Similar to fogging systems, pad and fan systems operate by evaporative cooling: the cooling happens due to usage of the heat in the greenhouse air 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. 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 of greenhouse:
The evaporative cooling lowers the greenhouse temperature and also affects the humidity inside the greenhouse.
Impact on resource use of greenhouse:
Depending on how much greenhouse area needs to be called, 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 pose problems in water-scarce areas.
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 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.
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.