Modeling Short Time Thermal Response Of Vertical Ground Heat Exchanger And Its Application Research

Ground source heat pump(GSHP)system is a new type of air conditioning system using shallow geothermal resources.It has a vast potential for future development as a renewable energy usage technology.Because of the advantages of green,energy-saving,pre-environment and renewable,in recent ten years GSHP system had been developed and applied rapidly in China.The ground heat exchanger(GHE)is an important component of GSHP system which determines the performance and cost of the overall system.The short time thermal response model of GHE is the base of the system design,operation control and performance optimization of GSHP system.The thermal physical parameters of soil are the most important parameters for the design of GHE.The short time GHE model can model the heat transfer in heat build-up stage so that it is very suitable for short term thermal response test(TRT).So it is very important and valuable to model the heat transfer for short time response of GHE.The composite medium infinite line source model(CMILS)was an attractive model to describe the short time thermal processes of GHE.However,this model was very complicated and had a lower computation efficiency which limited its practical applications.In this thesis,an equivalent diameter method to the CMILS model was introduced and a new analytical model was developed.The new model had a simple form and its calculation speed was fast.The short time performance of the new model was verified by comparison with three noteworthy analytical solutions,a reported experimental data from Oklahoma State University and a numerical model which was used to validate the short time performance of CMILS model.The influence of different equivalent diameter method was studied by using the new model,and a new method to calculate the equivalent radius was proposed,which was more accurate than other methods.The new method took into account the geometry of the borehole and the thermal properties of grout and soil.Simulation showed that the average fluid temperature calculated by the new method matched better with experimental data than other methods.Increasing the thermal conductivity of the grout would cause the equivalent radius to become smaller,and the influence of thermal diffusivity of the grout and the soil on the equivalent radius was very small.The impact of the shank spacing of U-tube legs was studied through the new analytical model explicitly.Shank spacing had a tremendous influence on the heat exchange between U-pipes and soil.The form of U-tube legs close to the borehole wall was the ideal configuration,but difficult to be installed in reality.Engineers should try their best to find a way to realize the installation.This thesis developed a new method for calculation of the short time g-functions using the new model.The advantage of the new method was that it was simple in calculation with comparable accuracy.Detailed comparisons were carried out between the new method,g-function calculated by other analytical solutions,g-function calculated by experimental data,short time step g-function of Yavuzturk and long time g-function of Eskilson.Simulation studies showed that the short time g-functions calculated by the new method matched nicely with the experimental data than other analytical models.The new method overcame the shortcoming of the short time g-function of Yavuztuk which became negative when the non-dimensional time was very small as the total resistance was subtracted.According to the requirement of the in-suit TRT to the portable equipment,an integrated circuit board for measurement and control was developed.The digital thermometer DS18B20 integrated 1-wire bus measured the temperatures.The flow was measured by the Scan IF module which was used to measure linear or rotational motion integrated in MSP430 of TI.The special chip of ADE7755 produced by Analog Device measured the electrical power.In addition,the USB interface and micro printer were designed to record and store data.The whole measurement and control system met the accuracy requirements for the field test of the national standard "Technical code for ground source heat pump system(GB50366-2009)".The impact of different measurement parameters on the result of TRT was studied.The measurement errors of average fluid temperature had small influence on the result of TRT.But the influence of heat flux error was bigger.It would impact the estimation values of the thermal conductivities of grout and soil(kg and ks).The error of the initial soil temperature also impacted the estimation values of kg and ks.When the difference between the average fluid temperature and the initial soil temperature was above 5℃,the influence of the initial soil temperature error to ks would become smaller,but still larger to kg.So the initial soil temperature T0 could be treated as an estimated parameter.And then taking the estimation value of T0 to recalculate the value of kg,the influence of T0 error on the kg could be greatly reduced.It could be considered as a criterion of the minim duration of TRT that the difference between the average fluid temperature and the initial soil temperature was above 6℃.Finally,two-step parameter estimation procedure(TSPEP)applied to TRT data was improved.Bozzoli et al.provided that the first step used the parameter estimation procedure in the early transient regime to restore the grout thermal conductivity kg and volumetric heat capacity Cg.In the second step,the parameter estimation procedure was applied to the late transient regime to restore the soil thermal conductivity ks and volumetric heat capacity Cs.According to the study of this thesis,the measurement method and error of initial soil temperature T0 and the measurement error of heat flux ql had great impact on the estimation results.The improved TSPEP was proposed by the analysis of the parameter sensitivities.In the early transient regime the first step was to estimate the value of kg,Cg and T0.The second step was to calculate the value of ks,Cs and qi using the late transient regime data.The improved TSPEP was validated through simulated noisy data and reference experimental data.