Research On The Double Heat Source Coupling Characteristics Of Solar-Ground Source Heat Pump And Heat Transfer Enhancement Of Underground Thermal Energy Storage

With the social progress and improvement of living standards, the importance of energy is becoming increasingly obvious. Social sustainable development requires that energy can be utilized sustainably. The solar-ground source heat pump system (SGSHPS) makes the best use of solar energy, which acts one of the system’s main heat sources, has incomparable advantages over ground-source heat pump system (GSHPS), so it has broad application prospects. The double heat sources coupling characteristics and the enhancement of underground energy storage heat transfer of SGSHPS were studied through experimental research, TRNSYS and MATLAB simulations in this paper.In the aspect of the experiments, experiments of single GSHP operation mode, combined operation mode, tank heat storage operation mode and soil heat storage operation mode were conducted in the SGSHPS experimental system. Furthermore, the performance coefficient of the unit, inlet and outlet temperatures of buried pipes, system heat relations, solar collecting efficiency and double heat sources coupling characteristics were analyzed and compared, and the optimum operation condition was obtained under different load characteristics. The results indicate that in combined operation mode, solar and geothermal energy are dynamically coupled, and the average COP and solar collection efficiencies are 3.61,51.5 and 3.48,38.2% for working condition 1 and working condition 2, respectively. In tank heat storage operation mode, the COP of the unit can be significantly improved and the outlet temperature of buried pipes of working condition 2 is 1.85℃ higher than working condition 1. Therefore, the decrease of soil temperature of condition 2 is smaller and more conducive to the system operation for the next day. Soil heat storage operation mode contributes to the restoration of soil temperature, and the soil temperature recovery rates and solar collecting efficiencies are 56.8,47.8 and 13.82,41.5% for working condition 1 and working condition 2, respectively. From synthetically analyzing experimental results, it is indicated that the average COP are 3.91 and 3.8 for working condition 2 of tank heat storage operation mode and soil heat storage operation mode, respectively, so the two working conditions are good choices for the building in which heating is needed only during night and the load is large. Moreover, for the building that needs heating only during daytime, working condition 1 of combined operation mode is suitable.In the aspect of theory, different SGSHPS simulation models were established based on TRNSYS transient simulation platform and the dynamic simulation calculation was performed under different working conditions for 10 years in this article. The variations of average soil temperature, unit performance coefficient and the collecting efficiency with operation time were analyzed and compared. The results show that for the buildings that heat dominated, the SGSHPS can keep the soil thermal equilibrium and the declining rates of soil temperature after 10 years are 29.1,13.1,17.1 and 8.6% for single GSHP operation mode, combined operation mode, water tank heat storage operation mode and alternate operation mode respectively, and the corresponding average unit COP during whole heating season are 4.5,4.87,4.7 and 4.78 respectively. The average collecting efficiencies and the proportion of heat source burdened by solar energy are 59.4,45.1; 44.8,19.3 and 62.6,60.4% for combined operation mode, water tank heat storage operation mode and alternate operation mode respectively. For combined operation mode, the results of component optimization indicate that the suitable collector area is 140 m2, tank volume is 2.8 m3 and number of borehole should be adjusted to 9.In order to further discuss the measures for strengthening heat transmission during the process of the underground energy storage and release in solar energy-U buried pipes, the enhancement of underground energy storage heat transfer of SGSHPS was studied through MATLAB simulation in this paper. Based on the numerical solution of the model, the influences of soil type, inlet temperature of buried pipes and energy storage allocation on soil heat storage were analyzed. The results indicate that the larger the thermal diffusivity is, the greater the ability of soil temperature tends to be consistent and the greater soil heat storage capacity is. The soil temperature and heat storage capacity are improved with the increasing inlet temperature of buried pipes. The way which stores heat with low inlet temperature at inner region and high inlet temperature at outside region has no obvious advantage in terms of heat storage, but the heat diffusion is reduced significantly and the average soil temperature of heat storage area is improved. After the yearly operation process of heat extract and release, the center temperature granite is the lowest with the largest thermal diffusivity, followed by sand and clay.This research can provide an experimental and theoretical foundation for further research of the double heat sources coupling characteristics and the enhancement of underground energy storage of SGSHPS. It can also provide guidance for SGSHP practical engineering application.