Abstract:
A system and method utilizing compressed gas according to which the gas is compressed at a location above ground and transported to an underwater location. The gas is stored at the underwater location and later returned from the underwater location to the above-ground location for utilization as energy.

Description:
BACKGROUND 
     This invention relates to a compressed gas utilization system and method and, more particularly, to such a system and method in which the compressed gas is stored in a sub-sea environment and later utilized as energy. 
     Compressed air energy storage (CAES) systems are generally known, and are for the purpose of storing energy, in the form of compressed gas, and later utilizing this stored potential energy for such purposes as the generation of electrical power. Typically, the CAES systems use electrical power purchased at low cost during off-peak periods to compress gas for storage. During periods of peak power demand, the potential energy in the stored gas is used to produce electrical power, which may be sole at a premium rate. 
     These systems can be used in a stand-alone mode for generating electrical power connected in a power grid, or they can be used with a conventional electrical power generating plant connected in a power grid, or the like. In the latter case, the power generated by the CAES system can be utilized as an adjunct to the power normally generated by the conventional power generating plant, usually during relatively high load conditions. CAES systems can also be used for balancing, optimizing, and enhancing the reliability of power grids and associated base-loaded power generating plants. Also, CAES systems can create spinning reserves or standby generating capacity, and can come on line in a relatively short time to take up a power load in the event a power generating plant on the grid malfunctions. Further, CAES systems can balance the power grid by taking and saving excess power, and can make up extra demand without a ramp up required by conventional power generating plants. Still further, CAES systems can improve the availability of renewable resource power by storing excess power and generating power when the renewable resource power is unavailable or inadequate. 
     A typical CAES system, or plant, includes a compression train in which a motor-driven compressor compresses a gas, such as air. The compressed gas is then transferred to, and stored at, a storage site, usually at a remote location, for later use at which time it is transferred back to an expansion side of the CAES plant. During the expansion cycle, the compressed gas is expanded through a conventional expansion train that may include high pressure and/or low pressure turbines that drive an electrical power generator to generate electrical power. In these arrangements, a fuel gas is often burned with the expanding gas to raise the temperature of the gas and improve the efficiency of the system 
     However, known CAES plants utilize underground storage facilities for the compressed gas, along with piping systems to connect the storage facility to the compression and expansion sides of the CAES plant. This severely limits the site location due to the dependence on an acceptable geology for underground storage location. Also, the underground storage facility is usually located a considerable distance from the power generation or power consumption areas, resulting in transmission costs, losses and related expenses. Furthermore, underground storage facilities are susceptible to earthquake damage. 
     Therefore what is needed is a system of the above type for storing the gas that avoids the above problems. To this end, an embodiment of the present invention is directed to a sub-sea energy storage system which provides a significant improvement over the previous systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view depicting the system of the present invention. 
         FIG. 2  is a diagrammatic view of the control/monitoring system for the system of FIG.  1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a system according to an embodiment of the invention which includes a plant  10  having a compression side  10   a  that includes a conventional motor-driven compression train and associated equipment (not shown) for compressing a gas, such as ambient air. The plant  10  also has an expansion side  10   b  in which the compressed gas is expanded through a conventional expansion train that includes high pressure and low pressure turbines that drive an electrical power generator to generate electrical power. It is understood that during operation of the expansion side  10   b  of the plant  10 , the gas can be burned with fuel to improve the efficiency of the plant. Since the turbines, the compression and expansion trains, and the power generator are conventional they are not shown nor will they be described in further detail. 
     The plant  10  is located on the ground surface in the vicinity of a coastline near an adjacent water source such as a lake, sea, or ocean (hereinafter referred to as “sea”) having a sea floor SF that drops off in height as it extends from the coastline. A piping system  12  is connected between the plant  10  and a manifold  14  resting underwater below the sea level SL on the sea floor SF, and at a distance from the coastline. It is understood that the piping system  12  includes at least one pipe that connects an outlet on the compression side  10   a  of the plant  10  to an inlet on the manifold  14 , and at least one pipe that connects an outlet on the manifold to an inlet on the expansion side  10   b  of the plant. It can be appreciated that the piping system can include branch pipes, valves, etc. (not shown) to enable these connections to be made. The piping system  12  and the manifold  14  are commercially available devices commonly used in offshore piping systems for oil or gas applications. 
     A storage vessel  16  is mounted to the sea floor SF in the vicinity of the manifold  14 . The vessel  16  is fabricated from a flexible material, such as a plastic, fabric, or similar material, that can collapse but does not stretch, and defines a fixed maximum closed volume. Although not shown in the drawings, it is understood that a suitable inlet and outlet are provided on the manifold  14  and the vessel  16  which can be controlled by valves in a conventional manner. 
     A conduit  20  connects the outlet of the manifold  14  to the inlet of the vessel as well as the outlet of the vessel to the inlet of the manifold so that the gas flow between the manifold and the vessel can be controlled. To this end it is understood that the conduit  20  can be provided with branch end portions and valving (not shown) to make the above connections. Although the vessel  16  is shown substantially cylindrical in shape with rounded ends, it is understood that this shape can vary, as will be discussed. 
     A mooring system  22  is provided that supports the vessel  16  slightly above the sea floor SF with the axis of the vessel extending substantially horizontally. The mooring system  22  is conventional and, as such, can, for example, be in the form of a piling system, an anchor system, a dead weight system, a combination of same, or the like. 
     When the flexible vessel  16  is inflated with the stored gas, and it is desired to release the gas from the vessel, the above-mentioned outlet valve associated with the vessel is opened and the hydrostatic pressure acting on the vessel causes a compression of the vessel to force the stored gas out from the vessel and into the conduit  20 . The volume of the vessel  16  and the depth of the vessel below the sea level SL are determined so that this hydrostatic pressure acting on the vessel enables the gas to be discharged from the vessel at a substantially constant discharge pressure as the volume of the gas in the storage vessel decreases. In particular, the volume of the vessel  16  is determined by the combination of the depth of the vessel, the amount of electrical power to be generated by the plant  10 , and the run time of the power generation cycle; while the depth of the vessel  16  is determined by the operating pressure of the plant and the volume of the vessel. The discharged gas passes through the conduit  20  and into the manifold  14  for return to the plant  10  via the piping system  12 . 
     Although only one storage vessel  16  is shown in  FIG. 1 , it is understood that a plurality of vessels can be provided, in which case the manifold  14  would be connected to each vessel. 
     A monitoring and control unit  24  is located on the ground surface and is adapted to monitor the conditions of the plant  10 , the piping system  12 , the conduit  20 , the manifold  14 , and/or the storage vessel  16 , and control the operation of same. In particular, and referring to  FIG. 2 , the unit  24  is electrically connected to five sensors  26  which are associated with the plant  10 , the piping system  12 , the conduit  20 , the manifold  14 , and the vessel  16 , respectively. The sensors  26  sense and monitor the volume, pressure and other parameters of the gas in the plant  10 , the piping system  12 , the conduit  20 , the manifold  14 , and/or the storage vessel  16  and send corresponding output signals to the unit  24 . Also, it is understood that the above-mentioned valves can be operated in any conventional manner, and that the control unit  24  controls the operation of the valves to selectively control the flow of the gas through the piping system  12  from the compression side  10   a  of the plant  10  to the manifold  14 , from the manifold to the vessel  16 , from the vessel back to the manifold, and from the manifold to the expansion side  10   b  of the plant. 
     The unit  24  receives the signals from the sensors  26  and includes a microprocessor, or other computing device, to control the flow of the gas through the piping system  12  and the conduit  20  in the above manner. The unit  24  also can be adapted to monitor other parameters, such as the volume of gas stored in the vessel  16 , the electrical power used to compress the gas in the plant, etc. Since this type of monitoring and control system is conventional, it will not be described in further detail. 
     In operation, the compression side  10   a  of the plant  10  receives a gas, such as air, and compresses it in the manner discussed above, before the gas flows to the manifold  14  via the piping system  12 , under the control of the control unit  24 . The manifold  14  directs the compressed gas into the storage vessel  16  at a flow rate that produces a pressure greater than the hydrostatic pressure exerted on the vessel. The vessel  16  is initially in a collapsed condition but inflates due to the presence of the compressed gas. This gas flow continues until tension is placed on the wall of the vessel, as measured by a strain gauge, or the like, which indicates that the vessel  16  is fully inflated at which time the gas flow is terminated so that there is minimum or no tensile stress on the vessel insuring that it will not be stretched. 
     When it is desired to release the gas from the vessel  16 , the above-mentioned outlet valve associated with the vessel is opened and the hydrostatic pressure acting on the vessel causes a compression of the vessel to force the stored gas out from the vessel and into the conduit  20 . The volume of the vessel  16  and the depth of the vessel below the sea level SL are determined in the manner discussed above so that the hydrostatic pressure acting on the vessel enables the gas to be discharged from the vessel at a substantially constant discharge pressure as the volume of the gas in the storage vessel decreases. The gas discharged from the vessel  16  passes via the conduit  20 , the manifold  14 , and the piping system to the expansion side  10   b  of the plant  10  for generating electrical power in the manner discussed above. 
     This system thus lends itself to the uses set forth above, including compressing and storing the gas during relatively low load conditions when the cost of electricity to compress the gas is relatively low, while permitting the stored compressed gas from the storage vessel  16  to be used in generating electricity during relatively high load conditions when the cost of the energy is relatively high. Also, due to the fact that the gas is discharged from the vessel  16  at a substantially constant discharge pressure as the volume of the gas in the vessel decreases, as described above, the efficiency is increased while the required overall storage volume is reduced. Further, the system enjoys a reduced susceptibility to earthquake damage and post-compression cooling of the gas due to the low temperature of the sea. This is all achieved while overcoming the drawbacks of the other underground storage facilities discussed above. 
     It is understood that variations may be made in the foregoing without departing from the scope of the invention. For example, the shape and orientation of the storage vessel  16  may be varied from that shown in the drawings as long as the pressure differential (or pressure swing) along the height (or diameter) of the vessel is limited so that a substantially constant discharge pressure is obtained during system operation, as discussed above. Also, a plurality of vessels  16  can be used, in which case the manifold  14  would be adapted to distribute the compressed gas to the vessels simultaneously or sequentially, and the operation would be the same as described above. Further, the manifold  14  can be eliminated and the gas transferred directly to the vessel  16 , especially if only one vessel is used. Moreover, the gas stored in the vessel  16  can be utilized in manners other than the generation of electrical power. 
     It is also understood that when the expression “gas” is used in this application, it is intended to cover all types of gas, including air, natural gas, and the like. For example, natural gas can be stored in the above manner and utilized to provide fuel for burners on the expansion side  10   b  of the plant  10 , as discussed above. Still further, it is understood that the piping system  12  and the conduit  20  can be used to transfer the compressed gas from the compression side  10   a  of the plant  10  to the manifold  14  and to the vessel  16 , respectively, and another conduit and piping system can be used to transfer the stored gas from the vessel and the manifold, respectively, to the expansion side  10   b  of the plant. 
     Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many other modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.