Research On The Thermal Performance Evaluation Of Ground Source Heat Pump Systems With Thermal Imbalance

The increasingly severe global warming and energy crisis have made it urgent to explore appropriate renewable energy for building use,particularly for the heating ventilation and air-conditioning(HVAC)systems.The ground source heat pump(GSHP)system is such an energy efficient and environmentally friendly device that capitalizes on the shallow geothermal energy to heat up or cool down buildings,which is achieved through the ground heat exchanger(GHE)system.For GSHP systems with the severe imbalance between the ground heat extraction and injection,they may encounter the thermal imbalance problem,in which the accumulative heat or cold could increase or decrease the temperature of the ground,and further cause serious issues such as system performance deterioration and system failure.Meanwhile,the GSHP system is simultaneously affected by various influential factors,which can be divided into the “internal factors”(geological structure and GHE layout)and “external factors”(heat load input)regarding the GHE system.Based on the research gap of the existed research on GSHP systems,the study aims to provide a comprehensive analysis via experimental and numerical methods.Various GHE heat transfer models were proposed and compared,and their applications on a single tube or GHE field for short-or long-term simulation were elaborated in detail.Based on the validated numerical models,influences of various factors(including the geological stratification,system operation mode,heat load,boundary conditions and size of the calculation domain)on the heat transfer process were analyzed.Furthermore,long-term operation of a large-scale GSHP system with thermal imbalance was analyzed and evaluated,and a three-level evaluation indices system was proposed.The main work and conclusions arising from this study are summarized as follows:Validated against the experimental data from a laboratory device designed with the similarity theory,thermal performances of GHEs in a complex geological structure predicted by three numerical models were compared.Among which,the 3D model is the most accurate one.Since the influence of load distribution is more significant in a stratified subsurface,it is essential to consider the variable heat load along the length of GHEs for those in complex geological structure.Based on the equivalent model with the variable heat load along the length of GHE,a reliable and feasible method to calculate the long-term performance of large-scale ground source heat pump system was proposed.Based on some typical geological structures in the hot summer and cold winter area,the influence of various factors(including the geological structure,heat loads,GHE scale and boundary conditions)on the short-or long-term heat transfer process of different scale of GHE systems were investigated and summarized.Moreover,calculation methods based on multi-factor coupling were proposed to predict the thermal performance of the subsurface.Besides the thermal properties,the effect of ground stratification on the heat transfer prediction could also be affected by the heat loads and boundary conditions.Specifically,a more significant impact of ground stratification could be found during the operation period with considerable load input(continuous operation and higher load input),and those accessible to the far-field boundaries(material with higher thermal diffusivities,longer operation time and smaller computation domain),especially for the systems with thermal imbalance.If the intermittent load was inputted with the same amount while a different pattern,the ground temperature would keep the same.Therefore,it could serve as a method to simplify the calculation of the intermittent operation with faster calculation and accuracy guaranteed.To overcome the drawbacks of the existing evaluation methods,a three-level evaluation indices system,including the qualification,efficiency and performance evaluations,was proposed.The evaluation approach could reflect the characteristics of the system loads and ground temperature distributions.Furthermore,the effect of the thermal imbalance ratio on the system thermal performance predictions was investigated,and the ratio is more suitable to evaluate systems with similar scales.The system performance varies with different scales,operation modes,and geological structures.Therefore,the thermal imbalance ratio is not recommended to be the only parameter to evaluate and limit the application of the GSHP systems.This study proposed several numerical methods and investigated the influences of various factors on the system thermal performance experimentally and numerically.The methods to calculate and evaluate the thermal performances of GSHP systems with thermal imbalance were introduced and elaborated in detail.This study can contribute towards the thermal imbalanced system performance assessment and optimization for the practical engineering and standards,and provide theoretical and technical guidance to enhance the applications of the ground source heat pump technology.