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Dale's 24 hour, SOLAR, food dehydrator.  RSS feed

 
Dale Hodgins
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I see that the most common problems afflicting solar dehydration, are slow speed and mold. ( mould to speakers of the Queen's English. My spell checker is set to "American Pidgin")

l have arrived, like a knight in shining --- gourd shell armor, sewn together with horse hair , to lay this problem to rest once and for all.

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The problem lies in the fact that, other than in polar regions, the sun doesn't give round the clock service.

I called an emergency meeting with all of the various deities last night and they were quite adamant that changing this would cause more problems than it would solve. There was snickering. I've sworn to do my best, with existing day length. -------- ( They want everybody to stop praying for rain and try key line and other means of water catchment.)
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What we need here, is some thermal mass, to continue providing updrafts of nice warm air, throughout the night.
--- The device needs to reside on a deck or patio, which is adjacent to a wall made of cob, brick, stone ... or other materials with high thermal mass. The available wall space, should be quite a bit larger than the dehydrator.

A frontal profile, about the shape of a door, sounds right. It would have hinged glazing, for easy access from the front. The rear, which has cloth weather stripping is placed tightly against the smooth wall surface. The wall sections not covered, will heat in the sun. The contraption moves sideways along the wall, either with wheels, or ideally, suspended from a barn door type track. Those things are very strong. It could be set up, so that the weight of the machine presses it against the wall, for a good seal. When moving, it would be pulled slightly away from the abrasive surface.

A mass wall will reach a point of maximum heat saturation, late in the afternoon on most days.
Once this temperature is achieved, an insulating blanket could be placed over it to preserve heat until it is required. Blankets are suspended from the track. More than one wall section could be insulated. At any time during the day, the machine could be moved to a warmer or cooler backing.

The thing is going to run well, until the sun is off of it, and then it will slowly shut down.
Move it to an insulated cob panel in the evening. Let the kids stay up to watch the late, late show, with the promise that they'll move the machine again at 3 am. It will keep cooking until the sun returns.

I believe that mold development, is mostly related to down time, when the air stops moving. In all but the worst weather conditions, we're going to keep the air moving 24/7.
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I'll run this by the guys, at our monthly meeting next Saturday, to see what they think. I'm going to press them on the issues of disease and pestilence, but don't hold your breath. They're a tough crowd.
 
Dale Hodgins
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Six months since this one began. I've come up with a few more ideas.

Insulated rock pits are a proven strategy for storing solar collected heat for later use. When used outdoors for solar drying, a rock pit could be very simple. Picture an insulated box about the size and configuration of a kid's sandbox. This box is filled with dark colored rocks. A glass lid is attached with no leaks. Those rocks will get hot as hell. An insulated pipe with a shut off valve runs up to the solar dehydrator. A one way vent allows air to enter the stone storage on the lower part of the opposite side whenever the valve to the dehydrator is opened. Hot air will flow by convection and air entering the rock storage will be drawn across the rocks.

An insulated lid or blanket should go on in the evening and be removed in the morning.

Operation --- Open the valve when heat is needed at night. Close it in the morning when the dehydrator will work from direct solar gain. Don't forget to put the blanket on at night and remove it in the morning. That's it.

There may be times when air from the collector is too hot. It could be run through a simple jet pump and mixed with cool outside air as it enters the unit. With proper sizing of thermal storage, solar powered food dehydration can be a 24/7 enterprise. It will be faster with far less spoilage.

Tonight, I'll link to a dehydrator made from pallets and cob for built in thermal mass.
 
William Bronson
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Hey Dale, how about a junked chest freezer, lined with cement or cob with a salvaged sliding glass door on top?
 
Dale Hodgins
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Yes, I considered that. This can certainly work for the dehydrator, I'd prefer an upright freezer. The rock storage could get hot enough to melt the plastic bits and then plastic vapours would flow over the food. Also, the depth of a chest freezer might be fine for a rock storage with its own solar panel, but is far too deep when used as a collector. Even one a foot deep would have temperature stratification. At three feet, the lower portion would be much cooler. I suppose that a really old one with metal sheeting inside would work. Most would need an insulation upgrade. The ratio of collection surface to heat losing sides is less than ideal. With the sand box shape, there's more collection area. A freezer has sides that are about 3 times the area of the top.

Old freezers are perfect rodent protection for dry feed storage. They could probably be useful in a root cellar too.
 
Abe Connally
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Location: Chihuahua Desert
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Here's how we made ours: http://www.velacreations.com/food/preservation/item/36.html



We never have an issue with mold. Things typically dry in 2 days in that dryer.

I think to add a thermal heat storage, you'll need to drastically increase the area of the solar collection to make up for the losses in storing the heat.

Solar dehydration is not really about heat, it's about increased air velocity. Heat gives you a chimney effect that increases the air velocity (and raises it's temperature a bit, too) to move more moisture out of the food. The problem with using night air is that the night air is cold and because of this, already has a lot of moisture in it. Heating it up will allow it hold a bit more moisture, but you'll likely see it cooling your thermal storage (at least around the air inlets) and depositing moisture (condensation) at this point. So, over time, your thermal storage will start holding moisture, too, which is NOT what you want.

You don't want a big flat plate for a solar dryer, but more of an inclined plate to increase air velocity.

An alternative method is to add a taller chimney to your solar dehydrator design to increase the airflow during the day and dry things faster.

 
Dale Hodgins
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Night air may have reached the dew point and be wet, but since dew is deposited on surfaces, there is actually less total moisture in a given volume of air. Moisture can't build up in the rock bed. Cold night air is heated to well beyond the dew point. I'm glad you brought this up, as it has led me to a slight design improvement. A dew fence or more precisely a dew bag could be placed over the inlet so that any mist or dew present in the night air can condense and run to the ground or to storage. The drier air will now take less heat from the storage and it will be more thirsty. Outside air holds the least moisture just before dawn. It has had all night to cool and deposit dew. When there is a little wind and grass or other surfaces for it to gather on, more is deposited. The artificial wind and the screen use this principal.

This heat from storage can also be used to increase air flow in the evening before the air has cooled much and become moist. If fast moving air from the hot storage were run through a simple jet pump, it could be used to push massive amounts of air through the drier. This would be regular outside air that has not drawn heat from storage. A rough test of a breath operated jet leads me to believe that it would work at about 3 to 1. That is, for every gallon of heated air which rises from storage, another three gallons of air can be pushed along. It all depends on what velocity can be achieved in the stream coming out of storage. Within a couple hours of sundown, it would not be advantageous to use this air since it is headed for the dew point. We should then only draw air from the heat storage. If it ever gets cold enough to reach the dew point, it will not flow. There is no way that this could blow very moist air over the items being dried. It's not a fan. It will only flow when the air inside is hot and dry. Without purposely introducing liquid, there is no way that the effluent air could be hot and wet. If it gets cold, it stops flowing.

Abe --- I mentioned your Rapidobe system here. It's about filling used pallets with dirt or rock to form mass walls. http://www.permies.com/t/33345/frugality/Endless-supply-free-pallets#261605
 
Abe Connally
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Location: Chihuahua Desert
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I wouldn't worry about the air being below the dew point, but the surfaces of the rocks/thermal mass at the air inlet, which will be the coldest part. If water is deposited there, then you are adding water to your thermal mass, which is not a good thing at all (you want this mass super dry).

Instead of a dew fence, you might consider running the incoming air through thermal mass (gravel or rocks) that is OUTSIDE of the thermal storage, and separated by a vapor barrier. This will have a lot more capacity for dew, and could greatly improve things.

I think the concept is sound, but it may need some tweaking, and it would be interesting to see what temperature difference you can achieve in the thermal storage (higher difference means higher air velocity potential)
 
Dale Hodgins
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Yes, since thinking of adding a jet pump, I'm more concerned about velocity to move non storage air. I don't currently have any product to dry. I do need to bring down some trees. I have a hot slope where I could build a 24 hr solar kiln. It could be a version of the cob pallet drier that I suggest in Courtland's pallet thread.

This is the problem with these thought experiments. Now I have to get out the chainsaw and the shovel.
 
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