Study On The Heat Transfer Mechanism And Parameter Estimation Method For Ground Heat Exchangers (GHEs) In Ground-Coupled Heat Pump Systems (GCHPs)

The ground-coupled heat pump systems(GCHPs)have attracted global attention for their high energy efficiency and low green gas emissions.The thermal properties of the ground and grout are the key parameters for a typical GCHP system as they are the basic information of the GCHPs and of great importance for the GCHPs operation performance.To improve the GCHPs performance,the ground and grout thermal properties,including the ground thermal conductivity,the grout thermal conductivity,the ground thermal diffusivity,the grout thermal diffusivity and the borehole thermal resistance,should be precisely estimated.As suggested by ASHRAE handbook,the in-situ Thermal Response Test(TRT)should be operated to estimate the ground and grout thermal properties when the GCHPs are projected.Although the conventional parameter estimation method for TRT offers a relatively accurate result,there are three potential issues that are important for enhancing the parameter estimation accuracy but have not been addressed.Firstly,the fluid temperature profile along the U-pipe varies with length,but it is treated as linear distributed for the conventional parameter estimation method.Secondly,the grout thermal properties are ignored for the heat transfer model used in the TRT,thus the grout cannot be estimated by the TRT.Furthermore,the heat transfer model ignored the grout thermal capacity is incapable of modeling short-time response of Ground Heat Exchangers(GHEs)which is significant for energy consumption analysis of GCHPs,hybrid GSHPs design and in-situ TRT of GHEs.Thirdly,optimal iterative algorithm for the conventional parameter estimation method is unstable and depends strongly on the initial guess value.This thesis aims to address the above issues by developing a set of new thermal parameter estimation methods,proposing a transient quasi-3D full time scale line source model and updating the optimal iterative algorithm.The details are list as follows.Firstly,to deal with the variation of fluid temperature profile approximation,this thesis proposes a new approach,titled as p(t)-linear average method,to estimate the ground thermal parameters as well as the p values at the different sampling times.The proposed method has been evaluated using the data collected from an in situ thermal response test(TRT)in Hunan Univesity.It is found that the proposed method leads to a 6.31%reduction of the borehole thermal resistance when compared to the conventional arithmetic mean temperature method(the p-linear average model with p = 1).Besides,compared to the theoretical results,the maximum relative error in the borehole thermal resistance for the conventional arithmetic mean temperature method is as high as 40.69%.In contrast,the maximum relative error for the p(t)-linear average method is less than 3%for all the typical practical cases.To enhance the physical meanings of p(t)-linear average method,the p(t)-linear average method also been improved by the functional relationship between the theoretical heat transfer model and the measured inlet and outlet fluid temperature.By comparing with Distributed TRT results which is serverd as the benchmark,both the p(t)-linear average method and improved p(t)-linear average method can be considered as an accurate parameter estimation method.Secondly,the short-term behavior of the GCHP system is very important for the energy performance of GCHPs,Hybrid GCHPs design and in-situ TRT.This thesis proposes a new transient heat transfer model under time-dependent and time-independent heat load.Compared with the equivalent diameter transient heat transfer model,the inlet pipe and the outlet pipe of the U-pipe are analyzed separately.Besides that,the measured borehole thermal resistance is take into account to improve the calculated transient borehole thermal resistance,which may be affected by the borehole configuration.The Sandbox experiment developed by Beier is employed to validate the prediction accuracy of the proposed model in this thesis.Compared with the measured values,the proposed method offers a good temperature prediction,and the relative errors for both the fluid and ground temperature are not larger than 5%.If 0.5 ℃ is taken as the acceptable maximum absolute error for the test,the accuracy of the fluid temperature predicted by the proposed model can be accepted after 45 min.Furthermore,the transient heat transfer model under time-varying heat flux is also proposed in this thesis.Compared with the time-independent heat flux,the fluid and the ground temperature profile should be simulated by the superposition principle of Duhamel Theorem.To minimize the computation time,the Fast Fourier Transform Algorithm is employed.The time-dependent transient heat transfer model is verified by the intermittent operation GCHP experiment platform in Hebei Engineering University.The result shows that both the predicted fluid and U-pipe wall temperature are in good agreement with the experiment results for most of the time,and the relative error is under 5%.Finally,with the proposed transient heat transfer model,five ground and grout thermal properties,including the ground thermal conductivity,the grout thermal conductivity,the ground thermal diffusivity,the grout thermal diffusivity and the borehole thermal resistance,are firstly estimated in this thesis.For a multi-parameter(more than three parameters)estimation process,the conventional optimal iterative algorithm will result in either a poor estimation performance or its performance strongly depended by initial guess values.With Genetic Algorithm,a new integrated parameter estimation method for five ground and grout thermal properties is proposed in this thesis.Compared with the conventional optimal iterative algorithm,the proposed method offers a better thermal parameters estimation results in both the robust and the estimation accuracy.In conclusion,the transient quasi-3D entire scale line source model and the parameter estimation method are effective methods,which are able to offer theoretical support for the control strategy development for GCHP systems and for the determination of the borehole distance and length.The innovative works in this thesis mainly includes the following points:1)The functional relationship between the temperature profile along the U-pipe and the parameter estimation method was solved,and the p(t)-linear average method and the improved p(t)-linear average method to estimate the thermal properties of borehole heat exchangers was proposed.2)The transient heat transfer model for the time-dependent heat load and time-independent heat load was proposed.3)The Genetic Algorithm was firstly employed to estimated five ground and grout thermal properties including the ground thermal conductivity,the grout thermal conductivity,the ground thermal diffusivity.