Solar Dehydrator Movie and Build Plans!

So, I just built my first solar dehydrator in August. It's loosely modeled after the ACT-one you guys built a few years ago, and now I've also made a smaller, demo-model for some classes I am teaching on the theme of building for the apocalypse. I plan on building a few more this winter, and gifting them for the holidays and such, but want to improve my design as I go.

A few questions that invariably (a)rise like hot air, is why do I have the sun trap (or heat collector) box coming in the top and the false wall outlet at the bottom, rather than the more common model of having the heat collector come in at the bottom and have the outlet at the top? The way I figure, what I'm really creating is a wind tunnel by using the zeroth law of thermodynamics (forget the abc of it, hot air follows cold is the takeaway).
With the air intake at the top, I can make a longer heat collector box, so the temperature can increase as compared to a shorter one that goes into the drying chamber, which makes for warmer air --> bigger difference in temperature --> the air moves faster through the drying chamber.

I've constricted the gap between the heat collector box and the drying chamber, so the air will move faster once it goes through the drying racks, same as wind or water speeds up through any narrow passage. The gap is as wide as the trays holding the produce, but is only about an inch tall, while the box is more than the double.

Also, hot air holds more humidity than colder air, so I assume that as the air stream loses temperature, while flowing down the chamber, water will precipitate out and theoretically pool at the bottom of the drying chamber, or if the temperature is still high enough, humidity will escape with the rest of the air through the fake wall.

Basically, what is the benefit of having the hot air intake at the top and the outlet of colder air at the bottom? It seems intuitively right, but I feel I don't have enough arguments to convince others while discussing the design choices I've made.

As I understand it, it is the wind tunnel effect, or the air streaming through the drying racks, that dehydrates the food and not a high temperature in there, which would cook the food rather than drying it.  Thus, I've made a slanted roof on top of my dehydrator to protect it from and rain, and from the sun heating up the inside of the drying chamber. Still, I notice that the produce on the top trays dries a lot faster than on the lower trays. Is this due to higher temperatures at the top, or that the air slows down as it works its way to the bottom and out through the chimney / false wall?

Lastly, someone suggested having rocks at the bottom of the dehydrator to make the bottom of the drying chamber warmer, and thus the drying more even throughout the chamber. Has anyone tried this? I've tried to make my dehydrators as light as possible, so this doesn't necessarily appeal to me, but I am willing to try it if it might improve the design and shorten drying times.

Your educated guesses or empirically formulated hypotheses are most welcome, and the sooner the better! Thanks!

Kate McRae wrote:So, I just built my first solar dehydrator in August. It's loosely modeled after the ACT-one you guys built a few years ago, and now I've also made a smaller, demo-model for some classes I am teaching on the theme of building for the apocalypse. I plan on building a few more this winter, and gifting them for the holidays and such, but want to improve my design as I go.

A few questions that invariably (a)rise like hot air, is why do I have the sun trap (or heat collector) box coming in the top and the false wall outlet at the bottom, rather than the more common model of having the heat collector come in at the bottom and have the outlet at the top? The way I figure, what I'm really creating is a wind tunnel by using the zeroth law of thermodynamics (forget the abc of it, hot air follows cold is the takeaway).
With the air intake at the top, I can make a longer heat collector box, so the temperature can increase as compared to a shorter one that goes into the drying chamber, which makes for warmer air --> bigger difference in temperature --> the air moves faster through the drying chamber.

I've constricted the gap between the heat collector box and the drying chamber, so the air will move faster once it goes through the drying racks, same as wind or water speeds up through any narrow passage. The gap is as wide as the trays holding the produce, but is only about an inch tall, while the box is more than the double.

Also, hot air holds more humidity than colder air, so I assume that as the air stream loses temperature, while flowing down the chamber, water will precipitate out and theoretically pool at the bottom of the drying chamber, or if the temperature is still high enough, humidity will escape with the rest of the air through the fake wall.

Basically, what is the benefit of having the hot air intake at the top and the outlet of colder air at the bottom? It seems intuitively right, but I feel I don't have enough arguments to convince others while discussing the design choices I've made.

As I understand it, it is the wind tunnel effect, or the air streaming through the drying racks, that dehydrates the food and not a high temperature in there, which would cook the food rather than drying it.  Thus, I've made a slanted roof on top of my dehydrator to protect it from and rain, and from the sun heating up the inside of the drying chamber. Still, I notice that the produce on the top trays dries a lot faster than on the lower trays. Is this due to higher temperatures at the top, or that the air slows down as it works its way to the bottom and out through the chimney / false wall?

Lastly, someone suggested having rocks at the bottom of the dehydrator to make the bottom of the drying chamber warmer, and thus the drying more even throughout the chamber. Has anyone tried this? I've tried to make my dehydrators as light as possible, so this doesn't necessarily appeal to me, but I am willing to try it if it might improve the design and shorten drying times.

Your educated guesses or empirically formulated hypotheses are most welcome, and the sooner the better! Thanks!

I have also wondered about this. What you are suggesting seems much more logical to me. Have you gotten a respons from someone else?

I think you are spot on.

As for the rocks in the solar dryer. I saw that there Uncle Mud made one with rocket assist and it has rocks inside.....

Air can only absorb a finite amount of moisture before it is saturated. At that point it condenses on cool surfaces. As the air goes through your box and picks up moisture from the food it gets closer to that point. Imagine the air as a sponge (kindof). As you slowly wet the sponge it will absorb water very fast at first. After a while it will slow down as it gets wetter, or more saturated. Just like air does. So as the air moves through the box it will always dry the food faster at the first trays it moves across.

Norbert Muller wrote:Air can only absorb a finite amount of moisture before it is saturated. At that point it condenses on cool surfaces. As the air goes through your box and picks up moisture from the food it gets closer to that point. Imagine the air as a sponge (kindof). As you slowly wet the sponge it will absorb water very fast at first. After a while it will slow down as it gets wetter, or more saturated. Just like air does. So as the air moves through the box it will always dry the food faster at the first trays it moves across.

Hey Norbert,

thanks so much for your response, that makes perfect sense! I kind of reached a similar conclusion, and took the dehydrator apart last fall.
I definitely have too many shelves for the air volume, and will tackle that in a couple different ways.
First, I will add one or two solar powered computer fans to assist with the air speed and volume flowing through.

Second, one of the mistakes I made was combining the design of the two dehydrators at Wheaton Labs (or Basecamp).
The one with the roof tilted backwards was the one with the additional RMH in it, so it probably made sense for that.  
For mine, however, the chimney should of course be at the highest point of the roof to create the best draft, i.e. the whole roof pitch should be forward if the double wall chimney is in the back. So, I will flip it around, put the air intake slot highest up on the lower side, and the double wall chimney as the highest side, probably with an additional height above the proper roof. I'll post some pictures once it's done.

Third, I will be more cautious about how much I fill the dehydrator.
We had an extremely wet fall last year, and I think that this far north (58°N+) this system works best in the height of summer with early harvests, rather than the rapidly shortening days of fall, especially with the coastal humidity. Perhaps I can build a trombe wall on a house and dry more produce there, instead. I saw a massive one a few years back, and it was extremely efficient at drying stuff. Felt like you were in a wind tunnel when you walked into the building...

I don't know about the rocks though, since this is a mobile unit, but I am putting a third wheel on it, and rebuilding the undercarriage for better balance, so it's easier to move around.

I might also add some light insulation to the walls, to decrease the chances of condensation within the drying box. Any thought on that?

On the upside, I have learned so much from actually building and trying this, rather than just reading about it and pondering how to build it.