A Lesson In Aquaponics
Part 2: Meet Your New Friend, Enthalpy
By Jeremiah Robinson, Madison, Wisconsin
In the March/April issue, I introduced you to my little homestead in the city, and claimed that it was possible to grow year-round in a small space — in a cold climate — using aquaponics (fish and plants together). In this issue we get into the technical and theoretical details about the forces of creation that make it possible. And we make a new friend.
Today we get to learn about the most fun subject in all of thermodynamics. Can you guess what it is?
You’re right. It’s enthalpy!
When we talk about enthalpy, we mean the total thermal (heat-related) energy contained by a substance. In aquaponics, we’re talking about the heat contained by your water, your grow beds, the air in your greenhouse, and so on. It relates to temperature, but there’s more to it than. And it’s a big deal.
It’s crucial to understand enthalpy if you want to save energy in your cold climate aquaponics system. To illustrate why I think you and enthalpy should become friends, I’d like to give you a simple quiz.
1. How many BTUs does it take to raise 10 pounds of water from 40 to 50°F (4 to 10°C)?
2. How many BTUs does it take to raise 10 pounds of water from 30 to 40°F (-1 to 4°C)?
(Hint: The answer to these questions is not the same. See end of article for the big reveal.)
The reason that the answer to question two so greatly exceeds question one is that to get from 30°F to 40°F water, you can’t just raise the temperature. You have to change the state from solid to liquid by providing the ice with the heat of fusion.
While raising one pound of water 1°F takes one BTU, changing one pound 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 (-0.5 to 0°C) as it does to heat from 32 to 176°F (0 to 80°C).
When you apply heat to a material to raise its temperature, we call that sensible heat. When you apply heat to change its state, we call that latent heat. The total (thermal) enthalpy of a material includes both its latent and sensible heat, or: Enthalpy = Latent Heat + Sensible Heat.
So far we’ve talked about solid and liquid water, and that’s fairly simple (oh sure, easy for me to say). Well, hold on to your hats. Things get complicated when you start talking about water vapor.
You might have thought that 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 the liquid 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 they would shoot off into space. When they cooled off they would fall back down and “condense” into the earth. 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. Some examples:
• At 100 percent RH (relative humidity), the same amount of water evaporates from a surface as condenses on it.
• At 10 percent RH and 100°F air temperature, when you sweat you don’t feel it because the water evaporates from your body so quickly. You do get kind of salty, though.
• A tray of water at 80°F (27°C), surrounded by air 80°F and 50 percent RH, about one pound (0.45 Kg) of water will evaporate per square foot (0.09 m2) of surface area 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 (0.91 metric tons) of heat on its way up, and releases it all on its way down when it condenses. Half of this heat comes from the air, and half from the surface (i.e. the water or your greenhouse plastic). This heat, called the heat of evaporation, equals a whopping 970 BTU/lb.
WHAT’S A BTU?
In the United States, we measure heat in British Thermal Units, or BTU. One BTU is defined as the amount of energy it takes to raise one pound of water 1 degree Fahrenheit (°F). In Metric, 1 BTU = 1060 Joules and 1 pound = 0.45 Kg.
APPLYING ENTHALPY IN AQUAPONICS
First of all, you should look carefully at every place in your system where water is exposed to air. Any exposed liquid surface area offers a place where water can evaporate, taking its 970 BTU per pound along with it. This is also the reason why I sometimes have to heat my fish tanks to keep them at 80°F (27°C) when the outdoor air is 90°F (32°C) and dry.
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 to the inside of your greenhouse plastic. If the outdoor temperature is below zero and you have some ice, don’t worry about it. However, if you still get ice when the outdoor temperature is 15, then you have an enthalpy problem.
To resolve your evaporation enthalpy problem, you need to do two things:
1. Reduce the surface area of water in contact with 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 trouble.
2. 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 dewpoint, 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 aquaponics system.
Now that you’ve resolved all your evaporation issues, you should turn to the remaining 144 BTU/lb (334 kj/kg) that results from melting and freezing. This side of 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 until it’s redhot, then let it cool for 20 minutes and touch it with your hands. But I wouldn’t do the same thing with a concrete pan — it’ll burn you! 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-gallon drum of water contains 458 pounds of water. To reduce that drum’s water from 32 to 31 degrees would release 65,894 BTUs. That’s more than some furnaces, and we’re talking about one drum.
Cold water is thermal mass on steroids. If thermal mass were NASCAR, cold water would be Richard Petty.
Even better, the temperature we’re trying to maintain in greenhouses for cold-weather crops is often at or near 32 (0°C). Having a huge quantity of thermal mass that really, really doesn’t want to drop below 32 will help with that. A lot.
Quiz Answers: 1. 100 BTU; 2. 1540 BTU
A LESSON IN A POEM:
If you want to live life without care,
Keep your water away from the air.
To save yourself some aquaponic cash,
Store your heat in liquid water thermal mass.
— Jeremiah Robinson