sensor 124 at the outlet 122 of the compressor 120; a pressure sensor 128 at the inlet 126 of the compressor 120; a temperature sensor 129 at the inlet 126 of the compressor 120; a second heat exchanger 130 connected to an inlet 126 of the compressor 120; a temperature sensor 136 at the inlet 132 of the second heat exchanger 130; a temperature sensor 138 at the outlet 134 of the second heat exchanger 130; a valve 140, such as a throttle valve, having an inlet 142 connected to an outlet 119 of the first heat exchanger 110 and an outlet 144 connected to an inlet 139 of the second heat exchanger 130; a temperature sensor 146 at the inlet 142 of the valve 140; and an electrical power meter 150 connected to the compressor 120.

In some embodiments, the pressure sensor 128 at the inlet 126 of the compressor 120a is the only pressure sensor 128 of the heat pump 100 and is utilized for all pressure measurements for the heat pump. Figure 1 b illustrates a prior art heat pump 101 . The prior art heat pump 101 has at least two pressure sensors 121 , 128. Furthermore, the heat pump 101 is provided with a flowmeter (not shown) for measuring heat. As the heat pump 100 only has one pressure sensor, the technical complexity (and the cost) of the heat pump 100 may be reduced. Furthermore, the heat pump 100 may be smaller than the prior art heat pumps.

Figure 2 illustrates a method 200 for the heat pump 100. The heat pump 100 comprises a first heat exchanger 110 and a compressor 120 connected to the first heat exchanger 110. The method comprises determining 210 an inlet temperature Tin of a fluid at an inlet 112 of the first heat exchanger 110. Furthermore, the method 200 comprises determining 220 an outlet temperature Tout of the fluid at an outlet 114 of the first heat exchanger 110. Optionally, the method 200 comprises determining 222 a difference between the fluid temperature at the inlet 112 and the outlet 114 of the first heat exchanger 110, i.e., a temperature difference AT. In some embodiments, the method comprises acquiring 225 the characteristics of the first heat exchanger 110 from a first lookup table, LUT, or from a first set of lookup tables. Each of the LLIT’s in the first set of LLIT’s may be associated with a particular type of heat exchanger. Then, the LUT, which corresponds to the type the first heat exchanger is, is selected. The data for the LUT’s may be derived empirically for each type of heat exchanger. The characteristics of a heat exchanger depends on whether the heat exchanger has parallel flow or counter flow, how large the surface area of the wall between the two fluids between which heat is exchanged is, and/or any fins or corrugations present in one or both directions of the flow inside the heat exchanger. Moreover, the method 200 comprises determining 230 a condensation temperature Tc for a refrigerant based on the determined inlet and outlet temperatures Tin, Tout and based on characteristics of the first heat exchanger 110. In some embodiments, the method 200 comprises acquiring 235 the characteristics of the refrigerant from a second lookup table or from a second set of lookup tables. Each of the LUT’s in the second set of LUT’s may be associated with a particular type of refrigerant, e.g., R134a. Then, the LUT, which corresponds to the type the refrigerant is, is selected. The method 200 comprises determining 240 a gas pressure P1 at an outlet 122 of the compressor 120 based on the determined condensation temperature Tc and based on characteristics of the refrigerant. In some embodiments, the method 200 comprises determining 250 a first heat power transferred at the first heat exchanger 110 based on an electrical power measured by an electrical power meter 150, based on one or more temperatures measured by one or more temperature sensors 124, 129, 136, 138, 146, based on the determined gas pressure P1 , based on a pressure P2 measured by a pressure sensor 128 at the inlet 126 of the compressor 120 and based on characteristics of the refrigerant. In some embodiments, the method 200 comprises determining 260 a second heat power transferred at the second heat exchanger 130 based on an electrical power measured by an electrical power meter 150, based on one or more temperatures measured by one or more temperature sensors 124, 129, 136, 138, 146, based on the determined gas pressure P1 , based on a pressure P2 measured by a pressure sensor 128 at the inlet 126 of the compressor 120 and based on characteristics of the refrigerant. The determining of first and/or second heat power may be performed as indicated in WO 1987/005097 A1.

In some embodiments, the method 200 comprises determining 270 a third heat power transferred by applying an offset to the first or the second transferred heat power, e.g., reducing the first or the second transferred heat power with the offset. The offset may be determined based on one or more temperatures measured by one or more temperature sensors 124, 129, 136, 138, 146, based on the determined gas pressure P1 , based on the pressure P2 measured by the pressure sensor 128 at the inlet 126 of the compressor 120 and based on characteristics of the refrigerant. In some embodiments, the method 200 comprises acquiring 265 the offset from a third lookup table or from a third set of lookup tables. Each of the LLIT’s in the third set of LLIT’s may be associated with a particular temperature or a set of temperatures and/or pressures. Then, the LUT, which corresponds to the measured temperature(s) and/or pressure(s) is selected. Furthermore, the offset may correspond to a maximum error margin, i.e. , the largest measurement error that can occur, based on information about measurement accuracies of the temperature and pressure sensors utilized. In some embodiments, a number of samples of estimated heat power transferred is taken over a time period and filtered, such as averaged or averaged with different weights for the different samples, in order to obtain an accuracy measure of the heat power transferred. The accurate measure may be a standard deviation or a variance around a mean heat power transferred over a certain time period. The accuracy may then be obtained as a confidence interval such that the heat power transferred is in the range defined by the estimated mean heat power transferred +/- 3 standard deviations (with 99.7%). In some embodiment the accuracy may be determined as an offset value from the estimated mean power outtake over a first time period, e.g., the offset value may be 3 standard deviations. The accuracy information of the heat power transferred from a plurality of heat pumps may be transferred to an energy grid control unit controlling the entire energy grid, and by having accuracy measures of the heat power transferred from respective heat pump, a more accurate control, such as an optimized control, of the energy grid can be made, thus saving energy. Furthermore, by applying an offset, the heat power transferred is not overestimated, i.e. , the estimated heat power transferred is always lower than or equal to the actual heat power transferred. Thus, the energy grid owner avoids the risk of charging the energy costumer for more than the actual heat power transferred. Thus, the heat pump 100 does not need any flowmeter and therefore the heat pump 100 may be made smaller and/or less complex. In some embodiments, a control unit or the control circuitry 160 comprises the first, second and/or third lookup tables and/or the first, second and/or third sets of lookup tables.

Figure 3 illustrating the heat pump 100 as an apparatus 300 with control circuitry. The control circuitry performs the actions 310, 320, 330, 340 as described in connection with figure 1 . Furthermore, in some embodiments, the control circuitry performs one or more of the actions 325, 335, 350, 360, 365, 370 in the same, corresponding or a similar manner as the method steps 225, 235, 250, 260, 265, 270 described above in connection with figure 2. Figure 3 illustrates example method steps implemented in the apparatus 300. The apparatus 300 comprises controlling circuitry. The controlling circuitry may be one or more processors, such as the control circuitry 160. The controlling circuitry is configured to cause determination 310 of an inlet temperature Tin of a fluid at an inlet 112 of the first heat exchanger 110. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a first determination unit (e.g., first determining circuitry or a first determiner). Furthermore, the controlling circuitry is configured to cause determination 320 of an outlet temperature Tout of the fluid at an outlet 114 of the first heat exchanger 110. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a second determination unit (e.g., second determining circuitry or a second determiner). Moreover, optionally the controlling circuitry is configured to cause determination 322 of a difference between the fluid temperature at the inlet 112 and the outlet 114 of the first heat exchanger 110, i.e. , a temperature difference AT. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a third determination unit (e.g., third determining circuitry or a third determiner). The controlling circuitry is optionally configured to cause acquisition 325 of the characteristics of the first heat exchanger 110 from a first lookup table, LUT, or from a first set of lookup tables. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a first acquisition unit (e.g., first acquiring circuitry or a first acquirer).

Furthermore, the controlling circuitry is configured to cause determination 330 of a condensation temperature Tc for a refrigerant based on the determined inlet and outlet temperatures Tin, Tout and based on characteristics of the first heat exchanger 110. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a fourth determination unit (e.g., fourth determining circuitry or a fourth determiner). In some embodiments, the controlling circuitry is configured to cause acquisition 335 of the characteristics of the refrigerant from a second lookup table or from a second set of lookup tables. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a second acquisition unit (e.g., second acquiring circuitry or a second acquirer). Moreover, the controlling circuitry is configured to cause determination 340 of a gas pressure P1 at an outlet 122 of the compressor 120 based on the determined condensation temperature Tc and based on characteristics of the refrigerant. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a fifth determination unit (e.g., fifth determining circuitry or a fifth determiner). In some embodiments, the controlling circuitry is configured to cause determination 350 of a first heat power transferred at the first heat exchanger 110 based on an electrical power measured by an electrical power meter 150, based on one or more temperatures measured by one or more temperature sensors 124, 129, 136, 138, 146, based on the determined gas pressure P1 , based on a pressure P2 measured by a pressure sensor 128 at the inlet 126 of the compressor 120 and based on characteristics of the refrigerant. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a sixth determination unit (e.g., sixth determining circuitry or a sixth determiner). In some embodiments, the controlling circuitry is configured to cause determination 360 of a second heat power transferred at the second heat exchanger 130 based on an electrical power measured by an electrical power meter 150, based on one or more temperatures measured by one or more temperature sensors 124, 129, 136, 138, 146, based on the determined gas pressure P1 , based on a pressure P2 measured by a pressure sensor 128 at the inlet 126 of the compressor 120 and based on characteristics of the refrigerant. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a seventh determination unit (e.g., seventh determining circuitry or a seventh determiner). In some embodiments, the controlling circuitry is configured to cause determination 370 of a third heat power transferred by applying an offset to the first or the second transferred heat power, e.g., reducing the first or the second transferred heat power with the offset. The offset may be determined based on one or more temperatures measured by one or more temperature sensors 124, 129, 136, 138, 146, based on the determined gas pressure P1 , based on the pressure P2 measured by the pressure sensor 128 at the inlet 126 of the compressor 120 and based on characteristics of the refrigerant. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) an eighth determination unit (e.g., eighth determining circuitry or an eighth determiner). In some embodiments, the controlling circuitry is configured to cause acquisition 365 of the offset from a third lookup table or from a third set of lookup tables. To this end, the controlling circuitry may be associated with (e.g., operatively connectable, or connected, to) a third acquisition unit (e.g., third acquiring circuitry or a third acquirer).

Figure 4 is a schematic drawing illustrating an example computer readable medium in the form of a compact disc (CD) ROM 400. Alternatively, the computer readable medium is a universal serial bus, USB, flash drive or other flash memory. The computer readable medium has stored thereon a computer program comprising program instructions. The computer program is loadable into a data processing unit (PROC) 420, which may be a processor/control circuitry 160 or another processing unit comprised in or otherwise associated with the heat pump 100. When loaded into the data processing unit, the computer program may be stored in a memory (MEM) 430 associated with (connectable or connected to) or comprised in the data processing unit. The memory may be data storage unit. According to some embodiments, the computer program may, when loaded into and run by the data processing unit, cause execution of method steps according to, for example, any of the methods illustrated in Figure 2 or otherwise (e.g., in claims 4-9) described herein.

Figure 5 illustrates how the condensation temperature Tc for a refrigerant depends on the temperature difference AT, i.e. , the difference between the fluid temperatures at an inlet and an outlet of a heat exchanger for different fluid temperatures at the inlet. As can be seen from figure 5, the condensation temperature Tc increases with increasing fluid temperatures at the inlet and with increasing temperature differences AT. Furthermore, as can be seen from figure 5, the condensation temperature Tc for a particular refrigerant may be specified if the fluid temperatures at the inlet of the heat exchanger and the temperature difference AT are known (or if the fluid temperatures at the inlet and the outlet of the heat exchanger are known since the temperature difference AT then can be calculated). Although figure 5 shows the characteristics of one particular heat exchanger and other heat exchangers may have different characteristics, the general principle is valid for all or for a large number of heat exchangers and in general the condensation temperature Tc for a particular refrigerant may be specified if the fluid temperatures at the inlet of the heat exchanger and the temperature difference AT are known.

Figure 6 is a refrigerant diagram of a refrigerant, R134a, and illustrates how a gas pressure at an outlet of a compressor depends on a condensation temperature of the refrigerant and on characteristics of the refrigerant. As can be seen in figure 6, if the condensation temperature Tc is known, e.g., 60 degrees C or 80 degrees C, the pressure at the outlet of the compressor can be specified, e.g., as the pressure at the horizontal part of the condensation temperature line of the known condensation temperature Tc. Although figure 6 shows the characteristics of one particular refrigerant, R134a, and other refrigerants may have different characteristics, the general principle is valid for all or for a large number of refrigerants and in general the gas pressure for a particular refrigerant at an outlet of a compressor may be specified if the condensation temperature Tc is known.

Generally, all terms used herein are to be interpreted according to their ordinary meaning in the relevant technical field, unless a different meaning is clearly given and/or is implied from the context in which it is used. Reference has been made herein to various embodiments. However, a person skilled in the art would recognize numerous variations to the described embodiments that would still fall within the scope of the claims. For example, the method embodiments described herein discloses example methods through steps being performed in a certain order. However, it is recognized that these sequences of events may take place in another order without departing from the scope of the claims. Furthermore, some method steps may be performed in parallel even though they have been described as being performed in sequence. Thus, the steps of any methods disclosed herein do not have to be performed in the exact order disclosed, unless a step is explicitly described as following or preceding another step and/or where it is implicit that a step must follow or precede another step. In the same manner, it should be noted that in the description of embodiments, the partition of functional blocks into particular units is by no means intended as limiting. Contrarily, these partitions are merely examples. Functional blocks described herein as one unit may be split into two or more units. Furthermore, functional blocks described herein as being implemented as two or more units may be merged into fewer e.g., a single) unit. Any feature of any of the embodiments/aspects disclosed herein may be applied to any other embodiment/aspect, wherever suitable. Likewise, any advantage of any of the embodiments may apply to any other embodiments, and vice versa. Hence, it should be understood that the details of the described embodiments are merely examples brought forward for illustrative purposes, and that all variations that fall within the scope of the claims are intended to be embraced therein.