Abstract :
A model for fan-ventilated greenhouse cooling is presented in which the primary heat transfer surfaces (cover/structure, canopy and floor) are represented as three parallel planes. Validation of the model was accomplished using data collected over 14 days. Agreement was good, with canopy temperatures over-predicted by only 0·1%, air temperatures in the canopy under-predicted by 0·5%, humidity of the canopy air under-predicted by 1·6% and transpiration rates under-predicted by 1·4%. Simulation runs suggest that when evaporative pad cooling is not used, little advantage is derived from increasing airflow rates beyond about 0·05 m3 m−2 s−1. When evaporative pad cooling is used, however, both air and canopy temperatures decline with increasing airflow rates up to 0·13 m3 m−2 s−1, the highest level considered. Increasing canopy size is predicted to be more influential in reducing air temperatures when evaporative pad cooling is used than when it is not, but its effect on canopy temperature is expected to be approximately the same whether or not evaporative pad cooling is used. With no evaporative pad cooling, the evapotranspiration coefficient (i.e., the ratio of energy used for transpiration to incoming solar energy) is predicted to range from 1·75 for an outside temperature of 36·8°C and an outside humidity ratios of 3·3 g kg−1 to 0·8 for an outside humidity ratio of 29·9 g kg−1 at the same temperature. With evaporative pad cooling, the coefficient is predicted to range from 0·6 to 0·8 at the same outside temperature and the same range of outside humidity ratios.
