Abstract:
To save weight and to enhance safety, a hydrogen tank for supplying an engine with hydrogen is attached externally to an aircraft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims priority to PCT application No. PCT/EP2011/003796 filed Jul. 28, 2011, which claims the benefit of the filing date of European Patent Application No. 10171070.5 filed Jul. 28, 2010, the disclosures of which applications are hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to the field of aircrafts. In particular, the invention relates to a hydrogen tank for an aircraft, an aircraft, a hydrogen supply system, a method for supplying hydrogen to an aircraft and a use of a hydrogen tank with an aircraft. 
       BACKGROUND 
       [0003]    Most aircrafts use gas turbine engines. A gas turbine engine may include a core having a compressor fixedly joined to a turbine by a core rotor extending axially there between. 
         [0004]    At least one combustor chamber, for example an annular combustor or a plurality of combustor chambers distributed around the core, may be disposed between the compressor and the turbine and may include fuel injectors. As a rule, conventional gas turbine engines are supplied with hydrocarbon fuel like kerosene. 
         [0005]    Generally, gas turbines may emit undesirable compounds such as nitrous oxide components (NOx) and carbon-containing compounds. It may be desirable to decrease various emissions as much as possible so that selected compounds may not enter the atmosphere. In particular, it may be desirable to reduce NOx emissions to a substantially low level. 
         [0006]    Injecting hydrogen into a combustion chamber of an engine of an aircraft helps to reduce the exhaust gas emissions of the combustion chamber. Besides reducing emissions, for example CO2, NOx, CO, HC, the hydrogen combustion enhances the thrust of the engine. This allows for reducing the size of the combustion chamber and an optimization of the design of the combustion chamber and the engine to enhance cruise conditions. 
         [0007]    For example, hydrogen may be supplied from an on-board fuel cell which may be supplied with hydrocarbon fuel, for example kerosene, that is also used for supplying the engine of the aircraft. Further, the hydrogen from the fuel cell may be buffered in an on-board hydrogen tank. 
         [0008]    However, this design may increase the weight of the aircraft, since the on-board fuel cell and the on-board hydrogen tank as well as the tubing interconnecting the hydrogen tank and the engine contribute additional weight to the aircraft. 
         [0009]    For example, the fuel cell and the hydrogen tank may be situated in the fuselage of the aircraft and may be interconnected with the engine by the tubing that transports the hydrogen to the engine of the aircraft that may be situated under the wing or near the aft of the aircraft. 
         [0010]    This may cause safety issues in view of hydrogen generated and stored within the aircraft. For example, authorities may not accept highly explosive hydrogen on-board of a passenger aircraft. 
         [0011]    The NACA research memorandum “Liquid hydrogen as a jet fuel for high-altitude aircraft” from Silverstein &amp; Hall relates to the use of hydrogen as jet fuel for military aircrafts. It mentions that hydrogen may be stored in drop tanks. 
       SUMMARY 
       [0012]    It is an object of the invention to provide an aircraft that is more ecological friendly than conventional aircrafts without increasing the weight and the safety of the aircraft. 
         [0013]    This objective is achieved by the subject-matter of the in dependent claims. Further exemplary embodiments are evident from the dependent claims. 
         [0014]    A first aspect of the invention relates to a hydrogen tank for an aircraft. 
         [0015]    According to an embodiment of the invention, the hydrogen tank is an external hydrogen tank. In other words, the hydrogen tank is adapted to be attached to an outside of the aircraft, for example it may be outside of the hull of the aircraft. 
         [0016]    In contrast to an external hydrogen tank, an internal hydrogen tank is situated within the aircraft. For example, an internal hydrogen tank may be attached to the fuselage of the aircraft inside of the aircraft and an external hydrogen tank may be attached to the fuselage of the aircraft outside of the aircraft. 
         [0017]    Further, an external hydrogen tank may be adapted to be ex posed to the environment of the aircraft. For example, the hydrogen tank may be designed not to corrode outside of the aircraft, if it is exposed to mist and rain. Further, the external hydrogen tank may be streamlined and may have low resistance against the flow of air, for example, when the aircraft is cruising. For example, the hydrogen tank may be a robust metallic tank. 
         [0018]    Further, the external hydrogen tank may be mechanical attached to the fuselage of the aircraft. In particular, the hydrogen tank may be adapted to be attached to a wing or a tail of the aircraft, for example the wing struts or the aft fuselage. 
         [0019]    For example, the hydrogen tank may be connected to the air craft very close to one of the engines. In this way, the length of the tubing or fuel lines between the tank and the engine may be very short. 
         [0020]    According to an embodiment of the invention, the hydrogen tank is adapted for providing hydrogen to an engine of the aircraft. For example, the hydrogen tank may have an opening that may be connected with a tubing of the aircraft that is connected with an engine of the aircraft. 
         [0021]    In other words, to save weight und to enhance safety, the hydrogen tank for supplying an engine with hydrogen is attached externally to the aircraft. 
         [0022]    According to an embodiment of the invention, the hydrogen tank comprises a tank connection element for releasably coupling the hydrogen tank to the aircraft. On the one hand, the hydrogen tank may be adapted to be mechanically connected to the outside of the aircraft. On the other hand, the hydrogen tank may be adapted to be connected with a tubing or with pipes or fuel lines of the aircraft for supplying hydrogen to an engine of the aircraft. 
         [0023]    It has to be understood, that a releaseable coupled hydrogen tank may be easily decoupled from the aircraft and coupled with the aircraft. For example, the attachment system of the hydrogen tank to the aircraft may be similar to that of external tanks of fighter aircrafts. In this way, the hydrogen tank may be connected and disconnected from the aircraft without opening or closing screws. A tank connection element may be adapted to be engaged with an aircraft connection element. For example, the tank connection element and the aircraft connection element form together a plug mechanism or a lock mechanism that easily may be opened or closed, for example by an actuator that, for example, may be activated by the pilot. 
         [0024]    According to an embodiment of the invention, the hydrogen tank is adapted to be dropped from the aircraft, in particular, when the aircraft is flying. For example, the hydrogen tank may comprise a security valve that closes the opening of the hydrogen tank, when the hydrogen tank is disconnected from the aircraft. This may assure that no hydrogen left in the hydrogen tank may leave the hydrogen tank in the vicinity of an engine of the aircraft. 
         [0025]    The hydrogen tank may be adapted for being reused after drop-off. 
         [0026]    Further, the hydrogen tank may comprise an outer hull that is adapted for receiving impact forces that are generated, when the hydrogen tank drops to the ground, for example when the hydrogen tank is dropped from the aircraft from an altitude up to 50 m, up to 100 m or up to 150 m. 
         [0027]    Alternatively or additionally, the hydrogen tank may comprise a parachute that is adapted to open after the release of the tank from the aircraft. Due to the parachute, the hydrogen tank may float to the ground without big impact forces. 
         [0028]    A further aspect of the invention relates to an aircraft. 
         [0029]    According to an embodiment of the invention, the aircraft comprises an aircraft connection element for attaching a hydrogen tank as described in the above and the following to the aircraft. For example, the aircraft connection element may be situated on an outer surface of the aircraft and/or may be connected with the fuselage of the aircraft for mechanically supporting the hydrogen tank. The aircraft connection element may be adapted to engage a tank connection element of the hydrogen tank. 
         [0030]    For example, there may be differently designed hydrogen tanks and aircrafts each of which comprise the same tank connection element and aircraft connection element, respectively. In this way, different hydrogen tanks may be used for different aircrafts, for example dependent on the situation. For example, when the aircraft is starting from an airport with a relative short runway, much hydrogen may be injected into the engine of the aircraft and a big hydrogen tank that is adapted to store a big amount of hydrogen may be used. Further, a hydrogen tank used for a first aircraft may be used with a second aircraft after being refilled. 
         [0031]    According to an embodiment of the invention, the aircraft connection element comprises a mechanical connection for supporting the hydrogen tank and/or a fuel connection for connecting the hydrogen tank to a tubing or a hydrogen line of the aircraft connected with the engine of the aircraft. This may have the advantage, that with only one working step, both connections may be connected. 
         [0032]    According to an embodiment of the invention, the aircraft connection element comprises an actuator element for disconnecting the hydrogen tank from the aircraft. The actuator element may be controlled by a controller of the aircraft. For example, the controller may be activated by the pilot of the aircraft, for example with a switch in the cockpit. Alternatively or additionally, the controller may be activated from an external source like the tower of an airport. For example, when the pilot or the personnel of the tower sees that the aircraft has reached the altitude at which the hydrogen tank should be dropped, the pilot or the personnel may activate the controller and the hydrogen tank is disconnected from the aircraft and will be dropped to the ground. It may also be possible that the dropping of the hydrogen tank is triggered automatically, when air airplane reaches a predefined altitude or when the airplane passes over a dropping area. 
         [0033]    For example, such an actuator element may be a lock mechanism with a bar that is moved by a motor or by the magnetic forces of a solenoid. 
         [0034]    According to an embodiment of the invention, the aircraft comprises an engine which is adapted to be supplied with hydrogen from the hydrogen tank. In a normal cruise mode, the engine may be adapted to be supplied with hydrocarbon fuel like kerosene. For enhancing the power of the engine and for reducing unwanted substances in the exhaust, which may cause pollution of the environment, the engine may be additionally supplied with hydrogen from the hydrogen tank. To this end, the engine may comprise hydrogen injectors that are adapted to inject hydrogen into the combustion chamber of the engine. 
         [0035]    According to an embodiment of the invention, the aircraft is a passenger aircraft. In particular for passenger aircrafts the safety is enhanced for the passengers, if during the normal cruise and the landing no large amounts of hydrogen are on board of the aircraft. 
         [0036]    According to an embodiment of the invention, the aircraft may be an airplane or a helicopter. In particular big airplanes, for example passenger airplanes for more than 50 passengers, and helicopters normally have an engine on the basis of a gas turbine. 
         [0037]    A further aspect of the invention is a hydrogen supply system. 
         [0038]    According to an embodiment of the invention, the hydrogen supply system comprises a plurality of hydrogen tanks as described in the above and the following, at least one aircraft as described in the above and in the following and a refilling station for refilling hydrogen tanks. 
         [0039]    For example, the hydrogen tanks may be re-usable or disposable tanks. In the filling or refilling station, the hydrogen tanks are filled with liquid hydrogen and transported to the aircraft. after that, at least one filled hydrogen tank is fixed or attached to the aircraft, for example to the wing or the aft fuselage. During taxiing, i.e. the movement of the aircraft on the ground, and in the initial takeoff phase, H2 or hydrogen is injected into the engine of the aircraft, for example into the combustion chamber of the engine. In particular during taxiing and takeoff, the hydrogen injection into the engine will cause a hydrogen combustion inside the engine which enhances the trust of the engine at taxiing and takeoff. Additionally, when hydrogen is injected in a low power mode of the aircraft, for example during taxiing at an airport, the unwanted emissions of the engines may be reduced. 
         [0040]    When the aircraft reaches the end of the runway or after the start of the aircraft the hydrogen tanks may be empty or may only contain only a small amount of hydrogen and may be dropped to the ground, for example they may be released by the pilot or an autonomous external trigger. 
         [0041]    According to an embodiment of the invention, the hydrogen supply system comprises a dropping area for receiving one of the hydrogen tanks dropped from the aircraft, for example, at the end of the runway there may be a specific area with a soft underground, for example a sandpit. It may be possible, that the at least one hydrogen tank or all of the hydrogen tanks of an aircraft are released from the air-craft, such that they all fall into the dropping area of the hydrogen supply system. After being dropped, the hydrogen tanks may be collected from the dropping area to be disposed or transported to the refilling station to be refilled. 
         [0042]    A further aspect of the invention relates to a method for supplying hydrogen to an aircraft. 
         [0043]    According to an embodiment of the invention, the method comprises the step of: attaching a hydrogen tank to an outside of the aircraft. During the attaching, the hydrogen tank may be mechanically connected to the aircraft and/or may be connected with an engine of the aircraft, such that hydrogen from the hydrogen tank may be injected into the engine. 
         [0044]    According to an embodiment of the invention, the method comprises the step of: injecting hydrogen into an engine of the aircraft. In particular, the injection may take place during taxiing of the aircraft and during takeoff of the aircraft, in particular during the initial phase of takeoff. 
         [0045]    According to an embodiment of the invention, the method comprises the step of: releasing the hydrogen tank from the aircraft. For example, the releasing may be triggered internally with respect to the aircraft, for example by a pilot of the aircraft or by a controller of the aircraft. Alternatively or additionally, the releasing may also be triggered externally with respect to the aircraft, for example by a radio signal, e.g. by a signal from the tower of an airport. 
         [0046]    According to an embodiment of the invention, the method comprises the step of: collecting the hydrogen tank in a dropping area. For example, the hydrogen tank may be released at the end of the runway the aircraft is lifting off, such that the hydrogen tank drops or falls into the dropping area. 
         [0047]    According to an embodiment of the invention, the method comprises the step of: refilling the hydrogen tank in a refilling station. For example, the hydrogen tank may be are-usable tank. On the other end, it is possible that the hydrogen tank is disposable and is discarded after the collection in the dropping area. 
         [0048]    According to an embodiment of the invention, the method comprises the step of: attaching the hydrogen tank to a further aircraft. After the refilling of the hydrogen tank, the hydrogen tank may be transported to a further aircraft, that may not be the same aircraft the hydrogen tank has been dropped from and may be attached to this further aircraft. 
         [0049]    A further aspect of the invention relates to a use of an external hydrogen tank with an aircraft. 
         [0050]    To summarize, the advantages of an external hydrogen tank of an aircraft may be: Firstly, H2 or hydrogen may be injected when it is most efficient, for example during taxiing and takeoff. Secondly, no hydrogen may be on-board during flight, i.e. there may be lower safety concerns of authorities. Thirdly, the hydrogen tanks may be attached very close to the engines of the aircraft, for example jet engines. This may result in lower additional weight caused by the hydrogen supply facilities of the aircraft. 
         [0051]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0052]    Below, embodiments of the present invention are described in more detail with reference to the attached drawings. 
           [0053]      FIG. 1  shows a schematic cross-sectional view of an aircraft according to an exemplary embodiment of the invention. 
           [0054]      FIG. 2  shows fuel supply facilities of an aircraft according to an exemplary embodiment of the invention. 
           [0055]      FIGS. 3   a ,  3   b  and  3   c  show views from different directions of an aircraft according to an exemplary embodiment of the invention. 
           [0056]      FIGS. 4   a  and  4   b  show different views of an aircraft according to an exemplary embodiment of the invention. 
           [0057]      FIG. 5  schematically shows a hydrogen supply system according to an exemplary embodiment of the invention and method steps for supplying hydrogen to an aircraft according to an exemplary embodiment of the invention. 
       
    
    
       [0058]    The reference symbols used in the drawings, and their meanings, are listed in summary form in the list of reference symbols. In principle, identical parts are provided with the same reference symbols in the figures. It should be noted that the figures are schematically and not to scale. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0059]      FIG. 1  shows a cross-sectional view of an aircraft  10 , which may be an passenger airplane  10 . The aircraft  10  comprises an engine  12  attached to the bottom side  14  of a wing  16  of the airplane  10 . An external hydrogen tank  18  is attached to the bottom side  14  of the wing  16  near the engine  12 . The hydrogen tank  18  is connected to the airplane  10  outside of to the engine  12  with respect to central body  20  of the aircraft  10 . I.e. the hydrogen tank  18  is more outside attached to the wing  16  as the engine  12 . 
         [0060]    Within the wing  16  there is an internal fuel tank  22  for supplying hydrocarbon fuel over a hydrocarbon fuel line  24  to the engine  12 . The hydrogen tank  18  is connected to a hydrogen fuel line  26  for supplying the engine  12  with hydrogen. The hydrocarbon fuel line  24  and the hydrogen fuel line  26  are situated within the wing  16 . 
         [0061]    Further, the aircraft  10  comprises an actuator  28  for disconnecting the hydrogen tank  18  from the wing  16  of the aircraft  10 . Actuator  28  also may be used for connecting the hydrogen tank  18  to the wing  16  of the aircraft  10 . 
         [0062]    The aircraft  10  further comprises a controller  30 , that may be situated in the cockpit of the aircraft  10 , which is connected over control lines  32  to the actuator  28 , to a valve  34  in the hydrocarbon fuel line  24  and to a valve  36  in the hydrogen fuel line  26 . 
         [0063]      FIG. 2  shows hydrocarbon fuel and hydrogen supply facilities of the aircraft  10  of  FIG. 1 . The engine  12  of the aircraft  10  comprises a combustion chamber  38  with hydrocarbon fuel injectors  40  and hydrogen injectors  42 . Inside the combustion chamber  38  either the hydrocarbon fuel or the hydrogen or a mixture thereof may be combusted. The hot gas generated by the combustion is supplied to a turbine  40  which generates the thrust of the engine  12 . 
         [0064]    The hydrocarbon fuel injectors  42  are connected with the hydrocarbon fuel line  24 . The controller  30  is adapted to control the opening of the valve  34  over the control line  32   a  to control the amount of hydrocarbon fuel injected into the combustion chamber  38 . Additionally, the controller  30  is adapted to control the opening of the valve  36  in the hydrogen fuel line  26  which is connected to the hydrogen injectors  44 , for controlling the amount of hydrogen injected into the combustion chamber  38 . By controlling the opening states of the valves  34 ,  36 , the controller  30  can set the mixing relationship of hydrocarbon fuel and hydrogen and the overall amount of this mixed fuel for the engine  12 . 
         [0065]    The external hydrogen tank  18  is connected to the bottom side  14  of the wing  16  via connection mechanism, that comprises a connection element  46  for connecting the interior of the tank  18  with the hydrogen line  26  and a lock mechanism  48  for mechanically coupling the hydrogen tank  18  to the aircraft  10 . The controller  30  is adapted to control the actuator  28  over the control line  32   c  to open and close the lock mechanism  48  for coupling or decoupling the hydrogen tank  18  from or to the aircraft  10 . It may also be possible, that the locking mechanism  48  is a mechanism that engages, when the hydrogen tank  18  is moved to its position under the aircraft wing  16  and that may be released by activating the actuator  28 . The connection element  46  comprises a plug and socket connection that may be simply interconnected by positioning the hydrogen tank  18  below the wing  16 . The connection element  46  may be automatically released, when the hydrogen tank  18  is disconnected from the aircraft and drops to the ground. 
         [0066]    The hydrogen tank  18  is designed in a streamlined form as indicated in  FIG. 2 , for example by round ends, and may further have a hull  50  that is adapted to withstand the impact forces, when the tank  18  is dropped to the ground. 
         [0067]      FIGS. 3   a ,  3   b  and  3   c  shows an aircraft  10  from the front, from the above and from the side. The aircraft  10  has two engines  12  and two hydrogen tanks  18  that are fixed to the aircraft closely to the engines  12 . 
         [0068]    In the embodiments shown in  FIGS. 1 ,  3   a ,  3   b  and  3   c , the hydrogen tanks  18  are fixed or attached to the underside of the wings  16  of the aircraft  10  in a position more distant from the central body  20  than the engines  12 . 
         [0069]      FIGS. 4   a  and  4   b  show an embodiment of an aircraft  10 ′ with engines  12  attached to the aft fuselage  52  or the tail  52  of the aircraft  10 ′. Hydrogen tanks  18  are attached to the aft fuselage  52  or tail  52  below the engines  12  but in the vicinity of the engines  12 , i.e. close to the engines  12 . 
         [0070]    In other words, the engines  12  and the hydrogen tanks  18  are connected to the rearward end of the central body  20  of the aircraft  10 ′. To get an impression of the dimensions of the aircraft  10 ′ in the form of a passenger airplane  10 ′, the airplane  10 ′ has a wing span a of 32.9 m, a rudder span b of 12.2 m, an overall length c of 45.1 m and a height d of 9.0 m. 
         [0071]      FIG. 5  shows a schematic view of a hydrogen supply system  80  that may be used in connection with an airport  82 . The hydrogen supply system  80  comprises a plurality of hydrogen tanks  18 , at least one aircraft  10  and a refilling station  84 . 
         [0072]    In a step S 1  hydrogen tanks  18  filled with hydrogen are fixed or attached to an aircraft  10 , for example during refueling of the aircraft  10  with conventional hydrocarbon fuel. 
         [0073]    In a step S 2 , the hydrogen is injected into the engines of the aircraft  10  during taxiing of the aircraft over the area of the airport  82 . 
         [0074]    In a step S 3 , further hydrogen is injected into the engines  12  of the aircraft  10  during the initial takeoff phase of the aircraft  10 . 
         [0075]    In a step S 4 , after the takeoff of the aircraft  10  the hydrogen tanks  18 , which then may be empty, are decoupled and released from the aircraft  10  and drop to the ground into a prepared area  86 , for example a sandpit  86 , that may be situated at the end of a runway of the airport  82 . 
         [0076]    In a step S 5  the aircraft  10  is driven with conventional propulsion, i.e. by the engines  12  supplied only with hydrocarbon fuel. 
         [0077]    In a step S 6 , the empty or nearly empty hydrogen tanks are collected from the sandpit  86  and are transported to the hydrogen refilling station  84 . Alternatively, if the hydrogen tanks  18  are not re-usable tanks, they also may be discarded. 
         [0078]    In a step S 7 , the hydrogen tanks  18  are refilled, transported to a further airplane  20  and attached to the further airplane  10 . 
         [0079]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or controller or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
           10 ,  10 ′ aircraft 
           12  engine 
           14  bottom side of wing 
           16  wing 
           18  hydrogen tank 
           20  central body 
           22  hydrocarbon fuel tank 
           24  hydrocarbon fuel line 
           26  hydrogen fuel line 
           28  actuator 
           30  controller 
           32  control lines 
           32   a ,  32   b ,  32   c  control line 
           34  valve 
           36  valve 
           38  combustion chamber 
           40  turbine 
           42  hydrocarbon fuel injectors 
           44  hydrogen injectors 
           46  connection element for hydrogen line 
           48  mechanical connection element 
           50  hull 
           52  aft fuselage, tail 
           80  hydrogen supply system 
           82  airport 
           84  refilling station 
           86  dropping area