In his follow-up to Aquaponics for the Frozen Tundra: Part I, which covered the importance of insulating and air sealing your system, Jeremiah returns with a lesson on enthalpy and its implications for gardening in frosty temperatures.
Last month I threw down a gauntlet, challenging Maximum Yield readers to come up with a way to grow outdoors in a cold climate without using a lot of energy. I offered some hints about how to do it, focusing on air sealing and insulation. This month we move into more theoretical territory. Better get your thinking cap—you’re going to need it!
Now, meet Enthalpy, Queen of Thermodynamics. Queen Enthalpy is wise and unyielding. Benevolent if you pay her homage, but if you fail to do so, she will smite all your living creatures. However, if you want to grow using aquaponics in the cold, Queen Enthalpy is your only hope. Bestowing gifts of solar radiation and British thermal units (BTUs), Queen Enthalpy repays your homage by protecting your plants and fish on frigid nights.
What the heck am I talking about? When I talk about enthalpy, I mean the total thermal (heat-related) energy contained by a substance. In aquaponics, I’m talking about the heat contained by water, grow beds, the air in the greenhouse, etc. Enthalpy relates to temperature, but there’s more to her than that. And she’s a big deal.
To illustrate why I think you should honor and serve your Queen, I present a simple quiz. But before we begin, note that in the United States, heat is measured in BTUs and one BTU is defined as the amount of energy it takes to raise 1 lb. of water by 1°F. Now, here is the quiz:
- How many BTUs does it take to raise 10 lbs. of water from 40 to 50°F?
- How many BTUs does it take to raise 10 lbs. of water from 30 to 40°F?
Be warned, the answer to these questions is not the same (see the end of the article for the big reveal). The reason the answer to question two greatly exceeds question one is that to get from 30 to 40°F water, you can’t just raise the temperature.
You have to change the state from a solid to a liquid by providing the ice with the heat of fusion. In other words, you have to honor the Queen. While raising 1 lb. of water by 1°F takes 1 BTU, changing 1 lb. of ice to liquid water takes 144 BTUs. In other words, it takes the same amount of energy to heat water from 31 to 32°F as it does to heat it from 32 to 176°F.
When applying heat to a material to raise its temperature, it’s called sensible heat. When applying heat to change its state, it’s called latent heat. The total (thermal) enthalpy of a material includes both its latent and sensible heat. The equation looks like this: enthalpy = latent heat + sensible heat.
So far I’ve talked about solid and liquid water, and that’s fairly simple (oh sure, easy for me to say). Well, hold onto your hats, things get complicated when you start talking about water vapor. You might have thought we don’t have to worry about enthalpy with regard to the liquid-to-vapor transition because we won’t be boiling our fish. But you’d be wrong. Molecules vibrate.
Molecules near the surface of a liquid can sometimes vibrate enough to launch themselves off of the liquid and into the air. This is called evaporation. Molecules crashing into any material and sticking to it is called condensation.
If people evaporated, it would mean that every time anyone got really angry we would shoot off into space. When we cooled off we would fall back down and “condense” into the earth. This might have interesting implications for our culture, but I digress. The conditions that cause molecules to launch include the temperature of the water and the temperature and relative humidity (RH) of the air.
Here are two examples: At 100% RH, the same amount of water evaporates from a surface as condenses on it. At 10% RH and 100°F air temperature, when you sweat you don’t really feel it because the water evaporates from your body so quickly. You do get kind of salty, though. At 80°F air and water temperature, and 50% RH, about 1 lb. of water will evaporate per square foot of surface per day.
The thing about this whole launching and condensing process is that it involves a lot of energy. Each launching molecule absorbs a ton of heat on its way up, and releases it all when it condenses. Half of this heat comes from the air, and half from the surface (i.e. the water or greenhouse plastic). This heat, called the heat of evaporation, equals a whopping 970 BTU/lb. Thermodynamics is a magical realm, and Queen Enthalpy rules it with an iron fist.
Enthalpy In Aquaponics
If you want to honor the Queen, thereby invoking her protection on cold nights, this section tells you how. Any location where liquid water is exposed to air offers a place where water can evaporate, taking its 970 BTU/lb. along with it. This matters especially in the middle of a cold winter, when RH approaches 20%. It is also the reason why you sometimes have to heat your tanks to keep them at 80°F when the outdoor temperature is 90°F.
Evaporation makes trouble for all of us in aquaponics, but here’s how you can tell if it’s a major problem for you. Each morning in winter, check to see how much ice has frozen on the inside of your greenhouse roof. If the outdoor temperature is below 0°F and you have some ice, don’t worry about it. However, if you still get ice when the outdoor temperature is 15°F, then you have not given Enthalpy her due. To honor Queen Enthalpy, you need to do two things:
Reduce the surface area of water in contact with dry air. This includes your fish tanks, grow beds and the surface of your plant leaves. It also includes the inside of your flood and drain beds because when you drain them, they fill with air that can’t wait to evaporate all the water left on the huge surface area of your media. Dry air is not your friend. Enthalpy hates dry air.
Insulate and air seal the exterior of your greenhouse. Use double- or triple-wall glazing. Insulate the north, east and west sides. If you can, insulate the top of your greenhouse on the north side. All of this serves to raise the temperature of the greenhouse surfaces above the dew point, or the surface temperature at which vapor will condense. Every drop of condensation that forms in your greenhouse is another drop that has to come out of your system.
Now that you’ve resolved all your evaporation issues and honored the Queen in this way, you should turn to the remaining 144 BTU/lb. that results from melting and freezing. For this, enthalpy will likely work in your favor. Not only does it make it difficult for the water in your system to freeze, but it also gives you a great way to store energy. It stores this energy in something called thermal mass.
Thermal mass signifies an object’s ability to store heat. Metal, for example, stores heat very poorly. You can heat a pan to red hot, then let it cool for 20 minutes and touch it with your hands. But don’t do the same thing with a concrete pan—it’ll burn you good!
True, the only reason you’d ever find a concrete pan is because you wanted to do a thermal mass experiment, but still. Thermal mass basically correlates with weight. Heavy things store more heat.
But there is one exception to this rule, and it relates to enthalpy. Liquids near their freezing point contain an incredibly large amount of thermal mass. A 55-gal. drum holds 458 lbs. of water. Reducing that drum’s water temperature from 32 to 31°F would release 65,894 BTUs. That’s more than some furnaces, and I’m talking about one drum.
Cold water is thermal mass on steroids. If thermal mass were snowboarding, cold water would be Shaun White. Enthalpy likes thermal mass. Even better, the temperature we’re trying to maintain in greenhouses is typically at or near 32°F. Having a huge quantity of thermal mass that really, really doesn’t want to drop below 32°F will help with that. A lot.
If you want to grow in a place where it gets cold in winter, you need to pay special attention to enthalpy, realizing that the vast majority of heat storage and heat loss happens—not by changing the temperature of materials like water—but by changing its phase from solid to liquid to gas.
Managing these phases is the key to managing heat when it’s cold. If it’s dry and cold and your plants are green, you’d better pay some homage to your Queen. To save yourself some aquaponic cash, store your heat in liquid water thermal mass.
- 100 BTUs
- 1,540 BTUs