PASSIVE ALTERNATOR DEPRESSURIZATION AND COOLING SYSTEM

Abstract

A pressure reduction system may include an alternator with a casing and a rotor positioned, at least in part, within a cavity defined by the casing. The pressure reduction system may also include a mass management system that includes a control tank configured to be maintained at a tank pressure lower than a cavity pressure within the cavity of the alternator, thereby forming a pressure differential. A first transfer conduit may transfer a working fluid from the cavity of the alternator to the control tank via the pressure differential. The mass management system may be positioned at an elevation above the alternator, and include a refrigeration loop configured to cool the working fluid contained within the control tank. A second transfer conduit may fluidly couple the alternator and the mass management system, and may transfer the cooled working fluid from the control tank to the cavity via gravitational force.

Description

This application claims the benefit of U.S. Prov. Appl. No. 62/093,544, filed Dec. 18, 2015. This application is incorporated herein by reference in its entirety to the extent consistent with the present application.

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 use heat exchanger devices to capture and recycle waste heat back into the process. However, the capturing and recycling of waste heat is generally infeasible by industrial processes that use high temperatures, have insufficient mass flow, or include other unfavorable conditions.

Waste heat can be converted into useful energy by a variety of 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 use waste heat to generate steam for driving a turbine or other type of expansion device. The turbine is then connected to an electric generator, such as an alternator, which is used to supply electricity to an electrical bus or grid (e.g., an alternating current bus) that usually has a varying load or demand over time.

In certain circumstances, such as, for example, peak demand, alternators and associated components thereof (e.g., rotor, stator, and bearings) may be susceptible to overheating. To eliminate or reduce such overheating, methods employed have included cooling the alternator by using a blower or a fan to circulate gas or fluid through an external heat exchanger then through the alternator. However, using such cooling components (e.g., blower/fan, heat exchanger, piping, and valves) generally incurs additional expenses, increases installation and maintenance time, and creates a larger footprint.

Alternators may also be susceptible to over pressurization of the alternator cavity, which may occur when additional working fluid from the expansion device leaks past the bearings and seals encasing the rotor of the alternator. Over pressurization of the alternator often results in reduced efficiency, and in some instances, complete shutdown of the alternator.

What is needed, then, is a system for use in a heat engine system that efficiently cools the alternator and efficiently reduces pressure within the alternator as needed.

In one embodiment, a pressure reduction system may include an alternator. The alternator may include a casing and a rotor positioned, at least in part, within a cavity defined by the casing. The pressure reduction system may also include a mass management system having a control tank configured to be maintained at a tank pressure lower than a cavity pressure within the cavity of the alternator, thereby forming a pressure differential therebetween. A first transfer conduit may be configured to transfer a working fluid from the cavity of the alternator to the control tank via the pressure differential.

In another embodiment, a cooling system may include an alternator having a casing and a rotor positioned, at least in part, in a cavity defined by the casing. The cooling system may also include a mass management system having a control tank configured to be positioned at an elevation above the alternator. The control tank may include a refrigeration loop configured to cool a working fluid contained within the control tank. The cooling system may include a first transfer conduit fluidly coupling the alternator and the mass management system, and the first transfer conduit may be configured to transfer the working fluid from the cavity to the control tank. The cooling system may also include a second transfer conduit fluidly coupling the alternator and the mass management system, and the second transfer conduit may be configured to transfer the cooled working fluid from the control tank to the cavity via gravitational force.

In another embodiment, a heat engine system may include an expansion device in a working fluid circuit, and the expansion device may be configured to receive a working fluid at an expansion device inlet at a high pressure. The expansion device may output the working fluid at a low pressure, and further convert a pressure drop in the working fluid to mechanical energy. The heat engine system may include an alternator fluidly coupled to the expansion device. The alternator may convert the mechanical energy to electrical energy, and include a casing and a rotor positioned at least in part in a cavity defined within the casing. The cavity of the alternator may further be configured to receive a portion of the working fluid from the expansion device. The heat engine system may include a mass management system that includes a control tank configured to be maintained at a tank pressure substantially lower than a cavity pressure within the cavity to form a pressure differential therebetween. A first transfer conduit may be configured to transfer the working fluid from the cavity of the alternator to the control tank via the pressure differential. The heat engine system may include a pump fluidly coupled to the expansion device and configured to receive the working fluid at a low pressure and output the working fluid at a high pressure. A recuperator may be fluidly coupled to the pump and configured to heat the working fluid exiting the pump. The heat engine system may further include a waste heat exchanger fluidly coupled to the recuperator. The waste heat exchanger may be configured to further heat the working fluid after exiting the recuperator and before entering the expansion device.

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 depicts an exemplary heat engine system including a system for depressurizing and cooling the alternator, according to one or more embodiments disclosed herein.

FIG. 2 depicts an exemplary system for depressurizing and cooling an alternator, according to one or more embodiments disclosed herein.

FIG. 3 is a graph depicting fluid friction loss and refrigeration work as a function of pressure in an alternator, according to one or more embodiments disclosed herein.

It is to be understood that the following disclosure describes several exemplary embodiments for implementing different features, structures, or functions of the invention. 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 invention. Additionally, the present disclosure may repeat reference numerals and/or letters in the various exemplary embodiments and across the Figures provided herein. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various exemplary embodiments and/or configurations discussed in the various Figures. Moreover, the formation of a first feature over or on a second feature in the 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 invention, 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. It should also be appreciated that the term “about,” as used herein, in conjunction with a numeral refers to a value that is +/−5% (inclusive) of that numeral, +/−10% (inclusive) of that numeral, or +/−15% (inclusive) of that numeral. It should further be appreciated that when a numerical range is disclosed herein, any numerical value falling within the range is also specifically disclosed. 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.

Embodiments of the disclosure generally provide a system for cooling and/or reducing pressure in an alternator. One or more embodiments of the disclosure also provide a heat engine system including the system for cooling and/or reducing pressure in the alternator.

FIG. 1 depicts a heat engine system 10 that includes a system 100 for heating and cooling an alternator 105. The heat engine system 10 may also be referred to as a thermal engine system, an electrical generation system, a waste heat or other heat recovery system, and/or a thermal to electrical energy system, as described in one of more embodiments herein. The heat engine system 10 may include a waste heat system 12 and a power generation system 220 coupled to and in thermal communication with each other via a working fluid circuit 202. The working fluid circuit 202 may contain the working fluid (e.g., sc-CO₂) and may have a high pressure side and a low pressure side, which will be described herein. A heat source stream 11 may flow through heat exchangers 20 and 30 disposed within the waste heat system 12. Each of the heat exchangers 20 and 30, independently, may be fluidly coupled to and in thermal communication with the high pressure side of the working fluid circuit 202, configured to be fluidly coupled to and in thermal communication with a heat source stream 11, and configured to transfer thermal energy from the heat source stream 11 to the working fluid within the high pressure side of the working fluid circuit 202. Thermal energy may be absorbed by the working fluid within the working fluid circuit 202 and converted to mechanical energy by flowing the heated working fluid through one or more expanders or turbines.

The heat engine system 10 may further include at least one pump, such as a turbopump 260, disposed within the working fluid circuit 202 and fluidly coupled between the low pressure side and the high pressure side of the working fluid circuit 202. The turbopump 260 may be configured to circulate and to pressurize the working fluid throughout the working fluid circuit 202. The turbopump 260 may include a pump portion 262 coupled with a turbine 264. The low pressure side of the working fluid circuit 202 extends from an outlet of the turbine 264 to the inlet of the pump portion 262 of the turbopump 260. The high pressure side of the working fluid circuit 202 extends from the inlet of the pump portion 262 to the outlet of the turbine 264.

The turbine 264 of the turbopump 260 may be fluidly coupled to the working fluid circuit 202 downstream of the heat exchanger 20 and the pump portion 262 of the turbopump 260 may be fluidly coupled to the working fluid circuit 202 upstream of the heat exchanger 20. In one embodiment, the turbine 264 may be downstream of multiple heat exchangers, such as heat exchanger 20 and 30, within the working fluid circuit 202. In one example, the turbine 264 may be configured to receive and be powered by the working fluid passing through and absorbing thermal energy from the heat exchanger 20. In one example, the turbine 264 may be configured to receive and be powered by the working fluid passing through and absorbing thermal energy from more than one heat exchanger, such as heat exchangers 20 and 30. The turbopump 260 may further include a driveshaft 267 coupled between the turbine 264 and the pump portion 262.

The turbine 264 may be fluidly coupled to and in thermal communication with the working fluid, and configured to convert thermal energy to mechanical energy by a pressure drop in the working fluid flowing between the high and the low pressure sides of the working fluid circuit 202. An alternator 105 may be coupled to the turbine 264 and configured to convert the mechanical energy into electrical energy. A power outlet may be electrically coupled to the alternator 105 and configured to transfer the electrical energy from the alternator 105 to an electrical grid. The power generation system 220 may further include a driveshaft 230 coupled between the turbine 264 and the alternator 105. In one embodiment, the driveshaft 267 may be integral with the driveshaft 230, or may be a solitary driveshaft. The power generation system 220 may further contain a bearing housing 238 which substantially encompasses or encloses the bearings disposed within the power generation system 220.

Exemplary structures of the bearing housing 238 may completely or substantially encompass or enclose the bearings as well as all or part of turbines, generators, pumps, driveshafts, or other components shown or not shown for the heat engine system 10. The bearing housing 238 may completely or partially include structures, chambers, cases, housings, such as turbine housings, generator housings, driveshaft housings, driveshafts that contain bearings, gearbox housings, derivatives thereof, or combinations thereof. FIG. 1 depicts the bearing housing 238 containing all or a portion of the turbine 264, the alternator 105, the driveshafts 230 and 267, and the pump portion 262 of the power generation system 220. In some examples, the housing of the turbine 264 may be coupled to and/or forms a portion of the bearing housing 238.

FIG. 2 shows the alternator 105 in more detail, and also depicts the system 100 for depressurizing and cooling the alternator 105. The alternator 105 may include a casing 110 defining a cavity 115 in which, at least in part, a rotor 125 is positioned and configured to spin at high speed. The rotor 125 may be integral with the driveshafts 230 or 267, or the rotor 125 may form a solitary driveshaft with the driveshafts 230 and/or 267. In one embodiment, the rotor 125 may have a rotational speed between about 20,000 RPM and about 40,000 RPM. The cavity 115 may contain a working fluid, which in one embodiment, may be or include carbon dioxide. Further, in one embodiment, the working fluid may be carbon dioxide and at least a portion of the working fluid may be in a supercritical state. However, other working fluids including, but not limited to ammonia and a combination of working fluids, are contemplated. The working fluid in the cavity 115 may be contained within the alternator 105 by a shaft seal 120 positioned between the rotor 125 and the casing 110 at one end of the alternator 105. The shaft seal 120 may be a labyrinth seal, a double seal, a dynamically pressure balanced seal, a dry gas seal, or any other sealing mechanism configured to reduce leakage flow of the working fluid into or out of the casing 110.

The system 100 for depressurizing and cooling the alternator 105 may include a mass management system 150 configured to control the pressure and temperature within the cavity 115 of the alternator 105. As discussed in more detail below, the mass management system 150 may include a control tank 155 configured to receive and store working fluid from the alternator 105 and, in addition, to disperse working fluid to the alternator 105. The control tank 155 may be maintained at a relatively low pressure, such as, for example, about 0.5 MPa to about 2 MPa.

The mass management system 150 may include a closed refrigeration loop 160 positioned, at least in part, within the control tank 155 in order to maintain a low pressure of the working fluid within the control tank 155. The refrigeration loop 160 may include a cool fluid source 161, which may be water, seawater, nitrogen, or any other fluid, that may flow through a conduit into the control tank 155. The cooled fluid may flow through a condenser 162 to further condense the cooled fluid. After condensing the cooled fluid, the cooled fluid may flow through a heat exchanger 163, wherein heat is transferred from the working fluid to the cooled fluid, and wherein the fluid flows into a compressor 164 and out to the cooled fluid source. The closed refrigeration loop 160 may therefore cool the working fluid contained within the control tank 155. The pressure of the control tank 155 may be lower than the pressure within the alternator 105, which may be, for example, around about 0.5 MPa to about 11 MPa. In one embodiment, the control tank 155 may be positioned at an elevation above the alternator 105.

The system 100 may include a first transfer conduit 130 fluidly coupling the alternator 105 and the mass management system 150, thereby forming a first fluid passageway therebetween. More specifically, the first transfer conduit 130 may fluidly couple the cavity 115 of the alternator 105 and the control tank 155. In one embodiment, the first transfer conduit 130 may provide the first fluid passageway between a lower portion of the alternator 105 and an upper portion of the control tank 155.

The first transfer conduit 130 may include a valve 135 positioned within the first fluid passageway between the alternator 105 and the control tank 155 and configured to control fluid flow therebetween. As such, the valve 135 may prevent or throttle the flow of working fluid between the alternator 105 and the control tank 155. In one embodiment, the valve 135 may be a check valve to prevent working fluid from flowing from the control tank 155 to the alternator 105 via the first transfer conduit 130. The system 100 may also include a heat exchanger 140 fluidly coupled with the first transfer conduit 130 and configured to cool the working fluid moving from the alternator 105 to the control tank 155 prior to the working fluid entering the control tank 155 in order to reduce the cooling duty of the closed refrigeration loop 160.

As previously discussed, the control tank 155 may be maintained at a lower pressure than the cavity 115 of the alternator 105. Therefore, working fluid may flow through the first transfer conduit 130, from the alternator 105 to the control tank 155, based on a positive pressure differential. Such flow may be passive, or in other words, without aid of a pump or other like equipment. In addition, the positive pressure differential may allow working fluid to be transferred from the alternator 105 to the control tank 155 to optimize the operation of the alternator 105. For example, the pressure within the cavity 115 may increase as working fluid from the turbine 264 leaks past the shaft seal 120 into the cavity 115. As shown in FIG. 3, a higher fluid pressure within the cavity 115 results in greater power loss within the alternator 105. However, in embodiments of the system 100 described herein, as working fluid leaks past the turbine 264 shaft seal 120 and into the cavity 115, working fluid may flow from the alternator 105 to the control tank 155 via the first transfer conduit 130 based on the pressure differential between the cavity 115 and the control tank 155 to prevent additional power loss. In one embodiment, the flow rate of the working fluid from the cavity 115 to the control tank 155 may be about 600 grams per second. The flow rate may be dependent on, amongst other factors, the leak rate of working fluid from the turbine 264 to the alternator 105.

The system 100 further includes a second transfer conduit 165 fluidly coupling the mass management system 150 and the alternator 105. More specifically, the second transfer conduit 165 may fluidly couple the control tank 155 and the cavity 115 of the alternator 105 to form a second fluid passageway therebetween. In one embodiment, the second transfer conduit 165 may form the second fluid passageway between a lower portion of the control tank 155 and an upper portion of the alternator 105. The second transfer conduit 165 may include a valve 170 positioned within the second fluid passageway between the control tank 155 and the alternator 105 and configured to control fluid flow therebetween. As such, the valve 170 may prevent or throttle the flow of working fluid between the control tank 155 and the alternator 105. In one embodiment, the valve 170 may be a check valve to prevent fluid from flowing from the alternator 105 to the control tank 155 via the second transfer conduit 165.

As discussed, the control tank 155 may be positioned at an elevation above the alternator 105, such that working fluid may be gravity fed via the second transfer conduit 165 from the control tank 155 to the cavity 115 of the alternator 105. In one embodiment, the working fluid may exit the control tank 155 at a flow rate of about 500 grams per second. In other embodiments, the flow rate of the working fluid exiting the control tank 155 may be greater or lesser depending on, amongst other factors the windage within the cavity 115 and the elevation of the control tank 155 above the alternator 105. Further, because the working fluid within the control tank 155 may be cooled by the refrigeration loop 160, the working fluid flowing from the control tank 155 to the alternator 105 may cool the alternator 105, which may be heated by the fluid friction (windage) generated by the rotation of the rotor 125 within the cavity 115 of the alternator 105. Because the control tank 155 may be vertically positioned at an elevation above the alternator 105, the cooling of the alternator 105 may be accomplished in a passive manner.

The system 100 may also include a return conduit 175 fluidly coupled with the second transfer conduit 165 at a location 185 between the control tank 155 and the alternator 105. The return conduit 175 may be configured to transfer working fluid from the second transfer conduit 165 to the heat engine system 10. A transfer pump 180 may be fluidly coupled with the return conduit 175 and configured to maintain a relatively constant amount of mass in the heat engine system 10 by transferring the working fluid from the mass management system 150 to the heat engine system 10 at a flow rate relatively equal to the flow rate of the working fluid entering the alternator 105 through the shaft seal 120 from the heat engine system 10. In one embodiment, the flow rate of the working fluid through the return conduit 175 may be about 100 grams per second. In other embodiments, the flow rate of the working fluid through the return conduit 175 may be greater or lesser depending, amongst other factors, on the leak rate of working fluid from the turbine 264 to the alternator 105.

In operation, the rotor 125, which may be driven by the turbine 264, may rotate at a high speed, e.g., about 20,000 RPM to about 40,000 RPM, within the cavity 115 at least partially filled with working fluid. In operation, as the rotor 125 rotates, working fluid may leak past the shaft seal 120 and into the cavity 115 from the turbine 264 of the heat engine system 10. The additional working fluid entering the cavity 115 may result in an increase of pressure within the cavity 115. Further, the rotation of the rotor 125 may induce fluid friction which leads to heating, or windage. As shown in FIG. 3, power loss within the alternator 105 increases as fluid pressure increases within the cavity 115 inducing windage. Accordingly, if the rotor 125 continues to rotate within the cavity 115 without intervention, the temperature within the alternator 105 will increase, which may lead to overheating. However, in the system 100 provided herein, as working fluid leaks into the cavity 115, working fluid may flow from the alternator 105 to the control tank 155 via the first transfer conduit 130 based on the pressure differential between the cavity 115 and the control tank 155.

The working fluid in the control tank 155 may be cooled by the mass management system 150 via the refrigeration loop 160. Because the control tank 155 may be positioned at an elevation above the alternator 105, the cooled working fluid may be transferred to the alternator 105 via the second transfer conduit 165 by gravitational force, thereby cooling the alternator 105. In addition, as working fluid is added to the system 100 from leakage through the shaft seal 120, working fluid may be returned back to the heat engine system 10 via the return conduit 175 and the transfer pump 180 as the working fluid flows out of the control tank 155.

Turning back to the heat engine system 10 illustrated in FIG. 1, the heat engine system 10 may further include at least one recuperator 216 fluidly coupled to the working fluid circuit and operative to transfer thermal energy between the high and low pressure sides of the working fluid circuit 202. In some examples, the recuperator 216 may be configured to transfer the thermal energy from the low pressure side to the high pressure side. The heat engine system 10 may further include a cooler 274 in thermal communication with the working fluid contained in the low pressure side of the working fluid circuit 202 and configured to remove thermal energy from the working fluid in the low pressure side. In some examples, the cooler 274 may be a condenser configured to control a temperature of the working fluid in the low pressure side of the working fluid circuit 202 by transferring thermal energy from the working fluid in the low pressure side to a cooling loop outside of the working fluid circuit 202.

In one embodiment, the cooler 274 may circulate a coolant from a cooling circuit 200 to cool the working fluid contained in the low pressure side of the working fluid circuit 202. In one embodiment, the coolant circulating through the cooling circuit 200 may be water, such as freshwater. A pump 210 may be disposed within the cooling circuit 200 to circulate the coolant through the cooling circuit 200. A cooler 215 may also be disposed within the cooling circuit 200 to transfer thermal energy from the coolant moving through the cooling circuit 200. In one embodiment, the cooler 215 may circulate seawater to transfer thermal energy from the coolant to the seawater. For example, seawater may enter the cooler 215 via an inlet line 212, and seawater may exit the cooler 215 via an outlet line 214.

The heat engine system 10 may also include another mass management system (MMS) 270 fluidly coupled to the working fluid circuit 202. The MMS 270 may include a mass control tank 286 fluidly coupled to the low pressure side of the working fluid circuit 202 and configured to receive, store, and deliver the working fluid. The mass control tank 286 and the working fluid circuit 202 may share the working fluid (e.g., carbon dioxide) such that the mass control tank 286 may receive, store, and disperse the working fluid during various operational steps of the heat engine system 10. In one embodiment, the mass control tank 286 may receive additional working fluid via a feed line inlet 288.

The MMS 270 may include an inventory return line 72 fluidly coupled to and between the mass control tank 286 and the low pressure side of the working fluid circuit 202, such as downstream of the condenser 274. As depicted in FIG. 1, a fluid line 68 may be fluidly coupled with and extend from the outlet of the condenser 274, and the inventory return line 72 may be fluidly coupled to and extend from the fluid line 68 to the mass control tank 286. The MMS 270 may also include a pump 70 fluidly coupled to the mass control tank 286 and configured to transfer the working fluid from the mass control tank 286 to the low pressure side of the working fluid circuit 202 by an inventory supply line 82. Accordingly, the MMS 270 may receive the working fluid from the working fluid circuit 202, store the working fluid for subsequent use, and deliver the working fluid into the working fluid circuit 202.

It is contemplated that the mass management system 270 for use with the pump portion 282 of the heat engine system 10 may be combined with the mass management system 150 for use with the alternator 105, as described above. To that extent, the mass control tank 286 of the heat engine system 10 for use with the pump portion 282 and the control tank 155 of the mass management system 150 for use with the alternator 105 may be combined into a single tank. The single tank may control the addition and/or removal of working fluid to the high pressure side of the heat engine system 10, the low pressure side of the heat engine system 10, and/or the alternator cavity 115.

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.

Claims

1. A pressure reduction system comprising:

an alternator comprising a casing and a rotor positioned, at least in part, within a cavity defined by the casing;

a mass management system comprising a control tank configured to be maintained at a tank pressure lower than a cavity pressure within the cavity to form a pressure differential therebetween; and

a first transfer conduit configured to transfer a working fluid from the cavity of the alternator to the control tank via the pressure differential.

2. The pressure reduction system of claim 1, further comprising a second transfer conduit configured to transfer the working fluid from the control tank to the cavity.

3. The pressure reduction system of claim 1, wherein the control tank comprises a closed refrigeration loop configured to cool the working fluid.

4. The pressure reduction system of claim 1, further comprising a heat exchanger configured to cool the working fluid prior to the working fluid entering the control tank.

5. The pressure reduction system of claim 2, further comprising:

a first valve configured to control a flow of the working fluid through the first transfer conduit; and

a second valve configured to control a flow of the working fluid through the second transfer conduit.

6. The pressure reduction system of claim 2, further comprising:

a return conduit fluidly coupled with the second transfer conduit between the control tank and the alternator; and

a pump fluidly coupled with the return line conduit and configured to transfer the working fluid out of the pressure reduction system.

7. The pressure reduction system of claim 1, wherein the control tank is configured to be maintained at a tank pressure between about 0.5 MPa and about 2 MPa.

8. The pressure reduction system of claim 7, wherein the cavity is configured to maintain a cavity pressure between about 0.5 MPa and about 11 MPa.

9. A cooling system comprising:

an alternator comprising a casing and a rotor positioned, at least in part, in a cavity defined by the casing;

a mass management system comprising a control tank configured to be positioned at an elevation above the alternator, the control tank comprising a refrigeration loop configured to cool a working fluid contained within the control tank;

a first transfer conduit fluidly coupling the alternator and the mass management system and configured to transfer the working fluid from the cavity to the control tank; and

a second transfer conduit fluidly coupling the alternator and the mass management system and configured to transfer the cooled working fluid from the control tank to the cavity via gravitational force.

10. The cooling system of claim 9, wherein the refrigeration loop is closed.

11. The cooling system of claim 9, further comprising a heat exchanger fluidly coupled with the first transfer conduit and configured to cool the working fluid prior to the working fluid entering the control tank.

12. The cooling system of claim 9, wherein the control tank is configured to be maintained at a tank pressure substantially lower than a cavity pressure within the cavity of the alternator.

13. The cooling system of claim 9, further comprising:

a return conduit fluidly coupled with the second transfer conduit between the control tank and the alternator; and

a pump fluidly coupled with the return line conduit and configured to transfer the working fluid out of the pressure reduction system.

14. The cooling system of claim 9, wherein the working fluid comprises carbon dioxide.

15. A heat engine system, comprising:

an expansion device in a working fluid circuit, the expansion device configured to receive a working fluid at an expansion device inlet at a high pressure and to output the working fluid at a low pressure, and wherein the expansion device converts a pressure drop in the working fluid to mechanical energy;

an alternator fluidly coupled to the expansion device, the alternator converting the mechanical energy to electrical energy, the alternator comprising a casing and a rotor positioned at least in part in a cavity defined within the casing, the cavity further configured to receive a portion of the working fluid from the expansion device;

a mass management system comprising a control tank configured to be maintained at a tank pressure substantially lower than a cavity pressure within the cavity to form a pressure differential therebetween;

a first transfer conduit configured to transfer the working fluid from the cavity to the control tank via the pressure differential;

a pump fluidly coupled to the expansion device and configured to receive the working fluid at a low pressure and output the working fluid at a high pressure;

a recuperator fluidly coupled to the pump and configured to heat the working fluid exiting the pump; and

a waste heat exchanger fluidly coupled to the recuperator and configured to further heat the working fluid after exiting the recuperator and before entering the expansion device.

16. The heat engine system of claim 15, further comprising a second transfer conduit configured to transfer the working fluid from the control tank to the cavity of the alternator.

17. The system of claim 16, further comprising:

a return conduit fluidly coupled with the second transfer conduit between the control tank and the alternator; and

a transfer pump configured to transfer the working fluid out of the control tank to a location in the working fluid circuit between the pump and the expansion device.

18. The system of claim 17, wherein the cavity is configured to receive the portion of the working fluid at a leak rate, and the transfer pump is configured to transfer the working fluid to the location in the working fluid circuit between the pump and the expansion device at a rate substantially equal to the leak rate.

19. The system of claim 17, wherein the mass management system comprises:

a third transfer conduit configured to transfer the working fluid between the control tank and a location upstream of the expansion device; and

a fourth transfer conduit configured to transfer the working fluid between the control tank and a location upstream of the pump.

20. The system of claim 17, further comprising a second mass management system comprising:

a second control tank;

a third transfer conduit configured to transfer the working fluid between the second control tank and a location upstream of the expansion device; and

a fourth transfer conduit configured to transfer the working fluid between the second control tank and a location upstream of the pump.