The characteristics of the thermoelectric device are influenced by the external
heat transfer law. The external irreversibility is caused by the finite rate heat transfer between the thermoelectric generator and its heat reservoirs. When the heat transfer law is nonlinear then there are optimal working electrical currents and optimal ratio of thermal conductance allocations corresponding to the maximum power output and maximum efficiency. Since there are a variety of heat exchangers for thermoelectric device, the heat transfer laws are various and different from each other.