基于温湿度异布的日光温室冬季主动通风策略设计与验证

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

针对日光温室冬季自然通风热量损失大、降湿效率低的问题,该研究在揭示室内温湿度空间异布特征基础上,提出主动通风策略,提高保温降湿效能。通过搭建包含28个温湿度传感器的日光温室物联网监测平台,深入分析了室内温湿度空间分布规律。结果显示,温室上下区域积温差值可达300℃,白天日照时段的相对湿度差值达20个百分点,且呈现温度上高下低、湿度上低下高的空间异布特征,自然通风模式下室内上部高温低湿空气优先与室外干冷空气置换导致其保温降湿效能低下。基于此,该研究提出了主动通风排湿策略,利用安装于温室底部的轴流风机改变气流方向,强迫下部区域低温高湿空气从风机口优先向外排出,使上部区域高温低湿空气逆向沉积保留在室内,有效排湿的同时降低热量损耗。主动通风实地试验结果显示,相比于受室内外气候影响可控性差的自然通风,主动通风率可在0~30 m^(3)/(m^(2)·h)之间无级调节,有利于通风的精准控制;晴朗、多云、阴雨3种典型天气下主动通风的日平均温湿比均高于自然通风,体现出较好的气象适应能力和稳定性;其中,晴朗天气下日均温度可提高2.0~2.7℃,日均相对湿度可降低15~17个百分点,日均温湿比可提高31.1%~32.9%,保温排湿效能改善明显。同时,主动通风策略的投入产出比为2.62,能够以较低的投入获得理想的经济收益,经济可行性较好。该研究所提出的主动通风排湿策略可以有效提高保温排湿效能,技术合理性和经济可行性良好,可为日光温室冬季微气候调控提供理论参考和解决方案。

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 5300 m^(3)/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-meterlong 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 m^(3)/(m^(2)·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.