Improvements in the Water Retention Characteristics and Thermophysical Parameters of Backfill Material in Ground Source Heat Pumps by a Molecular Sieve

Tingting Luo,Chen Wang,Yixia Chen,Peng Pei,Dingyi Hao

doi:10.3390/en15051801

Abstract

The thermophysical properties of backfill material (BM) in a heat exchange borehole significantly influence the heat exchange effect of ground source heat pumps (GSHPs). Several treatments such as compaction and adding bentonite, cement, and fine sands are often used to improve the thermophysical properties. In this study, a 3A molecular sieve (3A-MS), a type of porous material, was added to the BM to enhance its water maintaining capacity. Three types of backfill materials with different additive contents, named as BM-0, BM-1, and BM-2, were examined. The variation of the BM properties such as the soil–water characteristic curve (SWCC), thermal conductivity, specific heat capacity, and thermal diffusivity with the groundwater content were investigated through a series of experiments and simulations. A scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), and the BET method for specific surface area pore size analysis were used to characterize the material. The results indicated that the specific heat capacity improved with the water content whereas the thermal conductivity and thermal diffusivity decreased with the water content. The variation of the buried pipe outlet temperature with the change of the thermal physical parameters of the BM were researched by a numerical simulation and theoretical calculations; the results showed that BM-2 could raise the heat transfer rate per meter by 45.9% in summer and 118.4% in winter compared with the backfill materials without groundwater (NW). The research results provide theoretical support for the improvement of BM for ground source heat pump projects where abundant groundwater is available.

Highlights

Shallow geothermal energy is one of the most promising energy resources in China, given its unlimited distribution and recyclability [1,2]
By adding 0%, 5%, and 10% of a 3A molecular sieve (3A-MS) into backfill material (BM), the specific heat capacity of the material improved with the raising of the water content but the growth was different
Compared with BM-0, the specific heat capacity of BM-2 could be heightened by 12.4%

Summary

Introduction

Shallow geothermal energy is one of the most promising energy resources in China, given its unlimited distribution and recyclability [1,2]. Due to the effect of hydrogeology, porosity, and other factors of rock and soil materials, there are many restrictions to its practical design, construction, and operation [3]. Common issues with GSHPs include a thermal imbalance, the depletion of the heat exchange capacity of the borehole, and a decrease in the system efficiency. As the direct contact medium between the U pipe and the stratum, the backfill material (BM) in the heat exchange borehole determines the heat exchange performance between the buried pipe and the surrounding rock and soil, and prevents the penetration of the surface water into the ground through the heat exchange borehole [4,5]. There are two approaches to enhance the ability of the thermal transfer for backfill materials. On the one hand, increasing the thermal conductivity is the common way to acquire a higher heat transfer capacity. Kamal [6] demonstrated that the length of a U pipe could be decreased by 20% with wet sand–bentonite compared with dry sand–

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