Overview and outlook of thermal processes in geothermal energy extraction

This vision article accompanies a Special Issue of Applied Thermal Engineering dedicated to Heat Transfer in Geothermal Energy Extraction. This issue contains original research articles selected for publication in Applied Thermal Engineering, all connected by a focus on processes, technologies and systems for the exploitation of geothermal energy. Geothermal resources, which can be found at depths ranging from a few m to several km below the earth's surface, are amongst the most promising renewable, zero carbon, and clean energy sources. Geothermal energy utilization technologies can be mainly classed into four categories: ground source heat pumps, energy geo-structures, enhanced geothermal systems, and closed-loop geothermal systems. Ground source heat pumps are the most well-developed geothermal technology for shallow depths, and have been widely used for building cooling and heating applications. Current and future heat pump research is focused on the coupling and hybridization of different heat pump configurations, using supplementary energy sources and phase change materials as storage media to overcome the thermal imbalance of the ground. Energy geo-structures, including energy piles, energy walls and energy tunnels, are an emerging and promising geothermal technology, with current research mainly focusing on simulating the effects of influencing factors on heat extraction effectiveness. Enhanced geothermal systems are a technology that can be used to extract deep geothermal energy for power generation and direct use. This technology has attracted significant renewed attention in the last decade. Future research should focus on the heat transfer mechanisms in high temperature rock fractures, and how the layout of inlet and outlet wells and different heat transfer fluids – such as CO2 – affect performance. Closed-loop geothermal systems, including coaxial and U-shaped systems, have no direct physical contact between working fluid and the reservoir, and may overcome some shortcomings of enhanced geothermal systems, such as leakage of the working fluid and water–rock interactions; however, further research is urgently needed on understanding the thermal recovery performance of these systems.