A heat exchanger system (150) is described that includes an inlet (202) and an outlet (204) for a first fluid and a heat exchanger between the inlet (202) and the outlet (206) wherein the first fluid circulates, wherein the heat exchanger comprises at least one deflector (230, 232, 234, 236, 238, 430, 432) to guide the flow of a second fluid. A method is also described to exchange heat between a first and a second fluid using free convection velocity field to create forced convection in the heat exchanger of a heat exchanger system (150). A method to exchange heat between a first and a second fluid comprising providing a heat exchanger system (150) between the first and the second fluids, said heat exchanger system (150) comprising a heat exchanger wherein the first fluid circulates and increasing the flow turbulences of a second fluid around the heat exchanger.