Patent Publication Number: US-2022213818-A1

Title: Dual-cycle system for combined-cycle power plant

Description:
TECHNICAL FIELD 
     This document pertains generally, but not by way of limitation, to combined-cycle power plants utilizing a gas turbine engine, a heat recovery steam generator, and a steam turbine. More specifically, but not by way of limitation, the present application relates to systems for increasing the efficiency of combined-cycle power plants via addition of a secondary cycle, such as that utilizing liquefied natural gas cold energy. 
     BACKGROUND 
     In a gas turbine combined-cycle (GTCC) power plant, a gas turbine engine can be operated to directly generate electricity with a generator using shaft power. Hot exhaust gas of the gas turbine engine can additionally be used to generate steam within a heat recovery steam generator (HRSG) that can be used to rotate a steam turbine shaft to further produce electricity. 
     Natural gas is frequently used in GTCC power plants as fuel for gas turbine engines. Natural gas is the second largest source of energy globally and is expected to remain in that position for the foreseeable future. A major component of the natural gas market is liquefied natural gas (LNG) which is used to transport natural gas worldwide. Typically, LNG is currently regasified through open rack vaporizers using heat from seawater at receiving terminals where the LNG is received. The regasification process results in localized cooling of the seawater, which presents environmental challenges including negative impacts on marine life. 
     Organic Rankine Cycles (ORCs) have been used to take advantage of cold energy available in LNG, using seawater as a heat source. However, such systems can be limited in their application. 
     Examples of liquid natural gas regasification and expansion systems are described in U.S. Pat. No. 9,903,232 to Amir et al.; U.S. Pat. No. 6,116,031 to Minta et al; and U.S. Pat. No. 4,320,303 to Ooka et al. 
     OVERVIEW 
     The present inventor has recognized, among other things, that problems to he solved in GTCC power plants can include inefficient utilization of the inherent cold energy from LNG. A significant amount of energy is consumed to cool and liquefy natural gas for producing low-temperature (about −160° C.) LNG that can be readily stored and transported. The inherent cold energy/energy available from the low-temperature LNG is not being effectively utilized during regasifi cation. 
     The present subject matter can help provide a solution to this problem and other problems, such as by using an Organic Rankine Cycle (ORC) to utilize low pressure water from a heat recovery steam generator (HRSG) as a heat source and LNG as a cold sink. In parallel, direct natural gas expansion cycle also produces electricity by expanding the pressurized and regasified fuel. The combination of an ORC cycle and an fuel expansion cycle (direct natural gas expansion cycle) into a dual-cycle system can be utilized to power an additional turbine for generating electricity, improving the overall efficiency of a GTCC power plant. 
     In an example, a gas turbine combined-cycle power plant can comprise a gas turbine engine, a heat recovery steam generator, a steam turbine, a fuel regasification system and a fuel expansion turbine (also referred to herein collectively as a “fuel regasification and expansion system”). The gas turbine engine can comprise a compressor for generating compressed air, a combustor that can receive a fuel and the compressed air to produce combustion gas, and a turbine for receiving the combustion gas and generating exhaust gas. The heat recovery steam generator can be configured to generate steam from water utilizing heat from the exhaust gas. The steam turbine can be configured to produce power from steam generated by the heat recovery steam generator. The fuel regasification system can be configured to be in fluid communication with and disposed upstream of the combustor for converting the fluid from a liquid to a gas. The fuel expansion turbine can be configured to be in fluid communication with and disposed downstream of the fuel regasification process for producing power from gasified fuel. 
     In another example, an Organic Rankine Cycle (ORC) system for operation with a gas turbine combined-cycle power plant can comprise a fluid pump for pumping a fluid, an ORC turbine in fluid communication with and disposed downstream form the fluid pump for expanding the fluid, a regasification system for a fuel configured to cool the fluid between an outlet of the ORC turbine and an inlet of the pump, a first heat exchanger positioned between an outlet of the pump and an inlet of the ORC turbine to heat the fluid with heat from a heat recovery steam generator of the gas turbine combined-cycle power plant, and a fuel expansion turbine to produce power from the regasified fuel before it enters a gas turbine engine of the gas turbine combined-cycle power plant. 
     In an additional example, a method of operating a gas turbine combined-cycle power plant can comprise circulating a working fluid through a closed loop using a working pump, heating the working fluid with a first heat exchanger using heat from the gas turbine combined-cycle power plant, expanding the heated working fluid through a working fluid turbine, condensing the working fluid leaving the turbine with a liquid fuel regasification process, expanding gas fuel through a fuel turbine, and generating electrical power with the working fluid turbine and the fuel turbine. 
     This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a conventional Gas Turbine Combined Cycle (GTCC) power plant operating a gas turbine in conjunction with a Heat Recovery Steam Generator (HRSG) and steam turbine. 
         FIG. 2  is a schematic diagram illustrating a Gas Turbine Combined Cycle (GTCC) power plant of the present application having a dual-cycle system using a working fluid turbine and natural gas turbine to generate additional power. 
         FIG. 3  is a schematic diagram illustrating a dual-cycle system incorporating the ORC system of  FIG. 2  and a liquid natural gas (LNG) regasification and expansion system. 
         FIG. 4  is a graph showing a temperature-entropy (T-s) diagram of the ORC system and the LNG regasification and expansion system cycles of  FIG. 3 . 
         FIG. 5  is a line diagram illustrating steps of a method for operating the ORC system and the LNG regasification and expansion system of  FIG. 3 . 
     
    
    
     In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram illustrating a conventional Gas Turbine Combined Cycle (GTCC) power plant  10  having gas turbine engine (GTE)  12 , Heat Recovery Steam Generator (HRSG)  14  and steam turbine  16 . GTE  12  can be used in conjunction with electrical generator  18 , and steam turbine  16  can be used in conjunction with electrical generator  20 . Power plant  10  can also include condenser  22 , fuel gas heater  30 , condensate pump  40  and feedwater pump  42 . HRSG  14  can include low pressure section  44 , intermediate pressure section  46  and high pressure section  48 . Condenser  22  can form part of a cooling system and can comprise a surface condenser with seawater once-through cooling. GTE  12  can include compressor  50 , combustor  52  and turbine  54 . Steam turbine  16  can include IP/HP spool  56  and LP spool  58 . 
     As will be discussed in greater detail below with reference to  FIGS. 2 and 3 , water can be supplied from HRSG  14  to provide heat exchanging functions with an Organic Rankine Cycle (ORC) system (ORC system  70  of  FIG. 3 ) and a Liquid Natural Gas (LNG) regasification and expansion system (LNG regasification and expansion system  72  of  FIG. 3 ). The operation of GTCC power plant  10  is described with reference to  FIG. 1  operating without ORC system  70  and LNG regasification and expansion system  72 . 
     Ambient air A can enter compressor  50 . The compressed air is fed to combustor  52  and mixed with fuel from fuel source  60 , which can be a source of natural gas or regasified LNG. The compressed air from compressor  50  is mixed with the fuel for combustion in combustor  52  to produce high energy gas for turning turbine  54 . Rotation of turbine  54  is used to produce rotational shaft power to drive compressor  50  and electrical generator  18 . Exhaust gas E is directed to HRSG  14 , where exhaust gas E interacts with appropriate water/steam piping in high pressure section  48 , intermediate pressure section  46  and low pressure section  44  to produce steam. The steam is routed to IP/HP spool  56  and LP spool  58  of steam turbine  16  via steam lines  61 C,  61 B and  61 A to produce rotational shaft power to operate electrical generator  20 . Exhaust gas E can exit HRSG  14  utilizing any appropriate venting means, such as a stack. HRSG  14  can additionally include appropriate means for conditioning exhaust gas E to remove potentially environmentally hazardous materials. For example, HRSG  14  can include a Selective Catalytic Reduction (SCR) emissions reduction unit. 
     Water from HRSG  14  can also be used to perform fuel heating at fuel gas heater  30  with water line  66 A, as is shown by arrows X-X, and water can then be returned to low pressure section  44  via lines  66 C and  66 D. 
     Heat remaining in flue gas downstream of low pressure section  44  of HRSG  14  is typically wasted, resulting only in an increase of the temperature of exhaust gas E exiting HRSG  14 . In the present disclosure, ORC system  70  ( FIG. 3 ) can be connected in thermal communication with HRSG  14  and low-temperature LNG from regasification and expansion system  72  ( FIG. 3 ) to turn one or more additional turbines for generating electrical power. 
       FIG. 2  is a schematic diagram illustrating a Gas Turbine Combined Cycle (GTCC) power plant  10  of  FIG. 1  modified according to the present disclosure to include ORC system  70  ( FIG. 3 ) that uses water from HRSG  14  as a heat source and. Liquified Natural Gas (LNG) from regasification and expansion system  72  ( FIG. 3 ) as a cold sink.  FIG. 2  utilizes the same reference numbers where appropriate to indicate the same or functionally equivalent components as  FIG. 1 , with new reference numbers are added to indicate additional components. 
     In particular, lines  74 A and  74 B are added to connect first heat exchanger  76  and second heat exchanger  78  into operation of HRSG  14 . In the illustrated example, heat exchangers  76  and  78  are shown connected in parallel. to other examples, heat exchangers  76  and  78  can be connected in series, with either one being configured to be first. As discussed with reference to  FIG. 3 , first heat exchanger  76  can comprise a portion of ORC system  70  and second heat exchanger  78  can comprise a portion of LNG regasification and expansion system  72 . ORC system  70  and LNG regasification and expansion system  72  together comprise dual-cycle system  80  that can be integrated into operating with GTCC power plant  10 , as shown in  FIG. 2 , to increase the overall efficiency and output of GTCC power plant  10 . 
     Line  74 A can be positioned to extract low pressure water from HRSG  14  at low pressure section  44 . In other examples, line  74 A can be connected to intermediate pressure section  46  or high pressure section  48 . In examples, line  74 A can be configured to extract steam from HRSG  14 . Additional low pressure water in line  74 A from low pressure section  44  contains heat that is otherwise wasted if it is not produced and utilized. ORC system  70  and regasification and expansion system  72  can utilize this readily available heat source, without impacting the performance of GTCC power plant  10 , to generate additional power and increase the overall efficiency of GTCC power plant  10 . Line  74 B can return the low pressure water that has been cooled by ORC system  70  and regassification and expansion system  72  in heat exchangers  76  and  78  to an inlet of low pressure section  44  to further cool exhaust gas E before exhaust gas E leaves HRSG  14  and is vented to atmosphere. 
       FIG. 3  is a schematic diagram illustrating dual-cycle system  80  including ORC system  70  and regasification and expansion system  72 . In an example, ORC system  70 , propane may be used as a working fluid, and ORC system  70  can include working fluid pump  82 , fourth heat exchanger (functioning as a recuperator)  84 , first heat exchanger (functioning as a propane superheater)  76 , working fluid turbine  86  and third heat exchanger (functioning as a propane condenser)  88 . Regasification and expansion system  72  can comprise fuel source  60 , fuel pump  90 , third heat exchanger (functioning as a fuel vaporizer and also herein referred to as a “gasification heat exchanger”)  88 , second heat exchanger (functioning as a fuel superheater)  78  and fuel turbine  92 . Working fluid turbine  86  and fuel turbine  92  can be configured to drive generator  94 . Regasification and expansion system  72  can be fluidly coupled to fuel gas heater  30  and combustor  52 . 
     As compared to the system of  FIG. 1 , additional power can be generated using working fluid turbine  86  and fuel turbine  92 . In ORC system  70 , heat energy can be extracted from GTCC power plant  10  from low pressure section  44  of HRSG  14  at heat exchanger  76 . Heat exchanger  88  can be used as a cold sink to condense the working fluid. Furthermore, in the regasification and expansion system  72 , heat energy can be extracted from GTCC power plant  10  from low pressure section  44  of HRSG  14  at heat exchanger  78 , which can increase the temperature of fuel fed to fuel turbine  92 . The dual-cycle system  80  can reduce temperature of exhaust gas E ( FIG. 2 ) leaving the HRSG. Because LNG has improved fuel quality (relative to standard natural gas) and does not contain Sulphur, it is acceptable for the stack temperature of the system of  FIG. 2  to be lower than a conventional GTCC power plant, such as that of  FIG. 1 . 
     In an embodiment, the working fluid of ORC system  70  can be propane (C 3 H 8 ). However, in other embodiments, other fluids can be used. For example, various organic compounds can be used. In other embodiments, CO 2 , hydro-carbon fluids, ammonia (NH 3 ) and H 2 S can be used. Although other fluids may yield increased thermal efficiency, propane is commonly used in the industry. 
       FIG. 3  has been provided with parenthetical reference numbers ( 1 )-( 13 ) to identify locations within dual-cycle system  80 . Locations ( 1 )-( 13 ) are described with reference to  FIG. 3  to discuss the operation of system  80 . Locations ( 1 )-( 13 ) are also mapped to a temperature-entropy (T-s) diagram in  FIG. 4  and a process flow chart in  FIG. 5 . 
     Low pressure water is extracted from HRSG  14  at location ( 1 ). This low-pressure water can be provided to first heat exchanger  76  and second heat exchanger  78  in parallel as shown in  FIG. 2 . After this low-pressure water has been cooled in heat exchangers  76  and  78 , e.g., after heat has been extracted from the low pressure water to increase the temperature of the working fluid in ORC system  70  and the fuel of regasification and expansion system  72 , the low-pressure water can be returned to HRSG  14  at location ( 2 ). 
     ORC system  70  can start at third heat exchanger  88 , which can function as a condenser for ORC system  70  and a gasifier for regasification and expansion system  72 . At third heat exchanger  88 , propane gas can be condensed to a liquid at location ( 3 ) and can flow into working fluid pump  82 . The liquid propane can be pumped by pump  82  to a higher pressure at ( 4 ) and then heated to a higher temperature using recuperator  84  at ( 5 ). First heat exchanger  76  can gasify and superheat the propane at ( 6 ). The superheated propane can then continue to working fluid turbine  86  where the superheated propane can be expanded at ( 7 ). Finally, the propane can pass through recuperator  84  where it is cooled at ( 8 ) before returning to third heat exchanger  88  where the propane is condensed to a liquid. 
     Liquid natural gas from fuel source  60  can flow to pump  90  at ( 9 ). Pump  90  can increase the temperature and pressure of the liquid natural gas at ( 10 ). Next, the liquid natural gas can flow through third heat exchanger  88  where it can vaporize at ( 11 ). The vaporized natural gas can then be superheated in second heat exchanger  78  at ( 12 ). Fuel turbine  92  can then be used to expand the superheated natural gas at ( 13 ). Finally, the natural gas is passed through fuel gas heater  30  and then into combustor  52  for combustion in gas turbine engine  12  ( FIG. 2 ). 
     Working fluid turbine  86  and fuel turbine  92  can be used to extract energy from the working fluid (e.g., propane) and the fuel (e.g. natural gas), respectively. In examples, turbines  86  and  92  can be coupled to a common shaft to drive a single generator, such as generator  94 . In other examples, each of turbine  86  and  92  can be provided with a separate output shaft for driving separate independent electrical generators. 
     The operation of GTCC power plant  10 , ORC system  70  and fuel regasification and expansion system  72  can be modeled with software, and in an example GTCC system  10  was modeled using GTPro software and dual-cycle system  80  was modeled with Ebsilon software. An exemplary power plant for modeling purposes can include an arrangement of two 2-on-1 GTCC power islands using advanced-class gas turbines. The steam bottoming cycle is based on a typical HRSG arrangement which features three pressure levels (RP, IP and LP) with reheat. The simulation was based on typical ambient conditions in Caribbean regions: 1.013 bar, dry bulb temperature of 28° C., and relative humidity of 85%. It was assumed that LNG consists of pure methane (CH 4 ). 
     Two cases were simulated. In the first Base Case, conventional GTCC power plant  10  of  FIG. 1  was simulated using liquid natural gas (LNG) fuel, using GTPro software. In the second Improved Case, modified GTCC power plant  10  of  FIG. 2  was simulated using LNG fuel, and dual-cycle system  80  with ORC system  70  and regasification and expansion system  72 . The simulation results indicated that a 0.73% points plant net efficiency (LHV) increase can be achieved. 
     The Improved Case ( FIG. 2 ) results in no negative impact to the output of GTCC system  10 , relative to the Base Case ( FIG. 1 ). As such, the additional power produced by generator  94  can be obtained at little or no cost. 
     In the Improved Case of the present application, the stack temperature of HRSG 14 can be lower than a conventional combined cycle. For the simulated cases, the stack temperature can be reduced to about 60° C. Such a temperature is acceptable because: A) LNG is considered as being a “Sulphur free” fuel, so concern related to the flue gas dewpoint is mitigated; and B) it is still higher than minimum flue gas temperature for discharging to the stack with adequate buoyancy (50° C., typical). 
       FIG. 4  is a graph showing a temperature-entropy (T-s) diagram of low pressure water from HRSG  14  between locations ( 1 ) and ( 2 ), ORC system  70  and regasification and expansion system  72  of  FIG. 3 .  FIG. 4  indicates that, by utilizing the “free” heat energy available between locations ( 1 ) and ( 2 ) in HRSG  14  and the cold sink available from the liquid natural gas, such as at fuel source  60 , ORC system  72  can be driven to obtain shaft power at turbine  86 . Furthermore, the liquid natural gas can be heated with both ORC system  70  and the water from HRSG  14  between ( 1 ) and ( 2 ) to drive fuel turbine  92 . Temperature of the natural gas provided to the fuel gas heater  30  (downstream of the fuel turbine  92 ) in inventive embodiments such as depicted by FIG. 2  is substantially the same as the temperature of natural gas provided to the fuel gas heater  30  by typical LNG gasification systems as depicted by  FIG. 1 . 
       FIG. 5  is a line diagram illustrating steps of method  100  for operating dual-cycle system  80  of  FIG. 3 . At step  102 , an organic working fluid can be circulated through a closed-circuit loop using a pump, such as pump  82 . At step  104 , organic working fluid leaving the pump  82  can be heated by recuperator  84 , using heat from another portion of ORC system  70 . At step  106 , the organic working fluid can be superheated with first heat exchanger  76  using heat from HRSG  14 . At step  108 , the superheated and gasified working fluid can be expanded with turbine  86 . At step  110 , the expanded working fluid can be passed through recuperator  84  for cooling. At step  112 , the working fluid can be condensed into a liquid using third heat exchanger  88  before returning to pump  82 . 
     At step  114 , fuel can be pumped from fuel source  60  using pump  90 . The fuel can be pumped to third heat exchanger  88 , where, at step  116 , the liquid fuel can be heated and gasified. At step  118 , the gasified fuel can be superheated using second heat exchanger  78 . At step  120 , the fuel can be expanded in turbine  92 . At step  122 , the fuel can pass into combustor  52  ( FIG. 2 ), such as after passing through fuel gas heater  30 , for combustion. 
     Operation of ORC system  70  and regasification and expansion system  72  together as dual-cycle system  80  can be used to generate electricity with turbines  92  and  86  at steps  124  and  126 , respectively. 
     The systems and methods of the present application result in a significant performance improvement that can he achieved by application of a dual-cycle in a LNG-fueled GTCC power plant. ORC system  70  can utilize a recuperator to effectively redistribute heat within ORC system  70  to improve the performance of regasification and expansion system  72  and ORC  70 . Such operation of ORC system  70  and regasification and expansion system  72  can allow the dual-cycle system  80  to power turbines that can be used to generate additional electricity, thereby improving the overall efficiency of the LNG-fueled GTCC power plant. In addition, an environmental benefit can be achieved by avoiding the cooling of seawater in the LNG regasification process. 
     Various Notes &amp; Examples 
     Example 1 can include or use subject matter such as a gas turbine combined-cycle power plant comprising a gas turbine engine comprising a compressor for generating compressed air, a combustor that can receive a fuel and the compressed air to produce combustion gas and a turbine for receiving the combustion gas and generating exhaust gas; a heat recovery steam generator for generating steam from water utilizing heat from the exhaust gas; a steam turbine for producing power from the steam generated by the heat recovery steam generator; a fuel regasification system for converting the fuel from a liquid to a gas before entering the combustor; and; and a fuel expansion turbine in fluid communication with and disposed downstream of the fuel regasification system for producing power from gasified fuel. 
     Example 2 can include, or can optionally be combined with the subject matter of Example 1, to optionally include an Organic Rankine Cycle (ORC) system configured to vaporize liquid fuel entering the fuel regasification and expansion system. 
     Example 3 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 or 2 to optionally include an ORC comprising a fluid pump for pumping a fluid, an ORC turbine in fluid communication with and disposed downstream of the pump for expanding the fluid, a first ORC heat exchanger in fluid communication with and positioned between the pump and the ORC turbine to heat the fluid with low pressure water from the heat recovery steam generator and a cooling source in fluid communication with and disposed between the ORC turbine and the pump for cooling the fluid. 
     Example 4 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 3 to optionally include a recuperator positioned between the fluid pump and the first ORC heat exchanger to exchange heat between the fluid flowing from the fluid pump and the fluid flowing from the ORC turbine. 
     Example 5 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 4 to optionally include a fluid comprising propane. 
     Example 6 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 5 to optionally include a cooling source comprising liquid fuel from the fuel regasifi cation and expansion system. 
     Example 7 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 6 to optionally include a fuel regasification and expansion system comprising a fuel pump for receiving liquefied fuel, a third ORC heat exchanger in fluid communication with and disposed downstream from the fuel pump, the third ORC heat exchanger configured to function as a condenser for the ORC system, and a second ORC heat exchanger for heating gasified fuel flowing from the third ORC heat exchanger. 
     Example 8 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 7 to optionally include a fuel heat exchanger that can transfer heat from low pressure water from the heat recovery steam generator to gasified fuel. 
     Example 9 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 8 to optionally include a liquid fuel comprising liquified natural gas. 
     Example 10 can include or use subject matter such as an Organic Rankine Cycle (ORC) system for operation with a gas turbine combined-cycle power plant that can comprise a fluid pump for pumping a fluid, an ORC turbine in fluid communication with and disposed downstream from the fluid pump for expanding the fluid, a regasification and expansion system for a fuel configured to cool the fluid between an outlet of the ORC turbine and an inlet of the pump, a first heat exchanger positioned between an outlet of the pump and an inlet of the ORC turbine to heat the fluid with heat from a heat recovery steam generator of the gas turbine combined-cycle power plant, and a fuel expansion turbine to produce power from the fuel before it enters a gas turbine engine of the gas turbine combined-cycle power plant. 
     Example 11 can include, or can optionally be combined with the subject matter of Example 10, to optionally include a recuperator positioned between an outlet of the fluid pump and an inlet of the first heat exchanger to exchange heat between the fluid leaving the fluid pump and the fluid leaving the ORC turbine. 
     Example 12 can include, or can optionally be combined with the subject matter of one or any combination of Examples 10 or 11 to optionally include a second heat exchanger in thermal communication with the fuel and the heat recovery steam generator. 
     Example 13 can include, or can optionally be combined with the subject matter of one or any combination of Examples 10 through 12 to optionally include a second heat exchanger that is configured to heat the fuel with low pressure water from the heat recovery steam generator. 
     Example 14 can include, or can optionally be combined with the subject matter of one or any combination of Examples 10 through 13 to optionally include a third heat exchanger in thermal communication with the fuel and the fluid to transfer heat from the fluid to vaporize the fuel. 
     Example 15 can include, or can optionally be combined with the subject matter of one or any combination of Examples 10 through 14 to optionally include a fuel regasification and expansion system that can comprise a fuel pump for receiving liquefied fuel, a third heat exchanger disposed downstream of an din fluid communication with the fuel pump, a second heat exchanger disposed downstream of and in fluid communication with the third heat exchanger and the fuel turbine to receive fuel from the second heat exchanger. 
     Example 16 can include or use subject matter such as a method of operating a gas turbine combined-cycle power plant comprising circulating a working fluid through a closed loop using a working pump, heating the working fluid with a first heat exchanger using heat from the gas turbine combined-cycle power plant, expanding the heated working fluid through a working fluid turbine, condensing the working fluid leaving the turbine with a fuel regasification and expansion system, expanding gas fuel of the fuel regasification and expansion system through a fuel turbine and generating electrical power with the working fluid turbine and the fuel turbine. 
     Example 17 can include, or can optionally be combined with the subject matter of Example 16, to optionally include cooling the working fluid leaving the working fluid turbine with a recuperator receiving working fluid from the working pump. 
     Example 18 can include, or can optionally be combined with the subject matter of one or any combination of Examples 16 or 17 to optionally include heating the working fluid with a first external heat source by heating the working fluid with water from a heat recovery steam generator of the gas turbine combined-cycle power plant. 
     Example 19 can include, or can optionally be combined with the subject matter of one or any combination of Examples 16 through 18 to optionally include heating the fuel using a second heat exchanger in thermal communication with the water from the heat recovery steam generator. 
     Example 20 can include, or can optionally be combined with the subject matter of one or any combination of Examples 16 through 19 to optionally include cooling the fluid leaving the turbine with a fuel regasification and expansion system by pumping liquefied natural gas with a fuel pump through a regasification heat exchanger in thermal communication with the working fluid upstream of the working pump, transferring heat from the working fluid to the liquefied natural gas in the regasification heat exchanger to gasify the liquefied natural gas and condense the working fluid, heating the gasified natural gas in the second heat exchanger and providing the gasified natural gas to a gas turbine of the gas turbine combined-cycle power plant. 
     Each of these non-limiting examples can stand on its own, or can be combined in various permutations or combinations with one or more of the other examples. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventor also contemplates examples in which only those elements shown or described are provided. Moreover, the present inventor also contemplates examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
     In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, in an example, the code can be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAM), read only memories (ROMs), and the like. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.