Cool Your Greenhouse With Enthalpy: Part 3

[Jeremiah Robinson]

This post first appeared on coldweatheraquaponics.com.

Over the past two posts on 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 (38°C) your greenhouse will still get get hot.

But don't despair!  We resourceful aquapons always have a few more tricks up our sleeves.  This week 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.

CWA: How do you keep your greenhouse cool?
Everybody: Roll up the sides and turn on the fan.
CWA: But what if it doesn't work?
Everybody: 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 (2°C) of the outdoor temperature.

Going much beyond this is a losing proposition energy-wise.  You could put a 52" cattle barn exhaust fan in your 8 x 12 greenhouse, but it would literally shear the leaves off your plants and plastic from the walls.  It still won't get the temperature one degree below 100°F if it's 100°F outside, though it will make your utility meter spin like a Dervish!

Using passive ventilation, putting vents in the roof of your greenhouse works well if you only need minimal ventilation such as in a non-sunny climate like the northwest.  But when it's sunny you won't get you anywhere near 5°F (2°C) above 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 older sister, Evaporation.

Evaporation

As you learned in Fundamentals #2, changing the phase of a substance (eg. gas to liquid) removes a great deal more heat from it than changing its temperature.  In a greenhouse, the substance best suited to changing phase is water.

When it evaporates, water absorbs 970 Btu/lb (2256 Kj/kG).  This is great deal of energy.  Half of it comes from the air, half from the body of water that it evaporated out of.

For example, if you can evaporate 1 lb (0.45 kg) of water in a 20 x 40 ft. (6 x 12 m.) greenhouse from a 1000 gallon (3785L) aquaponics system, you will lower the air temperature by 4 degrees and the water temperature by 0.05 degrees.

The real kicker here is that it doesn't matter what the outside temperature is.  It could be 100°F (38°C) outside and 50°F (38°C) inside.  Evaporate a pound of water and you'll get the same temperature drop.

In the cold, we want to avoid evaporation.  In the heat, we want to encourage it. To do this, expose as much water to the air as possible.

Our plants help us with this through their process of transpiration, which is water evaporating through their leaves. We encourage transpiration by including large leafy plants in our systems at all times when it's hot out.  Spritzing water on plant leaves encourages transpiration all the more, though spritzer valves are notoriously difficult to keep clean if you have hard water.

To encourage more evaporation in other ways, we increase the surface area of water exposed to air.  You can do this by removing the tops to your grow beds and your fish tanks.  However, remember that we still want to prevent radiation from getting into our thermal mass (i.e. the water and media in our systems).  To overcome this dilemma, we raise the grow bed and fish tank 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.

The Feud

You can't put Ventilation and Evaporation in the same room.  They fight.

The problem is that the air which you cooled using evaporation will - through ventilation - leave the greenhouse and be replaced with (presumably warm) outside air.

On the other hand, without some ventilation, your greenhouse will become so humid that 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), you move air slowly through the greenhouse, and quickly over the surface of the water and the plants.  This will maximize transpiration and evaporation, thus cooling your greenhouse down.

A Trick

There is another highly effective trick to get the sisters to work together to cool (and heat) your greenhouse.   But you'll have to wait till next week to learn about it :)