U.S. patent application number 13/927538 was filed with the patent office on 2014-01-23 for vehicle and storage lng systems.
The applicant listed for this patent is Global Cooling, Inc.. Invention is credited to David M. Berchowitz.
Application Number | 20140020408 13/927538 |
Document ID | / |
Family ID | 49945407 |
Filed Date | 2014-01-23 |
United States Patent
Application |
20140020408 |
Kind Code |
A1 |
Berchowitz; David M. |
January 23, 2014 |
VEHICLE AND STORAGE LNG SYSTEMS
Abstract
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.
Inventors: |
Berchowitz; David M.;
(Athens, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Global Cooling, Inc. |
Athens |
OH |
US |
|
|
Family ID: |
49945407 |
Appl. No.: |
13/927538 |
Filed: |
June 26, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61674588 |
Jul 23, 2012 |
|
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|
Current U.S.
Class: |
62/6 |
Current CPC
Class: |
F17C 2201/0157 20130101;
F17C 2250/043 20130101; F17C 2250/0626 20130101; F17C 2270/0178
20130101; F17C 2203/0391 20130101; F17C 2203/0308 20130101; F17C
2250/032 20130101; F17C 2223/0161 20130101; F25B 9/14 20130101;
F17C 2265/066 20130101; F17C 2250/0439 20130101; F17C 2201/058
20130101; F17C 2225/0161 20130101; F17C 2201/056 20130101; F17C
2265/034 20130101 |
Class at
Publication: |
62/6 |
International
Class: |
F25B 9/14 20060101
F25B009/14 |
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.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] 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
[0002] (Not Applicable)
REFERENCE TO AN APPENDIX
[0003] (Not Applicable)
BACKGROUND OF THE INVENTION
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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
[0009] 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.
[0010] 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.
[0011] 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
[0012] FIG. 1 is a view in perspective of an automobile on which an
embodiment of the invention is installed.
[0013] FIG. 2 is a view in section of the LNG on-board storage tank
illustrated in FIG. 1 with the section taken substantially along
the line 2-2 of FIG. 1.
[0014] FIG. 3 is a view in section of the fuel pumping apparatus
illustrated in FIG. 2 with the section taken substantially along
the line 3-3 of FIG. 2.
[0015] FIG. 4 is an enlarged view in perspective of a portion of
FIG. 1 illustrating the engine, the vaporizer and an air intake
fuel supply system embodying the present invention.
[0016] FIG. 5 is a view in perspective of a home refueling system
embodying the present invention.
[0017] FIG. 6 is a graph showing the relationship, within a sealed
container of LNG, of internal pressure as a function of temperature
and also the relationship of energy consumption by a Stirling
cooler arrangement embodying the invention as a function of
temperature.
[0018] FIG. 7 is a view in axial section of an example of a free
piston Stirling cryocooler that can be used in embodiments of the
invention, this example being driven by a prime mover that is an
electromagnetic linear motor.
[0019] FIG. 8 is a view in axial section of an example of a free
piston Stirling cryocooler that can be used in embodiments of the
invention, this example being driven by a prime mover that is a
Stirling cycle engine.
[0020] FIG. 9 is a diagrammatic view illustrating another example
of a free piston Stirling cryocooler that can be used in
embodiments of the invention, this example being driven by both an
electromagnetic linear motor and a Stirling cycle engine which are
intended to be used in the alternative.
[0021] FIG. 10 is a block diagram illustrating a negative feedback
control system arranged to control the pressure within the LNG
container.
[0022] 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
[0023] A first part of this invention is directed to effectively
storing LNG on board cars. This is achieved by:
[0024] 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.
[0025] 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.
[0026] 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:
[0027] 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.
[0028] b. A special purpose coupling that attaches the car tank
either to the natural gas line or to the second vacuum Dewar
tank.
[0029] 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.
[0030] Referring to FIG. 1, an automobile 10 is shown with an LNG
tank 12 surrounded by a vacuum isolating secondary shell 14. There
is vacuum insulation between the tank 12 and the secondary shell 14
that would also include radiation shields and/or a proprietary
insulation material like Aerogel. The design goal is to reduce the
heat leak to a few Watts, say 3 Watts under typical hot ambient
temperatures. Heat leaks at entrance points and supports would be
reduced by use of low thermal conductivity materials. A Stirling
cryocooler 16 is situated so that its cold-end penetrates the LNG
tank 12 through a vacuum coupling 18. The purpose of the Stirling
cryocooler is to re-liquefy boil-off gas within the tank 12. At
steady state, the cooling capacity of the Stirling cryocooler would
just offset the heat leak into the tank. A well-designed Stirling
cryocooler would have a coefficient of performance at -162.degree.
C. of about 0.13 so the required input power for the prime mover
that drives the Stirling cryocooler will be less than 25 W for this
example. Improvements in the thermal insulation of the LNG tank
will have strong returns on net power required by the Stirling
cryocooler. Powering the Stirling cryocooler would be by an
on-board battery that would be charged by the alternator on the
vehicle engine, or alternatively or in combination, a solar panel
placed on the roof or other convenient location on the vehicle. A
small Stirling engine that utilizes the LNG fuel can also be used
to drive an alternator for charging the on-board battery that
powers the cryocooler or for being the prime mover driving the
Stirling cooler. A duplex Stirling engine and cooler combination,
in which the engine and cryocooler are integrated and use a common
piston, is an attractive alternative.
[0031] 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 (FIG. 4)
that convey vaporized fuel to fuel injectors into the engine 28. A
fuel pump 30 situated at the base of the fuel tank 12 provides fuel
pressure to drive the LNG through the fuel line 20. However, an
alternative means for providing fuel pressure is to operate the
Stirling cryocooler 16 at a liquefaction temperature so that the
saturation pressure maintained in the tank 12 is just sufficient to
provide the required fuel pressure for driving the LNG fuel to the
engine 28. Such control is possible with a free-piston Stirling
cryocooler. A gaseous fill point 32 has a fill tube 42 leading to
the tank 12 so that natural gas may be provided directly to the
system from a domestic gas supply for liquefaction by the on-board
Stirling cryocooler 16. Liquefaction by this method would be slow
and therefore only suitable for overnight or extended idle
situations. A faster method of refueling would be to provide LNG
directly into the tank 12 via inlet 34. The fill inlet 34 is
available for refueling from properly equipped fuelling stations or
from a home refueling station as shown in FIG. 5.
[0032] FIG. 2 shows some details of the construction of the
on-board vacuum insulated tank 12 and location of the Stirling
cryocooler 16. A condensing heat exchanger 36 is attached to the
cold-end of the cryocooler 16. The "cold end" of a Stirling cooler
is the part of the cooler that is intended to accept heat and
thereby cool a mass that is near or in contact with the cold end,
in this case natural gas vapor. The heat is pumped by the Stirling
cooler to its warm end exteriorly of the tank 12 where the heat is
rejected, ordinarily into the atmosphere. The cold end preferably
includes a heat exchanger surface that is designed to increase the
heat transfer from the cooled mass into the cold end. For
transferring heat from the gas, the heat exchanger surface is
formed to have a large surface area, in this case the illustrated
fins on the condensing heat exchanger 36.
[0033] 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.
[0034] FIG. 3 shows a means to pump the LNG fuel by use of heat.
This pump would be installed at the base of the fuel tank 12. By
adding controlled or modulated electrical energy at the cartridge
heater 52, the LNG will immediately begin to vaporize and raise the
pressure in chamber 54. The chamber 54 is provided with an inlet
check valve 56 and outlet check valve 58 arranged to allow fuel
flow in one direction in the valve arrangement that is common with
piston pumps. The pressure in the chamber 54 resulting from heating
and vaporizing the LNG in the chamber 54 will close valve 56 and
open valve 58 allowing mixed vapor and liquid fuel to enter the
outlet plenum 60. In this manner the outlet plenum eventually
achieves sufficient pressurization to deliver LNG fuel through fuel
line 20. The location of the entrance to the fuel line should be
set so that it is always in the liquid phase of the LNG as opposed
to the vapor regime. This will ensure that the cold LNG will
condense vapor resulting from heating the LNG in the fuel pump 30
so that liquid is delivered at the entrance to the fuel line
20.
[0035] 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 FIG. 4. The LNG fuel enters the vaporizer
22 through fuel line 20. The vaporizer 22 is physically located
within the air intake plenum 24. The illustrated vaporizer has
gas-conveying passages in pipes or conduits 62 that are interposed
as part of the fuel supply conduit network between the gas supply
and the engine combustion chambers. The LNG delivered to the
vaporizer 22 expands and vaporizes in the vaporizer conduits 62.
This vaporization both prepares the LNG for combustion and also
absorbs heat from surrounding surfaces. Consequently, the vaporizer
is formed as a heat exchanger and cools and therefore compresses
the incoming air. For this purpose, the vaporizer has heat
exchanger fins 64 on the exterior of the vaporizer. The fins 64 are
longitudinally aligned along the direction of incoming air flow
through air intake plenum 24 for transferring heat from incoming
air through the air intake plenum 24 to the combustible gas that is
vaporizing in the vaporizer. By locating the vaporizer within the
air inlet plenum 24, the inlet air to the engine is cooled down by
the vaporization energy of the LNG via heat exchanger 22. Since the
cooled inlet air is denser than it would otherwise be, the
volumetric efficiency of the engine is improved leading to better
power output. Once the fuel is vaporized, it is fed to the fuel
rails at 26 that feed the fuel injectors 66 that control intake of
the fuel-air mixture into the engine cylinders.
[0036] A home refueling station operates in a manner similar to the
vehicle system. Referring to FIG. 5, a Stirling cryocooler 70, of
somewhat higher capacity than vehicle tank cryocooler 16, is placed
so that a condensing heat exchanger 72, that is attached to its
cold end, is exposed to the natural gas vapor in a tank 74. The
Stirling cryocooler 70 is operated at a saturation temperature that
results in a slight over pressure in order to provide positive
pressure for refueling the vehicle. Tank 74 is vacuum insulated by
the vacuum containing outer shell 76. The Stirling cryocooler 70
penetrates shell 76 by way of a vacuum coupling 78. A heat reject
fan 80 carries away the heat rejected from the Stirling cryocooler
at 70. LNG fuel is carried out of the tank 74 by line 82 to an LNG
outlet port 84 where a vacuum insulated fuel delivery line may be
attached for delivery of fuel to a vehicle. Natural gas is fed from
the main domestic gas line to the tank 74 by line 86 where it is
condensed on the heat exchanger 72 and drips down into the liquid
phase LNG. This line 86 is equipped with a standard safety valve 88
and a shut-off valve 90. That allows an outlet from the domestic
gas distribution system to provide gas that is liquefied in the
tank 74 for replenishing the vehicle fuel supply. By having a
stationary home refueling tank in addition to a vehicle fuel tank,
natural gas can be slowly liquefied over long periods of time
without requiring the presence of the vehicle. During the
liquefaction of natural gas from the domestic supply, the pressure
within the tank 74 would need to be maintained by the cryocooler
control at a pressure that is at or near atmospheric pressure in
order to allow natural gas to flow into the tank 74 because the
incoming natural gas pressure is only slightly above atmospheric
pressure. The entire assembly sits on a stable base 92. In the
event of loss of power to the Stirling cryocooler 70, the pressure
in tank 74 would rise due to boil-off of the LNG. In this event,
the vaporized gas would be returned to the main gas line due to
over pressurization. If this is not allowed by local ordinance or
other safety rule, the gas could be blown-off through a safety vent
to a place where it could be safely combusted. The gas boil-off
would be extremely low due to the very low heat leak anticipated as
a result of the vacuum insulation. Any damage to the vacuum
insulation would signal a safety alarm so that the gas company or
other provider of the natural gas can take appropriate action.
Powering options for the home refueling station would be mains
electric power but it could be a Stirling engine fueled by the
natural gas fuel source, or a duplex configuration as already
described.
[0037] 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 FIG. 7. In this case it is a beta
free-piston, balanced machine driven in reciprocation by an
electromagnetic linear motor power by an alternating current. A
gamma configuration of the kind disclosed in patent application
U.S. Ser. No. 12/828,387 would also be satisfactory for this
purpose. Key requirements are a cold head 94, a forced air reject
system 96 in order to run the machine at the lowest possible reject
temperature, a linear motor 98 in order to control the cooling rate
so that energy consumption can be minimized during normal operation
and, if direct gas liquefaction is used, to increase the cooling
capacity by simply increasing the drive voltage on the linear motor
98. A vibration balancing system 100 is also essential in order to
avoid unpleasant noise and vibration being transmitted to the
vehicle or home refueling station.
[0038] FIG. 8 shows a Stirling engine 102 integrated with a
Stirling cryocooler 104 in an arrangement in which the Stirling
engine part drives the Stirling cryocooler part and is known in the
prior art as the duplex configuration. LNG fuel enters the engine
102 at a fuel inlet 106 and flows into a combustor 108 where it is
combusted to provide heat input to the engine 102 so the engine
section will power the cryocooler 104 section. This could be an
option for providing power to the Stirling cryocooler that would
not require a battery or other on board power source other than the
already present LNG fuel. The LNG fuel that is incoming at inlet
106 is converted directly into mechanical energy by the Stirling
engine section 102 which in turn drives the cryocooler section 104
that then provides the cooling energy at the cold head 110.
[0039] FIG. 7 illustrates a Stirling cryocooler which has a linear
motor as its prime mover and FIG. 8 illustrates a Stirling
cryocooler which has a Stirling engine as its prime mover. It can
be desirable in some situations to have available the option of
driving the Stirling cryocooler with either one of two alternative
power sources, electric or the available natural gas. FIG. 9
illustrates that a linear motor prime mover 112 and a Stirling
engine prime mover 114 can both be drivingly linked to a Stirling
cryocooler 116 for driving the latter. The Stirling engine 114 is
connected to receive combustible gas from the LNG fuel container
for powering the Stirling cooler 116 and the linear motor 112 is
connected to an AC power source 120 for also driving the Stirling
cooler 116.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] FIG. 6 illustrates the principles that are applied to
control technology for the present invention. In an enclosed
container of a liquefied gas, the pressure in the container is,
under most conditions, the saturation vapor pressure of the gas.
Saturation is when the number of molecules leaving the liquid
surface (vaporizing) equals the number of molecules returning to
the liquid surface (condensing). Saturation pressure is the vapor
pressure when the saturation condition exists. At saturation the
two phases are in equilibrium.
[0047] The saturation vapor pressure in a closed container is a
function of temperature. This is illustrated in FIG. 6. The curve
122 shows the relationship of temperature to the saturation vapor
pressure for LNG. The observation that is important to the present
invention is that, as the saturation vapor pressure increases, the
temperature increases. This means that the higher the pressure
within the container, the higher is the temperature at which the
saturation condition exists. The first important consequence is
that the higher the pressure within the container, the higher the
temperature at which a cold surface at the cold end of the Stirling
cryocooler is able to liquefy vapor within the container. In order
to liquefy vapor within the container, the Stirling cryocooler must
lift heat through the temperature differential from the cold
interior temperature within the container to the warmer ambient
temperature surrounding the container. The higher the liquefaction
temperature of the saturated LNG vapor, the closer the liquefaction
temperature is to the ambient temperature and therefore the smaller
the temperature differential through which the Stirling cryocooler
must lift heat. Of course the smaller the temperature differential
the less work the Stirling cryocooler must do and therefore the
less energy it consumes to do it. So the second important
consequence is that the higher the liquefaction temperature, the
less required cooling power (energy per unit of time to pump a unit
of heat from the cold end to the warm end of the cryocooler) is
needed to liquefy the LNG vapor. The curve 124 of FIG. 6
illustrates the relationship of the temperature of the LNG within
the container to the liquefaction energy requirement which is shown
as a normalized scale.
[0048] For example, looking at FIG. 6 curve 122, at a temperature
of about -165.degree. C., the pressure within the container is
approximately atmospheric pressure (approximately 1 bar absolute).
Looking at FIG. 6, its curve 124 and the normalized energy scale on
the right side of FIG. 6, the energy required to lift heat from
-165.degree. C. to 0.degree. C. is approximately 1.00. However, at
a temperature of about -125.degree. C. (pressure approximately 10
bar absolute), the normalized energy requirement is approximately
0.60. In other words liquefying the LNG vapor within a container at
-125.degree. C. and 10 bar requires only approximately 60% of the
energy that is required to liquefy LNG vapor at -165.degree. C. and
50 bar. If the container is constructed to retain the LNG at 25
bar, the gas temperature at that saturation vapor pressure would be
about -110.degree. C. At that temperature and pressure, liquefying
the LNG vapor within a container would require only approximately
half of the energy that is required to liquefy LNG vapor at
-165.degree. C. and 50 bar.
[0049] 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.
[0050] 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.
[0051] 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 FIG. 6, that
means that the cryocooler can operate approximately in the range of
-140.degree. C. to -108.degree. C. As also seen in FIG. 6,
operating an embodiment of the invention at a pressure of 20 bar
allows the cryocooler to consume only about half of the energy it
would consume if the pressure were 50 bar. The curve 124 of FIG. 6
illustrates the dramatic reduction in energy consumption that is
gained by increasing the temperature at which the LNG is
stored.
[0052] FIG. 10 illustrates a feedback control 128 for controlling
the Stirling cycle cooler 130 in a manner that maintains a designer
selected temperature or pressure within the highly insulated LNG
container 132. The Stirling cycle cooler 130 is mounted to the
container 132 and extends through a wall of the container 132 to a
cold end 134 of the cooler. The cold end 134 of the cooler 130 is
located in the vapor phase 136 above the liquid surface 138. The
Stirling cycle cooler 130 is driven by a prime mover 140, the
output power of which is controllably variable at a control input
142 for varying the heat transfer rate of the Stirling cycle cooler
130.
[0053] 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
FIG. 6, temperature and pressure have a direct correlation
indicated by the curve 122 so the control of either one is control
of the other. Consequently, in order to control the pressure in the
container, either the pressure can be directly sensed or the
temperature can be sensed and used to determine the pressure.
Because there is a direct correlation between temperature and
pressure, a mathematical equation or a look up table can be used to
convert a sensed temperature to the pressure in the container. For
the same reason, the feedback control can modulate the cooler heat
transfer rate to drive the temperature to a desired temperature and
therefore to the desired pressure.
[0054] 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.
[0055] As an alternative, the temperature or pressure sensor may
alternatively be positioned at 156 within the vapor phase 136. The
arrangement of FIG. 10 is the same basic arrangement for both the
on-board vehicle fuel tank of FIGS. 1 and 2 and the home refueling
station illustrated in FIG. 5. An outlet 158 for liquid phase LNG
leads to a vehicle engine in the case of an on-board fuel tank or
to the on-board fuel tank in the case of a home refueling station.
An inlet 160 for gaseous natural gas permits gas from a
conventional domestic source to be liquefied in the container 132
whether the container is an on-board fuel tank or a home refueling
station.
[0056] 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.
[0057] 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.
* * * * *