| For more than ten years,shallow ground tube heat source technology has been widely used.However,due to various reasons,there are insufficient space for buried pipes and unbalanced underground heating and cooling loads,especially the lack of underground heat replenishment in the northern region,which has caused problems such as limited application of the technology or poor application results.The mid-deep buried tube heat pump heating system is a new method of geothermal energy utilization.Its prominent feature is that the borehole depth is generally 1000 m-3000 m.The deficiencies of the ground-source heat pump system of the shallow underground pipe are more suitable for the heating in the cold regions of the north in winter or the construction of the narrow underground pipe space.The application of this technology is still in its infancy,especially the heat transfer analysis of mid-deep underground heat exchangers and the analysis of the water system between underground pipes and heat pump units,which need to be further optimized.This article focuses on the research and discussion of this issue in order to improve the heat exchange and water system efficiency of medium and deep buried pipes.Mathematical models of heat exchange and water system for mid-deep underground pipes are established.The model includes modules such as casing heat exchangers,heat pump units,and water pumps.Using analytical solutions,numerical analysis and the principle of superposition,the temperature fields in the borehole,the soil heat storage body and the entire heat exchange body are obtained.Based on the DOE-2 semi-empirical formula,the fitting mathematical description of the heat pump unit characteristic curve is obtained;the mathematical model of the working efficiency and heat dissipation of the circulating water pump is proposed.In-depth analysis of the factors affecting the heat transfer of buried pipes.Theoretical analysis shows that: borehole depth,ground temperature gradient,soil thermal conductivity,and buried pipe flow are the main factors that affect the heat transfer of buried pipes.The drilling depth,ground temperature gradient,and soil thermal conductivity increase,and the average heat transfer effect per unit depth of the buried pipe increases.The calculation results show that under normal operating conditions(drilling depth of 2000 m),the maximum heattransfer of buried pipes under different factors is about 110 W/m～220 W/m;For the casing-type buried tube heat exchanger,the heat exchange effect of the process form of external entry and internal exit is better than that of internal entry and external exit.Analysis and optimization of the heating operation mode of the underground heat exchanger.The circulating water in the buried tube heat exchanger is directly supplied to the end heating equipment(referred to as the direct supply mode;the circulating water supplied to the end heating equipment after the temperature is increased by the heat pump is referred to as the intermittent supply mode).The energy saving effect of the direct supply mode increases;on the contrary,the energy efficiency ratio of the supply mode decreases and the energy saving effect decreases.The simulation results of multiple operating conditions show that:Under the simulation parameters,Direct supply modelHeat load accounts for 50%,The running time accounts for about 41%,and the overall energy efficiency ratio(COP)Higher.The COP extreme values of direct supply mode and intermittent supply mode reached 9.75 and 5.32 respectively.Quantitative analysis of the operation time of direct supply mode and its main influencing factors.The length of operation of the buried pipe direct supply mode depends on the temperature of the buried pipe outlet water.Borehole depth,ground temperature gradient and soil thermal conductivityIt is the main factor that affects the temperature of the buried pipe outlet.Under simulated conditions(ground temperature gradient 0.03 ℃/m,soil thermal conductivity 1.5 W/m·K),The drilling depth is increased from 1200 m to 1400 m,the operating time of direct supply mode is increased by 78%;when the drilling depth(1400 m)and soil thermal conductivity(1.5W/m·K)are unchanged,the geothermal gradient is from0.02 ℃/m is increased to 0.04 ℃/m,and the operating time of the buried pipe direct supply mode is increased by 4.77 times.Average heat transfer capacity per hole depth of mid-deep buried pipes obtained from the above studyMid-deepUnderground pipe direct supply mode bears the results of heating load ratio and analysis and optimization of its running time.It has guiding significance for the rational selection of drilling depth and underground pipe operation mode,and effectively improving the comprehensive energy efficiency ratio of mid-deep underground heat pump heating system. |
