Research On Heat Storage And Release Simulation Of North Wall In Chinese Solar Greenhouse And Structure Optimization

The Chinese solar greenhouse is a typical greenhouse type in China, which is low cost,energy-saving and economic benefits. As an important building envelope, the walls play a great role inthe heat storage and heat preservation, which is the key character different from other types ofgreenhouse. However, the Multivariate quantitative relationship between the effective energy storageand release in walls and wall thickness and materials used for wall structure is still unclear due to thelimit effective studies on the law of temperature variation and the dynamic heat transfer. Therefore, aone-dimensional difference model for unsteady heat transfer of wall was established by the finitedifference method. The model was verified and the mechanism of heat transfer and storage wasanalyzed deeply. Heat transfer in the multiplayer walls was analyzed systematically based on thecombination of the heat transfer model in walls and the thermal environment model of the solargreenhouse, which could provide theory support for optimizing the design of wall structure. The mainstudies and results were as follows:1）A one-dimens ional difference model for unsteady heat transfer of wall was established by thefinite difference method and the corresponding computer program was achieved with MATLABprocedures according to the one-dimensional heat transfer theory, then a simulated heat transferexperiment was conducted in a polar greenhouse with earthen walls in Shouguang, Shandong province,to test the model. The result showed the simulation result of the model was identical with that of thepolar greenhouse experiment with the average correlation coefficient of0.72.2）The result of study on the mechanism of heat storage and release in earthen walls confirmed theexistence of an effective heat storage layer. The temperature increased on the internal surface of earthenwalls because of the solar radiation and the temperature of stratified walls decreased layer by layerinside-out, as a result, the heat was stored up in the inner layers during the day. A portion of heat storagein inner walls was released gradually into the solar greenhouse to raise the temperature during the night,and the wall layer that could release heat into the greenhouse was defined as the effective heat storagelayer. Another portion of heat was transmitted into walls with relative low temperature by heatconduction and at the end of the transmission a fraction of heat was lost outside of the greenhouse. Thedata collected during December,2012and January,2013showed the thickness of effective storage layerwas0.26-0.39m in the3.00m-thick wall and proved the existence of a thermal-stable layer in the middleof wall (0.5-2.5m) in which changes in the temperature and heat flux was not observed.3）The results of the simulated experiment under typical weather condition showed the thickness of240mm could be considered as the recommended parameter for the heat storage material. A simulationexperiment about heat conduction process of17different composite walls of Chinese solar greenhousewas conducted with reference to the outdoor climatic parameter collected in December,2012inTongzhou district, Beijing. It turned out that the heat storage and release was mainly related with the material and thickness of inner side of the wall. The heat storage time was long and the quantity of heatrelease was relatively stable when the material with good thermal storage properties and a thicknessover120mm was used in the inner layer of the greenhouse wall. The thickness of240mm of concretematerial in the inner wall layer was optimal for the whole heat environment in the solar greenhouse, andthe significant increase in the temperature inside the greenhouse was not found with the thickness ofwall material increasing up to480mm.4）An optimized construction mode was obtained through the simulation experiment. It was a betterchoice to use materials with high thermal storage coefficient in the inner side of the greenhouse wallbecause the higher the specific heat capacity, the heat conductivity coefficient and the density of thematerial were, the better the ability of heat storage and release was, while materials with low heatconductivity coefficient but high specific heat capacity was better for the outside of the greenhousewall.