| With the continuous growth of domestic industrialization and the national economy, the heating demand in China is highly urgent. Therefore, the sharply increased coal-fired boilers have caused a serious environmental problem to a certain extent. Especially in the cold area the heating time is much longer compared the other areas in China which needs a large amount of coal in winter. Under this background, some kinds of renewable or sustainable energy are highly needed to instead of the traditional coal-fired boilers. Therefore, ground source heat pump (GSHP) technology has been widely used in recent years. However, some critical problems caused by the improper design, construction and commissioning have been exposed during the operation years and needed to be solved especially in cold area where the GSHP system operation may cause a severe problem.It is well known the common problem of the GSHP system used in cold area is thermal imbalance of the underground soil, which may cause a series of other problems. Therefore, it is necessary to investigate the current situation and propose the diagnostic evaluation methods for the existing GSHP system. According to the diagnostic problems, some reasonable energy-saving renovation measures should be pointed out to improve the system performance. This dissertation takes an existing GSHP system installed in a freight train station in the northern part of Shanxi Province as an investigation example. Firstly, a set of data collection system was designed and installed on the GSHP plant to collect the operating data. Secondly, the system performance and the thermal balance of the underground have been carried out on the basis of the measured operating data during a whole heating and cooling season. A series of research methods have been employed, such as the on-site diagnosis, real-time data measurement, data analysis and simulation and optimization. Finally, the following several problems of the existing system have been found according to the diagnosis results:(1) The annual underground thermal imbalance is quite serious. This is caused by the reason that the heating time is much longer and the heating load is significantly lager compared to the cooling season. In this project, the annual extraction heat in winter was over 3 times amount of the heat rejected during the summer. (2) The number of the buried boreholes is larger for cooling case and insufficient for heating case, which leads to a large fluid flow and small temperature difference in summer. On the other hand, the circulating pumps are oversized in order to reduce the temperature difference of the ground heat exchanger during the winter. (3) Lack of the necessary automatic monitoring equipment and improper management strategies also caused a high energy consumption and dangerous operation state.According to the measured data, the overall operating COP of the GSHP system was quite low, only 1.8 in cooling and 1.4 in heating case. The final energy consumption per square meter in winter reached 0.33GJ which is significantly higher than the local energy consumption standard of 0.24GJ per square meter.The existing GSHP system has been compared with the conventional system (the coal-fired boiler plus the chiller) and original system (heating only). The terminal heating/cooling effect, energy-saving efficiency, environmental benefit and economic benefit of the whole GSHP system have been evaluated. Meanwhile, the underground temperature distribution and the energy consumption have been simulated by means of the professional software of GS-1.0, and some possible energy-saving retrofit measures have been suggested according to the simulation results, as follows:(1)To increase the air source heat pump as an auxiliary heating source to solve the problem of thermal imbalance in the underground; and the optimal operation strategy of the hybrid system has been suggested;(2) For cooling mode in summer, since the number of the buried boreholes is over sized, it is suggested that only part of the boreholes take part in the heat rejection process. This can alleviate the effect of cooling accumulation in the underground. Furthermore, it can improve the efficiency of heat transfer in the tube with a higher;(3) The existing circulating pumps are suggested to optimize to further reduce the transmission energy consumption. The on-site management system should be standardized.It can be seen from the research results that only increasing the number of the buried boreholes cannot thoroughly solve the thermal imbalance of the ground heat exchanger used in cold area. The reasonable capacity of the boreholes should be designed based on the cooling load and the excess heating load should be taken by other supplemental heating source. In this case where there is no suitable heating source in winter except the geothermal energy, the seasonable heat storage can operate to release heat to the underground to alleviate the cooling accumulation during summer or transition seasons by means of air source heat pump or solar collectors. However, this kind of hybrid system may increase the overall energy consumption since the thermal storage equipment is in operation in some seasons.It is estimated that the overall operating cost may be reduced by 40% after the system will be retrofitted according to the aforementioned measures. The research results also provide a theoretical guideline and technical support for the GSHP system applications in cold area in China. |
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