NOVELTY - The temperature within an electrochemical fuel cell stack (10) is controlled by introducing a reactant fluid stream (34) containing both a heat transfer liquid (HTL) (42) and a reactant (32) in the fuel assembly.    USE - For controlling temperature within the electrochemical fuel cell.    ADVANTAGE - The fuel cell power generation system has an increased power density. The fuel cell stack produces more power compared to a conventional fuel cell stack with the same dimensions and can be made smaller but still can produce the same amount of power. The manufacturing steps are reduced by eliminating the separate cooling layers and their components e.g. fluid seals. The reliability is improved by reducing the number of components that may potentially require maintenance services.    DETAILED DESCRIPTION - Controlling the temperature within an electrochemical fuel cell comprises:    (i) introducing a reactant fluid stream containing a reactant and a heat transfer liquid to the first electrode (22), the reactant fluid stream contacting the first electrode;    (ii) removing a reactant fluid exhaust stream containing HTL from the first electrode; and    (iii) recirculating at least a portion of the HTL from the reactant fluid exhaust stream to the first electrode via a heat exchanger, where the temperature of HTL is controlled.    The fuel cell comprises an electrolyte interposed between two electrodes and a quantity of electrocatalyst disposed at an interface between the electrolyte and each of the two electrodes.    An INDEPENDENT CLAIM is also included for an electrochemical fuel cell power generation system comprising:    (a) fuel cell assemblies arranged in a stack, each assembly including first and second reactant fluid passages adjacent to the two electrodes, an anode and a cathode having an inlet and an outlet, respectively;    (b) a two reactants supply subsystem comprising two reactant supply manifolds which fluidly connect to first and second reactants fluid passage;    (c) HTL supply subsystem having a reservoir fluidly connected to first reactant fluid passage inlet, for directing first reactant fluid stream having HTL into the first fluid passage;    (d) a recirculation subsystem containing recirculation fluid passage fluidly connecting to the first reactant fluid passage outlet; and    (e) a heat exchanger disposed in the recirculation passage between first reactant fluid passage outlet.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic illustration of a fuel cell power generation system.    Fuel cell stack (10)    Electrode (22)    Reactant (32)    Reactant fluid stream (34)    Heat transfer liquid (42)