A system having at least two cryogenic containers for providing a fluid

Abstract

The invention relates to a system for providing a fluid, comprising at least a first and a second cryogenic container for storing the fluid, wherein the system comprises a first retrieval line connecting to the first cryogenic container for retrieving a first mass flow (M1) of fluid and a second retrieval line connecting to the second cryogenic container for retrieving a second mass flow (M2) of fluid, wherein the system comprises means, which are configured to establish two mass flows (M1, M2) of different dimensions such that in a first operational mode a hold time of the two cryogenic containers converges upon retrieval and/or in a second operational mode the hold time of the two cryogenic containers essentially decreases at the same rate if the hold times of the two cryogenic containers are essentially equal.

Claims

1.-15. (canceled)

16. A system for providing a fluid, comprising at least a first and a second cryogenic container for storing the fluid, wherein the system comprises a first retrieval line connecting to the first cryogenic container for retrieving a first mass flow (M1) of fluid and a second retrieval line connecting to the second cryogenic container for retrieving a second mass flow (M2) of fluid, characterized in that the system comprises means, which are configured to establish two mass flows (M1, M2) of different dimensions such that in a first operational mode a hold time of the two cryogenic containers converges upon retrieval and/or in a second operational mode the hold time of the two cryogenic containers essentially decreases at the same rate if the hold times of the two cryogenic containers are essentially equal, wherein the hold time is the period of time from a termination of retrieval until that point of time, when the pressure in the cryogenic container (7, 8) reaches a predefined threshold.

17. A system according to claim 16, wherein the first cryogenic container has a container volume (V1), which is larger than a container volume (V2) of the second cryogenic container.

18. A system according to claim 16, wherein the first cryogenic container has a container volume (V1), which is equal to a container volume (V2) of the second cryogenic container, wherein the retrieval lines have a different flow resistance.

19. A system according to claim 16, wherein the means comprise a control unit, which is configured to regulate the first and/or the second mass flow (M1, M2).

20. A system according to claim 19, wherein the control unit is configured to retrieve in the first operational mode fluid only from the first cryogenic container until the hold time of the first cryogenic containers substantially corresponds to the hold time of the second cryogenic container.

21. A system according to claim 19, wherein the control unit is configured to retrieve in the first operational mode only such an amount of fluid from the second cryogenic container such that the hold time of the second cryogenic container remains constant and to retrieve the remaining fluid from the first cryogenic container.

22. A system according to claim 21, wherein the system is configured to retrieve in the first operational mode fluid only in a gaseous state from the second cryogenic container.

23. A system according to claim 16, wherein the control unit is configured to switch from the first into the second operational mode upon reaching the same hold time of the two cryogenic containers for the first time.

24. A system according to claim 19, wherein the control unit is configured to select, when retrieving the fluid from the first and/or the second cryogenic container, between a retrieval of the fluid in a liquid phase and/or a retrieval of the fluid in a gaseous phase.

25. A system according to claim 19, wherein the system may be transferred into a third operational condition and wherein the system in the third operational mode is configured to establish during operation a different operating pressure in the two cryogenic containers and to retrieve fluid only from the first or only from the second cryogenic container.

26. A system according to claim 19, wherein the system may be transferred into a fourth operational condition if the pressure in the two cryogenic containers lies between an operating pressure and the threshold mentioned and wherein the system in the fourth operational mode is configured to select the two mass flows (M1, M2) in such a way that the pressure in the two cryogenic containers is reduced to the operating pressure and, thereby, the hold time of the two cryogenic containers converges and/or increased essentially at the same rate upon retrieval.

27. A system according to claim 26, wherein fluid in the fourth operational mode is only retrieved in the gaseous phase from the cryogenic containers.

28. A system according to claim 19, wherein the control unit comprises a computing unit, which is configured to calculate the current hold time of the first and/or of the second cryogenic container.

29. system (1) according to claim 19, wherein the system comprises a first valve in the first retrieval line and a second valve in the second retrieval line, and wherein the control unit is configured to control the valves to adjust the first and the second mass flow (M1, M2).

30. A system according to claim 19, wherein the system comprises a measuring unit, which is configured to measure a current fill level of the fluid and/or a current operating pressure of the fluid in the first cryogenic container and/or in the second cryogenic container and to send it to the control unit.

Description

[0029] Advantageous and non-limiting embodiments of the invention are explained in greater detail below by way of the drawings.

[0030] FIG. 1 shows a motor vehicle, on which the system according to the invention is mounted.

[0031] FIG. 2 shows a diagram, in which the hold time is plotted in relation to a current container fill level of two different cryogenic containers.

[0032] FIG. 1 shows a motor vehicle 1 having a support frame 2 and two axles 3, 4. A cryogenic container 7, 8 is mounted on each side 5, 6 of the support frame 2 between the axles 3, 4. The cryogenic containers 7, 8 each store fluid, for example liquefied natural gas, which has also been known to those skilled in the art as LNG (“Liquid Natural Gas”). The fluid is present in the cryogenic containers 7, 8 in both liquid as well as gaseous state. If the cryogenic containers 7, 8 are used in connection with a motor vehicle 1, the stored fluid may, for example, serve as fuel for an engine of the motor vehicle 1. In other embodiments, however, the cryogenic containers 7, 8 could also be provided in other fields of application.

[0033] In the present system 1, the first cryogenic container 7 has a container volume V1, which is larger than a container volume V2 of the second cryogenic container 8. Such configurations may be useful, for example, if different amounts of installation space are available on the two sides 5, 6 of the motor vehicle 1.

[0034] When the system 1 is in operation, the pressure in the cryogenic containers 7, 8 is, for example, between 6 and 8 bar. This pressure may be regulated, for example, by retrieving fluid or by a heat exchanger projecting into the respective cryogenic container. However, as soon as the system 1 is no longer in operation, i.e. is switched off, the pressure in the cryogenic containers 7, 8 will increase steadily due to a constant heat input into the cryogenic containers 7, 8.

[0035] In order to prevent excessive pressure in the cryogenic containers 7, 8 and thus failure thereof, both the first cryogenic container 7 and the second cryogenic container 8 each have a pressure control valve 9, 10, which is connected directly or indirectly to the respective cryogenic container 7, 8 via a connection line. The pressure control valves 9, 10 are activated at a predetermined pressure, which is for example 16 bar, thereby discharging fluid in a gaseous state. Usually, both pressure control valves 9, 10 are activated at the same predetermined pressure, wherein there may also be provided that they may be activated at different pressures.

[0036] The period of time from the termination of retrieval until the point of time when the pressure in the cryogenic container 7, 8 reaches a predefined threshold value is referred to as the so-called hold time. It is understood that the hold time of the two cryogenic containers 7, 8 should be as high as possible, as discharged fluid represents an economic loss and an environmental impact.

[0037] The hold time of the respective cryogenic container 7, 8 is calculated, among other things, from the container volume V1, V2, as a larger container surface also means a larger heat input. Furthermore, the hold time depends on the current volume of fluid in the cryogenic container 7, 8 and the pressure difference between the activation pressure of the respective pressure control valve 9, 10 and the operating pressure present in the respective cryogenic container 7, 8 at the point of termination of retrieval. For the calculation of the hold time, there may be assumed a predetermined ambient temperature of the cryogenic containers 7, 8 or a predetermined heat input into the cryogenic container 7, 8, respectively. However, the ratio of the hold time of the two cryogenic containers 7, 8 depends only insignificantly on the ambient temperature.

[0038] FIG. 2 shows a diagram, in which the current volume of fluid in relation to the total container volume is plotted on the horizontal axis and the hold time in days on the vertical axis. HT1 presents the course of the hold time of a first cryogenic container 7 having a container volume V1 of 5001 l. HT2 presents the course of the hold time of a cryogenic container 8 having a container volume V2 of 300 l. It is obvious that the first cryogenic container 7 having the larger container volume V1 has a longer hold time than the second cryogenic container 8 having the smaller container volume V2 at the same fill level. In the example depicted, a minimum hold time of four days was selected.

[0039] In order to retrieve fluid from the cryogenic containers 7, 8 for operation, the system 1 comprises a first retrieval line 11 connecting to the first cryogenic container 7 for retrieving a first mass flow M1 of fluid as well as a second retrieval line 12 connecting to the second cryogenic container 8 for retrieving a second mass flow M2 of fluid.

[0040] The system 1 according to the invention comprises means 13, which are configured to establish the two mass flows M1, M2 of different dimensions. This is used with the aim of reaching a possibly long time following the termination of retrieval, at which one of the two pressure control valves 9, 10 will be activated. This is achieved when the hold time of the two cryogenic containers 7, 8 is essentially equal during retrieval.

[0041] The means 13 may, for example, be configured as a control unit 14, which controls the mass flows M1, M2. This may be achieved, for example, via valves 15, 16, which are arranged in each of the retrieval lines 11, 12 and are controlled by the control unit 14. In alternative embodiments, however, the means 13 may also comprise only a rigid throttle in one of the retrieval lines 11, 12.

[0042] In order to determine how large the mass flows should be in the first or second operational mode, respectively, the control unit 14 may comprise a computing unit, which is configured to calculate the current hold time of the first and/or of the second cryogenic container 7, 8. For this purpose, the system 1 may in particular comprise a measuring device, which is configured to measure a current volume of the fluid and/or a current operating pressure of the fluid in the first cryogenic container 7 and/or in the second cryogenic container 8 and to send this to the control unit 14. Other values could also be measured and sent to the control unit 14 in order to control the mass flows M1, M2 even more efficiently.

[0043] Alternatively, the control unit 14 may also carry out the control of the mass flows M1, M2 without direct measurements of the fluid in the cryogenic containers 7, 8, for example by controlling the mass flows M1, M2 according to a predetermined scheme, for example also depending on the retrieval time or an expected retrieval volume, respectively, of the fluid.

[0044] The system 1 may be operated in a first operational mode and/or in a second operational mode by way of the means 13 mentioned, by way of which the mass flows M1, M2 may be established of different sizes. In the first operational mode, the mass flows M1, M2 are adjusted in such a way that the hold time of the two cryogenic containers 7, 8 converges. In the second operational mode, the mass flows M1, M2 may be adjusted such that the hold time of the two cryogenic containers 7, 8 decreases substantially at the same rate when the hold times of the two cryogenic containers 7, 8 are substantially equal.

[0045] In principle, there might be provided that the system 1 is only operated in the first or only in the second operational mode. For example, if the system 1 is only operated in the first operational mode and the mass flows M1, M2 are set to the same value after reaching an equal hold time, the hold time will again diverge. In some cases, however, this divergence may be accepted, for example if a simplified control is to be achieved. The system 1 could also be reset to the first operational mode if the divergence exceeds a threshold value.

[0046] If the system 1 is configured to be operated in both the first and second operational condition, the system 1 is preferably first operated in the first operational condition until the hold time of the two cryogenic containers 7, 8 is substantially equal. Thereafter, the system 2 is operated in the second operational condition such that the hold time of the two cryogenic containers 7, 8 decreases substantially at the same rate. If the hold time of the two cryogenic containers 7, 8 deviates again, for example due to external influences, it is possible to switch back to the first operational mode until the hold time of the two cryogenic containers 7, 8 is again of an equal dimension and then switch to the second operational mode.

[0047] As a rule, fluid is retrieved in the first and/or in the second operational condition in the liquid phase in order to achieve the highest possible power. Fluid, however, may also be retrieved in the gaseous phase in the first and/or second operational condition, whereby an economiser function may be achieved, for example. When setting the mass flows M1, M2, the control unit 14 may also select whether fluid is to be retrieved in the gaseous phase or in the liquid phase in order to achieve a converging or constant, respectively, hold time.

[0048] With regard to FIG. 2, this will now be explained by way of a practical example. If both cryogenic containers 7, 8 are full, fluid would first be retrieved only from the 500 l cryogenic container 7 until this has a remaining hold time of 7.2 days corresponding to the hold time of the full 300 l cryogenic container 8. In general, however, even in the first operational mode, just enough fluid, preferably in the gaseous phase, may be retrieved from the small cryogenic container 8 to keep the pressure therein constant, thus achieving a maximum hold time of this cryogenic container 8.

[0049] For example, a full cryogenic container 7 with 500 l holds approx. 160 kg of LNG. A 300 l cryogenic container 8, on the other hand, holds approx. 95 kg of LNG. In the first operational mode, the first 65 kg are only retrieved from the 500 l cryogenic container 7. During this time, just enough is retrieved from the 300 l cryogenic container 8 to keep the pressure constant.

[0050] If the travel is continued after the 65 kg of LNG have been retrieved, there will take place a switch into the second operational mode and the retrieval rates of the two cryogenic containers 7, 8 will be adjusted such that both cryogenic containers 7, 8 have the same remaining hold time.

[0051] Subsequently, the hold time is reduced by one day if approx. 18.5 kg LNG is removed from the 500 l cryogenic container 7 and approx. 16.7 kg LNG is retrieved from the 300 l cryogenic container 8. Thus, while complying with this retrieval ratio, the 500 l cryogenic container 7 may be operated up to a residual quantity of approx. 34 kg LNG (fill level 13%) and the 300 l cryogenic container 8 up to a residual quantity of 42 kg (fill level 37%). At these fill levels, the minimum hold time of four days is achieved.

[0052] In total, this results in a usable LNG content of 160 kg-34 kg=126 kg for the 500 l cryogenic container 7. For the 300 l cryogenic container 8, this results in a usable LNG content of 95 kg-42 kg=53 kg. In total, the system 1 has a usable LNG content of 126 kg+53 kg=179 kg.

[0053] Without adjusting the mass flows M1, M2, only two times 53 kg=106 kg may be used for the remaining hold time of four days, as each additional mass retrieved reduces the hold time of the smaller cryogenic container 8 to fewer than four days.

[0054] A further increase of the usable cryogenic container content may be achieved if, furthermore, a third operational state is provided. In this case, the smaller cryogenic container 8 may be reduced to a lower pressure in order to achieve a hold time of four days again due to the larger pressure interval thus created. This is possible if, towards the end of the planned travel, the motor in part-load operation is supplied with lower pressures from the smaller cryogenic container 8. For this purpose, the fuel must be retrieved either from the one or the other cryogenic container 7, 8, since a simultaneous retrieval would lead to a pressure balance between the cryogenic containers 7, 8. In order to control from which cryogenic container 7, 8 fluid is retrieved, a travel distance profile may be used, which may be read from a pre-recorded map, for example.

[0055] If, for example, the pressure of the 300 l cryogenic container is reduced from 8 bar to 6 bar in the third operational mode, the residual mass for 4 days hold time is reduced by 16 kg from 42 kg to 26 kg and the total usable mass increases from 179 kg to 195 kg.

[0056] The third operational mode may either be controlled by the control unit 14 mentioned above or by a separate control unit, which may be connected to the control unit 14 mentioned above. In particular, heat exchangers that project into the respective cryogenic container 7, 8 may be controlled to set the pressure.

[0057] There could also be provided a fourth operational mode, which is selected, for example, as a start mode when the pressure in one or in both cryogenic containers 7, 8 is above a desired operating pressure. In prior art, as much fluid as possible is retrieved from both cryogenic containers 7, 8 for this purpose such that the operating pressure is reached as quickly as possible. In the present fourth operational mode, the mass flows M1, M2 in this fourth operational mode may be of different dimensions, on the one hand, in order to reach the operating pressure quickly, but, on the other hand, also in order to achieve a converging or constant, respectively, hold time. In the fourth operational mode, fluid is usually retrieved in the gaseous phase. However, fluid could also be retrieved in the liquid phase if this helps to achieve a converging or constant, respectively, hold time.

[0058] In further embodiments, the cryogenic containers 7, 8 may also have the same volume V1, V2. Setting different mass flows M1, M2 may be advantageous if the fill level of the two cryogenic containers is of a different dimension, as this will result in a different hold time of the two cryogenic containers 7, 8. In embodiments with cryogenic containers 7, 8 of different dimensions, there is usually not provided a second operational mode, i.e. in some embodiments the second operational mode is only provided if the cryogenic containers have a different container volume V1, V2.