Solar Food Dryers?

  Anyone got plans for a durable one (not the plastic sheeting kind - I tried that with no success)? I've got plenty of sun, so I see no need to buy a $200 high quality electrical dryer.

I don't have any plans, but I don't think you need them.  should be easy enough to rig something up.

I think it would be a good idea to separate the food you're drying from the solar collector so your food isn't degraded by the sunlight (unless your drying shiitake mushrooms).  so make a solar air heater that leads to a dark box with the trays of food.  maybe something along the lines of the soda can heater mentioned in the alternative energy thread for the heater.  folks have used old refrigerators and upright freezers as the dark box portion.  a small box made of plywood could work, too, if you don't need refrigerator-sized capacity.

some folks put a chimney in the top of the refrigerator and feed the hot air in at the bottom.  other folks reverse that.  both designs work.

I once checked out some really great books from the library that had ideas and building plans for solar dehydrators.  Maybe you want to check with your library?

Here's one we made.  Works really well, as long as the sun is shining.  Does not work at all on cloudy days. 



Raisins made with fall harvested grapes took a month to dry because of intermittent rain clouds...but they did eventually dry! 

I use it to dry seeds, herbs, roots.  We dried almost a gallon of cherries in June, a couple gallons of sliced apples in September.  Tried to make fruit leather by spreading apple butter on a ghee-smeared screen...the thicker parts peeled off but the thinner parts had to be soaked off the screen.  Next time! 

marina i have to say ive seen that pic before but it inspires me every time. i had a similar design setup in my head but yours is awesome. are there no problems with drying in the summer? we get no clouds or rain from like june to october. i was thinking of adding a possible smoker assembly at the bottom of mine so when it gets too cold, or its not sunny i can use it to smoke jerky, hot peppers, or anything else. i also had the idea of having on a rotating platform, so it can either move with some sort of mechanical action through the day or an electric motor. i plan on building it this spring i hope i have time to.

If you want a solar food dryer that doesn't expose the food to direct sunlight (although you should dry all mushrooms exposed to sunlight as their vitamin D content is dramatically increased), works on partially sunny days (depending on what you're drying), does not require any high tech crutches, and has worked in the humid upper Midwest for over 25 years, check out our design at http://www.geopathfinder.com/9473.html



The dryers built in a stacked tray, cabinet format that mimics an electric dryer but utilizing a solar collector to provide the heat, do not work as well because all the moisture is being pushed up through several trays of food.  Since the sun is not providing energy 24/7 on any dryer, you need to make optimum use of the energy available during the day.  However, complicating the design with fans or having to track the sun's path violates the K.I.S.S. approach that we take to design (keep it simple, stupid).

soil - it dries things really well in the summer, average temperature is between 100 and 110.  We accidentally burned tomatoes with it one year....that was sad, but it was 110 ambient temp that day, so maybe that had something to do with it. 

The trays up at the top actually dry faster, contrary to what you might think (with moisture moving upward thru the trays).  I think the box being black helps heat the air in there even without the solar collector doing it's thing, and the top portion of the cabinet is hotter, thus drying things up top faster. 

I think incorporating a smoker is a really good idea.  I hope you have the time too! 

I'd love to figure out a design for a dual solar/wood dehydrator so that raisins in the fall (when we have intermittent cloudy days) don't take quite so dang long to finish. 

Here are a couple from this thread:



And really old school:



And a video:

[youtube]http://www.youtube.com/watch?v=5U1JgAkW-xY[/youtube]

[youtube]http://www.youtube.com/watch?v=5U1JgAkW-xY[/youtube]

I am currently building a dryer much like marinajade's.   Like most of my projects, overthought overbuilt.   My collector panel is on a pivot so that I can adjust the angle per month.   The whole thing is on a frame and will have wheels so I can relocate it with my garden tractor.  The design also incorporates a port to accept ducted smoke from my big green egg.  There are a few issues that I am still concerned about though...


1.  I want the added benefit of the box being black but the thought of paint is unappealing.  Was thinking maybe a homemade concoction of black walnut stain?

2.  Because I am using roughcut lumber I expect in time to develop small gaps as the wood dries.  I could use food grade silicone but would rather not.  Maybe cotton and wax chinking?

3.  for optimal drying how hot shout it be?
 

three solar dehydrators.

The first solar dehydrator is shown by robert and marina at dell artimus farm.  The solar heat comes from a heated panel at the bottom, and there is a black chimney at the top that creates a draw.  They use  a stainless steel screen.  The dryer is a year and a half old.  They have dried beans, flowers, cherries, grapes (raisins), kale, walnuts and apples.    They tried some tomatoes, but those ended up as pig food.

Matt at feral farm shows a "down draft solar dehydrator."  The solar heat enters at the top and then goes down.  Because as it gathers moisture, the solar heated air gets heavier.    He has nettles in there.

Mark Vander Meer, of  wildland conservation service in Missoula, Montana shows off his solar food dehydrator still loaded with dried plums.  Those plums have been in there all fall, winter and most of the spring.  He talks about trying to dry fruit with electric food dehydrators and how expensive that was.  This solar dehydrator also uses the down draft technique.  He says plums take three days and apples take a day and a half.

These are all passive systems.  There are no fans.

Do you have any schematics for how to build that last one? That seems really interesting! I would love to try building one of those before my berries start fruiting!

We made a solar dehydrator like Mark VanDeMeer's but with the high humidity we have almost year round here in Florida it just does not work...we finally caved and bought an electric dehydrator...it was an expensive one but costs less than .07 cents per hour to run and is pretty effecient.

On the downdraft ones,the entrance opening should be the same area as the exaust opening I believe.Sinking air in the actual chamber is a result of both gaining moisture and cooling.

Hi, this is Marina, I rejoined finally. 

The tough thing is getting it to be airtight.  I think that's why it's probably smart to chop a hole in an old fridge or something, then you only have the one junction to worry about sealing, whereas when you build a box you have to be concerned about every single crack.  Ours could be waaaaaay tighter, that's the main criticism I have.  But it works just fine for our needs. 

Then again we live in a very hot and dry climate, we can dry things just by leaving them on the counter......but the thing we built is faster, cleaner, and keeps the flies off your fruit while it dries (that's another big important reason to seal up cracks - fruit fly maggots hatching out of food in the winter would be a real bummer).

1.  I want the added benefit of the box being black but the thought of paint is unappealing.  Was thinking maybe a homemade concoction of black walnut stain?

That sounds cool!  Um.....what about soot rubbed into the wood?  might be messy if you tough it later though....

2.  Because I am using roughcut lumber I expect in time to develop small gaps as the wood dries.  I could use food grade silicone but would rather not.  Maybe cotton and wax chinking?

  We used rolled up wool blanket pieces, mostly cause we had a lot of little scraps left over from another project, and the thickness of the fabric helps fill in wide gaps.  I'd be concerned about wax getting too soft in the heat.  Like i just said, this is a challenging aspect of these contraptions. 

3.  for optimal drying how hot shout it be?

We've had a thermometer in several parts of ours.  It gets really really hot here in the summer, and on a 110 degree day the top trays get up towards 125-130, the bottom trays are more like 110-115.  I think electric dryers stay below 120.  Some people are concerned with their food being exposed to temps more than 110 or even lower.....when the air temperature outside exceeds that I don't know how you'd go about drying food and keeping it cool enough to meet raw foodist standards.  Luckily, we do not care. 

thomas wrote:
1.  I want the added benefit of the box being black but the thought of paint is unappealing.  Was thinking maybe a homemade concoction of black walnut stain?

dellartemis wrote:
That sounds cool!  Um.....what about soot rubbed into the wood?  might be messy if you tough it later though.... 

First off, I am very impressed by all these designs, much more sophisticated than the cardboard chimney we built last year to demonstrate the concept.

I have no improvements on these designs.
But I've been interested in color and paints for a while, so I thought I'd chip in on that aspect.

1) Paint anyway. First off, for most of us, high-temp paints should work just fine.  Set up the system and run it for a few hot days with no food, until there is no odor of off-gassing paint, and then start drying your food.  You can use stoveblack (charcoal or soot mixed into your choice of grease) or modern Rustoleum enamel-type paints that are designed for the temperatures your dehydrator might reach.  Any paint is likely to offgas less than black plastic.

2) DIY non-toxic paint: Check with your local hardware store on ingredients in ordinary paint if you want minimal VOC (offgassing) content.  If needed you can get a few squirts of a straight black pigment from the hardware store or an art supplier. (Bone black or ivory black are forms of charcoal,  Mars black is an iron oxide, neither is particularly toxic as paint pigments go). 
You can mix the pigment yourself with a water-based binder such as milk casien or egg yolk and water.  Or, go for a natural, non-toxic, but smelly oil paint with food-grade linseed oil (AKA flaxseed oil).  Linseed oil will take at least a week to dry, more in cool weather or with low airflow, so plan to set it out a while before adding food.  Warning: I was taught that charcoal pigment can keep linseed oil paints from curing properly, they might stay tacky. 

A couple other non-toxic options for black color on wood:
3) Ink or pencil: The soot idea is not bad, you could also use graphite.  Both are used as pigments e.g. for India Ink, and I believe you can get food-grade soy-based black ink also. 
Paints could be more of a fire risk, where inks might not be as problematic.  Water-based inks or paints will be much quicker, but may be harder to make permanent or very dark. 
Any of the above will be flammable, and oil-based paints may increase the flammability of your wood.

Non-Flammable and Minimal Offgassing:

4) Cast-Iron Black: How about the blackness of a cast-iron skillet?  You could use a flat sheet of steel or iron, doesn't have to be very thick (panels from an old appliance might work, if you can expose a non-enameled side).  Sand or burn off any paint/enamel, oil with any food-grade oil such as you would use for seasoning cast iron, and heat if needed over a fire.

5) Pyro / Blackbody solution:
Take any metal panel, heat it over an open fire, until you have enough soot deposited on it to increase its heat absorbtion.

6) Inert Rocks: You could also consider using any naturally black material, like local stone, a thin layer of black sand, or dark sandpaper.

You folks are a wealth of info.  So a few updates on the build.

The pivot for the collector is done and will allow the collector panel to rotate from 40 to 80 degrees.  With a small modification I can get it to 32 but since I wont be drying anything in december I may not.  Have experienced less than expected shrinkage with the rough saw wood so far.  I love the idea of using old wool for chinking.  After seeing the down draft styles I am tinkering with a slight modification that will allow my plenum to operate in both air flow set ups.  Be interesting to test.    Since mime will also be a cold smoke chamber I tested it with smoke from the Big green egg, worked well and the thing smells fastastic hickory smoked.  Now if the kids would sleep later so I can make some progress. next up collector,wheels,tractor connection and racks.

I just bought milk paint from the realmilkpaint company....Just mix with water and paint. It took 2 coats to get a good black for me, now I Just have to test. As far as ease of use, it was fantastic, it dries really quick though. I was painting out in the sun in the morning and when I put down some paint you could see it evaporating off. This was during the second coat because the surface was so hot. Ill report back.

well now for the old school redneck way to do it.

ive never used anything but a elec. dehydrator. but my ex-wifes grandparents used to tell us about drying apples in the rear window of an old un usable car.  them old cars had huge back windows.  theyd lay the apples up in the back window.  keep the windows rolled up.  this built up the heat and kept the bees and wasps off the apples.  i cant image there was any air flow. but i guess the sheer heat of it all would dry the fruit.  i never saw it done.  just what i was told

Do the downdraft designs actually drop moisture at the bottom? And if so, how does the solar chimney still have enough pull? I can't wrap my head around it and I'm not interested enough to actually purchase a design.

It's been surprisingly hard to find a drawing of a downdraft design on the internet.

Yes. http://tinyurl.com/79vxwsy

Hope this is helpful.

Dusty
http://farm-dreams.com

Well, I looked at it but it's not a downdraft system in the same way as the ones in Paul's video. Those have a solar chimney that pulls the air through. I'm trying to figure out if the solar chimney draws even the moist air out or if moisture tends to collect below the food racks.

Hi I was wondering about the dehydrators on the youtube video. We would like to build one like the last one, my question is this, I live in michigan, will this work for me? also i saw a bit of mention to it being used as a smoker. Is there a place here or where ever that has good solid info on drying meats this way? It would be along the lines as using a cold smoke house correct ?, where the smoke enters from the outside, rather than being cooked? I think these are fabulous!

Somehow my old post has become thomas hatfield.. been away awhile. so some progress.

sealed everything from the outside with food grade silicone. Using batten board allowed me to seal the edge of the batten, so the stuff is 1" away from the inside. Also in similiar thinking once everything was caulked from the outside I painted regular exterior paint. Walmart had dark green oops paint for $7.50. The inside is still rough sawn wood. I thing the thickness of the wood will keep any nasty gases or chemicals on the outside. The positive internal pressure created by the heat/smoke should help also. Shingled it with "soda" can shingles I cut awhile back. Its slow going because I got TOO much going on. Maybe another month or so. The thing is supper heavy and though the wheels work fine I think bigger is better. The pivot was very difficult and time consuming to make, if adjusting the panel by month makes a huge difference it may be worth it. If not it was just an exercise in complex cutting,fitting and problem solving. I will try to get some pics up then its nicer outside.

Your ideas look new and fresh here, I just read about it in article and here you are implementing it. I am also working on my solar calculator when it will finished , I will post pics of my set ups.

Here's mine, simple and easy to build: http://www.velacreations.com/solarfooddryer.html

Check out "Appalachian" solar dehydrators, free plans pdf, tested and true, have made and used two of them, wore one out!! Easy to build and cheap to boot.
http://tec.appstate.edu/sites/default/files/HPImprovingSolarFoodDryers.pdf

Tim Meyer wrote:Check out "Appalachian" solar dehydrators, free plans pdf, tested and true, have made and used two of them, wore one out!! Easy to build and cheap to boot.
http://tec.appstate.edu/sites/default/files/HPImprovingSolarFoodDryers.pdf

Weren't the various downdraft designs essentially an improvement on the type shown in this link, as the warm air doesn't have to fight itself to exit the unit?

Im trying a downdraft type where its just a basic box with an open bottom. The food sits on a screen towards the top, and right above that I have alluminum sheet metal painted black and it is sealed to a piece of glass. There is about 2 inches between the black plate and the glass, the size is about 24 x 30. I put a thermometer in there during the sunny days, granted its only March but its been in the 60's and 70's here. I can't seem to get the temperature very high in there. I thought the downdraft type relies more on radient heat on the product not heating the air. Any ideas why mine isnt heating up very much?

Does anyone know where to find or purchase downdraft solar dehydrator plans?
I would like to build mine out of metal. I got the idea from "D acres" video on youtube. They built a separate compartment in their dehydrator where they can have a small smoldering fire. That way In the cooler month's in the fall or on cloudy days they have a way to get the extra warmth.
With hearing the responses from others it sounds like having a way to extend the months I can use the dehydrator is a valuable gain for the future.
The other aspects I would like is for it to be somewhat portable and have wheels on the bottom, an adjustable collector panel, and stacked shelf formation - so plans that have roughly these aspects would be useful to me.
Thanks
Laura (CT)

The best solar dryer I had was the northern Sacramento valley. Summer temperatures 110 with a steady wind up or down the valley. Apples pealed and sliced by a hand cranked machine would dry on racks before they could turn brown on a screened porch.
Some years later when back in Washington State, I decided to make a solar dehydrator. Materials I had available: scraps of 3/8 paneling, reclaimed 2x4, strips of wood that my father always saved when he ripped a board, role of plastic intended to make temporary inside storm windows, corrugated cardboard box material. I sorted out two 8' 2x4 for the back, two 4' 2x4 for the front and four 2' 2x4 for the top and bottom. With these I built a box with the paneling on the inside except for the sides where it went on the outside, one side for the door and the other nailed sollid for bracing. When assembled it looked like a big chair.

I wanted a side flow of air so during the assembly I cut slits with the circular saw in the paneling just below the cleats that would hold the drying racks front and back and at 3/4 inch intervals across the top. I used corrugated cardboard on the back of the front legs and from the bottom of the box to the back of the riser and up to the top. Could have used more paneling but this was just an experiment. I then painted inside of the front and top and riser black. I then used the plastic to cover the front, top and riser. From the scrap strips I framed screens to fit on the cleats in the box. I put them in with the screens up so that the air entering the slit would pass over the food to dry.

Operation then was to orient the front to the sun and the air heated between the front legs would be pulled up from the bottom and enter the slits and pass over the food. Hot air from the top would be pulled through the slits and drop down through the food. By the way the space between the riser and the top of the top 2x4s needs to be closed so that the air has to pass through the box to reach the riser. The heat generated in the riser then pulled the moist air through the back slits and out the top of the riser. This seems to meet the criteria of allowing the air cooled by evaporation to drop but still crating side flow across the food. I was not aware of the downflow problem at the time. I was trying to imitate the air flow in the circular tray system of the electric system I was using at the time which blew the heated air up the outside and pulled it down the center collomb. The disadvantage of the electric unit was it was re using a high percentage of the moist air to conserve heat.

The solar dehydrator worked reasonably well without trying to optimise for solar angle and it was cheap to make. I did put wheels on the back legs so that I could bring it under the care port at night and for storage. I would bring the trays in and put them on a clothes dryer rack by the wood stove at night if they were not finished.

Hope that gives you some ideas for your project.

2) DIY non-toxic paint: Check with your local hardware store on ingredients in ordinary paint if you want minimal VOC (offgassing) content. If needed you can get a few squirts of a straight black pigment from the hardware store or an art supplier. (Bone black or ivory black are forms of charcoal, Mars black is an iron oxide, neither is particularly toxic as paint pigments go).
You can mix the pigment yourself with a water-based binder such as milk casien or egg yolk and water. Or, go for a natural, non-toxic, but smelly oil paint with food-grade linseed oil (AKA flaxseed oil). Linseed oil will take at least a week to dry, more in cool weather or with low airflow, so plan to set it out a while before adding food. Warning: I was taught that charcoal pigment can keep linseed oil paints from curing properly, they might stay tacky.

That is good information Erica.

I am a weldor, and we work with steel all the time. I would think that steel in direct sunlight gets pretty hot, so hot it can not be touched. A simple temperature thermometer stick (wax with specific melting points) could give a good indication of just how hot. If the heat was trapped under glass I am sure the temperature would go up. I think unpainted sheet steel would be better and longer lasting than any available paint. No toxicity. The cost is higher. In california the cost of a 1/16" thick 4' x 8' sheet can exceed $90 dollars. Yet well worth it. The steel should not leave a flavor or smell if it is cleaned from having the oils on it from shipment. Sheet steel is very easy to work with. Tin snips, rivets etc. There are two primary grades of sheet steel hot rolled "black iron oxide coating from being hot rolled" less exspensive; and cold rolled : shinny steel surface highly reflective can rust very quickly, higher cost. I would think the hot rolled would be better, although both get very hot very quickly. The cold rolled has to be managed with more care to keep it shinny and to keep it from oxidizing. So my recomendation is Hot rolled sheet steel 1/16". there are thinner gauges but as it gets thinner it gets More exspensive as it gets thicker it gets more exspensive. 1/16 is in a weired position of cost. have a great day.

Hello!

I've been trying to make Mark Vander Meer's model since the video came out. We recently received a $200 donation for it, so we built it... except we went with tongue-and-groove local hardwood boards instead of plywood, as everybody advised me against it (I live in Costa Rica and the whether would make it warp too soon). I also didn't use clear material for the solar collector, because it wasn't available, if it had been, it would have been too expensive, but mostly because in another video a guy said that a flat surface would have reflected most of the sun rays, so he used some showerglass-looking plexi glass, and that's what I did, too.

The main difference is that we ran out of wood, so instead of a false wall all the way up, I made a short false wall that ends up into a 4" stove pipe that doesn't go straight up, as I put my dehydrator under the roof of our solar panel rack for fear of it overheating in the tropical sun. So the pipe goes back around to the South side, getting nice and heated in the sun, thus sucking up the cooler air from the bottom of the dehydrator... in theory, because I certainly don't feel the same draft that Paul Wheaton could feel with the door open. I even sat in it with a tea light and the flame wouldn't even flicker.

Any suggestions? I calculated the volume of air the false wall would have contained and my pipe contraption exceeds that. Is the opening of the stove pipe just too small? Meaning, it's not about the volume, but about the area? I did that, thinking of air wells, or solar trombs, or whatever they are called; you know those solar/wind turbines that look like upside-down funnels.

Let me see if I can post pictures.

Any input is welcome, but tomorrow I'll start adjusting the holes. If the area of a 4" pipe is 12.5 square inches, I'll make that the size of the opening between the solar collector and the cabinet, and between the cabinet and the false wall. I guess too bad, because it would reduce the amount of air coming in, but at least it would flow. It's easier than making a whole false wall chimney. Am I making sense?

Well, I can't post pictures at present, due to sucky connection.

Sergio Santoro wrote:Well, I can't post pictures at present, due to sucky connection.

It's a bit hard to follow without pictures. But I'm noticing that you mentioned protecting the contraption from direct sun, and being in a hot humid climate rather than a warm dry one (Mark van der Meer is in Montana, I believe, and they get a fair amount of sunny days and cool nights throughout the growing season).

In the contexts where I work with downdraft flow, there are a lot of relative "ifs" involved. Relative temperature, relative humidity. Air doesn't naturally go up - but hotter air will generally do so. Cooler air will naturally flow downward. If you are trying to draw cool air upward while heating it from above, a pure-and-simple upward chimney is probably your best bet. It's the initial pre-heating, and then a heat-loss process like evaporation (or a box that is shaded and cooler than the air intake chute) that would allow air to go downward through the food.
There's some controversy over whether the added moisture would make the air heavier - water vapor H20 is actually a lighter-weight molecule than air's main components N2 and 02 - hydrogens are tiny.
So if the water evaporating off the food displaces some of the air, the air would expand a bit and might become slightly less dense. This is balanced by the air being cooled dramatically when it evaporates water from the food. In Mark's climate, summer air can be very dry - less than 10% moisture on many days - and the water is probably pulled off the food relatively quickly, changing the relative humidity relatively fast.

In a hot humid climate, I imagine the evaporation rates, and the overall change in density due to cooling, evaporation, and increased humidity, will be different. If there is no heated up-ramp, and the air is near 100% humidity as it enters the box, then there would be no reason for it to flow downward after passing some of the food - it would not be evaporating moisture very fast, and it would not become cooler or denser.

In the tropics your big problem is drying the air, not necessarily heating the food. But heat dries the air somewhat - makes the relative humidity lower for the same weight content of water. Your big challenge will be to heat the air enough, or find some other way to achieve drying / lower relative humidity, without accidentally burning the food.

The other thing that could result in much lower draft in these downdraft heaters is any air leaks whatsoever. A leaky box would tend to stagnate or draft upwards, not downwards.

-EricaW

I have yet to find a method that works in high humidity other than smoking was done in the PNW. People that have 10% humidity don't need to do anything but keep the bugs off for a day. But high humidity is just impossible to dry, you can't get drier than the air.

You need to cool the air first to remove water, like an a.c. does. Pull the air from a rock well or other cool, condensing area, then heat it.

Funny this thread got unearthed after 5 years, and now that I'm thinking of upgrading the one I never posted pictures of.

It was an ordeal. I could barely follow the design of the top-draft model. I ran out of money before I could make the chimney wall, so I used some stove pipes, that went around the solar collector side, painted black.... I never got a draft. So now it's just a hot box, with a fan on the ceiling circulating the air coming in from the solar collector. I couldn't make the walls out of plywood, as we concluded one rainy season would be enough to warp and trash the plywood, which is exactly what's happening to the floor of the solar collector. So we used tongue-and-groove boards, and I sealed every crack with sylicone, and I used washers on the door... still, air and bugs could go in an out. Even geckos! Everything is screened in, I was so surprised to see so many critters. I went for the top-draft model, because the guy showed he had the last of the prunes from the previous year still in there! Don't they get a winter? No rain?

Anyway, we have so many pineapples and bananas coming constantly and I'b really grateful to find out the best way to dehydrate them. Also, seasonal stuff, like mangoes, come right when the rains come! We have a micro-hydro system in the river, and solar panels; so, I thought of using incandescent light bulbs as a source of heat at night or when it rains. A propane tank wouldn't last too long and would make that dehydrated fruit very expensive, even if we were to sell it.

Any help or insight would be welcome. Thanks!

And here I am back again. My solar dehydrator is falling apart. The Tropics are just too brutal, and we are just too poor; I think we cut too many corners to save money and the materials we used are disintegrating under the tropical sun and rain. So, I'll keep it short and sweet. First of all, we have a better internet connection, so I'll upload pictures of our current model at last. I've been looking for solar dehydrators for the Tropics or for humid climates, and all I find is very disappointing, such as dehydrators where the solar collector and the drying chamber are the same, with the fruit drying in direct light (no, thanks), or they have a huge solar collector and the drying chamber is as big as a dog house. I need something close to my current model, if not bigger, as I'll be having more and more bananas and pineapples (drought permitting). Right now I have 8 stacked 2'x3' trays. I looked into drying tunnels, but they take a lot of space. I wouldn't mind a walk-in model. As you can see from the pictures, I went with the top-draft model, the third in Paul Wheaton's video. I even corresponded with the guy in the video, who pretty much cut me short after too many questions and after giving me the design. I never got the darn top draft, so I got a top fan pointing down. During the rainy season, or even before the rains, the air is so moist I have to use a propane burner on the lowest setting to keep stuff from rotting. I don't know how people in North America can forget their prunes in there from the year before. Doesn't it rain there, too?
So, here I am, begging again for pointers, insights and help. I love permies.com, but it seems to be mostly North-America-centered (but please do prove me wrong!), and Geoff Lawton's forum may take days before I get an answer. Still, they are both great resources that can save a lot of time and help lots of people. Thanks, and keep it up!

I do not understand your collector design. for a down draft the air should only enter the dryer box at the top. Light entering the box all down the side toward the collector indicates it is open to the collector. that wall should be solid up to above the top shelf otherwise the warm moist air just keeps recirculating into the solar collector. Are the sides of the collector allowing the sun in? If that triangle of the collector contained some heat storing material that allowed the air to pass through then the air could enter a screened opening on the bottom near the dryer box and and pick up heat stored during the day to continue circulating after the sun sets. You might have to open the bottom of the chimney stack at night to allow the air cooled by the drying fruit to exit the bottom. I would put the fan at the bottom blowing out the chimney to get the circulation started
So if the air has been circulating from the dryer box back into the solar collector that is the first thing to correct.

Hi Sergio,
Where is the air supposed to exhaust from? I don't see any holes at the bottom of the drying chamber.