A STUDY OF THE HEAT FLUX OF U-PIPE GROUND HEAT EXCHANGER

Ground heat exchanger (GHE) coupled heat pumps constitute an alternative system for the air conditioning of buildings, with the vertical U-pipe being the most popular. With the aid of GHE, heat is exchanged between the fluid of the primary circuit of the heat pump and ground. Therefore, the designing and dimensioning of GHE are sensitive to the installation cost and the overall performance of the system, which makes it critical to calculate the heat flux per meter of borehole (HFPMB) that GHE can handle. Recently, significant research effort has been devoted in quantifying the effect some properties (ground, drilling and the ground temperature) have on the GHE performance so that the HFPMB can be calculated accurately according to the specific site of installation. This paper presents a calculation algorithm, by which the factors of impact on HFPMB in vertical GHE is analyzed. Firstly, heat transfer analysis of boreholes in vertical GHE is made according to its configuration, especially; the thermal resistance is deduced by the quasi-three-dimensional model, which is crucial to calculate HFPMB. Secondly, a suitable calculation algorithm is developed, which determines HFPMB as a function of the duration of the load. Iterative algorithm is applied in the solution for HFPMB. Based on the finite line source model, the borehole wall temperature as a medium parameter is calculated corresponding to a supposed HFPMB. In succession, the other HFPMB can be calculated by analysis on the heat transfer between fluid in U-pipe and the borehole wall corresponding to the resistance of the specific configuration under constant inlet water temperature of GHE. Until the difference between the two HFPMB is less than 1e-6, the iterative calculation finished and the calculated HFPMB can be determined in MATLAB. Many parameters are applied during the iterative calculation, but the computational time is short and the physical concept is legible. According to the calculated results from the algorithm, the impacts of inlet water temperature, water flow rate and borehole depth on HFPMB are analyzed corresponding to different inputs respectively. The iterative algorithm of HFPMB presented. in this paper can be applied to calculate accurately the dimensions of GHE according to ground parameters, and analysis on the three factors is helpful to choose capacity of heat pump and circulating pump, which can make the system operate efficiently and minimize the adverse effects on the installation cost of GCHP.