Solar Food Dryers
Larisa Welk and Lucien Holy
Starting with a request from a reader for an inexpensive solar food dryer, this project has grown into a valuable learning experience. Here are two types of solar food dryers we would like to share with you. One is for humid climates, two others for dryer climates.Solar Food Dryer for Humid ClimatesLarisa WelkWe dehydrate nearly all our food from our 1/4 acre garden except tomato sauce, salsa, pickles, sauerkraut, juices, and some fruit sauces. We also root cellar spuds, roots, squash, etc. Who needs a freezer? Our pantry is crammed with organic, nourishing foods for our simple, a la Nearings cuisine.For years I 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. Some didn't work at all if not tracked periodically during the day. It was with this background that the "idea light" came on in my head.Cat on a Hot Tin Roof TheoryOne day 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 lying around and a corrugated metal roof built over our old trailer-house. Using a ladder to get to the roof, I put the screen down first and put the food on it. I wanted to keep the sun off the food itself 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 lying around with the screen.Later that afternoon I thought I'd see how it was doing. The greens in the "dryer" were still quite limp when 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 as if 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. The Basic Design PrinciplesI found through experimenting that the primary ingredients for this dryer were: ¥ corrugated, galvanized metal roofing ¥ screen ¥ black, porous cloth ¥ glazing ¥ slope.The sun shines through the clear glazing onto the black cloth, heating up the air space under the glazing. The corrugated metal provides air spaces under the screen for the warm, moisture laden air to move. The air moves passively upward along the slope, carrying away the moisture from under the trays of food. The galvanized metal also gets hot and reflects heat back onto the food. This combination really gets the job done. The Deluxe Super DryerUsing these basic principles, I built a 4 foot x 12 foot , waist high "shed" (I store extra wood under this roof). The 4 foot width enables me to reach easily from either side. You could make this wider if you wanted. The roof pitch is approximately 15 degrees. The legs are treated wood and stick into the ground about 6-8 inch . Next I built twelve 2 foot x2 foot screens made from 1 inch x2 inch pine and 1/4 inch hardware cloth (this size of screen is easy to handle. They were originally 2 foot x 4 foot and I cut them in half).The glazing is Kalwall¨ Sunlite¨ and is the most expensive part of the system (it holds up better than glass in hail storms and weighs less). My neighbor has since built a dryer and used acrylic glazing. It was much cheaper but time will tell which material lasts longer. The framework for the glazing is attached to the dryer with T-strap hinges on both the north and south sides. These were made into loose pin hinges so you can openthe dryer from either side by pulling the pins and lifting the lid. A prop stick holds the lid open.For cloth I've found polyester double knits resist fading better than natural fibers (at last, a worthwhile use for this stuff). Be sure to hem the edges so you won't end up with fuzz or fibers in your food.I use fiberglass screen on the trays to keep the food from contacting the galvanized hardware cloth and also over the top of the food to keep it from sticking to the black cloth. I cut the screen double the size of each tray so it can be folded over the food. Stainless steel screen would be the best but I don't know of an economical source. If I used it I would still probably use hardware cloth underneath for rigidity and because having removable screen facilitates pouring food into containers and makes cleanup easier. What It Can DoEven in Minnesota the sun can dry all of these foods easily: apples, green beans, peas, corn, cabbage, broccoli, cauliflower, peppers, kale or any greens, herbs, melon, fruit leathers, strawberries & other berries, plums, beets, onions, mushrooms, squash, eggplant, tomatoes, asparagus, celery, bananas, etc. The dryer can also be used to crisp bean pods for threshing, small grains before storing, and to dry corn before shelling and grinding. When using the dryer this way, I do not use the black cloth since I do not want these items to get too hot (I save seed from my beans and corn). TechniquesWhen using a solar dryer, an accurate weather forecast to ensure proper timing is essential. Really wet foods (corn, melon, strawberries, etc.) will take at least two good days of full sun. The first day is the most critical. The food needs to get dry enough to coast through the night before finishing off the next day. Sometimes food will not be finished until the third day or longer, depending on the weather. If food is nearly dry, a raining spell will only postpone the process but the food won't spoil. Greens and herbs will be done in one day. My definition of "dry" is crispy for all vegetables, though fruits can remain somewhat pliable.Foods need to be cut in uniform pieces for best drying. For example, you'll need to dry celery stalks separately from the leaves. Placement in the dryer is important also since the warm, moist air rises. Foods entering their second day in the dryer should be below freshly cut up foods. Herbs can always go lower where it is not quite as hot. Foods dry faster if stirred once or twice although this isn't absolutely necessary. Melons and other sticky foods should be peeled from the screens when partially dry and flipped before they become permanently bonded.The only foods I steam blanch are sweet corn, peas, green beans, and asparagus. Because of the length of time it takes to pick and prepare the 18 to 24 dozen ears of corn we normally do in one batch, we pick it in the evening and steam blanch it immediately. I spread the ears out all overthe kitchen counters to cool for the night. Early the next morning I cut the already somewhat shriveled kernels from the cobs and have it all out into the dryer before the sun starts it work. If I started in the morning with picking, it would take until about 1:00 pm for all the corn to be blanched, cut, and into the dryer Ð too late for corn in this humid climate. Be sure to put away your dried goodies before the evening dew has remoistened them, but do allow the foods to cool off if you bring them in during the heat of the day. Store dried foods in airtight containers (a good use for all those extra canning jars you won't be needing) in a cool, dark place. ImprovementsIn eight years of use, there are a couple of improvements I would make in this design. I would build all the trays and glazing framework out of cedar instead of pine. Half of the original dryer has been rebuilt so far since the pine didn't hold up, even though the wood was painted with linseed oil. Furthermore, I would make the slope of the unit adjustable so it wouldwork better later into the fall when the sun is lower in the sky. Other than that, this dryer has been a real workhorse. Some of my neighbors use the dryer on my off days so it is often filled to capacity. With nearly 48 square feet of tray space, it can preserve enough food for a very large family or a group of smaller families.ATTENTION, PLACE DIAGRAM OF HUMID CLIMATE DRYER HERE. Solar Food Dryer for Hot ClimatesLucien HolySome older books on food dehydration recommend sulfiting even though it is now known to be very bad for asthma sufferers. Besides, another name for sodium bisulfite is "Sani-flush" toilet bowl cleaner! Yummy! Another treatment is sulphur dioxide created by burning sulphur. That is very polluting and breathing the fumes can damage your respiratorysystem. Treatment with ascorbic acid (Vitamin C), citrus juice, or nothing is more to my taste.The problem with many solar food dryers is that they are often solar ovens with vents, one design even has reflectors. If it looks like an oven, then on a good day it will become an oven. A solar oven is compact, tightly sealed and reaches up to 300¡+F. Even simple box ovens go over 200¡F. In contrast, the requirements for food dehydration are a constant change of air, roomy interior, and a temperature of under 120¡F (the temperature at which nutrient loss begins) with little or no chance of reaching cooking temperature. After all, food drying is a long process, and you don't want to constantly monitor and adjust the unit to avoid ruining the food through excess heat. Direct sunlight on the food is undesirable as it tends to bleach out color and flavor and dry unevenly.TAPThe solar device that does these things is not a solar oven, but a Thermosyphon Air Panel (TAP), which is a vertical solar air heater. My final designs are based on a separate TAP collector and dryer box. A box is the ideal shape for the dryer section, and is easily modified. Oven-like designs result in cramped space, poor ventilation, uneven temperatures, and odd shelf arrangements.Cardboard solar ovens, popularized by Joseph Radabaugh's book "Heaven's Flame" proves the practicality of this type of construction. Most old solar dryer plans require 50 or more hours of work, a shop, and money. Worse yet, they work well for someone, somewhere, but I havefound that dehydrators must be designed for a particular set of conditions and uses.You can quickly make a simple mockup with cardboard boxes and Saran Wrap¨ glazing (staple or tape on) and using a thermometer, and quickly arrive at a new design that works under your conditions. You can for example, enlarge your collector in a few minutes with a razor, tape, and cardboard.CollectorI use a collector about twice the size of my dryer section. One advantage of using a separate TAP is that the area ratio can be anything you need. InsulationIn a hot climate you don't need insulation because the temperature difference between the 110¡ - 120¡ inside air and the outside in the sun is very little. In a cool sunny area insulation will improve performance. You can use corrugated cardboard or use a double box with the space filled with wadded newspaper. Since the insulation is on the outside you may also use hard foam.GlazingI use Saran Wrap¨ glazing for my experiments because it costs 2¢ per sq.ft., is easy to apply, heat resistant, and food safe. Just tape or staple it to your collector. Oddly enough, it worked so well that it became my standard glazing, even for box ovens at 220¡F! It is very thin and very clear and passes more light than the usual glass, Plexiglas¨, Kalwall¨and Sunlite¨, etc. For oven use apply it with a loose fit because it shrinks when heated, Saran Wrap's¨ 'cling' quality makes it unnecessary to tape the 11 1/2 inch wide sheets together, just overlap them one inch. Air Flow Moving air by the thermosyphon method requires a vertical layout. If you want a really large collector, like 4 foot x 8 foot , then it can get rather awkward. If I were to go to a really large unit I would use a horizontal collector with positive air flow provided by a solar-powered fan. These are available in several sizes and are not expensive. Solar vents are perfect because they produce airflow in direct proportion to sunlight. I have built a small unit of that type because, stored vertically, it only takes up 1 sq. ft. of area in my apartmentTapeThe best tape for solar use is aluminum duct tape like Reflectex¨. By the way, this tape makes quick, easy, durable reflectors. Just apply rows to the backing until it is covered.ShelvingI'll leave that up to you, to suit your needs, if you use the usual aluminum screen, then you don't need a frame for most sizes. Do not use lemon juice on your food to be dried if you use aluminum screens. Use non-toxic materials, wood dowels or strips, netting or cheese cloth, etc. Remember, it must allow air flow.Passive Food DryerThis passive design uses two L=23 inch W=13 inch D=10.5 inch cardboard boxes with dryer dimensions of L=13 inches W=10.5 inches D=10 inches . The one disadvantage of corrugated cardboard construction is that it deteriorates when exposed to the elements, especially moisture. I brush on 50/50 polyurethane varnish and thinner, this not only water proofs and preserves the cardboard, but saturates it, bonding the fibers together for a very durable material. Cure well in the sun before using.