Experimental And Research On Thermal Energy Storage Of Underground Concrete Pile

Energy is one of five elements that support our daily life. The discovery and use of High-quality energy sources is indispensable to the development of human society. Energy is the roll booster of human culture. Energy is very important strategic material in all countries contemporarily. Currently, oil, gas and coal are the three kinds of traditional energy sources, which still play an dominant role in the global energy consumption. According to authoritative data, provided by "BP World Energy Statistics 2009", by the end of 2008, the World's proved oil storage capacity is 170.8 billion tons. By 2008 production basis, the oil can be exploited in 42 years. Based on the same calculation, gas and coal mine can be used 60.4 years and 122 years respectively. However, with the reduction of storage capacity, the exploitation becomes more difficult, the value of exploration becomes less. If we take into account the enlarged economic scale, population growth and other factors, the life span of traditional energy exploration will be shorter.The building energy consumption takes an big role in the traditional energy source consumption. Based on the survey, HVAC energy consumption accounts for 65% of building energy consumption. Supposing building energy consumption is 35% in total, HVAC energy consumption ratio is as high as 22.75 %. So people in the world pay great attention to the reduction of traditional energy consumption, to the usage of thermoelectric waste thermal and to the usage of new energy sources to replace the traditional energy sources. Especially the usage of combined solar energy and geothermal energy is considered to replace the traditional energy sources to satisfy the building energy consumption. Solar energy not only could be used directly, but also could be stored and used gradually.It is a technical problem on how to storage thermal energy efficiently and environmentally. At present, there are mainly three kinds of methods on underground energy storage: sensible thermal storage, latent thermal storage and thermo-chemical thermal storage. Sensible thermal storage makes use of the characteristics of thermal storage regenerator which thermal storage changes along with temperature. Latent thermal storage takes advantage of characteristics of absorbing or releasing a large number of thermal energy when the material occurred phase transition. Thermo-chemical thermal storage utilizes reversible chemical reaction to store energy, which usually uses some inorganic oxide hydration. The concrete methods of sensible thermal storage are as followed: artificial water tank, underground aquifer, underground natural rock and underground soil, etc. The advantages of sensible thermal storage are convenient selection and low cost of storage materials. While its disadvantages are low storage density, heave weight and large size of thermal storage system, wide range of temperature during input and output energy, and unstable thermal flow. Therefore, its application is limited. The merits of latent thermal storage are high storage density, approximate constant temperature in the process of input and output energy, high storage efficiency and small size of system. The disadvantages of latent thermal storage are that the system requires two separated liquid circulation loop, high thermal diffusivity and thermal capacity of phase change material. In addition, the thermal exchanger must be designed specially in order to ensure the coordination of solidification rate and thermal speed. The change materials are prone to cold and crystal liquid separation during the process of phase change. The thermo-chemical thermal storage has large thermal storage capacity and could stores for long term at ambient temperature. While its disadvantages are as followed: complex reaction process, demand of catalyst sometimes, high system security, low efficiency, high operation and maintenance requirements. Thus, it is difficult to form large-scale thermal storage system.This paper describes high-temperature concrete for thermal energy storage, aiming to resolve the issues of low efficient thermal energy by underground soil storage. Through theoretical analysis, the mathematical and physical models of internal thermal storage of underground concrete thermal storage pile were built. According to a large number of domestic and foreign references, by adding granite, basalt which can improve the special thermal and graphite which can improve the thermal conductivity, the concrete thermal storage materials have been improved. Ground thermal storage structure test platform and underground concrete storage thermal test platform have been built. In this paper, the influence of intermittent heating method and continuous heating method on internal temperature of concrete thermal storage pile and the influence of different sizes, different insulation thickness, thermal conductivity of different concrete, different initial thermal flux on internal temperature of concrete thermal storage pile has been studied. With the help of ANSYS, the research result has provided the basic data and theoretical guidance for choosing reasonable thermal energy storage materials and proper designing underground thermal energy storage structure. In this paper, theoretical research, testing and comparative analysis, simulation has been carried out on the research of underground thermal storage in concrete, have come to the following specific conclusions:1. The concrete thermal energy storing pile with high-temperature refractory aluminates cement as gelatins material, has little deformation at high-temperature, can improve the efficiency of thermal storage, and the thermal storage with the concrete pile is feasible.2. The rock fragments of specific large thermal capacity and the graphite can effectively improve thermal conductivity. 3. The structure of the ground thermal storage test showed with the same temperature , the greater volume of thermal storage body, the better thermal storage; and the insulation layer is not the thicker the better, the insulation layer has little effect above 70℃, but the thicker the better under 45℃.4. The thermal storing concrete pile with diameter 1m, height 3m, insulation layer 0.1m, temperature discrepancy 55℃in and out of the pile, which will take more than 1000 hours to lost thermal to make get the original temperature. Relative to the thermal exchanger, the lost thermal influence radios is 2.25m.5. ANSYS shows: with the same thermal energy, that the thermal in the concrete pile with the discontinuous heating mode is less than the continuous heating mode. The discontinuous heating test show that the temperature improved rate higher, and the thermal of outside utilization ratio higher than the continuous heating mode.6. The lower thermal conductivity, the better thermal storage effect. But too low thermal conductivity may affect the thermal absorb rapid. Generally speaking, the thermal conductivity between 1 W/(mk)～1.5 W/(mk) is better for the storage pile with lower diameter and smaller volume.7. If only change the thermal flow density, the ratio of temperature changes and lost thermal energy equal to the ratio of original thermal flow density.8. Small space between thermal exchange pipes, only affects the temperature circled by those pipes. But the temperature in the center will fall rapidly after heating. The storage hours equals to others.9. With the same thermal energy, the difference of thermal storing pile diameter do not affect the storing time; but if the temperature is lower than 45℃, it affects more, the larger diameter, the longer storing time.This paper is about high-temperature thermal energy storage concrete and insulation material applied to the underground thermal storage, which effectively improve the efficiency of underground thermal storage. In this paper, taking use of the ANSYS to analyze the effect of thermal storage with the different initial conditions. We optimize the structure by the side insulation thickness on thermal storage, proposed reasonable structural model of thermal storage. In short, by those innovations we can receive a new way of thinking and method of thermal storage underground.