Patent Publication Number: US-11047264-B2

Title: Power generation system and method with partially recuperated flow path

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/495,349, filed Apr. 24, 2017 which is a continuation of U.S. patent application Ser. No. 14/632,672, filed Feb. 26, 2015, now U.S. Pat. No. 9,657,599, entitled Power Generation System And Method With Partially Recuperated Flow Path, that claims priority to and the benefit of U.S. Provisional Application No. 61/966,574, filed Feb. 26, 2014, the entire contents of which are incorporated by reference into this application in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a power generation system and related methods that use supercritical fluids, and in particular, to a power generation system and related methods where a portion of the supercritical fluid is recuperated. 
     BACKGROUND 
     Traditionally, thermodynamic power generation cycles, such as the Brayton cycle, employ an ideal gas, such as atmospheric air. Such cycles are typically open in the sense that after the air flows through the components of the cycle, it is exhausted back to atmosphere at a relatively high temperature so that a considerable amount heat generated by the combustion of fuel is lost from the cycle. A common approach to capturing and utilizing waste heat in a Brayton cycle is to use a recuperator to extract heat from the turbine exhaust gas and transfer it, via a heat exchanger, to the air discharging from the compressor. Since such heat transfer raises the temperature of the air entering the combustor, less fuel is required to achieve the desired turbine inlet temperature. The result is improved thermal efficiencies for the overall thermodynamic cycle. However, even in such recuperated cycles, the thermal efficiency is limited by the fact that the turbine exhaust gas temperature can never be cooled below that of the compressor discharge air, since heat can only flow from a high temperature source to a low temperature sink. More recently, interest has arisen concerning the use of supercritical fluids, such as supercritical carbon dioxide (SCO2), in closed thermodynamic power generation cycles. One such prior art system  1  is illustrated in  FIG. 1 . 
     As shown in  FIG. 1 , the prior art power generation system  1  includes compressors, turbines, combustors and heat exchangers arranged in a first Brayton cycle  402 , in which the working fluid is a supercritical fluid, and a second Brayton cycle  404 , in which the working fluid is ambient air. The system  1  therefore includes an SCO2 cycle flow path  406  and air breathing cycle flow path  423 , which may be separate from each other. 
     In  FIG. 1 , the flow of SCO2 along flow path  406  is as follows. Initially, a stream A of supercritical fluid is supplied to the inlet of a compressor  408 . The supercritical fluid enters the inlet of the compressor  408  after it has been cooled and expanded to a temperature and pressure that is close to its critical point. The supercritical fluid is supplemented by a supercritical fluid source  431 . After compression in the compressor  408 , the stream B of SCO2 is heated in a cross cycle heat exchanger  410 , which is connected to the SCO2 flow path  406  and air breathing flow path  423 . The stream C of heated SCO2 from the heat exchanger  410  is then directed to the inlet of a turbine  412 , where the SCO2 is expanded and produces shaft power that drives both the SCO2 compressor  408  and an output device  416  by shaft  417 . The output device  416  can be a turboprop, turbofan, gearbox or generator. After expansion in the turbine  412 , the stream D of SCO2 is cooled in a second cross cycle heat exchanger  418 , also connected to the SCO2 flow path  406  and air breathing flow path  423 . The stream A of cooled SCO2 is returned to the inlet of the compressor  408  via the flow path  406 . In the air breathing Brayton cycle  404 , initially, ambient air  411  is supplied to a compressor  420 . The stream E of compressed air from the compressor  420  is then heated in the heat exchanger  418  by the transfer of heat from the SCO2 after the SCO2 has been expanded in the turbine  412 . The stream F of heated compressed air is then directed to a combustor  424 . The combustor  424  receives a stream  427  of fuel, such as jet fuel, diesel fuel, natural gas, or bio-fuel, is introduced by a fuel controller  428  and combusted in the air so as to produce hot combustion gas. The stream G of the combustion gas from the combustor  424  is directed to the heat exchanger  410  where heat is transferred to the SCO2, as discussed above. After exiting the heat exchanger  410 , the stream H of combustion gas is expanded in a turbine  426 , which produces power to drive the air compressor  420 , via shaft  421 . After expansion in the turbine  426 , the combustion gas I is exhausted to atmosphere. 
     While the supercritical-ambient fluid cycle power generation system  1  shown in  FIG. 1  can be advantageous, the heat exchangers required to transfer heat between the supercritical fluid cycle and the ambient cycle may be large, expensive, and impractical to implement. More effectively managing flow cycles can improve heat transfer efficiency in power generation systems that employ supercritical fluid cycles. 
     SUMMARY 
     An aspect of the present disclosure is a method for generating power in a system that includes a supercritical fluid cycle having a supercritical fluid flowing therethrough, an air-breathing cycle having air flowing therethrough that does not mix with the flow of the supercritical fluid. The method includes the step of directing air along the air-breathing cycle to flow through a plurality of heat exchangers. The method includes compressing the supercritical fluid in a supercritical fluid compressor along the supercritical fluid cycle and splitting the supercritical fluid discharged from the supercritical fluid compressor into first and second discharge streams of compressed supercritical fluid, such that the first discharge stream of compressed supercritical fluid flows through a recuperating heat exchanger. The method includes mixing the supercritical fluid discharged from the recuperating heat exchanger with the second discharge stream of compressed supercritical fluid and directing a mixture of compressed supercritical fluid through one of the plurality of heat exchangers arranged and into an inlet of a supercritical fluid turbine, such that heat from the air along the air-breathing cycle is transferred to the mixture of compressed supercritical fluid. The method includes splitting the supercritical fluid discharged from the supercritical fluid turbine into a first and second discharge streams of expanded supercritical fluid such that the first discharge stream of expanded supercritical fluid flows through the recuperating heat exchanger so as to heat the first discharge stream of compressed supercritical fluid. In addition, the method includes mixing the expanded supercritical fluid discharged from the recuperating heat exchanger with the second discharge stream of expanded supercritical fluid. The mixture of expanded supercritical fluid is directed toward the inlet of the supercritical compressor, wherein heat from the mixture of expanded supercritical fluid is transferred to the air of the air-breathing cycle, thereby cooling the mixture of expanded supercritical fluid to approximately its critical point. 
     Another aspect of the present disclosure is a system configured to generate power. The system includes a supercritical fluid cycle. The supercritical fluid cycle includes a supercritical fluid compressor configured to receive and compress a supercritical fluid, a supercritical fluid turbine configured to receive and expand the supercritical fluid, and a recuperating heat exchanger configured to receive discharge streams from the supercritical fluid compressor and the supercritical fluid turbine. The system also includes an air breathing cycle configured to heat air flowing along the air breathing cycle. The system further includes a plurality of heat exchangers arranged so that supercritical fluid from the supercritical fluid cycle and air from the air breathing cycle passes therethrough but does not intermix. The system is configured to: 1) split the supercritical fluid discharged from the supercritical fluid compressor into first and second discharge streams of compressed supercritical fluid, such that a) the first discharge stream of compressed supercritical fluid flows through the recuperating heat exchanger, and b) the second discharge stream of compressed supercritical fluid flows through one set of the plurality of heat exchangers; and 2) split the supercritical fluid discharged from the supercritical fluid turbine into a first and second discharge streams of expanded supercritical fluid such that a) the first discharge stream of expanded supercritical fluid flows through the recuperating heat exchanger, and b) the second discharge stream of expanded supercritical fluid flows through a different set of the plurality of heat exchangers. Heat from the first discharge stream of expanded supercritical fluid is transferred to the first discharge stream of the compressed supercritical fluid in the recuperating heat exchanger. 
     Another aspect of the present disclosure is a system configured to generate power. The system includes a supercritical fluid cycle. The supercritical fluid cycle includes a supercritical fluid compressor configured to receive and compress a supercritical fluid, a supercritical fluid turbine configured to receive and expand the supercritical fluid, and a recuperating heat exchanger configured to receive discharge streams from the supercritical fluid compressor and the supercritical fluid turbine. The system also includes an air breathing cycle configured to heat air flowing along the air breathing cycle. The system also includes a plurality of heat exchangers arranged so that supercritical fluid from the supercritical fluid cycle and air from the an air breathing cycle passes therethrough but does not intermix, wherein a first heat exchanger of the plurality of heat exchangers is arranged to feed into an inlet of the supercritical fluid turbine, and a second heat exchanger of the plurality of heat exchangers is arranged to feed into an inlet of the supercritical fluid compressor. The first heat exchanger has a first heat capacity rate and the second heat exchanger has a second heat capacity rate that is substantially different than the first heat capacity rate. Further, the system is configured to: 1) split the supercritical fluid discharged from the supercritical fluid compressor into first and second discharge streams of compressed supercritical fluid, such that a) the first discharge stream of compressed supercritical fluid flows through the recuperating heat exchanger, and b) the second discharge stream of compressed supercritical fluid flows through the first heat exchanger of the plurality of heat exchangers; and 2) split the supercritical fluid discharged from the supercritical fluid turbine into a first and second discharge streams of expanded supercritical fluid such that a) the first discharge stream of expanded supercritical fluid flows through the recuperating heat exchanger, and b) the second discharge stream of expanded supercritical fluid flows through the second heat exchanger of the plurality of heat exchangers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of an aspect, are better understood when read in conjunction with the appended diagrammatic drawings. For the purpose of illustrating the invention, the drawings show an aspect that is presently preferred. The invention is not limited, however, to the specific instrumentalities disclosed in the drawings. In the drawings: 
         FIG. 1  is a schematic diagram of a prior art power generation system incorporating a supercritical fluid; 
         FIG. 2  is a schematic diagram of a power generation system according to an aspect of the disclosure; 
         FIG. 3  is a schematic diagram of a power generation system according to another aspect of the disclosure; 
         FIG. 4  is a schematic diagram of a power generation system according to another aspect of the disclosure; 
         FIG. 5  is a chart illustrating heat exchanger capacity rate ratios for supercritical fluid and air according to the prior art power generation system illustrated in  FIG. 1 ; 
         FIG. 6  is a chart showing delta temperature between supercritical flow and air flow along a heat exchanger from inlet-to-exit as a function of fin location according to the prior art system illustrated in  FIG. 1 ; 
         FIG. 7  is a chart illustrating temperature as a function of fin station in a first heat exchanger according to an aspect of the disclosure; and 
         FIG. 8  is a chart illustrating temperature as a function of fin station in a third heat exchanger along a partially recuperated cycle according to an aspect of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  is a schematic diagram of a power generation system  100  according to an aspect of the disclosure. The power generation system  100  includes a first closed Brayton cycle  102 , in which the working fluid may be a supercritical fluid, and a second open Brayton cycle  104 , in which the working fluid may be ambient air. The first Brayton cycle  102  and the second Brayton cycle  104  include a supercritical fluid flow path  106  and an air fluid flow path  108 , respectively. The flow paths  106  and  108  are, in one aspect, separate so that little or no mixing occurs between the supercritical fluid and air between the two flow paths  106  and  108 . 
     The power generation system  100  includes compressors, turbines, one or more combustors, and a plurality of heat exchangers connected along the flow paths  106  and  108 . The heat exchangers include a plurality of cross-cycle heat exchangers  132 ,  134 ,  136 , and  138 . As used herein, the term “cross cycle heat exchanger” refers to a heat exchanger that receives air or both air and combustion gas from the air breathing cycle  104  as well as a supercritical fluid from the supercritical fluid cycle  102  and transfers heat between the fluids in the two cycles. Furthermore, the power generation system  100  includes a recuperating heat exchanger  130  along the supercritical fluid flow path  106 . As used herein, the term “recuperating heat exchanger” refers to heat transfers between the supercritical fluid discharged from the SCO2 turbine and the supercritical fluid discharged from the SCO2 compressor in the supercritical fluid cycle  102 . The power generation system  100  also may include valves  122 , flow meters  140 , mixing junctions  124 , and one or more controllers configured to control operation of the system  100 . 
     Initially, a stream 2 of supercritical fluid is supplied to the inlet of a compressor  110 , which may be an axial, radial, reciprocating or the like type compressor. The compressor  110  may be referred to as first SCO2 compressor  110 . The compressor  110  includes a shaft  112  operably connected to a turbine  114 . The turbine  114  may be referred to as first SCO2 turbine  114 . The flow meter  140  along the stream 2 measures a flow rate of the supercritical fluid supplied to the compressor inlet. The flow meter  140  facilities control of total SCO2 mass in the supercritical fluid cycle  102  as well as transient flow behavior. In one aspect, the supercritical fluid enters the inlet of the SCO2 compressor  110  after it has been cooled and expanded, as discussed below, to a temperature and pressure that is close to its critical point. The term “supercritical fluid” refers to a fluid in which distinct liquid and gaseous phases do not exist, and term “critical point” of a supercritical fluid refers to the lowest temperature and pressure at which the substance can be said to be in a supercritical state. The terms “critical temperature” and “critical pressure” refer to the temperature and pressure at the critical point. For carbon dioxide, the critical point is approximately 304.2° K and 7.35 MPa. In one aspect, the supercritical fluid entering the compressor  110  is cooled to within at least ±2° K of its critical point. In a further aspect, the supercritical fluid entering the compressor  110  is cooled to within ±1° K of its critical point. In yet another aspect, the supercritical fluid entering the compressor  110  is cooled to within ±0.2° K of its critical point. 
     After compression in the SCO2 compressor  110 , the discharge stream 4 of the supercritical fluid is split into first and second portions as first and second discharge streams 6 and 8. The streams 6 and 8 may be referred to herein as compressor discharge streams 6 and 8. The split permits the first portion of the discharge stream 4 from the compressor  110  to be recuperated and the remaining portion to be heated directly by a series of heat exchangers  134  and  132  by air fluid cycling through the flow path  108 . As illustrated, the discharge stream 4 is split via valve  122   a  which can be in electronic communication with a controller (not shown). The controller operates or actuates the valve  122   a  to direct flow through the flow path  106  as needed. In one aspect, the valve  122   a  is configured to direct between 55% to about 75% of the discharge stream 4 into the first discharge stream 6. The balance of the flow of the discharge stream 4 is directed to the second discharge stream 8. In another aspect, the valve  122   a  is configured to direct about 67% of the discharge stream 4 into the first discharge stream 6. 
     The first discharge stream 6 of the supercritical fluid is directed to the recuperating heat exchanger  130  where heat is transferred from the heated SCO2 exiting turbine  116  to the first discharge stream 6. The stream 19 of the heated SCO2 discharged from the recuperating heat exchanger  130  is directed to the junction  124   a  and mixed with the stream 10 of heated SCO2 that exits the cross-cycle heat exchanger  134 . 
     The second discharge stream 8 from the SCO2 compressor  110  is directed to the cross cycle heat exchanger  134 . In the cross cycle heat exchanger  134 , the heat from the combustion gas in flow path  108  is transferred to the second discharge stream 8 of SCO2. The stream 10 discharged from heat exchanger  134  mixes with stream 19 of SCO2 from recuperating heat exchanger  130  at junction  124   a , as discussed above. The junction  124   a  may be joint that is connected to conduits or it may include a mixing apparatus. 
     The mixed stream 12 is supplied to the cross cycle heat exchanger  132 . In the cross cycle heat exchanger  132 , heat is transferred from the combustion gas in the flow path  108  to the mixed stream of SCO2. The cross cycle heat exchanger  132  discharges the stream 14 of heated SCO2. 
     The stream 14 of heated SCO2 from the heat exchanger  132  is directed to the inlet of the first SCO2 turbine  114 . The first SCO2 turbine  114  may be an axial, radial, mixed flow, or the like type turbine. The first SCO2 turbine  114  expands the SCO2 and produces shaft power that drives the SCO2 compressor  110 , via shaft  112 . After expansion in the first SCO2 turbine  114 , the stream 15 is cycled through a second SCO2 turbine  116  that produces shaft power for a generator  120 , via the shaft  118 . The generator  120  can provide output power for the system  100 . In an alternate aspect, the cycle  102  may include one turbine  114  with the shaft  118  connected to the turbine  114  and the generator  120 . In such an aspect, the discharge stream 16 would discharge from the turbine  114  into a valve  122   b.    
     The discharge stream 16 from the second SCO2 turbine  116  may be split into second and first portions as the discharge stream 18 and the discharge stream 22. The discharge stream 18 and the discharge stream 22 may be referred to as second and first discharge streams 18 and 22. As illustrated, the valve  122   b  can spilt the discharge stream 16 into the second and first discharge streams 18 and 22. The controller operates or actuates the valve  122   b . In one aspect, the valve  122   b  is configured to direct between 70% to about 90% of the discharge stream 16 into the first discharge stream 22. The balance of the flow of the discharge stream 16 is directed to the second discharge stream 18. In another aspect, the valve  122   b  is configured to direct about 80% of the discharge stream 16 into the first discharge stream 22. Regardless of how the SCO2 turbine discharge stream 16 is spilt, the second discharge stream 18 is directed to the cross cycle heat exchanger  136  and cooled by the flow of air passing through the heat exchanger  136  along the flow path  108 . 
     The first discharge stream 22 is directed to the recuperating heat exchanger  130 , where heat from the discharge stream 22 is transferred to first discharged stream 6 from the SCO2 compressor  110 . In other words, the recuperating heat exchanger  130  cools the discharge stream 22 of SCO2. The discharge stream 24 of the cooled SCO2 from the recuperating heat exchanger  130  is mixed with an incoming stream 20 from the heat exchanger  136  at a junction  124   b . From the junction  124   b , the mixed stream 26 is directed to the cross-cycle heat exchanger  138  which may be optional). For instance, mixed stream 26 may be directed directly to the compressor  110 . As noted above, in the cross-cycle heat exchanger  138 , heat from the mixed stream 26 of SCO2 is transferred to the flow path  108  of the air cycle  104 . The stream 28 of cooled SCO2 is directed through a cooler  126  (which may be optional) and is returned to the inlet of the SCO2 compressor  110  as stream 2. Additional SCO2 from a supply  109  can be introduced into the stream 2 of SCO2 directed to the SCO2 compressor  110  to make up for any leakage of SCO2 from the system. In any event, the SCO2 stream 2 is returned to the inlet of the compressor  110  and the steps of compressing-heating-expanding-cooling are repeated. 
     Continuing with  FIG. 2 , the air breathing cycle  104  portion of the overall system  100  forms an open flow path  108 . Initially, ambient air  101  is supplied to an air breathing compressor  150  which may be an axial, radial reciprocating, or like type compressor. The compressor  150  includes a shaft  152  operably connected to a turbine  154 . The stream 30 of compressed air from the compressor  150  is then heated in the heat exchanger  138  (which may be optional) by the transfer of heat from the mixed stream 26 of SCO2 discharged from the turbine  116  via the heat exchangers  130  and  136  as discussed above. The stream 32 of heated compressed air is then directed to the heat exchanger  136 , where heat from the stream 18 of SCO2 (from SCO2 turbine  116 ) is transferred to the stream 32 of compressed air. The discharge stream 34 is the directed to the combustor  158 . The combustor  158  raises the temperature of the compressed air stream 34 above the required temperature at the turbine inlet of turbine  154 . The compressor  150  can operate via shaft  152  powered by turbine  154 . The combustor  158  can receive a stream of fuel  103 , such as fossil fuels or other fuel type. The combustor  158  can operate by means of a solar collector or nuclear reactor to produce system heat or some may other heat source of heat including combustion of waste, biomass, or bio-derived fuels. The discharge stream 36 of the combustion gas from the combustor  158  may be directed to the turbine  154 , where it is expanded. The stream 40 of expanded hot combustion gas is directed to the heat exchanger  132 , where heat is transferred from the hot combustion gas to the mixed stream 12 of SCO2 discussed above. After exiting the heat exchanger  132 , the stream 41 of hot combustion gas is directed to the heat exchanger  134 , where heat is transferred from the hot combustion gas to the discharge stream 8 of SCO2 from the SCO2 compressor  110 , as discussed above. The discharge stream  107  of the heat exchanger  134  may be exhausted into atmosphere. 
     In operation, the power generation system  100  will be described with reference to predicted results. For instance, the heat capacity rate can be determined by multiplying mass flow rate times the specific heat Cp, or mdot*Cp. The heat exchangers  136  and  134  have mis-matched heat capacity rates because they operate in the regime of temperatures where supercritical fluid, such as SCO2, has a more linear and flat specific heat Cp curve. See for example  FIG. 4 . Because the heat capacity rates at these locations are not well matched, an air mass flow rate in the air breathing cycle  104  can be lower compared to prior art system  1  shown in  FIG. 1 . An aspect of the present disclosure includes storing heat by creating a large difference in temperature ranges of the two flows and mis-matching the heat capacity rate, which can avoid the heat pinch point problem associated with the prior art system. In one example, the supercritical fluid cycle  102  in the power generation system  100  can have a mass flow rate between about 30 and 35 Kg/sec. The air cycle  104  in the power generation system  100  can have a mass flow rate between about 7.5 and about 16.0 Kg/sec. However, the mass flow rates stated herein are not considered limiting. They may be higher or lower than the ranges provided. Furthermore, the power generation system  100  is configured to have a ratio of air mass flow rate to supercritical fluid mass flow rate of between about 0.25 and 0.50. In one aspect, the ratio of the mass flow rates is approximately 0.30. Accordingly, the mass flow rates for the air in the air-breathing cycle  104  are generally lower compared to typical power generation systems. In just one example, the air mass flow rates are about 75% below the air mass flow rates in a prior art power generation system  1 , such as the aspect shown in  FIG. 1  and described above. Reduced air mass flow can result in a substantial reduction in heat exchanger size, footprint, cost, weight, parasitic power requirements and the like. 
     Turning to  FIG. 3 , which is a schematic diagram of a power generation system  200  according to another aspect of disclosure configured to generate power and heat. The power generation system  200  is similar to the aspect shown in  FIG. 2 , and includes a first or supercritical fluid cycle  202  and a second or air-breathing cycle  204 . The first and second cycles  202  and  204  include a supercritical fluid flow path  206  and an air fluid flow path  208 , respectively, that are in one aspect separate from each other such the supercritical fluid and air does not intermix. Furthermore, the power generation system  200  includes compressors, turbines, one or more combustors, at least one recuperating heat exchanger  230 , a plurality of cross-cycle heat exchangers  232 ,  234 ,  236 , and  238 , as well as valves  222 , flow meters, mixing junctions  224 , and one or more a controllers configured to control operation of the system. 
     Initially, a stream 42 of supercritical fluid is supplied to the inlet of a compressor  210 . The compressor  210 , sometimes referred to as the first SCO2 compressor  210 , includes a shaft  212  operably connected to the first turbine  214 , also referred to as the first SCO2 turbine  214 . An optional flow meter (not shown) can be used measure the flow rate of the fluid supplied to the compressor inlet. The stream 42 of the supercritical fluid enters the inlet of the compressor  110  after it has been cooled and expanded to a temperature and pressure that is close to its critical point. 
     After compression in the compressor  210 , the stream 44 of supercritical fluid is split into first and second portions as streams 46 and 48. The streams 46 and 48 may be referred to as first and second discharge streams 46 and 48, respectively. A valve  222   a  can split the stream 44 into the first and second discharge streams 46 and 48. The first discharge stream 46 of the supercritical fluid is supplied to the recuperating heat exchanger  230 . In the recuperating heat exchanger  230 , heat is transferred from the heated SCO2 discharge from a turbine  216  to the first discharge stream 46 from the SCO2 compressor  210 . The stream 50 of heated SCO2 discharged from heat exchanger  230  is directed to a junction  224   a  and mixed with the stream 74 of heated SCO2 from a cross-cycle heat exchanger  234 . 
     The second discharge stream 48 is directed to a valve  222   b , which directs the stream 70 through an optional heat exchanger  233  and into the cross cycle heat exchanger  234 . Exchanger  233  can be used to capture waste heat from avionics and weapons systems that are installed in moving platforms like aircraft, surface vessels, etc. The system  200  may not include heat exchanger  233  in every application or implementation. In the cross cycle heat exchanger  234 , the heat is transferred from the combustion gas in the flow path  208  to the discharge stream 70 of SCO2. The stream 74 discharged from heat exchanger  234  mixes with stream 50 at a junction  224   a . The junction  224   a  may be a joint or may include a mixing apparatus. The stream 51 is supplied to another junction  224   b  and combined with the discharge stream 72 from a cooler  219 . The valve  222   b  also may direct a portion of a second discharge stream 48 to the cooler  219  disposed along a shaft  218 . The discharge stream 72 from the cooler  219  is routed to the junction  224   b  combined with the stream 51 into mixed the stream 52. The mixed stream 52 is supplied to the cross cycle heat exchanger  232 . In the cross cycle heat exchanger  232 , heat from the combustion gas in the flow path  108  is transferred to the mixed stream 52. The discharge stream 54 of heated SCO2 from the cross cycle heat exchanger  232  is directed to the inlet of the first SCO2 turbine  214 . 
     The first SCO2 turbine  214  expands the SCO2 and produces shaft power that drives the SCO2 compressor  210 , via shaft  212 . After expansion in the first SCO2 turbine  214 , the stream 56 is cycled through a second SCO2 turbine  216  that produces shaft power for a generator  220 , via the shaft  218 . The generator  220  can provide output power for the system  200 . Alternatively, the stream 56 can bypass the turbine  216 . As illustrated, a valve  222   c  divides the stream 56 into a stream 57 directed toward the turbine  216  and the stream 58 directed toward the heat exchangers  130  and  236 . The stream 59 discharged from the turbine  216  flows to a junction  224   c  and is combined with the stream 58 to define a discharge stream 60. 
     The discharge stream 60 is directed to a valve  222   d , which splits the discharge stream 60 from the turbine  216  into a second discharge stream 62 and a first discharge stream 66. The second discharge stream 62 is directed to a cross cycle heat exchanger  236  and heated by the flow of air along a flow path  208  through the heat exchanger  236 . The discharge stream 64 discharged from the heat exchanger  236  is directed toward the heat exchanger  238 . The first discharge stream 66 of SCO2 is directed to the recuperating heat exchanger  230 , where its heat is transferred to the first discharge stream 4 of SCO2 from the SCO2 compressor  210 . The discharge stream 68 from the recuperating heat exchanger  230  is mixed with a discharge stream 64 from the heat exchanger  236  at a junction  224   d , forming a mixed stream 69. The mixed stream 69 of SCO2 is directed to the heat exchanger  238 , where heat from the SCO2 fluid is transferred to compressed air along the flow path  208  of the air cycle  204 . The stream 28 of cooled SCO2 is directed through a cooler  226  (which may be optional) and is returned to the inlet of the SCO2 compressor  210  via the flow path  206 . A water input  225   a  may supply water to a cooler  226 . The output stream  225   b  of the cooler  226  is heated water, which can be used as a heat source. Additional SCO2 from a supply  207  can be introduced into the stream 42 of SCO2 directed to the compressor  210  to make up for any leakage of SCO2 from the system. In any event, the SCO2 stream  202  is returned to the inlet of the compressor  210  and the steps of compressing-heating-expanding-cooling are repeated. In an alternative aspect, yet another heat exchanger  239   a  is placed along stream 68. A water input  239   b  may supply water to exchange  239   a . The output stream  239   c  of the heat exchanger  239   a  is heated water which can be used as a heat source for district heating. District heating generally requires water temps of 180 F or better, including heat exchanger  239   a  can help ensure output stream temperature of about 180 F or better, as opposed to system  200  that include only the cooler  226 . Accordingly, the system  200  may include cooler  226  or heat exchanger  239   a . In still other alternatives, the system  200  can include both cooler  226  and heat exchanger  239   a.    
     Continuing with  FIG. 3 , the air breathing cycle  104  portion of the overall system  200  forms open flow path  208 . Initially, ambient air  201  is supplied to a forced draft fan  250  which may be axial, radial, reciprocating, or similar type compressor. The forced draft fan  250  is driven by shaft  252  powered by a power source  254 . The power source  254  can be a motor. The stream 80 of compressed air from the forced draft fan  250  is then heated in the heat exchanger  238  by the transfer of heat from the mixed stream 69 of SCO2 (discharged from turbine  216  and cooled in the heat exchanger  230  and  236 ). The air stream 82 of heated compressed air is then directed the heat exchanger  236 , where heat from the second discharge stream 62 of heated SCO2 is transferred to an air stream 82. The air stream 84 is fed to a combustor  258  into which a fuel  203  (such as a fossil fuel, heat from solar conductor, nuclear reactor, or the like is supplied) is introduced by a fuel controller and combusted in the air so as to produce hot combustion gas. The stream 86 of the combustion gas from the combustor  258  is directed to a heat exchanger  232 , where heat is transferred from the stream 86 of hot combustion gas to the mixed stream 52 of SCO2 discussed above. The stream 88 of hot combustion gas directed to the heat exchanger  234 , where heat is transferred from the hot combustion gas to the stream 74 of compressed SCO2, as discussed above. The discharge stream 90 of the heat exchanger  234  may be directed to an induced draft fan  260 , which may be a compressor. The induced draft fan  260  may be connected to a shaft  262 , which is powered by a power source  264 , such as a motor. The stream of gas may be exhausted from the induced draft fan  260  to atmosphere. The purpose of both forced draft fan  250  and induced draft fan  260  is to drive flow through the heat exchangers and combustor and to overcome the pressure drop associated with them. It should be appreciated that the forced draft fan  250  may not be needed based on the type of fuel burnt in the combustor. For instance, a forced draft fan  250  is useful when it is desirable for the combustion zone to be sub-atmospheric pressure in the case of burning biomass where fuel is introduced through an open door. If, however, the combustor can be pressurized, as in the case of burning fossil fuels, the induced fan  260  is not necessary. 
     In operation and as described above with respect to the system  100 , the heat exchangers  236  and  234  have mis-matched heat capacity rates because they both operate in the regime of temperatures where the supercritical fluid has a more linear and flat heat capacity rate curve. Because the heat capacity rates at these locations are not well matched, an air mass flow rate in the air breathing cycle  204  can be lower compared to prior art system  1  shown in  FIG. 1 . An aspect of the present disclosure includes storing heat by creating a large difference in temperature ranges of the two flows and mis-matching the heat capacity rate, which can avoid the heat pinch point problem associated with the prior art system. In one example, the supercritical fluid cycle  202  in the power generation system  200  can have a mass flow rate between about 30 and 35 Kg/sec. The air cycle  204  in the power generation system  200  can have a mass flow rate between about 7.5 and about 16.0 Kg/sec. However, the mass flow rates stated herein are not considered limiting. They may be higher or lower than the ranges provided. Furthermore, the power generation system  200  is configured to have a ratio of air mass flow rate to supercritical fluid mass flow rate of between about 0.25 and 0.50. In one aspect, the ratio of the mass flow rates is approximately 0.30. Accordingly, the mass flow rates for the air in the air-breathing cycle  204  are generally lower compared to typical power generation systems. In just one example, the air mass flow rates are about 75% below the air mass flow rates in a prior art power generation system  1 . 
     Turning to  FIG. 4 , which is a schematic diagram of a power generation system  300  according to another aspect of disclosure. The power generation system  400  is substantially similar to the power generation system  100  shown in  FIG. 2  and described above. The description below will use the same reference numbers to identify elements that are common between power generation system  100  and power generation system  300 . Accordingly, the power generation system  300  a supercritical fluid cycle  402  and an air breathing cycle  404 . Furthermore, the power generation system  300  includes compressors, turbines, one or more combustors, and a plurality of heat exchangers connected along the flow paths  306  and  308 . The heat exchangers include a plurality of cross-cycle heat exchangers  132 ,  134  and  136  along the flow path  308 , and a recuperating heat exchanger  130  along the supercritical fluid flow path  306 . The power generation system  300  also may include valves  122 , flow meters  140 , mixing junctions  124 , and one or more controllers configured to control operation of the system  300 . As noted above, the power generation system  300  operates substantially similar to the power generation system  100 . 
     In accordance with the alternative aspect of the disclosure, however, the power generation system  300  does not include a terminal heat exchanger  138  that discharges stream 28 toward the inlet of the compressor  110  (see  FIG. 2 ). In accordance with the power generation system  300 , the valve  122   b  divides the discharge stream 16 from the second SCO2 turbine  116  into second discharge stream  318  and a first discharge stream  322 . In one aspect, the controller operates or actuates the valve  122   b  to direct between 70% to about 90% of the discharge stream 16 into the first discharge stream  322 . The balance of the flow of the discharge stream 16 is directed to the second discharge stream  318 . In another aspect, the valve  122   b  is configured to direct about 80% of the discharge stream 16 into the first discharge stream  322 . Regardless of how the SCO2 turbine discharge stream 16 is spilt, the second discharge stream  318  is directed to the cross cycle heat exchanger  136  and cooled by the flow of air passing through the heat exchanger  136  along the flow path  408 . 
     The first discharge stream  322  is directed to the recuperating heat exchanger  130 , where heat from the discharge stream  322  is transferred to first discharged stream 6 from the SCO2 compressor  110 . The discharge stream  324  of the cooled SCO2 from the recuperating heat exchanger  130  is mixed with an incoming stream 20 from the heat exchanger  136  at a junction  124   b . From the junction  124   b , the mixed stream  328  is directed to the compressor  110 . As illustrated, the stream  328  of cooled SCO2 is directed through a cooler  126  (which may be optional) and is returned to the inlet of the SCO2 compressor  110  as stream 2. In any event, the SCO2 stream 2 is returned to the inlet of the compressor  110  and the steps of compressing-heating-expanding-cooling are repeated. 
     Continuing with  FIG. 4 , the air breathing cycle  304  portion of the overall system  300  forms an open flow path  408 . Initially, ambient air  101  is supplied to an air breathing compressor  150 . The stream 30 of compressed air from the compressor  150  is directed to heat exchanger  136  and is heated by the transfer of heat from the stream  318  of SCO2 discharged from the turbine  116 . The discharge stream 34 is the directed to the combustor  158 . The discharge stream 36 of the combustion gas from the combustor  158  may be directed to the turbine  154 , where it is expanded. The stream 40 of expanded hot combustion gas is directed to the heat exchanger  132 , where heat is transferred from the hot combustion gas to the mixed stream 12 of SCO2 as discussed above. After exiting the heat exchanger  132 , the stream 41 of hot combustion gas is directed to the heat exchanger  134 , where heat is transferred from the hot combustion gas to the discharge stream 8 of SCO2 from the SCO2 compressor  110 . The discharge stream  107  of the heat exchanger  134  may be exhausted into atmosphere. 
     The power generation system  300  requires fewer cross-cycle heat exchangers compared to other aspects of the present disclosure. Furthermore, it should be appreciated that the power generation system  200  can be implemented without the need for heat exchanger  238 . In such an example, the stream 69 is directed to directly to the optional cooler and then the inlet of compressor  210 . Furthermore, on the air-breathing cycle  204 , the discharge stream 80 is directed into heat exchanger  236  and the cycle continues as disrobed above. 
     The power generation systems  100 ,  200 , and  300  described above have several advantages over typical supercritical power generation systems and/or or other non-supercritical fluid based systems. Reduced heat exchanger size, improved thermal efficiency, and lower thermal signature at exhaust are a few notable improvements. The alternative heat exchanger flow strategy—whereby SCO2 discharge flows from the SCO2 compressor and SCO2 turbine are spilt—mitigates a so-called heat exchanger “pinch point” in the prior art system  1 . More specifically, the prior art system  1  has a variable heat capacity mismatch at heat exchanger  418  ( FIG. 1 ) on the low pressure side. The variable mismatch is based on a mismatch between the heat capacity rate of the air and SCO2 flows that are exchanging heat in the heat exchanger  418 . For instance, as shown in  FIG. 4 , the air in the heat exchanger  418  has a fairly linear heat capacity rate curve across its operating temperatures. The supercritical fluid, however, has a spike in the heat capacity rate at the lower temperature range where the SCO2 discharge end of the heat exchanger  418  operates. The effect of this spike in heat capacity rate is illustrated in  FIG. 6 .  FIG. 6  displays delta temperature (ΔT) between the inlet and exit ends of the heat exchanger  418  for both SCO2 flow and air flow as function of S-fins from the SCO2 inlet. The different curves are different sized heat exchangers. For instance, the curve “100-Sfin Hx” would indicate a larger heat exchanger compared to the heat exchanger associated with the “50-Sfin Hx” curve. As noted above, the spike heat capacity rate of SCO2 at lower temperature at the SCO2 discharge end indicates that the heat exchanger length should be increased in order to create effective heat transfer. But as shown in  FIG. 6 , a relative low ΔT observed from 50-fins location to about the 100-fins location relative to the SCO2 inlet end of heat exchanger for the 100-Sfin HX curve. This suggests that the section of the heat exchanger which is doing the least amount of heat transfer is actually elongated. The result is that the prior art system  1  requires fairly large heat exchangers with limited or low performance and sometimes high loss of pressure, which can be detrimental to the system performance. Furthermore, large approach temperatures at either end of the heat exchanger  418  illustrate that a significant amount of heat is left un-transferred. 
     As described above, the power generation systems  100 ,  200 ,  300  splits the discharge flows of SCO2 from the SCO2 compressor  110 ,  210 , and the SCO2 turbine  116 ,  216 , between: A) the recuperating heat exchanger  130 ,  230 , and B) heat exchangers that feed into the respective inlets of the SCO2 turbine and SCO2 compressor. This split, in conjunction with the arrangement of the air-breathing cycle  104 ,  204 ,  304  results in airflow stream (see stream 40 in  FIGS. 2 and 4  and stream 86 in  FIG. 3 ) on the inlet side of the SCO2 turbines with a temperature above the desired temperature of SCO2 stream at inlet of SCO2 turbine  114 ,  214 . Furthermore, the splitting of SCO2 turbine discharge flow and SCO2 compressor discharge flow allows for the intentional mismatch of heat capacity rate at heat exchanger  138 ,  238  and heat exchanger  132 ,  232 , as in  FIGS. 2 and 3 . For the power generation system  300  shown in  FIG. 4 , this intentional mismatch of heat capacity rate would be between heat exchanger  136  and heat exchanger  132 . This in turn, permits somewhat large approach temperatures at the hot end of the heat exchanger  132 ,  232  and the cool end of heat exchanger  138 ,  238  in  FIGS. 2 and 3  and cool end of heat exchanger  136  in  FIG. 4 . The large approach temperatures alleviate the “pinch point” issue for these particular heat exchangers as used in prior art systems. For the power generation systems  100  and  200  shown in  FIGS. 2 and 3 , the heat exchangers  134 ,  234  and  136 ,  236 , however, have fairly well matched heat capacity rates because they operate in a range where SCO2 has a more linear Cp curve. In any event, the high approach temperatures at heat exchanger  132 ,  232  and heat exchanger  138 ,  238  increase the amount of heat exchanged per unit area of heat exchanger, further reducing heat exchanger size. And in at least some instances allows the elimination of the heat exchanger  138 ,  238 , as in the power generation system  300  shown in  FIG. 4 . 
     System heat can be added to by means of combustion of fossil fuels, a solar collector, a nuclear reactor, and/or similar heat source, thereby raising the temperature of the air flow to a value above the required high temperature at the inlet of the SCO2 turbines. Furthermore, because the heated combustion gas passes the majority of its heat to SCO2 streams via the heat exchangers  134 ,  234  and  132 ,  232 , very low exhaust gas temperatures result and thus reductions in thermal signature for applications where this is important, e.g., such as military applications. And because of the low compressibility factor associated with supercritical fluids, the discharge temperature from the SCO2 compressor is comparatively low and therefore ideal for receiving the heat energy from the heated combustion gas at the heat exchanger  134 ,  234  and the SCO2 discharge flow at the recuperating heat exchanger  130 ,  230 . These attributes result is high thermal efficiency of the system. 
     In alternative aspects, a power generation system includes more than one supercritical fluid cycle. In one example, the power generation system can include first and second supercritical fluid cycles, whereby one or both of the first and second supercritical fluid cycles spilt the SCO2 discharge from the SCO2 turbine and SCO2 compressor between A) a recuperating heat exchanger like  130 ,  230 , and B) respective heat exchanger in-line with inlets of a SCO2 turbine and a SCO2 compressor. In still other alternative aspects, a power generation system includes one or more air breathing cycles. In still other aspects, the air breathing cycle can include one or more reheat cycles. In still other aspects, a power generation system includes a vacuum cycle with one or more SCO2 cycles. In still other aspects, a power generation system includes steam injection. In still other aspects a power generation system includes a bottoming cycle using the low pressure discharge stream of heat exchangers  130 , 230  as a heat source. 
     Furthermore, the power generation system  100 ,  200 ,  300  an include various SCO2 and air breathing cycles as disclosed in U.S. Patent App. Pub. No. 2013/0180259 (the 259 publication) in combination with the alternative flow strategy as described herein. The disclosure of the SCO2 and the air breathing cycles in the 259 publication that are not inconsistent with the flow strategies as described above are incorporated herein by reference in its entirety. 
     In another alternative aspect, the power generation system  100 ,  200  as described herein includes a SCO2 turbine assembly that includes an eddy current torque coupling as disclosed in the 259 publication. The disclosure of the eddy current torque coupling in the 259 publication is incorporated by reference into this application in its entirety. 
     Applications for the power generation systems  100 ,  200 ,  300  include but are not limited to aircraft engines (such as turbo-fan, turbo-prop, or turbo-shaft engines), ground based electric power generators, naval propulsion systems, ground transportation engines, etc. Furthermore, other applications can include power and heat generation, such as steam and hot water. The systems can be used for any other application where shaft power is required. 
     The foregoing description is provided for the purpose of explanation and is not to be construed as limiting the invention. While the invention has been described with reference to preferred aspects or preferred methods, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Furthermore, although the invention has been described herein with reference to particular structure, methods, and aspects, the invention is not intended to be limited to the particulars disclosed herein, as the invention extends to all structures, methods and uses that are within the scope of the appended claims. Those skilled in the relevant art, having the benefit of the teachings of this specification, may effect numerous modifications to the invention as described herein, and changes may be made without departing from the scope and spirit of the invention as defined by the appended claims.