Fig. 12From: Effect of groundwater forced seepage on heat transfer characteristics of borehole heat exchangersDynamic change of the average outlet water temperature (\(\bar{T}\)o) and the average heat transfer efficiency (\(\bar{E}\) = \(\bar{Q}\)/\(\bar{Q}\) =|\(\bar{T}\)i − \(\bar{T}\)o|/|\(\bar{T}\)i − \(\bar{T}\)s0|) of the BHEs in the fifth operation cycle: a the cooling stage; b the heating stageBack to article page