A heat transfer fluid system for a vehicle comprising an electric energy supply system such as a fuel cell system and/or a battery system, said heat transfer fluid system comprising a heat transfer fluid circuit arranged for controlling the temperature of said electric energy supply system by means of a heat transfer fluid, and a heat transfer fluid storage tank defining a tank volume for said heat transfer fluid, said heat transfer fluid storage tank comprising a partition dividing said tank volume into a first volume (V1) and a second volume (V2), and a biasing device , wherein - said partition is movable to/from any position between a minimum heat storage position corresponding to a minimum second volume (V2), and a maximum heat storage position corresponding to a maximum second volume (V2), wherein - movement of said partition away from said minimum heat storage position corresponds to storing energy in said biasing device , and movement towards said minimum heat storage position corresponds to releasing energy from said biasing device ; and said heat transfer fluid system further comprising - a valve assembly (52-57) comprising one or more valves , said valve assembly (52-57) being controllable to provide an electric energy supply system heating state of said heat transfer fluid system wherein release of energy from said biasing device implies a flow of heat transfer fluid from said second volume (V2) to said first volume (V1) via said heat transfer fluid circuit for heating said electric energy supply system . a heat transfer fluid storage tank defining a tank volume for said heat transfer fluid, said heat transfer fluid storage tank comprising a partition dividing said tank volume into a first volume (V1) and a second volume (V2), and a biasing device , wherein - said partition is movable to/from any position between a minimum heat storage position corresponding to a minimum second volume (V2), and a maximum heat storage position corresponding to a maximum second volume (V2), wherein - movement of said partition away from said minimum heat storage position corresponds to storing energy in said biasing device , and movement towards said minimum heat storage position corresponds to releasing energy from said biasing device ; and - said partition is movable to/from any position between a minimum heat storage position corresponding to a minimum second volume (V2), and a maximum heat storage position corresponding to a maximum second volume (V2), wherein - movement of said partition away from said minimum heat storage position corresponds to storing energy in said biasing device , and movement towards said minimum heat storage position corresponds to releasing energy from said biasing device ; and said heat transfer fluid system further comprising - a valve assembly (52-57) comprising one or more valves , said valve assembly (52-57) being controllable to provide an electric energy supply system heating state of said heat transfer fluid system wherein release of energy from said biasing device implies a flow of heat transfer fluid from said second volume (V2) to said first volume (V1) via said heat transfer fluid circuit for heating said electric energy supply system . - a valve assembly (52-57) comprising one or more valves , said valve assembly (52-57) being controllable to provide an electric energy supply system heating state of said heat transfer fluid system wherein release of energy from said biasing device implies a flow of heat transfer fluid from said second volume (V2) to said first volume (V1) via said heat transfer fluid circuit for heating said electric energy supply system . <EOS>