With the rapid development of science and technology, the production of a large number of high-efficiency heat pumps has led to the active use of shallow geothermal energy with lower development cost than deep geothermal heat in many countries.
U-type GSHE (ground source heat exchanger) is a system used for shallow geothermal energy development where the heat exchanger is embedded in the ground and the geothermal energy is extracted from the heat exchange between the ground and the water flowing through the pipe.
Heat transfer models for mathematically modeling U-type GSHE and several experimental and numerical simulation methods have been proposed.
Previous studies used laboratory and field experimental data for comparative analysis, focusing on simulation calculations to enhance the performance and heat exchange efficiency of single well modes such as U-type ground source heat pump (GSHP) system, standing column well (SCW) geothermal heat pump system, and forced external circulation single well (FECSW) geothermal heat pump system. In addition, the simulation calculations and experimental results were compared, limited to individual modes, and three modes have never been studied in relation to one another.
In order to study and introduce these modes to realistic conditions, the heat exchange characteristics of individual modes under different environmental conditions should be studied and the heat extraction efficiency of each mode should be compared.
Choe Tok Gi, an institute head at the Faculty of Earth Science and Technology, has designed a 1:20 scaled model and made 10 experimental scenarios considering different experimental conditions such as soil moisture, groundwater convection, heat conduction, abstraction and injection interval selection. Then, he has conducted heat extraction experiments according to the scenarios and compared the effectiveness.
The results show that in the U-type mode it is reasonable to use porous casing or gravel-filled materials because there is no possibility of groundwater contamination in the case of water without antifreeze, in the standing column well mode the convective heat exchange between the circulating water and surrounding ground should be increased as much as possible to increase the geothermal extraction, and in the forced external circulation single well (FECSW) mode it is effective to inject water into the top of the borehole during heating and into the bottom during cooling to generate convection.
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