| With the massive consumption of fossil fuel and progressively serious environmental issues in our country,sustainable development has become the goal of our country’s efforts.In order to achieve this goal,our country proposes to vigorously develop renewable energy.Because of its advantages of wide range of use,stable operation,energy saving and environmental protection benefits,ground source heat pump has become the focus of research and key promotion technology in the direction of building energy conservation in recent years.The ground borehole heat exchangers(referred to as the ground borehole)are the core component of the ground source heat pump.At present,most of its related research is aimed at the U-pipe ground heat exchangers,and the coaxial borehole heat exchanger(CBHE)is becoming a research topic due to its good heat transfer performance and wide application range.Among them,the research on heat transfer model of CBHE and the measurement of geothermal properties are the core tasks of the design of CBHE system.Therefore,the heat transfer model of CBHE and the estimation method of thermal properties of ground are studied and analyzed in this paper.Firstly,the three-dimensional numerical model of CBHE is established by using modeling on GAMBIT software and numerical calculation on FIUENT software.In order to verify the three-dimensional numerical model,it is compared with the distributed thermal response test results(i.e.,experimental values).It can be seen that the error of inlet and outlet fluid temperatures of the three-dimensional numerical model are large in the short period of time,and gradually decreases with the increase of the simulation.Among them,the errors of inlet and outlet fluid temperature after 78 hours are 0.13 ℃ and 0.06 ℃,respectively.And this verified the feasibility of the three-dimensional numerical model.In order to overcome the shortcomings of the exist models of CBHE,a new semi-analytical heat transfer model of CBHE is established.The semi-analytical mode adopts the thermal resistance and capacity model to analyze two-dimensional(radial and axial)heat transfer in the borehole,and adopts the infinite cylindrical-source model to analyze one-dimensional radial heat transfer outside the borehole,which couples the two models through the borehole wall temperature and heat flow.In order to verify the feasibility of the semi-analytical heat transfer model,it is compared with the experimental date,other model and the established three-dimensional numerical model.The results show that the calculated results of the fluid temperature distribution of the semi-analytical model after a short time and a long time are in good agreement with the experimental dates.The average absolute errors of the calculated inlet and outlet fluid temperatures are 0.09 ℃ and 0.06 ℃,respectively,and this errors are smaller than other models.The calculation results of the semi-analytical heat transfer model and the three-dimensional numerical model are also in good agreement,At 80 hours,the errors between inlet and outlet fluid temperatures are both0.02 ℃.Therefore,the semi-analytical heat transfer model established in this paper has high accuracy both in comparison with experimental dates and the three-dimensional numerical model simulation results.Based on the semi-analytical heat transfer model established above,the influence of the heat capacities of each part of the borehole(fluid,pipes and grout)on the performance of CBHE are systematically analyzed,and the influence of the heat capacity of each part of borehole on the performance of CBHE are also analyzed under different borehole radius.The results show that the influence of the heat capacity of each part of the borehole on the heat transfer performance of the ground borehole heat exchangers are basically the same,and gradually decreases with the increasing of simulation time.Among the heat capacities of different parts of the borehole,the influence of the fluid heat capacity on the performance of CBHE is the greatest,and the influence of the pipe heat capacities is the least.In addition,when with the increase of borehole radius,the influence of borehole heat capacity is also greater and greater.On the basis of the above semi-analytical heat transfer model,a new method for estimating soil thermal properties based on thermal response test data is established.The traditional soil thermal properties estimation method assumes the average temperature of the fluid at the inlet and outlet as the average temperature of the fluid in the ground borehole,and uses the infinite line-source model to fit the slope of the function of the average temperature of the inlet and outlet fluids with respect to the natural logarithm of time,and then the soil thermal properties are estimated.And this method used a more accurate semi-analytical heat transfer model to match the thermal response test data,and used different objective functions to estimate the soil thermal conductivity and volumetric heat capacity,among which the objective function for estimating the soil volumetric heat capacity is the root mean square error of the inlet and outlet fluid temperatures,and the objective function for estimating the soil thermal conductivity is the absolute error of the slope of the fitting function of the average temperature of the inlet and outlet fluids with respect to the natural logarithm of time.In order to verify the feasibility of the estimation method,a three-dimensional numerical model is used to simulate a thermal response test experiment,and the simulated thermal response test results are used as the input conditions of the method to estimate the soil thermal properties.The results show that the relative errors of the soil thermal conductivity and volumetric heat capacity estimated by this method are0.61% and-4.69%,respectively.The relative error of the soil thermal conductivity estimated by traditional soil thermal properties estimation method is 2.62%,thus proving the feasibility of the method. |
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