Last week we learned about the four types of heat transfer. This week, we begin to put them to use. More specifically, this week we talk about radiation.
A day without electromagnetic radiation is like a day without sunshine. – Unknown
In cold climates, we put up greenhouses to trap radiation. The plastic covering allows radiant heat from the sun to pass through, heating up the air molecules and objects inside the greenhouse, and evaporating water. At night, the warm objects release this energy and the water condenses, keeping the greenhouse warm enough for plants to survive inside.
Brilliant! What about summer?
In summer the greenhouse covering still traps radiant heat from the sun, storing it in air, objects, and evaporating water. But in summer, this is the problem rather than the goal. With my greenhouse still sealed up last March, before I hooked up the fan, a string of warm sunny days drove the temperature to 140 degrees F. This killed my tomatoes and all my other plants except basil and hot peppers (which thrived). Radiation is powerful. Because of this, controlling radiation serves as our first line of defense against the overheating greenhouse.
We control radiation in three ways.
- Keeping it out of the greenhouse altogether.
- Keeping it off the things we want to stay cool.
- Making sure it bounces right back up to the sky.
Since time immemorial when Adam and Steve put up the first greenhouse in the first garden, they had to deal with radiation. But Steve knew about these kinds of things and so he made the first ever greenhouse improvement: shade cloth. Drive by any commercial greenhouse in any part of the world that experiences summer, where they grow anything other than hot peppers, and you will invariably find shade cloth.
Shade cloth comes in knitted and woven varieties and block from 30% to 90% of sunlight. If you grow only ferns and moss, go for the 90%! Most other plants need sunlight, unfortunately, so you should choose 40-50% blockage for most greenhouse applications. I use 40% because my greenhouse doesn’t have full sun to begin with. White works best, though I mostly see black. So probably color doesn’t matter that much. Assuming you’re now blocking 40-50% of the sun’s radiation, you’re well on your way to an effective greenhouse cooling strategy.
For that remaining 50-60% of sunlight, the goal is to keep it from heating up the things we want to keep cool. This includes plants, fish tanks, and grow beds.
Plants you can’t do much about. I would not recommend painting them white. Why not? No good reason, just heard it doesn’t work well. Maybe it affects the taste or something.
Fish tanks and grow beds we protect by covering them. Many folks leave their grow beds and fish tanks open to the sky in summer. Bad idea. All that sunlight absorbs right into the media, walls, the water, and the fish themselves. Covering them with a radiant barrier sends this radiation right back from whence it came and keeps it from heating up your system.
Rigid foam insulation works well over grow beds. Cut holes in it for your plants. Leave it there in winter for it’s insulating value. Note that covering your grow beds and fish tanks prevents evaporation. Evaporation is another strategy for keeping cool. There are ways to mitigate this, which we’ll learn in the next post.
Some other important items to keep heat off include those objects with the potential to store heat, such as your passive solar design features that we talk about here all the time. For example, your fifty stacked water-filled black-painted metal barrels, designed to absorb heat in winter, will still absorb that heat in summer. Cover these items with a radiant barrier in summer.
A radiant barrier consists of, basically, anything that reflects radiation such as foil, reflective paint (usually light colored), and the moon. A sheet of reflective mylar or foil-faced insulation works especially well as a radiant barrier. These also work well for covering fish tanks and grow beds, as discussed above.
The bare earth or your cement floor perform a lot of thermal mass storage as well, though we may not use them intentionally for this purpose. Covering the earth or the floor with something reflective helps a lot. With a radiant barrier covering your thermal mass storage, the radiation particles that entered your greenhouse magically reflect right back up to the sky from whence they came. This frees up your thermal mass storage to store coolness from the nighttime and release that during the day when it’s hot (I know, coolness isn’t really a thing from a thermodynamic perspective. Just roll with it).
In summary, to manage radiation in your greenhouse in summer, remember the three rules: Keep Out, Keep Off, and Bounce Back. Next week we learn about managing the heat that does get stuck into our greenhouse, using enthalpy.