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This paper presents a new analytical model to study the thermal behavior of borehole heat exchangers (BHE) in short time periods. Transient heat transfer considered inside the borehole and at the ground around the borehole, transient heat conduction is considered inside the borehole and ground around the borehole. For this purpose, the analytical solution has been developed in two stages. First, a new analytical equation is provided for the short-time thermal response of the BHE (dimensionless G-function). In the second phase, the outlet temperature calculation using the G-function is described. Inside the borehole, the analogy between thermal and electrical conduction is used for derivation heat balance equations. For this purpose, a new equivalent thermal network for modeling of the heat transfer inside the borehole is developed. In ground around the borehole, the conduction equation in the radial direction is considered. The governing equations are solved by Laplace transform. Finally, the mean fluid temperature and short thermal response of the BHE is computed. Then in the second phase, the outlet temperature in the on and off times of the system is calculated using the G-function. The solution of the proposed analytical model is compared with experimental measurements. Results show that the outlet temperature of the analytical model matches very well with the reference experimental measurements.

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In the present study, the direct utilization of borehole as a heat sink for both residential and office building is investigated in Tabriz city. The effect of external wall insulation and windows glazing is studied in the form of four cases and the performance of the ground sink direct cooling system is investigated for these cases. The borehole design depth is calculated by analytical method. Both sample residential and office buildings are investigated. The borehole design depth depends on the quality of the building design and its heat emission. The results show that using double glazed windows, compared to single glazed windows reduces the borehole design depth by about 10 percent. Also the utilization of insulation in external walls and roof decreases the borehole design depth more than half compared to buildings without insulation. Finally, the potential of the ground sink direct cooling in sample residential and office buildings is investigated for four cases. The results show that by using ground sink direct cooling system, thermal comfort is satisfied in almost all of the cooling hours in both sample residential and office buildings.