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Evaporative and Passive Cooling in a Greenhouse

Keeping a greenhouse cool throughout the summer heat can be a difficult task. To keep the plants within a greenhouse happy and healthy, a horticulturist must maintain a temperature in his or her greenhouse that is suitable for plant growth. Extreme temperatures can cause myriad of problems for greenhouse hobbyists, including stunted growth, root pathogens, wilting and even death.

Many greenhouse growers don’t realize there are easy and inexpensive ways to help keep the temperature within a greenhouse cooler. In some cases, passive cooling techniques (cooling without energy consumption) are enough to keep a greenhouse within the desired temperature range. Sometimes the combination of a passive cooling technique and an evaporative cooling method is required to maintain temperatures conducive to accelerated plant growth. In the majority of situations, implementing passive cooling techniques will only help a grower reduce the greenhouse’s heat signature, which automatically reduces the energy costs of any additional cooling equipment. Put another way, passive cooling techniques help make cooling a greenhouse as efficient and cost effective as possible.

Passive Cooling Techniques

As previously mentioned, passive cooling techniques can be the key to most efficiently cooling a greenhouse. Most passive cooling techniques are pretty straightforward, but will generally require a little forethought when setting up a greenhouse. It is a good idea to figure out how the greenhouse will be kept cool before beginning construction. Three passive cooling techniques that can greatly affect the overall temperature of a greenhouse are convection, the use of a shade cloth, and strategically locating the greenhouse.

Convection

Convection is the physical movement of a warm gas or liquid to a colder location. Greenhouses will naturally trap heat so it is advantageous to the gardener to use convection as a way of dispersing that captured heat. Convection is commonly used in greenhouses in remote areas or in greenhouses that do not have access to electricity. To use convection most efficiently, a grower needs to place vents near the top of the greenhouse from which the hot air will escape. As the hot air escapes out of the top of the greenhouse, it is replaced with cool air that enters from the lower portion of the greenhouse. This natural exchange of hot air for cool air can significantly reduce the temperature within a greenhouse.

Shade Cloth

A shade cloth is a fabric-like material that is placed next to the walls and/or ceiling of a greenhouse to help block some of the sun’s radiant energy from entering the greenhouse. As its name suggests, a shade cloth’s purpose is to create shade within a greenhouse. Many growers use shade cloth to protect sensitive plants from hot spots, but a shade cloth can also be used as an effective passive cooling device. Shade cloth can be made from a few different materials, such as aluminum alloys, polyester, or nylon.

Shade cloth is sold by the percentage of shading it provides. Higher or lower percentages are available, but the shade cloths mainly used by horticulturists will shade 50%, 60%, or 70% of the sunlight. The higher the percentage shaded, the more protection and the cooler the greenhouse will be. There are also white or reflective exterior shade cloths which are even more effective for cooling as they reflect a higher amount of the solar heat away from the greenhouse.

Physical Location of the Greenhouse

Where a greenhouse is built or positioned will have a dramatic effect on the greenhouse’s average temperature. Many growers believe a south-facing greenhouse is the best option. Although a south-facing greenhouse will receive the most sunlight, it will also be exposed to the most heat. Many greenhouse experts actually recommend a west-facing greenhouse over a south-facing greenhouse. A west-facing greenhouse will still get plenty of sunlight for healthy plant growth, but it will automatically operate at cooler temperatures than a south-facing greenhouse in the same geographical location.  If a gardener is planning on having a south-facing greenhouse, he or she will most likely need to implement some additional cooling techniques to maintain optimal temperature conditions. All in all, a west-facing greenhouse offers the most effective balance of direct sunlight and temperature control.

Evaporative Cooling Techniques

In addition to passive cooling techniques, one of the most common and efficient ways to cool a greenhouse is through evaporative cooling. In fact, plants use their own evaporative cooling process, which we refer to as transpiration. Similar to humans sweating, a plant transpires moisture through its stomata. Transpiration in plants has two main functions: to move water and minerals to the leaves and also to cool the plant. Mechanical evaporative cooling techniques mimic transpiration and create an evaporative cooling effect that alters the greenhouse’s environmental conditions. There are a few different ways to utilize evaporative cooling in a greenhouse. The two most common evaporative cooling systems used in greenhouses are wet wall systems and portable evaporative coolers.

Wet Wall Systems

One of the most popular and efficient ways to cool large, commercial greenhouses is with a wet wall system. A wet wall system consists of cooling pads encased in large aluminum housing. The aluminum housing appears similar in appearance to a honey comb (frame with multiple holes). The honey comb design allows air to pass over the cooling pads which are kept drenched with water. As air is drawn into the greenhouse, it passes through the wet pad and is cooled. This system works in a way that resembles a radiator cooling the engine of a car. The powered fans, which are placed on the wall opposite the wet wall system, are the driving forces of the air movement. Motorized shutters are positioned over the wet wall system to allow air to enter the greenhouse. These shutters are usually controlled by a thermostatic controller. This means they will open and close depending on the temperature. Wet wall systems are fairly expensive and require more extensive plumbing which is why they are generally reserved for large, commercial greenhouse applications.

Portable Evaporative Coolers

Similar to portable air-conditioners, portable evaporative coolers are self-contained, cooling devices that are commonly used by greenhouse hobbyists. Portable evaporative coolers contain a similar pad to that used in a wet wall system. A mechanical fan draws air over the wet pad to create an evaporative cooling effect. For a portable evaporative cooler to work most efficiently, it should have access to fresh air. This is why portable evaporative coolers work best when placed near the greenhouse entry or a fresh air intake vent. Like the wet wall systems, portable evaporative coolers are thermostatically controlled. Unlike the wet wall systems, portable evaporative coolers can easily be stored under a bench or somewhere out of the way when not in use. Portable evaporative coolers offer small greenhouse growers a great way to maintain cooler temperatures during the hot summer months of the growing season.

Fogger or Humidifier

For some small greenhouses, a fogger or humidifier can serve as an evaporative cooling system. A fogger is a device that emits water in a fine fog. These devices can be placed directly behind a circulating fan so the water-cooled air is blown around the greenhouse. As the water evaporates, it also creates a cooling effect. A humidifier found at a big box store can be set up in a similar fashion. When placed behind a circulating fan, the cool, moist air will be blown across the greenhouse for an evaporative cooling effect. Both foggers and humidifiers emit water droplets so small that the plants within the greenhouse do not get wet. Instead, the water quickly evaporates and cools the greenhouse’s climate in the process. Foggers and humidifying systems should not be confused with mist systems. Misting systems emit larger sized water droplet that will settle on plant leaves and actually make the plant wet. This extra moisture, combined with warm temperatures, can cause a whole new series of problems, including pathogenic molds and fungi.

Passive cooling offers an inexpensive way to reduce a greenhouse’s heat signature. In some cases, passive cooling on its own will be enough to keep a greenhouse in the desired temperature range. In other cases, when mechanical evaporative cooling or mechanical fans are also required, passive cooling techniques can still increase overall efficiency and, in turn, a grower’s return on investment. Gardeners who implement evaporative cooling techniques are essentially mimicking the plant’s natural cooling method known as transpiration. Not only are evaporative cooling techniques effective, but they are also efficient ways to maintain healthy growth in a greenhouse even during the mid-summer heat. Ultimately, the geographical location, the type of greenhouse, and the crop being grown should all be considered when determining which cooling technique or which combination of cooling techniques a horticulturist should use.

Eric Hopper resides in Michigan’s beautiful Upper Peninsula where he enjoys gardening and pursuing sustainability. He is a Garden & Greenhouse senior editor and can be contacted at Ehop@GardenAndGreenhouse.net.

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