Solar Food Dryer

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Introduction: Solar Food Dryer

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If you're growing your own fruit and vegetables, or just trying to eat the things that are produced locally, you come across one obvious problem: when something's in season, you have more than you can handle, and then there's nothing for the rest of the year. So the obvious solution is to preserve your food when you have it in abundance. Dehydration is an excellent preservation technique that's easy to do and that maintains a lot more of the original nutrients than canning or freezing.
 
However, a couple of years ago when we started to look around for a dehydrator to buy, we were sorely disappointed at what was available. A unit that could process any decent amount of produce was several hundred dollars, and they were all electric. We live off-grid, with solar and wind powering our home, so energy efficiency is a serious consideration for us. But it's also just common sense - why waste electricity on something when you have a perfectly good sun outside the window that can do the job just as well. So we got to working on making our own dryer, using the sun as the heat source, and it turned out not to be that hard or expensive. Within one weekend, we had the unit finished and ready to go, and we have been using it ever since.

So, what's the concept of a solar dryer? It's simple: move warm air over thinly sliced food.  The warmer the air, the more moisture it can remove from the food.  However, you don't want the air to move too quickly, as that will cause the temperature to decrease. Our design creates just enough air movement and warmth to dry food quickly.
 
The food is on trays, which sit behind a transparent polycarbonate sheeting.  Below the trays, there is a metal shelf, painted black, that serves as a heat absorber.  As heated air rises through the food, cool air is drawn in through the bottom vent, and the heated, moisture laden air flows out the exhaust at the top.
 
Because the dryer is something we plan to use for many years to come, we decided to make ours out of metal. If you do not have access to a welder, you can make the frame out of wood, but will have to adjust these plans accordingly. 

Step 1: Materials and Tools

Materials
  • 40 ft of 1" square tubing
  • 16 ft x 3 ft sheet metal
  • 2 ft x 8 ft polycarbonate greenhouse panel
  • 2 hinges
  • latch
  • silicon
  • 11 pieces of 8 ft long 1" x 2" lumber
  • 16 ft x 2 ft food-safe screen
  • 2 thin wooden moulding, 48” long
  • self tapping metal screws
  • wood screws
 
Tools
  • welder
  • metal chop saw
  • drill
  • tin snips
  • tape measure and marker
  • framing square
  • wood saw
  • box cutter

Step 2: The Frame

You can make the frame any size you want, but I settled on 48 inches long by 18 inches wide. This was the size that I could cover from one piece of 24”x96” polycarbonate.
  1. Cut 3 pieces of square tubing 46” long, and 2 pieces of square tubing 48” long. These will be the horizontal pieces of the frame.
  2. Cut the 4 upright pieces of square tubing, 2 at 34” and 2 at 22”.
  3. Place the two 34” uprights on a level surface. On each upright piece, make one mark 3” from the bottom and another mark 10” from the top. Place a 46” horizontal piece of square tubing in between the two uprights, below the 3” mark, and weld it in place. Place another horizontal piece in between the two uprights, above the 10” mark, and weld it in place. This is the front (or door) panel.
  4. Weld the two 22” verticals in between the two 48” horizontals, making a big rectangle. Make sure you put the horizontals above and below the uprights, not in between them, so that everything will come out to be 48” long. This is the back panel.
  5. With the back and front panels welded, it is time to lay out the sides. Stand the back panel up vertically on a level surface. Cut two pieces of square tubing 20 ½” long, and butt them up against the back panel, one on each side. Stand the front panel up vertically on top of the other end of the side horizontals. Clamp or tie the two vertical panels to something to keep them upright. Square the bottom side pieces with the uprights and clamp them all in place.
  6. Now you need to cut two pieces of square tubing that will connect the top of the front panel to the top of the back panel. Because the front is taller than the back, these two pieces will be angled. Hold a piece of square tubing roughly in place and mark it so that it will fit in between the front and back panel. Repeat for the other side. If your panels are level and square, these two pieces should be the same.
  7. Clamp these two top bars in place, in between the front and back panels, and then weld the four clamped joints, checking square periodically.
  8. Lie the unit down, so that the taller front (door) panel is on the floor and the bottom is facing you. Insert the last 46” horizontal piece of square tubing into the bottom panel, about 4 inches up from the front panel. This will be the bottom vent.

Step 3: The Door

  1. Cut 2 pieces of square tubing 47” long, and 2 pieces of square tubing 19 ½” long.
  2. Place these pieces over the opening of the front (door) panel to check for size. Make sure you put the horizontals above and below the uprights, not in between them, so that it will be 47” long and 21 1/2” tall.  You want the door panel to overlap the frame about 1/4” on all sides. Weld the door pieces together.
  3. Cut a piece of sheet metal 47” x 21” and attach it to the frame you just welded.  This is now the door. You will attach it after everything else is completed.

Step 4: The Covering

  1. Before we can start covering our frame, we need to get the tray supports installed while there is still plenty of room. Start by cutting 10 pieces of 1X2 lumber, 21” long.
  2. On the back panel, mark 4” up from the bottom bar on the uprights.  Proceed to mark the upright at 4” intervals. Now, mark the uprights on the front panel, starting at 4” from the bottom, and then at 4” intervals. You should have 5 marks on each upright.
  3. Attach the pieces of 1X2 to the sides, between the front and back panels at each mark. The boards should be on the inside of the dryer frame.
  4. Cut a piece of sheet metal for the heat absorber, 48” by 18”.  The absorber sits on top of the lowest board on the sides and the bottom square tubing of the door panel. You will need to cut out the front corners to fit around the upright square tubing. Screw this piece of metal in place. The absorber sheet doesn't go all the way to the back panel, there should be a gap between of about 3 ½”.
  5. Cut the polycarbonate in half, to give you two, 2' X 4' pieces. Use a razor -blade box cutter to score and then slice the polycarbonate.
  6. Clamp each piece to the outside of your welded frame, one on top, and one on the back panel.  Make sure they cover the frame well. Do not clamp so tightly that the polycarbonate breaks.
  7. Predrill holes around the perimeter of the polycarbonate for screws, and using self tapping metal screws, attach the polycarbonate to the metal frame.
  8. Cut pieces of galvanized sheet metal to cover the sides. It's easiest if you cut a piece of sheet metal 21” wide, and 35” long (for the sides), and then trace the frame out on the metal to get an exact fit.
  9. Cut a piece of sheet metal 18” wide x 48” long to cover the bottom.  There will be a 4” gap on the front (door) panel side of the bottom plate. This gap will become the entrance vent.
  10. Cut a 4” wide and 48” long piece of sheet metal and attach it with sheet metal screws below the door opening on the front panel. Cut a 8” wide and 48” long piece of sheet metal and attach it with sheet metal screws above the door opening on the front panel. There should be a 2” gap at the very top of the front (door) panel for the exit vent.
  11. Cut 2 pieces of screen, 6” wide and 50” long. This screen will cover the entrance and exit vents to prevent insects from entering the dryer.
  12. Cut two strips of molding, one 48” long and the other 46” long. Staple one of the pieces of screen to the 48” piece molding, along the edge of the screen. Using a few small screws, attach this molding to the underside of the top polycarbonate panel, so that the screen hangs down the front (door) panel. Attach the sides of this screen to the frame using one screw on each side.
  13. Place the other piece of molding in between the uprights of the front (door) panel on the inside of the sheet metal at the top. Screw it in place and staple the loose end of the screen to it.
  14. Attach the bottom screen with a few screws on the side bars to hold the screen in place.
  15. Screw the pieces of sheet metal on the bottom and around the door opening  Make sure the screen lies in between the sheet metal and frame for a secure and insect-tight fit.
  16. Silicon all edges and seams of the sheet metal and polycarbonate panels.
  17. Paint the entire inside black, but especially the heat absorber and sides.

Step 5: The Trays/Screens

  1. Cut 8 pieces of 1X2 lumber, 46” long.
  2. Cut 12 pieces of 1X2 lumber, 19 ½” long
  3. Screw and glue the lumber together to form 46” by 21 ½” rectangles, with a support board in the middle. Make sure the rectangles are nice and square.
  4. Cut 4 pieces of screen 54 inches by 28 inches long.
  5. Fold screen around the edges and staple to the boards of the rectangles, pulling tight as you go around the tray.
  6. Attach two screws for guides on the underside of each tray, one on each side. The screws should be about 1” from the end of the tray. Don't screw them in all the way, leave about 1/2” of the screw sticking out.  These guides will make it easier to slide the tray in and out of the dryer.

Step 6: Using

Attach the door with hinges and a latch.

You now have a brand new solar food dryer.  To begin with, make sure the dryer sits in the sun for a few days to allow any fumes from the paint and silicon to escape.

Test the dryer using 2 trays at a time, then increase to 4 trays if the weather is clear and dry.  Slice food as thin as possible (1/4”) to hasten drying time. We like to keep an oven thermometer in the dyer to see the temperature inside the dryer.  Ideally, you want it to be about 130 degrees F.  If the temperature is lower than this, you can reduce the size of the bottom vent with a piece of cloth.

Foods can be rehydrated before using, if needed. You can dry leaves, roots, fruit, vegetables, nuts, fruit pulp, meat and anything else you can think of.

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79 Comments

Stunning you did a great job.. I love it. I am going to try a smaller version of this.
Thanks for sharing.

I love it. wonderful idea. :) Much cheaper than using an indoor dehydrator that's loud and expensive to run all day and night.

Wow that's an impressive idea. Great work on that solar food dryer.

I think the simplicity of your design has confused people.

First: you specify greenhouse polycarbonate panels. Panels like this: http://www.greenhousemegastore.com/product/polycar... have the UV protection built in so complaints about sunlight are unwarranted.

Second: I don't think folks get the principles of the solar chimney. The temperature differential between the top and bottom of your drier pulls air up through your exhaust. We use a similar technique to keep electronics (Mounted to the side of a house) from overheating. However, in your case, you aren't isolating the air flow you are using the trays as baffles to direct the circulation around the food. Not enough circulation may be why you require a clear, dry day to make 4 trays work.

Third: The humidity in some places might make your drier less efficient, but the principle obviously works--otherwise Native American cultures in like this example of an Ojibwa farmer in Minnesota (Humid!) wouldn't have been able to dry corn.

Ojibwa-Farmer-near-Cass-Lake-Minn.-Drying-a-Harvest-circa-1920-courtesy-Minnesota-Historical-Society.jpg

I think you are confusing chemical "UV protection" within plastic glazing, which retards breakdown of the polymers, with UV penetration through the glazing. The UV-A still penetrates and hits the food beneath clear glazing. Heat and light both break down enzymes and some vitamins. If you want to dry food you need either heat or a partial vacuum. But light is easy to subtract from the design.

Possibly. That's a good point that I could be missing. I went back to the link I provided and pulled the product description:
  • ThermaGlas offers 99.5% UV protection.
  • ..coupled with integrated UV resistant co-extruded layer yields long service periods backed up by a 10 year limited warranty 
The first statement could be interpreted either way. I would take that as protecting against transmission, but it could be misleading.

The second statement is certainly the protection against breakdown that you state.

However, at the end, is this final statement:

ThermaGlas blocks ultraviolet rays that burn plants and living tissues, and transmits only beneficial radiation

They don't distinguish between UVA or UVB so I submitted a question asking for clarification.

Nice execution but a flawed design. Since hot air rises, your solar collector should be below the food trays. This design heats the air space on top and then vents it without passing over the food.

air doesn't heat directly. this design heats the black metal plate at the bottom, which heats the air and creates a draft. drying is about air movement more than air temperature.

I think you'll find that your food indeed does get heated. But the more densely you stack your trays the less sunlight the collector "sees". Ideally a dryer would be equally efficient no matter the loading, but that seldom occurs in practice. In our flat radiant design the trays can be loaded densely edge-to-edge as long as the food is one layer thick. Drying is about getting food dry in an efficient manner, before it rots in place. The amount of heat, where it is generated, how much is generated, how it can be regulated for different situations, and how it gets removed seem to be the roots of our disagreement on design.

Drying food in Texas is WAY different than drying food here in Minnesota. Challenge your design by using it in the humid upper Midwest and you'll see how ineffective it really is. Pulling lots of semi-warmed air through the food simply moves more ambient humidity around it, slowing drying times. While temps in an empty flat dryer can reach 150F at the hottest point, that's not a real-world situation. Radiant heat from the collector heats the food, yes, as does any dryer. The whole point is to dry the food, not just heat the air, and radiant heat is far more effective than convected heat+humidity since it actually drives humidity out of the food. Granted, the flat design isn't the smallest, but we use the space below it for storage. After testing many designs and solar drying for over 30 years I don't need a lecture on what works or doesn't, I'm well aware. What works in Texas just won't cut it here.