| With the development of the society,the problem of global energy shortage becomes more and more serious.The development and utilization of renewable energy has become the primary task of the whole world.Building energy consumption is one of the primary source of global carbon emissions.In the field of building energy conservation and emissions reduction,ground source heat pump heating technology with deep borehole heat exchanger（DBHE）has gradually become one of the clean heating options in northern China under China’s dual carbon target.The key to the popularization and application of this technology is to establish the heat transfer model of the DBHE and to analysis the heat transfer performances of the DBHE and the equipment located overground.At present,the research on this technology at home and abroad is mainly focused on the heat transfer analysis and experimental research of the DBHE.However,few studies on the performance analysis of direct heating of the DBHE coupled to the heating terminals are conducted.The DBHE can realize the function of direct heating for the building with radiant floor heating terminal if the outlet temperature of the DBHE is higher than 30℃.The direct heating mode has a significantly higher operation efficiency compared to heat pump heating mode.In this dissertation,a heat transfer model of DBHE based on the varied heat flux was proposed,and the corresponding heat transfer performance of the DBHE was analyzed.Based on the developed heat transfer model,a simulation model of the heat pump heating system with DBHE was developed in the platform of TRNSYS,which incorporates the heat pump units,water pumps,and hourly heating load.Then,the influences of different parameters on the system performance were studied.The results obtained from the study can provide some theoretical suggestions for popularizing heat pump heating technology with DBHE.The main research contents are as follows:（1）Based on the infinite line heat source model,an integrated heat transfer model of DBHE,considering the circulating fluid and the underground simultaneously,was established.The analytical solution of fluid temperature under varied heat flux was obtained by using the superposition principle and the Laplace transformation.Considering that the building heating load changes with time,the time-varying heat flow was approximately represented by a series of continuous heat pulses,and the temperature response formula for the variable load situation was also obtained.（2）Based on the developed model of DBHE,the thermal performance of the DBHE was analyzed.This dissertation mainly focused on the effects of borehole depth,geothermal gradient,soil thermal conductivity and circulating fluid mass flow rate on the maximum heat transfer rate and outlet fluid temperature of DBHE.The simulated results showed that the maximum heat transfer rate increases with the increases of the borehole depth,geothermal gradient,soil thermal conductivity,and circulating fluid mass flow rate.When the average heat transfer rate per meter is constant,the borehole depth has an approximate linear relationship with the outlet water temperature of DBHE.When the total heat transfer rate is constant,the final outlet water temperature of DBHE increases gradually with the increase of the borehole depth,but the increasing rate decreases gradually.The influence of the geothermal gradient on the outlet water temperature of the DBHE is also approximately linear.The outlet water temperature increases with the increase of soil thermal conductivity and mass flow rate,but the increase rate is gradually slow.When the geothermal gradient is 0.02 ～ 0.04 ℃/m and the soil thermal conductivity is 1.5 ～ 3.0 W/（m·K）,the maximum heat exchange transfer rate of the DBHE with borehole depth of 1000 ～ 2000 m is about 53 ～ 148 W/m.（3）Based on TRNSYS,a case study for a building located in Jinan was carried out.Firstly,a heat pump heating system with DBHE was established.Then,a control strategy for the joint control of load and water supply temperature was proposed as well as two interactive operation modes of the DBHE heat pump heating system,namely,the direct heating mode of DBHE and the indirect heating mode of the heat pump.The effects of borehole depth,geothermal gradient,soil thermal conductivity and fluid flow patterns on the operation of the system were simulated and analyzed comprehensively.The economic analysis of the two working conditions close to the reality was also carried out.The simulation results showed that the greater the borehole depth,the greater the proportion of peak heating load was supplied by the direct heating mode of the DBHE.When the borehole depth is 2140 m,the direct heating mode of the DBHE can bear all the building heating load.The effect of geothermal gradient on the running time and the proportion of the peak heating load supplied by the direct heating mode were obvious,while the soil thermal conductivity was relatively small.The minimum borehole depth that can fully supply all the heating loads in direct heating mode was simulated under different geothermal gradients.The direct heaitng operation time in the pattern of“annular in and internal out” was 19.4% longer than that in the mode of “internal in and annular out”.Finally,the economy analysis of the heating system was conducted.In the case with the following conditions: the borehole depth is 1500 m,the geothermal gradient is0.030 ℃/m and the DBHE direct supply bears the loads under 50% of the peak heating load,the system operation cost is about 10.02 RMB per square floor meter,which is 57.7% less than the gas-fired boiler heating system;When the borehole depth is 1600 m and the geothermal gradient is 0.035 ℃/m,the DBHE direct heating bears the loads under 60% of the peak heating load,the average COP of the DBHE direct heating mode can reach 8.63,and the system operation cost is about 8.23 RMB per square floor meter,saving 65.3% compared with the gas-fired boiler heating system.The results prove that the direct heating system of DBHE has a high annual operation economic benefit. |
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