The Design and Build of the Giant Solar Food Dehydrator (1 hour and 21 minutes HD)

Have both black paints held up equally well over time?  (Talking about the milk & vinegar vs strained yogurt in the ATC model.)

Kimi Iszikala wrote:Have both black paints held up equally well over time?  (Talking about the milk & vinegar vs strained yogurt in the ATC model.)

holding up fine so far!

Hi!

I just watched it and i would like to add something.

Ii tried one similar to hot aussie drier but it didn't perform well in my place -too much morning humidity, rainfall etc... But i remembered my research about updraft and downdraft driers...

The thing i found was interesting: the air with more humidity in it is lighter than the one with less humidity (relative humidity). The gas law from chemistry explains that because at the same pressure there can only be certain amount of molecules present in the same volume. Since the molecules of water vapour are lighter than average weight of molecules in the air, the drier air is denser (or heavier per volume unit).

https://www.omnicalculator.com/physics/air-density gives a simple calculation of air density:

few calculations:
at 50C and 20% density is 1,08 kg/m3
at 50C and 50% density is 1,07 kg/m3
at 50C and 70% density is 1,05 kg/m3
at 40C and 70% density is 1,10 kg/m3

The faster the airflow through the drier, the lesser is the cooling effect and the lesser is the humidifying effect - my conclusion would be that faster air flow saves it all, but it need some more solar gain surfaces to get that air faster to the appropriate temperature...

I would say that in this particular example there is not much of an information but maybe for the future projects it might come handy: humid air is lighter than dry air - contrary to obvious conclusion...

Thinking of it i can't help to think about rocket stoves with low temperature exhaust - now it looks like humidity in them helps to add upwards draft even if cooled to the temperature of the surrounding areas... But the CO2 highers the density of exhaust.

There is also an interesting point:
"Humid air containing water molecules as liquid - droplets - may be more dense than dry air or humid air containing water only as vapor.  "
I found that one here: https://www.engineeringtoolbox.com/molecular-mass-air-d_679.html

Pretty complex problems and with so many variables it is hard to walk safely on the edge -

Regards,
Klemen

I think the big test is to have three of these side-by-side.   And then load all three with the same food and find out which one dries faster.  

Frankly, I think that the glass part should be steeper.  I think that would help a lot.  

Another feature for improvement is to add a 4-inch rocket mass heater under the ramp.  Maybe there can be an 8-inch thick mass under the ramp so you can heat the mass, but it doesn't heat the air too much.   And then when the fire is out, the mass is still something like 100 degrees and keeps pushing warm air through the system.   So in the fall when you have 12 hours of daylight and 12 hours of night, then you get 24 hours of drying time.  

Another feature for improvement is to add a 4-inch rocket mass heater under the ramp.  Maybe there can be an 8-inch thick mass under the ramp so you can heat the mass, but it doesn't heat the air too much.   And then when the fire is out, the mass is still something like 100 degrees and keeps pushing warm air through the system.   So in the fall when you have 12 hours of daylight and 12 hours of night, then you get 24 hours of drying time.

In our country i don't count on longer periods of sunny weather, especially not in foggy autumn days.
If i would build a drier of a larger size, i don't like the long burning of a 4-incher to get enough heat. Our local heritages used slow and dirty burning to heat large driers overnight or even longer - this i don't like either... Maybe there could be a solution with a small batchbox and a small bell with fat wall. I would make a bell with metal fins pointing from inside the mass towards the inside of the bell. This would make a larger and a more effective ISA that could store thermal energy quicker into the thickness of the bell's walls. Emitting that energy from the outer side towards the drier would be slow because of the fat cob walls...

Burn some kilos of wood in one batch and then walk away for a day would be really convenient... If only air inlets could be closed automatically ;-)


Regards,
Klemen

So I'm curious about the upshot of the bigger design... does it seem like the secondary solar collector provides the boost that you expected?  Is it worth putting on a secondary collector, or not necessary?

I have another question about the design differences between the two collectors... the Missoula collector has the area under the collector enclosed with glass.  Does that seem to make any difference?  Do you think there's any advantage to enclosing that area, or is it unnecessary?

I'll be building my dehydrator in high desert so I'm thinking I might not need the extra boost.  It is quite arid where I am, but it does get cold at night.

Thanks for all this!

Kimi

Another feature for improvement is to add a 4-inch rocket mass heater under the ramp.  Maybe there can be an 8-inch thick mass under the ramp so you can heat the mass, but it doesn't heat the air too much.   And then when the fire is out, the mass is still something like 100 degrees and keeps pushing warm air through the system.   So in the fall when you have 12 hours of daylight and 12 hours of night, then you get 24 hours of drying time.  

Paul,
perhaps a solar pond would work to increase the drying time. The intake air could be piped through the very salty water at the bottom of the pond. It would likely have to be a large pond to duct enough air to supply the dehydrator

Or, maybe some well placed reflecting material?

Would it be feasible to char the wood under the glass instead of using the paint? Or would that be problematic?

jessie franks wrote:Would it be feasible to char the wood under the glass instead of using the paint? Or would that be problematic?

When i was young we put a candle under the piece of glass to make it black - then we used it to look the solar eclipse - the sun was dark reddish in color... Darkened glass (soot on top) was really totally dark and matte.... I think that could work...
I would think that charring of the wood might add some additonal flour to a dehydrated food because charred wood is not soot only but also partially charred wood.... But if there is only soot layer it might be a better option...


Regards,
Klemen

jessie franks wrote:Would it be feasible to char the wood under the glass instead of using the paint? Or would that be problematic?

Just to be sure, please try it and share your results.

We've done some wood burning stuff here and the weather washes it all away in about two years.  But since the char would be under glass, it might last decades?

paul wheaton wrote:I think the big test is to have three of these side-by-side.   And then load all three with the same food and find out which one dries faster.  

Frankly, I think that the glass part should be steeper.  I think that would help a lot.  

Another feature for improvement is to add a 4-inch rocket mass heater under the ramp.  Maybe there can be an 8-inch thick mass under the ramp so you can heat the mass, but it doesn't heat the air too much.   And then when the fire is out, the mass is still something like 100 degrees and keeps pushing warm air through the system.   So in the fall when you have 12 hours of daylight and 12 hours of night, then you get 24 hours of drying time.

paul wheaton wrote:Or, maybe some well placed reflecting material?  

jessie franks wrote:Would it be feasible to char the wood under the glass instead of using the paint? Or would that be problematic?  

I think folks should *might want to* consider their latitude, and what time of year their drying needs are greatest, to determine the angle of the glass. For Northern U.S. it does feel like it could be steeper. I think the ideal is local latitude plus the tilt of the Earth (for Winter solar gain = Lat. + 23.4 degrees (tropic of Cancer/Capricorn)).
Possibly a super-deluxe, albeit complicated, version would be adjustable tilt for maximum gain.

If the ramp surface itself was cob, it would BE the mass. It might even hold a solar charge of heat from the day ahead of a drying session...

I like the charred wood option as a non-toxic blackening, basically the same as black paint, but no binder required!

Something from the solar-air-heating collectors dept. is to use a few layers of dark colored aluminum window screen, arranged such that the air path flows through the screens, transferring the heat. This is in addition to the black inner surfaces.

Awesome food for thought as I plan out my build!

In  the Wheaton design with the open sides it seems the drying cabinet would cool the air too quick slowing the evaporation process. Was this changed in a later design? Closing the front of the cabinet under the glass would be a nominal cost but it seems that boxing in the sides would be best.

It does seem like insulating the ramp would be an improvement.

Rental worked perfectly (used paypal) - looks like it's all good from here

Rental worked perfectly for me as well (used paypal also )

BTW LOVE LOVE LOVE this idea of rental of the great content on the site - truly inspired

Conrad Farmer wrote:BTW LOVE LOVE LOVE this idea of rental of the great content on the site - truly inspired

Thanks!  I made a thread so we can talk about it more:

https://permies.com/t/137260/rental-video-content

Now is the time to report if the rental worked.

Conrad?  Jan?  You rented it and the 24 hours expired.  You should now NOT be able to watch the movie.  Is that accurate?

Hello Paul,

I can't see the video being available anymore on this page for me. I can only see purchasing the 3 day offer. Seems like it works fine :)

Cheers

Kimi Iszikala wrote:Is it worth putting on a secondary collector, or not necessary?

I believe the secondary solar collector is obsolete because... "where does the new air come from?"

Kenneth Elwell wrote:I think folks should *might want to* consider their latitude, and what time of year their drying needs are greatest, to determine the angle of the glass. For Northern U.S. it does feel like it could be steeper. I think the ideal is local latitude plus the tilt of the Earth (for Winter solar gain = Lat. + 23.4 degrees (tropic of Cancer/Capricorn)).
Possibly a super-deluxe, albeit complicated, version would be adjustable tilt for maximum gain.

I like the idea of having an equation to find a desired slope. I believe there is a sweet spot that relates more to the air flow science than it does the heat gain science. I believe more slope information will be discovered when Wheaton Labs alters their design at a future ATC.

Davin Hoyt wrote:

Kenneth Elwell wrote:I think folks should *might want to* consider their latitude, and what time of year their drying needs are greatest, to determine the angle of the glass. For Northern U.S. it does feel like it could be steeper. I think the ideal is local latitude plus the tilt of the Earth (for Winter solar gain = Lat. + 23.4 degrees (tropic of Cancer/Capricorn)).
Possibly a super-deluxe, albeit complicated, version would be adjustable tilt for maximum gain.

I like the idea of having an equation to find a desired slope. I believe there is a sweet spot that relates more to the air flow science than it does the heat gain science. I believe more slope information will be discovered when Wheaton Labs alters their design at a future ATC.

It could be Earth tilt on a calendar lookup (useful for adjusting for the day/month of intended use), plus one's local latitude. I agree that the air flow is primary, and the ultimate heat gain secondary. Adjustments for more equatorial latitudes might really frustrate the air flow...being much shallower angles.

I think it will be interesting to find out if fine-tuning for incidence on the glass by latitude and day/week/month of peak use has a measurable benefit. If it does, it may result in an earlier startup in morning, and run later in the afternoon, or work on partly sunny days. (tracking would add a bit more as well)
I'll probably build mine to be adjustable within equinoxes and summer solstice at my latitude (42.5 N (~4.5*south of Wheaton Labs)). I've already scrounged the materials, just need the time...

"If the ramp surface itself was cob, it would BE the mass. It might even hold a solar charge of heat from the day ahead of a drying session... "  this seems like a really good idea to me.  wonder though how much heat storage would really be achieved with a layer of cob on the inside bottom of the ramp....

Are either of the videos actually available for download so you can watch offline? I have very little access to internet and I needed to download so I can watch it at home off-line.

Zowe Smith wrote:Are either of the videos actually available for download so you can watch offline? I have very little access to internet and I needed to download so I can watch it at home off-line.

Either?  Two?

Zowe Smith wrote:Are either of the videos actually available for download so you can watch offline? I have very little access to internet and I needed to download so I can watch it at home off-line.

Hi Zowe,

The best I could find was this link to view online:

https://permies.com/t/165657/minute-long-documentary-design-build

I can't find a way to download though, so I'm sorry if this is of no help to you.

P.S. Welcome to Permies

Thank you for this gift.