This invention relates to a fuel cell system comprising a fuel cell stack, external circuit electrical leads coupled to the fuel cell electrodes, and a thermal enclosure assembly enclosing the stack. The stack includes a plurality of inner tubular solid oxide fuel cells, the inside of the inner fuel cells being fluidly couplable to a first reactant source, and a porous support matrix embedding the inner fuel cells and being fluidly couplable to a second reactant source such that a second reactant is flowable through the matrix and to the outer surface of the embedded fuel cells. The stack may also include an outer tubular solid oxide fuel cell surrounding the matrix and the inner fuel cells such that the second reactant is flowable through the matrix and to an inside surface of the outer fuel cell and wherein the outer surface of the outer fuel cell is fluidly couplable to a first reactant source.