An energy storage device for storing thermal energy, with a charging circuit for a working gas, is provided, having a compressor, heat accumulator and expansion turbine, the compressor and expansion turbine arranged on a common shaft, and the compressor connected on the outlet side to the inlet of the expansion turbine via a first line for the working gas, the heat accumulator wired into the first line, wherein the compressor is connected on the inlet side to a line, which is open to the atmosphere, and the expansion turbine is connected on the outlet side to a line, which is open to the atmosphere such that a circuit open to the ambient air is formed, wherein the expansion turbine is connected to the heat accumulator via a line for a hot gas such that the working gas in the expansion turbine can be heated by heat from the heat accumulator.