Sandbox test setups for evaluating the performance of borehole heat exchangers: a review

Geothermal energy has been increasingly gaining attention with the global transition to sustainable energy. One critical component of utilizing geothermal energy is the borehole heat exchanger (BHE), which can extract thermal energy from or inject it into the ground. Sandbox tests are often used to assess the performance of BHEs and to validate the analytical and numerical models for BHEs. However, until now, there has been no design standard for sandbox tests; a systematic review of sandbox test setups for BHE applications is notably absent from the existing literature. To address this need, a comprehensive overview was conducted for sandbox test setups used for simulating and assessing BHEs. First, the physical components of the sandbox test were introduced, including the sandbox frame, heat exchanger pipes, filling materials, seepage conditions, grouting materials, and insulation layers. In addition, the setups of the physical components of existing sandbox tests were reviewed. Furthermore, the methods to determine sandbox size and experiment duration for sandbox tests were discussed. Finally, key issues were indentified in current sandbox setups, and new insights were given to address these challenges, providing new perspectives for designing sandbox tests and thus promoting geothermal energy research.