Similar to fogging systems, pad and fan systems operate by evaporative cooling: the cooling happens due to usage of as the heat in the greenhouse air is used to evaporate water to water vapour. These systems consist of fans on one wall, and cooling pads with water circulation on the opposite wall. The fans pull in air from outside the greenhouse, which runs through cooling pads: as the water on the cooling pads evaporates using the energy from the air, the air becomes cooler and humid as it blows over the greenhouse crops and is expelled via the fans on the opposite wall.
The cooling that can be achieved with this method depends on the humidity of the air outside the greenhouse , and it works best for hot, dry air- the drier the air, the bigger the cooling effect. As the humidity of the outside air increases, the extent of achievable evaporative cooling is lesser.
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Depending on how much greenhouse area needs to be calledcooled, considerable energy may be required to move a huge volume of air across the greenhouse.
Another resource which is consumed in large quantities is water, which may can pose problems in hot and dry, water-scarce areas where such cooling might typically be required.
Possible steps towards sustainable, energy-efficient greenhouses
The energy demand for the fans can be supplied by renewable resources.
The evaporated water might be recovered, for instance To effectively limit the cooling power of the pad-and-fan system to the interior of the greenhouse, rather than cooling the outside air, natural ventilation should be avoided.
Possible improvements to water-use efficiency, such as recovery of the evaporated water by using cooling elements outside the greenhouse , powered by renewable resource. Another option to improve the water-use efficiency might be to use desalinated water, again powered by renewable energy.or the use of desalinated water using renewable energy sources, are not yet feasible in terms of energy or cost efficiency.
The energy demand for the fans could be supplied by renewable resources. Moreover, the active cooling using pads and fans could be coupled with more passive means of temperature regulation, such as using near-infrared (NIR) reflecting cover materials.
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