The following is an excerpt from The Carbon Free Home: 36 Remodeling Projects to Help Kick the Fossil-Fuel Habit by Stephen & Rebekah Hren. It has been adapted for the Web.
Evaporative Cooling Box
Project Time: Afternoon.
Cost: Inexpensive ($5–50, depending on materials used).
Energy Saved: High. Can be high if used to replace an electric refrigerator. Can also quickly be made during camping trips or blackouts when no other refrigeration is available.
Ease of Use: Medium. Food temperature will be uneven, and in summertime the box will be considerably warmer than a mechanical refrigerator, resulting in quicker spoilage.
Maintenance Level: High. Pan must be kept filled with water. If kept outside, animal intrusion is a concern. The water will eventually degrade the wood and metal in the structure. If used for long periods of time, the covering cloth will need to be laundered regularly and will eventually degrade.
Skill Levels: Carpentry: Basic.
Materials: A large pan, wood or bamboo for shelving and basic framing, clean cloth or burlap, screen, nails or screws, old bike tires, staples.
Tools: Hammer or drill, saw, scissors, stapler.
Easily the most adaptable of all food-cooling contraptions is an evaporative cooler. Taking advantage of the fact that as water evaporates it draws heat out of the air, this type of cooler can be constructed in a variety of ways from the slapdash to the sturdy. The basic premise is to keep food in the shade and covered with, but not touching, a wet cloth. The cloth is kept wet through capillary action by having its edges hang in a pan of water. The pan of water also helps keep away ants and roaches. The breezier the location, the better, as this will increase the evaporation rate and keep the temperature down.
Traditional evaporative coolers. This is the basic principle of the zeer pot, commonly used in West Africa for keeping fruits and vegetables fresh. In the zeer pot, a smaller clay pot fits inside a larger one. The gap between the two pots is filled with sand and then with water on a regular basis (usually once or twice a day). Fruit, veggies, and leftovers are placed in the smaller pot, which is covered with a lid or cloth. The water in between the pots slowly evaporates, keeping the food cool. Another example is the coolgardie safe, an evaporative cooler that was quite common in Australia at the beginning of the twentieth century. Both these systems work best in drier climates.
Impromptu coolers. The simplest method of creating an evaporative cooler is with found objects around the house. It would be easy to combine a couple of garden pots of different sizes and some sand to make an effective small cooler similar to the zeer pot. Be forewarned that some ceramic garden pots have lead in the glaze, which could potentially spread to any stored food.
Building the box. For longer-term use or for more space, building a screened box with scrap wood or bamboo with a functioning door could be a big help. The principles are similar. The food sits on shelves above a pan of water covered with cloth. This can be kept in the home, out of the sun in a breezy location. The basic structure should look like figure 5.4.
A basic box frame is built, of any size and with the desired shelf arrangement; shelves should be made of slats or other material to promote air circulation and maintain even temperature and humidity. The two sides, top, and back can be kept bug-free by stapling on screen, as the cloth alone is usually not completely effective in keeping out flies and other bugs. Note that ungalvanized screen will rust relatively rapidly, so use stainless steel or nylon.
The door. The door is made separately. Crossbracing will help ensure durability. Staple the screen to the door. Hinge the door on the top of the cooler, or leave it disconnected and hold it in place with rotating pieces of wood or rubber hoops made from cut-up strips of spent bike tires. The door can be left free of cloth. A flap of additional cloth can be left on the top of the evaporative cooler to cover the door when it is closed.
Variations on the theme. There could be many impromptu variations on this theme, based on one’s resources, needs, and time. It’s important to remember to take the simple steps for food storage described in the beginning of this chapter, because generally speaking an evaporative cooler will reduce the interior temperature only by about 15 degrees F (8 degrees C) compared to the ambient air temperature. Often this is well above the desired constant 40 degrees F (4 degrees C) of conventional refrigerators. Nevertheless, as the graph in the introduction relates, even a 15- degree F reduction will help inhibit the populations of fast-growing mesophilic bacteria that can spoil food quickly.
Image courtesy of EnergyBulletin.net.