Study On Operating Characteristic Of Hybrid Ground Source Heat Pump System With Cooling Tower

The hybrid ground source heat pump system with cooling tower solved the problem of the tradional ground source heat pump,in order to make it better and widly used,it is necessary that did a futher study for this system.Firstly,the thesis built the model of heat exchanger,and the three-dimensional steady heat transfer performance of single U-shaped and multi-buried pipes was analyzed by the CFD numerical simulation method under different parameters,such as flow velocity,inlet water temperature of the buried pipe,soil temperature and the thermophysical properties of backfill materials.The results shown that the thermal performance of the single-ground heat exchanger(GHE)was proportional to the flow velocity and inlet temperature in the summer,but it was decreased along with the soil temperature grown.In winter,the performance of GHE was proportional to the flow velocity and soil temperature,inversely proportional to inlet temperature.For muilt-GHEs,the overall trend change under different influence factors was same as single-GHE.However,there was impact among GHEs with the number of GHE increased,which reduced the performance of GHE.For example,in the summer,the soil temperature was 17℃,the thermal conductivity was 1.5 W/m·K,the inlet flow rate was 0.5m/s,the fluid inlet temperature was 36℃,the heat transfer capacity of single-GHE was 4.55 k W,higher than 50% muilt-GHEs t.Secondly,the thesis set up a small GSHP system with cooling tower bench,and the Siemens WinCC software was used to design control system,whitch realized the functions of pump valve start and stop control,experimental data monitoring and fault alarming.The experimental method was applied for three operation models in different intermittent time and different flow rate.The three models were separate operation of the cooling tower,separate operation of the ground buried pipes,and joint operation of the both.The result shown that the COP of unit was most high in the model which the ground source heat pump and cooling tower joint operation by 1:1 flow rate.However,the COP of whole system was best in the joint operation in 2:1 time rate of the ground source heat pump and cooling tower.On the basis of the research and analysis above,the article came to some conclusions that the stable and efficient of GSHP system with cooling tower requires a combination of the three kinds of operation mode,and the single mode of operation could not meet the requirements.Finally,the method of response test was used to determine the parameters of the controller,and the Matlab/Simulink software was applied for control simulating in different operation model and different control algorithm.The simulation results shown that the Smith predictive PID control has a better control effect compared with traditional PID control and Fuzzy PID control.This control algorithm was applicable to a variety of operating modes,such as separate operation of the cooling tower,separate operation of the ground buried pipes,and joint operation of the both.The controller not only has a high controlling accuracy,but also has the characteristic of rapid adjustment with small overshoot.So,this control method was feasible for ground source heat pump system with cooling tower.