There are myriad of situations where active cooling of a hot object is done by forced convection cooling using a liquid as a working fluid. However, no electric energy has so far been recovered from such situations even though an existence of a heat flow along a temperature gradient means an irreversible and continuous loss of exergy of thermal energy. To address this unresolved issue, we previously invented and demonstrated a new concept of technology in which thermo-electrochemical conversion has been integrated into forced convection cooling; we acquired important findings and demonstrated the viability of this concept in our previous reports. However, detailed influences of the geometry of the inter-electrode channel formed by two electrodes−cathode and anode−on the thermocell performance were not understood well. Therefore, in the present study, we carry out numerical studies by thermo-fluid simulations, all of which are in highly laminar regime. Consequently, we found that (i) a small inter-electrode spacing, which results in an enhanced heat transfer coefficient and thus is advantageous for cooling, does not diminish the interelectrode temperature difference (ΔT) if the thickness of the thermal boundary layer at the channel exit is properly set, and that (ii) the use of a parallel-fin type cathode not only diminishes ΔT but also deteriorates the cooling performance because of the increased hydraulic diameter, when compared under the same electrolyte flow rate. Therefore, for the present purpose, a rectangular inter-electrode channel formed by two plain electrodes with a narrow inter-electrode gap gives the best performances both in terms of thermoelectrochemical power generation and cooling of the hot surface, partially because of its advantage in the reduced diffusion distance required for redox species that have to be transported between the electrodes.