Investigation On Backfilling Materials And Underground Heat Transfer Process In Ground Source Heat Pump System

Ground Source Heat Pump system (GSHP) is a green environmental protection air conditioning technology, in which low temperature geothermal resources have been used for heating, cooling and hot water supply. The research object of this study is to design the formula and evaluate the performance of backfilling materials and investigate on the heat transfer process between buried heat exchanger and the surrounding soil in GSHP. The main research contents consist of three parts:screening and assessment of proper formula of backfilling materials mainly focus on thermal conductivity, workability and consolidation strength; simulation of true heat transfer process in ground heat exchanger on the informed heat transfer models and self-build GSHP simulation test-bed under different operating conditions; demonstration on performance of the backfilling materials and the reliability of heat transfer models on GSHP simulation test-bed and selecting model correction coefficient. Thus, the aim of this research is to provide scientific proof for the exploitation of backfilling materials and engineering design of GSHP.Firstly, the influence of water/cement ratio, sand/cement ratio, replacement rate of natural river sand, replacement rate of cement and incorporation rate of slurry on workability, thermal conductivity and consolidation strength of cement-based grouts were studied systematically, and the relationship among performance, porosity and micro structure of backfilling materials had been conducted. Results showed that the addition of coarse slag and slurry could reduce the economic cost with the guarantee of performance. Then a set of vertical U-shaped buried tube GSHP simulation test-bed had been set up to simulate the heat transfer process in the drilling hole and the external soil with one dimensional heat conduction model and one dimensional Kelvin line source model, respectively. Through analysis on influence of cycle water flow, water temperature and backfilling materials on the heat exchange process, the performance evaluation of backfilling materials and inspection of heat transfer model had been carried out. The results showed that the relationship between average fluid temperature and unit heat exchange capacity could be fitted by quadratic function. The GSHP simulation test-bed had reliability in evaluating the performance of backfilling materials. In addition, the heat exchange capacity in the drilling hole calculated by one dimensional heat conduction model was less than the total heat exchange capacity and a correction coefficient of1.06was recommended. The temperature of the drilling hole wall had liner rule with the change of Int, and the longer the running time continued the stronger the liner rule acted.