The invention relates to a cooling system in a fuel cell electric vehicle. A first chamber is configured to contain relatively hot fluid. A second chamber is configured to contain relatively cold fluid. The ratio of cooling power/fan power of a positive displacement device at a heat exchanger is monitored and thermal energy transfer between the coolant and said chambers is controlled based on said ratio. When the ratio is above a pre-defined value or value range, then thermal energy from the first chamber is provided to the coolant in the coolant circuit and is passed into the heat exchanger, after which part of the thermal energy of the cooled coolant leaving the heat exchanger is provided to and stored in the second chamber. The stored cold thermal energy is released from the second chamber when said ratio is below the pre-define value or value range. The invention also relates to method of controlling a cooling system.