Dynamic Simulation Of U-tube Ground Heat Exchanger

In the design of ground source heat pump system, the predication of ground heat exchanger performance is very important, while the predication requires accurate and practical mathematical model.At present, the models which are commonly used are linear source analytical model and cylinder source analytical model. These two analytical models are based on the constant heat flux. For the variable heat flux conditions in practice, the method of convolution (time superposition) must be adopted. If time superposition method used for long term simulation (such as 30 years) calculation, it is time consuming.Based on the concept of thermal resistance and heat capacity, a transient one-dimensional explicit numerical model of ground heat exchanger is built and two algorithms are provided in this thesis. This model is suitable for any heat load conditions. The heat load or inlet temperature can be used as the inputs for calculation.In constant heat flux case (80W/m), the borehole wall temperatures are simulated by using analytical model and numerical model. The results show that the one-dimensional numerical model is consistent with the cylinder source analytical model; the linear source model differs from the cylinder source analytical model, especially at the initial stage.The numerical model is validated by using four groups of field heat response testing data. Given that the load of the heater is constant, the outlet temperature is calculated by one-dimensional numerical model by taking the measured inlet temperature as inputs. Comparisons show that simulated values are in good agreement with measures values of outlet temperature . The maximum deviations of four groups of data are 1.6℃, 0.6℃, 0.8℃and 0.6℃, separately.In variable heat flux case, linear source analytical model, cylinder source analytical model and numerical model are used to simulate one-year operation, separately. Comparisons show that the one-dimensional numerical model is consistent with the cylinder source analytical model , the one-dimensional numerical model is faster than the other two analytical models. The time required to simulate the one year operation of heat exchanger by the numerical model is less than 1min.While applying the one-dimensional numerical model, the recommended space step is 0.033 to 0.067m, and the recommended time step is 60s.Based on the one-dimensional numerical model, a software for ground heat exchanger simulation is developed. On the basis of whole year dynamic load of the building, the inlet and outlet water temperatures, and the temperatures of rocks at the surroundings of heat exchanging holes can be simulated hourly, which will provide basis for the predication of long term operation performance of ground heat exchanger. The calculation speed of this software is quick. This software is suitable for the selection of engineering schemes, and it has practical values.