Mining machine and energy storage system for same

Abstract

A mobile mining machine includes a plurality of traction elements, a plurality of motors, a power source in electrical communication with the plurality of motors, and an energy storage system in electrical communication with the plurality of motors and the power source. Each of the motors is coupled to an associated one of the plurality of traction elements. Each of the motors is driven by the associated traction element in a first mode, and drives the associated traction element in a second mode. The energy storage system includes a shaft, a rotor secured to the shaft, a stator extending around the rotor, and a flywheel coupled to the shaft for rotation therewith. In the first mode, rotation of the motors causes rotation of the flywheel to store kinetic energy. In the second mode, rotation of the rotor and the flywheel discharges kinetic energy to drive the motors.

Claims

1. A mobile mining machine comprising: a plurality of traction elements; a plurality of motors, each motor being coupled to an associated one of the plurality of traction elements, each motor configured to be driven by the associated traction element in a first mode, each motor configured to drive the associated traction element in a second mode; a switched reluctance motor in electrical communication with the plurality of motors; and an energy storage system in electrical communication with the plurality of motors and the power source, the energy storage system including a shaft defining a shaft axis, a rotor secured to the shaft, a stator extending around the rotor and around the shaft axis, and a flywheel coupled to the shaft for rotation therewith, wherein, in the first mode, rotation of the plurality of motors causes rotation of the flywheel to store kinetic energy, wherein, in the second mode, rotation of the rotor and the flywheel discharges kinetic energy to drive the plurality of motors.

2. The mobile mining machine of claim 1, further comprising a bi-directional electrical bus providing electrical communication between the motors, the power source, and the energy storage system; and a plurality of power converters, each power converter associated with one of the motors, the power source, and the energy storage system.

3. The mobile mining machine of claim 2, wherein in the first mode the rotation of the motors transmits electrical energy to the energy storage system via the bus, the electrical energy causing rotation of the flywheel, wherein in the second mode rotation of the rotor and the flywheel is converted to electrical energy and transmitted to the motors via the bus.

4. The mobile mining machine of claim 1, further comprising a plurality of braking mechanisms, each of the braking mechanisms associated with one of the motors, wherein when the speed of the motor is reduced, the motor supplies energy to the energy storage system.

5. The mobile mining machine of claim 4, wherein, in the first mode, when the energy storage system reaches a maximum storage capacity, additional energy created by the rotation of the plurality of motors is transmitted to one of the braking mechanisms and dissipated as heat.

6. The mobile mining machine of claim 1, wherein the switched reluctance motor is coupled to one of more hydraulic pumps for driving auxiliary actuators.

7. The mobile mining machine of claim 1, wherein, in the first mode, the switched reluctance motor may receive energy from the plurality of motors, the switched reluctance motor driving an engine at a constant speed to reduce fuel consumption.

8. The mobile mining machine of claim 1, wherein the flywheel may rotate at least 3000 revolutions per minute, and wherein the energy storage system outputs energy of at least approximately 1 megajoule.

9. The mobile mining machine of claim 1, wherein the flywheel includes a first portion and a second portion, the first portion extending radially outward from the shaft and defining a periphery spaced apart from the shaft, the second portion connected to the periphery and extending axially along a portion of the shaft.

10. The mobile mining machine of claim 9, wherein the second portion extends around the rotor and the stator.

11. A mobile haulage vehicle comprising: a chassis; a boom including a first end pivotably coupled to the chassis and a second end; an attachment coupled to the second end of the boom; and a drive system including, a bi-directional electrical bus; a plurality of traction elements supporting the chassis; a plurality of motors, each motor being coupled to an associated one of the plurality of traction elements and in electrical communication with the bus, each motor configured to be driven by the associated traction element in a first mode, each motor configured to drive the associated traction element in a second mode; a switched reluctance motor in electrical communication with the plurality of motors via the bus; and an energy storage system in electrical communication with the plurality of motors and the switched reluctance motor via the bus, the energy storage system including a housing secured to the chassis, a shaft, a rotor secured to the shaft, a stator, and a flywheel coupled to the shaft for rotation therewith, the shaft defining a shaft axis and supported for rotation relative to the housing, the stator extending around the rotor and around the shaft axis, wherein, in the first mode, rotation of the plurality of motors transmits electrical energy to the energy storage system via the bus, the electrical energy driving rotation of the flywheel to store kinetic energy, wherein, in the second mode, rotation of the rotor and the flywheel transmits electrical energy to the motors via the bus, driving the plurality of motors.

12. The mobile haulage vehicle of claim 11, further comprising a plurality of power converters, each power converter providing electrical communication between the bus and one of the energy storage system, the switched reluctance motor, and one of the motors.

13. The mobile haulage vehicle of claim 11, further comprising a plurality of braking mechanisms, each of the braking mechanisms associated with one of the motors, wherein when the speed of the motor is reduced, the motor supplies energy to the energy storage system.

14. The mobile haulage vehicle of claim 11, wherein, in the first mode, when the energy storage system reaches a maximum storage capacity, additional energy created by the rotation of the plurality of motors is transmitted to one of the braking mechanisms and dissipated as heat.

15. The mobile haulage vehicle of claim 11, wherein the drive system further includes an engine coupled to the switched reluctance motor.

16. The mobile haulage vehicle of claim 11, wherein the switched reluctance motor is coupled to at least one hydraulic pump for driving at least one auxiliary actuator.

17. The mobile haulage vehicle of claim 11, wherein, in the first mode, the switched reluctance motor may receive energy from the plurality of motors, the switched reluctance motor driving an engine at a constant speed to reduce fuel consumption.

18. The mobile haulage vehicle of claim 11, wherein the flywheel may rotate between approximately 0 revolutions per minute and approximately 6500 revolutions per minute, and wherein the energy storage system outputs energy of up to approximately 3 megajoules.

19. A drive system for a haulage vehicle, the drive system comprising: a bi-directional electrical bus; a plurality of wheels; a plurality of motors, each motor being coupled to an associated one of the plurality of wheels and in electrical communication with the bus, each motor configured to be driven by the associated wheel in a first mode, each motor configured to drive the associated wheel in a second mode; a plurality of power converters, each power converter providing electrical communication between the bus and one of the motors; a switched reluctance motor in electrical communication with the plurality of motors via the bus, the switched reluctance motor coupled to at least one hydraulic pump for driving at least one auxiliary actuator; an engine coupled to the switched reluctance motor; and an energy storage system in electrical communication with the plurality of motors and the switched reluctance motor via the bus, the energy storage system including a housing, a shaft defining a shaft axis and supported for rotation relative to the housing, a rotor secured to the shaft, a stator, and a flywheel coupled to the shaft for rotation about the shaft axis, the stator extending around the rotor and around the shaft axis.

20. The drive system of claim 19, wherein in a first mode the rotation of the motors transmits electrical energy to the energy storage system and causes rotation of the flywheel, wherein in a second mode rotation of the rotor and the flywheel discharges energy to drive the plurality of motors.

21. The drive system of claim 19, further comprising a plurality of braking mechanisms, each of the braking mechanisms associated with one of the motors, wherein when the speed of one of the motors is reduced, the one motor supplies energy to the energy storage system.

22. The drive system of claim 19, wherein, in at least one mode of operation, the switched reluctance motor may receive energy from the plurality of motors, the switched reluctance motor driving the engine at a constant speed to reduce fuel consumption.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of a mining machine according to one embodiment.

(2) FIG. 2 is a perspective view of a portion of the mining machine of FIG. 1.

(3) FIG. 3 is a plan view of the mining machine of FIG. 1.

(4) FIG. 4 is a plan view of a drive train of the mining machine of FIG. 1.

(5) FIG. 5A is a schematic view of a drive train.

(6) FIG. 5B is a schematic view of potential power transmission paths in the drive train of FIG. 2A.

(7) FIG. 5C is a schematic view of a drive train during a charging mode.

(8) FIG. 5D is a schematic view of a drive train during a discharge mode of an energy storage device.

(9) FIG. 5E is a schematic view of a drive train during a drive mode.

(10) FIG. 5F is a schematic view of a drive train during a light braking mode.

(11) FIG. 5G is a schematic view of a drive train during a heavy braking and charging mode.

(12) FIG. 5H is a schematic view of a drive train during a heavy braking mode without charging an energy storage device.

(13) FIG. 6 is a perspective view of an energy storage system.

(14) FIG. 7 is a side section view of the energy storage system of FIG. 6 viewed along section 7-7.

(15) FIG. 8 is an exploded view of the energy storage system of FIG. 6.

(16) FIG. 9 is an enlarged view of section 9-9 of the side section view of FIG. 7.

(17) FIG. 10 is an enlarged view of section 10-10 of the side section view of FIG. 7.

(18) FIG. 11 is a side view of an energy storage device according to another embodiment.

(19) FIG. 12 is a side view of an energy storage device according to another embodiment.

(20) FIG. 13 is an end view of the energy storage device of FIG. 12.

DETAILED DESCRIPTION

(21) Before any embodiments are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings.

(22) In addition, it should be understood that embodiments may include hardware, software, and electronic components or modules that, for purposes of discussion, may be illustrated and described as if the majority of the components were implemented solely in hardware. However, one of ordinary skill in the art, and based on a reading of this detailed description, would recognize that, in at least one embodiment, aspects may be implemented in software (e.g., stored on non-transitory computer-readable medium) executable by one or more processing units, such as a microprocessor and/or an application specific integrated circuits (“ASICs”). As such, it should be noted that a plurality of hardware and software based devices, as well as a plurality of different structural components may be utilized to implement the invention. For example, “controllers” described in the specification can include one or more processing units, one or more computer-readable medium modules, one or more input/output interfaces, and various connections (e.g., a system bus) connecting the components.

(23) FIGS. 1-3 illustrate a mining machine 100 according to one embodiment. In the illustrated embodiment, the mining machine 100 is a load-haul-dump (“LHD”) machine. The machine 100 can be an underground mining machine (e.g., a continuous miner, a haulage system, a longwall shearer, a loader, etc.) or a surface mining machine (e.g., a wheel loader, a hybrid shovel, a dragline miner, etc.). In the illustrated embodiment, the mining machine 100 further includes a chassis 102, boom 104 having a first end 106 coupled to the chassis 102 and a second end 108 coupled to an attachment 112 (e.g., a bucket). In the illustrated embodiment, the chassis 102 also includes an operator cab 114. The mining machine 100 further includes traction elements, such as wheels 110, rotatably coupled to the chassis 102 and supporting the chassis 102 for movement over the ground. As shown in FIG. 2, a kinetic energy storage system (“KESS”) or energy storage device 135 is supported on the chassis 102. In the illustrated embodiment, the energy storage device 135 is positioned proximate an end of the chassis 102 opposite the attachment 112.

(24) FIG. 4 illustrates the primary components of the drive system or drive train 118 of the mining machine 100. The drive train 118 may include an engine 115, a generator 120, a power converter 125, motors 130, and the energy storage device 135. In some embodiments, the machine 10 may include multiple power converters, multiple motors, and/or multiple energy storage devices. The engine 115 provides power, in the form of mechanical energy, to the generator 120. In some embodiments, the engine 115 is a diesel engine. In some embodiments, the engine 115 provides an average power output of 180 horsepower (“Hp”) and a peak power output of 300 Hp. As discussed in further detail below, the energy storage device 135 can be used as a power averaging device, discharging stored energy during periods of peak power demand. The energy storage device 135 may supplement power supplied by the engine 115 in order to reduce the need to operate the engine 115 at peak power output.

(25) The generator 120 converts mechanical energy received from the engine 115 into electrical energy. In some embodiments, the generator 120 is a switched reluctance (“SR”) motor/generator. In other embodiments, the generator 120 is another type of direct current (“DC”) motor/generator. In other embodiments, the generator 120 is an alternating current (“AC”) motor/generator. In some embodiments, the generator 120 can also be used as a motor that increases the revolutions per minute (“RPM”) of the engine 115 (e.g., as an energy storage mechanism used separately or in combination with the energy storage device 135 described below).

(26) FIG. 5A schematically illustrates the components of a drive train for the mining machine 100. In the illustrated embodiment, the generator 120 transmits power to a converter 125 that converts a received energy into a second energy via a bus 140 (e.g., a DC bus). The bus 140 is in communication with additional converters 125, each of which transmits the second energy output to a traction motor 130. The converters 125 can be configured to transmit energy through the bus 140 or to receive power from the bus 140. The traction motors 130 convert electrical energy into rotational energy or torque to drive the wheels 110 (FIG. 2A) or other components of the mining machine 100. In some embodiments, the motors 130 include a motor for each wheel 110 of the machine 100. Each traction motor 130 is associated with a braking grid 142 that converts kinetic energy from the traction motor 130 into thermal energy when brakes are applied to slow down the machine 100. In the illustrated embodiment of mining machine 100, the motors 130 include a left-front (“LF”) motor 130a, a right-front motor (“RF”) 130b, a left-rear (“LR”) motor 130c, and a right-rear (“RR”) motor 130d. The motors 130 are used to propel (forward and reverse), brake (forward and reverse), and control tire slip.

(27) In some embodiments, one or more of the motors 130 are switched-reluctance (“SR”) motors. In such an embodiment, the SR motor may provide full torque at stall (i.e., when the output rotational speed is zero) while consuming a small percentage of the power output of the engine 115, which saves fuel consumption and reduces emissions. It should be understood that in other embodiments, the mining machine 100 can include fewer or additional motors.

(28) Referring to FIG. 5A, the generator 120 is also in communication with one or more components of the mining machine 100. These components may operate other aspects of the machine 100 (e.g., actuating a loading bucket or driving a cutter head). For example, in some embodiments, the generator 120 converts electrical energy to mechanical energy that drives one or more hydraulic components 132 (e.g., pumps and/or valves). The hydraulic components 132 supply hydraulic energy to the hydraulic systems such as actuators 134. The hydraulic systems can perform hoisting, steering, rotating, and/or other auxiliary functions of the mining machine 100. The hydraulic components 132 may also operate parasitic components 136, such as a cooling fan.

(29) In one embodiment, the energy storage device 135 may be charged by capturing braking energy from the traction system and/or by receiving power from the engine 115 and generator 120 during times of low power demand. The energy storage device 135 receives and stores electrical energy from the generator 120 via the bus 140. The energy storage device 135 also outputs stored electrical energy to other components of the mining machine 100 (e.g., the converters 125, the motors 130, a hydraulic system, etc.). In operation, each energy storage device 135 is configured to store electrical energy when there is available (i.e., excess) power from the engine 115 and output stored energy when energy demand is greater than the engine 115 can provide. In some embodiments, the energy storage device 135 includes a SR motor/generator (e.g., variable speed SR motor/generator).

(30) In one embodiment, the primary energy source for the energy storage device 135 is the traction system. When the components (e.g., the wheels 110 and motors 130) of the traction system are braking or slowing down, the energy of the slowing wheels is transmitted to the energy storage device 135 and stored as rotational energy in an inertial mass (i.e., flywheel 180).

(31) FIG. 5B illustrates various potential power transmission paths through the drive train 118. For example, the generator 120 and engine 115 can provide power to the hydraulic pumps 132, and the generator 120 can also receive energy from the bus 140 (e.g., when the traction system is braking). Also, each motor 130 can receive energy from the bus 140 and supply energy to the bus 140. Similarly, the energy storage device 135 can receive energy from the bus 140 and supply energy to the bus 140. In some embodiments, each motor 130 may include a mechanical brake (not shown). When a controller detects that the mechanical brake of the motor 130 is engaged, the speed of the motor 130 is retarded or reduced to inhibit propulsion of the machine. Braking mechanisms (e.g., braking grid resistors 142) may receive energy from the bus 140 and dissipate the energy as heat.

(32) FIG. 5C illustrates a power flow path through the drive train 118 when the energy storage system 135 is charged. Power supplied by the generator 120 is provided to the bus 140, which transmits power to the energy storage device 135. In some embodiments, the energy storage device 135 is charged during start-up of the machine 100. The energy storage device 135 may be charged during times of low load on the generator 120 (i.e., the generator 120 receives surplus energy from the engine 115 than is required to operate the traction motors 130 or the other components of the machine 100).

(33) FIG. 5D illustrates a power flow path through the drive train 118 when the traction motors 130 are driven to propel the machine 100. The energy storage device 135 can discharge and transmit power to the bus 140, which transmits the power to the motors 130 to drive the wheels 110. In some embodiments, the energy storage device 135 acts as the primary or master power source for the motors 130 and provides all of the energy required to drive the motors 130. If the energy storage device 135 cannot supply all of the energy required by the motors 130, the generator 120 and engine 115 supply additional power to the bus 140 that can be consumed by the motors 130. In this arrangement the energy storage device 135 is the primary power supply for the motors 130 and the generator 120 provides auxiliary or backup power.

(34) In one embodiment, the energy storage device 135 is a more responsive power source than the generator 120. The drive train 118 relies on the most responsive power source first, allowing the traction system to accelerate and decelerate faster than a conventional drive system. Furthermore, using the energy storage system 135 as the primary energy source reduces the need to operate the engine 115 at its full output. Rather, using the energy storage device 135 as the primary power source to the traction system allows the engine 115 to operate at a steadier output, thereby reducing fuel consumption, engine output requirements, and engine wear 115.

(35) In another mode of operation, shown in FIG. 5E, the drive train 118 may operate the traction motors 130 without using the energy storage device 135. That is, the energy supplied to the motors 130 via the bus 140 is supplied solely by the generator 120. This mode may be implemented when the energy storage device 135 is not charged, is malfunctioning, or is not present.

(36) FIGS. 5F-5H illustrate power flow paths when the machine 100 is braking and the motors 130 act as generators supplying electrical energy to the bus 140. During light braking (FIG. 5D), the energy supplied by the motors 108 can be supplied to the generator 120. The generator 120 can use the received energy to speed up the drive line between the generator 120 and the hydraulic pumps 132 (e.g., to speed up the engine 115 to a set speed at which fuel injectors are programmed to cease delivering fuel to the engine 115). In some situations, this mode of operation reduces engine fuel consumption (e.g., to operate at zero fuel or near-zero fuel levels).

(37) During heavy braking, shown in FIG. 5G, the motors 130 may generate more energy than the energy generated during light braking. Therefore, the energy generated by the motors 130 and supplied to the bus 140 may be transmitted to both the generator 120 and to charging the energy storage device 135. In another mode (FIG. 5H), the motors 130 may perform heavy braking without charging the energy storage device 135 (e.g., the energy storage device 135 is full, malfunctioning, or not present). Although some of the power supplied to the bus 140 from the motors 130 is transmitted to the generator 120, additional or excess energy can be supplied to one or more of the braking grids 142 to dissipate the energy as heat.

(38) Other modes of operation can be used with the energy storage device 135. For example, in some embodiments, the generator 120 can be used as the primary power source of the traction system and the energy storage device 135 can provide backup power. A controller can be incorporated and programmed to control the energy storage device 135 based on the operating speed of the traction system.

(39) Referring now to FIG. 6, the energy storage device 135 includes a housing 165 having feet 170 mounted on the chassis 102 (FIG. 1). The housing 165 also includes a junction box 168 in communication with the generator 102 (FIG. 4). As shown in FIGS. 7 and 8, the energy storage device 135 further includes a shaft 175, a flywheel 180 coupled to the shaft 175, and a motor stator 185 including coils 188. The shaft 175 extends through the housing 165 and includes a first end 190 and a second end 195. A shaft axis 200 extends between the first end 190 and the second end 195. Each end 190, 195 of the shaft 175 is supported for rotation relative to the housing 165 by bearings 205 (see also FIGS. 9 and 10). In the illustrated embodiment, the bearings 205 are double ball bearings. A lamination stack 210 forms a rotor and is secured to the outer surface of the shaft 175 proximate the first end 190. In the illustrated embodiment, the flywheel 180 is axially spaced apart from the rotor 210.

(40) Referring to FIGS. 7 and 8, the motor stator 185 is secured within the housing 165 and extends around the lamination stack 210. The flywheel 180 is positioned within the housing 165. The flywheel 180 is secured to the shaft 175 proximate the second end 195, such that the flywheel 180 is spaced apart from the stator 185 along the axis 200. In the illustrated embodiment, the flywheel 180 is positioned between the bearings 205. That is, the second end 195 of the shaft 175 extends beyond the flywheel 180 and is supported for rotation by a bearing 205b. The rotation of the flywheel 180 and the operation of the machine induces a gyroscopic load on the bearings, and this load is related to the distance between the bearings and the gyroscopic load. Increasing the distance between the flywheel and the bearings reduces the resultant load on the bearings.

(41) In conventional energy storage systems, larger energy storage capacity requires larger masses for the flywheel/storage component. Increasing the mass of the flywheel 180 increases the gyroscopic loads on the bearings. The configuration of the flywheel 180 with respect to the bearings 205 reduces the gyroscopic loads applied to the bearings 205 during operation. This allows a larger inertial mass, which in turn increases the energy storage capacity of the device 135. Increasing the energy storage capacity reduces the demand for engine power. In some embodiments, the increased storage capacity reduces the required engine output power by 50%.

(42) The flywheel 180 stores kinetic energy in the form of rotational energy. The energy storage device 135 is configured to receive electrical energy and output rotational energy, as well as to receive rotational energy and output electrical energy. In some embodiments, the flywheel 180 is capable of rotating at speeds between approximately 0 revolutions per minute (rpm) and approximately 6,500 rpm. In some embodiments, the maximum rotational speed of the flywheel 180 is between approximately 3,000 rpm and approximately 10,000 rpm. In some embodiments, the maximum rotational speed of the flywheel 180 is between approximately 5,000 rpm and approximately 8,000 rpm. In some embodiments, the maximum rotational speed of the flywheel is approximately 6,500 rpm. Also, in some embodiments, the maximum energy storage and discharge capacity of the energy storage device 135 is between approximately 1 megajoule and approximately 15 megajoules. In some embodiments, the maximum energy storage and discharge capacity of the energy storage device 135 is between approximately 2 megajoules and approximately 7 megajoules. In some embodiments, the maximum energy storage and discharge capacity of the energy storage device 135 is approximately 3 megajoules.

(43) In operation, the energy storage device 135 may receive electrical energy from, e.g., the generator 120. The electrical energy in the stator 185 induces the rotor shaft 175 to rotate about the shaft axis 200, thereby rotating the flywheel 180 and storing kinetic energy in the form of rotational energy in the flywheel 165. To discharge or extract the stored energy (i.e., to send electrical energy out of the energy storage device 135), the rotation of flywheel 180 is used to rotate the rotor shaft 175. Rotation of the rotor 175 in this manner acts as a generator to induce a current in the stator 185, thereby converting rotational energy into electrical energy. The electrical energy can be provided to other components of the mining machine 100, such as the motors 130. In some embodiments, when the energy storage device 135 is used in the mining machine 100, one of the converters 125 that would normally serve the generator 120 becomes the converter for the energy storage device 135.

(44) FIG. 1 illustrates an energy storage device 535 according to another embodiment. A flywheel 580 is formed as a cylindrical member, such that the flywheel 580 includes a first or web portion 582 coupled to the shaft 175 and extending radially outwardly from the axis 200 of the shaft 175. The web portion 582 includes an outer periphery. The flywheel 580 further includes a cylindrical portion 584 extending from the periphery of web portion 582 along the axis 200 of the shaft 175. In the illustrated embodiment, the cylindrical portion 584 extends around the rotor lamination stack 610 and the stator 585, and the cylindrical portion 584 extends along the length of the rotor and stator assembly. In other embodiments, the cylindrical portion 584 may have a different length compared to the rotor and stator assembly. In some embodiments, the stator 585 is secured to an end wall 172 of the housing 165. This configuration increases the power density of the energy storage device per unit of mass.

(45) FIGS. 12 and 13 illustrate an energy storage device 935 according to another embodiment. The rotor and flywheel are formed as a single assembly 975 rotating about a stationary stator core 985. The stator core 985 is positioned circumferentially within the rotor/flywheel assembly 975. The stator core 985 is supported on a shaft 982, and the rotor/flywheel assembly 975 is supported for rotation relative to the shaft 982 by bearings 1005. This configuration concentrates the inertial mass at an outer perimeter of the motor, thereby providing, among other things, more efficient energy storage per unit of mass and volume.

(46) Although some aspects have been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of one or more independent aspects as described.