Numerical Simulations of Heat Transfer in Underground Thermal Energy Storage (UTES) System: CFD Approach

This study presents a numerical analysis of heat transfer phenomena within an underground thermal energy storage system using Computational Fluid Dynamics (CFD). Three scenarios were analysed to examine heat transfer mechanisms: (i) Influx of hot water – Hot water enters the chamber, displacing colder water and transferring heat to both the surrounding soil and the outgoing water. The soil, acting as a thermal reservoir, absorbs heat with minimal temperature variation, except in regions near the chamber; (ii) Natural cooling – With the chamber’s inlets and outlets sealed, the water gradually cools through interaction with the colder surrounding soil. The focus is on the initial cooling rate, which determines how long the water remains above a certain temperature before reaching thermal equilibrium; (iii) Heat extraction via a heat exchanger – A submerged heat exchanger extracts heat from the chamber water while the surrounding soil compensates for the loss. The efficiency of this process depends on the dimensions and operational parameters of the heat exchanger. The results indicate that in all scenarios, the system maintains stable thermal stratification, with warmer water staying in the upper layers while cooler water settles due to density differences. The interaction between water and soil plays a crucial role – acting as a heat source during extraction and as a reservoir during cooling.