The present disclosure relates to an
energy storage tank (10) defining a tank volume for said
heat transfer fluid, and comprising a partition (12) dividing said tank volume into a first volume (V1) and a second volume (V2), wherein said partition (12) is movable to / from any position between a minimum
energy storage position corresponding to a minimum second volume (V2), and a maximum
energy storage position corresponding to a maximum second volume (V2), said energy
storage tank further comprising a biasing device (18) being arranged such that movement of the partition (12) away from said minimum energy storage position corresponds to storing energy in said biasing device (18), and movement towards the minimum energy storage position corresponds to releasing energy from said biasing device (18); said energy storage
system further comprising a
heat transfer fluid duct (20) connecting said first volume (V1) and said second volume (V2), and a
heat transfer arrangement (40) arranged for altering the temperature of the transfer fluid from a first temperature when in the first volume (V1) to a second temperature when in the second volume (V2), and said energy storage
system (100) further comprising a
fluid control assembly (30) for providing a pressure implying a flow of said
heat transfer fluid from said first volume (V1) to said second volume (V2) via said
heat transfer fluid duct (20).