| In the last decades, great concern has mounted about the ground source heat pumps. Ground source energy is generally regarded as a benign energy source, particularly when compared to nuclear, coal and oil; however, experiences show that there are some problems associated with its exploitation and utilization. History shows that hiding or ignoring such problems is, in the long-term, counterproductive to development of an industry because it leads to a loss of confidence in that industry by the public, regulatory, and financial sectors. If our aim is to further the use of ground source energy, then possible problems should be clearly identified, and countermeasures devised and adopted to avoid or minimize their negative impacts. Energy storage is one of most important strategies to settle the unbalanced energy utilization in extreme climates, for the technology of thermal energy storage is one of the shortcuts that make the energy reproducible and efficiently. The general higher efficiency and more real-time energy supply can be accomplished by temporary or long-term energy storage.Underground thermal energy storage is a complicated process that includes energy absorbing, accumulation, storage and exploitation. The underground temperature field in the different cases of thermal energy storage is discussed, and the optimized energy storage discipline is concluded in this paper.The elements, such as original ground temperature, aquifer, thermal conductivity, density, heat diffusivity, which affect the efficiency of ground source heat pumps are also discussed in the paper. The theory model of ground heat exchanger is also deeply discussed. The heat and mass exchanges in ground are simplified by the distributed resistance model, which assumes that there exists everywhere a local balance between pressure and resistance forces in ground according to Fourier principle and Fick law and Darcy law.In some extreme climates such as northeast of china the temperature is very low in winter, and the cold climate last for a long time (the heating period is more than 5 months), it is necessary for the ground source heat pumps to run under 0℃, which will result in freezing and thawing of the ground around the underground heat exchanger, and then the phenomenon of potential heat should be in considered. The corresponding model is built up to deal with this condition.The effect of different heat exchanger structures in large scale boreholes to the efficiency of energy storage is researched by the simulation. The results shown that borehole equipped with the monolithic heat exchanger which is made of low thermal conductivity materials, such as PTFE (polytetrafluoethylene), will degenerate the energy storage efficiency, while the borehole equipped with the integrated style heat exchanger which is made of low thermal conductivity materials has a better efficiency and lower short circuit between the two legs of the heat exchanger, however, the borehole equipped with the U-type heat exchanger ,which is made of low thermal conductivity materials, and backfilled with high thermal conductivity materials such as granite has best efficiency during the energy storage and largest short circuit between the two legs of the heat exchanger. The distance between the legs of the heat exchange in the borehole also discussed in the paper, which shown that as the distance larger the energy storage efficiency better. The integrated style ground heat exchanger should be the better one because it can make sure distance between the legs of the heat exchanger to be constant and be the largest in borehole, while the U-type heat exchanger can not keep the distance of the legs to be constant because of the pliability of the pipe. This integrated-style ground heat exchanger is an innovation of the paper. Large scale underground energy storage system is simulated in 96meters×96 meters field by the 8×8 boreholes with 6meters long between two holes. The assumptions are only thermal conductivity happened in the storage system. The simulated results show that the energy storage efficiency in granite is better than in clay, for the thermal conductivity of granite is higher than clay's. That is the media temperature should be higher for the clay than for granite to get same load per area of the borehole.The results also show that if the conductivity of the ground is constant, the storage efficiency will be better when the volume specific heat increases. However, the lower specific heat will result in higher temperature difference between borehole and the fare field, and the affected field will be larger. Comparing the energy storage in all of the 64 boreholes with those in one another 32 boreholes, the former model has lower but more uniform temperature field if the both model have same total load. And the temperature field of clay is more uneven than that of granite, and the results imply that there should be more boreholes to be energy storage for the clay than for granite at the same load, so that the temperature at the borehole wall of the clay can be decreased greatly which decreases the necessary for higher energy quality. And less number of boreholes is necessary for granite to energy storage, so the energy can be more efficiently stored around the boreholes. In the condition of unbalanced period energy utilization, the energy storage strategy is very important, and the results of simulation show that energy storage in center field is much better than in whole field, because the former can make the whole temperature field more uniform, and can utilize the energy in far field, and improve the center temperature field.The simulation results of aquifer show that underground water has significant impact on the temperature field. When the permeating speed increases, the temperature field changes much more, and the energy storage efficiency degenerates much more. And the arrangement of the boreholes is important in the condition of high permeating speed, the reasonable arrangement can decrease interfere between the upriver and downriver boreholes. Even though the underground water flow has negative impact on the energy storage, it has positive impact on the heat exchanger, because the water carries more energy away from the borehole, and increases the temperature difference between the borehole wall and far field, and decreases the maximal borehole temperature, and then improves the performance.The freezing and thawing simulations show that if the underground water volume percent increase, the potential heat of water contributes more to the system, and this is benefit to decrease the borehole scale. The equivalent pressure specific heat increases several times by considering the potential heat of underground water in the range of freezing and thawing temperature. So when the ground temperature will decrease slower at freezing and thawing temperature, and then the temperature difference between unfrozen and frozen area will increase, that will facilitate the heat transfer.A multi-functional test bed was built up to analyze the underground energy storage. Carbon cyber was used as the heat source because of its fascinating heat performance. The rock soil was used as the ground because it's lower thermal conductivity and smaller granule which will decrease the impacts of environment. The temperature field of the test bed was tested by 26 thermal couples.Four types of energy storage have been run on the test-bed, i.e. basic energy storage mode (15 watts per hole running for 6 hours and recovering 23 hours); period energy storage mode (15watts per hole running 1 hour and stop 2 hours for 6 periods and recovering to 29hours); focus energy storage mode (25 watts for center holes and 5 watts for peripheral holes running 9 hours and recovering to 29hours); and thermal shield energy storage mode (25 watts for center holes running 9 hours and 2.7 watts for peripheral holes running 1000 minutes and recovering to 29hours). The experimental results show that reasonable thermal shield energy storage can improve the efficiency of underground thermal energy storage obviously.The experimental results show that high power will dramatically increase the borehole wall temperature which corresponds to the high quality media. Thermal shield is very important in the condition of ground energy storage.Even though the freezing process can increase the utilization of the underground energy, the potential risk of damage the underground heat exchanger increase. When the water around the heat exchanger freezing the volume will increase and produce stress around it, which will deform the structure of the heat exchanger and may damage it. The model experiment shown the saturated sand will deform the plastic pipe during the freezing process. |
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