STRUCTURE

CROSS-REFERNCE TO RELATED APPLICATION(S)

[0001] The instant PCT Patent Application claims priority to the following United States Patent Applications: U.S. Provisional Patent Application No. 61/221,487 filed June 29, 2009; U.S. nonprovisional patent application No. 12/695,922 filed January 28, 2010, and U.S.

nonprovisional patent application no. 12/730,549 filed March 24, 2010.

BACKGROUND

[0002] Air compressed to 300 bar has energy density comparable to that of lead-acid batteries and other energy storage technologies. One source of compressed air is wind.

[0003] It is known that the efficiency of power generation from wind, improves with increased height of elevation of the fan blades of the wind turbine from the ground. Such elevation, however, requires provision of a large, fixed structure of sufficient mechanical strength to safely support the relatively heavy structure of the turbine, including the blades, under a variety of wind conditions.

[0004] The expense of constructing and maintaining such a support structure is an inherent expense of the system, detracting from the overall profitability of the wind generation device. Accordingly, there is a need in the art for novel structures and methods for supporting a wind turbine.

SUMMARY

[0005] An energy storage and recovery system employs air compressed utilizing power from an operating wind turbine. This compressed air is stored within one or more chambers of a structure supporting the wind turbine above the ground. By functioning as both a physical support and as a vessel for storing compressed air, the relative contribution of the support structure to the overall cost of the energy storage and recovery system may be reduced, thereby improving economic realization for the combined turbine/support apparatus. In certain embodiments, expansion forces of the compressed air stored within the chamber may be relied upon to augment the physical stability of a support structure, further reducing material costs of the support structure.

[0006] An embodiment of a method in accordance with the present invention comprises storing compressed gas generated from power of an operating wind turbine, within a chamber defined by walls of a structure supporting the wind turbine.

[0007] An embodiment of an apparatus in accordance with the present invention comprises a support structure configured to elevate a wind turbine above the ground, the support structure comprising walls defining a chamber configured to be in fluid communication with a gas compressor operated by the wind turbine, the chamber also configured to store gas compressed by the compressor.

[0008] An embodiment of an apparatus in accordance with the present invention comprises an energy storage system comprising a wind turbine, a gas compressor configured to be operated by the wind turbine, and a support structure configured to elevate the wind turbine above the ground, the support structure comprising walls defining a chamber in fluid communication with the gas compressor, the chamber configured to store gas compressed by the gas compressor. A generator is configured to generate electrical power from expansion of compressed gas flowed from the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a simplified schematic representation of an embodiment of a system in accordance with the present invention.

[0010] Figure IA shows a simplified top view of one embodiment of a planetary gear system which could be used in embodiments of the present invention. Figure IAA shows a simplified cross-sectional view of the planetary gear system of Figure IA taken along line IA- IA'.

[0011] Figure 2 is a simplified schematic representation of an alternative embodiment of a system in accordance with the present invention.

[0012] Figure 3 is a simplified schematic representation of an alternative embodiment of a system in accordance with the present invention.

[0013] Figure 3A is a simplified schematic representation of an alternative embodiment of a system in accordance with the present invention. [0014] Figure 4 is a simplified schematic representation of an alternative embodiment of a system in accordance with the present invention.

[0015] Figure 5 is a simplified schematic representation of an alternative embodiment of a system in accordance with the present invention. [0016] Figure 6 is a schematic representation of the first embodiment of a compressed air energy storage system in accordance with the present invention, that is a single-stage, single- acting energy storage system using liquid mist to effect heat exchange.

[0017] Figure 7 is a block diagram of a second embodiment of a compressed air energy storage system showing how multiple stages are incorporated into a complete system in accordance with the present invention.

[0018] Figure 8 is a block diagram of a multi-stage compressed air energy system that utilizes a hydraulic motor as its mechanism for conveying and receiving mechanical power.

[0019] Figure 9 is a schematic representation of a one single-acting stage that uses liquid mist to effect heat exchange in a multi-stage compressed air energy storage system in accordance with the present invention.

[0020] Figure 10 is a schematic representation of one double-acting stage in a multi-stage compressed air energy storage system in accordance with the present invention.

[0021] Figure 11 is a schematic representation of a single-acting stage in a multi-stage compressed air energy storage system, in accordance with the present invention, using multiple cylinder devices.

[0022] Figure 12 shows a simplified view of a computer system suitable for use in connection with the methods and systems of the embodiments of the present invention.

[0023] Figure 12A is an illustration of basic subsystems in the computer system of Figure 16.

[0024] Figure 13 is an embodiment of a block diagram showing inputs and outputs to a controller responsible for controlling operation of various elements of an apparatus according to the present invention.

[0025] Figures 14A-14F show operation of the controller to control the timing of various valves. [0026] Figures 15A-C show the configuration of an apparatus during steps of a compression cycle according to an embodiment of the present invention.

[0027] Figures 16A-C show the configuration of an apparatus during steps of an expansion cycle according to an embodiment of the present invention. [0028] Figure 17 shows an alternative embodiment of an apparatus in accordance with the present invention.

[0029] While certain drawings and systems depicted herein may be configured using standard symbols, the drawings have been prepared in a more general manner to reflect the variety implementations that may be realized from different embodiments. DETAILED DESCRIPTION

[0030] As previously described, a wind turbine operates to capture wind energy more effectively the higher it is elevated above the ground. In particular, wind speed is roughly proportional to the seventh root of the height. Power is proportional to the cube of the wind speed, and also proportional to the area of the wind turbine. A greater height, H, could theoretically allow a larger diameter turbine, giving area proportional to H ² and power proportional to H ^x , with x perhaps as great as 2 ³ Z ₇ . The support structure is thus a necessary element of the system. According to embodiments of the present invention, this support structure can perform the further duty of housing one or more chambers or vessels configured to receive and store compressed air generated from output of the wind turbine.

[0031] Such a support structure for a wind turbine is initially well suited for this task, as it is typically formed from an exterior shell that encloses an interior space. This structure provides the desired mechanical support for the wind turbine at the top, while not consuming the large amount of material and avoiding the heavy weight that would otherwise be associated with an entirely solid supporting structure.

[0032] Figure 1 shows a simplified schematic view of an embodiment of a system in accordance with the present invention. Specifically, system 100 comprises a nacelle 101 that is positioned on top of support tower 106. Nacelle 101 includes a wind turbine 102 having rotatable blades 104.

[0033] Nacelle 101 may be in rotatable communication (indicated by arrow 120) with support tower 106 through joint 111, thereby allowing the blades of the wind turbine to be oriented to face the direction of the prevailing wind. An example of a wind turbine suitable for use in accordance with embodiment of the present invention is the model 1.5 sle turbine available from the General Electric Company of Fairfield, Connecticut.

[0034] Upon exposure to wind 108, the blades 104 of the turbine 102 turn, thereby converting the power of the wind into energy that is output on linkage 105. Linkage 105 may be

mechanical, hydraulic, or pneumatic in nature.

[0035] Linkage 105 is in turn in physical communication with a motor/generator 114 through gear system 112 and linkage 103. Gear system 112 is also in physical communication with compressor/expander element 116 through linkage 107. Linkages 103 and 107 may be mechanical, hydraulic, or pneumatic in nature.

[0036] The gear system may be configured to permit movement of all linkages at the same time, in a subtractive or additive manner. The gear system may also be configured to

accommodate movement of fewer than all of the linkages. In certain embodiments, a planetary gear system may be well-suited to perform these tasks.

[0037] Compressed gas storage chamber 118 is defined within the walls 118a of the support tower. Compressor/expander 116 is in fluid communication with storage chamber 118 through conduit 109.

[0038] Several modes of operation of system 100 are now described. In one mode of operation, the wind is blowing, and demand for power on the grid is high. Under these conditions, substantially all of the energy output from rotation of the blades of the turbine, is communicated through linkages 105 and 103 and gear system 112 to motor/generator 114 that is acting as a generator. Electrical power generated by motor/generator 114 is in turn

communicated through conduit 113 to be output onto the grid for consumption. The

compressor/expander 116 is not operated in this mode.

[0039] In another mode of operation, the wind is blowing but demand for power is not as high. Under these conditions, a portion of the energy output from rotation of the blades of the turbine is converted into electrical power through elements 105, 112, 103, and 114 as described above.

[0040] Moreover, some portion of the energy output from the operating turbine is also communicated through linkages 105 and 107 and gear system 112 to operate

compressor/expander 116 that is functioning as a compressor. Compressor/expander 116 functions to intake air, compress that air, and then flow the compressed air into the storage chamber 118 located in the support tower. As described below, energy that is stored in the form of this compressed air can later be recovered to produce useful work. [0041] Specifically, in another mode of operation of system 100, the compressor/expander 116 is configured to operate as an expander. In this mode, compressed air from the storage chamber is flowed through conduit 109 into the expander 116, where it is allowed to expand. Expansion of the air drives a moveable element that is in physical communication with linkage 107. One example of such a moveable element is a piston that is positioned within a cylinder of the compressor/expander 116.

[0042] The energy of actuated linkage 107 is in turn communicated through gear system 112 and linkage 103 to motor/generator 114 that is acting as a generator. Electrical power generated by motor/generator as a result of actuation of linkage 103, may in turn be output to the power grid through conduit 113.

[0043] In the mode of operation just described, the wind may or may not be blowing. If the wind is blowing, the energy output by the compressor/expander 116 may be combined in the gear system with the energy output by the turbine 112. The combined energy from these sources (wind, compressed air) may then be communicated by gear system 112 through linkage 103 to motor/generator 114.

[0044] In still another mode of operation, the wind may not be blowing and power demand is low. Under these conditions, the compressor/expander 116 may operate as a compressor. The motor/generator 114 operates as a motor, drawing power off of the grid to actuate the compressor/expander 116 (functioning as a compressor) through linkages 103 and 107 and gear system 112. This mode of operation allows excess power from the grid to be consumed to replenish the compressed air stored in the chamber 118 for consumption at a later time.

[0045] Embodiments of systems which provide for the efficient storage and recovery of energy as compressed gas, are described in the U.S. Provisional Patent Application No. 61/221,487 filed June 29, 2009, and in the U.S. nonprovisional patent application No. 12/695,922 filed January 28, 2010, both of which are incorporated by reference in their entireties herein for all purposes. However, embodiments of the present invention are not limited to use with these or any other particular designs of compressed air storage and recovery systems. Also incorporated by reference in its entirety herein for all purposes, is the provisional patent application no.

61/294,396, filed January 12, 2010.

[0046] As previously mentioned, certain embodiments of the present invention may favorably employ a planetary gear system to allow the transfer of mechanical energy between different elements of the system. In particular, such a planetary gear system may offer the flexibility to accommodate different relative motions between the linkages in the various modes of operation described above.

[0047] Figure IA shows a simplified top view of one embodiment of a planetary gear system which could be used in embodiments of the present invention. Figure IAA shows a simplified cross-sectional view of the planetary gear system of Figure IA taken along line IA- IA'.

[0048] Specifically, planetary gear system 150 comprises a ring gear 152 having a first set of teeth 154 on an outer periphery, and having a second set of teeth 156 on an inner portion. Ring gear 152 is engaged with, and moveable in either direction relative to, three other gear assemblies.

[0049] In particular, first gear assembly 140 comprises side gear 142 that is positioned outside of ring gear 152, and is fixed to rotatable shaft 141 which serves as a first linkage to the planetary gear system. The teeth of side gear 142 are in mechanical communication with the teeth 154 located on the outer periphery of the ring gear. Rotation of shaft 141 in either direction will translate into a corresponding movement of ring gear 152.

[0050] A second gear assembly 158 comprises a central (sun) gear 160 that is positioned inside of ring gear 152. Central gear 160 is fixed to rotatable shaft 162 which serves as a second linkage to the planetary gear system.

[0051] Third gear assembly 165 allows central gear 160 to be in mechanical communication with the second set of teeth 156 of ring gear 152. In particular, third gear assembly 165 comprises a plurality of (planet) gears 164 that are in free rotational communication through respective pins 167 with a (planet carrier) plate 166. Plate 166 is fixed to a third shaft 168 serving as a third linkage to the planetary gear system.

[0052] The planetary gear system 150 of Figures IA- IAA provides mechanical

communication with three rotatable linkages 141, 162, and 168. Each of these linkages may be in physical communication with the various other elements of the system, for example the wind turbine, a generator, a motor, a motor/generator, a compressor, an expander, or a

compressor/expander.

[0053] The planetary gear system 150 permits movement of all of the linkages at the same time, in a subtractive or additive manner. For example where the wind is blowing, energy from the turbine linkage may be distributed to drive both the linkage to a generator and the linkage to a compressor. In another example, where the wind is blowing and demand for energy is high, the planetary gear system permits output of the turbine linkage to be combined with output of an expander linkage, to drive the linkage to the generator.

[0054] Moreover, the planetary gear system is also configured to accommodate movement of fewer than all of the linkages. For example, rotation of shaft 141 may result in the rotation of shaft 162 or vice-versa, where shaft 168 is prevented from rotating. Similarly, rotation of shaft 141 may result in the rotation of only shaft 168 and vice-versa, or rotation of shaft 162 may result in the rotation of only shaft 168 and vice-versa. This configuration allows for mechanical energy to be selectively communicated between only two elements of the system, for example where the wind turbine is stationary and it is desired to operate a compressor based upon output of a motor.

[0055] Returning to Figure 1 , certain embodiments of compressed gas storage and recovery systems according to the present invention may offer a number of potentially desirable characteristics. First, the system leverages equipment that may be present in an existing wind turbine system. That is, the compressed air energy storage and recovery system may utilize the same electrical generator that is used to output power from the wind turbine onto the grid. Such use of the generator to generate electrical power both from the wind and from the stored compressed air, reduces the cost of the overall system.

[0056] Another potential benefit associated with the embodiment of Figure 1 is improved efficiency of power generation. Specifically, the mechanical energy output by the rotating wind turbine blades, is able to be communicated in mechanical form to the compressor without the need for conversion into another form (such as electrical energy). By utilizing the output of the power source (the wind turbine) in its native mechanical form, the efficiency of transfer of that power into compressed air may be enhanced.

[0057] Still another potential benefit associated with the embodiment of Figure 1 is a reduced number of components. In particular, two of the elements of the system perform dual functions. Specifically, the motor/generator can operate as a motor and as a generator, and the

compressor/expander can operate as a compressor or an expander. This eliminates the need for separate, dedicated elements for performing each of these functions.

[0058] Still another potential benefit of the embodiment of Figure 1 is relative simplicity of the linkages connecting various elements with moving parts. Specifically, in the embodiment of Figure 1 , the turbine, the gear system, the motor/generator, and the compressor/expander are all located in the nacelle. Such a configuration offers the benefit of compatibility with a rotational connection between a nacelle and the underlying support structure. In particular, none of the linkages between the elements needs to traverse the rotating joint, and thus the linkages do not need to accommodate relative motion between the nacelle and support structure. Such a configuration allows the design and operation of those linkages to be substantially simplified. [0059] According to alternative embodiments, however, one or more of the gear system, the compressor/expander, and the motor/generator may be positioned outside of the nacelle. Figure 2 shows a simplified view of such an alternative embodiment of a system 200 in accordance with the present invention.

[0060] In this embodiment, while the turbine 202 is positioned in the nacelle 201, the gear system 212, compressor/expander 216, and motor generator 214 are located at the base of the tower 206. This placement is made possible by the use of an elongated linkage 205 running between turbine 202 and gear system 212. Elongated linkage 205 may be mechanical, hydraulic, or pneumatic in nature.

[0061] The design of the embodiment of Figure 2 may offer some additional complexity, in that the linkage 205 traverses rotating joint 211 and accordingly must be able to accommodate relative motion of the turbine 202 relative to the gear system 212. Some of this complexity may be reduced by considering that linkage 205 is limited to communicating energy in only one direction (from the turbine to the gear system).

[0062] Moreover, the cost of complexity associated with having linkage 205 traverse rotating joint 211, may be offset by the ease of access to the motor/generator, compressor/expander, and gear system. Specifically, these elements include a large number of moving parts and are subject to wear. Positioning these elements at the base of the tower (rather than at the top) facilitates access for purposes of inspection and maintenance, thereby reducing cost.

[0063] Still other embodiments are possible. For example, while Figure 2 shows the gear system, motor/generator, and compressor/expander elements as being housed within the support structure, this is not required. In other embodiments, one or more of these elements could be located outside of the support structure, and still communicate with the wind turbine through a linkage extending from the support tower. In such embodiments, conduits for compressed air and for electricity, and mechanical, hydraulic, or pneumatic linkages could provide for the necessary communication between system elements.

[0064] Embodiments of the present invention are not limited to the particular elements described above. For example, while Figures 1 and 2 show compressed gas storage system comprising compressor/expander elements and motor/generator elements having combined functionality, this is not required by the present invention.

[0065] Figure 3 shows an alternative embodiment a system 300 according to the present invention, utilizing separate, dedicated compressor 350, dedicated expander 316, dedicated motor 354, and dedicated generator 314 elements. Such an embodiment may be useful to adapt an existing wind turbine to accommodate a compressed gas storage system.

[0066] Specifically, pre-existing packages for wind turbines may feature the dedicated generator element 314 in communication with the turbine 302 through gear system 312 and linkages 303 and 305. Generator 314, however, is not designed to also exhibit functionality as a motor.

[0067] To such an existing configuration, a dedicated expander 316, a dedicated compressor 350, a dedicated motor 354, linkages 307 and 373, and conduit 370 may be added to incorporate a compressed gas storage system. In one embodiment, a dedicated expander 316 may be positioned in the nacelle 301 in communication with the gear system 312 through linkage 307. Dedicated expander 316 is in fluid communication with a top portion of the compressed gas storage chamber 318 defined within the walls 306a of support tower 306 through conduit 309.

[0068] Dedicated compressor 350 and a dedicated motor 354 are readily included, for example at or near the base of the support tower, thereby facilitating access to these elements. Dedicated compressor 350 is in fluid communication with storage chamber 318 through conduit 370, and in physical communication with dedicated motor 354 through linkage 372. Dedicated motor 354 is in turn in electronic communication with the generator and/or grid to receive power to operate the compressor to replenish the supply of compressed gas stored in the chamber 318.

[0069] As shown in Figure 3, this embodiment may further include an optional elongated mechanical, hydraulic, or pneumatic linkage 374 extending between the gear system 312 in the nacelle 301, and the dedicated compressor 350 located outside of the nacelle 301. Such a linkage would allow the dedicated compressor to be directly operated by the output of the turbine, avoiding losses associated with converting mechanical into electrical form by the dedicated generator, and re-converting the electrical power back into mechanical form by the dedicated motor in order to operate the compressor.

[0070] Figure 3 A shows a simplified view of yet another embodiment of a system in accordance with the present invention. In the embodiment of the system 380 of Figure 3 A, only the turbine 382, linkage 383, and dedicated compressor 386 elements are located in the nacelle 381 that is positioned atop support tower 396. Dedicated compressor 386 is in communication with the turbine through linkage 383 (which may be mechanical, hydraulic, or pneumatic), which serves to drive compression of air by the dedicated compressor. Compressed air output by the dedicated compressor is flowed through conduit 389 across joint 391 into chamber 398 present in the support tower 396.

[0071] The remaining elements are positioned outside of the nacelle, either in the support tower, or alternatively outside of the support tower. For example, a dedicated expander or expander/compressor 388 is in communication with the chamber 398 defined within walls 396a, to receive compressed air through conduit 393. Element 388 is configured to allow expansion of the compressed air, and to communicate energy recovered from this expansion through linkage 392 to generator or generator/motor 384. Element 384 in turn operates to generate electricity that is fed onto the grid.

[0072] The embodiment of Figure 3 A can also function to store energy off of the grid. Where element 384 is a generator/motor and element 388 is an expander/compressor, element 384 may operate as a motor to drive element 388 operating as a compressor, such that air is compressed and flowed into chamber 398 for storage and later recovery.

[0073] The embodiment of Figure 3 A offers a potential advantage in that power is transported from the top to the bottom of the tower utilizing the chamber, without requiring a separate elongated linkage or conduit. Another possible advantage of the embodiment of Figure 3 A is a reduction in the weight at the top of the tower. While this embodiment may incur losses where the mechanical power output of the turbine is converted first into compressed air and then back into mechanical power for driving the generator, such losses may be offset by a reduction in weight at the top of the tower, allowing the tower to be higher and to access more wind power.

[0074] The present invention is not limited to a support structure having any particular shape. In the particular embodiments shown in Figures 1 and 2, the support structure exhibits a cross- sectional shape that varies along its length. For example, the support structure 106 is wide at its base, and then tapers to a point at which it meets the wind turbine. By allocating material to where it will best serve the supporting function, such a design minimizes materials and reduces cost.

[0075] However, the present invention also encompasses supporting structures having other shapes. For example, Figure 4 shows a support structure 400 comprising a hollow tube having a circular or elliptical cross section that is substantially uniform. The walls 400a of this hollow tube 400 in turn define a chamber 402 for storing compressed gas. While possibly utilizing more mass, such a tube is a simpler structure that is employed for a various applications in many other industries. Accordingly, such a tube is likely available at a relatively low price that may offset any greater material cost.

[0076] Still further alternative embodiments are possible. For example, in certain

embodiments a support structure may be designed to take advantage of the forces exerted by the compressed air stored therein, in order to impart additional stability to the support structure. [0077] Thus, Figure 5 shows an embodiment wherein the support structure 500 comprises a portion 506a having thinner walls 506b exhibiting less inherent strength than those of the prior embodiments. This reduced strength may be attributable to one or more factors, including but not limited to, use of a different design or shape for the support, use of a reduced amount of material in the support, or use of a different material in the support.

[0078] According to embodiments of the present invention, however, any reduction in the inherent strength of the support structure 506 may be offset by expansion forces 524 exerted by the compressed air 526 that is contained within the chamber 518. Specifically, in a manner analogous to the stiffening of walls of an inflated balloon, the expansion force of the compressed air may contribute additional strength to the support structure. This expansion effect is shown grossly exaggerated in Figure 5, for purposes of illustration.

[0079] One possible application for such a design, employs a support structure that is fabricated from a material that is capable of at least some flexion, for example carbon fiber. In such an embodiment, expansion forces from the compressed air within the chamber of a flexible support member, may act against the walls of the chamber, thereby stiffening it and contributing to the structural stability of that support. Such a support structure could alternatively be formed from other materials, and remain within the scope of the present invention.

[0080] A design incorporating carbon fiber could offer even further advantages. For example, carbon fiber structures may exhibit enhanced strength in particular dimensions, depending upon the manner of their fabrication. Thus, a carbon fiber support structure could be fabricated to exhibit strength and/or flexion in particular dimensions, for example those in which the expansion forces of the compressed air are expected to operate, and/or dimension in which the support is expected to experience external stress (e.g. a prevailing wind direction).

[0081] Of course, a design taking advantage of expansion forces of the stored compressed air, would need to exhibit sufficient inherent strength in the face of expected (and unexpected) changes in the quantity of compressed air stored therein, as that compressed air is drawn away and allowed to expand for energy recovery. Nevertheless, expansion forces associated with minimal amounts of compressed air remaining within the support structure, could impart sufficient stability to support structure to reduce its cost of manufacture and maintenance.

[0082] The following is a description of certain embodiments of apparatuses for compressed gas energy storage and recovery systems which may employ a support structure of a wind turbine for storage of compressed gas. The present invention is not limited to this, or any other specific system. [0083] Certain embodiments of the present invention relate generally to energy storage systems, and more particularly, relates to energy storage systems that utilize compressed air as the energy storage medium, comprising an air compression/expansion mechanism, a heat exchanger, and one or more air storage tanks. [0084] According to embodiments of the present invention, a compressed-air energy storage system is provided comprising a reversible mechanism to compress and expand air, one or more compressed air storage tanks, a control system, one or more heat exchangers, and, in certain embodiments of the invention, a motor-generator.

[0085] The reversible air compressor-expander uses mechanical power to compress air (when it is acting as a compressor) and converts the energy stored in compressed air to mechanical power (when it is acting as an expander). The compressor-expander comprises one or more stages, each stage consisting of pressure vessel (the "pressure cell") partially filled with water or other liquid. In some embodiments, the pressure vessel communicates with one or more cylinder devices to exchange air and liquid with the cylinder chamber(s) thereof. Suitable valving allows air to enter and leave the pressure cell and cylinder device, if present, under electronic control.

[0086] The cylinder device referred to above may be constructed in one of several ways. In one specific embodiment, it can have a piston connected to a piston rod, so that mechanical power coming in or out of the cylinder device is transmitted by this piston rod. In another configuration, the cylinder device can contain hydraulic liquid, in which case the liquid is driven by the pressure of the expanding air, transmitting power out of the cylinder device in that way. In such a configuration, the hydraulic liquid can interact with the air directly, or a diaphragm across the diameter of the cylinder device can separate the air from the liquid.

[0087] In low-pressure stages, liquid is pumped through an atomizing nozzle into the pressure cell or, in certain embodiments, the cylinder device during the expansion or compression stroke to facilitate heat exchange. The amount of liquid entering the chamber is sufficient to absorb (during compression) or release (during expansion) all the heat associated with the compression or expansion process, allowing those processes to proceed near-isothermally. This liquid is then returned to the pressure cell during the non-power phase of the stroke, where it can exchange heat with the external environment via a conventional heat exchanger. This allows the compression or expansion to occur at high efficiency. [0088] Operation of embodiments according the present invention may be characterized by a magnitude of temperature change of the gas being compressed or expanded. According to one embodiment, during a compression cycle the gas may experience an increase in temperate of 100 degrees Celsius or less, or a temperature increase of 60 degrees Celsius or less. In some embodiments, during an expansion cycle, the gas may experience a decrease in temperature of 100 degrees Celsius or less, 15 degrees Celsius or less, or 11 degrees Celsius or less -nearing the freezing point of water from an initial point of room temperature.

[0089] Instead of injecting liquid via a nozzle, as described above, air may be bubbled though a quantity of liquid in one or more of the cylinder devices in order to facilitate heat exchange. This approach is preferred at high pressures.

[0090] During expansion, the valve timing is controlled electronically so that only so much air as is required to expand by the desired expansion ratio is admitted to the cylinder device. This volume changes as the storage tank depletes, so that the valve timing must be adjusted dynamically. [0091] The volume of the cylinder chambers (if present) and pressure cells increases from the high to low pressure stages. In other specific embodiments of the invention, rather than having cylinder chambers of different volumes, a plurality of cylinder devices is provided with chambers of the same volume are used, their total volume equating to the required larger volume.

[0092] During compression, a motor or other source of shaft torque drives the pistons or creates the hydraulic pressure via a pump which compresses the air in the cylinder device.

During expansion, the reverse is true. Expanding air drives the piston or hydraulic liquid, sending mechanical power out of the system. This mechanical power can be converted to or from electrical power using a conventional motor-generator.

[0093] While the present invention will be described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention. It will be noted here that for a better understanding, like components are designated by like reference numerals throughout the various figures. [0094] Sinsle-Stase System

[0095] Figure 6 depicts the simplest embodiment of the compressed air energy storage system 620 of the present invention, and illustrates many of the important principles. Briefly, some of these principles which improve upon current compressed air energy storage system designs include mixing a liquid with the air to facilitate heat exchange during compression and expansion, thereby improving the efficiency of the process, and applying the same mechanism for both compressing and expanding air. Lastly, by controlling the valve timing electronically, the highest possible work output from a given volume of compressed air can be obtained.

[0096] As best shown in Figure 6, the energy storage system 620 includes a cylinder device 621 defining a chamber 622 formed for reciprocating receipt of a piston device 623 or the like therein. The compressed air energy storage system 620 also includes a pressure cell 625 which when taken together with the cylinder device 621, as a unit, form a one stage reversible compression/expansion mechanism (i.e., a one-stage 624). There is an air filter 626, a liquid-air separator 627, and a liquid tank 628, containing a liquid 649d fluidly connected to the compression/expansion mechanism 624 on the low pressure side via pipes 630 and 631 , respectively. On the high pressure side, an air storage tank or tanks 632 is connected to the pressure cell 625 via input pipe 633 and output pipe 634. A plurality of two-way, two position valves 635-643 are provided, along with two output nozzles 611 and 644. This particular embodiment also includes liquid pumps 646 and 647. It will be appreciated, however, that if the elevation of the liquid tank 628 is higher than that of the cylinder device 621, water will feed into the cylinder device by gravity, eliminating the need for pump 646.

[0097] Briefly, atmospheric air enters the system via pipe 610, passes through the filter 626 and enters the cylinder chamber 622 of cylinder device 621, via pipe 630, where it is compressed by the action of piston 623, by hydraulic pressure, or by other mechanical approaches. Before compression begins, a liquid mist is introduced into the chamber 622 of the cylinder device 621 using an atomizing nozzle 644, via pipe 648 from the pressure cell 625. This liquid may be water, oil, or any appropriate liquid 649f from the pressure cell having sufficient high heat capacity properties. The system preferably operates at substantially ambient temperature, so that liquids capable of withstanding high temperatures are not required. The primary function of the liquid mist is to absorb the heat generated during compression of the air in the cylinder chamber. The predetermined quantity of mist injected into the chamber during each compression stroke, thus, is that required to absorb all the heat generated during that stroke. As the mist condenses, it collects as a body of liquid 649e in the cylinder chamber 622.

[0098] The compressed air/liquid mixture is then transferred into the pressure cell 625 through outlet nozzle 611, via pipe 651. In the pressure cell 625, the transferred mixture exchanges the captured heat generated by compression to a body of liquid 49f contained in the cell. The air bubbles up through the liquid and on to the top of the pressure cell, and then proceeds to the air storage tank 632, via pipe 633.

[0099] The expansion cycle is essentially the reverse process of the compression cycle. Air leaves the air storage tank 632, via pipe 634, bubbling up through the liquid 649f in the pressure cell 625, enters the chamber 622 of cylinder device 621, via pipe 655, where it drives piston 623 or other mechanical linkage. Once again, liquid mist is introduced into the cylinder chamber 622, via outlet nozzle 644 and pipe 648, during expansion to keep a substantially constant temperature in the cylinder chamber during the expansion process. When the air expansion is complete, the spent air and mist pass through an air-liquid separator 627 so that the separated liquid can be reused. Finally, the air is exhausted to the atmosphere via pipe 610.

[0100] The liquid 649f contained in the pressure cell 625 is continually circulated through the heat exchanger 652 to remove the heat generated during compression or to add the heat to the chamber to be absorbed during expansion. This circulating liquid in turn exchanges heat with a thermal reservoir external to the system (e.g. the atmosphere, a pond, etc.) via a conventional air or water-cooled heat exchanger (not shown in this figure). The circulating liquid is conveyed to and from that external heat exchanger via pipes 653 and 654 communicating with internal heat exchanger 652.

[0101] The apparatus of Figure 6 further includes a controller/processor 694 in electronic communication with a computer-readable storage device 692, which may be of any design, including but not limited to those based on semiconductor principles, or magnetic or optical storage principles. Controller 694 is shown as being in electronic communication with a universe of active elements in the system, including but not limited to valves, pumps, chambers, nozzles, and sensors. Specific examples of sensors utilized by the system include but are not limited to pressure sensors (P) 698, 684, and 674, temperature sensors (T) 690, 688, 686, and 676, humidity sensor (H) 696, volume sensors (V) 682 and 672, and flow rate sensor 699. [0102] As described in detail below, based upon input received from one or more system elements, and also possibly values calculated from those inputs, controller/processor 4 may dynamically control operation of the system to achieve one or more objectives, including but not limited to maximized or controlled efficiency of conversion of stored energy into useful work; maximized, minimized, or controlled power output; an expected power output; an expected output speed of a rotating shaft in communication with the piston; an expected output torque of a rotating shaft in communication with the piston; an expected input speed of a rotating shaft in communication with the piston; an expected input torque of a rotating shaft in communication with the piston; a maximum output speed of a rotating shaft in communication with the piston; a maximum output torque of a rotating shaft in communication with the piston; a minimum output speed of a rotating shaft in communication with the piston; a minimum output torque of a rotating shaft in communication with the piston; a maximum input speed of a rotating shaft in communication with the piston; a maximum input torque of a rotating shaft in communication with the piston; a minimum input speed of a rotating shaft in communication with the piston; a minimum input torque of a rotating shaft in communication with the piston; or a maximum expected temperature difference of air at each stage.

[0103] The compression cycle for this single-stage system proceeds as follows:

[0104] During step 1 of the compression cycle, liquid 649d is added to the chamber 622 of the cylinder device 621 from the liquid tank 628 (collecting as body of liquid 649e) such that, when the piston 623 reaches top dead center (TDC), the dead volume in the cylinder device is zero. This will only have to be done occasionally, so that this step is omitted on the great majority of cycles.

[0105] During step 2 of the compression cycle, liquid mist from pressure cell 625 is pumped, via pump 647, into the cylinder chamber 622, via pipe 648 and nozzle 644. The selected quantity of mist is sufficient to absorb the heat generated during the compression step (step 3). The volume fraction of liquid must sufficiently low enough that the droplets will not

substantially fuse together, thus reducing the effective surface area available for heat exchange (that is, the interface between air and liquid). Typically, the pressure differential between the pressure cell 625 and the chamber 622 of the cylinder device 621 is sufficiently high so that the operation of pump 647 is not required.

[0106] During step 3 of the compression cycle, the piston 623 is driven upward by a crankshaft (not shown) coupled to a piston rod 619, by hydraulic pressure, or by some other mechanical structure, compressing the air and mist contained in the cylinder chamber.

[0107] Step 4 of the compression cycle begins when the air pressure inside the cylinder chamber 622 is substantially equal to the pressure inside the pressure cell 625, at which point outlet valve 638 opens, allowing compressed air to flow from the cylinder chamber to the pressure cell. Because of the liquid added to the cylinder device during step 1 of the

compression cycle, substantially all the air in the cylinder chamber can be pushed out during this step. The compressed air is introduced into the pressure cell 625 through an inlet nozzle 611, along with any entrained mist, creating fine bubbles so that the heat generated during

compression will exchange with the liquid 649f in the cell rapidly. [0108] During step 5 of the compression cycle, the piston 623 is pulled down allowing low- pressure air to refill it, via valve 636 and pipe 630. The above table shows valve 639 as being closed during this step, and shows pump 647 as being off during this step 5. However, this is not required. In other embodiments valve 639 could be open and pump 647 could be on, during the step 5 such that mist is introduced into the cylinder chamber as it is refilled with air. [0109] The expansion cycle for this single-stage system proceeds as follows:

[0110] During step 1 of the expansion cycle, liquid is added to the cylinder chamber from the liquid tank 628 to eliminate dead volume in the system. This will be required only rarely, as mentioned above. Similar to the compression cycle, the pump 646 can be eliminated if the liquid tank 628 is oriented at an elevation higher than that of the chamber of cylinder device 621.

[0111] During step 2 of the expansion cycle, a pre-determined amount of air, Vo, is added to the chamber of the cylinder device by opening inlet valve 637 for the correct interval, which is dependent on the pressure of the air in the pressure cell and the desired expansion ratio. The Vo required is the total cylinder device volume divided by the desired expansion ratio. For a single stage system, that ratio is less than or equal to the pressure of air in the air storage tank in atmospheres. At the same time air is being introduced into the cylinder chamber 622, liquid mist from the pressure cell is being pumped (via pump 647) through inlet nozzle 644 into the cylinder chamber. If a sufficient pressure differential exists between the pressure cell 625 and the cylinder device 621, pump 647 is not required. Once the pressure inside of the cylinder chamber is sufficiently high, valve 637 is closed. The piston 623 is urged in the direction of BDC beginning with this step, transmitting power out of the system via a crankshaft, hydraulic pressure, or other mechanical structure. [0112] During step 3 of the expansion cycle, the air introduced in step 2 is allowed to expand in the chamber 622. Liquid mist also continues to be pumped into the chamber 622 through nozzle 644. The predetermined total amount of mist introduced is that required to add enough heat to the system to keep the temperature substantially constant during air expansion. The piston 623 is driven to the bottom of the cylinder device during this step.

[0113] It will be appreciated that this two-step expansion process (a quantity of air Vo introduced in the first step - step 2 - and then allowed to expand in the second step - step 3) allows the system to extract substantially all the energy available in the compressed air.

[0114] During step 4 of the expansion cycle, the crankshaft or other mechanical linkage moves the piston 619 back up to top dead-center (TDC), exhausting the spent air and liquid mist from the cylinder device. The power required to drive the piston comes from the momentum of the system and/or from the motion of other out-of-phase pistons. The exhausted air passes through an air- liquid separator, and the liquid that is separated out is returned to the liquid tank 628.

[0115] It will be appreciated that in accordance with the present invention, at any given time, energy is either being stored or delivered. The two processes are never carried out

simultaneously. As a result, the same mechanism can be used for both compression and expansion, reducing system cost, size and complexity. This is also the situation with all of the other embodiments of the present invention to be described below.

[0116] Multi-Stase System

[0117] When a larger compression/expansion ratio is required than can be accommodated by the mechanical or hydraulic approach by which mechanical power is conveyed to and from the system, then multiple stages should be utilized. A multi-stage compressed air energy storage system 720 with three stages (i.e., first stage 724a, second stage 724b and third stage 724c) is illustrated in schematic form in Figure 7. Systems with more or fewer stages are constructed similarly. Note that, in all figures that follow, when the letters a, b, and c are used with a number designation (e.g. 725a), they refer to elements in an individual stage of a multi-stage energy storage system 720.

[0118] In accordance with the present invention, each stage may typically have substantially the same expansion ratio. A stage's expansion ratio, r i, is the Nth root of the overall expansion ratio. That is, [0119] r » "W

[0120] Where R is the overall expansion ratio and N is the number of stages. It will be appreciated, however, that the different stages can have different expansion ratios, so long as the product of the expansion ratios of all of the stages is i?- That is, in a three-stage system, for example:

[0121] K ₁ X r ₂ X r ₃ = R.

[0122] In order for the mass flow rate through each stage to be substantially the, the lower pressure stages will need to have cylinder chambers with greater displacements. In a multi-stage system, the relative displacements of the cylinder chambers are governed by the following equation:

[0123] ^ ⁼ ^SF

[0124] Where K is the volume of the i ^th cylinder device, and Vf is the total displacement of the system (that is, the sum of the displacements of all of the cylinder devices).

[0125] As an example, suppose that the total displacement of a three-stage system is one liter. If the stroke length of each piston is substantially the same and substantially equal to the bore (diameter) of the final cylinder chamber, then the volumes of the three cylinder chambers are about 19 cm ³ , 127 cm ³ , and 854 cm ³ . The bores are about 1.54 cm, 3.96 cm, and 10.3 cm, with a stroke length of about 10.3 cm for all three. The lowest-pressure cylinder device is the largest and the highest-pressure cylinder device the smallest. [0126] Figure 8 is a schematic representation of how three stages 824a, 824b and 824c could be coupled to a hydraulic system (e.g., a hydraulic motor 857 and six hydraulic cylinders 861al - 861c2) to produce continuous near-uniform power output. Each compressed-air-driven piston 823al - 823c2 of each corresponding compressed-air driven cylinder device 82 IaI - 821c2 is coupled via a respective piston rod 819al - 819c2 to a corresponding piston 860al - 860c2 of a respective hydraulic cylinder device 86 IaI - 861c2.

[0127] The chambers of the air-driven cylinder devices 821al - 821c2 vary in displacement as described above. The chambers of the hydraulic cylinder devices 86 IaI - 861c2, however, are substantially identical in displacement. Because the force generated by each air-driven piston is substantially the same across the three stages, each hydraulic cylinder device provides substantially the same pressure to the hydraulic motor 857. Note that, in this configuration, the two air-driven pistons 82 IaI, 821a2 that comprise a given stage (e.g. the first stage 824a) operate 180 degrees out of phase with each other. [0128] Stages Using Liquid Mist to Effect Heat Exchange in a Multi-Stage System

[0129] If a stage is single-acting and uses liquid mist to effect heat exchange, it operates according to the scheme described in the section titled Single-Stage System above. Each single- acting stage of a multi-stage system (e.g., the second stage 724b of Figure 7) is illustrated schematically in Figure 9. In this configuration, air passes to a cylinder chamber 922b of the second stage 924b illustrated from the pressure cell 25a of the next-lower-pressure stage (e.g., first stage 724a) during compression, and to the pressure cell of the next- lower-pressure stage during expansion, via pipe 692a/990b. Liquid passes to and from the pressure cell 725a of the next-lower-pressure stage via pipe 693a/991b.

[0130] In contrast, air passes from pressure cell 925b of the stage illustrated (e.g., the second stage 924b) to the chamber of the cylinder device of the next higher-pressure stage (e.g., the third stage 724c) during compression and from the chamber of the cylinder device of the next higher- pressure stage during expansion via pipe 992b/690c. It will be appreciated that the air compression/expansion mechanism (i.e., second stage 924b) illustrated is precisely the same as the central elements (the cylinder device 621 and the pressure cell 625 of the first stage 624) shown in Figure 6, with the exception that, in Figure 9, there is a pipe 993b that conveys liquid from the pressure cell of one stage to the chamber of the cylinder device of the next higher- pressure stage. Pipe 993b is not required for the highest-pressure stage; hence, it doesn't appear in the diagrams such as Figure 6 of single-stage configurations.

[0131] If the stage illustrated is the lowest-pressure-stage (e.g., first stage 724a in the embodiment of Figure 7), then line 690a passes air to an air-liquid separator (e.g., separator 627 in Figure 6) during the expansion cycle and from an air filter (e.g., filter 626 in Figure 6) during the compression cycle. Similarly, if the stage illustrated is the lowest-pressure stage, then line 691a communicates liquid to and from the liquid tank. If the stage illustrated is the highest- pressure-stage (e.g., the third stage 724c), then air is conveyed to and from the air tank (e.g., air tank 632 in Figure 6) via pipe 692c. [0132] Multiple Phases

[0133] The systems as described so far represent a single phase embodiment. That is, all pistons operate together over the course of one cycle. During expansion, for example, this produces a varying amount of mechanical work output during one half of the cycle and requires some work input during the other half of the cycle. Such work input may be facilitated by the use of a flywheel (not shown).

[0134] To smooth out the power output over the course of one cycle and reduce the flywheel requirements, in one embodiment, multiple systems phases may be employed. N sets of pistons thus may be operated 360/N degrees apart. For example, four complete sets of pistons may be operated 90 degrees out of phase, smoothing the output power and effecting self-starting and a preferential direction of operation. Note that valves connecting cylinder devices to a pressure cell are only opened during less than one-half of a cycle, so it is possible to share a pressure cell between two phases 180 degrees apart.

[0135] If TV phases are used, and N is even, pairs of phases are 180 degrees apart and may be implemented using double-acting pistons. Figure 10 illustrates a double-acting stage that uses liquid mist to effect heat exchange. Each half of the piston operates according the protocol outlined in the section Single Stage System, but 180 degrees out of phase.

[0136] As described above in connection with Figure 6, the apparatus of Figure 10 further includes a controller/processor 1097 in electronic communication with a computer-readable storage device 1095, which may be of any design, including but not limited to those based on semiconductor principles, or magnetic or optical storage principles. Controller 1097 is shown as being in electronic communication with a universe of active elements in the system, including but not limited to valves, pumps, chambers, nozzles, and sensors. Specific examples of sensors utilized by the system include but are not limited to pressure sensors (P), temperature sensors (T), humidity sensor (H), and volume sensors (V).

[0137] The compression cycle for the double-acting stage illustrated in Figure 10 proceeds as follows:

[0138] Note that step 5 is unnecessary, in some specific embodiments, and can be omitted in the great majority of cycles since the liquid levels in the piston remain substantially the same across long periods of operation.

[0139] In contrast, the expansion cycle for the double-acting stage illustrated in Figure 10 proceeds as follows:

[0140] Note that, as with compression, step 5 is rarely necessary and can be omitted in the great majority of cycles.

[0141] Stages with Multiple Cylinder devices

[0142] If it is desirable that all the cylinder devices in a multi-stage system 720 be of substantially similar size, the larger (lower-pressure) cylinder devices may be divided up into two or more smaller cylinder devices communicating in parallel. An example of such a stage is illustrated in Figure 11, which is an alternative embodiment of the stage of embodiment of Figure 9. In this configuration, four substantially similar cylinder devices 112IbI-1121 b4 share a single pressure cell 1125b containing body of liquid 1149b. However, if it is desirable to operate the cylinder devices out of phase with each other so that the system as a whole may convey power more uniformly, separate pressure cells will be required for each cylinder device. As mentioned above, the exception is cylinder devices that are 180 degrees out of phase, which then may share a common pressure cell.

[0143] Referring back to the embodiment of Figure 11, each cylinder device 112IbI-1121b4 operates according to the scheme used for the mist-type system described in the Single-Stage System section above.

[0144] Multi-cylinder device stages may be single or double-acting, and may use either liquid mist or bubbles to effect heat exchange. A multi-stage system may have some stages with a single cylinder device and others with multiple cylinder devices.

[0145] System Configurations

[0146] It will be understood that a plurality of energy storage system embodiments, designed in accordance with this invention, are possible. These energy storage system may be single or multi-stage. Stages may be single-cylinder device or multi-cylinder device. Heat exchange may be effected via liquid mist or via bubbles. Power may be conveyed in and out of the system via any of the at least four methods described in the previous section. Each possible configuration has advantages for a specific application or set of design priorities. It would not be practicable to describe every one of these configurations here, but it is intended that the information given should be sufficient for one practiced in the art to configure any of these possible energy storage systems as required.

[0147] Note that all the configurations described herein use and generate power in mechanical form, be it hydraulic pressure or the reciprocating action of a piston. In most applications, however, the requirement will be for the storage of electrical energy. In that case, a generator, along with appropriate power conditioning electronics, must be added to convert the mechanical power supplied by the system during expansion to electrical power. Similarly, the mechanical power required by the system during compression must be supplied by a motor. Since compression and expansion are never done simultaneously, a motor-generator may be used to perform both functions. If the energy storage system utilizes a hydraulic motor or a hydro turbine, then the shaft of that device connects directly or via a gearbox to the motor-generator. If the energy storage system utilizes reciprocating pistons, then a crankshaft or other mechanical linkage that can convert reciprocating motion to shaft torque is required. [0148] Use of Waste Heat During Expansion

[0149] In order to operate isothermally, the tendency of air to cool as it expands while doing work (i.e. by pushing a piston or displacing hydraulic liquid) must be counteracted by heat exchange with the ambient air or with a body of water (e.g. a stream or lake). If, however, some other source of heat is available - for example, hot water from a steam condenser - it may be used advantageously during the expansion cycle. In Figure 6, as described in the Single-Stage System section above, pipes 653 and 654 lead to an external heat exchanger. If those pipes are routed instead to a heat source, the efficiency of the expansion process can be increased dramatically. [0150] Because the system operates substantially at or near ambient temperature, the source of heat need only be a few degrees above ambient in order to be useful in this regard. The heat source must, however, have sufficient thermal mass to supply all the heat required to keep the expansion process at or above ambient temperature throughout the cycle.

[0151] As described in detail above, embodiments of systems and methods for storing and recovering energy according to the present invention are particularly suited for implementation in conjunction with a host computer including a processor and a computer-readable storage medium. Such a processor and computer-readable storage medium may be embedded in the apparatus, and/or may be controlled or monitored through external input/output devices. Figure 12 is a simplified diagram of a computing device for processing information according to an embodiment of the present invention. This diagram is merely an example, which should not limit the scope of the claims herein. One of ordinary skill in the art would recognize many other variations, modifications, and alternatives. Embodiments according to the present invention can be implemented in a single application program such as a browser, or can be implemented as multiple programs in a distributed computing environment, such as a workstation, personal computer or a remote terminal in a client server relationship.

[0152] Figure 12 shows computer system 1210 including display device 1220, display screen 1230, cabinet 1240, keyboard 1250, and mouse 1270. Mouse 1270 and keyboard 1250 are representative "user input devices." Mouse 1270 includes buttons 1280 for selection of buttons on a graphical user interface device. Other examples of user input devices are a touch screen, light pen, track ball, data glove, microphone, and so forth. Figure 12 is representative of but one type of system for embodying the present invention. It will be readily apparent to one of ordinary skill in the art that many system types and configurations are suitable for use in conjunction with the present invention. In a preferred embodiment, computer system 1210 includes a Pentium™ class based computer, running Windows™ XP™ or Windows 7™ operating system by Microsoft Corporation. However, the apparatus is easily adapted to other operating systems and architectures by those of ordinary skill in the art without departing from the scope of the present invention.

[0153] As noted, mouse 1270 can have one or more buttons such as buttons 1280. Cabinet 1240 houses familiar computer components such as disk drives, a processor, storage device, etc. Storage devices include, but are not limited to, disk drives, magnetic tape, solid-state memory, bubble memory, etc. Cabinet 1240 can include additional hardware such as input/output (I/O) interface cards for connecting computer system 1210 to external devices external storage, other computers or additional peripherals, further described below.

[0154] Figure 12A is an illustration of basic subsystems in computer system 1210 of Figure 12. This diagram is merely an illustration and should not limit the scope of the claims herein. One of ordinary skill in the art will recognize other variations, modifications, and alternatives. In certain embodiments, the subsystems are interconnected via a system bus 1275. Additional subsystems such as a printer 1274, keyboard 1278, fixed disk 1279, monitor 1276, which is coupled to display adapter 1282, and others are shown. Peripherals and input/output (I/O) devices, which couple to I/O controller 1271, can be connected to the computer system by any number of approaches known in the art, such as serial port 1277. For example, serial port 1277 can be used to connect the computer system to a modem 1281, which in turn connects to a wide area network such as the Internet, a mouse input device, or a scanner. The interconnection via system bus allows central processor 1273 to communicate with each subsystem and to control the execution of instructions from system memory 1272 or the fixed disk 1279, as well as the exchange of information between subsystems. Other arrangements of subsystems and interconnections are readily achievable by those of ordinary skill in the art. System memory, and the fixed disk are examples of tangible media for storage of computer programs, other types of tangible media include floppy disks, removable hard disks, optical storage media such as CD-ROMS and bar codes, and semiconductor memories such as flash memory, read-only-memories (ROM), and battery backed memory. [0155] Figure 13 is a schematic diagram showing the relationship between the

processor/controller, and the various inputs received, functions performed, and outputs produced by the processor controller. As indicated, the processor may control various operational properties of the apparatus, based upon one or more inputs. [0156] An example of such an operational parameter that may be controlled is the timing of opening and closing of a valve allowing the inlet of air to the cylinder during an expansion cycle. Figures 14A-C are simplified and enlarged views of the cylinder 622 of the single-stage system of Figure 6, undergoing an expansion cycle as described previously.

[0157] Specifically, during step 2 of the expansion cycle, a pre-determined amount of air Vo, is added to the chamber from the pressure cell, by opening valve 637 for a controlled interval of time. This amount of air Vo is calculated such that when the piston reaches the end of the expansion stroke, a desired pressure within the chamber will be achieved.

[0158] In certain cases, this desired pressure will approximately equal that of the next lower pressure stage, or atmospheric pressure if the stage is the lowest pressure stage or is the only stage. Thus at the end of the expansion stroke, the energy in the initial air volume Vo has been fully expended, and little or no energy is wasted in moving that expanded air to the next lower pressure stage.

[0159] To achieve this goal, valve 637 is opened only for so long as to allow the desired amount of air (Vo) to enter the chamber, and thereafter in steps 3-4 (Figures 14B-C), valve 637 is maintained closed. In certain embodiments, the desired pressure within the chamber may be within 1 psi, within 5 psi, within 10 psi, or within 20 psi of the pressure of the next lower stage.

[0160] In other embodiments, the controller/processor may control valve 637 to cause it to admit an initial volume of air that is greater than Vo. Such instructions may be given, for example, when greater power is desired from a given expansion cycle, at the expense of efficiency of energy recovery.

[0161] Timing of opening and closing of valves may also be carefully controlled during compression. For example, as shown in Figures 14D-E, in the steps 2 and 3 of the table corresponding to the addition of mist and compression, the valve 638 between the cylinder device and the pressure cell remains closed, and pressure builds up within the cylinder. [0162] In conventional compressor apparatuses, accumulated compressed air is contained within the vessel by a check valve, that is designed to mechanically open in response to a threshold pressure. Such use of the energy of the compressed air to actuate a check valve, detracts from the efficiency of recovery of energy from the air for performing useful work. [0163] By contrast, as shown in Figure 14F, embodiments of the present invention may utilize the controller/processor to precisely open valve 638 under the desired conditions, for example where the built-up pressure in the cylinder exceeds the pressure in the pressure cell by a certain amount. In this manner, energy from the compressed air within the cylinder is not consumed by the valve opening process, and efficiency of energy recovery is enhanced. Embodiments of valve types that may be subject to control to allow compressed air to flow out of a cylinder include but are not limited to pilot valves, cam-operated poppet valves, rotary valves, hydraulically actuated valves, and electronically actuated valves.

[0164] While the timing of operation of valves 637 and 638 of the single stage apparatus may be controlled as described above, it should be appreciated that valves in other embodiments may be similarly controlled. Examples of such valves include but are not limited to valves 937b and 938b of FIG. 9, valves 1037bl, 1038bl, 1037b2 and 1038b2 ofFIG. 10, and the valves 1137bl-4 and 1138b 1-4 that are shown in FIG. 11.

[0165] Another example of a system parameter that can be controlled by the processor, is the amount of liquid introduced into the chamber. Based upon one or more values such as pressure, humidity, calculated efficiency, and others, an amount of liquid that is introduced into the chamber during compression or expansion, can be carefully controlled to maintain efficiency of operation. For example, where an amount of air greater than Vo is inlet into the chamber during an expansion cycle, additional liquid may need to be introduced in order to maintain the temperature of that expanding air within a desired temperature range. [0166] The present invention is not limited to those particular embodiments described above. Other methods and apparatuses may fall within the scope of the invention. For example, the step of adding liquid to a cylinder device is not required during every cycle. In addition, liquid may be added to the chamber at the same time air is being inlet.

[0167] Accordingly, the following table describes steps in an embodiment of a compression cycle for a single-stage system utilizing liquid mist to effect heat exchange, as shown in connection with Figures 15A-C, where similar elements as in Figure 6 are shown:

[0168] The corresponding expansion cycle where liquid is introduced at the same time as air, is shown in the table below, in connection with Figures 16A-C:

[0169] Variations on the specific embodiments describe above, are possible. For example, in some embodiments, a plurality of pistons may be in communication with a common chamber. In other embodiments, a multistage apparatus may not include a separate pressure cell.

[0170] For example, in the embodiment of Figure 17, the stages are connected directly together through a heat exchanger, rather than through a pressure cell as in the embodiment of Figure 9. The relative phases of the cycles in the two stages must be carefully controlled so that when Stage 1 is performing an exhaust step, Stage 2 is performing an intake step (during compression). When Stage 2 is performing an exhaust step, Stage 1 is performing an intake step (during expansion).

[0171] The timing is controlled so the pressures on either side of heat exchanger 1724 are substantially the same when valves 1737 and 1758 are open. Liquid for spray nozzle 1744 is supplied from an excess water in cylinder 1722 by opening valve 1736 and turning on pump 1732. Similarly, liquid for spray nozzle 1764 is supplied from an excess water in cylinder 1786 by opening valve 1738 and turning on pump 1734. Such precise timing during operation may be achieved with the operation of a controller/processor that is communication with a plurality of the system elements, as has been previously described. [0172] The present invention is not limited to the embodiments specifically described above. For example, while water has been described as the liquid that is injected into air as a mist, other liquids could be utilized and fall within the scope of the present invention. Examples of liquids that could be used include polypropylene glycol, polyethylene glycol, and alcohols.

[0173] The following claims relate to compression. [0174] 1. A method for storing energy, the method comprising: introducing a first quantity of air at a first temperature into a first chamber; in a compression cycle, subjecting the first quantity of air to compression by a first piston coupled to the first chamber; injecting a first determined quantity of fluid into the first quantity of air to absorb thermal energy generated by the compression cycle and thereby maintain the first quantity of air in a first temperature range during the compression; and transferring at least a portion of the first quantity of air to a first pressure cell.

[0175] 2. The method of claim 1 wherein the first determined quantity of fluid is based upon one or more control parameters. [0176] 3. The method of claim 2 wherein the control parameter is calculated for the compression cycle from a measured physical property.

[0177] 4. The method of claim 2 wherein the control parameter comprises a maximum increase in a temperature of the first quantity of air during compression. [0178] 5. The method of claim 2 wherein the control parameter comprises an amount of the fluid present in liquid form inside the chamber.

[0179] 6. The method of claim 2 wherein the control parameter comprises an efficiency.

[0180] 7. The method of claim 2 wherein the control parameter comprises a power input to the piston.

[0181] 8. The method of claim 2 wherein the control parameter comprises a speed of the piston.

[0182] 9. The method of claim 2 wherein the control parameter comprises a force on the piston. [0183] 10. The method of claim 1 wherein the piston is solid, liquid, or a combination of solid and liquid.

[0184] 11. The method of claim 1 wherein the first temperature range is reflected by a change in a temperature of the first quantity of air from a first temperature to a second temperature below a boiling point of the fluid. [0185] 12. The method of claim 11 wherein the fluid comprises water.

[0186] 13. The method of claim 12 wherein the first temperature range is about 60 degrees Celsius or less.

[0187] 14. The method of claim 1 wherein the first determined quantity of fluid is injected by spraying or misting. [0188] 15. The method of claim 1 wherein the thermal energy transferred from the first quantity of air to the first determined quantity of fluid is facilitated by bubbling air through a liquid.

[0189] 16. The method of claim 1 further comprising transferring compressed air within the pressure cell to a storage tank. [0190] The following claims relate to compression and expansion. [0191] 17. The method of claim 1 further comprising: in an expansion cycle, transferring a second quantity of air from the first pressure cell to the first chamber; allowing the second quantity of air to expand and drive the first piston; and injecting a second determined quantity of fluid into the second quantity of air to provide thermal energy absorbed by the expanding air and thereby maintain the second quantity of air in a second temperature range during the expansion.

[0192] 18. The method of claim 17 further comprising generating electrical power from the driving of the first piston.

[0193] 19. The method of claim 17 wherein the second determined quantity of fluid is based upon a one or more control parameters.

[0194] 20. The method of claim 17 wherein the control parameter is calculated for the expansion cycle from a measured physical property.

[0195] 21. The method of claim 17 wherein the control parameter comprises a maximum decrease in a temperature of the second quantity of air during the expansion. [0196] 22. The method of claim 17 wherein the control parameter comprises an amount of the fluid present in liquid form inside the chamber.

[0197] 23. The method of claim 17 wherein the control parameter comprises an efficiency.

[0198] 24. The method of claim 17 wherein the control parameter comprises a power output by the first piston. [0199] 25. The method of claim 17 wherein the control parameter comprises a speed of the piston.

[0200] 26. The method of claim 17 wherein the control parameter comprises a force on the piston.

[0201] 27. The method of claim 17 wherein the first determined quantity of fluid is injected by spraying or misting. [0202] 28. The method of claim 17 wherein thermal energy is transferred from the second quantity of air to the second determined quantity of fluid facilitated by bubbling air through a liquid.

[0203] 29. The method of claim 17 wherein the fluid comprises water. [0204] 30. The method of claim 17 further comprising placing the chamber in communication with additional thermal energy during the expansion cycle.

[0205] 31. The method of claim 30 wherein the additional thermal energy is waste heat from another thermal source.

[0206] 32. The method of claim 17 wherein the second temperature range is reflected by a change in a temperature of the second quantity of air from a first temperature to a second temperature above a freezing point of the fluid.

[0207] 33. The method of claim 32 wherein the fluid comprises water.

[0208] 34. The method of claim 33 wherein the second temperature range is about 11 degrees Celsius or less. [0209] 34a. The method of claim 17 wherein at an end of an expansion stroke of the first piston, the second quantity of air is configured to produce a pressure on the first piston substantially equal to a desired pressure.

[0210] 34b. The method of claim 34a, wherein the desired pressure is an input pressure of the next lowest pressure stage, or is ambient pressure. [0211] 34c. The method of claim 34a wherein the desired pressure is calculated to maximize an efficiency of expansion.

[0212] 34d. The method of claim 34a wherein the desired pressure is calculated to produce a desired level of power output.

[0213] 34e. The method of claim 34a wherein the desired pressure is within approximately 5 psi of an input pressure of the next lowest pressure stage.

[0214] The following claims relate to multi-stage operation. [0215] 35. The method of claim 17 further comprising: providing a second chamber in selective fluid communication with the first pressure cell and with a second pressure cell; introducing from the first pressure cell, a third quantity of air at a second temperature into the second chamber; in a compression cycle of the second chamber, subjecting the third quantity of air to compression by a second piston coupled to the second chamber; injecting a third determined quantity of fluid into the third quantity of air to absorb thermal energy generated by the compression and thereby maintain the third quantity of air in a third temperature range during the compression; and transferring at least a portion of the third quantity of air to the second pressure cell. [0216] 36. The method of claim 35 further comprising: in an expansion cycle of the second chamber, transferring a fourth quantity of air from the second pressure cell to the second chamber; allowing the fourth quantity of air to expand and drive the second piston; injecting a fourth determined quantity of fluid into the fourth quantity of air to provide thermal energy absorbed by the expanding air and thereby maintain the fourth quantity of air in a fourth temperature range during the expansion; and transferring at least a portion of the fourth quantity of air from the second chamber to the first pressure cell.

[0217] The following claims relate to expansion.

[0218] 37. A method for releasing stored energy, the method comprising: in an expansion cycle, transferring a quantity of air from a pressure cell to a chamber having a piston disposed therein; allowing the quantity of air to expand and drive the piston; and injecting a determined quantity of fluid into the quantity of air to provide thermal energy absorbed by the expanding air and thereby maintain the quantity of air in a first temperature range during the expansion.

[0219] 38. The method of claim 37 wherein the determined quantity of fluid is based upon one or more control parameters.

[0220] 39. The method of claim 38 wherein the control parameter is calculated from a measured physical property.

[0221] 40. The method of claim 38 wherein the control parameter comprises a maximum decrease in a temperature of the quantity of air during the expansion. [0222] 41. The method of claim 38 wherein the control parameter comprises an amount of the fluid present in liquid form inside the chamber.

[0223] 42. The method of claim 38 wherein the control parameter comprises an efficiency.

[0224] 43. The method of claim 38 wherein the control parameter comprises a power input to the piston. [0225] 44. The method of claim 38 wherein the control parameter comprises a speed of the piston.

[0226] 45. The method of claim 38 wherein the control parameter comprises a force of the piston.

[0227] 46. The method of claim 38 wherein the piston is solid, liquid, or a combination of solid and liquid.

[0228] 47. The method of claim 38 wherein the fluid comprises water.

[0229] 48. The method of claim 38 wherein the first temperature range is reflected by a change in a temperature of the first quantity of air from a first temperature to a second temperature, the change less than a determined value. [0230] 49. The method of claim 48 wherein the lower temperature is greater than a freezing point of the fluid. [0231] 50. The method of claim 48 wherein the higher temperature is less than a boiling point of the fluid.

[0232] 51. The method of claim 38 wherein the first determined quantity of fluid is injected by spraying or misting. [0233] 52. The method of claim 38 wherein the thermal energy transferred from the quantity of air to the determined quantity of fluid is facilitated by bubbling air through a liquid.

[0234] 52a. The method of claim 37 wherein at an end of an expansion stroke of the piston, the quantity of air is configured to produce a pressure on the piston substantially equal to a desired pressure. [0235] 52b. The method of claim 37, wherein the desired pressure is an input pressure of the next lowest pressure stage, or is ambient pressure.

[0236] 52c. The method of claim 37 wherein the desired pressure is calculated to maximize an efficiency of expansion.

[0237] 52d. The method of claim 37 wherein the desired pressure is calculated to produce a desired level of power output.

[0238] 52e. The method of claim 37 wherein the desired pressure is within approximately 5 psi of an input pressure of the next lowest pressure stage.

[0239] The following claims relate to temperature difference during system operation. [0240] 53. A method comprising: providing an energy storage system comprising a pressure cell in selective fluid communication with a chamber having a moveable piston disposed therein; flowing air into the chamber; in a compression cycle, storing energy by placing the piston in communication with an energy source to compress the air within the chamber, and then transferring the compressed air to the pressure cell; and then in an expansion cycle, releasing energy by transferring air from the pressure cell back into the chamber while allowing the piston to move in response to expansion of air inside the chamber; monitoring an operational parameter of the compression cycle and/or the expansion cycle; and controlling the operational parameter to maintain a temperature of air in the chamber within a range.

[0241] 54. The method of claim 53 wherein determining an operational parameter comprises controlling an amount of a liquid introduced into the air within the chamber during the compression cycle.

[0242] 55. The method of claim 53 wherein the liquid comprises water.

[0243] 56. The method of claim 53 wherein determining an operational parameter comprises controlling an amount of a liquid introduced into the air within the chamber during the expansion cycle.

[0244] 57. The method of claim 56 wherein the liquid comprises water.

[0245] 58. The method of claim 53 wherein a lower bound of the range is greater than a freezing point of a liquid introduced into the air within the chamber.

[0246] 59. The method of claim 58 wherein the liquid comprises water. [0247] 60. The method of claim 53 wherein an upper bound of the range is lower than a boiling point of a liquid introduced into the air within the chamber.

[0248] 61. The method of claim 60 wherein the liquid comprises water.

[0249] 62. The method of claim 53 wherein determining an operational parameter comprises controlling a timing of the transfer of air from the pressure cell into the chamber during the expansion cycle.

[0250] 62a. The method of claim 62 wherein the timing is controlled such that at an end of an expansion stroke of the piston, the transferred air is configured to produce a desired pressure on the piston.

[0251] 62b. The method of claim 62a, wherein the desired pressure is an input pressure of the next lowest pressure stage, or is ambient pressure.

[0252] 62c. The method of claim 62a wherein the desired pressure is calculated to maximize an efficiency of expansion. [0253] 62d. The method of claim 62a wherein the desired pressure is calculated to produce a desired level of power output.

[0254] 62e. The method of claim 62a wherein the desired pressure is within approximately 5 psi of an input pressure of the next lowest pressure stage. [0255] 63. The method of claim 53 wherein determining an operational parameter comprises monitoring a pressure in the pressure cell.

[0256] 64. The method of claim 53 wherein determining an operational parameter comprises monitoring a pressure in the chamber.

[0257] 65. The method of claim 53 wherein determining an operational parameter comprises monitoring a temperature of the air in the chamber.

[0258] 66. The method of claim 53 wherein determining an operational parameter comprises monitoring a humidity of the air flowed into the chamber.

[0259] 67. The method of claim 53 wherein determining an operational parameter comprises monitoring a humidity of air exhausted from the chamber. [0260] 68. The method of claim 53 wherein determining an operational parameter comprises monitoring a power released during the expansion cycle.

[0261] 69. The method of claim 53 wherein determining an operational parameter comprises monitoring a position of the piston.

[0262] 70. The method of claim 53 wherein determining an operational parameter comprises monitoring a force on the piston.

[0263] 71. The method of claim 54 wherein determining an operational parameter comprises monitoring a temperature of the liquid.

[0264] 72. The method of claim 56 wherein determining an operational parameter comprises monitoring a temperature of the liquid. [0265] 73. The method of claim 54 wherein determining an operational parameter comprises monitoring a rate of flow of the liquid. [0266] 74. The method of claim 56 wherein determining an operational parameter comprises monitoring a rate of flow of the liquid.

[0267] 75. The method of claim 54 wherein determining an operational parameter comprises monitoring a level of the liquid in the chamber. [0268] 76. The method of claim 56 wherein determining an operational parameter comprises monitoring a level of the liquid in the chamber.

[0269] 77. The method of claim 54 wherein determining an operational parameter comprises monitoring a volume of the liquid in the chamber.

[0270] 78. The method of claim 56 wherein determining an operational parameter comprises monitoring a volume of the liquid in the chamber.

[0271] 79. The method of claim 53 wherein: the piston is in communication with a rotating shaft; and determining an operational parameter comprises monitoring a speed of the rotating shaft. [0272] 80. The method of claim 53 wherein: the piston is in communication with a rotating shaft; and determining an operational parameter comprises monitoring a torque of the rotating shaft.

[0273] 81. The method of claim 53 wherein the operational parameter is controlled based upon a derived parameter calculated from the monitored operational parameter.

[0274] 82. The method of claim 81 wherein the derived parameter is selected from the group comprising, an efficiency of power conversion, an expected power output, an expected output speed of a rotating shaft in communication with the piston, an expected output torque of a rotating shaft in communication with the piston, an expected input speed of a rotating shaft in communication with the piston, an expected input torque of a rotating shaft in communication with the piston, a maximum output speed of a rotating shaft in communication with the piston, a maximum output torque of a rotating shaft in communication with the piston, a minimum output speed of a rotating shaft in communication with the piston, a minimum output torque of a rotating shaft in communication with the piston, a maximum input speed of a rotating shaft in communication with the piston, a maximum input torque of a rotating shaft in communication with the piston, a minimum input speed of a rotating shaft in communication with the piston, a minimum input torque of a rotating shaft in communication with the piston, or a maximum expected temperature difference of air at each stage.

[0275] 83. The method of claim 53 wherein controlling the operational parameter comprises controlling a timing of the transfer of air from the chamber to the pressure cell during the compression cycle.

[0276] 84. The method of claim 53 wherein controlling the operational parameter comprises controlling a timing of the transfer of air from the pressure cell to the chamber during the expansion cycle. [0277] 85. The method of claim 54 wherein controlling the operational parameter comprises controlling a timing of a flow of liquid to the chamber.

[0278] 86. The method of claim 56 wherein controlling the operational parameter comprises controlling a timing of a flow of liquid to the chamber.

[0279] 87. The method of claim 53 wherein: during the compression cycle, the piston is in communication with a motor or a motor-generator; and controlling the operational parameter comprises controlling an amount of electrical power applied to the motor or the motor-generator.

[0280] 88. The method of claim 53 wherein: during the expansion cycle, the piston is in communication with a generator or a motor- generator; and controlling the operational parameter comprises controlling an electrical load applied to the generator or the motor-generator.

[0281] 89. The method of claim 54 wherein: the liquid is flowed to the chamber utilizing a pump; and controlling the operational parameter comprises controlling an amount of electrical power supplied to the pump. [0282] 90. The method of claim 56 wherein: the liquid is flowed to the chamber utilizing a pump; and controlling the operational parameter comprises controlling an amount of electrical power supplied to the pump. [0283] 91. The method of claim 53 wherein: liquid in the pressure cell is circulated through a heat exchanger that is in thermal

communication with a fan; and controlling the operational parameter comprises controlling an amount of electrical power supplied to the fan. [0284] 92. The method of claim 53 further comprising placing the chamber in communication with additional thermal energy during the expansion cycle.

[0285] 93. The method of claim 92 wherein the additional thermal energy is waste heat from another thermal source.

[0286] 94. The method of claim 53 wherein controlling the operational parameter comprises controlling a compression ratio.

[0287] 95. The method of claim 53 further comprising transferring compressed air within the pressure cell to a storage tank.

[0288] The following claims relate to a system.

[0289] 96. An energy storage and recovery system comprising: a first chamber having a moveable piston disposed therein and in selective communication with an energy source; a pressure cell in selective fluid communication with the first chamber through a first valve; an air source in selective fluid communication with the first chamber through a second valve; a liquid source in selective fluid communication with the first chamber through a third valve; and a controller in electronic communication with, and configured to operate, system elements in one of the following states: an intake step wherein the first valve is closed, the second valve is open, and the third valve may be open or closed; a compression step wherein the piston is in communication with the energy source, the first and second valves are closed, the third valve is open or closed, and then the first valve is opened upon compression of the air in the chamber by the piston, an expansion step wherein the piston is not in communication with the energy source, the first valve is opened, the second valve is closed, and the third valve may be open or closed, such that the air expands in the chamber to move the piston, and then the first valve is closed as the air continues to expand, and an exhaust step wherein the piston is not in communication with the energy source, the first valve is closed, the second valve is open, and the third valve may be open or closed; and; wherein the controller is configured to determine an operational parameter in order to maintain a temperature of the air in the first chamber within a range.

[0290] 97. The energy storage and recovery system of claim 96 wherein the moveable piston comprises a solid piston.

[0291] 98. The energy storage and recovery system of claim 96 wherein the moveable piston comprises a liquid piston.

[0292] 99. The energy storage and recovery system of claim 96 further comprising a sprayer configured to inject the liquid into the air within the chamber. [0293] 100. The energy storage and recovery system of claim 99 wherein the liquid comprises water.

[0294] 101. The energy storage and recovery system of claim 96 further comprising a bubbler configured to transfer heat between the liquid and air within the pressure cell.

[0295] 102. The energy storage and recovery system of claim 101 wherein the liquid comprises water.

[0296] 103. The energy storage and recovery system of claim 96 further comprising a sensor configured to detect a volume of liquid present within the chamber, the sensor in electronic communication with the controller and referenced to determine the operational parameter. [0297] 104. The energy storage and recovery system of claim 96 further comprising a sensor configured to detect a property selected from the group comprising, a pressure, a temperature, a humidity, a position of the piston, a force on the piston, a liquid flow rate, a liquid level, a liquid volume, a speed of a shaft driven by the piston, or a torque of the shaft driven by the piston, wherein the sensor is in electronic communication with the controller and referenced to determine the operational parameter.

[0298] 105. The energy storage and recovery system of claim 96 further comprising a power generator or motor-generator configured to be in selective communication with the piston during the expansion stroke. [0299] 106. The energy storage and recovery system of claim 96 wherein the chamber is configured to be in thermal communication with a thermal energy source.

[0300] 107. The energy storage and recovery system of claim 96 further comprising a storage tank configured to receive compressed air from the pressure cell.

[0301] 107a. The energy storage and recovery system of claim 96 wherein during the expansion the controller is configured to operate the first valve to inlet the air such that at an end of an expansion stroke of the piston, a pressure on the piston is substantially equal to a desired pressure.

[0302] 107b. The method of claim 107a, wherein the desired pressure is an input pressure of the next lowest pressure stage, or is ambient pressure. [0303] 107c. The method of claim 107a wherein the desired pressure is calculated to maximize an efficiency of expansion.

[0304] 107d. The method of claim 107a wherein the desired pressure is calculated to produce a desired level of power output.

[0305] 107e. The method of claim 107a wherein the desired pressure is within approximately 5 psi of an input pressure of the next lowest pressure stage.

[0306] The following claims relate to a system having multiple stages.

[0307] 108. The energy storage and recovery system of claim 96, further comprising: a second chamber having a moveable piston disposed therein and in selective communication with the energy source; and a second pressure cell in selective fluid communication with the second chamber through a fourth valve, in selective fluid communication with the first pressure cell through a fifth valve, the fourth and fifth valves in communication with and configured to be operated by the controller.

[0308] 109. The energy storage and recovery system of claim 96, further comprising a plurality of a second chamber and second pressure cell connected in series with the first chamber and first pressure cell, such that output from the first chamber is communicated to the second chamber.

[0309] The following claims relate to a processor.

[0310] 110. An apparatus for storing and recovering energy, the apparatus comprising: a host computer comprising a processor in electronic communication with a computer-readable storage medium, the computer readable storage medium having stored thereon one or more codes to instruct the processor to, receive a signal indicating a property of an energy storage and recovery system comprising a first chamber having a moveable piston disposed therein and in selective communication with an energy source, and a pressure cell in selective fluid communication with the first chamber, in response to the received signal, control an element of the energy storage and recovery system to maintain a temperature of air within the first chamber within a temperature range.

[0311 ] 111. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a pressure in the pressure cell.

[0312] 112. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a pressure in the first chamber. [0313] 113. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a temperature of the air in the first chamber. [0314] 114. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a temperature of the air in the pressure cell.

[0315] 115. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a humidity of the air inlet to the first chamber.

[0316] 116. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a power output.

[0317] 117. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a humidity of the air exhausted from the first chamber.

[0318] 118. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a position of the piston.

[0319] 119. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a force on the piston.

[0320] 120. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a temperature of liquid flowed to the chamber.

[0321] 121. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating rate of flow of liquid to the chamber.

[0322] 122. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a level of liquid in the chamber.

[0323] 123. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating volume of liquid in the chamber.

[0324] 124. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating a speed of a rotating shaft in communication with the piston. [0325] 125. The apparatus of claim 110 wherein the code stored on the computer readable storage medium is configured to receive the signal indicating torque of a rotating shaft in communication with the piston.

[0326] 126. The apparatus of claim 110 wherein in response to the received signal, the code stored on the computer readable storage medium is configured to instruct the processor to control a timing of a transfer of air from the chamber to the pressure cell during a compression cycle.

[0327] 126a. The apparatus of claim 110 wherein in response to the received signal, the code stored on the computer readable storage medium is configured to instruct the processor to control a timing of a transfer of air from the pressure cell to the chamber during an expansion cycle. [0328] 127. The apparatus of claim 110 wherein in response to the received signal, the code stored on the computer readable storage medium is configured to instruct the processor to control a timing of a transfer of liquid to the chamber.

[0329] 128. The apparatus of claim 110 wherein in response to the received signal, the code stored on the computer readable storage medium is configured to instruct the processor to control the amount of liquid transferred to the chamber.

[0330] 129. The apparatus of claim 110 wherein in response to the received signal, the code stored on the computer readable storage medium is configured to instruct the processor to control an electrical load applied to a generator or a motor-generator in communication with the piston, during an expansion cycle. [0331] 130. The apparatus of claim 110 wherein in response to the received signal, the code stored on the computer readable storage medium is configured to instruct the processor to control an electrical power applied to a motor or a motor-generator in communication with the piston, during a compression cycle.

[0332] 131. The apparatus of claim 110 wherein in response to the received signal, the code stored on the computer readable storage medium is configured to instruct the processor to control an electrical power applied to a pump to flow liquid into the chamber.

[0333] 132. The apparatus of claim 110 wherein in response to the received signal, the code stored on the computer readable storage medium is configured to instruct the processor to control an electrical power applied to fans associated with a heat exchanger configured to receive liquid from the pressure cell. [0334] 133. The apparatus of claim 110 wherein in response to the received signal, the code stored on the computer readable storage medium is configured to instruct the processor to control a compression ratio.

[0335] The following claims relate to a multi-stage system. [0336] 134. An energy storage and recovery system comprising: a first stage comprising a first element moveable to compress air in the first stage, the first stage in selective fluid communication with an ambient air supply through a first valve; a final stage comprising a second element moveable to compress air in the final stage, and moveable in response to expanding air within the final stage, the final stage in selective fluid communication with a compressed air storage tank through a second valve; a controller configured to determine an amount of liquid to be injected into the first stage or the final stage to maintain a temperature of air in the first stage or in the final stage within a temperature range; and a liquid source in communication with the controller and configured to inject the determined amount of liquid into the first stage or into the final stage.

[0337] 135. The energy storage and recovery system of claim 134, wherein the first moveable element is also moveable in response to expanding air within the first stage.

[0338] 136. The energy storage and recovery system of claim 134, wherein the first moveable element comprises a piston. [0339] 137. The energy storage and recovery system of claim 134, wherein the first moveable element comprises a screw.

[0340] 138. The energy storage and recovery system of claim 134, wherein the first stage or the final stage comprises a pressure cell in selective fluid communication with a chamber.

[0341] 139. The energy storage and recovery system of claim 134, wherein the first stage is configured to transfer to, and receive compressed air from, the final stage through a third valve.

[0342] 140. The energy storage and recovery system of claim 139, wherein the first stage comprises a first chamber having a first piston disposed therein as the first moveable element, and the final stage comprises a second chamber having a second piston disposed therein as the second moveable element, the first and final stages lacking a pressure cell.

[0343] 141. The energy storage and recovery system of claim 134, further comprising an intermediate stage positioned in series and in selective fluid communication between the first stage and the final stage, the intermediate stage comprising a third element moveable to compress air in the intermediate stage, and moveable in response to expanding air within the intermediate stage.

[0344] 142. The energy storage and recovery system of claim 141, wherein the first moveable element is also moveable in response to expanding air within the first stage. [0345] 143. The energy storage and recovery system of claim 142, wherein the first stage comprises a first chamber having a first piston disposed therein as the first moveable element, and the intermediate stage comprises a second chamber having a second piston disposed therein as the third moveable element.

[0346] 144. The energy storage and recovery system of claim 141, wherein the intermediate stage comprises a first chamber having a first piston disposed therein as the third moveable element, and the final stage comprises a second chamber having a second piston disposed therein as the second moveable element.

[0347] 145. The energy storage and recovery system of claim 141, wherein the first stage, the intermediate stage, or the final stage comprises a chamber in selective fluid communication with a pressure cell.

[0348] 146. The energy storage and recovery system of claim 141, wherein consecutive stages do not include a pressure cell.

[0349] 147. The energy storage and recovery system of claim 141, further comprising additional intermediate stages positioned in series between the first stage and the final stage. [0350] 148. The energy storage and recovery system of claim 134, wherein the second moveable element comprises a piston.

[0351] 149. The energy storage and recovery system of claim 148, wherein the second moveable element comprises a liquid piston. [0352] 150. The energy storage and recovery system of claim 148, wherein the second moveable element comprises a solid piston.

[0353] 151. The energy storage and recovery system of claim 134, wherein a compression ratio of the first stage is larger than a compression ratio of the final stage. [0354] 152. The energy storage and recovery system of claim 141, wherein a compression ratio of the first stage is larger than a compression ratio of the intermediate stage, and the compression ratio of the intermediate stage is greater than a compression ratio of the final stage.

[0355] 153. The energy storage and recovery system of claim 134, wherein the liquid comprises water. [0356] 154. A method of storing energy, the method comprising: receiving ambient air in a first stage; compressing the ambient air in the first stage; transferring compressed air to a final stage; further compressing air in the final stage; transferring the further compressed air from the final stage to a storage tank; and determining an operational parameter to maintain a temperature change of air in the first stage or in the second stage within a range during the compression or the further compression.

[0357] 155. The method of claim 154 wherein the determined operational parameter comprises a timing of opening or closing valves controlling movement of air into or out of the stages.

[0358] 156. The method of claim 154 wherein the determined operational parameter comprises an amount of liquid injected into the first stage or into the final stage during the compression or the further compression.

[0359] 157. The method of claim 154 wherein compressing the ambient air comprises placing a piston disposed within a chamber of the first stage, in communication with an energy source.

[0360] 158. The method of claim 154 wherein compressing the ambient air comprises placing a screw disposed within a chamber of the first stage, in communication with an energy source. [0361] 159. The method of claim 154 wherein compressed air is transferred to the final stage via an intermediate stage in which additional compression takes place.

[0362] 160. The method of claim 154 further comprising: transferring compressed air from the storage tank to the final stage; allowing the compressed air to expand and drive a first moveable element in the final stage; transferring air from the final stage to the first stage; allowing compressed air in the first stage to expand and drive a second moveable element in the first stage; and determining an operational parameter to maintain a temperature change of air in the first stage or in the second stage within a range, during expansion of air within the first stage or within the second stage.

[0363] 161. The method of claim 160 wherein the determined operational parameter comprises a timing of opening or closing valves controlling movement of air into or out of the stages. [0364] 162. The method of claim 160 wherein the determined operational parameter comprises an amount of liquid injected into the first stage or into the final stage during expansion of air within the first stage or the second stage.

[0365] 163. The method of claim 160 wherein the first moveable element comprises a piston.

[0366] 164. The method of claim 160 wherein the second moveable element comprises a piston.

[0367] 165. The method of claim 160 wherein air is transferred from the final stage to the first stage via an intermediate stage wherein further expansion of air takes place.