Comparing the performance of naturally ventilated and fan ventilated greenhouses

In order to evaluate the comparative performance of natural ventilation (NV) and fan-ventilation (FV) greenhouses, several aspects of cooling were considered: fog cooling and control in NV houses; thermal stratification in FV houses; environmental comparison of NV and FV houses and their suitability under a variety of climates.; Cooling efficiency and evaporation efficiency of low-pressure and high-pressure fogging systems were compared. Data show that the high-pressure system had greater evaporation efficiency and cooling efficiency than the low-pressure system. A control strategy, NCSU control, was proposed to control both the pump and the vents in a high-pressure fogging system for naturally ventilated greenhouses. NCSU control and four other control strategies were tested. It was found that NCSU control cycled the pump less frequently and resulted in higher cooling efficiency. NCSU control was shown to be able to maintain air temperature and relative humidity simultaneously.; Thermal stratification in a fan-ventilated house was studied. Experimental data suggested that the vertical air temperature variation increased with solar radiation. Lower ventilation rates, in conjunction with evaporative pad cooling, also created greater vertical air temperature variations. The presence of a canopy reduced air temperature variation. A model was developed to predict the air temperature distribution in the vertical direction. Simulations suggested that increased ventilation reduced air temperature more in the upper part of the greenhouse. Evaporative pad cooling reduced air temperature more in the lower part of the greenhouse.; The inside environment was compared between NV and FV houses. The spatial variation of air temperature of the FV houses was more than in the NV houses and the air temperature in the NV houses underwent less variation during the course of a day. Leaf temperature was higher than air temperature when evaporative pads were used. The reverse was true when evaporative pads were not used. The cooling abilities of fan-and-pad system and high-pressure fog with natural ventilation were comparable in terms of temperature reduction. Due to lower airflow in NV houses, inside enthalpy was higher in NV houses than in FV houses.; The suitability of fan and natural ventilation with evaporative cooling (evaporative pads and high-pressure fog) was investigated with a simulation model and the weather data of selected locations. It is suggested that the suitability of various cooling technologies depends on climate, evapotranspiration and the ventilation characteristics. The geographical distribution pattern of suitable cooling technologies was examined for the U.S.