| The soil temperature of the vertical U-tube ground heat exchanger(GHE) in a ground-coupled heat pump(GCHP) system has great influence on the performance and the economy of the whole heating and cooling system. As the system run continuously for a long time, the temperature field of the soil around buried pipe changes over time, which led to the performance degradation of underground heat exchanger and heat pump system and the increase of unit energy consumption. Intermittent operation of ground source heat pump system(GSHP) makes the temperature field of soil around underground heat exchanger get periodical recovery, and it led to the improvement of heat transfer, which strengthens the heat transfer capacity of buried pipe and the operating performance of system. In most cases, intermittent operation of GSHP is effectively controled by the unit start-stop ratio of heat pump.For the practical engineering,it determines the unit start-stop ratio of heat pump based on the characteristics of the actual load so that it has a certain randomness.On the premise of meet the building load and keep the unit enough running time,this paper study on the intermittent operation model of underground heat exchanger system for GSHP.Through reasonable and effective control to it,this operation model not only has a short recovery time, but also reduce the degree of the heat accumulation and improve the efficiency of the system.The measured object is GSHP of a demonstration building engineering in Chongqing. In initial state or a typical day or the transition recovery season of the winter and summer, the temperature of the soil around underground heat exchanger and the temperature of the inlet and outlet water is tested though the experimental system of the measured object. The three-dimensional heat transfer model of underground heat exchanger has been built by Fluent and its temperature of the soil and the water is validated by the measured data. According to the model, a comprehensive analysis of soil thermal recovery performance have been studied from three factors,namely,different soil coefficient of thermal conductivity and different soil heat capacity and different backfill material. At last, through the law of soil restoration, the partition running method for the exchanger system is presented.By simulating buried pipe system under the hourly load operation, the best partition mode operation and the plan and the partition running control strategy of GSHP is identified in this project.Experimental research shows that because of the large and the strong sustainability building load in the typical running day of winter and summer, the temperature of soil and inlet and outlet water are more stable.The accumulation of load had no effect on the buried pipe system.The soil temperature recovery are mainly concentrated in 4 ~ 5 days in the recovery season.The largest degree of temperature changes on the first day,The temperature recovery of the first day accounts for 46.7% ~ 51.3% of the entire transition seasons in winter, the temperature recovery of the first day accounts for 58.2% ~ 73.1% of the entire transition seasons in summer. Simulation results show that the soil temperature recovery rate deline as a exponential linear.The greater the soil coefficient of thermal conductivity, the faster the recovery.The greater the heat capacity of soil, the slower the soil restoration and vice versa. Within the scope of meetting the specification requirements, soil backfill material coefficient of thermal conductivity have no effect on soil thermal recovery. Based on the partition method, the buried pipe system of GSHP appropriates one kind of partition model and two kinds of operation plan. According to load characteristics, the load rate(0~25%, 25%50%, 50%~75%, 75%~100%), the early and late stage of the load capacity, the running time in simulation is divided into seven operation conditions. Hourly load simulation was calculated in winter and summer. Under the partition running model, GSHP can achieve energy saving effect. And the energy-saving rate is connected with building load sustainable and size of load and load bearing rate of single well. Under theoretical operating conditions in summer, soil temperature has a trend of the periodic increasing. The highest temperature of backwater is that operation plan 1 is higher than plan 3, and plan 3 is higher than plan 2.The average temperature of supply and return water is that operation plan 3 is higher than plan 1, and plan 1 is higher than plan 2. Under actual operation conditions in summer, the return water temperature in partition running plan 1 reduces 0.63 ~ 2.00℃ between the condition of 1~7. And the unit energy consumption reduces by 1.84%~5.69%. And the water-pump energy consumption increased by 1.75% ~ 10.18%.And the system total energy saving rate is 3.18%. Under actual operation conditions in winter, the return water temperature in partition running plan 1 reduces 0.32 ~ 1.86℃ between the condition of 2~6. And the unit energy consumption reduces by 1.19%~7.69%. And the water-pump energy consumption increased by 1.17% ~ 9.26%. And the system total energy saving rate is 4.85%.The optimal partition running strategy of buried pipe system throughout the year is that it appropriate using partition running plan 1 in summer condition, and using continuous running plan in condition 1 and 7 of winter, and using partition running plan 1 in condition 2 ~ 6 of winter. |
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