Greenhouses capture sunlight energy. Radiation trapped in the greenhouse will release heat as photons interact with matter and lose energy. While abundant sunshine promotes vigorous and rapid growth, the extreme heat makes utilizing cooling systems mandatory during the hot summer months, especially in Mediterranean climates such as Greece. Different cooling methods have both advantages and drawbacks. The two major technologies currently used for greenhouses are either evaporative cooling or chiller-based cooling.
When water evaporates, air molecules are slowed and cooled, and the result is that air temperature drops. Because of their simplicity and low power requirements, evaporative cooling technologies are certainly the most energy and cost efficient solutions for cooling greenhouse spaces.
One of the most common systems using evaporative cooling is the Wet Pad and Fan system. In this set up, large pads made from a porous, cardboard-like material are installed in the walls of one side of the greenhouse, while on the other side powerful exhaust fans are positioned, which are capable of replacing the volume of air in the greenhouse by 14 times each hour. Water is pumped through a pipe system to the top of the pads, where it is distributed evenly onto the pads. The pad material is structured in thin wave-like layers, maximizing surface area and H2O absorption. The moisture absorbed onto the pads then evaporates readily as the exhaust fans pull air through the porous material. The water bearing, cooler air spreads through the greenhouse evenly, with the help of oscillating air fans, takes in heat from the surroundings, and is evacuated by the extraction fans as freshly cooled air is pulled through the wet pads, continuing the process.
Another evaporative cooling method is the high pressure fog system. This set-up bypasses the need for wet pads. Instead, a very fine mist of H2O is emitted into the greenhouse space at extremely high pressure. This fog is readily evaporated, and the cooled air is distributed through the greenhouse by a combination of high-powered exhaust fans and air circulation fans, similar to the Pad and Fan system. The placement and quantity of fog emitters, exhaust fans, and air circulation fans determines how effective the system will be at evenly distributing the cooled air to the plant zone within the greenhouse.
While typically requiring a much lower energy input and easier and cheaper maintenance than chillers, evaporative cooling does raise the amount of water vapour contained in the greenhouse air, and thus increases humidity. Depending on the growth phase of the plants, excess humidity may cause mold, mildew or insects to attack the developing flowers. Therefore, monitoring humidity levels is important when using these techniques. Usually, in Greece, when it is hot, it is also dry, so the resulting increase in humidity will not be substantial enough to negatively effect the cannabis plants. In climates which are hot and also have high ambient humidity, the wet pads are not effective because the warm, humid air flowing through the pads will already be close to saturation, and will not be able to evaporate the cool water on the pads.
Chillers use a “working fluid”, or refrigerant, which undergoes a closed loop cycle of expansion, evaporation, compression and condensation, to extract and evacuate heat from the greenhouse. Air or water chilled units differ only in that either air or water is used to cool the hot refrigerant in the condensation phase.
A metering valve acts as the expansion device, controlling the flow of refrigerant into the evaporation chamber. As the refrigerant expands and evaporates due to lower pressure, it extracts heat, from a water or a water/glycol solution via a heat exchanger, a device in which the two fluids do not come into physical contact, but can exchange thermal energy. The water or water/glycol mixture originally gained this heat from the ambient hot air in the greenhouse. After the heat is absorbed by the refrigerant, it must be evacuated from the greenhouse environment, to keep the ambient air temperature down.
The now hot refrigerant is then further heated by the compressor, because increasing pressure causes an increase in thermal energy. Finally, the compressed refrigerant enters the condenser which is also a heat exchanger. At this stage, units can use either water or air (water or water/glycol is used to provide heat to the refrigerant in the evaporation phase in both types of chillers) to absorb the heat from and chill the refrigerant, allowing it to condense. The heat absorbed by the air or water is evacuated from the unit, outside of the greenhouse space, or is transferred to another system for use (such as dehumidification).
Air-chilled units typically use outside air to cool the refrigerant, however this is clearly not practical when the outside air temperatures are high. Water-chilled units are much more efficient in these circumstances. With water-based chillers, after the refrigerant releases heat to the water, the water is either evacuated into a cooling tower, where it expands and dissipates heat to the outside, or the heated water is used to provide thermal energy to power other greenhouse or facility systems. An example is the hot water can be used to warm air used to reactivate a desiccant dehumidifier adsorption wheel. The hot water can also be heated further, and used as input for a steam powered electric generator.
Although chiller-based cooling systems have the advantage of not increasing the humidity within the greenhouse, their high energy consumption, complicated and costly maintenance, and environmentally detrimental waste (refrigerant and water/glycol) must be taken into account. In hot and dry areas, evaporative cooling presents itself as the best option. However, for producers wishing to completely seal their pharmaceutical cultivation from the outside environment, water-based chillers are the only option which provide the type of cooling required to ensure a successful harvest.
By Angela Swift