Analysis of shared geothermal borefield configurations

Abstract

Geothermal systems have been used in ground source heat pumps applications for decades. They are gaining in interest in recent years through community and hybrid systems. Inconveniently, nearly all of the available geothermal borehole models are limited to the same inlet condition for each U-tubes and geothermal borefields models to the same inlet condition for each borehole. This research project proposes models and applications to shared and hybrid geothermal systems, in residential/solar application. The aim of the project is to improve the efficiency of heat transfer and storage of shared and hybrid geothermal boreholes and borefields by segregating their inlet conditions. The approach used in this research project can be divided in three parts: the ground model, the single U-tube model and double U-tube model. The ground model is a 2D diffusion Control Volume Finite Difference Method (CVFDM) model. The single U-tube fluid-toground analytical model is based on delta thermal resistance analogy, adding a resistance outside the borehole for the shape factor. The double U-tube model is an addition to an existing model that considers the angle, distance and flow direction of the U-tubes legs. It also considers different mass flowrate and specific heat of the fluid in each U-tube. In the applications, the heat pumps of 12 residential buildings circuit and 24 m2 of solar collectors per residential building circuit were coupled to different borehole and borefield configurations. Three main categories of configurations are: a mitigating loop mixing fluids uphill to the borefield, segregating the circuits in different boreholes in a borefield (independent boreholes) and in circuits of double U-tubes boreholes (independent circuits). The energy consumption of each heat pump over the three years period was 10 884 kWh for the base case without solar collectors. The mitigated loop saved 2.4%, as for the independent boreholes central configuration with 4.5 m and the staggered configuration with 3 m and 4.5 m. The independent circuit gave the best results with 6.4% savings for the 12 borehole heat exchangers case and 9.3% for the 24 borehole heat exchangers.

Date	26 May 2016
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Louis Lamarche (Supervisor) & Daniel Rousse (Co-supervisor)