Most parts of Indian peninsular region witness hot summer, characterised by high ambient temperature and high solar radiation. Radiation intensity puts substantial load on a greenhouse for good part of a climatic cycle. Mere shading does not serve the purpose and the shade-houses fail miserably in summer. A proper greenhouse, fully or partially covered, is therefore a need, particularly when commercial scale cultivation is attempted for plants that are sensitive to environmental parameters like radiation, temperature and relative humidity (RH). India has a very high growth potential in floriculture. But traditional open-field cultivation cannot support this potential business in floriculture, particularly in export market where stringent quality requirements are to be met. Growth and quality of most crops and flowers get affected when temperature exceeds 30ºC and RH drops below 50 per cent. It becomes necessary to remove excess heat load from the greenhouse and enhance RH during hot summer and thus evaporative cooling gets due consideration. When temperature is not at detrimental level, natural ventilation with appropriate shading could be employed. But natural ventilation fails to extract excess thermal load in high radiation periods and in order to maintain conducive microclimate inside the greenhouse, fan ventilation is resorted to. It is to be noted that, while shade nets are quite effective in restricting solar gain of a greenhouse, it also decreases the light transmittance, decreasing plant growth rate.
In evaporative cooling of air, heat as well as mass exchange occurs between air and water. The sensible heat of air is taken up by water, which in turn evaporates and thus, increases the moisture content of the air. As a result, air temperature decreases, the extent of cooling depends on the incoming air RH and the saturation efficiency of the cooling process. Dry air naturally has greater cooling potential than moist air. Thus, evaporative cooling is more suitable for greenhouses in hot and dry climatic regions.
Evaporative Cooling
Evaporative cooling methods which are normally used for greenhouse cultivation are fogging, misting and fan-pad cooling. All these are direct evaporative cooling methods as water is directly applied into the air by means of spray or wetted surface. During evaporation, water takes heat from the warm air, thus, reducing the dry bulb temperature.
Fan-Pad Systems
Fan-pad evaporative cooling (Figure 1) has been in use for over half a decade and is the most common system in closed greenhouses. An exhaust fan is fitted on one end of the greenhouse while a wetted pad is placed on the opposite end. A pump is used to circulate water over and through the wetted surfaces of the pad. Warm outside air is drawn through the pores and channels of the wetted pad when the fan is in operation and, as a result, the warm air loses its heat due to the evaporation of water.
Figure.1: Fan-pad evaporative cooling system
Cooling pads of varied size and geometry are available. Most commercial pads are made of cellulose sheets, structured to give a bee-hive look and air flow channels across the width of the pad. They have good saturation efficiency, about 85-90 per cent, thus, yielding exit air temperature close to the wet bulb temperature (WBT). The air flow velocity through the pad normally remains in the range of 0.5-3.0 m/s. For hot and dry ambient air, a temperature drop of 8-10C is achievable using such cooling pads. Fans are designed to give certain number of air changes per hour or per minute (ACH or ACM) for the greenhouse. An air change of 20-30 ACH could be sufficient for moderate situations but high heat gain situations would call for higher air changes equivalent to 1-1.5 ACM. Usually each span of greenhouse structure is provided with two induced flow fans with a combined capacity matching the desired air changes.
While fan-pad technology is quite matured and widely-used technique in greenhouse cooling, it has got certain disadvantages. There exists a longitudinal air temperature gradient inside the greenhouse and the pad end experiences a higher temperature as the air flows from pad end to the fan end. This restricts the length of greenhouse. The other disadvantage lies in the maintenance of the flow channels in the pads. Clogging of flow channels is a common problem with cooling pads and this occurs as a result of solid deposition on pad surfaces. Pad clogging reduces the saturation efficiency and thus, the pad effectiveness significantly. Unattended pad clogging often leads to insufficient heat removal from the greenhouse resulting in increased air temperature and reduced humidity inside the greenhouse.
To manage the temperature gradient along the length of a conventional fan-pad greenhouse, Misra and Ghosh proposed a longitudinally-distributed system (Figure 2), where cooling pads are placed on the side walls and fans are fitted at central part of the canopy ridge, thus, making a ridge ventilated fan-pad system. Their thermal model predicted that the average temperature in the plantation region inside the greenhouse could be uniformly maintained at a level 5-7C below the ambient temperature (for summer in Kolkata). This design envisaged canopy wall-aligned shade nets, as shown in Figure 2. Pad maintenance and replacement also become easier as each pad frame could be separately handled and could therefore be removed and replaced without affecting the operation of the system. The distributed system, however, leads to a wider water circulation system to cater to a distributed network of cooling pads.
Fig. 2: Longitudinally-distributed fan-pad cooling system
Fogging & Misting Systems
Fogging and misting systems rely on spraying of water into the greenhouse air. Sprayed droplets come in direct contact with air and heat and mass exchanges take place. Foggers are designed to spray finer droplets from high-pressure nozzles. These finer droplets readily disperse into the air and a good part of them evaporate early before they fall on plant foliage. The key feature of the fogging system is high water evaporation rate. Another worthwhile feature is that ventilation fans are no more a necessity and therefore fogging and misting systems find popularity among the growers who rely solely on natural ventilation for their greenhouses to operate. Such naturally-ventilated greenhouses are normally equipped with continuous side vents or roll-up curtains as well as roof vents or continuous roof opening to aid in ventilation and air flow through the greenhouse. Fogging system performance varies widely, depending on factors such as fogging pressure, nozzle orientation and distribution, ambient humidity and air flow. Cooling efficiency usually in the range of 50-80% is observed.
Fig. 3: Schematic of a fogging syste
By using fog-cooling, average inside air temperature in a greenhouse could be maintained 2-4C below the ambient, although higher temperature reduction in the range of 4-8C has also been found in many studies. When the foggers are put on, air temperature rapidly reduces and with efficient fogging nozzles a near-WBT temperature could be reached. But soon after foggers are put off, air temperature rapidly rises. It is observed (Figure 4) that within 3/4 minutes of fogging, air temperature almost stabilises. Spraying beyond 4 minutes hardly reduces air temperature any further, as by that time RH rises to 90 per cent. Prolonged fogging leads to moisture separation and deposition on plants and floor of greenhouse.
Fig.4: Effect of continuous fogging Fig.5: Sequential fogging with short spray
Thus, fogging cycle plays an important role in maintaining the average inside temperature within desired limits. Studies indicate that repeated operation in shorter fogging cycle improves the cooling effect and reduces water loss. Figure 5 shows the findings in an experiment with sequential fogging, maintaining a short spray time of 1.5 min only and varying the interval between consecutive sprays.
Other Evaporative Cooling
Roof evaporative cooling is another system where cooling is achieved by spraying water on the roof of the greenhouse. The water evaporates taking heat from roof and cools the roof surface. Here, roof cooling is independent of the internal humidity level of the greenhouse and the inside air is sensibly cooled because of a colder roof. Presence of a water film over the roof surface also reduces the transmissivity of the surface for solar radiation. Roof cooling can be combined with internal fogging to yield better result. External wetted shade cloths (Figure 6) can be used with roof cooling as proposed by Ghosal.
Fig.6: Schematic of roof cooling with wetted shade cloth
External evaporative cooling of incoming air by forcing it to flow over external wetted surfaces or a surrounding water body could effectively manage suitable microclimate inside the greenhouse. Main advantage of this system is that it could save the energy consumption of a conventional fan-pad system and it does not need a closed and covered greenhouse. This system can operate under natural ventilation mode and fans could also be employed to augment the ventilation rate. Such an evaporative cooling system has been proposed by Misra and Ghosh with dual ventilation mode of operation. They considered an external evaporative cooling of greenhouse air by providing surrounding shallow water ponds and floating wetted surfaces and developed thermal model to predict its performance. It was reported that greenhouse temperature could be reduced by 3-6C as compared to the ambient temperature and with assisted fan ventilation, such system could achieve performance close to that of fan-pad system.
Fig.7: Surrounding water body and floating wetted surfaces
Fig. 8: Circular greenhouse with central solar chimney
Open or partially covered greenhouses that rely on natural ventilation, cannot achieve high rate of ventilation or air changes that is desired for steady growth of plants throughout the climatic cycle. The ventilation rate depends more on the free wind velocity and direction of wind at the location of facility. To enhance the ventilation rate, Misra and Ghosh proposed a new design of circular greenhouse with attached double-wall solar chimney. Studies suggest that incorporation solar chimney can yield ventilation rate 3 to 4 times that of ordinary naturally-ventilated greenhouses. Besides, a circular geometry makes such installation independent of wind direction. Figure 8 shows the arrangement of circular greenhouse fitted with solar chimney at the centre. When the greenhouse is equipped with fogging system it could reduce the greenhouse temperature substantially lower than the ambient air temperature. They reported that greenhouse temperatures could be reduced by 4-6C from the ambient temperature if a fogging cycle of 1.5 min spray time with 2 min interval period is used.
Two-stage evaporative cooling system comprising of a direct evaporative cooling (with cooling pad) and indirect evaporative cooling consisting (heat exchanger) has also been proposed and studied for greenhouse application. Seawater greenhouses also rely on evaporative cooling, using sea water as the working fluid. Such systems could be effectively combined with water desalination for the generation of fresh water in coastal regions.
Conclusion
This article presented an overview of evaporative cooling techniques that are applied in greenhouse systems. Selection of appropriate cooling system depends mainly on local environmental condition as well as greenhouse design and construction. Relative advantages and shortcomings of the different techniques have also been discussed. Some improvement options that can be applied in specific situations have also been discussed.
