How to Cool Your Greenhouse: Evaporate Your Problems Away

In a previous article on greenhouse cooling, we learned about the four methods of heat transfer and how to manage radiation in a greenhouse. With your solar radiation trained like an alpine St. Bernard, you’ve limited the heat entering your greenhouse to the bare minimum needed for plant growth, and kept out, kept off and bounced back the rest.

Despite all this, when outdoor temperatures hit 100°F, your greenhouse will still get hot. But don’t despair! We resourceful growers always have a few more tricks up our sleeves. This month we learn about the two fighting sisters of greenhouse cooling: ventilation and evaporation.

Ventilation

This is the one greenhouse cooling strategy that everybody knows about:

Question: How do you keep your greenhouse cool?

Answer: Roll up the sides and turn on the fan.

Question: But what if that doesn’t work?

Answer: You get a bigger fan.

This works to a point. Increasing the ventilation rate allows you to bring your greenhouse temperature, on a sunny day, down to within 5°F of the outdoor temperature. Going much beyond this is a losing proposition energy-wise. You could put a 52-in. exhaust fan in your 8- by 12-ft. greenhouse. While it would literally shear the leaves off your plants and plastic from the walls, it still wouldn’t get the temperature one degree below 100°F if it’s 100°F outside, although it will make your utility meter spin like a dervish!

Using passive ventilation—putting vents in the ridge of the roof of your greenhouse—works well if you only need minimal ventilation such as in a non-sunny climate. But when it’s sunny, you won’t get anywhere near the outdoor temperature. With active ventilation (fans), putting the fans on the roof helps a bit, but not as much as you’d think. The problem with ventilation concerns this ongoing feud. You see, ventilation is always trying to thwart the efforts of her more powerful sister—evaporation.

Evaporation

Changing the phase of a substance (e.g. gas to liquid) removes a great deal more heat from it than changing its temperature. In a greenhouse, the substance best-suited to changing phases is water. When water evaporates, it absorbs 970 btu/lb.

This is a great deal of energy. Half of it comes from the air, half of it from the body of water it evaporated out of. For example, if you evaporate 1 lb. of water in a 20- by 40-ft. greenhouse from a 1,000-gal. hydroponic system, you will lower the air temperature by 4°F and the water temperature by 0.05°F.

The real kicker here is that it doesn’t matter what the outside temperature is. It could be 100°F outside and 50°F inside. Evaporate 1 lb. of water and you’ll get the same temperature drop.

During cold weather, we want to avoid evaporation, but when it’s hot, we want to encourage it. To do this, expose as much water to the air as possible. Our plants help us with this through the transpiration process, which is water evaporating through their leaves.

We encourage transpiration by including large leafy plants in our gardens when it’s hot out. Spritzing water on plant leaves encourages transpiration all the more, although spritzer valves are notoriously difficult to keep clean if you have hard water.

To encourage evaporation in other ways, we can increase the surface area of water exposed to air by removing the covers on our grow beds and reservoirs. However, we still want to prevent radiation from getting into the water and media in our systems.

To overcome this dilemma, raise the grow bed and reservoir covers a small amount above the water/media surface and blow air over the water with small fans. This warm, moving air will evaporate water and cool your greenhouse.

A more standard method for reducing greenhouse temperature is using a cooling pad, which is essentially a large sponge you insert into a hole at one end of your greenhouse, then drip water onto. The fans at the other end suck air through the sponge, evaporating it into the greenhouse air. One farmer near me claims he keeps his greenhouse at 75°F when it’s 100°F outside using this method.

The Feud

You can’t put ventilation and evaporation in the same room. They fight. The problem is the air you cooled using evaporation will, with the help of ventilation, leave the greenhouse and be replaced with warm outside air. On the other hand, without some ventilation, your greenhouse will become so humid, you won’t be able to evaporate any more water.

This requires some balance. To cool below the outdoor temperature, with solar radiation controlled as best you can, move air slowly through the greenhouse, and quickly over the surface of the water and the plants (or cooling pad). This will maximize transpiration and evaporation, thus cooling your greenhouse down.

A Neat Trick

There’s one more trick that can make a big difference in your greenhouse temperatures—getting ventilation and evaporation to work together. Ventilation and evaporation work together using something called a “climate battery.” The climate battery fans draw the warm moist air from the greenhouse into pipes buried in the ground under the greenhouse. Once there, the cool sides of the tubing cause moisture to condense. This dries out the air in the same way a dehumidifier does.

Once dry, the air is primed to absorb more moisture from the water, further reducing greenhouse air and water temperatures. The magic of the climate battery is that the earth has a near-infinite ability to absorb heat. This is the principle that ground-source (geothermal) heating and cooling is based on.

At night, if the air temperature drops below the ground temperature, then some of that condensed water re-evaporates and the climate battery is recharged. You can absorb so much heat in the summer from cooling that it will be stored for months and can help you heat the greenhouse in the winter.

Summary

In conclusion, you can use the following ventilation and evaporation strategies to keep your greenhouse cool:

• Ventilate to keep your greenhouse only a few degrees warmer than outdoor temperatures. Ridge venting can be more efficient than end-wall venting.
• Evaporate water inside your greenhouse to lower the temperature below the outdoor air temperature. You can do this using moving air though plant transpiration, evaporation from your grow beds and reservoirs, and by passing air through a cooling pad.
• Transfer heat from your greenhouse air to the earth using a climate battery, which allows you to store it until winter—when you’ll need it!

Using these strategies will allow you to raise all manner of crops in the heat of summer in your greenhouse.