How To Make Your Own Solar Food Dehydrator
Taken with permission from GeoPathfinder
Not everything can be eaten fresh. Some food may need to be stored for later consumption. Reducing the amount of energy you use for food storage will further shrink the carbon footprint that you have already reduced by eating locally, growing your food biologically, and replacing your fossil-fueled tools with something more task-appropriate--like a garden fork.
Of course, food that is fresh is best, but our Minnesota winters challenge that approach. The next best thing to harvesting just before your meal, is to have food that is "live stored"--food that keeps itself. All you have to do is provide the right microclimate for the veggies and fruits that have this hibernation factor built into their biological cycle.
Next in our nutrient-saving, energy-saving hierarchy is food drying, or dehydration. Solar-dried foods don't require fossil-fueled energy inputs to store them, unlike refrigerators and freezers. Just a tightly sealed glass jar will do!
A Solar Food Dryer That Works!
Over the years I've tried about every solar dryer design imaginable. The only common factor in all those attempts was their very limited usefulness here in the humid upper Midwest. None of them could reliably turn food into a non-moldy finished product, unlike the many successful electric models I had built for myself and friends. Some didn't work at all if not tracked periodically during the day. Others were simply too slow, exposed the food to sunlight, or relied on electric backup at night.
This is our finished solar dryer, 12x4-feet, with 2x2-foot stainless steel screens framed in 2x2-inch cedar, and one of the 4x4 foot heat-generating solar collector panels raised for access. Sunlight shines through the clear glazing, lowering the frequency of the light to make more infrared (heating via the greenhouse effect). Moving inward, it then hits a black-painted aluminum sheet, heating the metal. The back side of the black aluminum re-radiates heat onto the food below, causing the food to heat up and lose moisture while keeping sunlight off the food itself. Moisture given off by the heated food passively flows up the sloped air channels under the food screens. And the galvanized steel roofing, with raised ribs forming the air channels, reflects heat back up toward the food, improving overall efficiency. It's not the prettiest dryer, not the most compact, and not the cheapest. It simply does what it is intended to do very well by following the rule I always tell people, "You can't fight physics!"
This photo shows a side view of a model solar dryer--only 2x2 feet. You can see one of the loose-pin hinges connecting the solar collector to the base on the north and south sides. The stainless-steel food screen is at the bottom of the removable, 2x2-inch cedar-rimmed tray in the middle of the stack. And you can see the air space formed below it by the ribs in the galvanized roofing. More below!
The Hot Tin Roof Theory. 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 onto 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
* Corrugated, galvanized metal roofing tilted toward the sun
The sun shines through the clear glazing onto the black surface, causing it to heat up. This heat is radiated from the black surface and onto the food screen below it. The shiny, sloped metal roofing that everything rests upon reflects heat back up toward the food. Also, its corrugations provide an airspace under the screen for moisture-laden air to circulate up the slope and out of the dryer by natural convection. The food is not exposed to sunlight and retains its color. This combination of design factors 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
Building the Deluxe Super Dryer:
Using the basic principles and design criteria established from our experimentation, we built a 4x12-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 2x2-inch treated wood and stick into the ground about 6 to 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 4x4 feet with four 2x2-foot trays. You can make more modules of this size later if you need to add capacity.
Our dryer holds twelve 2x2-foot screens made from mitered cedar 2x2s, with a deck screw in each corner. The lower inside edge of the frame is cut away so that the screen is recessed. 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 keeps food particles from getting stuck between the wood frame and the screen. A less costly screen option is to use food-grade polypropylene screening (available from www.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 and cans, galvanized metal, and aluminum screening or cookware.
The cover framework can also be constructed from 2x2-inch mitered cedar. The glazing 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.
Glazings in summary:
|Kalwall, Filon, Lascolite, etc.||FRP, fiberglass-reinforced polyester||Durable, light, impact-resistant, good light transmission, surface degrades over time and yellows slightly, rough surface of old glazing requires occasional scrubbing to keep it clear, easily cut|
|Glass||Glass||Fragile, heavy, variable impact resistance, great light transmission, no surface degradation or yellowing over time, cheap if recycled, hard to cut to size|
|Lexan, etc.||Polycarbonate||Very durable, light, highly impact resistant, great light transmission, no visible yellowing or surface degradation, expensive, easily cut to size|
|Plexiglass, etc.||Acrylic||Somewhat fragile, light, poor impact resistance, good initial light transmission, cracks and yellows with exposure to sun and heat, fairly inexpensive, hard to cut without cracking|
|Thin films||Polyethylene, etc.||Fragile, very light, fairly impact resistant, great light transmission, degrades quickly in sunlight unless cross-linked polymer, UV-resistant material is used, cheap if recycled from commercial greenhouses but can be pricey otherwise, very easily cut to size|
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.
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
* Removeable 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 wall on the north side of the dryer to increase solar gain at higher latitudes
* Brown or blue paint on the collector's metal sheet to avoid over-heating nearer the equator
* East and west sides enclosed to prevent excessive cooling by cross-flow winds
* The collector can also serve as a coldframe cover if the black, heat generating material is removable or separate
Note: The above is all excerpted from "A Pantry Full of Sunshine", by Larisa Walk, Copyright 2002, available as either a download or as a physical copy on our Home Page.
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?
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.
These are sliced red sweet peppers ready for a day of sunshine. They can also be cut using a potato frencher, which makes 1/2-inch pieces instead of strips.
These are drying tomatoes. The variety is Principe Borghese, which is available commercially or through Seed Saver's Exchange. We cut them in half and squeeze out the juicy seed cavity (you can drink the innards), then place them skin-side down on a stainless-steel cookie sheet until somewhat dry. Then move them to the stainless screen.
Ah, the middle of summer, and we're up to our eyeballs in sweet corn! Even growing an open-pollinated variety that doesn't mature equally, we still have a hard time eating more than a dozen ears per day when they really start maturing quickly. Here we see about 150 ears, picked the night before and steam-blanched, cooled overnight on the kitchen counter, and cut off the cobs the next morning. The overnight cooling actually starts the drying process. And if we tried to do the whole process in the morning, we wouldn't have it out into the dryer until noon. That would be a terrible waste of sunshine!
This is a closeup of two trays of sweet corn. The top one is freshly loaded, sitting at the top of the dryer so it gets the maximum warmth on its first day. The bottom tray has been drying for one day and is almost done. The variety we're growing is called Tru Gold, and it's available commercially as Organic seed. We use it in soups, stews, and just rehydrated with water. As with all processed foods, it's not as nutritious as fresh corn, and not as fresh-tasting as frozen corn, but Summer doesn't last long up here, and frozen corn uses more energy to keep it cold than it could ever provide to your body! Depending on how hot and sunny the weather is, the sugars in the corn could end up more or less carmelized, making the flavor actually sweeter and more intense.
This shows part of the solar dryer filled with apple chunks from wild apple trees growing near our home. They've been cut in half, cored, and run through the French-fry cone on the food chopping attachment of a Kitchen-Aid mixer. Note that we have left more airspace between pieces on the lower screens since the dryer will not be as hot there. In late September, with our lowering sun angle and very cool nights, they take two or three sunny days to fully dry.
Here you can see a typical mid-October dryer load, shot from the South. At the top, in the warmest spot, is a tray of freshly cut mushrooms. Below them is a batch that has dried for a day on cookie sheets and is transferred to cotton kitchen towels to allow better air flow, to catch small pieces that might go through the dryer's stainless screens, and to free up cookie sheets for the next fresh batch. We start them on cookie sheets because the spores given off before they begin drying can stain the cloth.
Below them are white oak acorns, drying in the coolest spot to ready them for shelling. The nut meats will be boiled to remove some bitter tannins, then laid out at the top of the dryer. When dry, they can be ground into nut flour or chopped up for use in pancakes, breads, and so on. But white oaks only produce seeds every 6 to 10 years, so be prepared!