Study On The Summer Intermittent Operation Model Of Underground Heat Exchanger For Ground-couple Source Heat Pump System In Chongqing

With the rapid development and a wide range of applications of soil source heatpump,the change rule of the running performance of soil source heat pump system foryears become the focus.The physical properties of the soil around the buried pipes arefixed. As the system run continuously, heat constant is absorbed or released by the soilcontinually and the temperature field of the soil around buried pipe changes over time,which led to the performance degradation of underground heat exchanger and heatpump system and the increase of unit energy consumption. Intermittent operation of soilsource heat pump system makes the soil store or release heat intermittently throughcontrolling the running time reasonably. It makes the temperature field of soil aroundunderground heat exchanger get periodical recovery,and it led to the improvement ofheat transfer,which enhance the heat transfer capacity of buried pipe and the operatingperformance of system.It determines the unit start-stop ratio of heat pump based on thecharacteristics of the actual load so that it has a certain randomness.On the premise ofmeet the building load,this paper study on the summer intermittent operation model ofunderground heat exchanger for soil source heat pump system on non-full loads.Thisoperation model not only meet the demand of construction load, but also has soilintermittent recovery.The measured object is the soil source heat pump system of a demonstrationbuilding engineering in Chongqing.The temperature of the soil around underground heatexchanger atthe different depth on the actual operation situation is tested though theexperimental system of the measured object.Moreover the recovery characteristics ofthe soil around underground heat exchanger are analyzed.The three-dimensional heattransfer model of underground heat exchanger has been built by fluent and its rationalityis validated by the measured data.The summer intermittent operation model ofunderground heat exchanger on non-full loads have been built based on the recoverycharacteristics of the soil around underground heat exchanger.And the soil temperaturedistributions and outlet temperature changes of the protoplasmic backfill buried pipe atdifferent load on different intermittent operation model have been simulated The bestoperating models on different non-full load have been get though comparing theinfluence of different operation model on the soil temperature distribution and the energy consumption of the unit.It is suggested that the recovery characteristics of the soil around underground heatexchanger at different depth are roughly same：The temperature drop in2days before ofthe whole recovery process is largest. The temperature drop of the first day accounts for58.15%~92.52%of the entire transition seasons, and the temperature drop of thesecond day accounts for3.82~11.93%of the entire transition seasons, the temperaturedrop of the remaining time accounts for0.14%~5.42%of the entire transitionseasons.Three the summer operation model based on the three pipe-groups of themeasured object have been built：Model1is that only one pipe-group runs and thesingle underground heat exchanger is on the operating model that runs1days andrestores2day repeatedly. Model2is that two pipe-groups run and the singleunderground heat exchanger is on the operating model that runs2days and restores1day repeatedly. Model3is that all pipe-groups run and the single underground heatexchanger is in a working day all the time.It is suggested that the flow of single pipe is smaller on continuous operationmodel than the intermittent operation model so that the rises of soil temperature andoutlet temperature of pipe are smaller in a day.It is suggested that the soil around pipegets better recovery on intermittent operation model than the continuous operationmodel so that the rises of soil temperature and outlet temperature of pipe and the energyconsumption of the unit are smaller day by day when the load of a single undergroundheat exchanger is not more than its full load.However,the best operating model shouldbeen built be combined with the construction demand for air conditioning load and therunning time of the unit prolonged when the load of a single underground heatexchanger is more than its full load on intermittent operation model. The greater theredundant part of the load, the smaller the significance of the intermittent operation ofthe underground heat exchanger is.Through the simulation analysis,the best operating models on different non-fullload of the soil source heat pump system of a demonstration building engineering havebeen get at the early stage of the building full load carrying section.The best operatingmodel is model1when the building load rate is25%.The best operating model is model2when the building load rate is50%.The best operating model is model2when thebuilding load rate is75%. Through the simulation analysis, the best operating models on different non-fullload of the soil source heat pump system of a demonstration building engineering havebeen get at the later stage of the building full load carrying section.The best operatingmodel is model2when the building load rate is75%.The best operating model is model1when the building load rate is50%.The best operating model is model1when thebuilding load rate is25%.