Sometimes you need a deadline to get motivated.
I want this done no later than close-of-business (COB) tomorrow.
No particular reason for COB, I just like the way it sounds.
Background
… so this morning, my wife emails me a link to some article in Wirecutter. You should read this, she says. I skim the thing, and I’m like, seriously? So I do the math, and sure enough, the latest and greatest Nesco food dehydrator draws more power per day than my central AC.
Non-Calculation: Even if I run this appliance outside (and so have no additional air-conditioning load in the house), it draws 1000 watts, or 24 KWH/day. (Which, based on apple-drying times as listed in the article, is about the amount of time I’d expect it to take to dry some trays of tomatoes.) Golly, that seems like just about enough to run my HVAC system. On average. Based on my bills. Rough cut.
Food-drying was already on my shit-list, energy wise. Now the latest model of my dryer uses more than twice the energy per hour? And is praised for its modestly higher speed, no less!
Damnation, the next thing they’ll be telling me is that more horsepower makes for a faster car.
Nobody cares about this high energy use, if this device is only used a day or two a year. But if you’re going to use this frequently, a thousand watts is a lot of draw, for an appliance that is to be turned on and left on for long stretches of time.
I took this as a sign that it’s time for me to make an indirect solar food dryer. I’m now motivated. I made a direct food dryer, but it doesn’t hack it, here in humid Virginia. Time to step up my game.
Fast and cheap
A standard engineering aphorism goes something like this: Of good, cheap, and fast, under ideal circumstances, you can choose two out of three.
With that in mind:
First I’m first going to condense the scholarly literature on solar air heaters down to two practical rules.
Then I’m going to give a list of solar air heater configurations I have come across, in the context of these rules.
Finally, I’ll describe two ways this project could progress: A rigid clear-topped box, or a pair of clear/black tarps taped to form tube. The advantage of the latter, if it works, is both cheapness and easy storage. Both of which allow it to be made with much larger solar collecting area than the rigid box, for the same investment in time and materials.
No clue if I can make either one work adequately.
Knowing the right buzzwords is key
I’m trying to build a solar food dryer. Google that, and you can get any number of home-made designs for such a device. From those I get methods, but no coherent understanding. Do this, do that, hot air ensues.
By contrast, Google solar air heater an entire engineering and scientific literature opens up. Because, at the very least, this is an important topic for third-world or developing-world agriculture.
The epiphany was realizing that an indirect solar food dryer is just a solar air heater, whose hot-air output is used to dry food.
And that a direct solar food dryer in fact is a simple flat-plate solar collector, that just happens to have food sitting on or above that plate. And so is the lowest-of-the low in terms of efficiency. For the difference between direct and indirect solar food dryers, see Post G23-057
The entire scientific literature on plate-type solar air heater efficiency, summarized: Maximize contact area and roughness.
We’re talking about heating air by passing it through some kind of a box that is heated by the sun. I’m going to restrict this further to include only “glazed” solar collectors, that is, the top of the box is clear and lets in sunlight.
So, picture a big black box, clear top, ventilated in some fashion.
How do you get the most hot air (heat energy) to come out of that box?
It’s all about what you put inside the box, how you drive air through it, and maybe, how well you insulate it.
Rule 1: The greater the contact between solar-heated material and air, the better.
Rule 2: Rougher the solar-heated material, the better. That is, the more micro-air-tubulence you generate, on that heated material, the better.
There’s a countervailing factor in primitive locations, the need for powered ventilation. With enough contact and roughness, you need to insert an electric fan in the design to get adequate air. My feeling is, I probably want a fan in the design, regardless. “Naturally aspirated” equipment is so last-century.
At one end of the scale, the least-efficient solar air heater is a plain flat-plate collector, that is, a clear-topped box with a smooth black bottom. That has the least contact between hot surface and air, and nothing to create turbulence. This might typically convert 15% of the incoming solar energy to hot air.
Somewhere near the middle of the scale comes the “box with layers of window screening” or wire mesh. The screening is the solar-heated element, and air is forced to flow through that screening as it passes through the box. Designs of this sort might typically convert 40% to 50% of the incoming solar energy to hot air.
Counter-intuitively, you don’t need to use metal window screening. Plastic or fiberglass screening appears to work just fine. It took me a while to get my mind around that, but it makes sense. You (mostly) aren’t trying to conduct heat within or along the wires of the screening. Instead, you are directly heating the very outermost surface of the screen wires with sunlight. And then stripping that heat off that outermost surface, into the passing air. In that context, the heat conductivity of the screen itself only helps if the passing air cannot capture all the heat from the sun-lit areas. In that case, the shaded areas of the screening will be heated by conduction. Thus you would expect heat-conducting wire screening to be somewhat more efficient, but not vastly more efficient, than non-conducting plastic/fiberglass screening.
At the upper end of the scale, some designs claim 90% conversion of incoming sunlight to energy embodied in heated air. But methods for these are esoteric. I ain’t going there.
Practical models from the literature
To improve on the simple flat-plate solar collector, you can:
- Pass the air both above and below the solar-heated plate. This gives 2x the metal/air interface of the simple flat plate.
- Do that, and corrugate that plate to yield even more surface area. Try corrugated metal roofing (~2.8x metal-air area), or string together soda cans (possibly as much as 6.28x the metal-air area, depending on how hot the shaded portion of the can gets. (My estimates, from typical geometry).
- And/or add conductive metal projections to the plate, typically done as small fins/bumps (adds both greater interface area, but mostly, micro-turbulent air flow. Sometimes the bumps are called turbulators).
- Achieve the same sort of effect by passing air through layers of black screening or metal mesh. Provides both high contact area and turbulence, but note that screens are directly heated by the sun.
- Take that to its logical conclusion and pass air through porous heat-conducting media (e.g., steel wool in firm contact with the solar-heated metal plate). Provides both high contact area and turbulence, but likely requires powered ventilation.
- Extras: Insulate the box, maybe double-glaze the box (to prevent heat loss).
- Extras: Pre-heat the air with ventilated double-glazing, that is, by passing it within the double glazing, before dumping it into the main heating chamber.
I think that’s adequate for a discussion of using solar air heaters to dry food. There are, for example, evacuated tube solar air heaters, transpired air solar heaters (for warming incoming air for a building), and so on. None of which seems on-point.
My best two options.
Option 1: A diagonal-screening solar air heater.
This is a coffin-sized box, with a clear lid, arranged so that air flowing from one end to the other must pass through several layers of black window screen. That’s achieved by mounting the screen on the diagonal, from top to bottom of the box. Below is a view of one, with the clear top removed. The box at the top is where the food to be dried is placed.
Source: Mother Earth News/Dennis Scanlin.
This design is all over the internet, and traces back to the Mother Earth News/Dennis Scanlin solar food solar food dryer, published in 2014.
There is no doubt that this design works well, but it involves a considerable amount of fabrication. It also results in having a fairly large piece of equipment to store.
Option 2: A cheap, roll-up solar air heater.
I can’t show you plans for this one, as I made it up on the fly. The idea is to:
- Tape a 3′ wide strip of clear tarp on top of a 3′ wide strip of black tarp, forming a clear-topped tube (say) 15′ long.
- Stuff that tube with scrunched-up dark plastic mesh. (In my case, green produce sacks.)
- Maybe add a sheet of radiant barrier to the back, doubling up the black tarp material.
- Tape a fan inside the bottom opening, tape a piece of duct inside the top.
Picture a gigantic toothpaste tube. Instead of toothpaste, there’s scrunched-up plastic produce sacks. Instead of the cap, there’s a piece of flexible duct. Instead of the sealed bottom, there’s a 20″ window fan taped to the tarp-tube. And one side off the tube is clear.
Then connect this solar air heater to a box designed to dry food. And there’s your indirect solar food dryer.
The big advantage of this, other than being easy to create, is that I should be able to roll it up to store it. So I don’t have yet another big, rarely used piece of equipment to store. (Table saw, I’m looking at you.)
The other big advantage is laid out in Econ 101 below. It’s so cheap and flimsy that if you have the scraps on hand, you can make a very large collector area for very little time and money.
Stuff nobody needs to know.
Interlude for a brief power calculation.
Is it worth making a durable solar air heater, and using that to heat my house when it’s not being used to dry food?
No. Not even close. Any feasible solar air heater I could build (30 square feet, say) would offset less than 2 KWH/day of electrical use by my heat pumps. On a sunny day.
Calculation follows.
In Post G23-057, I came up with a figure of about 60 watts of energy per square foot, on average, flowing into a solar collector, over a sunny, clear eight-hour summer day, at my latitude.
This collector will be about 30 square feet in area (two screen doors’ worth). So there will be about (60 x 30 =) 1800 watts, on average, flowing into the tops of the solar air heater collection boxes.
If these collectors are at best 50% efficient — which will take some work to achieve, that would provide a usable 900 watts of heat on average, flowing over the course of the eight-hour summer peak daylight hours.
But in winter, solar input has ah, half maybe of what it is in summer. I’d have to look it up, but that’s close, based on calculators for solar power systems. So in winter, I’d expect that 30-square-foot solar air heater to provide about 450 watts of heat energy, average, over an eight-hour day, or maybe 3.6 KWH of heat. Which, because my heat pump is over COP 3, would take less than 1.2 KWH of electricity.
That’s rounding error in my wintertime heating energy use. If I build a high-quality solar air heater, I should build it because that makes the solar food drying easier.
A brief Econ 101 interlude.
The scientific literature focuses on the technical efficiency at which a device is able to convert sunlight to hot air. In some crude sense, it’s almost a horse-race to see whose device is more technically efficient. As a consequence, you’ll see some fairly exotic stuff being done.
But the practical question is more like finding the cheapest, or least effort, or most convenient, or most reliable way to use the sun to dry food effectively. Using materials and techniques that are within your skill set.
And that’s where the tarp-tube solar collector came from. If you can make a collector cheap enough per square foot, you can afford to make a really big one. Then, even if it’s not the most technically efficient design, it might be the economically efficient design.
A quick visit to the world of the future, or, why not just put in some solar panels?
Imagine a world where you didn’t have to depend on the sun and weather in order to preserve your food. A world where you could simply call up a hot, dry breeze with the flick of a switch. And use that to dry your foods, any day, any time.
My point is, for the home gardener, using the energy of the sun, directly, to dry vegetables, is quaint. It’s a throwback to a different time. Fact is, electric dehydrators don’t cost all that much, and work pretty well. Near as I can tell (prior posts) the results taste the same as true sun-dried tomatoes.
My only gripe is that “table-top” electric food dehydration consumes a lot of electricity. And there seems to be no way around that. And now, the arms race is on, for offering higher-powered models.
Logically, I have to answer the obvious question: Why not just offset your food dryer use by hanging up some solar panels and generating electricity?
The short answer is that electricity is fungible. In other words, the solar panels produce electricity. And the food dryer uses it. But any linkage between the electric food dryer and the solar panels is entirely imaginary. It’s all in my mind.
The fact is, producing electricity reduces my carbon footprint. Consuming electricity increases it. And if I could install those solar panels, and then NOT use that electric dryer, I’d reduce my carbon footprint even more.
Upshot: Regardless of whether or not I install solar panels, eliminating my use of the electric food dryer reduces my carbon footprint.
But suppose I went ahead and installed enough photovoltaics to offset my food dehydrating electricity use. How much would I need to install? Without showing my math, a single 100-watt solar panel, run year round, would generate about 140 kilowatt-hours of electricity, which would run my 450- watt Nesco dehydrator for two weeks straight. (More than I use it in a typical garden season.) That, based on the solar energy calculator run by the National Renewable Energy Labs.
Separately, it would violate local electrical and fire code to do cheap D-I-Y plug in solar panels. That is, a couple of solar panels and a grid-tie inverter, plugged into a standard outlet in my house. The problem isn’t that this won’t work. It would work just fine. The problem isn’t the safety of linemen working on the utility lines when the power is out. Those grid-tie micro-inverters don’t produce voltage when the power is out. The problem is that I could, in theory, overload my home wiring without blowing the breaker. The circuit breakers in a home system only respond to the current flowing off the grid, into (and out of) a circuit. If you inject current directly into the circuit, downstream of the breaker, and then plug in a lot of current-using devices, you could, in theory, have some parts of that circuit carrying more current then they are rated for, without tripping the breaker.
At root, that part of the building code is there to prevent me from killing myself with stupidity. An admirable goal. I guess.