Structural design and operational performance analysis of an integrated heat pipe-based medium- and low-temperature geothermal thermoelectric generator

Medium- and low-temperature geothermal resources are globally abundant and widely distributed, and thus form an essential part of distributed low-carbon energy systems. The conventional organic Rankine cycle (ORC) and Kalina cycle (KC), however, suffer from system complexity, high maintenance cost, and the risk of working-fluid leakage, all of which hinder their deployment in distributed and small-scale applications. To develop a structurally compact, low-maintenance, and modularly scalable solid-state route for direct heat-to-electricity conversion, this paper proposes an integrated heat pipe-based geothermal thermoelectric generator (HP-GTEG) tailored to medium and low-temperature applications. The system compactly integrates a multi-pipe heat source comprising one central main pipe and eight circumferential branch pipes, an array of 16 sintered-copper-wick heat pipes embedded in an octagonal aluminium-based module, and a pair of arc-shaped water-cooling plates attached to the upper and lower surfaces of each module. Sixteen commercial Bi2Te3 thermoelectric modules (TEMs) are mounted on each module, and 20 modules are stacked axially to form a generation array containing 320 TEMs in total. A three-dimensional CFD–thermoelectric one-way coupled numerical model is established, in which each heat pipe is represented as an anisotropic equivalent high-thermal-conductivity solid derived from a five-segment series thermal-resistance reduction, and its range of applicability is explicitly defined. The influences of the hot- and cold-side inlet velocities, the hot- and cold-side inlet temperatures, and the heat-pipe effective thermal conductivity on the generation performance are systematically investigated. Three dedicated analyses the main-versus branch-pipe flow distribution, the with-versus without-heat-pipe comparison, and the pumping-power penalty—are presented to provide a quantitative evaluation of the principal design variables. The overall reliability of the model is confirmed by a dual cross-check against a single-module steady-state experiment with 16 actual heat pipes and a microchannel TEG benchmark from the literature. The results show that raising the hot-side inlet temperature from 323.15 K to 363.15 K increases the system output power from 57.91 W to 265.84 W and the conversion efficiency from 0.655 % to 1.403 %, making the hot-side inlet temperature the most influential controllable variable. Each 10 K increase in the cold-side inlet temperature induces an approximately linear reduction of about 21.7 % in the output power, which constitutes the principal constraint on temperature-difference maintenance. Once the heat-pipe effective thermal conductivity exceeds 8000 W m−1 K−1, the system response saturates, providing a reasonable upper bound for heat-pipe selection. With the pumping-power penalty included, the electric-to-pumping-power ratio reaches 21 at the calibration operating point and attains a maximum of 151 within the practical flow-velocity range, demonstrating that the electrical gain substantially exceeds the pumping-power penalty. The present study provides a set of quantitative design references for compact direct thermoelectric conversion in medium- and low-temperature geothermal and industrial waste-heat applications.