Research On The Dynamic Operating Characteristics Of Ground Source Heat Pump System Considering The Soil Freezing

In recent years,the energy consumption of the buildings in China has been increasing,accounting for about 30%of total energy consumption in our country,which is in the second place in the world.Energy saving and emission reduction are imperative in the critical period of the global energy transition to lower-carbon energy sources.The ground source heat pump(GSHP),applying renewable energy for heating and cooling,is an energy saving technology and has been increasingly widely used in the world.The heat transfer characteristics between borehole heat exchanger(BHE)and the surrounding soil are the key factors that influence the performance of the GSHP.During the heating period in cold area,the soil around the BHE will be frozen during the heat period when the soil heat extracted by the GSHP.However,the complexity of soil composition makes it difficult to study the effect of the frozen soil on the performance of GSHPS,especially under the action of dynamic load.Therefore,there are few researches considering the effect of the frozing soil on the heat transfer of BHE and the performance of GSHPS.Therefore,taking into account the complex geological conditions in practical engineering,this paper studies the influence of the frozen soil on the performance of GSHPS under the dynamic load on the basis of the related studies considering soil freezing at home and abroad.Firstly,taking a building in Harbin as the research object,the annual dynamic heat load of the building was calculated.Corresponding to the heat load,the size and arrangement of BHEs were designed and the heat pump units were also selected.Heat capacity method was used to deal with the process of soil freezing and heat transfer.A full-scale dynamic simulation platform of the GSHPS under typical geological conditions was established to realize the dynamic couple of heat exchange performance between heat pump units and underground buried pipe groups.Secondly,the dynamic operation and performance of GSHPS under two different conditions:considering and non-considering the soil freezing were simulated respectively.The soil temperature field,the temperature of inlet and outlet fluid,unit energy consumption,unit performance coefficient and system performance coefficient were compared to analyze the annual variation.Considering freezing makes the central heating borehole temperature higher than without considering the freezing 1.32K,the COP of the heating unit presents two stages:the heating medium-term consideration of freezing unit COP higher than not considering freezing,which is higher 0.12 than not considering freezing,the heating end is the opposite.Finally,the effect of soil freezing,soil type,porosity,stratification,stratified porosity,groundwater seepage,the velocity of heat transfer medium in tube and the backfill material on the dynamic operation performance of heat pump unit and the temperature distribution of soil under the conditions that the soil freezing was or wasn’t considered were studied respectively.By comparison,the influence of soil freezing on the performance of the GSHPS under different geological conditions,construction conditions and operating conditions was studied.The influence of different factors on the soil freezing and soil temperature field and the performance of the unit have a greater impact,when the seepage velocity increases,the COP of the unit gradually increases,and the effect of the freezing on the heat transfer efficiency of the system decreases gradually.The COP of 1.0m/d is 0.28 higher than the minimum unit without seepage.