A Comprehensive Study of Heat Transfer Characteristics of Shallow Closed-Loop Geothermal Energy Systems*

Decarbonisation of heat is essential to reduce CO₂ emissions and can be achieved by utilizing clean and stable geothermal energy systems for building and space heating. This paper presents a comparative study on the heat transfer characteristics of two different shallow closed-loop ground heat exchangers (GHEs): a single vertical U-tube and a gravity-assisted thermosyphon heat pipe (THP). The study harnesses shallow geothermal energy resources. A heat transfer model using existing correlations of heat transfer coefficients investigates the thermal performance of these GHEs. For the U-tube GHE, an energy conservation model with sensible heat transfer evaluates the heat extraction rate using a single-phase fluid, water. For the THP, a phase change heat transfer model considers pool boiling, evaporation, and condensation using different working fluids (ammonia, propane, methanol, R134a). The model is validated with experimental data. The results, analyzed as the THP to U-tube heat extraction rate ratio, indicate that THPs with ammonia have a higher heat transfer rate due to phase change, compared to the U-tube system. This study aids in optimizing geothermal heat pipes, advancing sustainable energy technologies.