Supercritical working fluid circuit with a turbo pump and a start pump in series configuration

Aspects of the invention provided herein include heat engine systems, methods for generating electricity, and methods for starting a turbo pump. In some configurations, the heat engine system contains a start pump and a turbo pump disposed in series along a working fluid circuit and configured to circulate a working fluid within the working fluid circuit. The start pump may have a pump portion coupled to a motor-driven portion and the turbo pump may have a pump portion coupled to a drive turbine. In one configuration, the pump portion of the start pump is fluidly coupled to the working fluid circuit downstream of and in series with the pump portion of the turbo pump. In another configuration, the pump portion of the start pump is fluidly coupled to the working fluid circuit upstream of and in series with the pump portion of the turbo pump.

BACKGROUND

Waste heat is often created as a byproduct of industrial processes where flowing streams of high-temperature liquids, gases, or fluids must be exhausted into the environment or removed in some way in an effort to maintain the operating temperatures of the industrial process equipment. Some industrial processes utilize heat exchanger devices to capture and recycle waste heat back into the process via other process streams. However, the capturing and recycling of waste heat is generally infeasible by industrial processes that utilize high temperatures or have insufficient mass flow or other unfavorable conditions.

Waste heat can be converted into useful energy by a variety of turbine generator or heat engine systems that employ thermodynamic methods, such as Rankine cycles. Rankine cycles and similar thermodynamic methods are typically steam-based processes that recover and utilize waste heat to generate steam for driving a turbine, turbo, or other expander connected to an electric generator, a pump, or other device.

An organic Rankine cycle utilizes a lower boiling-point working fluid, instead of water, during a traditional Rankine cycle. Exemplary lower boiling-point working fluids include hydrocarbons, such as light hydrocarbons (e.g., propane or butane) and halogenated hydrocarbon, such as hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) (e.g., R245fa). More recently, in view of issues such as thermal instability, toxicity, flammability, and production cost of the lower boiling-point working fluids, some thermodynamic cycles have been modified to circulate non-hydrocarbon working fluids, such as ammonia.

A pump or compressor is generally required to pressurize and circulate the working fluid throughout the working fluid circuit. The pump is typically a motor-driven pump, however, such 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. A turbo pump is a device that utilizes a drive turbine to power a rotodynamic pump. Replacing the motor-driven pump with a turbo pump eliminates one or more of these issues, but at the same time introduces problems of starting and achieving steady-state operation the turbo pump, which relies on the circulation of heated working fluid through the drive turbine for proper operation. Unless the turbo pump is provided with a successful start sequence, the turbo pump will not be able to circulate enough fluid to properly function and attain steady-state operation.

What is needed, therefore, is a heat engine system and method of operating a waste heat recovery thermodynamic cycle that provides a successful start sequence adapted to start a turbo pump and reach a steady-state of operating the system with the turbo pump.

SUMMARY

Embodiments of the invention generally provide a heat engine system and a method for generating electricity. In some embodiments, the heat engine system contains a start pump and a turbo pump disposed in series along a working fluid circuit and configured to circulate a working fluid within the working fluid circuit. The start pump may have a pump portion coupled to a motor-driven portion (e.g., mechanical or electric motor) and the turbo pump may have a pump portion coupled to a drive turbine. In one embodiment, the pump portion of the start pump is fluidly coupled to the working fluid circuit downstream of and in series with the pump portion of the turbo pump. In another embodiment, the pump portion of the start pump is fluidly coupled to the working fluid circuit upstream of and in series with the pump portion of the turbo pump.

The heat engine system and the method for generating electricity are configured to efficiently generate valuable electrical energy from thermal energy, such as a heated stream (e.g., a waste heat stream). The heat engine system utilizes a working fluid in a supercritical state (e.g., sc-CO2) and/or a subcritical state (e.g., sub-CO2) contained within a working fluid circuit for capturing or otherwise absorbing thermal energy of the waste heat stream with one or more heat exchangers. The thermal energy is transformed to mechanical energy by a power turbine and subsequently transformed to electrical energy by the power generator coupled to the power turbine. The heat engine system contains several integrated sub-systems managed by a process control system for maximizing the efficiency of the heat engine system while generating electricity.

In one embodiment disclosed herein, a heat engine system for generating electricity contains a turbo pump having a pump portion operatively coupled to a drive turbine, such that the pump portion may be fluidly coupled to a working fluid circuit and configured to circulate a working fluid through the working fluid circuit and the working fluid has a first mass flow and a second mass flow within the working fluid circuit. The heat engine system further contains a first heat exchanger fluidly coupled to and in thermal communication with the working fluid circuit, fluidly coupled to and in thermal communication with a heat source stream, and configured to transfer thermal energy from the heat source stream to the first mass flow of the working fluid. The heat engine system also contains a power turbine fluidly coupled to and in thermal communication with the working fluid circuit, disposed downstream of the first heat exchanger, and configured to convert thermal energy to mechanical energy by a pressure drop in the first mass flow of the working fluid flowing through the power turbine and a power generator coupled to the power turbine and configured to convert the mechanical energy into electrical energy. The heat engine system further contains a start pump having a pump portion operatively coupled to a motor and configured to circulate the working fluid within the working fluid circuit, such that the pump portion of the start pump and the pump portion of the turbo pump are fluidly coupled in series to the working fluid circuit.

In one exemplary configuration, the pump portion of the start pump is fluidly coupled to the working fluid circuit downstream of and in series with the pump portion of the turbo pump. Therefore, an outlet of the pump portion of the turbo pump may be fluidly coupled to and serially upstream of an inlet of the pump portion of the start pump. In another exemplary configuration, the pump portion of the start pump is fluidly coupled to the working fluid circuit upstream of and in series with the pump portion of the turbo pump. Therefore, an inlet of the pump portion of the turbo pump may be fluidly coupled to and serially downstream of an outlet of the pump portion of the start pump.

In some embodiments, the heat engine system further contains 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. In some examples, the first recuperator may be configured to transfer residual thermal energy from the first mass flow to the second mass flow before the second mass flow is expanded in the drive turbine. The first recuperator may be configured to transfer residual thermal energy from the first mass flow discharged from the power turbine to the first mass flow directed to the first heat exchanger. The second recuperator may be configured to transfer residual thermal energy from the second mass flow discharged from the drive turbine to the second mass flow directed to a second heat exchanger.

In some embodiments, the heat engine system further contains a second heat exchanger fluidly coupled to and in thermal communication with the working fluid circuit, disposed in series with the first heat exchanger along the working fluid circuit, fluidly coupled to and in thermal communication with the heat source stream, and configured to transfer thermal energy from the heat source stream to the second mass flow of the working fluid. The second heat exchanger may be in thermal communication with the heat source stream and in fluid communication with the pump portion of the turbo pump and the pump portion of the start pump. In many examples described herein, the working fluid contains carbon dioxide and at least a portion of the working fluid circuit contains the working fluid in a supercritical state.

In another embodiment, the heat engine system further contains a first recirculation line fluidly coupling the pump portion of the turbo pump with a low pressure side of the working fluid circuit, a second recirculation line fluidly coupling the pump portion of the start pump with the low pressure side of the working fluid circuit, a first bypass valve arranged in the first recirculation line, and a second bypass valve arranged in the second recirculation line.

In other embodiments disclosed herein, a heat engine system for generating electricity contains a turbo pump configured to circulate a working fluid throughout the working fluid circuit and contains a pump portion operatively coupled to a drive turbine. In some examples, the turbo pump is hermetically-sealed within a casing. The heat engine system also contains a start pump arranged in series with the turbo pump along the working fluid circuit. The heat engine system further contains a first check valve arranged in the working fluid circuit downstream of the pump portion of the turbo pump, and a second check valve arranged in the working fluid circuit downstream of the pump portion of the start pump and fluidly coupled to the first check valve.

The heat engine system further contains a power turbine fluidly coupled to both the pump portion of the turbo pump and the pump portion of the start pump, a first recirculation line fluidly coupling the pump portion of the turbo pump with a low pressure side of the working fluid circuit, and a second recirculation line fluidly coupling the pump portion of the start pump with the low pressure side of the working fluid circuit. In some configurations, the heat engine system contains a first recuperator fluidly coupled to the power turbine and a second recuperator fluidly coupled to the drive turbine. In some examples, the heat engine system contains a third recuperator fluidly coupled to the second recuperator, wherein the first, second, and third recuperators are disposed in series along the working fluid circuit.

The heat engine system further contains a condenser fluidly coupled to both the pump portion of the turbo pump and the pump portion of the start pump. Also, the heat engine system further contains first, second, and third heat exchangers disposed in series and in thermal communication with a heat source stream and disposed in series and in thermal communication with the working fluid circuit.

In other embodiments disclosed herein, a method for starting a turbo pump in a heat engine system and/or generating electricity with the heat engine system is provided and includes circulating a working fluid within a working fluid circuit by a start pump and transferring thermal energy from a heat source stream to the working fluid by a first heat exchanger fluidly coupled to and in thermal communication with the working fluid circuit. Generally, the working fluid has a first mass flow and a second mass flow within the working fluid circuit and at least a portion of the working fluid circuit contains the working fluid in a supercritical state. The method further includes flowing the working fluid into a drive turbine of a turbo pump and expanding the working fluid while converting the thermal energy from the working fluid to mechanical energy of the drive turbine and driving a pump portion of the turbo pump by the mechanical energy of the drive turbine. The pump portion may be coupled to the drive turbine and the working fluid may be circulated within the working fluid circuit by the turbo pump. The method also includes diverting the working fluid discharged from the pump portion of the turbo pump into a first recirculation line fluidly communicating the pump portion of the turbo pump with a low pressure side of the working fluid circuit and closing a first bypass valve arranged in the first recirculation line as the turbo pump reaches a self-sustaining speed of operation. The method further includes deactivating the start pump and opening a second bypass valve arranged in a second recirculation line fluidly communicating the start pump with the low pressure side of the working fluid circuit, and diverting the working fluid discharged from the start pump into the second recirculation line. Also, the method includes flowing the working fluid into a power turbine and converting the thermal energy from the working fluid to mechanical energy of the power turbine and converting the mechanical energy of the power turbine into electrical energy by a power generator coupled to the power turbine.

In some embodiments, the method includes circulating the working fluid in the working fluid circuit with the start pump is preceded by closing a shut-off valve to divert the working fluid around a power turbine arranged in the working fluid circuit. In other embodiments, the method further includes opening the shut-off valve once the turbo pump reaches the self-sustaining speed of operation, thereby directing the working fluid into the power turbine, expanding the working fluid in the power turbine, and driving a power generator operatively coupled to the power turbine to generate electrical power. In other embodiments, the method further includes opening the shut-off valve once the turbo pump reaches the self-sustaining speed of operation, directing the working fluid into a second heat exchanger fluidly coupled to the power turbine and in thermal communication with the heat source stream, transferring additional thermal energy from the heat source stream to the working fluid in the second heat exchanger, expanding the working fluid received from the second heat exchanger in the power turbine, and driving a power generator operatively coupled to the power turbine, whereby the power generator is operable to generate electrical power.

In some embodiments, the method also includes opening the shut-off valve once the turbo pump reaches the self-sustaining speed of operation, directing the working fluid into a second heat exchanger in thermal communication with the heat source stream, the first and second heat exchangers being arranged in series in the heat source stream, directing the working fluid from the second heat exchanger into a third heat exchanger fluidly coupled to the power turbine and in thermal communication with the heat source stream, the first, second, and third heat exchangers being arranged in series in the heat source stream, transferring additional thermal energy from the heat source stream to the working fluid in the third heat exchanger, expanding the working fluid received from the third heat exchanger in the power turbine, and driving a power generator operatively coupled to the power turbine, whereby the power generator is operable to generate electrical power.

DETAILED DESCRIPTION

FIGS. 1A and 1Bdepict simplified schematics of heat engine systems100aand100b, respectively, which may also be referred to as thermal heat engines, power generation devices, heat recovery systems, and/or heat to electricity systems. Heat engine systems100aand100bmay encompass one or more elements of a Rankine thermodynamic cycle configured to produce power (e.g., electricity) from a wide range of thermal sources. The terms “thermal engine” or “heat engine” as used herein generally refer to an 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.

Heat engine systems100aand100bgenerally have at least one heat exchanger103and a power turbine110fluidly coupled to and in thermal communication with a working fluid circuit102containing a working fluid. In some configurations, the heat engine systems100aand100bcontain a single heat exchanger103. However, in other configurations, the heat engine systems100aand100bcontain two, three, or more heat exchangers103fluidly coupled to the working fluid circuit102and configured to be fluidly coupled to a heat source stream90(e.g., waste heat stream flowing from a waste heat source). The power turbine110may be any type of expansion device, such as an expander or a turbine, and may be operatively coupled to an alternator, a power generator112, or other device or system configured to receive shaft work produced by the power turbine110and generate electricity. The power turbine110has an inlet for receiving the working fluid flowing through a control valve133from the heat exchangers103in the high pressure side of the working fluid circuit102. The power turbine110also has an outlet for releasing the working fluid into the low pressure side of the working fluid circuit102. The control valve133may be operatively configured to control the flow of working fluid from the heat exchangers103to an inlet of the power turbine110.

The heat engine systems100aand100bfurther contain several pumps, such as a turbo pump124and a start pump129, disposed within the working fluid circuit102. Each of the turbo pump124and the start pump129is fluidly coupled between the low pressure side and the high pressure side of the working fluid circuit102. Specifically, a pump portion104and a drive turbine116of the turbo pump124and a pump portion128of the start pump129are each fluidly coupled independently between the low pressure side and the high pressure side of the working fluid circuit102. The turbo pump124and the start pump129may be operative to circulate and pressurize the working fluid throughout the working fluid circuit102. The start pump129may be utilized to initially pressurize and circulate the working fluid in the working fluid circuit102. Once a predetermined pressure, temperature, and/or flowrate of the working fluid is obtained within the working fluid circuit102, the start pump129may be taken off line, idled, or turned off and the turbo pump124utilized to circulate the working fluid while generating electricity.

FIGS. 1A and 1Bdepict the turbo pump124and the start pump129fluidly coupled in series to the working fluid circuit102, such that the pump portion104of the turbo pump124and the pump portion128of the start pump129are fluidly coupled in series to the working fluid circuit102. In one embodiment,FIG. 1Adepicts the pump portion104of the turbo pump124fluidly coupled upstream of the pump portion128of the start pump129, such that the working fluid may flow from the condenser122, through the pump portion104of the turbo pump124, then serially through the pump portion128of the start pump129, and subsequently to the power turbine110. In another embodiment,FIG. 1Bdepicts the pump portion128of the start pump129fluidly coupled upstream of the pump portion104of the turbo pump124, such that the working fluid may flow from the condenser122, through the pump portion128of the start pump129, then serially through the pump portion104of the turbo pump124, and subsequently to the power turbine110.

The start pump129may be a motorized pump, such as an electric motorized pump, a mechanical motorized pump, or other type of pump. Generally, the start pump129may be a variable frequency motorized drive pump and contains the pump portion128and a motor-driven portion130. The motor-driven portion130of the start pump129contains a motor and a drive including a drive shaft and optional gears (not shown). In some examples, the motor-driven portion130has a variable frequency drive, such that the speed of the motor may be regulated by the drive. The motor-driven portion130may be powered by an external electric source.

The pump portion128of the start pump129may be driven by the motor-driven portion130coupled thereto. In one embodiment, as depicted inFIG. 1A, the pump portion128of the start pump129has an inlet for receiving the working fluid from an outlet of the pump portion104of the turbo pump124. The pump portion128of the start pump129also has an outlet for releasing the working fluid into the working fluid circuit102upstream of the power turbine110. In another embodiment, as depicted inFIG. 1B, the pump portion128of the start pump129has an inlet for receiving the working fluid from the low pressure side of the working fluid circuit102, such as from the condenser122. The pump portion128of the start pump129also has an outlet for releasing the working fluid into the working fluid circuit102upstream of the pump portion104of the turbo pump124.

The turbo pump124is generally a turbo/turbine-driven pump or compressor and utilized to pressurize and circulate the working fluid throughout the working fluid circuit102. The turbo pump124contains the pump portion104and the drive turbine116coupled together by a drive shaft123and optional gearbox. The pump portion104of the turbo pump124may be driven by the drive shaft123coupled to the drive turbine116.

The drive turbine116of the turbo pump124may be any type of expansion device, such as an expander or a turbine, and may be operatively coupled to the pump portion104, or other compressor/pump device configured to receive shaft work produced by the drive turbine116. The drive turbine116may be driven by heated and pressurized working fluid, such as the working fluid heated by the heat exchangers103. The drive turbine116has an inlet for receiving the working fluid flowing through a control valve143from the heat exchangers103in the high pressure side of the working fluid circuit102. The drive turbine116also has an outlet for releasing the working fluid into the low pressure side of the working fluid circuit102. The control valve143may be operatively configured to control the flow of working fluid from the heat exchangers103to the inlet of the drive turbine116.

In one embodiment, as depicted inFIG. 1A, the pump portion104of the turbo pump124has an inlet configured to receive the working fluid from the low pressure side of the working fluid circuit102, such as downstream of the condenser122. The pump portion104of the turbo pump124has an outlet for releasing the working fluid into the working fluid circuit102upstream of the pump portion128of the start pump129. In addition, the pump portion128of the start pump129has an inlet configured to receive the working fluid from an outlet of the pump portion104of the turbo pump124.

In another embodiment, as depicted inFIG. 1B, the pump portion128of the start pump129has an inlet configured to receive the working fluid from the low pressure side of the working fluid circuit102, such as downstream of the condenser122. The pump portion128of the start pump129has an outlet for releasing the working fluid into the working fluid circuit102upstream of the pump portion104of the turbo pump124. Also, the pump portion104of the turbo pump124has an inlet configured to receive the working fluid from an outlet of the pump portion128of the start pump129.

The pump portion128of the start pump129is configured to circulate and/or pressurize the working fluid within the working fluid circuit102during a warm-up process. The pump portion128of the start pump129is configured in series with the pump portion104of the turbo pump124. In one example, illustrated inFIG. 1A, the heat engine system100ahas a suction line127fluidly coupled to and disposed between the discharge line105of the pump portion104and the pump portion128. The suction line127provides flow from the pump portion104and the pump portion128. In another example, illustrated inFIG. 1B, the heat engine system100bhas a line131fluidly coupled to and disposed between the pump portion104and the pump portion128. The line131provides flow from the pump portion104and the pump portion128. Start pump129may operate until the mass flow rate and temperature of the second mass flow m2is sufficient to operate the turbo pump124in a self-sustaining mode.

In one embodiment, the turbo pump124is hermetically-sealed within housing or casing126such that shaft seals are not needed along the drive shaft123between the pump portion104and drive turbine116. Eliminating shaft seals may be advantageous since it contributes to a decrease in capital costs for the heat engine system100aor100b. Also, hermetically-sealing the turbo pump124with the casing126presents significant savings by eliminating overboard working fluid leakage. In other embodiments, however, the turbo pump124need not be hermetically-sealed.

In one or more embodiments, the working fluid within the working fluid circuit102of the heat engine system100aor100bcontains carbon dioxide. It should be noted that use of the term carbon dioxide is not intended to be limited to carbon dioxide of any particular type, purity, or grade. For example, industrial grade carbon dioxide 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 carbon dioxide mixture enabling the combination to be pumped in a liquid state to high pressure with less energy input than required to compress carbon dioxide. In other embodiments, the working fluid may be a combination of carbon dioxide and one or more other miscible fluids. In yet other 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.

The use of the term “working fluid” is not intended to limit the state or phase of matter of the working fluid. For instance, the working fluid or portions of the working fluid may be in a liquid phase, a gas phase, a fluid phase, a subcritical state, a supercritical state, or any other phase or state at any one or more points within the working fluid circuit102, the heat engine systems100aor100b, or thermodynamic cycle. In one or more embodiments, the working fluid may be in a supercritical state over certain portions of the working fluid circuit102(e.g., a high pressure side), and may be in a supercritical state or a subcritical state at other portions the working fluid circuit102(e.g., 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 circuit102.

In a combined state, and as will be used herein, the working fluid may be characterized as m1+m2, where m1is a first mass flow and m2is a second mass flow, but where each mass flow m1, m2is part of the same working fluid mass being circulated throughout the working fluid circuit102. The combined working fluids m1+m2from pump portion104of the turbo pump124are directed to the heat exchangers103. The first mass flow m1is directed to power turbine110to drive power generator112. The second mass flow m2is directed from the heat exchangers102back to the drive turbine116of the turbo pump124to provide the energy needed to drive the pump portion104. After passing through the power turbine110and the drive turbine116, the first and second mass flows are combined and directed to the condenser122and back to the turbo pump124and the cycle is started anew.

Steady-state operation of the turbo pump124is at least partially dependent on the mass flow and temperature of the second mass flow m2expanded within the drive turbine116. Until the mass flow rate and temperature of the second mass flow m2is sufficiently increased, the drive turbine116cannot adequately drive the pump portion104in self-sustaining operation. Accordingly, at start-up of the heat engine system100a, and until the turbo pump124“ramps-up” and is able to adequately circulate the working fluid, the heat engine system100aor100butilizes a start pump129to circulate the working fluid within the working fluid circuit102.

To facilitate the start sequence of the turbo pump124, heat engine systems100aand100bmay further include a series of check valves, bypass valves, and/or shut-off valves arranged at predetermined locations throughout the working fluid circuit102. These valves may work in concert to direct the working fluid into the appropriate conduits until steady-state operation of turbo pump124can be 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.

FIG. 1Adepicts a first check valve146arranged downstream of the pump portion104and a second check valve148arranged downstream of the pump portion128, as described in one embodiment.FIG. 1Bdepicts the first check valve146arranged downstream of the pump portion104, as described in one embodiment. The check valves146,148may be configured to prevent the working fluid from flowing upstream ofward the respective pump portions104,128during various stages of operation of the heat engine system100a. For instance, during start-up and ramp-up of the heat engine system100a, the start pump129creates an elevated head pressure downstream of the first check valve146(e.g., at point150) as compared to the low pressure at discharge line105of the pump portion104and the suction line127of the pump portion128, as depicted inFIG. 1A. Thus, the first check valve146prevents the high pressure working fluid discharged from the pump portion128from re-circulating toward the pump portion104and ensures that the working fluid flows into heat exchangers103.

Until the turbo pump124accelerates past the stall speed of the turbo pump124, where the pump portion104can adequately pump against the head pressure created by the start pump129, a first recirculation line152may be used to divert a portion of the low pressure working fluid discharged from the pump portion104. A first bypass valve154may be arranged in the first recirculation line152and may be fully or partially opened while the turbo pump124ramps up or otherwise increases speed to allow the low pressure working fluid to recirculate back to the working fluid circuit102, such as any point in the working fluid circuit102downstream of the heat exchangers103and before the pump portions104,128. In one embodiment, the first recirculation line152may fluidly couple the discharge of the pump portion104to the inlet of the condenser122.

Once the turbo pump124attains a self-sustaining speed, the bypass valve154in the first recirculation line152can be gradually closed. Gradually closing the bypass valve154will increase the fluid pressure at the discharge from the pump portion104and decrease the flow rate through the first recirculation line152. Eventually, once the turbo pump124reaches steady-state operating speeds, the bypass valve154may be fully closed and the entirety of the working fluid discharged from the pump portion104may be directed through the first check valve146. Also, once steady-state operating speeds are achieved, the start pump129becomes redundant and can therefore be deactivated. The heat engine systems100aand100bmay have an automated control system (not shown) configured to regulate, operate, or otherwise control the valves and other components therein.

In another embodiment, as depicted inFIG. 1A, to facilitate the deactivation of the start pump129without causing damage to the start pump129, a second recirculation line158having a second bypass valve160is arranged therein may direct lower pressure working fluid discharged from the pump portion128to a low pressure side of the working fluid circuit102in the heat engine system100a. The low pressure side of the working fluid circuit102may be any point in the working fluid circuit102downstream of the heat exchangers103and before the pump portions104,128. The second bypass valve160is generally closed during start-up and ramp-up so as to direct all the working fluid discharged from the pump portion128through the second check valve148. However, as the start pump129powers down, the head pressure past the second check valve148becomes greater than the pump portion128discharge pressure. In order to provide relief to the pump portion128, the second bypass valve160may be gradually opened to allow working fluid to escape to the low pressure side of the working fluid circuit. Eventually the second bypass valve160may be completely opened as the speed of the pump portion128slows to a stop.

Connecting the start pump129in series with the turbo pump124allows the pressure generated by the start pump129to act cumulatively with the pressure generated by the turbo pump124until self-sustaining conditions are achieved. When compared to a start pump connected in parallel with a turbo pump, the start pump129connected in series supplies the same flow rate but at a much lower pressure differential. The start pump129does not have to generate as much pressure differential as the turbo pump124. Therefore, the power requirement to operate the pump portion128is reduced such that a smaller motor-driven portion130may be utilized to operate the pump portion128.

In some embodiments disclosed herein, the start pump129and the turbo pump124may be fluidly coupled in series along the working fluid circuit202, whereas the pump portion104of the turbo pump124is disposed upstream of the pump portion128of the start pump129, as depicted inFIG. 1A. Such serial configuration of the turbo pump124and the start pump129provides a reduction of the power demand for the start pump129by efficiently increasing the pressure within the working fluid circuit102while self-sustaining the turbo pump124during a warm-up or start-up process.

In other embodiments disclosed herein, the start pump129and the turbo pump124are fluidly coupled in series along the working fluid circuit202, whereas the pump portion128of the start pump129is disposed upstream of the pump portion104of the turbo pump124, as depicted inFIG. 1B. Such serial configuration of the start pump129and the turbo pump124provides a reduction of the pressure demand for the start pump129. Therefore, the start pump129may also function as a low speed booster pump to mitigate risk of cavitation to the turbo pump124. The functionality of a low speed booster pump enables higher cycle power by operating closer to saturation without cavitation thus increasing the turbine pressure ratio.

In one or more embodiments disclosed herein, both of the heat engine systems100a(FIG. 1A) and the heat engine system100b(FIG. 1B) contain the turbo pump124having the pump portion104operatively coupled to the drive turbine116, such that the pump portion104is fluidly coupled to the working fluid circuit102and configured to circulate a working fluid through the working fluid circuit102. The working fluid may have a first mass flow, m1, and a second mass flow, m2, within the working fluid circuit102. The heat engine systems100aand100bmay have one, two, three, or more heat exchangers103fluidly coupled to and in thermal communication with the working fluid circuit102, fluidly coupled to and in thermal communication with the heat source stream90(e.g., waste heat stream flowing from a waste heat source), and configured to transfer thermal energy from the heat source stream90to the first mass flow of the working fluid within the working fluid circuit102. The heat engine systems100aand100balso have the power generator112coupled to the power turbine110. The power turbine110is fluidly coupled to and in thermal communication with the working fluid circuit102and disposed downstream of the first heat exchanger103. The power turbine110is generally configured to convert thermal energy to mechanical energy by a pressure drop in the first mass flow of the working fluid flowing through the power turbine110. The power generator112may be substituted with an alternator other device configured to convert the mechanical energy into electrical energy.

The heat engine systems100aand100bfurther contain the start pump129having the pump portion128operatively coupled to the motor-driven portion130and configured to circulate the working fluid within the working fluid circuit102. For example, the pump portion128of the start pump129and the pump portion104of the turbo pump124may be fluidly coupled in series to the working fluid circuit102.

In one exemplary configuration, as depicted inFIG. 1A, the pump portion128of the start pump129is fluidly coupled to the working fluid circuit102downstream of and in series with the pump portion104of the turbo pump124. Therefore, the heat engine system100ahas an outlet of the pump portion104of the turbo pump124that may be fluidly coupled to and serially upstream of an inlet of the pump portion128of the start pump129. In another exemplary configuration, as depicted inFIG. 1B, the pump portion128of the start pump129is fluidly coupled to the working fluid circuit102upstream of and in series with the pump portion104of the turbo pump124. Therefore, the heat engine system100bhas an inlet of the pump portion104of the turbo pump124that may be fluidly coupled to and serially downstream of an outlet of the pump portion128of the start pump129.

In some embodiments, the heat engine systems100aand100bfurther contain a first recuperator or condenser, such as condenser122, fluidly coupled to the power turbine110and configured to receive the first mass flow discharged from the power turbine110. The heat engine systems100aand100bmay also contain a second recuperator or condenser (not shown) fluidly coupled to the drive turbine116, such that the drive turbine116may be configured to receive and expand the second mass flow and discharge the second mass flow into the additional recuperator or condenser. In some examples, the recuperator or condenser122may be configured to transfer residual thermal energy from the first mass flow to the second mass flow before the second mass flow is expanded in the drive turbine116. The recuperator or condenser122may be configured to transfer residual thermal energy from the first mass flow discharged from the power turbine110to the first mass flow directed to the first heat exchanger103. The additional recuperator or condenser may be configured to transfer residual thermal energy from the second mass flow discharged from the drive turbine116to the second mass flow directed to a second heat exchanger, such as contained within the first heat exchanger103.

In some embodiments, the heat engine system100aand100bfurther contain a second heat exchanger103fluidly coupled to and in thermal communication with the working fluid circuit102and disposed in series with the first heat exchanger103along the working fluid circuit102. The second heat exchanger103may be fluidly coupled to and in thermal communication with the heat source stream90and configured to transfer thermal energy from the heat source stream90to the second mass flow of the working fluid. The second heat exchanger103may be in thermal communication with the heat source stream90and in fluid communication with the pump portion104of the turbo pump124and the pump portion128of the start pump129. In some embodiments described herein, the heat engine system100aor100bcontains first, second, and third heat exchangers, such as the heat exchangers103, disposed in series and in thermal communication with the heat source stream90by the working fluid within the working fluid circuit102. Also, the heat exchangers103may be disposed in series, parallel, or a combination thereof and in thermal communication by the working fluid within the working fluid circuit102. In many examples described herein, the working fluid contains carbon dioxide and at least a portion of the working fluid circuit102, such as the high pressure side, contains the working fluid in a supercritical state.

In another embodiment, the heat engine systems100aand100bfurther contain a first recirculation line152and a first bypass valve154disposed therein. The first recirculation line152may be fluidly coupled to the pump portion104of the turbo pump124on the low pressure side of the working fluid circuit102. Also, the heat engine system100ahas a second recirculation line158and a second bypass valve160disposed therein, as depicted inFIG. 1A. The second recirculation line158may be fluidly coupled to the pump portion128of the start pump129on the low pressure side of the working fluid circuit102.

In other embodiments disclosed herein, the heat engine systems100aand100bcontain the turbo pump124configured to circulate a working fluid throughout the working fluid circuit102and the pump portion104operatively coupled to the drive turbine116. In some examples, the turbo pump124is hermetically-sealed within a casing. The heat engine systems100aand100balso contain the start pump129arranged in series with the turbo pump124along the working fluid circuit102. The heat engine systems100aand100bgenerally have a first check valve146arranged in the working fluid circuit102downstream of the pump portion104of the turbo pump124. The heat engine system100aalso has a second check valve148arranged in the working fluid circuit102downstream of the pump portion128of the start pump129and fluidly coupled to the first check valve146.

The heat engine systems100aand100bfurther contain the power turbine110fluidly coupled to both the pump portion104of the turbo pump124and the pump portion128of the start pump129, a first recirculation line152fluidly coupling the pump portion104with a low pressure side of the working fluid circuit102. In some configurations, the heat engine system100aor100bmay contain a recuperator or condenser122fluidly coupled downstream of the power turbine110and an additional recuperator or condenser (not shown) fluidly coupled to the drive turbine116. In other configurations, the heat engine system100aor100bmay contain a third recuperator or condenser fluidly coupled to the additional recuperator or condenser, wherein the first, second, and third recuperator or condensers are disposed in series along the working fluid circuit102.

In other embodiments disclosed herein, a method for starting the turbo pump124in the heat engine system100a,100band/or generating electricity with the heat engine system100a,100bis provided and includes circulating a working fluid within the working fluid circuit102by a start pump and transferring thermal energy from the heat source stream90to the working fluid by the first heat exchanger103fluidly coupled to and in thermal communication with the working fluid circuit102. Generally, the working fluid has a first mass flow and a second mass flow within the working fluid circuit102and at least a portion of the working fluid circuit contains the working fluid in a supercritical state. The method further includes flowing the working fluid into the drive turbine116of the turbo pump124and expanding the working fluid while converting the thermal energy from the working fluid to mechanical energy of the drive turbine116and driving the pump portion104of the turbo pump124by the mechanical energy of the drive turbine116. The pump portion104may be coupled to the drive turbine116and the working fluid may be circulated within the working fluid circuit102by the turbo pump124. The method also includes diverting the working fluid discharged from the pump portion104of the turbo pump124into a first recirculation line152fluidly communicating the pump portion104of the turbo pump124with a low pressure side of the working fluid circuit102and closing a first bypass valve154arranged in the first recirculation line152as the turbo pump124reaches a self-sustaining speed of operation.

In other embodiments, the heat engine system100amay be utilized while performing several methods disclosed herein. The method may further include deactivating the start pump129in the heat engine system100aand opening the second bypass valve160arranged in the second recirculation line158fluidly communicating the start pump129with the low pressure side of the working fluid circuit102and diverting the working fluid discharged from the start pump129into the second recirculation line158. Also, the method further includes flowing the working fluid into the power turbine110and converting the thermal energy from the working fluid to mechanical energy of the power turbine110and converting the mechanical energy of the power turbine110into electrical energy by the power generator112coupled to the power turbine110.

In some embodiments, the method includes circulating the working fluid in the working fluid circuit102with the start pump129is preceded by closing a shut-off valve to divert the working fluid around the power turbine110arranged in the working fluid circuit102. In other embodiments, the method further includes opening the shut-off valve once the turbo pump124reaches the self-sustaining speed of operation, thereby directing the working fluid into the power turbine110, expanding the working fluid in the power turbine110, and driving the power generator112operatively coupled to the power turbine110to generate electrical power. In other embodiments, the method further includes opening the shut-off valve or the control valve133once the turbo pump124reaches the self-sustaining speed of operation, directing the working fluid into the second heat exchanger103fluidly coupled to the power turbine110and in thermal communication with the heat source stream90, transferring additional thermal energy from the heat source stream90to the working fluid in the second heat exchanger103, expanding the working fluid received from the second heat exchanger103in the power turbine110, and driving the power generator112operatively coupled to the power turbine110, whereby the power generator112is operable to generate electrical power.

In some embodiments, the method also includes opening the shut-off valve once the turbo pump124reaches the self-sustaining speed of operation, directing the working fluid into a second heat exchanger in thermal communication with the heat source stream90, the first and second heat exchangers, within the heat exchangers103, being arranged in series in the heat source stream90, directing the working fluid from the second heat exchanger into a third heat exchanger fluidly coupled to the power turbine110and in thermal communication with the heat source stream90, the first, second, and third heat exchangers, within the heat exchangers103, being arranged in series in the heat source stream90, transferring additional thermal energy from the heat source stream90to the working fluid in the third heat exchanger, expanding the working fluid received from the third heat exchanger in the power turbine110, and driving the power generator112operatively coupled to the power turbine110, whereby the power generator112is operable to generate electrical power.

FIG. 2depicts an exemplary heat engine system101configured as a closed-loop thermodynamic cycle and operated to circulate a working fluid throughout a working fluid circuit105. Heat engine system101illustrates further detail and may be similar in several respects to the heat engine system100adescribed above. Accordingly, the heat engine system101may be further understood with reference toFIGS. 1A-1B, where like numerals indicate like components that will not be described again in detail. The heat engine system101may 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 Appl. No. PCT/US11/29486, entitled “Heat Engines with Cascade Cycles,” filed on Mar. 22, 2011, and published as WO 2011/119650, the contents of which are hereby incorporated by reference. The working fluid circuit105generally contains a variety of conduits adapted to interconnect the various components of the heat engine system101. Although the heat engine system101may be characterized as a closed-loop cycle, the heat engine system101as a whole may or may not be hermetically-sealed such that no amount of working fluid is leaked into the surrounding environment. The heat engine system101generally has an automated control system (not shown) configured to regulate, operate, or otherwise control the valves and other components therein.

Heat engine system101includes a heat exchanger108that is in thermal communication with a heat source stream Qin. The heat source stream Qinmay derive thermal energy from a variety of high temperature sources. For example, the heat source stream Qinmay be a waste heat stream such as, but not limited to, gas turbine exhaust, process stream exhaust, other combustion product exhaust streams, such as furnace or boiler exhaust streams, or other heated stream flowing from a one or more heat sources. Accordingly, the thermodynamic cycle or heat engine system101may 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 stream Qinmay derive thermal energy from renewable sources of thermal energy such as, but not limited to, solar thermal and geothermal sources.

While the heat source stream Qinmay be a fluid stream of the high temperature source itself, in other embodiments the heat source stream Qinmay be a thermal fluid in contact with the high temperature source. The thermal fluid may deliver the thermal energy to the waste heat exchanger108to transfer the energy to the working fluid in the circuit105.

After being discharged from the pump portion104, the combined working fluid m1+m2is split into the first and second mass flows m1and m2, respectively, at point106in the working fluid circuit105. The first mass flow m1is directed to a heat exchanger108in thermal communication with a heat source stream Qin. The respective mass flows m1and m2may be controlled by the user, control system, or by the configuration of the system, as desired.

A power turbine110is arranged downstream of the heat exchanger108for receiving and expanding the first mass flow m1discharged from the heat exchanger108. The power turbine110is operatively coupled to an alternator, power generator112, or other device or system configured to receive shaft work. The power generator112converts the mechanical work generated by the power turbine110into usable electrical power.

The power turbine110discharges the first mass flow m1into a first recuperator114fluidly coupled downstream thereof. The first recuperator114may be configured to transfer residual thermal energy in the first mass flow m1to the second mass flow m2which also passes through the first recuperator114. Consequently, the temperature of the first mass flow m1is decreased and the temperature of the second mass flow m2is increased. The second mass flow m2may be subsequently expanded in a drive turbine116.

The drive turbine116discharges the second mass flow m2into a second recuperator118fluidly coupled downstream thereof. The second recuperator118may be configured to transfer residual thermal energy from the second mass flow m2to the combined working fluid m1+m2originally discharged from the pump portion104. The mass flows m1, m2discharged from each recuperator114,118, respectively, are recombined at point120in the working fluid circuit102and then returned to a lower temperature state at a condenser122. After passing through the condenser122, the combined working fluid m1+m2is returned to the pump portion104and the cycle is started anew.

The recuperators114,118and the condenser122may 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 exchanger108, recuperators114,118, and/or the condenser122may 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.

In one or more embodiments, the heat source stream Qinmay be at a temperature of approximately 200° C., or a temperature at which the turbo pump124is able to achieve self-sustaining operation. As can be appreciated, higher heat source stream 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 carbon dioxide injection upstream of the drive turbine116.

To facilitate the start sequence of the turbo pump124, the heat engine system101may further include a series of check valves, bypass valves, and/or shut-off valves arranged at predetermined locations throughout the circuit105. These valves may work in concert to direct the working fluid into the appropriate conduits until the steady-state operation of turbo pump124is 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 valve132arranged upstream from the power turbine110may be closed during the start-up and/or ramp-up of the heat engine system101. Consequently, after being heated in the heat exchanger108, the first mass flow m1is diverted around the power turbine110via a first diverter line134and a second diverter line138. A bypass valve140is arranged in the second diverter line138and a check valve142is arranged in the first diverter line134. The portion of working fluid circulated through the first diverter line134may be used to preheat the second mass flow m2in the first recuperator114. A check valve144allows the second mass flow m2to flow through to the first recuperator114. The portion of the working fluid circulated through the second diverter line138is combined with the second mass flow m2discharged from the first recuperator114and injected into the drive turbine116in a high-temperature condition.

Once the turbo pump124reaches steady-state operating speeds, and even once a self-sustaining speed is achieved, the shut-off valve132arranged upstream from the power turbine110may be opened and the bypass valve140may be simultaneously closed. As a result, the heated stream of first mass flow m1may be directed through the power turbine110to commence generation of electrical power.

FIG. 3depicts an exemplary heat engine system200configured with a parallel-type heat engine cycle, according to one or more embodiments disclosed herein. The heat engine system200may be similar in several respects to the heat engine systems100a,100b, and101described above. Accordingly, the heat engine system200may be further understood with reference toFIGS. 1A,1B, and2, where like numerals indicate like components that will not be described again in detail. As with the heat engine system100adescribed above, the heat engine system200inFIG. 3may be used to convert thermal energy to work by thermal expansion of a working fluid mass flowing through a working fluid circuit202. The heat engine system200, however, may be characterized as a parallel-type Rankine thermodynamic cycle.

Specifically, the working fluid circuit202may include a first heat exchanger204and a second heat exchanger206arranged in thermal communication with the heat source stream Qin. The first and second heat exchangers204,206may correspond generally to the heat exchanger108described above with reference toFIG. 2. For example, in one embodiment, the first and second heat exchangers204,206may be first and second stages, respectively, of a single or combined heat exchanger. The first heat exchanger204may serve as a high temperature heat exchanger (e.g., a higher temperature relative to the second heat exchanger206) adapted to receive initial thermal energy from the heat source stream Qin. The second heat exchanger206may then receive additional thermal energy from the heat source stream Qinvia a serial connection downstream of the first heat exchanger204. The heat exchangers204,206are arranged in series with the heat source stream Qin, but in parallel in the working fluid circuit202.

The first heat exchanger204may be fluidly coupled to the power turbine110and the second heat exchanger206may be fluidly coupled to the drive turbine116. In turn, the power turbine110is fluidly coupled to the first recuperator114and the drive turbine116is fluidly coupled to the second recuperator118. The recuperators114,118may be arranged in series on a low temperature side of the circuit202and in parallel on a high temperature side of the circuit202. For example, the high temperature side of the circuit202includes the portions of the circuit202arranged downstream of each recuperator114,118where the working fluid is directed to the heat exchangers204,206. The low temperature side of the circuit202includes the portions of the circuit202downstream of each recuperator114,118where the working fluid is directed away from the heat exchangers204,206.

The turbo pump124is also included in the working fluid circuit202, where the pump portion104is operatively coupled to the drive turbine116via the drive shaft123(indicated by the dashed line), as described above. The pump portion104is shown separated from the drive turbine116only for ease of viewing and describing the circuit202. Indeed, although not specifically illustrated, it will be appreciated that both the pump portion104and the drive turbine116may be hermetically-sealed within the casing126(FIG. 1). The start pump129facilitates the start sequence for the turbo pump124during start-up of the heat engine system200and ramp-up of the turbo pump124. Once steady-state operation of the turbo pump124is reached, the start pump129may be deactivated.

The power turbine110may operate at a higher relative temperature (e.g., higher turbine inlet temperature) than the drive turbine116, due to the temperature drop of the heat source stream Qinexperienced across the first heat exchanger204. The power turbine110and the drive turbine116may each be configured to operate at the same or substantially the same inlet pressure. The low-pressure discharge mass flow exiting each recuperator114,118may be directed through the condenser122to be cooled for return to the low temperature side of the circuit202and to either the main or start pump portions104,128, depending on the stage of operation.

During steady-state operation of the heat engine system200, the turbo pump124circulates all of the working fluid throughout the circuit202using the pump portion104, and the start pump129does not generally operate nor is needed. The first bypass valve154in the first recirculation line152is fully closed and the working fluid is separated into the first and second mass flows m1, m2at point210. The first mass flow m1is directed through the first heat exchanger204and subsequently expanded in the power turbine110to generate electrical power via the power generator112. Following the power turbine110, the first mass flow m1passes through the first recuperator114and transfers residual thermal energy to the first mass flow m1as the first mass flow m1is directed toward the first heat exchanger204.

The second mass flow m2is directed through the second heat exchanger206and subsequently expanded in the drive turbine116to drive the pump portion104via the drive shaft123. Following the drive turbine116, the second mass flow m2passes through the second recuperator118to transfer residual thermal energy to the second mass flow m2as the second mass flow m2courses toward the second heat exchanger206. The second mass flow m2is then re-combined with the first mass flow m1and the combined mass flow m1+m2is subsequently cooled in the condenser122and directed back to the pump portion104to commence the fluid loop anew.

During the start-up of the heat engine system200or ramp-up of the turbo pump124, the start pump129may be engaged and operated to start spinning the turbo pump124. To help facilitate this start-up or ramp-up, a shut-off valve214arranged downstream of point210is initially closed such that no working fluid is directed to the first heat exchanger204or otherwise expanded in the power turbine110. Rather, all the working fluid discharged from the pump portion128is directed through a valve215to the second heat exchanger206and the drive turbine116. The heated working fluid expands in the drive turbine116and drives the pump portion104, thereby commencing operation of the turbo pump124.

The head pressure generated by the pump portion128of the turbo pump124near point210prevents the low pressure working fluid discharged from the pump portion104during ramp-up from traversing the first check valve146. Until the pump portion104is able to accelerate past the stall speed of the turbo pump124, the first bypass valve154in the first recirculation line152may be fully opened to recirculate the low pressure working fluid back to a low pressure point in the working fluid circuit202, such as at point156adjacent the inlet of the condenser122. The inlet of pump portion128is in fluid communication with the first recirculation line152at a point upstream of the first bypass valve154. Once the turbo pump124reaches a self-sustaining speed, the bypass valve154may be gradually closed to increase the discharge pressure of the pump portion104and also decrease the flow rate through the first recirculation line152. Once the turbo pump124reaches steady-state operation, and even once a self-sustaining speed is achieved, the shut-off valve214may be gradually opened, thereby allowing the first mass flow m1to be expanded in the power turbine110to commence generating electrical energy. The heat engine system200generally has an automated control system (not shown) configured to regulate, operate, or otherwise control the valves and other components therein.

The start pump129can gradually be powered down and deactivated with the turbo pump124operating at steady-state operating speeds. Deactivating the start pump129may include simultaneously opening the second bypass valve160arranged in the second recirculation line158. The second bypass valve160allows the increasingly lower pressure working fluid discharged from the pump portion128to escape to the low pressure side of the working fluid circuit (e.g., point156). Eventually the second bypass valve160may be completely opened as the speed of the pump portion128slows to a stop and the second check valve148prevents working fluid discharged by the pump portion104from advancing toward the discharge of the pump portion128. At steady-state, the turbo pump124continuously pressurizes the working fluid circuit202in order to drive both the drive turbine116and the power turbine110.

FIG. 4depicts a schematic of a heat engine system300configured with a parallel-type heat engine cycle, according to one or more embodiments disclosed herein. The heat engine system300may be similar in some respects to the above-described the heat engine systems100a,100b,101, and200, and therefore, may be best understood with reference toFIGS. 1A,1B,2, and3, respectively, where like numerals correspond to like elements that will not be described again. The heat engine system300includes a working fluid circuit302utilizing a third heat exchanger304also in thermal communication with the heat source stream Qin. The heat exchangers204,206, and304are arranged in series with the heat source stream Qin, but arranged in parallel in the working fluid circuit302.

The turbo pump124(e.g., the combination of the pump portion104and the drive turbine116operatively coupled via the drive shaft123) is arranged and configured to operate in series with the start pump129, especially during the start-up of the heat engine system300and the ramp-up of the turbo pump124. During steady-state operation of the heat engine system300, the start pump129does not generally operate. Instead, the pump portion104solely discharges the working fluid that is subsequently separated into first and second mass flows m1, m2, respectively, at point306. The third heat exchanger304may be configured to transfer thermal energy from the heat source stream Qinto the first mass flow m1flowing therethrough. The first mass flow m1is then directed to the first heat exchanger204and the power turbine110for expansion power generation. Following expansion in the power turbine110, the first mass flow m1passes through the first recuperator114to transfer residual thermal energy to the first mass flow m1discharged from the third heat exchanger304and coursing toward the first heat exchanger204.

The second mass flow m2is directed through the valve215, the second recuperator118, the second heat exchanger206, and subsequently expanded in the drive turbine116to drive the pump portion104. After being discharged from the drive turbine116, the second mass flow m2merges with the first mass flow m1at point308. The combined mass flow m1+m2thereafter passes through the second recuperator118to provide residual thermal energy to the second mass flow m2as the second mass flow m2courses toward the second heat exchanger206.

During the start-up of the heat engine system300and/or the ramp-up of the turbo pump124, the pump portion128draws working fluid from the first bypass line152and circulates the working fluid to commence spinning of the turbo pump124. The shut-off valve214may be initially closed to prevent working fluid from circulating through the first and third heat exchangers204,304and being expanded in the power turbine110. The working fluid discharged from the pump portion128is directed through the second heat exchanger206and drive turbine116. The heated working fluid expands in the drive turbine116and drives the pump portion104, thereby commencing operation of the turbo pump124.

Until the discharge pressure of the pump portion104of the turbo pump124accelerates past the stall speed of the turbo pump124and can withstand the head pressure generated by the pump portion128of the start pump129, any working fluid discharged from the pump portion104is either directed toward the pump portion128or recirculated via the first recirculation line152back to a low pressure point in the working fluid circuit202(e.g., point156). Once the turbo pump124becomes self-sustaining, the bypass valve154may be gradually closed to increase the pump portion104discharge pressure and decrease the flow rate in the first recirculation line152. Then, the shut-off valve214may also be gradually opened to begin circulation of the first mass flow m1through the power turbine110to generate electrical energy. Subsequently, the start pump129in the heat engine system300may be gradually deactivated while simultaneously opening the second bypass valve160arranged in the second recirculation line158. Eventually the second bypass valve160is completely opened and the pump portion128can be slowed to a stop. The heat engine system300generally has an automated control system (not shown) configured to regulate, operate, or otherwise control the valves and other components therein.

FIG. 5depicts a schematic of a heat engine system400configured with another parallel-type heat engine cycle, according to one or more embodiments disclosed herein. The heat engine system400may be similar to the heat engine system300, and as such, may be best understood with reference toFIG. 3where like numerals correspond to like elements that will not be described again. The working fluid circuit402depicted inFIG. 5is substantially similar to the working fluid circuit302depicted inFIG. 4but with the exception of an additional, third recuperator404. The third recuperator404may be adapted to extract additional thermal energy from the combined mass flow m1+m2discharged from the second recuperator118. Accordingly, the working fluid in the first mass flow m1entering the third heat exchanger304may be preheated in the third recuperator404prior to receiving thermal energy transferred from the heat source stream Qin.

As illustrated, the recuperators114,118, and404may operate as separate heat exchanging devices. In other embodiments, however, the recuperators114,118, and404may be combined as a single, integral recuperator. Steady-state operation, system start-up, and turbo pump124ramp-up may operate substantially similar as described above inFIG. 3, and therefore will not be described again.

Each of the described systems inFIGS. 1A-5may 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 and related components (e.g., turbines110,116, recuperators114,118,404, condensers122, pump portions104,128, and/or other components) in a consolidated, single unit. An exemplary waste heat engine skid is described and illustrated in commonly assigned U.S. application Ser. No. 12/631,412, entitled “Thermal Energy Conversion Device,” filed on Dec. 9, 2009, and published as US 2011-0185729, wherein the contents are hereby incorporated by reference to the extent consistent with the present disclosure.

FIG. 6is a flowchart of a method500for starting a turbo pump in a heat engine system having a thermodynamic working fluid circuit utilized during operation, according to one or more embodiments disclosed herein. The method500includes circulating a working fluid in the working fluid circuit with a start pump that is connected in series with the turbo pump, as at502. The start 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 stream. Thermal energy is transferred to the working fluid from the heat source stream in the first heat exchanger, as at504. The method500further includes expanding the working fluid in a drive turbine, as at506. The drive turbine is fluidly coupled to the first heat exchanger, and the drive turbine is operatively coupled to a pump portion, such that the combination of the drive turbine and pump portion is the turbo pump.

The pump portion is driven with the drive turbine, as at508. Until the pump portion accelerates past the stall point of the pump, the working fluid discharged from the pump portion is diverted to the start pump or into a first recirculation line, as at510. The first recirculation line may fluidly communicate the pump portion 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 turbo pump reaches a self-sustaining speed of operation, the first bypass valve may gradually begin to close, as at512. Consequently, the pump portion begins circulating the working fluid discharged from the pump portion through the working fluid circuit, as at514.

The method500may also include deactivating the start pump and opening a second bypass valve arranged in a second recirculation line, as at516. The second recirculation line may fluidly communicate the start pump with the low pressure side of the working fluid circuit. The low pressure working fluid discharged from the start pump may be diverted into the second recirculation line until the start pump comes to a stop, as at518.