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Solar Food Dehydrator Design?  RSS feed

 
Tyler Cullender
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Hey folks,

I'm trying to wrap my brain around solar dehydrator designs. I've done a bit of research and I like the Appalachian State design that provides a long tunnel that pulls air from the bottom and lifts it up through the food trays. However, I would really like to build something that is capable of drying sauce type items. I want my solar dehydrator to be just as versatile as an electric one, and that means the ability to make items like tomato paste, fruit leather, and other items that begin in a fairly liquidy state. My concern is that most of the solar dehydrator plans I've seen focus on pulling air from below that rises over the racks of food. Is this adequate for liquid food items that require a solid tray? I've heard of designs that pull air in from the top and bring it down over the food -- I haven't been able to find a design like this, but it seems to me like this would be more appropriate for drying liquid items. Any insight or links you have would be much appreciated!

On a related note, I would like to try making black garlic, but I don't like the idea of leaving an oven or rice cooker on for the ~40 days required for the garlic to ferment. Has anyone use solar dehydration to make black garlic? My concern is that the temperature would dip too much at night, so I'm also interested in insulating the dehydrator as much as is reasonable.
 
Blaine Lindsey
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how about a thin volcanic rock slab if you have a local rockshop in town. or some other type of thin yet heat absorbing platform so the heat coming from below wouldnt be that bad because it would help to heat up the volcanic rock enough to capture just enough heat to help process the liquid (i have one made of mostly silica, they used to use them in ancient days to solar cook bread) + you can make it so the hot air stays in the dehydrating chamber longer by controlling how/where it flows out! create some type of oven environment where the hot air that enters cant escape easily or until you release it? Im trying to make one too!
 
Judith Browning
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Hi, Tyler, I am searching for efficient solar food dehydrator designs also.
Could you explain black garlic? Maybe post how to do it here? I would love to know more fermented food possibilities. Thanks.

Blaine, I wonder about soap stone since it is used to cook on? I like your idea of something to hold the heat.
 
Tyler Cullender
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I agree that a heat-retaining material like rock or concrete would be great. The design issue that I'm having a bigger problem conceiving is making a top-flow chamber, such that warm air is pulled in from the top and flows over the food. I've heard this design mentioned, but have never seen one or plans for one. If people have experience drying sauces or pastes using solar dehydrators, I would love to hear input, and weather a top-down airflow is necessary.

Black garlic involves fermenting the whole bulbs at high temperatures (~140 F) for a long period of time (30-40 days). The technique was first practiced in Korea, I think. I've found instructions for making it online using a rice cooker or an oven, or a homemade heating box like this one: http://adamkapela.com/2012/02/28/dyi-black-garlic/

 
laura sharpe
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there is good reason you never hear of the top down heat design, it could not possibly be a passive system as heat naturally rises.

I have had good luck using normal dehydration methods for tomato dehydration all the way down to a flake. I would line my screens in the solar dehydrator with teflon sheets and not pack the dehydrator too full.

I never made fruit leather as I do not like them .
 
S Bengi
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I have seen a top down solar dehydrator.
It uses the same black slide to heat the air then it enters at the top.
The air then falls as it cools evaporating water once it get to the bottom another solar chimney from the bottom to the top is employed thus heating the water heavy air again sending it upward.
http://www.youtube.com/watch?v=oVTcnCuX2Qc&feature=youtu.be&t=2m22s
 
laura sharpe
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S Bengi wrote:I have seen a top down solar dehydrator.
It uses the same black slide to heat the air then it enters at the top.
The air then falls as it cools evaporating water once it get to the bottom another solar chimney from the bottom to the top is employed thus heating the water heavy air again sending it upward.
http://www.youtube.com/watch?v=oVTcnCuX2Qc&feature=youtu.be&t=2m22s


the intake of the air in that unit is pushing air out the bottom. In fact, air full of water is NOT heavier than air without moisture, in fact it is generally lighter. This particular design would make the dehydrated stuff warmer, not always a desired effect.

But I maintain that it is a mistake to think that top down drying is better for moist items, the draft is needed to move the moist air way from the foods and to move more dry air in. None the less, it seems both units work most of the time. http://www.permies.com/t/16003/passive-solar/Home-Solar-Dryer-Dehydrator-workie

 
Gary Kimmons
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I'm curious if any of you folks out there know where there is a source for inexpensive (?!) stainless steel screen for a passive dehydrator application? Also are there safe alternatives to stainless steel?
 
Abe Connally
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This is our design. It works great!

http://www.velacreations.com/food/preservation/item/36.html

 
Peter Ellis
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laura sharpe wrote:there is good reason you never hear of the top down heat design, it could not possibly be a passive system as heat naturally rises.

I have had good luck using normal dehydration methods for tomato dehydration all the way down to a flake. I would line my screens in the solar dehydrator with teflon sheets and not pack the dehydrator too full.

I never made fruit leather as I do not like them .


Absolutes are difficult. One does hear about top down designs in solar dehydrators, and they can be passive systems, taking advantage of the fact that heat does naturally rise. You have to consider the system as a whole, and recognize how the draft will work.

The heating portion utilizes the natural tendency of hot air to rise, drawing in unheated air at the bottom and dumping hot air into the dehydration chamber at the apex of both the heating chamber and the dehydration chamber. The dehydration chamber is pretty much airtight except for the opening at the top for hot air to enter and at the bottom for it to leave.

But hot air rises, so it won't go down? Yes, hot air rises. But it can't go back down through the heating chamber, so it has to go someplace else. That exit vent at the bottom of the dehydration chamber goes into a tall, flat, chimney. The chimney can even be painted black on the exterior face so that it gets some solar gain, but it's not entirely necessary. The hot air, which cannot go back down through the heating chamber due to the hot air rising up through that space, does not have a choice but to go through the system and out at the highest point, where the chimney opens to atmosphere.

It works, it's a passive system, it gives top down flow. Whether it works any better or worse than a system with a bottom to top air flow I cannot say.
 
Brian Knight
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In case you missed Appalachian State's research PDF:

http://appropriatetec.appstate.edu/sites/appropriatetec.appstate.edu/files/HPImprovingSolarFoodDryers.pdf
 
thomas rubino
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Hi All; I desined and built my solar food dryer several years ago and it has worked flawlessly since. Imagine a south facing sloped metal roof low to the ground , 8 feet wide and only 2 feet long . now set 4, 2x2 trays built with food safe screening on the roof, a small ledge on the bottom side keeps them in place. Build an 8x2' frame , cover one side with any avalable sheet metal, painted black on both sides . Now cover the other side with a clear glass, or a greenhouse panel. Set frame with metal on bottom over top of food trays and atach with long tee hinges to bottom roof frame. The top panel now higes off the bottom and the food trays are free to lift in and out. This works by the sun heating the black metal thru the glass wich then radiates its heat thru the drying trays and the sloping metal roofing underneith pulls the heat thru and sucks it up and away. This works awsome! herbs are dyed in about 4 hours , tomatos take a few days, everthing is in the dark so you get brite green herbs. No electricity required ! However the sun is required as to many rainy days and things will mold. One important thing when drying food is only use stainless steel or food safe plastic screening for your drying trays, DO NOT use window screening. I used plastic on mine and I found that I needed to put three runs of stainless tie wire across the bottom of the drying tray to keep the plastic from saggin down with heavy loads. As you can see from the pictures I used all scrap lumber and only bought the greenhouse plastic panel for apx. 20.00 and the food safe screening for about 25.00. My neibors used my desine and having more $ than me used all top grade lumber and stainless steel screen to make theirs (very nice looking ) and they are using it to make jerky ! Mine may look a little funky but it works just as well as a fancy one.
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Hugo Rozzo
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Hello everybody,i'd like to put in my 2 cents. I saw the video of Paul Wheaton above and i combined a bit what i saw of the different methods in the video. I went for a top-down model, spray-painted the PVC chimney black. The cupboard itself with all the lovely fruities in them white, cause the hot air is to cool in there i figured, to drop toward the entry of the chimney. And i insulated the back of the inlet for maximum heating with a left over multilayer space insulation. It has gotten bigger uglier and heavier then i had anticipated, all neighbors are curious.
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Dan Grubbs
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What if one would run a series of black pipe in a zig-zag pattern in the solar collector of the dehydrators and used it to heat up some water for warm water needs. Would that take away too much heat from the air inside the collector? Was thinking it might be a cool idea to make these dehydrators dual purpose "appliances" for our needs. Thoughts?
 
tom Brue
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Is anyone using solar dehydrators in the gulf coast or other very humid areas? I saw mention on one of the radiant heat designs (black metal over single layer trays) about the high humidities in the Midwest. Down here is Houston, the humidity is almost always around 80% or more. If you walk outside and the humidity is oddly low, it's a miracle blessing! Those are "sick" days from work!

Anyway, would the solar driers work if the incoming works ready 80-90% saturated? Would they just be less efficient? Or not dry fast enough? I'm wondering if it's even worth building one here.
 
Angelika Maier
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A very important feature of such a dryer is that it is foldable. You will need the dryer in autumn and maybe in summer, but the rest of the year it is simply a nuisance unless you have a huge barn garage or shed and that tends to be always full. And I would like the food dryer to remain in neat condition, maybe packed away in a wooden box, it is the food I dry with it and I don't want to eat last years dust or clean an hour before use. Just my 5 cents and why I haven't got one.
 
Michael Mendez
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Hi my name is Michael Mendez, I am a senior at Vandegrift High School, and I am a Life Scout in Boy Scouts working on my eagle project. I am trying to build a passive solar veggie dehydrator (in the style of the large outhouse looking ones) for Community, First!, a program with Mobile Loaves and Fishes to get homeless people into homes in a self-sustained village, in order for this community to preserve their food. I was wondering if anyone had any plans/ drawings that I could go off of in order to build this. My email is m.d.mendez1996@gmail.com , thanks!
 
Violet Heart
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I live in a hot dry climate. and the best solar dehydrator so far is a greenhouse. as we dont use the greenhouse in the summer, it is the perfect place to dry fruit and veggies! it keeps the cooler air off at night.
 
Erica Wisner
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tom Brue wrote:Is anyone using solar dehydrators in the gulf coast or other very humid areas? I saw mention on one of the radiant heat designs (black metal over single layer trays) about the high humidities in the Midwest. Down here is Houston, the humidity is almost always around 80% or more. If you walk outside and the humidity is oddly low, it's a miracle blessing! Those are "sick" days from work!

Anyway, would the solar driers work if the incoming works ready 80-90% saturated? Would they just be less efficient? Or not dry fast enough? I'm wondering if it's even worth building one here.


Please, Gulf States cooks and gardeners, or those from the humid Southeast, reply with practical tips!

Do you dehydrate? Does dehydrated food keep, or do you have to seal or freeze it afterwards anyway?

If you currently keep and use dehydrated foods such as raisins, sundried tomatoes, tomato paste, etc, and you grow these foods in your garden, then it might be worth doing a solar dehydrator to produce your own.

What follows here is speculation, as I've spent most of my life in Western states with dry summers.
We often get summer relative humidity below 10%, rising to above 25% only at night, which creates extreme fire danger. But food dries pretty darn quick no matter where you put it.
In winter, the same west-coast climates typically have 75% to 100% humidity. Keeping buildings, clothing, and dehydrated food from going moldy requires good design for ventilation, air-sealing during summer, and/or supplemental or passive-solar heat.

You can affect humidity by heating the air.
When air gets warmer, all other things being equal, the relative humidity should decrease. (the gulf heating up and releasing more moisture into the air in summer is the sort of thing that is not "equal" everywhere.)
The question is: how hot would you need to get your air in order to have a low enough humidity to dehydrate the food?
To deal with your humid climate, you will basically be cooking your foods hotter while drying them. Herbs can be more sensitive to heat than, say, tomatoes or meat jerky.


Here's a graph showing how much water is in the air at 100% and at 50% relative humidity.

Citation: "Relative Humidity" by ​English Wikipedia user GregBenson. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Relative_Humidity.png#/media/File:Relative_Humidity.png.

To take an example temperature that most of us have experienced, 80 F would be 27 C, or so.

30 C is about 86 F.
40 C is 104 F, fever temp, a warm-ish bath, or time to find some shade to avoid heatstroke. (Hot bath might be 110 F / 45 C).
50 C is 122 F, a safety threshold to avoid scalding for tap water; for dry surfaces it's hot but comfortable to touch; as a weather condition it could be lethal to be outside too long.
60 C would be 140 F, quite hot, and above about 65 C / 150 F it's too hot to touch.
... You are in some danger of burning or caramelizing things if they bake long enough at these temps.
100 C is 212 F, the boiling point of water. Wet foods like soup or paste won't burn at this temp as long as they remain wet, but the edges of uneven-thickness pieces could carmelize or degrade if they stay this hot after they are mostly dry.
Most ovens bake at 300 to 450 F, or 150 to 230 C. Expect browning and softening of cell structures at these temperatures; herbs will lose color and flavor, but other foods may concentrate and sweeten.

Hold a ruler or bookmark up to make a horizontal line along the graph.
If you were 100% humidity at 27 C (80 F), then you raised the temperature of that same air to 40 C (104 F), you'd be less than 50% humidity. If you can raise it further, to 50 C (122 F), you'd be at less than 25% humidity if I'm reading the graph right. Most things will dehydrate under those conditions, with enough air movement.

However, a dehydrator that gets to 120 F on a hazy day when it's 80 degrees out could become dangerously hot on a sunny day when it's 105 F to begin with. I might think about including a reflector (like those car window-shades) to block some sun from the dehydrator as needed.

Other tricks for high-humidity situations:
Physical or chemical moisture absorbers:
A friend with leaky car windows kept a bag of cat litter in her car to help de-fog the windows in our rainy season.
(Clean, unused, bentonite clay cat litter without perfumes and dyes. Pond, well, and oil-drilling companies also use a rougher grade of bentonite clay for their processes if cat litter squicks you out.)
Rock salt will also absorb some humidity, though it's more corrosive to store. There are chemical dehumidifiers that absorb even more.
When the clay or salt is too moist to be useful any more, you can dry it again in a kitchen oven or solar oven to drive off the moisture and re-use it. (Don't try this with chemical dehumidifiers unless they come with instructions to do so.)
A bag or tray of cat-litter in the air intake, to absorb moisture from incoming air before it is heated, could be enough to get good results from any common dehydrator design.

Air movement helps - if the same air remains over wet food and doesn't move, it will get saturated with moisture and no more water will evaporate. But if the air is already saturated, the same thing happens. Changing the temperature of the air helps adjust the saturation, but air movement will generally make a bigger difference than temperature once it's working at all. Most of the successful designs include a lot of air movement. At your working temperatures, expect the hot air to continue moving upward even while collecting moisture from the food. Consider a bigger solar chimney to accelerate air movement - this might help protect food from burning by letting you keep the temps a little lower while removing moisture faster.

Freeze-dried air - if you have a way to cool the air, so it can't hold as much moisture, then warm it back up, you can get the same de-humidifying effect at lower temperatures. Some foods will readily dehydrate in the freezer; if you can seal them well enough to avoid off flavors, you might get better results in the freezer than outdoors. An indoor dehydrator that takes advantage of air-conditioned air could also be an asset - maybe consider putting your dehydrator next to the air outlet vents from your house, if you use AC. (If you dehydrate indoors your AC would have to work harder, just like it does when you cook or make sauces indoors. But it could still be a tactic worth considering if the food is delicious enough to justify indoor cooking.)
Likewise, your dehydrator might work better in winter, when air temperatures and total moisture content of the air is lower.

Surface area: if you are able to get successful but slow dehydration, cutting the food smaller or drying thinner layers of pastes will increase the surface area and help speed moisture removal. Consistent size chunks will also help things dry at the same rate. It's easy to burn the edges, or the thin spots on a tomato slice, when drying with higher temperatures.


Is it worth it?
What would you be gaining from doing this?

I spent a couple months in San Antonio last year (August-October), and had a tiny cupboard and a single fridge drawer to cook from. We had to check the cupboard frequently to eliminate spoiled foods.
Storing dehydrated foods may be an even bigger concern than passive dehydration - if you just leave them out, they'll go moldy or slowly compost.
Glass jars might work if you get the moisture low enough, or you might add salt or sugar or smoke to deter molds and insect pests. I brought some dehydrated tomatoes from home in Zip-loc bags and they stored fine for a month or more.
If you are going to have to pressure-can or freeze the food to keep it, it might not be worth doing the separate step of dehydrating, except to make foods you really like or to reduce your storage volume. Canning in an outdoor kitchen might be more effective for bulk storage, and you might want to save up for emergency power to keep the fridge or freezer running.

However, if I was living from the garden and wanted to keep my fuel needs down, I might still find it worth using a solar dehydrator or solar oven to make tomato paste, fruit leather, or fruit 'candies' like raisins, dried apricots, and dried pears. Some foods are treats when dehydrated, even if you don't dry them all the way to the crispy potato-chip-like texture that's required for preservation.

You also have the luxury of growing more foods year-round than some of the northern climates, so your total volume of preserved food doesn't need to be as much. You might be able to do a smaller dehydrator and use it indoors.

If you do try it, please share the results!
Yours,
EKW
 
Erica Wisner
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Here's the relative humidity graph with 27 C marked out. The vertical line shows 27 C, the horizontal line shows what moisture content you'd have if you heated up that 27 C (80 F) air to a hotter temperature.

I'm attaching it, so it may not be as easy to see for all users. But it might help.
RelativeHumidity-27C.png
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Modified drawing- Relative Humidity graph with 27 C marked out.
 
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