Patent Application
20130327029
In various embodiments, the present invention relates to hydraulics, pneumatics, power generation, and energy storage, and more particularly, to compressed-gas energy-storage systems using pneumatic and/or hydraulic cylinders.
Storing energy in the form of compressed gas has a long history and components tend to be well tested, reliable, and have long lifetimes. The general principle of compressed-gas energy storage (CAES) is that generated energy (e.g., electric energy) is used to compress gas (e.g., air), thus converting the original energy to pressure potential energy; this potential energy is later recovered in a useful form (e.g., converted back to electricity) via gas expansion coupled to an appropriate mechanism. Advantages of compressed-gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
If expansion occurs slowly relative to the rate of heat exchange between the gas and its environment, then the gas remains at approximately constant temperature as it expands. This process is termed “isothermal” expansion. Isothermal expansion of a quantity of gas stored at a given temperature recovers approximately three times more work than would “adiabatic expansion,” that is, one in which no heat is exchanged between the gas and its environment, because the expansion happens rapidly or in an insulated chamber. Gas may also be compressed isothermally or adiabatically.
An ideally isothermal energy-storage cycle of compression, storage, and expansion would have 100% thermodynamic efficiency. An ideally adiabatic energy-storage cycle would also have 100% thermodynamic efficiency, but there are many practical disadvantages to the adiabatic approach. These include the production of higher temperature and pressure extremes within the system, heat loss during the storage period, and inability to exploit environmental (e.g., cogenerative) heat sources and sinks during compression and expansion, respectively. In an isothermal system, the cost of adding a heat-exchange system is traded against resolving the difficulties of the adiabatic approach. In either case, mechanical energy from expanding gas must usually be converted to electrical energy before use.
An efficient and novel design for storing energy in the form of compressed gas utilizing near isothermal gas compression and expansion has been shown and described in U.S. patent application Ser. Nos. 12/421,057 (the '057 application) and 12/639,703 (the '703 application), the disclosures of which are hereby incorporated herein by reference in their entireties. The '057 and '703 applications disclose systems and methods for expanding gas isothermally in staged hydraulic/pneumatic cylinders and intensifiers over a large pressure range in order to generate electrical energy when required. Mechanical energy from the expanding gas is used to drive a hydraulic pump/motor subsystem that produces electricity.
Additionally, in various systems disclosed in the '057 and '703 applications, reciprocal motion is produced during recovery of energy from storage by expansion of gas in the cylinders. This reciprocal motion may be converted to electricity by a variety of means, for example as disclosed in U.S. Provisional Patent Application Nos. 61/257,583 (the '583 application), 61/287,938 (the '938 application), and 61/310,070 (the '070 application), the disclosures of which are hereby incorporated herein by reference in their entireties.
The ability of such systems to either store energy (i.e., use energy to compress gas into a storage reservoir) or produce energy (i.e., expand gas from a storage reservoir to release energy) will be apparent to any person reasonably familiar with the principles of electrical and pneumatic machines.
Various embodiments described in the '057 application involve several energy conversion stages: during compression, electrical energy is converted to rotary motion in an electric motor, then converted to hydraulic fluid flow in a hydraulic pump, then converted to linear motion of a piston in a hydraulic-pneumatic cylinder assembly, then converted to mechanical potential energy in the form of compressed gas.
Conversely, during retrieval of energy from storage by gas expansion, the potential energy of pressurized gas is converted to linear motion of a piston in a hydraulic-pneumatic cylinder assembly, then converted to hydraulic fluid flow through a hydraulic motor to produce rotary mechanical motion, then converted to electricity using a rotary electric generator.
Both these processes—storage and retrieval of energy—present opportunities for improvement of efficiency, reliability, and cost-effectiveness. One such opportunity is created by the fact that the pressure in any pressurized gas-storage reservoir tends to decrease as gas is released from it. Moreover, when discrete quantities or installments of gas are released into the pneumatic side of a pneumatic-hydraulic intensifier for the purpose of driving its piston, as described in the '057 application, the force acting on the piston declines as the installment of gas expands. The result, in a system where the hydraulic fluid pressurized by the intensifier is use to drive a hydraulic motor/pump, is variable hydraulic pressure driving the motor/pump. For a fixed-displacement hydraulic motor/pump whose shaft is affixed to that of an electric motor/generator, this will result in variable electrical power output from the system. This is disadvantageous because (a) it is desirable that the power output of an energy storage system be approximately constant (b) a hydraulic motor/pump or electric motor/generator runs most efficiently over a limited power range. Widely varying hydraulic pressure is therefore intrinsically undesirable. A variable-displacement hydraulic motor may be used to achieve constant power output despite varying hydraulic pressure over a certain range of pressures, yet the pressure range must still be limited to maximize efficiency.
Another opportunity is presented by the fact that pneumatic-hydraulic intensifier cylinders that may be utilized in systems described in the '057 and '703 applications may be custom-designed and built, and may therefore be difficult to service and maintain. Energy-storage systems utilizing more standard components that enable more efficient maintenance through, e.g., straightforward access to seals, would increase up-time and decrease total cost-of-ownership.
Embodiments of the present invention enable the delivery of hydraulic flow to a motor/generator combination over a narrower pressure range in systems utilizing inexpensive, conventional components that are more easily maintained. Such embodiments may be incorporated in the above-referenced systems and methods described in the patent applications incorporated herein by reference above. For example, various embodiments of the invention relate to the incorporation into an energy storage system (such as those described in the '057 application) of distinct pneumatic and hydraulic free-piston cylinders, mechanically coupled to each other by some appropriate armature, rather than a single pneumatic-hydraulic intensifier.
At least three advantages accrue to such arrangements. First, components that transfer heat to and from the gas being expanded (or compressed) are naturally separated from the hydraulic circuit. Second, by mechanically coupling multiple pneumatic cylinders and/or multiple hydraulic cylinders so as to add (or share) forces produced by (or acting on) the cylinders, the hydraulic pressure range may be narrowed, allowing more efficient operation of the hydraulic motor/pump and the other benefits noted above. Third, maintenance on gland seals is easier on separated hydraulic and pneumatic cylinders than in a coaxial mated double-acting intensifier wherein the gland seal is located between two cylinders and is not easily accessible.
In compressed-gas energy storage systems in accordance with various embodiments of the invention, gas is stored at high pressure (e.g., approximately 3000 pounds per square inch (psi)). In one embodiment, this gas is expanded into a cylindrical chamber containing a piston or other mechanism that separates the gas on one side of the chamber from the other, preventing gas movement from one chamber to the other while allowing the transfer of force/pressure from one chamber to the next. A shaft attached to the piston is attached to a beam or other appropriate armature by which it communicates force to the shaft of a hydraulic cylinder, also divided into two chambers by a piston. The active area of the piston of the hydraulic cylinder is smaller than the area of the pneumatic piston, resulting in an intensification of pressure (i.e., ratio of pressure in the chamber undergoing compression in the hydraulic cylinder to the pressure in the chamber undergoing expansion in the pneumatic cylinder) proportional to the difference in piston areas.
The hydraulic fluid pressurized by the hydraulic cylinder may be used to turn a hydraulic motor/pump, either fixed-displacement or variable-displacement, whose shaft may be affixed to that of a rotary electric motor/generator in order to produce electricity.
In other embodiments, the expansion of the gas occurs in multiple stages, using low- and high-pressure pneumatic cylinders. For example, in the case of two pneumatic cylinders, high-pressure gas is expanded in a high pressure pneumatic cylinder from a maximum pressure (e.g., approximately 3000 pounds per square inch gauge) to some mid-pressure (e.g., approximately 300 psig); then this mid-pressure gas is further expanded (e.g., approximately 300 psig to approximately 30 psig) in a separate low-pressure cylinder. These two stages may be tied to a common shaft or armature that communicates force to the shaft of a hydraulic cylinder as for the single-pneumatic-cylinder instance described above.
When each of the two pneumatic pistons reaches the limit of its range of motion, valves or other mechanisms may be adjusted to direct higher-pressure gas to and vent lower-pressure gas from the cylinder's two chambers so as to produce piston motion in the opposite direction. In double-acting devices of this type, there is no withdrawal stroke or unpowered stroke: the stroke is powered in both directions.
The chambers of the hydraulic cylinder being driven by the pneumatic cylinders may be similarly adjusted by valves or other mechanisms to produce pressurized hydraulic fluid during the return stroke. Moreover, check valves or other mechanisms may be arranged so that regardless of which chamber of the hydraulic cylinder is producing pressurized fluid, a hydraulic motor/pump is driven in the same sense of rotation by that fluid. The rotating hydraulic motor/pump and electrical motor/generator in such a system do not reverse their direction of spin when piston motion reverses, so that with the addition of an short-term-energy-storage device such as a flywheel, the resulting system can be made generate electricity continuously (i.e., without interruption during piston reversal).
A decreased range of hydraulic pressures, with consequently increased motor/pump and motor/generator efficiencies, may be obtained by using multiple hydraulic cylinders. In various embodiments, two hydraulic cylinders are used. These two cylinders are connected to the aforementioned armature communicating force with the pneumatic cylinder(s). The chambers of the two hydraulic cylinders are attached to valves, lines, and other mechanisms in such a manner that either cylinder may, with appropriate adjustments, be set to present no resistance as its shaft is moved (i.e., compress no fluid).
Consider an exemplary system of the type described above, driven by a single pneumatic cylinder. Assume that a quantity of high-pressure gas has been introduced into one chamber of that cylinder. As the gas begins to expand, moving the piston, force is communicated by the piston shaft and the armature to the piston shafts of the two hydraulic cylinders. At any point in the expansion, the hydraulic pressure will be equal to the force divided by the acting hydraulic piston area. At the beginning of a stroke, the gas in the pneumatic cylinder has only begun to expand, it is producing maximum force; this force (ignoring frictional losses) acts on the combined total piston area of the hydraulic cylinders, producing a certain hydraulic output pressure, HPmax.
As the gas in the pneumatic cylinder continues to expand, it exerts decreasing force. Consequently, the pressure developed in the compression chamber of the active cylinders decreases. At a certain point in the process, the valves and other mechanisms attached to one of the hydraulic cylinders is adjusted so that fluid can flow freely between its two chambers and thus offers no resistance to the motion of the piston (ignoring frictional losses). The effective piston area driven by the force developed by the pneumatic cylinder thus decreases from the piston area of both hydraulic cylinders to the piston area of one of the hydraulic cylinders. With this decrease of area comes an increase in output hydraulic pressure for a given force. If this switching point is chosen carefully the hydraulic output pressure immediately after the switch returns to HPmax. (For the example of two identical hydraulic cylinders the switching pressure would be at the half pressure point.)
As the gas in the pneumatic cylinder continues to expand, the pressure developed by the hydraulic cylinder decreases. As the pneumatic cylinder reaches the end of its stroke, the force developed is at a minimum and so is the hydraulic output pressure, HPmin.
For an appropriately chosen ratio of hydraulic cylinder piston areas, the hydraulic pressure range HR=HPmax/HPmin achieved using two hydraulic cylinders will be the square root of the range HR achieved with a single pneumatic cylinder. The proof of this assertion is as follows.
Let a given output hydraulic pressure range HR1 from high pressure HPmax to low pressure HPmin, namely HR1=HPmax/HPmin, be subdivided into two pressure ranges of equal magnitude HR2. The first range is from HPmax down to some intermediate pressure HPI and the second is from HPI down to HPmin. Thus, HR2=HPmax/HPI=HPI/HPmin. From this identity of ratios, HPI=(HPma/HPmin)1/2. Substituting for HPI in HR2=HPmax/HPI, we obtain HR2=HPmax/(HPmax/HPmin)1/2=(HPmax/HPmin)1/2=HR11/2.
Since HPmax is determined (for a given maximum force developed by the pneumatic cylinder) by the combined piston areas of the two hydraulic cylinders (HA1+HA2), whereas HPI is determined jointly by the choice of when (i.e., at what force level, as force declines) to deactivate the second cylinder and by the area of the single acting cylinder HA1, it is clearly possible to choose the switching force point and HA1 so as to produce the desired intermediate output pressure. It may be similarly shown that with appropriate cylinder sizing and choice of switching points, the addition of a third cylinder/stage will reduce the operating pressure range as the cube root, and so forth. In general, N appropriately sized cylinders can reduce an original operating pressure range HR1 to HR11/N.
By similar reasoning, dividing the air expansion into multiple stages facilitates further reduction in the hydraulic pressure range. For M appropriately sized pneumatic cylinders (i.e., pneumatic air stages) for a given expansion, the original pneumatic operating pressure range PR1 of a single stroke can be reduced to PR11/M. Since for a given hydraulic cylinder arrangement the output hydraulic pressure range is directly proportional to the pneumatic operating pressure range for each stroke, simultaneously combining M pneumatic cylinders with N hydraulic cylinders can realize a pressure range reduction to the 1/(N×M) power.
To achieve maximum efficiency it is desired that gas expansion be as near isothermal as possible. Gas undergoing expansion tends to cool, while gas undergoing compression tends to heat. Several modifications to the systems already described so as to approximate isothermal expansion can be employed. In one approach, also described in the '703 application, droplets of a liquid (e.g., water) are sprayed into the side of the double-acting pneumatic cylinder (or cylinders) presently undergoing compression to expedite heat transfer to/from the gas. Droplets may be used to either warm gas undergoing expansion or to cool gas undergoing compression. If the rate of heat exchange is sufficient, an isothermal process is approximated.
Additional heat transfer subsystems are described in the U.S. patent application Ser. No. 12/481,235 (the '235 application), the disclosure of which is hereby incorporated by reference herein in its entirety. The '235 application discloses that gas undergoing either compression or expansion may be directed, continuously or in installments, through a heat-exchange subsystem. The heat-exchange subsystem either rejects heat to the environment (to cool gas undergoing compression) or absorbs heat from the environment (to warm gas undergoing expansion). Again, if the rate of heat exchange is sufficient, an isothermal process is approximated.
Any implementation of this invention employing multiple pneumatic cylinders or multiple hydraulic cylinders such as that described in the above paragraphs may be co-implemented with either of the optional heat-transfer mechanisms described above.
Various other embodiments of the present invention counteract, in a manner that minimizes friction and wear, forces that arise when two or more hydraulic and pneumatic cylinders in a compressed-gas energy storage and conversion system are attached to a common frame and the distal ends of their piston shafts are attached to a common beam, as described above.
When two or more free-piston cylinders, each oriented with their piston movement in the same direction, are attached to a common rigid, stationary frame and the distal ends of their pistons are attached to a common rigid, mobile beam, the forces acting along the piston shafts of the several cylinders will not, in general, be equal in magnitude. Additionally, the forces may result in deformation of the frame, beam, and other components. The resulting imbalance of forces and deformations during operation may apply side loads and/or rotational torques to parts of the system that may be damaged or degraded as a result. For example, piston rods may snap if subjected to excessive torque, and seals may be damaged or wear rapidly if subjected to uneven side displacement and loads. Moreover, side loads and torques may increase friction, diminishing system efficiency. It is, therefore, desirable to manage unbalanced forces and deformations in such a system so as to minimize friction and other losses and to reduce undesirable forces acting on vulnerable components (e.g., seals, piston rods).
For any given set of hydraulic and pneumatic cylinders, oriented and mounted as described above, with known operating pressures and linear speeds, one or more optimal arrangements may be determined that will minimize important peak or average operating values such as torques, deflections, and/or frictional losses. In general, close clustering of the cylinders tends to minimize deflections for a given beam thickness. As well, for identical cylinders operating over identical pressures and speeds, location of cylinders mirrored around the center axis typically will eliminate net torques and thus reduce frictions. In other instances, if the cylinders are mounted so that their central axes of motion all lie in a plane (e.g., cylinders are aligned in a single row), then unwanted forces tend to act almost exclusively in that single plane, restricting the dimensionality of the unwanted forces to two.
Further, when the moving beam reaches the end of its range of linear motion during either direction of motion of the cylinder pistons, an abrupt collision with the frame or some component communicating with the frame may occur before the piston reverses its direction of motion. The collision tends to dissipate kinetic energy, reducing system efficiency, and its suddenness, transmitted through the system as a shock, may accelerate wear to certain components (e.g., seals) or create excessive acoustic noise. Embodiments of the invention provide for managing these unwanted forces of collision as well as the unwanted torques and side loads already described.
Generally, embodiments that address these detrimental or unwanted forces include up to four different techniques or features. First, cylinders may be arranged to minimize important peak or average operating values such as torques, deflections, and/or frictional losses. Second, rollers (e.g., track rollers, linear guides, cam followers) may be mounted on the rigid, moving beam and roll vertically along grooves, tracks, or channels formed in the body of the frame. The rollers allow the beam to move with low friction and are positioned so that any torques applied to the beam by unbalanced piston forces are transmitted to the frame by the rollers, while keeping rotation and/or deformation of the beam within acceptable limits. This, in turn, reduces off-axis forces at the points where the pistons attach to the beam. Third, deflection of the rods and cylinders may be minimized by using a beam design (e.g., an I-beam section for a linear arrangement) that adequately resists deformation in the cylinder plane and reducing transmission to pistons of torque in the cylinder plane by attaching each piston to the beam using a revolute joint (pin joint). Fourth, stroke-reversal forces may be managed by springs (e.g., nitrogen springs) positioned so that at each stroke endpoint, the beam bounces non-dissipatively, rather than colliding with the frame or some component attached thereto.
The systems described herein may also be improved via the elimination (or substantial reduction) of air dead space therein. Herein, the terms “air dead space” or “dead space” refer to any volume within the components of a pneumatic system—including but not restricted to lines, storage vessels, cylinders, and valves—that at some point in the operation of the system is filled with gas at a pressure significantly lower than other gas which is about to be introduced into that volume for the purpose of doing work. At other points in system operation, the same physical volume within a given device may not constitute dead space.
Air dead space tends to reduce the amount of work available from a quantity of high-pressure gas brought into communication therewith. This loss of potential energy may be termed a “coupling loss.” For example, if gas is to be introduced into a cylinder through a valve for the purpose of performing work by pushing against a piston within the cylinder, and a chamber or volume exists adjacent the piston that is filled with low-pressure gas at the time the valve is opened, the high-pressure gas entering the chamber is immediately reduced in pressure during free expansion and mixing with the low-pressure gas and, therefore, performs less mechanical work upon the piston. The low-pressure volume in such an example constitutes air dead space. Dead space may also appear within that portion of a valve mechanism that communicates with the cylinder interior, or within a tube or line connecting a valve to the cylinder interior. Energy losses due to pneumatically communicating dead spaces tend to be additive.
Various systems and methods for reducing air dead space are described in U.S. Provisional Patent Application No. 61/322,115 (the '115 application), the disclosure of which is hereby incorporated by reference herein in its entirety. The '115 application discloses actively filling dead volumes (e.g., valve space, cylinder head space, and connecting hoses) with a mostly incompressible liquid, such as water, rather than with gas throughout an expansion and compression cycle of a compressed-air storage and recovery system.
Another approach to minimizing air dead volume is by designing components to minimize unused volume within valves, cylinders, pistons, and the like. One area for reduction of dead volume is in the connection of piping between cylinders. Embodiments of the present invention further reduce dead volume by locating paired air volumes together such that only a single manifold block resides between active air compartments. For example, in a two-stage gas compressor/expander, the high and low pressure cylinders are mounted back to back with a manifold block disposed in between.
All of the mechanisms described above for converting potential energy in compressed gas to electrical energy, including the heat-exchange mechanisms, can, if appropriately designed, be operated in reverse to store electrical energy as potential energy in compressed gas. Since the accuracy of this statement will be apparent to any person reasonably familiar with the principles of electrical machines, pneumatics, and the principles of thermodynamics, the operation of these mechanisms to store energy rather than to recover it from storage will not be described. Such operation is, however, explicitly encompassed within embodiments of this invention.
In one aspect, embodiments of the invention feature a system for energy storage and recover via expansion and compression of a gas, which includes first and second pneumatic cylinder assemblies. Each of the pneumatic cylinder assemblies includes or consists essentially of (i) a first compartment, (ii) a second compartment, (iii) a piston, slidably disposed within the cylinder assembly, separating the compartments, and (iv) a piston rod coupled to the piston and extending outside the first compartment. The piston rods of the pneumatic cylinder assemblies are mechanically coupled, and the pneumatic cylinder assemblies are coupled in series pneumatically, thereby reducing the force range produced during expansion or compression of a gas within the pneumatic cylinder assemblies. The pneumatic cylinder assemblies may be mechanically coupled in parallel such that, during a single stroke, their piston rods move in the same direction.
Embodiments of the invention may include one or more of the following, in any of a variety of combinations. The system may include a first hydraulic cylinder assembly and, fluidly coupled thereto such that a hydraulic fluid flows therebetween, a hydraulic motor/pump. The first hydraulic cylinder assembly may include or consist essentially of (i) a first compartment, (ii) a second compartment, (iii) a piston, slidably disposed within the cylinder assembly, separating the compartments, and (iv) a piston rod coupled to the piston, extending outside the first compartment, and mechanically coupled to the piston rods of the first and second pneumatic cylinder assemblies. The system may include a second hydraulic cylinder assembly fluidly coupled to the hydraulic motor/pump such that the hydraulic fluid flows therebetween. The second hydraulic cylinder assembly may include or consist essentially of (i) a first compartment, (ii) a second compartment, (iii) a piston, slidably disposed within the cylinder assembly, separating the compartments, and (iv) a piston rod coupled to the piston, extending outside the first compartment, and mechanically coupled to the piston rod of the first hydraulic cylinder assembly. The first and second hydraulic cylinder assemblies may be mechanically coupled in parallel such that, during a single stroke, their piston rods move in the same direction. The system may include a mechanism for selectively fluidly coupling the first and second compartments of the first hydraulic cylinder assembly, thereby reducing a pressure range of the hydraulic fluid flowing to the hydraulic motor/pump.
The system may include a second hydraulic cylinder assembly that includes or consists essentially of (i) a first compartment, (ii) a second compartment, and (iii) a piston, slidably disposed within the cylinder assembly, separating the compartments. The first hydraulic cylinder assembly may be telescopically disposed within the second hydraulic cylinder assembly and coupled to the piston of the second hydraulic cylinder assembly.
The system may include an armature coupled to the piston rods of the first and second pneumatic cylinder assemblies, thereby mechanically coupling the piston rods. The armature may include or consist essentially of a crankshaft assembly. A heat-transfer subsystem may be in fluid communication with at least one of the pneumatic cylinder assemblies. The heat-transfer subsystem may include a circulation apparatus for circulating a heat-transfer fluid through at least one compartment of at least one of the pneumatic cylinder assemblies. The heat-transfer subsystem may include a mechanism, e.g., a spray head and/or a spray rod, disposed within at least one compartment of at least one of the pneumatic cylinder assemblies for introducing the heat-transfer fluid. The heat-transfer subsystem may include a circulation apparatus and a heat exchanger, the circulation apparatus configured to circulate gas from at least one compartment of at least one of the pneumatic cylinder assemblies through the heat exchanger and back to the at least one compartment.
The system may include a manifold block on which the first and second pneumatic cylinder assemblies are mounted, and a connection between the first and second pneumatic cylinder assemblies may extend through the manifold block and have a length minimizing potential dead space between the first and second pneumatic cylinder assemblies. The first and second cylinder assemblies may be mounted on a first side of the manifold block. The first cylinder assembly may be mounted on a first side of the manifold block, and the second cylinder assembly may be mounted on a second side of the manifold block opposite the first side. During expansion or compression of gas, the piston of the first pneumatic cylinder assembly may move toward the manifold block and the piston of the second pneumatic cylinder assembly may move away from the manifold block.
The system may include (i) a frame assembly on which the first and second pneumatic cylinder assemblies are mounted, and (ii) a beam assembly, slidably coupled to the frame assembly, that mechanically couples the piston rods of the first and second pneumatic cylinder assemblies. The system may include a roller assembly disposed on the beam assembly for slidably coupling the beam assembly to the frame assembly, the roller assembly counteracting forces and torques transmitted between the first and second pneumatic cylinder assemblies and the beam assembly. The frame assembly may include a horizontal top support configured for mounting each pneumatic cylinder assembly thereto, and at least two vertical supports coupled to the horizontal top support, each of the vertical supports defining a channel for receiving a portion of the beam assembly. At least one additional cylinder assembly (e.g., a pneumatic cylinder assembly or a hydraulic cylinder assembly) may be mounted on the frame assembly. The first and second pneumatic cylinder assemblies and the at least one additional cylinder assembly may be aligned in a single row. Cylinder assemblies that each have substantially identical operating characteristics may be equally spaced about and disposed equidistant from a common central axis of the frame assembly.
In another aspect, embodiments of the invention feature a system for energy storage and recover via expansion and compression of a gas that includes a manifold block and first and second pneumatic cylinder assemblies mounted on the manifold block. Each of the pneumatic cylinder assemblies includes or consists essentially of (i) a first compartment, (ii) a second compartment, (iii) a piston, slidably disposed within the cylinder assembly, separating the compartments, and (iv) a piston rod coupled to the piston and extending outside the first compartment. A first platen is coupled to the piston rod of the first pneumatic cylinder assembly, and a second platen is coupled to the piston rod of the second pneumatic cylinder assembly. The second compartments of the pneumatic cylinder assemblies are selectively fluidly coupled via a connection disposed in the manifold block. During expansion or compression of a gas within the pneumatic cylinder assemblies, the first and second platens move reciprocally.
Embodiments of the invention may include one or more of the following, in any of a variety of combinations. The connection may have a length minimizing potential dead space between the first and second pneumatic cylinder assemblies. The first and second pneumatic cylinder assemblies may be mounted to a second manifold block, and the piston rods of the first and second pneumatic cylinder assemblies may extend through the second manifold block. The first compartments of the pneumatic cylinder assemblies may be selectively fluidly coupled via a second connection disposed in the second manifold block. The second connection may have a length minimizing potential dead space between the first and second pneumatic cylinder assemblies.
In a further aspect, embodiments of the invention feature a method for energy storage and recovery. Gas is expanded and/or compressed within a plurality of pneumatic cylinder assemblies that are coupled in series pneumatically, thereby reducing the range of force produced by or acting on the pneumatic cylinder assemblies during expansion or compression of the gas. The force may be transmitted between the pneumatic cylinder assemblies and at least one hydraulic cylinder assembly (e.g., a plurality of hydraulic cylinder assemblies) fluidly connected to a hydraulic motor/pump. One of the hydraulic cylinder assemblies may be disabled to decrease the range of hydraulic pressure produced by or acting on the hydraulic cylinder assemblies. The force may be transmitted between the pneumatic cylinder assemblies and a crankshaft coupled to a rotary motor/generator. The gas may be maintained at a substantially constant temperature during the expansion or compression.
These and other objects, along with advantages and features of the invention, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. As used herein, the term “substantially” means±10%, and, in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. Herein, the terms “liquid” and “water” refer to any substantially incompressible liquid, and the terms “gas” and “air” are used interchangeably.
In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:
The above-described cylinder embodiments may be utilized in a variety of energy-storage and recovery systems, as disclosed herein.
As shown, the system includes a pneumatic cylinder 701 divided into two compartments 702 and 703 by a piston 704. The cylinder 701, which is shown in a horizontal orientation in this illustrative embodiment but may be arbitrarily oriented, has one or more gas circulation ports 705 which are connected via piping 706 and valves 707 and 708 to a compressed-gas reservoir 709. The pneumatic cylinder 701 is connected via piping 710, 711 and valves 712, 713 to a second pneumatic cylinder 714 operating at a lower pressure than the first. Both cylinders 701, 714 are typically double-acting, and, as shown, are attached in series (pneumatically) and in parallel (mechanically). (Series attachment of the two cylinders means that gas from the lower-pressure compartment of the high-pressure cylinder is directed to the higher-pressure compartment of the low-pressure cylinder.)
Pressurized gas from the reservoir 709 drives the piston 704 of the double-acting high-pressure cylinder 701. Intermediate-pressure gas from the lower-pressure side 703 of the high-pressure cylinder 701 is conveyed through valve 712 to the higher-pressure chamber 715 of the lower-pressure cylinder 714. Gas is conveyed from the lower-pressure chamber 716 of the lower-pressure cylinder 714 through a valve 717 to a vent 718.
One primary function of this arrangement is to reduce the range of pressures over which the cylinders jointly operate. Note that as used herein the terms “pipe,” “piping” and the like shall refer to one or more conduits that are rated to carry gas or liquid between two points. Thus the singular term should be taken to include a plurality of parallel conduits where appropriate.
The piston shafts 719, 720 of the two cylinders act jointly to move a bar or armature 721 in the direction indicated by the arrow 722. The armature 721 is also connected to the piston shaft 723 of a hydraulic cylinder 724. The piston 725 of the hydraulic cylinder 724, impelled by the armature 721, compresses hydraulic fluid in the chamber 726. This pressurized hydraulic fluid is conveyed through piping 727 to an arrangement of check valves 728 that allow the fluid to flow in one direction (shown by arrows) through a hydraulic motor/pump, either fixed-displacement or variable-displacement, whose shaft drives an electric motor/generator. For convenience, the combination of hydraulic pump/motor and electric motor/generator is here shown as a single hydraulic power unit 729.
Hydraulic fluid at lessened pressure is conducted from the output of the hydraulic motor/pump to the lower-pressure chamber 730 of the hydraulic cylinder through a hydraulic circulation port 731.
Reference is now made to
The piston shafts 719, 720 of the two cylinders act jointly to move the armature 721 in the direction indicated by arrow 802. The armature 721 is also connected to the piston shaft 723 of a hydraulic cylinder 724. The piston 725 of the hydraulic cylinder 724, impelled by the armature 721, compresses hydraulic fluid in the chamber 730. This pressurized hydraulic fluid is conveyed through piping 803 to the aforementioned arrangement of check values 728 and hydraulic power unit 729. Hydraulic fluid at lessened pressure is conducted from the output of the hydraulic motor/pump to the lower-pressure chamber 726 of the hydraulic cylinder.
As shown, the stroke volumes of the two chambers of the hydraulic cylinder differ by the volume of the shaft 723. The resulting imbalance in fluid volumes expelled from the cylinder during the two stroke directions shown in
Reference is now made to
The pneumatic cylinder shaft 909 moves a bar or armature 910 in the direction indicated by the arrow 911. The armature 910 is also connected to the piston shafts 912, 913 of the double-acting hydraulic cylinders 902, 903.
In the state of operation shown in
The fluid in the high-pressure chambers of the two hydraulic cylinders 902, 903 is at a single pressure, and the fluid in the low-pressure chambers 916, 917 is also at a single pressure. In effect, the two cylinders 902, 903 act as a single cylinder whose piston area is the sum of the piston areas of the two cylinders and whose operating pressure, for a given driving force from the pneumatic piston 901, is proportionately lower than that of either cylinder 902 or cylinder 903 acting alone.
Reference is now made to
In effect, the acting hydraulic cylinder 902 has a smaller piston area providing a higher hydraulic pressure for a given force, than the state shown in
Reference is now made to
As in
The fluid in the high-pressure chambers of the two hydraulic cylinders 902, 903 is at a single pressure, and the fluid in the low-pressure chambers 1104, 1105 is also at a single pressure. In effect, the two cylinders 902, 903 act as a single cylinder whose piston area is the sum of the piston areas of the two cylinders and whose operating pressure, for a given driving force from the pneumatic cylinder 901, is proportionately lower than that of either cylinder 902 or cylinder 903 acting alone.
Reference is now made to
In effect, the acting hydraulic cylinder 902 has a smaller piston area providing a higher hydraulic pressure for a given force, than the state shown in
Additionally, valving may be added to cylinder 902 such that it may be disabled in order to provide another effective hydraulic piston area (considering that cylinders 902 and 903 have different diameters, at least in the depicted embodiment) to somewhat further reduce the hydraulic fluid range for a given pneumatic pressure range. Likewise, additional hydraulic cylinders with valve arrangements may be added to substantially further reduce the hydraulic fluid range for a given pneumatic pressure range.
Reference is now made to
The hydraulic cylinder shaft 1309 moves a bar or armature 1310 in the direction indicated by the arrow 1311. The armature 1310 is also connected to the piston shaft 1312 of the double-acting hydraulic cylinder 1302.
In the state of operation shown, the entire narrow cylinder 1302 acts as the shaft of the piston 1313 of the wider cylinder 1303. The piston 1313, cylinder 1302, and shaft 1312 of the hydraulic cylinder 1303 are moved in the indicated direction by the armature 1310. Compressed hydraulic fluid from the higher-pressure chamber 1314 of the larger diameter cylinder 1303 passes through a valve 1315 to the aforementioned arrangement of check values 728 and hydraulic power unit 729, producing electricity. Hydraulic fluid at lessened pressure is conducted from the output of the hydraulic pump/motor 729 through valve 1316 to the lower-pressure chamber 1317 of the hydraulic cylinder 1303.
In this state of operation, the piston 1318 of the narrower cylinder 1302 remains stationary with respect to cylinder 1302, and no fluid flows into or out of either of its chambers 1319, 1320.
Reference is now made to
In this manner, the effective piston area on the hydraulic side is changed during the pneumatic expansion, narrowing the hydraulic pressure range for a given pneumatic pressure range.
Reference is now made to
Compressed hydraulic fluid from the higher-pressure chamber 1317 of the wider cylinder 1303 passes through valve 1316 to the aforementioned arrangement of check values 728 and hydraulic power unit 729, producing electricity. Hydraulic fluid at lessened pressure is conducted from the output of the hydraulic pump/motor 729 through valve 1315 to the lower-pressure chamber 1314 of the hydraulic cylinder 1303.
In yet another state of operation of the illustrative embodiment of the invention shown in
The spray arrangement for heat exchange and/or the external heat-exchanger arrangement described in the above-incorporated '703 and '235 applications may be adapted to the pneumatic cylinders described herein, enabling approximately isothermal expansion of the gas in the high-pressure reservoir. Moreover, these identical exemplary embodiments may be operated as a compressor (not shown) rather than as a generator (shown). Finally, the principle of adding cylinders operating at progressively lower pressures in series (pneumatic and/or hydraulic) and in parallel or telescoped fashion (mechanically) may be carried out via two or more cylinders on the pneumatic side, the hydraulic side, or both.
The cylinder assemblies coupled to a rigid armature described above may be utilized in a variety of energy storage and recovery systems. Such systems may be designed so as to minimize deleterious friction and to balance the forces acting thereon to improve efficiency and performance. Further, such systems may be designed so as to minimize dead space therein, as described below.
As shown in
The basic arrangement of the cylinder assemblies may vary to suit a particular application and the various arrangements provide a variety of advantages. For example, as shown in
The distal ends of the rods are attached to a beam assembly 140 slidably coupled to the frame 1605. The pistons of the cylinder assemblies act upon the beam assembly, which is free to move vertically within the frame assembly. In one embodiment, the beam assembly 1640 is a rigid I-beam. The distal ends of the rods are attached to the beam assembly 1640 via revolute joints 1635, which reduce transmission to the pistons of moments or torques arising from deformations of the beam assembly 1640. Each revolute joint 1635 consists essentially of a clevis attached to an end of a rod 1630, an eye mounting bracket, and a pin joint, and rotates freely in the cylinder plane.
The system 1600 further includes roller assemblies 1645 that slidably couple the beam assembly 1640 to the frame assembly 1605 to ensure stable beam position. In this illustrative embodiment, sixteen track rollers 1645 are used to prevent the beam assembly 1640 from rotating in the cylinder plane, while allowing it to move vertically with low friction. Only four track rollers 1645 are shown in
The system 1600 may also include two air springs 1650 mounted on the underside of the frame's horizontal member 1625 with their pistons pointing down. The springs 1650 cushion any impacts arising between the beam assembly 1640 and frame assembly 1605 as the beam assembly 1640 travels vertically within the frame assembly 1605. The beam assembly 1640 rebounds from the springs 1650 at the extreme or turnaround point of an upward piston stroke.
The beam assembly 1640 is shown in greater detail in
Also shown in
Additional embodiments of the invention employ different component and frame proportions, different numbers and placements of hydraulic and pneumatic cylinders, different numbers and types of rollers, and different types of revolute joints. For example, V-notch rollers may be employed, running on complementary V tracks attached to the frame 1605. Such rollers are able to bear axial loads as well as transverse loads, such as those shown in
As shown in
Each cylinder body 2010, 2020 includes a first chamber or compartment 2016, 2026 and a second chamber or compartment 2018, 2028. The first cylinder compartments 2016, 2026 are disposed between their respective pistons 2012, 2022 and the manifold block 2030 and are sealed against leakage of pressurized air between the first and second compartments by a piston seal (not shown), such that gas may be compressed or expanded within the first compartments 2016, 2026 by moving their respective pistons 2012, 2022. The second cylinder compartments 2018, 2028, which are disposed farthest from the manifold block 2030, are typically unpressurized.
One advantage of this arrangement is that the high and low pressure cylinder compartments 2016, 2026 are in close proximity to one another and separated only by the manifold block 2030. In this way, during a multiple-stage compression or expansion, non-cylinder space (dead space) between the cylinder bodies 2010, 2020 is minimized. Additionally, any necessary valves may be mounted within the manifold block 2030, thereby reducing complexity related to a separate set of cylinder heads, valve manifold blocks, and piping.
The system 2000 shown in
As previously discussed, the cylinders 2010, 2020 may also include heat transfer subsystems for expediting heat transfer to the expanding or compressing gas. The heat transfer subsystems may include a spray head mounted on the bottom of piston 2022 for introducing a liquid spray into first compartment 2026 of the low pressure cylinder 2020 and at the bottom of the manifold block 2030 for introducing a liquid spray into the first compartment 2016 of the high pressure cylinder 2010. Such implementations are described in the '703 application. The rods 2014, 2024 may be hollow so as to pass water piping and/or electrical wiring to/from the pistons 2012, 2022. Spray rods may be used in lieu of spray heads, also as described in the '703 application. In addition, pressurized gas may be drawn from first compartments 2016, 2026 through heat exchangers as described in the '235 application.
Dead space within system 2000 may also be minimized in configurations in which cylinder bodies 2010, 2010 are mounted on the same side of manifold block 2030, as shown in
In various embodiments, system 2000 may incorporate double-acting cylinders and thus pressurize and/or recover gas during both upward and downward motion of their respective pistons. As shown in
Reference is now made to
The two pneumatic cylinder assemblies 2110, 2120 are identical in function to cylinder assembly 2001 of system 2000 described with respect to
As shown, the hydraulic cylinder assembly 2160 includes a first hydraulic cylinder body 2170 and a second hydraulic cylinder body 2180 that are mounted on the common manifold block 2130. The hydraulic cylinder bodies 2170, 2180 are mounted to the manifold block 2130 in such a manner as to be sealed against leakage of pressurized fluid between the cylinder bodies and the manifold block 2130 (e.g., flange mounted with an O-ring seal or threaded with sealing compound). The cylinder bodies 2170, 2180 each contain a piston 2172, 2182 and piston rod 2174, 2184 extending therefrom. The cylinder compartments 2176, 2186 between the pistons 2172, 2182 and the manifold block 2130 are sealed against leakage of pressurized fluid by piston seals (not shown), such that fluid may be pressurized by piston force or by pressurized flow from a hydraulic pump (not shown). The cylinder compartments 2178, 2188 farthest from the manifold block 2130 are typically unpressurized. The hydraulic cylinder assembly 2160 acts as a double-acting cylinder with fluid inlet and outlet ports 2190, 2192 formed in the manifold block 2130. The ports 2190, 2192 may be connected through a valve assembly to a hydraulic pump/motor (not shown) that allows for hydraulically harnessing work from expansion in the pneumatic cylinder assemblies 2110, 2120 and using hydraulic work by the hydraulic motor/pump to compress gas in the pneumatic cylinder assemblies 2110, 2120.
The second pneumatic cylinder assembly 2120 is mounted in an inverted fashion with respect to the first pneumatic cylinder assembly 2110. The piston rods 2102a, 2102b, 2104a, 2104b for the cylinder assemblies 2110, 2120 are attached to the common beam or platen 2140a, 2140b and operated out of phase with one another such that when high-pressure gas is expanding in the narrower high-pressure cylinder 2112 in the first pneumatic cylinder assembly 2110, lower-pressure gas is also expanding in the wider low-pressure cylinder 2124 in the second pneumatic cylinder assembly 2120. In this manner, the forces from the high pressure expansion in the first pneumatic cylinder assembly 2110 and the low pressure expansion in second pneumatic cylinder assembly 2120 are collectively applied to beam 2140b. Beam 2140b is attached rigidly to beam 2140a through tie rods 2142a, 2142b or other means, such that as expansion occurs in cylinder 2112, air in cylinder 2122 expands into cylinder 2124 and low pressure cylinder 2114 of the first pneumatic cylinder assembly 2110 is reset. Additionally, force from the expansion in cylinders 2112, 2124 is transmitted to hydraulic cylinder 2170, pressurizing fluid in hydraulic cylinder compartment 2176, and allowing the work from the expansions to be harnessed hydraulically. Similar to
The mechanical crankshaft assembly 2210 consists essentially of a rotary shaft 2220 attached to a rotary machine such as an electric motor/generator (not shown). During expansion of air in the pneumatic cylinder assemblies 2110, 2120, up/down motion of the platen structure 2140a, 2140b, 2142a, 2142b pushes and pulls the connecting rod 2230. The connecting rod 2230 is attached to the platen 2140a by a pin joint 2232, or other revolute coupling, such that force is transmitted to a crank 2234 through the connecting rod 2230, but the connecting rod 2230 is free to rotate around the axis of the pin joint 2232. As the connecting rod 2230 is pushed and pulled by up/down motion of the platen structure 2140a, 2140b, 2142a, 2142b, the crank 2234 is rotated around the axis of the rotary shaft 2220. The connecting rod 2230 is connected to the crank 2234 by another pin joint 2236.
The mechanical crankshaft assembly 2210 is an illustration of one exemplary mechanism to convert the up/down motion of the platen into rotary motion of a shaft 2220. Other such mechanisms for converting reciprocal motion to rotary motion are contemplated and considered within the scope of the invention.
The hydraulic cylinder assemblies 2310, 2320 are identical in construction to the hydraulic cylinder assembly 2160 described with respect to
In
In the exemplary systems and methods described with respect to
The system also includes a first set of two identical pneumatic cylinder assemblies 2430a, 2430b that are also operated in parallel and connected together in such a way as to act as a single pneumatic cylinder assembly. The first set of pneumatic cylinder assemblies 2430a, 2430b are operated in the same manner as pneumatic cylinder assembly 2110 described with respect to
The system 2400 further includes a second set of two identical pneumatic cylinder assemblies 2440a, 2440b that are operated in parallel and connected together in such a way as to act as a single pneumatic cylinder assembly. The second set of pneumatic cylinder assemblies 2440a, 2440b are operated in the same manner as pneumatic cylinder assembly 2120 described with respect to
Generally, the systems described herein may be operated in both an expansion mode and in the reverse compression mode as part of a full-cycle energy storage system with high efficiency. For example, the systems may be operated as both compressor and expander, storing electricity in the form of the potential energy of compressed gas and producing electricity from the potential energy of compressed gas. Alternatively, the systems may be operated independently as compressors or expanders.
In addition, the mechanisms shown in
As described above, various embodiments of the invention feature heat exchange with gas being compressed and/or expanded to improve efficiency thereof and facilitate, e.g., substantially isothermal compression and/or expansion.
With reference now to the heat transfer subsystem 2518, as shown, the cylinder 2500 has one or more gas circulation output ports 2520 that are connected via piping 2522 to a gas circulator 2524. The gas circulator 2524 may be a conventional or customized low-head pneumatic pump, fan, or any other device for circulating gas. The gas circulator 2524 is preferably sealed and rated for operation at the pressures contemplated within the gas chamber 2502. Thus, the gas circulator 2524 creates a flow (arrow 2526) of gas up the piping 2522 and therethrough. The gas circulator 2524 may be powered by electricity from a power source or by another drive mechanism, such as a fluid motor. The mass-flow speed and on/off functions of the circulator 2524 may be controlled by a controller 2528 acting on the power source for the circulator 2524. The controller 2528 may be a software and/or hardware-based system that carries out the heat-exchange procedures described herein. The output of the gas circulator 2524 is connected via a pipe 2528 to a gas input 2530 of a heat exchanger 2532.
The heat exchanger 2532 of the illustrative embodiment may be any acceptable design that allows energy to be efficiently transferred to and from a high-pressure gas flow contained within a pressure conduit to another mass flow (e.g., fluid). The rate of heat exchange is based at least in part on the relative flow rates of the gas and fluid, the exchange surface area between the gas and fluid, and the thermal conductivity of the interface therebetween. For example, the gas flow is heated in the heat exchanger 2532 by the fluid counter-flow 2534 (arrows 2536), which enters the fluid input 2538 of heat exchanger 2532 at ambient temperature and exits the heat exchanger 2532 at the fluid exit 2540 equal or approximately equal in temperature to the gas in piping 2528. The gas flow at gas exit 2542 of heat exchanger 2532 is at ambient or approximately ambient temperature, and returns via piping 2544 through one or more gas circulation input ports 2546 to gas chamber 2502. By “ambient” it is meant the temperature of the surrounding environment, or another desired temperature at which efficient performance of the system may be achieved. The ambient-temperature gas reentering the cylinder's gas chamber 2502 at the circulation input ports 2546 mixes with the gas in the gas chamber 2502, thereby bringing the temperature of the fluid in the gas chamber 2502 closer to ambient temperature.
The controller 2528 manages the rate of heat exchange based, for example, on the prevailing temperature (T) of the gas contained within the gas chamber 2502 using a temperature sensor 2548 of conventional design that thermally communicates with the gas within the chamber 2502. The sensor 2548 may be placed at any location along the cylinder including a location that is at, or adjacent to, the heat exchanger gas input port 2520. The controller 2528 reads the value T from the cylinder sensor and may compare it to an ambient temperature value (TA) derived from a sensor 2550 located somewhere within the system environment. When T is greater than TA, the heat transfer subsystem 2518 is directed to move gas (by powering the circulator 2524) therethrough at a rate that may be partly dependent upon the temperature differential (e.g., so that the exchange does not overshoot or undershoot the desired setting). Additional sensors may be located at various locations within the heat exchange subsystem to provide additional telemetry that may be used by a more complex control algorithm. For example, the output gas temperature (TO) from the heat exchanger may measured by a sensor 2552 that is placed upstream of the outlet port 2546.
The heat exchanger's fluid circuit may be filled with water, a coolant mixture, and/or any acceptable heat-transfer medium. In alternative embodiments, a gas, such as air or refrigerant, is used as the heat-transfer medium. In general, the fluid is routed by conduits to a large reservoir of such fluid in a closed or open loop. One example of an open loop is a well or body of water from which ambient water is drawn and the exhaust water is delivered to a different location, for example, downstream in a river. In a closed loop embodiment, a cooling tower may cycle the water through the air for return to the heat exchanger. Likewise, water may pass through a submerged or buried coil of continuous piping where a counter heat-exchange occurs to return the fluid flow to ambient before it returns to the heat exchanger for another cycle.
Stored compressed gas in pressure vessels, not shown but indicated by 2614, is admitted via valve 2616 into the cylinder 2600 through air port 2618. As the compressed gas expands into the cylinder 2600, fluid (e.g., gas or hydraulic fluid) is forced out through fluid port 2620 as indicated by 2622. During expansion (or compression), heat exchange liquid (e.g., water) may be drawn from a reservoir 2624 by a circulator, such as a pump 2610, through a liquid-to-liquid heat exchanger 2612, which may be a shell-and-tube type with an input 2626 and an output 2628 from the shell running to an environmental heat exchanger or to a source of process heat, cold water, or other external heat exchange medium.
As shown in
Stored compressed gas in pressure vessels, not shown but indicated by 2714, is admitted via valve 2716 into the cylinder 2700 through air port 2718. As the compressed gas expands into the cylinder 2700, fluid (e.g., gas or hydraulic fluid) is forced out through fluid port 2720 as indicated by 2722. During expansion (or compression), heat exchange liquid (e.g., water) may be drawn from a reservoir 2724 by a circulator, such as a pump 2710, through a liquid-to-liquid heat exchanger 2712, which may be a tube-in-shell setup with an input 2726 and an output 2728 from the shell running to an environmental heat exchanger or to a source of process heat, cold water, or other external heat exchange medium. As indicated in
The terms and expressions employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.
This application is a continuation of U.S. patent application Ser. No. 12/966,855, filed on Dec. 13, 2010, which is a continuation of U.S. patent application Ser. No. 12/879,595, filed on Sep. 10, 2010, which claims the benefit of and priority to U.S. Provisional Patent Application No. 61/241,568, filed Sep. 11, 2009; U.S. Provisional Patent Application No. 61/251,965, filed Oct. 15, 2009; U.S. Provisional Patent Application No. 61/318,060, filed Mar. 26, 2010; and U.S. Provisional Patent Application No. 61/326,453, filed Apr. 21, 2010; the entire disclosure of each of which is hereby incorporated herein by reference.
This invention was made with government support under IIP-0810590 and IIP-0923633 awarded by the NSF. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61241568 | Sep 2009 | US | |
| 61251965 | Oct 2009 | US | |
| 61318060 | Mar 2010 | US | |
| 61326453 | Apr 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13351367 | Jan 2012 | US |
| Child | 13910643 | US | |
| Parent | 12966855 | Dec 2010 | US |
| Child | 13351367 | US | |
| Parent | 12879595 | Sep 2010 | US |
| Child | 12966855 | US |
