A thermoelectric stack composed of arrays of Bi-Te alloy thermoelectric converter (TEC) modules is considered for geothermal heat conversion. The TEC modules consist of Al₂O₃ plates with surface 30×30 mm² and 127 p-type (Bi_0.2Sb_0.8) ₂Te₃ and n-type Bi₂(Te_0.96Se _0.04)₃ thermoelement pairs, each having a cross-section of 1.05×1.05 mm², and with a figure-of-merit of 1 and a heat-to-electricity conversion efficiency of ∼5%. A heat transfer model is formulated to couple conduction in the thermoelements with convection between the Al₂O₃ plates and the water flow in counter-flow channel configuration. The calculated open-circuit voltages are compared to those resulting from the mean temperature differences across the TEC modules computed by CFD. The investigated parameters are: hot water inlet and outlet temperatures (373-413 K and 323-363 K, respectively), stack length (300-1500 mm), thermoelement length (1-4 mm) and hot channel heights (0.2-2 mm). The heat transfer model is then applied to optimize a 1 kW_el stack with hot water inlet at 393 K and outlet at 353 K for either maximum heat-to-electricity conversion efficiency of 2.9% or minimum size of 0.0044 m³.