Edit: See Post G22-015. Skip the drying racks, just place the food directly on the floor of the tote. Replace the ventilation “chimney” with a computer fan. With those changes, two days in the sun produced perfectly dry potato slices.
Last fall I came up with what I hoped would be a cheap and simple solar food dryer capable of drying tomatoes in the humid climate of Virginia.
Not this one (Post G21-048):
This one (Post G21-049):
That worked — some. But it didn’t work well enough to be usable as-is. It needs some improvement.
This year, my aim is to see what it takes to make that simple solar dehydrator work. The definition of “work” is to dry three pounds of tomatoes, completely, in two sunny days. With the tomatoes being dry enough after the first day that it doesn’t appear risky to leave them overnight.
This post only shows the construction of Version 2 of that dehydrator. The test has to wait until tomorrow, on account of the weather.
What’s the point of making one?
The point is that I’d like to have a small, effective solar food dehydrator. I can’t buy one, for a reasonable amount of money. And plans available on line are either overkill, or require a lot of fabrication –– and then leave you with yet-another large single-purpose object that has to be stored for most of the year.
Dried tomatoes were one of the biggest hits of last year’s garden. My wife likes them, her relatives like them, and it’s an easy way to preserve a lot of tomatoes with little effort. Wash them, slice them, dry them, and bag them. It’s zero waste and little work.
That said, it takes a lot of energy to dry tomatoes with an electric food dehydrator. A standard electric food dehydrator is arguably the most energy-intensive way to preserve tomatoes. That’s by my calculation, as shown in Post G22-010.
For my four-tray Nesco dehydrator, it took about two kilowatt-hours of electricity to dry a pound of tomatoes. Drying an entire batch (four trays) outdoors, in the Virginia summer, took more electricity than two loads of laundry run through a dryer.
I can almost surely reduce that some by running that dryer at full capacity. The standard model comes with four trays, but you can actually use up to 12 trays. I’ve bought another eight, and if I use that this year, I’ll use it with 12 trays. That should reduce the electricity needed per pound of fresh tomatoes. But that’s still a fair amount of fossil fuel use to create what is, in essence, a home-made luxury good.
Solar food drying has been used for millennia. But that’s mostly or exclusively in arid climates. Simple open-air drying is not recommended for a humid climate such as that of Virginia. The food will likely spoil before it dries. Just read what the University of Minnesota extension service says about sun-drying produce.
As far as true “solar food dehydrators” go — ones that gather and trap the sun’s rays in some sort of enclosure — the Dehytray was the only one for sale that seemed like a good fit for use with a home garden. But that’s really designed for use in lesser-developed nations, and is quite expensive for a limited amount of drying capacity.
Beyond that one example, there seems to be no market for this type of a product. The best I could find was an out-of-production kit for a home solar dehydrator (reference). There was also one no-longer-produced solar oven that advertised itself as being able to do food dehydrating (reference). And this rather odd plastic device, also no longer produced (reference).
The only small “solar” food dryers sold commercially are simple open-air dryers. They are net baskets designed to spread out your food so that air may circulate around it. Those won’t work in a humid climate.
You can easily find plans for a large, effective D-I-Y solar dryer, such as this one from Mother Earth News. (A cheaper version of the same general design can be found at this reference.) Other D-I-Y plans either look like they won’t work in a humid climate (i.e., air-flow only), look like they won’t work, period (e.g., cardboard and aluminum foil), or look like they involve as much fabrication as the Mother Earth News dryer (custom wooden boxes with glass fronts and ventilation).
But I want something small and simple. Something that doesn’t require hours of fabrication and multiple trips to the hardware store. And something that I can just toss on a shelf at the end of the garden season. And that doesn’t cost an arm and a leg.
Insight: Why build a big, clear food-safe box when you can just buy one?
So far, my total contribution to this topic is to realize that, for $18*, you can buy a food-safe clear plastic 60-quart under-bed tote, from Sterilite. That’s a lot easier than fabricating something.
* That was last year. Looks like I can’t find that for under $25 this year.
I still think that’s a useful insight. So, I’m going to continue to refine a design based on using a 60 quarter Sterilite under-bed tote.
Last year’s attempt — call it tote-based food dehydrator, version 1, pictured above — was a failure. It only removed about a pound of water per sunny day. I need it to remove at least three times that much to make it useful, in the sense of being able to dry a single layer of tomatoes in one day.
So the question is, how do I triple the efficiency of the plastic tote food dehydrator, Version 1?
In the autopsy (Post G21-050), I identified inadequate air flow as the likely main culprit. And not inadequate heating power. I just redid the calculations and came up with the same result. The amount of sunlight impinging on the surface of the box should be more than enough to evaporate three pounds of water.
Plus, just glance at the one successful version of a device like this — the Dehytray — and notice just how much ventilation that has. The bottom is a grille, the sides have ventilation slots, the top has adjustable vent openings.
Source: Dehytray
So both theory and practice suggest that I need more ventilation.
But.
But first, from a cleanliness and insect-resistance standpoint, I really like the idea of the bottom of the tote-based dehydrator being a one-piece plastic “tub”. That is, a single piece of molded plastic with no holes in it. That’s why I put all the ventilation through the top of version 1. I really don’t want to put the ventilation in via the bottom and the sides.
But second, I like the fact that this is entirely passive solar, no electricity used. Clearly I could add (e.g.) a computer fan to force the ventilation through the box. The electrical use would be minimal, but that would tether this to an extension cord run from inside the house. If possible, I would like this to be a free-standing design.
But third, when I do the math, it seems like I ought to be able to get adequate ventilation passively, as long as I set this up correctly. I’ll spare you the math, but in order to:
- remove three pounds of water,
- in six hours of prime drying time,
- through the 2″ diameter exit chimney,
- at 80 percent relative humidity and 120 F,
I need to move the air at just over one MPH. That would remove the required 750 cubic feet of air, which, at 120F and 80% relative humidity, would contain three pounds of water.
One MPH is the breeze you feel on your face when you walk slowly. I didn’t measure it, but by feel at least, the tote-based dehydrator already puts out that much air.
Tote-based food dehydrator (TBFD), version 2.
In TBFD version 1, I crammed the box full of wet tomatoes, and the ventilation was more-or-less based on a hope and a prayer.
For TBFD version 2, I’m making the following changes:
1: All the insulation now goes on the outside of the box. Instead of lining the box with Reflectix (r) insulation, I’ve now wrapped the outside of the tote with a layer of black cloth and Reflextix.
2: Single layer of produce, spread evenly over the entire cross-section of the box. I thought about my options, and by far the easiest way to achieve that, with adequate ventilation, was to buy a set of cooling racks that just fit the box. Target had these on close-out for $6.
Edit: Skip this, see note at top of posting.
As a result, the inside of the box is just smooth polypropylene, and those wire racks.
3: Direct the ventilation. Finally, I took a couple of pieces of the cheap plastic tubing I am using (it’s a bulb shatter guard for a linear T12 fluorescent light), and used them to lead the cool intake air under the cooling-rack grates. The hot air, as before, exhausts up a plastic tube chimney. The idea is that the air will have to flow through the racks of drying produce as it moves through the box. (That’s the idea, anyway).
Here’s the top of the box, inverted, showing the air intakes. These now fit down into the box, in the space between the end of the cooling racks and the end of the tote, as shown above.
Edit: Nowhere near enough ventilation. Replace the tall chimney with a computer fan.
Recap of construction
Let me recap the construction of the tote-based food dryer, version 2.
You will need:
- A Sterilite 60-quart under-bed tote, e.g., $25 at Target (reference). Any shallow tote with a clear top would do, but I use Sterilite because Sterilite (r) says that all of their products are made with food-safe plastics.
Maybe 4′ of some short of cheap, thin plastic tubing, roughly 2″ diameter (such as this).You could just as easily use the cores from paper towel rolls, taped together.One set of short cooling racks. I used these (Target).- A Skil knife or hole saw for cutting holes in the top of the tote.
- Some sort of insulation (optional). E.g., bubble wrap, Refletix, cardboard. Some black material to go under that insulation, to absorb sunlight.
- Some small pieces of screening or open-weave cloth, to keep the bugs out of the ventilation holes (optional).
- Tape to hold things in place.
- A computer fan.
All in, given that I can’t find that tote any cheaper than $25 now, this should cost just under $40 to build.
Effort is minimal. Tape the insulation to the outside of the tote, cut some holes in the top. Cut and tape the ventilation tubes into place over those holes. Short tubes protrude down into the box, to bring in fresh air. The long “chimney” tube sticks out of the top of the box, to help form the thermal siphon that will draw the warm humid air out. Computer fan draws the air through and out of the box.
Load one layer of produce on the racks the floor of the tote, put the lid on, prop this up so that it faces the sun, turn the fan on, and wait.
Addendum: Other methods?
Drying food really isn’t rocket science. Pass enough air, with low enough relative humidity, over the food, and the food will give up its moisture to the air. The cooler the food and the air, and the higher the relative humidity, the longer that takes. The warmer the food and the air, and the lower the relative humidity, the less time that takes.
There is always the intriguing option of unheated forced-air drying. As long as you start out with reasonably dry air, you can create perfectly fine dried food just by blowing a stream of air across it.
It’s almost as if I could dry food just as well by sitting it on top of a running box fan. As long as the air circulated by the fan was reasonably dry. And, sure enough, you can do that. Alton Brown (of Good Eats) makes dried fruit using a box fan. At least one person has replicated that and says it works well.
It’s just that outdoor air in Virginia summer is anything but dry. So I’m pretty sure I’d have to do this indoors.
But at that point, you’re kidding yourself about energy use if you only look at the energy used to move air over the food. In effect, you use the house AC, to remove the moisture that you put in the air, by using a box fan to dry your produce. That might still be reasonably energy-efficient, but there’s no easy way for me to tell.
Another approach that appears in the DIY dryers is heated-box forced air drying. For some approaches, the idea was to create a black cabinet, with fan-driven circulation, with a small amount of ventilation. The idea is presumably that the fan circulates the hot air, and moisture slowly escapes.
Really, all that does is replace the heating element of a standard electric dehydrator with solar heating.
There’s a lot to be said for this, as the main point of heating the air — in this method, or in a standard food dehydrator — is to reduce the relative humidity. (That is, to increase the amount of water the air can hold, compared to the amount that’s already in the air.) A typical awful August afternoon around here might be 85 F at 75% relative humidity. But if you heat the air to (say) 120F, the relative humidity of that air drops to 25%.
A third type of dryer is a solar indirect dryer, as with the Mother Earth News dryer cited above. Unlike a direct solar dryer, where the sun shines onto the food, in an indirect dryer, the solar collection area is separate from the hot-air drying area, and you just pipe the hot air into the cabinet holding the food to be dried. That way you have the option to provide more heating power than, and plausibly greater air flow, than if insist on a one-to-one match between the solar collection area and the produce drying area.
But those tend to be large, sturdily-built piece of equipment. E.g., the Mother Earth News dryer is really more than I think I need. It’s certainly more than I would want to store.
For now, I’m going to continue down the path that I’ve started down: A clear ready-made box with some air circulation, what I’m calling the tote-based food dehydrator (TBFD). TBFD V1 worked some, but not nearly well enough. Tomorrow I’ll test TBFD V2. If that doesn’t work, I’ll add a small computer fan to aid air circulation, and try again.
And if that doesn’t work, I’ll rethink the whole thing.
