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

  • 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
  • welder
  • metal chop saw
  • drill
  • tin snips
  • tape measure and marker
  • framing square
  • wood saw
  • box cutter
Stunning you did a great job.. I love it. I am going to try a smaller version of this. <br>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.
<p>I think the simplicity of your design has confused people.</p><p>First: you specify greenhouse polycarbonate panels. Panels like this: <a href="http://www.greenhousemegastore.com/product/polycarbonate-panel-10mm-clear/greenhouse-polycarbonate-sheets" rel="nofollow">http://www.greenhousemegastore.com/product/polycar...</a> have the UV protection built in so complaints about sunlight are unwarranted.</p><p>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. </p><p>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.</p>
<p>I think you are confusing chemical &quot;UV protection&quot; 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.</p>
Possibly. That's a good point that I could be missing. I went back to the link I provided and pulled the product description:<br> <ul> <li> ThermaGlas offers 99.5% UV protection. <li> ..coupled with integrated UV resistant co-extruded layer yields long service periods backed up by a 10 year limited warranty&nbsp; </ul> The first statement could be interpreted either way. I would take that as protecting against transmission, but it could be misleading.<br> <br> The second statement is certainly the protection against breakdown that you state.<br> <br> However, at the end, is this final statement: <blockquote> <p> ThermaGlas blocks ultraviolet rays that burn plants and living tissues, and transmits only beneficial radiation</p> </blockquote> <p> They don't distinguish between UVA or UVB so I submitted a question asking for clarification.</p>
<p>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. </p>
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.
<p>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 &quot;sees&quot;. 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. </p>
<p>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.</p>
<p>Obviously you have never been in East Texas; Try our beautiful mid summer days in the high 90's with 80% humidity and a due point about the same. Now west TX a whole different world</p>
<p>East Texas is where our humidity comes from! We got the remnants of a dead hurricane from there back in August of 2007 that parked here for two days and dropped 44 inches of rain (2 summer's worth) in 36 hours.</p>
<p>Hi, </p><p>I live in Michigan which is also humid, and most of the time cold, and I was wondering if you have any suggestions for a food dehydrator. After reading your comment it made me realize that Michigan is only sunny, really, during the summer. Do you have any suggestions and/or ideas for a food dehydrator in Michigan?</p>
<p>The other thing about Michigan is that food only grows there in the Summer, when it's sunny. So I'd suggest a solar dryer, of course, instead of a plug-in electric. The solar box designs work fairly well where it's sunny, hot, and dry. But where it's cooler, sunny to partly cloudy, and very humid <a href="http://www.geopathfinder.com/Solar-Food-Drying.html" rel="nofollow">the radiant design</a> works far better. But you don't have to build anything except drying screens if you have a parked vehicle with its largest window facing south. Put some black cloth over the screened food to generate heat and protect it from direct sunlight, then place it in the vehicle where the sun can hit the cloth. Crack a couple of windows down an inch to let humidity out. Voila! A solar dryer.</p>
<p>Looks like the Fodor design from Mother Earth News. It lets UV-A hit the food, which destroys nutrients and color, and it mimics an electric box dryer in that humidity stagnates between screen layers and must be removed. The collector area to screen area ratio is much too small for this to work in humid or cool areas, but it might work well where it's warm and dry. We tested similar designs and many others back in the mid 80's before developing the radiant solar dryer found here: http://www.geopathfinder.com/Solar-Food-Drying.html . It's also found here on Instuctables. It has been in use world-wide since 1985, customized for many climates and crops. It doesn't let sunlight hit the food, it works even where it's cool and humid, it uses truly food-safe stainless steel screens, it doesn't require tracking to follow the sun, and food can even be left inside overnight for 2-day drying without spoilage.</p>
I've studied that design before, and while it is interesting, it is extremely bulky for the amount of food that can be dried. Also, by heating the food, nutrients are also destroyed, most commonly sensitive vitamins, like Vitamin C.<br><br>The whole point of a solar food dryer is to heat air so that it can hold more moisture. Heating the food is not the goal, nor is it desirable. Also, with a good design, the hot air moves through the food with a higher velocity, because of the solar chimney effect.<br><br>The flat design does not allow for the heated air to contact the food, nor significantly increase air velocity, so drying is mostly done by heat from the metal plate. It's more akin to a solar cooker, than a solar dryer. While I have no doubts it works, I am not convinced that it would work better than a chimney and hot air device like ours.<br><br>We rarely have to leave food overnight in our design, because it dries quickly, but we can leave it in there, even on cloudy days, with no spoilage or insect issues. No tracking is required, either. We can fit at least 4 times the food for the same footprint as the flat design and perform equally (most likely better) than a flat design.
<p>There seem to be some strange ideas about what makes an efficient dryer.</p><p>Warmer air CAN hold more moisture, but it doesn't, because the same amount of moisture that was in the colder air is now in a larger volume of expanded warmer air. Which means it's drier than before. Water vapor doesn't get created out of nothing.</p><p>Not only is the absolute humidity lower in the warmed air, the relative humidity is lower yet. And that's why your house can get very dry in the winter: dry cold air is made even drier because it's heated in your home.</p><p>No matter how humid the cool air is, when you warm the same air, without any input of water, it will be drier than before. That's real physics.</p><p>Further, dryers are not supposed to cook the food. You want to warm the air, not the food, and let convection do the drying. Heating the food is considered an UNwanted side effect of drying. It saps efficiency and reduces nutrient content.</p><p>This system would be much more efficient if it were modified so the panels just warm the air, then let the warmed air pass through a set of stacked screens at the top to do the drying. There is no need for the sun (or plates) to add radiant warmth directly to the food. </p>
<p>You need added heat to draw moisture from the food. You can get that by heating air and moving it around food through convection, forced or passive. And you can do so by direct or indirect radiant heating of the food itself. Either way, the food heats and moisture moves from the food to the air. Try a real-world experiment and build a few different dryers. The unit pictured isn't the best one out there and it's not the worst either. Obviously a dryer isn't intended as a cooker. Temperatures should be limited. And most designs can be quickly and easily modified to match the food, its thickness, the weather conditions, and the ambient temp by using various densities of shade cloth over the collector area to damp down excessive heating on clear, hot days while drying thin, easy-to-dry foods. But first the dryer must generate enough heat for more marginal drying days. Because on cloudier days, drying thicker slices of wet foods, you'll end up with a stinking moldy mess in a box dryer. I ran into that with several designs back in the 80s before we designed the indirect radiant dryer. And even with a hotter dryer, in the past 30+ years we've had to use our wood-fired sauna as a dryer a few times on days when the weather forecasters just got it completely wrong!</p>
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<p>I've wanted to dry fruit/veg and had an idea to place the trays in a box with air filters at the top and bottom. The box would use the hot air in the attic as the drying source. In Toronto attic temps can easily reach 50C and generally it's pretty dry in the summer due to sun radiating heat through the roof for 12+ hrs a day. The attic's insulation is fiberglass with a thick layer of blown cellulose on top. Preventing any contamination is my only worry.</p>
<p>with all the trays loaded and the unti in the sun and the temp inside it at 130... how long does it take to dryout the fruit? (apples) </p>
<p>Outside temp. as of 9:30 am 6/28/2016 in Bullhead City, AZ 138 degrees.</p>
<p>amazzzzing! wish I had welder!!</p>
<p>The sunlight should only be used to heat up the air, the radiation itself should be kept away from the food. The chamber should be dark and dry. For better air circulation you could mount some PC Fans powered by solar cells.</p><p>It just appears not to be...</p>
<p>Where is the exhaust at the top?</p>
<p>You guys are wonderful. I enjoyed this and exploring your blog. This project like your other work is a creative mix of inexpensive and fit for the purpose. You rightly say air movement is the major factor. I'm wondering if a black painted &quot;chimney&quot; added to the air outlet, might help. A small pop-can furnace (even laying flat) at the inlet end could raise the heat nicely if that is ever necessary</p>
Uhmm. How much do you think will it cost if we use wood instead of metal?
<p>It would be cheaper to buy wood than metal tubing. No reason it couldn't be made out of wood, especially if you didn't have access to a welder. Go to the hardware store and you can get an idea of the price difference.</p>
<p>I think using a white paint on the outside is counter productive. Paint is black and it will work even better.</p><p>Nice job otherwise and done very neatly.</p><p>Thanks</p>
you don't want it to heat up too much. We're trying to create an airflow inside the dryer to dry the food, not cook it.
<p>It's cool ( actually Hot ) idea . Brilliant work .</p>
B R I L L I A N T ! <br>
Hey congratulations on being a finalist in the weekend projects contest!
Thanks a lot. What a great turn-out for the competition! I think we lucked out to get onto the final 26.
Did you know you can make one from Boxes and aluminum Foil????
Cool thing! <br> <br>A few suggestions if i may? <br>- To increase the ammount of heat collected, paint all non-transparent sides black on the outside (useless if you want to insulate those sides). <br>- To minimise the heatloss, add insulation like styrofoam to all non transparent side on the inside (Useless if you want those sides to collect heat as well). <br> <br>I think i will use this basic idea and try to make a mini-one on teh cheap and simple for drying Biltong. <br>
we're not trying to collect heat. it's a dryer, not an oven. We want to collect heat in a specific area to create a flow of air through the dryer. Increased air movement is the main factor when drying food. If you heat too much, you can partially cook the food.
but, there are some foods (meats, especially chicken &amp; pork) that really NEED to be heated to a specific temperature for a certain length of time....and for meats, that is WELL over the 118F mentioned above.
dried meat is not cooked. chicken and pork are typically not used for jerky.
It may not be as commonplace to use chicken or pork, but it is done. Dehydrating chicken and pork is a great way to add a little diversity to backpacking meals. Packets of dehydrated meals can be QUITE spendy, and its not that hard to make your own.<br><br>Beef and venison (as long as you are sure the processing of the animal was done in very sanitary conditions) can be eaten raw with no ill effects, so the temps for those don't have to be as high. Chicken &amp; pork is a whole 'nuther story.
if you are going to be drying foods that require pasteurization, I think it is better to do that outside of the dehydrator.
Not necessarily, the sun is capable of providing the temps needed. You can boil water in a solar cooker. People just need to be aware of the temps necessary....to be safe.<br><br>I have an &quot;on-the-grid&quot; dehydrator, and it is thermostatically controlled....it is VERY possible for you terrific design to be 'tweaked' to provide the temps the I have to pay the utility company for.<br><br>Your design is great.<br>
yeah, I agree, we have a solar cooker as well, and it cooks at 400F. But, I think it is better to separate those functions, and not have the dryer and cooker as one machine. Low temp drying is much better for fruits and vegetables.
You are right. My bad ;)<br>
To dry biltong you would need a constant flow of air and preferably room tempreture to cool.. you could slice the meat into thin strips after marination and then slicing before spicing and placing on wire racks.. but you'll have to ad a fan to help with the constant circulation of air.
Careful to not over-spice your meat
The air doesn't need to be warm, and it can't move too fast. What you want is plenty of fast moving DRY air, below say, 120 degrees F. You want it cool enough to dry without cooking, and quick moving enough to dry the moisture before spoilage. This rig is a great concept, but I think I'd want more airflow to get rid of the humid air that's going to collect in the box as the food drys. Alton Brown did a couple detailed episodes on his show about dehydrating foods. One on making jerky, and one on drying fruits. And I think maybe an herb drying episode. They're on YouTube, check them out.
warm air can hold more moisture than cool air, that's why we try and increase the temperature. <br> <br>this unit works well for us, and dries most foods in a day or 2.

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