Synergistic improvement of Seebeck coefficient and power density of an aqueous thermocell using natural convection for low-grade heat utilization

•Performance of thermocells under different electrode orientations is investigated.•Effects of natural convection of electrolyte are correlated to electrode orientations.•Se of thermocell at θ = 90° is 18.5% higher than that of θ = 0° without natural convection.•Pmax of thermocell at θ = 135° is 744% higher than that of θ = 0° without natural convection.•Natural convection has trade-off effects on internal resistance and thermal resistance.