for Waste Heat Recovery

Cross -Reference to Related Applications

[0001] This application claims benefit of U.S. Util. Appl. No. 14/051 ,433, entitled "Supercritical Carbon Dioxide Power Cycle for Waste Heat Recovery," and filed October 1 0, 201 3 and of U.S. Prov. Appl. No. 61 /712,907, entitled "Supercritical Carbon Dioxide Power Cycle for Waste Heat Recovery," and filed October 1 2, 201 2, which is incorporated herein by reference in its entirety to the extent consistent with the present application.

Background

[0002] Heat is often created as a byproduct of industrial processes and is discharged when liquids, solids, and/or gasses that contain such heat are exhausted into the environment or otherwise removed from the process. This heat removal may be necessary to avoid exceeding safe and efficient operating temperatures in the industrial process equipment or may be inherent as exhaust in open cycles. Useful thermal energy is generally lost when this heat is not recovered or recycled during such processes. Accordingly, industrial processes often use heat exchanging devices to recover the heat and recycle much of the thermal energy back into the process or provide combined cycles, utilizing this thermal energy to power secondary heat engine cycles.

[0003] Waste heat recovery can be significantly limited by a variety of factors. For example, the exhaust stream may be reduced to low-grade (e.g., low temperature) heat, from which economical energy extraction is difficult, or the heat may otherwise be difficult to recover. Accordingly, the unrecovered heat is discharged as "waste heat," typically via a stack or through exchange with water or another cooling medium. Moreover, in other settings, heat is available from renewable sources of thermal energy, such as heat from the sun or geothermal sources, which may be concentrated or otherwise manipulated. [0004] In multiple-cycle systems, waste heat is converted to useful energy via two or more components coupled to the waste heat source in multiple locations. While multiple-cycle systems are successfully employed in some operating environments, generally, multiple-cycle systems have limited efficiencies in most operating environments. In some applications, the waste heat conditions (e.g., temperature) can fluctuate, such that the waste heat conditions are temporarily outside the optimal operating range of the multiple-cycle systems. Coupling multiple, discrete cycle systems is one solution. However, multiple independent cycle systems introduce greater system complexity due to the increased number of system components, especially when the system includes additional turbo- or turbine components. Such multiple independent cycle systems are complex and have increased control and maintenance requirements, as well as additional expenses and footprint demands.

[0005] Therefore, there is a need for a heat engine system and a method for recovering energy, such that the system and method have an optimized operating range for a heat recovery power cycle, minimized complexity, and maximized efficiency for recovering thermal energy and producing mechanical energy and/or electrical energy.

Summary

[0006] Embodiments of the invention generally provide heat engine systems and methods for recovering energy, such as by producing mechanical energy and/or generating electrical energy, from a wide range of thermal sources, such as a waste heat source. In one or more exemplary embodiments disclosed herein, a heat engine system contains a working fluid within a working fluid circuit having a high pressure side and a low pressure side. The working fluid generally contains carbon dioxide and at least a portion of the working fluid circuit contains the working fluid in a supercritical state. The heat engine system further contains a first heat exchanger and a second heat exchanger, such that each of the first and second heat exchangers is fluidly coupled to and in thermal communication with the high pressure side of the working fluid circuit, configured to be fluidly coupled to and in thermal communication with a heat source stream (e.g., a waste heat stream), and configured to transfer thermal energy from the heat source stream to the working fluid within the working fluid circuit. The heat engine system also contains a first expander fluidly coupled to and downstream of the first heat exchanger on the high pressure side of the working fluid circuit and a second expander fluidly coupled to and downstream of the second heat exchanger on the high pressure side of the working fluid circuit.

[0007] The heat engine system further contains a first recuperator and a second recuperator fluidly coupled to the working fluid circuit. The first recuperator may be fluidly coupled to and downstream of the first expander on the low pressure side of the working fluid circuit and fluidly coupled to and upstream of the first heat exchanger on the high pressure side of the working fluid circuit. In some embodiments, the first recuperator may be configured to transfer thermal energy from the working fluid received from the first expander to the working fluid received from the first and second pumps when the system is in the dual-cycle mode. The second recuperator may be fluidly coupled to and downstream of the second expander on the low pressure side of the working fluid circuit and fluidly coupled to and upstream of the second heat exchanger on the high pressure side of the working fluid circuit. In some embodiments, the second recuperator may be configured to transfer thermal energy from the working fluid received from the second expander to the working fluid received from the first pump when the system is in dual-cycle mode and is inactive when the system is in the single-cycle mode.

[0008] The heat engine system further contains a condenser, a first pump, and a second pump fluidly coupled to the working fluid circuit. The condenser may be fluidly coupled to and downstream of the first and second recuperators on the low pressure side of the working fluid circuit. The condenser may be configured to remove thermal energy from the working fluid passing through the low pressure side of the working fluid circuit. The condenser may also be configured to control or regulate the temperature of the working fluid circulating through the working fluid circuit. The first pump may be fluidly coupled to and downstream of the condenser on the low pressure side of the working fluid circuit and fluidly coupled to and upstream of the first and second recuperators on the high pressure side of the working fluid circuit. The second pump may be fluidly coupled to and downstream of the condenser on the low pressure side of the working fluid circuit and fluidly coupled to and upstream of the first recuperator on the high pressure side of the working fluid circuit. In some exemplary embodiments, the second pump may be a turbopump, the second expander may be a drive turbine, and the drive turbine may be coupled to the turbopump and operable to drive the turbopump when the heat engine system is in the dual-cycle mode.

[0009] In some exemplary embodiments, the heat engine system further contains a plurality of valves operatively coupled to the working fluid circuit and configured to switch the heat engine system between a dual-cycle mode and a single-cycle mode. In the dual-cycle mode, the first and second heat exchangers and the first and second pumps are active as the working fluid is circulated throughout the working fluid circuit. However, in the single-cycle mode, the first heat exchanger and the first expander are active and at least the second heat exchanger and the second pump are inactive as the working fluid is circulated throughout the working fluid circuit.

[0010] In some examples, the plurality of valves may include a valve disposed between the condenser and the second pump, wherein the valve is closed during the single-cycle mode of the heat engine system and the valve is open when the heat engine system is in the dual-cycle mode. In other examples, the plurality of valves may include a valve disposed between the first pump and the first recuperator, the valve may be configured to prohibit flow of the working fluid from the first pump to the first recuperator when the heat engine system is in the dual-cycle mode and to allow fluid flow therebetween during the single-cycle mode of the heat engine system.

[0011] In other exemplary embodiments, the plurality of valves may include five or more valves operatively coupled to the working fluid circuit for controlling the flow of the working fluid. A first valve may be operatively coupled to the high pressure side of the working fluid circuit and disposed downstream of the first pump and upstream of the second recuperator. A second valve may be operatively coupled to the low pressure side of the working fluid circuit and disposed downstream of the second recuperator and upstream of the condenser. A third valve may be operatively coupled to the high pressure side of the working fluid circuit and disposed downstream of the first pump and upstream of the first recuperator. A fourth valve may be operatively coupled to the high pressure side of the working fluid circuit and disposed downstream of the second pump and upstream of the first recuperator. A fifth valve may be operatively coupled to the low pressure side of the working fluid circuit and disposed downstream of the condenser and upstream of the second pump.

[0012] In some examples, the working fluid from the low pressure side of the first recuperator and the working fluid from the low pressure side of the second recuperator combine at a point on the low pressure side of the working fluid circuit, such that the point is disposed upstream of the condenser and downstream of the second valve. In some configurations, each of the first, second, fourth, and fifth valves may be in an opened-position and the third valve may be in a closed-position when the heat engine system is in the dual-cycle mode. Alternatively, during the single-cycle mode of the heat engine system, each of the first, second, fourth, and fifth valves may be in a closed-position and the third valve may be in an opened-position.

[0013] In other embodiments disclosed herein, the plurality of valves may be configured to actuate in response to a change in temperature of the heat source stream. For example, when the temperature of the heat source stream becomes less than a threshold value, the plurality of valves may be configured to switch the system to the single-cycle mode. Also, when the temperature of the heat source stream becomes equal to or greater than the threshold value, the plurality of valves may be configured to switch the system to the dual-cycle mode.

[0014] In other embodiments disclosed herein, the plurality of valves may be configured to switch the system between the dual-cycle mode and the single-cycle mode, such that in the dual-cycle mode, the plurality of valves may be configured to direct the working fluid from the condenser to the first and second pumps, and subsequently, direct the working fluid from the first pump to the second heat exchanger and/or direct the working fluid from the second pump to the first heat exchanger. In the single-cycle mode, the plurality of valves may be configured to direct the working fluid from the condenser to the first pump and from the first pump to the first heat exchanger, and to substantially cut-off or stop the flow of the working fluid to the second pump, the second heat exchanger, and the second expander. [0015] In one or more embodiments disclosed herein, a method for recovering energy from a heat source (e.g., waste heat source) is provided and includes operating a heat engine system in a dual-cycle mode and subsequently switching the heat engine system from the dual-cycle mode to a single-cycle mode. In the dual-cycle mode, the method includes operating the heat engine system by heating a first mass flow of a working fluid in the first heat exchanger fluidly coupled to and in thermal communication with a working fluid circuit and a heat source stream and expanding the first mass flow in a first expander fluidly coupled to the first heat exchanger via the working fluid circuit. The first heat exchanger may be configured to transfer thermal energy from the heat source stream to the first mass flow of the working fluid within the working fluid circuit. In many exemplary embodiments, the working fluid contains carbon dioxide and at least a portion of the working fluid circuit contains the working fluid in a supercritical state.

[0016] Also, in the dual-cycle mode, the method includes heating a second mass flow of the working fluid in the second heat exchanger fluidly coupled to and in thermal communication with the working fluid circuit and the heat source stream and expanding the second mass flow in a second expander fluidly coupled to the second heat exchanger via the working fluid circuit. The second heat exchanger may be configured to transfer thermal energy from the heat source stream to the second mass flow of the working fluid within the working fluid circuit. The method further includes, in the dual- cycle mode, at least partially condensing the first and second mass flows in one or more condensers fluidly coupled to the working fluid circuit, pressurizing the first mass flow in a first pump fluidly coupled to the condenser via the working fluid circuit, and pressurizing the second mass flow in a second pump fluidly coupled to the condenser via the working fluid circuit.

[0017] In the single-cycle mode, the method includes operating the heat engine system by de-activating the second heat exchanger, the second expander, and the second pump, directing the working fluid from the condenser to the first pump, and directing the working fluid from the first pump to the first heat exchanger. The method may include de-activating the second recuperator and directing the working fluid from the second pump to the first recuperator while switching to the single-cycle mode. [0018] In other embodiments, the method includes operating the heat engine system in the dual-cycle mode by further transferring heat via the first recuperator from the first mass flow downstream of the first expander and upstream of the condenser to the first mass flow downstream of the second pump and upstream of the first heat exchanger, transferring heat via the second recuperator from the second mass flow downstream of the second expander and upstream of the condenser to the second mass flow downstream of the first pump and upstream of the second heat exchanger, and switching to the single-cycle mode further includes de-activating the second recuperator and directing the working fluid from the second pump to the first recuperator.

[0019] In some embodiments, the method further includes monitoring a temperature of the heat source stream, operating the heat engine system in the dual-cycle mode when the temperature is equal to or greater than a threshold value, and subsequently, operating the heat engine system in the single-cycle mode when the temperature is less than the threshold value. In some examples, the threshold value of the temperature of the heat source stream is within a range from about 300°C to about 400°C, such as about 350°C. In one aspect, the method may include automatically switching from operating the heat engine system in the dual-cycle mode to operating the heat engine system in the single-cycle mode with a programmable controller once the temperature is less than the threshold value. In another aspect, the method may include manually switching from operating the heat engine system in the dual-cycle mode to operating the heat engine system in the single-cycle mode once the temperature is less than the threshold value.

Brief Description of the Drawings

[0020] Embodiments of the present disclosure are best understood from the following detailed description when read with the accompanying Figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

[0021] Figure 1 schematically illustrates a heat engine system, operating in dual-cycle mode, according to exemplary embodiments described herein. [0022] Figure 2 schematically illustrates the heat engine of Figure 1 , operating in single- cycle mode, according to exemplary embodiments described herein.

[0023] Figure 3 illustrates a flowchart of a method for extracting energy from heat source, according to exemplary embodiments described herein.

Detailed Description

[0024] Embodiments of the invention generally provide heat engine systems and methods for recovering energy (e.g., generating electricity) with such heat engine systems. Figures 1 and 2 schematically illustrate a heat engine system 1 00, according to an exemplary embodiment described herein. The heat engine system 1 00 is flexible and operates efficiently over a wide range of conditions of the heat source or stream (e.g., waste heat source or stream) from which the heat engine system 1 00 extracts energy. As will be discussed in further detail below, Figure 1 illustrates the heat engine system 100 in dual-cycle mode, while Figure 2 illustrates the heat engine system 1 00 in single-cycle mode. The dual-cycle mode may be particularly suitable for use with heat sources having temperatures greater than a predetermined threshold value, while the single-cycle mode may be particularly useful with heat sources having temperatures less than the threshold value. In some examples, the threshold value of the temperature of the heat source and/or the heat source stream is within a range from about 300°C to about 400°C, such as about 350°C. Since the heat engine system 1 00 is capable of switching between the two modes of operation, for example, back-and- forth without limitation, the heat engine system 1 00 may operate at an increased efficiency over a broader range of heat source temperatures as compared to other heat engines. Although referred to herein as "dual-cycle" and "single-cycle" modes, it will be appreciated that the dual-cycle mode can include three or more cycles operating at once, and the single-cycle mode is intended to be indicative of a reduced number of active cycles, as compared to "dual-cycle" mode, but can include one or more cycles operating at once.

[0025] Referring now specifically to Figure 1 , the heat engine system 1 00 contains a first heat exchanger 1 02 and a second heat exchanger 1 04 fluidly coupled to and in thermal communication with a heat source stream 105, such as a waste heat stream. The heat source stream 1 05 may flow from or otherwise be derived from a heat source 1 06, such as a waste heat source or other source of thermal energy. In an exemplary embodiment, the first and second heat exchangers 1 02, 104 are coupled in series with respect to the heat source stream 1 05, such that the first heat exchanger 102 is disposed upstream of the second heat exchanger 104 along the heat source stream 1 05. Therefore, the first heat exchanger 1 02 generally receives the heat source stream 1 05 at a temperature greater than the temperature of the heat source stream 105 received by the second heat exchanger 104 since a portion of the thermal energy or heat was recovered by the first heat exchanger 1 02 prior to the heat source stream 105 flowing to the second heat exchanger 104.

[0026] The first and second heat exchangers 1 02, 104 may be or include one or more of suitable types of heat exchangers, for example, shell-and-tubes, plates, fins, printed circuits, combinations thereof, and/or any others, without limitation. Furthermore, it will be appreciated that additional heat exchangers may be employed and/or the first and second heat exchangers 1 02, 1 04 may be provided as different sections of a common heat exchanging unit. Since the first heat exchanger 1 02 may be exposed to the heat source stream 1 05 at greater temperatures, a greater amount of recovered thermal energy may be available for conversion to useful power by the expansion devices coupled to the first heat exchanger 1 02, relative to the recovered thermal energy available for conversion by the expansion devices coupled to the second heat exchanger 1 04.

[0027] The heat engine system 1 00 further contains a working fluid circuit 1 1 0, which is fluidly coupled to the first and second heat exchangers 1 02, 104. The working fluid circuit 1 10 may be configured to provide working fluid to and receive heated working fluid from one or both of the first and second heat exchangers 102, 104 as part of a first or "primary" circuit 1 12 and a second or "secondary" circuit 1 1 4. The primary and secondary circuits 1 1 2, 1 1 4 may thus enable collection of thermal energy from the heat source via the first and second heat exchangers 102, 1 04, for conversion into mechanical and/or electrical energy downstream. [0028] The working fluid may be or contain carbon dioxide (C0 ₂ ) and mixtures containing carbon dioxide. Carbon dioxide as a working fluid for power generating cycles has many advantages as a working fluid, such as non-toxicity, non-flammability, easy availability, and relatively inexpensive. Due in part to its relatively high working pressure, a carbon dioxide system can be built that is much more compact than systems using other working fluids. The high density and volumetric heat capacity of carbon dioxide with respect to other working fluids makes carbon dioxide more "energy dense" meaning that the size of all system components can be considerably reduced without losing performance. It should be noted that use of the terms carbon dioxide (C0 ₂ ), supercritical carbon dioxide (sc-C0 ₂ ), or subcritical carbon dioxide (sub-C0 ₂ ) is not intended to be limited to carbon dioxide of any particular type, source, purity, or grade. For example, industrial grade carbon dioxide may be contained in and/or used as the working fluid without departing from the scope of the disclosure.

[0029] The working fluid circuit 1 1 0 contains the working fluid and has a high pressure side and a low pressure side. In exemplary embodiments, the working fluid contained in the working fluid circuit 1 10 is carbon dioxide or substantially contains carbon dioxide and may be in a supercritical state (e.g., sc-C0 ₂ ) and/or a subcritical state (e.g., sub- C0 ₂ ). In one example, the carbon dioxide working fluid contained within at least a portion of the high pressure side of the working fluid circuit 1 1 0 is in a supercritical state and the carbon dioxide working fluid contained within the low pressure side of the working fluid circuit 1 10 is in a subcritical state and/or supercritical state.

[0030] In other exemplary embodiments, the working fluid in the working fluid circuit 1 10 may be a binary, ternary, or other working fluid blend. The working fluid blend or combination can be selected for the unique attributes possessed by the fluid combination within a heat recovery system, as described herein. For example, one such fluid combination includes a liquid absorbent and carbon dioxide mixture enabling the combined fluid to be pumped in a liquid state to high pressure with less energy input than required to compress carbon dioxide. In another exemplary embodiment, the working fluid may be a combination of supercritical carbon dioxide (sc-C0 ₂ ), subcritical carbon dioxide (sub-C0 ₂ ), and/or one or more other miscible fluids or chemical compounds. In yet other exemplary embodiments, the working fluid may be a combination of carbon dioxide and propane, or carbon dioxide and ammonia, without departing from the scope of the disclosure.

[0031] The use of the term "working fluid" is not intended to limit the state or phase of matter of the working fluid or components of the working fluid. For instance, the working fluid or portions of the working fluid may be in a fluid phase, a gas phase, a supercritical state, a subcritical state, or any other phase or state at any one or more points within the heat engine system 100 or fluid cycle. The working fluid may be in a supercritical state over certain portions of the working fluid circuit 1 1 0 (e.g., the high pressure side), and in a subcritical state or a supercritical state over other portions of the working fluid circuit 1 1 0 (e.g., the low pressure side). In other exemplary embodiments, the entire working fluid circuit 1 10 may be operated and controlled such that the working fluid is in a supercritical or subcritical state during the entire execution of the working fluid circuit 1 10.

[0032] The heat source 1 06 and/or the heat source stream 1 05 may derive thermal energy from a variety of high-temperature sources. For example, the heat source stream 1 05 may be a waste heat stream such as, but not limited to, gas turbine exhaust, process stream exhaust, or other combustion product exhaust streams, such as furnace or boiler exhaust streams. Accordingly, the heat engine system 1 00 may be configured to transform waste heat or other thermal energy into electricity for applications ranging from bottom cycling in gas turbines, stationary diesel engine gensets, industrial waste heat recovery (e.g., in refineries and compression stations), and hybrid alternatives to the internal combustion engine. In other exemplary embodiments, the heat source 1 06 may derive thermal energy from renewable sources of thermal energy such as, but not limited to, a solar thermal source and a geothermal source. While the heat source 1 06 and/or the heat source stream 1 05 may be a fluid stream of the high temperature source itself, in other exemplary embodiments, the heat source 106 and/or the heat source stream 105 may be a thermal fluid in contact with the high temperature source. Thermal energy may be transferred from the thermal fluid to the first and second heat exchangers 1 02, 104, and further be transferred from the first and second heat exchangers 1 02, 104 to the working fluid in the working fluid circuit [0033] In various exemplary embodiments, the initial temperature of the heat source 106 and/or the heat source stream 1 05 entering the heat engine system 1 00 may be within a range from about 400°C (about 752°F) to about 650°C (about 1 ,202°F) or greater. However, the working fluid circuit 1 10 containing the working fluid (e.g. , sc-C0 ₂ ) disclosed herein is flexible with respect to the temperature of the heat source stream and thus may be configured to efficiently extract energy from the heat source stream at lesser temperatures, for example, at a temperature of about 400°C (about 752°F) or less, such as about 350°C (about 662°F) or less, such as about 300°C (about 572°F) or less. Accordingly, the heat engine system 100 may include any sensors in or proximal to the heat source stream, for example, to determine the temperature, or another relevant condition (e.g., mass flow rate or pressure) of the heat source stream, to determine whether single or dual-cycle mode is more advantageous.

[0034] In an exemplary embodiment, the heat engine system 1 00 includes a power turbine 1 16, which may also be referred to as a first expander, as part of the primary circuit 1 1 2. The power turbine 1 1 6 is fluidly coupled to the first heat exchanger 1 02 via the primary circuit 1 1 2 and receives fluid from the first heat exchanger 1 02. The power turbine 1 1 6 may be any suitable type of expansion device, such as, for example, a single or multistage impulse or reaction turbine. Further, the power turbine 1 1 6 may be representative of multiple discrete turbines, which cooperate to expand the working fluid provided from the first heat exchanger 102, whether in series or in parallel. The power turbine 1 16 may be disposed between the high pressure side and the low pressure side of the working fluid circuit 1 1 0 and fluidly coupled to and in thermal communication with the working fluid. The power turbine 1 1 6 may be configured to convert thermal energy to mechanical energy by a pressure drop in the working fluid flowing between the high and the low pressure sides of the working fluid circuit 1 10.

[0035] The power turbine 1 1 6 is generally coupled to a generator 1 13 via a shaft 1 1 5, such that the power turbine 1 1 6 rotates the shaft 1 15 and the generator 1 1 3 converts such rotation into electricity. Therefore, the generator 1 1 3 may be configured to convert the mechanical energy from the power turbine 1 16 into electrical energy. Also, the generator 1 13 may be generally electrically coupled to an electrical grid (not shown) and configured to transfer the electrical energy to the electrical grid. It will be appreciated that speed -altering devices, such as gear boxes (not shown), may be employed in such a connection between or with the power turbine 1 1 6, the shaft 1 15, and/or the generator 1 13, or the power turbine 1 16 may be directly coupled to the generator 1 13.

[0036] The heat engine system 100 also contains a first recuperator 1 18, which is fluidly coupled to the power turbine 1 1 6 and receives working fluid therefrom, as part of the primary circuit 1 12. The first recuperator 1 18 may be any suitable heat exchanger or set of heat exchangers, and may serve to transfer heat remaining in the working fluid downstream of the power turbine 1 16 after expansion. For example, the first recuperator 1 1 8 may include one or more plate, fin, shell-and-tube, printed circuit, or other types of heat exchanger, whether in parallel or in series.

[0037] The heat engine system 1 00 also contains one or more condensers 1 20 fluidly coupled to the first recuperator 1 1 8 and configured to receive the working fluid therefrom. The condenser 120 may be, for example, a standard air or water-cooled condenser but may also be a trim cooler, adsorption chiller, mechanical chiller, a combination thereof, and/or the like. The condenser 120 may additionally or instead include one or more compressors, intercoolers, aftercoolers, or the like, which are configured to chill the working fluid, for example, in high ambient temperature regions and/or during summer months. Examples of systems that can be provided for use as the condenser 120 include the condensing systems disclosed in commonly assigned U.S. Appl. No. 13/290,735, filed November 7, 201 1 , and published as U.S. Pub. No. 201 3/01 13221 , which is incorporated herein by reference in its entirety to the extent consistent with the present application.

[0038] The heat engine system 1 00 also contains a first pump 126 as part of the primary circuit 1 1 2 and/or the secondary circuit 1 1 4. The first pump 1 26 may a motor-driven pump or a turbine-driven pump and may be of any suitable design or size, may include multiple pumps, and may be configured to operate with a reduced flow rate and/or reduced pressure head as compared to a second pump 1 17. A reduced flow rate of the working fluid may be desired since less thermal energy may be available for extraction from the heat source stream during a startup process or a shutdown process. Furthermore, the first pump 126 may operate as a starter pump. Accordingly, during startup of the heat engine system 1 00, the first pump 1 26 may operate to power the drive turbine 122 to begin the operation of the second pump 1 1 7.

[0039] The first pump 126 may be fluidly coupled to the working fluid circuit 1 1 0 upstream of the first recuperator 1 1 8 and upstream of the second recuperator 1 28 to provide working fluid at increased pressure and/or flowrate. In one embodiment, the heat engine system 100 may be configured to utilize the first pump 1 26 as part of the primary circuit 1 12. The working fluid may be flowed from the first pump 126, through the third valve 136, through the high pressure side of the first recuperator 1 18, and then supplied back to the first heat exchanger 102, closing the loop on the primary circuit 1 12. In another embodiment, the heat engine system 1 00 may be configured to utilize the first pump 126 as part of the secondary circuit 1 14. The working fluid may be flowed from the first pump 126, through the first valve 130, through the high pressure side of the second recuperator 1 28, and then supplied back to the second heat exchanger 104, closing the loop on the secondary circuit 1 1 4.

[0040] Therefore, the primary circuit 1 12 may be configured to provide the working fluid to circulate in a cycle, whereby the working fluid exits the outlet of the first heat exchanger 1 02, flows through the power turbine throttle valve 1 50, flows through the power turbine 1 16, flows through the low pressure side (or cooling side) of the first recuperator 1 1 8, flows through point 134, flows through the condenser 1 20, flows through the first pump 126, flows through the third valve 1 36, flows through the high pressure side (or heating side) of the first recuperator 1 1 8, and enters the inlet of the first heat exchanger 1 02 to complete the cycle of the primary circuit 1 1 2.

[0041] In another exemplary embodiment described herein, when sufficient thermal energy is available from the heat source 1 06 and the heat source stream 105, the secondary circuit 1 1 4 may be active and configured to support the operation of the primary circuit 1 1 2, for example, by driving a turbopump, such as the second pump 1 1 7. To that end, the heat engine system 1 00 contains the drive turbine 1 22, which is fluidly coupled to the second heat exchanger 104 and may be configured to receive working fluid therefrom, as part of the secondary circuit 1 1 4. The drive turbine 1 22 may be any suitable axial or radial, single or multistage, impulse or reaction turbine, or any such turbines acting in series or in parallel. Further, the drive turbine 122 may be mechanically linked to a turbopump, such as the second pump 1 17 via a shaft 1 24, for example, such that the rotation of the drive turbine 1 22 causes rotation of the second pump 1 1 7. In some exemplary embodiments, the drive turbine 122 may additionally or instead drive other components of the heat engine system 100 or other systems (not shown), may power a generator, and/or may be electrically coupled to one or more motors configured to drive any other device.

[0042] The heat engine system 1 00 may also include a second recuperator 1 28, as part of the secondary circuit 1 1 4, which is fluidly coupled to the drive turbine 1 22 and configured to receive working fluid therefrom in the secondary circuit 1 1 4. The second recuperator 128 may be any suitable heat exchanger or set of heat exchangers, and may serve to transfer heat remaining in the working fluid downstream of the drive turbine 122 after expansion. For example, the second recuperator 1 28 may include one or more plates, fins, shell-and-tubes, printed circuits, or other types of heat exchanger, whether in parallel or in series.

[0043] The second recuperator 1 28 may be fluidly coupled with the condenser 120 via the working fluid circuit 1 10. The low pressure side or cooling side of the second recuperator 128 may be fluidly coupled downstream of the drive turbine 1 22 and upstream of the condenser 1 20. The high pressure side or heating side of the second recuperator 128 may be fluidly coupled downstream of the first pump 1 26 and upstream of the second heat exchanger 104. Accordingly, the condenser 1 20 may receive a combined flow of working fluid from both the first and second recuperators 1 1 8, 1 28. In another exemplary embodiment, the condenser 120 may receive separate flows from the first and second recuperators 1 18, 128 and may mix the flows in the condenser 120. In other exemplary embodiments, the condenser 1 20 may be representative of two condensers, which may maintain the flows as separate streams, without departing from the scope of the disclosure. In the illustrated exemplary embodiment, the primary and secondary circuits 1 12, 1 1 4 may be described as being "overlapping" with respect to the condenser 1 20, as the condenser 120 is part of both the primary and secondary circuits 1 12, 1 14. [0044] The heat engine system 1 00 further includes a second pump 1 17 as part of the secondary circuit 1 1 4 during dual-cycle mode of operation. The second pump 1 1 7 may be fluidly coupled to and disposed downstream of the condenser 120 on the low pressure side of the working fluid circuit 1 1 0, such that the outlet of the condenser 1 20 is upstream of the inlet of the second pump 1 17. Also, the second pump 1 1 7 may be fluidly coupled to and disposed upstream of the first recuperator 1 18 on the high pressure side of the working fluid circuit 1 1 0, such that the inlet of the first recuperator 1 18 is upstream of the outlet of the second pump 1 17.

[0045] The second pump 1 17 may be configured to receive at least a portion of the working fluid condensed in the condenser 120, as part of the secondary circuit 1 14 during the dual-cycle mode of operation. The second pump 1 1 7 may be any suitable turbopump or a component of a turbopump, such as a centrifugal turbopump, which is suitable to pressurize the working fluid, for example, in liquid form, at a desired flow rate to a desired pressure. In one or more embodiments, the second pump 1 1 7 may be a turbopump and may be powered by an expander or turbine, such as a drive turbine 1 22. In one specific exemplary embodiment, the second pump 1 1 7 may be a component of a turbopump unit 1 08 and coupled to the drive turbine 122 by the shaft 1 24, as depicted in Figures 1 and 2. However, in other embodiments, the second pump 1 1 7 may be at least partially driven by the power turbine 1 1 6 (not shown). In an alternative embodiment, instead of being coupled to and driven by the drive turbine 1 22 or another turbine, the second pump 1 1 7 may be coupled to and driven by an electric motor, a gas or diesel engine, or any other suitable device.

[0046] Therefore, the secondary circuit 1 1 4 provides the working fluid to circulate in a cycle, whereby the working fluid exits the outlet of the second heat exchanger 1 04, flows through the turbo pump throttle valve 152, flows through the drive turbine 1 22, flows through the low pressure side (or cooling side) of the second recuperator 1 28, flows through the second valve 132, flows through the condenser 1 20, flows through the fifth valve 142, flows through the second pump 1 17, flows through the fourth valve 1 40, and then is discharged into the primary circuit 1 12 at the point 1 34 on the working fluid circuit 1 10 downstream of the third valve 136 and upstream of the high pressure side of the first recuperator 1 1 8. From the primary circuit 1 1 2, upon setting the third valve 1 36 and the fifth valve 1 42 in closed-positions and the first valve 130 in an opened-position, the secondary circuit 1 1 4 further provides that the working fluid flows through the first pump 1 26, flows through the first valve 1 30, flows through the high pressure side of the second recuperator 1 28, and then supplied back to the second heat exchanger 1 04, closing the loop on the secondary circuit 1 1 4.

[0047] The heat engine system 100 contains a variety of components fluidly coupled to the working fluid circuit 1 1 0, as depicted in Figures 1 and 2. The working fluid circuit 1 10 contains high and low pressure sides during actual operation of the heat engine system 100. Generally, the portions of the high pressure side of the working fluid circuit 1 10 are disposed downstream of the pumps, such as the first pump 1 26 and the second pump 1 1 7, and upstream of the turbines, such as the power turbine 1 16 and the drive turbine 122. Inversely, the portions of the low pressure side of the working fluid circuit 1 10 are disposed downstream of the turbines, such as the power turbine 1 1 6 and the drive turbine 122, and upstream of the pumps, such as the first pump 1 26 and the second pump 1 1 7.

[0048] In an exemplary embodiment, a first portion of the high pressure side of the working fluid circuit 1 1 0 may extend from the first pump 1 26, through the first valve 1 30, through the second recuperator 128, through the second heat exchanger 1 04, through the turbo pump throttle valve 152, and into the drive turbine 1 22. In another exemplary embodiment, a second portion of the high pressure side of the working fluid circuit 1 1 0 may extend from the second pump 1 17, through the fourth valve 1 40, through the first recuperator 1 18, through the first heat exchanger 1 02, through the power turbine throttle valve 1 50, and into the power turbine 1 1 6. In another exemplary embodiment, a first portion of the low pressure side of the working fluid circuit 1 10 may extend from the drive turbine 1 22, through the second recuperator 1 28, through the second valve 132, through the condenser 1 20, and either into the first pump 1 26 and/or through the fifth valve 1 42, and into the second pump 1 1 7. In another exemplary embodiment, a second portion of the low pressure side of the working fluid circuit 1 10 may extend from the power turbine 1 16, through the first recuperator 1 1 8, through the condenser 120, and either into the first pump 126 and/or through the fifth valve 142, and into the second pump 1 1 7. [0049] Some components of the heat engine system 100 may be fluidly coupled to both the high and low pressure sides, such as the turbines, the pumps, and the recuperators. Therefore, the low pressure side or the high pressure side of a particular component refers to the respective low or high pressure side of the working fluid circuit 1 1 0 fluidly coupled to the component. For example, the low pressure side (or cooling side) of the second recuperator 128 refers to the inlet and the outlet on the second recuperator 1 28 fluidly coupled to the low pressure side of the working fluid circuit 1 1 0. In another example, the high pressure side of the power turbine 1 16 refers to the inlet on the power turbine 1 16 fluidly coupled to the high pressure side of the working fluid circuit 1 10 and the low pressure side of the power turbine 1 1 6 refers to the outlet on the power turbine 1 16 fluidly coupled to the low pressure side of the working fluid circuit 1 1 0.

[0050] The heat engine system 1 00 also contains a plurality of valves operable to control the mode of operation of the heat engine system 100. The plurality of valves may include five or more valves. For example, the heat engine system 1 00 contains a first valve 130, a second valve 132, a third valve 1 36, a fourth valve 140, and a fifth valve 1 42. In an exemplary embodiment, the first valve 130 may be operatively coupled to the high pressure side of the working fluid circuit 1 1 0 and may be disposed downstream of the first pump 1 26 and upstream of the second recuperator 1 28. The second valve 1 32 may be operatively coupled to the low pressure side of the working fluid circuit 1 1 0 in the secondary circuit 1 14 and may be disposed downstream of the second recuperator 128 and upstream of the condenser 120. Further, in embodiments of the heat engine system 100 in which the primary and secondary circuits 1 1 2, 1 1 4 overlap to share the condenser 120, the second valve 132 may be disposed upstream of the point 1 34 where the primary and secondary circuits 1 12, 1 1 4 combine, mix, or otherwise come together upstream of the condenser 120. The third valve 1 36 may be operatively coupled to the high pressure side of the working fluid circuit 1 1 0 and may be disposed downstream of the first pump 1 26 and upstream of the first recuperator 1 18. The fourth valve 1 40 may be operatively coupled to the high pressure side of the working fluid circuit 1 10 and may be disposed downstream of the second pump 1 1 7 and upstream of the first recuperator 1 1 8. The fifth valve 1 42 may be operatively coupled to the low pressure side of the working fluid circuit 1 1 0 and may be disposed downstream of the condenser 120 and upstream of the second pump 1 1 7.

[0051] Figure 1 illustrates a dual-cycle mode of operation, according to an exemplary embodiment of the heat engine system 1 00. In dual-cycle mode, both the primary and secondary circuits 1 1 2, 1 1 4 are active, with a first mass flow "m-ι" of working fluid coursing through the primary circuit 1 12, a second mass flow "m ₂ " of working fluid coursing through the secondary circuit 1 14, and a combined flow "m-i +m ₂ " thereof coursing through overlapping sections of the primary and secondary circuits 1 1 2, 1 14, as indicated.

[0052] During the dual-cycle mode of operation, in the primary circuit 1 1 2, the first mass flow m-i of the working fluid recovers energy from the higher-grade heat coursing through the first heat exchanger 102. This heat recovery transitions the first mass flow m-i of the working fluid from an intermediate-temperature, high-pressure working fluid provided to the first heat exchanger 102 during steady-state operation to a high- temperature, high-pressure first mass flow m-ι of the working fluid exiting the first heat exchanger 1 02. In an exemplary embodiment, the working fluid may be at least partially in a supercritical state when exiting the first heat exchanger 1 02.

[0053] The high-temperature, high-pressure (e.g., supercritical state/phase) first mass flow mi is directed in the primary circuit 1 12 from the first heat exchanger 102 to the power turbine 1 16. At least a portion of the thermal energy stored in the high- temperature, high-pressure first mass flow ΠΓΗ is converted to mechanical energy in the power turbine 1 1 6 by expansion of the working fluid. In some examples, the power turbine 1 16 and the generator 1 1 3 may be coupled together and the generator 1 1 3 may be configured to convert the mechanical energy into electrical energy, which can be used to power other equipment, provided to a grid, a bus, or the like. In the power turbine 1 1 6, the pressure, and, to a certain extent, the temperature of the first mass flow mi of the working fluid is reduced; however, the temperature still remains generally in a high temperature range of the primary circuit 1 12. Accordingly, the first mass flow m-i of the working fluid exiting the power turbine 1 16 is a low-pressure, high-temperature working fluid. The low-pressure, high-temperature first mass flow ΓΤΗ of the working fluid may be at least partially in gas phase.

[0054] The low-pressure, high-temperature first mass flow ITH of the working fluid is then directed to the first recuperator 1 1 8. The first recuperator 1 1 8 is coupled to the primary circuit 1 1 2 downstream of the power turbine 1 1 6 on the low-pressure side and upstream of the first heat exchanger 1 02 on the high-pressure side. Accordingly, a portion of the heat remaining in the first mass flow ΓΤΗ of the working fluid exiting from the power turbine 1 1 6 is transferred to a low-temperature, high-pressure first mass flow mi of the working fluid, upstream of the first heat exchanger 1 02. As such, the first recuperator 1 18 acts as a pre-heater for the first mass flow ITH proceeding to the first heat exchanger 1 02, thereby providing the intermediate temperature, high-pressure first mass flow ITH of the working fluid thereto. Further, the first recuperator 1 18 acts as a pre-cooler for the first mass flow ITH of the working fluid proceeding to the condenser 1 20, thereby providing an intermediate-temperature, low-pressure first mass flow ITH of the working fluid thereto.

[0055] Upstream of or within the condenser 120, the intermediate-temperature, low- pressure first mass flow m may be combined with an intermediate-temperature, low- pressure second mass flow m ₂ of the working fluid. However, whether combined or not, the first mass flow ITH may proceed to the condenser 120 for further cooling and, for example, at least partial phase change to a liquid. In an exemplary embodiment, the combined mass flow m ₁ +m ₂ of the working fluid is directed to the condenser 1 20, and subsequently split back into the two mass flows ITH, m ₂ as the working fluid is directed to the discrete portions of the primary and secondary circuits 1 1 2, 1 1 4.

[0056] The condenser 1 20 reduces the temperature of the working fluid, resulting in a low-pressure, low-temperature working fluid, which may be at least partially condensed into liquid phase. In dual-cycle mode, the first mass flow ITH of the low-pressure, low- temperature working fluid is split from the combined mass flow mi +m ₂ and passed from the condenser 1 20 to the second pump 1 1 7 for pressurization. The second pump 1 1 7 may add a nominal amount of heat to the first mass flow ΓΤΗ of the working fluid, but is provided primarily to increase the pressure thereof. Accordingly, the first mass flow ITH of the working fluid exiting the second pump 1 17 is a high-pressure, low-temperature working fluid. The first mass flow m-i of the working fluid is then directed to the first recuperator 1 18, for heat transfer with the high-temperature, low-pressure first mass flow m-i of the working fluid, downstream of the power turbine 1 16. The first mass flow mi of the working fluid exiting the first recuperator 1 1 8 as an intermediate-temperature, high-pressure first mass flow m of the working fluid, and is directed to the first heat exchanger 102, thereby closing the loop of the primary circuit 1 1 2.

[0057] During dual-cycle mode, as shown in Figure 1 , the second mass flow m ₂ of combined flow m-i+m ₂ working fluid from the condenser 120 is split off and directed into the secondary circuit 1 1 4. The second mass flow m ₂ may be directed to the first pump 1 26, for example. The first pump 126 may heat the fluid to a certain extent; however, the primary purpose of the first pump 126 is to pressurize the working fluid. Accordingly, the second mass flow m ₂ of the working fluid exiting the first pump 1 26 is a low-temperature, high-pressure second mass flow m ₂ of the working fluid.

[0058] The low-temperature, high-pressure second mass flow m ₂ of the working fluid is then routed to the second recuperator 1 28 for preheating. The second recuperator 1 28 is coupled to the secondary circuit 1 14 downstream of the first pump 1 26 on the high- pressure side, upstream of the second heat exchanger 1 04 on the high-pressure side, and downstream of the drive turbine 1 22 on the low-pressure side. The second mass flow m ₂ of the working fluid from the first pump 126 is preheated in the recuperator 1 28 to provide an intermediate-temperature, high-pressure second mass flow m ₂ of the working fluid to the second heat exchanger 104.

[0059] The second mass flow m ₂ of the working fluid in the second heat exchanger 1 04 is heated to provide a high-temperature, high-pressure second mass flow m ₂ of the working fluid. In an exemplary embodiment, the second mass flow m ₂ of the working fluid exiting the second heat exchanger 104 may be in a supercritical state. The high- temperature, high-pressure second mass flow m ₂ of the working fluid may then be directed to the drive turbine 122 for expansion to drive the second pump 1 17, for example, thus closing the loop on the secondary circuit 1 14. [0060] During dual-cycle mode, the first, second, fourth, and fifth valves 130, 132, 1 40, 1 42 may be open (each valve in an opened-position), while the third valve 136 may be closed (valve in a closed-position), as shown in an exemplary embodiment. As indicated by the solid lines depicting fluid conduits therebetween, the first, second, fourth, and fifth valves 1 30, 132, 1 40, 1 42 - in opened-positions - allow fluid communication therethrough. As such, the first pump 126 is in fluid communication with the second recuperator 1 28 via the first valve 1 30, and the second recuperator 128 is in fluid communication with the condenser 1 20 via the second valve 1 32. Further, the second pump 1 1 7 is in fluid communication with the first recuperator 1 1 8 via the fourth valve 1 40, and the condenser 120 is in fluid communication with the second pump 1 1 7 via the fifth valve 1 42. In contrast, as depicted by the dashed line for conduit 1 38, although they are fluidly coupled as the term is used herein, fluid communication between the first pump 126 and the first recuperator 1 1 8 is generally prohibited by the third valve 1 36 in a closed-position.

[0061] Such configuration of the valves 130, 132, 1 36, 140, 1 42 maintains the separation of the discrete portions of the primary and secondary circuits 1 1 2, 1 1 4 upstream and downstream of, for example, the condenser 120. Accordingly, the secondary circuit 1 1 4 may be operable to recover thermal energy from the heat source stream 105 in the second heat exchanger 1 04 and employ such thermal energy to, for example, power the drive turbine 1 22, which drives the second pump 1 17 of the primary circuit 1 1 2. The primary circuit 1 12, in turn, may recover a greater amount of thermal energy from the heat source stream 1 05 in the first heat exchanger 1 02, as compared to the thermal energy recovered by the secondary circuit 1 1 4 in the second heat exchanger 104, and may convert the thermal energy into shaft rotation and/or electricity as an end-product for the heat engine system 1 00.

[0062] Figure 2 schematically depicts the heat engine system 1 00 of Figure 1 , but with the opened/closed-positions of the valves 1 30, 1 32, 1 36, 140, 142 being changed to provide the single-cycle mode of operation for the heat engine system 100, according to an exemplary embodiment. In the single-cycle mode of operation, the heat engine system 100 may be utilized with less or a reduced number of active components and conduits of the working fluid circuit 1 1 0 than in the dual-cycle mode of operation. Active components and conduits contain the working fluid flowing or otherwise passing therethrough during normal operation of the heat engine system 100. Inactive components and conduits have a reduced flow or lack flow of the working fluid passing therethrough during normal operation of the heat engine system 1 00. The inactive components and conduits are indicated in Figure 2 by dashed lines, according to one exemplary embodiment among many contemplated. More particularly, the flow of the working fluid to the second heat exchanger 1 04 may be substantially cut-off in the single-cycle mode, thereby de-activating the second heat exchanger 1 04. The flow of the working fluid to the second heat exchanger 104 may be initially cut-off due to reduced temperature of the heat source stream 105 from the heat source 1 06, component failure, or for other reasons. In one configuration, the heat engine system 1 00 may include a sensor (not shown) which may monitor the temperature of the heat source stream 1 05, for example, as the heat source stream 105 enters the first heat exchanger 1 02. Once the sensor reads or otherwise measures a temperature of less than a threshold value, for example, the heat engine system 100 may be switched, either manually or automatically with a programmable controller, to operate in single- cycle mode. Once the temperature becomes equal to or greater than the threshold value, the heat engine system 100 may be switched back to the dual-cycle mode. In some embodiments, the threshold value of the temperature of the heat source and/or the heat source stream 1 05 may be within a range from about 300°C (about 572°F) to about 400°C (about 752°F), more narrowly within a range from about 320°C (about 608°F) to about 380°C (about 716°F), and more narrowly within a range from about 340°C (about 644°F) to about 360°C (about 680°F), for example, about 350°C (about 662°F).

[0063] As indicated, the first heat exchanger 1 02 may be active, while the second heat exchanger 104 is inactive or de-activated. Thus, splitting of the combined flow of the working fluid to feed both heat exchangers 1 02, 1 04, described herein for the dual-cycle mode of operation, may no longer be required and a single mass flow "m" of the working fluid to the first heat exchanger 102 may develop. Additionally, flow of the working fluid to the drive turbine 122 and the second recuperator 128 may also be cut- off or stopped, as the working fluid flows may be provided to recover thermal energy via the second heat exchanger 1 04, as discussed above, which is now inactive.

[0064] Since the drive turbine 1 22, powered by thermal energy recovered in the second heat exchanger 1 04 during the dual-cycle mode of operation, is also inactive or deactivated during the single-cycle mode of operation, the second pump 1 1 7 may lack a driver. Accordingly, the second pump 1 17 may be isolated and deactivated via closure of the fourth and fifth valves 140, 1 42. However, as is known for thermodynamic cycles, the working fluid in the active primary circuit 1 12 requires pressurization, which, in the single-cycle mode of operation, may be provided by the first pump 1 26. By closure of the fifth valve 1 42 and opening of the third valve 136, the working fluid is directed from the condenser 1 20 and to the first pump 126 for pressurization. Thereafter, the working fluid proceeds to the first recuperator 1 1 8 and then to the first heat exchanger 102.

[0065] Although described as two-way control valves, it will be appreciated that the valves 130, 1 32, 136, 140, 1 42 may be provided by any suitable type of valve. For example, the second and fourth valves 1 32, 140 may function to stop back-flow into inactive portions of the heat engine system 100. More particularly, in an exemplary embodiment, the fifth valve 1 42 prevents fluid from flowing through the second pump 1 17 and to the fourth valve 1 40, while the first valve 1 30 prevents fluid from flowing through the second recuperator 1 28, second heat exchanger 1 04, and drive turbine 122 to the second valve 132. The function of the second and fourth valves 1 32, 1 40, thus, is to prevent reverse flow into the inactive components. As such, the second and fourth valves 132, 1 40 may be one-way check valves. Furthermore, in another configuration, the first and third valves 130, 1 36, for example, may be combined and replaced with a three-way valve, without departing from the scope of the disclosure. Since a single three-way valve may effectively provide the function of two two-way valves, reference to the first and third valves 1 30, 136 together is to be construed to literally include a single three-way valve, or a valve with greater than three ways (e.g., four-way), that provides the function described herein.

[0066] The heat engine system 1 00 further contains a power turbine throttle valve 1 50 fluidly coupled to the working fluid circuit 1 10 upstream of the inlet of the power turbine 1 16 and downstream of the outlet of the first heat exchanger 1 02. The power turbine throttle valve 150 may be configured to modulate, adjust, or otherwise control the flowrate of the working fluid passing into the power turbine 1 16, thereby providing control of the power turbine 1 16 and the amount of work energy produced by the power turbine 1 1 6. Also, the heat engine system 1 00 further contains a turbo pump throttle valve 1 52 fluidly coupled to the working fluid circuit 1 1 0 upstream of the inlet of the drive turbine 122 of the turbopump unit 1 08 and downstream of the outlet of the second heat exchanger 1 04. The turbo pump throttle valve 152 may be configured to modulate, adjust, or otherwise control the flowrate of the working fluid passing into the drive turbine 1 22, thereby providing control of the drive turbine 1 22 and the amount of work energy produced by the drive turbine 1 22. The power turbine throttle valve 1 50 and the turbo pump throttle valve 152 may be independently controlled by the process control system (not shown) that is communicably connected, wired and/or wirelessly, with the power turbine throttle valve 1 50, the turbo pump throttle valve 152, and other components and parts of the heat engine system 1 00.

[0067] Figure 3 illustrates a flowchart of a method 200 for extracting energy from heat source stream. The method 200 may proceed by operation of one or more embodiments of the heat engine system 1 00, as described herein with reference to Figures 1 and/or 2 and may thus be best understood with continued reference thereto. The method 200 may include operating a heat engine system in a dual-cycle mode, as at 202. The method 200 may further include sensing the temperature or another condition of heat source stream fed to the system, as at 204, for example, as the heat source stream is fed into a first heat exchanger, which is thermally coupled to the heat source (e.g., waste heat source or stream). This may occur prior to, during, or after initiation of operation of the dual-cycle mode at 202. If the temperature of the heat source stream is less than a threshold value, the method 200 may switch the system to operate in a single-cycle mode, as at 206. In some examples, the threshold value of the temperature may be within a range from about 300°C to about 400°C, more narrowly within a range from about 320°C to about 380°C, and more narrowly within a range from about 340°C to about 360°C, such as about 350°C. The sensing at 204 may be iterative, may be polled on a time delay, may operate on an alarm, trigger, or interrupt basis to alert a controller coupled to the system, or may simply result in a display to an operator, who may then toggle the system to the appropriate operating cycle.

[0068] Operating the heat engine system in dual-cycle mode, as at 202, may include heating a first mass flow of working fluid in the first heat exchanger thermally coupled to a heat source, as at 302. Operating at 202 may also include expanding the first mass flow in a first expander, as at 304. Operating at 202 may also include heating a second mass flow of working fluid in a second heat exchanger thermally coupled to the heat source, as at 306. Operating at 202 may further include expanding the second mass flow in a second expander, as at 308. Additionally, operating at 202 may include at least partially condensing the first and second mass flows in one or more condensers, as at 310. Operating at 202 may include pressurizing the first mass flow in a first pump, as at 31 2. Operating at 202 may also include pressurizing the second mass flow in a second pump, as at 314.

[0069] In an exemplary embodiment, operating at 202 may include transferring heat from the first mass flow downstream of the first expander and upstream of the condenser to the first mass flow downstream of the first pump and upstream of the first heat exchanger. Further, operating at 202 may also include transferring heat from the second mass flow downstream of the second expander and upstream of the condenser to the second mass flow downstream of the second pump and upstream of the second heat exchanger.

[0070] Switching at 204 may include de-activating the second heat exchanger, the second expander, and the first pump, as at 402. Switching at 204 may also include directing the working fluid from the condenser to the second pump, as at 404. Switching at 204 may also include directing the working fluid from the first pump to the first heat exchanger, as at 406. In embodiments including first and second recuperators, switching at 204 may also include de-activating the second recuperator and directing the working fluid from the second pump to the first recuperator.

Exemplary Embodiments

[0071] In one or more exemplary embodiments disclosed herein, as depicted in Figure 1 and 2, a heat engine system 100 contains a working fluid within a working fluid circuit 1 10 having a high pressure side and a low pressure side. The working fluid generally contains carbon dioxide and at least a portion of the working fluid circuit 1 1 0 contains the working fluid in a supercritical state. The heat engine system 1 00 further contains a first heat exchanger 102 and a second heat exchanger 1 04, such that each of the first and second heat exchangers 1 02, 1 04 may be fluidly coupled to and in thermal communication with the high pressure side of the working fluid circuit 1 1 0, configured to be fluidly coupled to and in thermal communication with a heat source stream 105 (e.g., a waste heat stream), and configured to transfer thermal energy from the heat source stream 1 05 to the working fluid within the working fluid circuit 1 1 0. The heat source stream 1 05 may flow from or otherwise be derived from a heat source 106, such as a waste heat source or other source of thermal energy. The heat engine system 100 also contains a first expander, such as a power turbine 1 16, fluidly coupled to and disposed downstream of the first heat exchanger 102 on the high pressure side of the working fluid circuit 1 1 0 and a second expander, such as a drive turbine 122, fluidly coupled to and disposed downstream of the second heat exchanger 104 on the high pressure side of the working fluid circuit 1 1 0.

[0072] The heat engine system 100 further contains a first recuperator 1 18 and a second recuperator 128 fluidly coupled to the working fluid circuit 1 10. The first recuperator 1 18 may be fluidly coupled to and disposed downstream of the power turbine 1 16 on the low pressure side of the working fluid circuit 1 10 and fluidly coupled to and disposed upstream of the first heat exchanger 1 02 on the high pressure side of the working fluid circuit 1 10. In some embodiments, the first recuperator 1 1 8 may be configured to transfer thermal energy from the working fluid received from the power turbine 1 1 6 to the working fluid received from the first and second pumps 1 26, 1 1 7 when the heat engine system 100 is in the dual-cycle mode. The second recuperator 1 28 may be fluidly coupled to and disposed downstream of the drive turbine 1 22 on the low pressure side of the working fluid circuit 1 1 0 and fluidly coupled to and disposed upstream of the second heat exchanger 1 04 on the high pressure side of the working fluid circuit 1 1 0. In some embodiments, the second recuperator 1 28 may be configured to transfer thermal energy from the working fluid received from the drive turbine 1 22 to the working fluid received from the first pump 1 26 when the heat engine system 100 is in dual-cycle mode and is inactive when the heat engine system 1 00 is in the single- cycle mode.

[0073] The heat engine system 1 00 further contains a condenser 120, a first pump 1 26, and a second pump 1 1 7 fluidly coupled to the working fluid circuit 1 10. The condenser 1 20 may be fluidly coupled to and disposed downstream of the first recuperator 1 1 8 and the second recuperator 1 28 on the low pressure side of the working fluid circuit 1 1 0. The condenser 1 20 may be configured to remove thermal energy from the working fluid passing through the low pressure side of the working fluid circuit 1 10. The condenser 1 20 may also be configured to control or regulate the temperature of the working fluid circulating through the working fluid circuit 1 10. The first pump 126 may be fluidly coupled to and disposed downstream of the condenser 1 20 on the low pressure side of the working fluid circuit 1 10 and fluidly coupled to and disposed upstream of the first recuperator 1 18 and the second recuperator 128 on the high pressure side of the working fluid circuit 1 10. The second pump 1 17 may be fluidly coupled to and disposed downstream of the condenser 120 on the low pressure side of the working fluid circuit 1 10 and fluidly coupled to and disposed upstream of the first recuperator 1 1 8 on the high pressure side of the working fluid circuit 1 1 0. In some exemplary embodiments, the second pump 1 17 may be a turbopump, the second expander may be the drive turbine 1 22, and the drive turbine 1 22 may be coupled to the turbopump and operable to drive the turbopump when the heat engine system 1 00 is in the dual-cycle mode.

[0074] In some exemplary embodiments, the heat engine system 100 further contains a plurality of valves operatively coupled to the working fluid circuit 1 10 and configured to switch the heat engine system 100 between a dual-cycle mode and a single-cycle mode. In the dual-cycle mode, the first and second heat exchangers 1 02, 104 and the first and second pumps 1 26, 1 17 are active as the working fluid is circulated throughout the working fluid circuit 1 10. However, in the single-cycle mode, the first heat exchanger 1 02 and the power turbine 1 16 are active and at least the second heat exchanger 104 and the second pump 1 1 7 are inactive as the working fluid is circulated throughout the working fluid circuit 1 10. [0075] In other exemplary embodiments, the plurality of valves may include five or more valves operatively coupled to the working fluid circuit 1 1 0 for controlling the flow of the working fluid. A first valve 130 may be operatively coupled to the high pressure side of the working fluid circuit 1 10 and disposed downstream of the first pump 1 26 and upstream of the second recuperator 128. A second valve 132 may be operatively coupled to the low pressure side of the working fluid circuit 1 1 0 and disposed downstream of the second recuperator 128 and upstream of the condenser 120. A third valve 136 may be operatively coupled to the high pressure side of the working fluid circuit 1 10 and disposed downstream of the first pump 126 and upstream of the first recuperator 1 1 8. A fourth valve 140 may be operatively coupled to the high pressure side of the working fluid circuit 1 10 and disposed downstream of the second pump 1 17 and upstream of the first recuperator 1 18. A fifth valve 142 may be operatively coupled to the low pressure side of the working fluid circuit 1 10 and disposed downstream of the condenser 120 and upstream of the second pump 1 17.

[0076] In some examples, the plurality of valves may include a valve, such as the fourth valve 140, disposed between the condenser 120 and the second pump 1 17, wherein the fourth valve 140 is closed when the heat engine system 100 is in the single-cycle mode and the fourth valve 140 is open when the heat engine system 100 is in the dual- cycle mode. In other examples, the plurality of valves may include a valve, such as the third valve 136, disposed between the first pump 126 and the first recuperator 1 1 8, the third valve 136 may be configured to prohibit flow of the working fluid from the first pump 1 26 to the first recuperator 1 18 when the heat engine system 100 is in the dual-cycle mode and to allow fluid flow therebetween when the heat engine system 100 is in the single-cycle mode.

[0077] In some examples, the working fluid from the low pressure side of the first recuperator 1 18 and the working fluid from the low pressure side of the second recuperator 1 28 combine at a point 134 on the low pressure side of the working fluid circuit 1 10, such that the point 1 34 may be disposed upstream of the condenser 120 and downstream of the second valve 132. In some configurations, each of the first, second, fourth, and fifth valves 130, 132, 1 40, 1 42 may be in an opened-position and the third valve 136 may be in a closed-position when the heat engine system 100 is in the dual-cycle mode. Alternatively, when the heat engine system 1 00 is in the single- cycle mode, each of the first, second, fourth, and fifth valves 130, 132, 1 40, 1 42 may be in a closed-position and the third valve 1 36 may be in an opened-position.

[0078] In other embodiments disclosed herein, the plurality of valves may be configured to actuate in response to a change in temperature of the heat source stream 1 05. For example, when the temperature of the heat source stream 1 05 becomes less than a threshold value, the plurality of valves may be configured to switch the heat engine system 1 00 to the single-cycle mode. Also, when the temperature of the heat source stream 1 05 becomes equal to or greater than the threshold value, the plurality of valves may be configured to switch the heat engine system 1 00 to the dual-cycle mode. In some examples, the threshold value of the temperature of the heat source stream 105 is within a range from about 300°C to about 400°C, such as about 350°C.

[0079] In other embodiments disclosed herein, the plurality of valves may be configured to switch the heat engine system 1 00 between the dual-cycle mode and the single-cycle mode, such that in the dual-cycle mode, the plurality of valves may be configured to direct the working fluid from the condenser 1 20 to the first and second pumps 1 26, 1 17, and subsequently, direct the working fluid from the first pump 126 to the second heat exchanger 1 04 and/or direct the working fluid from the second pump 1 17 to the first heat exchanger 1 02. In the single-cycle mode, the plurality of valves may be configured to direct the working fluid from the condenser 120 to the first pump 126 and from the first pump 1 26 to the first heat exchanger 1 02, and to substantially cut-off or stop the flow of the working fluid to the second pump 1 17, the second heat exchanger 1 04, and the drive turbine 122.

[0080] In one or more embodiments disclosed herein, a method for recovering energy from a heat source (e.g., waste heat source) is provided and includes operating a heat engine system 100 in a dual-cycle mode and subsequently switching the heat engine system 1 00 from the dual-cycle mode to a single-cycle mode. In the dual-cycle mode, the method includes operating the heat engine system 1 00 by heating a first mass flow of a working fluid in the first heat exchanger 102 fluidly coupled to and in thermal communication with a working fluid circuit 1 10 and a heat source stream 105 and expanding the first mass flow in a power turbine 1 16 fluidly coupled to the first heat exchanger 1 02 via the working fluid circuit 1 10. The first heat exchanger 102 may be configured to transfer thermal energy from the heat source stream 1 05 to the first mass flow of the working fluid within the working fluid circuit 1 1 0. In many exemplary embodiments, the working fluid contains carbon dioxide and at least a portion of the working fluid circuit 1 10 contains the working fluid in a supercritical state.

[0081] Also, in the dual-cycle mode, the method includes heating a second mass flow of the working fluid in the second heat exchanger 1 04 fluidly coupled to and in thermal communication with the working fluid circuit 1 10 and the heat source stream 105 and expanding the second mass flow in a second expander, such as the drive turbine 122, fluidly coupled to the second heat exchanger 104 via the working fluid circuit 1 1 0. The second heat exchanger 104 may be configured to transfer thermal energy from the heat source stream 1 05 to the second mass flow of the working fluid within the working fluid circuit 1 1 0. The method further includes, in the dual-cycle mode, at least partially condensing the first and second mass flows in one or more condensers, such as the condenser 1 20, fluidly coupled to the working fluid circuit 1 1 0, pressurizing the first mass flow in a first pump 126 fluidly coupled to the condenser 120 via the working fluid circuit 1 1 0, and pressurizing the second mass flow in a second pump 1 1 7 fluidly coupled to the condenser 120 via the working fluid circuit 1 10.

[0082] In the single-cycle mode, the method includes operating the heat engine system 1 00 by de-activating the second heat exchanger 104, the drive turbine 122, and the second pump 1 17, directing the working fluid from the condenser 1 20 to the first pump 1 26, and directing the working fluid from the first pump 1 26 to the first heat exchanger 1 02. The method may include de-activating the second recuperator 128 and directing the working fluid from the second pump 1 17 to the first recuperator 1 1 8 while switching to the single-cycle mode.

[0083] In other embodiments, the method includes operating the heat engine system 1 00 in the dual-cycle mode by further transferring heat via the first recuperator 1 18 from the first mass flow "m-i " downstream of the power turbine 1 1 6 and upstream of the condenser 120 to the first mass flow m-i downstream of the second pump 1 1 7 and upstream of the first heat exchanger 1 02, transferring heat via the second recuperator 1 28 from the second mass flow "m ₂ " downstream of the drive turbine 1 22 and upstream of the condenser 1 20 to the second mass flow m ₂ downstream of the first pump 126 and upstream of the second heat exchanger 1 04, and switching to the single-cycle mode further includes de-activating the second recuperator 1 28 and directing the working fluid from the second pump 1 17 to the first recuperator 1 1 8.

[0084] In some embodiments, the method further includes monitoring a temperature of the heat source stream 1 05, operating the heat engine system 100 in the dual-cycle mode when the temperature is equal to or greater than a threshold value, and subsequently, operating the heat engine system 1 00 in the single-cycle mode when the temperature is less than the threshold value. In some examples, the threshold value of the temperature of the heat source stream 105 is within a range from about 300°C to about 400°C, such as about 350°C. In one aspect, the method may include automatically switching from operating the heat engine system 100 in the dual-cycle mode to operating the heat engine system 100 in the single-cycle mode with a programmable controller once the temperature is less than the threshold value. In another aspect, the method may include manually switching from operating the heat engine system 100 in the dual-cycle mode to operating the heat engine system 1 00 in the single-cycle mode once the temperature is less than the threshold value.

[0085] It is to be understood that the present disclosure describes several exemplary embodiments for implementing different features, structures, or functions of the disclosure. Exemplary embodiments of components, arrangements, and configurations are described herein to simplify the present disclosure; however, these exemplary embodiments are provided merely as examples and are not intended to limit the scope of the invention. Additionally, the present disclosure may repeat reference numerals and/or letters in the various exemplary embodiments and across the Figures provided herein. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various exemplary embodiments and/or configurations discussed in the various Figures. Moreover, the formation of a first feature over or on a second feature in the present disclosure may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Finally, the exemplary embodiments described herein may be combined in any combination of ways, e.g., any element from one exemplary embodiment may be used in any other exemplary embodiment without departing from the scope of the disclosure.

[0086] Additionally, certain terms are used throughout the written description and claims for referring to particular components. As one skilled in the art will appreciate, various entities may refer to the same component by different names, and as such, the naming convention for the elements described herein is not intended to limit the scope of the disclosure, unless otherwise specifically defined herein. Further, the naming convention used herein is not intended to distinguish between components that differ in name but not function. Further, in the written description and the claims, the terms "including," "containing," and "comprising" are used in an open-ended fashion, and thus should be interpreted to mean "including, but not limited to". All numerical values in this disclosure may be exact or approximate values unless otherwise specifically stated. Accordingly, various embodiments of the disclosure may deviate from the numbers, values, and ranges disclosed herein without departing from the intended scope. Furthermore, as it is used in the claims or specification, the term "or" is intended to encompass both exclusive and inclusive cases, i.e., "A or B" is intended to be synonymous with "at least one of A and B," unless otherwise expressly specified herein.

[0087] The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.