Abstract:
A water generation system for the generation of water on board an aircraft, in one example, may be used for the cooling of the fuel cell exhaust air cabin air which is drawn off through the condenser due to the pressure difference between the cabin pressure and ambient pressure and may not require extensive cooling circuits or pumps.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of the filing date of German Patent Application No. 10 2006 034 814.1 filed Jul. 27, 2006 and of U.S. Provisional Patent Application No. 60/820,493 filed Jul. 27, 2006, the disclosure of which applications is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The field relates to the generation of water on board aircraft. In particular, the field relates to a water generation system for the generation of water on board an aircraft, a condenser for condensing water from an exhaust gas from a fuel cell device on board an aircraft, the use of a water generation system of this kind, an aircraft and a method for the generation of water on board an aircraft. 
       BACKGROUND OF THE INVENTION 
       [0003]    In aircraft, fuel cell arrangements can be used for the recovery of water from the fuel cell exhaust gas. Hereby, it is necessary to condense out the water contained in the exhaust gas flow water vapor by means of a condenser. 
         [0004]    For the condensation of the water vapor, a cooling circuit may be provided to which two heat exchangers are coupled. This may involve a primary heat exchanger (PHE) and a secondary heat exchanger (SHE). 
         [0005]    The condensation is performed hereby by indirect cooling by means of coolant, which is finally cooled by external air (see  FIG. 1 ). This indirect cooling is necessary to prevent icing of the primary heat exchanger due to direct contact with external air, the temperature of which can be significantly below the freezing point of water. However, this arrangement may be complex and associated with a high overall system mass. 
       SUMMARY OF THE INVENTION 
       [0006]    According to an embodiment of the water generation system, a water generation system for the generation of water on board an aircraft comprises a fuel cell device, a condenser for condensing water from an exhaust gas of the fuel cell device and an outlet, wherein the condenser is designed to cool the exhaust gas with cabin air and wherein the outlet is designed to discharge the cabin air to the environment of the aircraft when the cabin air has flowed through the condenser. 
         [0007]    Thus, two cooling circuits may no longer have to be used. In addition, a pump or the like may no longer be necessary. The condenser is no longer (indirectly) cooled by external air but instead cooled by the on-board cabin air which is drawn through the outlet through the condenser. To do this, the outlet may be connected to the outer environment of the aircraft so that a pressure drop occurs when the aircraft is at cruising altitude which draws the cabin air through the condenser. 
         [0008]    According to an embodiment of the water generation system, the condenser comprises at least one separation volume, whereby the separation volume is designed to change the direction of flow of the exhaust gas and for the separation of water. 
         [0009]    The separated water may accumulate in the separation volume. The change in the direction of flow of the exhaust gas in the separation volume may also increase the rate of separation. 
         [0010]    According to an embodiment of the water generation system, the separation volume is arranged at one side of the water generation system. 
         [0011]    Obviously, a plurality of separation volumes may be each arranged at one of the sides of the water generation system. For example, the exhaust flow is guided through corresponding lines from the one side of the condenser to the other side and then arrives at a corresponding separation volume. Here, the flow is deflected and then flows through corresponding further lines to the other side of the condenser into a further separation volume. This process may be repeated several times until sufficient water has been condensed out or separated. 
         [0012]    According to an embodiment of the water generation system, the water generation system comprises a perforated separating element through which the water accumulated in the separation volume may flow into an adjacent separation volume. 
         [0013]    The two adjacent separation volumes are arranged one on top of the other for example, such that the water in the higher volume flows through the separating element (for example, a perforated partition) due to its weight force and/or due to the pressure difference between the upper region of the condenser and the lower region of the condenser in the underlying separation volume. 
         [0014]    According to an embodiment of the water generation system, the water generation system further comprises two independent openings to remove the separated water. 
         [0015]    These openings are each arranged in a side region of the condenser, for example. The first opening may be used, for example, to remove water which has accumulated at the left side of the condenser and the second opening may be used, for example, to remove water, which has accumulated on the right side of the condenser. Obviously, further openings for the removal of water may be provided. 
         [0016]    According to an embodiment of the water generation system, the condenser is designed as a tubular heat exchanger comprising a plurality of lines for transporting the exhaust air. For example, the lines are arranged in different planes which lie one on top of another so that a whole battery of lines extending in parallel results. These lines have a tubular type design, for example, but could also have other cross sections, for example rectangular or square cross sections. 
         [0017]    According to an embodiment of the water generation system, the directions of flow of the exhaust gas in the first plane and the second plane correspond, whereby planes arranged thereunder have an opposite direction of flow. 
         [0018]    For example, the exhaust gas in three tube planes lying directly one on top of the other flows in the one direction, while it flows in the opposite direction in the three tube planes lying thereunder, etc. 
         [0019]    According to an embodiment of the water generation system, the condenser is designed as a plate heat exchanger. 
         [0020]    Other heat exchangers may be provided with a corresponding outlet, which discharges the cabin air from the aircraft into the environment, may also be used. 
         [0021]    According to an embodiment of the water generation system, the outlet comprises a cabin air outflow valve. 
         [0022]    This may enable the through-flow rate of the cabin cooling air to be controlled. If no through-flow is desired, the valve may be closed. 
         [0023]    According to an embodiment of the water generation system, the water generation system further comprises an inlet to let the exhaust gas into the condenser. The inlet is hereby designed for connection to the fuel cell device. It may also be possible to provide a plurality of inlets to which a plurality of fuel cells can be connected. For example, the inlet has a large area so that the fuel cell exhaust gas is distributed as uniformly as possible when flowing into the condenser. 
         [0024]    According to an embodiment of the water generation system, cabin air and exhaust gas flow against each other in the condenser. 
         [0025]    For example, the cabin exhaust air introduced into the top left of condenser and brought out of the condenser at the bottom right. On the other hand, the cabin cooling air is supplied to the bottom side of the condenser and drawn out at the upper side of the condenser. 
         [0026]    According to an embodiment of the water generation system, a condenser for condensing water from an exhaust gas from a fuel cell device on board an aircraft comprises an outlet, wherein the condenser is designed to cool the exhaust gas with cabin air and wherein the outlet is designed to discharge the cabin air into the environment of the aircraft when the cabin air has flowed through the condenser. 
         [0027]    The condenser may therefore be connected to a fuel cell device in the aircraft. It is also connected to the cabin air outflow valve of the aircraft such that cabin air can be drawn through the condenser. Pumps or extra cooling devices or coolant may not be required. 
         [0028]    According to an embodiment of the water generation system, an aircraft with a water generation system as described above is provided. Therefore, water may be generated on board the aircraft without a plurality of cooling circuits being required for this. After suitable processing, the condensed water can be used as drinking water during the flight as drinking water which enables, for example, the fill level of the water tank to be reduced on take-off. 
         [0029]    Also provided is a method for the generation of water on board an aircraft in which exhaust gas from a fuel cell device is fed into a condenser. Cabin air is also fed into the condenser and the exhaust gas is cooled by the cabin air so that it condenses. The cabin air is subsequently discharged into the environment of the aircraft. 
         [0030]    Further examples of the water generation system may be found in the subclaims. 
         [0031]    The following describes preferred examples of the water generation system with reference to the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0032]      FIG. 1  shows a schematic representation of indirect cooling by means of coolant. 
           [0033]      FIG. 2  shows a schematic representation of an example of a water generation system on board an aircraft. 
           [0034]      FIG. 3  shows a schematic representation of a condenser for the condensation of water vapor from fuel cell exhaust gas by means of direct cooling by cabin air according to an example of a water generation system. 
           [0035]      FIG. 4  shows a schematic structure of a tubular heat exchanger according to an example of a water generation system. 
           [0036]      FIG. 5  shows a schematic representation of the air guidance through a water generation system according to an example of a water generation system. 
       
    
    
       [0037]    The representations in the figures are schematic and not to scale. In the following description of the figures, the same reference numbers are used for the same or similar elements. 
       DETAILED DESCRIPTION 
       [0038]    The examples described and drawings rendered are illustrative and are not to be read as limiting the scope of the invention as it is defined by the appended claims. 
         [0039]      FIG. 1  shows a concept for indirect cooling by means of coolant. Hereby, a primary heat exchanger  101  is provided through which the fuel cell exhaust gas passes. The fuel cell exhaust gas enters the primary heat exchanger  101  through line  202  and leaves it through the outlet line  203 . 
         [0040]    Also provided is a secondary heat exchanger  303  comprising an inlet line for external air  301  and an outlet line for external air  302 . At cruising altitude, the external air may have an effective temperature of approximately −20° C. or lower. 
         [0041]    Provided between the primary heat exchangers  101  and the secondary heat exchangers  303  is a coolant circuit  304 ,  305  in order to ensure a heat connection between the two heat exchangers  101 ,  303  and on the other hand to prevent the icing of the primary heat exchanger  101 . The cooling of the primary heat exchanger  101  results in the formation of water or condensate which can be discharged via the line  204 . 
         [0042]    The air-conditioning and ventilation technology already provided in civilian aircraft may also be used for the condensation of water vapor from fuel cell exhaust gases in order to simply and efficiently to achieve on-board water generation from fuel cell exhaust gases. For this, the heat exchanger/condenser can be arranged in a suitable manner in front of the cabin air outflow valve  102  so that, before being discharged outside, the air flows through the heat exchanger. 
         [0043]      FIG. 2  shows an example of a water generation system for the direct cooling of water vapor from fuel cell exhaust gas by means of outflowing cabin air. The water generation system, in this example, comprises a condenser  101 , a fuel cell system  108  and at least one outlet  102 . 
         [0044]    A part of the air from the cabin  105  enters the lower region  106  of the aircraft fuselage. The region  106  is, for example, the cargo hold. However, the water generation system can also be arranged wholly or partially in the cabin. The lower region  106  and the cabin  105  are separated from each other by the floor  107 . 
         [0045]    In this example, the temperature of the cabin air may be approximately 20° C. with an absolute cabin pressure of approximately 750 millibar (abs). The air mass flow may be approximately 1.4 kg s −1  per cabin air outflow valve  102 . 
         [0046]    The fuel cell  108  and condenser  101  are connected to each other by an exhaust gas line  109 . A water store  110  for liquid water which is connected to the condenser, may be provided. 
         [0047]    The cabin air outflow valve  102  can comprise a valve flap  111  which is electronically controllable so that the cabin air through-flow rate can be adjusted as desired. 
         [0048]    After passing through the condenser  101 , the cabin air  104  is discharged from the aircraft fuselage  103 . 
         [0049]    Therefore, heat may be discharged from the water-vapor-containing fuel cell exhaust gas by means of heat exchange into the outflowing cabin air which results in the condensation of water vapor in the fuel cell exhaust gas. After suitable processing, the condensed water may be used as drinking water during the flight which enables, for example, the fill level of the water tank to be reduced on take-off. 
         [0050]      FIG. 3  shows a schematic representation of a condenser for the condensation of water vapor from fuel cell exhaust gas by means of direct cooling by cabin air according to an example of a water generation system. The cooling cabin air  201  enters the condenser  101  and, when the fuel cell exhaust air has been cooled, leaves this through the outlet  104  via the cabin air outflow valve. The fuel cell exhaust gas  202  also enters the condenser  101 , is cooled as appropriate and leaves the condenser  101  through the outlet  203 . The condensed water is discharged via the line  204 . 
         [0051]    The following describes an example of an embodiment for condensation by means of cabin air with reference to an exhaust gas mass flow from an 100 kW PEMFC (proton exchange membrane fuel cell). Input values and calculation and design data are shown in the following tables and diagrams. 
         [0000]    Table 1 shows a definition of a PEMFC exhaust gas: 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
               
             
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Composition of the air fed to the condenser for 
               
               
                   
                 lambda 2 (air ratio representing the oxygen content): 
               
             
          
           
               
                   
                 Value 
                 Unit 
                 Value 
                 Unit 
                 Value 
                 Unit 
               
               
                   
                   
               
             
          
           
               
                 Overall 
                 100 
                 Mol % 
                 100 
                 Mass % 
                 0.17276 
                 kg s −1   
               
               
                 Dry air 
                 85.0641 
                 Mol % 
                 90.1192 
                 Mass % 
                 0.15569 
                 kg s −1   
               
               
                 H 2 O 
                 14.9359 
                 Mol % 
                 9.8808 
                 Mass % 
                 0.01707 
                 kg s −1   
               
             
          
           
               
                 Additional parameters for the precise layout and design: 
               
             
          
           
               
                 Operating pressure of the exhaust gas fed 
                 1.0 bar 
               
               
                 to the condenser (abs): 
               
               
                 Operating temperature of the exhaust gas 
                 54.1° C. (=dew point + 0.1) 
               
               
                 fed to the condenser (abs): 
               
               
                 Desired condensation efficiency: 
                 50% (of the absolute water 
               
               
                   
                 component) 
               
               
                   
               
             
          
         
       
     
         [0052]    Table 2 shows detailed layout and design parameters for a PEMFC tubular heat exchanger, cooled with cabin air (50% condensation efficiency). The condensation takes place in the tubes where the exhaust gas flow takes place. The outsides of tubes are cooled by cabin air: 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Description 
                 Type/Value 
                 Dimension 
               
               
                   
               
             
             
               
                 Tube arrangement 
                 Staggered 
                 — 
               
               
                   
                 arrangement 
                   
               
               
                 Type of flow 
                 Cross- 
                 — 
               
               
                   
                 countercurrent 
               
               
                 Type of lines 
                 Smooth tubes 
                 — 
               
               
                 Outer diameter of the tubes 
                 12.5 
                 mm 
               
               
                 Wall thickness of the tubes 
                 0.5 
                 mm 
               
               
                 Longitudinal division of the tubes 
                 18.00 
                 mm 
               
               
                 Transverse division of the tubes 
                 18.00 
                 mm 
               
               
                 Length of the heat exchanger tubes 
                 450.00 
                 mm 
               
               
                 Number of rows through which the 
                 3 
                 — 
               
               
                 flow passes 
               
               
                 Number of tubes in the direction 
                 39 
                 — 
               
               
                 of flow 
               
               
                 Number of tubes per plane 
                 23 
                 — 
               
               
                 (transversal) 
               
               
                 Wall thickness of the condenser 
                 0.3 
                 mm 
               
               
                 housing 
               
               
                   
               
             
          
         
       
     
         [0053]      FIG. 4  shows a schematic detailed view of a tubular heat exchanger  400 , such as may be used, for example, for a PEMFC fuel cell arrangement. The cooling is performed by cabin air with a condensation efficiency of, for example, 50%. 
         [0054]    The exhaust gas enters at the upper left side  403  of the condenser  400  through inlets  402  with a variable design. Hereby, the inlets  402  may be designed with respect to their number, diameter, cross section in such a way that the exhaust gas is distributed as uniformly as possible when flowing into the condenser  400 . The water generation system has a “triple flow” design, that is in each case, three tubes or tube planes extend in the direction of flow in order to achieve a sufficiently low flow rate of the exhaust gas. The exhaust gas is a low density medium so that lower flow rates have a positive impact on the condensation process (higher dwell time). 
         [0055]    At each end of a row of tubes, the exhaust gas (including the condensate) flows into a separation volume (eg  404 ,  405 ,  406 ), in that the exhaust gas reverse its direction of flow and simultaneously condensate (water) can be separated onto the walls. The separated water can flow downward through the perforated partitions (eg  408 ,  409 ). The water which accumulates in the separation volumes  404 ,  405 ,  406  seals the perforated partitions  408 ,  409  toward the bottom so that the exhaust gases are unable to flow through the partitions  408 ,  409  but instead have to enter the continuing tubes. 
         [0056]    The gravitational force and the pressure difference between the separation volumes support the water mass flow downward. 
         [0057]    When the exhaust gas has crossed the cabin air cooling flow 23 times, the exhaust gas leaves the condenser  400  at the bottom right end  407  through corresponding outlet openings  410 . The water may be removed at two independent outflow openings  411 ,  412  on the floor. 
         [0058]    The condenser may also be designed as a plate heat exchanger. Lower cabin air temperatures increase the condensation efficiency or enable a reduction in the size of the condenser with the same condensation efficiency. 
         [0059]    23 tubes are arranged, for example, in each plane (symbolised by the arrow  413 ). The number of tube planes arranged one on top of the other (symbolised by the arrow  414 ) is, for example, 39, which results in 13 deflections (separation volumes) of the cabin air cooling flow. The length  415  of an individual heat exchanger tube is, for example, 450 mm. 
         [0060]    Table 3 shows relevant operating data for the PEMFC tubular heat exchanger, cooled with cabin air (50% condensation efficiency): 
         [0000]    
       
         
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Parameters 
                 Symbol 
                 Value 
                 Unit 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 General parameters: 
               
             
          
           
               
                 Cooling efficiency 
                 Q 
                 22.27 
                 kW 
               
               
                 Overall (average) heat 
                 k(eff, mean) 
                 70.54 
                 W m −2  K −1   
               
               
                 transfer coefficient 
               
               
                 Heat transfer coefficient 
                 α(o) 
                 223.58 
                 W m −2  K −1   
               
               
                 outside the tubes 
               
               
                 Heat transfer coefficient 
                 α(i) 
                 112.33 
                 W m −2  K −1   
               
               
                 inside the tubes 
               
               
                 Thermal conductivity of 
                 λ 
                 23 
                 W m −2  K −1   
               
               
                 the tube walls 
               
               
                 Heat exchanging surface 
                 A 
                 15.85 
                 m 2   
               
             
          
           
               
                 Exhaust gas parameters (=inside the tubes): 
               
             
          
           
               
                 Inlet temperature 
                 T(in) 
                 54.1 
                 ° C. 
               
               
                 Inlet pressure (abs) 
                 ρ(in) 
                 1.0 
                 bar 
               
               
                 Total inlet gas mass 
                 m(in) 
                 0.17276 
                 kg s −1   
               
               
                 flow 
               
               
                 Absolute inlet water 
                 m(in, H 2 O) 
                 0.01707 
                 kg s −1   
               
               
                 mass flow 
               
               
                 Inlet dew point 
                 DP 
                 54.0 
                 ° C. 
               
               
                 Outlet temperature 
                 T(out) 
                 40.4 
                 ° C. 
               
               
                 (gas and water) 
               
               
                 Outlet mass flow of 
                 m(out, g) 
                 0.16396 
                 kg s −1   
               
               
                 the gas 
               
               
                 Outlet mass flow of 
                 m(out, H 2 O) 
                 0.0088 
                 kg s −1   
               
               
                 the condensed water 
               
               
                 Absolute condensation 
                 E(cond) 
                 51.6 
                 % 
               
               
                 efficiency 
               
               
                 Pressure drop 
                 Δp(gas) 
                 58.4 
                 mbar 
               
               
                 Mean gas velocity 
                 ν(gas) 
                 24.17 
                 m s −1   
               
             
          
           
               
                 Cabin air parameters (absolute cabin pressure: 750 mbar) 
               
             
          
           
               
                 Inlet temperature 
                 T(in) 
                 20.0 
                 ° C. 
               
               
                 Outlet temperature 
                 T(out) 
                 35.8 
                 ° C. 
               
               
                 Mass flow 
                 m(air) 
                 1.4 
                 kg s −1   
               
               
                 Pressure drop 
                 Δp(air) 
                 52.1 
                 mbar 
               
               
                 Air velocity 
                 ν(air) 
                 29.37 
                 m s −1   
               
               
                   
               
             
          
         
       
     
         [0061]    Table 4 shows masses and volumes for the PEMFC tubular heat exchanger cooled with cabin air (50% condensation efficiency): 
         [0000]    
       
         
               
             
               
               
             
               
               
             
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 General data 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Total volume of the condenser 
                 Approximately 176.1 L (=792 × 
               
               
                   
                 530 × 419.5 mm 3 ) 
               
               
                 Total number of heat 
                 897 (=39 × 23) 
               
               
                 exchanger tubes 
               
               
                 Total length of the 
                 403.65 m (=897 × 450 mm) 
               
               
                 heat exchanger tubes 
               
               
                 Internal total volume 
                 Approximately 41.92 L (403650 × 
               
               
                 of the heat exchanger tubes 
                 103.86 mm 3 ) 
               
               
                 Total volume of the metal 
                 Approximately 7.61 L (=403650 × 
               
               
                 of the heat exchanger tubes 
                 18.85 mm 3 ) 
               
               
                 Total volume of the metal 
                 0.37 L 
               
               
                 of the four plates arranged 
                 (=[450 + 450 + 419.5 + 
               
               
                 directly around the condenser 
                 419.5] × 702 × 0.3 mm 3 ) 
               
               
                 Total volume of the metal 
                 Approximately 0.3 L 
               
               
                 of the additional housing 
                 (=2 × [792 × 419.5 × 0.3 mm 3 ] + 
               
               
                 (outer wall, 
                 13 × [419.5 × 40 × 0.3 mm 3 ] + 2 × 
               
               
                 water-separator, frame) 
                 [90 × 40 + 90 × 40 + 90 × 419.5 + 
               
               
                   
                 40 × 419.5] × 0.3 mm 3 ] 
               
               
                   
               
             
          
           
               
                   
                 Masses 
               
             
          
           
               
                   
                 High-grade steel 
                 Aluminium 
               
               
                   
                 (density: 7850 kg m −3)   
                 (density: 2700 kg m −3 ) 
               
               
                   
               
               
                 Overall mass of the 
                 Approximately 59.8 kg 
                 Approximately 20.6 kg 
               
               
                 metal of the heat 
               
               
                 exchanger tubes 
               
               
                 Overall mass of the 
                 Approximately 2.9 kg 
                 Approximately 1.0 kg 
               
               
                 metal of the 
               
               
                 metal plates, 
               
               
                 directly abutting 
               
               
                 the condenser 
               
               
                 Overall mass of the 
                 Approximately 2.4 kg 
                 Approximately 0.8 kg 
               
               
                 metal of the housing 
                   
                   
               
               
                 Overall mass of 
                 Approximately 65.1 kg 
                 Approximately 22.4 kg 
               
               
                 the condenser 
               
               
                   
               
             
          
         
       
     
         [0062]    In the case of titanium (density: 4510 kg m 3 ), the condenser weights approximately 37.3 kg. In the case of zirconium (density: 6500 kg m 3 ), the condenser weights approximately 53.8 kg. 
         [0063]    Obviously, larger or smaller condensers may be used. 
         [0064]      FIG. 5  shows a schematic representation of the air guidance through an example of a water generation system. For integration in the aircraft, a countercurrent may occur between the cabin air flow and fuel cell exhaust gas and nevertheless, both the water and the cabin air are able to flow effectively “downward.” A countercurrent of this kind may increase the effectiveness of the heat exchanger. This may be achieved by a special air conduction  505  which is shown in  FIG. 5 . While the exhaust gas from “top”  501  to “bottom”  502  (as symbolised by the arrow  509 ) and hence the condensate removal  503 ,  504  is encouraged, the cabin air  508  initially flows through the condenser “upward”, which enables the countercurrent in the condenser. The cabin air is then guided back downward by a suitable device  505  (see arrow  507 ). There, it is able to flow outside through the cabin air outflow valve  102 . In addition to this, the rest of the exhaust gas can also be guided outside via the cabin air outflow valve  102  (see arrow  502 ). 
         [0065]      FIG. 6  shows a flow diagram of a method according to an example of a water generation system. In Step  1 , exhaust gas from a fuel cell device is introduced into the condenser. In addition to this, cabin air in introduced into the condenser. In Step  2 , the exhaust gas is cooled by the cabin air. In Step  4 , water is condensed from the exhaust gas and can be fed into the aircraft&#39;s water circuit. In Step  5 , the cabin air is discharged together with the exhaust gas into the environment of the aircraft when the cabin air and the exhaust gas have flowed through the condenser. 
         [0066]    In addition, reference is made to the fact that “comprising” does not exclude any other elements or steps and “one” does not exclude a plurality. Furthermore, reference is made to the fact that features or steps which are described with reference to one of the above examples of embodiments can also be used in combination with other features or steps of the other examples of embodiments described above. Reference numbers in the claims should not be seen as a restriction.