Patent Publication Number: US-2012043221-A1

Title: Method and Device for Generating and Storing Hydrogen

Description:
FIELD OF THE INVENTION 
     The invention relates to a method and a device for generating and storing hydrogen, particularly for providing a hydrogen source for fuel cells, the method comprising the steps of generating gaseous hydrogen in an electrolyser, transporting the gaseous hydrogen to the metal hydride store and feeding the metal hydride store with the hydrogen. 
     BACKGROUND OF THE INVENTION 
     Hydrogen storage via a metal hydride, e.g. LaNi or TiZr based alloys with AB2 or AB5 structure respectively is a known method for providing a hydrogen supply for consumers such as fuel cells. Metal hydrides, however, are very sensitive to water in both a liquid and vapour form. Hydrogen generated by electrolysis typically contains fractions of water which must be eliminated before reaching the metal hydride store. 
     It is known, e.g. from U.S. Pat. No. 5,964,089, FIG. 3, to eliminate the water fraction by a liquid water trap, a condensing coil and a chemical drier. For a complete elimination of any liquid, these elements need to be broadly dimensioned. It is an object of the invention to obtain a nearby complete elimination in a simpler and volume saving way. 
     SUMMARY OF THE INVENTION 
     To this end, the method of the invention is characterized in that the gaseous hydrogen is first transported under a first pressure of at least 1 MPa absolute, in the following all pressure data are absolute pressure, which pressure is then reduced by a pressure adjusting means to a second, designated pressure lower to the first pressure, the second pressure being the feeding pressure to the metal hydride store. Further the device of the invention, comprising an electrolyser, a metal hydride hydrogen store and a conduit connecting both these elements, is characterized in that into the conduit a pressure adjusting means is integrated. 
     The invention is based on the fact that the saturation water vapour partial pressure depends on the temperature but keeps materially constant if the gas pressure of the mixture containing the water vapour is increased. Upon electrolysing, it is possible to generate the hydrogen under a high pressure directly at the electrode by controlling parameters such as voltage, especially if a polymer-electrolyte membrane (PEM) electrolyser is used. A typical electrolysis temperature is 60° C.; then, if the hydrogen is produced under about 0.1 MPa, for 100 molecules of gas, 20 water molecules are present and if the hydrogen is produced under about 1 MPa, for 100 molecules of gas, 2 water molecules are present. On the other hand, the metal hydride hydrogen store at the beginning of charging exhibits a counter pressure of about 0.1 kPa which rises during charging. If the hydrogen-water mixture is fed into the metal hydride store as generated with 0.1 MPa, the metal hydride will soon be spoiled. 
     The gas mixture under the first pressure is advantageously poor of water molecules, whether generated under the high pressure or afterwards compressed, and under the second pressure which is the counter pressure of the store, still profits of the low content of water molecules. 
     Additional dehydrating means such as water vapour separators in the form of a chemical absorber can advantageously be added. The time intervals for a replacement of the chemical absorber are large due to the poor total water vapour produced by the electrolyser. Further a liquid water separator such as a condenser working through cooling to ambient temperature or below can be integrated. The danger of system blocking by freezing is negligible due to the low water content as a result of the high first pressure. The liquid water separator contains a gas collecting area and a liquid collecting area, the latter comprising a liquid level sensor controlling a liquid exhaust which contains a pressure reducer and a valve. The collected liquid can be exhausted or recycled to the electrolysis cell or cells. This can guarantee a continuous operation. 
     According to the invention the pressure adjusting means, in particular a pressure reduction valve, can preferably be a pressure retention valve keeping the pressure at its input side at the first pressure of at least 1 MPa, materially irrespective of the second pressure at its output side. 
     Further, a first pressure sensitive switch between the electrolyser and the pressure adjusting means is integrated as a safety device to check whether the electrolyser produces a certain minimum pressure; if not, a system failure is assumed and the complete system is shut down. And a second pressure sensitive switch between the pressure adjusting means and the metal hydride store is integrated to switch off the electrolysis upon reaching a certain back pressure of the store that depends on the level of charging. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 drawings. 
         FIG. 1  shows the principle of the invention. 
         FIG. 2  shows a block diagram of one embodiment of the invention. 
         FIG. 3  shows a section view of a pressure adjusting valve. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     According to  FIG. 1 , in a stack of PEM electrolysis cells  1 , in the drawing for simplicity shown as a single cell, a cathode compartment  2  and an anode compartment  3  are separated by a polymer electrolyte membrane (PEM)  4 . To the anode compartment  3 , in operation water is added. The compartments contain electrodes to which dc is supplied, whereupon at the anode in compartment  3  oxygen is produced and at the cathode in compartment  2  hydrogen is produced, which is the used product of the electrolysis cells  1 . The hydrogen is generated under a pressure of at least 1 MPa, preferably 2.5 to 4 MPa under which pressure hardly any water vapour, escapes with the hydrogen. In this case, hardly any contaminating water will emerge in the cathode compartment  2 . 
     From this compartment  2 , a first conduit  5  conducts the hydrogen, nearly without any water content, under the original high pressure to a pressure adjusting device  11 , which, in the described example, is a pressure reduction valve. The output of device  11  is connected to a second conduit  12  which leads the hydrogen under a reduced pressure to a metal hydride store  13  where the hydrogen is chemically stored. The metal hydride store  13  develops a counter pressure which is for typically 0.1 MPa at the beginning of charging, and 2 MPa at the end thereof. The difference between the first pressure in the first conduit  5  and the second pressure in the second conduit  12  is maintained by the pressure adjusting device  11  during preferred all stages of charging. Thus, the condition of the hydrogen before reaching device  11  is, preferred constantly, under a higher pressure than needed for the charging and so keeping the water content nearby zero. 
     If the PEM cell  1  delivers a pressure not sufficient for the delivering hydrogen almost without water, a construction is possible using a compressor in the output conduit  5 . After this compressor, the gas stream is moderately dry. 
       FIG. 2  shows a block diagram of one embodiment of the invention. The electrolysis cell  1  containing the compartments  2  and  3  and the PEM  4  is connected to a water tank  17  via a pump  18  feeding the water to be dissociated under the control of an electronic control device  19  to which the pump is connected via a control line  20 . The water is pumped through the anode compartment  3  back to the water tank  17  via a back water line  21  together with the reaction product oxygen. The power supply is managed via a supply unit  22  fed by the control device  19 . A stack control line  23  serves for managing the stack of electrolysis cells depicted as cell  1 . The water tank  17  has a water supply inlet  25  and a water level sensor  26  which is connected to the electronic control device  19  via a control line  27 . Further it has an oxygen outlet  28 . 
     The hydrogen escaping from the cathode compartment  2  through the first conduit  5  and still containing traces of water vapour is first cooled in a radiator  32  to ambient temperature, thereby condensing part of the residual water, and then is led to the water collecting tank  33 . The upper part of tank  33  is optionally cooled under ambient temperature, preferably by using Peltier elements. Even a cooling under 0° C. is possible. Due to the low water content as result of the high first pressure the danger of system blocking by freezing is negligible. 
     In the lower part of tank  33  liquid water is collected and its level is controlled by a level sensor  34  activating an electromagnetic valve  35  when a water level is reached. Between the tank  33  and the valve  35  a pressure reducer  36  is inserted, since in the tank  33  the high pressure of the cathode compartment  2  is still present. The water dispensed from the tank  33  is recycled to the tank  17  via recycling line  37  or purged out to the environment via purging line  38 . 
     The hydrogen leaving the tank  33  via a conduit which is still designated first conduit  5  passes a safety valve  43  which opens e.g. at 0.3 MPa above the first pressure and then enters a chemical absorber  44 , known in the art, for finally dehydrating the hydrogen. The hydrogen leaving the chemical absorber  44  passes a pressure sensitive switch  45  which signalizes to the control device  19  if the first pressure is falling below a first threshold e.g. of 2 to 2.5 MPa. If the pressure falls below this threshold after a certain starting period, this is interpreted as a leakage failure of the system and the control device  19  will shut down the system. 
     The conduit  5  then leads into the pressure adjusting device  11 , still under the first pressure of the cathode compartment  3 . The pressure adjusting device  11  reduces the pressure to the pressure momentarily demanded by the metal hydride store  13 . Several constructions for such device  11  are possible, the preferred example uses a pressure reduction valve, described in  FIG. 3  below. 
     The outlet of the device  11  is the second conduit  12  leading the hydrogen under the second pressure to the metal hydride store  13 . A pressure sensitive switch  48  checks the pressure in conduit  12  for finding out the end of the charging step, signalizing this to the control device  19  and then switching off the system. As mentioned, the back pressure of the store  13  increases while charging goes on and also is dependent on the temperature of the store. For this reason, after a first switch off, the temperature will decrease lowering also the back pressure, which has the consequence that the switch  48  causes a restart of the system, which sequence may occur several times. 
     Into the second conduit  12  according to the described example a conduit flushing valve  49  is inserted to be used for a controlled flushing after an extended non-use. The metal hydride store  13  is detachably connected to the conduit  12  for being exchanged after complete charging. A mounting sensor  50  checks the correct assembly of the metal hydride store  13  to the conduit  12 , and signalizes it to the control device  19 . 
     The construction of  FIG. 3  is a basic type of a back-pressure valve. The conduit  5  is connected to a hydrogen input  55  and the conduit  12  is connected to the hydrogen output  56 . These ports  55  and  56  are situated within a cylinder block  57  wherein a piston  58  is slidably disposed in a bore and is pressed by a helical spring  59  until a seal  60  in the front surface of the piston  58  impinges on raised-faced flange  65  communicating via a duct  66  with the output  56  while the space surrounding the flange  65  and limited by the block  57  and the piston  58  communicates via a duct  67  with the input  55 . Thereby, between the flange  65  and the seal  60  a circular ring shaped sealing area  68  is formed. The opposite surface of the piston  58  communicates via a hole  69  with the surroundings thus atmospheric pressure is applied to the backside of the piston. Between the ducts  66  and  67 , the pressure difference between the higher first pressure and the lower second pressure exists, both pressures together shifting the piston  58  against the force of the spring  59 , thereby opening a gap between the elements  60  and  65  which acts as a choke through which the hydrogen passes to the conduit  12  under the second pressure. The second pressure is determined by the metal hydride store  13  and its charging status. 
     REFERENCE LIST 
     
         
           1  PEM electrolysis cell 
           2  cathode compartment 
           3  anode compartment 
           4  polymer electrolyte membrane (PEM) 
           5  first conduit 
           11  pressure adjusting device 
           12  second conduit 
           13  metal hydride store 
           17  water tank 
           18  pump 
           19  electronic control device 
           20  control line 
           21  back water line 
           22  supply unit 
           23  stack control line 
           25  water supply inlet 
           26  water level sensor 
           27  control line 
           28  oxygen outlet 
           32  radiator 
           33  water collecting tank 
           34  level sensor 
           35  valve 
           36  pressure reducer 
           37  recycling line 
           38  purging line 
           43  safety valve 
           44  chemical absorber 
           45  pressure sensitive switch 
           48  pressure sensitive switch 
           49  conduit flushing valve 
           50  mounting sensor 
           55  hydrogen input 
           56  hydrogen output 
           57  cylinder block 
           58  piston 
           59  spring 
           60  seal 
           65  raised-face flansh 
           66  duct 
           67  duct 
           68  sealing area 
           69  hole