A lot of gardeners I know love a good challenge. So let me throw down a gauntlet. I challenge you to build an aquaponic system (fish and plants together) that meets the following criteria:
- Provides space for 150 plants at 8-in. spacing
- Holds 100 fish at 2-lb. each
- Allows you to grow outdoors
- Handles nighttime temperatures of -25°F
- Uses less than 1,400 kWh of electricity annually, and no other fuel sources
- Costs less than $4,000 to build
Sound impossible? Over the next three issues, I will walk you through four design strategies that make it possible to grow outdoors when it’s cold enough to freeze your pecans off. This month we talk about my Winter Aquaponics Design Strategy No. 1. In the immortal words of The Offspring, you’ve got to keep ’em separated.
Your Fish Factory
The inspiration for this design came from heavy industry. As an engineer working with energy efficiency in factories, I noticed something striking—factories are not like houses. In houses we heat air. Because we move from one room to another and want to stay comfortable everywhere we go, we keep our whole house warm. This generally requires heating the air in the house above a certain set point, say 68°F.
On the other hand, factories only heat the things that need heating, and they keep those things as separated from the rest of the factory as possible. For example, I visited a paper factory in Wisconsin with a process that requires heating a chemical solution to 4,000°F. A few feet away, another process requires cooling another material down to 50°F. Keeping these processes thermally separated required some serious engineering.
Most people who grow using aquaponics outdoors in the cold (usually in a greenhouse) treat their system like a house—they heat the air. But when you really get down to it, aquaponics is more like a factory than a house.
There are different factory processes involved in aquaponics. I call them thermal zones. I’ll tell you about each of them and their requirements, and then we’ll learn how to separate them thermally.
Fish Zone: The fish live here, in the water. The requirements depend on the type of fish. With the fastest-growing kind of fish—red Nile tilapia—this zone should stay at above 80°F for maximum growth. For other kinds of fish, temperatures vary greatly. For example, Arctic char find their happy place around 40°F.
Transport Zone: This zone moves the fish waste between the other zones. The transport zone includes any solids filtration devices, such as swirl or suspended solids filters. The only requirement for this zone is that the water shouldn’t freeze.
Nitrification Zone: This zone converts fish waste into the epic nutrients that create the massive growth aquaponics is famous for. The organisms living in this zone vary in their effectiveness based on temperature, providing the maximum nutrient conversion rate at 85°F, continuing to nitrify at reduced rates until they hit 32°F, when they stop. Below 32°F, the organisms don’t die, but sort of get stuck in one place (the ice). The main thermal requirement of the nitrification zone is that temperatures don’t change too quickly because the warm-water organisms go to sleep faster than the cold-water ones wake up, and vice-versa.
Root Zone: The thermal requirements of this zone depend on the types of plants grown. Tomatoes, for example, grow slowly with root temperatures below 70°F while spinach does just fine down to 40°F. On the other hand, many root diseases, such as pythium, wreak the most havoc at warm temperatures. Temperature swings also matter to roots, and some plants cannot tolerate dramatic changes.
Leaf Zone: This varies even more dramatically than the root zone does. Some plants require temperatures that never drop below 50°F, while others tolerate temperatures as low as 5°F without serious damage. Humidity also matters a great deal, as we’ll learn next month.
Everything Else: This includes everything outside the aquaponic system. In a greenhouse, it includes the area above and around the plants. It includes the area you walk around in and where you store fish feed and tools. From a thermal perspective, the only time this area has any requirements at all is when you’re there, which generally only occurs during the day. Unless, of course, your aquaponic system includes a hot tub (which I highly recommend).
Put Up Barriers In Your Relationships
Has anyone ever heard the following phrase from a significant other? “I feel like we’re growing apart—you’ve been putting up barriers in our relationship.” It’s one of the classic phrases of Generation X. “Putting up barriers” puts an end to a lot of relationships. I won’t presume to give you dating advice, but I can tell you this: in winter aquaponics, you need to put up some barriers.
These fall into two major categories: insulation and air sealing. Adding insulation to each component protects it from heat loss through the walls of that component. In some cases, you can make the walls themselves out of insulation. Air sealing takes a bit more explanation, some of which we’ll get into next month. For now, you just need to know that any time water comes in contact with air, including through your leaves, which evaporate water using transpiration, a lot of energy leaves your system.
Fish Zone: The best way to insulate and air seal your fish zone is to build it out of an existing, highly insulated and air-sealed container. The best container on Earth was invented in 1834 and has been improving ever since then, through 180 years of engineering. Many of us have one in our homes. Can you guess what it is? That’s right—a freezer! A chest freezer, modified through the use of potable, water-safe epoxy paint, makes an incredibly well-insulated fish tank. The lid and gaskets maintain an almost-perfect air seal, minimizing water-air contact.
Transport Zone: Assuming you’re using PVC pipes rather than open channels, air sealing is largely under control. Insulate the pipes using large-diameter commercial pipe insulation that you can purchase at your local plumbing supply house. Because many of these supply stores refuse to serve homeowners, you may have to come up with a business name (your name & sons plumbing tends to work well) and walk in with a serious expression and work boots. Bring along a section of pipe to make sure you get the right size. Alternatively, you can buy large-diameter swim noodles.
Nitrification Zone: You can insulate filters, including nitrification filters or biofilters, by surrounding them with rigid insulation (flexible insulation becomes useless when wet). This includes pink or blue board insulation and spray foam. I use foil-faced board insulation, which I wrap around the filters (mine are round) by cutting through one side of the foil but not the other to make it fit around a curved surface, then gluing it to the filter itself. To make it pretty and weatherproof, you can surround the insulation with flexible plastic such as a piece of shower surround panel. The recommendations from this section also apply to any solids filters.
Root Zone: For this zone, the easiest way to insulate and air seal is to build the entire grow bed out of insulation with a structural frame, using a potable, water-safe pond liner to prevent leaks. Using another sheet of insulation for the top, with holes cut into it for insertion of net pots, allows for air sealing and insulation of that portion. I recommend allowing the insulation to rest on the edges rather than float on the water, as it creates a better air seal that way.
Leaf Zone: This zone creates the largest challenge for insulating and air sealing. Most growers don’t even try. However, when you set to work on it creatively, it’s not that hard. The simplest method involves creating Eliot Coleman-style low tunnels over the grow beds. Using a fabric-style low tunnel provides insulation but no air sealing. It also blocks a significant portion of the sunlight. A plastic low tunnel would provide some of both insulation and air sealing, while letting more light through. You can air seal by fastening it to the grow beds or creating a removable low tunnel with its own structure, but it’s essential to allow yourself a way to open it up during the day and on warm nights so plants can transpire when temperatures rise above 40°F.
Everything Else: The only reasons to insulate or air seal the rest of your greenhouse are as follows: desire for comfort when you’re in there planting, harvesting, hanging out or feeding your fish, and to provide a space in which to add thermal storage, which you’ll learn about next month. This last point flies in the face of most existing aquaponic design strategies, which focus nearly all their attention on air sealing and insulating the Everything Else zone, which, in my not-really-all-that-humble opinion, is a waste.
In Aquaponics for the Frozen Tundra: Part II, I talk about the magic of thermodynamics and a mysterious new word that makes all the difference in the cold.