Abstract:
The electrolysis occurs as a high-pressure electrolyzer, oxygen being produced on one side and hydrogen on the other side, with corresponding pressure. The gases may optionally be stored without additional compression. The PEM fuel cell process is used in reverse for the process. It is advantageous that excess energy may be used by wind power plants. In the associated device, a high-pressure electrolyzer ( 1 ) is present which is operated using environmentally friendly air power. Due to the improved operating point of the high-pressure electrolyzer, improved economy results for the generation process compared to the prior art, in particular for hydrogen as an energy storage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Application of International Application No. PCT/EP2010/060520 filed Jul. 21, 2010, which designates the United States of America, and claims priority to German Patent Application No. 10 2009 035 440.9 filed Jul. 31, 2009. The contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a method for generating hydrogen and oxygen, wherein use is made of the excess energy from wind power installations in particular. The invention further relates to an associated device for performing the method, in which a PEM fuel cell is used as an electrolysis unit. 
       BACKGROUND 
       [0003]    By reversing the fuel cell process, a fuel cell can be used to produce hydrogen on one side and oxygen on the other side. The fuel cell then functions as an electrolysis unit and must be supplied with electrical power. The electrical power from e.g. wind power installations can be used for this purpose. 
         [0004]    In wind power installations, the generated electrical power is used directly on site as wind energy to generate current. In the absence of wind energy, other power stations have to replace the wind power installation. If the effective wind power increases, the installation must be turned off or the energy must be reduced by wide distribution in the event of excess generation. Both of these reduce the efficiency factor of the installation. 
         [0005]    The electrolysis equipment which is currently used for generating hydrogen on one side and oxygen on the other side comprises devices that normally work at atmospheric pressure. One application of such devices is e.g. the use of the hydrogen as corrosion protection in pipe systems in the exclusion area of nuclear power stations. 
       SUMMARY 
       [0006]    By contrast, according to various embodiments, a method and an associated device can be provided by means of which in particular hydrogen can be generated as a process gas having a high energy content, wherein the hydrogen can be used as an energy store or as a synthesis gas for other industrial installations. In this case, it is intended in particular to use the excess energy from wind power installations, wherein said energy is not compromised by CO 2 . 
         [0007]    According to an embodiment, in a method for generating hydrogen and oxygen, water undergoes electrochemical electrolysis, wherein the electrolysis takes place at pressures exceeding atmospheric pressure, wherein PEM high-pressure electrolysis is employed. 
         [0008]    According to a further embodiment, the high-pressure electrolysis may take place at pressures &gt;10 bar, in particular &gt;100 bar. According to a further embodiment, the PEM high-pressure electrolysis may be operated in the temperature range 5° C. to 100° C. According to a further embodiment, the resulting gases can be stored in a high-pressure storage facility. According to a further embodiment, the resulting gases can be stored in gas bottles. According to a further embodiment, the hydrogen can be used as a source of energy. According to a further embodiment, excess energy from wind power installations can be used to operate the high-pressure electrolysis. According to a further embodiment, the overall CO 2  balance can be improved by using environmentally friendly energy. According to a further embodiment, the high-pressure electrolysis may generate a precompression of approximately 10 bar and the further compression can be done by a mechanical compressor. According to a further embodiment, the generated gas can be purified. 
         [0009]    According to another embodiment, a device for performing the method as described above, may comprise an electrolysis unit for converting electrical energy into energy in the form of gas, wherein the electrolysis unit is a high-pressure electrolysis unit from which the generated gases are conveyed into the pressure containers, wherein the high-pressure electrolysis unit comprises a fully enclosed PEM high-pressure electrolysis. 
         [0010]    According to a further embodiment of the device, a further compressor is connected in series to the electrolysis unit. According to a further embodiment of the device, gas bottles can be provided for the immediate transfer of the generated gases. According to a further embodiment of the device, units can be provided for purification of the generated gases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Further details and advantages are derived from the following description of figures, these relating to exemplary embodiments. 
           [0012]      FIG. 1  shows a block schematic diagram of a high-pressure electrolysis installation comprising gas purification and the associated storage tanks, and 
           [0013]      FIG. 2  shows a graphical representation of the operating voltage of a cell used in  FIG. 1  as a function of the current density. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    According to various embodiments, it is possible easily to utilize the excess energy that occurs in wind power installations. This excess energy is stored in the form of hydrogen in particular, wherein said storage is comparatively simple due to the use of a high-pressure electrolysis unit. In particular, PEM high-pressure electrolysis can be used for this purpose, wherein pressures greater than 10 bar can be achieved. In the associated device, the filling of gas storage units by means of suitable equipment and compressors is considerably simplified in this case, and can even be omitted. Use of a high-pressure cell as a device has the crucial advantage that the cell can be operated at higher current densities as the process pressure increases. This has been tested in practice specifically for PEM high-pressure cells. 
         [0015]    According to various embodiments, the application of the known electrolysis is realized in the high-pressure range, i.e. specifically the water circuit, the electrolysis cell and the gas separation facility are realized in such a way that the equipment units are designed as pressure containers. Consequently, it is advantageously possible to achieve an operating pressure of up to 110 bar. The operating pressure can be increased even further by means of design measures, however. 
         [0016]    One advantage of the high-pressure electrolysis installation described here is the direct generation of the high gas pressure without additional compressors. Alternatively, the high-pressure electrolysis can merely precompress the gas, wherein the pressure is subsequently increased further by compressors in a simple and economical manner. Since the quantity of gas that is generated depends directly on the current density, which can be selected, the possibility of selecting higher current densities at higher pressures is a further advantage of the high-pressure system. 
         [0017]    The use of excess current makes it particularly beneficial to use electrolysis for generating hydrogen in particular. The current that is otherwise unused can now be used to generate gas, and the gas that is produced can be sold on to e.g. chemical industries or the like. 
         [0018]    In summary, PEM electrolysis offers the advantage that it can be operated in a highly dynamic manner which is not possible using other electrolysis systems. Other electrolysis systems have to be started up at considerable expense in each case, in order to achieve relatively steady operating conditions. 
         [0019]    A so-called high-pressure electrolysis unit is designated as  1  in  FIG. 1 . Such a high-pressure electrolysis unit performs electrolysis of water by applying a voltage at the electrodes, thereby delivering gaseous oxygen on one side and gaseous hydrogen on the other side. 
         [0020]    The electrolysis is achieved by applying an electrical voltage to the corresponding fuel cell, thereby creating an energy converter. This means that power in the form of electrical energy is converted into a process gas as an energy store. Conversely, an electrolysis unit can also be used for generating power in the form of electrical energy, the term fuel cell being applicable in this case. 
         [0021]    An MEA unit (membrane electrode assembly) is assumed in the case of a so-called PEM fuel cell (polymer electrolyte membrane), wherein the PEM fuel cell has already been widely tested in practice. A catalytic converter is still required for such PEM fuel cells. 
         [0022]    When using the PEM fuel cell as an electrolysis unit, electrical energy must be supplied as mentioned above. Wind power installations are increasingly being used for this purpose, particularly when excess energy which cannot be fed directly into the electricity network occurs in the wind power installation. In this case, electrical power that is generated by the wind power installation can be used to generate hydrogen as a source of energy. 
         [0023]    In the present case, high-pressure electrolysis is effected by designing the fuel cell as a high-pressure device. In this case, the hydrogen that is generated on one side and the oxygen that is generated on the other side are obtained at a corresponding pressure. For example, a pressure of up to 110 bar is generated. 
         [0024]    Since the gases are not generated at the desired pressure in the electrolysis unit  1 , a compressor  5  can be connected in series and compress the oxygen and the hydrogen in a suitable manner. Furthermore, the gases that are generated in this way, i.e. the oxygen on one side and the hydrogen on the other side, are conditioned by units for gas purification and then supplied to a separate tank. This means that the oxygen is stored at a corresponding pressure in the tank  10  and the hydrogen is stored at a corresponding pressure in the tank  20 . The gases from the tanks  10 ,  20  can be transferred into bottles  11 ,  21  as operating gases or process gases for the chemical industry or for other purposes. 
         [0025]    Using the process described here, the gases that have already been compressed as a result of the generation process can easily be conditioned as appropriate and optionally monitored by existing gas analysis systems. The storage of the gases can take place in the pressure-resistant gas tanks  10 ,  20 . Gas bottles can then be filled by means of suitable transfer devices, wherein direct filling of gas bottles without the use of gas tanks as a buffer is also possible. 
         [0026]    The latter installation design is intended for discontinuous operation in particular. This means that the installation only operates when excess current from the wind power installation is available, at night or at other times of low demand for current. 
         [0027]    If further compression of the generated gases is desired, this is simplified by the precompression that is already present. For example, an increase in pressure from 20 bar to 200 bar (the usual gas bottle pressure) is possible and can be achieved using a single-stage compressor. In addition to the technical simplification, such an approach allows costs to be reduced by half in comparison with multi-stage compression systems as described in the prior art, which have to achieve an increase in pressure from 1 bar to 200 bar. In particular, the possibility is established of adjusting the pressure ranges of the individual components (e.g. electrolysis unit and compressor) relative to each other, such that an economical and effective installation design is achieved. 
         [0028]    It can be shown that the operation of the electrolysis equipment at higher operating pressures has a positive influence on the working point of the system. This is explained in detail with reference to  FIG. 2 . 
         [0029]    In  FIG. 2 , the current density in mA/cm 2  is plotted on the X-axis and the voltage in volts is plotted on the Y-axis. By way of example, measured values from trials at atmospheric pressure are shown in comparison with measured values from trials at 100 bar, resulting in graphs  25  and  26 . 
         [0030]    In summary,  FIG. 2  shows that an increase in the process pressure produces a change in the characteristic curve for voltage relative to current density. The resulting equilibrium voltage becomes slightly higher as a function of the increasing pressure, but the saturation of the water with gas bubbles commences later due to the compression of the gas bubbles here. The installation therefore works more effectively at higher pressures and consequently can be operated using higher current densities. The gas yield of the installation is therefore improved while the input power remains the same.