Assembly for storing thermal energy and method for its operation

The thermal energy storage plant has a working gas circuit comprising a thermal energy fluid machine (13), heat accumulator (14), a thermal energy fluid machine (15) and a cold storage device (16) that are connected to each other by a line (11) for the working gas. The thermal energy fluid machine is a working machine and the thermal energy fluid machine is a combustion engine when viewed in flow direction of the working gas from the heat accumulator to the thermal energy fluid machine. A low temperature heat accummulator is provided before the thermal energy fluid machine (13). Independent claims are included for the following: (1) a method for storage of thermal energy; and (2) a method for conversion of thermal energy.