Measurment of the thermal performance of a Borehole Heat Exchanger while injecting air bubbles in the groundwater

Abstract

The most common way to exchange heat with the ground in Ground Source Heat Pump (GSHP) applications is with borehole heat exchangers (energy col-lectors in vertical wells). These boreholes contain the pipe with the secondary fluid of the GSHP and they are often filled with natural groundwater. It has been recently discovered that injecting air bubbles in the groundwater side of the boreholes increases the efficiency of the heat transfer. The aim of this thesis is to analyze the thermal changes in the borehole and the surrounding ground when bubbles are injected in the groundwater. Experiments are carried out through a distributed thermal response test along the borehole using two differ-ent rates of bubble injection. Temperature profiles of the secondary fluid and the groundwater are analyzed and calculations on the thermal resistances inside the borehole and the conductivity of the ground are made. Moreover, the validity of the line source conduction model is discussed under the above mentioned circumstances.