Analysis of the energy efficiency of medium-depth cased ground source heat pump system with buried pipe
DATE:2024-08-20
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The results show that: the inlet water temperature and geotechnical thermal conductivity have an important influence on the system energy efficiency, and the thermal conductivity of the backfill material has a smaller influence; increasing the circulation flow rate increases the heat extraction power, but the system energy efficiency also decreases, when the circulation flow rate is increased from 8.33 kg/s to 10.56 kg/s, the heat extraction power is increased by 4.95%, and the energy efficiency decreases by 1.23%; increasing the diameter of the outer pipe, and the thermal conductivity of the geotechnical and backfill materials can improve the system heat transfer performance; the thermal conductivity of the geotechnical and backfill materials can increase the system heat transfer performance. Increasing the diameter of the outer pipe, the thermal conductivity of the rock and backfill materials can improve the energy efficiency of the system, but after reaching a certain value, the effect on the energy efficiency improvement is not significant. This study is of reference significance for the long-term stable operation and energy efficiency improvement of the medium-deep cased ground source heat pump system. Geothermal energy, as a renewable energy source, has the advantages of environmental protection, wide distribution and large storage capacity, and has been widely used in the construction field. Geothermal energy is categorized into shallow geothermal energy and medium-depth geothermal energy according to its depth, and the technology of developing geothermal energy by using shallow geothermal heat pump system is relatively mature, but the shallow geothermal heat pump system suffers from the problem of year-round cooling and heating imbalance, and its application has been limited to some extent. In order to solve the problems brought by the shallow ground source heat pump system, in recent years, the medium and deep ground source heat pump system has been developed rapidly.
The core component of the medium-deep ground source heat pump system is the medium-deep cased buried tube heat exchanger, and scholars at home and abroad have made a lot of research on it. Using Fortran program, Du et al. focused on the thermal conductivity of backfill materials and the influence of different thermophysical parameters of multiple soil layers on the nominal heat extraction, confirming that it is preferable to choose backfill materials with poor thermal conductivity for shallow ground source heat pumps and backfill materials with good thermal conductivity for deep ground source heat pumps. Jiang Guoxin studied the middle and deep geothermal energy of abandoned oil wells, and confirmed that the heat extraction capacity of middle and deep cased submerged pipe heat exchanger increases gradually with the increase of flow rate, and when the flow rate is more than 5 kg/s, the enhancement effect of flow rate on the heat exchanger's heat extraction capacity is not obvious.Li et al. verified a negative correlation between the inlet water temperature and middle and deep cased submerged pipe heat exchanger's heat extraction capacity. In addition, Liu et al. found that reducing the diameter of the inner pipe can effectively reduce the heat loss of the medium-depth cased buried pipe heat exchanger. Xu Bao investigated the effect of quantitatively increasing the pipe diameter of the outer pipe on the heat extraction of the medium-depth cased buried pipe heat exchanger. Gao Jie et al. combined the earth heat flow with the form of layered distribution of geotechnical thermal conductivity and found that when the geotechnical thermal conductivity increases with depth, it is favorable to the increase of the heat exchange capacity of the buried pipe. Han Ershuai et al. analyzed the effect of geothermal temperature gradient on the COP of a heat pump unit after 30 years of system operation, and the study showed that the larger the geothermal temperature gradient, the higher the proportion of the unit's COP decreases. Huang Shuai et al. showed that when the thermal conductivity of the backfill material increased to a certain value, the effect on the heat extraction of the medium-depth cased underground pipe heat exchanger was small. It can be seen from the above analysis that most of the existing studies focus on the influence of design parameters on the heat extraction performance of medium-depth cased buried pipe heat exchangers, but there are fewer studies on the influence of different factors on the energy efficiency of the system. In the actual project, various factors on the buried tube heat extraction performance at the same time, the energy efficiency of the system will also have an impact. For example, increasing the flow rate will enhance the heat extraction power of the buried pipe, but the increase in flow rate will lead to a decrease in the outlet water temperature of the buried pipe, which will in turn reduce the energy efficiency of the heat pump unit. Therefore, there is a need to reveal the impact of changes in various factors on energy efficiency and to provide a theoretical basis for improving the energy efficiency of the system.
To this end, this paper establishes a coupled heat transfer model of medium-depth ground-source pipe heat exchanger and geotechnical soil, based on the finite-difference method of discrete solving, and adopts demonstration project data for validation, to quantify the effects of circulating flow rate, outer pipe diameter, geotechnical soil thermal conductivity, inlet water temperature, and thermal conductivity of backfill material on the energy efficiency of the system. This study is of reference significance for the design and energy efficiency improvement of medium and deep ground source heat pump systems.
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The core component of the medium-deep ground source heat pump system is the medium-deep cased buried tube heat exchanger, and scholars at home and abroad have made a lot of research on it. Using Fortran program, Du et al. focused on the thermal conductivity of backfill materials and the influence of different thermophysical parameters of multiple soil layers on the nominal heat extraction, confirming that it is preferable to choose backfill materials with poor thermal conductivity for shallow ground source heat pumps and backfill materials with good thermal conductivity for deep ground source heat pumps. Jiang Guoxin studied the middle and deep geothermal energy of abandoned oil wells, and confirmed that the heat extraction capacity of middle and deep cased submerged pipe heat exchanger increases gradually with the increase of flow rate, and when the flow rate is more than 5 kg/s, the enhancement effect of flow rate on the heat exchanger's heat extraction capacity is not obvious.Li et al. verified a negative correlation between the inlet water temperature and middle and deep cased submerged pipe heat exchanger's heat extraction capacity. In addition, Liu et al. found that reducing the diameter of the inner pipe can effectively reduce the heat loss of the medium-depth cased buried pipe heat exchanger. Xu Bao investigated the effect of quantitatively increasing the pipe diameter of the outer pipe on the heat extraction of the medium-depth cased buried pipe heat exchanger. Gao Jie et al. combined the earth heat flow with the form of layered distribution of geotechnical thermal conductivity and found that when the geotechnical thermal conductivity increases with depth, it is favorable to the increase of the heat exchange capacity of the buried pipe. Han Ershuai et al. analyzed the effect of geothermal temperature gradient on the COP of a heat pump unit after 30 years of system operation, and the study showed that the larger the geothermal temperature gradient, the higher the proportion of the unit's COP decreases. Huang Shuai et al. showed that when the thermal conductivity of the backfill material increased to a certain value, the effect on the heat extraction of the medium-depth cased underground pipe heat exchanger was small. It can be seen from the above analysis that most of the existing studies focus on the influence of design parameters on the heat extraction performance of medium-depth cased buried pipe heat exchangers, but there are fewer studies on the influence of different factors on the energy efficiency of the system. In the actual project, various factors on the buried tube heat extraction performance at the same time, the energy efficiency of the system will also have an impact. For example, increasing the flow rate will enhance the heat extraction power of the buried pipe, but the increase in flow rate will lead to a decrease in the outlet water temperature of the buried pipe, which will in turn reduce the energy efficiency of the heat pump unit. Therefore, there is a need to reveal the impact of changes in various factors on energy efficiency and to provide a theoretical basis for improving the energy efficiency of the system.
To this end, this paper establishes a coupled heat transfer model of medium-depth ground-source pipe heat exchanger and geotechnical soil, based on the finite-difference method of discrete solving, and adopts demonstration project data for validation, to quantify the effects of circulating flow rate, outer pipe diameter, geotechnical soil thermal conductivity, inlet water temperature, and thermal conductivity of backfill material on the energy efficiency of the system. This study is of reference significance for the design and energy efficiency improvement of medium and deep ground source heat pump systems.