Greenhouse Cooling: Evaporative Pad & Fan versus Natural 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. It therefore goes without saying that an effective greenhouse cooling system is one of the most important considerations for greenhouse growers.
Depending on your external climate, greenhouse volume and how much air needs to be changed, there are various approaches for achieving optimum ventilation. These solutions are either passive (natural) or forced (mechanical).
In this article we will be comparing the forced evaporative pad and fan cooling system to natural greenhouse ventilation.

Forced Ventilation with Evaporative Pad & Fan Cooling
Pad & Fan

The evaporative pad and fan cooling system works in the following way:
An exhaust fan sucks warm air out of the greenhouse creating air flow over evaporative cooling pads. These pads consist of radiators over which water is run. As the water evaporates, it absorbs heat and the air vented into the greenhouse is cooled.
This is a popular method for cooling greenhouses and can reduce temperatures significantly (between 5-10 °C depending on the relative humidity) but it has various disadvantages:
1. Energy is required to power the fans and depending on how much air needs to be moved, it can be an expensive exercise affecting profitability. The fans have to run throughout the whole day and during the entire year. 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 °C within minutes). Solar energy can be used as an alternative, but the upfront costs associated with powering a large commercial operation can also be considerable.
2. Incorporating a pad and fan cooling system comes with increased maintenance costs.
3. Maintaining uniform air temperatures throughout the greenhouse is essential for growing high yield crops. 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. This results in an inconsistent temperature gradient throughout the greenhouse, thus negatively affecting overall yields.
4. 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 means that the pad and fan system is mainly effective in dryer climates.
5. The increased humidity in the greenhouse area closer to the pad side increases the risk of plant diseases.
6. Water consumption is higher for pad and fan cooling when compared to natural ventilation used in tandem with high pressure fogging.

Natural, Passive Ventilation with High Pressure Fogging
Natural Ventilation

Natural ventilation is the result of pressure differences created by wind and temperature. 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. This makes it the cheapest method for cooling greenhouses.
Natural ventilation creates a better internal climate for greenhouses, especially in humid, tropical and subtropical regions. Ventilation openings can be optimized in order to cool the entire greenhouse, even in low wind speed conditions. The ventilation areas should at least be 25-30% of the greenhouse floor area for most of our local South African regions.
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.
This setup takes advantage of the following:

Wind Ventilation
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 is made possible through strategic vent placement.

Ventilation Caused by Convection
Warmer, more humid air inside the greenhouse expands and rises causing internal pressure. For pressure to be released, the warm air is vented and then displaced by cooler air from outside the greenhouses.
Taller greenhouses also enhance the effect of buoyancy. It allows for warmer air to be higher than plant level and also reduces the risk of quick air temperature changes that can affect optimal plant growth.

Enhanced Natural Ventilation
In some cases where warmer climates are involved, you may struggle to get your internal greenhouse temperature close to external temperatures. However, this can easily be mitigated by incorporating shading screens or roll up sidewalls on the windward side of your greenhouse.
With the correct greenhouse design, ventilation can provide temperature control to prevent the extreme build-up of heat during the summer months, thereby controlling excessive humidity and ensuring adequate air exchange occurs to manage carbon dioxide and oxygen levels.

High Pressure Fogging (HPF)
If the above-mentioned natural ventilation options are not sufficient for cooling the greenhouse, they can be used in conjunction with high pressure fogging as a more efficient cooling alternative to evaporative pad and fan systems. A fogging system releases tiny water droplets into the air through high pressure nozzles forming a mist or fog. This air gets evaporated which is enhanced by the natural ventilation system’s air movement. As is the case with pad and fan cooling, this evaporation cools the air. Documented in a recent research paper, experiments were done on naturally ventilated greenhouses in Pietermaritzburg, KZN. It was found that by using HPF, the internal temperature can also be reduced by up to 10 °C (E. du Plessis, 2016). It should be noted that this system requires very good quality water to prevent blockages.

Automated control with Hoogendoorn Growth Management

Each method of ventilation we have described above can be controlled by both the iSii process computer and the latest IIVO process computer of Hoogendoorn Growth Management. Both computers are extremely accurate and the best solution for every automated cooling option in any type of professional greenhouse.

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