I know what you’re thinking. You’re thinking “Hey, dummy, that’s what the microwave is for.”
And while I acknowledge the truthiness of that statement, my excuse is that I baked my briefs in this manner for Science.
This post focuses on a seemingly simple question: Why is an electric food dehydrator such an incredibly energy-intensive way to preserve food?
The answer is unsatisfying. In roughly equal parts:
- Evaporating water is energy-intensive
- Evaporation water out of food is even more so.
- My particular dehydrator is somewhat inefficient.
My bottom line is that drying a pound of wet produce, in my electric dryer, under optimal conditions, takes about 2 kilowatt-hours of electricity. And there ain’t much I can do about that, other than coming up with a solar dryer that will function in my climate. My first attempt at that turned out mediocre (Post G22-015).
Part 1: My food dryer is an energy hog.
For those of you who’ve never run across one of these, a home food dehydrator (or dryer) is basically a hair dryer hooked up to a box. You fill the box with food, you blow warm air over it, and some time later, you end up with a box of shelf-stable dehydrated food. E.g., Meat goes in, jerky comes out. Tomato slices go in, dried tomatoes come out.
The actual designs are more elegant. Fan, heating element, a housing, some trays. Maybe a thermostat and/or timer. But hair dryer and a box is pretty much the gist of it.
There is a bigger issue here, in that drying crops is a big part of commercial food production. Finding energy-efficient ways to do that is an ongoing challenge, particularly in the lesser-developed world. Once you get to that scale, you see far more efficient and sophisticated solutions. But I’m talking about small-scale food drying as an alternative to canning or freezing food from the home garden. Table-top drying, if you will.
First, my table-top food dryer is an energy hog compared to other methods of home food preservation.
Source: Post G22-010
Second, my food dryer is also vastly less efficient than other appliances whose function is to evaporate water. Per pound of water evaporated, it consumes about three times as much energy as my clothes dryer or room humidifier. (Assuming you’re smart enough to do the math right for the humidifier, Post #1669.)
Source: Home Depot.
Calculation: I figure the totally-maxed-out 12 trays in my Nesco dehumidifier might begin as about 12 pounds of wet (i.e., fresh) produce. Which, because these things are about 95-something percent water, equates to (roughly) 12 pounds of water. The Nesco draws 450 watts, so running it for the 24 hours required to dry tomatoes is going to consume (450 watts x 24 hours /1000 =~) 11 kilowatt-hours of electricity.
By contrast, on 11 KWH of electricity, I could dry three full loads of laundry, using an old-style resistance-heat tumble dryer. At about 3.5 KWH per load. Each load liberating about 10 pounds of water (per Post #910). Evaporating somewhere around 30 pounds of water.
And, in fact, fully loaded, the Nesco unit took about 36 hours to get the last tomatoes dry. That would equate to about 16 KWH, or enough to dry 4.5 loads of laundry, or 45 pounds of water, which means that, for this batch, the food dehydrator actually used more than four times as much electricity, per pound of water evaporated, compared to a clothes dryer.)
And that’s a genuine puzzle. What is a clothes dryer, if not a bigger hair drier, hooked up to a bigger box? So why does the same technology — blow some hot air, pick up moisture, exhaust it — take three times as much energy per pound of water extracted, for the food dryer as opposed to the clothes dryer?
There’s no clear reason based on first principles or basic physics. The energy required to evaporate water — the heat of vaporization — is roughly the same for any water temperatures you’d find in the home.
Source: Engineering Toolbox, annotations in red are mine.
Part 2: Sacrificing my undies for Science
Why does it take so much energy for my table-top food dehydrator to dry out food? Competing hypotheses are:
Hypothesis 1: Food is difficult to dry, period. Anybody who has dried food knows that as food dries, it dries ever-more-slowly. Taking the last 10% of the water out takes much longer than taking the first 10% of water out. This means that you have to run the dehydrator for an extended period of time to get the food down to a dryness level that will (e.g.) prevent the growth of mold.
This is generally explained in terms of humectants — substances in the food that trap and hold water. Things like sugars and starches. Unlike pure water, these substances want to stay gummy and pliable, rather than drying entirely.
If that’s the prime driver of high electricity use, then there ain’t much I can do about it. If I want to dry food, to the point where it is shelf-stable, using an electric food dehydrator, I’m going to use a lot of electricity.
Hypothesis 2: My Nesco dryer is simply an inefficient design. Alternatively, maybe the dehydrator I use is just a poor design. It’s pretty crude, as I explain in the next section.
If that’s true — if the Nesco is simply bad at evaporating water — then I could plausibly reduce the energy cost of food drying by getting a better table-top food dryer.
The experiment: To try to parse that out, I tried drying cloth, soaked with clean water, in my Nesco dehydrator. This takes the “food” out of the equation, and presumably puts the Nesco dryer on the same footing as my clothes dryer.
I cut up a three pairs of men’s briefs, soaked them in water, loaded them into my Nesco food dehydrator, and let that run for two hours. By comparing the weight of the cotton/poly cloth before and after, I know the weight of water evaporated. A Kill-a-watt meter gives me the total KWH consumed. In the end, I will know, under ideal conditions, how efficiently the Nesco machine can evaporate water.
The results: The Nesco food dehydrator is, in fact, a relatively inefficient device for evaporating water. Where the resistance-heat tumble drier converts about 84% of the total energy use into water removal, the Nesco only converted about 48%. Given the one-off nature of this test, let me express it in simple round numbers. Under ideal conditions, my Nesco food dehydrator uses twice as much electricity as a clothes dryer, per pound of water removed.
Conclusion: When I do the math (in round numbers), my food dehydrator energy use can be split into three roughly equal parts. (Where energy use not explained by the first two is attributed to the general difficulty of drying food, as opposed to drying clothes.)
- It takes energy to evaporate water, even if done efficiently.
- The Nesco dehydrator isn’t very efficient.
- And food is harder to dry than clothes.
Part 3: What I learned from overloading my Nesco food dryer
The bottom line of this section is that if you want to dry a little bit of produce, the Nesco will work just fine, if somewhat inefficiently. But if you want to dry a lot of food, this Nesco unit just can’t handle it. A large load of wet produce highlights all of the limitations of the device.
I have an actual, practical reason for looking into this. Over the weekend, my wife and I dried a big load of fruit and tomatoes. (Or, fruit, if you want to be pedantic about it.) This is the first time we’ve maxed out our cylindrical Nesco food dehydrator, using a stack of 12 drying racks filled with produce. As pictured above.
Doing that large a batch was a mistake, as it turns out. Nesco says that you can use up to 12 trays at once. But 12 trays of tomatoes and peaches was way more than the Nesco could handle well. And this near-failure made me face up to the high energy cost of running that device, and the severe limitations on its performance.
1 Energy hog. For one thing, we ran this indoors. I don’t normally do that, but the act of dehydrating peaches turns your food dehydrator into God’s own air freshener. The house has never smelled so nice. For another thing, it took almost 36 hours to finish drying the tomatoes, which is about half-a-day longer than normal. Even with that, some where not fully dry.
In effect, we ran a 450-watt space heater for a day-and-a-half straight, then used roughly the same amount of electricity on top of that, for the house AC to remove the resulting heat and humidity. (See Post G22-010 for details of the AC energy use question, for my house.) Using a Kill-a-watt meter to measure the dehydrator’s energy use, I estimate that in total we used about 30 KWH of electricity, to dry maybe 12 pounds of produce. Somewhat less than half of that was due to the fact that we chose to run the dryer indoors.
Extras for experts: If I run this outdoors, on a typical Virginia August day, I calculate that, despite the humidity outside, heating the air to 135F reduces the relative humidity to around 10%. So running this outside, in the summer, works just fine. Running it inside, for this batch, was a choice, not a requirement.
2: How a Nesco food dehydrator works. The Nesco dehydrator has a fan, heating element, and thermostat in the lid. It has three air intakes. Two, under the handles, draw in a small amount of fresh air. One, on the underside of the lid, recirculates hot air from the interior of the dryer. The fan then pushes the heated air into the cylinder of trays. It pushed hot air down between the double walls of the trays. That hot air then seeps out around the rim of each tray. Finally, some portion of the hot, humid air passes out the vents in the base.
3: Limitation 1, the thermostat has little to do with the temperature of the food. I realize this should be obvious. The same is true for, say, a kitchen oven. You’re setting the air temperature, not the food temperature.
The issue is that when you overload the dehydrator, it takes hours for the food to begin approaching the temperature shown on the thermostat. The upshot is that while I thought I was drying my tomatoes at 135F, for about the first half-day, the air exiting the device never exceeded 100F. The tomatoes were held at an unsafe temperature, for an extended period of time, and some of them appear to have spoiled in the process.
4: Limitation 2: Single-speed device. Set at 135F, the heater runs almost continuously. The amount of ventilation is determined entirely by the size of the intake and exhaust vents. The upshot of all that is that, regardless of the size of the load or how wet or dry it is, the device runs at a single speed, and a single ventilation setting.
Normally, I don’t notice a problem. Overloaded, it was clear that the small flow of air out of the device was super-saturated. The water immediately condensed on my kitchen countertop. At the start of the drying cycle, I would have liked to have run the device with much more ventilation, to carry of the high humidity load. Conversely, once the food is mostly dry, I’d have liked to have reduced the ventilation, in order to save energy. But in fact, I can’t do either.
5: Limitation 3: Not much air flow over the food. Until I actually inspected it, I had assumed that the Nesco blew hot air down onto the food. So that, when I heard the fairly loud fan, I could assume that a lot of air was passing over the trays. But in fact, that couldn’t possibly work, because it would dry the top tray long before the bottom tray got done.
Instead, to achieve even drying, hot air more-or-less seeps into the interior of the dryer by passing through the very thin spaces between the trays. As a result, you can get fairly even drying without having to move the trays around. But it’s not as if there’s a lot of rapid air movement within the dehydrator. Other dehydrators, by contrast, emphasize the fact that they work like a convection oven — a fan blows a fairly strong stream of hot air over the food.
Conclusion
When all is said and done, I’m not sure I have any significantly better options for an electric food dryer.
First, I’m never going to load up 12 trays at once, unless the food to be dried is already relatively dry, and cut thin. For example, I’m sure I could dry a dozen trays of basil leaves. But it was just foolish to try to dry a dozen trays of wet produce like tomatoes and peaches.
Second, I’m glad that I follow USDA guidelines when making jerky. These days, the USDA-approved methods boil down to cooking the meat first, to kill potentially pathogenic bacteria, and then drying it. That seemed like overkill, but when it comes to food preservation, I do what the USDA says to do. Now that I know the food in the dryer be well below the thermostat setting, for an extended period of time, pre-cooking meat for jerky that now seems like a common-sense precaution.
Third, drying tomatoes in this Nesco dryer takes far more energy than I would like. But because I don’t dry much food over the course of a year, that’s more of an annoyance than a serious problem. I might do a couple of batches of tomatoes, and a couple of batches of jerky. In the grand scheme of things, the energy required hardly matters.
Finally, I doubt I can buy a table-top dryer that’s significantly better. About a third of the energy is straight-up physics. That’s the heat of vaporization of the water that was removed. Maybe a third of it is due to the inefficiency of the Nesco dehydrator. And the rest — that’s probably because food is just plain hard to dry. And, near as I can tell, all of the low-end dryers are crude single-speed devices, like the Nesco. Factors that could improve efficiency — for example, running the intake and exhaust air streams through a heat exchanger — would only be available on large, commercial units. Ditto for heating and dehydrating the air using a heat pump, instead of resistance heating.
The bottom line is that unless I’m willing to gin up a solar food dryer that works well in my humid climate, I’ll just have to live with it.
