GREENHOUSE COOLING When greenhouse ventilation is not sufficient for cooling the greenhouse to a desired temperature other cooling techniques have to be applied. The large quantity of heat that has to be removed from greenhouses, especially in warm regions, precludes the possibility of using mechanical refrigeration. Therefore, all commercial greenhouses that utilize a cooling system use evaporative cooling which is much cheaper. Evaporative cooling is based on the conversion of sensible heat into latent heat. When non-saturated air comes in contact with free moisture and the two are thermally isolated from outside heat source, there is transfer of mass and heat. Because the vapour pressure of the free water is higher than that of the unsaturated air, water transfers in response to the difference in vapour pressure. The transfer involves a change of state from liquid to vapour, requiring heat for vaporization. This heat comes from the sensible heat of the air and the water resulting in a drop of temperature in both of them. Since no outside heat is added during this process, it can be assumed that the total heat (enthalpy) of the air does not change. The lowest temperature the air can get is the wet bulb temperature (when the air is completely saturated). Under practical greenhouse conditions, however, the air does not become completely saturated. In this process the wet bulb of the temperature is remained constant, the dry bulb temperature is lowered and the relative humidity is increased. The efficiency of the cooling unit is the ratio between the change in saturation actually achieved to the potential change in saturation.
Several systems, can satisfactorily provide cooling by evaporating water: sprinkling, fan and pad and fogging or misting.
Sprinkling Spraying water onto a surface of the roof and the canopy using sprinklers enlarges the free surface of the water and hence increases the evaporation rate. The evaporation process causes cooling of the canopy and of the air in the immediate vicinity, in accordance with the local microclimate. The advantage lies in the low cost. The main disadvantage of this method is the creation of conditions favorable to the development of fungal diseases. Also, sprinkling usually results in scalding and deposition of precipitates on the surfaces of the leaves and the fruits, especially when water quality is poor.
Fan and Pad Outside air is sucked into the greenhouse through the wet pad, and is thus humidified and cooled. From the wet pad, this air flows through the greenhouse, absorbing heat and water vapor, and is removed by the fans at the opposite end. The advantages of this method lie in its simplicity of operation and control and also in that it does not entail any risk of wetting the foliage. The main disadvantages are: relatively high cost; lack of uniformity of the climatic conditions, which are characterized by rising temperature and water vapour (due to solar heating and transpiration of the plants) along the length of the structure and in the flow direction; electric power failure transforms the greenhouse into a heat trap; low cooling effect compared with a fogging system; and waste of water – to prevent blockage of the wet pad, water-bleed is necessary.
This method is based on supplying water in the form of the smallest possible drops (in the fog range – diameter 2-60 mm) so as to enhance the heat and mass exchange between the water and the air. This is because (for a given quantity of water) the surface area of the water in contact with the air increases in a direct relationship to the diminution in the diameter of the drops. Also characteristic of drops in this size range is that the frictional forces arising from movement of the drops through the air are relatively large, so that the terminal velocity of the falling drops is low, which results in a long residence time, allowing complete evaporation of the drops. Furthermore, because of their small size these drops are properly carried by the airflow. These combined characteristics ensure highly efficient evaporation of the water, while keeping the foliage dry. The high efficiency is because, in addition to the evaporation of water to cool the air that enters the greenhouse (similarly to the wet pad), it is possible to evaporate water in quantities sufficient to match the energy absorbed in the greenhouse. Most fogging systems are based on high-pressure nozzles, characterized by low cost and high cooling effect.