Abstract:
A two stage expansion and single stage combustor compressed air energy storage cycle that employs a heat exchanger to raise the temperature of the compressed air before it enters a high pressure expander. A combustor heats the exhaust from the high pressure expander and creates a working gas to drive a low pressure expander. The exhaust from the low pressure expander is supplied to the heat exchanger to raise the temperature of the compressed incoming air. A portion of the exhaust of the high pressure expander is cooled and employed in a cooling circuit within the low pressure expander. A starter valve in the compressed air input circuit finely tunes the incoming air during startup. The startup air flow is heated by an auxiliary duct burner until the low pressure turbine exhaust reaches operating temperature and is sufficient to heat the incoming air under normal operating conditions.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to compressed air energy storage (CAES) turbomachinery cycle systems and more particularly to such systems that employ a recuperator to heat the incoming compressed air with heat recovered from the turbine exhaust. 
     BACKGROUND OF THE INVENTION 
     CAES power plants have become effective contributors to a utilities generation mix as a source of peaking or intermediate energy and spinning reserve. CAES plants store off-peak energy from relatively inexpensive energy sources such as coal and nuclear base load plants by compressing air into storage devices such as underground caverns or reservoirs. Underground storage can be developed in hard rock, bedded salt, salt dome or acquifer media. 
     Following off-peak storage, the air is subsequently withdrawn from storage, heated, combined with fuel and combusted and expanded through expanders, i.e., turbines, to provide needed peak/intermediate power. Since inexpensive off-peak energy is used to compress the air, the need for premium fuels, such as natural gas and imported oil, is reduced by as much as about two-thirds compared with conventional gas turbines. 
     Compressors and turbines in CAES plants are each connected to a synchronous electrical machine such as a generator/motor device through respective clutches, permitting operation either solely of the compressors or solely of the turbines during the appropriate selected time periods. During off-peak periods (i.e., nights and weekends), the compressor train is driven through its clutch by the generator/motor. In this scheme the generator/motor functions as a motor, drawing power from a power grid. The compressed air is then cooled and delivered to underground storage. 
     During peak/intermediate periods, with the turbine clutch engaged, air is withdrawn from storage and then heated and expanded through a turbine to provide power to drive the generator/motor. In this scheme, the generator/motor functions as a generator, providing power to a power grid. To improve the CAES efficiency, waste heat from a low pressure turbine exhaust is used to preheat high pressure turbine inlet air in a recuperator. The compression process in a CAES plant is characterized by a much higher overall compression ratio than traditionally experienced in conventional gas turbines. This requires multistage compression with intercoolers in order to improve CAES plant efficiency. 
     The turbomachinery associated with a convention CAES plant has high pressure and low pressure turbines with high pressure and low pressure combustors, respectively. Fuel is mixed with compressed air and combusted at essentially constant pressure in these combustors, thus producing mixtures of products of combustion with high temperatures. The high temperature mixtures are then expanded in series through the high pressure and low pressure turbines, thereby performing work. Each turbine generally has an optimum expansion ratio (i.e., ratio of turbine input pressure to turbine output pressure) resulting in the highest possible efficiency for a specific turbine inlet temperature. The efficiency and optimum pressure ratio increase with increasing turbine inlet temperatures. 
     Turbine trains used in CAES systems have an overall expansion ratio which is the product of expansion ratios of the individual turbines which are serially connected. The overall expansion ratio of a turbine train comprising high and low pressure turbines is the ratio of turbine train input pressure (to a high pressure turbine) to turbine train output pressure (exhaust from a low pressure turbine), and generally ranges for CAES applications from 20 to a 100 or more. 
     Due to generally high air storage pressures, CAES plants are subject to high operating pressures unless the intake compressed air is throttled to a lower pressure. As pointed out in U.S. Pat. No. 4,885,912 issued Dec. 12, 1989, throttling the pressure from 60 bar and above, that may be encountered in high pressure turbines of CAES systems, is inefficient due to the energy of stored pressure in the compressed air that is lost. As the patent recognizes, one solution is to develop high pressure combustors which are yet unproven. A second solution proposed by the patent is to eliminate the high pressure combustor and transmit the heated compressed air from the recuperator directly to the high pressure turbine. While this latter proposal is efficient under operating conditions it does create some instabilities and under startup conditions it creates some inefficiencies. 
     Furthermore, to further improve the efficiency of the process it is desirable to run the low pressure turbine as hot as its materials will permit. Increasing the heat of the working gas in the low pressure turbine not only increases the efficiency of the low pressure turbine cycle but also increases the temperature of the exhaust gas and, through the heat recovered in the recuperator, the temperature of the compressed air entering the high pressure turbine, thus improving the efficiency of the high pressure turbine as well. 
     Accordingly, it is an object of this invention to improve the startup conditions of the CAES system that does not employ a high pressure combustor. Additionally, it is a further object of this invention to improve the efficiency of the CAES system by enabling the low pressure turbine to handle a higher temperature working gas. 
     SUMMARY OF THE INVENTION 
     A two-stage expansion and single stage combustor compressed air energy storage cycle is disclosed that employs a heat exchanger to raise the temperature of the compressed air before it enters a high pressure expander. A portion of the exhausted air from the high pressure expander is cooled and redirected to cool a number of the turbine components. The remaining air exhausted from the high pressure expander is directed through a combustor where it is combined with fuel and burned to produce a hot gas which drives a low pressure turbine, the second stage of the expansion cycle. The exhaust from the low pressure turbine is fed back through the heat exchanger where a portion of its heat is transferred to the incoming compressed air. A startup arrangement and method is also described that uses a duct burner to heat the incoming compressed air until the low turbine exhaust temperature reaches operating conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the invention will become more readily apparent with reference to the following description of the invention in which: 
     FIG. 1 is a schematic illustration of a compressed air energy cycle employing the components of this invention including a first embodiment for extracting a portion of the compressed air to cool the low pressure turbine components; 
     FIG. 2 is a schematic illustration of a compressed air energy storage cycle employing the components of this invention including a second embodiment for extracting a portion of the compressed air to cool the low pressure turbine components; 
     FIG. 3 is a schematic illustration of the compressed air energy cycle illustrated in FIG. 2 further including a starter valve and duct burner to enhance startup conditions; and 
     FIG. 4 is a schematic illustration of a compressed air energy cycle employing the components of this invention illustrated in FIG. 1 further including a fifth high pressure turbine air extraction line for cooling the first stage of the low pressure turbine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A recuperated compressed air energy storage (CAES) cycle using a single stage combustor and two stage expansion for power generation constructed in accordance with this invention is illustrated in FIG.  1 . The CAES system employs a large cavern  20  for storing high pressure air generated during a low power demand period. The high pressure air is then expanded to generate electricity during a high power demand period. During low demand periods, the turbine power not being employed to drive the generators, drives compressors  12  and  16  to compress the air to cavern specifications. Coolers  14  and  18  are disposed within the compressed air path respectively downstream of the compressors  12  and  16  to reduce the compressed air temperature to satisfy the requirements of the cavern  20  set by the cavern operator. FIG. 1 shows a double cooling circuit  14  and  18  in series with the compressed air flow path, while FIG. 2 shows a single cooling circuit  14 . The extent of cooling required will be dictated by the cavern specifications. The cooling required can be achieved through the use of one or more cooling towers containing the cooling circuits. During high power demand periods the high pressure air from the cavern  20  can then be expanded through a turbine to generate electricity. The air released from the cavern is cool and at a high pressure. This cool and high pressure air presents difficulties for starting and sustaining combustion in a combustion turbine system. The current invention overcomes these difficulties by using a recuperator  28  instead of a high pressure combustor to warm the cold air before it is conveyed to the high pressure expander or turbine  30 . The recuperator  28  can also be provided with a duct burner  70 , as illustrated in FIG.  3  and described hereafter, for ease of starting. 
     FIG. 1 shows a recuperated CAES cycle at a base load condition for a typical power generation application. At low power demand, the compressor system  12  and  16  pumps the ambient air  10  into a large cavern  20 . At peak power demand periods, a main throttle valve  22  can be opened to convey the stored compressed air in the cavern  20  to a recuperator  28 . The main throttle valve  22  is designed to reduce the pressure and temperature of the air from the cavern to an optimal condition before entering the recuperator  28 . The air is heated in the recuperator (heat exchanger)  28  by the exhaust gas  64  from a low pressure expander  62 . The compressed air then flows through a high pressure expander  30  to further reduce the air temperature and pressure, and in the process, generate mechanical power. The power developed from the high pressure expander  30  runs on the same shaft as the low pressure expander or combustion turbine  62  to drive an electric power generator figuratively illustrated in FIG. 1 by the block labeled “G”. 
     The air  56  exiting the high pressure expander  30  flows into a low pressure combustor  60 . Part of the air  31  exiting the high pressure expander  30  is bled into a mixing manifold  41  through proportioning valves  32 ,  34 ,  36  and  38  that are respectively coupled with mixing valves  46 ,  44 ,  42  and  40 , which tap into the air  24  being diverted downstream of the main throttle valve  22  coming from the cavern  20 . The air  24  from the cavern is then proportioned through each of the valves  40 ,  42 ,  44  and  46  to obtain the desired flow rate along with the corresponding component of air  31  diverted downstream of the high pressure turbine  30  through proportioning valves  38 ,  36 ,  34  and  32 . In this way the air  24  from the cavern  20  is used to cool the air  31  diverted from the high pressure turbine  30  in such proportions to give the desired temperature, pressure and flow rate to cool various components of the low pressure turbine  62  through the cooling conduits  48 ,  50 ,  52  and  54 . The pressure and flow rate of the cooling air for each of the respective turbine components has to be sufficient to assure that the cooling air is expelled into the working gas for power augmentation. 
     The air  56  that flows into the low pressure combustor  60  is mixed with fuel  58  to generate a high temperature working gas. The working gas from the combustor  60  then enters the low pressure expander  62  to develop mechanical power for driving the electrical generator “G”. The hot gas  64  exiting the low pressure expander  62  then flows into the recuperator  28  and is placed in heat exchange relationship with the incoming air  26  from the cavern  20 . Some of the heat in the exhaust gas  64  is recovered in the recuperator and transferred to the cold air  26  from the cavern  20 . The exhaust gas from the recuperator  28  then exits into the ambient air and thus completes the thermodynamic cycle. 
     FIG. 2 illustrates another embodiment of this invention in which the air  31  bled from the high pressure expander  30  exhaust is diverted back to the recuperator where it is placed in heat exchange relationship with the incoming air  26  from the cavern  20 . Preferably, this third heat transfer circuit within the recuperator  28  is placed upstream of the incoming air  26  from the heat transfer circuit that places the low pressure turbine exhaust  64  in heat transfer relationship with the incoming air  26 . It should be noted that like reference characters refer to corresponding elements in the several figures. The recuperator  28  is an energy (heat) exchange device and the variation in temperature in various parts of the recuperator is quite large, varying as much as 1,000° F., (537.8° C.). Therefore, the recuperator  28  can be used as a heat sink. The routing of the air  31  exhausted from the high pressure turbine  30  back to the recuperator  28  takes advantage of using the recuperator  28  as a heat sink rather than employing the more commonly used method illustrated in FIG. 1 to achieve the desired temperatures, pressures and flow rates, to cool the various components of the low pressure turbine  62 . Though not shown, valves can be placed in each of the respective cooling lines  48 ,  50 ,  52  and  54  to proportion the cooling air  31  among the several cooling lines to achieve the desired parameters in the manner taught in FIG.  1 . While FIG. 2 shows one cooling circuit within a cooling tower  14  intermediate of the intake compressors  12  and  16  as compared to the two cooling circuits  14  and  18  illustrated in FIG. 1, in all other respects, except as noted for the cooling circuit, the design of the CAES cycle illustrated in FIG. 2 is the same as FIG.  1 . As previously mentioned, the pressure P 7  and temperature T 7  are dictated by the cavern specifications and will vary from application to application. 
     FIG. 3 illustrates a third embodiment of this invention, which is a modification of the design illustrated in FIG. 2, to enhance startup of the system. An atmospheric duct burner  70  fed by a blower  68  is placed in a heating circuit  72  in heat exchange relationship with the incoming air  26 . A control system  78  can sense the temperature of the low pressure turbine  62  exhaust  64  entering the recuperator  28  and control the duct burner  70  so that the duct burner can gradually be reduced in temperature as more heat is transferred from the low pressure expander  62  exhaust  64 . A starter valve  23  is also placed in parallel with the main throttle valve  22 . Though not unique to this embodiment, FIG. 3 also shows a bleeder valve  33  that controls the amount of air  31  diverted from the exhaust of the high pressure expander  30 . In all other respects, the system illustrated in FIG. 3 is identical to that illustrated in FIG.  2 . 
     On startup of the system illustrated in FIG. 3 the duct burner  70  is ignited and the blower  68  is activated to drive ambient air  66  through the heating circuit  72  where it is exhausted on the other side of the recuperator  28  to the ambient atmosphere. Once the recuperator gets hot, the control system  78  can open the starter valve  23  in the cavern  20  outlet. The starter valve  23  has a much higher pressure ratio or pressure drop than the main throttle valve  22 . The air from the starter valve  23  is at a lower flow rate, pressure, and temperature. This air flows into the recuperator  28  and is heated by the atmospheric burner  70 . This warm air flows through the high pressure expander  30  to spin the shaft and into the low pressure combustor  60  to establish a favorable condition for the combustor  60  to ignite. Once the low pressure combustor  60  is ignited, the fuel flow  58  will be increased and bring the shaft speed to synchronize with the electrical power grid. The exhaust gas  64  from the low pressure expander  62  will heat up gradually to its operating temperature. At that point the duct burner  70  can be turned off. The main throttle valve  22  is gradually opened and the starter valve  23  can be closed. The loading procedure is initiated by increasing the air flow  26  from the cavern  20  and simultaneously increasing the fuel flow  58  into the low pressure combustor  60 . 
     Bleeding of a portion of the air exhausted from the high pressure turbine  31  for cooling the low pressure turbine  62  components becomes increasingly important as turbine firing temperatures become higher to increase their efficiencies. The selection of temperatures and pressures of the air for cooling the turbine components also becomes an important consideration for optimum system performance. The temperatures T, pressures P, flow rate G and enthalpy H at various points in this system are illustrated in FIGS. 1 and 2 by their corresponding subscripts and exemplary values for those parameters can be found in the following Table  1 . 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 PARAMETER 
                 US 
                 SI 
               
               
                   
                   
               
             
             
               
                   
                 T 1   
                 87° F. 
                 30.56° C. 
               
               
                   
                 P 1   
                 1500 psia 
                 10342 kPa 
               
               
                   
                 T 2   
                 −37° F. 
                 −38.33° C. 
               
               
                   
                 G c1   
                 1008.7 lb/sec 
                 457.54 kg/sec 
               
               
                   
                 G c2   
                 38.19 lb/sec 
                 17.32 kg/sec 
               
               
                   
                 G c3   
                 27.89 lb/sec 
                 12.65 kg/sec 
               
               
                   
                 G c4   
                 5.26 lb/sec 
                 2.39 kg/sec 
               
               
                   
                 G c5   
                 2.77 lb/sec 
                 1.26 kg/sec 
               
               
                   
                 G c6   
                 2.77 lb/sec 
                 1.26 kg/sec 
               
               
                   
                 P 2   
                 610 psia 
                 4206 Kpa 
               
               
                   
                 G 2   
                 970.5 lb/sec 
                 440.21 kg/sec 
               
               
                   
                 T 3   
                 1050° F. 
                 565.56° C. 
               
               
                   
                 P 3   
                 602 psia 
                 4151 kPa 
               
               
                   
                 G 3   
                 970.5 lb/sec 
                 440.21 kg/sec 
               
               
                   
                 T 4   
                 742.8° F. 
                 394.89° C. 
               
               
                   
                 P 4   
                 231.8 psia 
                 1598 kPa 
               
               
                   
                 G 4   
                 893.1 lb/sec 
                 405.10 kg/sec 
               
               
                   
                 G F1   
                 21.9 lb/sec 
                 9.93 kg/sec 
               
               
                   
                 T 5   
                 1104.5° F. 
                 595.83° C. 
               
               
                   
                 P 5   
                 14.98 psia 
                 103 kPa 
               
               
                   
                 G 5   
                 1030.6 lb/sec 
                 467.47 kg/sec 
               
               
                   
                 T 6   
                 144° F. 
                 62.22° C. 
               
               
                   
                 P 6   
                 14.7 psia 
                 101 kPa 
               
               
                   
                 G 6   
                 1030.6 lb/sec 
                 467.47 kg/sec 
               
               
                   
                 P 7   
                 1500 psia 
                 10342 kPa 
               
               
                   
                 T 7   
                 200° F. 
                 93.33° C. 
               
               
                   
                 T 8   
                 50° F. 
                 10° C. 
               
               
                   
                 P 8   
                 610 psia 
                 4206 kPa 
               
               
                   
                 G 8   
                 1008.7 lb/sec 
                 457.54 kg/sec 
               
               
                   
                 T 9   
                 1050° F. 
                 565.56° C. 
               
               
                   
                 P 9   
                 602 psia 
                 4150 kPa 
               
               
                   
                 G 9   
                 1008.7 lb/sec 
                 457.54 kg/sec 
               
               
                   
                 G c7   
                 115.6 lb/sec 
                 52.44 kg/sec 
               
               
                   
                 T 10   
                 742.8° F. 
                 394.89° C. 
               
               
                   
                 P 10   
                 231.8 psia 
                 1598 kPa 
               
               
                   
                 G 10   
                 893.1 lb/sec 
                 405.1 kg/sec 
               
               
                   
                 G F2   
                 21.9 lb/sec 
                 9.930 kg/sec 
               
               
                   
                 T 11   
                 360.6° F. 
                 182.56° C. 
               
               
                   
                 P 11   
                 66.3 psia 
                 457 kPa 
               
               
                   
                 G 11   
                 4.55 lb/sec 
                 2.064 kg/sec 
               
               
                   
                 T 12   
                 531.8° F. 
                 277.67° C. 
               
               
                   
                 P 12   
                 144.3 psia 
                 995 kPa 
               
               
                   
                 G 12   
                 9.96 lb/sec 
                 4.52 kg/sec 
               
               
                   
                 T 13   
                 643.8° F. 
                 339.89° C. 
               
               
                   
                 P 13   
                 179.6 psia 
                 1238 kPa 
               
               
                   
                 G 13   
                 40.2 lb/sec 
                 18.23 keg/sec 
               
               
                   
                 T 14   
                 391.8° F. 
                 199.89° C. 
               
               
                   
                 P 14   
                 219.5 psia 
                 1513 kPa 
               
               
                   
                 G 14   
                 60.92 lb/sec 
                 27.63 kg/sec 
               
               
                   
                 T 15   
                 2575.7° F. 
                 1413.17° C. 
               
               
                   
                 P 15   
                 223.0 psia 
                 1537 kPa 
               
               
                   
                 G 15   
                 915.0 lb/sec 
                 415.04 kg/sec 
               
               
                   
                 T 16   
                 1104.5° F. 
                 595.83° C. 
               
               
                   
                 P 16   
                 14.98 psia 
                 103 kPa 
               
               
                   
                 G 16   
                 1030.6 lb/sec 
                 467.47 kg/sec 
               
               
                   
                 T 17   
                 200° F. 
                 93.33° C. 
               
               
                   
                 P 17   
                 14.7 psia 
                 101 kPa 
               
               
                   
                 G 17   
                 1030.6 lb/sec 
                 467.47 kg/sec 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 4 illustrates another embodiment of this invention, which is a variant of the embodiment illustrated in FIG.  1 . Cold incoming pressurized air from the cavern  20 , at approximately 600 psia (4137 kPa), is extracted out of the main line  26  supplying cold air to the recuperator  28  and eventually to the high pressure expander  30 . The extracted air  24  is taken to a header  25  and distributed into four streams  48 ,  50 ,  52  and  54 . The four streams  48 ,  50 ,  52  and  54  are respectively provided with control valves  40 ,  42 ,  44  and  46  and instrumentation  55  to measure the temperature, pressure and flow of the cold air. Three of the streams  48 ,  50  and  52  provide air that will be mixed with the hot air  31  bled from the air  56  exiting the high pressure expander  30  to provide cooling air for the nozzles of the  2   nd ,  3   nd  and  4   th  stages of the low pressure expander  62 . The fourth stream  54  provides air that mixes with the hot air to provide for the rotor air cooling of the low pressure expander  62 . 
     The air exiting the recuperator is run through the high pressure expander  30  and directed to the combustor  60  for heating and expanding in the low pressure expander  62 , as described with respect to the embodiment of FIG.  1 . In the embodiment shown in FIG. 4, the portion of the air  31  bled from the high pressure expander  30  is taken to a header and distributed into five streams. Each stream is provided with a corresponding proportioning control valve  321 ,  34 ,  36 ,  38  and  39  and instrumentation  55  to measure the temperature, flow and pressure of the hot air. 
     The four streams of cold air from the mixing valves  40 ,  42 ,  44  and  46  are mixed with the four steams of the hot air from the proportioning valves  32 ,  34 ,  36  and  38  in proportions to meet the flow and temperature requirements of the cooling air. The valve control system  80  is set in such a way that the proportioning control valves  32 ,  34 ,  36  and  38  on the hot air  31  bled from the high pressure expander exhaust stream  56  provides the flow to meet the flow requirements as dictated by the low pressure expander cooling flow requirements and the control mixing valves  40 ,  42 ,  44 , and  46  on the cold air streams provide air for meeting the temperature requirements as dictated by the low pressure expander cooling temperature requirements, or vice versa depending on the temperature of the cold air coming in from the cavern  20 . The  5   th  hot air stream  47  controlled by proportioning valve  39  provides cooling air directly to the  1   st  stage nozzle of the low pressure expander  62 . 
     In addition, it is to be understood that the invention can be carried out by different equipment and devices and that various modifications, both as to equipment details and operating procedures and parameters can be effected without departing from the scope of the claimed invention. For example, the duct burner  70  can be incorporated directly into the conduit through which the air  26  passes through the recuperator  28  as figuratively shown by reference characters  74  and  76  in FIG. 3, obviating the need for the blower  68 . This arrangement would work equally well provided the flow rate does not extinguish the burner flame.