VEHICLE AND STORAGE LNG SYSTEMS
LNG, for use as a motor vehicle fuel, is stored in a manner that does not require massive tanks, eliminates evaporative loss and reduces refrigeration energy consumption. A Stirling cryocooler extends through a wall of a highly insulated, relatively low pressure container to its cold end located in the vapor phase above the liquid surface. The pressure or temperature of the LNG is sensed and applied to a feedback control that modulates the heat transfer rate of the Stirling cryocooler so that LNG vapor is liquefied at a rate to maintain a desired pressure and temperature within the container. Maintaining a superatmospheric pressure in the container reduces the energy consumption required for re-liquefaction of the LNG vapor. The apparatus is also usable for liquefaction of natural gas for refueling vehicles from the ubiquitous consumer level domestic gas distribution system.
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No. 61/674,588 filed Jul. 23, 2012 The above prior application is hereby incorporated by reference.
STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT
REFERENCE TO AN APPENDIX
BACKGROUND OF THE INVENTION
This invention generally relates to providing a practical means for using as a motor vehicle fuel a liquefied combustible gas, such as liquefied natural gas (LNG) or other fuels that can be liquefied by cooling. More particularly, the invention relates to an apparatus and method for long term storage of such liquefied combustible gases in a manner that avoids fuel loss by evaporation into the atmosphere without requiring high pressure fuel containers that are associated with compressed gases and does so at a low energy cost and is practical for both a vehicle fuel container and small consumer sized fuel supply containers for refueling. The invention also relates to the liquefaction of natural gas at consumers' homes, improving the efficiency of burning the fuel and to a manner of moving the liquefied gas out of a container.
Manufacturers of gasoline powered cars are required to improve emissions and efficiency while under market pressure of ever increasing fuel prices. This has led to the development of new technologies together with their associated compromises. The hybrid vehicle reduces emissions and increases fuel efficiency by utilizing an electric motor to recover braking energy and to avoid idling losses. However, the battery is an expensive and bulky item that leads to poor space utilization. Driving performance is often compromised and the cost premium is in most cases recovered by reduced operating costs over periods of greater than 3 years. All-electric vehicles take advantage of high electric motor efficiency to obtain low operating costs. Unfortunately, battery energy storage density is poor leading to poor range. Charging is currently only practical at home and a special high current electric system is required to do this effectively. The cost premium is high leading to long payback periods that may exceed the life of the vehicle. Recent developments of the Diesel engine have allowed extraordinary gains in fuel efficiency while maintaining decent performance. However, high fuel costs offset the efficiency advantage and emission controls and amelioration systems are expensive. Compressed natural gas (CNG) vehicles enjoy low fuel costs but suffer from reduced range due to the low energy content of the fuel per unit volume and also lower power due to poor volumetric efficiency. In addition, the need for a large and heavy high-pressure fuel tank reduces trunk volume. Refueling is only possible at stations that offer CNG.
Liquefied natural gas (LNG) has been used as a fuel for motor vehicles. LNG offers the reduced cost of natural gas and the significantly lower emissions that are available from CNG. LNG is stored in highly insulated tanks at atmospheric pressure and therefore does not require the large mass that is necessary to retain a high pressure gas. LNG has been a practical option for large trucks when making a long distance run. An advantage of LNG is that it does not require the heavy high pressure tanks that are required to store CNG at pressures on the order of 3000 psi to 3600 psi. Another advantage is that LNG is more than twice as dense as CNG and therefore has more than twice the energy density. However, one problem encountered with LNG arises because LNG is stored on board vehicles and in stationery supply tanks at cryogenic temperatures in containers that are vented to the atmosphere. During the evaporation of the LNG from its container, the heat of vaporization helps maintain the low temperature required to maintain the LNG in its liquid phase. However, the evaporation also represents a fuel loss. Consequently, the use of LNG as a motor vehicle fuel is practical if the fuel is consumed in a sufficiently short time period that the fuel lost by evaporation (boil off) in that time period is small enough to keep costs reasonable. Because autos sit unused for long periods of time, during which there is evaporation loss, LNG is not practical for vehicles that are inactive for long periods of time, which is the case for passenger cars and small trucks.
LNG would become an attractive alternative to gasoline-powered vehicles and a practical fuel for cars and small trucks if it could be stored at a relatively low pressure without evaporative loss, if the equipment for doing so were relatively inexpensive to purchase and to operate and if the vehicle owner had a readily available manner of refueling the vehicle, especially from the currently commonly available domestic supply distribution system of natural gas for home heating. If these obstacles could be overcome and implemented quickly on a large scale, that would permit car owners to obtain the advantages of reduced emissions and of lower fuel and operating costs from the use of LNG.
It is, therefore, an object and purpose of the invention to provide a manner of inexpensively and rapidly overcoming these obstacles.
BRIEF SUMMARY OF THE INVENTION
Disclosed is a system that allows vehicles to effectively use liquid natural gas (LNG) or other appropriately cooled liquefied gases even when there are substantial non-use periods where previous systems would have been subject to boil-off.
The basic concept of the invention is the combination of (1) a highly insulated LNG container that is capable of retaining the LNG under pressure, but not anywhere near the pressure required for CNG, (2) a Stirling cryocooler with its cold head extending through a container wall into the upper portion of the container which is occupied with natural gas vapor so that the vapor can condense on the cold head or a heat exchanger attached to the cold head and drip back down into the portion of the container occupied by liquid phase LNG, and (3) a negative feedback type of control system that senses the temperature or pressure within the container and modulates the rate of heat transfer by the cryocooler from the cold head to the exterior of the container in order to maintain a desired pressure within the container. Preferably, the control system is capable of selectively maintaining any of three pressure conditions. In one pressure condition, the controls system maintains a pressure which is a maximum pressure that the LNG container can safely withstand so that the LNG is confined to the container, rather than being vented to the atmosphere, which allows the cryocooler cold head to be maintained at the highest possible temperature and thereby minimize the power consumption of a prime mover that drives the cryocooler. In a second and lower pressure condition, the pressure in the LNG container is maintained at a pressure that is appropriate for propelling the LNG to the engine instead of pumping the LNG with a fuel pump. In a third and still lower pressure condition, the pressure is maintained at a pressure that allows the flow into the container of natural gas from a domestic gas supply so that the gas is condensed on the cold end or heat exchanger of the cryocooler and liquefied for refilling the container.
Another aspect of the invention is heating a small portion of LNG in a chamber mounted to the LNG container so that the heated LNG is vaporized to a pressure suitable for propelling the LNG to the engine. Yet another aspect of the invention is to include a Stirling engine as a prime mover driving the cryocooler and fueling the Stirling engine with the LNG from the LNG container. A further aspect of the invention is to position an LNG vaporizer, which vaporizes the LNG for introduction into the vehicle engine, within the air intake plenum of the vehicle engine and provide external heat exchanger fins on the surface of the vaporizer so that the heat of vaporization of the vaporizing LNG is used to cool and thereby compress the combustion supporting air that is being drawn into the vehicle engine.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
DETAILED DESCRIPTION OF THE INVENTION
A first part of this invention is directed to effectively storing LNG on board cars. This is achieved by:
a. Utilizing vacuum insulation in the form of a Dewar tank and multilayer radiation shields so that heat leakage is kept to an absolute minimum. It is anticipated that the heat leak into the fuel tank can be reduced to a few Watts.
b. In order to remove all net heat transfer to the stored LNG, a small Stirling cryocooler will be used to re-liquefy the vapor from boil-off. The cryocooler will consume electrical power at a rate of about 30W or less. When being driven, the power can be provided from the car. When the vehicle is stationary or parked, sufficient power must come from a secondary source. This can be a battery, an electrical hook up, a small solar panel, a small Stirling engine or a combination of these.
A second part of the invention is the refueling system. Since LNG terminals are not widespread and it would take time to develop such refueling infrastructure, it would be convenient to be able to use home natural gas availability. This will be achieved by:
a. Utilizing a small Stirling cryocooler to liquefy the natural gas on site. In one embodiment, this may be the same Stirling cryocooler on board the vehicle in which case the LNG is simply stored in the vehicle tank. In a second embodiment, this would be a second Stirling cryocooler of somewhat greater capacity that liquefies the natural gas into a second stationary vacuum insulated Dewar stored in a convenient location such as a home garage.
b. A special purpose coupling that attaches the car tank either to the natural gas line or to the second vacuum Dewar tank.
A third part of this invention is the engine system. This will be arranged so that the LNG heat of vaporization is used to increase the volumetric efficiency of the engine by cooling the inlet air to the engine. By this process, the engine maximum power will be increased over what is possible with CNG systems.
A vacuum insulated fuel line 20 carries the liquid fuel to a fuel vaporizer 22 situated in an air intake plenum 24. The vaporized or gaseous fuel is then fed to the fuel rails 26 (
The secondary shell 14 is a vacuum enclosure that surrounds the LNG tank 12. The Stirling cryocooler cold-end enters into the LNG tank 12 via a vacuum coupling 38. A low thermal conductivity penetration or vacuum coupling at 41 allows vacuum or thermally insulated fuel line 20 to enter into the LNG tank 12. The fuel line 20 takes fuel from fuel pump 30 or from just a sump in the same location if the fuel delivery pressure is controlled by the Stirling cryocooler as already described. A vacuum or thermally insulated gas line 42 provides a connection for gaseous natural gas at the natural gas inlet fill point 32. A similar vacuum or thermally insulated LNG inlet line 44 with connection inlet 34 provides a means for refueling directly from an LNG source such as a home refueling station. Low thermal conductivity supports 46 keep the LNG vacuum tank 12 separated from the secondary shell vacuum enclosure 14. Heat reject fan 40 carries the rejected heat away from the Stirling cryocooler 16 via its plenum 50.
Once the LNG fuel leaves the vacuum insulated tank 12, it needs to be vaporized before it is useful. A detailed view of the vaporizer 22 is shown in
A home refueling station operates in a manner similar to the vehicle system. Referring to
Those skilled in the free piston Stirling engine and cryocooler art are aware that there are a large and diverse variety of such Stirling machines known in the prior art. The present invention involves the use of a Stirling cryocooler but the invention is not the design of any particular Stirling cryocooler. However and by way of example, a preferred embodiment of a Stirling cryocooler is shown in
A free piston Stirling cryocooler, which of course operates with the Stirling cycle, is believed to be preferred for use in embodiments of the invention. However, it is believed that a pulse tube cryocooler, which also operates in accordance with the Stirling cycle, can alternatively be used. Consequently, either type of Stirling cryocooler can be used in the above described embodiments of the invention.
Though the above are embodiments of using Stirling cryocoolers to provide practical LNG fuel systems for vehicles, other embodiments are possible and are considered part of this invention. For example, the system described shows a condensing heat exchanger for re-liquefying boil-off. An alternative would be to use a thermosiphon heat transport system whereby the vacuum insulated tank walls are cooled to offset any net heat leakage. This method is employed by applicant in the use of Stirling cryocoolers to provide cooling to ultra-low temperature freezers. See U.S. Pat. Nos. 6,550,255 and 7,073,567.
From the above description of the preferred embodiment, it can be seen that the invention is an apparatus for storing a liquefied combustible gas in a thermally insulated container that is sealable from the atmosphere and capable of superatmospheric (above atmospheric) pressurization. The container can be an on-board fuel tank for a vehicle or a home storage tank for storing a liquefied combustible gas for refueling. The gas includes a liquid phase and a vapor phase located above the liquid phase with the phases separated by the surface of the liquid. A Stirling cycle cooler is mounted to the container and extends through a wall of the container to the cold end of the cooler. The cold end of the cooler is located in the vapor phase above the liquid surface and preferably has a heat transfer facilitating surface of the type commonly used on heat exchangers.
The Stirling cycle cooler is driven by a prime mover that has a variable power output. The power output of the prime mover can be varied to vary the heat transfer rate of the Stirling cycle cooler and thereby control the temperature of the cryocooler' s cold end. One common type of prime mover is an electromagnetic linear motor that is mechanically linked to drive the Stirling cooler. The voltage applied to a linear motor can be varied to vary to power of the cryocooler. As known to those in the art, the stroke and power of linear motor is controlled by a control system that varies the voltage amplitude applied to the armature windings of the linear motor. Such control systems have a control input for controlling that voltage amplitude or alternatively, the armature windings themselves can be considered a combined power and control input. A Stirling engine can additionally or alternatively be mechanically linked to drive the Stirling cooler and advantageously connected to receive combustible gas from the LNG container for powering the Stirling engine.
The invention also has a temperature sensor or a pressure sensor, or both, positioned to sense temperature or pressure within the container. The sensor or sensors have an output for communicating its sensed temperature or pressure to a control system. A temperature sensor is preferably positioned in the liquid phase and a pressure sensor is preferably positioned in the vapor phase.
Embodiments of the invention use a feedback control for controlling the pressure within the container. The feedback control is designed by applying well known control principles to the following principles of the invention. The typical modern control is a digital data processor that has a stored program for operating according to its control algorithm. The control drives the Stirling cryocooler at a heat pumping rate that maintains the pressure within the container at a desired pressure. The control modulates the Stirling cooler's rate of heat transfer from the vapor phase, thereby controlling the rate of liquefaction of the LNG vapor in the container and thereby maintains the pressure within the container at a desired pressure above atmospheric pressure. As will be seen, the pressure can be controlled by sensing either the pressure or temperature within the container.
The saturation vapor pressure in a closed container is a function of temperature. This is illustrated in
For example, looking at
The result of the above principles is that it is desirable to store the LNG at the highest possible safe pressure for which the container is designed in order to store the LNG at the highest possible temperature at which the saturation condition exists because this minimizes the energy consumed for re-liquefaction of the LNG by the Stirling cryocooler in the enclosed container. This result creates the opportunity for storing the LNG in a manner that avoids the need to vent, and therefore waste, some of the LNG to the atmosphere in order to maintain the LNG in a liquid phase. By containing the LNG at a superatmospheric pressure, the energy consumed by the cryocooler in the re-liquefaction of the LNG vapor can be made low enough to make the invention economically practical and attractive. The higher the saturation vapor pressure and temperature at which the LNG is maintained in the container, the less energy that is consumed by the cryocooler of embodiments of the invention. The pressure and temperature within the container is determined by the relationship of (1) the heat coming into the tank by both conduction through the container walls and the heat generated by any heater within the container to (2) the heat pumped out of the tank by the Stirling cryocooler. The Stirling cryocooler need only maintain an equilibrium between those opposite heat transfers.
It is apparent to those skilled in the art that the design of an embodiment of the invention requires typical engineering trade-offs between the container and the cryocooler. By designing the container for a higher safe maximum pressure and by designing the container with greater thermal insulation, a cryocooler with a lower cooling power capacity can be used. However, the greater the pressure capacity and thermal insulation of the container, the greater its cost and weight. A designer must choose the balance of these factors for a particular implementation of the principles of the invention.
Nonetheless, the invention offers significant advantages over the equipment used for CNG. A typical CNG container is pressurized to approximately 200 to 250 bar for storing the CNG. With the present invention, the pressure within the container can be far less than required for CNG. Consequently, a container of considerably less mass may be used than required for storing CNG. More desirably the pressure in a container embodying the present invention will be in the range of 5 bar to 20 bar and most preferably around 10 bar. As seen by the graph of
A temperature or pressure sensor 144 is positioned to sense the temperature and pressure within the container 132. An output 146 of the temperature or pressure sensor 144 is connected to the control's summing junction 148 which is an input of the feedback control 128 for communicating the sensed temperature or pressure to the control 128 and operating as its feedback loop. As seen from
An output 150 of the control 128 is connected to the control input 142 of the prime mover 140. The selection of an appropriate forward transfer function 152 and the manner in which the negative feedback control 128 operates to drive the temperature or pressure within the container to a set point that is input at a set point input 154 are well known to those skilled in the art.
As an alternative, the temperature or pressure sensor may alternatively be positioned at 156 within the vapor phase 136. The arrangement of
In the operation of embodiments of the invention, heat is transferred from a location in the vapor phase 136 to outside the container 132 by the Stirling cryocooler 130. This is accomplished by cooling a surface in contact with the vapor phase to a temperature below the temperature of the vapor phase. The temperature or pressure or both within the container is or are sensed and the rate of transferring heat from the vapor phase is modulated in response to the sensed temperature or pressure to maintain the pressure within the container at a desired pressure above atmospheric pressure. In order to maximize the benefit of the invention, the rate of transferring heat from the vapor phase is modulated to a rate that maintains the pressure within the container at the maximum rated safe pressure for the container. That allows the combustible gas to be stored at the warmest safe temperature and thereby minimize the power required for transferring heat from the vapor phase to outside the container.
This detailed description in connection with the drawings is intended principally as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the designs, functions, means, and methods of implementing the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be adopted without departing from the invention or scope of the following claims.
1. An apparatus for storing a liquefied combustible gas in a thermally insulated container that is sealable from the atmosphere and capable of superatmospheric pressurization, the gas including a liquid phase and a vapor phase above the liquid phase and separated by a surface of the liquid, the apparatus comprising:
- (a) a Stirling cycle cooler mounted to the container and extending through a wall of the container to a cold end of the cooler, the cold end of the cooler being located in the vapor phase above the liquid surface, the Stirling cycle cooler being driven by a prime mover that has a variable power output and is controllable at a control input for varying the heat transfer rate of the Stirling cycle cooler from the cold end to a heat rejector that is external to the container;
- (b) a temperature or pressure sensor positioned to sense the temperature or pressure within the container and having an output for communicating its sensed temperature or pressure;
- (c) a Stirling cycle cooler feedback control for controlling the temperature or pressure within the container, the control having an input connected to the output of the sensor and an output connected to the control input of the prime mover and adapted for variably controlling the temperature or pressure within the container.
2. An apparatus in accordance with claim 1 wherein the sensor is a temperature sensor positioned in the liquid phase.
3. An apparatus in accordance with claim 1 wherein the sensor is a pressure sensor positioned in the vapor phase.
4. An apparatus in accordance with claim 1 wherein a heater is mounted within the container, the heat output of the heater being variably controllable at a heater control input connected to an output of the Stirling cycle cooler feedback control, the control adapted to transfer heat to the liquefied gas in the container for increasing the gas pressure within the container.
5. An apparatus in accordance with claim 1 wherein the prime mover comprises an electromagnetic linear motor mechanically linked to drive the Stirling cooler and a Stirling engine also mechanically linked to drive the Stirling cooler and connected to receive combustible gas from the container for powering the Stirling engine
6. An apparatus in accordance with claim 1 wherein the control modulates the Stirling cooler's rate of heat transfer from the vapor phase and thereby maintains the pressure within the container at a desired pressure above atmospheric pressure.
7. An apparatus in accordance with claim 6 wherein the control maintains the pressure within the container in the range from above atmospheric pressure to 20 bar absolute for storing the combustible liquefied gas.
8. An apparatus in accordance with claim 6 wherein the control maintains the pressure within the container in the range from above atmospheric pressure to 2 bar absolute for transporting the LNG out of the container.
9. An apparatus in accordance with claim 1 and further comprising a fuel pumping apparatus mounted to the container, the pump having a vaporizing chamber, a heater mounted for supplying heat to the vaporizing chamber, an inlet check valve and an outlet check valve arranged to allow flow of liquid phase into the chamber and from the chamber through an outlet conduit to a vehicle engine, the outlet conduit being located at a position for contacting contained LNG in its liquid phase.
10. A method for maintaining a liquefied combustible gas in an insulated container, the gas including a liquid phase and a vapor phase above the liquid phase that are separated by a surface of the liquid phase, the method comprising:
- (a) condensing vapor phase by transferring heat from a location in the vapor phase to outside the container, the transfer including cooling a surface in contact with the vapor phase to a temperature below the temperature of the vapor phase;
- (b) sensing the temperature or pressure within the container; and
- (c) modulating the rate of transferring heat from the vapor phase in response to the sensed temperature or pressure to maintain the pressure within the container at a desired pressure above atmospheric pressure.
11. A method in accordance with claim 10 and more particularly comprising modulating the rate of transferring heat from the vapor phase at a rate that maintains the pressure within the container at a maximum safe pressure for the container in order to store the combustible gas at the warmest safe temperature and thereby minimize power required for transferring heat from the vapor phase to outside the container.
12. A method in accordance with claim 11 wherein the pressure maintained within the container is in the range from above atmospheric pressure to 20 bar absolute for storing the combustible liquefied gas.
13. A method in accordance with claim 10 and more particularly comprising modulating the rate of transferring heat from the vapor phase at a rate that maintains the pressure within the container at desired pressure for driving the gas out of the container.
14. A method in accordance with claim 13 wherein the pressure is maintained in the range from above atmospheric pressure to 2 bar absolute.
15. A method in accordance with claim 13 and further comprising heating the gas within the container for elevating the pressure to the desired pressure.
16. An apparatus for compressing combustion-supporting air flowing into an internal combustion engine through an air intake plenum, the engine being fueled by a supply of liquefied combustible gas that is conveyed through a conduit network into engine combustion chambers, the apparatus comprising:
- a combustible gas vaporizer physically located within the air intake plenum and having gas-conveying passages that are part of the conduit network, the gas-conveying passages being interposed between the gas supply and the engine combustion chambers, the vaporizer being adapted to allow expansion within the gas-conveying passages of the liquefied combustible gas, the vaporizer having heat exchanger fins on the exterior of the vaporizer, the fins being longitudinally aligned along the air flow plenum for transferring heat from incoming air through the air intake plenum to the combustible gas vaporizing in the vaporizer.
Filed: Jun 26, 2013
Publication Date: Jan 23, 2014
Inventor: David M. Berchowitz (Athens, OH)
Application Number: 13/927,538
International Classification: F25B 9/14 (20060101);