Study On The Effect Of Seepage On Heat Transfer Performance Of Rock And Soil Mass And Optimization Of Heat Transfer Efficiency Of Heat Pump System

Shallow geothermal energy has the advantages of huge reserves,wide distribution,green,low-carbon,renewable,stable and efficient,and is a practical and competitive clean energy.The main way to develop and utilize shallow geothermal energy is the ground-coupled heat pump technology.Borehole heat exchangers are the heat sources of ground-coupled heat pump system,and its heat transfer characteristics are affected by factors such as groundwater seepage,stratum distribution,geotechnical thermophysical properties,and operating conditions of borehole heat exchangers.In coastal areas,where rivers and lakes are widely distributed,groundwater seepage has a significant impact on the heat transfer performance of borehole heat exchangers and the efficiency of heat pump systems.There are many existing studies on the optimization of heat transfer efficiency of borehole heat exchangers.However,as a multi component system,only optimizing the borehole heat exchangers may lead to a decrease in the efficiency of the heat pump unit,making it difficult to achieve performance optimization of the entire heat pump system.There are few studies on the performance optimization of ground-coupled heat pump system that comprehensively consider the heat exchange efficiency of borehole heat exchangers,the energy efficiency of heat pump units,and the pressure loss of heat exchange fluid under seepage conditions.Therefore,it is of great significance to conduct research on the heat transfer characteristics between layered group borehole heat exchangers and rock soil mass under seepage conditions,as well as the performance optimization of ground-coupled heat pump system,for guiding the design and operation of ground-coupled heat pump system.In this thesis,theoretical research on heat transfer and fluid dynamics borehole heat exchangers and rock soil mass with or without seepage has been carried out.Based on the infinite linear heat source model and the superposition principle,an analytical formula for the average temperature at the inlet and outlet of borehole heat exchangers under intermittent operation has been derived.Based on theoretical research,the impact mechanism of groundwater seepage on the ground temperature field is revealed by monitoring the site temperature of an actual ground-coupled heat pump system and comparing it with theoretical calculation values.Based on an actual project,a heat transfer model of a layered group borehole heat exchangers considering the influence of groundwater seepage was established.And the evolution characteristics of the ground temperature field in the seepage and non seepage layers were analyzed.The heat transfer laws of the borehole heat exchangers under continuous and intermittent operation conditions were obtained.The sensitivity factors of the heat transfer characteristics of the borehole heat exchangers were analyzed.Based on the concept of maximum theoretical energy efficiency of heat pumps,the theoretical relationship between heat transfer of borehole heat exchangers,the energy efficiency of heat pump units and the average temperature at the user end is established.Using the established numerical model as a sampling tool,the sample point parameters and response values were obtained from actual engineering data.Kriging approximation models were established that could respectively indicate the heat generation capacity of the borehole heat exchangers,the pressure loss of the heat exchange fluid,and the energy efficiency ratio of the heat pump unit.The optimal combination of the comprehensive performance of the ground-coupled heat pump system is obtained by optimizing the approximate model with multiple factors and multiple objective functions using a non dominated sorting genetic algorithm.The main conclusions obtained are as follows:(1)Based on the principle of constant total heat exchange per day,the actual heat extraction or heat release process of a unit length borehole heat exchangers is simplified to a rectangular heat extraction or heat release pulse.The pulse size is the designed heat extraction or heat release per unit length borehole heat exchangers,and the pulse action time is the equivalent full load heat release or heat release hours per day.The heat load during the entire heat exchange season can be simplified into several rectangular pulse loads.On this basis,combined with the infinite linear heat source model and the superposition principle,an analytical formula for the average temperature at the inlet and outlet of the borehole heat exchangers under intermittent operation conditions is derived,and the concept of equivalent operation duration is introduced to equate long-term intermittent operation to a period of continuous operation.(2)The study area has a variable temperature layer 15 m below the ground surface,and its temperature field varies seasonally with air temperature.There is a serious imbalance in the heat extraction and release of borehole heat exchangers,with an imbalance rate of up to 77.85%.Theoretically,it will cause the ground temperature to rise by 1.88 ℃ after a heating and cooling cycle,but in reality,it only rises by 0.12 ℃,mainly because groundwater seepage maintains the stability of the ground temperature.(3)Groundwater seepage can effectively improve the heat transfer efficiency of borehole heat exchangers.When the seepage layer thickness is 40 m,the average seepage velocity is 400m/y,and the seepage temperature is equal to the initial average temperature of rock and soil,the heat transfer efficiency is increased by 41.3% compared to the case without seepage.The heat transfer efficiency of borehole heat exchangers is most sensitive to the thickness of the seepage layer,followed by seepage temperature,seepage velocity,and inlet temperature of the borehole heat exchangers.The sensitivity to initial ground temperature,inlet flow velocity of the borehole heat exchangers,borehole spacing,and thermal conductivity of the seepage layer is relatively low,and the sensitivity to the direction of seepage is the weakest.(4)Based on the maximum theoretical energy efficiency concept of an ideal heat pump,a theoretical relationship between heat transfer of borehole heat exchangers,energy efficiency of a heat pump unit,and average temperature at the user end is established considering the influence of seepage.Combining Kriging approximation model and non dominated sequencing genetic algorithm,it is possible to achieve an optimal combination scheme that comprehensively considers various components of a ground source heat pump system: heat generation capacity of borehole heat exchangers,energy efficiency ratio of heat pump units,and pressure loss of heat exchange fluid.This can provide theoretical support and suggestions for the design and operation of ground source heat pump systems.