Fan and Pad Evaporative Cooling System for Greenhouses: Evaluation of a Numerical and Analytical Model

Abstract

An experimental greenhouse equipped with a fan-and-pad evaporative cooling system is analyzed using two different models. The first model consists of a numerical simulation approach applying a commercial CFD code. The main aspects of evaporative cooling systems, in terms of heat and mass transfer, and both the external and internal climatic conditions, were integrated to set up the numerical model.The crop (tomato) was simulated using the equivalent porous medium approach by the addition of a momentum and energy source term. The temperature and humidity of the incoming air, the operational characteristics of exhaust fans, and the pressure drop occurring in the pad were specified to set up the CFD model.The second model considers the greenhouse as a heat exchanger. Based on greenhouse structural characteristics, external climatic conditions, pad efficiency, and ventilation rate, the air temperature distribution is predicted.The results concerning the air temperature, provided by both the numerical and analytical models, were validated by experimental measurements obtained at a height of 1.2 m above the ground in the middle of the crop canopy. The correlation coefficient (R²) between computational results and experimental data was of the order of 0.96 for the numerical model and 0.77 for the analytical model, with average percentage errors of 3.5% and 7.6%, respectively.The analytical model proved to be a useful and simple evaluation tool, whereas the numerical model provides a more accurate overview of the airflow in the greenhouse. The results show that the fan-and-pad evaporative cooling system can be effectively parameterized in numerical terms in order to improve the system’s efficiency.