Thermal energy recovery systems include a piston assembly including a primary cylinder adapted to receive vapor; a single-acting secondary cylinder/piston assembly extending from opposite ends of the primary cylinder; a primary piston disposed for displacement in the primary cylinder; first and second secondary pistons disposed for displacement in the secondary cylinder/piston; and a piston connecting member connecting the first and second secondary pistons to the primary piston. Alternatively, a secondary piston is of the type of a double-acting piston for a more compact reciprocating function to reduce piston friction losses. Metering valves regulate the vapor pressure being introduced into displacement volume chambers at a constant pressure. A working fluid pressure-tank/accumulator/transfer-conduit is in communication with the displacement volume chambers to help regulate pressure of the working fluid. A working fluid transfer conduit forms integrally with the working fluid pressure-tank/accumulator to reduce fluid friction losses.