CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in International Patent Application No. PCT/EP2011/004804 filed on Sep. 26, 2011 and Danish Patent Application No. PA 2010 00874 filed Sep. 27, 2010. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a heat exchanger arrangement comprising a heat exchanger, the heat exchanger having a primary side connectable to a fluid circulation system, and a secondary side exposed to a gas. 
         [0003]    Furthermore the invention relates to a heat pump system comprising a circuit for circulating a fluid, the circuit comprising a fluid driving section, a warm side heat exchanger, an expansion valve, and a cold side heat exchanger. 
       BACKGROUND 
       [0004]    In a heat pump system a refrigerant which can be in liquid and in vapor phase is circulated through a circuit. In compressor driven systems, the vapor is compressed by the compressor. The temperature of the vapor rises. The hot vapor is guided through the warm side heat exchanger. Heat emitted from the warm side heat exchanger can be used for space—or tap water heating. The vapor cools down and changes its phase to a liquid. The liquid is allowed to expand in the expansion valve. The expanded liquid is guided through the cold side heat exchanger. In the cold side heat exchanger the liquid adsorbs heat from air and evaporates. Other systems like absorption or adsorption heat pump systems run without a compressor but still have a cold side heat exchanger and a warm side heat exchanger. 
         [0005]    Usually a fan or another driving means is used in order to drive enough air through the cold side heat exchanger. However, such a fan requires additional energy. Furthermore, the fan is noisy. 
       SUMMARY 
       [0006]    The problem underlying the present invention is to operate the heat exchanger with little energy consumption. 
         [0007]    This object is solved by a heat exchanger arrangement comprising a heat exchanger having a primary side connectable to a fluid circulation system and a secondary side exposed to a gas, wherein the secondary side is connected to a duct extending downwardly in the direction of gravity. 
         [0008]    In particular the secondary side of the heat exchanger is connected to a duct extending downwardly in the direction of gravity. 
         [0009]    When heat is absorbed from the air in the cold side heat exchanger the air cools down and the cold air has a higher density than the surrounding air. The idea is to use the potential energy of this heavy air to drive the air flow through the heat exchanger. By means of the duct the maximum available potential energy to drive the air through the heat exchanger is increased. Without the duct the maximum available potential energy comes from the thickness of the exchanger itself. However, if a duct is used the air cooled down by the heat exchanger cannot escape directly to the ambient or surrounding air but falls down in form of a volume which is limited in a direction perpendicular to its movement by the duct. Therefore, a greater volume of cold air is kept together which increases the potential energy. 
         [0010]    In a preferred embodiment the duct extends in vertical direction. Therefore, the gravity can act on the cold air without any deviation. 
         [0011]    Preferably the duct has a cross section corresponding to an area covering the secondary side of the heat exchanger. In this case all air flowing through the heat exchanger is used to form the volume of cold air which in turn drives the air through the heat exchanger. 
         [0012]    In an advantageous embodiment the cross section decreases in a direction away from the heat exchanger. In this embodiment the air escaping from the duct is accelerated. The suction force driving the air through the heat exchanger is increased. This can be used to drive more air through the heat exchanger than with a constant cross section. 
         [0013]    Preferably the duct has at least over a part of its length a circular cross section. The relation between the cross section and the length of the surrounding wall in circumferential direction is an optimum in the section of the duct which has a circular cross section. Therefore, the air in the duct is less thermally influenced by the ambient temperature. 
         [0014]    Preferably the duct comprises walls having a thermal insulation. This is an alternative or additional feature to avoid heating of the air within the duct by warmer air outside of the duct. Since the temperature difference between the air inside the duct and the ambient air outside the duct is not very high a rather small thermal insulation is sufficient. 
         [0015]    Preferably the duct has a length of at least 0.5 m. The longer the duct the more potential energy is available. The length of at least 0.5 m relates to the vertical length of the duct in direction of gravity. 
         [0016]    In a preferred embodiment the secondary side is free of fluid driving devices. The duct allows an operation with less fan energy, i.e. a smaller fan can be used or a fan which is driven with less power. In an optimum configuration the duct is sufficient to drive enough air through the secondary side of the heat exchanger. 
         [0017]    Preferably a spacer arrangement is arranged at the outlet of the duct. The spacer arrangement prevents closing of the duct, in particular keeps a sufficient distance to a base plate on which the duct is erected. 
         [0018]    The problem is solved with a heat pump system comprising a circuit for circulating a fluid, the circuit comprising a fluid driven section, a warm side heat exchanger, an expansion valve, and a cold side heat exchanger, wherein the cold side heat exchangers is part of a heat exchanger arrangement. 
         [0019]    The air (or any other gas or liquid) is driven through the cold side heat exchanger with the help of the potential energy of the air which is increased when the duct is used. 
         [0020]    Preferably the cold side heat exchanger is exposed to outdoor ambient air. In this case the outdoor ambient air can be used as heat source for the heat pump system. However, there is less or even no power necessary for driving the air through the cold side heat exchanger. This leads to a very good efficiency of the heat pump system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    In the following, preferred embodiments of the invention will be described in further details with reference to the drawing, in which: 
           [0022]      FIG. 1  is a schematical illustration of a heat pump system, 
           [0023]      FIG. 2  shows a first embodiment of a heat exchanger arrangement, 
           [0024]      FIG. 3  shows a second embodiment of a heat exchanger arrangement, and 
           [0025]      FIG. 4  shows a third embodiment of a heat exchanger arrangement. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  shows schematically a heat pump system  1  comprising a circuit  2  for circulating a refrigerant. The refrigerant is a fluid which can have a liquid phase and a gaseous phase within the circuit  2 . The state of the fluid depends on temperature and pressure conditions within certain sections of the circuit  2 . 
         [0027]    The circuit  2  comprises a compressor  3  operating as fluid driving section compressing the gaseous fluid. The energy necessary for this compression is taken from an electrical power supply for example. During compression the temperature of the fluid increases. 
         [0028]    The compressor  3  is followed by a condenser  4  in which heat  5  is emitted to e. g. tap water, radiators, floor heating or the like. In the condenser  4  the fluid is cooled and leaves the condenser as a liquid. 
         [0029]    The liquid is passed through an expansion valve  6  which allows the fluid to expand. After leaving the expansion valve  6  the fluid has a lower pressure. 
         [0030]    The liquid under a lower pressure runs through an evaporator  7 . In the evaporator  7  heat  8  is absorbed from ambient air so that the fluid evaporates. The vapor is again passed to the compressor  3 . 
         [0031]    The system illustrated in  FIG. 1  can be replaced by any other heat pump system having a cold side heat exchanger and a warm side heat exchanger, e.g. an absorption heat pump system or an adsorption heat pump system both operating without compressor. 
         [0032]    The condenser  4  is a warm side heat exchanger and the evaporator  7  is a cold side heat exchanger. 
         [0033]    The following description is directed to the cold side heat exchanger. 
         [0034]      FIG. 2  shows the cold side heat exchanger  7 . The cold side heat exchanger  7  has a first connection  9  which is or can be connected to the expansion valve  6  and a second connection  10  which is or can be connected to the compressor  3 . In this example, the heat exchanger  7  comprises a plurality of pipe sections  11  through which the fluid of the circuit  2  flows. Furthermore, the heat exchanger comprises a plurality of fins  12  which are in heat conducting connection with the pipe sections  11  so that heat can be transmitted from the fins  12  to the pipe sections  11 . Other types of heat exchangers can be used as well, e.g. “microchannel” heat exchangers where the fluid passes through thin and flat tubes with approximately rectangular cross section. 
         [0035]    A stream of air  13  (symbolized by an arrow) should be directed through the heat exchanger  7  in order that heat is transmitted from the air  13  to the fluid passing through the heat exchanger  7 . This heat is necessary to evaporate the fluid in the heat exchanger  7 . 
         [0036]    However, the heat exchanger  7  exhibits a certain flow resistance against the stream of air  13  so that usually a fan is necessary to drive the air  13  to the heat exchanger  7 . 
         [0037]    According to the embodiment described this fan can be omitted or at least driven with less power so that the overall power consumption of the heat exchanger  7  is decreased. 
         [0038]    To this end the heat exchanger  7  is connected with a duct  14 . 
         [0039]    The duct  14  is arranged below the heat exchanger  7  in the direction of gravity. Preferably it is directed in the direction of gravity. The cross section of the duct  14  corresponds to the area of the heat exchanger  7  through which air  13  flows during the operation. 
         [0040]    At the bottom of the duct  14  a spacer  15  is arranged so that an opening  16  remains which cannot be closed inadvertently. 
         [0041]    The duct comprises walls  17  which have a thermal insulation. Therefore, a heat exchange between the air in the interior of the duct  14  and the ambient air is reduced to a minimum. 
         [0042]    The heat exchanger  7  uses gravity or natural convection. 
         [0043]    Ambient air  13  is getting in contact with the heat exchanger  7  and is cooled down by the heat exchanger  7 . Heat is transferred from the air  13  to the fluid in the circuit  2 . When the air gets colder the density of the air  13  increases. Therefore, the now colder air  13  will fall down through the heat exchanger  7  and into the duct  14  placed under the heat exchanger  7 . The cool and heavy air remains in the duct  14  moving downwardly and drawing ambient air  13  through the heat exchanger  7 . 
         [0044]    The duct  14  has an effective length of at least 0.5 m. The effective length is the length in the direction of gravity. It is more preferred that the length is greater, for example 1 m, 1.5 m or 2 m or even more. The greater the length of the duct  14  the more potential energy is available and the better is the efficiency of the heat exchanger  7 . A fan can be operated with lower power consumption or a fan can be completely omitted. 
         [0045]    In the embodiment of  FIG. 2  the duct has the same cross section as the area of the heat exchanger  7  through which the ambient air  13  passes. In other words, if this area of the heat exchanger  7  is rectangular the duct  14  also has a rectangular cross section. 
         [0046]      FIG. 3  shows an alternative embodiment.  FIG. 3   a  shows a side elevation and  FIG. 3   b  shows a view from the bottom. The same elements as in  FIG. 2  are marked with the same reference numerals. 
         [0047]    In  FIG. 3  the duct  14  has a rather large part of it&#39;s length in which ( FIG. 3   b ) the cross section of the duct  14  is circular. The area of the cross section of the duct  14  is the same as the effective area of the heat exchanger  7  so that there is basically no change in the flow condition of the ambient air  13  through the heat exchanger  7  and the duct  14 . However, the duct  14  has a wall  17  which is shorter in circumferential direction than with a rectangular cross section as in  FIG. 2 . In this way the risk of a heat transfer from the ambient air to the air inside the duct  14  is further reduced. A transition section  18  is provided to give a smooth transition from the area of the heat exchanger  7  to the duct  14 . 
         [0048]      FIG. 4  shows a third embodiment. The same elements are designated with the same reference numerals. 
         [0049]    The duct  14  still has a circular cross section. However, the wall  17  are inclined inwardly in the direction of gravity. So the duct  14  forms a cone having an opening  19  forming a kind of nozzle. The opening  19  has a much smaller cross section than the top of the duct  14 . 
         [0050]    The effect of the cone form of the duct  14  that air in the duct  14  is accelerated in a direction downwards. Thus, the suction power of the duct  14  is increased and the efficiency of the heat exchanger  7  is further increased. 
         [0051]    A reduction of the effective area of the duct  14  can of course also be used in connection with the embodiment shown in  FIG. 2 . In this case the cone has a rectangular section. 
         [0052]    Although various embodiments of the present invention have been described and shown, the invention is not restricted thereto, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims.
A heat exchanger arrangement is given comprising a heat exchanger ( 7), the heat exchanger ( 7) having a primary side connectable to a fluid circulation system and a secondary side exposed to a gas ( 13). This heat exchanger arrangement should be operated with little energy consumption. To this end the secondary side is connected to a duct ( 14) extending downwardly in the direction of gravity, when the duct ( 14) is connected to a cold side heat exchanger ( 7), and upwardly, when the duct is connected to a warm side heat exchanger.