Heat transfer process of soil wall in Chinese solar greenhouse and its theoretical simplification methods
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Abstract
Abstract: Soil wall of the Chinese solar greenhouse (hereafter referred to as "solar greenhouse") has problems of occupying large area and damaging the cultivation land. The simplification of soil wall, which means decreasing the thickness and soil use of the soil wall, becomes very important. The purpose of this study is to develop simplification methods of soil wall. A simplification wall with less soil use was proposed based on the measured temperature of soil wall and analysis of feasibility of those methods. The tested solar greenhouse was located in Yongqing county, Lanfang city, Hebei province (116°44′ E, 36°27′ N). It is 50 m long and 10 m wide. The top and bottom thicknesses of the soil wall were 2.0 and 5.3 m, respectively. Its average thickness was 3.6 m. The test period was from Dec. 01, 2013 to Mar. 30, 2014. During that time, the tested solar greenhouse was used to growing cucumber with surface irrigation. The heat insulation sheet of the solar greenhouse was rolled up and down at 8:30 am and 5:00 pm daily, respectively. The wind vent was open if the indoor air temperature was high during daytime. The indoor and outdoor air temperatures, solar irradiating on the inner surface of the wall, the temperatures in the soil wall and indoor soil were measured continuously at a time interval of 10 min. The data collected in a typical cloudy day ( 08:30 am of Dec. 29, 2013 to 08:30 am of Dec. 30, 2013) and a typical sunny day (08:30 am of Jan. 16, 2014 to 08:30 am of Jan. 17, 2014) were used to study the heat transfer pattern of the soil wall. Based on the measured temperature in the soil wall, the soil wall can be divided into heat storage layer and heat insulation layer. The heat storage layer had large temperature fluctuation and can be used for storing heat during daytime and release heat into the solar greenhouse during nighttime. The temperature of the heat insulation layer was lower than that of the heat storage layer and mainly used to prevent the heat in the heat storage layer from losing. Under the test conditions, the thicknesses of heat storage and insulation layers were 47 cm and 313 cm, respectively. Considering that the heat resistance of the heat insulation layer equals that of 7 cm polystyrene board, a composite wall with 7 cm polystyrene board and 47 cm rammed soil in the direction from exterior to interior was proposed. The results showed that under same conditions, the differences between the measured inner surface temperature of the soil wall and the simulated inner surface temperature of the composite wall was less than 5% in both sunny and cloudy days. The application of the polystyrene board can reduce the thickness and occupied area of soil wall by 85.0% and 89.8%, respectively in comparison with the conventional soil wall. On the other hand, the heats released by the indoor soil during the nights of sunny and cloudy day were 1.3 and 2.9 times more compared to those released by the soil wall. According to the simulation results, by increasing the 20 cm surface soil temperature from 17.0℃ to 23℃, the heat released by the indoor soil during nighttime were more than the measured heat released by both soil wall and indoor soil. In this case, the soil wall can be replaced by the wall build with thermal insulation material only. The thickness of soil wall can be further decreased. We concluded that the soil wall can be simplified by the following methods: 1) building its heat insulation layer with thermal insulation material, or 2) building the wall with thermal insulation material and increasing the indoor soil temperature.
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