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

A system comprising:

• an underwater vehicle comprising a body and fins projecting from the body;
• the underwater vehicle also comprising a power generator comprising:
• first and second tanks each configured to receive and store a refrigerant under pressure;
• 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;
• and
• first and second hydraulic drives associated with the first and second tanks, respectively, each hydraulic drive comprising:
• 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;
• wherein the channel of each hydraulic drive has a cross-sectional area that is less than a cross-sectional area of the associated tank.

An apparatus comprising:

• a cavity configured to receive a material;
• and
• a hydraulic drive comprising:
• 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;
• wherein the channel of the hydraulic drive has a cross-sectional area that is less than a cross-sectional area of the cavity.