Process for refilling a gas tank and gas supply system

Abstract

A process for filling a gas tank made from a gas tank material with gas is provided, which process comprises the following steps: a) setting (S10) a nominal gas filling rate such that the tank is substantially completely filled within a predetermined filling time from a predetermined initial gas pressure value, b) determining (S20), assuming hot case tank conditions, a maximum mass-averaged gas filling temperature that will be reached at the end of the filling process, when filling the gas tank for the predetermined filling time with the nominal gas filling rate, c) selecting (S30) a target gas filling temperature not greater than the maximum mass-averaged gas filling temperature, d) cooling (S40) the gas to be supplied to the gas tank to the target gas filling temperature, e) starting the supply of gas to the gas tank, f) determining (S50) the actual mass-averaged gas filling temperature of the gas supplied to the tank, g) estimating (S60) an end-of-fill gas pressure from the actual mass-averaged gas filling temperature assuming cold case tank conditions, and h) terminating (S70) the supply of gas to the gas tank when the actual pressure of the gas tank is equal to the lower of the end-of-fill gas pressure and a maximum final fill pressure.

Claims

1. A process for filling a gas tank made from a gas tank material with gas, the process comprising the steps: a) setting a nominal gas filling rate such that the gas tank is substantially completely filled within a predetermined filling time from a predetermined initial gas pressure value, b) determining, assuming hot case tank conditions, a maximum mass-averaged gas filling temperature that will be reached at an end of a filling process, when filling the gas tank for the predetermined filling time with the nominal gas filling rate, c) selecting a target gas filling temperature not greater than the maximum mass-averaged gas filling temperature, d) cooling the gas to be supplied to the gas tank to the target gas filling temperature, e) starting supply of gas to the gas tank, f) determining an actual mass-averaged gas filling temperature of the gas supplied to the gas tank, g) estimating an end-of-fill gas pressure from the actual mass-averaged gas filling temperature assuming cold case tank conditions, and h) terminating the supply of gas to the gas tank when an actual pressure of the gas tank is equal to a lower of the end-of-fill gas pressure and a maximum final fill pressure.

2. The process according to claim 1, wherein the step b) further comprises the steps: ba) determining the maximum mass-averaged gas filling temperature by means of a thermal behavior simulation tool which predicts a temperature increase of the gas in the gas tank and/or the gas tank material by iterations; and bb) providing a look-up table of the maximum mass-averaged gas filling temperature for each possible ambient temperature and for each possible value of initial gas pressure in the gas tank.

3. The process according to claim 2, wherein the step b) further comprises the steps: bc) determining, on the basis of an analytical model including adiabatic boundary conditions at the transition from the gas volume of the gas tank to the gas tank material, a relation between enthalpy of the gas added to the gas tank with a specific filling rate and a temperature of the gas in the gas tank and/or of the gas tank material at a time by which the gas has been added to the gas tank, assuming the hot case tank conditions; and bd) determining the maximum mass-averaged gas filling temperature from a predetermined maximum allowable temperature of the gas in the gas tank or the gas tank material at the end of the filling process, assuming the hot case tank conditions and using the relation determined in the step bc).

4. The process according to claim 3, wherein the step b) further comprise be) determining, on the basis of the analytical model including adiabatic boundary conditions at the transition from the gas volume of the gas tank to the gas tank material, a relation between temperature increase of the gas in the gas tank under the adiabatic boundary conditions and temperature increase of the gas tank material and/or of the gas in the gas tank; and bf) determining the maximum mass-averaged gas filling temperature from the predetermined maximum allowable temperature of the gas in the gas tank or of the gas tank material at the end of the filling process, assuming the hot case tank conditions and using the relation determined in the step be).

5. The process according to claim 4, wherein the step b) further comprises bg) determining a maximum allowable gas temperature under the adiabatic boundary conditions from a maximum allowable temperature increase of the gas tank material which is obtainable assuming the hot case tank conditions from the predetermined maximum allowable temperature of the gas tank material at the end of the filling process when filling the gas tank for the predetermined filling time with the nominal gas filling rate; bh) determining an initial specific internal energy of the gas in the gas tank for the hot case tank conditions; bi) determining, assuming that the gas tank is filled to a predetermined state of charge, under the adiabatic boundary conditions, a maximum specific internal energy of the gas for the determined maximum allowable gas temperature under the adiabatic boundary conditions; bj) determining, by applying the first law of thermodynamics, a maximum specific enthalpy of the gas to be added to the gas tank such that the specific internal energy of the gas at the end of the filling process does not exceed the maximum specific internal energy of the gas; and bk) determining the maximum mass-averaged gas filling temperature on the basis of the maximum specific enthalpy.

6. The process according to claim 4, wherein a relation between a gas temperature increase in the gas tank under the adiabatic boundary conditions and a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material is defined by the following equation:
T.sub.J/T.sub.A=a.sub.JP.sub.i+b.sub.J, where T.sub.A is a temperature increase of the gas in the gas tank under the adiabatic boundary conditions, T.sub.J is a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material with J denoting the gas or the gas tank material, a.sub.J and b.sub.J are fixed parameters specific to tank characteristics and a predetermined filling time, and P.sub.i is an initial gas pressure in the gas tank.

7. The process according to claim 4, wherein a relation between a gas temperature increase in the gas tank under the adiabatic boundary conditions and a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material is defined by the following equation: $T_J/T_A=\left[\left({\mathrm{aa}}_1/{}_3+{\mathrm{ba}}_1\right)\log \left({}_1/c_1{}_3\right)+\left({\mathrm{aa}}_0/{}_3+{\mathrm{ba}}_0\right)\right]/{\left({}_2/c_1{}_3\right)}^{0.5}+\left[\left({\mathrm{ab}}_1/{}_3+{\mathrm{bb}}_1\right)\log \left({}_1/c_1{}_3\right)+\left({\mathrm{ab}}_0/{}_3+{\mathrm{bb}}_0\right)\right]$ where T.sub.A is a temperature increase of gas in the gas tank under the adiabatic boundary conditions, T.sub.J is a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material with J denoting the gas or the gas tank material, and .sub.1, .sub.2 and .sub.3 are parameters defined as .sub.1=T.sub.ft.sub.f.sup.0.5/P.sub.i, .sub.2=hT.sub.ft.sub.f/P.sub.i, .sub.3=P.sub.n/P.sub.i, with T.sup.f denoting a gas filling temperature, P.sub.i denoting an initial gas pressure in the gas tank, h denoting a gas-to-tank material heat transfer coefficient, t.sub.f denoting a predetermined filling time, P.sub.n denoting a nominal fill pressure, denoting a tank material effusivity and denoting a tank vessel volume to internal area ratio, and aa.sub.1, ba.sub.1, aa.sub.0, ba.sub.0, ab.sub.1, bb.sub.1, ab.sub.0, bb.sub.0, and c.sub.1 are constants.

8. The process according to claim 1, wherein the step f) further comprises: fa) measuring an actual temperature of the gas supplied to the gas tank, and fb) estimating the actual mass-averaged gas filling temperature on the basis of the amount of gas supplied to the gas tank in each time interval since starting the supply of gas to the gas tank and the respectively measured temperature of the gas supplied.

9. The process according to claim 1, wherein the step g) further comprises: ga) determining the end-of-fill gas pressure by means of a thermal behavior simulation tool predicts a temperature increase of the gas in the gas tank and/or of the gas tank material by iterations; and gb) providing a look-up table for the end-of-fill gas pressure for each possible value of applied mass-averaged gas filling temperatures and for each possible value of initial gas pressures in the gas tank.

10. The process according to claim 9, wherein determining the end-of-fill gas pressure in step g) comprises the following steps: gc) determining, on the basis of an analytical model including adiabatic boundary conditions at the transition from the gas volume of the gas tank to the gas tank material, a relation between temperature increase of the gas in the gas tank under the adiabatic boundary conditions and a temperature increase of the gas tank material and/or of the gas in the gas tank; gd) estimating, assuming that the maximum mass-averaged gas filling temperature is the actual mass-averaged gas filling temperature at the end of the filling process and by applying the first law of thermodynamics, the end-of-fill gas pressure based on the relation determined in the step gc) such that a predetermined state of charge in the gas tank is achieved at the end of the filling process.

11. The process according to claim 10, wherein a relation between a gas temperature increase in the gas tank under the adiabatic boundary conditions and a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material is defined by the following equation:
T.sub.J/T.sub.A=a.sub.JP.sub.i+b.sub.J, where T.sub.A is a temperature increase of the gas in the gas tank under the adiabatic boundary conditions, T.sub.J is a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material with J denoting gas or gas tank material, a.sub.J and b.sub.J are fixed parameters specific to tank characteristics and the predetermined filling time, and P.sub.i is an initial gas pressure in the gas tank.

12. The process according to claim 10, wherein a relation between a gas temperature increase in the gas tank under the adiabatic boundary conditions and a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material is defined by the following equation: $T_J/T_A=\left[\left({\mathrm{aa}}_1/{}_3+{\mathrm{ba}}_1\right)\log \left({}_1/c_1{}_3\right)+\left({\mathrm{aa}}_0/{}_3+{\mathrm{ba}}_0\right)\right]/{\left({}_2/c_1{}_3\right)}^{0.5}+\left[\left({\mathrm{ab}}_1/{}_3+{\mathrm{bb}}_1\right)\log \left({}_1/c_1{}_3\right)+\left({\mathrm{ab}}_0/{}_3+{\mathrm{bb}}_0\right)\right]$ where T.sub.A is a temperature increase of gas in the gas tank under the adiabatic boundary conditions, T.sub.J is a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material with J denoting gas or gas tank material, and .sub.1, .sub.2 and .sub.3 are parameters defined as .sub.1=T.sub.ft.sub.f.sup.0.5/P.sub.i, .sub.2=hT.sub.ft.sup.f/P.sub.i, .sub.3=P.sub.n/P.sub.i, with T.sub.f denoting a gas filling temperature, P.sub.i denoting an initial gas pressure in the gas tank, h denoting a gas-to-tank material heat transfer coefficient, t.sub.f denoting a predetermined filling time, P.sub.n denoting a nominal fill pressure, denoting a tank material effusivity and denoting a tank vessel volume to internal area ratio, and aa.sub.1, ba.sub.1, aa.sub.0, ba.sub.0, ab.sub.1, bb.sub.1, ab.sub.0, bb.sub.0, and c.sub.1 are constants.

13. The process according to claim 1, wherein the step g) further comprises determining whether information transmitted from the gas tank to be filled is available and, if the information is available, determining a state of charge in the gas tank to be achieved at the end of the filling process based on the information, and determining the end-of-fill gas pressure and/or the maximum final fill pressure further based on the state of charge determined.

14. The process according to claim 5, wherein a relation between a gas temperature increase in the gas tank under the adiabatic boundary conditions and a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material is defined by the following equation:
T.sub.J/T.sub.A=a.sub.JP.sub.i+b.sub.J, where T.sub.A is a temperature increase of the gas in the gas tank under the adiabatic boundary conditions, T.sub.J is a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material with J denoting gas or gas tank material, a.sub.J and b.sub.J are fixed parameters specific to tank characteristics and a predetermined filling time, and P.sub.i is an initial gas pressure in the gas tank.

15. The process according to claim 5, wherein a relation between a gas temperature increase in the gas tank under the adiabatic boundary conditions and a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material is defined by the following equation: $T_J/T_A=\left[\left({\mathrm{aa}}_1/{}_3+{\mathrm{ba}}_1\right)\log \left({}_1/c_1{}_3\right)+\left({\mathrm{aa}}_0/{}_3+{\mathrm{ba}}_0\right)\right]/{\left({}_2/c_1{}_3\right)}^{0.5}+\left[\left({\mathrm{ab}}_1/{}_3+{\mathrm{bb}}_1\right)\log \left({}_1/c_1{}_3\right)+\left({\mathrm{ab}}_0/{}_3+{\mathrm{bb}}_0\right)\right]$ where T.sub.A is a temperature increase of gas in the gas tank under the adiabatic boundary conditions, T.sub.f is a temperature increase of the gas in the gas tank or a temperature increase of the gas tank material with J denoting gas or gas tank material, and .sub.1, .sub.2 and .sub.3 are parameters defined as .sub.1=T.sub.ft.sub.f.sup.0.5/P.sub.i, .sub.2=hT.sub.ft.sup.f/P.sub.i, .sub.3=P.sub.n/P.sub.i, with T.sub.f denoting a gas filling temperature, P.sub.i denoting an initial gas pressure in the gas tank, h denoting a gas-to-tank material heat transfer coefficient, t.sub.f denoting a predetermined filling time, P.sub.n denoting a nominal fill pressure, denoting a tank material effusivity and denoting a tank vessel volume to internal area ratio, and aa.sub.1, ba.sub.1, aa.sub.0, ba.sub.0, ab.sub.1, bb.sub.1, ab.sub.0, bb.sub.0, and c.sub.1 are constants.

16. The process according to claim 5, wherein the step g) further comprises determining whether information transmitted from the gas tank to be filled is available and, if the information is available, determining a state of charge in the gas tank to be achieved at the end of the filling process based on the information, and determining the end-of-fill gas pressure and/or the maximum final fill pressure further based on the state of charge determined.

17. The process according to claim 8, wherein the step g) further comprises determining whether information transmitted from the gas tank to be filled is available and, if the information is available, determining a state of charge in the gas tank to be achieved at the end of the filling process based on the information, and determining the end-of-fill gas pressure and/or the maximum final fill pressure further based on the state of charge determined.

18. The process according to claim 10, wherein the step g) further comprises determining whether information transmitted from the gas tank to be filled is available and, if the information is available, determining a state of charge in the gas tank to be achieved at the end of the filling process based on the information, and determining the end-of-fill gas pressure and/or the maximum final fill pressure further based on the state of charge determined.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 shows the strict limits required in the standard protocol on the filling temperature and the filling pressure.

(2) FIG. 2 illustrates, for a given initial gas tank temperature, the maximum mass-averaged gas filling temperature which enables the temperature of the gas in the tank and of the liner material of the gas tank not to exceed 85 C. at the end of the filling process for a Type 4 hydrogen gas tank with the initial gas pressure of the gas in the tank being 0.5 MPa and the tank being filled for 90 seconds.

(3) FIG. 3A illustrates the dependency of the ratio between gas temperature increase under the adiabatic conditions and temperature increase of the gas or the gas tank material on the gas filling temperature.

(4) FIG. 3B illustrates the dependency of the ratio between gas temperature increase under the adiabatic conditions and temperature increase of the gas or the gas tank material on the initial gas tank temperature.

(5) FIG. 3C illustrates the dependency of the ratio between gas temperature increase under the adiabatic conditions and temperature increase of the gas or the gas tank material on the initial gas pressure in the tank.

(6) FIG. 4 shows exemplarily an experimental validation of a model according to an embodiment of the present invention by comparing gas/liner temperature at the end of a filling process computed using the model and actually measured for a Type 3 hydrogen gas tank.

(7) FIG. 5 shows a block diagram of an embodiment of the process for filling a gas tank according to the present invention. For the sake of simplicity, the embodiment is explained with an example of a hydrogen gas tank of Type 4.

(8) FIG. 6 shows a block diagram of another embodiment of the process for filling a gas tank according to the present invention. For the sake of simplicity, the embodiment is explained with an example of a hydrogen gas tank of Type 4.

DESCRIPTION OF EMBODIMENTS

(9) FIG. 4 exemplarily shows an experimental validation of a model according to an embodiment of the present invention.

(10) In FIG. 4, the temperature of the gas and of the gas tank material (i.e. liner) at the completion of filling processes, i.e. when the predetermined filling time of 3 minutes ended for a Type 3 hydrogen tank having volume of 401, actually measured and calculated using a model according to an embodiment of the present invention are demonstrated for various initial gas tank temperature (i.e. various ambient temperature) and for three different gas filling temperature profiles, namely, filling with hydrogen having a constant filling temperature of 20 C. which is denoted with Tgas for the gas temperature and Tlin for the liner temperature; filling with hydrogen having a temperature of 0 C. for the first 1.5 minutes and then with hydrogen having a temperature of 40 C. for the next 1.5 minutes, denoted with Tg[0/40] for the gas temperature and T1[0/40] for the liner temperature; filling with hydrogen having a temperature of 40 C. for the first 1.5 minutes and then with hydrogen having a temperature of 0 C. for the next 1.5 minutes, denoted with Tg[40/0] for the gas temperature and T1[40/0] for the liner temperature. In the calculation to obtain the gas or the liner temperature, the average filling temperature is used as a constant mass average gas filling temperature for the sake of simplicity, and Equations (1) and (3) above are applied with the fixed parameters a.sub.J and b.sub.J being determined by linear regression using data available to the public.

(11) As can be seen from FIG. 4, the calculated temperature agrees with the measured temperature sufficiently well. This provides not only the confirmation to the applicability of the model, but also to the observation that for a given tank design and given initial conditions, only the average filling temperature (i.e. the enthalpy input) needs to be controlled to prevent a predetermined maximum allowable gas or gas tank material temperature from being exceeded, but not the temperature profile throughout the filling process.

(12) FIG. 5 shows a block diagram of an embodiment of the process for filling a gas tank according to the present invention. For the sake of simplicity, the embodiment is explained with an example of a hydrogen gas tank of Type 4. The embodiment is applicable to other gas to be filled in a gas tank as well as other types of gas tank such as Type 2 or Type 3.

(13) According to FIG. 5, the process starts with fixing a nominal filling rate which may be applied to any gas filling process (step S10).

(14) The nominal filling rate may be determined as the filling rate which provides the tank being substantially completely filled within a predetermined filling time t.sub.fill, e.g. 3 minutes from a predetermined initial gas pressure value, e.g. 0.5 MPa.

(15) In step S20, the hot case conditions may be assumed and the initial gas tank temperature and the initial gas pressure of the tank may be set accordingly for computing the mass-averaged gas filling temperature so as to avoid the overheating of the gas tank at the end of the filling process. Thus, it may be conservatively assumed that the temperature of the gas in the gas tank is higher than the ambient temperature by a certain value, e.g. 10 C. The ambient temperature may be measured by a gas refilling station. Further, for taking into account the possibility that the gas tank may have undergone an aborted fill just before the fill being carried out, the initial gas tank pressure value may be set to be a lowest allowable one, e.g. 0.5 MPa, in order to determine a maximum filling temperature for the fill as if it was the continuation of a hypothetical aborted fill starting from the lowest possible initial tank pressure.

(16) Then, under the hot case conditions, the maximum mass-averaged gas filling temperature MAT.sub.max that may be applied such that the material temperature may not exceed the maximum allowable temperature, e.g. 85 C. at the end of filling process may be determined.

(17) The maximum mass-averaged gas filling temperature MAT.sub.max may be determined by means of any known thermal behavior simulation tool which allows to predict the temperature increase of the gas in the tank and/or gas tank material by iterations: The simulation tool may comprise a numerical model calculating the temperature of the gas together with the transient heat conduction into the wall of the gas tank vessel, assuming homogeneous temperature of the gas and unidirectional heat transfer in the wall, e.g. composed of two layers of the liner and the wrapping in composite material. For each possible constant filling temperature and the ambient temperature, the final temperature of the gas in the tank and/or gas tank material may be computed using the thermal behavior simulation tool for the tank type and a look-up table for the computation may be provided. By constraining the final temperature to be at the highest 85 C., the allowable maximum mass-averaged gas filling temperature MAT.sub.max may be chosen.

(18) Alternatively or additionally, the maximum mass-averaged gas filling temperature can be determined on the basis of an analytical model including adiabatic boundary conditions at the transition from the gas volume of the gas tank to the gas tank material, based on the relation between a gas temperature increase in the tank under the adiabatic boundary conditions and a temperature increase of the gas or the gas tank material which may be described by the following equation:
[Math. 5]
T.sub.J/T.sub.A=a.sub.JP.sub.i+b.sub.J,(5)

(19) where T.sub.A is temperature increase of the gas in the tank under the adiabatic boundary conditions, T.sub.J is temperature increase of the gas or the gas tank material, with J denoting gas or gas tank material, a.sub.J and b.sub.J are fixed parameters specific to tank characteristics and the predetermined filling time t.sub.fill, and P.sub.i is an initial gas pressure in the gas tank.

(20) The fixed parameters a.sub.J and b.sub.J may be determined for the gas and/or the liner material by linear regression on the temperature predictions provided by a thermal behaviour simulation tool applied to the Type 4 hydrogen tank, for the nominal filling rate selected.

(21) Using Equation (5) and the hot case conditions, the maximum allowable adiabatic temperature increase may be determined from the maximum allowable liner temperature increase (e.g. 65 C. if the initial temperature considered in the hot case conditions is 20 C. and the maximum allowable liner temperature at the end of filling process is 85 C.), providing the maximum allowable gas temperature assuming the adiabatic boundary conditions.

(22) Then, the specific internal energy value of the gas in the tank may be determined for the initial conditions using a linear approximation which provides the specific internal energy in function of gas temperature and gas pressure of the tank and the data available to the public on temperature of the gas tank material and the gas in the gas tank during a gas refilling process. Also, the specific internal energy value of the gas in the tank after the tank is filled to a state of charge of 100% under the adiabatic boundary conditions may be determined.

(23) Next, applying the first law of thermodynamics, the maximum specific enthalpy of the gas to be added to the gas tank under the adiabatic boundary conditions which provides the determined specific internal energy value of the gas in the tank after the tank is filled to the state of charge of 100% may be determined.

(24) Finally, using a linear approximation of the specific enthalpy of the gas in function of temperature and pressure, and considering pressure drop in the piping down-stream of the point where the filling temperature is measured, as well as the thermal capacity of tank system piping resulting in a certain amount of energy input to the gas (which depends on the filling temperature), the maximum mass-averaged gas filling temperature MAT.sub.max may be determined providing the maximum specific enthalpy of the gas added at the vessel inlet of the tank.

(25) After the maximum mass-averaged gas filling temperature is determined, in step S30, a target gas filling temperature MAT.sub.target may be selected which is equal to or lower than the maximum mass-averaged gas filling temperature MAT.sub.max. The target gas filling temperature MAT.sub.target may be selected by taking the actual conditions of the gas supply system, e.g. in the gas refilling station into account. For instance, the ambient temperature in the gas refilling station may be so low that the target gas filling temperature MAT.sub.max cannot be reached for the filling gas without heating the gas; it may be practical to select a target gas filling temperature MAT.sub.target lower than the maximum mass-averaged gas filling temperature MAT.sub.max. The gas to be supplied to the gas tank may be then cooled to the target gas filling temperature MAT.sub.target in step S40 and supply of the gas cooled to the gas tank may be initiated.

(26) In step S50, the mass averaged gas filling temperature actually provided MAT may be determined by measuring the actual temperature of the gas supplied to the tank.

(27) In step S60, cold case conditions may be assumed and the initial gas tank temperature and the initial gas pressure in the tank may be set accordingly for determining the end-of-fill pressure so as to avoid the overfill of the gas tank at the end of the filling process. As a lower initial temperature may allow supply of more gas, it may be conservatively assumed that the initial gas tank temperature is a predetermined lowest value, e.g. 40 C. The initial pressure may be set as a value measured, e.g. by a pressure sensor installed in the gas tank or the gas supply system. Then, the end-of-fill pressure P.sub.fill to be applied for reaching a predetermined maximum allowable state of charge, e.g. 107%, in the cold case conditions may be determined from the actual mass-averaged gas filling temperature MAT.

(28) The end-of-fill-pressure may be determined by means of any known thermal behavior simulation tool which allows to predict the temperature increase of the gas and/or gas tank material by iterations. The simulation tool may be a numerical model calculating the temperature of the gas together with the transient heat conduction into the wall of the gas tank vessel, assuming homogeneous temperature of the gas and unidirectional heat transfer in the wall, e.g. composed of two layers of the liner and the wrapping in composite material. A look-up table may be prepared in advance providing the end-of-fill pressure P.sub.fill for each possible value of applied mass-averaged gas filling temperatures and for each possible value of initial gas pressures in the gas tank. From the look-up table, the end-of-fill-pressure may be determined for a given tank Type, the applied mass-averaged gas filling temperature and the initial gas pressure in the tank measured.

(29) Alternatively or additionally, the end-of-fill-pressure can be determined on the basis of an analytical model including adiabatic boundary conditions at the transition from the gas volume of the gas tank to the gas tank material, based on the relation between a gas temperature increase in the tank under the adiabatic boundary conditions and a temperature increase of the gas or the gas tank material which may be described by the following equation:
[Math. 6]
T.sub.J/T.sub.A=a.sub.JP.sub.i+b.sub.J,(6)

(30) where T.sub.A is a gas temperature increase in the tank under the adiabatic boundary conditions, T.sub.J is a temperature increase of the gas or the gas tank material, with J indicating gas or gas tank material, a.sub.J and b.sub.J are fixed parameters specific to tank characteristics and the predetermined filling time, and P.sub.i is an initial gas pressure in the gas tank.

(31) The fixed parameters a.sub.J and b.sub.J of Equation (6) may be determined for the cold case conditions by linear regression on the temperature predictions provided by a thermal behaviour simulation tool applied to this design, for the selected filling rate, e.g. nominal fill in 3 minutes.

(32) Then, assuming that the mass averaged filling temperature at the end of fill is the currently measured value of the mass-averaged gas filling temperature MAT, the end-of-fill pressure P.sub.fill providing a target end-of-fill state of charge, e.g. 107%, may be determined.

(33) After the P.sub.fill is determined, the supply of gas may be terminated in step S70, when the gas pressure in the tank reaches the end-of-fill pressure P.sub.fill or a predetermined maximum filling pressure, e.g. 87.5 MPa, whichever is lower.

(34) FIG. 6 shows a block diagram of another embodiment of the process for filling a gas tank according to the present invention. For the sake of simplicity, the embodiment is explained with an example of a hydrogen gas tank of Type 4. However, the embodiment is applicable to other gas to be filled in a gas tank as well as other types of gas tanks such as Type 2 or Type 3 tanks. The steps corresponding to the steps according to the first embodiment are designated by the same reference numerals as in FIG. 5, however multiplied by 10. Moreover, the steps of the gas filling process in FIG. 6 are described only insofar as they differ from the steps of the gas filling process of FIG. 5. For the remaining description, it is hereby explicitly referred to the description of FIG. 5.

(35) The filling process shown in FIG. 6 differs from the filling process in FIG. 5 in that the process may further comprise the step of determining whether or not information transmitted from the gas tank to be filled is available, e.g. to the gas supply system.

(36) In step S600, the end-of-fill pressure P.sub.fill to be applied is determined from the actual mass-averaged gas filling temperature MAT. The step S600 may comprise three sub-steps, namely, S610, S620 and S632 or S634:

(37) In step S610, the cold case conditions may be assumed and the initial gas tank temperature and the initial gas pressure in the tank may be set accordingly. In particular, the initial gas tank temperature in the tank may be set to be 40 C. and the initial gas pressure in the tank may be set as the value measured, e.g. by a pressure sensor measuring the counter pressure needed to start injecting gas into the gas tank.

(38) In step S620, it may be determined whether information from the gas tank to be filled is available at the gas supply system, i.e. whether there is a communication between the gas tank and the gas supply system. If the information is determined to be available (step S632), a state of charge in the gas tank to be achieved at the end of filling process may be set based on the information. Alternatively, the state of charge value in the gas tank to be achieved may be set to be a maximum allowable value, e.g. 120%, i.e. a safety measure for ensuring termination of the process even if the information transmitted from the gas tank is wrong. The maximum final fill pressure P.sub.fmax may be determined based on the state of charge which was set.

(39) If information is determined not available (step S634), the state of charge in the gas tank to be achieved at the end of filling process may be set more conservatively, e.g 107%. The maximum final fill pressure P.sub.fmax may be determined based on the state of charge set.

(40) In step S700, the supply of gas is terminated. The step S700 may comprise two substeps S710 and S722 or S724.

(41) In step S710, it may be determined whether information from the gas tank to be filled is available or not.

(42) If the information is available, in step S722, the supply of gas may be terminated when the state of charge becomes 100%. The actual gas pressure of the gas tank at the end of the filling process P.sub.fin may be lower than the maximum final fill pressure P.sub.finmax.

(43) If the information is determined not available, in step S724, the supply of gas may be continued until the actual pressure of the gas tank becomes equal to the maximum final fill pressure P.sub.finmax As a result, the state of charge value at the end of filling process may be a value between 92% and 107%.