Climate Control: Cooling

Why It's Important:
1. To control temperature and humidity
2. To avoid plant stress
3. To prevent over-irrigation
4. To ensure flower set

​The selection of cooling systems depends on the amount of heat gained from the outside, as well as any heat generating equipment located within the indoor farm. Heat gain is a function of facility size and orientation, local climate, envelope materials, and sources of internal heat gain (eg. lights, people, equipment).  Electricity will be required to operate mechanical ventilation and cooling systems.  Renewable energy, generated from photovoltaics and wind turbines, can help offset the cost of energy.  Water will also be required when using evaporative cooling systems.  To reduce the use of potable water use rainwater harvesting and water recycling from the irrigation system.  The HVAC system can also be engineered to remove water vapor from the exhaust air and reuse it for evaporative cooling. 

For greenhouses, the cooling requirements are driven by solar radiation through the glazing, as well as the outdoor temperature.  For vertical farms and indoor gardens, cooling is dictated by heats given off by the lamps, as well as moisture transpired by the plants (see Humidity Control).  The paragraphs to follow describe the typical systems used for both greenhouses and closed plant production facilities.

Greenhouses

Ventilation
Typically, the first line of defense against over-heating the greenhouse is to ventilate it.  Although shade curtains block out solar heat gain from the greenhouse, they are typically used as the last line of defense for cooling because they also reduce the amount of sunlight reaching the plants.  Shading can be accomplished with a cloth-like material that is either draped over the greenhouse cover or installed inside the greenhouse and suspended over the plant canopy.  Shading materials can be purchased to block anywhere between 10-60% of solar radiation, and be selective for plant-specific wavelengths of light. External shades have the benefit of preventing solar heat gain from entering the greenhouse, but must be removed seasonally.


Shading
Although shade curtains block out solar heat gain from the greenhouse, they are typically used as the last line of defense for cooling because they also reduce the amount of sunlight reaching the plants.  Shading can be accomplished with a cloth-like material that is either draped over the greenhouse cover or installed inside the greenhouse and suspended over the plant canopy.  Shading materials can be purchased to block anywhere between 10-60% of solar radiation, and be selective for plant-specific wavelengths of light. External shades have the benefit of preventing solar heat gain from entering the greenhouse, but must be removed seasonally.

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Evaporative Cooling
When ventilation is inadequate to control greenhouse temperatures, evaporative cooling can be used to further reduce indoor air temperatures.  Evaporative cooling systems work by using hot air to evaporate water, simultaneously reducing air temperature and increasing the air moisture content (humidity).  Evaporative cooling systems work particularly well in hot, arid climates.  However, because water tends to be in short supply in hot, arid regions, evaporative cooling systems and greenhouse should be designed to reduce water use demands.

To learn more about Evaporative Cooling read the fact sheet "Evaporative Cooling in Semi-Arid Climates", published by University of Arizona's Cooperative Extension.

Pad-and-Fan (“Wet Wall”) System (P&F)
Fans at one end of the greenhouse pull outside air into the greenhouse through a wetted pad, reducing the temperature and increasing the moisture content of the entering air before it is in the greenhouse.  Typically, fans are located at one end of the greenhouse and “suck” outside air in through the wetted pads located at the opposite end of the greenhouse.  Other pad and fan configurations can also be designed.

High-Pressure-Fog (HPF) and Low-Pressure-Mist (LPM) Systems
A series of nozzles inject droplets of water directly into the greenhouse air, typically above the crop canopy, and relies on the greenhouse air itself to evaporate the water, reduce air temperature, and increase humidity. Ventilation is achieved with mechanical fans or passive vents.  High-pressure-fog systems inject very fine water droplets at pressures generally between 1000-1500 psi.  Low-pressure misting systems inject larger water droplets at pressures generally below 100 psi.  The small droplets produced by the HPF system allow evaporation to occur more  quickly and reduce the risk of water droplets to form on the plants. 

Wet wall used for pad-and-fan cooling of a greenhouse.

High-pressure fog (HPF) sprayed over tomato plants in a greenhouse.

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Vertical Farms and Indoor Grows

​Refrigerant-Based Systems
Refrigeration-based systems are traditionally used for air conditioning buildings and homes. These systems can be designed to use 100% outside air or recirculate the indoor air, and can provide heating and cooling at many locations or just one.  Although there are many efficient models for these systems, they can use a lot of energy to remove the heat and moisture from an indoor facility due to the use of compressors for refrigeration. These systems may also be well suited for mixed-mode applications, where the HVAC system is serving both plant and human environments. Several types of systems are available, including packaged DX units, split-system heat pumps, and chilled water based systems.

Mini-split AC units installed outside an indoor growing facility.

Indirect Evaporative Cooling (IEC) System
An indirect evaporative cooling system works by passing air across a heat exchanger filled with water (sometimes refrigerant) to cool the air without humidification.  Like P&F System, the heat exchanger will be located at one end of the greenhouse and a fan will draw air through it and into the greenhouse. IEC systems may be a good option in humid climates where 5-10F of cooling is desirable. IEC may also be combined with a direct evaporative cooling (such as P&F) to provide humidification and additional cooling, if needed. 

Packaged rooftop DX unit being installed on the roof of an indoor growing facility.