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Pictured below is my design for an aquaponic walipini and a walipini cold frame.

Walipini: a partially submerged greenhouse that takes advantage of the mass of surrounding earth to hold heat and maintain a warm environment year round. The size and shape of the pit is directly related to the amount of heat it can store.

Aquaponic: you feed fish with cheap bulk fish food, they produce waste material, the bacteria and worms break down the waste material into food that the plants thrive on. The fish can actually be a decorative variety or just plain goldfish if you don't want to eat them, or they can be a source of meat if you want to grow tilapia or trout or bass or something. As opposed to a hydroponic system (just water, no fish) where you add chemical plant food to the water and grow plants. The chemicals are expensive and the process is technical.

Some of the design considerations included tradeoffs for ground based grow beds vs elevated. Ground based are less expensive, easier to construct, allows for full rear wall hanging garden, and can have higher density of useable surface area. Elevated trays can be worked more easily, allow under-bed storage, and have better lighting. I am going with two ground based grow beds.

Another tradeoff was depth. 3' deep is easier to dig and has better lighting for floor based beds, but the glass profile had to be built with several angles. This presented more surface area for heat loss and less depth for geothermal capacity. I opted for 4' deep, which allowed me to make a straight glass roof, also allowing for deeper and narrower space for better geothermal properties and less glass for heat loss. It does have the tradeoff of less headroom for the forward grow bed, which I offset by using part of that area for the fish tank. I also used that area for the tank because it gets less direct sunlight.

Sinking the fish tank into the ground and placing it in a far corner allows for maximum geothermal mass to keep the tank at an even temperature. The tank will be constructed by digging that area of the walipini deeper into the ground and placing a surface frame with a pond liner into the space. The pressure of the water plus the top frame will hold back any possible cave in into the tank.

The grow beds will be sectioned into five 6' long partitions, and a six position index valve will be used to cycle through the grow beds and the rear hanging garden. Only 16 cubic feet of water will be used at a time to flood each bed to a depth of 10", a 9" fluctuation in total water depth in the fish tank.

The dark red lines are the angle of the sun in Iowa at mid-summer, fall/spring, and mid-winter. The blue lines are the reflected light path of the winter sun to illuminate the front beds.

The walipini cold frame pictured below will have room for 12 standard transplant flats. The window is double panes of glass with large gauge bubble wrap between them for improved insulation. The sides will be treated and painted wood with aluminum foil glued to the inside surfaces to reflect all available sunlight to the plants. The dry well in the bottom is a pit with a well perforated 5 gallon bucket in the center surrounded with gravel.

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