One day (in 1985) I needed to dry a bunch of greens and the current solar dryer was full (a couple of handfuls was all it could handle). I had an old window screen laying around and a corrugated metal roof built over our old mobile home. Using a ladder to get on the roof, I put the screen down first and put the food on it. I wanted to warm the food while keeping the sunlight out so I covered it with a piece of black cloth. Then to keep everything from blowing away or being bothered by flies, I covered it with the storm window that was laying around with the screen.
Later that afternoon I thought I'd check up on the experiment. The greens in the old dryer were still quite limp so I crawled up the ladder to take a look at the stuff on the roof. Much to my surprise, the roof-top greens were crispy dry! It looked like I had finally stumbled on something that worked. I tried several other foods on the roof before I was convinced enough of the design to build a unit at ground level for easier access.
Basic Design Principles
I found through experimenting that the primary ingredients for this idea were:
glazing (glass or greenhouse plastics) to seal out rain and raise interior temperatures
black surface over the food (fabric or metal) to keep sunlight off the food
food-safe screen to hold the food and allow moisture movement outward
corrugated, galvanized, metal roofing tilted for airflow and aimed toward the sun.
So this is the combination of design factors that also met the criteria of what I thought would be the ideal solar dryer:
utilizes passive solar energy,
has no moving parts
no tracking required to follow the sun
food not exposed to sunlight *
spacious enough to accommodate large pickings
moderate temperatures to dry quickly but not overheat
easy to use and clean
absolutely must work reliably, even in high humidity
not necessary to remove partially dry food from the dryer overnight **
stable in windy locations
mostly "critter" and bug-proof (if there are bears in your area all bets are off!)
*The only exception to the "no sunlight" rule that we know of is mushrooms. Some species of mushrooms can be dried using sunlight exposed directly (no glass or plastic shielding them from UV light) on the spore side to produce loads of extra vitamin D. Lentinus (shiitake) mushrooms can have 10 times or more their normal, already high levels, making them almost a supplement instead of a food, and making vitamin D overdose a possibility among mushroom lovers! Check out Paul Stamets' books on mushroom culture for more details.
** There are a couple of exceptions depending on where you are located and what you are attempting to dry. If you have bears around and they are hungry (when aren't they?) you either need some MAJOR fencing or you should just bring the "bear bait" indoors. And if you are drying something intensely sweet, ants may discover your food by climbing up the dryer legs. We suggest either applying ant-repellents such as orange or peppermint essential oil to the dryer legs, or making "moats" using cut-off plastic jug tops, trimmed to fit the neck around each leg, mounted upside-down, caulked with silicone, then filled with water.
Building the Radiant Super Dryer:
For the complete building process, you can find a PDF of the details at:
Using the basic principles and design criteria established from our experimentation, we built a 4-by-12 foot, waist-high "shed". The metal roofing on this shed has corrugations that run north to south. The roof angle is approximately 12-15 degrees from horizontal and slopes toward the south (in the northern hemisphere). This gives enough slant so the warm air will rise but not so much that the food will slide downhill. The mini "shed" space underneath can be used to store firewood or garden accessories. The 4-foot width enables food to be reached from either side yet is wide enough to achieve sufficient hot air flow. The legs are 2-by-2 inch treated wood and stick into the ground about 6-12 inches. Metal fence T-posts attached to the wooden frame with U-bolts are a good alternative if you don't like treated wood. Additional leg bracing may be necessary if your dryer is large. A good "starter" size is 4 by 4 feet with four 2-by-2 foot trays. You can make more modules of this size later if you need to add capacity.
Our dryer holds twelve 2-by-2 foot screens made from mitered cedar 2-by-2's, with a deck screw in each corner. The lower inside edge of the frame is cut away so that the screen is recessed a bit from the outside edge. This eliminates the possibility of screen edges snagging clothes when you lean against the dryer. The screen is stainless steel which, although costly, is easy to clean, provides a non-toxic surface for the food, and should last a lifetime. A bead of high-temp, food-grade, silicone caulk (any of the "100% silicone" caulks without added "mildew resistance" biocides will do) keeps food particles from getting stuck between the wood frame and the screen. A less costly but controversial screen option is "food-grade polypropylene" screening (available, among other sources, from Dryit.com). You will need to provide support under this material to prevent sagging. You could use galvanized metal fence wire or hardware cloth for this as the food would not come into direct contact with the metal. My original experiments used fiberglass window screen, but I have since discovered that it is coated with polyvinyl chloride that is stabilized with several substances, one of which may be lead. Please do not place food in direct contact with any material that is not absolutely food safe. This would include plastic garbage bags/cans, galvanized metal, and aluminum screening or cookware.
And while some plastics (polypropylene, HDPE, nylon) are advertised as "food-safe" that may be just a self-serving boast. With just what IS currently known about xenoestrogens and plastics we prefer the metal. Some bloggers, without an actual basis in data, have questioned the purity of Chinese-made stainless steel screening, implying that it might contain some impurities. Metallurgically, type 304 or 316 stainless steel is made to a strict formula. No matter which chemical impurities, found (in tiny amounts), might possibly react with your food on a metal screen, at least you know that they might actually be necessary as an enzyme-supporting mineral (in tiny amounts) in the human diet. There is no dietary requirement for plastics!
The cover framework can also be constructed from 2-by-2 inch mitered cedar. The glazing we most often used is Kalwall brand, 40 mil, fiberglass reinforced polyester. It holds up better than glass in hail storms and weighs much less. My neighbor built a dryer but used acrylic glazing. The acrylic was much cheaper initially but needed replacement after three years of use due to the formation of numerous cracks. New dryers made by friends and neighbors use locally available clear, corrugated fiberglass greenhouse glazing. It is much less expensive than Kalwall while appearing to be just as durable. Our favorite glazing, and the one we currently use, is corrugated polycarbonate. It is the most durable, yellows the least over time, and remains the least brittle with constant exposure to sunlight. The end channels can either be sealed, using silicone caulk or the foam strips sold for the purpose, or they can be left open. The dryer works well in either case, although sealing the ends may keep the black collector sheet somewhat cleaner.
We have received a number of technical questions about the physics of the glazing, in terms of both necessity and function. When sunlight passes through a light-transparent material it changes a bit. Most texts speak of light "slowing down" when passing through air, water , clear glazing, etc. In reality light does not slow down even though it appears to. Light always travels at "light speed", relative to an observer, but when it passes through a transparent medium other than a complete vacuum its frequency shifts downward (the "red shift"). Supposedly the light regains its frequency on the other side of the glazing, but no energy conversion is 100% efficient, and some of the downward shift still remains. This makes more visible light out of ultraviolet (UV) light and more infrared (heat) out of visible light. So the glazing not only keeps the wind from pulling heat off the black absorber sheet by convection (wind), it also actually helps create more heat for the absorber to radiate downward onto the food.
And if you think you can simply dry foods in open sunlight or with a mere plastic/glass covering, guess again! Some solar dryer designs actually depend on sunlight hitting the food to generate heat (the Fodor design in Mother Earth News publications). Plastic/glass cuts out UV-B radiation but not UV-A, so a layer of black metal or cloth is a necessity to keep nutrient levels as high as possible.
Instead of the black cloth used in the original experiments we now use a layer of thin aluminum sheet, painted with "Barbeque Black" paint on both sides, stapled to the underside of the cover framework. The top is black to absorb the sun's heat and the bottom is black to re-radiate that heat onto the food. You could use steel sheets (28 gauge or lighter - not galvanized so paint will adhere better) or used "printer plates", but aluminum "flashing" is easy to find in 2-foot widths. The black metal heat collector is more convenient to use, less messy, and probably less expensive in the long run than black cloth.
One way to improve the collector's efficiency is to use a "selective surface" or "Black Chrome" paint on the top layer of the metal sheet. These paints absorb sunlight just like common black paint but have less emissivity at lower wavelengths. This means that they re-radiate less heat back upward into space once they get the metal hot. At a typical temperature of 150F on the metal this could mean a 35% efficiency increase. But it comes at a much higher cost and you can only use it on the top side. If you use it on both sides it will reduce the amount of heat radiating toward the food. And if you are at a high altitude or low latitude the added efficiency will lead to higher temperatures which are more likely to burn your food.
The cover framework is attached to the dryer base with T-strap hinges on both the north and south sides. These were made into loose-pin hinges so you can open the dryer from either side by pulling the pins and lifting the lid on the opposing side. A bent steel rod can be inserted into the hinges to prop the lid open. Right angle brackets are fastened to the south edge of the roofing below each screen (where there is a hinge placed) to keep the trays of food from sliding off the downhill side.
All of the cedar is coated with a homemade sealant: melt 1 pound of paraffin wax, remove from heat and vigorously stir in 3 quarts of linseed oil and 1 cup of gum spirits turpentine. When cool it looks like crystallized honey. Apply liberally with a brush. This is also great stuff for windowsills or exterior trim where you want water repellancy. Recoat in subsequent years when needed. The legs and under-framework need no other sealants if you use the treated wood option. ACQ treated lumber is the newest option and supposedly more environmentally safe.
Improvements and Alternatives
a shelf or table nearby as a handy work space
removable or folding legs to facilitate winter storage
1-inch square aluminum framework for the collector instead of wood
adjustable slope to accommodate seasonal sun angle changes or for high latitudes a reflective white/silver wall on the back side of the dryer to increase solar gain at higher latitudes
brown, green, red, or blue paint on the collector's metal sheet to avoid over-heating foods when the dryer is used nearer the equator or at higher altitudes, or use an open-weave shading cloth or window screen over the collector to reduce solar input in hot-sun areas, giving you the option of removing it on cloudy or partly cloudy days to bring the temperatures back up
east and west sides enclosed or shielded to prevent excessive cooling by cross-flowing winds under the food screens
the collector can also serve as a cold-frame cover if the black, heat generating material is removable or separate
other cheaper screen materials could be substituted as long as they are safe for direct food contact.
Keep in mind that YOU CAN DO THIS.
You can substitute what you have for what we've used. You can use recycled materials. You can use what's most readily available for building materials in your area. You can even use your parked car as a food dryer. Just set up screens, cover with black cloth, park with the biggest windows facing the Equator, put the screens in the sun's path, and leave a window open a bit to let the moisture out. This is a much better use for a car than driving it, but why not do both since you're already doing one?
In response to a number of questions about dried food storage, we store all of our dried and canned food in glass canning jars, in a cool, dark room (our pantry). Plastics are not a reliable barrier against moisture and dried vegetables must be kept completely dry (fruits aren't as picky - they can be somewhat pliable and still store well). We actually use old blue-green quarts and half-gallons found at rummage sales, along with the old-style rubber rings and zinc-coated covers. This helps to keep the light out even when the jars are out of the pantry. Both light and heat work against the nutrient value of the contents, so any way that you can reduce them improves storage time.