Research On Variable-period Model Of Heat Exchanger Of Ground Source Heat Pump

The heat transfer process of heat exchanger of ground source heat pump is a complex non-steady-state process, involving a number of parameters. This paper mainly considers the impact of the unbalanced and dynamic load of ground-source heat pump.For different time scales, different spatial regions of heat transfer process, established an underground heat exchanger heat transfer model and verified the model by using numerical simulation or experimental methods, and analyzed the relevant parameters. Obtained the following main results:1. A new method—slab method is proposed in the paper, which means measuring thermal conductivity of soil in one-dimensional steady heat conduction process. Compared with conventional methods the described slab method offers numerous advantages, such as easy for specimen preparation, less interference factors, higher reliability of measuring accuracy and low cost. Experimentally studies the thermal conductivity of soil under different water ratio and dry density conditions using the slab method. Obtained the following rules:as dry density and water ratio increases, the thermal conductivity of soil increases.The variation of thermal conductivities of soil got in this experiment is well consistent with the results of thermal probe method. In the end, deduces two experimental correlations for thermal conductivity of soil by data fitting, which are easy for project use.2. Analyzed the failure mechanism of ground source heat pump heat exchanger, and divided the failure types into the following three types:short-term, medium-term and long-term failure, and explained the difference between the three types.3. Design engineers usually evaluate the length of ground heat exchangers by the peak load, without considering the dynamic characteristics of actual load and resulting in a slightly larger design value. This paper put forward a new design method of the best heat load, and studied the temperature response of borehole wall under 5 different types of dynamic loads by using numerical simulation and field experiments methods. The results show that:1)Under a periodical pulsed heat generation, the temperature of borehole wall has a fluctuant rising tendency; 2) At any moment, the temperature response of dynamic load is lower than that of the best design load, that is to say the design load is equivalent to the dynamic loads, thus the validity of this design method is proved.4. This medium-term model of heat exchanger used the improved Finite line-source model. It is based on the principle of superposition,and the design load is optimized dynamically.The improved heat exchanger medium-term model is easy to calculate and solves the thermal interference problem of the buried pipes. This method has significant advantages compared with the traditional design.5. Finite cylinder-source model is put forward to study the performance of underground heat exchanger after a long-time running. The model considers both the heat-affected problem of the buried pipes and the dynamic characteristic of the heat load.The method is convenient and high precision.6. Using Variable-period model of heat exchanger of ground source heat pump, mentioned in the foregoing sections, this paper presents a new design method of geothermal heat exchanger,the new method can solve the unbalanced-heat problem in the design of the heat exchanger.7. This paper presents a new indicator-ground source heat pump's average efficiency factor,using this indicator,we can objectively evaluate the operating efficiency of source heat pump system in an extended period.