Patent Grant
8365777
This invention relates to an apparatus and a method for filling a vessel, such as a fuel tank, with a compressed gas, such as hydrogen, safely over a minimum period of time without overheating the vessel.
Gaseous hydrogen is expected to supplant liquid hydrocarbon fuels, such as gasoline and diesel, as the fuel of choice for automobiles, trucks and buses due to its obvious environmental advantage. Hydrogen burns cleanly and does not produce greenhouse gases such as CO and CO2 as a combustion by-product.
The practical considerations associated with the large scale storage and dispensing of gaseous hydrogen fuel for motor vehicles presents different problems from those associated with the handling of liquid fuels due to the gaseous nature of the hydrogen. One problem, as yet not satisfactorily addressed, is that of filling fuel tanks of various capacities, different pressures, and different ullage with gaseous hydrogen safely, over a time period comparable with the time required to fill a fuel tank of comparable energy capacity with a liquid fuel.
Safety demands that a tank not be filled too fast to avoid overheating. Additionally, any charging of a tank with compressed gas will cause the gas within the tank to heat up, increasing its pressure, the volume being fixed. The increase in gas temperature and pressure within a tank can prevent the tank from being filled to capacity when the tank is filled to a maximum working pressure. After filling, the heat dissipates to the ambient, cooling the gas and reducing its pressure within the tank. Once cool, the tank can accept more gas (up to its maximum working pressure) requiring that the tank be “topped off” by charging it with additional gas so that it is filled to capacity. Depending upon the rate of fill, the tank may need to be topped off several times before it is completely filled. These steps of filling and cooling are impractical and require too much time compared with filling of a tank with liquid.
The invention concerns a method of filling a vessel with a gas, the method comprising:
In one embodiment, measuring comprises measuring the instantaneous gas pressure within the vessel, the gas pressure within the vessel being used to adjust the flow rate to achieve the predetermined rise rate of the gas pressure within the vessel. In another embodiment, measuring comprises measuring the instantaneous gas temperature within the vessel, the gas temperature within the vessel being used to adjust the flow rate to achieve the predetermined rise rate of the gas pressure within the vessel. In yet another embodiment, measuring comprises measuring the ambient temperature surrounding the vessel, the ambient temperature being used to adjust the flow rate to achieve the predetermined rise rate of the gas pressure within the vessel. In still another embodiment, measuring comprises measuring the temperature of the gas before compressing the gas into the vessel, the temperature of the gas before the compressing into the vessel being used to adjust the flow rate to achieve the predetermined rise rate of the gas pressure within the vessel.
The invention also encompasses an apparatus for filling a vessel with a gas. The apparatus comprises a source of the gas and a compressing means for compressing the gas. The compressing means has a variable mass flow rate and an inlet and an outlet. A first conduit connects the source and the inlet, and a second conduit connects the outlet and the vessel. A controller controls the mass flow rate of the compressing means. A transducer measures the value of a parameter selected from the group consisting of gas pressure, gas temperature, ambient temperature and combinations thereof. The controller receives electrical signals from the transducer and uses the electrical signals to control the mass flow rate of the compressing means to fill the vessel from the source.
Flow of fluid (liquid 15 or gas 14) from source 16 to the compressing means inlet 18 is controlled by a valve 24 positioned within the conduit 22 between the source and the compressing means. Compressing means 20 has an outlet 26 which may be connected in fluid communication with the vessel 12 using a conduit 28. If liquid 15 is supplied from the compressing means 20 to the conduit 28 then a vaporizer 29 is in fluid communication with the conduit 28 to change the state of the fluid to the gas 14. For practical applications the conduit 28 has a coupling 30 which is sealingly connectable to and disconnectable from the vessel 12. Flow of liquid 15 or gas 14 from the compressing means outlet 26 may be controlled by a valve 32 in the conduit 28. The source 16 may also be connected to the vessel directly using a conduit 34. A valve 36 in the conduit 34 is used to control flow of gas 14 directly from the source 16 to the vessel 12. Note that conduit 34 may share the coupling 30 with conduit 28 for connection to the vessel 12.
The apparatus may comprise one or more pressure and/or temperature transducers, for example, pressure transducer 38 which measures the gas pressure supplied from the source in conduit 22, pressure transducers 40 and 42, which measure the gas pressure at the inlet 18 and outlet 26, respectively, of the compressing means 20, and pressure transducer 44, which measures the gas pressure within conduits 28 and 34 as delivered to the vessel 12. Temperature transducers include transducer 46, which measures the ambient temperature surrounding the vessel 12, transducer 48, which measures the temperature of the gas within the vessel 12, and transducer 50, which measures the temperature of the gas within conduits 28 and 34 before entering vessel 12. Each pressure transducer 38, 40, 42 and 44 generates electrical signals indicative of pressure at their respective measurement points. Similarly the temperature transducers 46, 48 and 50 generate electrical signals indicative of the temperature at their respective measurement points. Note that the transducers need not be located exactly at the point where the measurement is desired, as it is possible to infer the temperature or pressure at a location, for example, within the vessel 12, by measuring the temperature on the outside surface of the vessel, and the pressure in a conduit hydraulically connected to the vessel. It is further understood that every transducer disclosed herein is not necessarily used simultaneously in the apparatus, the exemplary embodiment shown in
The signals generated by the transducers are transmitted to a controller 52 which is also in communication with valves 24, 32 and 36 as well as the variable flow rate compressing means 20. The controller 52, which may be for example, a programmable logic controller or other microprocessor based device, runs resident software and uses the signals from the transducers in a feed-back loop to open and close the valves and vary the flow rate of the compressing means (for example by varying its speed) according to an algorithm to adjust the flow rate of gas 14 to the vessel 12 to increase the gas pressure within the vessel at a predetermined rate of change as described below. A user interface 54, for example, a keyboard and display monitor, provides communication with the controller permitting the apparatus 10 to be operated to dispense gas from the source to the vessel.
In a practical embodiment the apparatus 10 may serve as a fueling station for dispensing hydrogen fuel to vehicles, the fuel tank comprising the vessel 12. Vehicle tanks have a maximum allowed pressure of about 875 bar. It is desired to maintain the vessel at or below 85° C. to avoid overheating. In such a station the gas source 16 comprises a liquid hydrogen storage tank having a capacity between about 5,700 liters and about 171,000 liters. Hydrogen gas is stored in the reservoir at a pressure between about 0 and about 90 MPa. (Alternately, the gas source 16 may comprise, for example, a steam methane reformer, an electrolysis apparatus or a hydrogen pipeline.) The valves may be pneumatic plug valves which are actuatable by the controller. The pressure transducers may be, for example, piezo-electric transducers. The temperature transducers may comprise thermistors. The compressing means operates to provide a mass flow rate variable between about 0.1 and about 10 kg/min.
With reference also to
Various parameters and associated algorithms may be used by the controller to achieve the desired gas pressure change rate within the vessel 12. For example, the gas pressure may be increased at a constant rate. For practical applications, for example for hydrogen gas filling a fuel tank, the constant increase rate may range between about 2.5 MPa/min and about 35 MPa/min. In another algorithm the gas pressure in the vessel increases at an increasing rate. There is also the possibility to increase the pressure at a decreasing rate.
In one embodiment, the controller receives pressure measurements from the pressure transducer 44 which measures values of the gas pressure within the vessel 12. The controller adjusts the compressing means' flow rate using the gas pressure values to achieve and maintain the desired pressure rise rate according to the feedback relationship:
Compressing Means' Flow Rate(n+1)=Compressing Means' Flow Rate(n)+(Target Pressure Rise Rate−Measured Pressure Rise Rate)×K
Wherein:
Compressing Means' Flow Rate(n) is the actual volume flow rate at which the compressor or pump is moving fluid and is proportional to the speed at which a compressor or pump is run during the just completed time interval;
Compressing Means' Flow Rate(n+1) is the actual volume flow rate at which the compressor or pump is moving fluid and is proportional to the speed at which a compressor or pump is run during the next time interval;
Target Pressure Rise Rate is the desired pressure rise rate based on current conditions and filling strategy;
Measured Pressure Rise Rate is the actual pressure rise measured during the just completed time interval; and
K is the gain, or the rate at which the controller responds to stimulus.
The compressing means' flow rate may be measured as a mass flow rate in grams/second and the pressure rise rates may be measured in MPa/min. This yields the units associated with the gain K as grams/second per MPa/min for consistency of the functional relationship. The gain K will be specific to a particular apparatus and is determined experimentally consistent with control system practices.
In another embodiment, the controller receives temperature measurements from the temperature transducer 48 which measures values of the gas temperature within the vessel 12. The controller adjusts the compressing means' flow rate using the gas temperature values, for example based upon the functional relation:
When gas temperature within the vessel is less than 65.6° C. the pressure rise rate is maintained at 35 MPa/min; when the gas temperature in the vessel exceeds 65.6° C. the pressure rise rate is reduced according to the formula: pressure rise rate=(85−gas temperature)×0.555.
In another embodiment, the controller receives temperature measurements from the temperature transducer 46 which measures ambient temperature values surrounding the vessel 12. The controller adjusts the compressing means' flow rate using the ambient temperature values, for example based upon the functional relation:
If ambient temperature<15° C. then the pressure rise rate=10 MPa/min; if 15°<ambient temperature<30° C. then the pressure rise rate=7.5 MPa/min; if ambient temperatur>30° C. then pressure rise rate=5 MPa/min.
In another embodiment, the controller receives temperature measurements from the temperature transducer 50 which measures values of the gas temperature before it is compressed into the vessel 12. The controller adjusts the compressing means' flow rate using the gas temperature values, for example based upon the functional relation:
If gas temperature<15° C. then the pressure rise rate=10 MPa/min; if 15°<gas temperature<30° C. then the pressure rise rate=7.5 MPa/min; if gas temperature>30° C. then the pressure rise rate=5 MPa/min.
When the gas source 16 comprises a reservoir of sufficient pressure and capacity, for example at a pressure between about 15 MPa and about 70 MPa and a capacity between about 40 liters and about 1,000 liters, it is feasible to initially fill the vessel 12 by coupling the source 16 directly to the vessel. As shown in
The method and apparatus according to the invention allows gas vessels to be filled safely and in a reasonable period of time without the fear of overheating and its consequences.
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