U.N. FAO GAP Report Recommends Active Dehumidification for Vegetable Greenhouses
In a new Food and Agriculture Organization report, discussing Good Agricultural Practices (GAP) for greenhouse vegetable crops, researcher Ly Minh Cuong recommends active dehumidification.
This recommendation is made to solve the two major problems greenhouse humidity poses – diseases and molds, and high energy consumption.
Humidity Is a Major Problem for Greenhouse Growers
High humidity is inevitable in a greenhouse environment. This is especially problematic from dusk to dawn, when there’s no sunlight, and temperatures outside drop.
“Condensation is heaviest in greenhouses from sunset to several hours after sunrise. During daylight hours, there is sufficient heating from solar radiation to minimize or prevent condensation, except on very cold, cloudy days.”
During these hours, high humidity can lead to condensation in the greenhouse, creating a hotbed for some of the most common greenhouse diseases.
“Condensation is a symptom of high humidity and can cause significant problems (e.g. germination of fungal pathogen spores, including Botrytis and powdery mildew.) Condensation can be a major problem – at certain times of the year, impossible to avoid entirely.”
Why Ventilation Doesn’t Provide an Effective and Efficient Solution
Traditionally, greenhouse growers use a combination of ventilation and heating. This can provide an adequate solution during the day. However, it can quickly become very difficult to reach optimal humidity levels, and the energy costs can skyrocket.
“When the ventilation required to reduce the temperature is less than that needed to remove moisture from the air, dehumidification consumes energy. Warm greenhouse air is replaced by cold dry outside air, lowering the temperature in the greenhouse.”
In the scenario described above, ventilating in order to reduce humidity has a negative impact, by lowering the temperature in the greenhouse. Growers who rely on ventilation combat this with heating, which can become very costly, especially as energy prices rise.
Active Condensation-Based Dehumidification Is Effective and Efficient
The report mentions active dehumidification, based on condensation, such as DryGair, as an effective solution to reduce humidity.
“Wet humid air is forced to a cold surface located inside the greenhouse. Condensation occurs on the cold surface, the water is collected and can be reused, and the absolute humidity of the wet greenhouse air is reduced.”
It continues to detail the energy efficiency of such systems:
“One metre of finned pipe used at a temperature of 5°C can remove 54 g of vapour per hour from air at a temperature of 20°C and with 80 percent relative humidity.”
DryGair DG-12 dehumidifiers extract 48 liters of water per hour, at 18°C and 80% RH, running on 12 kW of electricity. At 4 liters per kW, that leads to an average of 50% savings, compared to traditional ventilation.
Dehumidification GAP Recommendations
The FAO report closes out the section on humidity by providing the following recommendations:
When should dehumidification take place?
- Dusk: Reduce humidity to 70–80% as night falls to prevent condensation.
- Dawn: Reduce humidity to prevent condensation, and jumpstart transpiration as the sun rises.
Dehumidification – GAP recommendations
- Remove any excess sources of water in the greenhouse.
- Open the windows or the door to the greenhouse and allow excess moisture to escape.
- Utilize greenhouse fans to improve air circulation.
- Purchase a humidity controller or a dehumidifier for use in the greenhouse.
- Use thermal screens at night to prevent radiative heat loss from plant surfaces.
- Place radiant heat sources near the crop to keep plant surfaces slightly warmer than air.
