Similar to fogging systems, pad and fan systems operate by evaporative cooling: the cooling happens 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 - 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.
Pad and fan cooled greenhouses typically show a temperature gradient across the greenhouse, because the cooled air closest to the cooling pads warms up on moving across the greenhouse by radiation from the sun, soil and plants. The non-uniform temperature gradient could affect overall yields, and it may not be beneficial to have greenhouses that exceed 50 metres in the direction of air flow. Moreover, the extent to which humidity in the greenhouse can be controlled is also low, and the high air humidity close to the pads might also pose a disease risk to the plants.
Care should be taken to maintain the fans well, as well as to maintain the permeability of the pads by avoiding blocking them with salt.
Contribution to energy balance and resource use of greenhouses:
The evaporative cooling lowers the greenhouse temperature and also affects the humidity inside the greenhouse.
Depending on how much greenhouse area needs to be cooled, 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 can pose problems in hot and dry, water-scarce areas where such cooling might typically be required.
Possible steps towards sustainable, energy-efficient greenhouses
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 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.