HEAT TRANSFER ANALYSIS OF ENERGY PILES WITH PARALLEL CONNECTED U-TUBES

A theoretical model for the analysis of a heat transfer process of energy piles with parallel connected U-tubes is presented and validated by the numerical simulated results. The model can take into account the vertical heat convection of the circulated fluid in the U-tube and the thermal disturbance between the two branches. Using the model, the influence of the U-tube arrangement on pile heat transfer efficiency is analyzed. The results show that the increase of U-tubes can effectively enhance pile heat transfer efficiency when there are only a few U-tubes in the pile. However, when the number of U-tubes reaches a certain value the influence will be small. With the same number of U-tubes, heat transfer efficiency will reach its highest value when all the branches of the U-tubes are equidistantly located. It seems that exchanging the inlet branch and the outlet branch of a U-tube has no effect on the pile heat performance. With the same velocity for the circulated fluid, pile heat transfer efficiency will increase by enlarging the diameters of the U-tubes.