Abstract:
A heat exchanger may include a heat exchange surface partially coated with a heat-conducting layer. The heat exchange surface may include a plurality of contact regions coated with the heat-conducting layer and a plurality of insulating regions that are not coated with the heat-conducting layer. The heat exchange surface may further include a degree of coverage of the heat-conducting layer that varies to compensate at least one of at least one hot spot and at least one cold spot. The at least one hot spot and the at least one cold spot may be included within at least one of the heat exchange surface and a plurality of energy storage elements of an energy store that contacts the heat exchange surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to German Patent Application No. DE 10 2014 218 923.3, filed on Sep. 19, 2014, and International Patent Application No. PCT/EP2015/071007, filed on Sep. 15, 2015, both of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a heat exchanger for controlling the temperature of energy storage elements and/or of power electronics, for example of an energy store, having a heat exchange surface which is placed on the energy store. The invention further relates to an energy store having at least one such heat exchanger, and the production of such a heat exchanger. 
       BACKGROUND 
       [0003]    In modern hybrid and electric cars, energy stores, for example lithium-ion batteries, are used. In order to guarantee an optimum performance and long lifespan of the energy stores, an efficient heat management is necessary, which makes it possible to operate energy storage elements, for example battery cells, of the energy store in a particular temperature window. Under load, the energy storage elements must be cooled actively and homogeneously, and in the cold season they must be heated accordingly. For this, the reliable contact between the energy storage elements and a heat exchanger is necessary. In addition, the temperature gradient occurring on the heat exchanger should be equalized and not transferred to the battery. In particular in the case of low volume flows in a coolant circuit of the energy store, relatively high temperature gradients occur on the heat exchanger. Thereby, the energy storage elements are cooled at different intensity, which leads to a temperature difference between the energy storage elements. Such a temperature difference is unfavourable, because it has a negative effect on the lifespan and the efficiency of the battery. 
         [0004]    Likewise, a non-homogeneous tensioning of the heat exchanger with the energy store can cause a non-homogeneous temperature in the energy store. 
         [0005]    From DE 10 2011 084 002 A1 a heat exchanger is known with a thermal transition device, which is composed of several layers. A thermal insulation layer is applied with varying thickness on the heat exchanger. Over this, a tolerance equalizing layer of a compressible material is applied, which equalizes the different layer thicknesses of the thermal insulation layer and unevenness of the cell module. Thereby, a heat conductivity, varying depending on location, between the heat exchanger and the energy storage elements which are to be cooled is achieved. A disadvantage in this solution is that two layers must be applied on the heat exchanger, which increases the production expenditure. 
       SUMMARY 
       [0006]    The present invention is based on the problem of providing a possibility to reduce a temperature difference between energy storage elements of an energy store at a favourable cost. 
         [0007]    This problem is solved according to the invention by the subjects of the independent claim(s). Advantageous further developments are the subject of the dependent claims. 
         [0008]    The invention is based on the general idea of applying a heat-conducting layer partially onto the heat exchange surface of the heat exchanger, so that a contact takes place between the heat exchanger and the energy storage elements of the energy store via the heat-conducting layer. Through the only partial application of the heat-conducting material on the heat exchange surface, insulating regions form, in which an air gap is present between the heat exchange surface and the energy storage elements. By variation of the ratio between coated and uncoated regions, the thermal resistance between the heat exchanger and the energy storage elements can be adjusted. It is expedient here that the heat exchange surface is coated partially with a layer having a heat-conducting material, and that the heat exchange surface has contact regions which are provided with the heat-conducting layer and has insulating regions which are not provided with the heat-conducting layer. Through the formation of the contact regions and insulating regions, the thermal resistance between the heat exchanger and the energy storage elements can be adapted locally, so that despite a temperature gradient in the heat exchanger, the energy storage elements only have small temperature differences with respect to one another. Through the fact that air is used as insulation for the insulating regions, the applying of only one layer influencing the heat transmission is necessary, so that the production of such a heat exchanger is simple and favourably priced. If an electrical insulation between the heat exchanger and the energy store is necessary, a covering electrically insulating layer can be applied on the heat exchange surface, onto which layer the heat-conducting layer is then partially applied. 
         [0009]    It is advantageous if the contact regions are provided for a defined thermal contact between the heat exchanger and the energy storage elements. Thereby, the heat transmission between the heat exchanger and the energy storage elements can take place via the contact regions when the heat exchanger is placed onto the energy store. In this way, the thermal resistance between the heat exchanger and the energy store can be adjusted in a targeted manner. 
         [0010]    It is particularly advantageous that the contact regions are configured so as to be flat and round, in particular circular and/or oval, and/or angled, in particular four-sided and/or hexagonal. In this way, the contact regions can be applied in a uniformly distributed manner on the heat exchange surface. Furthermore, by variation of the size of the respective contact regions, a ratio between the insulating regions and the contact regions can be influenced. 
         [0011]    It is particularly advantageous that a degree of coverage, which indicates the ratio between an area of the contact regions and an area of the insulating regions, varies in sections. The heat transmission between the heat exchanger and the energy storage elements takes place both via the contact regions and also via the insulating regions in the form of a parallel circuit. Whereas the heat transmission via the contact regions is very good, the heat transmission in the insulating regions via the air gap is distinctly less. The degree of coverage therefore determines the thermal resistance between the heat exchanger and the energy storage elements. Through the variation of the degree of coverage, accordingly also the thermal resistance between the heat exchanger and the energy storage elements varies. In this way, temperature variations of the heat exchanger can be equalized, so that the temperature differences between the energy storage elements are small. 
         [0012]    In particular, the degree of coverage varies in a longitudinal direction of the heat exchanger, therefore in a main flow direction of a fluid which flows through the heat exchanger. Thereby, the variation of the degree of coverage takes place in the direction of a temperature gradient of the heat exchanger, which occurs during operation. 
         [0013]    A favourable variant makes provision that the degree of coverage of the heat exchange surface varies in a longitudinal direction of the heat exchanger and in a transverse direction of the heat exchanger. Thereby, a temperature non-homogeneity in the energy storage elements can also be prevented in a direction transversely to the flow direction. 
         [0014]    In a favourable manner, the degree of coverage of the heat exchange surface is adapted locally to a desired temperature difference between the heat exchanger and the energy storage elements, such that the degree of coverage is smaller, the higher a desired temperature difference is between the heat exchanger and the energy storage elements. This means that in regions in which a small temperature difference is desired, the degree of coverage is greater than in regions in which a greater temperature difference is desired. The aim is to achieve as uniform a temperature distribution as possible within the energy store. In order to achieve an approximately constant temperature in the energy store, the heat flow out from the energy store into the heat exchanger must be at least approximately constant or respectively must correspond to the waste heat energy flow. Owing to the temperature gradient which occurs in the heat exchanger, and which where applicable can only be reduced with great effort, a small thermal resistance, therefore a high degree of coverage, should be present in regions in which the driving temperature difference between the heat exchanger and the energy store is small, whereas in regions in which the temperature difference is, or respectively should be, greater, a higher thermal resistance is necessary. Through the adapting of the degree of coverage to the desired temperature differences, it can therefore be achieved that the heat flow from the energy store to the heat exchanger or the other way round, over the surface of the heat exchanger is constant, so that only small temperature differences occur between the individual energy storage elements. 
         [0015]    A further favourable variant makes provision that the degree of coverage of the heat exchange surface varies in order to compensate at least one hot spot and/or a cold spot of the heat exchanger and/or of the energy storage elements. If, for example, the temperature of a hot spot is greater than that of the energy store, the degree of coverage can be selected to be very small. Therefore, substantially only insulating regions are arranged on the respective region of the heat exchange surface of the heat exchanger, so that additional heat is not introduced into the energy storage elements which are to be cooled. If, however, in the region of the hot spots the temperature of the heat exchanger is still less than the desired temperature of the energy storage elements, the degree of coverage can also be selected to be very high, in order to achieve a sufficient cooling of the energy storage elements despite the hot spot. Accordingly, hot spots or cold spots in the energy store can also be equalized by an adapting of the degree of coverage. 
         [0016]    A preferred solution makes provision that the heat exchanger has a fluid channel through which a fluid can be directed, in order to control the temperature of the energy storage elements, and that at an inflow side of the heat exchanger, at which the fluid flows into the heat exchanger, the degree of coverage is smaller than at an outflow side of the heat exchanger, at which the fluid flows out from the heat exchanger. At the inflow side of the heat exchanger, the fluid arrives freshly temperature-controlled and thereby has a greater temperature difference to the energy storage elements than at the outflow side of the heat exchanger. For this reason, a smaller degree of coverage is necessary at the inflow side of the heat exchanger than at the outflow side of the heat exchanger. At the outflow side of the heat exchanger, the temperature of the fluid and thereby the temperature of the heat exchanger has generally drawn near to the temperature of the energy store, so that the temperature difference there between the heat exchanger and the energy storage elements is less. The variable degree of coverage therefore has a positive effect both during cooling and also heating of the energy storage elements. 
         [0017]    In addition, the above-mentioned problem is solved by an energy store with energy storage elements and with at least one heat exchanger according to the above description, which rests on the energy storage elements, in order to control the temperature of the energy storage elements, wherein a contact area between the energy storage elements and the heat exchanger is formed by the heat-conducting layer applied to the heat exchanger. Therefore, the advantages of the heat exchanger are transferred to the energy store, to the above description of which reference is made in this respect. 
         [0018]    An advantageous variant makes provision that the degree of coverage of the heat exchange surface varies, in order to compensate non-homogeneous contact pressures between the heat exchanger and the energy storage elements. If the layer thickness of the heat-conducting layer is different locally owing to a non-homogeneous tensioning, the thermal connection resistance is thereby also altered. This can be taken into consideration in the configuring of the degree of coverage, in order to equalize such alterations. 
         [0019]    An advantageous solution makes provision that the contact regions which are coated with the heat-conducting material, and the insulating regions, which are not coated with the heat-conducting material, alternate in a transverse direction of the heat exchanger, so that in the insulating regions an air gap is formed between the heat exchanger and the energy storage elements. Through the fact that the contact regions and the insulating regions alternate, the contact regions keep the energy storage elements at a distance from the heat exchange surface of the heat exchanger, so that in the insulating regions an air gap is formed between the heat exchange surface of the heat exchanger and the energy storage elements. 
         [0020]    Advantageously, the heat transmission between the heat exchanger and the energy storage elements takes place in a parallel manner via the contact regions covered by heat-conducting material and via the air gaps formed at the insulating regions. Thereby, both the contact regions and also the insulating regions contribute to the heat transmission, so that the thermal resistance can be influenced by variation of the ratios of the areas of the insulating regions and contact regions. 
         [0021]    It is particularly preferred that the contact regions have structural sizes which are smaller than the structural sizes of the energy storage elements. In this way, it can be prevented that, owing to the partial application of the heat-conducting layer, temperature differences occur between the energy storage elements. Through the smaller structural size of the contact regions and insulating regions compared to the structural size of the energy storage elements, the heat transmission between the heat exchanger and respectively an energy storage element takes place always via at least one contact region and at least one insulating region. 
         [0022]    In particular, the structural sizes of the contact regions are measured parallel to the heat transmission surface and can be for example the edge length, the diameter, the length of semi-axes or of the diagonals or similar dimensions of the contact regions. The structural size of the energy storage elements is observed here in the same direction, therefore parallel to the heat exchange surface and is provided for example by a cell diameter of the energy storage elements. 
         [0023]    A favourable solution makes provision that an average thermal resistance varies in sections between the heat exchanger and the energy storage elements. In this way, a temperature difference occurring between the heat exchanger and the energy storage elements can be influenced in a targeted manner, in order to equalize, for example, a temperature gradient present in the heat exchanger. 
         [0024]    The average thermal resistance is averaged here for example over an area which corresponds to the structural size of the energy storage elements. Thus, the average thermal resistance can vary over the heat exchange surface and can be relevant for an individual energy storage element. 
         [0025]    The average thermal resistance varies in particular in the flow direction of a fluid which flows through the heat exchanger, therefore in longitudinal direction of the heat exchanger. Thereby, the average thermal resistance varies in the direction of a temperature gradient of the heat exchanger which occurs during operation. 
         [0026]    The above-mentioned problem is further solved by a method for the production of a heat exchanger according to the above description for an energy store according to the above description, wherein the contact regions are printed onto the heat exchange surface of the heat exchanger or are glued on as film pre-cut parts. In this way, the heat-conducting layer can be applied in a very simple and very cost-efficient way, so that the heat exchanger is extremely favourable to produce. 
         [0027]    A particularly favourable variant makes provision that with a determining of the degree of coverage of the heat exchange surface, hot spots and/or cold spots of the heat exchanger and/or of the energy storage elements are taken into consideration. Thereby, an even more uniform temperature can be achieved in the energy storage elements. 
         [0028]    A particularly advantageous variant makes provision that with a determining of the degree of coverage of the heat exchange surface, non-homogeneous contact pressures between the heat exchanger and the energy storage elements are taken into consideration. The contact pressure between the heat exchanger and the energy storage elements brings about a compression of the heat-conducting layer. When the contact pressure is not equal, different layer thicknesses and therefore different thermal resistances occur. In order to achieve a uniform contact pressure over the entire surface, a great effort would have to be made. It is therefore favourable to equalize the effect of the non-homogeneous contact pressures by variation of the degree of coverage. 
         [0029]    Further important features and advantages of the invention will emerge from the subclaims, from the drawings and from the associated figure description with the aid of the drawings. 
         [0030]    It shall be understood that the features named above and to be explained further below are able to be used not only in the respectively indicated combination, but also in other combinations or in isolation, without departing from the scope of the present invention. 
         [0031]    Preferred example embodiments of the invention are presented in the drawings and are explained in further detail in the following description, wherein the same reference numbers refer to identical or similar or functionally identical components. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    There are shown, respectively diagrammatically: 
           [0033]      FIG. 1  a sectional illustration through an energy store with a heat exchanger, 
           [0034]      FIG. 2  a diagram, wherein a temperature of the heat exchanger and a temperature of the energy store are presented in a location-dependent manner in the direction of a flow direction of a fluid in the heat exchanger, 
           [0035]      FIG. 3  a diagram, wherein a degree of coverage of a heat exchange surface and a thermal resistance between the heat exchanger and the energy store are presented in a location-dependent manner in the direction of a flow direction of a fluid in the heat exchanger, 
           [0036]      FIG. 4  a top view onto a heat exchange surface of the heat exchanger, 
           [0037]      FIG. 5  a top view onto a second variant of the heat exchanger, 
           [0038]      FIG. 6  a top view onto the heat exchange surface of the heat exchanger of  FIG. 4 , 
           [0039]      FIG. 7  a sectional illustration of an energy store with two heat exchangers, 
           [0040]      FIG. 8  a top view onto a heat exchange surface of a third variant of the heat exchanger, and 
           [0041]      FIG. 9  a top view onto a heat exchange surface of a fourth variant of the heat exchanger. 
       
    
    
     DETAILED DESCRIPTION 
       [0042]    An energy store  10  illustrated in  FIG. 1  has a plurality of energy storage elements  12 , and a heat exchanger  14 , which rests on the energy storage elements  12 , in order to control the temperature of the energy storage elements  12 . For this, the heat exchanger  14  has a heat exchange surface  16 , by which the heat exchanger  14  rests on the energy storage elements  12 , and a fluid channel  18 , through which a fluid  20  can be directed through the heat exchanger  14 , in order to control the temperature thereof and therefore the energy storage elements  12 . 
         [0043]    It shall be understood that the heat exchanger  14  can also be used in order to control the temperature of other elements, such as power electronics, for example. In the following, by way of example only the temperature control of energy storage elements is illustrated. 
         [0044]    The energy store  10  is used for example in motor vehicles, in particular hybrid motor vehicles or electric vehicles. In order to be able to store the necessary electrical energy for this, the energy store  10  has the energy storage elements  12  which are, for example, electrochemical storage elements such as lithium-ion accumulator batteries. Generally, capacitors, in particular electrochemical double-layer capacitors, are also conceivable. 
         [0045]    Such energy stores have a limited temperature window, in which the operation of the energy storage elements can take place properly. At lower temperatures, owing to the lower chemical reaction dynamics, the efficiency of the energy storage elements is limited. And temperatures which are too high lead to a quicker ageing or to the direct damage of the energy storage elements. For this reason, it is advantageous if the temperature differences between the individual energy storage elements  12  are small, because in this way the temperature of all energy storage elements  12  can be kept more easily in the permitted temperature window. 
         [0046]    The temperature control of the energy storage elements  12  takes place by the heat exchanger  14 , the temperature of which is controlled by the fluid  20 , in which it is flowed through by the fluid. Here, the fluid  20  flows through a fluid inlet  22  on an inflow side  24  of the heat exchanger  14  into the heat exchanger  14  and flows out from the heat exchanger  14  again through a fluid outlet  26  at an outflow side  28  of the heat exchanger  14 . Depending on whether the fluid  20  is to heat or cool the energy store  10 , the fluid  20  at the inflow side  24  of the heat exchanger  14  is warmer or colder than the heat exchanger  14 , in any case the fluid  20  at the inflow side  24  of the heat exchanger  14  has a higher temperature difference to the heat exchanger  14  than at the outflow side  28  of the heat exchanger  14 , because the fluid, on flowing through the heat exchanger, was able to emit or receive thermal energy. Thereby, a temperature gradient also occurs in the heat exchanger  14 . The inflow side  24  and the outflow side  28  are spaced apart from one another in a longitudinal direction  17  of the heat exchanger. Consequently, a flow direction  21  of the fluid in the heat exchanger  14  is substantially parallel to the longitudinal direction  17 . A local flow direction of the fluid can of course deviate therefrom, if for example the fluid channel  18  runs in a meandering manner. 
         [0047]    As illustrated for example in  FIG. 2 , during cooling a temperature  23  of the heat exchanger  14  at the inflow side  24  of the heat exchanger  14  is lower than at an outflow side  28  of the heat exchanger  14 . As a temperature  25  of the energy store  10  is to be as constant as possible, a temperature difference  27  between the heat exchanger  14  and the energy storage elements  12  at the inflow side  24  is greater than at the outflow side  28 . Thereby, the temperature gradient in the heat exchanger  14  can be transferred to the energy storage elements  12 , which would be unfavourable, as described above. Accordingly, during heating by means of the heat exchanger  14  also the temperature difference  27  at the inflow side  24  is greater than at the outflow side  28 . 
         [0048]    In order to keep the temperature gradient small within the heat exchanger  14 , the flow speed of the fluid  20  could be increased, so that a small temperature difference  27  is achieved between the inflow side  24  and the outflow side  28  of the heat exchanger  14 . However, this would lead to an increased energy consumption for the cooling. In contrast thereto, the energy consumption for the cooling could be reduced, if the flow speed of the fluid  20  is reduced and thereby a higher temperature difference occurs between the inflow side  24  and the outflow side  28 , because through the higher temperature difference per unit of volume of the fluid  20  more thermal energy can be transported towards the heat exchanger  14  or away from the heat exchanger. 
         [0049]    In order to reduce the effects of the temperature gradient of the heat exchanger  14 , the heat exchange surface  16  is partially covered by a layer  29  having a heat-conducting material. Here, contact regions  30  are provided with the heat-conducting layer  29 , and insulating regions  32  are not provided with the heat-conducting layer  29 . When the heat exchanger  14  is now placed on the energy storage elements  12 , only the contact regions  30  of the heat exchanger  14  which are coated with heat-conducting material are in contact with the energy storage elements  14 . By variation of the sizes of the contact regions  30  and of the insulating regions  32 , in particular by variation of a degree of coverage  34 , which indicates the ratio between the areas of the contact regions  30  and of the insulating regions  32 , a thermal resistance  36  can be adjusted between the heat exchanger  14  and the energy store  10 , therefore to the energy storage elements  12 . 
         [0050]    Through the selection of small structural sizes of the contact regions  30 , therefore with a use of a plurality of contact regions  30 , an average thermal resistance  36  between the heat exchanger  14  and the energy store  10  can be adjusted in sections. Preferably, the structural size  30  of the contact regions  30  is so small that the heat transmission from the heat exchanger  14  takes place over several contact regions  30 , for example three, to an individual energy storage element  12 . Thereby, for each energy storage element  12 , the degree of coverage  34  and therefore the thermal resistance  36  can be adjusted individually, in order to keep the respective individual energy storage elements  12  in the usable temperature window despite the temperature gradient in the heat exchanger  14 . 
         [0051]    For example, the degree of coverage  34  in the region of the inflow side  24  is smaller than in the region of the outflow side  28 . Thereby, the thermal resistance  36  in the region of the inflow side  24  is greater than in the region of the outflow side  28  (cf.  FIG. 3 ). Therefore it can be achieved that the thermal energy transmission from the energy storage elements  12  to the heat exchanger  14  in the region of the inflow side  24  is approximately equally great as the thermal energy transmission of the energy storage elements  12  to the heat exchanger  14  in the region of the outflow side  28 . In the region of the inflow side  24 , the temperature difference  27  between the heat exchanger  14  and an energy storage element  12  arranged there is greater than in the region of the outflow side  28 . Consequently, at the inflow side  24  a higher thermal resistance  36  is necessary than at the outflow side  28 , in order to achieve the same thermal flow density from the energy store  10  to the heat exchanger  14 . 
         [0052]    The contact regions  30  are formed by the partially applied heat-conducting layer. Accordingly, these are configured so as to be flat. Furthermore, different shapes are conceivable. In particular, angled or round shapes are conceivable. Four-sided or hexagonal shapes are particularly advantageous, because these shapes can be arranged in a row adjacent to one another on a surface. 
         [0053]    The contact regions  30  are arranged spaced apart from one another and are separated from one another by the insulating regions  32 . The insulating regions  32  are constructed in particular contiguously and are interrupted by the contact regions  30 . As illustrated for example in  FIG. 4 , the insulating regions  32  are configured entirely continguously. However, depending on the arrangement of the contact regions  30 , it is also conceivable that several groups of insulating regions  32  are configured separate from one another. 
         [0054]    The degree of coverage  34  of the heat exchange surface  16  can be influenced in particular by the number of contact regions  30 . Furthermore, it can also be influenced by the size of the contact regions  30 . 
         [0055]    As illustrated for example in  FIGS. 5 and 6 , the degree of coverage  34  can also be adapted, so that so-called hot spots and/or cold spots  40 , the temperature of which deviates very locally and therefore intensively from the average temperature, are equalized. In particular, hot spots and/or cold spots  40  in the energy store, therefore also in the heat exchanger  14 , can be equalized. For this, the degree of coverage  34  of the heat exchange surface  16  can be varied both in the longitudinal direction  17  and also in a transverse direction  19  running transversely to the longitudinal direction  17 . 
         [0056]    In addition, as is illustrated for example in  FIGS. 8 and 9 , provision can be made to equalize the effect of non-uniform contact pressures between the energy storage elements  12  and the heat exchange surface  16  to the thermal resistance. The contact pressure brings about a compression of the heat-conducting layer  29  or respectively of the contact regions  30 , so that the thermal resistance reduces owing to the reduced thickness. If now the contact pressure is not constant over the entire heat exchange surface  16 , this leads to deviations of the achieved thermal resistance and therefore to undesired temperature deviations in the energy storage elements  12 . 
         [0057]    This can likewise be equalized by a variation of the degree of coverage  34 . This can take place, as illustrated for example in  FIG. 8 , in that through a higher degree of coverage, the supporting effect of the contact regions  30  is improved and therefore the compression is reduced. Alternatively, provision can also be made, as illustrated for example in  FIG. 9 , to reduce the degree of coverage  23  in regions with higher contact pressure, in order to equalize the thermal resistance which has decreased through the compression of the heat-conducting layer  29 . 
         [0058]    It shall be understood that by variation of the degree of coverage, any desired combination of the above-mentioned compensations can be equalized with a combined coverage pattern. Thus, for example, both the temperature gradient in the heat exchanger  14  and also hot spots and/or cold spots  40  can be equalized. In addition, it is also possible to equalize a non-uniform tensioning between the energy storage elements  12  and the heat exchanger  14 . 
         [0059]    In addition, it is also possible to use several heat exchangers  14  in an energy store  10 , which heat exchangers are connected fluidically in series, for example. In such a variant, the degree of coverage  34  of the two heat exchangers  15  can be adapted individually, so that despite the temperature differences between the two heat exchangers  14 , an at least approximately constant temperature can be achieved within the energy store  10 . 
         [0060]    The contact regions  30  can be applied onto the heat exchange surface  16  for example by a printing method, such as for example screen printing or stencil printing. Furthermore, the contact regions can be applied by means of film pre-cut parts onto the heat exchange surface  16 .