Abstract:
A hydrogen generator and methods of generating hydrogen are described in which a hydride is hydrolysed in a reaction chamber in an exothermic reaction by the addition of liquid water to be vaporized in the reaction chamber, and/or the added water is controlled in relation to the hydrogen generated according to a value x defined by equations, which, for a divalent metal, reads: 
       MH 2 +xH 2 O→(2-x)MO+(x-1)M(OH) 2 +2H 2 ,
 
     wherein M symbolizing the divalent metal and x is preferred in the interval of 1.2 to 1.3. The equation as well as the values of x depend on the sort of hydride, i.e. whether a 1-, 2-, 3- or 4-valent hydride is applied. The hydrogen generator has a reaction chamber and an operably coupled water storage vessel acting also as condenser for recycled water.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/SG2014/000103, filed Mar. 4, 2014, which claims priority to Singapore Application No. 201302296-7, filed Mar. 25, 2013, both of which are hereby specifically incorporated by reference herein in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method of generating hydrogen from a hydride being hydrolysed in a reaction chamber in an exothermic reaction by the addition of water and to a hydrogen generator for performing this method comprising a reaction chamber, a water supply inlet into the reaction chamber and a hydrogen outlet from the reaction chamber. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is known that hydrogen can be generated by hydrolysis of a hydride reacting with water, the hydride being for example a metal hydride or a metal borohydride. 
         [0004]    Hydrogen generators can be useful for supplying fuel cells or hydrogen combustion engines with the necessary hydrogen. Generating the hydrogen by a chemical reaction as stated above enables relatively compact and safe generators if compared with hydrogen tanks under high pressure. It is necessary to have some control of the hydrogen generation dependent on the demand of the consuming device, respectively on the effective power demand of the load, by a system which itself should be simple and tightly arranged. 
         [0005]    From GB 970420, a system is known that produces hydrogen from an exothermic reaction of magnesium hydride with water, wherein the pressure of the hydrogen is sensed to control the operation of a pump, defining the water supply rate. The hydrogen powers a fuel cell which, as a by-product, produces waste water that, according to this prior art, is used to feed the hydrogen production in the reactor. 
         [0006]    From US 2008/0075987 A1, another hydrogen generator as stated above is known which also is controlled on demand by controlling the amount of steam injected into the reaction chamber. The amount of steam reacting with the magnesium hydride is proportional to the amount of hydrogen gas generated. For start-up, electric heaters in some sort of boiler are used for heating the water to vaporize before it enters the reaction chamber. After start-up the water is vaporized in the boiler by heat recovered from the exothermic reaction in the reaction chamber; the boiler could be provided as an integral component of the reaction chamber. 
         [0007]    In the prior art, there is the problem of controlling the water supply rate with reference to the different parameters and of system complexity. 
       SUMMARY 
       [0008]    It is an object of the invention to provide a method and an apparatus for generating hydrogen in terms of optimization of reaction controlling. 
         [0009]    To attain this object, according to a first aspect of the invention, the water, by its water supply rate, is supplied as liquid to be vaporized in the reaction chamber, i.e. after a starting phase the liquid water entering the reaction chamber is vaporized instantly by the heat originated by the reaction, and an external boiler is avoided. 
         [0010]    According to a second aspect of the invention, the hydride which can be the hydride of a 1-valent metal, e.g. lithium hydride, or a 2-valent metal, e.g. magnesium hydride, or a 3-valent metal, e.g. aluminum hydride, or a 4-valent metal, e.g. titanium (IV) hydride, a stoichiometric factor x in the following equations shall be controlled within the below mentioned intervals by adjusting the reaction conditions, in particular the water supply rate, in relation to the hydrogen generated, the value x being defined by the following four equations: 
         [0000]      MH+ x H 2 O→(1- x )M 2 O+(2 x -1)MOH+H 2,  
 
         [0000]      M symbolizing a 1-valent metal, x being in the interval of 0.51 to 0.9; 
         [0000]      MH 2   +x H 2 O→(2- x )MO+( x -1)M(OH) 2 +2H 2 ,
 
         [0000]      M symbolizing a divalent metal, x being in the interval of 1.02 to 1.8; 
         [0000]      MH 3   +x H 2 O→(1⅓ x )M 2 O 3 +(⅔ x -1)M(OH) 3 +3H 2 ,
 
         [0000]      M symbolizing a 3-valent metal, x being in the interval of 1.5 to 3; 
         [0000]      MH 4   +x H 2 O→(2½ x )MO 2 +(½ x -1)M(OH) 4 +4H 2 ,
 
         [0000]      M symbolizing a 4-valent metal, x being in the interval of 2.04 to 3.6, 
         [0000]      divalent metal M and particularly magnesium being preferred. 
         [0011]    The solid reaction products may be formed as two discrete substances or as one substance having the atomic ratios corresponding to the sum of the oxide and the hydroxide product. 
         [0012]    The stoichiometric factor x defines the water consumption for a given quantity of generated hydrogen. As can be seen from the equation, the higher the value x, the lower is the hydrogen yield per unit of supplied water, the hydrogen atoms disappearing in the compound M(OH) x , in the preferred embodiment M(OH) 2 . The factor x can be looked upon as averaged relative water consumption from the start of the reaction to any desired point in time until completion, or as an interval or a momentary parameter. In this document, if not mentioned differently, x shall be interpreted as average stoichiometric factor of the reaction from the start to the completion. Completion is herewith defined as the reaction status where 90% of the initial amount of hydride is converted. 
         [0013]    For the divalent metal hydride, a preferable range of x is in the interval of 1.1 to 1.45, and even more preferably in the interval of 1.2 to 1.3. Especially for applications with critical weight and/or volume of the hydrogen generator, e.g. portable devices or devices to be used in aerial vehicles, a low value of x causes at a given desired hydrogen capacity low weight and volume of the necessary water. 
         [0014]    However, as mentioned, x can as well have the meaning of a measurable momentary parameter x(t) which characterizes the quality of the reaction at any given time. x(t) can be straightforwardly calculated from measurements of momentary water supply flow and momentary hydrogen flow. The latter is in good accuracy proportional to the current of the fuel cell and thus is easy to measure. The resulting value and the history of former values indicate whether a correction of the reaction conditions is necessary or not. 
         [0015]    Similarly as in the prior art, a preferred hydride is magnesium hydride (MgH 2 ), preferably having the form of a granular material, e.g. a powder, with an average particle size of less than 50 μm and more preferably of not more than 10 μm and a granular packing density between 0.4 g/cm 3  and 0.7 g/cm 3 . Such granular material guarantees sufficient gas permeability for hydrogen and water vapor all through the reaction. The magnesium hydride and the magnesium by-products after the reaction do not expand significantly and remain in the granular form. Therefore the magnesium hydride may be mildly compressed in the reactor chamber in order to achieve larger hydrogen content per volume of the reaction chamber. For the value x being in the preferred range, a potential control mechanism is to set the hydrogen production flow rate to a predefined value not exceeding 9 normal liter (1013 mbar at 0° C.) per kg of still un-reacted MgH 2  contained in the reactor and more preferable being less than 3.5 normal liter per kg of MgH 2 . 
         [0016]    The control strategy and the set parameters of the reaction conditions in order to achieve the claimed stoichiometric factor x depend on the type of metal hydride, the reactor design and the balance of plant, the main variable parameter being the water supply rate. The water supply rate is controlled, e.g. through regulating a feeding pump, under consideration of the value x, and by influencing the temperature in the reaction chamber, i.e. if not otherwise mentioned in this document the average temperature all over in the reaction chamber, in dependency of the pressure of the hydrogen generated and/or the water vapour partial pressure in the reaction chamber. The value of x tends towards 1 if the water vapor partial pressure is low and the temperature is high; vice versa x approaches 2 for high water vapor partial pressure and low temperature. The water vapor partial pressure is a main factor which determines the reaction speed and thus the hydrogen output. A useful temperature range for the operation as average temperature in the reaction chamber is 80° C. to 400° C., preferably 220° C. to 350° C. 
         [0017]    If the total pressure, i.e. the pressure of hydrogen and water vapor at the hydrogen outlet of the reaction chamber, rises, the value of x approaches 2, which means that more water is necessary for the production of the hydrogen. Rising of the total pressure leads to a higher value of x. 
         [0018]    For starting of the reaction or at low hydrogen demand of the consuming device, favourably the reaction chamber is heated up to a starting temperature of 80° C. to 100° C., either electrically by means of resistance wires, or chemically by reacting phosphorous-pent-oxide deposited in the reaction chamber and water in a two-step exothermic reaction comprising a first step of dissolving the phosphorous-pent-oxide to form phosphoric acid and a second step of reacting the phosphoric acid with the metal hydride to form hydrogen and phosphates. 
         [0019]    In case of electric pre-heating the electric energy needed is by far less than the electric energy which can be generated by using the hydrogen e.g. in a fuel cell; thus several consequent start-stops can be supported during the process of one completion of the hydride. A chemical heating with phosphorous-pent-oxide allows one time start-up pre-heating only, but doesn&#39;t need electricity. 
         [0020]    For carrying out the above method, a hydrogen generator is appropriate, comprising the reaction chamber, a water supply inlet into the reaction chamber, and a hydrogen outlet from the reaction chamber, wherein in series with the hydrogen outlet within the reaction chamber a porous material permeable to hydrogen, but not permeable to the hydride is arranged. This porous material, e.g. carbon cloth, glasfibre fleece etc., supports the hydrogen collection and transfer, prevents blocking of outlets of the reaction chamber by the hydride and the escape of some hydride and furthermore provides for at least partial thermal insulation. Advantageously a further porous material in series with the water supply inlet is arranged within the reaction chamber, that is permeable to steam but not permeable to the hydride and preferably also not permeable to liquid. It prevents a blocking of inlets of the reaction chamber and, if impermeable to liquid, prohibits the contact of liquid water with the hydride. 
         [0021]    Particularly for hydrogen generators not having a temperature control, it is of advantage if the reaction chamber is surrounded by a non-linear thermal insulation, having a high thermal resistance at low temperature and a low thermal resistance at high temperature, such as vacuum insulation, permeable to thermal radiation. An insulation which causes highly non-linear heat losses means that for magnesium hydride, in the temperature range below 180° C. a good isolation property is given that avoids the necessity of additional heating of the reaction chamber, while in the range above 350° C. inferior insulation property is given, to avoid a self-decomposition of the magnesium hydride. 
         [0022]    For portable or aerospace applications, orientation insensitivity is important. This means that the system has to work independently of the direction of gravity relative to the system. The reaction chamber is to be made uncritical in this respect by the hydride being fixed in there; however if the hydrogen generator comprises a water storage vessel, orientation insensitivity is important. To solve this object, the water storage vessel having an inlet connected to the hydrogen outlet of the reaction chamber, a water outlet connected via a pump to the water supply inlet into the reaction chamber and a hydrogen outlet, wherein the water storage vessel has a volume lager than the amount of water to be stored, and the hydrogen outlet is connected to a hydrogen collecting body within the vessel, having a porous wall which is permeable to hydrogen and impermeable to water; and the water outlet is connected to a hose open at its far end and having a weight at its far end, the hose being as long as to reach the most distant area in the vessel. This guarantees that in all orientations of the system the hydrogen collecting body reaches a space free of water and the water outlet always reaches a space under water. The hydrogen collecting body can preferably consist of a tube of porous polytetrafluoroethylene, the tube being formed as a coil penetrating through the complete length of the vessel. The water storage vessel also acts as a condenser; together with hydrogen also residual steam leaves the reaction chamber and is collected in the water storage vessel. The steam condenses there and is added to the water already stored within the vessel. 
         [0023]    Also for light and portable applications that require a compact electrical power supply, it is of advantage to connect a fuel cell as consuming device to the hydrogen system. In this case water can as well be recycled from the cathode exhaust gas and/or even anode exhaust gas of this fuel cell. A double recycling system for water from the residual water in the hydrogen flow of the reaction chamber and from the product water of the fuel cell is thereby provided. 
         [0024]    Since the maximum favorable hydrogen generation rate depends on the remaining MgH 2  in the reactor, at the beginning of the reaction more hydrogen can be generated than towards the end. For supplying a load which is powered by the fuel cell system with sufficient power at any time, the use of a buffer storage is of advantage. This buffer can be a simple gas storage device which however adds substantial volume and weight to the system. More favorable is the use of an electrical buffer, i.e. a rechargeable battery, provided that the dimensioning of the electric power output of the fuel cell is higher than the maximum power demand of the load. The buffer balances the hydrogen flow rate of the hydrogen generator against the power demand of the load and thus the electric power output of the fuel cell. 
         [0025]    To achieve a lightweight and compact fuel cell system it is further of advantage to recycle the product water of the fuel cell in the water storage vessel to be applied to the reaction chamber. In the case of the hydride being a divalent metal hydride like magnesium hydride, and the value x is 2 (x=2), two mol of water in the hydrolysis reaction produce two mol of hydrogen which, if used in a fuel cell generate the two mol of water again. So water consumption and water generation of the hydrogen generator in combination with a fuel cell are theoretically balanced. However water cannot be recovered fully from the fuel cell because water vapor leaves the cell with the reactant flows, in particular the air flow, but also with the hydrogen flow, if a hydrogen purge mechanism is applied. Therefore in practice typically only 60%-70% of the water produced by the fuel cell can be recovered for the hydrogen generator. Thus, if water balanced operation and thus high system energy density is desired, according to the invention control of x to the preferred values is carried out. 
         [0026]    For easy handling of the hydrogen generator as well as safe and fast recharging, the reaction chamber or additionally the water storage vessel are designed as replaceable cartridge, recyclable or disposable. 
         [0027]    To ensure the operational capability of the hydrogen generator in a wide temperature range, a requirement e.g. for military aerospace applications, an anti-freeze is added to the water within the water storage vessel. The applied additive is selected as not to block the reactor and not to cause severe side reactions within the reactor, and furthermore is not to emit substances which could poison the fuel cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The foregoing and further objects, features and advantages of the present invention will become apparent from the following description of a preferred embodiment with reference to the accompanying schematic drawings. 
           [0029]      FIG. 1  shows a fuel cell system being supplied with hydrogen from a hydrogen generator according to the invention; 
           [0030]      FIG. 2  is a longitudinal section view of a first realisation of a cylindrical reaction chamber of the hydrogen generator; 
           [0031]      FIG. 3  is a longitudinal section view of a second realisation of a reaction chamber of the hydrogen generator; 
           [0032]      FIG. 4  is a longitudinal section view of a cylindrical water storage vessel of the hydrogen generator. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]      FIG. 1  shows a fuel cell system  1  comprising a fuel cell  2  and a hydrogen generator  3 . The fuel cell  2  is coupled to the hydrogen generator  3  via a pressure reduction valve  4  inserted into a hydrogen outlet  5  and via a control and electric output cable  6 . 
         [0034]    In the hydrogen generator  3  the main component is a reaction chamber  10 , comprising a water supply inlet  11  and a hydrogen outlet  13 . The water supply inlet is coupled via a water line  14  and a pump  15  to a water storage vessel  16 ; the water line  14  optionally also contains a water pre-heater  17  using the waste heat from the hydrogen generation reaction. The water storage vessel  16  also is connected via a hydrogen line  20  to the hydrogen outlet  13  of the reaction chamber  10 . To the hydrogen line  20 , a pressure sensor  21  and an excess pressure valve  22  are connected. 
         [0035]    The control and electric output cable  6  is connected to an electronic board  23 , carrying a control unit  24 , a battery  25 , a power output connection  26  to a load and optionally short-circuit switches and further equipment (not shown) usual to control the operation of the fuel cell  2 . The control unit  24  has a connection (not shown) to the pressure sensor  21  or to the pressure reduction valve  4 , a control cable  27  to the reaction chamber  10  and a further connection (not shown) to the pump  15 . At the reaction chamber  10 , the control cable  27  is connected to sensors, in particular to temperature sensors (not shown). 
         [0036]    To overcome the complex handling of refilling the reaction chamber  10  with the hydride, a replaceable cartridge  28 , comprising the reaction chamber  10  and the water storage vessel  16  is provided, having quick connectors (not shown) to all relevant media and electric contacts of the fuel cell system  1 . This cartridge  28  is after use professionally recycled or disposed. 
         [0037]    The exothermic reaction taking place in the reaction chamber  10  is defined by the following equation: 
         [0000]      H 2   +x H 2 O→(2- x )MO+( x -1)M(OH) 2 +2H 2  
 
         [0000]      which is a combination of the following two equations known from GB 970,420 and from US 2008/0075987 A1: 
         [0000]      H 2 +H 2 O→MO+2H 2  
 
         [0000]      MH 2 +2H 2 O→M(OH) 2 +2H 2  
 
         [0000]    wherein M symbolizes a divalent metal and the value x determines the relation between the latter two equations and can be described to be a water consumption value. Upon use of a divalent metal hydride, for optimal effectiveness the value x is preferred to be controlled to be from 1.2 to 1.3. 
         [0038]    For magnesium hydride MgH 2  as divalent metal hydride, liquid water from the water storage vessel  16  is injected via the water supply inlet  11  into the reaction chamber  10 , the water supply rate being determined by the pump  15 . Hydrogen resulting from the exothermic reaction, and residual water vapour not consumed by the reaction leave the reaction chamber  10  via the hydrogen outlet  13  and are forwarded via the hydrogen line  20  to the water storage vessel  16 , which, for the water vapour, acts also as a condenser and thus recycles the water. An additional condenser (not shown) somewhere along line  20  can also be used to condense and recycle residual water. The hydrogen entering the storage vessel  16  leaves this vessel  16  via the hydrogen outlet  5  and is fed to the fuel cell  2 . 
         [0039]    At fast varying load of fuel cell  2  or toward the end of the hydrogen production, e.g. limited by the amount of fuel, problems can arise in adjusting the required hydrogen flow or hydrogen pressure for the fuel cell  2 . This problem can be solved by using a gas buffer volume, as known in the art. This however increases size and weight of the system dramatically. Therefore, for compact systems, an electric buffer, i.e. the battery  25 , can be used. 
         [0040]    To achieve the value x being in the preferred range of 1.2 to 1.3, the control functions of the fuel cell system  1  is shown hereafter. 
         [0041]    Before starting water supply, the reactor shall be heated to at least 80C, preferred are 100° C. At regular operation, the reactor temperature shall be controlled to be in the range 220° C. to 350° C. A temperature close to 400° C. and above causes substantial self-decomposition of MgH 2  which results in uncontrollable hydrogen generation even if the water supply is stopped. This has to be avoided. Too low a temperature, however, favors the formation of Mg(OH) 2  and causes a large stoichiometric factor x, and thus, large relative water consumption and consequently, considering the larger water store to be provided, finally a high system weight. 
         [0042]    Although the reaction can be quite fast, if the water vapour partial pressure and thus the total pressure are high, it is not favorable for low relative water consumption to approach the limits of reaction rate. The larger the water vapour partial pressure is at given temperature, the larger the value x will be, and vice versa. Since the water vapour partial pressure in the reactor is not easily detectable, it is more convenient to limit the hydrogen production rate to be seen from the hydrogen flow rate in the hydrogen outlet  5 . The flow rate should not exceed 9 normal liters (1013 mbar at 0° C.) per kg of still un-reacted MgH 2  contained in the reactor and more preferably should be less than 3.5 normal liters per kg of MgH 2 . The lower the hydrogen flow per unit mass of MgH 2  is, the lower is the related relative water consumption value x. 
         [0043]    For supplying the load, which is powered by the fuel cell system  1  via the power output connection  26 , with the sufficient power at any time, the battery  25  has to function as electric buffer. If the load needs less energy than supplied by the fuel cell  2 , especially at the beginning of the hydrogen production, excess energy is stored in the battery  25 . Towards the end of the hydrogen production, the hydrogen flow reduces and thus also the power output from the fuel cell  2  should be reduced in order to prevent unfavorable large values of x. Missing power can then be supplied to the load by the battery  25 . 
         [0044]      FIG. 2  is a longitudinal section view of a first realisation of a reaction chamber  10 . It has the water supply inlet  11  and the pipe connections (two being shown in the drawing) constituting the hydrogen outlet  13 . It comprises MgH 2  as granular hydride  30  with a particle size of 5 μm and a density of 0.5 g/cm 3 , filled in a cylindrical housing  31  through which the inlets and outlets  11  and  13  penetrate. The water supply inlet  11  continues into the housing  31  as an axial evaporation tube  32 , perforated here uniformly by a lot of holes  33  (some ones only being shown), wrapped by a coat  34  of porous material permeable to steam but not permeable to the granules of hydride  30  and not permeable to liquid water, such materials being known in the art. A woven, hydrophobic carbon cloth such as is usually used for preparing gas diffusion electrodes for fuel cells having a thickness of 0.35 mm to 0.55 mm is appropriate for this purpose. The coat  34  thus prevents a congestion of the holes  33  by the granular hydride  30  as well as a contact of the hydride  30  with liquid water; the latter avoids the development of slurry which can block the hydrogen outlet  13 . The inner wall of the housing  31  is covered by a lining  35 , again of a porous material permeable to hydrogen but not permeable to the granules of the hydride  30 . The reaction chamber  10  further contains an electric resistor coil  36  allowing heating of the reaction chamber  10 . A temperature sensor  37  is connected via the cable  27  ( FIG. 1 ) to the control unit  24 . 
         [0045]    The liquid water entering the evaporation tube  32  is evaporated in the tube by the heat resulting from the exothermic reaction and in the areas immediately behind the holes  33 , the resulting steam reacting with the hydride  30 . The hydrogen originated by this reaction penetrates through the hydride  30 , which with increasing time contains more and more the by-products MO and M(OH) 2 , and the lining  35  that leads the hydrogen to the pipe connections of the hydrogen outlet  13 . 
         [0046]    Before start of the reaction, the temperature in the reaction chamber  10  is too low as to convert the liquid water into steam. Therefore, in such cold phases, signalled by the temperature sensor  37 , electric heating is carried out by means of the coil  36 . This heating also rises the temperature of hydride  30  so as to avoid condensation of steam in the reaction chamber  10 . Electric heating will also be favourable in case of low load, thus low hydrogen demand, thus low water supply rate and finally low hydrogen pressure in the hydrogen line  20 . Under such conditions, the temperature falls below a threshold value, e.g. 100° C. to 80° C., which is signalled to the control unit  24  which supplies the resistance wire in coil  36  with heating current from the battery  25 , thereby keeping the temperature in a useful range. 
         [0047]      FIG. 3  is a longitudinal section view of a second realisation of a reaction chamber  10 . The housing  31 , which in this example can be cylindrical or block-shaped, is filled with the hydride  30  and is axially penetrated by the steam and the produced hydrogen. It has the water supply inlet  11 , the hydrogen outlet  13  and the temperature sensor  37 . Corresponding to the coat  34  and the lining  35  of  FIG. 2 , the present reaction chamber contains a layer of porous material  41  at the water supply inlet  11  and a further layer of porous material  42  at the hydrogen outlet  13 . Other than in the embodiment of  FIG. 2 , there are no heating coils but phosphorous-pent-oxide (P 2 O 5 ) is placed in the layer  41 . P 2 O 5  is an oxide of the element phosphor, a hygroscopic powder that, if combined with water, reacts in a strong exothermic reaction to phosphoric acid (H 3 PO 4 ). Upon start of the water supply, the heat released by the exothermic reaction is used to pre-heat the reaction chamber  10 . In a second step the H 3 PO 4  reacts with MgH 2  to form H 2  and phosphates. The phosphates coagulate to particles, but in a tolerable extent because of the small amount of the P 2 O 5 . 
         [0048]    The operating temperature, i.e. the average reaction chamber temperature, for achieving a reasonable performance is rather uncritical, ranging for MgH 2  from 220° C. to 350° C.; it is noted, that in some specific areas of the reaction chamber the temperature may differ significantly. It may thus be allowable not to control the temperature in a closed loop control, but to design the heat losses of the reaction chamber  10  according to the power demand and profile of the specific application. According to  FIG. 3 , non-linear heat insulation is provided, having a high thermal resistance at low temperatures and a low thermal resistance at high temperature. The heat loss thereby is small at low reactor temperatures, e.g. 180° C. and rises disproportionally if the reaction chamber temperature rises to 350° C. and higher. Vacuum insulation, mirrored or not mirrored depending on reactor size and average hydrogen flow demand, fulfills this requirement. Since the heat transfer is not governed mainly by conduction, which is proportional to the temperature difference, but by radiation through a vacuum gap, the heat loss can be proportional to the reactor temperature to the fourth power (T 4 ). Thus, in  FIG. 3 , the housing  31  consists of a double wall cylinder with vacuum  43  in between. If the above described insulation is not sufficient for avoiding excess temperatures, known cooling techniques can be applied. 
         [0049]    Further, at critical temperature of the reaction chamber  10  above 350° C. to 380° C. self-decomposition of the MgH 2  starts in an uncontrollable way and metallic magnesium is produced (MgH 2 →Mg+H 2 ). This reaction is endothermic, and cools down the reaction chamber  10  by itself. The desired production of hydrogen is not controllable until the lower temperature is reached again but the endothermic nature of this reaction provides at least the basic safety feature of preventing thermal runaway. 
         [0050]      FIG. 4  is a longitudinal section view of a cylindrical water storage vessel  16 , for applications where the orientation has to be insensitive. The vessel  16  is filled with water up to a water level  46  leaving a sufficient air volume above. The water level  46  in the depicted example is approximately ⅔ of the entire vessel  16 . The hydrogen outlet  5  continues inside the vessel  16  as tube  47 , thus constituting a hydrogen collecting body. The tube  47  consists of expanded, porous and hydrophobic polytetrafluoroethylene (PTFE), that is permeable for gaseous but not for liquid media. The PTFE tube  47  is arranged closed to the walls of the vessel  16 , penetrates through the complete volume of the vessel  16  and approximately reaches the bottom of the vessel  16 . 
         [0051]    The water line  14  continuous inside the vessel  16  as hose  48 , i.e. a very flexible plastic tube, with an open end and, at its open end, a high density weight body  49 , e.g. a metallic weight, the hose  48  having at least the length of the longitudinal extension of the vessel  16 . Thus, in any orientation, the water to be extracted from the vessel  16  is always taken from under the level  46  and the hydrogen to be extracted from the vessel  16  is always taken from above the level  46 . 
         [0052]    Water can as well be extracted and recycled from the cathode exhaust gas and/or even anode exhaust gas of the fuel cell  2 . The water generated by the fuel cell  2  is theoretically enough to replace the water consumed by the hydrogen generator  3 , in practice the water losses of the fuel cell  2  make the control of x to values smaller than 1.4 inevitable if water balanced operation and thus high system energy density is required. 
       REFERENCE LIST 
       [0000]    
       
           1  fuel cell system 
           2  fuel cell 
           3  hydrogen generator 
           4  pressure reduction valve 
           5  hydrogen outlet 
           6  control and electric output cable 
           10  reaction chamber 
           11  water supply inlet 
           13  hydrogen outlet 
           14  water line 
           15  pump 
           16  water storage vessel 
           17  water pre-heater 
           20  hydrogen line 
           21  pressure sensor 
           22  excess pressure valve 
           23  electronic board 
           24  control unit 
           25  battery 
           26  power output connection 
           27  control cable 
           28  replaceable cartridge 
           29  granular hydride 
           30  housing 
           31  evaporation tube 
           32  holes 
           33  coat 
           34  lining 
           35  coil 
           36  temperature sensor 
           41  layer of porous material 
           42  layer of porous material 
           43  vacuum 
           46  level 
           47  porous PTFE tube 
           48  hose 
           49  high density weight body