| In the history development of human society, every major advance of the human civilization is closely related to the improvement of energy, the development and utilization of energy has greatly promoted the development of the human society productivity and economy. However, the energy not only brought the technological progress and economic development, but also encountered a series of unavoidable problems, such as:energy shortages, energy security, waste of resources and environmental pollution which was caused by the utilization of energy, these problems are a serious threat to the human survival and development.China has become an enormous energy production and consumption country. The sustained growth of economic demand more production and consumption of energy. According to the report of Chinese Academy of Geological Sciences, it is reported that in the next 20～30 years, the supply of available resources of China will not be sustainable. In the next 20 years, the total demand of oil, natural gas and other resources would be at least 2～5 times more than now if China realized the industrialization. At the same time, the total energy consumption and the backwardness of the coal-based energy consumption structure leaded to serious environmental pollution. Furthermore, SO2 and CO2 in China's emissions have taken respectively first and second place in the world. In the future, China will take inevitably responsible for obligation of Greenhouse gas reduction. Under the situation of growing shortage of fossil energy in the world, the development of new energy and renewable energy for sustainable energy security is an inevitable choice to relieve the energy pressure and the only way to reduce environmental pollution.China is located in the northern hemisphere; a vast territory, which has a very rich solar energy resource. The development and utilization of solar energy resources have a significant and far-reaching significance to renewable energy development and environmental protection in China. However, solar energy is not only impacted by the changes of day and night and season, but also constrained by some random factors, such as:clouds or rain. The solar energy could not be used effectively because of significant instability and discontinuity. In order to solve these problems, this paper built solar energy storage and extracting test platform in the underground concrete, and then, the variable laws of underground temperature field which were during the process of solar energy storage and extraction were also studied. The research mainly included following several aspects:Firstly, the solar energy storage and extracting test platform was set based on concrete thermal energy storage pile, synchronous tracking solar collector system and heat pumps. Its running processes of working condition, data acquisition and automatic control systems conform to the design requirements, which lay the foundation on different conditions for the analysis of variation of underground temperature field.Secondly, the transient heat transfer model of heat exchanger within the concrete pile was established on the basis of finite element method. And then, the variable laws of underground temperature field were simulated under different running conditions. During the period of solar energy storage in daytime, the temperature of heat exchanger fluctuated obviously, the increasing range of average temperature was 0.86℃/h. Along with the increasing spread of thermal, the temperature gradient decreased gradually. When energy storage stopped, the temperature of heat exchanger decreased, but the temperature in the edge of concrete pile increased constantly. However, the temperature increasing amplitude in the stage is less than heating stage, their temperature rose respectively 0.42℃/h and 0.26℃/h.In stage of heat extraction, the variation of underground temperature field was influenced by heat extraction model, heat flux, thermal conductivity and specific heat of concrete and pipe distance. The decrease amplitude of heat exchanger was proportional to the increment of heat flux, but the thermal effect radius of different heat flux was little difference. Enlarged thermal conductivity and specific heat of concrete could make the temperature gradient of concrete pile decreased, which was conductive to heat extraction. Little pipe spacing would lead to heat unevenly distributed within the concrete piles. In this time, the temperature of heat exchanger could not be effectively supplement, which detrimental to the long-term operation of heat extraction. Therefore, pipe distance should not be too small; it was 0.4m for small volume concrete pile in this experiment.In addition, the distribution of underground temperature field was influenced by different heat extraction model, which affected the heat transfer characteristics of the system. Taking intermittent heat extraction, the temperature of heat exchanger could be supplement in time from around concrete, which made heat well distributed and improved the efficiency of heat extraction. During the stage of intermittent heat extraction, the decrease amplitude of concrete was 0.6℃/h. Moreover, the thermal effect radius of intermittent heat extraction model was longer than continued heat extraction model.At last, in order to study the operating characteristics of solar energy storage and extraction in underground concrete and the variable laws of underground temperature field under the different conditions of heat extraction, the experiments of solar energy storage and extraction in underground concrete were carried out. During solar energy storage, the average efficiency of focusing synchronous tracking solar collector was between 49.9%～55.4%. In addition, comparing different heat extraction model, it could be seen that temperature decreased rate of intermittent heat extraction model was higher than continued heat extraction model. The average decreased amplitude was respectively 0.61℃/h and 0.26℃/h at the center of concrete. The average decreased amplitude was respectively 0.58℃/h and 0.43℃/h at the edge of concrete. Moreover, the total heat extraction amount and heat extraction efficiency of intermittent heat extraction model were significantly higher than continued heat extraction model. The total heat extraction amounts were 280.498MJ and 218.326MJ, and the heat extraction efficiency were 94.37% and 73.46%.The technology of solar energy extraction in underground concrete could be effectively solve the problems of solar energy in collection, storage and trans-utilization. It is beneficial to give full play to the source of advantage and the use of underground space. It provides a new heating mode for north area. In addition, it receives good economic and social benefits from reducing pollution and carbon dioxide.
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