Studies On Variation Of Soil Wall Temperature And Heat Flux In Sunken Solar Greenhouse

In recent years, in order to increase the sunlight greenhouse thermal insulationperformance, sunken solar greenhouse with thick soil wall was appeared, and this type ofsolar greenhouse was used widely in northern China. However, because of the lack oftheoretical guidance, and this type of solar greenhouse had common problems of too thick soilwall, high damage in cultivated soil layer and lower land utilization rate, so system researchsshould be carried out on the wall and confirmed reasonable constructing parameter of soilwall, and theoretical guidance was provided for practical production.Therefore, from December2011to March2012, representative sunken solar greenhousewas selected for experiment in Shouguang city of Shandong Province. Consecutive test wasconducted on the temperature and heat flux of different depth of the soil backwall,temperature of inner wall at different height, temperature and heat flux of different depth ofsoil in the greenhouse, and indoor and outdoor air temperature and solar radiation. Variationof temperature and heat flux of the wall and their correlation with indoor and outdoor airtemperature and radiation, and variation of temperature of inner wall at different height wereanalyzed, on this base, variation of temperature and heat flux of wall and soil in thegreenhouse were compared. The main research results were as follows:(1) Variation of temperature and heat flux of soil wall was confirmed in winter.In winter, temperature of each layer decreased first and then creased, in accordance withseasonal variation trend of indoor and outdoor air temperature. Overall, wall temperaturecontinued to decline from the inner surface to the outer surface at thickness direction.Temperature and heat flux of inner surface and outer covering surface of the wall varied overa wide range, with the same variation trend of indoor and outdoor air temperature respectively.The amplitude of variation of temperature and heat flux decreased gradually from shallowlayers to deep layers of the wall, and the soil layers of the inner wall with higher rangeabilityof temperature and heat flux were more than that of the outer wall. In successive cloudy days,temperature of each layer of inner wall declined at different degrees, and the layers releasingheat to the indoor deepened constantly, whereas the heat in layers of outer wall mainlyreleased to the outdoor.(2) Relation among temperature and heat flux of soil wall and indoor and outdoor airtemperature and solar radiation was found out.Temperature of inner and outer surface of wall had significant correlation with airtemperature and solar radiation, which had higher correlation with air temperature than with solar radiation, so the wall temperature was affected by air temperature higher than by solarradiation. In the wall, each layer except surface had no direct correlation with air temperatureand solar radiation, however, they had higher correlation with each other, and the neighboringlayers had highest correlation, with the increase of distance, the correlation decreased betweentwo layers.Heat flux of inner wall surface had significant positive correlation with air temperatureand solar radiation, which had higher correlation with indoor solar radiation than with indoorair temperature, so heat flux of inner wall was affected by indoor solar radiation significantly;however, outer surface of covering and the outer wall surface had significant negativecorrelation with outdoor air temperature and solar radiation, which had higher correlationwith air temperature than with solar radiation, so heat flux of outer wall was affected byoutdoor air temperature significantly. Except surface, heat flux of each layer of inner andouter wall had no direct correlation with solar radiation.With increase of depth, the correlation between heat flux of the inner surface to depth0.4m and air temperature decreased, and other layers in the wall had no direct correlation with airtemperature, therefore air temperature and solar radiation mainly influenced on the heat fluxof wall surface. Heat flux of each layer in the inner or outer wall except surface layers hadhigher correlation with each other, and the neighboring layers had highest correlation, and thecorrelation decreased between two layers with increase of distance.(3) Variation of temperature at different height of inner wall was confirmed.In winter, temperature of different height in the inner wall decreased first and then creased,in accordance with seasonal variation trend of indoor air temperature. Overall, walltemperature continued to decline from the surface to deeper layers at different height, andsurface temperature of different height was higher than indoor air temperature，so differentsections of the wall had significant role in maintaining indoor air temperature.With the increase of height, temperature of the same depth continued to decline, thereforeheat flux transferred from the lower section to the upper section continuously. In successivecloudy days in January, the layers of the upper wall releasing heat to the indoor reached0.6m,however the layers of the central and lower wall releasing heat to the indoor reached1.0m, sothe central and lower sections of wall contributed to indoor air temperature more than that ofthe upper section.(4) Variation of temperature and heat flux of wall and soil at different depth in thegreenhouse was compared. In winter, temperature of wall and soil in the greenhouse decreased first and thenincreased, in accordance with changing trend of indoor air temperature. Temperature of walland soil surface varied over a wide range, and the temperature differential between wall andsoil surface on sunny days was significantly larger than that on cloudy days. The rangeabilityof wall and soil temperature continued to decrease with the increase of depth, and variationrange of wall temperature was higher than that of soil at the same depth.Heat flux of wall and soil surface varied over a wide range, and rangeability of heat fluxdecreased with depth increasing. On sunny days, variation range of wall and soil heat fluxwas significantly larger than that on cloudy days, and the time of heat absorption on sunnydays was longer than that on cloudy days. Heat flux of wall and soil surface had largedifference on sunny days, however little difference on cloudy days. Regardless of wall or soil,the layer releasing heat to the indoor was0.2m depth on sunny days, however0.2~0.4m oncloudy days.No matter sunny or cloudy days, heat absorption and release of per unit wall were largerthan that of soil, on the whole, total heat absorption and release of soil larger than that of wall.Heat absorption of per unit wall and soil had significant correlation with accumulated solarradiation, and heat release of wall and soil had significant role in maintaining indoor airtemperature.(5) The suitable thickness of soil wall of sunken solar greenhouse was proposed inShouguang regionAccording to the variation of temperature and heat flux in the wall, the wall could bedivided into the heat storage layer, the transitional layer, and the cold resistant layer frominside to outside, and their thickness was0.8~1.0m,2.2~2.6m and0.4~0.6m, respectively,under the condition of the outer surface with covering. Without considering the transitionallayer, the suitable thickness of the soil wall was1.4~1.6m in Shouguang City.