Numerical simulation of heat extraction by a coaxial ground heat exchanger under freezing conditions

TL;DR

This paper introduces a finite-difference model for simulating heat transfer in a coaxial ground heat exchanger, accounting for ground freezing effects, and analyzes different operational modes to assess temperature sensitivities.

Contribution

It presents a novel numerical model that incorporates ground moisture freezing effects into heat exchanger simulations, highlighting the impact on temperature predictions.

Findings

Freezing effects cause up to 35% difference in outlet temperature predictions.

The model accurately simulates stationary, transient, and controlled operation modes.

Ground moisture freezing significantly influences heat transfer estimates.

Abstract

A new finite-difference model of heat transfer inside a shallow coaxial ground heat exchanger and in the surrounding layered soil is presented, taking into account the freezing of ground moisture. Three modes of heat exchanger operation are numerically simulated: stationary mode, transient mode and controlled mode. In the stationary mode, estimates of the sensitivity of the heat carrier fluid outlet temperature to changes in the heat exchanger parameters are calculated. In all modes, close attention is paid to demonstrating the difference in the results at a negative temperature of the fluid, calculated with and without taking into account the freezing of ground moisture. It is shown that this difference, caused by the zero-curtain effect, can range from 10% in the stationary mode to 35% in the control mode.