Thermal fluids are served to give the greater ΔT to the thermoelectric (TE) junctions. For the larger power generation using a limited number of modules and the limited amounts of thermal fluids, the multi-layered TE panels can save the occupied space effectively. Because the temperatures of the fluids vary along the planer TE panels due to heat exchange through the panels, the directions of fluids is important to obtain the larger output power, P, from a TE power generator. The methods to stack a few long planer TE panels and to determine the fluid directions are mathematically studied from the steady-state heat exchange. P for various kinds of fluid directions are calculated. P commonly shows the maximum against the panel length because of the balance between the internal resistance and ΔT. This maximum of P can be further maximized by choosing the counter flow. The non-dimensional analysis predicts that two fluids should flow in counterwise. When two series of fluids are used, the circulation method is another key issue for the optimal TE design.