Patent Application
20060005895
This invention relates to a re-fueling station for vehicles. More particularly, the invention relates to a re-fueling station that can supply either liquefied gas or compressed gas, as required by the vehicle, and a method of operating such a station. While not wishing to be limited to any particular fuel gas, natural gas shall be used as a convenient example, and references to the fuel hereafter will be to liquefied natural gas (LNG) and compressed natural gas (CNG). Those skilled in the art will understand that a different liquefied fuel gas such as hydrogen may be substituted for natural gas without deviating from the spirit of the disclosed invention.
Natural gas has been used as a fuel for piston engine driven vehicles for over fifty years. The desire to improve efficiency and reduce pollution is causing continual change and improvements in the available technology. Some companies are also researching the use of other gaseous fuels, such as hydrogen, as a substitute for liquid fuels.
Some vehicles are designed with fuel systems that store compressed gas in pressure vessels. For example, CNG is commonly stored at ambient temperatures at pressures up to 3600 pounds per square inch (24,925 kPa). CNG can be stored at higher pressures, but this adds to the weight of the storage tanks because they need to be designed and certified for such higher pressures.
Because the energy density of liquefied gas is much greater than that of compressed gas, vehicles designed for longer range sometimes employ fuel systems that store liquefied gas at cryogenic temperatures in special thermally insulated tanks. For example, LNG is normally stored at temperatures of between about −240° F. and −175° F. (about −150° C. and −115° C.), hereinafter generally referred to as “cryogenic temperatures”, and at pressures of between about 15 and 200 psig (204 and 1477 kPa). LNG storage tanks mounted on vehicles can store fuel for several days under common operating conditions. For vehicles in regular use, storing fuel at cryogenic temperatures is not a problem.
Despite the longtime use of natural gas fueled vehicles, these vehicles represent only a small fraction of the total number of vehicles currently in use and compared to the vast number of gasoline and diesel re-fueling stations, there remains a relatively small number of liquefied gas re-fueling stations. Conventional natural gas re-fueling stations are typically designed for supplying only one of LNG or CNG. When a re-fueling station is intended to serve a fleet of vehicles, the fleet can be standardized to use only LNG or only CNG. However, for re-fueling stations that are intended to serve the general public, or a plurality of commercial fleets, there is a need for a re-fueling station that can supply either LNG or CNG.
Since LNG is stored at low pressures compared to CNG, LNG re-fueling stations deliver fuel at relatively low pressures. For cryogenic fluids, centrifugal pumps are suitable for operating within the typical pressure ranges and are capable of operating with high flow rates. Centrifugal pumps designed for cryogenic fluids offer reasonable efficiency in addition to being relatively inexpensive.
Centrifugal pumps typically require fuel to be supplied to the pump suction with a positive value for the net suction head (NSH), which is defined as the difference between the pump inlet pressure and the inlet saturation pressure (expressed in terms of head). NSH is positive as long as the pump inlet pressure is greater than the inlet saturation pressure. Conversely, NSH can be negative when pump inlet pressure is less than the inlet saturation pressure.
Other LNG re-fueling stations use a pressure transfer system where vapor pressure within the LNG storage tank is controlled to provide the means for displacing the LNG from the storage tank. However, such pressure transfer systems result in extra heat being introduced into the storage tank, and may require additional equipment to prevent over-pressurization of the LNG storage tank. For example, some pressure transfer systems further comprise equipment for refrigerating and/or re-condensing vapor, and/or rely on higher quantities of gas being removed through pressure relief systems.
Another disadvantage with a pressure transfer system is that fuel delivery can be delayed since it takes time to build pressure within the storage tank.
On the other hand, CNG re-fueling stations typically employ positive displacement compressors and a cascading CNG storage system for delivering relatively high-pressure gas. Even though conventional CNG compressors operate at relatively high speeds, flow rates are typically relatively low. A cascading CNG storage system is typically used to ensure an adequate supply of high-pressure gas to fill an average-sized vehicle fuel tank in an acceptable amount of time.
The divergent operating conditions between re-fueling stations for LNG (low pressure with high mass flow rate) and CNG (high pressure with low mass flow rate) have presented a challenge for designing a simple re-fueling station capable of delivering both LNG and CNG, especially when it is desirable to have a system with only one fuel pump or compressor for quickly dispensing either LNG or CNG.
U.S. Pat. No. 5,315,831 issued 21 May 1994 (the '831 patent) discloses a combined LNG and CNG fueling station. Vapor pressure in the cryogenic tank is employed to deliver LNG to the dispenser and a natural gas fueled internal combustion engine is employed to drive a fuel pump while providing heat to a heat exchanger for producing CNG. In some embodiments, pressure within the cryogenic storage tank is relieved by bleeding gas from the storage tank into the fuel supply system for the internal combustion engine.
Accordingly, the '831 patent discloses a pressure transfer system for delivering LNG from the re-fueling station. However, as already noted, there are disadvantages associated with a pressure transfer system, such as more frequent venting from the LNG storage tank when pressure within the storage tank exceeds a predetermined maximum pressure. Venting from the LNG storage tank results in wasted natural gas.
In other arrangements, to avoid frequent venting, refrigeration equipment may be employed for re-condensing the natural gas or at least cooling the gas to collapse some of the pressure within the LNG storage tank. However, such arrangements add to the complexity of the system in addition to increased capital and operational costs.
A combined liquefied gas and compressed gas re-fueling station is provided for selectively dispensing fuel in the form of liquefied gas or compressed gas, and provides cost-effectiveness and versatility compared to conventional re-fueling stations. The combined liquefied gas and compressed gas re-fueling station comprises:
The positive displacement fuel pump is preferably a reciprocating piston fuel pump that can pump liquefied gas, vapor, or a mixture of liquefied gas and vapor. An example of a preferred embodiment of a reciprocating piston fuel pump is described in the Applicant's U.S. Pat. No. 5,884,488. This type of fuel pump is operable with a negative net suction head and this allows greater flexibility in locating the fuel pump in relation to the storage tank, and this facilitates re-fueling station arrangements where the storage tank is buried underground. The fuel pump is preferably a double-acting fuel pump.
In a preferred arrangement, the reciprocating piston fuel pump is selectively operable in a low speed mode when fuel flow is directed to the first dispenser to deliver compressed gas; and, in a high speed mode when fuel flow is directed to the second dispenser to deliver liquefied gas, whereby the fuel pump operates with a higher number of cycles per minute compared to when the fuel pump is operated in the low speed mode. The fuel pump is driven by at least one hydraulic cylinder.
For example, in a preferred embodiment, one of two separate hydraulic cylinders, each with a different diameter is selected to drive the pump. With this embodiment, the high speed and low speed operating modes can be efficiently met with a single hydraulic pump. For example, the smaller hydraulic cylinder, which has a smaller displaced volume, can be used for operating the fuel pump at faster speeds for delivering liquefied gas, which is delivered to a relatively low-pressure vessel, and the larger hydraulic cylinder, which has a larger displaced volume, can be used for operating the fuel pump at slower speeds for delivering compressed gas, which is delivered to a relatively high-pressure vessel. The larger hydraulic cylinder is idle when the smaller hydraulic cylinder is driving the fuel pump, and vice versa. Because the power requirements for the fuel pump correlate to the product of fluid pressure and fluid mass flow rate, a single hydraulic pump can be used to satisfy both operating modes, namely the low speed mode for delivering compressed gas at high pressure and a low mass flow rate, and the high speed mode for delivering compressed gas at low pressure and a high mass flow rate.
Advantageously, fuel pump speed may be changed to selectively operate in a high-speed mode or a low speed mode, using one hydraulic pump that supplies hydraulic fluid to one of the two hydraulic cylinders, while the other hydraulic cylinder is idle.
For additional versatility, the hydraulic pump may be a variable speed hydraulic pump. By controlling the speed of the hydraulic pump further modulation of fuel pump speed is possible. An example where this might be advantageous is a re-fueling station that has a plurality of liquefied gas or compressed gas dispensers that may or may not be all activated at the same time.
The reciprocating piston fuel pump preferably comprises:
The displaced volume of the first compression chamber is preferably larger than the displaced volume of the second compression chamber, and more preferably, the displaced volume of the first compression chamber is about two times the displaced volume of the second compression chamber.
The fuel pump piston assembly comprises the piston and the piston shaft. To reduce the piping between the first and second compression chambers, the one-way transfer valve and the fluid passage between the first and second compression chambers are preferably disposed within the piston assembly.
A vertical or inclined alignment of the piston shaft is preferred so that the suction inlet for the fuel pump may be disposed within a sump and fuel that leaks from the compression chambers can flow back into the sump under the influence of gravity. The vertically aligned or inclined fuel pump preferably further comprises a fluid recovery chamber above the first and second compression chambers for collecting fuel and returning it to a sump. The fuel may be returned to the sump from the recovery chamber through an open drain port located near the bottom of the recovery chamber.
The re-fueling station may further comprise an accumulator vessel disposed between the heat exchanger and the first dispenser. However, because the mass flow capacity of the disclosed fuel pump system can be designed to satisfy desired flow rates for re-fueling stations, a cascade system is not required, and even the accumulator vessel may be rendered optional.
A method is provided of operating a re-fueling station to selectively supply liquefied gas or compressed gas. The method comprises:
In a preferred method the fuel pump is operable at speeds between 5 and 30 cycles per minute. In a particular embodiment, the fuel pump operates between about five and twelve cycles per minute when the low speed mode is selected and at between about ten and twenty cycles per minute when the high-speed mode is selected. In another embodiment the fuel pump operates at about six cycles per minute when the low speed mode is selected and at about eighteen cycles per minute when the high-speed mode is selected.
Another embodiment provides a method of operating a re-fueling station to selectively supply liquefied gas or compressed gas; this method comprising:
In all methods, the hydraulic pump system preferably comprises a single hydraulic pump, for reduced capital costs and lower maintenance costs. However, a plurality of hydraulic pumps may also be employed without departing from the spirit of this invention. For example, a re-fueling station may employ a stand-by hydraulic pump, or a tandem arrangement, depending upon the needs of the re-fueling station.
The drawings illustrate specific embodiments of the invention, but should not be construed as restricting the scope of the invention:
Referring to
In a preferred embodiment, LNG storage tank 100 is buried underground. As noted above, since LNG is stored at cryogenic temperatures (typically less than −175° F. (−115° C.) for LNG), an advantage of burying LNG storage tank 100 compared to a tank situated above ground, is that there is much less temperature variation around underground LNG storage tank 100. Another advantage is that an underground storage tank conserves more space above ground for improved accessibility of vehicles to the dispensers. Building codes also typically require less distance between an underground storage tank and an adjacent property, compared to an above-ground storage tank. LNG storage tank 100 preferably has a double wall with a vacuum applied in the space between the walls to provide further thermal insulation.
Fuel pump unit 110 comprises a positive displacement fuel pump disposed within a sump. Preferred arrangements for a reciprocating piston fuel pump are shown in
Fuel pump unit 110 further comprises a flow diverter, which can be controlled to direct fuel to one of the LNG dispenser or the CNG dispenser. Preferably, the activation of the LNG dispenser and/or the CNG dispenser automatically controls the flow diverter so that fuel is directed from the fuel pump discharge to the activated dispenser.
If the fuel pump is activated and it requires cooling to lower its temperature to the desired operating temperature for supplying fuel, a cool down procedure is initiated. A cool down procedure is required, for example, whenever the fuel pump has not been used for a period of time and the passages and chambers through which the LNG flows have become warmer than cryogenic temperatures.
To cool down fuel pump unit 110, LNG is supplied from LNG storage tank 100. LNG vaporizes as it cools fuel pump unit 110 and the associated piping between LNG storage tank 100 and fuel pump unit 110. The vaporized LNG is returned to LNG storage tank 100. Preferably the vapor is returned to the top of the tank to temporarily raise vapor pressure within the LNG storage tank 100, helping to push more LNG from storage tank 100 to cool fuel pump unit 110. When vapor is no longer being introduced into LNG storage tank 100, a thermal equilibrium is eventually reached within the storage tank and vapor pressure declines when some of the vapor re-condenses after being cooled by exposure to the LNG in the bottom of the LNG storage tank 100.
When the LNG dispenser is activated, the demand placed upon fuel pump unit 110 is for a high mass flow rate at a relatively low pressure. To satisfy this demand, the fuel pump preferably operates in a high-speed mode.
In a preferred arrangement for the flow diverter, the discharge from the fuel pump is connected to a tee or a “Y” with a first branch of leading to LNG dispenser 120 and a second branch leading ultimately to CNG dispenser 140. Associated with the first branch is a shut off valve that is preferably automatically opened when LNG dispenser 120 is activated. The shut off valve automatically closes when LNG dispenser 120 is shut down.
Associated with the second branch is a one-way valve that only allows fuel to flow from the flow diverter towards CNG dispenser 140. Downstream of the one-way valve is the high pressure CNG dispensing system and the one-way valve prevents high-pressure fuel from flowing back into fuel pump unit 110. The high pressure CNG downstream from the one-way valve also prevents fuel from flowing into the second branch when the shut off valve is open because the fuel will flow into the first branch where the fuel pressure is much lower.
If the CNG dispenser is activated, the shut off valve remains closed and fuel is forced through the one-way valve. Under these conditions, the demand placed upon the fuel pump is for a high discharge pressure and mass flow rate need not be as high compared to when LNG dispenser 120 is activated.
Without departing from the spirit of arrangement described above, the diverter may employ other valve arrangements. For example, a three-way valve could be substituted for the tee or “Y”, the shut off valve, and the one-way valve. In one position, the three-way valve diverts fuel to LNG dispenser 120, and in a second position, the three-way valve diverts fuel to CNG dispenser 140. The three-way valve may be manually actuated or actuated by an actuator that is controlled by a remotely located switch or controller.
An advantage of the disclosed re-fueling station arrangement is that a positive displacement fuel pump can be sized to provide acceptable re-fueling rates for both LNG and CNG dispensers without the CNG dispensing system requiring a cascading arrangement or an accumulator vessel. As described in greater detail below, the positive displacement fuel pump is preferably a reciprocating piston fuel pump that can operate at lower speeds for delivering CNG at high pressure and low mass flow rates, and at high speeds for delivering LNG at relatively low pressures and relatively high mass flow rates. While an accumulator vessel can be added to the disclosed arrangement, it is not necessary for operation within commercially acceptable parameters because of the versatility of the fuel pump when operated in the disclosed manner. Operating without an accumulator vessel can help to reduce the costs of the overall system. For example, some of the features that enable the elimination of the accumulator include the fuel pump speed control using two hydraulic cylinders, which adds to the versatility of the flow rate through the fuel pump, and the double-acting fuel pump design, which allows continuous fuel discharge from the fuel pump.
Heat exchanger 130 and odorizer 135 are conventional components of known design.
With reference now to
Another difference between the other illustrated embodiments is that instead of separate LNG and CNG dispensers, the embodiment of
With reference now to
During an extension stroke (shown in
Because fuel is discharged during both the retraction and extension strokes, the fuel pump operates as a “double-acting” pump.
In the embodiment of
With reference to
During a retraction stroke piston 530 moves to expand the volume of first chamber 510 and fuel from the sump is drawn into fuel pump 500 through one-way inlet 505. One-way pass through valve 515 is closed during the retraction stroke and fuel within second chamber 520 is pushed through fuel pump discharge 525 by advancing piston 530. During an extension stroke, fuel flows from first chamber 510 through one-way pass-through valve 515 and into second chamber 520. One-way inlet 505 is closed during the extension stroke. As with fuel pump 400, because of the differential volume between the first and second chambers, fuel is discharged through fuel pump discharge 525 during both the retraction and extension piston strokes.
Fuel pump 500 further comprises fuel recovery port 532. Fuel that leaks from second chamber 520 into the space above is drained therefrom and back into the sump through fuel recovery port 532.
As noted above, the desired fuel pump speed may be changed depending on whether fuel is being delivered to the CNG dispenser or the LNG dispenser. The dual hydraulic drive arrangement shown in
When fuel is being delivered to the CNG dispenser, larger hydraulic piston 545b is selected, (as shown in
Hydraulic fluid that leaks from the hydraulic cylinders is captured and recovered through drain pipe 550.
While not illustrated in the Figures, the exterior of the fuel pump, sump, piping, valves and dispensers that handle LNG at cryogenic temperatures are thermally insulated to prevent heat transfer into the system.
If the pump has been idle for a period of time, it may need to be cooled prior to supplying fuel to the CNG or LNG dispenser. During cool down procedures, LNG entering the fuel pump vaporizes until the fuel pump is cooled to cryogenic temperatures. During the cool down period, because of the vaporization of the LNG, the fuel pump operates with a much reduced mass flowrate and the fuel is recirculated to the LNG storage tank. A shorter cool down period may be achieved by driving fuel pump 500 during the cool down procedure with the smaller hydraulic cylinder, because this allows a faster pump speed and a higher mass flow rate.
As will be apparent to those skilled in the art in light of the foregoing disclosure, many alterations and modifications are possible in the practice of the present invention without departing from the scope thereof. Accordingly, the scope of the present invention is to be construed in accordance with the substance defined by the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2,401,926 | Sep 2002 | CA | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA03/01345 | 9/3/2003 | WO | 8/3/2005 |
