Modelling of longwave radiation exchange at greenhouse surfaces under all-sky conditions

Abstract

Accurate and detailed longwave radiation heat transfer models are necessary in modern protected cultivation especially for greenhouses. For this reason, the study focuses on modelling the longwave radiation exchange between glass-covered greenhouse surfaces and the sky taking into consideration representative test conditions. Apart from the surface design and the thermal properties of the cover, the key meteorological parameters influencing longwave radiation models include air temperature, cloudiness and relative humidity. To model the downwelling longwave radiation under all-sky conditions, an effective atmospheric emissivity is required, which depends on the cloudiness of the sky. To achieve this, 10 typical clear-sky atmospheric emissivity equations were selected from the literature and their performances assessed. From comparative statistics, the Sugita and Brutsaert (1993) model produced the best results for emissivity estimations both at night and during the day. To work with the model a cloudiness factor was derived from analyzed weather maps and with cloudiness octas (eighths) assigned by weather watchers. The best simulation results for both the downwelling and the upwelling longwave radiation were obtained with the map-based cloudiness factor. However, the errors related to the model performance with the two cloudiness prediction approaches were not significantly different. The thermal emissions were weighted according to the computed view factors and these enhanced delivery of reliable results in the simulation models. The most sensitive parameters in the longwave radiation models were found to be the air temperature and the surface-to-air temperature difference (sensitivity index SI > 1). Furthermore, the SI of the models with respect to the cloudiness was always higher than with respect to the relative humidity. Overall, precise measurement or estimation of atmospheric parameters is important in accurate modelling of the exterior longwave radiation exchange.