Experimental analysis of Geothermal Passive Thermoelectric Generators combining water and methanol based heat exchangers

Thermoelectric generators are a highly suitable alternative for applications that require robustness, reliability and low maintenance, such as power supply in remote volcanic regions and extreme environments. However, in order to broaden the scope of this technology, it is essential to adapt its design and performance to variable operating conditions. In this context, the present study has developed an innovative system of geothermal passive thermoelectric generators based on a phase change heat exchanger, aimed at the continuous generation of electricity from low-enthalpy geothermal sources, specifically volcanic fumaroles. Experimental tests have evaluated different heat exchanger configurations under a wide range of operating conditions, including different ambient temperatures and wind speeds. The analysis included the power produced, the conversion efficiency and the thermal resistance of the heat exchangers. On the hot side, a heat pipe-based exchanger was compared with a thermosyphon-based exchanger, with the former producing an average of 17% more electricity. To adapt the system to adverse meteorological conditions, both water and methanol were used as working fluids. Water was more efficient, with a 49% lower thermal resistance, resulting in a 17% increase in electricity generation. Nevertheless, when freezing occurs, the performance of the water-based system drops by 90%, while the methanol-based system continues to operate effectively. As a solution, a hybrid system combining water and methanol heat pipes on the cold side has been developed, achieving power outputs of up to 1 W and generating between 600% and 710% more power than the water-based system under freezing conditions. • Geothermal passive thermoelectric generators for extreme volcanic environments. • Experimental characterisation of generators with different heat exchanger configurations. • Innovative phase change heat exchanger design combining water and methanol heat pipes. • The hybrid prototype achieves a continuous power generation of 1 W per module.