| Solar greenhouses play a crucial role in ensuring the supply of fruits and vegetables during winter in northern China.In order to increase the indoor temperature,the greenhouse is closed most of the operating time in winter,which is easy to lead to high humidity and frequent pests and diseases.Ventilation is a low-cost moisture removal method often used in solar greenhouses in winter,and the natural ventilation with the opened upper vent is the mainstream ventilation mode used for solar greenhouses.This model has the advantages of simple structure,convenient operation,but there are also low moisture removal efficiency,heat loss and other shortcomings,resulting in the ventilation process of heat preservation and ventilation and moisture removal contradiction is prominent,not easy to reconcile.In view of the unclear inner mechanism of the low efficiency of natural ventilation heat preservation and moisture removal,and the lack of the ventilation strategy with efficient heat preservation and moisture removal efficiency,this paper quantitative analysis of the temperature and humidity spatial coupling heterogeneous law based on the optimally-constructed temperature and humidity monitoring system,explore the causes of the inefficiency of the natural ventilation moisture removal and heat loss,and propose the active ventilation strategy which is fittable for the characteristics of the coupled heterogeneity of the temperature and humidity.The paper mainly includes the following five research contents.(1)The optimization of a temperature and humidity monitoring system for solar greenhouses was proposed through a weighted Hilbert-Schmidt Independence Criterion(HSIC).In order to accurately depict the overall trend and heterogeneous distribution of temperature and humidity,a sensor optimization configuration strategy based on weighted HSIC was proposed.This strategy involved prioritizing algorithm according to the cooperative operation of primary and secondary sensors and the thresholds of monitoring Root Mean Squared Error(RMSE)and Information Gain Rate(IGR).Consequently,monitoring errors of temperature and humidity in the vertical direction were reduced to 0.23°C and 0.58%,respectively,with corresponding IGR of 4.8%and 8.7%.This optimized strategy not only mitigated sensor redundancy but also facilitated precise and comprehensive monitoring of temperature and humidity spatial heterogeneous distribution.(2)Quantitative study of temperature and humidity coupling heterogeneous distribution based on multi-field coupling CFD model was carried out.In order to quantitatively reveal the coupled heterogeneous distribution of temperature and humidity in the solar greenhouse and its influencing factors,a full-size solar greenhouse multi-field coupled CFD model was built.The multi-field coupling mechanism of temperature,humidity and airflow was analyzed.The coupled influence of thermal buoyancy and crop transpiration on the spatial heterogeneity of temperature and humidity was investigated,and it was clarified that the intensity of solar irradiation is the main factor driving the spatial and temporal evolution of the coupled heterogeneity of temperature and humidity.When the intensity of direct solar radiation is higher than 900W/m~2,the degree of indoor temperature and humidity coupling heterogeneity is significant,which provides a theoretical basis for the design of the subsequent active ventilation strategy.(3)Conducting research on the change of temperature and humidity under natural ventilation mode and the causes of inefficient air exchange.On the basis of the clear temperature and humidity coupling heterogeneity distribution,the ventilation efficiency evaluation system including ventilation rate,heat to moisture ratio,moisture content,enthalpy and other indicators was constructed.After measurement and calculation,the ventilation rate is about 4～7 m~3/m~2/h for only opening the upper vent,and the daily average indoor air temperature,moisture content,relative humidity,enthalpy,and heat-to-moisture ratio are16.2℃,9.7g/kg,84%,40.5k J/kg,and 0.64,respectively;then the experimental research on natural ventilation based on the CFD model was carried out.It was clarified that the indoor airflow field and temperature and humidity field were influenced by the ventilation parameters and environmental wind speed.Furthermore,it was revealed that the mismatch between the heat and mass transfer process of natural ventilation and the spatial heterogeneous distribution characteristics of temperature and humidity is an important cause of the low efficiency of natural ventilation for heat preservation and moisture removal in solar greenhouses.(4)The active ventilation strategy of solar greenhouse adapted to the coupled spatial heterogeneity of temperature and humidity was constructed.Based on the systematic coupled spatial heterogeneity of indoor temperature and humidity and the flow direction of the natural ventilation mode,it is proposed to use active ventilation to change the indoor airflow direction,for matching the heat and mass transfer process with the spatial heterogeneity of temperature and humidity in the ventilation process,which significantly enhances the heat preservation and moisture removal of the greenhouse.Furthermore,the response surface method is used to optimize the operational parameters of active ventilation mode,and the significant order is ventilation rate,solar radiation intensity,and wind speed.When the above parameters were 7.3m~3/m~2/h,912.4 W/m~2 and 0～1.9 m/s respectively,the ventilation effect was optimal,and the heat-to-moisture ratio could reach the maximum value of 1.46,which provided an important data reference for the real experiment.(5)The experimental verification and optimization of the active ventilation strategy for heat preservation and moisture removal efficiency were carried out.An active ventilation scheme for solar greenhouses was designed and deployed,with four axial fans installed at the bottom of the greenhouse as the power source to regulate the direction of indoor airflow,realizing the active control of indoor airflow and heat and mass transfer process.The test results show that the active ventilation rate can be adjusted steplessly from 0 to 30 m~3/m~2/h,which is more adaptive to the indoor and outdoor climate and conducive to the precise control of ventilation.Compared to natural ventilation,the average daily temperature,heat-to-moisture ratio and enthalpy of the active ventilation under sunny,cloudy and cloudy,is higher than,and relative humidity is lower,which reflects a better meteorological adaptive ability and stability.The average daily temperature under sunny weather is increased by 2℃～2.7℃,average daily relative humidity is reduced by 15%～17%,and the average daily heat-to-moisture ratio can be increased by 31%～39%,which improves the heat preservation and dehumidification efficiency significantly.At the same time,the return-to-investment ratio ROI of the active ventilation strategy is 2.62,which is capable of obtaining the ideal returns with lower investments,and the economic feasibility is good. |
PDF Full Text Request