Thickness determination of heat storage layer of wall in solar greenhouse

Abstract: A wall of the Chinese solar greenhouse (hereafter referred as "solar greenhouse") included the heat storage layer, heat insulation layer and heat preservation layer. The heat storage layer was part of the wall exposed to the indoor environment of solar greenhouse and mainly used to store and release heat during daytime and night time, respectively. The objective of this study was to develop methods to identify the heat storage layer of solar greenhouse wall and then estimate its width. An earth wall with the top and bottom widths of 2.0 and 5.3 m was used. Firstly, it was proposed that the heat storage layer of wall was identified as: 1) The indoor part of the wall, of which the temperature at the end of daytime was 1℃ higher than that at the start of daytime or at the end of night time; or, 2) The indoor part of the wall, of which the temperature fluctuation was over 1℃ during a day. Based on the measured earth wall temperature in a sunny day, the earth wall temperature at the end of daytime was higher than those at the start of daytime and at the end of night time. But with the increase in depth of earth wall, the wall temperature at the end of daytime got close to those at the start of daytime and at the end of night time. At the earth wall depth of 30 cm, the wall temperature at the end of daytime was only 1.0 and 0.4℃ higher than those at the start of daytime and at the end of night time, respectively. According to the first method, the width of heat storage layer was estimated as 30 cm. On the other hand, the temperature fluctuation of the wall decreased with the increase in the wall depth as well. The temperature fluctuation at the earth wall depth of 30 and 40 cm were 3.3 and 0.9℃, respectively. Based on the second method, the width of heat storage layer was estimated as 40 cm. It was noticed that the maximum temperature at the earth wall depth of 30 cm appeared during the period from 20:00 to 23:00. It was 1.4℃ higher than that at the start of daytime. The results indicated that the inner part of earth wall could store heat not only in the daytime, but also in the night time. Thus, the second method was more reasonable than the first. Secondly, a one-dimensional difference model was developed to simulate the temperature fluctuation of earth wall in a sunny day with two assumptions: 1) The heat flux through the homothermal section was zero; 2) The width of heat storage layer was firstly assumed as 60 cm. When the solar-energy absorbance factor of earth wall was 0.5, the simulated width of heat storage layer was 38.5 cm, which was close to that estimated with the tested value. By using different heat fluxes and assumed widths in the model, it was discovered that the effects of above two assumptions on the accuracy of simulated width of heat storage layer could be neglected. Hence, the proposed model can be applied for designing the wall of solar greenhouse.