I built a Walk Radiant Solar Dehydrator and I like it

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.

Excellent Mike, excellent.  Now I have another project to work on.  Maybe this winter...

I made some changes to the dehydrator yesterday to make it easier to move around.  

1.  I had the wheels too far forward.  This was to make it easier to lift but is was so easy that when you lifted the panel on the same side as the handle, you could pick up the whole unit.  So I moved them back about a foot.

2.  I had the metal posts that the unit sits upon on the low side (South) too long.  This was to make sure I had enough wheel clearance but it made the unit too top heavy when rolling over uneven ground.  I cut them 7" shorter and it's much more steady now.

3.  The handle was crap.  Now that the wheels are farther back, the handle was too wimpy to pick it up.  So I fixed that problem.

Now we're all set for a summer/fall of drying.  We've already dried some lovage, oregano and savory.

Now that you have had some experience, how is this design working out for you?

I think it's working as good or better than any solar dehydrator we could use.  I don't have any experience with other styles but I believe the comparison work the Walk's have done on it.  We've only done herbs this year so far.  But based on last year's productivity, it can crank out the food.  

If I were to build another one, I'd try to find lighter glass.  It isn't a big deal, the missus can lift it just fine.  But if it was lighter it would be nifty.

In the mid summer we don't rotate it to line up with the sun but in the fall we do like to pivot it to line up better.  So while it doesn't need wheels if you have a permanent spot for it, having a way to rotate it would be nice.

We have yet to use the adjustable angle option as well.  So feel free to skip that complexity

Wow! I rarely see those kind for drying tool here. Hope to see after you put some adjustments! Cheers!

I figure an update is in order.  We've basically planted it in one spot all summer (no rotating).  The wheels are handy to get it into the shed for the winter though.  It dries everything we've thrown at it.  Mushrooms, herbs and apples are the biggies.

This year we'll try drying berries on it.  Since it's angled we'll put them in the upper tray.

I did notice some wasps coming and going the last time we did apples so putting some light weight window screening on the four open edges might be an upgrade to put on the list.
'

i built something similar a few years ago using a window from a sliding door someone was throwing out.
2 wooden frames 1 for the glass and 1 for the base.
It gets hot enough to dry anything and kill all the bug eggs that the shop bought ones dont.
i think the highest temp I've seen it get too is around 150.
i wish i had thought about putting wheels on it though
time for a little modernization......

I'm curious where you got the perforated plate from. It looks easy to clean.

I got it at a garage sale.  I think it was from a seed cleaning part of a farm implement...

Because we live in a humid area I've been looking at building this type of dehydrator, and the food safe surface is at its core.
Because of that I'm planning mine around the size of a standard sheet pan.
They come in standard sizes, last forever, are reasonably priced and come in perforated versions.

Mike Haasl wrote:I figure an update is in order.  We've basically planted it in one spot all summer (no rotating).  The wheels are handy to get it into the shed for the winter though.  It dries everything we've thrown at it.  Mushrooms, herbs and apples are the biggies.
'

Hi Mike. I pick a lot of mushrooms in the late summer and fall and the dehydrator can run almost non stop for weeks. Id love a solar dehydrator, but its so wet and relatively cool here September and October I’m not sure how well it would work. We get good sun exposure, but it’s getting low in the sky that time of year. We’re in Michigan’s upper peninsula. Where are you at and do you think this design would work well for us?

I've been looking for a way to cook/dehydrate chicken manure because that's easier for me than composting it. I think this might just do the trick!

I believe I can skip the black panel under the glazing since I don't need the UV protection and the manure should be dark enough to absorb sunlight effectively.

Brody Ekberg wrote:

Id love a solar dehydrator, but its so wet and relatively cool here September and October I’m not sure how well it would work.

It's more work, but you could consider running the shrooms through the solar dehydrator during the day, then transferring them to the electric one for overnight to finish them.

Hey Brody, if the sun is out, yes it dries them down well.  I think you do want mushrooms to dry in the sun to pick up vitamin D also, I'm not sure if that can be done after they're dry or if it needs to happen before.  So I would time my harvest a bit around the forecast.  Later in the fall, I think being able to rotate and incline it does help.

Matt, the black metal helps radiate the heat down to the target, while the less hot, yet humid, air around the target rises out of the unit.  I think if you skip it, it might not work right.  I'm not sure though...

Matt Todd wrote:I've been looking for a way to cook/dehydrate chicken manure because that's easier for me than composting it. I think this might just do the trick!

I believe I can skip the black panel under the glazing since I don't need the UV protection and the manure should be dark enough to absorb sunlight effectively.

This sounds intriguing. Some would say it sounds gross. In the past, I've turned my own poop into biochar, so I'm not in the grossed-out category. Am I wrong to think that this would be a way to fast-track the utilization of chicken poop?

Michael Helmersson wrote: Am I wrong to think that this would be a way to fast-track the utilization of chicken poop?

That is the plan. After reading about commercial chicken farms dehydrating the manure, I was sold on the concept. It's supposed to eliminate the ammonia and pathogen concerns... and the labor of composting. I can't keep up with composting even my dozen chickens manure. And this will render it more "shelf-stable." Which is important to me because last time I horded buckets of poop waiting to add it all at once to a huglekultur, it went anaerobic in the buckets and stank up the whole neighborhood!

Mike Haasl wrote:Hey Brody, if the sun is out, yes it dries them down well.  I think you do want mushrooms to dry in the sun to pick up vitamin D also, I'm not sure if that can be done after they're dry or if it needs to happen before.  So I would time my harvest a bit around the forecast.  Later in the fall, I think being able to rotate and incline it does help.

I don’t have much control over timing the mushroom harvest. It basically is a matter of when do they grow and when do I find them. They tend to grow in humid and wet weather, not sunny dry weather, so it’s almost a perfect situation to not be suited for a solar dehydrator. Unless that dehydrator works in cool climates and less intense sun. But you’re right, rotating and changing angles according to the sun would probably help immensely.

Yeah, the weather's hard to plan for but it tends to be fairly sunny here in Sep and Oct.  I'm just a little bit south of you but not by far.  

There are some rocket stove powered food dryer designs that would work independently of sun exposure.
They could also kiln dry fuel or manure.

I've often thought a that a solar thermal evaporator would be a good way to deal with human generated grey and blackwater.
The problem would be keeping up with the influx.

I would be concerned by the amount of handling required to dry chicken poop in a radiant dryer .
In my set up layers of poop and bedding build up in the coop till it's full, then the soiled m7xtures goes into the run, where the chickens churn it into compost.
It may be a little hot still,but not too hot for worms, so  mild enough for a top dressing.
.

My dryer design works well for mushrooms even in early spring or late fall here in SE Minnesota. It doesn't take too much solar power to dry a low moisture product like 'shrooms. But it is better to put them out to dry (gill side up) in direct sunlight for a day or two to increase their vitamin D content many times over. Then into the solar dryer to finish them off. I seem to remember reading awhile back that Paul Stamets said that this process can also be done to already dried 'shrooms. I haven't tried that but I do know that if you lay out trays you need some window screening over them to keep the lightweight 'shrooms from blowing away. You can increase the efficiency of the dryer for more northerly regions by locating the dryer on the south side of a white wall to get a bit more solar reflected down onto the glazing. Also changing the tilt (within reason) to get a better angle for sun penetration of the glazing. You could get extreme with this and periodically track the sun by rotating the dryer during the day but I have never found that to be necessary in the 35+ years we've used this design. As to the chicken poop idea, dried chicken manure would be substantially a different end product than compost as you would cook out the volatile nitrogen and micro-organisms that would be present in a healthy compost. I wouldn't advise it unless you're aim is to bag and sell the finished product.

Mike Haasl wrote:......  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.....

Heya Mike,
I've got harvest food storage on my mind...

I don't expect you to remember 3 years ago but I am hoping.....do you remember how many gallons the dried apples tookup?  

...did you take pictures of the spacing of the chopped apples on the screen?  

Just checked with the missus.  We're guessing that a 5 gallon bucket of apples would turn into 3 gallons of sliced up pieces which would dry down to around 1 gallon.  Give or take 50%

Hey, I do have pictures.  Here's 12 lbs of apples before, during and after!

Mike Haasl wrote:Hey, I do have pictures.  Here's 12 lbs of apples before, during and after!

Thanks Mike!  

These kind of hard statistics make self sufficiency food planning possible.....    ....I once calculated an annual rice, bean, and oatmeal diet, with a few extras thrown in, to require 11 containers about 26" tall and 21" in diameter.   This would be one way to explain all the food cache large clay pots left in multiple places across the southwestern USA by the Anasazi

One can now see to have a quart of dried apples per month, one would need to do what you did four times: 20 gallons of apples, 6 days of drying and 12 jars to store them in.

I'm not sure I can make those Anasazi large pots without them cracking in the firing process....I could make ringed sections I suppose.

Thanks again for taking the time to dig out and post those pics!!!   ...very  valuable information from my perspective!

My pleasure, I took the photos for a PEP BB so they were very handy.  Please note that I'm not sure what the volume of the 12 lbs was.

Oh, and here's the BB submission for Dehydrate food with a solar food dehydrator

Do you know where I can get step by step instructions with pictures for this? I asked questions about that before I bought the book "feeding ourselves" which is why I believed it would have all that. It doesn't. It has a materials list and one very dark photo of the completed dehydrator. I'm not a happy camper.

I have my heart set on building and using the dehydrator this summer, so if anybody could point me in the right direction I'd appreciate it. It looks awesome.

Here is more info on the dehydrator from their website (solar dehydrator).  There's a downloadable pdf in there as well with more info.

I've been working on making 4 of these dryers.
2'x2'.
Bought the stainless screen from them.
Cedar from some used decking.
Got a stash of hinges,..
just need to find some free 2'x2' glass and hold downs for it.
I'm glad you posted this because I hadn't thought hard about putting wheels on it.

M James wrote:Do you know where I can get step by step instructions with pictures for this? I asked questions about that before I bought the book "feeding ourselves" which is why I believed it would have all that. It doesn't. It has a materials list and one very dark photo of the completed dehydrator. I'm not a happy camper.

I have my heart set on building and using the dehydrator this summer, so if anybody could point me in the right direction I'd appreciate it. It looks awesome.

Sorry our book left you unhappy. Here's a direct link to a PDF http://www.geopathfinder.com/DryerWorkshop5-2011.pdf on our website that has complete photos of a workshop build of our design. The details are left "vague" so that people will utilize locally available materials for the most part. Once the basic concept is understood there are all sorts of possibilities for scrounging. For instance, we've seen dryers of our design built with glass shower doors, patio doors, and auto glass as a substitute for the polycarbonate glazing. Bamboo woven trays instead of stainless screens. As long as it's food safe and utilizes the basic principles of radiant heat and passive airflow it will do the job. The basic size recommendation of 4'x4' is based on both the dimensions of commonly available materials and a size large enough to produce adequate heating. A 2'x2' dryer is too small. One that is 2'x4' is marginally effective. The north/south dimension should be kept to around 4' as bigger would heat excessively, although one person in Italy did make a longer north/south one that sat atop a stone wall but it was only 2' wide east/west and apparently was working well for him. But another person in the southern U.S. made our dryer with shower doors, long dimension north south, and he burned peaches. He rebuilt it with the shorter dimension north/south, cut the peaches thicker, and it worked fine in his climate. The design is adaptible to local conditions but may take some experimentation and refinement if being built in areas other than the upper Midwest where it was developed and in use for over 35 years. I hope this helps. We're always willing to answer questions as they arise.