Design Considerations for Borehole Thermal Energy Storage (BTES): A Review with Emphasis on Convective Heat Transfer

Figure 10

(a) A groundwater-filled, 80 m deep BHE in unfractured rock, with a length of surface casing protruding above ground level; the groundwater level is at the surface. (b) If heat is rejected to the BHE, the groundwater heats up and expands, rising into the surface casing. (c) If the casing is removed, the excess water runs away at the surface; if a hydraulically conductive fracture exists at depth in the borehole, cool water will be drawn into the borehole due to the reduced heat and will subsequently expand, creating a thermally driven groundwater flow up the borehole (a “thermosiphon” or thermal pump). (d) If a fracture exists at a higher level in the borehole, the warm groundwater can flow away via that fracture, creating a convection cell within the surrounding rock or sediment mass, with the borehole as a central convecting axis.