Design and verification of an active ventilation strategy using the heterogeneous distribution of temperature and humidity for solar greenhouses in winter

Solar greenhouses have been widely used to provide abundant fruits and vegetables in the cold regions of China in winter. The microclimate in the greenhouse is crucial to the crop quality and yield. But the greenhouse environment can often be regulated for dehumidification and carbon dioxide supplementation using low-cost natural ventilation. The low indoor temperatures have resulted from the significant heat loss. In this research, active ventilation was proposed to reduce the heat loss for the better suitability of winter greenhouse environments using the spatial distribution of temperature and humidity. Firstly, the differences in temperature and humidity were quantitatively analyzed in the different regions of the greenhouse. 28 sensors of temperature and humidity were deployed inside and outside the greenhouse. The data was collected for 115 days during winter and spring, respectively. A field test was performed on the winter greenhouses in Shandong Province, China. The results show that there were significant differences in temperature and humidity in the different areas. The upper-middle areas shared the higher temperatures and lower humidity, while the main growth area of the crops, i.e., the lower-middle area, had the lower temperatures and higher humidity. The maximum accumulated temperature difference between the upper and lower areas during winter can reach up to 300℃, which is 30% of the average accumulated temperature, and the relative humidity difference during daylight hours can be up to 20 percentage points. This heterogeneous distribution was attributed to the substantial heat dissipation and the low dehumidification efficiency that was caused by the replacement of warm, dry air at the top with outside air during natural ventilation. Secondly, an active ventilation dehumidification was proposed to utilize the temperature and humidity heterogeneous distribution. The axial fans were installed at the bottom of the greenhouse. The direction of airflow was adjusted to remove the cold and humid air from the bottom, while the top air was replenished with outside air, in order to realize the efficient convective exchange of natural ventilation. Furthermore, 4 axial fans were installed at the bottom of the greenhouse. The maximum ventilation capacity was achieved at 5 300 m³/h when each fan with a power of 550 W. The uniformity of the airflow field was also achieved inside the greenhouse during the ventilation process. A 70-meter-long corrugated pipe was laid along the east-west direction of the greenhouse, where evenly spaced openings faced the greenhouse. The wet and cold air was preferentially removed from the lower part of the greenhouse while retaining the dry and warm air in the upper part, in order to improve the dehumidification and insulation. Thirdly, an evaluation system was constructed for the insulation and dehumidification performance of the ventilation. The indicators included the ventilation rate, daily average temperature-humidity ratio, and return on investment. The active ventilation rates were finely adjusted from 0 to 30 m³/(m²∙h), compared with the less controllable natural ventilation subjected to the indoor and outdoor climates. The daily average temperature-humidity ratio with the active ventilation was consistently higher than that with the natural ventilation. The better meteorological adaptability and stability were obtained under different weather conditions, such as sunny, cloudy, and rainy. In particular, the daily average temperature-humidity ratio increased by 33.1%-32.9% in sunny, whereas, the absolute humidity in the air decreased by about 1 g/kg. The indoor temperature increased by about 2.0-2.7 ℃, indicating the better performance of the insulation and dehumidification. The active ventilation still provided significant dehumidification and thermal insulation performance in cloudy weather. The relative humidity was reduced by about 15 percentage points, whereas, the average temperature increased by about 2-3 ℃. However, both ventilation were affected by the outside climate. The average temperature was about 2 ℃ lower in the greenhouse, compared with sunny weather. While the relative humidity was about 5% higher. The active ventilation also shared significant thermal insulation in rainy weather, where the indoor temperature increased by about 2-5 ℃. The dehumidification reduced the relative humidity by about 9 percentage points, due to the high humidity of the outdoor climate. Therefore, active ventilation was achieved in the dehumidification and thermal insulation under different weather conditions. Finally, the average annual investment and expected returns from active ventilation were calculated to significantly improve the crop yield and quality. The return on investment (ROI) of the active ventilation strategy was 2.62 for the economic benefits with relatively low investment, indicating the economic feasibility. The active ventilation with the spatial distribution of temperature and humidity was efficiently achieved in the dehumidification and insulation, leading to higher returns under indoor temperatures. The findings can provide theoretical and practical implications for dehumidification and insulation in winter greenhouses.