WO2024177551A1

WO2024177551A1 - A buffer tank for a heating system and a heat transfer arrangement comprising the same

Info

Publication number: WO2024177551A1
Application number: PCT/SE2024/050148
Authority: WO
Inventors: Michael Lindahl; Bengt Lindoff
Original assignee: Qvantum Industries Ab
Priority date: 2023-02-20
Filing date: 2024-02-15
Publication date: 2024-08-29

Abstract

The disclosure relates to a buffer tank suitable for being fluidly connected to a heating system for a building and to house a buffer liquid which is circulated via said heating system to be heated, said buffer tank comprising: a vessel arranged to house the buffer liquid, a buffer liquid inlet structured and arranged to allow receiving buffer liquid to the buffer tank from the heating system, a stratification tube arranged such that an axial extension thereof is aligned substantially in parallel with the vertical extension of the vessel, said stratification tube having lateral side walls(s) which presents a plurality of through-openings, and a supply conduit fluidly interconnecting the buffer liquid inlet with the stratification tube, wherein the supply conduit has a flow speed reducing portion. The disclosure further relates to a heat transfer arrangement comprising said buffer tank.

Description

A BUFFER TANK FOR A HEATING SYSTEM AND A HEAT TRANSFER ARRANGEMENT COMPRISING THE SAME

Technical field

The present disclosure relates to a buffer tank suitable for being fluidly connected to a heating system for a building. The present disclosure further relates to a heat transfer arrangement comprising the same.

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, easier to control 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. Such heat pump-based systems are often combined with buffer tanks, also termed accumulator tanks, for providing a temporary storage of thermal energy. The heat pump-based system is configured to heat a buffer liquid stored in the buffer tank, typically water, by recirculating the buffer liquid via the heat pump-based system. The buffer tank thus provides instant access to thermal energy, which is convenient when heating tap water. Typically, tap water is heated by means of a heat exchanger arrangement between the buffer tank and a tap water circulation system.

A problem in the conventional systems known in the art is that the requirements of the tap water heating vis-a-vis the buffer tank is not always compatible with the operating conditions for typical heat pumps, which tends to reduce efficiency of the systems. Thus, there is a need in the art for an improvement in this area.

It is an object to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solve at least the above-mentioned problem.

It is an object of the disclosure to provide an efficient heat transfer arrangement for heating and/or cooling and/or providing tap water to different types of buildings.

Another object is to provide a flexible, but also adjustable, heat transfer arrangement.

Another object is to provide such a heat transfer arrangement which is environmentally friendly.

Another object is to provide such a heat transfer arrangement which is less time consuming to install and/or maintain.

It is also an object to provide a buffer tank which allows more efficient tap water heating.

It is also an object to provide a buffer tank which allows improved conditions for heat pump-based heating systems.

According to a first aspect there is provided a buffer tank suitable for being fluidly connected to a heating system for a building and to house a buffer liquid which is circulated via said heating system to be heated, said buffer tank comprising: a vessel which, when being arranged in its operating position in relation to the heating system, has a vertical extension between a bottom end and a top end to allow forming temperature stratification in the buffer liquid housed therein, a buffer liquid outlet structured and arranged to allow retrieving buffer liquid from the bottom end of the buffer tank to be heated by the heating system, a buffer liquid inlet structured and arranged to allow returning heated buffer liquid to the buffer tank, a stratification tube arranged such that an axial extension thereof is aligned substantially in parallel with the vertical extension of the vessel, said stratification tube having lateral side wall(s) which presents a plurality of through-openings, and a supply conduit fluidly interconnecting the buffer liquid inlet with the stratification tube, wherein the supply conduit has a flow speed reducing portion having a cross-sectional area, as seen transverse to a flow direction of the buffer liquid, which cross-sectional area is gradually increasing along the flow direction for reducing a flow speed of the buffer liquid prior to it entering the stratification tube.

The buffer tank may be advantageous as it allows adapting the temperature distribution of the buffer liquid supplied to the buffer tank to the temperature distribution of the buffer liquid which is already residing in the buffer tank. In other words, the buffer tank may provide a means to distribute buffer liquid of a specific temperature predominately in regions of the buffer tank having that same temperature. As readily appreciated by the person skilled in the art, this reduces the risk that the heating process via the heat pump-based heating system influences the temperature distribution of the buffer liquid within the buffer tank. This effect occurs due to the way the buffer liquid is introduced into the buffer tank via the supply conduit and the stratification tube. The buffer liquid having been heated by the heat pumpbased heating system enters the supply conduit and reduces its speed within the flow speed reducing portion. When the buffer liquid enters the stratification tube, which is disposed downstream of the flow speed reducing portion of the supply conduit, the axial speed of the buffer liquid is reduced to a level where its influence on the mixing process is minimized. The buffer liquid entering the stratification tube then will strive to move towards the region having a corresponding density. This is a result from thermal stratification, i.e. , that a liquid will naturally strive to be vertically distributed such that the density decreases as function of vertical position. Since a buffer liquid, such as e.g., water, has a density which monotonically decreases with increasing buffer liquid temperature in the operating temperature region of buffer tanks (it is noted that no buffer tank storing water is run close to the inflection point at 4 C°), the temperature stratification will ensure that the buffer liquid having the highest temperatures are located that the top end of the buffer tank, and that the buffer liquid having the lowest temperatures are located that the bottom end of the buffer tank. Ones the predominately warmer buffer liquid has been accumulated in the upper parts of the stratification tube, and the predominately colder buffer liquid has been accumulated in the lower parts of the stratification tube, the buffer liquid will strive to exit the stratification tube at the vertical position where its density matches the density of the buffer liquid already residing in the tank (i.e., where the temperature matches). This is achieved by the provision of the plurality of through-openings in the lateral side walls(s) of the stratification tube.

The improved distribution may be advantageous as will be described in what follows. The heated buffer liquid that is supplied to the buffer tank is the buffer liquid that has previously been retrieved from the bottom end of the buffer tank where the temperature can range from 20 C° to 50 C°. The buffer liquid is heated by being pumped past condenser(s) of the heat pump-based heating system which emit(s) heat and then the heated buffer liquid is supplied to the buffer tank. If, for example, the temperature set point of the buffer liquid heated by the heat pump-based heating system is 65 C°, it is desirable to supply this buffer liquid at the very top of the buffer tank which has the highest temperatures (ideally also 65 degrees, but due to heat losses to the surroundings and/or thermal heat retrieval for, e.g., a tap water heating system, it may be somewhat lower). However, if the effective power of the heat pump-based heating system is not sufficient to heat the buffer liquid pumped through the condenser(s) to the set point temperature of 65 C°, then the buffer liquid entering the buffer tank may have a considerably lower temperature than the temperature of the buffer liquid at the top end of the buffer tank. If this buffer liquid is supplied to the top end of the tank, it will result in a cooling of the buffer liquid in the buffer tank. By the provision of the supply conduit and the stratification tube, buffer liquid which has a lower temperature than the temperature at the top end of the buffer tank may naturally be allowed to distribute in lower levels of the buffer tank where the temperature is lower, thus improving thermal efficiency by reducing the entropy losses in the buffer tank. An advantage with the buffer tank is that it is self-adjustable. Thus, there is no need for actively controlling where buffer liquid is supplied to the tank, and there is no need to measure the temperature of the heated buffer liquid and/or the temperature of the buffer tank. Once installed, the buffer liquid supplied to the buffer tank will be distributed passively, purely governed by natural forces. Another advantage is that the buffer tank allows to handle large temperature transients in inlet buffer liquid temperature, such as the large temperature transients occurring during a starting phase of a heat pump. Even where large temperature transients occur in the buffer liquid entering the buffer tank, the supply conduit and stratification tube will ensure minimal influence of the stratification and entropy of the buffer liquid already present within the buffer tank.

Thus, the solution is very reliable as it does not rely on any auxiliary systems, sensors, controllers, power, or moving parts. Moreover, it is cheap and highly durable.

The vessel typically has a cylindrical geometry. This implies that a cross-section of the vessel along the vertical direction is circular. It is however also conceivable that the vessel has another shape, such as an elliptical cross section, or a square cross section. According to some embodiments, the buffer tank is structured and arranged such that the stratification tube extends within a top 2/3 of the vertical extension of the vessel. This region has been found to be especially efficient for the conditions typically encountered for the buffer tank when used together with a heat pump-based heating system. In particular, the lowermost 1/3 of the buffer tank typically houses temperature sensors used to trigger heating of the buffer tank by means of the heating system. By deliberately not extending the stratification tube into this region, the triggering has been found to be more reliable. However, also other embodiments are conceivable. For example, the buffer tank may be structured and arranged such that the stratification tube extends within a top 1/2 of the vertical extension of the vessel, or a top 3/4 of the vertical extension of the vessel.

The buffer tank may be structured and arranged such that the stratification tube extends to the top end of the buffer tank. This implies that the stratification tube may be in abutment with, or attached to, an inner wall of the buffer tank at the top end. It is however also conceivable that the buffer tank is structured and arranged such that the stratification tube extends towards, but not all the way to, the top end of the buffer tank. This implies that the stratification tube may be spaced from the inner wall of the buffer tank at the top end.

According to some embodiments, the supply conduit fluidly connects to the stratification tube at an intermediate portion thereof. This may be advantageous as it provides a two-way junction at the transition from the supply conduit and the stratification tube, which junction allows an improved temperature distribution within the stratification tube. The intermediate portion may be located symmetrically along the vertical extension of the stratification tube. This implies that the stratification tube may have an upper portion which extends upwardly from the intermediate portion, and one lower portion which extends downwardly from the intermediate portion, wherein the upper and lower portions have the same dimension along the axial extension of the stratification tube. It is however also conceivable that the intermediate portion is located asymmetrically along the vertical extension of the stratification tube, which implies that the upper and lower portions have different dimensions along the axial extension of the stratification tube.

According to some embodiments, a top end and/or a bottom end of the stratification tube is sealed. This may be advantageous as it increases the residence time for the heated buffer liquid within the stratification tube before it leaves the same, thereby further increasing the temperature distribution. It is conceivable that the top end but not the bottom end is sealed, or the bottom end but not the top end is sealed, or that both the top end and the bottom end are sealed. It is also conceivable that the top end and/or the bottom end is provided with a wall being semi-penetrable. For example, such a wall could also present a plurality of through-openings.

According to some embodiments, the vessel, the stratification tube and the supply conduit are made of the same material. This may be advantageous as it simplifies construction. By using the same material, the risk of corrosion is also reduced. The material may be steel. The material may for example be stainless steel. Alternatively, the material may be black steel. Black steel is understood to be a particular form of non-galvanized steel. Instead of undergoing galvanization for protection against corrosion, this type of steel goes through a chemical conversion process (blackening), which is used to create black iron oxide or magnetite.

According to some embodiments, at least some of the through- openings of the plurality of through-openings present flow-diverting means structured and arranged to increase a vertical flow velocity component of the buffer liquid exiting therethrough in a direction away from the supply conduit. This may be advantageous as it allows to further enhance the distribution of the supplied buffer liquid to the tank. By the provision of the flow-diverting means, buffer liquid that may have been forced out from the stratification tube a little too close to the supply conduit will obtain an extra push in the correct direction. The flow-diverting means may be protruding portions of the lateral wall(s) which are shaped so as to divert the flow in the vertical direction. Such flow-diverting means may resemble the apertures used on food graters. An advantage of providing such flow-diverting means is that it simplifies construction, since such flow-diverting means can be provided by cutting and forming a single piece of material.

According to some embodiments, the supply conduit is structured and arranged such that the stratification tube is aligned substantially coaxially with the buffer tank. With “coaxially” is herein meant that the buffer tank and the stratification tube will be arranged with respect to each other such that their respective symmetry axis are parallel with each other and coinciding. This may be advantageous as it allows to further enhance the distribution of the supplied buffer liquid to the tank. By providing the stratification tube centrally within the buffer tank, the residence time for buffer liquid exiting the stratification tube to reach its equilibrium position within the tank will be similar. It is however also conceivable to arrange the stratification tube off- axis, i.e. such that their respective symmetry axis are not coinciding.

According to some embodiments, the flow speed reducing portion is frustoconical and has a cone angle within the range 10 to 30 degrees, or 12 to 25 degrees, or 12 to 20, or 15 degrees. The frustoconical form is easier to manufacture and therefore cheaper. More importantly, the frustoconical form has been found to provide an efficient speed reducing function. It also provides a more compact solution. The referenced cone angles have been found to result in an appropriate speed reduction for the buffer liquid which passes the flow speed reducing portion. Cone angles in the range 12 to 20, and most preferably 15 degrees have been found to provide the most efficient speed reducing function. Ideally, the speed of the buffer liquid that enters the stratification tube should be close to zero, so as to minimize any unwanted liquid motion in the buffer liquid which may risk hampering the stratification process within the stratification tube.

According to some embodiments, an internal cross section of the stratification tube as seen transverse to its axial extension is within the range of 5-25%, or 10-20% of an internal cross section of the vessel as seen transverse to its axial extension of the stratification tube. This has been found to result in a good compromise between an efficient buffer liquid distribution formation within the stratification tube and an efficient buffer liquid distribution formation within the vessel but outside of the stratification tube.

According to some embodiments, an internal cross section of the stratification tube as seen transverse to its axial extension is at least 25 times larger than an internal cross section of the buffer liquid inlet. As readily appreciated by the person skilled in the art, the above-described area ratio will cause the axial flow speed within the stratification tube to be at least 25 times lower than the axial flow speed in the buffer liquid inlet. For typical axial flow speeds in the buffer liquid inlet within the range 0.4 to 2.5 m/s, this will result in axial flow speeds within the stratification tube within the range 16 to 100 mm/s.

For embodiments of the stratification tube having the supply conduit fluidly connecting to the stratification tube at an intermediate portion thereof, the above-described area ratio will cause the axial flow speed within the stratification tube to be at least 50 times lower than the axial flow speed in the buffer liquid inlet. For typical axial flow speeds in the buffer liquid inlet within the range 0.4 to 2.5 m/s, this will result in axial flow speeds within the stratification tube within the range 8 to 50 mm/s.

This area ratio has been found to result in an appropriately low flow rate for the buffer liquid within the stratification tube to allow an efficient buffer liquid distribution formation therein. Other ratios are however also conceivable. For example, the internal cross section of the stratification tube as seen transverse to its axial extension may be 15 to 25 times larger than an internal cross section of the buffer liquid inlet, or 20 to 30 times larger than an internal cross section of the buffer liquid inlet.

Turned around, when in use together with a heating system, such as a heat-pump based heating system, the internal cross section of the stratification tube as seen transverse to its axial extension may relate to an internal cross section of the buffer liquid inlet such that the axial flow speed within the stratification tube is less than 100 mm/s, or less than 70 mm/s, or less than 50 mm/s, or less than 40 mm/s, or less than 30 mm/s, or less than 20 mm/s, or less than 10 mm/s. The internal cross section of the stratification tube as seen transverse to its axial extension may be at least 16 times smaller than the internal cross section of the vessel. The internal cross section of the stratification tube as seen transverse to its axial extension may be between 16 and 36 times smaller than an internal cross section of the vessel.

According to some embodiments, the stratification tube is supported within the tank only by the supply conduit. This may be advantageous as it simplifies construction and allows less material stresses in the buffer tank during use.

According to a second aspect there is provided a heat transfer arrangement comprising: a heating system for a building including at least one heat pump, and a buffer tank according to the first aspect, wherein the heating system is fluidly connected to the buffer tank for circulating a buffer liquid housed in the buffer tank via the heating system to be heated.

By the term “heat pump” is herein means a system which includes at least a first heat exchanger, a compressor, a second heat exchanger, and an expander connected to each other along a refrigerant circulation path. The heat pump may be operated for heating, e.g., for moving thermal energy from outside of a building to the inside. The heat pump may alternatively be operated as a “cool pump” or “air conditioning device” e.g., for moving thermal energy from the inside to the outside of a building.

The provision of the buffer tank according to the first aspect in combination with a heating system for a building including at least one heat pump , i.e. , a heat pump-based heating system, may be synergistically advantageous, since, depending on the number of heat pumps, the transient time and the transient buffer liquid temperature at start may vary quite significantly during normal operation.

According to some embodiments, the at least one heat pump is an airliquid heat pump. By the term “air-liquid heat pump” is herein meant a heat pump which exchanges heat with air at one end, and with a liquid at the other end. Examples of such heat pumps for heating buildings are typically having the thermal energy exchange by air at the outside of the building and the thermal energy exchange to the liquid (often water) at the inside of the building.

The heating system may be based on a modular architecture. The heating system may include at least one modular air-liquid heat pump, wherein the at least one modular air-liquid heat pump is structured and arranged to be detachable from the heating system. The at least one modular air-liquid heat pump may be a plurality of modular air-liquid heat pumps operating in parallel.

According to some embodiments, the heating system includes at least one modular liquid-liquid heat pump, wherein the at least one modular liquidliquid heat pump is structured and arranged to be detachable from the heating system.

By the term “liquid-liquid heat pump” is herein meant a heat pump which exchanges heat with a first liquid at one end, and with a second liquid at the other end. Examples of such heat pumps for heating buildings are typically having the thermal energy exchange by a first liquid (often called brine) at the outside of the building and via a tubing system exchanging heat with the ground, and the thermal energy exchange to the second liquid (often water) at the inside of the building.

By the term “modular liquid-liquid heat pump”, also referred to herein as “liquid-liquid heat pump module”, or merely “heat pump module”, is here meant a liquid-liquid heat pump which is constructed as a single unit, or module. This allows for the modular liquid-liquid heat pump to be easily removed from the heat transfer arrangement, and subsequently replaced by another liquid-liquid heat pump.

By the term “detachable” is here meant that the modular liquid-liquid heat pump is removably arranged in the heat transfer arrangement. Put differently, at least one heat pump module of the plurality of heat pump modules is arranged in the arrangement in a way such that it is possible to remove and/or replace the heat pump module.

The at least one modular liquid-liquid heat pump may be structured and arranged to be portable. This implies that each of the at least one modular liquid-liquid heat pump falls within certain size restrictions. Each heat pump module of the plurality of heat pump modules may have a height between 25-45 cm. Each heat pump module of the plurality of heat pump modules may have a width between 15-35 cm. Each heat pump module of the plurality of heat pump modules may have a depth between 45-65 cm.

Each modular liquid-liquid heat pump may comprise: first inlet and outlet ports; second inlet and outlet ports; control means for controlling the heat pump module; and a refrigerant circulation path which includes the following entities connected to one another in sequence: a first heat exchanger unit fluidly connected to said first inlet and outlet ports; a compressor; a second heat exchanger unit fluidly connected to said second inlet and outlet ports; and an expander.

According to some embodiments, the at least one modular liquid-liquid heat pump is a plurality of modular liquid-liquid heat pumps operating in parallel.

The provision of a plurality of liquid-liquid heat pump modules operating in parallel may be advantageous as it provides for a flexible heat pump system. Thus, since each heat pump module of the plurality of heat pump modules is separated from and independent of the other heat pump modules, and that they are connected in parallel, it is possible to design and/or modify the arrangement in different ways. Thus, the different heat pump modules may have different maximum input power such that it is possible to tailor an arrangement based on different requirements, among other things, the size of the building and the required arrangement output power needed for heating and/or cooling and/or providing tap water to the building. This provides an efficient heat transfer arrangement for heating and/or cooling and/or providing tap water to different types of buildings.

By arranging the plurality of heat pump modules in parallel to each other, it is possible to continue operate the arrangement although one or more of the heat pump modules may broke or having other problems with running.

The provision of the buffer tank according to the disclosure is especially advantageous when combined with a plurality of modular liquidliquid heat pumps operating in parallel, because a plurality of modular liquidliquid heat pumps operating in parallel may cause a significantly steeper temperature gradient in the buffer tank as a result from the higher heating power. Consequently, the buffer tank of the disclosure which includes the inlet flow distributor may be especially advantageous since it minimizes the influence of the incoming liquid on the temperature distribution of the tank.

Effects and features of the second and third aspects are largely analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect and third aspects. It is further noted that the inventive concepts relate to all possible combinations of features unless explicitly stated otherwise.

A further scope of applicability of the present disclosure will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the disclosure, are given by way of illustration only, since various changes and modifications within the scope of the disclosure will become apparent to those skilled in the art from this detailed description.

Hence, it is to be understood that this disclosure is not limited to the particular component parts of the device described or steps of the methods described as such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It must 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 include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

Brief descriptions of the drawings

The disclosure will by way of example be described in more detail with reference to the appended schematic drawings, which shows presently preferred embodiments of the disclosure.

Figure 1 illustrates a heat transfer arrangement for heating and/or cooling and/or providing tap water to buildings or the like according to an example embodiment of the disclosure.

Figure 2 illustrates the heat transfer arrangement from Fig. 1 when one modular heat pump is detached from the arrangement.

Figure 3 illustrates a heat transfer arrangement for heating and/or cooling and/or providing tap water to buildings or the like according to another example embodiment of the disclosure.

Figure 4 illustrates a buffer tank comprising a stratification tube and a supply conduit according to an example embodiment of the disclosure.

Figure 5 illustrates the buffer tank of Fig. 4 in a cross-sectional side view.

Detailed description

The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the disclosure are shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the disclosure to the skilled person.

Figures 1 and 2 illustrates a modular liquid-liquid heat transfer arrangement 100 for heating and/or cooling and/or providing tap water to buildings or the like by way of example. Hereafter, the modular liquid-liquid heat transfer arrangement 100 may also be referred to as “heat transfer arrangement 100” or “arrangement 100”.

The heat transfer arrangement 100 comprises a heating system 10, which is indicated in Fig. 1. The heating system 10 comprises a cold side and a hot side. The heating system 10 is configured to transfer thermal energy from the cold side to the hot side. The cold side comprises first inlet and outlet junction pipes 111 , 112. The cold side is connected to a cold liquid side 101 via the first inlet and outlet junction pipes 111 , 112 thereby forming a cold side liquid recirculation path 103. The hot side comprises second inlet and outlet junction pipes 122, 121. The hot side is connected to a hot liquid side 102 via the second inlet and outlet junction pipes 122, 121 thereby forming a hot side liquid recirculation path 104.

The first inlet junction pipe 111 is configured to supply a cold side liquid (often termed: a brine) from the cold liquid side 101 to the heat transfer arrangement 100. The first outlet junction pipe 112 is configured to return the cold side liquid from the heat transfer arrangement 100 to the cold liquid side 101 , thereby forming the cold side liquid recirculation path 103. During heating, the cold side liquid has a higher temperature when supplied to the heat transfer arrangement 100 then when being returned therefrom to the cold liquid side 101 .

The second outlet junction pipe 121 is configured to supply a hot side liquid (typically: water) from the heat transfer arrangement 100 to the hot liquid side 102. The second inlet junction pipe 122 is configured to return the hot side liquid from the hot liquid side 102 to the heat transfer arrangement 100 thereby forming the hot side recirculation path 104. During heating, the hot side liquid has a lower temperature when supplied to the heat transfer arrangement 100 then when being returned therefrom to the hot liquid side 101.

The liquid-liquid heat transfer arrangement 100 may be a liquid-liquid heat pump arrangement configured to provide heat to the hot side liquid for heating the same. The liquid-liquid heat transfer arrangement 100 may be a liquid-liquid cool pump arrangement configured to remove heat from the cold side liquid for cooling the same.

For typical heating applications of the arrangement 100, the cold liquid side 101 may be 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. The cold liquid side 101 may be coupled to a downhole heat exchanger, or borehole heat exchanger. For typical heating applications of the arrangement 100, the hot liquid side 102 may be a heating system, such as radiators or tap water systems, in the building. The hot liquid side 102 will be described in detail later.

The heat transfer arrangement 100 further comprises three heat pump modules 130a, 130b, 130c. It should however be noted that, although not illustrated, the heat transfer arrangement 100 may comprise less than three heat pump modules 130a, 130b, 130c or more than three heat pump modules 130a, 130b, 130c. Each heat pump module 130a, 130b, 130c comprises first inlet and outlet ports 131a, 131 b and second inlet and outlet ports 132b, 132a. The first inlet and outlet ports 131a, 131 b are connected to the first inlet and outlet junction pipes 111 , 112, respectively. The second inlet and outlet ports 132b, 132a are connected to the second inlet and outlet junction pipes 122, 121 , respectively.

When the heat transfer arrangement 100 is in use, the three heat pump modules 130a, 130b, 130c are connected in parallel to each other. This is achieved by their respective first inlet and outlet ports 131a, 131 b which are connected to the first inlet and outlet junction pipes 111 , 112, respectively, and by their respective second inlet and outlet ports 132b, 132a which are connected to the second inlet and outlet junction pipes 122, 121 , respectively. Each heat pump module 130a, 130b, 130c further comprises a refrigerant recirculation loop 134. The refrigerant recirculation loop 134 comprises a first heat exchanger unit 135 and a second heat exchanger unit 137 as well as a compressor 136, and an expander 138. The first heat exchanger unit 135 is fluidly connected to the first inlet and outlet ports 131a, 131b. Thus, the first heat exchanger 135 is connected to the first inlet and outlet junction pipes 111 , 112 via the first inlet and outlet ports 131a, 131 b, respectively. The second heat exchanger unit 137 is fluidly connected to the second inlet and outlet ports 132b, 132a, Thus, the second heat exchanger unit 137 is connected to the second inlet and outlet junction pipes 122, 121 via the second inlet and outlet ports 132b, 132a, respectively.

The refrigerant circulation loop 134 preferably circulates a refrigerant through the first heat exchanger unit 135, the compressor 136, the second heat exchanger unit 137 and the expander 138. In the first heat exchanger unit 135, the refrigerant which is then predominately in a liquid phase and the cold side liquid are configured to exchange thermal energy between each other such that a temperature of the refrigerant increases, the refrigerant is vaporised into a gaseous phase and a temperature of the cold side liquid thereby decreases. The first heat exchanger is therefore often termed: the “evaporator”. The refrigerant is circulated from the first heat exchanger unit 135 to the compressor 136 which is configured to increase the pressure and thereby temperature of the refrigerant before supplying the refrigerant to the second heat exchanger unit 137. In the second heat exchanger unit 137, the refrigerant which is still in a gaseous phase and the hot side liquid is configured to exchange thermal energy between each other such that a temperature of the refrigerant decreases and the refrigerant condenses into a liquid phase transferring thermal heat to the hot side first liquid which temperature is thereby increased. The second heat exchanger is therefore often termed: the “condenser”. The refrigerant is then circulated from the second heat exchanger unit 137 to the expander 138 which is configured to reduce the pressure of the refrigerant. As the pressure drops, refrigerant starts to evaporate and the heat of evaporation is taken from the refrigerant itself which causes its temperature to drop and the result is a low- temperature, low-pressure mix of liquid and vapour which is then circulated into the first heat exchanger unit 135 where the cycle starts over again.

Although not illustrated, it should be noted that the arrangement 100 may comprise one or more sensors, such as temperature sensors and/or pressure sensors, one or more control valves, such as check valves, one or more flow rate controllers, such as pumps etc. This is however well known in the art and is therefore excluded from the figures in this context.

In addition to what have been discussed in connection with figure 1 , and as best illustrated in figure 2, each of the heat pump modules 130a, 130b, 130c are removably arranged in the arrangement 100, or put differently, may be detachable from the arrangement 100. Thus, as illustrated in Fig. 2, it is possible to disconnect the heat pump module 130c from the first inlet and outlet junction pipes 111 , 112 and from the second inlet and outlet junction pipes 122, 121 such that the heat pump module 130c may be removed from the arrangement 100. In this way, it is possible to remove or replace the heat pump module 130c if needed. Although not illustrated in Fig. 2, all heat pump modules 130a, 130b, 130c of the arrangement may be removably arranged in the arrangement 100, or detachable from the arrangement 100.

As illustrated in Figs 1 and 2, the heat transfer arrangement 100 further comprises, within the hot liquid side 102, a radiator system 160 (only schematically illustrated herein), a buffer tank 140 and a tap water heating system 170 for heating a tap water circuit 180. The arrangement 100 is thus configured to transfer heat from the heating system 10 to the radiator circuit 160 or to the tap water circuit 180. The arrangement 100 is thus using the same heating system 10 for the radiator circuit 160 (i.e. , floor heating/radiator systems) and for generating hot tap water to the tap water circuit 180. A switchable conduit system 150 is configured to fluidly connect the heating system 10 to the radiator circuit 160 or to fluidly connect the heating system 10 to the buffer tank 140. Thus, it should be noted that the heating system 10 is physically connected to the radiator circuit 160 and the buffer tank 140 at the same time. It should however be noted that the heating system 10 is only fluidly connected to one of the radiator circuit 160 and the buffer tank 140 at a particular time. When the heating system 10 is fluidly connected to the radiator circuit 160, it is preventing fluid communication with the buffer tank 140. When the heating system 10 is fluidly connected to the buffer tank 140, it is preventing fluid communication with the radiator circuit 160. The hot side liquid which is returned from the radiator circuit 160 or from the buffer tank 140 is entering into the heat pump modules 130a, 130b, 130c of the heating system 10 via the switchable conduit system 150 and the second inlet junction pipe 122.

The buffer tank 140 is configured to store a heat buffer liquid. For the now described arrangement 100, the heat buffer liquid is the previously described hot side liquid which is transported through the second outlet junction pipe 121 to the buffer tank 140. When describing the buffer tank 140, the term buffer liquid will be used.

The buffer tank 140 comprises a buffer liquid inlet 141a configured to receive buffer fluid exiting the heating system 10 and being supplied to the buffer tank 140, when the heating system 10 is fluidly connected to the buffer tank 140. In other words, the buffer liquid outlet 141a is structured and arranged to allow returning heated buffer liquid to the buffer tank 140 once the buffer liquid has been heated by the heating system 10. The buffer tank further comprises an inlet flow distributor 300 comprising a supply conduit 310 and a stratification tube 320. The supply conduit 310 fluidly interconnects the buffer liquid inlet 141a with the stratification tube 320. Thus, buffer liquid entering the buffer tank 140 through the buffer liquid inlet 141a will pass through the supply conduit 310 and then enter the stratification tube 320. The inlet flow distributor 300 will be further described later.

The buffer tank 140 further comprises a buffer liquid outlet 142b configured to return buffer liquid from the buffer tank 140 to the heating system 10, when the heating system 10 is fluidly connected to the buffer tank 140. In other words, the buffer liquid outlet 142b is structured and arranged to allow retrieving buffer liquid from the bottom end of the buffer tank 140 to be heated by the heating system 10. The buffer liquid inlet 141a is located in the upper parts of the buffer tank 140 and the buffer liquid outlet 142b is located in the lower parts of the buffer tank 140. The buffer liquid outlet 142b is extending inside the buffer tank so as to retrieve buffer liquid at the bottom of the buffer tank 140 where the temperature of the buffer fluid, due to the temperature stratification, is expected to be at its lowest. The position of the buffer liquid inlet 141a will be further described later.

The buffer tank 140 further comprises a further buffer liquid outlet 141 b and a further buffer liquid inlet 142a, both being connected to the tap water heat exchange circuit 170. The tap water heat exchange circuit 170 comprises a heat exchanger 171 and a circulation pump 172. The tap water heat exchange circuit 170 is arranged to retrieve heat buffer liquid from the buffer tank 140, via the further buffer liquid outlet 141 b, to a first side 171 a of the heat exchanger 171 . The tap water heat exchanger circuit 170 is further configured to return the retrieved heat buffer fluid from the first side 171a of the heat exchanger 171 to the buffer tank 140, via the further buffer liquid inlet 142a. The further buffer liquid outlet 141b is located in the upper parts of the buffer tank 140 and the further buffer liquid inlet 142a is located in the lower parts of the buffer tank 140. The further buffer liquid outlet 141 b is extending inside the buffer tank 140 so as to retrieve buffer liquid at the top of the buffer tank 140 where the temperature of the buffer fluid, due to the temperature stratification, is expected to be at its highest.

As depicted in Figs 1 and 2, the tap water heat exchanger circuit 170 is configured to return the retrieved heat buffer liquid from the first side 171a of the heat exchanger 171 to the buffer tank 140 via the circulation pump 172. A second side 171 b of the heat exchanger 171 is connected to the tap water circuit 180. The tap water circuit 180 is connected to the second side 171 b of the heat exchanger 171 via a domestic hot water supply line DHW and a cold water supply line CW. The domestic hot water supply line DHW is arranged for supply hot tap water from the tap water heat exchange circuit 170 to the tap water circuit 180. The cold water supply line CW is arranged for return tap water from the tap water circuit 180 to the tap water heat exchange circuit 170. A hot water circulation supply line HWC is connected to the cold water supply line CW. The hot water circulation supply line HWC is arranged for maintaining a constantly circulating base flow of hot tap water from the tap water circuit such that hot tap water is always available once the tap water circuit is activated.

As previously mentioned, the switchable conduit system 150 is configured to fluidly connect the heating system 10 to the radiator circuit 160 or to fluidly connect the heating system 10 to the buffer tank 140. As depicted in Fig. 1 , the switchable conduit system 150 comprises two valves 151 a, 151 b arranged to fluidly connect the heating system 10 to either the radiator circuit 160 or to the buffer tank 140. The valves 151 a, 151 b are arranged for controlling the liquid flow retrieved from the heating system 10 to the switchable conduit system 150 and for controlling the fluid flow entering the heating system 10 from the switchable conduit system 150.

Turning now to Fig. 3, another example embodiment of the heat transfer arrangement is illustrated, namely the heat transfer arrangement 200. The heat transfer arrangement 200 differs from the already described heat transfer arrangement 100 only in the heating system. Whereas the heat transfer arrangement 100 comprises the heating system 100 which is a modular liquid-liquid heat pump system based on the use of several heat pump modules in parallel, the heat transfer arrangement 200 instead includes the heating system 200 which comprises an air-liquid heat pump 230. The airliquid heat pump 230 exchanges heat with air on the cold side, typically outside of a building, and transfers the heat to the hot side. The only principal difference between a liquid-liquid heat pump of the heating system 10 and the air-liquid heat pump 230 of the heating system 20 is that the first heat exchanger unit 235 exchanges heat directly with the air which is constantly forced to flow by the heat exchanging surfaces of the first heat exchanger unit 235 by means of a fan 240.

Although not illustrated herein, it is also conceivable to provide a heat transfer arrangement comprising one or more air-liquid heat pump modules. Such modular arrangements may be provided with a plurality of modular airliquid heat pumps operating in parallel. It is stressed that many alternative heating systems are conceivable within the scope of the disclosure than described in detail herein. Heat pumpbased heating systems are preferred, but it is also conceivable to provide heating systems based on another principle, such as e.g., solar heating, electric heating, or boilers.

The buffer tank 140 will now be described in more detail with reference to Figs 4 and 5. The buffer tank 140 is suitable for being liquidly connected to a heating system 10, 20 for a building and to house a buffer liquid which is circulated via said heating system 10, 20 to be heated. The buffer tank 140 comprises a vessel 149 which, when being arranged in its operating position in relation to the heating system 10, 20, has a vertical extension VA between a bottom end 140a and a top end 140b of the vessel 149 to allow forming temperature stratification in the buffer liquid which is housed therein. As previously mentioned, buffer liquid is retrieved from the buffer tank 140 via the buffer liquid outlet 142b to be heated by the heating system 10, 20. Once heated, the buffer liquid will be returned to the buffer tank 140 via the buffer liquid inlet 141a and the inlet flow distributor 300. When required for heating tap water, buffer liquid will be retrieved from the buffer tank 140 via the further buffer liquid outlet 141 b once having transferred heat to the tap water heating system 170 once again returned to the tank 140 via the further buffer liquid inlet 142a.

The inlet flow distributor 300 comprises a supply conduit 310 and a stratification tube 320. The stratification tube 320 is arranged in the buffer tank 140 such that an axial extension SA thereof is aligned substantially in parallel with a vertical extension VA of the vessel 149. For the example embodiment of the inlet flow distributor 300 illustrated in Figs 4 and 5, the stratification tube 320 is structured and arranged to extend within a top 1/3 of the vertical extension VA of the vessel 149. This region has been found to be especially efficient for the conditions typically encountered for the buffer tank when used together with a heat pump-based heating system, such as the heating systems 10 and 20. However, also other embodiments are conceivable. For example, the buffer tank 140 may be structured and arranged such that the stratification tube 320 extends within a top 1/2 of the vertical extension VA of the vessel 149, or a top 1/4 of the vertical extension VA of the vessel 149. The vessel 149, the stratification tube 320 and the supply conduit 120 are for the example embodiment made of the same material, steel. It is however also conceivable that they are made of different materials. An internal cross section A1 of the stratification tube 320 as seen transverse to its axial extension SA may be within the range of 5-25%, or 10- 20% etc of an internal cross section A2 of the vessel 149 as seen transverse to its vertical extension VA.

The internal cross section A1 of the stratification tube 320 as seen transverse to its axial extension SA may be at least 25 times larger than an internal cross section A3 of the buffer liquid inlet 141a. As readily appreciated by the person skilled in the art, the above-described area ratio will cause the axial flow speed within the stratification tube 320 to be at least 25 times lower than the axial flow speed in the buffer liquid inlet 141a. For typical axial flow speeds in the buffer liquid inlet 141a within the range 0.4 to 2.5 m/s, this will result in axial flow speeds within the stratification tube 320 within the range 16 to 100 mm/s. For embodiments of the stratification tube 320 having the supply conduit fluidly connecting to the stratification tube 320 at an intermediate portion 320a thereof (see below), the above-described area ratio will cause the axial flow speed within the stratification tube 320 to be at least 50 times lower than the axial flow speed in the buffer liquid inlet 141a. For typical axial flow speeds in the buffer liquid inlet 141a within the range 0.4 to 2.5 m/s, this will result in axial flow speeds within the stratification tube 320 within the range 8 to 50 mm/s.

The stratification tube 320 is supported by the supply conduit 310 within the vessel 149. For this example embodiment, the stratification tube 320 is only supported within the buffer tank 140 by the supply conduit 310, but for other not shown embodiments, further support structures are used to support the stratification tube 320 within the vessel 149. The supply conduit 310 is structured and arranged such that the stratification tube 320 is aligned substantially coaxially with the buffer tank 140. In other words, the buffer tank 140 and the stratification tube 320 will be arranged with respect to each other such that their respective symmetry axis SA, VA are parallel with each other and coinciding. The supply conduit 310 fluidly connects to the stratification tube 320 at an intermediate portion 320a of the stratification tube 320. The intermediate portion 320a is for the example embodiment located symmetrically along the vertical extension SA of the stratification tube 320. The stratification tube 320 has an upper portion 320b which extends upwardly from the intermediate portion 320a, and a lower portion which extends downwardly from the intermediate portion 320c. The upper 320b and lower 320c portions have the same dimension along the axial extension of the stratification tube 320. For other not shown embodiments, the intermediate portion 320a may be located asymmetrically along the vertical extension SA of the stratification tube 320, leading to the upper 320b and lower 320c portions having different dimensions along the axial extension SA of the stratification tube 320.

The stratification tube 320 has lateral side walls 321 which presents a plurality of through-openings 330a, 330b. At least some of the through- openings 330a, 330b of the plurality of through-openings 330a, 330b present flow-diverting means 331a, 331 b structured and arranged to increase a vertical flow velocity component of the buffer liquid exiting therethrough. For the example embodiment, all through-openings 330a, 330b has flow-diverting means 331a, 331b. The through-openings 330a which are located on the upper portion 320b have flow-diverting means 331a structured and arranged to increase a vertical flow velocity component in the upward direction. The through-openings 330b which are located on the lower portion 320c have flow-diverting means 331 b structured and arranged to increase a vertical flow velocity component in the downward direction. This is best illustrated in Fig. 5 which illustrates the flow movement within the stratification tube 320. The flow-diverting means 331a, 331 b resembles the apertures used on food graters. An advantage of providing such flow-diverting means is that it simplifies construction, since such flow-diverting means can be provided by cutting and forming a single piece of material. The top end 322a and the bottom end 322b of the stratification tube 320 are both sealed. Thus, the buffer liquid which enters the buffer tank 140 through the inlet flow distributor 300 will leave the inlet flow distributor 300 solely through the plurality of through-openings 330a, 330b. For other embodiments not shown, the top end 322a and/or the bottom end 322b are open, or semi-penetrable. For example, the top end 322a and/or the bottom end 322b may be provided with a wall which present a plurality of further through-openings.

The supply conduit 310 fluidly interconnects the buffer liquid inlet 141a with the stratification tube 320. The supply conduit 310 has a flow speed reducing portion 312 which has a cross-sectional area A4, as seen transverse to a flow direction L of the buffer liquid, which cross-sectional area A4 is gradually increasing along the flow direction L for reducing a flow speed of the buffer liquid prior to it entering the stratification tube 320. For the example embodiment, the flow speed reducing portion 312 is frustoconical. The flow speed reducing portion 312 may have a cone angle within the range 10 to 30 degrees, or 12 to 25 degrees, or 12 to 20. For the example embodiment, the cone angle is 15 degrees. Prior to entering the flow speed reducing portion 312, the buffer liquid is transported through a straight portion 311 . The straight portion 311 connects with inner walls 148 of the vessel 149. The straight portion 311 may not be essential, and other not shown embodiments do not have a straight portion.

The stratification tube 320 may have an axial extension SA of 400 to 600 mm, 450 mm to 550 mm, 470 to 520 mm, 480 to 510 mm, or 500 mm. The stratification tube 320 may have a cylindrical cross-section A1 with a diameter of 50 to 200 mm, 70 to 150 mm, 80 to 120 mm or 90 to 110 mm, or 100 mm.

The internal cross section A3 of the buffer liquid inlet 141a may be 15 to 30 mm, or 17 to 25 ,mm or 18 to 22 mm, or 20 mm.

The cross-sectional area A4 of the flow speed reducing portion 312 may have a maximum diameter of 40 to 100 mm, or 50 to 80 mm, or 55 to 70 mm or 59 mm. The flow speed reducing portion 312 may have an axial extension as seen along the flow direction L being 100 to 200 mm, 120 to 180 mm, or 140 to 160 mm, or 150 mm.

Needless to say, the dimensions of the inlet flow distributor 300 may depend on parameters such as the dimensions of the vessel 149, the flow speed of the buffer liquid supplied therethrough, the power of the heating system 10, 20 etc. It is for example conceivable that the dimensions of the inlet flow distributor 300 scales with the dimensions of the vessel 149.

The purpose of the inlet flow distributor 300 is to distribute incoming buffer liquid at different heights in the buffer tank 140 dependent on its temperature. Thus, the inlet flow distributor 300 aims to minimize the influence of the buffer liquid supply from the heating system 10, 20 on the temperature distribution within the buffer tank 140.

As previously mentioned, thermal stratification, i.e. , that a liquid will naturally strive to be vertically distributed such that the density decreases as function of vertical position, will cause the buffer fluid stored within the buffer tank 140 to have an increasing temperature as function of vertical position from the bottom end 140a to the top end 140b. The temperature stratification is utilized to enhance efficiency of tap water heating by retrieving the buffer liquid with the highest temperature for heating the tap water (i.e. the buffer liquid retrieved via the further buffer liquid outlet 141 b from the top end 140b of the buffer tank 140) and return the same buffer liquid, after losing heat and hence decreasing temperature via heat exchange in the tap water heat exchange circuit 170, to the buffer tank 140 at a position with the lowest temperature (i.e. the buffer liquid returned via the further buffer liquid inlet 142a at the bottom end 140a of the buffer tank 140).

The effect may be of special importance when the heating system 10, 20 is heat pump-based, as will be explained in what follows. The heated buffer liquid that is supplied to the buffer tank 140 is the buffer liquid that has previously been retrieved from the bottom end 140a of the buffer tank where the temperature can range from 20 C° to 50 C°. The buffer liquid is heated by being pumped past second heat exchanger unit(s) 137 of the heat pump- based heating system 10, 20 and the heated buffer liquid is thereafter supplied to the buffer tank 140. If, for example, the temperature set point of the buffer liquid heated by the heat pump-based heating system is 65 C°, it is desirable to supply this buffer liquid at the top end 140b of the buffer tank 140 which has the highest temperatures (ideally also 65 degrees, but due to heat losses to the surroundings and/or thermal heat retrieval for, e.g., the tap water heating system 170, it may be somewhat lower). However, if the effective power of the heat pump-based heating system 10, 20 may not be sufficient to heat the buffer liquid pumped through the second heat exchanger unit(s) 137 to the set point temperature of 65 C°. For such a situation, the buffer liquid entering the buffer tank 140 may have a considerably lower temperature than the temperature of the buffer liquid at the top end 140b of the buffer tank 140. If this buffer liquid is supplied to the top end 140b of the buffer tank 140, it will result in a cooling of the buffer liquid at the top end 140b of the buffer tank 140.

By the provision of the inlet flow distributor 300, buffer liquid which has a lower temperature than the temperature at the top end 140b of the buffer tank 140 may naturally be allowed to distribute in lower levels of the buffer tank 140 where the temperature is lower, thus improving thermal efficiency by reducing the entropy losses in the buffer tank 140. In other words, the inlet flow distributor 300 allows adapting the temperature distribution of the buffer liquid supplied to the buffer tank 140 to the temperature distribution of the buffer liquid which is already residing in the buffer tank 140. The buffer tank 140 may thus provide a means to distribute buffer liquid of a specific temperature predominately in regions of the buffer tank 140 having that same temperature. As readily appreciated by the person skilled in the art, this reduces the risk that the heating process via the heat pump-based heating system influences the temperature distribution of the buffer liquid within the buffer tank 140, as detailed in the example given above.

The distribution effect occurs due to the way the buffer liquid is introduced into the buffer tank 140 via the supply conduit 310 and the stratification tube 320. The buffer liquid having been heated by the heat pump-based heating system 10, 20 enters the supply conduit 310 and reduces its speed within the flow speed reducing portion 312. When the buffer liquid enters the stratification tube 320, which is disposed downstream of the flow speed reducing portion 312 of the supply conduit 310, the axial speed of the buffer liquid is reduced to a level where its influence on the mixing process is minimized. The buffer liquid entering the stratification tube 320 will then strive to move towards the region having a corresponding density. This is a result from thermal stratification, i.e. , that a liquid will naturally strive to be vertically distributed such that the density decreases as function of vertical position. Since a buffer liquid, such as e.g., water, has a density which monotonically decreases with increasing buffer liquid temperature in the operating temperature region of buffer tanks (it is noted that no buffer tank storing water is run close to the inflection point at 4 C°), the temperature stratification will ensure that the buffer liquid having the highest temperatures are located that the top end 140b of the buffer tank 140, and that the buffer liquid having the lowest temperatures are located that the bottom end 140a of the buffer tank 140. Hence, the predominately warmer buffer liquid will accumulate in the upper parts of the stratification tube 320 (i.e., within the upper portion 320b), and the predominately colder buffer liquid will accumulate in the lower parts of the stratification tube 320 (i.e., within the lower portion 320c). This is illustrated in Fig. 5 by the thick arrows. The buffer liquid will then strive to exit the stratification tube 320 at the vertical position where its density matches the density of the buffer liquid already residing in the tank (i.e., where the temperature matches). This is achieved by the provision of the plurality of through-openings 330a, 330b in the lateral side walls(s) 321 of the stratification tube 320. The flow-diverting means 331a, 331 b allow to further enhance the distribution of the supplied buffer liquid to the tank. By the provision of the flow-diverting means 331a, 331b, buffer liquid that may have been forced out from the stratification tube 320 a little too close to the intermediate portion 320a will obtain an extra push in the correct direction. This is illustrated in Fig. 5 by the thin arrows. An advantage with the buffer tank is that it is self-adjustable. Thus, there is no need for actively controlling where buffer liquid is supplied to the tank, and there is no need to measure the temperature of the heated buffer liquid and/or the temperature of the buffer tank. Once installed, the buffer liquid supplied to the buffer tank will be distributed passively, purely governed by natural forces. Thus, the solution is very reliable as it does not rely on any auxiliary systems, sensors, controllers, power, or moving parts. Moreover, it is cheap and highly durable.

The person skilled in the art realizes that the present disclosure by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. A buffer tank suitable for being fluidly connected to a heating system for a building and to house a buffer liquid which is circulated via said heating system to be heated, said buffer tank comprising: a vessel which, when being arranged in its operating position in relation to the heating system, has a vertical extension between a bottom end and a top end to allow forming temperature stratification in the buffer liquid housed therein, a buffer liquid outlet structured and arranged to allow retrieving buffer liquid from the bottom end of the buffer tank to be heated by the heating system, a buffer liquid inlet structured and arranged to allow returning heated buffer liquid to the buffer tank, a stratification tube arranged such that an axial extension thereof is aligned substantially in parallel with the vertical extension of the vessel, said stratification tube having lateral side walls(s) which presents a plurality of through-openings, and a supply conduit liquidly interconnecting the buffer liquid inlet with the stratification tube, wherein the supply conduit has a flow speed reducing portion having a cross-sectional area, as seen transverse to a flow direction of the buffer liquid, which cross-sectional area is gradually increasing along the flow direction for reducing a flow speed of the buffer liquid prior to it entering the stratification tube.

2. The buffer tank according to claim 1 , wherein the buffer tank is structured and arranged such that the stratification tube extends within a top 2/3 of the vertical extension of the vessel.

3. The buffer tank according to claim 1 or 2, wherein the supply conduit fluidly connects to the stratification tube at an intermediate portion thereof.

4. The buffer tank according to any one of claims 1 to 3, wherein a top end and/or a bottom end of the stratification tube is sealed.

5. The buffer tank according to any one of claims 1 to 4, wherein the vessel, the stratification tube and the supply conduit are made of the same material.

6. The buffer tank according to any one of claims 1 to 5, wherein at least some of the through-openings of the plurality of through-openings present flow-diverting means structured and arranged to increase a vertical flow velocity component of the buffer liquid exiting therethrough in a direction away from the supply conduit.

7. The buffer tank according to any one of claims 1 to 6, wherein the supply conduit is structured and arranged such that the stratification tube is aligned substantially coaxially with the buffer tank.

8. The buffer tank according to any one of claims 1 to 7, wherein the flow speed reducing portion is frustoconical and has a cone angle within the range 10 to 30 degrees, or 12 to 25 degrees, or 12 to 20, or 15 degrees.

9. The buffer tank according to any one of claims 1 to 8, wherein an internal cross section of the stratification tube as seen transverse to its axial extension is within the range of 5-25%, or 10-20% of an internal cross section of the vessel as seen transverse to its vertical extension.

10. The buffer tank according to any one of claims 1 to 9, wherein an internal cross section of the stratification tube as seen transverse to its axial extension is at least 25 times larger than an internal cross section of the buffer liquid inlet.

11 . The buffer tank according to any one of claims 1 to 10, wherein the stratification tube is supported within the tank only by the supply conduit.

12. A heat transfer arrangement comprising: a heating system for a building including at least one heat pump, and a buffer tank according to any one of the claims 1 to 11 , wherein the heating system is fluidly connected to the buffer tank for circulating a buffer liquid housed in the buffer tank via the heating system to be heated.

13. The heat transfer arrangement according to claim 12, wherein the at least one heat pump is an air-liquid heat pump.

14. The heat transfer arrangement according to claim 12, wherein the heating system includes at least one modular liquid-liquid heat pump, wherein the at least one modular liquid-liquid heat pump is structured and arranged to be detachable from the heating system.

15. The heat transfer arrangement according to claim 14, wherein the at least one modular liquid-liquid heat pump is a plurality of modular liquidliquid heat pumps operating in parallel.