EP4646545A1

EP4646545A1 - Method for eliminating foreign gas

Info

Publication number: EP4646545A1
Application number: EP23833319.9A
Authority: EP
Inventors: Andreas Wezel; Michael Gries
Original assignee: Stellantis Auto SAS
Current assignee: Stellantis Auto SAS
Priority date: 2023-01-05
Filing date: 2023-12-13
Publication date: 2025-11-12
Also published as: WO2024146754A1; DE102023200073A1

Abstract

To eliminate foreign gas from a hydrogen tank of composite material, hydrogen is repeatedly fed into the tank (S1, S3, S5, S7) until a predetermined upper final pressure of below 50 bar is reached in the tank, and a mixture of hydrogen and the foreign gas thereby formed in the tank is let off (S2, S4, S6).

Description

METHOD FOR ELIMINATING FOREIGN GAS

D E S C R I P T I O N

The present invention relates to a method for eliminating foreign gas from a hydrogen tank.

Before a tank is filled with hydrogen and put into operation, foreign gas, typically atmospheric air or nitrogen included in the manufacture of the tank, must be removed therefrom, in order to ensure that the tank contains no gas mixture which is intrinsically explosive, and if applicable to ensure that the hydrogen of the tank has the necessary purity for an intended use such as, for instance, the supply of a fuel cell. In the case of self-supporting tanks, this can take place through the suctioning of the foreign gas before filling with hydrogen for the first time; however, the problem exists here that generally suctioning must take place via a shut-off valve having a small passage cross-section and therefore a long time passes until the pressure in the tank comes close to the final pressure of a pump used for suctioning. It is therefore usual to eliminate the foreign gas by hydrogen being fed into the tank several times until a predetermined upper final pressure is reached in the tank, and the gas mixture which is thereby formed in the tank is let off. The process can be repeated as often as necessary in order to guarantee a desired purity of the hydrogen in the tank.

Tanks in composite construction, as are used for motor vehicles with hydrogen drive, are generally not to fall below a predetermined internal pressure of several bar, in order to ensure that an inner lining which is hydrogen-tight, but typically is not sufficiently pressure-resistant on its own, is held pressed constantly against a surrounding, pressure-resistant casing. Therefore, the elimination of foreign gas in the case of such a tank takes place conventionally by hydrogen being fed therein until the tank has reached an upper final pressure of 150 to 200 bar, the gas mixture thereby formed in the tank is let off, until a lower final pressure of 20-30 bar is reached, and the whole is repeated until a required purity of the tank content of e.g. >99.9% is reached. The quantity of hydrogen consumed for this in the course of three feeding- and letting-off cycles corresponds to more than 400 times the tank volume, which causes considerable costs. The speed which is able to be reached during the feeding and letting off of gas is limited by the passage cross-section of the shut-off valve, so that in practice, with a pressure change rate of ca. 4 bar/min during feeding and during letting off, the eliminating of the residual gas and a filling of the tank with hydrogen for the first time up to a maximum pressure of 700 bar takes four hours or more.

It is an object of the invention to indicate a method by which the eliminating of foreign gas is possible in a shorter time and with less hydrogen consumption.

This problem is solved as a result of an aspect of the invention, by the final pressure being lower than 50 bar in a method for the eliminating of foreign gas from a hydrogen tank of composite material through repeated feeding of hydrogen into the tank until a predetermined upper final pressure is reached in the tank, and letting off a mixture of hydrogen and the foreign gas formed thereby in the tank. Thus, more feeding- and letting-off cycles can be carried out in a shorter time, and the foreign gas can be eliminated effectively more quickly.

The lower the upper final pressure is selected, the smaller the quantity of hydrogen is which must be used before foreign gas can be flushed out from the tank, and the shorter the time is which is taken until then. The upper final pressure should therefore be lower than or equal to 30 bar, preferably lower than or equal to 20 bar. On the other hand, in the case of an upper final pressure which is too low, the risk exists of an insufficient intermixing of the fed hydrogen with the gas present in the tank, so that in a letting-off, after reaching the upper final pressure, the possibility exists that the let off gas exists to a disproportionate part of the hydrogen which has just been fed, and the previously present gas remains behind in the tank. Therefore, an upper final pressure of 10 bar or thereabove is preferred.

The lower final pressure which is achieved on letting off should not amount to below 5 bar, in order to ensure that the pressure in the tank is sufficient in order to keep an inner lining of the tank in contact with a surrounding casing, even if the inner lining were to be under a tensile stress.

Whereas the lower final pressure may be sufficient in order to keep the lining in contact with the casing over a large area, it may nevertheless be low enough to enable a local lifting of the lining from a locally intensively curved wall region of the casing, and consequently to enable a stress equalization through local slipping of the lining. So that this slipping can be reversed when the pressure rises again, and finally is sufficient in order to also press the lining against the locally intensively curved wall region, the speed of the pressure rise should be limited, preferably to a maximum of 2 bar/min, better still a maximum of 1.5 bar/min.

As the difference between upper and lower final pressure in the method according to the invention is less than in the conventional method described above, the dilution of the foreign gas which is able to be achieved with each feeding/letting-off cycle is smaller than in the conventional method; because of this, the number of cycles should be greater and the feeding should preferably be repeated at least four times.

Further features and advantages of the invention will emerge from the following description of example embodiments with reference to the enclosed figures. There are shown:

Fig. 1 a schematic illustration of a hydrogen tank for a motor vehicle;

Fig. 2 a section through the wall of the tank; and

Fig. 3 a flow chart of the method according to the invention.

Fig. 1 shows in a schematic, partially cut-open view a tank 1 with a rigid casing 2 of fibre-reinforced, in particular carbon fibre-reinforced, plastic, a lining 3 in abutment with an inner side of the casing 2, and a shut-off valve 4. The tank 1 can be manufactured by firstly the lining 3 being pre-fabricated as a gas-tight bag, being connected with the valve 4 and, through feeding of a fluid under excess pressure, being brought into a form which is rigid enough in order to wind around the casing in the form of a band of reinforcement-fibre non-woven fabric or woven fabric, pre- impregnated with plastic, and, through melting of the plastic of layers of the nonwoven fabric or woven fabric lying over one another, to form a rigid hollow body. The excess pressure which used for this is considerably less than the later operating pressure of the tank, but can be sufficient to expand the material of the lining 3, so that after winding around with the band it is under tensile stress.

The inner surface of the casing can have unevenness, e.g. the inner surface along an edge 5 of the band 6 can form a groove 7, when the band 6, as shown in Fig. 2 in a section through the wall of the tank 1, is wound in scaled arrangement around the lining 3. When the tank 1 is under operating pressure, the lining 3, as illustrated by an outline in dashed lines, is also pressed in the groove 7 closely against the casing 2 and is thereby locally expanded. As a repeated expanding during the operation of the tank would strain the lining 3 and could lead to leakage, it is conventionally recommended not to allow the pressure in the tank 1 in operation to fall below a predetermined minimum pressure of several bar, which is sufficient in order to keep the lining 3 pressed closely in accordance with the course in the groove 7 illustrated by dashed lines.

However, this necessity does not exist as long as the tank has, however, never been brought to its maximum operating pressure of typically several 100 bar and the lining 3 has never been pressed into the form illustrated by dashed lines, but rather its course follows the thick drawn line in Fig. 2, at most slightly deflected into the groove 7. On the contrary here a repeated pressure change between an upper final pressure of 20-30 bar and a lower final pressure of ca. 5 bar can promote a displacement of material of the lining 3 from regions 8 lying over the entire area against the casing 2 to the groove 7, so that, when the tank is later brought to its maximum operating pressure, the lining 3 can nestle into the groove 7 with a slight expansion. In order to promote such a material displacement, it is expedient to keep the speed low at which the pressure in the tank is increased.

Fig. 3 shows the sequence of the method of eliminating of air from a new hydrogen tank, which is typically already mounted in a vehicle. From its manufacture, the tank is still filled with 5 bar N2; this pressure should not be fallen below, in order to keep the lining 2 in abutment with the casing 3. In a first step S1, 99.999% pure H2 is fed into the tank, until an upper threshold pressure of 20 bar is reached. At a pressure change rate of 1.5 bar/min, 10 min are required for this. Thereafter, the tank content consists of H2 at 75%. The gas mixture is let off until a lower threshold pressure of 5 bar is reached (S2). This takes a further 10 minutes, again at a pressure change rate of 1.5 bar/min. After a second feeding of H2 up to the upper threshold pressure of 20 bar and letting off to 5 bar (S3, S4), the hydrogen content amounts to 93.8%; after a third repetition of these steps (S5, S6), it is 98.4%. A fourth filling (S7) increases the hydrogen content to 99.6%. A renewed letting off can now be omitted; instead - after only 70 minutes of feeding and letting off at a low pressure change rate - filling can now be carried out at a normal hydrogen filling station with a higher pressure change rate up to the maximum pressure of 700 bar (S8), in order to achieve a purity of 99.99%. The entire process can be completed within just 2h, and the loss of H2 through the letting off corresponds to only ca. 40 bar.

REFERENCE NUMBERS tank casing lining shut-off valve edge band groove abutting region

Claims

C L A I M S

1. A method for eliminating foreign gas from a hydrogen tank (1) of composite material through repeated feeding (S1 , S3, S5, S7) of hydrogen into the tank (1) until a predetermined upper final pressure is reached in the tank (1), and letting off (S2, S4, S6) of a mixture, thereby formed in the tank, of hydrogen and the foreign gas, characterized in that the final pressure is lower than 50 bar.

2. The method according to Claim 1, in which the upper final pressure is lower than or equal to 30 bar, preferably lower than or equal to 20 bar, and/or higher than or equal to 10 bar.

3. The method according to Claim 1 or 2, in which the letting off (S2, S4, S6) takes place up to a lower final pressure of not below 4 bar, preferably 5 bar.

4. The method according to one of the preceding claims, in which the feeding (S1 , S3, S5, S7) takes place at a filling speed of a maximum of 2 bar/min, preferably a maximum of 1.5 bar/min.

5. The method according to one of the preceding claims, in which the feeding (S1, S3, S5, S7) is repeated at least four times.

6. The method according to one of the preceding claims, in which the tank (1) comprises a hydrogen-tight inner lining (3) and a casing (2), supporting the inner lining (3), of fibre-reinforced, in particular carbon fibre-reinforced, plastic.

7. The method according to one of the preceding claims, in which the tank (1) is a new tank of a motor vehicle with hydrogen drive.