Method and system for the real-time calculation of the amount of energy transported in a non-refrigerated, pressurised, liquefied natural gas tank

Abstract

Some embodiments of the presently disclosed subject matter relate to a method and system for the real-time calculation of the amount of residual chemical energy in a non-refrigerated, pressurised tank containing liquefied natural gas, without a composition of the liquefied natural gas having to be determined.

Claims

1. A method for real-time calculation of residual chemical energy E contained in a pressurised tank defined by its shape and its dimensions and containing a layer of liquefied natural gas, the layer of liquefied natural gas being defined at a given instant t, by its temperature T, its density ρ, and its level h in the tank, the method including an algorithm that, at a given instant t, comprises: acquiring the characteristic parameters of the layer of liquefied natural gas by measurement, of the level h of the layer of liquefied natural gas in the tank, using a level sensor, of the temperature T using a temperature sensor, and of the density ρ using a density sensor; and determining the total mass m.sub.t of the liquefied natural gas contained in the tank, wherein the algorithm, for each instant t, further comprises: calculating of the mass gross calorific value GCV.sub.mass of the liquefied natural gas using a function f taking as parameters the temperature and the density ρ of the liquid according to the formula:
GCV.sub.mass=f(T,ρ); and calculating of the residual chemical energy E according to the formula:
E=GCV.sub.mass*m.sub.t wherein the function f that connects the mass gross calorific value GCV.sub.mass to die parameters T and ρ is according to the formula:
f(T,ρ)=A(T)+B*p where, A is a constant value for a given temperature, and B is a constant independent of the composition.

2. The method according to claim 1, wherein either the algorithm is reiterated as requested by an operator using the tank, or the algorithm is carried out automatically, as soon as a given interval of time Δt has elapsed.

3. The method according to claim 1, wherein the determination of the total mass m.sub.t of liquefied natural gas contained in the tank is carried out via a direct measurement using a balance or strain gauges.

4. The method according to claim 1, wherein the determination of the total mass m.sub.t of liquefied natural gas contained in the tank is carried out via a calculation according to the formula:
m.sub.t=ρ*g(h) Where, h is the level of the layer of liquefied natural gas in the tank, ρ is the density of the liquefied natural gas, and g is a function linked to the shape of the tank.

5. A system for the real-time calculation, according to the method such as defined according to claim 1, the residual chemical energy E contained in a pressurised tank defined by its shape and its dimensions and containing a layer of liquefied natural gas, the layer of liquefied natural gas being defined at a given instant t, by its temperature T, its density ρ and its level h in the tank, the system comprising: a calculator intended to be connected to level, temperature, and density sensors of which the tank is provided with, the calculator being able to execute the algorithm of the method defined according to claim 1, and an MMI interface interacting with the calculator in order to report to the operator, the amount of residual chemical energy obtained by the algorithm of the method defined according to claim 1.

6. The system according to claim 5, which is an onboard system wherein the calculator is an onboard calculator connected to the level, temperature, and density sensors, the calculator being specifically designed to execute the algorithm of the method according to the invention, and the MMI interface is an onboard interface of the vehicle onboard dashboard type or an offset interface.

7. A vehicle comprising a pressurised tank containing a layer of liquefied natural gas and being provided with level, temperature and density sensors, the vehicle being characterised in that it includes a system such as defined according to claim 5.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the result of several measurements of the calorific value of the LNG according to the density of the liquid natural gas for a given temperature and composition.

(2) FIG. 2 shows the diagram of a particular embodiment of the measuring system according to the presently disclosed subject matter.

(3) FIG. 3 shows the drawing of an example of a non-refrigerated, pressurised tank that can be used in the framework of the presently disclosed subject matter (case of a cylindrical and horizontal tank), whereon are shown the various parameters making it possible to determine the function g(h) that makes it possible to calculate the mass of LNG contained in this tank.

(4) FIG. 4 shows the diagram of an example of a non-refrigerated, pressurised tank that can be used in the framework of the presently disclosed subject matter (case of a spherical tank), whereon are shown the various parameters making it possible to determine the function g(h) that makes it possible to calculate the mass of LNG contained in this tank.

(5) FIGS. 5 to 7 are screen captures of dashboards of a vehicle each transporting a tank of LNG that is cylindrical and horizontal, showing the input data used for the calculation of the residual chemical energy E according to the method of the presently disclosed subject matter, as well as the result of this calculation.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(6) FIG. 1 shows the result of a set of measurements of gross calorific value taken for different values of density of LNG at a given temperature (−160° C.). These measurement points can be connected satisfactorily (with a correlation coefficient R.sup.2=0.957) via a regression line that, in this particular case at −160° C., has for equation f(ρ)=0.0283ρ−0.7791. This equation f can therefore be used as a correlation function in order to determine the GCV.sub.mass of the LNG when the latter is at the temperature of −160° C.

(7) FIG. 2 shows the simplified diagram of a particular embodiment of the presently disclosed subject matter in the case where the tank 1 is cylindrical and vertical. When a measurement is taken, which can be done continuously, after a time interval Δt has elapsed or after an order from the operator 7, the density 4, temperature 3 and level 2 sensors present in the tank read the values of the temperature of the liquid, of the density as well as level of this liquid in the tank. This information is then sent to the calculator 5 wherein the operator 7 has entered beforehand, via a man-machine interface (MMI) 6, the shape of the tank 1 as well as the characteristic dimensions thereof, in this particular case its radius. This allows the calculator 5 to define the function g(h) used for the determination of the total mass m.sub.t of LNG contained in the tank.

(8) FIG. 3 shows the diagram of a cylindrical tank placed horizontally. In this case, the calculation of the volume of a layer of LNG in this tank is similar to calculating the area of a segment of a disc. The function g(h) is then:

(9) $g\left(h\right)=(R^2\times {\cos }^{-1}\left(\frac{R-h}{R}\right)-\left(R-h\right)\times \sqrt{\left(R^2-{\left(R-h\right)}^2\right)}\times L$

(10) If the tank is placed vertically, g(h) is then simply g(h)=S×R.sup.2×h

(11) FIG. 4 has a spherical tank. In this case, the calculation of the volume of a layer of LNG in this tank is similar to calculating a spherical cap. The function g(h) is then:

(12) $V_h=\pi \times {\left(2R-h\right)}^2\times \frac{\left(R+h\right)}{3}$

(13) Using this information, the calculator 5 then calculates the total mass m.sub.t of LNG contained in the tank 1 and the gross calorific value GCV.sub.mass of the LNG, with these values then allowing the calculator to obtain the value of the residual energy E contained in the tank at the time of the measurement. The value of the residual energy E can then be supplied to the operator via the MMI 6 or be reprocessed in order to obtain information that can be understood easily, such as the number of kilometres remaining. The presently disclosed subject matter is shown in more detail in the examples hereinafter.

EXAMPLES

Example 1

(14) This example shows the variability in the volume energy density of the LNG stored in a non-refrigerated reservoir.

(15) For this, through a calculation using the equation (1) of standard ISO 6976:1995, the residual chemical energy E is determined in a reservoir containing 600 L (i.e. 0.6 m.sup.3) of LNG in the case of a heavy and cold LNG (case a): balance at 3 bars) and in the case of an LNG of the same composition but light and hot (case b): balance at 14 bars).

(16) Case a) of a Heavy and Cold LNG (Balance 3 Bars)

(17) The hypothesis is made that the LNG has the following composition, indicated hereinafter in table 1.

(18) TABLE-US-00001 TABLE 1 Portion of the compound in the LNG as molar Compound percentages methane 88.034 ethane 8.243 propane 2.097 i-butane 0.294 n-butane 0.407 nitrogen 0.925 Combustion conditions: Combustion temperature T.sub.c=0° C., Pressure: 1.01325 bar, Mass GCV (T.sub.a)=14.99 kWh/kg, calculated according to the equation of standard ISO 6976:1995, Temperature of the LNG T=−147.07° C., and Density=443.7153 kg/m.sup.3.
E=0.6*density*GCV.sub.mass=3990kWh
Case b) of a Light and Hot LNG (Balance at 14 Bars)

(19) The LNG has the same composition as that given in table 2 hereinafter.

(20) TABLE-US-00002 TABLE 2 Portion of the compound in the LNG as molar Compound percentages methane 96.367 ethane 2.623 propane 0.689 i-butane 0.17 n-butane 0.15 nitrogen 0.01 Combustion conditions: Combustion temperature T.sub.c=0° C., and Pressure: 1.01325 bar, Mass GCV (T.sub.c)=15.37 kWh/kg calculated according to the equation of standard ISO 6976:1995, Temperature of the LNG T=−112.5° C., and Density=355.65 kg/m.sup.3.
E=0.6*density*GCV.sub.mass=3279kWh

(21) A difference is therefore observed of more than 17% between the energy values E calculated respectively in the cases a) and b). In other terms, for the same initial volume of LNG of 600 litres, this difference in energy can lead to a hundred kilometres travelled in addition if the LNG introduced into the reservoir is cold and heavy (case a), in relation to the number of kilometres travelled in the case b).

Example 2

(22) FIGS. 5 to 7 are screen captures of dashboards of a vehicle each transporting a tank of LNG that is cylindrical and horizontal, showing the input data used for the calculation of the residual chemical energy E according to the method of the presently disclosed subject matter, as well as the result of this calculation.

(23) In particular, FIG. 5 is a screen capture of a dashboard showing the input data that is specific to the tank: Shape: cylinder, arranged horizontally in the vehicle carrying it; Dimensions: length: 1.2 m; diameter: 0.7 m

(24) FIG. 6 is a screen capture of a dashboard showing the input data specific to the layer of LNG: temperature T: −152.2° C.; density ρ: 420.2 kg/m.sup.3; and level h: 0.501 m.