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

An experimental greenhouse equipped with fan and pad evaporative cooling is analysed using two different models. The first one 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 incoming air, the operational characteristics of exhaust fans and the pressure drop occuring 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 both by numerical and analytical model, were validated by experimental measurements obtained at a height level of 1.2 m above the ground in the middle of the crop canopy. The correlation coefficient (R-2) between computational results and experimental data was at the order of 0.96 for the numerical model and 0.77 for the analytical one, with average percentage error of 3.5% and 7.6%, respectively. The analytical model proved to be a useful simple evaluation tool, but the numerical one provides a more accurate overview of the air flow in the greenhouse showing that fan and pad evaporative cooling system could be effectively parameterized in numerical terms, in order to improve system's efficiency.