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

Figure 6

Average heat transfer fluid temperatures in a BHE array. The grey envelope in the middle of the diagram shows the range of heat transfer fluid temperature for a BTES where heat extraction and rejection are seasonally balanced (324 MWh/season, each across a 4-month extraction and rejection season). The envelope between the red dotted lines indicates the typical heat transfer fluid temperature range for a BTES where heat is being rejected year on year (324 MWh/season; maximum temperature in summer, minimum in winter); the envelope between the blue dotted lines indicates the range of heat transfer fluid temperature for a BTES where heat is being extracted year on year (324 MWh/season). In both diagrams, the green dotted line shows the estimated annual marginal heat accumulation in the BHE storage in the case of heat rejection only (calculated from the gradient of the heat rejection temperature curve, a rock volumetric heat capacity of 2.4 MJ/m³/K, and a BTES volume of 90,000 m³). (a) An array of 36 number, 100 m deep boreholes on a square grid at 6 m spacing; (b) an array of 81 number, 44.4 m deep boreholes on a square grid at 5.6 m spacing. (a) also shows the temperature curve for a comparable single 100 m deep borehole extracting or rejecting 9 MWh heat per 4-month season (i.e., 324 MWh/36). Initial ambient average ground .

(a)

(b)