Home made Solar Dryer / Dehydrator not workie so good... Help!

Hey all,

I have recently built a solar dryer unit in an attempt to overcome the use of the electric Dryer. I mainly tried to make it very similar to the last one featured in Pauls 3 solar dehydrators.

I made a short youtube that gives an overview of what it looks like.
The solar panel is from an old solar hot water service. The issue is that the air is just not drawing through the chamber. The hot humid air stays up the top and doesn't draw through....

The chimmney is made of stainless steel painted black, but only gets hot where the the sun shines on it. The Solar panel seems to be doing a great job of putting hot air into the top of the chamber, not sure about the speed of the air movement. I Hid in it with a full set of wet nappies on a sunny day and couldn't feel the air drawing out into the chimmny like it did in the last of the 3 solar dehydrators videos. At this stage I am open to all suggestions. My idea is to put fans hooked up to a solar panel on the threshold of the chamber / chimmney that would suck the hot air down and out, hopefully creating the draw on the solar panel and getting some consistent air movement. Other ideas was to increase chimmney height and increase contact area through the chimmney.

Let me know what you guys think.

Cheers

I would love to see this so I could help but there is something wrong with either your camera or the video. Its coming through as a narrow vertical band of video about 1-1/2" wide instead of a full picture. Very hard to see what is going on in there through such a narrow peephole. Can you re-shoot?

Try this one....

Notice the particular shake in the camera as I "miss" one of the stairs.

Neato! Have you thought about putting a photovaltic powered fan inside to move air? Also Painting the box itself [or even the roof ] black will raise the internal temperature significantly so you can still dry on not so sunny days.

Depending on temperatures, when there aren't any clothes inside you could have racks of meat dehydrating

Hey Nathan,

I have just bought a solar panel from a friend, some fans to do what you have suggested, just need to did a second hand car battery to power it now... I sure the carnivorous dogs that we have would enjoy the meat jerky, but their herbivore owners are very unlikely to be dehydrating the stuff ...

Any Ideas on how to increae draw of the chimney?

Scott Patrik wrote:
Any Ideas on how to increae draw of the chimney?

Increase the temperature of the chamber and airflow out the top. Your chimney is at the bottom and it is black, so when the sun hits it, the chimney heats up and it will suck air from your cabinet out. Problem is, your cabinet is also hot, and hot air rises, so the sucking of the chimney is counteracted by the sucking of the cabinet.

Port the top of your chimney back into your cabinet so it creates a loop. As the sun hits it, it heats the cooler air from the bottom, and spits it out up top nice and hot. [you can dehydrate vegetables and mushrooms, and store them long term, dehydrate onions and garlic and make seasonings. If you pump some serious heat in you can cook bread! Put a chair in there and some wetstones and you could use it like a sauna]

If you want portable heat [use it in your house when it is cold, you can build a solar convection heater like the one I describe in my blog post:
http://projectupcycle.blogspot.com/p/solar-convection-heating.html

The problem with the box heater is that it is probably made of wood and painted black. It does get hot and move air, but if you had a conductor [like aluminum cans with the tops and bottoms removed, painted black, and stacked on top of each other.] you could heat 70*f ambient air to 110*f at the head moving fast enough to flutter paper.

also why do you need a battery? as long as the sun is out your fan should be running, when the sun goes away, you don't need to be moving any more air. A battery in this system is irrelephant.

So aluminum cans increases contact area and therefore heats up more air, therefore faster movement... Is that the theory? The Black panel is made of steel, secured to the silver aluminum backing.

Would creating a loop not just make the overall air very humid and unable to release the moisture in the air. The dryers main purpose is to dry super absorbent cloth nappies that take up to 2 hours to dry in the electric dryer. My concern would be that creating a loop would create flow but not dry the clothes. I have definately wanted to utilize it as a food dehydrator, just not sure of it's food gradeness with silocone, liquid nails and "non-toxic" expanding foam stuff.

Iniatially I thought the battery was unneccersary too. So all I need to do is to connect the solar photovoltaic panel straight to the fans? or do I need to go via a regulator / controller? I have a regulator / controller that I can use... would you recommend putting the fans at the top of the chamber pointing down, or at the bottom of the chamber at the threshold, which would theorectically evacuate the air and hopefully draw hot moist air down and out, creating a draw for the hot air coming out of the solar panel. how likely is this to happen?

Cheers for all the info...

So aluminum cans increases contact area and therefore heats up more air, therefore faster movement... Is that the theory? The Black panel is made of steel, secured to the silver aluminum backing.

Yes, that and the air is being heated in smaller individual chambers with lots of turbulence over heated surfaces. You can create very hot air like this

Would creating a loop not just make the overall air very humid and unable to release the moisture in the air.

This shouldnt be a sealed structure, it should leak air in the bottom and out the top

I have definately wanted to utilize it as a food dehydrator, just not sure of it's food gradeness with silocone, liquid nails and "non-toxic" expanding foam stuff.

Let it heat up and run a bunch of clothes through it first, it will give these things a chance to off gas and get most of the nasties out, an easy way to tell if it has off gassed enough is to get some fish [10-15 rosy red minnows works best imho] feed them the first half of a can of flakes to keep them alive, with the second half of the flakes, spread them out on a cookie sheet and put them in the dehydrator chamber for a week. feed the fish the test flakes, if they all start dying, you probably shouldn't make human food out of it. If the fish live you are probably good to go.

So all I need to do is to connect the solar photovoltaic panel straight to the fans? or do I need to go via a regulator / controller?

probably not, your fan can probably handle however many watts your panel is rated for [unless you have some gargantuan panel] You can just wire positive to positive, ground to ground.


would you recommend putting the fans at the top of the chamber pointing down, or at the bottom of the chamber at the threshold

Top of the chamber pointing down and opposite your hot air out from your convection heaters, below the air leaks in the top. Air can still get out the top, and the super hot air from the convection heaters is circulated down and around your clothes/food, then the hot, now moist air can flow up and out of the top of the chamber as new dry air is pumped in from your convection heaters [the air holes in the bottom should be somewhere near your convection heater air in]



Cheers!

this is the one we use: http://www.velacreations.com/food/preservation/item/36-solar-food-dryer.html

It works fine without a chimney. There's lots of black surface area inside, so it heats up enough to create a draft.

Scott,

I'm just curious if you have worked the bugs out.

My first thoughts from your video was that your chimney just wasn't tall enough. Adding maybe 2ft pipe will create more draft.


I'm about to embark on my own Solar Dehydrator project.
I will start a new post soon.

Eric

the problem is the design delivers hot air to the top of the unit. The designers state that they believe moist air is heavier and sinks. The unfortunate truth is that wet air is NOT heavier so you have to force it to go down from the top through the food. Seal the dehydrator box well with caulking and such forcing the air from the solar warmers to travel down past the food and out the exhaust (hole) down at the bottom of the box. Make sure the solar panels are well faced to the sun so that it takes in lots of air.

I really could not make out the design totally, I assume the exhaust comes from the bottom of the box, this is as it should be. I also could not see to make sure but the air needs to come in the bottom of the solar panels.

While it is true that moist air is less dense than dry air at the same temperature, the phase change from liquid to gas removes some energy from the air. This loss of energy reduces the air temperature, thus increasing its density. The cooler/humid air will then sink. My downdraft dehydrator heats up to about 130 deg. F in the winter, and can dry cherry tomatoes cut in half in 2 days. I have yet to try it out in the summer months. I do have it sealed very tight with latches that pull the door shut. I get less air movement when it is empty or when the food has dried out.

a very decent point, yes evaporation cools air.

I really would like to line the two different types of dehydrators up to test effectiveness. There is certainly many factors in how well they would work. With many air leaks the downdraft would not work right but that is totally fixable.

I decided to calculate if a change in temperature would be enough to counter the change in humidity in relation to air density. Playing around with the numbers it turns out that it is mostly a wash. less than a 1% difference in density. It most likely makes little difference if you use a up or down draft design. I would need to measure exact temperatures and humidity levels to determine if my estimates were close.

I used:
20% humidity increased to 30%
115 deg F cooled to 100 deg F

Same pressure both cases

the air is really suppose to move faster than it would take to up the humidity by a 10 percent nor should the temperature drop much. I am fairly certain that in the design i saw the solar collector is large enough that the air forced up into the top to push air out the bottom. I agree such a design works, i can see it working. I was just shocked by the repeated assertion that wet air is heavier. Also the thought that the air should be in the drying chamber long enough to cool.

I think what we would find when drying in the downdraft type of box is there is more cooking and carmelization of the dried goods. This is not something many people would object to. This is all speculation on my part so please do not take it too seriously, i have been thinking of putting together a box when will take the solar input air either at the top or the bottom so i can examine the products from both. I have to store my dehydrator inside for the winter so the removable solar panel would make this process much easier .

Perhaps in the summer and fall I can display products from both if there is a difference.

What if you added a hollow section to the roof that also absorbed sunlight and piped it into an extended chimney up above the roof to create a syphon that could help to pull the air from the back chimney? Cheaper and less maintenance than solar panels.

Dale: I think you mean venturi or something, not siphon. You'd need a lot of careful calibration and airtight seals to have your roof updraft "pull" the chimney exhaust- but with a large enough chimney outlet it might help. It might also be possible to design a roof heater that boosts the temperature of the chimney, while keeping them separate, to accelerate that final updraft. I would want to see a working example rather than a concept diagram before taking someone's word on this - it requires balancing several different physical principles, and sometimes the real world shakes things out differently than intended.

Laura and Eric Reed: I think you have something there, pointing out that moist air is not necessarily denser (or lighter) than dry air - relative temperature and humidity both affect density. For passive draft using ambient incoming air humidity, air temps, and available solar energy, there's a lot of variation from day to day or even hour to hour. Designs that rely on balancing these factors may work distinctly better in some climates, or weather conditions, than others.

But I agree with Eric Ellison that a taller chimney may help. Multiple, taller, hotter chimneys may help. Solar chimneys work best in warm, dry climates, where they consistently reach temperatures well above the dew point of the moisture in the air - because a cool chimney can condense moisture, and air that hosts a fog of suspended water droplets can be distinctly denser than dry air at the same temperatures.

I also notice that the chimney comes out of the box on the same side as the solar panel goes in. (Is the solar panel actually built like a tube, with plenty of air flow? can't tell from the pictures. If not, there's your problem.)

With vertical hangers instead of racks, there may be more tendency for the air to divide and go downward more in one specific area (like the wall closest to the inlet and outlet), and skip the other areas. Changing the racks to run perpendicular to that wall might help; it would at least make each path roughly equally attractive in terms of pressure, instead of an obvious "fast track" and "slow lane."
The original design mentioned using a "false wall" to force the air to take the complete path. Perhaps in this case, a false bottom above the chimney, an inch or two below the bottom of the diapers, could force the air to take a longer path between intake and exit. I suppose you could do horizontal racks and lay each diaper flat, if the diapers are not bigger than the box, and shift the racks around so the air has to take a zig-zag path that passes by all of them.

In all cases, the passive downdraft will only work if the construction is airtight. Otherwise, everything will draw upwards: the chimney and solar box will flow upwards, and the main chamber will tend to stagnate with hot air escaping through any leaks in the top, and the lower part of the main chamber any leaks will serve as air intakes for both the main chamber and chimney.

This dehydrator design (downdraft) was presented on the theory that something about the process of dehydrating foods makes the air want to move downwards (be it moisture or the cooling that results from evaporating it). The other benefit is that the solar catcher (long, black-painted tray in most cases) is at a nice, steep angle. Angles over 45 degrees are a bit more effective at starting a draft, and this also positions the collector up out of the way - not leaning down where it might get buried in dew-damp grass or stepped on or run over with equipment by accident. So if you like the overall design, then it's worth sealing it and making the chimney taller to see if you can make it work better.

Another option might be to change the problem a bit.
Have you tried the old-fashioned large diapers that are thin cloth, folded several times?
These are easier to wash and dry than a single, quilted, super-absorbent modern diaper. The modern high-tech diaper designs pretty much assume you are using a high-heat tumble drier. If there are super-absorbent crystals involved, the diaper may not even fully dry unless it is sort of "baked" to a specific temperature range. A dehydrator might be about as effective as a clothes-line in the sun for pre-drying, with a finish cycle in the tumble-drier. Older diapers are kind of like a thicker version of cheesecloth: about a yard square, and there are a couple of different ways to fold them depending on the size of the baby. You end up with a soft, absorbant diaper made of multiple layers of fabric, which can be unfolded again for washing and drying.

Yours,
Erica W