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How to Build a Solar Food Dryer

Physics In Action!

There are many food dehydrators on the market or that you can build for yourself from purchased plans. Plug-ins or farm-size solar, many people have come to realize that food preservation by dehydration is better in many ways than canning or freezing. Dried food needs no sterile jars and caps, no water bath or pressure canning, uses no electricity for long term storage, and retains more flavor as well as nutrient value than any other method of preservation.

As Americans increasingly turn yards into garden space to grow some of their own fresh fruit and vegetables, interest in more personal-sized solar dehydrator technology is increasing. Such units are not difficult to build, and work as well or better than expensive, professionally crafted dryers without wasted space (or electricity). The thing for the do-it-yourself crowd to keep in mind as they design and build a solar dehydrator is that the technology is all about physics - thermodynamics, to be precise. Once the operations are understood, people can get creative with their designs!

A few of the things to keep in mind are:

1. Sunlight through glass will heat an enclosed space significantly. You will need a way to control interior temperature.

2. Sunlight will heat a black surface much hotter than a white surface. Black absorbs more heat.

3. Heat rises, cool air tends to sink. Convection chambers for dehydrators should make use of this fact to carry moisture away from the drying food.

A solar dryer I made last year uses these principles. I used a 2 x 2.5 foot window salvaged from an old barn. Scraped old paint off and repainted the top side flat black with exterior acrylic. Simply stripped and washed the underside, which faces the interior of the dryer. Stapled black Indian cotton to the frame on the interior side of the window as a sun shade so drying food is not discolored or burned.

Built a box of untreated pine 1 x 6 boards to fit the window frame and painted the exterior flat black. The color of the interior doesn't matter, I wanted to avoid possible chemical fumes because the interior gets to 150º or more during operation, so I left the wood raw. Attached the window to the box at the top end with heavy duty hinges, and put a hook and eye at the lower end so it can be closed tightly. Then I stapled window screen to the box bottom, fitting it tightly so flies and other bugs can't get in. Four little screws above the screen on the interior box sides hold an old refrigerator rack which just happens to fit perfectly. This is the open surface on which I put trays of prepared foods to dry.

The air chamber at the rear used 1 x 2 strips, tightly screwed to the longer side boards but not to the top or bottom boards. I then attached a piece of metal roofing to the back side of those, leaving the bottom and top of this flat chamber open so air can flow freely. The entire unit is 2 foot by 3 foot and just over 10 inches deep, fits nicely on a wooden table on my sunny front porch outside the kitchen. When it's in use I prop the top (hinge) end on a brick so that it's a few inches higher than the lower end, making use of that "heat rises" concept for this flow-through ventilation chamber.

What is being dried will determine whether or not you'll want to keep the window tightly closed for the greatest heat or prop it open a bit so the interior doesn't get too hot. There are good sites on the internet that have tables for this kind of thing as well as recommended moisture content that are useful. You want the food to dry, not bake!

While the food is drying the hot air and moisture is drawn by air flow into the metal-backed chamber and out the top. This evens out the temperature quite well and shortens drying time considerably. Even watery tomatoes will dry to leather in a few hours on a sunny day.

So get busy all you new veggie garden fans, and try your hand at preserving your own food!

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