A Comprehensive Review of Backfill Materials and Their Effects on Ground Heat Exchanger Performance

Abstract

Geothermal energy systems can help in achieving an environmentally friendly and more efficient energy utilization, as well as enhanced power generation and building heating/cooling, thereby making energy systems more sustainable. The role of the backfill material, which fills the space between a pipe and the surrounding soil, is important in the operation of ground heat exchangers. Among the review articles on parameters affecting ground heat exchanger performance published over the past eight years, only two discuss types of backfill materials, even though the importance of these materials is significant. However, no review has yet been published exclusively on the kinds of backfill materials used in ground heat exchangers. This article addresses this need by providing a comprehensive review of a variety of types of backfill materials and their effects on ground heat exchanger performance. For organizational purposes, the backfill materials are divided into two categories: conventional backfill materials (pure and mixed materials) and modern backfill materials (improved phase change materials). Both categories are described in detail. It is shown that bentonite has been used considerably as a conventional backfill material in ground heat exchangers, followed by silica sand and coarse/fine sand. Moreover, acid and shape-stabilized phase change materials have been applied mostly as modern backfill materials in ground heat exchangers. It is observed, generally, that conventional backfill materials are used more than modern backfill materials in ground heat exchangers. It should be noted that the data covered in this study are not from all the articles published in the last eight years, but rather from a subset based on specific criteria (i.e., English-language papers published in reputable journals). These articles were published by authors from numerous countries. The results may, as a consequence, have some corresponding limitations, but these are likely to be minor.

Facile Synthesis of Graphene from Waste Tire/Silica Hybrid Additives and Optimization Study for the Fabrication of Thermally Enhanced Cement Grouts

“…A grout with good rheological properties can provide good pumpability, less entrapped air, and consequently, lower permeability, less sensitivity to freeze, and thaw cycles and accordingly more durability and good thermal contact between the grout, the pipes and the surrounding underground formations that lead to higher thermal conductivity between the heat carrier fluid and the ground [37]. One of the most important parameters that affects the efficiency of the geothermal energy system is the thermal resistance of the heat-exchange boreholes that, in turn, depend on the thermal properties of the backfill [2]. The thermal resistivity of the grout, R g can be determined using the following Equation 1, where S b is the borehole shape factor and λ g is the thermal conductivity of the backfill grout in terms of [W•m −1 K −1 ] [38]:…”

“…In geothermal energy systems, the thermal conductivity of the grout used for backfilling the heat exchange boreholes and the pipes used in the loops for circulating the heat carrier fluid has been considered as an important issue for the improvement of the efficiency of the system. That is because the media influencing the heat exchange between the heat carrier fluid (in the loops) and the surrounding formations (i.e., soil or rock) include the pipe's wall and the backfill materials in the borehole [1,2]. The poor thermal conductivity of neat cementitious grout not only decreases the efficiency of the system performance but also influences thermal cracking of the used backfill grout due to the high-temperature gradient between the pipe and the surrounding ground during the heat injection or extraction process [3].…”

Assessment and Minimization of Potential Environmental Impacts of Ground Source Heat Pump (GSHP) Systems

“…These materials have been developed since the 1990s, combining cement, bentonite, silica sand, and other additives (graphite flakes etc.) to find a trade-off between low hydraulic conductivity, high thermal conductivity, and good mechanical resistance [66]. Tests on cement-sand grout samples conducted by Allan and Philippacopoulos (1998, [21]) reported hydraulic conductivities in the order of 10 −12 m/s, while higher values in the order of 10 −10 m/s were measured for grout-pipe specimens where the interface between the pipe and the grout is a weak point for the hydraulic sealing [22,23].…”

Investigation of the thermal behavior and energy consumption of refrigeration systems

“…The difference is then used to adjust some input to the process in order to bring the process measured value back to its desired set point [6]. PID controller makes rapid and precise control possible [7][8][9][10][11]. This is also true in reality, provided that the system is correctly defined, taking disturbances into consideration accurately.…”