Patent Application
20120043221
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.
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.
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.
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.
According to
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.
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
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
| Number | Date | Country | Kind |
|---|---|---|---|
| 201010256747.5 | Aug 2010 | CN | national |
