Post #G21-049: Simplest solar food dehydrator, works with Nesco dehydrator trays.

Posted on September 2, 2021

 

This started out to be a little treatise on solar dehydrators, ovens, and kilns. And I may yet write that.

But my goal, right now, is to make a small solar food dehydrator that will work with four 13.5″ diameter trays from my Nesco electric food dryer.  That’s “a batch of tomatoes” for me.  The upside of using those trays is that I can always finish off the drying using the electric food dehydrator.  It is, in effect, hybrid solar-electric food drying.

There are lots of plans for sturdy, well-made, functional solar dehydrators available.  And all of them were way more than I needed, and far more than I cared to fabricate.  I mean, why spend hours making something that you can buy for a few bucks?

In the end, by far the simplest and most cost-effective approach was to buy an $18 clear plastic tote from Sterilite (r).  That, plus a few bits of junk lying around the house, an hour of time, and I had a functional, sanitary, minimal-effort solar food dehydrator.

If the thought of drying your food in a plastic box turns you off, I’ll note up front that Sterilite (r) says that all of their products are made with food-safe plastics.

I’ll document this while my first batch of tomatoes is drying.  I’ll update this evening with an analysis of the results from one day’s drying in this solar dehydrator. The upshot is that this setup will partially dry trays of tomato slices in one day.  But it won’t dry them completely in one day.  In the humid Virginia climate, I’m not willing to let half-dried tomatoes sit overnight, so I moved the trays to the electric dehydrator and finished the job.  This simple dryer reduces but does not eliminate the use of electricity for drying produce.

Upon reflection, given that I’m going to have to use electricity as part of this process, it’s obviously smarter to start the tomatoes in the electric dryer, then finish the half-dried tomatoes in the solar dryer.  I haven’t done that yet, but I lay out the logic behind that in the “power calculations” section at the end of this posting.


Background 1:  Solar food drying works even in humid climates.

My rolling solar food dehydrator worked pretty well  (Post #G21-048).  A day in the sun greatly reduced the time needed to finish the drying in an electric dehydrator.  On the face of it, that little bit of solar “pre-drying” saved about 4 kilowatt-hours, or roughly the electricity needed to dry a large load of laundry.

If nothing else, I now feel better prepared for the zombie apocalypse.   If society collapses, there will be no lack of dead cars out here in the ‘burbs.  My future-dystopia food preservation plan is all set.

The real benefit from this test is that it shows that solar drying is possible in Virginia.  Many reputable websites (e.g., state extension service website) state plainly that solar drying is not practical in humid climates.  I now realize that statement probably applies to unassisted, open-air solar drying.  If you put your food in the equivalent of a solar oven, it will dry, even with the high relative humidity we face here in Virginia.

Drying vegetables in my car works, but it’s less than ideal.  Not just because it looks odd.  It’s inconvenient to load and unload.  The car only gets up to 115 F, whereas the preferred drying temperature for vegetables is 140 F.  And I might actually want to drive that car at some point.


Background 2:   This ain’t rocket science, but a bit of calculation is in order.

In theory, any enclosed object with clear cover could be used for solar drying.  At least to some extent.  One can, for example, use a cardboard box and a piece of plastic wrap to create a pizza-box solar oven.

The interesting thing about a solar dryer is that it’s not a solar oven or solar kiln.  Unlike an oven or kiln, you don’t want it to get very hot.  For drying vegetables, you’d prefer to keep it under 140F.  That means the demands on the materials, insulation, and so on are all relatively minimal.  For example, double-paned window glazing is almost unheard-of in a solar dryer.  You just don’t need it.

Under those conditions, I ought to be able to made an adequate solar dryer from any simple enclosed box.  It needs to have at least one clear face, and some ventilation. But it doesn’t otherwise have to be exceptionally well-built or well-insulated.

The size of the clear face of this box determines the energy input, which in turn determines how much food can be dried in a reasonable amount of time.  The bigger the window, the larger the mass of vegetables you can dry at once, within a reasonable time.

My typical batch size is about 6 pounds of tomatoes.  When I work through all the math (typical summer insolation in this area, losses through the clear window, energy required to boil a pound of water), I estimate that I need 3.5 square feet of window area merely to evaporate off six pounds of water in one sunny day.  Given the inevitable conduction-radiation-convection energy losses that any hot object has, surely I’ll need more than that.

But this tells me that I need a box with a window that’s at least 3.5 square feet.  If I hope to get these tomatoes done in a day or two.

There are a lot of excellent, well-engineered solar food dryers out there.  But almost all of them were a) big, and b) required a lot of fabrication if home-built.  They were, in effect, complete overkill for a person who wants to dry a few batches of garden vegetables per year.

One of the simplest you’ll see is this product, the Dehytray.  It really is nothing more than a black plastic box with a clear lid and vent holes.  It was developed to help third-world farmers preserve their crops more easily.  I found the simplicity (load it and put it out in the sun) appealing.

Source:  Dehytray

Were it not for the fact that these are $140 each, I’d have gone with that.  I’m sure this thing is built to last a lifetime in third-world conditions.  But as with most of the designs I’ve seen, this is overkill for my needs.

But the general concept — you can dry food in a plastic box with a clear lid — that’s the direction I’m headed.  I really don’t need anything fancier than that.


Building a food dehydrator using a clear plastic tote.

I chose this particular tote, below, as the basis for my food dehydrator.  It needed to be large enough to accommodate four dryer trays, and it needed to have a top of at least 3.5 square feet in area.

I went to about an hour’s worth of effort to build this.  But I bet I could just chuck the trays in, leave the lid ajar, and it would still dry the food.

What I’m trying to say is that if you start with a food-safe clear plastic box, it should be no surprise that it’s not very hard to build a working solar food dehydrator.

Source:  Target.

First, Sterilite (r) says that all of their products are made with food-safe plastics. Even with that, I’m a little concerned that the hoped-for 140F operating temperature might cause me problems. But they make everything from polypropylene and polyethylene.  A working temperature of 140F should cause no problems with those materials.

Second, I know this isn’t UV stabilized, so it will eventually rot from sun exposure.  But, again, I’ll be using this for a few days per year.  I’m sure I’ll get years of use out of it at that rate.

Third, it’s large enough to accommodate four 13.5″ diameter Nesco dryer trays, stacked two deep.  (Interior Dimensions at bottom: 31 1/2″ x 16 5/8″ x 6 3/8″).  By using those trays, I always have the option of finishing the drying process in my electric food dryer.

Fourth, the top is approximately four square feet.  Per my earlier calculation, in theory, on a clear day, at my latitude, that will let in enough energy to evaporate the six pounds of water required to dry my tomatoes.

Fifth, ventilation can be accomplished by cutting through the relatively soft plastic.  If I’m careful, I won’t really even ruin the tote for other uses during the rest of the year.

Sixth, beyond a couple of holes for ventilation, it needs:

  • Something dark and insulated for the bottom
  • Something reflective and insulated for the sides.
  • Something to prop it up so that the top faces into the sun.
  • Screened vent holes and other items to encourage air flow but keep bugs out.

I bought the tote yesterday, and put the dehydrator together in about an hour today, from stuff I had lying around the house.  Here are some views of the final product.

The body is the $18 tote, purchase from my local Target.

The shiny stuff is Reflextix (r) insulation.  More-or-less, it’s a tough, aluminized bubble wrap. I cut two sheets to size and folded the corners so that they cover the bottom and sides of the box.  The only reason I used that material is that I had a piece of it sitting around.  Anything that would insulate the bottom and sides, and would not taint the food, would work.

The spacers between the trays are pieces of furring strips.  Again, anything that won’t taint the food would do.  I’d shy away from using PVC pipe, owing to the potential for high temperatures, but I’m not sure there’s any science behind that.

Not shown, I lined the bottom of the inside with a cut-up brown paper grocery bag.  I originally thought I’d need that dark surface to capture sunlight.  As you can see, the box is full enough that not much sunlight makes it to the bottom.

 

Ventilation is the hard part.  I could not even make an educated guess for the amount of ventilation this needs.  But that’s easy enough to change if I got it wrong.

The only thing you want to be sure to include is a good-sized chimney of some sort.  That’s the plastic pipe at the top of the picture.  The “draw” of the chimney is what directs the flow of of air into the holes at the bottom, over the produce, and out top of the chimney.

For ventilation, I cut three holes in the top, roughly 1.5″ diameter, using a drill with a hole saw.  (You could just as easily cut rougher holes with a utility knife.)

I attached pieces of cloth netting over the lower two holes (using tape).

In the top one, I placed my chimney.  In this case, it’s a lightweight piece of plastic pipe.  (It’s a piece of a fluorescent tube bulb guard for a traditional T-12 fluorescent bulb).  Anything that’ll carry the hot air up, and can stand up to some moisture, would have worked.  I would bet that a piece of carboard, taped up to form a tube of some sort, would work just fine.  There’s really nothing to attach it to, so the lighter the better.  In this case, I made a “pipe collar” out of a couple of pieces of cardboard, ran the pipe through that, and taped the cardboard to the top of the tote.

To summarize the construction:

  • Start with an appropriately-sized Sterilite (r) tote.  I used the one depicted above.
  • Find something to insulate the sides and bottom a bit.  I used a couple of layers of Reflextix (r).  You need something that can take the humidity and a little bit of heat.  It’s not completely clear that you need to do this, but you guarantee a higher ultimate working temperature if you do.
  • Get four thin sticks about as long as the box, to use as spacers for the trays.
  • Cut three holes in the top, about 1.5″ in diameter.
    • Cover two of those with something to keep the bugs out.  I used pieces of tulle (cloth netting), taped in place.  Window screen would be fine.  Any sort of fine mesh would work..
    • Fit some sort of chimney in the third hole, maybe 2′ to 2.5′ long.  I used fluorescent bulb tube guard, but anything lightweight that will take the heat and moisture will do.  (The ideal chimney length is whatever will fit diagonally in your tote, so that you can stow it when you’re done. )
  • To fabricate the chimney, I cut a piece out of the side of the tube guard, at the bottom, so that air could get in even if the pipe rested on the bottom of the box.  And I slit a couple of pieces of cardboard to make “pipe collars”, slipped those over the tube, and taped the cardboard to the top of the box.  So my chimney rests on the bottom of the box, runs through a hole in the lid, and is held in place by a couple of pieces of cardboard, slit so they’d slip over the pipe.

None of that is hard.  Any sort of reasonable substitution is acceptable.  The only rules are that the materials need to stand up to moisture, stand up to a bit of heat, and can’t be made of things that might taint the food.


Operation so far.

I started out at 40 minutes before solar noon, ambient temperature 75 F, clear sunny day, 39 degrees north latitude.

It took 18 minutes for the interior of the box to reach 100 F.  After an hour, it seems to have stabilized at 118 F.  (I expect that to go up as the food dries, as much of the energy entering the box now should be absorbed in the conversion of water to water vapor).

There’s a steady stream of warm, moist air coming out the chimney.

This appears under-ventilated, given the size of the load.  There’s some visible condensation on the underside of the lid.  I probably need to add another chimney.

That said, this clearly works.  The 118 F is a little lower than I’d like, but well within the range of temperatures acceptable for dehydrating vegetables.  Water is in fact clearly being removed from the tomatoes, at a temperature that will not cook them.

I’ll come back to this in late afternoon to summarize how my six pounds of tomatoes slices are doing.

I now wish that I had weighed the trays before starting.  The next section of this posting is based solely on how the tomatoes looked.

By the end of the solar day, the tomatoes were only partially dried.  The top racks looked about half-done, with the thinner slices just starting to turn leathery.

By eye, they looked about the same as the trays that I left on the dashboard of my car for a day.  That would make sense, as the car got up to 115 F and the clear-tote solar dryer got up to about 118 F.

The bottom trays showed minimal evidence of drying.  The obvious lesson here is that I can’t stack the trays two deep in this cheap little solar dryer.

Even if I had just a single layer, it looks like it would take at least two days to get the tomato slices dry enough for storage.

This means that all-solar drying with this cheap little dryer depends on my willingness to leave the partially-dried produce overnight.  I’ll have to think about that some.

With the right climate, people surely do that.  Direct sun-drying of tomatoes — just laying them out on screens to dry in the sun — is said to take four days to two weeks, per The Spruce Eats.  Any directly sun-dried tomatoes have, by defintion, sat for a few days in a partially-dried state.

But the experts say that only works in an arid climate.  Here’s what the University of Minnesota extension service says about sun-drying produce (emphasis mine):

  • Sun-drying is not recommended in Minnesota due to our high humidity and cool night temperatures.
  • Foods dried in the sun can take 3-4 days to dry; if the humidity is high, as is generally the case in Minnesota, the food will mold before it dries.
  • Sun-drying requires constant exposure to direct sunlight during the day and a relative humidity of less than 20%. These conditions are found only in areas like the Sacramento Valley of California or in Arizona.

Given the high relative humidity inside the tote, and the high relative humidity outside in Virginia, the only way I’d consider doing that is bringing it into the house overnight.  But even with air conditioning, relative humidity in my house never gets close to 20% in the summer.

In short, I’m not going to start with raw tomatoes, use this solar dryer for a day, and leave the resulting tomatoes overnight.  They remained too wet for me to be comfortable doing that, even if I brought them inside.

As it stands, I put the trays into the Nesco dryer overnight, and they were all done the next morning.  I err on the side of caution when it comes to food safety, so that’s probably how I’ll end up using this.  Part-solar, part-electric drying.

Other types of solar dryers can get the job done in a day.  That’s the claim for the best indirect solar dryer — where a large glass-covered box is used to produce hot air, which is then piped over the produce.  With that piece of equipment (and maybe a lower-humidity climate), you can get solar drying done in a day. That’s just more equipment than I want to use for my occasional batch of dried vegetables or fruits.

So, in Virginia, with this dryer, I can reduce but not eliminate the use of electricity for making dried tomatoes.  Given how easy it is to use — stack the Nesco trays in and put the lid on — I’ll certainly keep using it.

In hindsight, it might actually be more energy-efficient to pre-dry the tomatoes in the electric dryer, and finish them in the solar dryer.  Rather than the other way around, as I did here.  See below.


Afterword:  Some power calculations.

In this trial, I deliberately used the same quantity of tomatoes that I would normally run through my electric dehydrator.  That works out to be about six pounds.  That means that, in theory, I need the same total energy input with this solar dryer as I would normally use with the electric dryer.

The Nesco dehydrator that I use runs at about 400 watts.  And I have to believe it was designed to give the optimum level of ventilation as it runs.  Last time I ran a batch of tomato slices through that, in the humid outdoor air of Virginia, it used 8 KWH of electricity, which means it took about 20 hours to dry the tomato slices completely.

This little solar dryer, by contrast, runs at about 250 watts, as near as I can calculate.  Starting with data on summertime ground-level insolation in my area, for an average summertime daylight hour (spanning four hours before solar noon to four hours after solar noon), I calculate that about 60 watts of energy per square foot makes it through the plastic, into the box.  For roughly four square feet of free area on the top, that works out to about 250 watts.

Not all of that energy goes into removing water.  Both dryers will suffer energy losses from conduction and convection.  The Nesco, being far more compact and (presumably) designed for optimal ventilation almost certainly has lower losses than the solar dryer.

Factoring in energy losses, this little solar dryer is at best half as powerful as the electric dryer I use.  (And, separately, it clearly had too little ventilation, as evidenced by condensation on the inside.)  So it’s no surprise that it could not dry the full batch of tomatoes in one solar day. 

Given the estimated power input, it’s right on the edge of plausible that this simple solar dryer could do half a batch in a day.  In other words, stack the trays one deep, not two deep as in this trial.  That means that two of these might possibly provide a complete substitute for my electric dryer.

Maybe a better possibility is to use this solar dryer as-is, but use it to finish the tomatoes, not start them.  If you’ve ever tracked the drying process, you know that as the food dries, the drying slows down.  In this scholarly article on food drying, the conventional wisdom is that two-thirds of the drying time is used to remove the last one-third of the water.   As a consequence, the last few hours of electric food drying are the least efficient, in terms of water removed per KWH of electricity used.

(As an aside, that same article also notes how energy-inefficient food drying is.  “Most common drying processes use extensive quantities of energy at relatively low efficiency. Energy-wise, drying is a wasteful water removal process …”.  That high energy use is the very thing that started me down this path.

Combine that with the observation that my little solar dryer was overwhelmed with the water from the raw tomatoes.  It was not sufficiently well-ventilated to avoid condensation from a full (four-tray, six pound) load of tomatoes.

My final observation is that the dryer the tomatoes are, the safer it is to leave them out overnight.  If I had a batch of tomatoes that was two-thirds dried — leathery, say, with no visible moisture — I’d consider leaving those out overnight.

When I put that together, I can almost certainly dry my tomatoes more efficiently by combining solar and electric drying in the reverse order of what I just did.  I should use the electric dryer first, remove the bulk of the moisture, then finish the half-dried tomatoes slowly in the solar dryer.  Maybe something like four hours in the electric dryer, followed by two days in the sun.

Let me summarize:

  • This solar dryer, as currently constructed, can’t dry a full load of tomatoes in a day.  It’s too low-powered.
  • I am not willing to let the partly-dried load of tomatoes sit overnight.  After a day in the sun, they were still too wet for me to be comfortable doing that.
  • Therefore, I will use some electricity as part of this drying process.  This is inherently a hybrid solar-electric system.
  • Upon reflection, given that I am going to use electricity, I should use that first.  It’s smarter to dry the raw tomatoes for a few hours in the electric dryer, then finish the partially dried tomatoes in the solar dryer over the course of one or two days.

That’s obviously the next thing to try.