Research On The Performance Of Water-Phase Change Material Tank Auxiliary Ground Source Heat Pump System In Underground Shelter

With the rapid development of underground rail transit dominated by subways,and underground municipal administration dominated by integrated pipe corridors,the development and utilization of underground space in Chinese cities have gradually become diversified,since the 21^st century.As part of the underground space,it is necessary to ensure the construction of civil underground shelters closely integrated with the development and utilization of underground space to realize the intensive development of urban protection systems and urban construction.According to the report of the 3^rd Civil underground shelter Conference of China,the general policy of civil underground shelter construction"Combination of Peace and War"was put forward.It is required to provide services for urban construction,economic construction,and emergency management in peacetime,to bring better social and economic benefits,and in addition to concealing personnel and materials in wartime to protect the safety of people’s lives and property.Concerning the principle of indoor thermal control,the air-conditioning system design for the civil underground shelter should meet the specific requirement in the peace mode and emergency mode.In this way,it should not only meet the different load requirements at both peacetime and wartime but also reduces the initial cost of the project.Once the emergency mode occurs,the cooling load dominates and rises sharply,which is significantly different from the load in the peace mode.In addition,to ensure the safety of property and life inside caused by infrared exposure,the civil underground shelter cannot be equipped without external heat emission equipment,which means it is not possible to directly discharge hot and cold air whose temperature differs greatly from the atmosphere temperature.Therefore,it is essential to propose an air conditioning system that meets the requirements of camouflage and energy saving.This thesis aims to find the solution to meet the cascade load requirement in peacetime and wartime and meet the requirement of infrared camouflage and energy-saving for the underground shelter.Based on the idea of cascaded utilization of geothermal energy and latent heat thermal energy storage technology,a water-PCM tank-assisted ground-source heat pump system is proposed.The design and operating characteristics of the hybrid system are studied specifically through theoretical analysis and simulation,aiming to provide theoretical support for the application.The entire research work was completed with the funding of the National Natural Science Foundation of China"Thermal control of underground shelter with different zones and cascaded load,meeting the Camouflage and Energy-saving Requirements"（51678488）.For the stage of designing the hybrid system,it is necessary to mention the following creative ideas.To take advantage of the thermal inertia of the ground and the constant temperature character of the soil with a certain buried depth,a buried water-PCM tank is proposed.Besides,to be convenient for mass production,modular water-PCM tanks are proposed.Additionally,to take full advantage of the initial excavation foundation,the conception of application of energy diaphragm wall in air-conditioned underground shelter are proposed.Afterward,based on the Transient System Simulation Program software（TRNSYS）,the simulation model of the hybrid system was established.Firstly,taking the underground shelter as an example.The shelter functions as a shopping center in peace mode while a shelter in emergency mode.The thermal load of the underground shelter at both peacetime and emergency mode is calculated.Moreover,load characteristics with meteorological parameters and ground temperature parameters of four representative cities from the cold zone,dry zone,hot and humid zone,and wet zone are compared.The design principle of a water-PCM tank-assisted ground-source heat pump system is proposed.In addition,to make full use of the excavation foundation in the early stage of underground engineering and reduce investment,the applicability and heat transfer characteristics of energy diaphragm wall technology in the underground shelter have been carried out.A mathematical model for the heat transfer of the buried pipe in the diaphragm wall is established.Different from previous research,the model calculated the heat convection at the inner wall surface to calculate the heat loss through the wall.Based on the mode,the feasibility of applying the energy diaphragm wall in the air-conditioned underground space is demonstrated considering the heat exchange effect and the attenuation of the wall’s natural cooling capacity.The study found that although the natural cooling capacity of the wall without a buried pipe is 16 times that of the energy diaphragm wall,the excess cooling capacity is less than 7%of the total heat dissipation of the buried pipe₄0)).It is applicable in air-conditioned underground engineering.Furthermore,the influence of the wall structure,hydrogeological conditions around the buried pipe,geometry and operating parameters of the buried pipe,and indoor air temperature on the heat transfer characteristics of the buried pipe are further analyzed.Additionally,this paper proposes a modular water-PCM tank as the cold source of the ground source heat pump system.The heat exchange and phase change proceed mathematical mode of the modular water-PCM model was established by MATLAB.The accuracy of the mathematical model was validated by the experiment result.By comparing the cooling effect of the water-PCM tank in two scenarios,it is concluded that the cooling capacity of the buried phase change water tank is 24.96%higher than that of the insulated water tank.Simultaneously,a specific analysis of the influencing factors considering c the water-PCM tank’s geometric dimensions,operating parameters,and phase change temperature is carried out.Corresponding geometry,thermophysical properties,and operation schemes of a modular water-PCM tanks suitable for underground space heat pumps are proposed.The result lays a foundation for the promotion and application of modular water-PCM tanks for load shifting.A mathematical model of the modular water-PCM tank matrix is established.Heat charging and natural cooling（passive cooling conditions）are analyzed.Taking the effective guarantee time and temperature drop requirements as targets,the optimal combination matrix of modular water-PCM tanks is proposed.Furthermore,based on the TRNSYS software,a ground-source heat pump system model was established for peace mode.Based on the analysis of the load characteristics of the civil underground shelter unit in Beijing,the standard design parameters of each component of the ground source heat pump system are determined.The operating characteristics of the hybrid system in peace mode are analyzed.Based on the phenomenon of high part-load rate,for energy saving,the desirable range of buried pipe length and loop flow rate in peace mode is proposed.Finally,given emergency mode,based on the basic principle of heat transfer and fluid mechanics,a water-PCM tank-assisted ground source heat pump system model was established in TRNSYS.On this basis,researches on the operating characteristics and optimization of the hybrid system were carried out.The effects of the combat readiness time,system control temperature,and phase change the temperature on the hybrid system performance are analyzed.Furthermore,using influencing factor analysis and orthogonal experiment（Taguchi）method,the influence of buried pipe length,modular water-PCM tank matrix,and the flow ratio of the two on the operation characteristics of the hybrid system are additionally analyzed.Taking the minimum investment per guarantee time as the optimization objective,the best combination scheme of the hybrid system under this load characteristic is proposed.The theoretical result of the thesis lays the foundation for the application and promotion of the hybrid system in practical engineering.