Method and equipment for transmission of energy by mechanical compression and / or relaxation of a quasi-isothermal gas

A compression and quasi-isothermal expansion of a gas (5), comprising - at least one container (B1, B2, B3, B4) containing a hydraulic fluid (6) and a gas (5); - a heat exchange fluid (10) in thermal contact with said gas (5) and / or with said hydraulic fluid (6), so as to maintain the temperature of said gas (5) at a substantially constant level during the compression or expansion of said gas (5); - a source of heat or cold capable of exchanging thermal energy with said heat exchange fluid (10); - compression means - relaxation capable mode "compression" of pumping hydraulic fluid (6), and in mode "relaxation" relax hydraulic fluid (6) generating mechanical energy.