Building a Solar Food Dehydrator
Today I’m going to show how I built this solar food dehydrator.
First we’ll start with making the base. The entire frame is made with 2 by 4 by 12 foot pieces which minimizes the scrap. The final base size is 36” wide by 48” deep. Depending on the width of the glazing, you may want to make yours slightly more narrow because the finished plywood will make the body about 36.5” wide. I decided to use construction adhesive at all the seams along the exterior to help to minimize air from leaking at the joints. I’m not sure if it really matters, but it’s just for peace of mind. The shell is made from 3/16” plywood, but there’s no problem using 1/4" if you can’t find 3/16”.
The rear wall acts as the chimney to help with the air draft through the dehydrator. At the top, I cut each of the 2-bys at a 45 degree angle so I can put a small roof over the vent to minimize rain from getting in the dehydrator. The plywood hangs over the end of the 2-bys by 2.5 inches so that it can be used to anchor the rear wall to the base section. The top of the sheet ends around 8 inches from to top. This is where the air will exit.
On the front of the chimney, I cut the plywood at an angle so that it would match the angle of the roof section. At the base of the chimney, I left it a couple of inches from the bottom to allow the air to flow a bit easier into the chimney. This entire section is also glued and nailed together.
Next is to attach the back chimney wall to the base. Just plywood holds the two sections together with a little glue and nails. It may not look pretty yet, but I usually prefer function over aesthetics. I then square the two sections and add a temporary cleat to maintain their positions.
The solar collector is next. On mine, I made it 73.5” long and it draws the cool air in from the back. However, most sheets of acrylic are around 70” long so you may want to make this a bit shorter and draw the air in from the bottom. The top section is cut at a 45 degree angle so that it will match against the back chimney. Instead of using a full sheet of plywood, I used to remaining sections of scraps. Since this joint is between the solar collector and the drying racks, it really didn’t matter if some air sneaks between this joint. There should be around 6” of space at the top end of the plywood so the hot air can get into the dehydrating area.
Before attaching the solar collector section, I added some rails along the rear chimney wall. These are just made from some scraps that I ripped down to be around ¾” square. They are spaced so the racks will be2” apart on center. I then made up some more rails that are attached to a 2 by 6 that I ripped a 45 degree angle along its edge. This will attach into the solar collector.
With the unit lying on its back, I attached the solar collector between the base and the chimney wall with some nails or decking screws. This triangle shape makes the unit very rigid and then the cleat can be removed.
To set the rails under the collector, I cheated just a little so I could make sure the drying racks would be level with the racks attached to the chimney wall. I took two pieces of scrap and measured the height of each rail and cut the scraps to that height. I then temporarily attached the scraps to the new rails so they acted as legs to hold them at the proper elevation. Then added some glue and screwed it into place through the collector’s plywood.
For one side of the collector I fastened on a piece of plywood. It doesn’t need any structural support so there are no studs built into it. At this point, I ran out of glue and I didn’t feel like going to the hardware store so I just attached it with nails.
When it’s complete, the dehydrator will weight a few hundred pounds and I wanted to be able to move it around and align it with the sun, so I made up an axle out of some scrap black pipe and found some old riding mower wheels. There’s always a good reason to save old junk like these wheels!
For the door, I made up a few brackets and anchored them into the base. They are tapered so when the door is resting on them, they will force to door tight against the frame. Then around the rest of the perimeter, I made up some little turn-knobs that hold the door in place. It only takes a few seconds to open and close the door. It’s not a perfect seal so at some point I may add a foam gasket in around the door frame.
One last piece is to add a little roof over the chimney so that it helps to keep some of the rain out of the dehydrator.
The entire thing is painted with flat black exterior latex paint. It took about two quarts to put on two coats. Even the sides should be painted so if the sun hits it, it will help to keep it warm.
This next part is optional depending on your location. Here in the northeast, we need to try to maximize our solar collection as much as possible. In sunnier areas, you should test your dehydrator to make sure it doesn’t get too hot whereas you start to cook your food instead of dehydrating it.
Take some expanded steel plaster lathe and spray paint both sides of it with flat black paint. It’s best to just paint a few sheets together to minimize your paint loss as it blows through the lathe. You can try to be complete, but don’t worry about getting 100% coverage. Then cut the sheets into pieces so that the width will fit into the cavity of the solar collector. To mark the cutting line, I just ran a piece of tape along the cut line since it was difficult to see a chalk line. Inside of each cavity install a cleat that runs from about 3/4" from the top of the cavity down to the bottom. I stacked 5 layers into each section to absorb as much light as possible. I couple of sheetrock screws hold them in place and I then touch them up one more time with some spray paint. As the air passes up through the collector, it is forced through each of these “solar sponges” and the heat that is absorbed into the metal is transferred into the air.
You can see as I stack each layer in the sun, less light is able to get through to the bottom. Even though these are flat black, some light does bounce off of the surface, but a percentage will hit the backside of the layer above it, helping to maximize the light absorption.
I also added one more wheel to the dehydrator. After attaching a cleat and painting it, I attached a swivel castor to it. It makes it far easier to push it around now!
Along the bottom opening, it’s a good idea to add some screening to keep the bugs from getting up inside the solar collector….unless you like dehydrated grasshopper!
To attach the sheet of acrylic, I drilled a bunch of holes through it and fastened it to the frame with some screws and washers. You want to make sure the holes are extra-large because the acrylic will expand and contract with the temperature changes. The best trick for drilling holes in acrylic is to run the drill backwards so that it will melt its way through the plastic. When screwing the acrylic down, don’t over tighten it so it can still move around a bit. I had to add one little extra piece along the bottom because I purchased the acrylic after I build the solar collector and had the wrong measurements….so you can learn from my mistakes!
The last part of the project is to make some drying racks. I used some scrap lumber and ripped it down to be about 3/4" square. Then I took some scrap plywood and made some corner brackets and glued and tacked them to each corner. Next I stretch some fiberglass screen over each rack and stapled it into place. After I ran out of the fiberglass screen, I found some pet screening. It’s a bit thicker and smoother and I’m finding that the food doesn’t stick to it much. I wanted to get some stainless steel screen, but it was far outside of the budget.
After waiting for a few days for the paint to fully cure, it’s finally time to give it a test drive and move it to its new home near the greenhouse. We happen to have some fresh peaches on our tree and sliced them up to about 1/4" slices. After a couple of days of drying at around 145 degrees, it was time for a taste test. Not many of the slices made it back to the house so I loaded it with a bigger batch.
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