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Solar Dehydrator in High Humidity

 
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Quick question.

Does anyone know if the solar food dehydrator concept is a valid idea for high humidity areas?  If it isn't directly valid could a few modifications be made to make it more valid?

My thoughts / concerns:
1.  Dehydration is done by differential in water content between the air and the product being dehydrated.  Basically you can't dry something more than the air entering the dehydrator.  In an area like Houston (where i currently live) would that be dry enough?  Average humidity is 75% morning humidity is 90% (according to google).  According to me it's always 150%, think Atlantis...but with 1% more air .  
2.  If it isn't dry enough could a duff or dehydration chamber be installed before the heating chamber that would absorb moisture and then allow the air to move on drier to re-absorb moisture from the food before venting.  The chamber would require baffles to ensure all the material is being contacted...and it would have to be large enough to allow a certain contact time.  If you get real creative...which might be my downfall...you could try to source some sort of dehydrating material that could be put back into the house with AC (humidity and AC seem to go hand in hand), let it dry out and then use it again...or put it in the oven for a short period of time.
3.  I know we get tons of solar gains and air movement.  If you watch the vents on most attics in the summer they are spinning fast to ditch the extra heat inside the attics...which get up to 120 deg F.  So i'm not really concerned with that part...However if a pre-drying chamber is required how would one balance residence time vs. pressure drop...don't want to choke off the air flow.  Maybe something like 200% the screened area or something like that?

Obviously this isn't my solar dehydrator design...this is from Permies and I claim no credit for it.  All I added was a duff filled rectangle with baffles on the air intake...if this isn't allowed please let me know and I'll make a new drawing (hoping to not have to do that as I'm not a great computer drawing guy...as I'm sure you can tell already).

Anyways hoping to get some good insight on solar dehydration in Atlantis

Thanks,
Colter

Modified-Solar-Dehydrator.png
[Thumbnail for Modified-Solar-Dehydrator.png]
 
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The 75% and 90% RH air probably has the same moisture content, it's just that the temp has risen.  Relative humidity is relative to the temperature.  Warmer air can hold more moisture.  The solar dehydrator raises the temp, thus lowering the relative humidity of the air inside.  I don't have the tables handy, but if your humidity was 75% at 95F, if the air in the dehydrator is 120F, your RH is about 31%.  

I've done a lot of attic work up here in Canada and I've seen attics at 170F on a few occasions.  If you're only seeing 120F in the attic, that's because you've got great ventilation.  The dehydrator isn't vented, so I wouldn't be surprised if it hits 150F, where your RH would be just over 11%.  

Regardless, even at 120F, with an RH of 31%, you'll have no trouble drying stuff.
 
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I live in Florida, and I gave up on this option as mold grows too too well here.


That said, I can run my Excalibur dehydrator via solar panels with a large 5 KWH battery.

Other options is to use a heat lamp with thermostats.

I gave up on a solar dehydrator because there are just to many rainy days here, and I want my food mold free.

 
Mart Hale
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Oh another idea I had but have not tested is to use a standard dehumidifier combined with a small space to be used as a dehydrator,   but the more I look into it the new freeze driers have now dropped down in price to about $2,000.    They do a much better job and may be a better option if you are doing bulk preservation.
 
Timothy Markus
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Did you use one of the solar dehydrators that Permies is selling plans for, or something else?

I'm glad to hear freeze dryers are coming down in price.  I've thought about that route a lot for sales.
 
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I think the increase in temperature leading to a decrease in relative humidity is the key.

I've had good luck with a totally different style of dehydrator that is designed for the humid midwest by Larisa and Bob Walk.  I think it would work well for your conditions.  It has much more glass per square foot of food.  Here's a thread to my build: Walk Radiant Solar Dehydrator. With your higher sun angles it should work very nicely.
 
Colter Schroeder
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Thanks for the replies everyone...somehow it didn't cross my mind that the relative humidity was changing with the increase in temperature...what a solid engineer I am .  Anyways I do like the idea of the walk dehydrator as it seems like it would have a smaller foot print and I could minimize it and keep it mobile even easier...I was debating how much smaller I could make the large one before it became ineffective.  Quick question.  Does the fact that the food is exposed to the sun rays have any effect on the quality of the food?  I had a quick look through the link you sent (will read the full thing later) and noticed that the food is exposed to the sunlight and they mentioned that some foods actually benefit from this UV exposure...is this true of most foods or are mushrooms the exception to the rule?

Thanks again for the ideas...when we decide what to create I'll create a link with lots of photos.
 
Timothy Markus
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Around here they give you a ring to remind you that, even though you're an engineer, you're likely to make a lot of boneheaded mistakes.  

I don't know about UV degradation, but I think that dehydrating leads to the least amount of nutritional loss in processing for storage, though maybe freezing is better if you're willing to pay for electricity.
 
Mike Haasl
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Yeah Colter, give it a read through.  It isn't immediately obvious but the food isn't in direct sunlight.  There's a layer of metal between the food and the glass.  So coming down from above, you'd encounter glass, air gap, tin, food, food tray, air gap, reflective metal roofing, then framework.  The distance from bottom to the hinge is important but you can make the units as narrow or wide as you want.  Their website has a lot of design info on it.
 
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Just want to add that the interior of a vehicle may work for dehydrating. Definitely worth a try to see if the heat beats out the humidity in your area.
 
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Our dryer design has worked in the humid midwest for over 35 years. You can read all about it at http://geopathfinder.com/Solar-Food-Drying.html
 
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Larisa, what amount of humidity is normal in your area? Such as what percent of humidity you have.  I'm in the humid midsouth, and am wondering if your area is similar to mine.

I am very familiar with your design. I've been admiring it over the last few years.
 
Mike Haasl
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I think I know about where Larisa lives and the dew point in that area is supposed to be between 65 and 72 degrees for the next 5 days.  Lows in the low 70s and high in the low 90s.  Humidity of 70-90% in the morning dropping to 50% in the afternoons.
 
Larisa Walk
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Mike hit it right about our upcoming weather forecast for the next few days. We've got melons and tomatoes in the dryer now with more to go in, but we've got a mostly cloudy day ahead so will wait to add in more until tomorrow. If we get a bit of solar gain today the current batches will be done by days end.
 
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I found I need additional heat source at night and cloudy days. Or bring food in. Not much but a small gas flame to keep the temp up a bit.
My plan is a solar oven type dehydrator with reflectors and thermocouple -fan, to blow out air when it becomes about 150*f. This should keep the food temp about 130f I figure.
 
Larisa Walk
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The key to using a passive solar food dryer without "crutches" is to achieve sufficient drying in the first day to allow the food to "coast" through the night until the sun resumes the process the next day. This works in humid Minnesota summers where the night time temps can be either quite warm (70-80*F) or cool (50-60*F) to cold (near freezing) with dew points that make most mornings quite wet. I think you should try to maximize the initial day of drying, then in subsiquent days perhaps you would need some shading on the glazing in your more equatorial location as you will need less heating as the moisture content of the food is removed. For foods that need more than a day of drying, the first day's forecast needs to be optimal and the second day (and beyond) is less crucial.
 
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