Optimization of back roof projection width and northern wall height in Chinese solar greenhouse

Abstract: The typical Chinese solar greenhouse, compared to other types of greenhouse, has the properties of low construction cost and low energy consumption when operating during winter days, which draws the attention of the researcher in the field of facility horticulture. In recent years, the structural parameters, such as span, ridge height and wall thickness in Chinese solar greenhouse have been extensively studied. However, the correlation relationship between reasonable back roof projection width (BRPW) and northern wall height (NWH) have always been inconsistent and require further investigation. In this study, a new method for calculating BRPW and NWH was obtained based on the reasonable periods where the northern wall surface receive direct solar radiation in different latitude regions. Firstly, the average monthly lowest temperature and the average monthly highest temperature were statistically analyzed in some cities from January to June in 1998-2012. The data showed that when the average monthly lowest temperature was close to or exceeded 0 ℃, the average monthly highest temperature was close to or exceeded 30 ℃. Therefore, combined with the traditional Chinese solar knowledge, it is suggested that when the average monthly lowest temperature was lower than 0 ℃, it was necessary to ensure that at least part of the northern wall can receive direct solar radiation between 10:00 to 14:00 each day. Based on this, the reasonable periods where the northern wall surface received direct solar radiation in Chinese solar greenhouse in different latitude regions were obtained. Then according to the change of the sun direct light in the reasonable periods, the method for calculating BRPW and NWH in Chinese solar greenhouse was obtained. The NWH of solar radiation, which were observed at 10:00 am on winter solstice, in the solar greenhouses with different spans, were calculated by using the typical structure parameters of Chinese solar greenhouse in previous studies. The effects of thermal environment analysis were in accordance with the results in literatures. The utilization of the method for modeling the NWH of solar radiation in the well-known Shouguang solar greenhouse in various date confirmed that the actual period where the northern wall accept the direct solar radiation was close to the reasonable period proposed in this study, further suggesting the feasibility of this method. The method has been used to calculate the front roof lighting angle, ridge height, BRPW and NWH in solar greenhouses with various spans in different cities in China. The ratio of BRPW to the span in solar greenhouse ranged from 0.04 to 0.23, which varied in in geographic locations and climatic conditions. When the climatic conditions in different areas were similar, the increase of latitude resulted in an increase of BRPW. The areas with lowest BRPW ratio (0.04 to 0.11) were located in Taiyuan, Xi’an, Lanzhou, Xining, Yinchuan and other low-latitude cold regions. These results suggested that the BRPW can be slightly decreased in cold areas and appropriately increased in warm regions. This method can provide theoretical guidance for optimal design of solar greenhouse in China.