Patent Description:
<CIT> describes a power generation system and related methods that use supercritical fluids, whereby a portion of the supercritical fluid is recuperated.

The foregoing background discussion is intended solely to aid the reader. It is not intended to limit the innovations described herein. Thus, the foregoing discussion should not be taken to indicate that any particular element of a prior system is unsuitable for use with the innovations described herein, nor is it intended to indicate that any element is essential in implementing the innovations described herein. The implementations and application of the innovations described herein are defined by the appended claims.

Closed-cycle turbomachines do not lend themselves readily to the scenario where compressed flow is injected. Closed systems are usually based on the Rankine cycle which requires a long cycle of heating of the working fluid to cause phase change in the boiler or primary heater. Additionally, using a starter motor in a closed-cycle system to rotate the compressor/turbine spool can sometimes result in the working fluid flowing through the cycle in the wrong direction resulting in a failed start, unless check valves are employed.

Where supercritical fluids are used as the working fluid in a closed-cycle turbomachine, it is possible to use a compressed stored supply of working fluid to start the turbomachine. Such a method requires one or more supply tanks charged to a pressure above the steady-state design operating pressure of the turbomachine, which adds to the cost, complexity and footprint of the complete machine. It is also possible to use a motor-driven compressor as a means of creating a pressure rise capable of starting the closed-cycle turbomachine. This method has several drawbacks. It requires an expensive high-speed motor, a high-pressure casing, and battery or facility power. In addition, sophisticated moving parts add to machine cost, are prone to needing service, and reduce reliability of the overall system.

Disclosed herein are methods and systems for starting a closed-cycle turbomachine that utilizes a supercritical fluid as a working fluid. In one exemplary method in a system for generating power utilizing a closed cycle with a working fluid, when the working fluid is in a supercritical state, the closed cycle including a turbomachine comprising a supercritical fluid compressor connected via a shaft to a supercritical fluid turbine, the method according to the present invention for starting the turbomachine comprises the steps of: charging a mass of working fluid that is not in the supercritical state in the closed cycle up to a starting mass that is greater than a design operating mass, wherein the design operating mass is the mass of working fluid in the closed cycle when operating at steady-state design point conditions; heating the working fluid; and venting a discharge mass of working fluid from the closed cycle at a location in the closed cycle that is on an exhaust side of the supercritical fluid turbine.

In some turbomachine applications, foil bearings are used to support a shaft connecting the compressor to the high-pressure turbine. In operation, once the shaft is spinning fast enough (i.e., at or above a critical speed), the working fluid creates a gap between the bearing and the shaft, such that no wear occurs. However, the bearing will contact the shaft during the turbomachine startup and shutdown when the shaft is rotating at speeds less than the critical speed. Thus, decelerating the turbomachine quickly during the shutdown process may have the desired effect of increasing bearing life.

Disclosed herein are methods not defined by the appended claims, for rapidly decelerating a turbomachine in a closed supercritical fluid cycle. In one exemplary method in a system for generating power utilizing a supercritical fluid in a closed cycle, the closed cycle including a turbomachine comprising a supercritical fluid compressor connected via a shaft to a supercritical fluid turbine and at least one heat exchanger disposed along the closed cycle and configured to reduce a temperature of the supercritical fluid entering an inlet of the compressor, a method for decelerating the turbomachine comprising the steps of: directing a flow of supercritical fluid from the compressor to an inlet of the turbine; directing the flow of supercritical fluid from the turbine to the at least one heat exchanger; directing the flow of supercritical fluid from the at least one heat exchanger to the inlet of the compressor; and reducing a flow of a cooling medium to the at least one heat exchanger, thereby allowing the temperature of the supercritical fluid entering the compressor inlet to increase, wherein the increased temperature of the supercritical fluid entering the compressor inlet causes the turbomachine to decelerate.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description section. Furthermore, the claimed subject matter is not constrained to limitations that solve any or all disadvantages noted in any part of this disclosure.

The foregoing summary, as well as the following detailed description, are better understood when read in conjunction with the appended drawings. In the drawing, for purposes of explanation, numerous specific details are set forth in order to provide an understanding of the variations in implementing the disclosed technology. However, the instant disclosure may take many different forms and should not be construed as limited to the specific examples disclosed in the drawings. When practical, like numbers refer to like elements throughout. In the drawings:.

The methods and systems as described herein may include components and configurations of the power generation systems disclosed in <CIT>, and <CIT>, the disclosures of which relating to the structure and function of the power generation systems are referenced by this present application.

In one aspect of the disclosure, a closed cycle includes a turbomachine that comprises a compressor connected via a shaft to a high-pressure turbine. The discharge start method comprises charging the closed cycle with an excess mass of a working fluid and heating the working fluid by an external means, thereby raising the temperature and pressure of the working fluid in the closed cycle. In one exemplary method, sufficient heat is added to raise the temperature of the working fluid to make it supercritical. A mass of working fluid is vented from the closed cycle at a location downstream of the high-pressure turbine. The resulting pressure drop causes the working fluid to flow through the closed cycle and rotate the high-pressure turbine wheel, which drives the compressor. The working fluid entering the compressor is in a supercritical state (i.e., at or above the critical point). Accordingly, compressed supercritical fluid is discharged from the compressor to flow through the closed cycle. The flow of compressed supercritical fluid is heated and directed to the high-pressure turbine inlet. The turbine expands the heated compressed supercritical fluid and exhausts expanded supercritical fluid, which is directed through the closed cycle back to the compressor. The process continues until a start is achieved. The discharge valve is closed when the desired mass of working fluid has been vented from the closed cycle.

<FIG> illustrates an exemplary method for starting a turbomachine in a power generation system that utilizes a supercritical fluid in a closed cycle. The turbomachine may comprise a core that includes a supercritical fluid compressor connected via a shaft to a first supercritical fluid turbine (also referred to herein as the high-pressure supercritical fluid turbine). The turbomachine may further comprise a second supercritical fluid turbine (referred to herein as a power turbine) that produces shaft power for an output device (e.g., generator, turboprop, turbofan, or gearbox). The power turbine may be positioned along the closed cycle on the exhaust side of the high- pressure supercritical fluid turbine. The term "supercritical fluid" refers to a fluid in which distinct liquid and gaseous phases do not exist, and the 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 <NUM>°K and <NUM> MPa.

Referring to <FIG>, step <NUM>, the closed cycle may be charged with a starting mass of a working fluid (e.g., carbon dioxide) that is greater than a design operating mass of working fluid, wherein the design operating mass of working fluid is the mass of the working fluid in the supercritical fluid cycle when the power generation system is operating at steady-state design point conditions. At this step, the working fluid that is charged into the closed cycle is below its critical point (i.e., it is not a supercritical fluid).

At step <NUM>, heat is added to the working fluid in the closed cycle. As discussed below, heat may be added to the working fluid in the closed cycle via various means. For example, the power generation system may include a second cycle configured to heat a second working fluid, and one or more heat exchangers configured to transfer heat from the second working fluid to the working fluid in the closed cycle. In one example, the second cycle may be an air breathing cycle that includes a combustor configured to burn fossil fuel, biomass, bio-derived fuels, or waste in air, thereby heating the air flowing along the air breathing cycle. The power generation system may then utilize one or more heat exchangers to transfer heat from the heated air to the working fluid in the closed system. Alternatively, the second cycle may produce system heat by means of a solar collector, a nuclear reactor, or Thermal Energy Storage (TES). One exemplary TES thermal storage medium is "thermal bricks" comprising a Miscibility Gap Alloy (MGA) that is capable of storing large amounts of energy as heat. Electrical power from a renewable source, such as solar or wind energy, may be directed to heating the MGA storage medium. The MGA based TES has environmental and cost benefits over traditional battery storage of electrical energy produced by renewable power sources (e.g., longer life span, non-toxic, and recyclable). The second cycle may utilize one or more heat exchangers to transfer heat from the second cycle to the working fluid in the closed system.

At step <NUM>, sufficient heat may be added to raise the temperature of the working fluid in the closed system to that which is required to make the working fluid supercritical. In another aspect of the disclosure, the working fluid may be heated such that the working fluid, located at an inlet of the supercritical fluid compressor, is at or above the critical point of the working fluid. Alternatively, the working fluid may be heated to within a predetermined temperature range of the critical temperature of the working fluid. For example, the working fluid may be heated to within <NUM> to <NUM> degrees Kelvin (°K) of its critical temperature. In another alternative, the working fluid may be heated to within <NUM> to <NUM>°K of its critical temperature. In yet another altemative, the working fluid may be heated to within <NUM> to <NUM>°K of its critical temperature. In yet another alternative, the working fluid may be heated to within <NUM> to <NUM>°K of its critical temperature. In yet another alternative, the supercritical fluid may be heated to within <NUM> to <NUM>°K of its critical temperature.

At step <NUM>, a discharge mass of working fluid is vented from a location along the closed cycle that is on an exhaust side of the high-pressure supercritical fluid turbine. The resulting pressure sink aft of the high-pressure turbine may cause the working fluid to flow along a flow path in the closed cycle. The flow path may direct the working fluid to flow through one or more heat exchangers where heat is added to the flow of working fluid via the second cycle (e.g., via heated air). The heated working fluid may then be directed to an inlet of the high-pressure supercritical fluid turbine, thereby initiating rotation of the high-pressure turbine wheel, which drives the compressor to supply additional compressed flow of the working fluid until a start is successfully achieved at step <NUM>.

At step <NUM>, venting the discharge mass may reduce the mass of working fluid in the closed cycle from the starting mass to the design operating mass. Alternatively, venting the discharge mass at step <NUM> may reduce the mass of working fluid in the closed system to an amount that is less than the starting mass but greater than the design operating mass. In another aspect, the venting step <NUM> may be used to reduce the mass of working fluid in the closed cycle to an amount that is less than the design operating mass. For example, it may be desirable to operate the power generation system for an extended time at operating speeds different from the design point. In that scenario, a second discharge mass of working fluid may be vented from the closed cycle to achieve a desired new operating mass, wherein the working fluid at the compressor inlet is close to the critical point, thereby optimizing the system efficiency.

At step <NUM>, the venting may be performed by opening a discharge valve located on the exhaust end of the high-pressure turbine. The discharge valve may be opened for a prescribed length of time to vent the desired amount of working fluid from the closed cycle. In addition, the rate at which the working fluid is encouraged to flow along the closed cycle flow path may be controlled by varying the opening of the discharge valve, thereby controlling the rate of acceleration of the turbomachine. Controlling the rate of acceleration of the turbomachine may have benefits, including managing radial loads on the radial bearings, as well as allowing an operator to transit rotor dynamically-unstable speeds at a rate which protects the turbomachine from damage.

At step <NUM>, the discharge mass of working fluid may be vented from the closed cycle into a containment vessel that is at a lower pressure than the pressure inside the closed cycle. Alternatively, the discharge mass may be vented to atmosphere.

The method may include optional step <NUM> to control a flow direction the working fluid through the closed cycle along the closed cycle flow path. For example, the power generation system may include a check valve located downstream from the discharge valve to encourage the majority of working fluid to flow through the closed cycle in the desired direction (e.g., from the compressor outlet to the high-pressure turbine inlet and from the high-pressure turbine outlet to the compressor inlet). The inventors have determined that a Tesla valve (also called a fluidic diode) is useful for performing this step. The Tesla valve has no moving parts and, therefore, does not decrease the reliability of the power generation system. Alternatively, the turbomachine may include a second supercritical fluid turbine (e.g., a power turbine) that may comprise one or more turbine nozzles configured to have fluidic diode characteristics that all discourage backflow of the working fluid in the closed cycle. Thus, the one or more power turbine nozzles may encourage the majority of working fluid to flow through the closed cycle in the desired direction.

In an alternative application, the discharge start method may comprise the steps of: step <NUM>, charging the closed cycle with a starting mass of working fluid; step <NUM>, heating the working fluid; and step <NUM>, venting a discharge mass of working fluid at a location on the exhaust side of the supercritical fluid turbine. In this exemplary method, the discharge mass may be vented from the closed cycle when the temperature of the working fluid is within a predetermined temperature range of the critical temperature of the working fluid, as discussed above. This would result in two phase flow of the working fluid along the closed cycle flow path for some period of time after the turbomachine start until sufficient heat is added to the working fluid via the one or more heat exchangers to raise the temperature of the working fluid to be at or above the critical point.

<FIG> is a schematic diagram of a power generation system <NUM> initially disclosed in the '<NUM> Patent <FIG> and column <NUM>, line <NUM> through column <NUM>, line <NUM>. The power generation system <NUM> illustrated in <FIG> and described herein includes additional features that may allow system <NUM> to implement the turbomachine discharge start method illustrated in <FIG>. Power generation system <NUM> includes a first closed Brayton cycle <NUM>, in which the working fluid may be a supercritical fluid, such as supercritical carbon dioxide (SCO2), and a second cycle <NUM> that is configured to add heat to the supercritical fluid cycle <NUM>. For example, the second cycle <NUM> may be an open Brayton cycle in which the working fluid may be air. The supercritical fluid cycle <NUM> includes a supercritical fluid flow path <NUM> and the air breathing cycle <NUM> includes an air flow path <NUM>. The flow paths <NUM>, <NUM> are separate so that little or no mixing occurs between the supercritical fluid and air.

Referring to <FIG>, system <NUM> includes at least one core disposed along supercritical fluid flow path <NUM>. Each core includes a supercritical fluid compressor <NUM> connected via a shaft <NUM> to a first supercritical fluid turbine <NUM>. The compressor <NUM>, shaft <NUM>, and turbine <NUM> may also be collectively referred to herein as a turbomachine. The compressor <NUM> may be an axial, radial, reciprocating or the like type of compressor. The turbine <NUM> may be referred to as the high-pressure turbine. Power generation system <NUM> may also include a second supercritical fluid turbine <NUM> (also referred to herein as a power turbine) connected via a shaft <NUM> to an output device <NUM> (e.g., generator, turboprop, turbofan, or a gearbox). The compressor <NUM> is configured to receive and compress a supercritical fluid and the turbine <NUM> is configured to receive and expand the supercritical fluid. The power generation system <NUM> may also include valves <NUM>, mixing junctions <NUM>, coolers <NUM>, couplings, flow meters (not shown), temperature and pressure sensors (not shown), and one or more controllers (not shown) configured to control the operation of system <NUM>.

The air breathing cycle <NUM> may include at least one combustor <NUM> arranged along air flow path <NUM> and configured to receive and combust a fossil fuel, biomass, bio-derived fuels, or waste in air, thereby heating the air flowing along the air flow path <NUM>.

Power generation system <NUM> may also include one or more cross cycle heat exchangers <NUM>, <NUM>, <NUM>, <NUM> arranged along flow paths <NUM>, <NUM>. As used herein, the term "cross cycle heat exchanger" refers to a heat exchanger that receives working fluids from two different cycles and transfers heat between the two respective working fluids. For example, cross cycle heat exchanger <NUM>, also referred to herein as the primary cross cycle heat exchanger, is arranged along flow paths <NUM> and <NUM>, and receives heated air exiting the combustor <NUM> along the air flow path <NUM> and supercritical fluid from the supercritical fluid flow path <NUM> and thereby transfers heat from the heated air to the supercritical fluid.

The power generation system <NUM> may also include a recuperating heat exchanger <NUM> arranged along the supercritical fluid flow path <NUM>. As used herein, the term "recuperating heat exchanger" refers to a heat exchanger arranged along a single cycle flow path that transfers heat between different portions of the working fluid flowing within the cycle. For example, recuperating heat exchanger <NUM> is arranged along supercritical fluid flow path <NUM> and receives a portion of the compressed supercritical fluid <NUM> discharged from the supercritical fluid compressor <NUM> and a portion of the hot expanded supercritical fluid <NUM> discharged from the supercritical fluid turbine <NUM>. In the recuperating heat exchanger <NUM>, heat is transferred from the portion of hot expanded supercritical fluid <NUM> to the portion of compressed supercritical fluid <NUM>, such that heated compressed supercritical fluid <NUM> and cooled supercritical fluid <NUM> exit recuperating heat exchanger <NUM>. Thus, the recuperating heat exchanger <NUM> enables heat transfers between different portions of the supercritical fluid flowing within the supercritical fluid cycle.

In an exemplary operation referring to <FIG> and <FIG>, the discharge start method comprises step <NUM> of charging the closed supercritical fluid cycle <NUM> with a starting mass of a working fluid (e.g., carbon dioxide) that is greater than a design operating mass of working fluid, wherein the design operating mass is the mass of carbon dioxide in the supercritical fluid cycle <NUM> when the power generation system <NUM> is operating at steady-state design point conditions. At this step, the carbon dioxide that is charged into the closed cycle <NUM> is below its critical point (i.e., it is not a supercritical fluid). Charging cycle <NUM> with a starting mass of carbon dioxide effectively makes the supercritical fluid cycle portion of power generation system <NUM> the pressure vessel that would normally be associated with a blowdown supply tank(s) of carbon dioxide required in an alternative method of starting a closed cycle turbomachine.

At step <NUM>, heat is added to the carbon dioxide in supercritical fluid cycle <NUM>. For example, heat may be added from a second cycle via cross cycle heat exchangers <NUM> and <NUM>. Air may be flowed along air flow path <NUM> through cross cycle heat exchangers <NUM>, <NUM>, <NUM>, <NUM> in the order of <NUM> → <NUM> → <NUM> → <NUM>. A combustor <NUM> configured to combust a fossil fuel in air, so as to heat the air flowing along air flow path <NUM>, may be arranged between cross cycle heat exchangers <NUM> and <NUM> such that the air entering <NUM> is hotter than the air exiting <NUM>. One or more circulation pumps (not shown) may be used to flow the carbon dioxide along supercritical fluid flow path <NUM> through cross cycle heat exchangers <NUM> and <NUM>, wherein heat is transferred from the heated air to the carbon dioxide. Alternatively, the carbon dioxide in the closed supercritical fluid cycle <NUM> may be heated via external heaters, such as infrared (IR) lamps or heat tape. In another alternative, the carbon dioxide in the closed supercritical fluid cycle <NUM> may be heated by a combination of external heaters and heat transfer from a second cycle via flowing through one or more cross cycle heat exchangers.

At step <NUM>, sufficient heat may be added to raise the temperature of the carbon dioxide in the supercritical fluid cycle <NUM> to that which is required to make the carbon dioxide supercritical (i.e., the carbon dioxide is at or above its critical point of approximately <NUM>°K and <NUM> MPa). In an alternative example, at step <NUM>, the carbon dioxide in the supercritical fluid cycle <NUM> may be heated such that the carbon dioxide, located at an inlet of the supercritical fluid compressor <NUM>, is at or above the critical point of carbon dioxide.

At step <NUM>, a discharge mass of carbon dioxide may be vented from a location along supercritical fluid cycle <NUM> that is on an exhaust side of the high-pressure supercritical fluid turbine <NUM>. For example, a discharge valve 222c may be located along supercritical flow path <NUM> after the high-pressure turbine <NUM> (e.g., along section <NUM>). Alternatively, a discharge valve 222c may be located along supercritical flow path <NUM> after the power turbine <NUM> (e.g., along section <NUM>). The discharge valve 222c may be in electronic communication with a controller (not shown). Discharge valve 222c may be opened for a prescribed length of time to vent the desired amount of carbon dioxide from supercritical fluid cycle <NUM>. For example, discharge valve 222c may be opened to vent a discharge mass of carbon dioxide that reduces the mass of carbon dioxide in the supercritical fluid cycle from the starting mass to the design operating mass. Alternatively, discharge valve 222c may be opened to vent and reduce the mass of carbon dioxide in the supercritical fluid cycle <NUM> to an amount that is less than the starting mass but greater than the design operating mass. At step <NUM>, the discharge mass of carbon dioxide may be vented from the closed cycle <NUM> into a containment vessel (not shown) that is at a lower pressure than the pressure inside the closed cycle. Alternatively, the discharge mass may be vented to atmosphere.

Opening discharge valve 222c may result in a pressure sink aft of the high-pressure turbine <NUM> that may cause the carbon dioxide to flow along supercritical flow path <NUM>. For example, the carbon dioxide may flow along sections <NUM> and <NUM> of supercritical flow path <NUM> and flow through cross cycle heat exchangers <NUM> and <NUM>, wherein heat is transferred to the carbon dioxide from the heated air flowing along air flow path <NUM>. The heated carbon dioxide is directed along section <NUM> to the inlet of the high-pressure turbine <NUM>, thereby initiating rotation of the high-pressure turbine wheel, which drives the compressor <NUM> via shaft <NUM>. The compressor <NUM> supplies additional compressed flow of carbon dioxide (which may be in a supercritical state) along section <NUM> of supercritical fluid flow path <NUM> until a start is successfully achieved at step <NUM>.

The rate at which the carbon dioxide fluid is encouraged to flow along supercritical flow path <NUM> may be controlled by varying the opening of the discharge valve 222c, thereby controlling the rate of acceleration of the turbomachine. Controlling the rate of acceleration of the turbomachine may have benefits, including managing radial loads on the radial bearings, as well as allowing an operator to transit rotor dynamically-unstable speeds at a rate which protects the turbomachine from damage.

The discharge start method may also include optional step <NUM> to control a flow direction of the working fluid through the closed supercritical fluid cycle <NUM> along supercritical flow path <NUM>. In an exemplary operation, working fluid may flow from the compressor outlet along sections <NUM>, <NUM>, and <NUM> to heat exchangers <NUM> and <NUM> and to the high-pressure turbine inlet via section <NUM>. And working fluid may flow from the high-pressure turbine outlet along sections <NUM>, <NUM>, and <NUM> to heat exchangers <NUM> and <NUM> and to the compressor inlet via section <NUM>. The supercritical fluid cycle <NUM> may include a check valve located downstream from the discharge valve that may help to encourage the majority of carbon dioxide to flow along the supercritical fluid flow path <NUM> in the desired direction. For example, a check valve may be located along section <NUM> of the supercritical flow path <NUM>. Alternatively, check valves may be located along sections <NUM> and <NUM> of the supercritical flow path <NUM>. In another alternative, power turbine <NUM> may comprise one or more turbine nozzles configured to discourage backflow of the working fluid along supercritical flow path <NUM> (e.g., flowing from the power turbine <NUM> along section <NUM> to the high-pressure turbine <NUM>).

In an alternative application, the discharge start method may comprise the steps of: step <NUM>, charging the closed cycle <NUM> with a starting mass of working fluid (e.g., carbon dioxide); step <NUM>, heating the working fluid; and step <NUM>, venting a discharge mass of working fluid from discharge valve 222c until a start is achieved. In this exemplary method, the discharge mass may be vented from the closed cycle <NUM> when the temperature of the working fluid is within a predetermined temperature range of the critical temperature of the working fluid (e.g., for carbon dioxide, approximately <NUM>°K and <NUM> MPa). For example, the carbon dioxide may be heated to within <NUM> to15°K of its critical temperature. In another alternative, the carbon dioxide may be heated to within <NUM> to <NUM>°K of its critical temperature. In yet another alternative, the carbon dioxide may be heated to within <NUM> to <NUM>°K of its critical temperature. In yet another alternative, the carbon dioxide may be heated to within <NUM> to <NUM>°K of its critical temperature. In yet another alternative, the supercritical fluid may be heated to within <NUM> to <NUM>°K of its critical temperature. This would result in two phase flow of the carbon dioxide along the closed cycle flow path <NUM> for some period of time after the turbomachine start until sufficient heat is added to the carbon dioxide via heat exchangers <NUM>, <NUM> to raise the temperature of the carbon dioxide to be at or above its critical point.

<FIG> is a schematic diagram of a power generation system <NUM> that may implement the turbomachine discharge start method illustrated in <FIG>. Power generation system <NUM> includes a first closed Brayton cycle <NUM>, in which the working fluid may be a supercritical fluid, such as supercritical carbon dioxide (SCO2), and a second cycle <NUM> that is configured to add heat to the supercritical fluid cycle <NUM>. The second cycle <NUM> is illustrated in simplified form. As discussed above, the second cycle may be an air breathing cycle that includes a combustor <NUM> configured to burn fossil fuel, biomass, bio-derived fuels, or waste in air, thereby heating the air flowing along the air breathing cycle. Alternatively, the second cycle may produce system heat by means of a solar collector, a nuclear reactor, or Thermal Energy Storage (TES). The second cycle may utilize one or more heat exchangers <NUM> to transfer heat from the second cycle to the working fluid in the closed system. The supercritical fluid cycle <NUM> includes a supercritical fluid flow path <NUM> and the second cycle <NUM> includes a thermal medium flow path <NUM>. The flow paths <NUM>, <NUM> are separate so that little or no mixing occurs between the supercritical fluid and the thermal medium.

Referring to <FIG>, system <NUM> includes at least one core disposed along supercritical fluid flow path <NUM>. Each core includes a supercritical fluid compressor <NUM> connected via a shaft <NUM> to a first supercritical fluid turbine <NUM>. The compressor <NUM>, shaft <NUM>, and turbine <NUM> may also be collectively referred to herein as a turbomachine. The compressor <NUM> may be an axial, radial, reciprocating or the like type of compressor. The turbine <NUM> may be referred to as the high-pressure turbine. Power generation system <NUM> may also include a second supercritical fluid turbine <NUM> (also referred to herein as a power turbine) connected via a shaft <NUM> to an output device <NUM> (e.g., generator, turboprop, turbofan, or a gearbox). The compressor <NUM> is configured to receive and compress a supercritical fluid and the turbine <NUM> is configured to receive and expand the supercritical fluid. The power generation system <NUM> may also include a discharge valve <NUM>, mixing junctions <NUM>, coolers <NUM>, couplings, flow meters (not shown), temperature and pressure sensors (not shown), and one or more controllers (not shown) configured to control the operation of system <NUM>.

Power generation system <NUM> may also include one or more cross cycle heat exchangers <NUM> arranged along flow paths <NUM>, <NUM>. For example, cross cycle heat exchanger <NUM> is arranged along flow paths <NUM> and <NUM> and receives the supercritical fluid from the supercritical fluid flow path <NUM> and transfers heat from the thermal medium in the second cycle <NUM> to the supercritical fluid.

The power generation system <NUM> may also include a recuperating heat exchanger <NUM> arranged along the supercritical fluid flow path <NUM>. Recuperating heat exchanger <NUM> may be arranged along supercritical fluid flow path <NUM> and receive the compressed supercritical fluid <NUM> discharged from the supercritical fluid compressor <NUM> and the hot expanded supercritical fluid <NUM> discharged from the supercritical fluid turbine <NUM>, <NUM>. In the recuperating heat exchanger <NUM>, heat is transferred from the hot expanded supercritical fluid <NUM> to the compressed supercritical fluid <NUM>, such that heated compressed supercritical fluid <NUM> and cooled supercritical fluid <NUM> exit recuperating heat exchanger <NUM>. Thus, the recuperating heat exchanger <NUM> enables heat transfers between different portions of the supercritical fluid flowing within the supercritical fluid cycle.

At step <NUM>, heat is added to the carbon dioxide in supercritical fluid cycle <NUM>. For example, heat may be added from a second cycle via cross cycle heat exchanger <NUM>. One or more circulation pumps (not shown) may be used to flow the carbon dioxide along supercritical fluid flow path <NUM> through cross cycle heat exchanger <NUM>, wherein heat is transferred from the second cycle to the carbon dioxide. By way of example, the second cycle <NUM> may include Thermal Energy Storage (TES). Cross cycle heat exchanger <NUM> may be embedded in the TES such that heat from the TES is transferred to the carbon dioxide as it flows along supercritical flow path <NUM> through heat exchanger <NUM>. Alternatively, the carbon dioxide in the closed supercritical fluid cycle <NUM> may be heated via external heaters, such as infrared (IR) lamps or heat tape. In another alternative, the carbon dioxide in the closed supercritical fluid cycle <NUM> may be heated by a combination of external heaters and heat transfer from the second cycle via flowing through one or more cross cycle heat exchangers <NUM>.

At step <NUM>, sufficient heat may be added to raise the temperature of the carbon dioxide in the supercritical fluid cycle <NUM> to that which is required to make the carbon dioxide supercritical (i.e., the carbon dioxide is at or above its critical point of approximately <NUM>°K and <NUM> MPa). In an alternative example, at step <NUM>, the carbon dioxide in the supercritical fluid cycle <NUM> may be heated such that the carbon dioxide, located at an inlet of the supercritical fluid compressor <NUM>, is at or above the critical point of carbon dioxide. Continuing with the previous example wherein the second cycle <NUM> utilizes TES, the heat transferred from the TES to the carbon dioxide via heat exchanger <NUM> is sufficient to maintain the temperature of the carbon dioxide to be at or above its critical temperature during steady-state operating conditions (i.e., the carbon dioxide flowing in the closed supercritical fluid cycle <NUM> is maintained in the supercritical state).

At step <NUM>, a discharge mass of carbon dioxide may be vented from a location along supercritical fluid cycle <NUM> that is on an exhaust side of the high-pressure supercritical fluid turbine <NUM>. For example, a discharge valve <NUM> may be located along supercritical flow path <NUM> after the high-pressure turbine <NUM> (e.g., along section <NUM>). Alternatively, a discharge valve <NUM> may be located along supercritical flow path <NUM> after the power turbine <NUM> (e.g., along section <NUM>. The discharge valve <NUM> may be in electronic communication with a controller (not shown). Discharge valve <NUM> may be opened for a prescribed length of time to vent the desired amount of carbon dioxide from supercritical fluid cycle <NUM>. For example, discharge valve <NUM> may be opened to vent a discharge mass of carbon dioxide that reduces the mass of carbon dioxide in the supercritical fluid cycle from the starting mass to the design operating mass. Alternatively, discharge valve <NUM> may be opened to vent and reduce the mass of carbon dioxide in the supercritical fluid cycle <NUM> to an amount that is less than the starting mass but greater than the design operating mass. At step <NUM>, the discharge mass of carbon dioxide may be vented from the closed cycle <NUM> into a containment vessel (not shown) that is at a lower pressure than the pressure inside the closed cycle. Alternatively, the discharge mass may be vented to atmosphere.

Opening discharge valve <NUM> may result in a pressure sink aft of the high-pressure turbine <NUM> that may cause the carbon dioxide to flow along supercritical flow path <NUM>. For example, the carbon dioxide may flow along sections <NUM>, <NUM>, <NUM>, <NUM> and <NUM> of supercritical flow path <NUM> and flow through cross cycle heat exchanger <NUM>, wherein heat is transferred to the carbon dioxide from the heated air flowing along air flow path <NUM>. The heated carbon dioxide is directed along section <NUM> to the inlet of the high-pressure turbine <NUM>, thereby initiating rotation of the high-pressure turbine wheel, which drives the compressor <NUM> via shaft <NUM>. The compressor <NUM> supplies additional compressed flow of carbon dioxide (which may be in a supercritical state) along section <NUM> of supercritical fluid flow path <NUM> until a start is successfully achieved at step <NUM>.

The rate at which the carbon dioxide fluid is encouraged to flow along supercritical flow path <NUM> may be controlled by varying the opening of the discharge valve <NUM>, thereby controlling the rate of acceleration of the turbomachine. Controlling the rate of acceleration of the turbomachine may have benefits, including managing radial loads on the radial bearings, as well as allowing an operator to transit rotor dynamically-unstable speeds at a rate which protects the turbomachine from damage.

The discharge start method may also include optional step <NUM> to control a flow direction of the working fluid through the closed supercritical fluid cycle <NUM> along supercritical flow path <NUM>. In an exemplary operation, working fluid may flow from the compressor outlet along sections <NUM> and <NUM> to heat exchanger <NUM> and to the high-pressure turbine inlet via section <NUM>. And the working fluid may flow from the high-pressure turbine outlet along sections <NUM>, <NUM>, <NUM>, <NUM> and <NUM> to recuperating heat exchanger <NUM> and to the compressor inlet via sections <NUM> and <NUM>. The supercritical fluid cycle <NUM> may include a check valve located downstream from the discharge valve that may help to encourage the majority of working fluid to flow along the supercritical fluid flow path <NUM> in the desired direction. For example, a check valve may be located along section <NUM> of the supercritical flow path <NUM>. Alternatively, check valves may be located along sections <NUM> and <NUM> of the supercritical flow path <NUM>. In another alternative, power turbine <NUM> may comprise one or more turbine nozzles configured to discourage backflow of the working fluid along supercritical flow path <NUM> (e.g., flowing from the power turbine <NUM> along section <NUM> to the high-pressure turbine <NUM>).

In an alternative application, the discharge start method may comprise the steps of: step <NUM>, charging the closed cycle <NUM> with a starting mass of working fluid (e.g., carbon dioxide); step <NUM>, heating the working fluid; and step <NUM>, venting a discharge mass of working fluid from discharge valve <NUM> until a start is achieved. In this exemplary method, the discharge mass may be vented from the closed cycle <NUM> when the temperature of the working fluid is within a predetermined temperature range of the critical temperature of the working fluid (e.g., for carbon dioxide, approximately <NUM>°K and <NUM> MPa). For example, the carbon dioxide may be heated to within <NUM> to <NUM>°K of its critical temperature. In another alternative, the carbon dioxide may be heated to within <NUM> to <NUM>°K of its critical temperature. In yet another alternative, the carbon dioxide may be heated to within <NUM> to <NUM>°K of its critical temperature. In yet another alternative, the carbon dioxide may be heated to within <NUM> to <NUM>°K of its critical temperature. In yet another alternative, the supercritical fluid may be heated to within <NUM> to <NUM>°K of its critical temperature. This would result in two phase flow of the carbon dioxide along the closed cycle flow path <NUM> for some period of time after the turbomachine start until sufficient heat is added to the carbon dioxide via heat exchanger <NUM> to raise the temperature of the carbon dioxide to be at or above its critical point.

<FIG> illustrates an exemplary foil bearing <NUM>, one or more of which may be used in a turbomachine to support a shaft <NUM> that connects a compressor and a high-speed turbine (e.g., <FIG>, compressor <NUM> and high-speed turbine <NUM>). The foil bearing <NUM> may include a bearing sleeve <NUM>, a bump foil <NUM> and a top foil <NUM>. The shaft <NUM> is supported by a compliant, spring-loaded top foil <NUM>. In operation, once shaft <NUM> is spinning fast enough (i.e., at or above a critical speed), the working fluid (e.g., supercritical carbon dioxide) pushes the top foil <NUM> away from the shaft <NUM> so that no contact occurs. The shaft <NUM> and top foil <NUM> are separated by a gap of high-pressure working fluid, which is generated by the shaft rotation that pulls working fluid into the bearing via viscosity effects. The relative high speed of the shaft with respect to the top foil is required to initiate the gap, and once the gap has been achieved, no wear occurs. Thus, the top foil <NUM> will contact the shaft <NUM> during the turbomachine startup and shutdown when the shaft is rotating at speeds less than the critical speed.

The top foil <NUM> may have a coating to protect the bearing while it contacts the shaft <NUM> during startup and shutdown. The inventors have determined that the rate at which the turbomachine is decelerated during the shutdown determines how much bearing wear occurs at each shutdown. Decelerating quickly will have the desired effect of increasing bearing life.

Referring to <FIG>, power generation system <NUM> includes at least one core disposed along supercritical fluid flow path <NUM>. Each core includes a supercritical fluid compressor <NUM> connected via a shaft <NUM> to a first supercritical fluid turbine <NUM>. The compressor <NUM>, shaft <NUM>, and turbine <NUM> may also be collectively referred to herein as a turbomachine. The compressor <NUM> may be an axial, radial, reciprocating or the like type of compressor. The turbine <NUM> may be referred to as the high-pressure turbine. The compressor <NUM> is configured to receive and compress a supercritical fluid and the turbine <NUM> is configured to receive and expand the supercritical fluid.

The power generation system <NUM> may also include a heat exchanger <NUM> disposed along supercritical flow path <NUM> upstream of the compressor <NUM>. The heat exchanger <NUM> may comprise a cooler. In normal operation, heat exchanger <NUM> may be configured to maintain the temperature of the supercritical fluid directed to an inlet of the compressor <NUM> to be at or slightly above the critical temperature of the supercritical fluid (e.g., within between <NUM> and <NUM>°K of its critical temperature). For example, the heat exchanger <NUM> may be configured to receive a flow of supercritical fluid and a flow of a cooling medium, and to transfer heat from the supercritical fluid to the cooling medium. In an exemplary operation, heat exchanger <NUM> may comprise a cooler that receives a flow of supercritical fluid <NUM> and a flow of water from water input 225a. Heat exchanger <NUM> may discharge a flow of heated water 225b and a cooled flow of supercritical fluid <NUM> that is directed to an inlet of the compressor <NUM>. Alternatively, heat exchanger <NUM> may receive a flow of air as the cooling medium.

The power generation system <NUM> may also include a heat exchanger <NUM> disposed along supercritical flow path <NUM> upstream of the compressor <NUM>. The heat exchanger <NUM> may comprise a cooler. In normal operation, heat exchanger <NUM> may be configured to maintain the temperature of the supercritical fluid directed to an inlet of the compressor <NUM> to be at or slightly above the critical temperature of the supercritical fluid (e.g., within between <NUM> and <NUM>°K of its critical temperature). For example, the heat exchanger <NUM> may be configured to receive a flow of supercritical fluid and a flow of a cooling medium, and to transfer heat from the supercritical fluid to the cooling medium. In an exemplary operation, heat exchanger <NUM> may comprise a cooler that receives a flow of supercritical fluid <NUM> and a flow of water from water input 325a. Heat exchanger <NUM> may discharge a flow of heated water 325b and a cooled flow of supercritical fluid <NUM> that is directed to an inlet of the compressor <NUM>. Alternatively, heat exchanger <NUM> may receive a flow of air as the cooling medium.

<FIG> illustrates an exemplary method for decelerating a turbomachine according to an aspect of the disclosure. Referring to <FIG> and <FIG>, at step <NUM>, a flow of supercritical fluid is directed from the compressor <NUM> along supercritical flow path <NUM> to an inlet of the turbine <NUM> (e.g., sections <NUM>, <NUM>). At step <NUM>, the flow of supercritical fluid is exhausted from the turbine <NUM> and directed along supercritical flow path <NUM> (e.g., sections <NUM>, <NUM>, <NUM>) to at least one heat exchanger <NUM>. At step <NUM>, the flow of supercritical fluid is directed along supercritical flow path <NUM> (e.g., section <NUM>) from the at least one heat exchanger <NUM> to an inlet of the compressor <NUM>. At step <NUM>, a flow of a cooling medium (e.g., water, air) directed to heat exchanger <NUM> is reduced. At step <NUM>, the reduced flow of cooling medium to heat exchanger <NUM> allows the temperature of the supercritical fluid directed to the compressor inlet to increase. At step <NUM>, the increased temperature of the supercritical fluid entering the compressor inlet has the effect of breaking the turbomachine thereby causing it to decelerate.

Claim 1:
In a system (<NUM>) for generating power utilizing a closed cycle (<NUM>) with a working fluid, when the working fluid is in a supercritical state, the closed cycle including a turbomachine comprising a supercritical fluid compressor (<NUM>) connected via a shaft (<NUM>) to a supercritical fluid turbine (<NUM>), a method for starting the turbomachine comprising the steps of:
a) charging (<NUM>) a mass of working fluid that is not in the supercritical state in the closed cycle up to a starting mass that is greater than a design operating mass, wherein the design operating mass is the mass of working fluid in the closed cycle when operating at steady-state design point conditions;
b) heating (<NUM>) the working fluid; and
c) venting (<NUM>) a discharge mass of working fluid from the closed cycle at a location (222c) in the closed cycle that is on an exhaust side of the supercritical fluid turbine.