An investigation into the performance of a very shallow borehole thermal energy store for a solar-assisted ground source heat pump

Ground-source heat pumps (GSHP) have been used in building applications to meet heating needs at lower energy costs. Likewise, solar thermal systems have been integrated into conventional GSHP to reduce the size of the ground heat exchanger and provide seasonal heat storage. So far, this technology has mainly been used in large commercial or residential buildings due to its high installation costs.

The present work focuses on an experimental and numerical investigation on the thermal performance of a very shallow borehole heat exchanger for a solar-assisted ground source heat pump (SAGSHP) for small-scale applications. For the experimental part, energy analysis of a small domestic SAGSHP was conducted. The main innovation of this system is the use of an array of 16 very shallow (1.5-metre depth) boreholes to store heat seasonally into an underground earth energy bank (EEB). In the numerical investigation, different models were developed to study the thermal performance of the natural soil as well as the thermal response of very shallow borehole heat exchangers.

A comparison in accuracy and time of simulation was carried out between the Infinite Line Source (ILS) model, the Infinite Cylindrical Source (ICS) model, and a new model developed by the author based on the Finite Difference Method (FDM). To consider in the model the interaction between adjacent boreholes as well as the impact of the natural soil temperature, the superposition technique was applied.

Results show that the proposed model based on the FDM is faster and more accurate than the conventional analytical models (ILS and ICS) to study the thermal response of very shallow boreholes. Likewise, dimensionless graphical representations (a.k.a. G-functions) were developed for the study of very shallow boreholes for different types of soil thermal properties and different borehole diameters.