passive solar dehydrator

[Ziptie]

So, not like I don't have enough projects right now. Not even going to dare mention this project to DH.

Thought I would put all the info I found and throw it out to everyone to see what the best design it.

Want to have a tall up right dehydrator. These designs look about the same but one has the hot air comming in from the bottom and going out the top. The other has the hot air comming into the top and going out the bottom. Which one do you think would give the greatest air flow?





http://www.instructables.com/id/Larg...ar-Dehydrator/


A youtube. Think the first one in the video is the best one.

YouTube

[LauraD]

Hot air naturally rises, so my first instinct would be to have the hot air come in the bottom and go out the top.

[SolarGary]

Hi,
Maybe "none of the above"

Dennis Scanlin at the at Appalachian State U did quite a bit of work on this with various student groups over the last decade or so and came up with the designs shown here:
http://www.motherearthnews.com/diy/s...#axzz35ya3tAYW

If that long link does not work, go to the Mother Earth website, and search for "Scanlin Best-Ever Solar Food Dehydrator Plans"
Full plans there.

A good detailed description of the tests he did here:
https://docs.google.com/viewer?url=h...FoodDryers.pdf

One of the conclusions they came up with is that flow through absorbers (like metal screen or lath) work significantly better than solid absorbers. So, this is something I'd try to include in your design.

The chimney on the one of the dryers you show seems like it might be helpful as long as its large enough in diameter not to restrict the flow.

These are all thermosyphon collectors, and the buoyancy forces that make the air flow through them are small, so, I'd think that the ones that make the air reverse path and flow down would not generate as much flow as the ones that just let the air keep rising. Also think that collectors that have more vertical rise produce more airflow.


Some more on solar dryers on my site here:
http://www.builditsolar.com/Projects...ing.htm#Drying

Gary

[gila_dog]

I borrowed a neighbor's old electric food dryer. It made lots of noise and ate lots of power, but eventually dried some apples. But I improved on the system a lot by loading up all its little drying trays and then just lining them up on the dashboard of my truck, parked out in the hot sun. In 3 days the apples were all dry and ready to store. Kinda redneck, but it sure works.

[Ziptie]

SolarGary-Thanks for all the info looks like I have some reading to do.

What do you think will happen if I make the box where the food is taller or wider.

gila_dog- Yah, hubby has a fit when I transport hay bales in in car. Also, not sure how well it would work with all the humidity we have here. Lots of condensation on the inside windshield of the car.

I was thinking this outdoor dehydrator would be nice for drying out tomato so making tomato paste wouldn’t take so long and animal feed stuff... duck weed,sorghum,sunflowers, etc.

That is one thing I liked on the first pics I posted, the large deep trays.

[SolarGary]

Originally Posted by Ziptie

SolarGary-Thanks for all the info looks like I have some reading to do.

What do you think will happen if I make the box where the food is taller or wider.

gila_dog- Yah, hubby has a fit when I transport hay bales in in car. Also, not sure how well it would work with all the humidity we have here. Lots of condensation on the inside windshield of the car.

I was thinking this outdoor dehydrator would be nice for drying out tomato so making tomato paste wouldn’t take so long and animal feed stuff... duck weed,sorghum,sunflowers, etc.

That is one thing I liked on the first pics I posted, the large deep trays.

Hi,
I'd guess that the total airflow would be a little more for larger trays, just because the flow area is larger and that reduces the flow resistance. But, this somewhat greater flow is spread out over more area, so an individual tomato slice would probably see less flow velocity past it. So, I think, the net result of a bigger tray would be that you could get more fruit per tray (just because its bigger) but that it would dry a bit slower. All a guess as I've never built or used one of these myself (yet).

Gary

[Forcast]

Mother Earth News june july 2014 has solar food dehydrator diy.

[BlackFeather]

I built a card board version of the one that says Costa Rica on it. I wanted to try a cheap version before I built a good version. It dried cherries very well as they come around the beginning of July when the sun is hot up here in the north. At the end of August when the tomatoes ripen in numbers, it didn't work so well as the sun is lower in the sky and the days are shorter. My tomatoes were covered by small gnats and didn't want to dry very well. I ended up buying an electric one but would like to build one that fits on the wood burning stove. I'm not sure how it would work in Iowa, you might want to experiment like I did with a inexpensive design before building a nice one to see how it works in your area. Your weather would be different, here we're between two lakes hence more humidity, and have probably more cloudy days than you have.

[Ziptie]

Good idea Blackfeather.

I would think that it would be good to have the solar collector part somewhat adjustable some way as the angle of the sun changes over the year.

The humidity is really bad here. The salt licks in the rabbit cages are dripping water. So I would want something that would have decent air flow.

Did you have problems with it cooling down at night and the food reabsorbing the moisture?

[Wolf mom]

A gal I know here in Arizona shuts her greenhouse down in the summer. She cleans it up and uses her start (starting plants) tables as drying tables.
The added benefit is if any bugs have gotten in the greenhouse over the past seasons when she is growing things, they die.

[unregistered5595]

Wolfmom, that is such a great idea. Thank you.
We were trying to figure out where to store the removable cold frames from the raised garden. Two sections 4 x 7 feet. Side view is a triangle, squared on the horizontal, and the vertical back, and the diagonal is the door, hinged at the top back, handle on the bottom.

We decided we'll put a dark tarp on the deck, place the cold frame over it, back is covered in clear plastic but we'll drape some silver tarp to the back to reflect the heat. Face the door to the south. (WI)
I'm excited to see how this works during July and August. It's hot and sunny in that location.

I'm usually running my dehydrator 24/7 August-Sept-Oct-Nov and it's costing us about $10 in electricity per month. Even if I can partially dry things in a passive dehydrator, it will save us time and money.

[BlackFeather]

to answer your question about reabsorption of moisture, not enough that I noticed.