Recent advances in experimental studies of energy geostructures: A comprehensive review

Energy geostructures (EGSs) couple geothermal heat exchangers with load-bearing ground structures such as pile foundations, tunnels, and diaphragm walls, simultaneously providing structural support and geothermal extraction or injection for ground source heat pumps. Since their proposal in the 1980s, EGSs have attracted growing interest for their efficient use of shallow geothermal energy and underground space, principally as energy piles (EPs), energy tunnels (ETs) and energy walls (EWs), and now stand at a pivotal transition from theoretical design to engineering practice. Although analytical and numerical models have been extensively reviewed, the equally critical body of experimental research remains poorly synthesised, impeding engineering deployment. Moreover, the pronounced developmental imbalance among EGS types, with EPs dominating while ETs and EWs lagging considerably behind, has received insufficient scholarly attention. This paper addresses these gaps through a systematic review of experimental research across three typical EGSs spanning field tests and laboratory experiments. Field tests, conducted under real geological and climatic conditions, capture the transient multiphysical behaviour, spanning thermal response tests, heat exchanger performance, thermo-mechanical behavior, and heat pump efficiency. Laboratory experiments, isolate governing mechanisms, probing thermal enhancement through novel materials and structural optimization, and resolving thermo-mechanical behaviour from interfacial to macroscopic scales. Taking EPs as the benchmark, this review analyses the technological, structural, and operational origins of the relative lag of ETs and EWs and distils them into recommendations for future development, offering actionable guidance for advancing all three forms and supporting the wider deployment of shallow geothermal energy.