Build a Solar Dehydrator


Like many of you, I suspect, we’ve got a few too many tomatoes at this time of the year. One of our favorite ways to preserve our modest harvest is with our solar dehydrator. There’s nothing like the taste of sun dried tomatoes, but unless you live in a very dry desert climate like Phoenix, Arizona you can’t just set fruit out in the sun and expect it to do anything but go moldy. In most places in the world, including here in Los Angeles, the relative humidity is too high to dry things out in the sun. Solar dehydrators work by increasing air flow to dry out the food. The one we built uses a clever strategy to get air moving without the use of electric fans such as you’d find in your typical store bought electric dehydrator.

Our solar dehydrator is constructed out of plywood and consists of a heat collector containing a black metal screen housed in a box with a clear plastic top. This screen heats up on a sunny day and feeds hot air into a wooden box above it. Vents at the bottom and the top of the contraption create an upward airflow through natural convection (hot air rises). You put the food on screen covered trays in the upper box. With sliced tomatoes it takes about two full days of drying and you have to take the food indoors at night to prevent mold from growing (a minor inconvenience). We built our dehydrator several years ago and have used it each season for tomatoes, figs and for making dried zucchini chips.

You can find plans for this “Appalachian Dehydrator”, designed by Appalachian State University’s Appropriate Technology Program, in the February/March 1997 issue of Home Power Magazine. The February/March 1999 issue of Home Power features a refinement of this plan, but we just built the original design and it works fine. The original plans and improvements to those plans are split between two articles: Issue #57 and Issue #69. You need to download and read through both before building this dehydrator. . Alternatively, the always excellent Build it Solar website has a whole bunch of solar dehydrator designs, including a nice cardboard version. And while you’re in the library there’s also a book by Eben Fodor, The Solar Food Dryer.

As an added bonus to the tinkerers out there, take almost any of these designs, remove the top box, stick it in a window and you’ve got a passive solar room heater.

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20 Comments

  1. Nice work.

    A question for you. Why is the collector area reflective mylar (or foil) instead of painted black. I would have thought that maximum heat gain was the goal for the collector.

    As a side note I’ve found that the commercial Sun Oven can do a okay job as a dehydrator when the lid is cracked about 1/2″. The upside is that it gets really hot (350F) so the dehydration for tomatoes takes about 3 hours. The downside is that it is a pretty small space.

    http://blog.holyscraphotsprings.com/2008/05/sun-dried-tomatoes.html

  2. Howdy Mikey,

    I think the idea is that the foil reflects sunlight on to the black screen material that is suspended in the box. You actually don’t want a dehydrator to get too hot as that will cook the food rather than dry it. Good to hear from you, as always. Hope other readers here check out your awesome blog.

  3. For simplicity and efficiency’s sake, I think a large piece of metal duct-work painted black may be better and simpler than the foil lined wooden box. The air will heat faster, but I don’t think it will cook the food since the hot air isn’t trapped. I think it would just speed the airflow and the dehydration. Do you think this has a chance of working in a place as humid as Miami?

  4. Anonymous,

    The engineers who designed this unit are in Appalachia, which is fairly humid. They tested several different designs before coming up with the black screen in reflective box configuration, again with humidity being one of the main concerns. It should work in Miami but I can’t say for sure.

  5. I think if you could add a small solar cell, to charge a little battery and fan overnight, you should be able to leave things in there overnight…

  6. >if you could add a small solar cell, to charge a little battery and fan overnight, you should be able to leave things in there overnight…

    I’m thinking a thermal mass would be more practical. It would mean letting the dryer warm up for half a day before using, perhaps…

    I’m not sure a fan would necessarily work. If there’s dew, for instance, the moving air will cool the food to the temperature of outside air, and keep pushing moist air past it.

  7. Soooo…let’s say that you actually *do* live in Phoenix, AZ. Would you need this, or would spreading produce out on cheesecloth on my rock yard work?

  8. @Anonymous in Phoenix: Yes, you can sun dry! I envy you this. All you need to do is wait for days where temps are in the 90′s (or above) and the humidity is low. And be sure to use cheesecloth on top of the food as well to keep flies off it.

    If you want to really get into it, window screens or screens you build for the purpose would be ideal for spreading the food on–they would allow for some bottom air flow and easy transport if you wanted to move the food.

    Take that one step further, and I can imagine building a shallow box made of screen, with a hinged screen lid, which would be completely bug proof and safe from wind gusts and birds and whatnot.

  9. Do you have any problems with ants getting into the food in this dehydrator? Also when you bring the food indoors at night do you refrigerate it?

  10. Anonymous–have not had trouble with ants, but I have had trouble with moth larvae. Food dehydrated outdoors in a rig like this should be frozen after drying to kill bug eggs. And, no I don’t refrigerate at night.

    • Sure–screens are great for outdoor pests. I think the contamination happens for us when we take the food in for the night. It has to come in when the sun goes down, or the cooling leads to condensation with leads to mold. (drying is often a multi-day process). We have occasional outbreaks of pantry moths in our kitchen, and no matter how “quiet” they seem, it only takes one to lay her eggs on one piece of food for a whole jar to be ruined. That’s why we freeze our food for 3 or 4 days before shelf storage, for extra safety.

  11. I am interested in the solar mass idea. Lining an area with brick then adding milk jugs full of water to the center may keep an up sweep of warm air going through the night. Anybody?

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  13. Hi there – I was looking at your blog entry on the solar dehydrator and the links to Issue 57 and Issue 69 are not functioning. Do you still have the downloads/information? I am preparing for building in a couple weeks and plan to start collecting material soon.
    Thanks so much,
    Bee

  14. Pingback: Food Storage and Survival Radio Episode 36: Introduction to Dehydrating | Going Forward

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