So, the genesis of the idea was to make a more advanced meat curing chamber. The first thing to do was to rough out the design ideas. I wanted to cure meat, so that helped set the chamber design conditions. We starting hanging meats to dry a long time ago in caves. Some caves were better than others for the job. Over time, we gained experience about what conditions were the best. Different products might have slightly different conditions for preferred drying, but they cluster pretty tightly around 55F (~13C) and 70-80% humidity.

During the drying process, the temperature in the curing chamber will be relatively stable. Once the meat is at the same temperature as the setpoint for the curing chamber, then the only sensible heating or cooling that will be required is what conducts through the walls of the chamber, or to heat or cool any air that leaks in. That perspective points out the necessity of doing a good job of sealing and insulating the curing chamber. From that perspective, a refrigerator is not a bad place to start. They are sealed up, and relatively well insulated.

A design temperature of 55F means that the chamber is going to need to be colder than most rooms. That means that unless this thing is kept outside, or in an unconditioned space it's going to need more cooling than heating. Identifying the cooling source is going to be key to the design.

A compressor-based refrigeration cycle is what is used in most refrigerators. It's a very efficient way to do cooling. Unfortunately, it's very expensive, and not terribly flexible. There's a compressor, multiple coils, expansion valves, lots of copper tubing, and refrigerant that (probably) requires a license to be able to deal with. The pressure of the refrigerant inside the system drives the temperature that it operates at. We'd have to add or remove refrigerant, or otherwise modify the compressor in order to reach a specific cooling temperature with this type of system. That's not an easily controllable system, and it's the reason that a standard refrigerator is not a good tool for a meat curing chamber.

The pressures that most consumer refrigerators run at produce a cold-side temperature that's close to freezing. The problem with that is that it produces surface temperatures that are too cold. Moisture will condense out of the air inside the refrigerator, just like it condenses on the sides of a glass of ice water. This brings down the humidity inside the refrigerator below what is needed for meat curing, which is why all the refrigerator-based systems require using a humidifier to add moisture back to the air, even though the meats are releasing pounds of moisture into the air,