Study On The Seasonal Soil Heat Storage Charactertistics In A Solar Ground Coupled Heat Pump System In Severe Cold Areas

Because the heating load of buildings in the severe cold area is very large, when the solar-ground coupled heat pump (SGCHP) system is used for space heating, the heat pump extracts much heat from the soil, which makes the soil temperature field recover difficultly after several years of operations and the heating coefficient of performance (COP) of the heat pump decline annually. This paper presents a new idea of using the existing installations in the system to collect the solar energy in non-heating seasons and the heat is injected into the natural soil through the ground heat exchanger. In winter, the heat is extracted by the heat pump for the space heating of buildings. The seasonal soil heat storage not only can recover and promote the soil temperature, but also can transfer the abundant solar energy in non-heating seasons to winter, which effectively increases the thermal utilization extension of the solar energy as a renewable energy source and has great energy saving significance.To find out the characteristics of the heat storage process systematically and exactly, the experimental heating system of SGCHP was established first. Then, the performances of the solar seasonal soil heat storage and the system heating in winter were monitored. Moreover, the underground soil temperature field variations were analyzed systematically. What is different from the traditional SGCHP system is that the underground vertical U-tube is not only the heat extraction installation, but also the heat storage installation, which has dual function. Based on this, the three-dimensional unsteady-state mathematical model of the multi-well ground heat exchanger which was composed of a number of vertical U-tubes was established in this paper. Furthermore, the extension function of User-Defined Function (UDF), provided by FLUENT software, was used to compile the model programs of solar collector and other equipment. They were dynamically loaded into the model of the multi-well ground heat exchanger and then the dynamic simulation of the solar seasonal soil heat storage process was realized. In addition, to study the heat balance problem of the heat extraction and heat storage of the underground soil, the heat storage system simulation models were extended to the heating system simulation models. The reliability and validity of the mathematical models established in this paper were validated by comparison between the simulation results and the measured data.Through the simulation, the influences on the soil heat storage characteristics caused by the pipe spacing, the pipe depth, the ground heat exchanger position that has buildings or not on the ground surface and the parameters of the soil as the heat storage medium were analyzed theoretically. Also, the influences on the soil heat storage characteristics caused by the solar collectors titled angle and the heat storage start temperature based on the load characteristics were analyzed. Through monitoring the operating parameters and the soil temperature field, the heat storage indexes: heat storage temperature difference, heat storage operating time, monthly and total heat storage capacity, heat storage energy efficient ratio, heat storage power, heat storage capacity of unit depth, etc. were studied theoretically.At last, from the heat extraction and temperature variation characteristics of the soil after the heat storage, which mainly include the heat extraction capacity, the heat extraction capacity of unit depth, inlet and outlet temperature of the evaporator, soil temperature field and the heat pump's COP, system heating COP and annual system's COP, the yearly variations of the soil heat storage and heat extraction characteristics under the condition of the entire heating system operating year in year out as well as the interaction between each other were discussed. In addition, through the yearly variations of the soil temperature field, the heat accumulation effect and heat balance status were analyzed. The results show that, when the operation control conditions is not changed, the soil temperature increases year by year, while the heat storage efficiency decreases year by year. During the initial 3 years, the both vary rapidly, and then the variation trend turns smoothly.The COP of the heat pump increases year by year, but the margin is small, which basically maintains at a higher level. Therefore, after 3 years of running, the heat storage capacity can be reduced appropriately, to maked the soil heat balance. The research can provide guidance for the system long-term running and popularization. The combination of SGCHP and solar seasonal soil heat storage overcomes the shortcoming of solar thermal utilization that is not matching with the season and expands the depth and width of solar thermal utilization. Through the solar seasonal soil heat storage, the temperature of the heat pump low-temperature heat source is raised and the COPs of the heat pump and the heating system are effectively increased, which has very significant energy saving effect. Now, under the great environment of increasingly serious environmental problem and the country's energetic development of the low-carbon industry, the system would certainly get considerable development and wide applications. The researches done in this paper can provide theoretical basis and technical support for the application of SGCHP system with solar seasonal soil heat storage in the future.