WO2013056295A1

WO2013056295A1 - System and method for refuelling of compressed gas pressure vessels

Info

Publication number: WO2013056295A1
Application number: PCT/AU2012/001249
Authority: WO
Inventors: Derek FEKETE; Paul Anthony Whiteman
Original assignee: Mosaic Technology Development Pty Ltd
Priority date: 2011-10-21
Filing date: 2012-10-16
Publication date: 2013-04-25

Abstract

A pressure vessel refuelling system enables reduced recompression heating in vehicle CNG pressure vessels during refuelling. The system includes a first pressure vessel and a second pressure vessel, each vessel having at least one opening for the entry and exit of a gas and a liquid. A liquid line establishes fluid communication between the first and second pressure vessels via the at least one opening in each of the first and second pressure vessels. A backpressure control device is operatively connected to the liquid line, whereby during refuelling of the first pressure vessel a gaseous fuel from a refuelling station flows into the first pressure vessel, and whereby a liquid in the first pressure vessel flows, under control of the backpressure control device, from the first pressure vessel through the liquid line to maintain a near constant pressure in the first pressure vessel.

Description

TITLE

SYSTEM AND METHOD FOR REFUELLING OF COMPRESSED GAS

PRESSURE VESSELS FIELD OF THE INVENTION

This invention relates generally to a compressed gas transfer system. In particular, the invention relates to a compressed natural gas transfer system including a method for refuelling of CNG cylinders that eliminates heat rise of CNG in the cylinder whilst retaining the simplicity of a single CNG hose refuelling connection.

BACKGROUND OF THE INVENTION

Natural gas fuels are relatively environmentally friendly for use in vehicles, and hence there is support by environmental groups and governments for the use of natural gas fuels in vehicle applications. Natural gas based fuels are commonly found in three forms: Compressed Natural Gas (CNG), Liquefied Natural Gas (LNG) and a derivative of natural gas called Liquefied Petroleum Gas (LPG).

Natural gas fuelled vehicles have impressive environmental credentials as they generally emit very low levels of SO2 (sulphur dioxide), soot and other particulate matter. Compared to gasoline and diesel powered vehicles, CO₂ (carbon dioxide) emissions of natural gas fuelled vehicles are often low due to a more favourable carbon-hydrogen ratio found in natural gas. Natural gas vehicles come in a variety of forms, from small cars to (more commonly) small trucks and buses. Natural gas fuels also provide engines with a longer service life and lower maintenance costs. Further, CNG is the least expensive alternative fuel when comparing equal amounts of fuel energy. Still further, natural gas fuels can be combined with other fuels, such as diesel, to provide similar benefits mentioned above.

A key factor limiting the use of natural gas in vehicles is the storage of the natural gas fuel. In the case of CNG and LNG, the fuel tanks are generally expensive, large and cumbersome relative to tanks required for conventional liquid fuels having equivalent energy content. In addition, the relative lack of wide availability of CNG and LNG refuelling facilities, and the cost of LNG, add further limitations on the use of natural gas as a motor vehicle fuel. Further, in the case of LNG, the cost and complexity of producing LNG and issues associated with storing a cryogenic liquid on a vehicle further limits the widespread adoption of this fuel.

Some of the above issues are mitigated when using LPG and this fuel is widely used in high mileage motor cars such as taxis. However, cost versus benefit comparisons are often hot favourable in the case of private motor cars. Issues associated with the size and shape of the fuel tank, the cost variability of LPG and the sometimes limited supply mean that LPG also has significant disadvantages that limit its widespread adoption. In summary, unless there is massive investment in a network of LNG plants around major transport hubs, CNG is the only feasible form of natural gas that is likely to be widely utilised in the near future.

Methods for delivering natural gas into an internal combustion engine can be broadly categorized into two main groups: Low pressure carburetted induction or manifold based injection:

The practice of introducing natural gas into the inlet of an internal combustion spark ignition engine is well known and is similar to practices used in LPG fuelled vehicles. When natural gas is introduced into a compression ignition engine with the combustion air during the intake stroke, the ignition characteristics of natural gas means that the level of liquid fuel substitution is somewhat limited. Another problem with this method concerns 'methane slippage' that results from the overlap of the timing of the inlet and exhaust valves, and/or non-combustion zones in the cylinder chamber typically in the piston-land gap. The result is a level of unburnt hydrocarbons in the engine exhaust that can negate most of the greenhouse gas emission benefits of using natural gas. High pressure direct injection (HPDI):

In a compression ignition engine fitted with high pressure direct injection, natural gas is injected into the cylinder at the top of the compression stroke with a small quantity of pilot diesel fuel, (typically between 3% and 5%) basically eliminating the potential for methane slippage and/or pre-ignition of the fuel-air mixture. A diesel engine operating on natural gas with high pressure direct injection retains the benefits of a high efficiency diesel engine, is able to achieve better than 95% displacement of the liquid fuel, and achieves significant reductions in greenhouse gas emissions and pollutants including sulphur dioxide, carbon dioxide, oxides of nitrogen and soot.

HPDI may also be utilised in spark ignition or thermal tip ignition engines.

However, high pressure direct injection requires natural gas to be supplied to an engine at a consistent high pressure (typically greater than 3000 psig). For LNG this is achieved through the use of a specially designed pump capable of operating at cryogenic temperatures while pressurising the fuel to the required pressure before it is vaporised to CNG. For natural gas stored as CNG it requires an expensive and complex gas compressor that must deliver natural gas at the required pressure from a range of pressures, typically between 10 psig (in a near empty CNG tank) and 3600 psig (in a full CNG tank). This means the ancillary equipment to the gas compressor must have the capability to reject the significant quantities of heat created by a range of compression ratios exceeding 100:1. This requires complex machinery and large gas intercoolers, consumes significant power, adds significant excess mass, and requires a large amount of space which is something not available on most vehicles.

While LNG has had some success as a liquid fuel replacement in some regions of the world, the lack of availability of LNG and its high cost means that in many regions of the world it is not feasible to use LNG. In the case of CNG, it also has had some success as a liquid fuel replacement but almost exclusively in spark ignition engines utilising the low pressure carburetted port injection induction technology: This application is popular in government bus fleets around the world where the cleaner burning natural fuel is used in a spark ignition engine fitted in place of a conventional diesel engine.

The unavailability of a system to maintain a high CNG pressure for direct injection engines means that CNG fuel storage systems for HPDI engines have not been considered practical, and many experts in the field have pursued LNG as the only viable natural gas fuel storage system that can be readily pumped/maintained at a high pressure as a liquid to meet the pressure requirements of direct injection.

In addition, the pressure to which composite CNG cylinders can be filled at a typical CNG re-fuelling station is limited due to the heat of compression overheating the cylinder being filled. This has typically meant that 245 BAR at 21degC (settled temperature) is the limit for composite CNG cylinder filling, and has become the standard adopted in many parts of the world including the US.

In the US, codes typically allow for filling to an overpressure of 1.25 times the pressure rating of the CNG cylinder provided it would subsequently settle to 245 BAR if cooled to 21 deg. C. The code also identifies in cylinder heating as having the potential to cause transient temperature excursions exceeding cylinder design parameters.

In Europe the relevant codes limit the maximum pressure in composite CNG cylinders during re-fuelling to 260 barg to ensure maximum design temperatures are not exceeded.

These limitations have meant that the currently available composite cylinders designed for 350 barg operating pressure and above cannot be utilised in conventional CNG re-fuelling systems. This means that the opportunity to utilise smaller CNG cylinders or to achieve increases in range for the same size cylinders, cannot be realised. International Patent Application Publication WO 2008/074075, titled "A Compressed Gas Transfer System", disclosed for the first time a liquid delivery system that enables the volume in a pressure vessel to be varied to maintain gas in the pressure vessel at a near constant pressure. That enables a CNG tank to be maintained at a consistent high pressure (e.g., greater than 3000 psig) while the tank is emptied to supply fuel to a high pressure direct injection engine. In addition it disclosed a method of fuel transfer onto the vehicle that held the vehicle tank at a relatively constant high pressure, enabling the fuel to transfer at high pressure, and eliminating the heat of compression in the CNG cylinder. However it required the transfer and return of liquid into and out of the vehicle fuel pack from an external source which meant conventional CNG single hose refuelling was not possible and refuelling times were substantially increased due to the need to fill the vehicle CNG cylinders with liquid prior refuelling with CNG.

OBJECT OF THE INVENTION

It is an object of some embodiments of the present invention to provide consumers with improvements and advantages over the above described prior art, and/or overcome and alleviate one or more of the above described disadvantages of the prior art, and/or provide a useful commercial choice.

SUMMARY OF THE INVENTION

In one form, although not necessarily the only or broadest form, the invention resides in a pressure vessel refuelling system comprising:

first and second pressure vessels, each vessel having at least one opening for the entry and exit of a gas and a liquid;

a liquid line establishing fluid communication between the first and second pressure vessels via the at least one opening in each of the first and second pressure vessels; and

a backpressure control device operatively connected to the liquid line;

whereby during refuelling of the first pressure vessel a gaseous fuel from a refuelling station flows into the first pressure vessel via the at least one opening in the first pressure vessel, and whereby a liquid in the first pressure vessel flows, under control of the backpressure control device, from the first pressure vessel through the liquid line to maintain a near constant pressure in the first pressure vessel.

Optionally, a volume of liquid maintained in the system is at least approximately equal to the volume of either the first or the second pressure vessel, whereby the volume of liquid maintained in the system is sufficient for refuelling to commence immediately upon connection of the system to the refuelling station without pumping or transferring additional liquid into the system.

Optionally, liquid flowing from a bottom of the first pressure vessel is cycled, via the back pressure control device, to the second pressure vessel while maintaining a relatively constant total volume of liquid inside the first and second pressure vessels.

Optionally, the system further comprises an additional plurality of pressure vessels.

Optionally, the system is located on a vehicle.

Optionally, the system further comprises a liquid reservoir, whereby if insufficient liquid is in the pressure vessels, additional liquid can be pumped into the pressure vessels from the reservoir prior to or during refuelling.

Optionally, liquid flowing from a last pressure vessel is returned, via the back pressure control device, to the liquid reservoir.

Optionally, the liquid is moved from the first pressure vessel, to the second pressure vessel via the back pressure control device using pressure from the incoming gaseous fuel.

Optionally, the liquid is returned to the liquid reservoir via the back pressure control device using pressure from the incoming gaseous fuel.

Optionally, the first pressure vessel includes a top end and a bottom end, and a first bottom float valve is in liquid communication with the bottom end of the first pressure vessel.

Optionally, the first pressure vessel includes a top end and a bottom end, and a first bottom float valve is in liquid communication with the liquid reservoir.

Optionally, the first pressure vessel is a gaseous fuel vessel located on a vehicle.

Optionally, the system further comprises a pump operatively connected to the liquid reservoir, whereby the liquid can flow from the liquid reservoir, through the liquid line, and into the first pressure vessel as gas is simultaneously released from the first pressure vessel, thereby maintaining a near constant pressure in the first pressure vessel.

Optionally, the first pressure vessel includes an expandable bladder that separates the liquid from the gaseous fuel inside the first pressure vessel.

Advantages of some embodiments of the present invention thus include a method and system for fast fill refuelling of CNG vehicles to pressures in excess of current CNG limits (such as 245BAR at 21 degC (settled temperature) with some composite cylinders). The refuelling process can be performed via a conventional single CNG refuelling connection, with the liquid contained in the plurality of pressure vessels providing the necessary fluid inventory to control the gaseous fuel filling process, and for the filling process to commence immediately upon connection to CNG refuelling source.

Some embodiments of the present invention eliminate the issue of in cylinder recompression heating inside vehicle CNG cylinders during refuelling, enabling consistent filling to a pressure vessel's standard ambient temperature pressure rating at design pressures significantly above 245 barg. This provides increased CNG storage and vehicle range or the opportunity to reduce storage vessel sizes.

A further advantage of some embodiments of the present invention, in eliminating the heat of compression inside the vessel being filled, is an increase in safety, particularly from the prevention of transient temperature excursions during re-filling, thereby allowing for the potential to redesign cylinders to be lower cost and lighter weight. BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention showing a compressed natural gas transfer system (referred to herein as a fuel package) during use and refuelling will be described with the reference to the accompany drawings in which:

FIG. 1 is a system diagram illustrating a fuel package, including four pressure vessels that are completely full of gas, according to an embodiment of the present invention.

FIG. 2 is a further system diagram illustrating the system of FIG. 1 , where one pressure vessel is partially emptied of gas, according to an embodiment of the present invention.

FIG. 3 is a further system diagram illustrating the system of FIG. 1 , where one pressure vessel is emptied of usable gas and filled to the maximum liquid level, according to an embodiment of the present invention.

FIG. 4 is a further system diagram illustrating the system of FIG. 1 , where the first pressure vessel contains only decompressing residual gas and a quantity of liquid and the second pressure vessel is supplying gas to the engine as additional liquid is injected, according to an embodiment of the present invention.

FIG. 5 is a further system diagram illustrating the system of FIG. 1 , where two pressure vessels are completely emptied of usable gas and a liquid is shown fully drained from the first emptied vessel while liquid is being drained from the second vessel and the third pressure vessel is supplying gas to the engine as additional liquid is injected, according to an embodiment of the present invention.

FIG. 6 is a further system diagram illustrating the system of FIG. 1 , where two pressure vessels are completely emptied of usable gas and a liquid is shown fully drained from the two emptied vessels while liquid is being drained from the third vessel and the fourth pressure vessel is supplying gas to the engine as additional liquid is injected, according to an embodiment of the present invention. FIG. 7 is a further system diagram illustrating the system of FIG. 1 , where three pressure vessels are emptied of usable gas and liquid is shown fully drained from two of the emptied vessels while liquid is being drained from the third emptied vessel and the fourth pressure vessel is supplying gas to the engine as additional liquid is injected, according to an embodiment of the present invention.

FIG. 8 is a further system diagram illustrating the system of FIG. 1 , where all four pressure vessels are completely emptied of usable gas and a liquid is shown fully drained from three of the emptied vessels, while the fourth emptied vessel is filled with liquid to the maximum liquid level at operating pressure and ready to be re-fuelled, according to an embodiment of the present invention.

FIG. 9 is a further system diagram illustrating the fuel package at the commencement of refuelling of the first cylinder to be refuelled, according to an embodiment of the present invention.

FIG. 10 is a further system diagram illustrating the fuel package, during refuelling of the first cylinder to be refuelled, with the liquid flowing through the back pressure valve into the second cylinder to be refuelled compressing the residual gas in that cylinder, according to an embodiment of the present invention.

FIG. 11 is a further system diagram illustrating the fuel package with refuelling of the first cylinder to be refuelled now complete, and the second cylinder to be filled is filled with liquid to the maximum liquid level having compressed the residual gas in that cylinder up to operating pressure, according to an embodiment of the present invention.

FIG. 12 is a further system diagram illustrating the fuel package during refuelling of the second cylinder being refuelled and depicting liquid from that cylinder flowing through the back pressure valve into the third cylinder to be refuelled, according to an embodiment of the present invention.

FIG. 13 is a further system diagram illustrating the fuel package during refuelling of the third cylinder to be refuelled, with the final cylinder to be refuelled filling with liquid while compressing the residual gas in that cylinder, according to an embodiment of the present invention.

FIG. 14 is a further system diagram illustrating the fuel package with refuelling of the third cylinder to be refuelled complete and the final cylinder to be refuelled is filled with liquid to the maximum liquid level having compressed the residual gas in that cylinder to operating pressure according to an embodiment of the present invention.

FIG. 15 is a further system diagram illustrating the fuel package with refuelling of the fourth cylinder to be refuelled underway with liquid flowing from that vessel through the back pressure valve to the liquid reservoir, according to an embodiment of the present invention.

FIG. 16 a further system diagram illustrating the fuel package with refuelling complete, according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Elements of the invention are illustrated in concise outline form in the drawings, showing only those specific details that are necessary to understanding the embodiments of the present invention, but so as not to clutter the disclosure with excessive detail that will be obvious to those of ordinary skill in the art in light of the present description.

In this patent specification, adjectives such as first and second, left and right, front and back, top and bottom, etc., are used solely to define one element from another element without necessarily requiring a specific relative position or sequence that is described by the adjectives. Words such as "comprises" or "includes" are not used to define an exclusive set of elements or method steps. Rather, such words merely define a minimum set of elements or method steps included in a particular embodiment of the present invention. It will be appreciated that the invention may be implemented in a variety of ways, and that this description is given by way of example only. Further, the CNG cylinders that supply gaseous fuel to the engine are synonymously referred to as tanks, vessels, pressure vessels, CNG cylinders and cylinders. FIG. 1 illustrates a compressed natural gas transfer system 10 that supplies gas at high pressure to a gas consuming engine 12. The gas consuming engine 12 is typically in the form of a vehicle engine such as a high pressure direct injection (HPDI) engine and accordingly the transfer system 10 is usually transportable. However, the transfer system 10 may be stationary and supply a gas consuming device in the form of a gas turbine or any plant or process requiring a stream of high pressure gas.

The compressed natural gas transfer system 10 includes a plurality of interconnected pressure vessels 14, 16, 18, 20 in the form of CNG cylinders. The cylinders are able to cater for different pressures as required. However, current pressure technology would reasonably allow operating pressures up to 350 bar and beyond. This is typical of the pressure at which compressed natural gas is supplied to an engine in a high pressure direct injection system. However, it should be appreciated that the rating and operating pressure of the pressure vessels 14, 16, 18, 20 can be varied depending upon the requirements of the gas consuming engine 12.

An engine gas line 22 connects the pressure vessel 20 to the gas consuming engine 12. An engine gas supply valve 24 fitted to engine gas line 22 is used to isolate the supply of gas from the pressure vessel 20 to the gas consuming engine 12. . An engine gas supply pressure regulator 26 is located along the engine gas line 22 to maintain the gas pressure to the gas consuming engine 12 within a desired range.

A liquid gas separator system 28 is also located along the gas line 22 to prevent liquids from the pressure vessel 20 entering the engine 12. The system 28 includes a separator vessel 30, a float valve 32, and a liquid dump valve 34.

A liquid reservoir 36 is connected to the pressure vessel 20 via a liquid delivery line 38 and a liquid return line 40. The liquid reservoir 36 is filled with liquid 42. The liquid 42 is typically water with salt or other antifreeze product being added to the water for use in low temperature environments. Liquid inlet valves 44, 46, 48, 50 are used to permit the delivery of liquid 42 from the liquid reservoir 36 to, respectively, the pressure vessels 14, 16, 18, 20 through the liquid delivery line 38. Liquid outlet valves 52, 54, 56, 58 and a liquid return header valves 59A and 59B are used to allow liquid 42 to return from, respectively, the pressure vessels 14, 16, 18, 20 through the liquid return line 40. During refuelling, the liquid outlet valves 52, 54, 56, 58 and inlet valves 44, 46, 48, 50 are also be used to transfer liquid between the pressure vessels 14, 16, 18, 20 via a back pressure valve 65 and liquid transfer valve 64.

A liquid pump 60 is located along the liquid delivery line 38 to pump liquid 42 from the liquid reservoir 36 to the pressure vessels 14, 16, 18, 20. A reservoir pump pressure controller 62 is connected to the reservoir pump 60 and the liquid delivery line 38 to ensure the desired pressures within the pressure vessels 14, 16, 18, 20 are maintained when supplying gas to the engine. The amount of liquid 42 being delivered to the pressure vessels 14, 16, 18, 20 can be controlled by cycling the liquid pump 60, which for example can be a variable speed pump, a variable displacement pump, a constant speed pump with or without spill valves or a combination thereof. The liquid 42 can be periodically replenished or replaced if necessary in the reservoir 36 via a filler cap on the reservoir 36 (not shown).

In use, the pressure vessels 14, 16, 18, 20 are filled with gas to a desired pressure as shown. Gas filling occurs through a gas refuelling valves 66, and 63, and gas valves 68, 70, 72, 76 associated, respectively, with pressure vessels 14, 16, 18, 20. A gas header valve 74 is attached between the top of vessels 14, 16, 18 and a bottom end of the vessel 20. As shown in FIG. 1 , all vessels 14, 16, 18, 20 are full of CNG, all liquid 42 is stored in the reservoir 36, and the engine 12 is off.

Referring to FIG. 2, consider that the engine gas header valve 24, gas header valve 74, gas valves 68, 70, 72 and 76, and the liquid inlet valve 44 are open. The pump 60 commences operation once pressure in gas line 22 upstream of pressure regulator 26 has dropped to a designated level. When the engine is started gas immediately flows from pressure vessel 20 through gas line 22 and to the engine 12. Gas also flows from the top end of vessels 14, 16, 18 through header valve 74 and into the bottom end of vessel 20. When the pressure in gas line 22 upstream of pressure regulator 26 has dropped to a designated point, liquid 42 is moved from the reservoir 36 by the liquid pump 60 through the liquid delivery line 38 and liquid inlet valve 44 into the pressure vessel 14 maintaining the pressure in the vessel. The gas pressure in vessel 14 also supports the pressure in vessels 16, 18 and 20. Using feedback from the pressure controller 62, liquid pump 60 maintains a liquid flow rate that maintains relatively constant pressure of the gas within the pressure vessels 14, 16, 18, 20. Optionally, the system 10 uses a non-variable speed pump that cycles the system 10 on and off between upper and lower pressure settings.

Referring to FIG. 3, when the gas in vessel 14 is almost all consumed, the liquid 42 reaches the bottom of a top float valve 78 in the vessel 14. A float 80 inside the top float valve 78 then floats upward with the rising level of the liquid 42 until the float 80 closes the top float valve 78. The configuration of the top float valve 78 results in a small quantity of residual gas remaining in the vessel 14 to provide the force to drive the liquid 42 back to the liquid tank 36 when required. Optionally if a top float valve 78 is not used, an inverted stand pipe can be fitted to the vessel 14 top fitting to achieve the same result. The action of the top float valve 78 closing means that the liquid 42 is prevented from flowing through the gas valve 68, which is then closed. This condition can be detected in a variety of ways, including for example by a sensor that senses closure of the top float valve 78, by detecting a spike in pressure at the pressure controller 62, or by a volumetric measurement of the liquid 42 that has been pumped from the liquid tank 36 into the vessel 14.

When the maximum liquid level in vessel 14 is reached the liquid inlet valve 44 and gas valve 68 are both closed and the liquid pump 60 is stopped with the compressed volume of gas in vessels 16,18 & 20 continue to supply gas from the vessel 20 and through line 22 to the engine 12.

Referring to FIG. 4, the liquid inlet valve 46 and the liquid outlet valve 52 have been opened. Liquid 42 is draining from the vessel 14 and returning to the liquid reservoir 36 through a restricted flow valve 59A and the liquid return header valve 59B in the liquid return line 40. Residual pressurised gas at the top of the vessel 14 assists in forcing the liquid 42 out of the vessel 14. Gas continues to flow at a high pressure from the vessel 16 into vessel 20 and through line 22 to the engine 12. As the gas in vessel 16 is consumed, the level of the liquid 42 in vessel 16 continues to rise, while the level of the liquid 42 in vessel 14 continues to fall. Those skilled in the art will thus appreciate that the total volume of the reservoir 36 does not need to significantly exceed the total volume of one of the vessels 14, 16, 18, 20. As described further below, by sequentially transferring the liquid 42 from the vessel 14, back to the reservoir 36, and then to the vessel 16, less volume of the liquid 42, and hence less mass of the liquid 42, is required to be stored in and transported with the CNG transfer system 10.

Referring to FIG. 5, the rising liquid level in the vessel 16 has closed the upper float valve 84, liquid inlet valve 46 and gas valve 70 have been closed and liquid outlet valve 54 and liquid return valve 59A have been opened and liquid is flowing from vessel 16 to the liquid tank 36. Liquid inlet valve 48 on vessel 18 has been opened and pump 60 has commenced injecting liquid 42 from the liquid tank 36 into the vessel 18 causing the float in lower float valve on vessel 18 to move upwards and allow the liquid into vessel 18. The increasing volume of liquid 42 in vessel 18 maintains the pressure in cylinders 18 and 20 and hence the gas supply pressure to the engine 12. Further, as the liquid 42 was fully drained from the vessel 14, a float 88 in a bottom float valve 90 floated down with the level of the liquid 42 and closed the bottom float valve 90.

Referring to FIG. 6, a similar process to that described in FIG. 5 is taking place with the recycling of the liquid 42 from the vessel 18, through the reservoir 36, and into the vessel 20 continues. Also similar, when the vessel 16 was emptied of the liquid 42, a float 92 in a bottom float valve 94 floated down with the level of the liquid 42 and closed the bottom float valve 94.

Referring to FIG. 7, vessel 14, 16 contain only residual gas at low pressure and all valves on these vessels are closed; vessel 18 also contains residual gas at low pressure but also contains a volume of liquid 42. Liquid return valve 56 is open allowing liquid 42 to flow from vessel 18 through liquid return header valves 59A and 59B to liquid tank 36. Simultaneously liquid pump 60 is pumping the liquid 42 from liquid tank 36 into the vessel 20 through liquid inlet valve 50, thereby maintaining a near constant pressure of the gas in the vessel 20.

Liquid return header valves 59A and 59B are cycled open and closed to maintain the combined total volume of liquid 42 in the pressure vessels 18 and 20 at a volume approximately equal to the volume of one pressure vessel. This means that the requisite volume of liquid to undertake the PLIGATS re-fuelling process is present within the vessels at all times.

Referring to FIG. 8, the usable gas in all four of the pressure vessels 14, 16, 18, 20 has been consumed by the engine 12. In vessel 18, a bottom float valve 98 has been closed by a float 96 moving down while in vessel 20 a top float valve 106 has been closed by a float 104. All valves on the system 10 are closed waiting refuelling.

However, more commonly the system 10 will be refuelled before all of the gas from vessel 20 is consumed. In such cases vessel 18 will retain a quantity of liquid equal in volume to the gas space in vessel 20 to enable the PLIGATS refuelling process described herein to function as described.

The float valve 32 in the liquid gas separator vessel 30 provides a secondary level of protection from liquid carry over from vessel 20 and into the engine 12.

The flow of gas from the vessel 20 can be stopped based on a volumetric calculation of the liquid 42 that has been injected into the vessel 20. Thus the top float valve 106 may be included only as a fail-safe mechanism. Also, according to an alternative embodiment of the present invention, those skilled in the art will appreciate that the top float valves 106, 100, 84, 78 all can be replaced by a single top float valve in fluid communication with the top ends of the pressure vessels 14, 16, 18, 20, such as the single top float valve 32 located in the separator tank 30 or in another suitable location close to the engine 12. Use of such a single top float valve also can effectively serve the fail-safe function of preventing liquid from entering the engine 12. Also, according to an alternative embodiment of- the invention, those skilled in the art will appreciate a bladder or such other isolation device could be used in the vessels to isolate the liquid from the gas rather than using float valves.

In the refuelling scenario, the vessels 14, 16, 18, 20 are refuelled at a nominal constant pressure, thus avoiding the heat of compression in the vessels 14, 16, 8, 20, Gas can be delivered from a conventional CNG dispenser through refuelling valve 66. The gas refuelling process is nominally performed in a reverse sequence to that in which the gas is used, starting with the last used of the vessels 14, 16, 18, 20, by cascading the liquid 42 back through the vessels 14, 16, 18, 20 via the liquid transfer valve 64 and liquid transfer back pressure valve 65, which controls the transfer of liquid 42 such as to provide a near constant pressure in the current vessel 14, 16, 18, 20 being refilled. The refuelling process starts with the last vessel 14, 16, 18, 20 that has been used.

The result is gas transfer occurs at near constant high pressure via the gas refuelling connection 66, and avoids traditional fast-fill CNG heating issues concerning heating and expansion of gas on transfer-which generally reduces the effective capacity of a vessel. Transferring the gas at high pressure also reduces gas velocities and largely eliminates decompression/re-compression on transfer. The bottom float valves 90, 94, 96, 108, close in sequence when the liquid 42 is evacuated from the vessel 14, 16, 18, 20 being filled and nominally causes a pressure spike which results in a transition to filling the next vessel 14, 16, 18, 20 in the sequence. Figures 9-16 describe the process of refuelling the empty system 10 with CNG.

Referring to FIG. 9, consider that the engine 12 is stopped and that the refuelling valves 66 and 63, gas valve 76, liquid inlet valve 48, and liquid outlet valve 58 are open, and refuelling is ready to commence.

Referring to FIG. 10, consider than refuelling is ready to commence as depicted in FIG 9 and liquid transfer valve 64 is opened to commence refuelling. Liquid 42 immediately flows from the bottom end of vessel 20, through liquid outlet valve 58, back pressure valve 65, liquid transfer valve 64, and liquid inlet valve 48 and into the bottom end of vessel 18. The pressure in vessel 20 drops immediately in response to the outflow of liquid 42 and gas immediately flows into pressure vessel 20 via gas line 22 from the refuelling valve 66. Gas can fill vessel 20 at near constant pressure, regulated by back pressure control valve 65, as the liquid 42 is displaced from vessel 20 into vessel 18.

Referring to FIG 11 , refuelling of vessel 20 is complete, float valve 108 has closed as the liquid 42 in vessel 20 has been fully displaced which caused a pressure transient that was detected by controller 62. Liquid outlet valve 58 has been closed and vessel 18 is now filled with liquid except for the residual gas at its top which has been re-pressured by the liquid to high pressure. Vessel 18 is ready to be refuelled.

Referring to FIG. 12, refuelling of vessel 18 is underway, gas valve 72, liquid return valve 56 and liquid inlet valve 46 are open with high pressure gas flowing through refuelling valve 66 and gas valve 72 into vessel 18 causing the liquid 42 in cylinder 18 to be displaced through liquid outlet valve 56 back pressure valve 65, liquid transfer valve 64 and liquid inlet vale 46 into vessel 16.

Referring to FIG. 13, refuelling of vessel 18 is complete, and refuelling of vessel 14 is underway, gas valve 70, liquid return valve 54 and liquid inlet valve 44 are open with high pressure gas flowing through refuelling valve 66 and gas valve 70 into vessel 16 causing the liquid 42 in cylinder 16 to be displaced through liquid outlet valve 54 back pressure valve 65, liquid transfer valve 64 and liquid inlet valve 44 into vessel 14.

Referring to FIG. 14, refuelling of vessel 16 is complete, but refuelling of vessel 14 is not yet underway. Vessel 14 is now filled with liquid except for the residual gas at its top which has been re-pressured by the liquid to high pressure. Vessel 14 is ready to be refuelled.

Referring to FIG. 15, refuelling of vessel 14 is underway; gas valve 68, liquid return valve 52 and liquid transfer valve 64 are open. Liquid 42 in cylinder 14 is being displaced through liquid outlet valve 52 through back pressure valve 65 and liquid transfer valve 64 into liquid tank 36. Note all gas valves and refuelling valve 63 remain open to allow all vessels to reach an equalised static pressure once all liquid has been displaced.

Referring to FIG. 16, refuelling is complete with all valves are closed and the system 10 is ready for use.

According to an alternative embodiment of the present invention, those having ordinary skill in the art will recognise that the bottom float valves 90, 94, 96, 108 need not be positioned inside of the vessels 14, 16, 18, 20, respectively, but may be positioned below and in fluid

communication with the vessels 14, 16, 18, 20, respectively. Also according to an alternative embodiment of the present invention, those having skill in the art will recognise that a bladder could be used to separate the liquid from the gas rather than use float valves.

The present invention thus provides a method and system for fast fill refuelling of CNG vehicles at pressures in excess of current code limits (such as approximately 245BAR with some composite cylinders). The refuelling process can be performed via a conventional single CNG refuelling connection, with the liquid contained in the plurality of pressure vessels providing the necessary fluid inventory to control the filling process and for the filling process to commence immediately upon connection to a CNG refuelling source.

Some embodiments of the present invention can eliminate the issue of in cylinder recompression heating in vehicle CNG cylinders during refuelling, enabling reliable filling to a pressure vessel's design capacity. Further, some embodiments of the present invention enable CNG vessels to fill to elevated pressures beyond limitations such as 245BAR, removing the limitation of heat on transfer and thus providing increased CNG storage and vehicle range.

The above description of various embodiments of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. As mentioned above, numerous alternatives and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art Accordingly, this patent specification is intended to embrace all alternatives, modifications and variations of the present invention that have been discussed herein, and other embodiments that fall within the spirit and scope of the above described invention.

Claims

1. A pressure vessel refuelling system, comprising:

a first pressure vessel and a second pressure vessel, each vessel having at least one opening for the entry and exit of a gas and a liquid; a liquid line establishing fluid communication between the first and second pressure vessels via the at least one opening in each of the first and second pressure vessels; and

a backpressure control device operatively connected to the liquid line;

2. The system of claim 1 , wherein a volume of liquid maintained in the system is at least approximately equal to the volume of either the first or the second pressure vessel, whereby the volume of liquid maintained in the system is sufficient for refuelling to commence immediately upon connection of the system to the refuelling station without pumping or transferring additional liquid into the system.

3. The system of claim 1 , wherein liquid flowing from a bottom of the first pressure vessel is cycled, via the back pressure control device, to the second pressure vessel while maintaining a relatively constant total volume of liquid inside the first and second pressure vessels.

4. The system of cjaim 1 , further comprising an additional plurality of pressure vessels.

5. The system of claim 1 , wherein the system is located on a vehicle.

6. The system of claim 1 , further comprising a liquid reservoir, whereby additional liquid can be pumped into the pressure vessels from the liquid reservoir prior to or during refuelling.

7. The system of claim 6, wherein liquid flowing from a last pressure vessel is returned, via the back pressure control device, to the liquid reservoir.

8. The system of claim 6, wherein the liquid is returned to the liquid reservoir via the back pressure control device using pressure from the incoming gaseous fuel.

9. The system of claim 6, wherein the first pressure vessel includes a top end and a bottom end, and a first bottom float valve is in liquid communication with the liquid reservoir.

10. The system of claim 1 , further comprising a pump operatively connected to the liquid reservoir, whereby the liquid can flow from the liquid reservoir, through the liquid line, and into the first pressure vessel as gas is simultaneously released from the first pressure vessel, thereby maintaining a near constant pressure in the first pressure vessel.

11. The system of claim 1 , wherein the liquid is moved from the first pressure vessel to the second pressure vessel via the back pressure control device using pressure from the incoming gaseous fuel.

12. The system of claim 1 , wherein the first pressure vessel includes a top end and a bottom end, and a first bottom float valve is in liquid communication with the bottom end of the first pressure vessel.

13. The system of claim 1 , wherein the first pressure vessel is a gaseous fuel vessel located on a vehicle.

14. The system of claim 1 , wherein the first pressure vessel includes an expandable bladder that separates the liquid from the gaseous fuel inside the first pressure vessel.