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The Design, Construction, and Use of an
Indirect, Through-Pass, Solar Food Dryer
Dennis Scanlin ©1997 Dennis Scanlin
Drying is our oldest method of food preservation. For several thousand years people have been preserving dates, figs, apricots, grapes, herbs, potatoes, corn, milk, meat, and fish by drying. Until canning was developed at the end of the 18th century, drying was virtually the only method of food preservation. It is still the most widely used method. Drying is an excellent way to preserve food and solar food dryers are an appropriate food preservation technology for a sustainable world.
Food scientists have found that by reducing the moisture content of food to between 10 and 20%, bacteria, yeast, mold and enzymes are all prevented from spoiling it. The flavor and most of the nutritional value is preserved and concentrated. Vegetables, fruits, meat, fish and herbs can all be dried and can be preserved for several years in many cases. They only have 1/3 to 1/6 the bulk of raw, canned or frozen foods and only weigh about 1/6 that of the fresh food product. They don’t require any special storage equipment and are easy to transport.
The solar dryer which will be described in this article is easy to build with locally available tools and materials (for the most part) for about $150 and operates simply by natural convection. It can dry a full load of fruit or vegetables (7–10 lbs) thinly sliced in two sunny to partly sunny days in our humid Appalachian climate or a smaller load in one good sunny day. Obviously the amount of sunshine and humidity will affect performance, with better performance on clear, sunny and less humid days. However, some drying does take
place on partly cloudy days and food can be dried in humid climates. The dryer was developed at Appalachian State University in the Department of Technology’s Appropriate Technology Program. Over the last 12 years we have designed, built, and tested quite a few dryers and this one has been our best. It was originally developed for the Honduras Solar Education Project, which Appalachian State implemented several years ago. The prototype for that project was constructed by Chuck Smith, a graduate student in the Technology Department. Amy Martin, another Appalachian student, constructed the modified and improved version depicted in this article. Solar dryers are a good way to introduce students to solar thermal energy technology. They have the same basic components as do all low temperature solar thermal energy conversion systems. They can be easily constructed at the school for small sums of money and in a fairly short amount of time, and they work very well. While conceptually a simple technology, solar drying is more complex than one might imagine and much still needs to be learned about it. Perfecting this technology
62 Home Power #57 • February / March 1997
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