Study On Compling Characteristics Of Composite Heat Pump Heating System

In cold areas, because of the large energy consumption in buildings and the low temperature in soil, the efficiency of a single ground source heat pump is usually low. According to local conditions, by using composite heat pump heating system, higher energy efficiency can be achieved. At present, the studies on hybrid heat pump system are focusing on small-scale simulation and research-based test-bed experiments. Lacking of verification of the actual project, often, makes the experimental data widely vary from the actual project.This thesis focuses on the climate, geological conditions and load variation of cold regions, studies on the compling characteristics of composite ground-source heat pump heating system, and validates the model and the correctness of simulation results by practical engineering experiment. The main research contents and results are as follows:The performance of underground heat exchanger is a key factor to the performance of heat pump. In this thesis, first of all, in order to make the simulation approach the realistic operating conditions, a heat transfer model of underground heat exchanger under variable heat flux is proposed. Secondly, to simplify the computational process and to save the operation time, a boundary element method is used in analyzing the thermal resistance of underground heat exchanger. Finally, a self-compiled software is used to simulate the performance of vertical U-shaped underground heat exchanger and the correctness of numerical simulation is verified by a thermal response test set. The results show that in order to reduce the influence of heat transfer between underground heat exchangers, the drilling space has to be more than 4 meters. Higher initial temperature and higher thermal conductivity of soil, higher thermal conductivity of tube material and higher thermal conductivity of backfill material in drilling are conducive to the performance of underground heat exchanger. Increasing the distance between drillings and increasing the distance between branch pipes in the drilling can reduce the influence between the underground heat exchanger and branch pipes. A double U-shaped pipe has better performance of heat transfer than a single U-shaped pipe, but the difference is less significant in the region where the soil temperature is lower than 10 centigrade. Consequently, in cold regions, the best form of underground heat exchanger is the single U pipe.In this thesis, the main works are carried out as follows:First of all, a mathematical model of all the parts of ground source heat pump system was established. Simulation program for the dynamic simulation of system performance was written and the mathematical model was verified by an experimental ground source heat pump system built in cold region. The results showed that in order to improve the system efficiency, the number of drillings should be properly increased and the temperature of supply water should be properly decreased, and intermittent operation mode should be used while the circulation pumps on both sides should be adjusted by using frequency convertion technique.Secondly, a mathematical model of all the parts of solar-ground source composite heat pump system was established. Simulation program for the dynamic simulation of system performance was written and the mathematical model was verified by an experimental solar source composite heat pump system built in cold region. The results showed that by using a solar collector system, the amount of drillings could be properly reduced. In the case of meeting the demand of the need of heat, properly reducing the temperature of supply water could raise the efficiency of the heat pump system. The system should adopt the intermittent operation mode, in order to raise the COP of the unit. In the period of solar collectors participating the system, the soil temperature and system efficiency were significantly improved.Thirdly, by using the self-built multi-condition heat pump test-bed, the control model and performance of subway exhaust heat-ground source composite heat pump system were studied. The results showed that when the thermal discharge of subway met the demand of heat of building, air source heat pump could work separately, and the average COP was 4.9. When no waste heat could be used, a soil source heat pump could be used for heating with the average COP of 3.8.Then, the research on water source heat pump system used in transfer station of oil field was done. A mathematical model of heat pump was proposed and the performance of the heat pump system was simulated. The results show that the performance index was in good agreement with the design conditions in winter, while in summer, a variable duty model should be used, the energy efficiency could be higher if the circulation pump applied the frequency conversion technique.On the basis of the methodology of economic evaluation, in the case of various fuel prices, the economical efficiency of three kinds of source heat pump system was compared with other heating modes. Some conclusions were achieved: the energy utilization of three kinds of heat pump system was the highest compared with other heating modes, and the running costs were the lowest. As the operating cost of heating system is highly affected by the fuel price, the heating plans should be considered carefully according to the actual situation.