Analysis of Heat Transfer of Different Utilization Ways in Shallow Geothermal Energy Applications

Abstract

Recently shallow geothermal energy has been increasingly utilized for its constant temperature, wide distribution, cleanliness, and environmental sustainability. Borehole Heat Exchangers (BHE), Water Source Heat Pumps (WSHP), Downhole Heat Exchangers (DHE), as well as Standing Column Wells (SCW) are all ways of developing shallow geothermal energy. This paper analyzes their respective heat transfer characteristics. The properties of grouting materials are essential in determining the thermal efficiency of Borehole Heat Exchangers, as heat is mainly transferred by conduction. Water Source Heat Pumps have better heat performance compared to BHE, due to their constant heat source temperature and heat transfer through convection. Downhole Heat Exchangers are often installed in shallow wells - water contained in tubes, exchanging heat with the source by a natural convection process, allowing for moderate stand-alone heating performance without a need for additional facilities. Downhole Heat Exchangers are recommended for use in conjunction with surface heat pump systems, especially in locations with low temperature aquifer wells. Standing Column Wells exhibit the characteristics of both BHE and DHE, the thermal efficiency of SCW is higher due to forced heat convection, which is most suitable for areas with abundant groundwater. Based on the above analyzed, this paper also discusses utilization mode selection methods with consideration for different local hydrologic geology conditions.