Thermodynamic cycle process performing transfer between mechanical and heat energies

A thermodynamic cycle process, performing transfer between mechanical and heat energies, by changing a state of a fluid, the process comprising:

• conducting an expansion of said fluid;
• retrieving an energy from said fluid expansion;
• and
• powering a liquid pump or a gas compressor with said retrieved energy, using a cyclic free piston expander which alternatively changes direction of said free piston sliding by alternatively:
• closing said fluidic communication between said both opposite sides of said free piston, to make the pressures applied respectively thereon different from each other, so that said free piston then slides in a first direction,
• opening a fluidic communication between both opposite sides of said free piston, to make the pressures applied respectively thereon equal to each other, so that said free piston then slides in a second direction opposite to said first direction,
• wherein said free piston is sliding, directly and mechanically, opening and closing, said fluidic communication, the free piston sliding within a sliding chamber,
• said first surface of said free piston is in an admission space of said sliding chamber having a first variable volume, and
• said second surface of said free piston is in an expansion space of said sliding chamber having a second variable volume, said second volume decreasing when said first volume increases and said second volume increasing when said first volume decreases.