Modelling greenhouse temperature by means of auto regressive models

In this study, it was investigated to what extent linear auto regressive models with external input (ARX) and auto regressive moving average models with external input (ARMAX) could be used to describe the inside air temperature of an unheated, naturally ventilated greenhouse under Western European conditions. Outside air temperature and relative humidity, global solar radiation, and cloudiness of the sky were used as the input variables. Firstly, different models were built for the first and middle week of each season. The models were suitable to describe the greenhouse temperature evolution satisfactorily, except for the ventilation periods, apparently due to the non-linear effect of ventilation strategies. It was also observed that ARX models performed better than ARMAX models. None of the input variables could be omitted from models for a complete year. It was found that the application of a single model structure for a complete year required frequent retuning. Retuning when the goodness of fit falls below a pre-set threshold, proved to be more efficient than retuning at fixed time intervals in maintaining high accuracy. (C) 2003 Silsoe Research Institute. All rights reserved.