A 1 kWe thermoelectric stack for geothermal power generation – Modeling and geometrical optimization

A thermoelectric stack comprising an array of Bi–Te based thermoelectric converter (TEC) modules is considered for geothermal heat conversion. Each TEC module consists of 127 (Bi0.2Sb0.8)2Te3/Bi2(Te0.96Se0.04)3 p/n-type thermoelement pairs, fastened by 30 × 30 mm2 Al2O3 plates. The thermoelement pairs have leg cross-section of 1.05 × 1.05 mm2, a figure-of-merit equal to 1, and a theoretical heat-to-electricity conversion efficiency of ∼5% when the module is operated at a temperature difference of 200 K. A temperature gradient across the thermoelement legs within an array is imposed via a Cu parallel-plate heat exchanger adhering to the Al2O3 plates and operating hot and cold water in counter-flow channel configuration. A heat transfer model coupling conduction through the thermoelement legs with convection to and from the Al2O3 plates is formulated to investigate the performance of the stack as function of the following parameters: hot water inlet and outlet temperatures (313–413 K and 303–393 K, respectively), stack length (300–1500 mm), thermoelement leg length (0.5–4 mm) and hot/cold channel heights (0.2–2 mm). The open-circuit voltages resulting from the temperature differences are within 3% mean relative error of those resulting from temperature differences computed via CFD. The heat transfer model is then applied to optimize a 1 kWe stack with hot water inlet and outlet temperatures of 413 K and 393 K, respectively, for either a maximum heat-to-electricity efficiency of 4.2% or for a minimum volume of 0.0021 m3.