Cool your Greenhouse with Enthalpy: Part 1

[Jeremiah Robinson]

Well, I had a revelation this week.

On Sunday I got heat exhaustion from gardening all weekend.  Laid out in bed all day Monday, I realized something profound.

It's not cold out anymore!  

Flowers are in full bloom; spinach is bolting; my greenhouse hit 100 last week with the fan running; and the water in my aquaponics threatens to break 70.

What do we do in cold weather aquaponics in bizarre times like this?

We talk about cooling!

Besides my epiphany, Jim's been nagging me for weeks now to write a post about cooling to save his precious summer trout.  I think he ought to eat them, but since he's the ACWA I figure I oughta be nice.  So here we go.

In Fundamentals #2 I explained how to heat your greenhouse with enthalpy.  This summer we use enthalpy to cool.

But first we need to learn about heat transfer, especially radiation.

On Earth, we transfer heat using four methods.

1. Conduction
2. Convection
3. Mass Transfer
4. Radiation

Conduction

In conduction, heat moves through solid surfaces or between them.

To demonstrate: go find a friend, 6' of bare metal wire, and start a fire.  Each of you grab an end and hold the middle over the fire.  Whoever lets go first pays for the wine.

In this example, heat travels through the wire, then between the wire and your finger.

The greater the difference in temperature in or between the surfaces, the more heat transfers.  Also, some things conduct better than others.

Most people don't use conduction to cool greenhouses.  Takes too much wire.

Convection

In convection, fluids absorb heat from a solid.  To demonstrate: stick a fan next to your electric stove.  Turn the stove on high, the fan on low.  If the air feels warm (air is a fluid), you've experienced convection!  And created a fire hazard.

The bigger the difference in temperature between the solid and fluid, the more heat transfers.  Also, the more surface area the more heat transfers.

Greenhouse cooling maximizes convection.

Mass Transfer

Mass transferpretty much speaks for itself.  To demonstrate: when you get back from some hard exercise, take off your clothes and put them in the freezer.

Guess what?

You're naked.

Now drink some cold water.  The clothes you took off were hot, while the water you drank was cold.  Replacing your hot clothes with cold water cools you off.  Now use the bathroom and put your frozen clothes on. That's more mass-transfer cooling-off.

The more clothes you take off and the more water you drink, the more heat transfers.  If your spouse finds  your clothes, tell him you were doing science!

We use mass transfer to cool greenhouses by taking in cooler outside air and expelling hot greenhouse air.  In aquaponics, we do it by adding ice cubes to our water, or doing water changes.

Radiation

Radiation is a bit more tricky. It involves transferring heat without touching at all.  To demonstrate: start a fire in your fireplace.  Put on a blindfold, stand in the middle of the room, and spin around till you're totally lost.  Point to the fireplace and take off your blindfold.

How did you know?

It's not because the air was warmer in that direction.  It was, but not much.  It's because the fire radiated heat particles at you.

In fact, everyone and everything radiates heat particles back and forth all the time.  "You over there, I just radiated some heat particles at you."  This is not a good pick-up line.

Radiation is a big deal.  The average surface temperature of a room makes a bigger difference in how warm/cold you feel than the temperature of the air.  It's the same reason we feel warm in sunlight and cool in shade.In greenhouse cooling, radiation is your enemy.  Plants need it to grow, but it'll turn your greenhouse into a solar oven.

Next post, we use these concepts to cool your greenhouse.