Comparative studies of patterns of variation of daily temperature differences inside the north wall in Chinese solar greenhouses of three different designs during cold periods

摘要

Three Chinese solar greenhouses were selected in Jinzhong city, Shanxi, in which the thickness of the bottom of the north wall was 6.6 m (for a digging-down greenhouse), 2 m (for a clay-wall greenhouse) and 0.5 m (for a brick-mixture wall greenhouse).The inner spans of the greenhouses were 15,10 and 9.25 m, respectively. The temperature inside the north wall (measuring points were set every 5 cm from the inner surface to the outside) was measured every 0.5 h at 1.5 m height above the ground during the cold period, and daily temperature differences (TemDIF) of each measuring point were analyzed. Our results showed that the heat exchange trends inside the north wall were basically the same, although the structure and thickness of the north walls weredifferent. According to the daily variation patterns of TemDIF in each layer, the north wall could be divided into three layers, a heat exchange layer, a heat buffer and a heat stability layer, which were located at 0-15,15-25 and beyond 25 cm depth, respectively, from the inner surface of the north wall to the outside. The TemDIF in these layers was greater than 5°C (up to 25°C), 2-5 and 0-2°C, respectively. The change point/inflection point between layers of heat exchange and heat stability was located at 17-22 cm based on our linear regression analyses. The daily TemDIFs at those points were not very different, which was related to the mode of heat exchange (heat conduction) of the north wall. From winter to spring, the location of the change point/inflection point did not change significantly, but the daily TemDIF rose a little. This increase was related to changes in solar elevation angle and ventilation. Based on our results, the suitable thickness of the north wall in a Chinese solar greenhouse in Jinzhong, Shanxi Province, and similar areas should be around 30 cm. The insulation layer can then potentially be replaced by a thinner but highly insulating material.