Thermal energy storage system comprising a combined heating and cooling machine and a method for using the thermal energy storage system

A thermal energy storage system comprising an energy storage (2) having a vertical temperature gradient and an internal combined heating and cooling machine (15),

• said internal combined heating and cooling machine (15) being adapted for retrieving fluid having a first temperature from the energy storage, while simultaneously returning heated fluid having a second higher temperature and cooled fluid having a third lower temperature, wherein said internal combined heating and cooling machine (15) comprises at least two heat pumps, wherein each heat pump comprises at least two compressors, and wherein said internal combined heating and cooling machine (15) is adapted for alternating the connection between said at least two compressors between series connection and parallel connection.