Role of temperature-dependent fluid property variations on energy pile performance

Abstract

The flow and heat exchange analyses of energy piles often assume constant fluid properties, which can lead to inaccurate performance predictions. This study addresses this limitation by investigating the role of temperature-dependent fluid property variations on the thermofluid performance of two field scale energy piles. Mathematical functions describing the effects of temperature on water density, viscosity, and specific heat capacity were developed and integrated into the physics-based Reynolds number and heat exchange calculations. This approach was used to assess the propagated errors in the flow and heat exchange calculations when temperature-dependent fluid property variations are ignored. Across the typical temperature range of 1–47 °C during heating, cooling, and cyclic operations, the dynamic viscosity showed the largest variation, up to 67%, resulting in substantial propagated errors of up to 73% in the Reynolds number and 17% in the friction factor, compared to constant property assessments at reference temperature of 20 °C. Incorporating viscosity variations into the design of energy piles is therefore essential to accurately classify turbulence, perform pressure loss calculations, select pumps, and size heat exchanger pipes. In contrast, water density and specific heat capacity varied by approximately 1%, introducing negligible propagated errors in the heat exchange rates. Therefore, constant values of water density and specific heat capacity are adequate for reliable evaluations of geothermal heat exchange rates. Grey relational analysis further showed that the dynamic viscosity and the inlet-outlet water temperature difference of the energy pile are the dominant factors influencing the Reynolds number and heat exchange rates, respectively.