For pressure to be released, the warm air is vented and then displaced by cooler air from outside the greenhouses. Warmer, more humid air inside the greenhouse expands and rises causing internal pressure. This is made possible through strategic vent placement. The wind blowing outside your greenhouse causes a pressure gradient between the windward and leeward side of the greenhouse creating a vacuum which results in natural air flow. This setup takes advantage of the following: It’s possible to get excellent, even cooling throughout your greenhouse if your greenhouse structure incorporates a well-designed natural ventilation system consisting of roof vents and side vents. The ventilation areas should at least be 25-30% of the greenhouse floor area for most of our local South African regions. Ventilation openings can be optimized in order to cool the entire greenhouse, even in low wind speed conditions. Natural ventilation creates a better internal climate for greenhouses, especially in humid, tropical and subtropical regions. This makes it the cheapest method for cooling greenhouses. It requires less energy when compared to pad and fan cooling and in some cases, where there are fixed ventilation openings, zero energy is needed. Natural ventilation is the result of pressure differences created by wind and temperature. Natural, Passive Ventilation with High Pressure Fogging Water consumption is higher for pad and fan cooling when compared to natural ventilation used in tandem with high pressure fogging. The increased humidity in the greenhouse area closer to the pad side increases the risk of plant diseases.Ħ. This means that the pad and fan system is mainly effective in dryer climates.ĥ. High external relative humidity limits how much water can be absorbed by the air during evaporation off of the pads which in turn limits cooling. This results in an inconsistent temperature gradient throughout the greenhouse, thus negatively affecting overall yields.Ĥ. Air is warmed by the sun and the radiation off the plants and soil as it moves from the pad side to the fan side of the greenhouse. Maintaining uniform air temperatures throughout the greenhouse is essential for growing high yield crops. Incorporating a pad and fan cooling system comes with increased maintenance costs.ģ. Solar energy can be used as an alternative, but the upfront costs associated with powering a large commercial operation can also be considerable.Ģ. The entire operation has to be placed on back-up power to prevent losses during power outages (in the event of a power outage, the internal climate can easily reach 50-60 ☌ within minutes). The fans have to run throughout the whole day and during the entire year. This is a popular method for cooling greenhouses and can reduce temperatures significantly (between 5-10 ☌ depending on the relative humidity) but it has various disadvantages:ġ.Ğnergy is required to power the fans and depending on how much air needs to be moved, it can be an expensive exercise affecting profitability. As the water evaporates, it absorbs heat and the air vented into the greenhouse is cooled.
These pads consist of radiators over which water is run.
The evaporative pad and fan cooling system works in the following way:Īn exhaust fan sucks warm air out of the greenhouse creating air flow over evaporative cooling pads. In this article we will be comparing the forced evaporative pad and fan cooling system to natural greenhouse ventilation.įorced Ventilation with Evaporative Pad & Fan Cooling These solutions are either passive (natural) or forced (mechanical). It therefore goes without saying that an effective greenhouse cooling system is one of the most important considerations for greenhouse growers.ĭepending on your external climate, greenhouse volume and how much air needs to be changed, there are various approaches for achieving optimum ventilation. High greenhouse temperatures are massively detrimental to optimum plant health and growth which obviously has a negative effect on commercial greenhouse farming yields and profitability.
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