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I built a Walk Radiant Solar Dehydrator and I like it  RSS feed

Posts: 1267
Location: Northern WI (zone 4)
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One of this summer's projects was to build a solar dehydrator.  After looking at plenty of plans I was hooked on the Walk dehydrator design.  Particularly because I attended a presentation by Larisa Walk at the Midwest Renewable Energy Association fair (highly recommended).

The things I liked were that it made sense for a humid, cool environment, it was designed in nearby Minnesota, I had the materials on hand and I could adapt it to be more efficient for our needs.

The plans and instructions and downloadable free pdf and tons of info is on her website Eldergrove Homestead.

The idea is that it uses the sun to heat up a sheet of metal that is protected with glazing. That sheet then transfers the heat to the food on trays below it.  Under the whole works is metal roofing that reflects some of the heat back at the food and allows for an inclined air passageway so the moist hot air can rise up and out of the unit.  The ratio of solar collector to food tray is about 1:1.  So it has a lot of horsepower.  The challenge is that it has to sit at an angle to let the air rise so if you're trying to dry something round it may want to roll down the tray. 

Any "downsides" have been experienced and adapted around by Larisa so please read her site if you get initially turned off by a challenge I experienced.

The plans are for a 4' by 4' unit that sits on legs.  I had two big pieces of glass that I used which required a 4.5' by 3.5' design.  I emailed Larisa/Bob and got a quick reply that I should orient them the tall way at my high latitude.  You gotta love great customer support when the product was free in the first place

Other things I did were:
  • I made the angle of the unit adjustable
  • I put the whole thing on wheels so I can store it, move it around to find the sun and change the angle during the day
  • Made double sized screens to get more drying tray area
  • Added a thermometer in the frame to see how hot it is running
  • Changed the screen frame to allow air to flow in above the screen (I think this is a good idea but I'll see if I get any feedback on it)

  • I think I spent about $20 on it since I had many of the materials.  I built the wagon part from pallet runners and mountain bike rims.  Eagle eyed observers may notice the wheels are a different diameter but it doesn't matter.  The glass was free, the wood for the dehydrator was 2x4's and other demolition scrap I had laying around.  The roofing underneath it was leftover from a project.  The metal heat reflector was purchased trim coil from the restore, as were the hinges.  I "borrowed" the thermometer from the kitchen and the missus hasn't noticed yet.  The screen material is some stainless steel seed cleaning mesh that I got from a scrap guy many years ago.  That was the biggest cost savings of the project.

    We put it into service a month ago and have used it several times.  A 5 gallon bucket of apples, once chopped up, will fit on the four trays and they dry in a day and a half.  We can dry herbs in a half a day.  I have some sprouted wheat berries on cookie sheets drying in it right now.  I love being able to turn it to line up with the sun.  We regularly have exit air temps of 100-105F.  I built it with regular pine (not cedar or PT) since I plan to move it under a roof for 10 months of the year.

    I'd highly recommend this design for people in less sunny, less hot places.  I'm sure it works everywhere but other designs may be better?

    Things I'd change if I were doing it again using heavy glass:
  • I'd shorten the support legs so the heavy drier unit is closer to the ground for easier transportation
  • The angle adjustment isn't strong enough to hold up the heavy glass so I'd drill cross holes and use pegs/bolts to hold the angle adjustment
  • I'd reconsider if I really wanted to use glass

  • Hopefully this inspires folks to build their own.  If you need stainless steel screen material, Larisa sells that on her website.
    [Thumbnail for DSC03772s.jpg]
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    Side view
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    Undercariage front leg
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    Undercariage adjusting leg
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    Adjusting leg top
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    Thermometer and hinges
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    Open for loading - two trays instead of four
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    Closeup of tray construction
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    Posts: 1793
    Location: Wisconsin, zone 4
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    Excellent Mike, excellent.  Now I have another project to work on.  Maybe this winter...
    Don't listen to Steve. Just read this tiny ad:
    Binge on 17 Seasons of Permaculture Design Monkeys!
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