Analysis of Radiative and Convective Heat Exchange at Greenhouse Walls

Abstract

A calculation method was developed to analyse the heat flows of a greenhouse wall by combining steady-state heat balance calculations and a simulation model of the airflow near the wall and the adjacent heating system. Heat exchange by radiation between the wall, heating pipes, interior and exterior of the greenhouse were taken into account by deriving radiation absorption factors from the geometry and optical properties of the surfaces. Convective heat transfer coefficients at the wall were determined with an airflow simulation program. The accuracy of the simulation modelling technique used, was validated by comparing simulation results of detailed problems with data from literature. The local convective heat transfer coefficient at the inner side of the wall of a standard Venlo-type greenhouse appears to vary with height between 4 and 9 Wm -2K -1 and is on average 6·7 Wm -2 K -1 under Dutch design conditions. The analysis showed that about 20% of the heat released by the heating pipes near the wall, is radiated towards and absorbed by the wall and about 30% of the total heat loss through the wall is compensated by radiative exchange from the greenhouse interior. Predictions of the vertical temperature distribution of a greenhouse wall in practice were within 1 to 2 K.