Energy storage system and method

Abstract

The energy storage system includes a pressure vessel exposed to ambient temperature and containing a condensable working fluid (CWF) at ambient temperature, a liquid reservoir in communication with one of the vessels and containing a non-vaporizable liquid in the reservoir and in the vessel, and a device for pumping liquid from the reservoir to the vessel. The CWF is compressible by direct contact with the liquid in the vessel and can be stored in a liquid state after being compressed to a saturation pressure. In one method, at least a portion of the liquid in the vessel is propelled by the CWF towards a turbine to generate power. In one embodiment, the module includes a first vessel having at least four ports, a second vessel at ambient temperature, and a flow control component operably connected to a corresponding conduit for selectively controlling the flow of the fluid.

Description

[Technical field]

[0001] The present invention relates to the field of mechanical energy storage, and more particularly to an energy storage system and method capable of releasing compressed gas for later use. [Background technology]

[0002] Energy collected from various sustainable energy sources, such as wind, solar, and wave power, is known to be stored in the form of compressed gas. During periods of demand, the compressed gas can be released from the storage vessel to generate electricity. Alternatively, the stored compressed gas can be used for various industrial applications.

[0003] Many Compressed Air Energy Storage (CAES) systems are known in the prior art. Compressed air has traditionally been stored in large and expensive underground caverns or underwater elastic balloons. CAES plants have large power ratings and storage capacities, but they suffer from several major drawbacks. First, air has a large heat of compression, with approximately 85% heat loss when compressed. As the temperature increases, less compressed air can be stored. Second, the pressure in the caverns where the compressed air is stored gradually decreases as additional air is released, negatively affecting the amount of power that a turbine driven by the released compressed air can generate.

[0004] Attempts have been made to reduce the heat of compression by charging and discharging the air isothermally, but the need to use heat exchangers to facilitate isothermal compression adds unnecessary cost to the storage facility.

[0005] Other CAES systems have used non-adiabatic (D-CAES), adiabatic (A-CAES), and liquid (LAES) air energy storage. Because the air temperature in these prior art systems is far from ambient, these prior art systems also require the use of expensive heat exchangers and rotating machinery, i.e., compressors and gas turbines.

[0006] It is an object of the present invention to provide a compressed gas energy storage system having improved energy density compared to prior art systems.

[0007] It is a further object of the present invention to provide a compressed gas energy storage system having improved system round trip efficiency over prior art systems.

[0008] It is a further object of the present invention to provide a compressed gas energy storage system and method that has reduced capital and operating costs compared to the prior art.

[0009] Other objects and advantages of the present invention will become apparent as the description proceeds. Summary of the Invention

[0010] 1. A multi-phase energy storage system comprising: a first pressure vessel having at least four ports with corresponding conduits each in fluid communication with the first pressure vessel; and a second pressure vessel configured with one or more ports in fluid communication with the first pressure vessel, the second pressure vessel being set at a temperature below ambient temperature, wherein a condensable working fluid (CWF) that is condensable at ambient temperature is introduceable into the first pressure vessel through a first port of the at least four ports, and is compressible within the first pressure vessel by direct contact with a liquid that is not vaporizable within the first and second pressure vessels and is introduceable into the first pressure vessel through a second port of the at least four ports, and a sufficient volume of the CWF or non-vaporizable liquid is introduced into the first pressure vessel through the second port of the at least four ports to provide a sufficient volume of the CWF or non-vaporizable liquid to be compressed within the first pressure vessel. an energy storage system, wherein after introduction into the pressure vessel, at least a portion of the compressed CWF can be transferred from the first pressure vessel through a third port of the at least four ports to the second pressure vessel through one of the one or more ports by direct contact with a non-vaporizable liquid, all or a majority of said CWF in the second pressure vessel can be stored in a liquid state in the second pressure vessel after being compressed to a saturation pressure and condensed, and at least a portion of the non-vaporizable liquid in the first pressure vessel can be propelled through a fourth port of the at least four ports by the compressed CWF discharged from the second pressure vessel, discharging stored energy.

[0011] In one aspect, the system further comprises a gas source in fluid communication with the first pressure vessel and containing the CWF, the CWF being deliverable from the gas source to the first pressure vessel.

[0012] In one aspect, the energy storage system further comprises at least one hydraulic turbine capable of being driven by a non-vaporizable liquid, and at least a portion of the non-vaporizable liquid in the first pressure vessel can be propelled toward the at least one hydraulic turbine by the compressed CWF released from the second pressure vessel.

[0013] In one aspect, the system further comprises a liquid reservoir in fluid communication with the first pressure vessel and containing a non-vaporizable liquid, and means for delivering the non-vaporizable liquid from the liquid reservoir to the first pressure vessel, the non-vaporizable liquid being introduceable into the first pressure vessel by the delivery means via a second port of the at least four ports.

[0014] It is understood that a "non-vaporizable liquid" may vaporize under other conditions. Under the conditions of the multi-phase energy storage system, particularly the first and second pressure vessels and, optionally, the liquid reservoir, the liquid, also referred to as the "transfer liquid," is not vaporizable.

[0015] In one embodiment, non-vaporizable liquid discharged from the at least one hydraulic turbine may be received in a liquid reservoir.

[0016] In one embodiment, the CWF is compressible and expandable substantially isothermally during direct contact with a non-vaporizable liquid in the first pressure vessel.

[0017] In one embodiment, the pumping means is at least one hydraulic pump for pumping non-vaporizable liquid from the liquid reservoir to the first pressure vessel.

[0018] In one aspect, the energy storage system further comprises at least one additional hydraulic pump for pumping non-vaporizable liquid from the first pressure vessel to the liquid reservoir.

[0019] In one embodiment, the CWF in the first pressure vessel is continuously and incrementally compressed while additional non-vaporizable liquid is introduced into the first pressure vessel.

[0020] In one aspect, the energy storage system further comprises a gas-liquid separator disposed between the first pressure vessel and the second pressure vessel to prevent flow of non-vaporizable liquid to the second pressure vessel, and may further comprise a liquid-liquid separator disposed between the first pressure vessel and the second pressure vessel to prevent flow of non-vaporizable liquid to the second pressure vessel.

[0021] In one aspect, the energy storage system further comprises a liquid-liquid separator disposed between the first pressure vessel and the second pressure vessel to prevent flow of non-vaporizable liquid to the second pressure vessel.

[0022] In one aspect, the energy storage system further comprises a third pressure vessel in fluid communication with the first pressure vessel, and prior to performing one of a charge cycle or a discharge cycle, the third pressure vessel is completely filled with a first fluid selected from a CWF gas or a non-vaporizable liquid and the first pressure vessel is completely filled with a second fluid selected from a CWF gas or a non-vaporizable liquid and different from the first fluid, and upon completion, the third pressure vessel is completely filled with the second fluid and the first pressure vessel is completely filled with the first fluid.

[0023] In one embodiment, the first pressure vessel is also exposed to ambient temperature.

[0024] In one aspect, an energy storage system includes a plurality of first pressure vessels.

[0025] In one embodiment, all of the first pressure vessels are in fluid communication with each other or selected first pressure vessels are in fluid communication with each other.

[0026] In one embodiment, the energy storage system includes a plurality of second pressure vessels.

[0027] In one embodiment, all of the second pressure vessels are in fluid communication with each other or selected second pressure vessels are in fluid communication with each other.

[0028] In one embodiment, the gas holder further contains a non-CWF gas that is mixable with the CWF, and the non-CWF gas is further compressible after liquefaction of the CWF.

[0029] In one aspect, the energy storage system further comprises a flow control component operatively connected to each of the corresponding conduits for selectively controlling the flow of fluid through the corresponding conduit.

[0030] In one embodiment, the second pressure vessel is exposed to ambient or subambient temperatures.

[0031] A method of generating electrical power from stored energy includes providing a first pressure vessel and a second pressure vessel, the second pressure vessel being in fluid communication with the first pressure vessel and set at a temperature below ambient temperature, substantially isothermally compressing in the first pressure vessel a condensable working fluid (CWF) that is condensable at ambient temperature in direct contact with a non-vaporizable liquid, transferring at least a portion of the compressed CWF from the first pressure vessel to the second pressure vessel in response to interaction with the non-vaporizable liquid, transferring an additional amount of the compressed CWF to the second pressure vessel, and compressing the CWF in the second pressure vessel at a temperature below ambient temperature. compressing all or a substantial portion of the first pressure vessel to a saturation pressure, condensing to produce a liquid CWF, propelling at least a portion of the non-vaporizable liquid in the first pressure vessel toward at least one hydraulic turbine with the compressed CWF discharged from the second pressure vessel, and rotating the at least one hydraulic turbine with the propelled non-vaporizable liquid to produce electrical power, wherein flow of non-vaporizable liquid into the second pressure vessel is prevented while at least a portion of the compressed CWF is transferred from the first pressure vessel to the second pressure vessel.

[0032] As referred to herein, the phrase "the flow of non-vaporizable liquid into the second pressure vessel is prevented" includes the possibility that only a negligible volume of non-vaporizable liquid is introduced into the second pressure vessel relative to the volume of liquid CWF in the second pressure vessel. The prevention of the flow of non-vaporizable liquid into the second pressure vessel may be accomplished by a suitable device such as a demister.

[0033] In one embodiment, the CWF is substantially isothermally compressed by operating a mixer unit in fluid communication with the first pressure vessel or the second pressure vessel, thereby eliminating temperature gradients within the CWF.

[0034] In one embodiment, the mixer unit is actuated in response to a sensed condition indicative of liquefaction of the CWF.

[0035] In one embodiment, at least a portion of the non-vaporizable liquid in the first pressure vessel is propelled by the compressed CWF discharged from the second pressure vessel toward at least one hydraulic turbine when the discharged compressed CWF is in a liquid state, a gas state, or a multi-phase state.

[0036] In one embodiment, the step of transferring at least a portion of the compressed CWF from the first pressure vessel to the second pressure vessel is performed during multiple charging cycles.

[0037] In one embodiment, the step of propelling at least a portion of the non-vaporizable liquid in the first pressure vessel with the compressed CWF discharged from the second pressure vessel is performed for multiple discharge cycles.

[0038] In one aspect, the method further includes the steps of providing a third pressure vessel in fluid communication with the first pressure vessel, prior to performing one of a charge cycle or a discharge cycle, the third pressure vessel being completely filled with a first fluid selected from a CWF gas or a non-vaporizable liquid and the first pressure vessel being completely filled with a second fluid different from the first fluid selected from a CWF gas or a non-vaporizable liquid, and performing one of a charge cycle or a discharge cycle such that, when completed, the third pressure vessel is completely filled with the second fluid and the first pressure vessel is completely filled with the first fluid.

[0039] In one aspect, the method further includes performing the other of the charge cycle or the discharge cycle when the third pressure vessel is fully filled with the second fluid and the first pressure vessel is fully filled with the first fluid.

[0040] In one embodiment, the compressed CWF discharged from the second pressure vessel is expanded substantially isothermally while in direct contact with the non-vaporizable liquid in the first pressure vessel.

[0041] The present invention relates to a pressure vessel having at least four ports with which a corresponding conduit is in fluid communication, a second pressure vessel having one or more ports and set at a temperature below ambient temperature, and a flow control component operatively connected to each of the corresponding conduits for selectively controlling a flow of fluid through the conduits, wherein a condensable working fluid (CWF) that is condensable at ambient temperature can be introduced into the first pressure vessel through a first port of the at least four ports, and a non-vaporizable liquid can be introduced into the first pressure vessel through a second port of the at least four ports, thereby causing substantially isothermal compression of the CWF upon direct contact with the non-vaporizable liquid, and a first pressure vessel having a first pressure of at least one of the ... A direct contact fluid transfer (DCFT) module, wherein after introducing a sufficient amount of additional non-vaporizable liquid into the vessel, at least a portion of the compressed CWF can be transferred from the first pressure vessel through a third port of the at least four ports to a second pressure vessel through one of the one or more ports upon direct contact with the non-vaporizable liquid, and after being compressed and condensed to a saturation pressure, can be stored in a liquid state in the second pressure vessel, and at least a portion of the non-vaporizable liquid in the first pressure vessel can be propelled through a fourth port of the at least four ports by the compressed CWF discharged from the second pressure vessel, discharging stored energy. [Brief description of the drawings]

[0042] [Figure 1] FIG. 1 is a schematic diagram illustrating one embodiment of a multi-phase energy storage system. [Diagram 2] FIG. 2 is a schematic diagram illustrating another embodiment of a multi-phase energy storage system. [Diagram 3] FIG. 3 is a flow chart illustrating the various steps involved during cycling through the charging mode, according to one embodiment. [Figure 4] FIG. 4 shows a method of gas delivery using suction flow. [Diagram 5] FIG. 5 is a flow chart illustrating the various steps associated with executing the discharge mode, according to one embodiment. [Figure 6]FIG. 6 is a schematic diagram illustrating another embodiment of a multi-phase energy storage system. [Figure 7] FIG. 7 is an exemplary pressure-volume diagram of the energy storage system of FIG. [Figure 8] FIG. 8 is an example temperature entropy diagram of the energy storage system of FIG. [Figure 9] FIG. 9 is a schematic diagram illustrating another embodiment of a multi-phase energy storage system showing the charging states. [Figure 10] FIG. 10 is a schematic diagram illustrating another embodiment of a multi-phase energy storage system, illustrating the operations performed during a charging cycle. [Figure 11] FIG. 11 is a schematic diagram of the multi-phase energy storage system of FIG. 10 illustrating the operations performed during a discharge cycle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] The multiphase compressed gas energy storage system of the present invention advantageously avoids the significant heat losses experienced by working fluids in prior art systems by employing a condensable working fluid (CWF) that is normally a gas but will condense at ambient temperature when sufficiently compressed. A liquid-based direct contact fluid transfer (DCFT) module is employed in conjunction with the CWF to ensure that the CWF undergoes a substantially isothermal phase change in both charge and discharge modes. Because the DCFT module can operate in conjunction with a transfer fluid that is reliably maintained in a liquid state, relatively inexpensive hydraulic equipment can be advantageously employed in conjunction with the flowing transfer fluid, thereby reducing system costs while improving system round trip efficiency (RTE).

[0044] As used herein, "direct contact" refers to a heat transfer process involving heat exchange between two fluids in physical contact with each other when the two fluids are at different temperatures. Also, "substantially isothermal" is defined as a condition where heat transfer occurs at a nearly constant temperature, with an absolute temperature difference between the final temperature and the initial temperature of 5% or less in an adiabatic heat transfer process having the same initial thermodynamic conditions as the given heat transfer process. Deviations from a purely isothermal heat transfer process are due to a lower than required heat transfer rate, a smaller than required heat transfer area, or a shorter time than is required for heat transfer to occur.

[0045] 1 illustrates generally one embodiment of a multiphase energy storage system, generally designated 1, capable of operating in both charge and discharge modes. Two fluids interact in system 1: the first is a multiphase CWF fluid and the second is a transfer fluid.

[0046] The multi-phase energy storage system 1 comprises a gas holder 2 exposed to ambient temperature for holding the CWF gas at low pressure close to atmospheric pressure. The gas holder 2 is an inexpensive large-capacity vessel, such as one separated by a low-strength membrane. The system 1 further comprises one or more compression tanks 4, which constitute at least a part of a DCFT module, in fluid communication with the gas holder 2, and one or more storage tanks 3, exposed to ambient temperature and in fluid communication with each compression tank 4, for storing high energy density liquid CWF at the end of the charging mode. The compression tank or tanks 4 may also be exposed to ambient temperature, but not limited to this. The storage tank or tanks 3 and the compression tank 4 are preferably pressure vessels capable of withstanding the relatively high pressure of the compressed CWF gas. A transfer liquid, such as hydraulic oil or water, may be exposed to air at atmospheric pressure and temperature, such that the transfer liquid is maintained in liquid phase when held in the liquid reservoir 5. A liquid reservoir 5 is in fluid communication with each compression tank 4, and also with a hydraulic feed pump 7 and a hydraulic turbine 6 used to utilize the stored gas energy in the discharge mode. The transfer liquid is optionally immiscible with the CWF fluid.

[0047] As shown in FIG. 2, the operating costs of the multi-phase energy storage system 1 can be reduced by powering not only the supply pump 7 but also the return pump, if used, during the charging mode with electrical energy generated from a renewable energy source 11, such as a photovoltaic system or a wind farm, or from other energy sources, such as a power plant. The electrical power generated by the renewable energy source 11 is mostly fed into the power grid 14. When the demand for electrical power is lower than the electrical power generated by the renewable energy source, the surplus electrical power can be fed to the pumps to charge the system. The surplus amount of electrical power generated by the renewable energy source 11 can be used to power the supply pump and the return pump. Similarly, the electrical power generated by the generator coupled to the hydraulic turbine 6 can be fed into the power grid 14. The system can also be powered by other means known to those skilled in the art under certain conditions.

[0048] FIG. 6 shows another embodiment of a multi-phase energy storage system, generally designated 21. System 21 is similar to system 1, with different components such as gas exhaust conduit 23, mixer 24, gas-liquid separator 26, and heat exchanger 28 configured with additional devices to form further embodiments offering advantageous features. With respect to energy storage system 21, liquid reservoir 25 is not exposed to the ambient air, but rather is in fluid communication with gas holder 2 by gas exhaust conduit 23, allowing any gas that is absorbed in the transfer liquid during charge or discharge mode and then withdrawn from the transfer liquid to flow back to the gas holder by a closed conduit circuit arrangement. The transfer liquid remains non-vaporizable under the given thermodynamic conditions of charge and discharge modes, where the pressure of the transfer liquid is significantly higher than its saturation pressure in a controlled temperature range.

[0049] FIG. 9 illustrates another embodiment of a multi-phase energy storage system, generally designated 71. The cost-effective system 71 does not require a storage tank and includes a single compression tank 74 exposed to ambient temperature that can hold the CWF and condense it while directly contacting a non-vaporizable transport liquid. A liquid reservoir 5 configured to receive the transport liquid is exposed to the atmosphere and atmospheric pressure and is in liquid communication with a liquid supply conduit to which a hydraulic pump 7 is operatively connected, and a turbine inlet conduit to which a hydraulic turbine 6 is operatively connected. The system 71 may also interface with a renewable energy source 11 and a power grid 14, as described with reference to FIG. 2.

[0050] If the mass of gas currently held in system 1 of FIG. 1 or system 71 of FIG. 9 is less than the minimum required for efficient operation to generate power from stored gas energy, an auxiliary gas tank (not shown) can be used to inject an auxiliary amount of gas into the compression tank or gas holder.

[0051] Charging Mode Referring initially to the multi-phase energy storage system 71 of FIG. 9, the transfer liquid is held in the liquid reservoir 5, the low pressure CWF is held in the compression tank 74, and all valves are closed. When the liquid supply valve is opened and the supply pump 7 is activated, the transfer liquid is pumped into the compression tank 74. The introduced transfer liquid reduces the available volume that the CWF can occupy in the compression tank 74, causing the CWF to compress. As additional transfer liquid is introduced, the CWF becomes more and more compressed until its pressure is higher than the saturation pressure and it condenses. The CWF is substantially isothermally compressed by direct contact with the transfer liquid. When the charging mode is terminated, the liquid supply valve is closed and the supply pump is stopped.

[0052] Referring to system 1 of FIG. 1 and system 21 of FIG. 6, in the pre-charged state, the CWF is in the gas phase and held in a gas holder 2 at atmospheric or slightly higher pressure, the transfer liquid is held in a liquid reservoir 5, one or more storage tanks 3 are pre-compressed with gas, and one or more compression tanks 4 are filled with atmospheric gas or are filled with the transfer liquid.

[0053] When the compression tank(s) 4 are filled with transfer liquid, the procedure described with reference to FIG. 4 is carried out to facilitate delivery of the transfer liquid to the liquid reservoir 5 while allowing gas to flow from the gas holder 2 to the compression tank(s) 4.

[0054] When there is a sufficient volume of gas in one or more compression tanks 4, the isolation valves 3B of at least one port to each storage tank 3 are opened to provide a combined internal volume shared by each compression tank 3 and the storage tank 4 via the terminal conduits 3A extending therebetween. Gaseous CWF entering from each compression tank 4 through the terminal conduits 3A is not discharged but flows through the isolation valves 3B before being received by the storage tank 3. When there are multiple storage tanks 3, the gaseous CWF flows in parallel through the corresponding isolation valves 3B and ports to the storage tanks, but not all isolation valves 3B are open at the same time.

[0055] After an initial portion of the CWF gas has been introduced into the one or more tanks 3 and 4, a liquid supply valve 1B is opened which is operably connected to a region of the liquid supply conduit 1A extending from the liquid reservoir 5 to the one or more compression tanks 4 and downstream of a supply pump 7 which is also operably connected to the liquid supply conduit 1A. The hydraulic supply pump 7 is activated, causing the transfer liquid to be pumped from the reservoir 5 to the one or more compression tanks 4.

[0056] The transfer liquid introduced into the one or more compression tanks 4 reduces the combined volume of the tanks 3 and 4 occupied by the CFW gas. As a result, the CFW gas is compressed inside the compression tanks 4. As more transfer liquid is introduced into the one or more compression tanks 4, the combined volume of the tanks 3 and 4 occupied by the CFW gas is reduced, and the CFW in both gas tanks 3 and 4 is additionally compressed. During compression, the CWF gas is cooled by direct contact with the transfer liquid, reducing the heat of compression, so that the CWF gas can be compressed at a substantially constant temperature. Eventually, the one or more compression tanks 4 are fully occupied by the transfer liquid, while the CWF gas that was in the one or more compression tanks 4 is transferred to the one or more storage tanks 3, and the CFW gas in the one or more storage tanks 3 is further compressed. The CFW received in the one or more storage tanks 3 is liquefied when compressed to at least the saturation pressure. All valves are then closed, and the charging mode cycle is completed.

[0057] If the pressure of the CWF received in the storage tank(s) 3 is lower than the saturation pressure, additional cycles of the charging mode may be performed. During each additional cycle, an additional gas portion is supplied to the compression tank 4, increasing the pressure of the stored CWF fluid.

[0058] FIG. 3 illustrates various steps associated with performing a charging mode cycle in conjunction with system 1 of FIG. 1 and system 21 of FIG. 6, according to one embodiment.

[0059] Once the compression tank(s) 4 have been filled with transfer liquid in step 29 to commence an additional cycle of the charging mode or alternatively a first cycle of the charging mode, a return valve 5B operatively connected to another conduit 5A, i.e. a return conduit, extending from the liquid reservoir 5 to the compression tank(s) 4 is opened in step 31 to pump the transfer liquid from the compression tank(s) 4 back to the reservoir 5. At the same time, a gas supply valve 4B is momentarily opened to cause a flow of a gas portion from the gas holder 2 to the compression tank(s) 4.

[0060] When the transfer liquid returns to the reservoir 5, all valves are closed. Then, in step 36, each isolation valve 3B is opened. Then, in step 37, the feed pump 7 is operated to feed the transfer liquid to the compression tank(s) 4. This causes the CWF gas fed from the gasholder to the compression tank(s) 4 in the current cycle to be compressed in step 39 and subsequently transferred by the transfer liquid to the storage tank(s) 3 in step 41. In step 43, the pressure of the stored CWF gas is increased by the introduction of additional CWF gas to the storage tank(s) 3. Finally, after one or more cycles, the pressure of the ambient CWF received in the storage tank(s) 3 is increased to the saturation pressure and liquefied in step 45.

[0061] If the compression tank(s) 4 are filled with gas at step 35, the charge mode cycle proceeds to step 36.

[0062] In another embodiment, gas is supplied to one or more compression tanks through the gas supply valve in step 33, and steps 37 and 39 are performed to compress the supplied gas in one or more compression tanks while closing the isolation valves at the inlet ports of each storage tank to increase the pressure applied in the compression tanks. The isolation valves are then opened, and the compressed fluid is transferred to the storage tank by the transfer liquid. The transferred fluid, which may be in a gas state or a two-phase gas-liquid state, undergoes a first stage compression in the compression tank and a second stage compression in the storage tank until it is completely liquefied.

[0063] In another embodiment shown in Figure 4, a portion of the gas is drawn from the gasholder by suction, for example when the pressure differential between the gasholder and the compression tank is relatively low. Such flow by suction is possible when the compression tank(s) are filled with transfer liquid in step 31. While the gas supply valve is open, a return pump is activated in step 44, operably connected to a return conduit extending between the compression tank(s) and a reservoir. The flow of transfer liquid through the return conduit in the direction of the reservoir creates a suction that draws gas from the gasholder into the compression tank(s) in step 48.

[0064] The volume of CWF fluid at a given pressure that can be stored in a liquid state is significantly smaller than when provided in a gaseous state, and thus the ability of systems 1, 21 and 71 to store CWF in a liquid state significantly increases the energy density, i.e., potential stored energy per volume, of the working fluid compared to prior art techniques that store working fluids in a gaseous state. The energy density achievable with CWF can also be advantageously increased by the reduced heat losses resulting from substantially isothermal compression and expansion. The capital cost of the small volume storage tank 3 is significantly lower than the larger volume storage tanks required to store compressed gas in a gaseous state.

[0065] As shown in system 21 of Figure 6, the energy density of the fluid stored in the one or more storage tanks 3 may be further increased if the gasholder 2 contains an additional low pressure gas, such as air or a non-condensable gas (NCG), that is non-CWF but miscible with the CWF. This mixed gas is compressed in step 39 and transferred to one or more storage tanks in step 41. After the CWF is liquefied in step 45, the non-CWF gas may be additionally compressed to further increase the energy density of the stored fluid, such as in response to the introduction of an additional volume of transfer liquid into compression tank 4. It will be appreciated that in systems 1 and 71, the non-CWF may also be mixed with the CWF to increase the energy density of the stored compressed fluid.

[0066] A high energy density of the CWF received in the storage tank(s) 3 is advantageous as it allows more energy to be released from the stored CWF in a discharge mode.

[0067] With reference to a multi-phase energy storage system 21, as shown in FIG. 6 according to another embodiment, the energy density of the CWF transferred to the storage tanks 3 can be further increased by blocking the transfer liquid from entering each storage tank, so that more CWF can occupy the storage tank interior and condense. The transfer liquid can be blocked by providing a gas-liquid separator 26 at the inlet of the port of the storage tank, or a liquid-liquid separator in case the CWF also condenses in the compression tank. The gas-liquid separator 26 can, for example, allow the transfer liquid to fall under the influence of gravity in the gas-liquid separator and return to the compression tank 4. The gas-liquid separator 26 can also be embodied by a buffer tank located above the height of each compression tank 4. The gas-liquid separator 26 or the liquid-liquid separator can be operatively connected to a conduit 27 extending from a dedicated outlet provided in the corresponding compression tank 4 to a terminal conduit 3A in fluid communication therewith.

[0068] When systems 1 and 21 employ a gas-liquid separator 26, the CFW can be isothermally compressed even though there is no direct contact between the transfer liquid and the CFW fluid within the storage tank(s) 3 as a result of the cooling effect of the ambient air or ground surrounding the storage tank(s) 3. Additionally, the CFW can be compressed slowly, for example over a period of hours, to reduce or completely eliminate the heat of compression.

[0069] Alternatively or additionally, isothermal compression may be ensured by a mixer, as shown diagrammatically in the multi-phase energy storage system 21 of FIG. 6 and the multi-phase energy storage system 71 of FIG. 9. The mixer unit 24B, which is placed in fluid communication with the corresponding storage tank 3, may be activated when the CWF starts to liquefy, for example when it reaches a two-phase gas-liquid state. The liquid and gas portions of the CWF may be mixed to provide a fluid with a substantially uniform temperature and heat transfer rate, and to eliminate temperature gradients in the stored CWF by absorbing the released heat of compression. The unit 24B may be activated in response to detection of a predetermined pressure or the presence of liquid in the corresponding storage tank 3 by an associated sensor. Similarly, the mixer unit 24A, which may be the same as or different from the unit 24B, may be in fluid communication with the corresponding compression tank 4.

[0070] The mixing device can take various forms. The mixing device, such as an agitator, may be rotatably attached to the corresponding tank. The agitator provides homogeneity of the stored fluid in terms of temperature, heat transfer rate, and composition, in order to mix any particles that may have precipitated from the compressed liquefied solution.

[0071] The mixing device may also be located external to the corresponding tank. In one embodiment, the mixing device is configured with a set of conduits and nozzles, whereby a fluid portion in one region of the tank is caused to flow through one of the conduits, such as in conjunction with a pump, and injected into another region of the tank via one or more nozzles. This arrangement may also facilitate circulation of transfer liquid between two or more compression tanks 4, which may be interconnected, or from one or more compression tanks 4 to one or more storage tanks 3, which may also be interconnected.

[0072] Also, to aid in achieving isothermal compression, a heat exchanger 28A in heat exchange relationship with one or more compression tanks 4, or a heat exchanger 28B in heat exchange relationship with one or more storage tanks 3 may be provided. Each heat exchanger may be of the shell-and-tube type, finned tube type, plate type heat exchanger type, air-cooled type, or other heat exchanger type known to those skilled in the art. The piping through which the cooling medium flows, if employed, may be shared with conduits associated with a mixer, such as where one or more valves are selectively opened and closed to permit flow to the heat exchanger or to the mixer unit. Two or more heat exchangers may be employed, and each may be of the same type or different types.

[0073] Discharge Mode The condensed CWF can remain stored under charging conditions for long periods of time until a discharge mode is initiated, which is typically initiated during periods of peak demand or when there is a shortage of energy availability, since the energy stored in the multi-phase energy storage system can be discharged whenever needed to generate electricity to be fed into the grid.

[0074] During the discharge mode, the transfer liquid is propelled by the high pressure liquid CWF towards the hydraulic turbine to generate power. The use of the transfer liquid to cause compression of the CWF during the charge mode and to be propelled by the CWF during the discharge mode advantageously allows a hydraulic turbine to be employed to generate power as opposed to gas turbines used in prior art compressed gas storage systems. Advantages of using a hydraulic turbine compared to a gas turbine include high efficiency, low rotational speed, easy maintenance, and low running costs.

[0075] These advantages are also related to the fact that the multi-phase energy storage system uses hydraulic pumps, as opposed to prior art compressed gas storage systems that use compressors to introduce compressed gas.

[0076] 9, the CWF liquid and high pressure transfer liquid are held in a compression tank 74 in anticipation of the subsequent initiation of a discharge mode. The CWF fluid has been compressed to its saturation pressure, which is significantly higher than the pressure under pre-charge conditions, and because the transfer liquid is held in the same closed compression tank 74 as the CWF liquid, the CWF liquid exerts a force on the transfer liquid that pressurizes the transfer liquid to the same saturation pressure as the CWF liquid. The pressurized transfer liquid in the compression tank 74 is at a pressure significantly higher than the atmospheric pressure to which the transfer liquid held in the liquid reservoir 5 is exposed.

[0077] When the turbine injector valve is opened to initiate the discharge mode, the high pressure difference between the high pressure transfer liquid in the compression tank 74 and the low pressure transfer liquid in the liquid reservoir 5 urges the high pressure transfer liquid to be discharged from the compression tank 74 with high enough kinetic energy to drive the hydraulic turbine 6 to generate power. In response to the discharge of the high pressure transfer liquid from the compression tank 74, the additional volume of the compression tank 74 not occupied by the transfer liquid becomes available to the CWF, so that the CWF can expand to a gas-liquid two-phase state. It should be noted that since the multi-phase CWF expands isobarically, the pressure of the CWF and the transfer liquid pressurized by the CWF does not change, thereby allowing the output of power to continue for a certain period of time. When the CWF becomes gaseous, the pressure of the high pressure transfer liquid decreases. Nevertheless, the pressure of the propelled transfer liquid is high enough to drive the hydraulic turbine 6 to generate power until it falls below a turbine-specific threshold. When the discharge mode is terminated, the turbine injector valve is closed and the liquid supply valve is opened.

[0078] The simplicity of the system 71 allows the transfer liquid to circulate in a piston-like manner between the liquid reservoir 5 and the compression tank 74 while alternating between charging and discharging modes, respectively, to generate additional power.

[0079] In the pre-charge state, with reference to system 1 of Figure 1 and system 21 of Figure 6, all valves are closed and the CWF fluid held in the one or more storage tanks 3 is in a liquid phase after being exposed to the ambient temperature of the storage tank and compressed to a relatively high pressure. Non-CWF gases, which if mixed together with the CWF fluid, are compressed to a pressure higher than the CWF liquid, are also held in the one or more storage tanks 3.

[0080] FIG. 5 illustrates various steps associated with performing a discharge mode according to one embodiment.

[0081] In the first step 51 of the discharge mode, if there is no transfer liquid in any of the compression tanks 4 after the last cycle of the charge mode, the return valve 5B is momentarily opened to allow the transfer liquid received in the reservoir 5 at a relatively high head value to flow through the return conduit 5B to fill the compression tank or tanks. Alternatively, the liquid supply valve 1B may be momentarily opened and the supply pump 7 may be operated to supply the required amount of transfer liquid to the compression tank or tanks 4. This step may be omitted if there is a sufficient volume of transfer liquid in the compression tank or tanks 4. Each compression tank may be equipped with a sensor, such as a capacitive level sensor or a float switch, and possibly also a pressure sensor, to determine when the transfer liquid is at a predetermined liquid level. The return valve 1B or the valve 5B is then closed to prevent the return flow of the transfer liquid.

[0082] In step 53, the turbine injector 2B and at least one storage tank isolation valve 3B are opened. As a result, the high pressure compressed liquid, including at least the CWF liquid, is discharged from the storage tank 3 in step 55 through the terminal conduit 3A with which the corresponding opened isolation valve 3B is in fluid communication by being exposed to the lower pressure in the compression tank 4, and expands slightly. The discharged CWF liquid is forced to contact the transfer liquid in each compression tank 4 in step 57, and as a result of the pressure difference between the transfer liquid in the liquid reservoir and the CWF liquid, the transfer liquid is propelled through the turbine inlet conduit 2A to which the turbine injector 2B is operatively connected. The transfer liquid can be propelled as long as the pressure difference between the transfer liquid, which is at the same pressure as the CWF and is propelled by the CWF, and the transfer liquid in the liquid reservoir 5 downstream of the hydraulic turbine 6 is greater than a predetermined threshold. As a result of the volume occupied by the transfer liquid in the combined tanks 3 and 4 in response to the movement of the transfer liquid, the CWF liquid can evaporate while expanding isobarically in step 59 and enter a gas-liquid two-phase state. The pressure of the multiphase CWF fluid and the pressure of the CWF when entirely in the gas phase are high enough to continue propelling the transfer liquid through the turbine inlet conduit 2A. Direct contact between the CWF gas and the transfer liquid reduces the temperature change of the CWF gas, allowing the CWF gas to expand substantially isothermally. This expansion process continues as more and more of the transfer liquid is propelled and the now entirely gaseous CWF fluid occupies an increased volume. The transfer liquid propelled through the turbine inlet conduit 2A thereby drives the hydraulic turbine 6 to generate power in step 61. The transfer liquid leaving the hydraulic turbine 6 flows to the reservoir 5 in step 63, raising the liquid level in the liquid reservoir. When the volume of transfer liquid remaining in the compression tank or tanks falls below the propellable volume, all valves are closed to complete the discharge mode cycle in step 65. Thereafter, in step 67, the gas supply valve 4B is opened and the pressure differential between the one or more compression tanks 4 and the gasholder 2 causes any gas remaining in the one or more compression tanks to flow back into the gasholder 2 through the gas supply conduit 4A.

[0083] Thus, the use of a phase-changing CWF fluid during the discharge mode does not reduce the amount of power that can be extracted; rather, the multiphase CWF fluid continues to propel the transfer fluid toward the hydraulic turbine to generate additional amounts of power. This power extraction capability allows for a reduction in the volume of compression and storage tanks, in addition to the benefits provided by condensed fluid.

[0084] If the pressure of the fluid, including at least the CWF, remaining in the storage tank(s) 3 is sufficiently greater than atmospheric pressure to facilitate propulsion of the transfer liquid, one or more cycles including the above steps may be repeated in a discharge mode, i.e., step 51 is performed to refill the transfer liquid from the reservoir 5 when the compression tank(s) 4 is sufficiently emptied, and the refilled transfer liquid is then propelled by the fluid discharged from the storage tank(s) in step 57. Additional cycles may be repeated in step 69 until the pressure of the gas in the storage tank(s) 3 drops to a pressure below the propulsible pressure capable of propelling the transfer liquid.

[0085] The sequence of the various steps in both the charge and discharge modes can be controlled manually in operating the various flow control devices, or automatically in response to the operation of the controller. Each step can be performed in response to time or sensed actions commanded by the controller. With the intervention of the controller, the charge and discharge steps can be synchronized to provide a continuous discharge operation.

[0086] By employing a large-capacity gas holder 2 and performing multiple charge / discharge cycles, the volume of one or more compression tanks 4 can be advantageously reduced, reducing costs. Depending on the need for gas energy storage, the systems 1 and 21 can be scaled up or down by changing the number or size of one or more tanks. When the system 1 or system 21 includes multiple storage tanks 3 and compression tanks 4, each storage tank and each compression tank may be connected to each other. For more efficient transfer operations, different steps may be performed simultaneously in association with two different tanks. For example, a liquid transfer may be flowed to the first compression tank 4 at the start of the next discharge cycle, while at the same time, a CWF may be expanded in the second compression tank in another discharge step to drive the liquid transfer and rotate a hydraulic turbine.

[0087] 7 and 8 illustrate various exemplary thermodynamic states of the CWF fluid. In state F, the CWF gas is introduced into the compression tank at a relatively low pressure, e.g., from a gas holder, and the pressure of the CWF gas steadily increases in response to the introduction of transfer liquid into the compression tank to the saturation pressure of the CWF in state G. The CWF undergoes a phase change to become a two-phase fluid in state G, and then undergoes isobaric condensation until it is fully liquefied and occupies a minimal volume in state H. Due to direct contact with the transfer liquid and the influence of the surroundings, the CWF isothermally compressed from state F to state H through the charge mode, and these states are reversed as the CWF fluid isothermally expands through the discharge mode from state H to state F.

[0088] Although only a single hydraulic turbine 6 is shown in Figures 1, 6 and 9, it is understood that two or more hydraulic turbines may be employed. If the pressure of the CWF propelling the transfer liquid is high enough as detected by the sensor, two or more stages of hydraulic turbines may be used to generate additional amounts of power. Different types of turbines may be used for each stage to maximize efficiency for the designed operating conditions of a particular stage, such as a given range of pressure drop, specific speed, or head. Alternatively, multiple turbines may be used in parallel. A controller receiving pressure measurements detected by the sensor may command flow control components, such as valves at the outlet of the first stage turbine, to direct the transfer liquid leaving the first stage turbine to the inlet of the second stage turbine to rotate the second stage turbine.

[0089] It will be appreciated that multiple feed pumps may be used in parallel or in series.

[0090] As described above in this specification, the energy storage system may be capable of alternating multiple charge and discharge cycles, with the charge cycle being repeatable when the pressure of the CWF is less than the saturation pressure, and the discharge cycle being repeatable when the pressure of the propulsion fluid, including at least the CWF, is sufficiently greater than atmospheric pressure to facilitate propulsion of the transport liquid.

[0091] 10 and 11 show another embodiment of a multi-phase energy storage system, generally designated 91. System 91 is a closed system comprising two or more DCFT modules, each of which may be in fluid communication with a gasholder, one or more storage tanks, a hydraulic turbine, and at least another DCFT module. Having two or more DCFT modules advantageously allows charge or discharge cycles to be performed repeatedly with minimal wait time between subsequent cycles.

[0092] The multi-phase energy storage system 91 comprises a gas holder 92 exposed to ambient temperature, which is shown to hold a low pressure CWF gas near atmospheric pressure and an additional low pressure non-CWF gas such as air or NCG, but may hold only CWF. The system 91 further comprises two or more DCFT modules, e.g. DCFT modules 93a and 93b, one or more storage tanks, e.g. three storage tanks 94a-94c, exposed to ambient temperature for storing CWF, in particular liquid CWF, or a mixture of liquid CWF and NCG that becomes highly energy dense at the end of a charging mode, at least one hydraulic turbine 96, a low pressure liquid reservoir 98 for receiving liquid discharged from the hydraulic turbine 96, and pumps 108 and 109. The DCFT modules 93a-93b may be referred to as the "first module" and the "second module", respectively, and are preferably pressure vessels capable of withstanding the relatively high pressure of the compressed CWF gas. The fluid held by gasholder 92 and flowable within the energy storage system may be referred to as a “CWF-based fluid.” A diagrammatically illustrated controller 107 in data communication with at least pumps 108 and 109 and valves synchronizes the flow of the transfer liquid and the CWF-based fluid.

[0093] 10 illustrates the operation of the system 91 in a charging mode. In the pre-charging state, all valves are closed. Also, the first DCFT module 93a is filled with a non-vaporizable transfer liquid, and the second DCFT module 93b and the storage tanks 94a-94c are filled with a CWF or a mixture of CWF and non-CWF gases that are maintained at a pre-compressed pressure in the vapor phase before being compressed by the transfer liquid. The first DCFT module 93a, the second DCFT module 93b, and the storage tanks 94a-94c may be pre-filled by known means such as pumps and valves, or any other suitable means.

[0094] A pressure control means 103B connected to the second DCFT module 93b is in data communication with the controller and ensures that a predetermined precompression pressure is reached inside the second module. The pressure control means 103B may be embodied by means known to those skilled in the art such as a pressure gauge or pressure regulator, and may optionally comprise an isothermal compressor for rapidly increasing the gas pressure inside the second module while minimizing the heat of compression. A similar pressure control means 103A may be connected to the first DCFT module 93a.

[0095] To start the first charging cycle, the liquid drain valve 101A of the first module, the liquid inlet valve 102B of the second module, the liquid supply valve 114, the gas inlet valve 111A of the first module, the gas inlet valve 111B of the second module, and the gas transfer valve 117B of the second module are opened. The gas inlet valve 111B of the second module is opened and in fluid communication with the gas holder 92, so that the pressure of the gas in the gas holder is also maintained at a predetermined precompression pressure. The first pump 108 is then operated to pump the transfer liquid from the first module 93a to the second module 93b, flowing in the direction of arrow A through a conduit 121 extending from a first port of the first module, flowing through a conduit 123 to which the first pump 108 and the liquid supply valve 114 are operatively connected, and flowing in the direction of arrow B through a conduit 124 extending from the end of the conduit 123 to a second port of the second module 93b.

[0096] The transfer liquid introduced into the second module 93b reduces the volume occupied by the retained gas within the second module 93b. As a result, the retained gas may be compressed within the second module 93b. During compression, the retained gas is cooled by direct contact with the transfer liquid, reducing the heat of compression, so that the retained gas may be substantially isothermally compressed. When the second module 93b is filled with the transfer liquid, the compressed CWF-based gas is urged to move and flow across the sixth port of the second module 93b, flowing in the direction of arrow C through conduit 127, and flowing in the direction of arrow D through conduit 128 and corresponding conduit 129 to the first port of one or more of the storage tanks 94a-94c. The compressed gas transferred from the second module 93b further compresses the CWF-based gas stored in each of the one or more of the storage tanks 94a-94c.

[0097] Substantially isothermal compression of the CWF-based fluid may be ensured by circulating the cooling transfer liquid located at the bottom of the second module 93b in a closed loop through conduits 136-138 by a second pump 109 across the fourth and fifth ports of the second module when isolation valve 116B operably connected to conduit 136 and isolation valve 118B operably connected to conduit 138 are open. The degree of cooling provided by the circulating transfer liquid may be augmented by a heat exchanger 104 in heat exchange relationship with conduit 137. Alternatively or additionally, a mixing device or spray nozzle used when isolation valves 116B and 118B are open may help ensure substantially isothermal compression.

[0098] In response to the pumping out of the transfer liquid from the first module 93a, a portion of the CWF-based gas held in the gas holder 92 flows in the direction of arrows E and F through conduits 131-133 and the third port of the first module to the first module 93a to maintain the non-liquid volume within the first module at a pre-compression pressure in anticipation of a subsequent charge cycle.

[0099] The first charging cycle ends when the second module 93b is completely filled with transfer fluid, the first module 93a is filled with CWF-based gas at a pre-compression pressure, and the one or more storage tanks 94a-94c are completely filled with CWF-based gas at a pressure higher than the initial pre-compression pressure after being compressed by the CWF-based gas transferred from the second module 93b, i.e., when no other fluid is received in the one or more storage tanks. All valves are then closed.

[0100] To begin the second charging cycle, the second module liquid drain valve 101B, the first module liquid inlet valve 102A, the liquid supply valve 114, the first module gas inlet valve 111A, the second module gas inlet valve 111B, and the first module gas transfer valve 117B are opened. The first pump 108 is then operated, and transfer liquid is pumped from the second module 93b to the first module 93a, flowing in the direction of arrow G through conduit 141 extending from the first port of the second module, through conduit 123, and in the direction of arrow H through conduit 144 extending from the end of conduit 123 to the second port of the first module 93a. The same process that took place in the first charging cycle is repeated in the second charging cycle. Thus, once the first module 93a is filled with transfer liquid, compressed gas is urged to travel and flow across the sixth port of the first module 93a, through conduit 147 in the direction of arrow I, and through conduit 128 and corresponding conduit 129 in the direction of arrow D to one or more of the storage tanks 94a-94c. The compressed gas transferred from the first module 93a further compresses the CWF-based gas stored in each of the one or more of the storage tanks 94a-94c.

[0101] The second charging cycle ends when the first module 93a is completely filled with transfer fluid, the second module 93b is filled with CWF-based gas at a pre-compression pressure, and the storage tank(s) 94a-94c are completely filled with CWF-based fluid at a pressure higher than the pressure reached at the end of the first charging cycle after being compressed by the CWF-based gas transferred from the first module 93a, and then all valves are closed.

[0102] The charging cycle can be repeated in a similar manner, so long as in each subsequent charging cycle the first and second modules are alternately filled with transfer liquid and CWF-based gas, respectively, at the precompression pressure. By shuttling the transfer liquid between the first and second modules during each subsequent charging cycle and replenishing the volume of the modules emptied of the transfer liquid with CWF-based gas from the gas holder, the two modules are ready for use in the subsequent charging cycle upon completion of the previous charging cycle. The charging process continues until the CWF-based fluid in the one or more storage tanks 94a-94c condenses and undergoes a phase change from gas to liquid. The liquid CWF-based fluid is stored in the one or more storage tanks 94a-94c until it is required to be discharged.

[0103] During any charging cycle, the stored CWF-based fluid may be subjected to a cooling, heating or mixing operation, as desired, in conjunction with suitable devices 122 upon opening of an isolation valve 119 operably connected to a conduit 126 in parallel fluid communication with the second and third ports of each of the storage tanks 94a-94c to facilitate circulation of the stored CWF-based fluid in a closed loop. Alternatively, the conduit 126 may be configured to deliver a cooling medium, such as water, that may flow through a cooling coil disposed within one or more of the storage tanks.

[0104] 11 illustrates the operation of system 91 in discharge mode. In the pre-discharge condition, all valves are closed and one module is fully filled with transfer liquid and the other module is fully filled with CWF-based gas at pre-compression pressure. In the following exemplary first discharge cycle, the first module 93a is fully filled with transfer liquid and the second module 93b is fully filled with CWF-based gas at pre-compression pressure.

[0105] To initiate a first discharge cycle, the first module liquid discharge valve 101A, the first module gas transfer valve 117A, and the turbine injector valve 151 are opened. A predetermined amount of CWF-based fluid, which may be controlled by momentarily opening the first module gas transfer valve 117A for a predetermined time or in response to a sensed value, is discharged from one or more storage tanks 94a-94c and flows in directions J and K through conduits 129, 128, and 147 until it is introduced inside the first module 93a. The discharged CWF-based fluid is forced into contact with the transfer liquid within the first module 93a, thereby pressurizing the transfer liquid to the same pressure as the discharged CWF-based fluid. As a result of the pressure difference between the turbine inlet, which is at a pressure substantially equal to the pressure of the pressurized transfer liquid, and the turbine outlet, which is exposed to the low pressure reservoir 98, the transfer liquid is propelled in directions L and M through conduits 121, 148, and 153 toward the hydraulic turbine 96. A conduit 153, to which a turbine injector 151 is operatively connected, extends from the junction between conduits 141 and 148 to reservoir 98. The transfer liquid drives hydraulic turbine 96 to generate electricity that can be supplied to an electrical grid. Reservoir 98 is in fluid communication with gasholder 92 via communication conduit 143 and is not exposed to the environment, or is in fluid communication with the ambient air, and receives the transfer liquid discharged from hydraulic turbine 96.

[0106] Upon discharge of the high pressure transfer liquid from the first module 93a, additional volume of the first module 93a not occupied by the transfer liquid becomes available for the CWF-based fluid, which may then expand to a two-phase state or entirely in the vapor phase. Direct contact between the CWF-based fluid and the transfer liquid reduces temperature change of the CWF-based fluid, allowing the CWF-based fluid to expand substantially isothermally.

[0107] Substantially isothermal expansion of the CWF-based fluid within the first module 93a may be ensured by circulating heated transfer liquid located at the bottom of the first module 93a in a closed loop across the fourth and fifth ports of the first module and through conduits 135, 137 and 139 by the second pump 109 when isolation valve 116A operably connected to conduit 135 and isolation valve 118A operably connected to conduit 139 are open while the CWF-based fluid undergoes a cooling process. Alternatively or additionally, a mixing device or spray nozzle used when isolation valves 116A and 118A are open may help ensure substantially isothermal expansion.

[0108] Simultaneously with the discharge of the high pressure transfer liquid from the first module 93a, the liquid inlet valve 102b of the second module, the gas outlet valve 113b of the second module and the return valve 156 of the second module are opened. Thus, while the transfer liquid flows into the reservoir 98, the received transfer liquid is sent by the first pump 108 to the second module 93b and flows in the N and O directions through the return conduit 158, the conduits 123 and 124 in anticipation of the next discharge cycle. In response, the CWF-based gas is moved by the sent transfer liquid from the seventh port of the second module 93b through the conduits 131 and 149 to the gas holder 92 in the P and Q directions. When the first module 93a is completely filled with the CWF-based gas at a predetermined low pressure and the second module 93b is completely filled with the transfer liquid at approximately atmospheric pressure, the first discharge cycle is completed. All valves are then closed.

[0109] To initiate a second discharge cycle, the liquid discharge valve 101B and the turbine injector valve 151 of the second module are opened. The gas transfer valve 117B of the second module is momentarily opened, causing a predetermined amount of CWF-based fluid to be released from one or more of the storage tanks 94a-94c and flow in directions J and R through the conduits 127-129, 128 until it is introduced into the interior of the second module 93b for a predetermined duration or until a sensed value is detected. Once the predetermined amount of CWF-based fluid has been introduced into the second module 93b, the valve 117b is closed. As a result, the transfer liquid in the second module 93b is propelled by the released CWF-based fluid through the conduits 141 and 153 in directions S and M towards the hydraulic turbine 96 to generate electricity that can be supplied to the power grid.

[0110] The same process carried out in the first discharge cycle is repeated in the second discharge cycle, but with respect to the other module. Thus, the transfer liquid flowing into the reservoir 98 is returned by the first pump 108 to the first module 93a in the N and T directions while the first module liquid inlet valve 102A, the first module gas outlet valve 113A and the return valve 156 are open, in anticipation of the next discharge cycle. Correspondingly, the CWF-based gas is moved by the pumped transfer liquid from the seventh port of the first module 93a through the conduits 131, 149 and 161 to the gas holder 92 in the U, P and Q directions. The second discharge cycle is ended when the second module 93b is completely filled with the CWF-based gas at a predetermined low pressure and the first module 93a is completely filled with the transfer liquid at approximately atmospheric pressure. All valves are then closed.

[0111] The discharge cycle may be repeated in a similar manner, so long as the first and second modules are alternately filled with transfer liquid and CWF-based gas at the predetermined low pressure for each subsequent discharge cycle. By shuttling the transfer liquid between the first and second modules during each subsequent discharge cycle and receiving the CWF-based gas in the module emptied of the transfer liquid from the storage tank(s), the two modules are ready for use in the subsequent discharge cycle at the end of the previous discharge cycle. The discharge process continues until the pressure of the CWF-based liquid in the storage tank(s) 94a-c drops to the predetermined pre-compression pressure.

[0112] As can be appreciated from the foregoing description, cost savings for mechanical energy storage and increased round trip efficiency of the overall system can be realized in the inventive system compared to prior art implementations by using conventional hydraulic equipment, i.e., hydraulic pumps during charge mode and hydraulic turbines during discharge mode. Cost savings are also achieved by avoiding the need for additional thermal energy storage typically required in the prior art to absorb the heat of compression, since the working fluid used in the inventive system is advantageously capable of undergoing substantially isothermal compression.

[0113] Although several embodiments of the invention have been described by way of example, it will be apparent that the invention can be implemented by means of numerous modifications, variations and adaptations, as well as the use of numerous equivalents or alternative solutions, that are within the scope of the art, without departing from the scope of the claims.

Claims

1. a) a first pressure vessel having at least four ports each in fluid communication with a corresponding conduit; b) a second pressure vessel configured with one or more ports capable of fluidly communicating with the first pressure vessel, the second pressure vessel being set at a temperature below ambient temperature; A multi-phase energy storage system comprising: a condensable working fluid (CWF) that is condensable at room temperature can be introduced into the first pressure vessel through a first port of the at least four ports and can be compressed in the first pressure vessel by direct contact with a liquid that cannot be vaporized in the first and second pressure vessels and can be introduced into the first pressure vessel through a second port of the at least four ports; After introducing a sufficient volume of the CWF or the non-vaporizable liquid into the first pressure vessel, at least a portion of the compressed CWF can be transferred from the first pressure vessel through a third port of the at least four ports to the second pressure vessel through one of the one or more ports by direct contact with the non-vaporizable liquid, and all or a majority of the CWF in the second pressure vessel can be stored in a liquid state in the second pressure vessel after being compressed and condensed to a saturation pressure; A multi-phase energy storage system, wherein at least a portion of the non-vaporizable liquid in the first pressure vessel can be propelled through a fourth of the at least four ports by the compressed CWF discharged from the second pressure vessel to discharge stored energy.

2. 2. The energy storage system of claim 1, further comprising: c) a gas source in fluid communication with the first pressure vessel and containing the CWF, the CWF being supplyable to the first pressure vessel from the gas source.

3. 2. The energy storage system of claim 1, further comprising at least one hydraulic turbine driveable by the non-vaporizable liquid, wherein at least a portion of the non-vaporizable liquid in the first pressure vessel is driveable toward the at least one hydraulic turbine by the compressed CWF discharged from the second pressure vessel.

4. a liquid reservoir in fluid communication with the first pressure vessel and containing the non-vaporizable liquid; and means for delivering the non-vaporizable liquid from the liquid reservoir to the first pressure vessel; 4. The energy storage system of claim 3, wherein the non-vaporizable liquid is introduceable into the first pressure vessel by the delivery means through a second port of the at least four ports.

5. The energy storage system of claim 4 , wherein the non-vaporizable liquid discharged from the at least one hydraulic turbine can be received in the liquid reservoir.

6. 2. The energy storage system of claim 1, wherein the CWF is substantially isothermally compressible and expandable during direct contact with the non-vaporizable liquid in the first pressure vessel.

7. 5. The energy storage system of claim 4, wherein said pumping means is at least one hydraulic pump for pumping said non-vaporizable liquid from said liquid reservoir to said first pressure vessel.

8. 8. The energy storage system of claim 7, further comprising at least one additional hydraulic pump for pumping the non-vaporizable liquid from the first pressure vessel to the liquid reservoir.

9. 10. The energy storage system of claim 1, wherein the CWF in the first pressure vessel is continuously and incrementally compressed while additional non-vaporizable liquid is introduced into the first pressure vessel.

10. 10. The energy storage system of claim 1, further comprising a liquid-vapor separator disposed between the first pressure vessel and the second pressure vessel to prevent flow of the non-vaporizable liquid into the second pressure vessel.

11. 11. The energy storage system of claim 10, further comprising a liquid-liquid separator disposed between the first pressure vessel and the second pressure vessel to prevent flow of the non-vaporizable liquid into the second pressure vessel.

12. 10. The energy storage system of claim 1, further comprising a liquid-liquid separator disposed between the first pressure vessel and the second pressure vessel to prevent flow of the non-vaporizable liquid into the second pressure vessel.

13. a third pressure vessel in fluid communication with the first pressure vessel; 2. The energy storage system of claim 1, wherein prior to performing one of a charge cycle or a discharge cycle, the third pressure vessel is completely filled with a first fluid selected from a CWF gas or the non-vaporizable liquid, and the first pressure vessel is completely filled with a second fluid selected from a CWF gas or the non-vaporizable liquid and different from the first fluid, and when completed, the third pressure vessel is completely filled with the second fluid and the first pressure vessel is completely filled with the first fluid.

14. 10. The energy storage system of claim 1, wherein the first pressure vessel is also exposed to ambient temperature.

15. The energy storage system of claim 1 comprising a plurality of the first pressure vessels.

16. 16. The energy storage system of claim 15, wherein all of the first pressure vessels are in fluid communication with each other or selected first pressure vessels are in fluid communication with each other.

17. The energy storage system of claim 1 comprising a plurality of the second pressure vessels.

18. 20. The energy storage system of claim 17, wherein all of the second pressure vessels are in fluid communication with each other or selected second pressure vessels are in fluid communication with each other.

19. 3. The energy storage system of claim 2, wherein the gas holder further contains a non-CWF gas that is miscible with the CWF, the non-CWF gas being further compressible after liquefaction of the CWF.

20. 10. The energy storage system of claim 1, further comprising a flow control component operatively connected to each of the corresponding conduits for selectively controlling fluid flow through the corresponding conduit.

21. 10. The energy storage system of claim 1, wherein the second pressure vessel is exposed to ambient or subambient temperatures.

22. 1. A method for generating electrical power from stored energy, comprising: a) providing a first pressure vessel and a second pressure vessel, the second pressure vessel being in fluid communication with the first pressure vessel and set at a temperature below ambient; b) substantially isothermally compressing a condensable working fluid (CWF) condensable at ambient temperature in direct contact with a non-vaporizable liquid in the first pressure vessel, and transferring at least a portion of the compressed CWF from the first pressure vessel to the second pressure vessel in response to interaction with the non-vaporizable liquid; c) transferring an additional amount of the compressed CWF to the second pressure vessel and compressing all or a majority of the CWF in the second pressure vessel to a saturation pressure and condensing to produce liquid CWF; d) propelling at least a portion of the non-vaporizable liquid in the first pressure vessel toward at least one hydraulic turbine with the compressed CWF discharged from the second pressure vessel; e) rotating the at least one hydraulic turbine with the propelled non-vaporizable liquid to generate electrical power; The method of claim 1, wherein flow of the non-vaporizable liquid into the second pressure vessel is prevented while at least a portion of the compressed CWF is transferred from the first pressure vessel to the second pressure vessel.

23. 23. The method of claim 22, wherein the CWF is substantially isothermally compressed by operating a mixer unit in fluid communication with the first pressure vessel or the second pressure vessel, thereby eliminating temperature gradients within the CWF.

24. 24. The method of claim 23, wherein the mixer unit is activated in response to a sensed condition indicative of liquefaction of the CWF.

25. 23. The method of claim 22, wherein at least a portion of the non-vaporizable liquid in the first pressure vessel is propelled by the compressed CWF discharged from the second pressure vessel toward the at least one hydraulic turbine when the discharged compressed CWF is in a liquid state, a gas state, or a multi-phase state.

26. 23. The method of claim 22, wherein the step of transferring at least a portion of the compressed CWF from the first pressure vessel to the second pressure vessel is performed during multiple charging cycles.

27. 27. The method of claim 26, wherein the step of propelling at least a portion of the non-vaporizable liquid in the first pressure vessel with the compressed CWF discharged from the second pressure vessel is performed for multiple discharge cycles.

28. i. providing a third pressure vessel in fluid communication with the first pressure vessel, the third pressure vessel being completely filled with a first fluid selected from a CWF gas or the non-vaporizable liquid and the first pressure vessel being completely filled with a second fluid different from the first fluid selected from a CWF gas or the non-vaporizable liquid prior to performing one of a charge cycle or a discharge cycle; 28. The method of claim 27, further comprising the step of performing said one of a charge cycle or a discharge cycle such that, when completed, said third pressure vessel is completely filled with said second fluid and said first pressure vessel is completely filled with said first fluid.

29. 30. The method of claim 28, further comprising performing the other of a charge cycle or a discharge cycle when the third pressure vessel is fully filled with the second fluid and the first pressure vessel is fully filled with the first fluid.

30. 30. The method of claim 28, wherein the compressed CWF discharged from the second pressure vessel expands substantially isothermally while in direct contact with the non-vaporizable liquid in the first pressure vessel.

31. a) a first pressure vessel having at least four ports each in fluid communication with a corresponding conduit; b) a second pressure vessel having one or more ports and set at a temperature below ambient temperature; and c) a flow control component operatively connected to each of said corresponding conduits for selectively controlling the flow of fluid therethrough; A condensable working fluid (CWF) that is condensable at room temperature is introduceable into the first pressure vessel through a first port of the at least four ports; a non-vaporizable liquid can be introduced into the first pressure vessel through a second port of the at least four ports, thereby causing substantially isothermal compression of the CWF upon direct contact with the non-vaporizable liquid; After introducing a sufficient amount of the non-vaporizable liquid into the first pressure vessel, at least a portion of the compressed CWF can be transferred from the first pressure vessel through a third port of the at least four ports to the second pressure vessel through one of the one or more ports in direct contact with the non-vaporizable liquid, and can be stored in a liquid state in the second pressure vessel after being compressed and condensed to a saturation pressure; A direct contact fluid transfer (DCFT) module, wherein at least a portion of the non-vaporizable liquid in the first pressure vessel can be propelled through a fourth of the at least four ports by the compressed CWF discharged from the second pressure vessel, discharging stored energy.

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