Study Of Thermal Response Model And Algorithm Of U-vertical Ground Heat Exchanger

In China, the building sector accounted for about 27.8% of the total energy consumption and is still increasing. On the other hand, the structure of energy consumption in China is dominated by coal consumption; and clean energies only represent a very small percentage. As a renewable energy technology, the ground-source heat pump (GSHP) technologies have increasingly attracted world-wide attention due to their advantages of energy-efficiency and environmental friendliness. The wildly application of GSHP systems are helpful for the building energy efficiency and the sustainable development of energy and environment. The initial cost, operating cost, operating performance and service life are very deeply influenced by the long-term performance of ground heat exchangers. A simple and fast algorithm is needed to predict analysis and evaluate the long-term performance of ground heat exchangers effectively. Thus, it is valuable to develop a fast algorithm of thermal response model.In this paper, the thermal response model and its algorithm are studied based on the system identification method, Z-conduction transfer function (CTF) method and harmonic response method. The fast Fourier transform (FFT) method is used to transform the heat flow which is perform on the U-bend ground heat exchanger (GHE). So the heat flow can be regard as a sum of a step heat flow and some sinusoidal heat flows. So the random nature single can be transformed to the specific signal base on the FFT method. Thus, only the temperature responses of the specific signals are needed to be analyzed in the follows. According to the contrastive analysis,the fast Fourier transform method is in high precision.The conduction transfer function method is the most popular tools currently, with fast calculation speed, available for the hourly thermal load calculation through building envelopes in the design, simulation and energy analysis of building heating, ventilation and air-conditioning (HVAC) systems. An identification model of ground heat exchanger thermodynamic system is developed based on the conduction transfer function method. Furthermore, an identification algorithm of ground heat exchanger thermodynamic system is presented based on the conduction transfer function method. In the identification algorithm, the iterative identification algorithm (LSI) is used as the identification criterion, and the numerical solution of finite line source model is adopted as the observed valve. Then, the temperature response can be easily calculated by using the Z transfer factors. This approach is very easy and simple to implement in programming and avoids the case of missing roots in finding the roots of characteristic equation. The examples and comparisons have demonstrated that this approach has high computation accuracy.The temperature responses of sinusoidal heat flows with different periods are analyzed base on the identification algorithm with the conduction transfer function method. It is found that this algorithm has high computation accuracy and speed when the period is short (T≤720h). However, to reach the same precision much more Z transfer factors are needed when the period is long (T>720h). Thus, the computation speed will be reduced. So the identification algorithm with the conduction transfer function method is not suitable for the long period sinusoidal heat flows. To solve the problem, an identification model of ground heat exchanger thermodynamic system is developed based on the harmonic response method. Furthermore, an identification algorithm of ground heat exchanger thermodynamic system is presented based on the harmonic response method. The examples and comparisons have demonstrated that this approach has high computation accuracy.Then, a fast algorithm with short time step is developed, base on the identification algorithm with the conduction transfer function method and harmonic response method, to calculate the thermal response model. Through the comparison with the g-function method, this fast algorithm with short time step shows great advantages of high precision and fast speed. With the calculate results, a model to calculate the fluid temperate in the ground heat exchanger is developed. The fluid temperate of a single ground heat exchanger is then calculated base on this model. According to the results, some suggestions are given to adjust design plan.At last, the temperature responses of multiple ground heat exchangers are analyzed base on the identification algorithm with the conduction transfer function method. The fluid temperate is then calculated base on the model which is developed in the chapter seven. Furthermore, the long-term performance of multiple ground heat exchangers is analyzed according to the fluid temperate, and some suggestions are given to adjust design plan.