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

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.