Research And Design Application Of Heat Transfer Law On The Ground Source Side Of Buried Pipe Ground Source Heat Pump

Reliable test and design methods are the key to the further application of ground source heat pumps(GSHPs).In the GB50366—2009 Technical code for ground-source heat pump system,the on-site thermal response test(TRT)is used to obtain thermal properties,and designers design the size of the ground heat exchanger(GHE)accordingly.Limited by the test conditions and theoretical models,there are three shortcomings in the thermal property test and the subsequent calculation of GHE:First,the heat exchange per unit length obtained by the TRT cannot accurately reflect the changing heat transfer capacity of soil,and there will be large errors when using this static index to calculate the size of GHE.Meanwhile,the process of estimating thermal property parameters from test data is complicated and and the results of different estimation methods have large deviations;Second,the value rp required for the calculation of additional thermal resistance in the GB50366 algorithm is uncertain,so the result may have a large error;Third,the GB50366 algorithm is a design method based on the worst-case scenario,and it does not take into account the uncertainty of its decision-making,such as the peak load value,which leads to the system design capacity is too large.The above problems objectively make it difficult to provide accurate thermal physical parameters through TRT,and it is also difficult to accurately calculate the size of GHE during design.Overcoming these shortcomings of the above traditional test and design methods,improving the accuracy of GHE length calculation has important significance to the application of GSHPs.The main purpose of this paper is to propose a test and design method for GSHP system.Based on the classical linear heat source model,the law of heat transfer on the ground source side of GSHP system was studied by combining theoretical deduction and experimental research.A ground-source side heat transfer model,test method,and a new method for calculating the length of the GHE ware presented,which provide theoretical guidance for the fast and accurate design of the GSHP system.Carrying out the calculation and analysis of engineering examples,and showing the specific process of applying the new method.The main research works are as follows:Firstly,theoretical study on the law of heat transfer on the ground source side.Based on the classical line heat source model,this paper establishes a mathematical model reflecting the heat transfer law of the ground source side through theoretical analysis and derivation.The model implies the thermal physical parameters into the correlation coefficients k1 and k2,and the heat transfer law of the ground source side is simplified as the relationship between "outlet temperature of GHE","injection heat flow and "running time".The dynamic calculation model of the GSHP system is formed by combining the heat pump model.Secondly,experimental study on the law of heat transfer on the ground source side.Conducting TRT experiments,the correctness of the model was verified by comparing the consistency of the calculated and measured values.The results show that the absolute error of less than ±0.2℃ and the relative error of less than 2%after 35 hours of operation by using the new model to estimate the outlet temperature of GHE.To study the heat transfer performance of GHE under different operation modes,continuous operation and intermittent operation experiments were carried out.The results show that under the same injection heat flow intensity or inlet temperature of GHE,the allowable injected heat flow of intermittent operation is larger than that of continuous operation in the equivalent running time.Therefore,the GHE size design needs to consider the problem of excessive system capacity caused by the calculation of continuous operation.Thirdly,a new test and design method for the buried-pipe GSHP system is proposed based on the theoretical and experimental results.Introducing three existing length design methods of GHE and their characteristics.This paper discusses the engineering applicability of the GB50366 algorithm and points out the possible error caused by the calculation uncertainty.Finally,a new method of testing and design was proposed based on the previous theoretical and experimental results,and the test and calculation steps in the design and application of the GSHP system were also given.Finally,analysis combined with an engineering example.This tudy took the design of a GSHP project of an office building in Yiyang City as an example,using GB50366 algorithm and the new method proposed to calculate the length of GHE.The results show that the size of GHE calculated by the two methods differs by 10.4%under cooling conditions and 4.8%under heating conditions when the same heat flow injected.The calculation results of the test and design method is smaller than those of the GB50366 algorithm,which are also closer to the calculation results of LoopLink PRO professional software.The method proposed in this study avoids the uncertain factors caused by estimating the thermal physical parameters in traditional TRT.And the length of GHE required by GSHP system can be estimated accurately and quickly given the temperature constraints of the GHE outlet.The new method for testing and design of GSHP proposed in this paper can provide an important theoretical direction for the optimal design of GSHP.