Energy storage device and energy storage method

The energy-storing device (1) has a compressor (4), a heat accumulator (5), and an expansion turbine (6). The compressor is connected to an inlet of the expansion turbine by a line (7) for a working gas (3). The heat accumulator is connected into the line. The compressor and the expansion turbine are arranged on a common shaft (14). The heat exchanger of the heat accumulator is designed such that the working gas largely matches the thermodynamic state variables of the working gas prior to entering the compressor. An independent claim is included for a method for storing thermal energy.