EP2132476A2

EP2132476A2 - Method for filling a pressurised reservoir provided for a cryogenic stored medium in particular hydrogen

Info

Publication number: EP2132476A2
Application number: EP08709217A
Authority: EP
Inventors: Tobias Brunner
Original assignee: Bayerische Motoren Werke AG
Current assignee: Bayerische Motoren Werke AG
Priority date: 2007-03-09
Filing date: 2008-02-26
Publication date: 2009-12-16
Also published as: US20090308083A1; WO2008110453A2; WO2008110453A3; DE102007011530A1

Abstract

The invention relates to a method for filling a cryogenic pressurised tank on a motor vehicle provided for cryogenic hydrogen, in which cryogenic hydrogen taken from a large reservoir in liquid form, essentially at atmospheric pressure with a corresponding saturation temperature, can be stored with absolute pressure values in the range of 150 bar or more. The hydrogen is adiabatically compressed with a cryopump after withdrawal from the large reservoir and delivered to the cryotank at a supercritical pressure (13bar or more). A recooling to ca 20k preferably occurs previously by running the compressed hydrogen through a heat exchanger arranged in the hydrogen stored in the large reservoir. The residual cryo medium in the cryo pressure tank can be removed from the cryo pressure tank and introduced into the larger reservoir before the filling with new stored medium.

Description

Method for filling a pressure accumulator provided for a cryogenic storage medium, in particular hydrogen

The invention relates to a method for filling a for a cryogenic storage medium, in particular cryogenic hydrogen, provided pressure accumulator, in particular a cryogenic pressure tank of a motor vehicle in which the extracted from a large reservoir substantially under ambient pressure with a corresponding saturation temperature in the liquid state cryogenic storage medium under absolute pressure values in the order of 150 bar or more can be stored. The prior art, in addition to the currently used refueling technology, which is applied, for example, in the vehicle "Hydrogen 7" of the present application and explained in the following paragraph, refer to DE 41 29 020 C2 and in particular to US 6,708,502 B1.

State of the art in the mentioned vehicle "Hydrogen 7", which is equipped with a so-called. Cryo-tank for storing cryogenic hydrogen (to supply the run as an engine Fzg. -Antriebsaggregats) is a "subcritical" storage tank (as a cryogenic tank ), which consists of a metallic inner tank, a metallic outer tank and an intermediate vacuum superinsulation to reduce the heat input into the inner tank. The typical operating pressure of this storage tank is between 1 bar absolute and 10 bar absolute, and the operating temperatures in the so-called "cold regular operation" are between 20 K (Kelvin) and about 30 K, ie the same contained in the storage tank or in the inner tank of cryogenic hydrogen has these physical values mentioned, which are in the pressure-density diagram of the hydrogen in the so-called. Subcritical range, on. The highest system storage densities shown so far are less than 30 grams of hydrogen per liter of system volume, to which, in addition to the cryogenic tank, all auxiliary systems necessary for the operation of the fuel supply system are counted. This corresponds to a volumetric system energy density of less than 1 kWh per liter of system volume.

The refueling of the cryogenic tank according to this prior art is carried out with cryogenic liquid hydrogen at pressures between 1 bar and 6 bar and corresponding saturation temperatures of the cryogenic hydrogen or with slight supercooling thereof. The currently maximum representable supercooling is in the order of 6 Kelvin as the difference of the saturation temperature at a pressure of 6 bar absolute and the saturation temperature at a pressure of 1 bar absolute. The physical storage densities are limited by the highest refueling pressure of approx. 6 bar absolute and the lowest possible hydrogen temperature of approx. 20 K and reach values of max. 71.5 g / l. Regular vehicle refueling is currently limited by the minimum pressure requirement of the vehicle drive unit on the one hand and by the lack of "overfillability" on the other hand - as a rule an approximately 80% to 95% liquid volume filling limit of the cryogenic tank is limited. Tank according to the current state of the art therefore does not achieve the above-mentioned maximum possible physical storage densities.

Known is the so-called. Boil-off problem of the current cryo-tanks, according to which, due to although minimal, but unavoidable heat input into the cryogenic tank in this pressure increase takes place, which by Blowing off gaseous hydrogen from the cryogenic tank must be reduced. The maximum lossless life of a current optimal cryo-tank is at a shutdown at operating pressure in the order of about 3 days, ie after this period, blowing off a small subset of stored hydrogen is unavoidable, which is not satisfactory in daily practice.

Further known prior art is the so-called. Cryo-pressure storage is, to which on the o.g. No. 6,708,502 B1, in which various types of isolated pressure accumulators for cryogenic storage media with inner and outer diffusion barriers, which encase a CFK inner tank, are described. According to this prior art, the so-called cryogenic pressure accumulator described can be filled with warm compressed gas at 350 bar and low storage capacity or alternatively with liquid hydrogen at low pressure of about 1 bar (absolute) with a higher storage capacity. When filled with liquid hydrogen, this known cryogenic pressure accumulator according to the description of a working pressure range of 1 bar (absolute) to 350 bar (absolute). The achievable physical storage densities are up to 71, 5 g / l, namely at 100% - filling at 1 bar absolute pressure. Systemic storage densities reach values of up to approx. 33 grams per liter of system volume or 1.1 kWh per liter of system volume.

Advantageously, the heat absorption capacity of said cryogenic accumulator with liquid hydrogen filling at 1 bar (absolute) and a possible pressure increase up to about 350 bar about 7 days per watt average heat input and per kg stored hydrogen. With a system according to this state of the art (with about 150 l of hydrogen, which corresponds to 10.7 kg with 1 bar filling and 10 W of maximum heat input), loss-free service lives of the order of 5 to 10 days are thus included maximum filling (with 10 kg of liquid hydrogen) and of about 30 days with medium filling (with 5 kg of liquid hydrogen) achievable, which is a significant increase over the above-mentioned prior art.

A disadvantage of this prior art, however, is that when filling at 1 bar absolute with the highest physical storage density, no pressure supply for an out of this cryogenic pressure accumulator with hydrogen to be supplied for combustion unit, eg. A vehicle drive unit or a fuel cell, anschleißend directly to a refueling is possible, since this or the hydrogen under slightly higher pressure, which is for today's said components in the order of at least 4 bar absolute, needed. A time-consuming and / or energy-intensive subsequent pressure increase in the vehicle would therefore be necessary for the operation of the drive unit or a fuel cell directly following refueling. Because of the lack of opportunities for a timely, energy-efficient increase in tank pressure or pressure on the way to the drive unit (or generally a consumer, which may be in addition to an internal combustion engine and a fuel cell) so the automotive boundary conditions are not met with the known Kryo pressure accumulator ,

Also results with such a cryogenic pressure tank a long-lasting refueling operation with high amounts of gas recirculation (with density jump) of liquid hydrogen in contact with superheated tank walls, ie there is a need for complete cooling of the cryogenic pressure tank from permanent uptake of liquid hydrogen , Furthermore, an accelerated pressure build-up in the cryogenic pressure tank can be determined by the tendency to thermal stratification. Finally, both such a cryopressure tank and its ancillary systems must be removed from the cryopressure tank Storage medium are exposed to be designed for two-phase operation of the storage medium, ie the cryogenic hydrogen, with the result that an (increased) fatigue due to boiling processes of the storage medium is taken into account.

An improved method for filling a cryogenic pressure tank according to the preamble of claim 1, with which at least one disadvantage of the prior art described in connection with US Pat. No. 6,708,502 B1 can be avoided, is the object of the present invention.

The solution to this problem is characterized in that the storage medium compacted after removal from the large reservoir and then introduced with supercritical pressure in the pressure accumulator, in particular cryogenic pressure tank. Advantageous developments are content of the dependent claims.

Basically, a refueling of the cryogenic accumulator with cryogenic storage medium is proposed at supercritical pressure by the from a so-called. Large reservoir, which is analogous to the previous liquid hydrocarbon fuels (such as gasoline or diesel) at a "gas station", substantially below If the said storage medium is hydrogen, it may preferably be compressed to a supercritical pressure of the order of magnitude of 13 bar or more (known per se) If the critical pressure for hydrogen is 12.8 bar.) With the completion of the filling process of the cryopressure tank, this results in a pressure level in the amount of said supercritical pressure value which is sufficiently high to reach a consumer, for example the one above called Fzg. -Antriebsaggregat od he a fuel cell easy and unproblematic to supply with the storage medium or hydrogen. The other disadvantages described above in connection with a cryogenic pressure accumulator as prior art can hereby be avoided or at least mitigated.

For the purposes of an advantageous development, the storage medium taken from the large storage container and preferably compressed by means of a cryogenic pump can be recooled substantially or as isobarically as possible, before it is introduced into the accumulator / cryopressure tank, i. By cooling, the temperature increase associated with the preceding compression is at least substantially reversed. In the case of hydrogen as a storage medium, preferably a recooling to a temperature in the order of 20 K. Advantageously, thereby the storage capacity of the cryopressor is significantly increased by the compressed hydrogen at supercritical pressure to the temperature level of subcritical liquid hydrogen (from 1 bar absolute and about 20 K) is cooled.

Regardless of the respective storage medium, it is particularly advantageous if the removed from the large reservoir and compressed to a supercritical pressure storage medium for recooling through a arranged in the large reservoir storage medium heat exchanger is performed. In this large Vorhaltsbehälter is on the one hand a sufficient cooling capacity available, on the other hand, the amount of heat introduced via this heat exchanger with regard to the compensation for the withdrawn amount of cryogenic storage medium is advantageous.

In the case of hydrogen as the cryogenic storage medium, the corresponding process steps in the appended FIG. 1, which are essentially chen a pressure-density diagram of the hydrogen shows, reproduced by way of example. Above the density in the unit "grams per liter (g / l)" is the associated absolute pressure in "bar" (= bara) is shown, which is explicitly pointed to the interruption in the ordinate, which is why the respective isotherms ( 293 K, 77 K, 50 K, 23 K, 20 K) are shown interrupted.

Liquid cryogenic hydrogen is thus withdrawn from a large storage tank (substantially below ambient pressure = 1 bar and at a temperature of 20 Kelvin) in a state according to point "a." This liquid cryogenic hydrogen is then as adiabatic as possible to a supercritical filling pressure of 20 bara compressed, so that the point "b" is reached. Preferably, as isobaric aftercooling or recooling then takes place essentially at the temperature of the liquid hydrogen present in the large reservoir, of the order of magnitude of about 20 Kelvin, so that a state according to point "c" has now been reached Alternatively, in order to obtain even higher densities, the hydrogen withdrawn from the large reservoir can be compressed from point "a" to, for example, 150 bara (point "d") and, in the second embodiment, to be filled into the cryopressor subsequently cooled as isobarically to about 20 K, whereby the point "e" is reached.

FIG. 2, which is explained below, shows a device for carrying out the method proposed here, by means of which this method will now be explained again in detail:

Reference numeral 1 denotes, for example, a large reservoir located at a gas station, in which cryogenic liquid hydrogen 41 Usually stored at ambient pressure (= 1 bara) and corresponding saturation temperature of 20.24 K. For this large reservoir 1, a marked by the reference numeral 12 cryopressure tank of a motor vehicle to be fueled, ie the latter is to be filled with cryogenic liquid hydrogen from the large Vorhaltsbehälter 1. For this purpose, a the cryosepressure tank 12 associated supply line 9, which is connected via a cold valve 10 with a filling line 1 1 opening within the cryogenic pressure tank 12, connected via a cryogenic pressure tank coupling 8 to a supply line 7 of the large reservoir 1.

However, said supply line 7, via which cryogenic hydrogen is ultimately discharged from the large storage tank 1, does not discharge directly into the cryogenic liquid hydrogen 24 stored in the large storage tank 1. Rather, the hydrogen 24 stored in the large storage tank 1 is discharged via a liquid removal line 2 taken from the large reservoir 1 and then in a preferably adiabatic cryogenic pump 3 to a supercritical pressure level, ie compressed or compressed over the pressure value 12.8 bara. Subsequently, by this compression or compression slightly heated liquid so-called cryogenic pressurized hydrogen either via valve 4 directly or via a valve 5, first through a heat exchanger 6 and then led thereto to the already mentioned supply line 7. In the heat exchanger 6, which is located within the large reservoir 1 in the stored therein cryogenic liquid hydrogen 24, which passed through the heat exchanger 6 so-called. Kryo-pressure hydrogen is substantially at the temperature level of the stored cryogenic liquid hydrogen, ie approximately cooled back to the above-mentioned saturation temperature of 20.24 K. Of course, the respective components, as far as necessary, are sufficiently isolated. Thus, the large reservoir 1 with the cryopump 3 and said valves 4, 5 is surrounded by an insulation 40. Of course, the supply line 7 and the clutch 8 and the supply line 9 are sufficiently isolated. Also, the cryogenic pressure tank 12 is provided as usual with a vacuum Superisolation 14, which surrounds the pressure tank 12, which receives the cryogenic hydrogen, and in turn in a pressure tank 12 correspondingly spaced enveloping vacuum-tight outer tank 13 is held.

If, at the beginning of a desired filling operation of the cryopressure tank 12, there is a residual amount of stored hydrogen that is not yet sufficiently relaxed and to increase the final refueling mass with warm gas content of the cryogenic pressure tank 12 before refilling, a pressure reduction can take place therein. ie a pressure equalization can be performed by at least a portion of said residual amount, i. generally residual storage medium, which / is usually in gaseous form, is discharged from the cryogenic pressure tank 12. This is done via a return gas line 15, which leads via a return gas valve 16 to an insulated line 17, the end of which has a return gas coupling 18.

An isolated feed line 19 can be connected to this return gas coupling 18, which feeds this residual gas back into the cryogenic liquid hydrogen 24 of the large storage container 1 via a so-called storage tank valve 20. By returning this residual gas or a part thereof, the pressure loss caused by the removal of liquid hydrogen for refueling in the large reservoir 1 can be at least partially compensated. In particular, in the case of exceeding the desired pressure in the large reservoir 1, excess residual gas via a valve 22 and a clutch 23 but also to a external consumers or recyclers, which may be, for example, a stationary fuel cell or a connected Drucktankpeichersys- system, are delivered.

The filling method presented here has the following advantages:

There are high storage densities achievable, for example, according to point "c" of Figure 1, a 3.3% higher density than when filling a cryogenic pressure accumulator with cryogenic liquid hydrogen at a pressure of 1 bar (absolute), or a 12.7% higher density than in today's conventional refueling according to the cited prior art with a maximum pressure of 4 bar (absolute) in a cryotank not designed for high pressure values When the method according to point "e" of FIG a 16.8% higher density when using a cryogenic pressure tank or a 31, 6% higher density than when refueling a customary today, not designed for high pressure values cryotank.

In particular, a pressure sufficient for the use of the cryogenic hydrogen (or storage medium) in an aggregate is available almost immediately following a filling of the cryogenic pressure tank. For example, a fuel cell requires a pressure level between 4 bar and 10 bar (absolute), while for a supercharged hydrogen internal combustion engine even a pressure level between 8 bar and 20 bar is needed.

Advantageously, a time-fast filling of the cryogenic pressure tank is possible, since no evaporation occurs with density jump and resulting rapid pressure increase when filling in a non-cold tank. This results in shorter refueling times and lower return gas volumes. gene, which are otherwise regarded by the gas station side as refueling losses. As stated here, an amount of residual gas recirculated to the filling station or into the large reservoir 1 can be used even more advantageously.

Finally, due to the presence of supercritical pressure, no phase transitions and a lower tendency to thermal stratification are observed. This results in both a lower material load of the cryopressure tank and its ancillary systems as well as a slower pressure build-up in the cryogenic pressure tank when the vehicle is parked for a long time and thus no hydrogen is removed.

In comparison to the first-mentioned prior art with a cryotank, which can absorb only slight overpressure, ie can be stored in the cryogenic hydrogen only up to a pressure level of about 4 bar, results with a inventively filled cryogenic pressure tank in addition the advantage a high loss-free life (over an average of 20 days) during which at most very small amounts of hydrogen must be released. This results from the fact that this cryopressure tank can hold a cryogenic storage medium up to pressure values of 300 bar or more: A clear improvement over simple cryotanks, which can only absorb slight overpressure, is already achieved with a cryopressure tank, the absolute pressure values can withstand the order of 150 bar, ie that the storage medium stored in the cryopressure tank can accept pressure values of up to 150 bar before a blow-off to reduce excess pressure values must be initiated. Advantageously, when using a cryogenic pressure tank instead of a simple practically not overpressure-resistant cryogenic tank, even after a long lossy service life depending on Abblasedruck always a sufficient amount of hydrogen in the tank, so that a vehicle equipped with this vehicle can still be moved sufficiently far. To ensure the highest storage density and sufficient pressure availability for the stored cryogenic hydrogen or generally for the cryogenic storage medium so hereby a quasi lossless cryopressure (cryogenic pressure accumulator) with sufficiently long loss-free life at the same sownevorgang free extraction operation, stand mode and especially refueling proposed which is made possible by the fact that a refueling of a cryogenic pressure tank with cryogenic storage medium is carried out at supercritical pressure, wherein it should be noted that quite a variety of details may be deviated from the above explanations, without departing from the content of the claims.

Claims

claims

1. A method for filling a for a cryogenic storage medium, in particular cryogenic hydrogen, provided pressure accumulator, in particular a cryogenic pressure tank (12) of a motor vehicle, in which the from a large reservoir (1) taken substantially under ambient pressure with corresponding saturation temperature in the liquid state Cryogenic storage medium (24) under absolute pressure values in the order of 150 bar or more can be stored, characterized in that the storage medium after removal from the large reservoir (1) compressed and then with supercritical pressure in the pressure accumulator, in particular cryo-pressure tank (12 ) is introduced.

2. The method according to claim 1, characterized in that the compressed storage medium is recooled before being introduced into the duck storage / cryopressure tank (12).

3. The method according to claim 2, characterized in that the storage medium for recooling through a in the large reservoir (1) mounted storage medium arranged heat exchanger (6) is guided.

4. Method according to one of the preceding claims, characterized in that the compression of the removed from the large reservoir storage medium by means of a cryopump (3).

5. The method according to any one of the preceding claims, characterized in that in the case of hydrogen storage medium as a compression to a supercritical pressure in the order of 13 bar or more.

6. The method according to any one of the preceding claims, characterized in that in the case of hydrogen storage medium as a re-cooling to a temperature in the order of 20 K takes place.

7. The method according to any one of the preceding claims, characterized in that prior to filling with new storage medium in the pressure storage / cryopressure tank (12) contained residual storage medium is discharged from the pressure accumulator.

8. The method according to claim 8, characterized in that the discharged residual storage medium in the large reservoir (1) is recycled.

9. The method according to claim 8, characterized in that the discharged residual storage medium is supplied to a consumer or a pressure tank storage system.