Patent Publication Number: US-2017349252-A1

Title: Hydraulic drives for use in charging systems, ballast systems, or other systems of underwater vehicles

Description:
TECHNICAL FIELD 
     This disclosure generally relates to underwater vehicles. More specifically, this disclosure relates to hydraulic drives for use in charging systems, ballast systems, or other systems of underwater vehicles. 
     BACKGROUND 
     Unmanned underwater vehicles (UUVs) can be used in a number of applications, such as undersea surveying, recovery, or surveillance operations. However, supplying adequate power to UUVs for prolonged operation can be problematic. For example, one prior approach simply tethers a UUV to a central power plant and supplies power to the UUV through the tether. However, this clearly limits the UUV&#39;s range and deployment, and it can prevent the UUV from being used in situations requiring independent or autonomous operation. Another prior approach uses expanding wax based on absorbed heat to generate power, but this approach provides power in very small amounts, typically limited to less than about 200 Watts (W) at a 2.2 Watt-hour (WHr) capacity. Yet another prior approach involves using fuel cells in a UUV to generate power, but fuel cells typically require large packages and substantial space. It is also often necessary or desirable to provide ballast systems on UUVs in order to help stabilize the UUVs and provide buoyance management during use. 
     SUMMARY 
     This disclosure provides hydraulic drives for use in charging systems, ballast systems, or other systems of underwater vehicles. 
     In a first embodiment, an apparatus includes first and second tanks each configured to receive and store a refrigerant under pressure. The apparatus also includes at least one generator configured to receive flows of the refrigerant between the tanks and to generate electrical power based on the flows of the refrigerant. The apparatus further includes first and second hydraulic drives associated with the first and second tanks, respectively. Each hydraulic drive includes a first piston within the associated tank, a channel fluidly coupled to the associated tank and configured to contain hydraulic fluid, and a second piston within the channel and configured to move within the channel in order to vary an amount of the hydraulic fluid within the associated tank and vary a position of the first piston within the associated tank. The channel of each hydraulic drive has a cross-sectional area that is less than a cross-sectional area of the associated tank. 
     In a second embodiment, a system includes an underwater vehicle having a body, fins projecting from the body, and a power generator. The power generator includes first and second tanks each configured to receive and store a refrigerant under pressure. The power generator also includes at least one generator configured to receive flows of the refrigerant between the tanks and to generate electrical power based on the flows of the refrigerant. The power generator further includes first and second hydraulic drives associated with the first and second tanks, respectively. Each hydraulic drive includes a first piston within the associated tank, a channel fluidly coupled to the associated tank and configured to contain hydraulic fluid, and a second piston within the channel and configured to move within the channel in order to vary an amount of the hydraulic fluid within the associated tank and vary a position of the first piston within the associated tank. The channel of each hydraulic drive has a cross-sectional area that is less than a cross-sectional area of the associated tank. 
     In a third embodiment, an apparatus includes a cavity configured to receive a material and a hydraulic drive. The hydraulic drive includes a first piston within the cavity, a channel fluidly coupled to the cavity and configured to contain hydraulic fluid, and a second piston within the channel and configured to move within the channel in order to vary an amount of the hydraulic fluid within the cavity, vary a position of the first piston within the cavity, and vary an amount of the material within the cavity. The channel of the hydraulic drive has a cross-sectional area that is less than a cross-sectional area of the cavity. 
     Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A through 1D  illustrate a first example underwater vehicle having one or more hydraulic drives in accordance with this disclosure; 
         FIGS. 2A through 2C  illustrate a second example underwater vehicle having one or more hydraulic drives in accordance with this disclosure; 
         FIG. 3  illustrates example components of an underwater vehicle having one or more hydraulic drives in accordance with this disclosure; 
         FIGS. 4 through 5G  illustrate an example charging system for periodically charging an underwater vehicle in accordance with this disclosure; 
         FIG. 6  illustrates an example method for periodically charging an underwater vehicle in accordance with this disclosure; 
         FIGS. 7A through 7C  illustrate an example ballast system for an underwater vehicle in accordance with this disclosure; and 
         FIG. 8  illustrates an example method for stabilizing an underwater vehicle using a ballast system in accordance with this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 8 , described below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any type of suitably arranged device or system. 
       FIGS. 1A through 1D  illustrate a first example underwater vehicle  100  having one or more hydraulic drives in accordance with this disclosure. In this example, the vehicle  100  denotes an unmanned underwater vehicle or other device that can function as both a buoy and a glider within an ocean or other body of water. The vehicle  100  could be used to support various functions, such as undersea surveying, recovery, or surveillance operations. 
     As shown in  FIGS. 1A and 1B , the vehicle  100  includes a body  102  having fins  104   a - 104   b  and wings  106 . The body  102  denotes any suitable structure configured to encase, protect, or otherwise contain other components of the vehicle  100 . The body  102  could be formed from any suitable material(s) and in any suitable manner. The body  102  can be formed so that the vehicle  100  is able to withstand extremely elevated pressures found at deep depths in an ocean or other body of water. In some embodiments, the body  102  could allow the vehicle  100  to operate at depths of up to 1,000 meters or more. 
     The fins  104   a - 104   b  denote projections from the body  102  that help to stabilize the body  102  during travel. Each of the fins  104   a - 104   b  could be formed from any suitable material(s) and in any suitable manner. Also, each of the fins  104   a - 104   b  could have any suitable size, shape, and dimensions. Further, at least some of the fins  104   a - 104   b  could be movable or adjustable to help alter the course of the body  102  and to steer the body  102  through water during travel. In addition, the numbers and positions of the fins  104   a - 104   b  shown here are examples only, and any numbers and positions of fins could be used to support desired operations of the vehicle  100 . 
     As described below, the underwater vehicle  100  can both ascend and descend within a body of water during use. In some embodiments, the fins  104   a  could be used to steer the vehicle  100  while ascending, and the fins  104   b  could be used to steer the vehicle  100  while descending. Moreover, when the vehicle  100  is ascending, the fins  104   a  can be used to control the pitch of the vehicle  100 , and a differential between the fins  104   a  can be used to control the roll of the vehicle  100 . Similarly, when the vehicle  100  is descending, the fins  104   b  can be used to control the pitch of the vehicle  100 , and a differential between the fins  104   b  can be used to control the roll of the vehicle  100 . 
     The wings  106  support gliding movement of the vehicle  100  underwater. The wings  106  are moveable to support different directions of travel. For example, the wings  106  are swept downward in  FIG. 1A  when the vehicle  100  is ascending, and the wings  106  are swept upward in  FIG. 1B  when the vehicle  100  is descending. In this way, the wings  106  help to facilitate easier or more rapid movement of the vehicle  100  while ascending or descending. Each of the wings  106  could be formed from any suitable material(s) and in any suitable manner. Also, each of the wings  106  could have any suitable size, shape, and dimensions. In addition, the number and positions of the wings  106  shown here are examples only, and any number and positions of wings could be used to support desired operations of the vehicle  100 . 
     The underwater vehicle  100  may further include one or more ballasts  108   a - 108   b , which help to control the center of gravity of the vehicle  100 . As described in more detail below, material can move within a power supply or other portion of the vehicle  100 , and that movement can alter the center of gravity of the vehicle  100 . Underwater gliders can be particularly susceptible to changes in their centers of gravity, so the vehicle  100  can adjust one or more of the ballasts  108   a - 108   b  as needed or desired (such as during ascent or descent) to maintain the center of gravity of the vehicle  100  substantially at a desired location. In some embodiments, the ballasts  108   a - 108   b  are located on opposite sides of the vehicle&#39;s power supply along a length of the vehicle  100 . Each ballast  108   a - 108   b  includes any suitable structure configured to modify the center of gravity of an underwater vehicle. Note that the number and positions of the ballasts  108   a - 108   b  shown here are examples only, and any number and positions of ballasts could be used in the vehicle  100 . 
       FIGS. 1C and 1D  illustrate different possible end views of the underwater vehicle  100 . In  FIG. 1C , the wings  106  are positioned and extend from the body  102  along a line through a center of the body  102 . In  FIG. 1D , the wings  106  are positioned and extend from the body  102  along a line tangential to the body  102 . In either case, the wings  106  can be stowed in a folded position where the wings  106  extend along the length of the body  102  and later unfolded before, during, or after deployment. 
       FIGS. 2A through 2C  illustrate a second example underwater vehicle  200  having one or more hydraulic drives in accordance with this disclosure. In this example, the vehicle  200  denotes an unmanned underwater vehicle or other device that can function as a buoy within an ocean or other body of water. The vehicle  200  could be used to support various functions, such as undersea surveying, recovery, or surveillance operations. 
     As shown in  FIGS. 2A through 2C , the underwater vehicle  200  includes a body  202  and fins  204   a - 204   b . The body  202  denotes any suitable structure configured to encase, protect, or otherwise contain other components of the vehicle  200 . The body  202  could be formed from any suitable material(s) and in any suitable manner. The fins  204   a - 204   b  denote projections from the body  202  that help to stabilize the body  202  during travel. Each of the fins  204   a - 204   b  could be formed from any suitable material(s) and in any suitable manner. Also, each of the fins  204   a - 204   b  could have any suitable size, shape, and dimensions. Further, at least some of the fins  204   a - 204   b  could be movable or adjustable to help alter the course of the body  202  and to steer the body  102  through water during travel. In addition, the numbers and positions of the fins  204   a - 204   b  shown here are examples only, and any numbers and positions of fins could be used to support desired operations of the vehicle  200 . The vehicle  200  may further include one or more ballasts  208   a - 208   b , which help to control the center of gravity of the vehicle  200 . 
     As can be seen in  FIGS. 2A through 2C , the underwater vehicle  200  lacks wings used to support gliding of the vehicle  200  through water. As a result, the vehicle  200  denotes a device that can function as a buoy but generally not as a glider within an ocean or other body of water. 
     In some embodiments, each underwater vehicle  100  or  200  shown in  FIGS. 1A through 2C  could remain generally vertical during normal operation. In this configuration, the vehicle  100  or  200  is generally operating as a buoy and can collect information or perform other tasks. Of course, exact vertical orientation is not required during operation of the vehicle  100  or  200 . During movement up and down within a body of water, the vehicle  100  or  200  can travel through the water to the surface or to a desired depth of the water. While submerged, the vehicle  100  or  200  could perform operations such as capturing various sensor measurements or searching for anomalies. Periodic surfacing of the vehicle  100  or  200  may allow the vehicle  100  or  200  to (among other things) transmit and receive data, verify its current location, and perform operations needed for power generation (note that the term “periodic” and its derivatives do not require action at a specific interval but merely that an action occurs repeatedly, possibly although not necessarily at a specific interval). After each surfacing, the vehicle  100  or  200  can re-submerge and, if needed, travel at an angle to a desired depth. The angle of travel may be based on the current location of the vehicle  100  or  200  and its desired location, which may allow the vehicle  100  or  200  to operate continuously or near-continuously at a desired station. 
     As described in more detail below, devices such as the vehicles  100  and  200  can include one or more systems that include one or more hydraulic drives. For example, each of the vehicles  100  and  200  could include a system that supports periodic charging of the vehicle, where hydraulic drives are used to push refrigerant from one tank to another through a turbine or other power generator. As another example, each of the ballasts  108   a - 108   b ,  208   a - 208   b  of the vehicles  100  and  200  could include a hydraulic drive to pull or push water into a cavity in order to alter the center of gravity of the vehicle. 
     Although  FIGS. 1A through 2C  illustrate examples of underwater vehicles  100  and  200  having one or more hydraulic drives, various changes may be made to  FIGS. 1A through 2C . For example, these figures illustrate example underwater vehicles only, and the periodic charging systems, ballast systems, and hydraulic drives described in this patent document could be used in any other suitable device or system. 
       FIG. 3  illustrates example components of an underwater vehicle  300  having one or more hydraulic drives in accordance with this disclosure. The underwater vehicle  300  could, for example, denote either of the underwater vehicles  100  and  200  described above. The components shown in  FIG. 3  could therefore denote internal or other components within either of the vehicles  100  and  200  that were not shown in  FIGS. 1A through 2C . 
     As shown in  FIG. 3 , the vehicle  300  includes at least one controller  302  and at least one memory  304 . The controller  302  controls the overall operation of the vehicle  300  and can denote any suitable hardware or combination of hardware and software/firmware for controlling the vehicle  300 . For example, the controller  302  could denote at least one processor configured to execute instructions obtained from the memory  304 . The controller  302  may include any suitable number(s) and type(s) of processors or other computing or control devices in any suitable arrangement. Example types of controllers  302  include microprocessors, microcontrollers, digital signal processors, field programmable gate arrays, application specific integrated circuits, and discrete circuitry. 
     The memory  304  stores data used, generated, or collected by the controller  302  or other components of the vehicle  300 . Each memory  304  represents any suitable structure(s) configured to store and facilitate retrieval of information (such as data, program code, and/or other suitable information on a temporary or permanent basis). Some examples of the memory  304  can include at least one random access memory, read only memory, Flash memory, or any other suitable volatile or non-volatile storage and retrieval device(s). 
     The vehicle  300  in this example also includes one or more sensor components  306 , one or more communication interfaces  308 , and one or more device actuators  310 . The sensor components  306  include sensors that could be used to sense any suitable characteristics of the vehicle  300  itself or the environment around the vehicle  300 . For example, the sensor components  306  could include a position sensor, such as a Global Positioning System (GPS) sensor, which can identify the position of the vehicle  300 . This could be used, for instance, to help make sure that the vehicle  300  is following a desired path or is maintaining its position at or near a desired location. The sensor components  306  could also include audio sensors for capturing audio signals, photodetectors or other cameras for capturing video signals or photographs, or any other or additional components for capturing any other or additional information. Each sensor component  306  includes any suitable structure for sensing one or more characteristics. 
     The communication interfaces  308  support interactions between the vehicle  300  and other devices or systems. For example, the communication interfaces  308  could include at least one radio frequency (RF) or other transceiver configured to communicate with one or more satellites, airplanes, ships, or other nearby or distant devices. The communication interfaces  308  allow the vehicle  300  to transmit data to one or more external destinations, such as information associated with data collected by the sensor components  306 . The communication interfaces  308  also allow the vehicle  300  to receive data from one or more external sources, such as instructions for other or additional operations to be performed by the vehicle  300  or instructions for controlling where the vehicle  300  operates. Each communication interface  308  includes any suitable structure(s) supporting communication with the vehicle  300 . 
     The device actuators  310  are used to adjust one or more operational aspects of the vehicle  300 . For example, the device actuators  310  could be used to move the fins  104   a - 104   b ,  204   a - 204   b  of the vehicle while the vehicle is ascending or descending. The device actuators  310  could also be used to control the positioning of the wings  106  to control whether the wings  106  are stowed or swept upward or downward (depending on the direction of travel). Each device actuator  310  includes any suitable structure for physically modifying one or more components of an underwater vehicle. 
     The vehicle  300  further includes a power generator  312 , a power conditioner  314 , and a power storage  316 . The power generator  312  generally operates to create electrical energy based on movement of the vehicle  300 . In particular, the power generator  312  can operate based on different water temperatures or water pressures that the vehicle  300  experiences over the course of its travel. The power generator  312  includes any suitable structure configured to generate electrical energy based on temperature or pressure. 
     The power conditioner  314  is configured to condition or convert the power generated by the power generator  312  into a suitable form for storage or use. For example, the power conditioner  314  could receive a direct current (DC) signal from the power generator  312 , filter the DC signal, and store power in the power storage  316  based on the DC signal. The power conditioner  314  could also receive power from the power storage  316  and convert the power into suitable voltage(s) and current(s) for other components of the vehicle  300 . The power conditioner  314  includes any suitable structure(s) for conditioning or converting electrical power. 
     The power storage  316  is used to store electrical power generated by the power generator  312  for later use. The power storage  316  denotes any suitable structure(s) for storing electrical power, such as one or more batteries or super-capacitors. 
     The vehicle  300  further includes one or more propulsion components  318 , which denote components used to physically move the vehicle  300  through water. The propulsion components  318  could denote one or more motors or other propulsion systems. In some embodiments, the propulsion components  318  could be used only when the vehicle  300  is traveling between a position at or near the surface and a desired depth. During other time periods, the propulsion components  318  could be deactivated. Of course, other embodiments could allow the propulsion components  318  to be used at other times, such as to help maintain the vehicle  300  at a desired location or to help move the propulsion components  318  to avoid observation or detection. 
     The power generated by the power generator  312  and the power stored in the power storage  316  can be supplied to any of the components in  FIG. 3 . For example, electrical power could be provided to the controller  302  and memory  304  to facilitate computations and instruction execution by the controller  302  and data storage/retrieval by the memory  304 . Electrical power could also be provided to the sensor components  306 , communication interfaces  308 , and device actuators  310  in order to support sensing, communication, and actuation operations. In addition, electrical power could be provided to the propulsion components  318  in order to support movement of the vehicle  300 . 
     As described in more detail below, the power generator  312  could include one or more hydraulic drives that operate based on elevated water pressures (and possibly temperature differentials) to force refrigerant through a turbine or other generator. Also or alternatively, one or more of the ballasts  108   a - 108   b ,  208   a - 208   b  could include one or more hydraulic drives. 
     Although  FIG. 3  illustrates one example of components of an underwater vehicle  300  having one or more hydraulic drives, various changes may be made to  FIG. 3 . For example, various components in  FIG. 3  could be combined, further subdivided, rearranged, or omitted or additional components could be added according to particular needs. 
       FIGS. 4 through 5G  illustrate an example charging system for periodically charging an underwater vehicle in accordance with this disclosure. In particular,  FIG. 4  illustrates an example charging system  400 , and  FIGS. 5A through 5G  illustrate example operations of the charging system  400 . This type of charging system could, for example, be implemented as the power generator  312  in the vehicle  300  of  FIG. 3 , although this type of charging system could be used in any other suitable device or system. 
     As shown in  FIG. 4 , the charging system  400  generally employs a Carnot-Brayton cycle involving two tanks  402  and  404 . A refrigerant  406  is transferred back and forth between the tanks  402  and  404  through a generator  408 . Each tank  402  or  404  is configured to hold the refrigerant  406  under pressure and to provide the refrigerant  406  through the generator  408  to the other tank  404  or  402 . When the refrigerant  406  passes through the generator  408 , the generator  408  generates electrical power. 
     Each tank  402  and  404  includes any suitable structure configured to hold a refrigerant under pressure. The refrigerant  406  includes any suitable fluid used to transfer heat between the tanks  402  and  404 , such as gaseous or liquid carbon dioxide. The generator  408  includes any suitable structure for generating electrical energy based on a flow of refrigerant, such as a Pelton turbine or a brushless DC (BLDC) generator. In particular embodiments, the generator  408  could include a vane motor, impulse, or axial flow turbine and a choked flow orifice. If implemented in this manner, different turbines could be used to generate power for refrigerant flows in different directions, with choked flow orifices on different sides of the different turbines. Alternatively, adjustable orifices could be used on opposing sides of a single turbine and opened or closed based on the direction of refrigerant flow. 
     The charging system  400  can also include multiple insulated water jackets  410  and  412 . Each insulated water jacket  410  and  412  receives and retains warmer or colder water in order to facilitate movement of the refrigerant  406  between the tanks  402  and  404 . For example, the tank  402  or  404  containing more refrigerant  406  can be surrounded by warmer water, increasing the pressure in that tank. Conversely, the tank  404  or  402  containing less refrigerant  406  can be surrounded by colder water, lowering the pressure in that tank. The pressure difference can be used to facilitate easier or more effective refrigerant transport between the tanks  402  and  404 . Each insulated water jacket  410  and  412  includes any suitable insulated structure configured to receive and retain water. The insulated water jackets  410  and  412  need not be pressurized and can be unpressurized containers. 
     Conduits  414  and  416  respectively couple the tanks  402  and  404  to the generator  408 . Each conduit  414  and  416  denotes any suitable passageway for a refrigerant. Each conduit  414  and  416  could be formed from any suitable material(s) and in any suitable manner. Note that while a single conduit  414  and  416  couples each tank  402  and  404  to the generator  408 , multiple conduits could also be used for each tank. For example, different conduits could be used to support refrigerant flows in different directions (and possibly coupled to different generators  408 ). 
     Valves  418  and  420  are used to control the flow of the refrigerant  406  through the conduits  414  and  416 . For example, the valve  418  controls whether the refrigerant  406  can enter or exit the tank  402  through the conduit  414 , and the valve  420  controls whether the refrigerant  406  can enter or exit the tank  404  through the conduit  416 . Each valve  418  and  420  denotes any suitable structure for controlling the flow of a refrigerant, such as a needle valve. 
     Additional valves  422 - 428  are included in the insulated water jackets  410  and  412  to control the flow of fresh water into and out of the insulated water jackets  410  and  412 . For example, when the vehicle  300  is located at or near the surface of a body of water, the valves  422 - 424  or  426 - 428  could be opened so that fresh warmer water can be drawn into the insulated water jacket  410  or  412 . When the vehicle  300  is located at a desired depth underwater, the valves  422 - 424  or  426 - 428  could be opened so that fresh colder water can be drawn into the insulated water jacket  410  or  412 . Although not shown, pumps or other mechanisms can be used to help pull water into or push water out of the insulated water jackets  410  and  412 . Also, although not shown, a water brake ram could be used to slow a vehicle&#39;s ascent or descent using water contained in the water jacket to be flushed. Each valve  422 - 428  denotes any suitable structure for controlling the flow of water into or out of an insulated water jacket. 
     The various valves  418 - 428  shown in  FIG. 4  could be controlled in any suitable manner. For example, in some embodiments, the controller  302  of an underwater vehicle  300  could control the valves  418 - 428  as part of the overall control of the vehicle  300 . 
     As shown in  FIG. 4 , each tank  402  and  404  is associated with a hydraulic drive  430  and  432 , respectively. Each hydraulic drive  430  and  432  is configured to use water pressure when the vehicle  300  dives underwater to help force the refrigerant  406  out of one of the tanks  402  and  404 . In this example, each hydraulic drive  430  or  432  includes a piston  434  within the associated tank  402  or  404 . The piston  434  is sealed with sides of the associated tank so that all or substantially all of the refrigerant  406  cannot pass the piston  434 . Each piston  434  denotes any suitable structure for moving within a tank and pushing a refrigerant. 
     Each hydraulic drive  430  or  432  also includes a channel  436  that contains a hydraulic fluid  438  and a movable piston structure  440 . The channel  436  is fluidly coupled to the associated tank  402  or  404  so that the hydraulic fluid  438  can move freely into and out of the tank. The amount of hydraulic fluid  438  forced into the associated tank  402  or  404  controls the position of the piston  434  in that tank, thereby controlling the amount of force being applied to the refrigerant  406  in that tank. The channel  436  denotes any suitable passageway configured to hold a hydraulic fluid. The hydraulic fluid  438  denotes any suitable material that can be used to apply force against a piston, such as an oil. 
     The movable piston structure  440  represents a structure that moves based on external pressure in order to increase or decrease the amount of hydraulic fluid  438  within the associated tank  402  or  404 . In this example, the movable piston structure  440  includes two small pistons  442   a - 442   b  attached by a connecting bar  444 , which helps to provide two seals between the hydraulic fluid  438  and an external environment. An interior space between the pistons  442   a - 442   b  could contain air or fluid, such as castor oil. However, any other suitable piston(s) could be used as the piston structure  440 . Although not shown, one or more stops could be used to control the range of possible motion of the movable piston structure  440 . 
     The piston structure  440  is moved using water  446 , which is allowed to enter the hydraulic drive  430  or  432  via a respective valve  448   a  or  448   b . The water  446  can enter a channel  436  when the associated valve  448   a  or  448   b  is opened and the vehicle  300  is underwater at a suitable depth. The elevated water pressure can force the water  446  into the channel  436 , pushing the associated piston structure  440  towards the associated tank  402  or  404  and pushing more of the hydraulic fluid  438  into the associated tank  402  or  404 . In contrast, the water  446  can exit a channel  436  when the associated valve  448   a  or  448   b  is opened and the vehicle  300  is at or near the surface of a water body. The decreased water pressure and warmer temperature can allow gas within the associated tank  402  or  404  to expand and force some of the hydraulic fluid  438  out of the associated tank  402  or  404 , pushing the associated piston structure  440  away from the associated tank  402  or  404  and forcing some of the water  446  out of the channel  436 . Each valve  448   a - 448   b  denotes any suitable structure for controlling the flow of water into and out of a channel. 
     Example operations of the charging system  400  are shown in  FIGS. 5A through 5G . As shown in  FIG. 5A , the charging system  400  has just completed a prior power generation cycle. Most or all of the refrigerant  406  is located in the tank  402 , and the valves  418  and  420  have been closed to prevent further transfer of refrigerant  406 . The valve  448   a  is closed and the valve  448   b  is opened, so there is more water  446  in the hydraulic drive  432  than in the hydraulic drive  430 . 
     As shown in  FIG. 5B , the vehicle  300  has ascended, and the valve  448   b  remains opened. The water in the water jackets  410  and  412  can be flushed and replaced with warmer water. The higher ambient temperature and/or the higher temperature of the warmer water in the water jacket  412  can heat the refrigerant  406  in the tank  404 , causing the refrigerant  406  to expand and push some of the water  446  out of the hydraulic drive  432  through the valve  448   b . In some embodiments, the pistons  434  in the hydraulic drives  430  and  432  could be located at substantially the same positions within the tanks  402  and  404  at this point. 
     The vehicle  300  can then close the valve  448   b  and dive to a desired depth, such as several hundred meters or more. Once at a desired depth, the water in the water jacket  412  can be flushed and replaced with colder water. Also, the valves  418 ,  420 , and  448   a  can be opened as shown in  FIG. 5C . The temperature differential between the tanks  402  and  404  (created in part by the temperature differential of the water in the water jackets  410  and  412 ) and the pressure created by the piston  434  in the tank  402  (caused by water pressure through the valve  448   a ) causes most or all of the refrigerant  406  to flow from the tank  402  into the tank  404  through the generator  408 , producing electrical power. Eventually, the charging system  400  reaches the state that is shown in  FIG. 5D , where the bulk of the refrigerant  406  has been transferred to the tank  404  and the piston  434  in the tank  402  has reached its maximum travel. 
     As this point, the same process can occur with the tanks reversed. As shown in  FIG. 5E , the vehicle  300  has ascended, and the valve  448   a  remains opened. The water in the water jackets  410  and  412  can be flushed and replaced with warmer water. The higher ambient temperature and/or the higher temperature of the warmer water in the water jacket  410  can heat the refrigerant  406  in the tank  402 , causing the refrigerant  406  to expand and push some of the water  446  out of the hydraulic drive  430  through the valve  448   a . In some embodiments, the pistons  434  in the hydraulic drives  430  and  432  could be located at substantially the same positions within the tanks  402  and  404  at this point. 
     The vehicle  300  can then close the valve  448   a  and dive to a desired depth, such as several hundred meters or more. Once at a desired depth, the water in the water jacket  410  can be flushed and replaced with colder water. Also, the valves  418 ,  420 , and  448   b  can be opened as shown in  FIG. 5F . The temperature differential between the tanks  402  and  404  (created in part by the temperature differential of the water in the water jackets  410  and  412 ) and the pressure created by the piston  434  in the tank  404  (caused by water pressure through the valve  448   b ) causes most or all of the refrigerant  406  to flow from the tank  404  into the tank  402  through the generator  408 , producing electrical power. Eventually, the charging system  400  reaches the state that is shown in  FIG. 5G , where the bulk of the refrigerant  406  has been transferred to the tank  402  and the piston  434  in the tank  404  has reached its maximum travel. This is the same condition shown in  FIG. 5A . 
     The amount of power generated during these cycles can vary based on a number of factors, such as the size of the tanks  402  and  404 , the amount of refrigerant  406  in the tanks  402  and  404 , the temperatures of the tanks  402  and  404 , and the amount of pressure applied by the pistons  434  in the tanks  402  and  404 . In some embodiments, the charging system  400  could use about twenty pounds of carbon dioxide, the warmer water temperature could be about 25° C., the colder water temperature could be about 5° C., and a pressure differential of up to 500 pounds per square inch (PSI) or more could be created between the tanks  402  and  404  at a depth of 1,000 meters. Given these parameters, the charging system  400  could generate about 430 kJ of energy per dive. 
     Conventional “extended endurance” underwater vehicles are often limited by their total energy carrying capacity or their recharge resources. Charging systems based solely on thermal energy conversion can be limited by thermal absorption rates and convective limitations of heat transfer. In situ properties of ocean travel also include pressures at depth and convective flows from depth changes. The charging system  400  therefore combines the various properties of ocean thermal energy transfer into a single energy cycle in a closed-loop system. This enables underwater vehicles possessing this cycle to cross large distances or produce large quantities of electrical power without external resources or replenishment. 
     As noted above, the charging system  400  generally employs a Carnot-Brayton cycle. The Carnot portion of the cycle uses the hydraulic pistons  434  driven by pressure at depth. Pressure at depth increases the forces applied to the refrigerant  406  by the pistons  434  in the tanks  402  and  404 , which occurs alternatively between the tanks  402  and  404 . The Brayton portion of the cycle involves the use of high-pressure gas expanding through a turbine, where heat is added to the gas through evaporative cooling, the gas is passed through a generator, and the gas is then cooled. 
     The charging system  400  therefore provides power generation based on variable and fixed volumes. The use of the hydraulic drives  430  and  432  helps to provide the charging system  400  with a mechanical advantage to help pressurize the refrigerant  406  while overcoming resistance of the receiving vessel. As a result, the charging system  400  supports a variable volume power system that multiplies water pressures via a hydraulic mechanical advantage. For instance, an advantage of 3:1 allows 400 PSI of water pressure to be converted to 1,200 PSI of pressure by a piston  434  against the refrigerant  406 , while an advantage of 4:1 allows 400 PSI of water pressure to be converted to 1,600 PSI of pressure by a piston  434  against the refrigerant  406 . The mechanical advantage defined by the ratio X: 1  indicates that a tank  402  or  404  is X times wider than the channel  436  of the associated hydraulic drive  430  or  432  or that the tank  402  or  404  has a cross-sectional area X times wider than a cross-sectional area of the channel  436  of the associated hydraulic drive  430  or  432  (where X is any whole or real number greater than one). This type of pressure can be suitable for use in gas transfer systems using carbon dioxide gas or other gaseous refrigerant  406 . Moreover, hydraulic lines can route forces for better packaging of the charging system  400 . 
     Although  FIGS. 4 through 5G  illustrate one example of a charging system  400  for periodically charging an underwater vehicle, various changes may be made to  FIGS. 4 through 5G . For example, various components in each figure could be combined, further subdivided, rearranged, or omitted or additional components could be added according to particular needs. Also, shapes, sizes, and dimensions of various components in these figures could vary as needed or desired. In addition, the use of the water jackets  410  and  412  may be optional, such as when the water pressures applied to the hydraulic drives  430  and  432  generate refrigerant flows adequate for power generation. 
       FIG. 6  illustrates an example method  600  for periodically charging an underwater vehicle in accordance with this disclosure. For ease of explanation, the method  600  is described with respect to the charging system  400  operating in the vehicle  300 . However, the method  600  could be used in any other suitable device or system. 
     As shown in  FIG. 6  and referring to components described in  FIGS. 4 through 5G , pistons of the charging system are placed in substantially equal positions at a higher depth at step  602 . This could include, for example, the charging system  400  pulling warmer water into the water jackets  410  and  412 . This could also include the warmer water or a warmer ambient temperature heating the refrigerant  406  in the tank  404 , causing the refrigerant  406  to expand and push at least some of the water  446  out of the hydraulic drive  432  through the valve  448   b.    
     The underwater vehicle dives to a lower depth at step  604 . This could include, for example, the charging system  400  closing the valve  448   b . This could also include the controller  302  of the vehicle  300  controlling the propulsion components  318  so that the vehicle  300  dives to a desired depth, such as up to 1,000 meters or more. A refrigerant is caused to flow from a first tank through a generator and into a second tank at step  606 . This could include, for example, the charging system  400  pulling colder water into the water jacket  412  and opening the valves  418  and  420 . This could also include the refrigerant  406  flowing from the tank  402  to the tank  404 . A first valve of a first hydraulic drive is opened to drive a first piston in the first tank using water at step  608 . This could include, for example, the charging system  400  opening the valve  448   a  to allow water  446  to enter the hydraulic drive  430 . This also includes the hydraulic drive  430  pushing the piston structure  440  towards the tank  402  to push the piston  434  up into the tank  402 , helping to force the refrigerant  406  out of the tank  402 . Electrical power is generated and stored and/or used at step  610 . This could include, for example, the charging system  400  using the generator  408  to generate DC power based on the flow of refrigerant  406  between the tanks  402  and  404 . The DC power can be provided to the power conditioner  314  and stored in the power storage  316  or used by the vehicle  300 . 
     The transfer of the refrigerant eventually stops or is prevented, and at some point the vehicle rises to a higher depth at step  612 . This could include, for example, the controller  302  of the vehicle  300  controlling the propulsion components  318  so that the vehicle  300  ascends to or near the surface of a body of water. Water is pushed out of the first hydraulic drive at step  614 . This could include, for example, the charging system  400  pulling warmer water into the water jackets  410  and  412 . This could also include the warmer water or a warmer ambient temperature heating the refrigerant  406  in the tank  402 , causing the refrigerant  406  to expand and push the water  446  out of the hydraulic drive  430  through the valve  448   a . This causes the pistons of the charging system to be placed in substantially equal positions at the higher depth at step  616 . 
     At this point, the process repeats with the tanks and water jackets reversed. The underwater vehicle dives to a lower depth at step  618 . The refrigerant is caused to flow from the second tank through a generator and into the first tank at step  620 . This could include, for example, the charging system  400  pulling colder water into the water jacket  410  and opening the valves  418  and  420 . This could also include the refrigerant  406  flowing from the tank  404  to the tank  402 . A second valve of a second hydraulic drive is opened to drive a second piston in the second tank using water at step  622 . This could include, for example, the charging system  400  opening the valve  448   b  to allow water  446  to enter the hydraulic drive  432 . This also includes the hydraulic drive  432  pushing the piston structure  440  towards the tank  404  to push the piston  434  up into the tank  404 , helping to force the refrigerant  406  out of the tank  404 . Electrical power is generated and stored and/or used at step  624 . This could include, for example, the charging system  400  using the generator  408  to generate DC power based on the flow of refrigerant  406  between the tanks  402  and  404 . The DC power can be provided to the power conditioner  314  and stored in the power storage  316  or used by the vehicle  300 . 
     At that point, the overall process can begin again by causing the underwater vehicle to ascend to a higher depth at step  626  and pushing the water out of the second hydraulic drive at step  628 . This could include, for example, the charging system  400  pulling warmer water into the water jackets  410  and  412 . This could also include the warmer water or a warmer ambient temperature heating the refrigerant  406  in the tank  402 , causing the refrigerant  406  to expand and push the water  446  out of the channel  436  of the hydraulic drive  430  through the valve  448   b . This causes the pistons of the charging system to be placed in substantially equal positions at the higher depth back at step  602 . 
     This process can be repeated any number of times as the vehicle  300  ascends and descends in a body of water. The interval of time between ascending and descending can be fixed or variable and could vary based on a number of factors. 
     Although  FIG. 6  illustrates one example of a method  600  for periodically charging an underwater vehicle, various changes may be made to  FIG. 6 . For example, while shown as a series of steps, various steps in  FIG. 6  could overlap, occur in parallel, occur in a different order, or occur any number of times. As a particular example, steps  606 - 610  or steps  620 - 624  generally overlap during the production of electrical power. 
       FIGS. 7A through 7C  illustrate an example ballast system  700  for an underwater vehicle in accordance with this disclosure. This type of ballast system could, for example, be implemented as any of the ballasts  108   a - 108   b ,  208   a - 208   b , although this type of ballast system could be used in any other suitable device or system. 
     As shown in  FIGS. 7A through 7C , the ballast system  700  generally operates to pull variable amounts of water  702  into a cavity  704 . The cavity  704  denotes any suitable structure configured to receive and hold fluid. This could be done to adjust an underwater vehicle&#39;s center of gravity, stabilize the underwater vehicle, or perform other functions. 
     A hydraulic drive  706  is used to adjust the amount of water  702  in the cavity  704 . In this example, the hydraulic drive  706  includes a piston  708  that is positioned within the cavity  704  and that pushes the water  702  out of and pulls the water  702  into the cavity  704 . The piston  708  can be sealed against the inner wall(s) of the cavity  704 . The piston  708  denotes any suitable structure for moving within a cavity. 
     The hydraulic drive  706  also includes a channel  710  that contains a hydraulic fluid  712  and a movable piston structure  714 . The channel  710  is fluidly coupled to the cavity  704  so that the hydraulic fluid  712  can move into and out of the cavity  704 . The amount of hydraulic fluid  712  forced into the cavity  704  controls the position of the piston  708  within the cavity  704 , thereby controlling the amount of water  702  in the cavity  704 . The channel  710  denotes any suitable passageway configured to hold a hydraulic fluid. The hydraulic fluid  712  denotes any suitable material that can be used to apply force against a piston, such as an oil. 
     The movable piston structure  714  represents a structure that moves in order to increase or decrease the amount of hydraulic fluid  712  within the cavity  704 . In this example, the movable piston structure  714  includes two small pistons  716   a - 716   b  attached by a connecting bar  718 . An interior space between the pistons  716   a - 716   b  is divided into multiple pressure volumes  720   a - 720   b  by a separator  722 . Each pressure volume  720   a - 720   b  denotes a space configured to receive a fluid (such as a gas or liquid) in order to move the piston structure  714 . The separator  722  denotes any suitable structure for separating a space into different volumes. 
     An end portion of the channel  710  includes at least one vent  724 . The vent  724  allows air within the end of the channel  710  to move into and out of channel  710  as the piston structure  714  moves. Each vent  724  includes an opening with any suitable size and shape, and any number and arrangement of vents can be used. 
     Valves  726  and  728  are fluidly coupled to the pressure volumes  720   a - 720   b  and to tanks  730  and  732 . The tanks  730  and  732  can be used to provide fluid to or receive fluid from the pressure volumes  720   a - 720   b . Typically, one of the tanks  730  and  732  functions as a source while another of the tanks  732  and  730  functions as a sink. These roles could be swapped over time, such as when the tanks  730  and  732  are implemented with water jackets that can receive warmer and colder water to alter the roles of the tanks  730  and  732 . Each of the valves  726  and  728  includes any suitable structure for controlling a flow of fluid into and out of a pressure volume. Each of the tanks  730  and  732  includes any suitable structure for holding a gas or other fluid used to adjust a position of a movable piston structure. The tanks  730  and  732  may or may not represent the same tanks  402  and  404  used in a charging system  400 . 
     In  FIG. 7A , the movable piston structure  714  may be located at or near its top extreme position, so the pressure volume  720   a  has been expanded in size and the pressure volume  720   b  has been reduced in size. In  FIG. 7B , the valves  726  and  728  have been opened, allowing fluid to flow from the tank  732  into the pressure volume  720   b . A colder temperature or pressure of the tank  730  or movement of the piston structure  714  pushes fluid out of the pressure volume  720   a  into the tank  730 . As a result, the piston structure  714  moves downward. The position in  FIG. 7B  could denote a neutral buoyance position of the ballast system  700 . In  FIG. 7C , the piston structure  714  may be located at or near its bottom extreme position, so the pressure volume  720   b  has been expanded in size and the pressure volume  720   a  has been reduced in size. A similar process could be repeated by pushing fluid into the pressure volume  720   a  and removing fluid from the pressure volume  720   b , allowing the movable piston structure  714  to move back up. Note that the movable piston structure  714  could be controlled to stop at any desired location between its extreme positions. 
     The hydraulic drive  706  is used to control the amount of material in a defined space here. The same type of control mechanism used above in the charging system  400  can be used in the ballast system  700 . The ballast system  700  therefore obtains a mechanical advantage using the hydraulic drive  706 . For example, an advantage of 2:1 allows 700 PSI of pressure to be converted to 1,400 PSI of pressure by the piston  708  against the water  702 , while an advantage of 3:1 allows 700 PSI of pressure to be converted to 2,100 PSI of pressure by the piston  708  against the water  702 . This allows the ballast system  700  to use smaller amounts of pressure even when a vehicle is under elevated pressure at large depths. The mechanical advantage defined by the ratio X:1 indicates that the cavity  704  is X times wider than the channel  710  or that the cavity  704  has a cross-sectional area X times wider than a cross-sectional area of the channel  710  (where X is any whole or real number greater than one). 
     The type of system shown in  FIGS. 7A through 7C  could be used in various ways in an underwater vehicle. For example, the ballast system  700  provides a variable volume buoyancy system through the use of the hydraulic drive  706 . Also, in some embodiments, the system  700  can have dual use as part of a topping cycle (with mechanical advantage) and a buoyancy system (also with mechanical advantage). 
     Although  FIGS. 7A through 7C  illustrate one example of a ballast system  700  for an underwater vehicle, various changes may be made to  FIGS. 7A through 7C . For example, various components in  FIGS. 7A through 7C  could be combined, further subdivided, rearranged, or omitted or additional components could be added according to particular needs. Also, shapes, sizes, and dimensions of various components in  FIGS. 7A through 7C  could vary as needed or desired. 
       FIG. 8  illustrates an example method  800  for stabilizing an underwater vehicle using a ballast system in accordance with this disclosure. For ease of explanation, the method  800  is described with respect to the ballast system  700  operating in the vehicle  300 . However, the method  800  could be used in any other suitable device or system. 
     As shown in  FIG. 8  and referring to components described in  FIGS. 7A through 7C , a desired change in a ballast level is identified at step  802 . This could include, for example, the controller  302  of the vehicle  300  identifying whether more or less mass is needed or desired in the cavity  704  of the ballast system  700 . This could be based on any suitable calculations, such as the vehicle&#39;s current depth or whether the vehicle  300  is traveling in a desired manner. 
     Higher and lower pressures are created in or applied to different pressure volumes within a hydraulic drive at step  804 . This could include, for example, the ballast system  700  opening the valves  726  and  728  to allow fluid to flow into and out of the appropriate pressure volumes  720   a - 720   b . This causes a hydraulic piston to change position within a cavity of a hydraulic drive at step  806 . This could include, for example, the changes in pressure within the pressure volumes  720   a - 720   b  causing the movable piston structure  714  to move. This alters an amount of hydraulic fluid within the cavity at step  808 , moves a ballast piston within the cavity at step  810 , and alters an amount of ballast within the cavity at step  812 . This could include, for example, the movable piston structure  714  altering an amount of the hydraulic fluid  712  within the cavity  704 , moving the piston  708  within the cavity  704  and changing an amount of water  702  within the cavity  704 . 
     Although  FIG. 8  illustrates one example of a method  800  for stabilizing an underwater vehicle using a ballast system, various changes may be made to  FIG. 8 . For example, while shown as a series of steps, various steps in  FIG. 8  could overlap, occur in parallel, occur in a different order, or occur any number of times. As a particular example, steps  804 - 812  generally overlap with one another. 
     In some embodiments, various functions described in this patent document are implemented or supported by a computer program that is formed from computer readable program code and that is embodied in a computer readable medium. The phrase “computer readable program code” includes any type of computer code, including source code, object code, and executable code. The phrase “computer readable medium” includes any type of medium capable of being accessed by a computer, such as read only memory (ROM), random access memory (RAM), a hard disk drive, a compact disc (CD), a digital video disc (DVD), or any other type of memory. A “non-transitory” computer readable medium excludes wired, wireless, optical, or other communication links that transport transitory electrical or other signals. A non-transitory computer readable medium includes media where data can be permanently stored and media where data can be stored and later overwritten, such as a rewritable optical disc or an erasable memory device. 
     It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The terms “application” and “program” refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, related data, or a portion thereof adapted for implementation in a suitable computer code (including source code, object code, or executable code). The term “communicate,” as well as derivatives thereof, encompasses both direct and indirect communication. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrase “associated with,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. The phrase “at least one of,” when used with a list of items, means that different combinations of one or more of the listed items may be used, and only one item in the list may be needed. For example, “at least one of: A, B, and C” includes any of the following combinations: A, B, C, A and B, A and C, B and C, and A and B and C. 
     The description in the present application should not be read as implying that any particular element, step, or function is an essential or critical element that must be included in the claim scope. The scope of patented subject matter is defined only by the allowed claims. Moreover, none of the claims is intended to invoke 35 U.S.C. §112(f) with respect to any of the appended claims or claim elements unless the exact words “means for” or “step for” are explicitly used in the particular claim, followed by a participle phrase identifying a function. Use of terms such as (but not limited to) “mechanism,” “module,” “device,” “unit,” “component,” “element,” “member,” “apparatus,” “machine,” “system,” “processor,” or “controller” within a claim is understood and intended to refer to structures known to those skilled in the relevant art, as further modified or enhanced by the features of the claims themselves, and is not intended to invoke 35 U.S.C. §112(f). 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the scope of this disclosure, as defined by the following claims.