Patent Publication Number: US-9410449-B2

Title: Driven starter pump and start sequence

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/205,082, entitled “Driven Starter Pump and Start Sequence,” and filed on Aug. 8, 2011, which claims benefit of U.S. Prov. Appl. No. 61/417,789, entitled “Parallel Cycle Heat Engines,” and filed on Nov. 29, 2010, and which claims priority to PCT Appl. No. US2011/029486, entitled “Heat Engines with Cascade Cycles,” and filed on Mar. 22, 2011, the contents of which are hereby incorporated by reference to the extent not inconsistent with the present disclosure. 
    
    
     BACKGROUND 
     Heat is often created as a byproduct of industrial processes where flowing streams of high-temperature liquids, solids, or gases must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Sometimes the industrial process can use heat exchanger devices to capture the heat and recycle it back into the process via other process streams. Other times it is not feasible to capture and recycle this heat either because its temperature is too high or it may contain insufficient mass flow. This heat is referred to as “waste” heat and is typically discharged directly into the environment or indirectly through a cooling medium, such as water or air. 
     This waste heat can be converted into useful work by a variety of turbine generator systems that employ well-known thermodynamic methods, such as the Rankine cycle. These thermodynamic methods are typically steam-based processes where the waste heat is recovered and used to generate steam from water in a boiler in order to drive a corresponding turbine. Organic Rankine cycles replace the water with a lower boiling-point working fluid, such as a light hydrocarbon like propane or butane, or a HCFC (e.g., R245fa) fluid. More recently, and in view of issues such as thermal instability, toxicity, or flammability of the lower boiling-point working fluids, some thermodynamic cycles have been modified to circulate more greenhouse-friendly and/or neutral working fluids, such as carbon dioxide or ammonia. 
     A pump is required to pressurize and circulate the working fluid throughout the working fluid circuit. The pump is typically a motor-driven pump, however, these pumps require costly shaft seals to prevent working fluid leakage and often require the implementation of a gearbox and a variable frequency drive which add to the overall cost and complexity of the system. Replacing the motor-driven pump with a turbopump eliminates one or more of these issues, but at the same time introduces problems of starting and “bootstrapping” the turbopump, which relies heavily on the circulation of heated working fluid for proper operation. Unless the turbopump is provided with a successful start sequence, the turbopump will not be able to bootstrap itself and thereafter attain steady-state operation. 
     What is needed, therefore, is a system and method of operating a waste heat recovery thermodynamic cycle that provides a successful start sequence adapted to start a turbopump and bring it to steady-state operation. 
     SUMMARY 
     Embodiments of the disclosure may provide a heat engine system for converting thermal energy into mechanical energy. The heat engine system may include a turbopump comprising a main pump operatively coupled to a drive turbine and hermetically-sealed within a casing, the main pump being configured to circulate a working fluid throughout a working fluid circuit, wherein the working fluid is separated in the working fluid circuit into a first mass flow and a second mass flow. The heat engine system may also include a first heat exchanger in fluid communication with the main pump and in thermal communication with a heat source, the first heat exchanger being configured to receive the first mass flow and transfer thermal energy from the heat source to the first mass flow. The heat engine system may further include a power turbine fluidly coupled to the first heat exchanger and configured to expand the first mass flow, a first recuperator fluidly coupled to the power turbine and configured to receive the first mass flow discharged from the power turbine, and a second recuperator fluidly coupled to the drive turbine, the drive turbine being configured to receive and expand the second mass flow and discharge the second mass flow into the second recuperator. Moreover, the heat engine system may include a starter pump arranged in parallel with the main pump in the working fluid circuit, a first recirculation line fluidly coupling the main pump with a low pressure side of the working fluid circuit and a second recirculation line fluidly coupling the starter pump with the low pressure side of the working fluid circuit. 
     Embodiments of the disclosure may further provide a method for starting a turbopump in a thermodynamic working fluid circuit. The exemplary method may include circulating a working fluid in the working fluid circuit with a starter pump, the starter pump being in fluid communication with a first heat exchanger that is in thermal communication with a heat source, transferring thermal energy to the working fluid from the heat source in the first heat exchanger, and expanding the working fluid in a drive turbine fluidly coupled to the first heat exchanger, the drive turbine being operatively coupled to a main pump, where the drive turbine and the main pump comprise the turbopump. The method may further include driving the main pump with the drive turbine, diverting the working fluid discharged from the main pump into a first recirculation line fluidly communicating the main pump with a low pressure side of the working fluid circuit, the first recirculation line having a first bypass valve arranged therein, and closing the first bypass valve as the turbopump reaches a self-sustaining speed of operation. The method may also include circulating the working fluid discharged from the main pump through the working fluid circuit, deactivating the starter pump and opening a second bypass valve arranged in a second recirculation line fluidly communicating the starter pump with the low pressure side of the working fluid circuit, and diverting the working fluid discharged from the starter pump into the second recirculation line. 
     Embodiments of the disclosure may further provide another exemplary heat engine system for converting thermal energy into mechanical energy. The heat engine system may include a turbopump including a main pump operatively coupled to a drive turbine and hermetically-sealed within a casing, the main pump being configured to circulate a working fluid throughout a working fluid circuit, a starter pump arranged in parallel with the main pump in the working fluid circuit, and a first check valve arranged in the working fluid circuit downstream from the main pump. The heat engine system may also include a second check valve arranged in the working fluid circuit downstream from the starter pump and fluidly coupled to the first check valve, a power turbine fluidly coupled to both the main pump and the starter pump, and a shut-off valve arranged in the working fluid circuit to divert the working fluid around the power turbine. The heat engine system may further include a first recirculation line fluidly coupling the main pump with a low pressure side of the working fluid circuit, and a second recirculation line fluidly coupling the starter pump with the low pressure side of the working fluid circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is 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. 
         FIG. 1  illustrates a schematic of a cascade thermodynamic waste heat recovery cycle, according to one or more embodiments disclosed. 
         FIG. 2  illustrates a schematic of a parallel heat engine cycle, according to one or more embodiments disclosed. 
         FIG. 3  illustrates a schematic of another parallel heat engine cycle, according to one or more embodiments disclosed. 
         FIG. 4  illustrates a schematic of another parallel heat engine cycle, according to one or more embodiments disclosed. 
         FIG. 5  is a flowchart of a method for starting a turbopump in a thermodynamic working fluid circuit, according to one or more embodiments disclosed. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure describes several exemplary embodiments for implementing different features, structures, or functions of the inventions. Exemplary embodiments of components, arrangements, and configurations are described below to simplify the present disclosure; however, these exemplary embodiments are provided merely as examples and are not intended to limit the scope of the inventions. 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 description that follows 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 presented below may be combined in any combination of ways, i.e., any element from one exemplary embodiment may be used in any other exemplary embodiment, without departing from the scope of the disclosure. 
     Additionally, certain terms are used throughout the following description and claims to refer 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 inventions, 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. Additionally, in the following discussion and in the claims, the terms “including” 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. 
       FIG. 1  illustrates an exemplary heat engine system  100 , which may also be referred to as a thermal engine, a power generation device, a heat or waste heat recovery system, and/or a heat to electricity system. The heat engine system  100  may encompass one or more elements of a Rankine thermodynamic cycle configured to produce power from a wide range of thermal sources. The terms “thermal engine” or “heat engine” as used herein generally refer to the equipment set that executes the various thermodynamic cycle embodiments described herein. The term “heat recovery system” generally refers to the thermal engine in cooperation with other equipment to deliver/remove heat to and from the thermal engine. 
     The heat engine system  100  may operate as a closed-loop thermodynamic cycle that circulates a working fluid throughout a working fluid circuit  102 . As illustrated, the heat engine system  100  may be characterized as a “cascade” thermodynamic cycle, where residual thermal energy from expanded working fluid is used to preheat additional working fluid before its respective expansion. Other exemplary cascade thermodynamic cycles that may also be implemented into the present disclosure may be found in PCT Pat. App. No. U.S.2011/29486, entitled “Heat Engines with Cascade Cycles,” filed on Mar. 22, 2011, and published as WO2011119650 (A2), the contents of which are hereby incorporated by reference. The working fluid circuit  102  is defined by a variety of conduits adapted to interconnect the various components of the heat engine system  100 . Although the heat engine system  100  may be characterized as a closed-loop cycle, the heat engine system  100  as a whole may or may not be hermetically-sealed such that no amount of working fluid is leaked into the surrounding environment. 
     In one or more embodiments, the working fluid used in the heat engine system  100  may be carbon dioxide (CO 2 ). It should be noted that use of the term CO 2  is not intended to be limited to CO 2  of any particular type, purity, or grade. For example, industrial grade CO 2  may be used without departing from the scope of the disclosure. In other embodiments, the working fluid may a binary, ternary, or other working fluid blend. For example, a working fluid combination can be selected for the unique attributes possessed by the combination within a heat recovery system, as described herein. One such fluid combination includes a liquid absorbent and CO 2  mixture enabling the combination to be pumped in a liquid state to high pressure with less energy input than required to compress CO 2 . In other embodiments, the working fluid may be a combination of CO 2  and one or more other miscible fluids. In yet other embodiments, the working fluid may be a combination of CO 2  and propane, or CO 2  and ammonia, without departing from the scope of the disclosure. 
     Use of the term “working fluid” is not intended to limit the state or phase of matter that the working fluid is in. For instance, the working fluid may be in a fluid phase, a gas phase, a supercritical phase, a subcritical state or any other phase or state at any one or more points within the heat engine system  100  or thermodynamic cycle. In one or more embodiments, the working fluid is in a supercritical state over certain portions of the heat engine system  100  (i.e., a high pressure side), and in a subcritical state at other portions of the heat engine system  100  (i.e., a low pressure side). In other embodiments, the entire thermodynamic cycle may be operated such that the working fluid is maintained in either a supercritical or subcritical state throughout the entire working fluid circuit  102 . 
     The heat engine system  100  may include a main pump  104  for pressurizing and circulating the working fluid throughout the working fluid circuit  102 . In its combined state, and as used herein, the working fluid may be characterized as m 1 +m 2 , where m 1  is a first mass flow and m 2  is a second mass flow, but where each mass flow m 1 , m 2  is part of the same working fluid mass coursing throughout the working fluid circuit  102 . 
     After being discharged from the main pump  104 , the combined working fluid m 1 +m 2  is split into the first and second mass flows m 1  and m 2 , respectively, at point  106  in the working fluid circuit  102 . The first mass flow m 1  is directed to a heat exchanger  108  in thermal communication with a heat source Q in . The heat exchanger  108  may be configured to increase the temperature of the first mass flow m 1 . The respective mass flows m 1 , m 2  may be controlled by the user, control system, or by the configuration of the system, as desired. 
     The heat source Q in  may derive thermal energy from a variety of high temperature sources. For example, the heat source Q in  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 thermodynamic cycle  100  may be configured to transform waste heat 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 embodiments, the heat source Q in  may derive thermal energy from renewable sources of thermal energy such as, but not limited to, solar thermal and geothermal sources. 
     While the heat source Q in  may be a fluid stream of the high temperature source itself, in other embodiments the heat source Q in  may be a thermal fluid in contact with the high temperature source. The thermal fluid may deliver the thermal energy to the waste heat exchanger  108  to transfer the energy to the working fluid in the circuit  100 . 
     A power turbine  110  is arranged downstream from the heat exchanger  108  for receiving and expanding the first mass flow m 1  discharged from the heat exchanger  108 . The power turbine  110  may be any type of expansion device, such as an expander or a turbine, and may be operatively coupled to an alternator, generator  112 , or other device or system configured to receive shaft work. The generator  112  converts the mechanical work generated by the power turbine  110  into usable electrical power. 
     The power turbine  110  discharges the first mass flow m 1  into a first recuperator  114  fluidly coupled downstream thereof. The first recuperator  114  may be configured to transfer residual thermal energy in the first mass flow m 1  to the second mass flow m 2  which also passes through the first recuperator  114 . Consequently, the temperature of the first mass flow m 1  is decreased and the temperature of the second mass flow m 2  is increased. The second mass flow m 2  may be subsequently expanded in a drive turbine  116 . 
     The drive turbine  116  discharges the second mass flow m 2  into a second recuperator  118  fluidly coupled downstream thereof. The second recuperator  118  may be configured to transfer residual thermal energy from the second mass flow m 2  to the combined working fluid m 1 +m 2  originally discharged from the main pump  104 . The mass flows m 1 , m 2  discharged from each recuperator  114 ,  118 , respectively, are recombined at point  120  in the circuit  102  and then returned to a lower temperature state at a condenser  122 . After passing through the condenser  122 , the combined working fluid m 1 +m 2  is returned to the main pump  104  and the cycle is started anew. 
     The recuperators  114 ,  118  and the condenser  122  may be any device adapted to reduce the temperature of the working fluid such as, but not limited to, a direct contact heat exchanger, a trim cooler, a mechanical refrigeration unit, and/or any combination thereof. The heat exchanger  108 , recuperators  114 ,  118 , and/or the condenser  122  may include or employ one or more printed circuit heat exchange panels. Such heat exchangers and/or panels are known in the art, and are described in U.S. Pat. Nos. 6,921,518; 7,022,294; and 7,033,553, the contents of which are incorporated by reference to the extent consistent with the present disclosure. 
     The pump  104  and drive turbine  116  may be operatively coupled via a common shaft  123 , thereby forming a direct-drive turbopump  124  where the drive turbine  116  expands working fluid to drive the main pump  104 . In one embodiment, the turbopump  124  is hermetically-sealed within a housing or casing  126  such that shaft seals are not needed along the shaft  123  between the main pump  104  and drive turbine  116 . Eliminating shaft seals may be advantageous since it contributes to a decrease in capital costs for the heat engine system  100 . Also, hermetically-sealing the turbopump  124  with the casing  126  presents significant savings by eliminating overboard working fluid leakage. In other embodiments, however, the turbopump  124  need not be hermetically-sealed. 
     Steady-state operation of the turbopump  124  is at least partially dependent on the mass flow and temperature of the second mass flow m 2  expanded within the drive turbine  116 . Until the mass flow and temperature of the second mass flow m 2  is sufficiently increased, the main pump  104  cannot adequately drive the drive turbine  116  in self-sustaining operation. Accordingly, at heat engine system  100  startup, and until the turbopump  124  “ramps-up” and is able to adequately circulate the working fluid on its own, the heat engine system  100  uses a starter pump  128  to circulate the working fluid. The starter pump  128  may be driven by a motor  130  and operate until the temperature of the second mass flow m 2  is sufficient such that the turbopump  124  can “bootstrap” itself into steady-state operation. 
     In one or more embodiments, the heat source Q in  may be at a temperature of approximately 200° C., or a temperature at which the turbopump  124  is able to bootstrap itself. As can be appreciated, higher heat source temperatures can be utilized, without departing from the scope of the disclosure. To keep thermally-induced stresses in a manageable range, however, the working fluid temperature can be “tempered” through the use of liquid CO 2  injection upstream of the drive turbine  116 . 
     To facilitate the start sequence of the turbopump  124 , the heat engine system  100  may further include a series of check valves, bypass valves, and/or shut-off valves arranged at predetermined locations throughout the circuit  102 . These valves may work in concert to direct the working fluid into the appropriate conduits until turbopump  124  steady-state operation is maintained. In one or more embodiments, the various valves may be automated or semi-automated motor-driven valves coupled to an automated control system (not shown). In other embodiments, the valves may be manually-adjustable or may be a combination of automated and manually-adjustable. 
     For example, a shut-off valve  132  arranged upstream of the power turbine  110  may be closed during heat engine system  100  startup and ramp-up. Consequently, after being heated in the heat exchanger  108 , the first mass flow m 1  is diverted around the power turbine  110  via a first diverter line  134  and a second diverter line  138 . A bypass valve  142  is arranged in the first diverter line  134  and a bypass valve  140  is arranged in the second diverter line  138 . The portion of working fluid circulated through the first diverter line  134  may be used to preheat the second mass flow m 2  in the first recuperator  114 . A check valve  144  allows the second mass flow m 2  to flow through to the first recuperator  114 . The portion of the working fluid circulated through the second diverter line  138  is combined with the second mass flow m 2  discharged from the first recuperator  114  and injected into the drive turbine  116  in its high-temperature condition. 
     A first check valve  146  may be arranged downstream from the main pump  104  and a second check valve  148  may be arranged downstream from the starter pump  128 . The check valves  146 ,  148  may be configured to prevent the working fluid from flowing upstream toward the respective pumps  104 ,  128  during various stages of operation of the heat engine system  100 . For instance, during startup and ramp-up the starter pump  128  creates an elevated head pressure downstream from the first check valve  146  (e.g., at point  150 ) as compared to the low pressure discharge of the main pump  104 . The first check valve  146  prevents the high pressure working fluid discharged from the starter pump  128  from circulating toward the main pump  104  and thereby impeding the operational progress of the turbopump  124  as it ramps up its speed. 
     Until the turbopump  124  accelerates past its stall speed, where the main pump  104  can adequately pump against the head pressure created by the starter pump  128 , a first recirculation line  152  may be used to divert the low pressure working fluid discharged from the main pump  104 . A first bypass valve  154  may be arranged in the first recirculation line  152  and may be fully or partially opened while the turbopump  124  ramps up its speed to allow the low pressure working fluid to recirculate back to a low pressure point in the working fluid circuit  102 , such as any point in the working fluid circuit  102  downstream of the power or drive turbines  110 ,  116  and upstream of the pumps  104 ,  128 . In one embodiment, the first recirculation line  152  may fluidly couple the discharge of the main pump  104  to the inlet of the condenser  122 , such as at point  156 . 
     Once the turbopump  124  attains a “bootstrapping” speed (i.e., a self-sustaining speed), the bypass valve  154  in the first recirculation line  152  can be gradually closed. Gradually closing the bypass valve  154  will increase the fluid pressure at the discharge from the main pump  104  and decrease the flow rate through the first recirculation line  152 . Eventually, once the turbopump  124  reaches steady-state operating speeds, the bypass valve  154  may be fully closed and the entirety of the working fluid discharged from the main pump  104  may be directed through the first check valve  146 . 
     Once the turbopump  124  reaches steady-state operating speeds, and even once a bootstrapped speed is achieved, the shut-off valve  132  arranged upstream from the power turbine  110  may be opened and the bypass valve  140  may be simultaneously closed. As a result, the heated stream of first mass flow m 1  may be directed through the power turbine  110  to commence generation of electrical power. 
     Also, once steady-state operating speeds are achieved the starter pump  128  becomes redundant and can therefore be deactivated. To facilitate this without causing damage to the starter pump  128 , a second recirculation line  158  having a second bypass valve  160  is arranged therein may direct lower pressure working fluid discharged from the starter pump  128  to a low pressure side of the working fluid circuit  102  (e.g., point  156 ). The low pressure side of the working fluid circuit  102  may be any point in the working fluid circuit  102  downstream of the power or drive turbines  110 ,  116  and upstream of the pumps  104 ,  128 . The second bypass valve  160  is generally closed during startup and ramp-up so as to direct all the working fluid discharged from the starter pump  128  through the second check valve  148 . However, as the starter pump  128  powers down, the head pressure past the second check valve  148  becomes greater than the starter pump  128  discharge pressure. In order to provide relief to the starter pump  128 , the second bypass valve  160  may be gradually opened to allow working fluid to escape to the low pressure side of the working fluid circuit. Eventually, the second bypass valve  160  is completely opened as the speed of the starter pump  128  slows to a stop. Again, the valving may be regulated through the implementation of an automated control system (not shown). 
     As will be appreciated by those skilled in the art, there are several advantages to the embodiments disclosed herein. For example, the turbopump  124  is able to circulate the fluid to not only generate electricity via the power turbine  110  but also use fluid energy remaining in the working fluid to drive the main pump  104  via the drive turbine  116 . Consequently, fluid energy is not required to be converted into mechanical work, then into electricity, and then back into mechanical work, as would be the case with a motor-driven pump. This reduces the required capacity of the generator  112  for the power turbine  110  and therefore provides cost saving on capital investment. Moreover, the turbopump  124  eliminates the need for a variable frequency drive and gearbox that would otherwise be needed for a motor-driven pump. Such components not only introduce energy loss terms and decrease overall system performance, but also increase capital costs and present additional points of failure in the heat engine system  100 . Also, the design of the drive turbine  116  and pump  104  can be matched to provide a high degree of performance from a physically small pump, providing cost advantages, small system footprint, and physical arrangement flexibility. 
     Referring now to  FIG. 2 , an exemplary heat engine system  200  is shown wherein heat engine system  200  may be similar in several respects to the heat engine system  100  described above. Accordingly, the heat engine system  200  may be further understood with reference to  FIG. 1 , where like numerals indicate like components that will not be described again in detail. As with the heat engine system  100  described above, the heat engine system  200  in  FIG. 2  may be used to convert thermal energy to work by thermal expansion of a working fluid mass flowing through a working fluid circuit  202 . The heat engine system  200 , however, may be characterized as a parallel-type Rankine thermodynamic cycle. 
     Specifically, the working fluid circuit  202  may include a first heat exchanger  204  and a second heat exchanger  206  arranged in thermal communication with the heat source Q in . The first and second heat exchangers  204 ,  206  may correspond generally to the heat exchanger  108  described above with reference to  FIG. 1 . For example, in one embodiment, the first and second heat exchangers  204 ,  206  may be first and second stages, respectively, of a single or combined heat exchanger. The first heat exchanger  204  may serve as a high temperature heat exchanger (e.g., a higher temperature relative to the second heat exchanger  206 ) adapted to receive initial thermal energy from the heat source Q in . The second heat exchanger  206  may then receive additional thermal energy from the heat source Q in  via a serial connection downstream from the first heat exchanger  204 . The heat exchangers  204 ,  206  are arranged in series with the heat source Q in , but in parallel in the working fluid circuit  202 . 
     The first heat exchanger  204  may be fluidly coupled to the power turbine  110  and the second heat exchanger  206  may be fluidly coupled to the drive turbine  116 . In turn, the power turbine  110  is fluidly coupled to the first recuperator  114  and the drive turbine  116  is fluidly coupled to the second recuperator  118 . The recuperators  114 ,  118  may be arranged in series on a low temperature side of the working fluid circuit  202  and in parallel on a high temperature side of the working fluid circuit  202 . For example, the high temperature side of the working fluid circuit  202  includes the portions of the working fluid circuit  202  arranged downstream from each recuperator  114 ,  118  where the working fluid is directed to the heat exchangers  204 ,  206 . The low temperature side of the working fluid circuit  202  includes the portions of the working fluid circuit  202  downstream from each recuperator  114 ,  118  where the working fluid is directed away from the heat exchangers  204 ,  206 . 
     The turbopump  124  is also included in the working fluid circuit  202 , where the main pump  104  is operatively coupled to the drive turbine  116  via the shaft  123  (indicated by the dashed line), as described above. The pump  104  is shown separated from the drive turbine  116  only for ease of viewing and describing the working fluid circuit  202 . Indeed, although not specifically illustrated, it will be appreciated that both the main pump  104  and the drive turbine  116  may be hermetically-sealed within the casing  126  ( FIG. 1 ). This also applies to  FIGS. 3 and 4  below. The starter pump  128  facilitates the start sequence for the turbopump  124  during startup of the heat engine system  200  and ramp-up of the turbopump  124 . Once steady-state operation of the turbopump  124  is reached, the starter pump  128  may be deactivated. 
     The power turbine  110  may operate at a higher relative temperature (e.g., higher turbine inlet temperature) than the drive turbine  116 , due to the temperature drop of the heat source Q in  experienced across the first heat exchanger  204 . Each turbine  110 ,  116 , however, may be configured to operate at the same or substantially the same inlet pressure. The low-pressure discharge mass flow exiting each recuperator  114 ,  118  may be directed through the condenser  122  to be cooled for return to the low temperature side of the working fluid circuit  202  and to either the main or starter pumps  104 ,  128 , depending on the stage of operation. 
     During steady-state operation of the heat engine system  200 , the turbopump  124  circulates all of the working fluid throughout the working fluid circuit  202  using the main pump  104 , and the starter pump  128  does not generally operate nor is needed. The first bypass valve  154  in the first recirculation line  152  is fully closed and the working fluid is separated into the first and second mass flows m 1 , m 2  at point  210 . The first mass flow m 1  is directed through the first heat exchanger  204  and subsequently expanded in the power turbine  110  to generate electrical power via the generator  112 . Following the power turbine  110 , the first mass flow m 1  passes through the first recuperator  114  and transfers residual thermal energy to the first mass flow m 1  as the first mass flow m 1  is directed toward the first heat exchanger  204 . 
     The second mass flow m 2  is directed through the second heat exchanger  206  and subsequently expanded in the drive turbine  116  to drive the main pump  104  via the shaft  123 . Following the drive turbine  116 , the second mass flow m 2  passes through the second recuperator  118  to transfer residual thermal energy to the second mass flow m 2  as the second mass flow m 2  courses toward the second heat exchanger  206 . The second mass flow m 2  is then re-combined with the first mass flow m 1  and the combined mass flow m 1 +m 2  is subsequently cooled in the condenser  122  and directed back to the main pump  104  to commence the fluid loop anew. 
     During startup of the heat engine system  200  or ramp-up of the turbopump  124 , the starter pump  128  is engaged and operates to start the turbopump  124  spinning. To help facilitate this, a shut-off valve  214  arranged downstream from point  210  is initially closed such that no working fluid is directed to the first heat exchanger  204  or otherwise expanded in the power turbine  110 . Rather, all the working fluid discharged from the starter pump  128  is directed through the second heat exchanger  206  and the drive turbine  116 . The heated working fluid expands in the drive turbine  116  and drives the main pump  104 , thereby commencing operation of the turbopump  124 . 
     The head pressure generated by the starter pump  128  near point  210  prevents the low pressure working fluid discharged from the main pump  104  during ramp-up from traversing the first check valve  146 . Until the main pump  104  is able to accelerate past its stall speed, the first bypass valve  154  in the first recirculation line  152  may be fully opened to recirculate the low pressure working fluid back to a low pressure point in the working fluid circuit  202 , such as at point  156  adjacent the inlet of the condenser  122 . Once the turbopump  124  reaches its “bootstrapped” speed (e.g., self-sustaining speed), the bypass valve  154  may be gradually closed to increase the discharge pressure of the main pump  104  and also decrease the flow rate through the first recirculation line  152 . Once the turbopump  124  reaches steady-state operation, and even once a bootstrapped speed is achieved, the shut-off valve  214  may be gradually opened, thereby allowing the first mass flow m 1  to be expanded in the power turbine  110  to commence generating electrical energy. Again, the valving may be regulated through the implementation of an automated control system (not shown). 
     With the turbopump  124  operating at steady-state operating speeds, the starter pump  128  can gradually be powered down and deactivated. Deactivating the starter pump  128  may include simultaneously opening the second bypass valve  160  arranged in the second recirculation line  158 . The second bypass valve  160  allows the increasingly lower pressure working fluid discharged from the starter pump  128  to escape to the low pressure side of the working fluid circuit (e.g., point  156 ). Eventually the second bypass valve  160  may be completely opened as the speed of the starter pump  128  slows to a stop and the second check valve  148  prevents working fluid discharged by the main pump  104  from advancing toward the discharge of the starter pump  128 . At steady-state, the turbopump  124  continuously pressurizes the working fluid circuit  202  in order to drive both the drive turbine  116  and the power turbine  110 . 
       FIG. 3  illustrates an exemplary parallel-type heat engine system  300 , which may be similar in some respects to the above-described heat engine systems  100  and  200 , and therefore, may be best understood with reference to  FIGS. 1 and 2 , where like numerals correspond to like elements that will not be described again. The heat engine system  300  includes a working fluid circuit  302  utilizing a third heat exchanger  304  also in thermal communication with the heat source Q in . The heat exchangers  204 ,  206 ,  304  are arranged in series with the heat source Q in , but arranged in parallel in the working fluid circuit  302 . 
     The turbopump  124  (i.e., the combination of the main pump  104  and the drive turbine  116  operatively coupled via the shaft  123 ) is arranged and configured to operate in parallel with the starter pump  128 , especially during heat engine system  300  startup and turbopump  124  ramp-up. During steady-state operation of the heat engine system  300 , the starter pump  128  does not generally operate. Instead, the main pump  104  solely discharges the working fluid that is subsequently separated into first and second mass flows m 1 , m 2 , respectively, at point  306 . The third heat exchanger  304  may be configured to transfer thermal energy from the heat source Q in  to the first mass flow m 1  flowing therethrough. The first mass flow m 1  is then directed to the first heat exchanger  204  and the power turbine  110  for expansion power generation. Following expansion in the power turbine  110 , the first mass flow m 1  passes through the first recuperator  114  to transfer residual thermal energy to the first mass flow m 1  discharged from the third heat exchanger  304  and coursing toward the first heat exchanger  204 . 
     The second mass flow m 2  is directed through the second heat exchanger  206  and subsequently expanded in the drive turbine  116  to drive the main pump  104 . After being discharged from the drive turbine  116 , the second mass flow m 2  merges with the first mass flow m 1  at point  308 . The combined mass flow m 1 +m 2  thereafter passes through the second recuperator  118  to provide residual thermal energy to the second mass flow m 2  as the second mass flow m 2  courses toward the second heat exchanger  206 . 
     During the heat engine system  300  startup and/or the turbopump  124  ramp-up, the starter pump  128  circulates the working fluid to commence the turbopump  124  spinning. The shut-off valve  214  may be initially closed to prevent working fluid from circulating through the first and third heat exchangers  204 ,  304  and being expanded in the power turbine  110 . The working fluid discharged from the starter pump  128  is directed through the second heat exchanger  206  and the drive turbine  116 . The heated working fluid expands in the drive turbine  116  and drives the main pump  104 , thereby commencing operation of the turbopump  124 . 
     Until the discharge pressure of the main pump  104  accelerates past its stall speed and can withstand the head pressure generated by the starter pump  128 , any working fluid discharged from the main pump  104  is generally recirculated via the first recirculation line  152  back to a low pressure point in the working fluid circuit  202  (e.g., point  156 ). Once the turbopump  124  becomes self-sustaining, the bypass valve  154  may be gradually closed to increase the main pump  104  discharge pressure and decrease the flow rate in the first recirculation line  152 . At that point, the shut-off valve  214  may also be gradually opened to begin circulation of the first mass flow m 1  through the power turbine  110  to generate electrical energy. Also, at this point the starter pump  128  can be gradually deactivated while simultaneously opening the second bypass valve  160  arranged in the second recirculation line  158 . Eventually the second bypass valve  160  is completely opened and the starter pump  128  can be slowed to a stop. Again, the valving may be regulated through the implementation of an automated control system (not shown). 
       FIG. 4  illustrates an exemplary parallel-type heat engine system  400 , wherein the heat engine system  400  may be similar to the system  300  above, and as such, may be best understood with reference to  FIG. 3  where like numerals correspond to like elements that will not be described again. The working fluid circuit  402  in  FIG. 4  is substantially similar to the working fluid circuit  302  of  FIG. 3  but with the exception of an additional, third recuperator  404  adapted to extract additional thermal energy from the combined mass flow m 1 +m 2  discharged from the second recuperator  118 . Accordingly, the temperature of the first mass flow m 1  entering the third heat exchanger  304  may be preheated in the third recuperator  404  prior to receiving thermal energy transferred from the heat source Q in . 
     As illustrated, the recuperators  114 ,  118 ,  404  may operate as separate heat exchanging devices. In other embodiments, however, the recuperators  114 ,  118 ,  404  may be combined as a single, integral recuperator. Steady-state operation, system startup, and turbopump  124  ramp-up may operate substantially similar as described above in  FIG. 3 , and therefore will not be described again. 
     Each of the described heat engine systems  100 ,  200 ,  300 , and  400 , as depicted in  FIGS. 1-4 , may be implemented in a variety of physical embodiments, including but not limited to fixed or integrated installations, or as a self-contained device such as a portable waste heat engine “skid.” The waste heat engine skid may be configured to arrange each working fluid circuit  102 ,  202 ,  302 , and  402  and related components (e.g., turbines  110 ,  116 , recuperators  114 ,  118 ,  404 , condenser  122 , pumps  104 ,  128 , etc.) in a consolidated, single unit. An exemplary waste heat engine skid is described and illustrated in U.S. application Ser. No. 12/631,412, entitled “Thermal Energy Conversion Device,” filed on Dec. 4, 2009, and published as U.S. 2011-0185729, the contents of which are hereby incorporated by reference to the extent consistent with the present disclosure. 
     Referring now to  FIG. 5 , illustrated is a flowchart of a method  500  for starting a turbopump in a thermodynamic working fluid circuit. The method  500  includes circulating a working fluid in the working fluid circuit with a starter pump, as at  502 . The starter pump may be in fluid communication with a first heat exchanger, and the first heat exchanger may be in thermal communication with a heat source. Thermal energy is transferred to the working fluid from the heat source in the first heat exchanger, as at  504 . The method  500  further includes expanding the working fluid in a drive turbine, as at  506 . The drive turbine is fluidly coupled to the first heat exchanger, and the drive turbine is operatively coupled to a main pump, such that the combination of the drive turbine and main pump is the turbopump. 
     The main pump is driven with the drive turbine, as at  508 . Until the main pump accelerates past its stall point, the working fluid discharged from the main pump is diverted into a first recirculation line, as at  510 . The first recirculation line may fluidly communicate the main pump with a low pressure side of the working fluid circuit. Moreover, a first bypass valve may be arranged in the first recirculation line. As the turbopump reaches a self-sustaining speed of operation, the first bypass valve may gradually begin to close, as at  512 . Consequently, the main pump begins circulating the working fluid discharged from the main pump through the working fluid circuit, as at  514 . 
     The method  500  may also include deactivating the starter pump and opening a second bypass valve arranged in a second recirculation line, as at  516 . The second recirculation line may fluidly communicate the starter pump with the low pressure side of the working fluid circuit. The low pressure working fluid discharged from the starter pump may be diverted into the second recirculation line until the starter pump comes to a stop, as at  518 . 
     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.