Building a Solar Dryer

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The best design will be ready to go when you are. It should be fairly light, easy to transport and have little or no setup time. And it should be compact and easy to store, either outside or in a garage or garden shed.

The Solar Food Dryer by Eben Foder will teach readers a new way of cooking and baking foods using a solar dryer. Learn to build a solar dryer to fit specific needs, follow the guidelines Foder provides. Find new and exciting recipes that can be used with a solar dryer. Find this excerpt in Chapter 3, “How Big a Dryer Do I Need?”

Portability, Capacity and Size

A major consideration in the design of a solar food dryer is the convenience factor. If your dryer is not easy to use, you may find that you’re simply not using it very much. There are probably a few old, homemade solar dryers sitting in storage that aren’t getting used because they are just too big and heavy to move, or too much trouble to set up. The best design will be ready to go when you are. It should be fairly light, easy to transport and have little or no setup time. And it should be compact and easy to store, either outside or in a garage or garden shed. I leave mine outside and just put a tarp over it when I know I won’t be using it for a while. It may sound appealing to build a dryer with a huge capacity. But a bigger capacity equals bulkier size, more weight, and extra cost. Don’t overbuild your dryer or you risk ending up with an unused piece of “yard art.” Consider how much food you are likely to have on hand at one time and how much time you want to spend preparing and loading the food. The 6-pound capacity of the SunWorks SFD is plenty for most home users. Processing and loading 6 pounds of tomatoes or apples takes about 30 to 45 minutes. Twelve pounds would take twice as long. Since you can dry a new load of food every two days, consider how much food your garden will produce in two days as a measure of how much capacity you might need. If you find that you don’t have enough capacity in your dryer, having a second dryer may be the best way to go.

Materials

To produce a quality food dryer, you’ll want to select materials that will hold up well in the outdoor environment. Any wood that contacts the ground (like the dryer legs) should be rot resistant, like cedar. Treated wood should not be used in any area that has contact with food, or that may come in contact with your hands during loading and unloading of the dryer. Untreated pine, hemlock or fir wood will last for many years outdoors, as long as it’s not in contact with the ground. Exterior-grade plywood is an excellent material for making a sturdy, lightweight cabinet.

The two materials of particular interest in the solar food dryer are the glazing and the food screens.

Glazing

Any clear or translucent material that transmits a high percentage of solar radiation is a candidate for solar food dryer glazing. Listed below are some of the possibilities.

Glass is one of the best solar glazing materials. A single pane of clear window glass transmits 86 to 92 percent of incident solar radiation, and filters out most of the higher frequency UV radiation. One of the best features of glass is that it’s easy to clean and does not tend to get scratched. It holds up well outdoors, does not yellow or become hazy and can last indefinitely, if not broken. It’s also readily available at most hardware stores and is about the lowest-cost material that can be used for a
durable application like a dryer. On the down side, glass is brittle and can break fairly easily.

It’s also heavy and has sharp edges. Tempered glass breaks without the sharp edges, but it costs more than regular untempered glass and isn’t significantly stronger.

Fiberglass-reinforced polyester is a thin, durable sheet that is lightweight and reasonably priced. This material has been widely used for solar collector applications. It transmits 84 to 90 percent of solar radiation. It’s translucent, so it won’t provide a good view of your food, if that is part of your plan. It’s very resistant to breakage. It is flexible with little structural strength so it may sag in the middle if not stretched tightly or supported, and will also expand when heated.

Polycarbonate sheet is transparent and may be the closest alternative to glass. This is the same material that most sunglasses and safety glasses are made from. It’s hard and tough, and won’t scratch easily. It’s a rigid material that doesn’t bend easily, and it’s lighter than glass. Polycarbonate is available in clear sheets for outdoor applications like greenhouses. It comes with a UV-resistant coating on one side that blocks most UV and protects the material from aging. It has a high thermal expansion (like many plastics), which can make mounting and sealing difficult. Proper mounting should allow for movement due to thermal expansion. This material is available at some hardware stores and is fairly expensive. Acrylic (Plexiglass) is available in clear sheets. It’s not nearly as strong or hard as polycarbonate, yet costs almost as much. It will scratch easily and yellows with time, reducing its performance for solar applications. It has similar thermal expansion characteristics to polycarbonate.

Plastic films are generally not sturdy or durable enough for solar dryer applications; however, some designs still call for them. They can provide a cheap and quick glazing material for experimenting with solar energy, or act as a temporary glazing until you can find something better. The best plastic film is probably polyester (Mylar). It’s clear, very tough, fairly heat tolerant and doesn’t stretch or sag. Some polyester films are designed especially for outdoor applications, so these would be the ones to consider. The most common plastic film is polyethylene. It is sometimes used for cold frames or as a covering for temporary greenhouses.

While cheap, polyethylene is not very durable and will fall apart within a couple of years.

Food Screens

The screens that the food rests on in the dryer must be made from a mesh that allows plenty of airflow. The screens should be made from an inert (non-reactive), food-safe material that can withstand dryer temperatures as high as 200° F

(93° C). Good screens should not stick to food and will be easy to clean. Furthermore, the material should not stretch or sag when heated and loaded with food. All together, this is a pretty tall order, and few materials can make the

grade. Certain popular screen materials are not suitable for food applications because they are reactive. Galvanized metal screen and wire mesh, for example, contain zinc and other metals that could oxidize and dissolve into the food. Aluminum screens may be suitable for many foods, but can react with the acids in

tomatoes. Polypropylene screens are available in food-safe materials and are strong and easy to clean. They come in various forms, ranging from a fine screen material that can be used as the main screen on drying racks, or as thicker, coarser screen inserts that can be placed on top of other drying racks.

Fiberglass window screens work well in food dryer applications but may not be safe for food contact. Fiberglass screens are made from woven fiberglass coated with vinyl. While vinyl is safely used in many food-contact applications, the plasticizers used with window screens are not FDA approved for food uses. If you are using fiberglass screen in your dryer, consider placing the food on polypropylene tray inserts.

Stainless steel screens are expensive and hard to work with in terms of cutting and bending. Stainless steel comes in many grades, depending on the alloys it is made with, so it’s important to use a food-grade stainless. Some grades may leach metals and flavors into foods.

More and more of the stainless screen material is made in China, so it’s difficult to be certain of alloy content. Another issue with stainless is that it is reflective, so it will bounce some solar energy back out of the dryer, if your design uses direct solar heating.

Screen options are rather limited right now, but with growing interest in solar food drying we can hope for more products to emerge in the near future. Other materials may be suitable for solar dryer screens, including nylon, polyester, Teflon-coated fiberglass and natural fibers.

Using Recycled Materials

Using recycled or reclaimed materials for constructing your solar dryer is a great way to reduce consumption of natural resources and save money at the same time. Window glazing and screens are likely to be the most expensive part of a solar dryer, but it’s possible to find them for next to nothing at recycling centers. If there is no recycling center nearby, try calling a window replacement or home weatherizing company.

Windows and screens from old aluminum storm windows and storm doors are best, but wood frames will work too. Always replace the old screen material with new material — preferably polypropylene screening — before using it in your dryer. Plywood scraps, which are useful in making the cabinet, are plentiful at construction

sites where exterior-grade sheathing is cut out for windows and doors. Avoid particle board, though, which is not as strong or durable. You may be able to find cedar boards left over from decking and fencing projects. Avoid using treated

wood in any part of the dryer that may contact food. More information on using recycled materials is provided with the instructions in Chapter 4.

Types of Solar Dryers

There is a wide range of solar dryer designs, and probably just about all of them work to some extent. Each design may have certain advantages.

So the question is not so much “Will this work?” as “How well will this work?” and “Is this the best dryer for my needs?” What distinguishes the various dryer designs most from one another is their relative performance and sophistication. But two other key factors are the cost and the time it takes to build them.

We can start by considering open-air sun drying. Sun drying merely requires a tray or rack for food that can be placed outdoors. This works fine for some applications, and there is little or no material cost involved. However, sun drying is relatively slow and results are unpredictable. Food is exposed to the elements (rain, wind and dust), and constant supervision is needed to make sure raccoons don’t run away with it, birds don’t poop on it, mice don’t munch on it, and ants, bees and flies are kept away.

Placing food on a rooftop, hanging it on a line, or making an elevated drying rack like the Native Americans used to do will solve some of these problems. It works, it’s just not the most convenient, efficient, secure or sanitary way to dry food. And the quality of the food may suffer if drying times are prolonged or weather changes.

The first stage in solar drying, in terms of design sophistication, is the “hot box” dryer in which the food is enclosed and protected in a box or tent with a clear covering. This design might resemble a cold frame or a miniature greenhouse. 

Tips on Using Recycled Materials

Recycled materials will generally produce excellent results and will save you quite a bit of expense and go even easier on the planet. Minor design modifications may be necessary to adjust dimensions to fit the materials available.

If you have a recycling center with building materials, look for aluminum windows and screens
that are about the same size as the ones specified here (or slightly smaller). These are the highest cost item in the solar dryer, so yield the biggest savings. Old aluminum storm doors and storm windows are a good source. Damaged screens can be repaired if the aluminum frames are in good condition. Wood-framed windows and screens will work too, but add weight to the dryer and
may need to be replaced sooner.

Make sure glass is clear and not tinted. It doesn’t need to be tempered. The glass should be about
the same size as the screens or slightly larger. Double glazing is not recommended because it adds
weight and isn’t necessary for good dryer performance.

The metal absorber plate can be made from any sheet metal that is thin and bendable, so check
scrap metal yards and recycling centers. Aluminum material is best, since it is lightweight and doesn’t
rust.

Scrap plywood is often available from construction sites and from your own past building projects.
Stick with 1/2”-thick material, if possible. Thinner material (down to 3/8”) will work, but is not as
sturdy. T1-11 plywood siding is fine, but don’t use any particle board, LP siding or similar composite
material. The 3/4” trim stock used here can also be made from straight, select 2×4’s, if you are handy
with a table saw. Recycled hardware will also work well, including handles, hinges, knobs, and latches.

Since it is unlikely that you will be able to find recycled glass and screens of the exact size specified
in these plans, you will need to adapt this design to what you have. Resize the width and depth
of the dryer cabinet to fit your two screens. Then add trim or plywood to the top of the cabinet to
make it fit your glass. Try to adhere to all the vertical measurements outlined here, especially for internal components and vent areas.

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Reprinted with Permission from The Solar Food Dryer and Published by New Society Publishers.