Investigation On Control Of Underground Thermal Energy Transport And Heat Transfer

With the increasing tension of energy and environment problems, renewable energy utilization is an effective way to solve the relative problems, shallow underground is considered as one of the most clean and environmentally friendly green energy technologies which can be used combination with underground storage (UTES) and ground-source heat pump (GSHP). It can achieve long-term and inter-seasonal storage which is conducive to apply waster heat, solar energy and valley electricity.Most of the studies about underground heat exchangers are based on the line heat source theory and cylindrical heat source theory which are almost about heat conduction while few of them take into account groundwater flow which always exists in underground heat transfer and pipe flow coupling of heat transfer. Particularly in large-scale underground heat exchangers researches are more insufficient. In addition, both the UTES and GSHP are involved in complex underground heat transfer and energy transfer which are influenced by many factors, such as thermal properties of soil, geological structure and natural conditions and underground geological structure, while the more important factor is the active control mode in the process of energy storage to more effectively control energy flow, and to make the energy transfer, diffusion and maintenance harmonious.Variable load conditions, dynamic group of underground heat transfer, heat transfer mechanism and dynamic control mechanism of seasonal underground storage are studied in this paper on the basis of numerical method of finite volume and combining with experiments in the field of National Science funded project of China-No. 50806028. By way of promotion and restrain of the thermal energy transfer, active control of temperature field profile control and energy flow control, underground thermal energy transfer control mechanism is established to promote the underground storage technology innovation and development.Heat transfer characteristics and performance of coaxial tubes underground heat exchanger were analyzed. A heat transfer mode coupling heat transfer of flow in pipe with heat transfer about groundwater seepage flow is advanced. Under the condition of groundwater seepage flow and no groundwater seepage flow in saturated soil, some primary influences on heat transfer, such as the size of buried tubes, the velocity of flow in the tube are analyzed to explore the varying-time characteristics.In the case of non-groundwater seepage in saturated soil, the results show that the pipe heat transfer rate per unit length is greater when the outer diameter of coaxial underground heat exchanger above a certain diameter, and the heat transfer between boreholes and soil is sufficient. Heat transfer rate per unit length is higher when the storage fluid velocity is in the range of 0.2 ~ 0.4m / s. If the velocity is higher than this range, the effect of enhanced heat transfer is no longer obvious.Under the condition of groundwater seepage flow in saturated soil, results show that the speed of groundwater seepage flow of groundwater is benefit for the heat transfer between underground heat exchanger and soil. Groundwater seepage flow of groundwater in saturated soil can increase the heat convection and improve the heat transfer effect. With the increase of speed of groundwater seepage, the unit heat transfer quantity of boreholes increases. At a certain speed of groundwater seepage, the increase of soil porosity can enhance the heat transfer between underground heat exchanger and soil, and with the increase of soil porosity, the effect of heat transfer enhances, also the time of system running smoothly is shorter. At the same time have the same quantity flow of groundwater through the pipe, the increase of speed of groundwater seepage flow is more effective than the increase of porosity in soil. If the soil porosity is smaller than 0.1 and the speed of groundwater seepage flow is very small, the heat transfer enhancement leading of groundwater is small.To further validate the numerical simulation model, the establishment of a large array of multi-heat sources experimental system is established to study dynamic control of heat transfer in energy storage process. A dynamic timing control, and focus on the dynamic sequence of three different dynamic control modes are studied to analyze temperature time-varying field. Temperature characteristics and effect law is also analyzed in the process of energy storage, energy diffusion and energy maintenance. Experiments show that the dynamic timing control of the storage has significant effect to energy storage. Under the condition of the same energy storage quantity, storage time and the experimental operating conditions, load dynamic timing control can achieve significant differences of heat storage capacity. It not only affects the energy storage period, but also affect energy maintenance period. Variable phase and amplitude will be beneficial to enhance the energy diffusion and energy maintenance, and provide effective method for optimizations of underground storage and heat transfer control. By comparison of experiment with simulation, simulation date agree with the experimental data well.According to the investigation of the heat transfer of energy diffusion and transport during the process of energy input, storage and extraction in UTES, a new conception, the energy flux was presented to evaluating the effect of thermal energy storage. Through scaling parameters and comparing the energy flux, by mean of the degree of heat diffusion the energy flow characteristics of a UTES system can be quantitatively analyzed and it leads to use UTES technology rationally by controlling heat diffusion. The different configuration of multi-borehole underground heat exchangers was calculated numerically, and the energy flow intensity in the period of maintaining energy stored in soil was compared by using the energy flux parameter as a criterion. Results show that the configuration of multi-borehole affects underground temperature distribution and it plays an important role in energy diffusion and transport during the process of energy input, storage and extraction. Monitoring position on energy flux can be selected form the outer bolehole center of 0.5 to 2.5 times spacing where the energy flux can fully reflect the effect of the energy storage and energy diffusion. Also research shows that 3m to 6m spacing between boreholes is appropriate when the temperature difference between flow in borehole and soil is small, otherwise when the temperature is high or the speed of groundwater seepage flow is large, around 6m spacing is appropriate to energy storage, diffusion and maintenance.The control strategies about the multi-borehole underground heat exchangers are discussed in the cases of groundwater seepage and non-groundwater seepage flow in saturated ground soil, and different heat source distribution and different load configuration in these cases were studied to evaluate the energy preservation and diffusion effect. Four control modes were studied under non-seepage. Results show that energy storage based on ring and hierarchical load mode about temperature is more conductive to energy diffuse and maintenance.On the condition of strong groundwater seepage flow, a new offset load control mode which contributes load of different temperature in the direction of groundwater seepage flow is put forward, and four different offset load control modes were studied. Researches show that offset load control mode is important to energy flow and underground storage. It is more efficient to enhance energy maintenance capacity. As the analysis show that offset load control mode and ring load control mode has the same input quantity of energy storage, but offset load control mode has more capacity of energy maintenance and energy extraction.Therefore, temperature and hierarchical loading mode based on fluid temperature and offset loading control could provide a new control concept of energy flow for heat transfer and UTES. It would give technological breakthroughs for underground energy utilization. Studies for intermittent periods of energy storage show that shorter interval of off-set heat source model is better for energy storage and energy maintenance than longer interval of offset heat source mode in the energy storage quantity, average temperature and maximum temperature..Comprehensive studies have shown that the splendid energy diffusion capacity is essential for high energy storage efficiency, but good energy maintenance capacity requires low energy diffuse to far boundary. This causes the conflicts and coordination problems of promotion and inhibition of energy diffusion in the process of underground energy storage. Load changes, load distribution of different temperature, offset load method and other means can achieve actively control of underground heat diffusion and enhance energy storage efficiency by studying the effect of dynamic heat transfer and energy flow of variable heat loads. Storage temperature weakened, energy transmission buffer, phase fluctuations delay of temperature field, relationships of period of storage time in process of hierarchical and offset energy storage mode are also searched by studied problems of the control and offset of temperature field, variability control and synergy of load of large-scale heat sources. The research will improve the control theory of underground storage and heat transfer, and promote the development and application of underground energy storage and heat transfer technology.