Beyond Fickian diffusion: Stefan-based modelling of coupled thermo-water vapour transport in geomembranes for pit thermal energy storage liners

Accurate prediction of water vapour transport through geomembranes is critical for ensuring the long-term performance of pit thermal energy storage (PTES) lining covers. Conventional assessments commonly rely on Fickian diffusion under ambient conditions, neglecting the elevated temperatures and high water vapour pressures encountered in service, where coupled diffusive–convective transport mechanisms become significant. This study investigates the temperature- and pressure-dependent water vapour transmission rates (WVTR) of high-density polyethylene (HDPE) and Ethylene Interpolymer Alloy (EIA) Polyvinyl chloride (PVC) geomembranes at temperatures ranging from 25 to 90 °C and vapour pressures from 20 to 70 kPa, replicating the elevated thermo-water vapour environments encountered in service. Under these conditions, Fick's law underestimated vapour fluxes by up to 73.8% due to the omission of the vapour–air counter-convective flow. In contrast, a Stefan-based framework that accounted for coupled diffusion and convection reproduced the experimental trends with substantially improved accuracy, reducing mean average errors to 0.9–2.1% for HDPE and 3.3–13.4% for EIA geomembranes. Resistance analysis within the Stefan framework further demonstrated HDPE's superior thermo-water vapour barrier performance at elevated temperatures. The results establish a mechanistically based modelling approach that moves beyond Fickian assumptions and supports performance-based design of geomembrane liners for high-temperature PTES applications.