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 and a second volume, 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, and a maximum heat storage position corresponding to a maximum second volume, 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 comprising one or more valves (52, 53, 54, 55, 56, 57), said valve assembly 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 to said first volume 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 and a second volume, 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, and a maximum heat storage position corresponding to a maximum second volume, 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, and a maximum heat storage position corresponding to a maximum second volume, 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 comprising one or more valves (52, 53, 54, 55, 56, 57), said valve assembly 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 to said first volume via said heat transfer fluid circuit for heating said electric energy supply system. - a valve assembly comprising one or more valves (52, 53, 54, 55, 56, 57), said valve assembly 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 to said first volume via said heat transfer fluid circuit for heating said electric energy supply system.