Temperature variations in energy storage layers in Chinese solar greenhouse walls

Chinese solar greenhouse (CSG) walls can be made of a single material or can be layered walls that are conceptually divided into three layers (from the inside to the outside) as the energy storage layer, the thermally stable layer and the thermal preservation layer. The temperature variations in the energy storage layer then greatly influence the thermal characteristics of CSG walls during the winter. This study compares the storage layer temperature variations inside single material walls during clear winter days predicted by an analytical solution of the one-dimensional (1D) transient conduction equation with the storage layer temperature distributions inside two-and three-layer layered walls predicted by a previously validated CFD model. This study used single material walls made of clay brick, gravel, clay with grass or rammed clay with the gravel having the largest thermal diffusivity (9.24×10-7m2/s) and the clay brick having the smallest thermal diffusivity (4.29×10-7m2/s). The single material wall temperature predictions show that for an inside wall surface temperature variation amplitude of 15℃and temperature variation amplitudes at the interface between the energy storage layer and the thermally stable layer of less than 0.1℃, the gravel wall had to be 0.25 m thicker than the clay brick wall. When the inside wall surface temperature variation amplitude was only 5℃, the gravel wall had to be 0.2 m thicker. Also, for interface wall temperature variation amplitudes of less than 0.1℃ and an inside wall surface temperature variation amplitude of 15℃, the clay brick wall had to be 0.54 m thick, but the wall had to be only 0.42 m thick for an inside wall surface temperature variation amplitude of 5℃. The model can predict the required thicknesses of the heat storage layer for any single material wall based on the interior wall temperature variations. The predicted temperature variations in a single material wall and the predicted storage layer thicknesses agree well with the measured values for a rammed clay wall. The simulation results for layered walls also show how the interior temperatures in the heat storage layer change with the insulation layer arrangement. For a total wall thickness of 0.6 m and heat storage layer thicknesses of 0.12, 0.24, 036 or 0.48 m, the temperatures across the energy storage layer decrease more quickly with the thicker energy storage layers. Additionally, the wall temperatures in the energy storage layer decrease more quickly in a layered wall than in a single material wall made of the same material since the energy storage layer thickness of 0.48 m was thinner than the single material wall thickness with the same material. Thus, the expected energy storage layer needs to be thicker than the thermal wavelength in the wall. When the total wall thickness changed from 0.6 to 0.72 m while the heat storage layer thickness was kept at 0.36 m, the average wall temperature in the thermal storage layer increased by up to 1.7℃. The results also show that the required thicknesses of the heat storage layer for layered walls depend on the insulation layer arrangement. The results for single material walls and layered walls give good guidance for wall thickness and wall composition selections for CSG.