A thermal energy storage system with environmental air exchange and a method of its operation

A thermal energy storage, TES, system (100) comprises a thermodynamic gas flow circuit containing air as a working fluid for transport of thermal energy through the TES system. The air is maintained in gas phase throughout the entire gas flow circuit, which is open to the environment. Air is received (7) from the environment and pressurized by a compressor (2), driven by a motor (1A), for use as a working fluid with increased temperature, followed by transfer of thermal energy to a first TES medium (5') during charging prior to release (7') of the working fluid to the environment again. In discharging cycles, the air is received from the environment and used as working fluid in reverse flow through the first TES medium (5') for being heated by the first TES medium prior to driving an expander (3), typically a turbine, which in turn drives an electrical generator (IB) for recovery of electrical energy.