Application Basic Research Of Soil Cold Storage With Seasonal Natural Energy In Severe Cold Climates

To reduce air conditioning energy consumption in summer and use the renewable energy source, a new type of soil cold storage system, which bases on the idea of seasonal natural cold energy transfer and is applicable to cold regions, is presented in this dissertation. Using the outdoor low temperature air as the cold energy and the underground soil as the cold storage device, the system is to store cold energy underground by U-tube heat exchanger and extract cold energy in summer for building air conditioning. Compared with the traditional way of short-time storage, this mode has a lot of benefits. It can avoid using expensive cold storage devices with large area and high cost; due to the heat exchanger and cold storage device underground, it runs stably and structures simply; finally, it has low operation cost and avoids refrigeration equipment and pollution. This system provides a new application field for natural cold air energy.Different from the traditional ground source heat pump system, firstly, the underground tube has a dual function that it not only serves as cold storage devices, but also behaves a role as exchanger for charging and discharging. Secondly, soil cold storage is the soil freezing process under artificial freezing effect, and during soil freezing, the soil physical properties change, which affect the heat transfer performance of underground tube. Meanwhile, soil freezing will lead to water gradient changes and cause soil moisture migration, which alters cold storage capacity of soil, and enhances its heat transfer capacity with the function of both heat conduction and convection. Besides, the soil, as storage medium, exchange heat with the air of soil surface and the surrounding natural soil, resulting in cold loss. Based on the above-mentioned, the mathematical model of the moisture-heat coupling and cold loss caused by temperature difference and pre-existing storage are established for the inner and outer tube in the vertical U-tube bundles; A new method called the backwards calculating time step method is presented to solve heat transfer problems of complex nonlinear phase change, and as a result, the iteration procedure can be simplified; In addition, by supplementing to indoor and outdoor heat exchanger heat transfer model, the model for the whole system is completely finished.Taking into account the sold storage before next year, it is impossible for soil temperature to be restored to its original level. The method, which keeps the internal energy of soil around tube at the end of charging period unchangeably compared with the first year, is used to adjust the cold-storage time during the second year. It can ensure the sustainable operation of the system. Through numerical simulation, this dissertation does a lot of analysis to the factors which influence on the operation performances of the system, such as soil moisture migration, tube distances, cold-storage time, soil type, soil with various water contents and heat exchanger area of indoor and outdoor heat exchanger etc, and research to the system cold loss of the first year and the second year and under different tube distances, cold-storage times and soil types during the three processes of cold storage, shutdown and cold extraction of the system. It supplies theory support and technology accumulation for the optimal design of system and the matching of the parameters. Take the two representative cities Shenyang and Changchun as an example. According to their climate characteristics the operation schemes of system are designed. Through the simulative analysis of operation process over one entire year, simulations show that the system achieved many better operation results if applied in severe cold region and supplied a part of cold energy for air conditioning if the system is applied in cold region.Hereby, the field experiment of soil cold storage system with seasonal natural energy is set up in Harbin while the test work in the continuous two years is implemented. This dissertation researches the charged quantities in winter, the discharged quantities in summer, the changing principle of inlet and outlet liquid temperature of tube and soil temperature in well, and puts forward the main factors of influencing on performance coefficient. The experimental results show that the system is feasible in severe cold region. The reliability and validity of the model established above are validated by comparing the simulation results and experimental data.In view of the numerical simulation of temperature fields of soil freezing and thawing, the phase change heat transfer problems are solved using the traditional methods and backwards calculating time step method presented in this dissertation, respectively. This new method, in contrast to the traditional methods, not only accurately calculates the temperature fields of soil freezing and thawing, but accelerates rate for adjusting the time step. It is an improvement over traditional methods, and can provide reference for solving other complex multi-dimensional problems involving phase change heat transfer.The system will supply a kind of cold storage technology of air conditioning with more efficient and cleaner for the 21st century buildings. At the same time, the researches in this dissertation provide theory support and demonstration effect for the system application.