Upshot: Direct solar food drying — putting your food out in the sun (with or without some clear cover) — is an inherently low-powered and slow way to dry food.
By contrast, indirect solar food drying — connecting a solar heat collector to a box full of food to be dried — can be much, much faster. That’s because you can increase the power of the device. Mostly, you can greatly increase the efficiency of the solar collector, relative to direct solar drying. Secondarily, you can also make it larger, if you choose — there’s no necessary relationship between solar collection area and the area covered by food.
And faster drying means lower taxes!!! Uh, no, I meant, faster drying means fewer days-in-a-row at the mercy of the weather.
The key, to all this new-found wisdom? Figuring out that a “box-with-clear-lid” food dryer is, technically speaking, a flat-plate solar collector. Then realizing that flat-plate is really inefficient, relative to other things I could make.
I need to make an indirect solar food dehydrator. And it only took me two or three years to figure this out.
In the interest of reducing TL;DR, I’m breaking this into two posts. This post is just the setup. Next post should be the actual construction and use, if any.
Background
I have made a few attempts at simple solar food dryers. The goal of which is to make dried tomatoes, using tomatoes from my garden.
I could just use an electric food dehydrator (like a normal person). The taste of dehydrated tomatoes is identical to sun-dried tomatoes, as far as my wife and I could tell. The drawback is that my food dehydrator uses a lot of electricity. Producing just a few ounces of dried tomatoes consumes about 10 kilowatt hours.
I could just build a solar dehydrator from a set of good, tested plans, such as this classic from Mother Earth News. In theory, that dehydrator can dry five pounds of produce in a day, or ten pounds in two warm, sunny days.
Source: Mother Earth News.
But solar dehydrators of this type — a big custom-made wooden box, with glazing and so on — seem like overkill, for my situation. They take a lot of materials and effort to build. And then you’re stuck with something the size of a small refrigerator. The one pictured above is 6′ tall and 7′ long.
By contrast, for drying a just few batches of tomatoes per year, I’m looking for something that doesn’t take much effort to put together, and that’s easy to store. Which is why I tried making my own.
I started with the West Virginia method (Post G21-048), because who doesn’t have a few junk cars disused vehicles sitting around the yard. This was really just to see whether or not tomatoes would dry, at all, given how humid Virginia is in the summer. This did, in fact, dry the tomatoes, somewhat. As well as amuse the neighbors.
Next, I tried a passively-ventilated clear plastic tote (Post G21-049). This is a Sterilite (r) 60-quart under-bed tote with a couple of screened holes for air intake, a plastic chimney for air exhaust, and some dark cloth as a solar absorber. It clearly didn’t have enough ventilation and/or should not have loaded it so fully, because I got a lot of condensation inside the box. But it dried the tomatoes, somewhat.
I then replaced the passive ventilation with a computer fan. That worked pretty well, as long as I didn’t load a lot of wet produce into the box. A couple of days in the sun dried quarter-inch potato slices to crispness (Post G22-015). So that one works pretty well, but you can only dry a couple of pounds of fairly dry (low water content) produce at a time. And it needs clear, hot, sunny days.
Finally, I tried traditional sun-drying. Just put the tomatoes out in the sun, put some netting over them, and let them be (Post G23-055 and 056). To my surprise, with a string of four good drying days (hot, sunny, breezy, and dry), this worked just fine. With this approach, you can dry as much produce as you want, at one time. But it’s rare to get a string of four perfect drying days here in humid Virginia.
All of these approaches worked, at least somewhat. The power-ventilated plastic tote, and traditional sun-drying, both worked well enough to be usable. But all of them have limitations. Too weird, or too little capacity, or too much reliance on perfect weather.
Maybe it’s time for a re-think.
Two key things that I completely misunderstood.
- Tomatoes are tougher than I thought.
- A simple ventilated box is an inefficient solar collector.
Tomatoes are tougher than I thought.
Above: Actual real picture of my tomato bed, wherein I harvest 30 pounds of tomatoes per square foot, annually. I promise. Source: Gencraft.com
My biggest mistake in all of this was in not understanding how long wet tomato slices could safely sit around. This is what I termed the “ick” factor. Can I really let wet tomato slices sit around for days, unrefrigerated? And then eat them? Ick.
But, as it turns out, under the right conditions, yes, you can do that. In the open air, it typically takes four days for tomato slices to dry fully. Over that time period, some combination of the acidity of the tomato, the salt sprinkled on top, the air movement, the loss of water, and the direct sunlight prevents the growth of any visible mold.
Above: Ancestral rain forest tomato, the prehistoric precursor of the modern grocery-store tomato. Source: Gencraft.com
Because I didn’t understand how robust drying tomato slices are, I rejected those earliest attempts for not drying the tomatoes fast enough. If, at the end of the day, the tomato slices still contained a lot of water, I deemed the experiment a failure, solely because I was squeamish about leaving wet tomato slices, un-refrigerated, overnight. When, in fact, that’s a normal part of drying tomatoes in anything but an electrically-heated food dehydrator.
In hindsight, I now know that drying tomatoes is a lot easier than I thought. Knowing what I now know, I’d now bet that anything from a car windshield to a lightly-loaded plastic tote (with computer fan) would, in fact, produce adequate dried tomatoes. As long as the weather cooperates, for long enough.
A simple ventilated box is an inefficient solar collector.
For my plastic-tote dehydrator, I calculated that over the course of a summer day (from four hours before solar noon to four hours after), an average of about 60 watts of energy per square foot flows into that box. That’s based on typical summer solar insolation at my latitude (about 37 degrees north).
That much is true. Near as I can tell, I messed up nothing material in that calculation.
But, in hindsight, it is a huge mistake to assume that the box actually captures and uses all of that solar power. Or even most of that solar power. In my original post, I multiplied the area of the top, by 60 watts per square foot, and under that incorrect assumption, I’d guess that this little solar dryer operated on about 250 watts of (solar) power, average.
Above: Imaginary power level of clear-tote solar collector. Source: Gencraft.com
That’s dead wrong.
Now that I’m up to speed, I know that the right term for that simple ventilated box is that it is a “flat plate solar collector”. That is, it’s a box with a clear top, a flat black bottom, with air flowing through it. Sitting in the sun.
That’s the simplest and crudest form of solar collector. And it’s the least efficient. An optimized flat-plate solar collector might capture 20 percent of the energy in sunlight and convert it to heat (reference). A better guess for this plastic tote might be on-order-of 15 percent efficient. If that.
So the actual power that this little solar dryer operates on is more like (250 watts in x 15% captured = ) ~40 watts. Over the course of an eight-hour day, the plastic-tote dehydrator harnesses maybe 320 watt-hours of solar energy. Call it a third of a kilowatt-hour (KWH) per day.
Above: Actual power of clear-tote solar collector. Source: Gencraft.com
Now that I know that, it all starts to make sense.
Based on the energy used by my Nesco electric dehydrator, it takes about 2 KWH of electricity to dry a pound of tomatoes to crispness. But, of that, it seemed that about a third of the energy was wasted by aspects of the Nesco design. So, accounting for those losses due to inefficiency, I’d guess that it takes a minimum of 1.4 KWH of energy to dry a pound of tomatoes to crispness.
‘Bout half that 1,4 theoretical minimum is the sheer heat of vaporization of water, pure physics. The other half I attributed it to food being harder to dry than pure water, owing to “humectants” such as sugars and starches. That apportionment arrived at via drying my underwear in my food dehydrator.
At that rate, it should take that little solar dehydrator about four days to reduce a pound of tomatoes to crisp dried tomatoes. Assuming it can get hot enough to produce crisp tomatoes.
When I asses the power correctly, that simple plastic-tote dehydrator probably isn’t going to be any faster than open-air sun-drying. It’s just too under-powered to dry food quickly.
Now that I’m willing to let those tomatoes sit around, wet, for days, I can see how long it takes that dryer to produce shelf-stable dried tomatoes under mediocre conditions. I am currently drying a batch of about two pounds of tomatoes, in that dryer. But today turned out to be partly cloudy, and the interior of the box drops to barely above ambient temperature when clouds cover the sun. Well see whether or not mold or sun wins this race.
Moreover, I now understand how the Mother Earth News solar dryer can work so quickly. They don’t use a simple flat-plate collector. Instead, their solar collector is carefully engineered to capture a far larger fraction of the solar energy entering it, by use of six plates of metal mesh, painted black, and positioned so that the air must flow through all six plates. That results in a vastly more efficient capture of the energy in sunlight.
How much more efficient? Hard for me to say. But the scholarly literature on solar collectors says that the fanciest, best designs approach 90% efficiency in converting sunlight to hot air. Or roughly six times as efficient as a flat-plate solar collector.
Direct versus indirect solar dryers.
Above: Solar heater interpreted as Rollaway folding bed. Source: Gencraft.com
All of the methods I’ve tried so far are direct solar drying methods. That is, the tomatoes to be dried are laid out in sunlight, with or without some type of clear covering over them. The clear covering being either a car windshield, or the top of the plastic tote.
This is an inherently slow way to dry food. That’s mostly because its low-powered. You only have as much power as the sunlight you manage to capture. The area of the “solar collector” is firmly tied to the area of the trays on which the food is drying. That means that a) you only capture energy in the area that you’ve spread your food over, and b) at best, you’ll capture that as efficiently as a flat-plat solar collector. (That is to say, not very efficiently at all).
By contrast, an indirect solar dryer separates the solar energy collection area from the racks of food. Basically, it’s a solar air heater of arbitrary size, tied to a box holding the food to be dehydrated.
Breaking those two functions apart — the trays holding the food, and the solar collector — allows you to increase the power of the device, relative to the amount of food being dried. That’s because you can a) capture solar energy in a much larger collector area than the space covered by your food, and b) you can use something far more efficient than a flat-plate collector to capture your solar energy.
Want your food to dry faster? Connect up a bigger or more-efficient (or both) solar air heater. Simple as that.
Above: The first solar heater successfully landed on Mars. Inherently inefficient due, at least in part, to the lack of air there. Source: Gencraft.com
Upshot: The main advantage of indirect drying is the potential for more input power, and hence more speed. Instead of requiring four consecutive sunny days with direct solar methods, I might be able to get dried tomato slices in just two days with a higher-powered indirect solar dryer. That would make it a lot more practical in my climate.
And hey, it only took me two years to figure this out. But that’s mostly because plans for solar dehydrators will tell you what to do. But none of them clearly explain why you are doing it. And/or, I was just to dumb to listen to what they were trying to tell me.
Above: The Mid-Century Modern solar heater. Source: Gencraft.com
