Abstract:
A method of operating a compressed air energy storage (CAES) system includes operating a compressor train of the CAES system, thereby compressing air. The method further includes, while operating the compressor train: inter-cooling a first portion of the compressed air; further compressing the inter-cooled first portion; after-cooling the further compressed first portion; supplying the after-cooled first portion to a storage vessel; supplying a second portion of the compressed air to a combustor; combusting the second portion; and operating a turbine train of the CAES system using the combusted second portion.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention generally relate to a compressed air energy system having variable generation modes. 
         [0003]    2. Description of the Related Art 
         [0004]    Compressed air energy storage (CAES) is a historically proven method of “storing” electric energy for later supply to the bulk power grid. The first utility scale CAES facility began service in 1978, in Huntorff, Germany, with a second plant in McIntosh, Ala., going into service in 1991. CAES plants compress air with an electric motor-driven compressor, injecting the air at high pressure into an underground storage cavern. Subsequently, when power is needed for the grid, high-pressure air from the cavern is routed through one or more expansion turbines, performing work and driving an electric generator, producing power for the grid. 
         [0005]    In these initial applications, CAES was implemented to produce cost-effective peak power by shifting low-cost, off-peak energy into high demand hours. One distinctive feature of these initial CAES installations was the use of a single electrical machine to function as a motor to drive the compression train, and as a generator when withdrawing air from the cavern. This was accomplished by linking the “motor/generator” via clutches to a compressor train and to the expanders. One clutch would always be engaged, with the other disengaged, allowing either function to be performed by this single electrical machine. The advantage of this design is the avoidance of an additional large electrical machine (i.e., a motor or a generator). 
       SUMMARY OF THE INVENTION 
       [0006]    Embodiments of the present invention generally relate to a compressed air energy system having variable generation modes. In one embodiment, a method of operating a compressed air energy storage (CAES) system includes operating a compressor train of the CAES system, thereby compressing air. The method further includes, while operating the compressor train: inter-cooling a first portion of the compressed air; further compressing the inter-cooled first portion; after-cooling the further compressed first portion; supplying the after-cooled first portion to a storage vessel; supplying a second portion of the compressed air to a combustor; combusting the second portion; and operating a turbine train of the CAES system using the combusted second portion. 
         [0007]    In another embodiment, a compressed air energy storage (CAES) system includes: an electric motor; a compressor train connected to the electric motor via a first drive shaft; an intercooler and aftercooler in fluid communication with the compressor train; an electric generator; a turbine train connected to the electric generator via a second drive shaft; a combustor in fluid communication with the turbine train; and a programmable logic controller (PLC). The PLC is operable to: divert a portion of air from the compressor train to the combustor at a first flow rate, supply fuel to the combustor at a second flow rate, and control the first flow rate and the second flow rate to operate the turbine train at or near minimum capacity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0009]      FIGS. 1A-1C  illustrate a compressed air energy storage (CAES) system operating in various modes, according to embodiments of the present invention.  FIG. 1A  illustrates the CAES system in a low power generation mode.  FIG. 1B  illustrates the CAES system in an emergency power generation mode.  FIG. 1C  illustrates the CAES system in a high power generation mode. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIGS. 1A-1C  illustrate a compressed air energy storage (CAES) system  1  operating in various modes, according to embodiments of the present invention.  FIG. 1A  illustrates the CAES system  1  in a low power generation mode. The CAES system  1  may include an electric motor  2   m,  an electric generator  2   g,  a compressor train  3   w,h,  a cooling system  4   c,  storage vessel  7 , one or more combustors  8   h,w,  a turbine train  9   h,w,  a recuperator  10 , a transformer  11 , a programmable logic controller (PLC)  15 , and one or more control valves  17   b,c,g.    
         [0011]    The storage vessel  7  may be a subterranean geological space, such as a salt dome, cavern, or mine. Alternatively, the storage vessel may be a pressure vessel located on the surface or underground. A wellbore  5   w  may provide communication between the storage vessel  7  and a wellhead  5   h.  A casing string  5   c  may be installed in the wellbore  5   w  by being hung from the wellhead  5   h  and cemented (not shown) in place. Once the casing string  4  has been deployed and cemented, a header  6  may be fastened to the wellhead  5   h.  The header  6  may include one or more shutoff valves, a flow cross, and a cap. An inlet valve  18   i  and an outlet valve  18   o  may each be fastened to a respective branch of the header flow cross. Each inlet and outlet valve  18   i,o  may be an automated shutoff valve having a powered actuator. The valve actuators may each be hydraulically, electrically, or pneumatically powered and may be in communication with the PLC  15  for operation of the respective inlet and outlet valves  18   i,o  by the PLC  15 . 
         [0012]    The transformer  11  may be connected to transmission lines of an electric grid  12 . The transformer  11  may be a one or more (three shown) phase transformer for stepping voltage supplied by the generator  2   g  from an output voltage to a substation or transmission line voltage. The transformer  11  may also step a substation or transmission line voltage from the power grid  12  to an input voltage for supplying the electric motor  2   m.  Alternatively, the CAES system  1  may include a first transformer for the electric motor  2   m  and a second transformer for the electric generator  2   g.  A first drive shaft  19   c  may connect a rotor of the electric motor  2   m  to a rotor of the compressor train  3   w,h  for torsional driving of the compressor train  3   w,h  by the electric motor  2   m.  A second drive shaft  19   t  may connect a rotor of the electric generator  2   g  to a rotor of the turbine train  9   h,w  for torsional driving of the electric generator  2   g  by the turbine train  9   h,w.    
         [0013]    The compressor train  3   w,h  may include two or more compressors connected in series, such as low pressure compressor  3   w  and high pressure compressor  3   h.  The low pressure compressor  3   w  may intake ambient air  20  and compress the ambient air  20 . The cooling system  4   c  may include a cooling tower  4   t,  an intercooler  4   i,  an aftercooler  4   a,  piping, circulation pumps, and a coolant, such as water. 
         [0014]    The compressed air  21  may be discharged from the low pressure compressor  3   w  to an inlet of a first tee  40   c  having a pair of outlet branches. A first portion  21   a  of the compressed air  21  may flow through a first branch of the first tee  40   c  to the intercooler  4   i.  A second portion  21   b  of the compressed air  21  may flow through a second branch of the first tee  40   c  to an inlet of the recuperator  10 . The intercooler  4   i  may transfer heat from the first portion  21   a  to the cooling tower  4   t.  The cooled compressed air  22  may be discharged from the intercooler  4   i  to an inlet of the high pressure compressor  3   h.  The high pressure compressor  3   h  may further compress the cooled compressed air  22 . The further compressed air  23  may be discharged from the high pressure compressor  3   h  to the aftercooler  4   a.  The aftercooler  4   a  may transfer heat from the further compressed air  23  to the cooling tower  4   t  such that a temperature of the further cooled and further compressed air  24  is suitable for discharge into the storage vessel  7 . The further cooled and further compressed air  24  may be discharged from the aftercooler  4   a  through the inlet valve  18   i,  down the casing string  5   c  and into the storage vessel  7 . 
         [0015]    To facilitate optimal control of the CAES system  1  by the PLC  15 , the CAES system  1  may include one or more sensors, such as one or more (five shown) pressure sensors  30   p,  one or more (five shown) temperature sensors  30   t,  one or more (two shown) tachometers  31 , one or more (four shown) flow meters  32 , a voltmeter  33   v  and an ammeter  33   i.  Each sensor  30   p,t - 33   v,i  may be in data communication with the PLC  15 . The PLC  15  may also be in communication with the grid operator via a network  16   n,  such as an intranet or the Internet, and a network interface, such as a modem  16   m.  The PLC  15  may also monitor the sensors  30   p,t - 33   v,i  to determine if any of the CAES equipment requires maintenance. 
         [0016]    The PLC  15  may maintain a charge pressure of the storage vessel  7  between a minimum and a maximum charge pressure. The maximum charge pressure may be determined from a depth  5   d  of a bottom (aka shoe) of the casing string  5   c.  The casing shoe depth  5   d  may be greater than or equal to about 1,000, about 1,500, about 2,000, about 2,500, about 3,000, or about 3,500 feet. The maximum charge pressure (in psia) may be based on a percentage of the casing shoe depth  5   d  (in feet), such as eighty-five percent of the casing shoe depth  5   d  (measured at the casing shoe depth  5   d,  slightly less if measured at the wellhead  5   h  due to head pressure). The minimum charge pressure (in psia) may also be based on a percentage of the casing shoe depth  5   d  (in feet), such as greater than or equal to: forty-five percent, fifty percent, sixty percent, seventy percent, or seventy-five percent of the casing shoe depth  5   d  (measured at the casing shoe depth  5   d,  slightly less if measured at the wellhead  5   h  due to head pressure). 
         [0017]    For example, for a casing shoe depth  5   d  equal to 3,750 feet, the maximum charge pressure may be 3,188 psia (at casing shoe depth  5   d,  about 2,850 psia at the wellhead  5   h ) and the minimum charge pressure may be 2,500 psia (at casing shoe depth  5   d,  about 2,300 psia at the wellhead  5   h ). Alternatively, the minimum charge pressure may be based on the required charge pressure to achieve the rated output of the turbine train  9   h,w  for a predetermined period of time, such as greater than or equal to one hour, two hours, four hours, eight hours, or twelve hours, and may be substantially greater than the required charge pressure to achieve the rated output. 
         [0018]    A flow rate of the second portion  21   b  of the compressed air  21  to the recuperator  10  may be regulated by a bypass control valve  17   b.  The recuperator  10  may preheat the second portion  21   b  of the compressed air  21 . The preheated second portion  21   w  of the compressed air  21  may be discharged from the recuperator  10  to an air inlet of a high pressure combustor  8   h.  The high pressure combustor  8   h  may also receive high pressure fuel gas  25   h  from a booster compressor  14 . The booster compressor  14  may be supplied by a fuel supply, such as a pipeline  13 . A flow rate of the high pressure fuel gas  25   h  may be regulated by the PLC  15  controlling operation of the booster compressor  14 . The high pressure fuel gas  25   h  may be natural gas, propane, butane, methane, or syngas. 
         [0019]    The high pressure combustor  8   h  may mix the high pressure fuel gas  25   h  with the preheated second portion  21   w  of the compressed air  21  and combust the mixture, thereby further heating the preheated second portion  21   w  of the compressed air  21 . The turbine train  9   h,w  may include two or more gas turbines connected in series, such as low pressure turbine  9   w  and high pressure turbine  9   h.  The heated exhaust gas  26  may be discharged from the high pressure combustor  8   h  to the high pressure turbine  9   h.  The high pressure turbine  9   h  may intake and expand the heated exhaust gas  26  while harnessing energy therefrom to drive the generator  2   g.  The expanded exhaust gas  27  may be discharged from the high pressure turbine  9   h  to a low pressure combustor  8   w.    
         [0020]    The low pressure combustor  8   w  may receive low pressure fuel gas  25   w  from the pipeline  13  via control valve  17   g.  The low pressure fuel gas  25   w  may be natural gas, propane, butane, methane, or syngas. A flow rate of the low pressure fuel gas  25   w  may be regulated by the PLC  15  controlling operation of the control valve  17   g.  The low pressure combustor  8   w  may mix the low pressure fuel gas  25   w  with the expanded exhaust gas  27  and combust the mixture, thereby reheating the expanded exhaust gas  27 . The reheated exhaust gas  28  may be discharged from the low pressure combustor  8   w  to the low pressure turbine  9   w.  The low pressure turbine  9   w  may intake and expand the reheated exhaust gas  28  while harnessing energy therefrom to drive the generator  2   g.  The flue gas  29  may be discharged from the low pressure turbine  9   w  to the recuperator  10 . The recuperator  10  may utilize residual heat from the flue gas  29  for preheating the second portion  21   b  of the compressed air  21 . The spent flue gas  29   s  may be discharged from the recuperator  10  to the atmosphere. 
         [0021]    In the low power generation mode, assuming the storage vessel  7  is depleted or substantially depleted, the PLC  15  may operate the compressor train  3   w,h  at or near rated capacity and the turbine train  9   h,w  at or near minimum capacity due to off-peak pricing of electricity by the grid operator. If/when the storage vessel  7  is recharged or nearly recharged, the PLC  15  may reduce compressor output to the flow rate necessary to operate the turbine train  9   h,w  at minimum capacity by control of the electric motor  2   m.  If/when the storage vessel  7  is fully recharged, the PLC  15  may shut the inlet valve  18   i.    
         [0022]    The PLC  15  may continue operating the turbine train  9   h,w  at minimum capacity for the duration of the low power generation mode such that the CAES system  1  may qualify for consideration as spinning reserve capacity by being able to rapidly increase output of the turbine train  9   h,w  to a requested, such as rated, capacity. The response time may be predetermined by the grid operator, such as less than or equal to five or ten minutes. The grid operator typically maintains spinning reserve capacity in case of generation or transmission outages. The CAES system  1  may be online when operating as spinning reserve capacity and may or may not be frequency responsive. Alternatively, the CAES system  1  may be operated as supplemental reserve capacity. The minimum capacity of the turbine train  9   h,w  may be substantially less than the rated capacity, such as less than or equal to one-tenth the rated capacity or ranging between one percent and five percent of the rated capacity. 
         [0023]    A rated output of the turbine train  9   h,w  may be based on the useful capacity of the storage vessel  7 . The useful capacity may be the difference between the maximum and minimum charge pressures of the storage vessel  7 . The turbine train  9   h,w  rated capacity may be designed to consume the useful storage vessel capacity within a predetermined period of time, such as: eight hours, twelve hours, eighteen hours, one day, two days, or four days. A rated capacity of the compressor train  3   w,h  may also correspond to the useful capacity of the storage vessel  7 . The rated output of the compressor train  3   w,h  may be designed to recharge the useful storage vessel capacity within a predetermined period of time, such as: six hours, eight hours, twelve hours, eighteen hours, one day, two days, or four days. 
         [0024]    A flow rate of the second portion  21   b  may be greater than or equal to the flow rate necessary to operate the turbine train  9   h,w  at minimum capacity. In terms relative to the flow rate of the compressed air  21  (at rated capacity of the compressor train  3   w,h ), the flow rate of the second portion  21   b  may range between: about one-eighth to about one-half, about one-sixth to about one-third, or be about one-fourth. A pressure of the second portion  21   b  may be greater than or equal to the pressure necessary to operate the turbine train  9   h,w  at minimum capacity, such as, for example, about one hundred psia, about one hundred fifty psia, or about two hundred psia, and substantially less than the minimum charge pressure of the storage vessel  7 , such as less than or equal to about one-third, about one-fourth, about one-fifth, about one-sixth, or about one-eighth of the minimum charge pressure. If the compressor train  3   w,h  includes one or more intermediate compressors (not shown), then the second portion  21   b  may be diverted from the compressor having an outlet pressure closest to and greater than the turbine train minimum operating pressure. For example, a four-compressor train may have the second portion  21   b  diverted from an outlet of the second compressor. 
         [0025]      FIG. 1B  illustrates the CAES system  1  in an emergency power generation mode. If the grid operator requests implementation of the spinning reserve capacity, the PLC  15  may rapidly increase output, such as to rated capacity, of the turbine train  9   h,w  within the predetermined period of time dictated by the grid operator. The PLC  15  may open the outlet valve  18   o,  thereby releasing the further cooled and further compressed stored air  24  from the storage vessel  7 . The further cooled and further compressed stored air  24  may exit the header flow cross and flow through the control valve  17   c.  As discussed above, the minimum charge pressure of the storage vessel  7  may be substantially greater than the pressure for rated operation of the turbine train  9   h,w  such that substantial expansion may occur through the control valve  17   c.    
         [0026]    The expanded released air  24   e  may be discharged from the control valve  17   c  to the recuperator  10  via a second tee  40   t.  The expanded released air  24   e  may substantially increase pressure at the second tee  40   t  relative to the discharge pressure of the low pressure compressor  3   w,  thereby shutting check valve  41 . The expanded released air  24   e  may be preheated by the recuperator  10  and the preheated released air  24   w  may be discharged to the high pressure combustor  8   h.  The PLC  15  may increase the flow rate of the booster compressor  14  and control valve  17   g  according to the flow rate of the preheated released air  24   w.  The PLC  15  may adjust operation of the compressor train  3   w,h  and/or the cooling system  4   c  in response to closing of the check valve  41 . Operation of the CAES system  1  in emergency mode may continue as required by the grid operator. 
         [0027]    Since the turbine train  9   h,w  and compressor train  3   w,h  are operated independently via the respective electric generator  2   g  and electric motor  2   m,  the PLC  15  may respond to various forms of requests by the grid operator by independently increasing or decreasing capacity of the compressor train  3   w,h  and turbine train  9   h,w.  Such requests may include frequency regulation, such as up-regulation or down-regulation, or other (in addition to spinning reserves) ancillary services. Additionally, instead of (or in addition to) increasing generation by the turbine train  9   h,w,  the PLC  15  may decrease consumption by the compressor train  3   w,h  to create an equivalent net (or cumulative) effect. 
         [0028]      FIG. 1C  illustrates the CAES system  1  in a high power generation mode. In response to increase of electricity price to peak-level, the PLC  15  may transition the turbine train  9   h,w  to operation from the storage vessel  7 , as discussed above. The PLC  15  may then shut down the compressor train  3   w,h  and close the inlet valve  18   i.  The PLC  15  may operate the turbine train  9   h,w  at full or partial capacity depending on the requirements of the grid operator. 
         [0029]    Alternatively, the CAES system may include a boiler or steam generator and a steam turbine train instead of the recuperator  10  for utilizing residual heat of the flue gas. 
         [0030]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.