Energy storage plant and process

An energy storage plant, comprising:

• a casing configured to store a working fluid other than atmospheric air in a gaseous phase;
• a tank configured to store said working fluid in a liquid phase;
• a compressor;
• a turbine;
• and
• heat exchangers configured to store thermal energy from the working fluid or to release thermal energy to the working fluid,
• wherein the casing is selectively in fluid communication with an inlet of the compressor or with an outlet of the turbine;
• wherein the heat exchangers are selectively in fluid communication with an outlet of the compressor or with an inlet of the turbine;
• wherein the tank is in fluid communication with the heat exchangers;
• wherein the plant is configured to perform a closed cyclic thermodynamic transformation, first in one direction in a charge configuration and then in an opposite direction in a discharge configuration, between said casing and said tank;
• wherein in the charge configuration the plant stores heat and pressure and in the discharge configuration generates energy;
• wherein the heat exchangers are configured to operate a sub-critical transformation of the working fluid;
• wherein the plant is configured to make the tank work at a constant pressure or substantial constant pressure and to make the casing work at a constant pressure or substantial constant pressure;
• wherein in the charge configuration the working fluid is accumulated in the tank in the liquid phase with a temperature close to a critical temperature and said critical temperature is close to the ambient temperature;
• wherein in the discharge configuration the working fluid is evaporated, expanded, and stored in the casing;
• wherein the casing is a pressure-balloon or has the structure of a gasometer so that the working fluid in said casing is in equilibrium of pressure with the atmosphere, with low or no overpressure, in any operating condition,
• wherein the heat exchangers comprise a primary heat exchanger and a secondary heat exchanger, the secondary heat exchanger being located between the primary heat exchanger and the tank or being integrated into the tank;
• and
• wherein, in the discharge configuration, the working fluid is heated through the heat stored by the primary heat exchanger and secondary heat exchanger.