2017 ATC solar food dehydrator

Update!

You can see the dehydrator plans HERE!

You can see the full video of the build HERE!




The 2017 ATC (Appropriate Technology Course) class taught by Tim Barker built an epic solar food dehydrator. Davin started documenting it in the 2017 Homesteaders PDC (permaculture design course) & ATC (appropriate tech course) thread and I thought I would copy his awesome posts and pictures here, so there could be a thread dedicated to this magnificent device.

For a simpler, less expensive and easier to make solar food dehydrator, see the awesome thread that Rob Griffin created:  2016 AT (appropriate technology) course solar dehydrator.

Davin's first solar food dehydrator post starts here:

Davin Hoyt wrote:Fred got a skid for the new Solar Dehydrator being built/finished up. The ATC group loaded and unloaded the Solar Dehydrator into place on the trailer, then, skid.

Another of Davin's posts from the 2017 PDC/ATC thread:

Davin Hoyt wrote:A lot of work was done on the Solar Dehydrator project today.

Davin's video post from the PDC/ATC thread:

Davin Hoyt wrote:Erica gives an overview of the Solar Dehydrator.

And another post:

Davin Hoyt wrote:Testing the Wheaton Labs ATC 2017 Solar Dehydrator.

Great to see the final result, I'm still watchjing the videos of the ATC to see the process. IS there any place where we can read more on the black paint Erica speaks about? I'm very interested in this for my dehydrator.

Thanks for sharing

Lorenzo Costa wrote:Great to see the final result, I'm still watchjing the videos of the ATC to see the process. IS there any place where we can read more on the black paint Erica speaks about? I'm very interested in this for my dehydrator.

Thanks for sharing

I'd be happy to write up the black paints.

Test Sample 0.1: Charring the Wood:
This darkened the wood slightly, but left it pretty shiny (the flat of the grain was very reflective).  Not an impressive heat absorber; there may be other reasons you might want to do this. (See Japanese charred-cedar siding).

Test Sample 0.2: Oxy Torch Soot:
An oxy-acetylene welder torch, if adjusted badly, will deposit a lot of velvety dark soot.  
This is nice and dark, but didn't stick to the wood very well, and we anticipated it might have problems with either wind or condensation over time (could end up blown up onto the backside of the glass, or washed away by internal "rain".

Version 1: Wheat paste and soot / charcoal: MASSIVE FAILURE.  

This paint worked OK on earthen plaster, but when heated under glass in the sun, it turned into little crumbles like the over-cooked edges of pancakes.  (In hindsight, not that big a surprise, but disappointing).  
Wheat paste is an "archival" adhesive for things like photo albums, meaning you can steam it off again without damaging most kinds of paper or wood.  Which means it is very eco-friendly, but not a very permanent paint.

Wheat paste: take 1 cup of water and put it in a quart pot to boil.  
Meanwhile take 1 cup of flour (or any starch you like) and 1 cup of cold water; mix well.  
When the water on the stove boils, take it off the heat, and whisk in your flour soup.  You may return it to the heat if needed - but very carefully - to achieve a translucent paste (kind of like a very boring pudding).  You don't want a stretchy dough; the paste is a consistency more like frosting.  
You can add salt to this paste if you want (to keep it a bit longer, with less risk of mold or yeast fermentation).  Refrigerate unused portions, and use within about 4 days; it's possible you could dry and rehydrate, but I think the quality would suffer.

We added pigment - in hindsight probably not enough - collected by vaccuming the inside of our rocket stoves, and when there wasn't enough, Tyler spent large amounts of time grinding down biochar to make more dark-grey pigment.
The correct ratio would probably be:
dilute the wheat paste 1:1 with cold water, then add another 2 to 4 parts moistened pigment.

After this wheat paste paint crumbled, we had to sand it all off to expose the wood again for Versions 2 and 3.

Version 2:  Nicko Brand Milk Paint (a classic lime-milk paint recipe):
2 gallons of skim milk
Sour with 2 cups white vinegar (or ACV, or lemon juice - anything that's 5% acidity)
Let sit for 2 hours up to overnight.

Prepare 1/2 cup of hydrated lime, moistened with water to a thick paste consistency.  If you are making a lighter paint color, you may want to moisten a whole jug or bucket of lime at this time, to use as pigment.  
(Precautions: While some people do handle type S lime with bare hands, I find it tends to give me alkali poisoning (hot, inflamed, achey hands, dry/itchy skin) or even blisters.  Wear nitrile or rubber gloves, a dust mask, and eye protection while opening lime bags and during the initial moistening process; if you want to ditch the PPE after that, clean up the outsides of the containers before removing PPE.
To remedy skin that gets exposed to lime, you can neutralize it with any mild acid- vinegar, lemon juice, even a soda pop or fruit juice.  I like to rinse off with one of these at the end of the work day, rather than waiting to find out if my skin starts itching.)

Strain the milk curds through a bandanna or several layers of cheesecloth.  (Butter muslin or an old bed sheet would also work.)  Keep the curds, feed the whey to some critters if you like.

Combine the curds (about 2 to 4 cups) with 1/2 cup of moistened lime, mix well.
This is your paint base.  
(Note: If used without pigment, this is not "paint" - it is a translucent binder, and will curl up and break apart like crazy as it dries.  
You can see this on one of the "eyes" on the dehydrator; I thought Nicko's base was a white paint, but it didn't have pigment yet.  The part I painted first, before I realized the problem and added pigment, has completely peeled free of the wood.  This area was painted over the old wheat base without adequate sanding, but even on a solid base this would likely produce a distinct 'crackle' effect if it stayed on the surface at all.  If you want a crackle effect, I would experiment with using less pigment - maybe 1 part pigment: 2 parts binder - but not zero pigment.)

To make paint, mix 1 part paint base with 1 to 3 parts pigment. (1:1, 1:2, or 1:3; good paint is mostly pigment).

This time for pigment, we used some powdered, activated charcoal that Jocelyn had handy - a beautiful velvety black.  The lime in the paint base made it a little bit grey, but it was still very dark.
This milk paint adhered well to the wood, and handled the heat of the sun much better than the wheat paint.  It did crack slightly when applied in direct sun; most paints will bond better if you let them dry in the shade before exposing to hot sun.

For interior/fine art painting, especially if you are mixing two or more shades of pigment, always pour the finished paint through a fine-mesh screen to remove pigment lumps before using the paint.


Version 3: Erica's Quick Yogurt Milk Paint

This is my personal take on milk paint, for those not blessed with an abundance of free/cheap skim milk.
Yogurt is a lot faster to prepare, may keep a bit longer if unused.  By my reckoning it's actually cheaper than using fresh milk, if you have to pay retail for the dairy ingredients.  (The opposite might be true on a dairy, where yogurt takes extra work.)

You do want an alkali for the best paint base. In this case I used borax, because I wanted to avoid any white pigment effects, but you could also use lime for ordinary colors / outdoor paint base.

Borax-water:
Mix about 1/2 cup Borax with 2-3 cups warm water.  
You want to dissolve a lot of the borax, but have a little bit left on the bottom that won't dissolve; this is a "saturated" borax solution, which we will call "Borax-water."  

Mix 1-2 cups nonfat plain yogurt: 1 cup borax-water.  This is your paint base.
Moisten the pigment.
Mix 1-3 parts pigment (2 to 6 cups): 1 part paint base (2 cups).

(I also did a batch where I mixed the pigment with the borax-water, a generous 1 to 1.5 cups pigment: 1 cup borax water, and then added about 1 cup yogurt.  It seemed to work just fine).

This came out velvety-black, a very "matte" paint, and adhered well to the wood.  It appeared to crack less than the 'classic' milk paint when exposed to sunlight during painting and drying.  


We painted each side of the solar collector with the two different milk paints.  
When Tim tested with the IR thermometer, the blacker yogurt paint registered about 4 degrees hotter than the dark-grey milk paint made with lime.


Both of these milk paints appear to be a pretty good indoor-outdoor paint.  
I would generally use borax indoors or in sheltered areas, and lime on weather-exposed outdoor surfaces, just because the Borax is more water-soluble and I don't know if it would leach out over time.
Borax is also known to suppress mold.  I've had good results with a lime-borax paint as part of the restoration process in a damp bathroom that tended to collect condensation and mold behind certain fixtures.

I might use the lime-based paint if I wanted a harder surface.  The small amount of cracking we saw might just indicate that we didn't use enough pigment; these binders go a LONG way.
After we finished the dehydrator, I mixed up the rest of Nicko's paint base into a white or off-white barn paint, and used it to paint about 4 panels behind the garage.  
I think the total coverage for these 2 gallons of milk was about 150 square feet of rough, bare wood.  
(I did have to make up one small yogurt batch to finish an area, and I did some re-coating with this yogurt paint, tinted off-white with some local clay, to get better color matching after some kind of oil/stain in the siding bled through the first/primer coat of milk paint.)

Special Effects:

You can wax or oil on top of either of these paints if you want a very wear-resistant finish, like for furniture.  However once you've waxed it, it's very hard to re-paint if you wanted to change the color, you'd basically have to sand back down to bare wood.
These paints can also be used, diluted, or with different concentrations of pigment, for very fun 'antiqueing' effects.  
Try diluting with a lot of water for a soft 'wash' or stain.  
Or after painting your base, use a richer top coat (dry it fast with heat if needed) for a 'crackle' effect.
(If you want a smooth, "normal" finish, of course don't do either of these things; use a lot of pigment and a little binder, and just enough water that you can paint effectively with your brushes or rollers.)

Milk paint with red iron oxide was the original "Barn Red" paint, and you can see how a dairy farm with the occasional batch of unsold or spoiled milk would find these recipes cheap and easy to use.

You can also make a very simple 'whitewash' by mixing moistened lime with straight milk, or with either curds or yogurt if you feel fancy.

Note that whitewash goes on translucent but dries white; it will take several coats to get an opaque white, and if you have damp problems on the wall you will need to fix them before the paint will dry and stay opaque.


Cleanup:
Since these milk paints set permanently, be sure to clean brushes promptly, and have a damp rag in your hand while painting to wipe off any splatters immediately.  
(Considerate painters mask or remove anything delicate near the job site, to avoid this problem.  Also, I was taught to dip paintbrushes in water at the beginning, and only use the ends of the bristles - don't dunk the metal part of the brush in paint if you can help it.)

All of the above paints clean up with water.  
The paints are non-toxic.  The ingredients are almost all food-grade, except for the alkali ingredients. Most of the alkali reacts with the casein as the paint is mixed and as it sets, but some residues may remain.
After drying, the lime paint should be about as toxic as sidewalk chalk.  
The borax paint may be slightly more toxic/prone to leaching, but that's part of its anti-mold properties.

As with any dairy product, refrigerate unused milk paint.  Or wheat paste.
It can probably keep in the fridge for up to a week.  (Letting it sit overnight may in fact improve the consistency, as long as you stir it well / screen it again before using it the next day.)

If you want to save some matching paint for future touch-ups, it's best to mix and store your pigment separate from the binder, and write down the proportions you used.  Then re-make the paint base fresh when needed.

This is another reason I like the yogurt base, it's simple to get your proportions consistent.  
You can mix all the other ingredients dry (including borax or dry lime), and keep a big batch of color-matching powdered paint base.  When you need it, moisten the desired amount of the pigments, and add the portion of yogurt right before use.

That is probably more than you ever wanted to know about milk paints!

Yours,
Erica

Where did you get the screens for the trays?

Adrien Lapointe wrote:Where did you get the screens for the trays?

Short answer: https://www.twpinc.com/wire-mesh-material/stainless-steel/14-mesh-woven-stainless-44

Long answer: Several posts about the screens started here: https://permies.com/wiki/240/61764/permaculture-projects/Homesteaders-PDC-permaculture-design-ATC#571958

Some quick thoughts here.

1.  Would it be possible to use walnut stain as the base for the paint instead of water?  It might improve color durability and darkness of the finish.

2.  In my area because prevailing winds are out of the north/northwest and are common during the drying season the outlet would need a short chimney that opened on all sides with a rain cap roof over it.  This design would be very easy to modify to accommodate that.

3.  Also true of this area is lots of dust during the drying season.  So I would raise the collector bottom end just a bit so I could pull the air flow from behind the collector panel.  Then add furnace filters(or oiled fabric filters) behind louvered panels to collect the dust using the collector itself as the roof.  If this wasn't done then some of the dryed food would be alkali flavored with a fine dust texture.

4.  I assume eventually the whole outside of the box will be painted black for that bit of heat gain?



Beyond that one other one that may not be in keeping with the ideals of this project.  Window screen collectors with shiny back boards seem to be winning the collector efficiency tests every where and would be very little more expensive to build.

Erica Wisner wrote:

Lorenzo Costa wrote:Great to see the final result, I'm still watchjing the videos of the ATC to see the process. IS there any place where we can read more on the black paint Erica speaks about? I'm very interested in this for my dehydrator.

Thanks for sharing

I'd be happy to write up the black paints.

Erica

Erica, thank you so much for sharing these natural paint recipes.  curious about the collector - does the depth of the solar collector matter much?  I see the one pictured is about 5 inches deep or so?  also wondering if you use glass rather than a plexi or other plastic material, so as to avoid any fumes from the plastic being absorbed by the food?

p.s. is this angle of the collector important?  what is best?

Peter Chan wrote: curious about the collector - does the depth of the solar collector matter much?  I see the one pictured is about 5 inches deep or so?  also wondering if you use glass rather than a plexi or other plastic material, so as to avoid any fumes from the plastic being absorbed by the food?

p.s. is this angle of the collector important?  what is best?

GLASS: This solar dehydrator has glass, not plexi-glass.

DEPTH: Although I have not run tests, I believe the volume of air (depth of collector) inside the solar collector does matter, and contributes to the velocity of air moving through the system. I believe the dehydration cabinet would need to be smaller for the solar collector to lose depth. The relationship between solar collector and dehydration cabinet has yet to be fully explored here. I would enjoy seeing a study for volume of air / velocity of air ratios.

ANGLE: The angle will also contribute to the velocity of the air. I was informed that the appropriate angle of solar collector ceiling is between 35 and 45 degrees. (Little Colorado Plateau Resource Conservation and Development Area, Inc.)

C. Letellier wrote:4.  I assume eventually the whole outside of the box will be painted black for that bit of heat gain?

No. Only the interior of the solar collector has been/will be painted. I believe we want the difference in temperature. At the back bottom of the dehydration cabinet, the air will need to rise and escape the system. I believe temperature difference will encourage air movement, and I fear that the increased heat would slow the system's air velocity. I have not done tests.

Davin Hoyt wrote:

C. Letellier wrote:4.  I assume eventually the whole outside of the box will be painted black for that bit of heat gain?

No. Only the interior of the solar collector has been/will be painted. I believe we want the difference in temperature. At the back bottom of the dehydration cabinet, the air will need to rise and escape the system. I believe temperature difference will encourage air movement, and I fear that the increased heat would slow the system's air velocity. I have not done tests.

I would argue that wanting that difference in temperature is exactly why the outside should be painted black.  This system is driven by the difference between inlet and outlet temperatures and the height differential between them.  Inside the box where no heat is being added the down flow and up flow temperatures will roughly equal each other for a given height.  You can find that discussion here on Permies in the thread discussing up flow vs down flow solar dehydrators.  Painting the box black should result in a greater difference between inlet and outlet temperatures should result in more air flow.  Which should improve performance very slightly.

We all know that painted wood is more durable in most circumstances.  So the box should probably be painted anyway.

I can only come up with 2 counter arguments.  

1.  The shaded side of the box radiates more heat because of black body radiation than is added to the sunny side so the temperature difference driving the systems is reduced.   Experience around passive solar would say this one is false.

2.  Black body radiation means more heat is lost at night faster cooling the box down faster reducing the drying during the dark.  Possible but given how poor a heat conductor wood is I am going to say unlikely.  Plus any losses should be made up by sunny day gains.

C. Letellier wrote:I would argue that wanting that difference in temperature is exactly why the outside should be painted black.

I am arguing that the conduction heat passing through the wood at point C and D (see image) would not be desired.

are there plans for this 2017 ATC solar dehydrator build?  i saw plans for something a few weeks ago, but can't find it now, and don't know if it was for this build or a previous version.

thanks.

Peter Chan wrote:are there plans for this 2017 ATC solar dehydrator build?

Link is at the top of this page.

My first glance at the thumbnail gave me the impression that this dehydrator was actually attached to the building behind it. Blowing it up, it's obviously not - BUT, depending on the orientation of the house, it looks like this could, particularly if the house was already being remodeled, or was not yet built, be revamped just a bit, to be attached, with the door opening into the house, to allow access, from inside. In that case, a vent released into the house (when not being used for is original intent) could also work as a bonus passive solar heat source. Thoughts?

Carla Burke wrote:it looks like this could, particularly if the house was already being remodeled, or was not yet built, be revamped just a bit, to be attached, with the door opening into the house

I have a sketch of a system similar to what you describe in relation to food drying (but not house warming). It has 1) a solar collector attached to the outside of an exterior wall, 2) a cabinet inside the exterior wall, and 3) a chimney vent out the top of the exterior wall.

Hi

What is range degrees reached inside Dryer? with a 20 to 30 centigrades in exterior.
Can be use to jerky beef ?

where can I get plans of this dehydrator to do my project

Best Regards
Oviel Figueroa

Oviel Figueroa wrote:What is range degrees reached inside Dryer? with a 20 to 30 centigrades in exterior.
Can be use to jerky beef ?

where can I get plans of this dehydrator to do my project

Quantities/tests have not been performed.

Link to all drawings: https://permies.com/wiki/davin

Davin Hoyt wrote:

Oviel Figueroa wrote:What is range degrees reached inside Dryer? with a 20 to 30 centigrades in exterior.
Can be use to jerky beef ?

where can I get plans of this dehydrator to do my project

Quantities/tests have not been performed.

Link to all drawings: https://permies.com/wiki/davin

Hi Davin
Someone has tried this dryer who can give his feedback
I'm almost ready to buy the plans to start my project

Thanks
Oviel Figueroa

Fred and the boots have been using the dehydrator regularly, Fred has stated he's dried out many pounds of fruit at a time and during the 2019 ATC I saw herbs were in it. The internal temp depends on how cloudy the day is, how long the sun is out and at what angle, the ambient air temps, and how much mass/moisture in the plants you're trying to dry out.

Hi! Any news concerning the ratio of air flow to air volume? My dehydrator will be an old wardrobe, so the interior is quite large compared to the part that is collecting sun. Do you think this will be a problem?

Tomke Roolfs wrote:Do you think this will be a problem?

No, I don't think it will be a problem. I think it will decrease the amount of hours it can operate. I believe a large solar collector should be desired, but your construction looks really nice. I would offer you ratio math if I had it. Looks great! Thanks for the pic!

After 6 years of iterations, Paul & the folks at Wheaton Labs have come up with the ultimate in giant solar food dehydrators... one that can operate without the sun (& without electricity). Presenting...the Rocket-Assist Solar Food Dehydrator! There is a brand new website to showcase all three solar food dehydrators at the Solar Food Dehydrator website

Many thanks Erica!

This is everything I wanted to know but didn't even know that I needed to know about exterior plasters.

How are ukrainian r m h project is finally getting up and running!

Hope to post more soon.

My Ukrainian volunteer team and I are starting as cobbers building an outdoor bench and table filled with war debris recycled.

We'll use your advice as the foundation for our finish.

Thanks for all you do!

Please let Ernie know that all his advice is laying the groundwork for the RMH system that will be starting next week.