Recent advances in various nanomaterials utilized in geothermal heat exchangers

Geothermal heat exchanger (GHE) can be used in energy system for greenhouse gas reduction, which has been recognized as one of important devices for renewable energy application. In order to enhance the GHE heat transfer efficacy, various nanomaterials are employed as the thermal fluids and the backfill materials in the energy systems. As a result, this paper reviews and summarizes the state-of-the-art of the GHEs with different nanomaterials (such as carbon nanotubes, graphene nanoplatelets, graphene oxide, silver, copper oxide, titanium dioxide, aluminium oxide and silicon dioxide suspended in based-fluid) and their ground tube configurations (like serpentine tube, spiral coil tube, twisted tape coil tube, swirl coil tube, tree-shaped bifurcation tube, louvered winglet tapes tube, coiled flow inverter tube and centre tapered wavy tape tube). Some critical aspects concerning influences of nanomaterial size, volume fraction, stability and sedimentation, and compatible base fluid and thermal properties on pumping power, soil temperature fluctuation, and heat transfer coefficient of the GHEs system, are produced for ameliorating geothermal energy system efficiency. Additionally, the effects of the ground buried tube structure on the GHEs system pressure drop, entropy production and long-term continuously operating are explored. Finally, the viewpoints, advices and future studies regarding the GHE corrosion and erosion, nanoparticle migration, magnetic and biological nanofluids utilizations, are deliberated to conquer the obstacles and challenges for the actual application. Hence, this comprehensive review will be beneficial for researchers, engineers and academics to comprehend the remarkable impact of various nanomaterials on heat transfer in the GHEs system.