Some math on solar dehydrators

So I'm really into the idea of building a solar dehydrator but I was asking myself how much money it could save, never mind plastic and electronics. I thought I would share my calculations as an encouragement for others to consider this.

My 5 tray electric food dehydrator uses roughly 250 W. That means if I run it for 24 hours it uses 6 kWh in a day. Where I live electricity is in the 7c/kWh ballpark, which I understand is quite a bit cheaper than many people have. That means 42 cents to run it for 24 hours.

Furthermore though 5 trays sounds like a lot, if I'm making apple chips I can only fit about 5 or 6 apples worth in the unit at once. It takes my dehydrator approximately 24 hours to sufficiently dry apple chips. I eat dried fruit like candy so this is not nearly enough. With all of the apples I get from my cousin's tree (not to mention the ones I intend to plant), I figure I could reasonably use at least 5 of these things just drying for personal use and I still may not make it through the winter on apple chips. So if I used 5 electric dehydrators (gasp) that would be $2.10 per day to run them.

While $2.10 per day doesn't seem like much, I imagine a day when I will dry much more than just apples. I could see myself running a dehydrator over 70 days per year. When you think of it this way, using a solar dehydrator can reasonably save over $150 per year. This is $150 per year that can be used to buy more seeds for more things to plant, grow, harvest, and dehydrate.

If you're drying anything for business scale, well, then I hope you see the potential for some significant savings here. If you're off grid I think there's no comparison.

I can't wait to build my own.

I would look at this a different way :-)
Firstly where I live I mostly use the machine in the autumn early winter it's often dull and damp here at that time of year so the idea of relying on a solar dryer particularly for any type of commercial operation not good plus the department of people making you sad might regard any drying as food preparation and demand the use of certified stainless steel etc etc so I would look out for this .
We use our dryer for pears ,apples and tomatoes and next year will hope to try cherries and plums too and am planting figs and apricots for future drying . I am also thinking about trying veg .
I would also advise you trying out different types of apples and pears etc as there are types that are more suited than others for drying .
We normally use our dryer over night at it helps keep the house warm and saves us having to put the fire on some nights . Ours sounds like it is larger than yours and can take about a dozen or so apples and has stainless steel trays

David Livingston wrote:I would look at this a different way :-)
Firstly where I live I mostly use the machine in the autumn early winter it's often dull and damp here at that time of year so the idea of relying on a solar dryer particularly for any type of commercial operation not good

This is a fair point that others may wish to consider. We happen to generally have a whole lot of sun during most of the drying season so I'm not so worried for us.

Here's a link to our solar dryer which is located in Minnesota. We've been sharing and teaching about this design for over 30 years and know of several built in Canada, so it should work well for you.
http://geopathfinder.com/Solar-Food-Drying.html

Fascinating!  I guess the photo showing the "hood up" is just for display...   So if this works better than convection, maybe that is because it only heats one side of the fruit, so it would dry from the top down, whereas with convection I suppose it would try to dry the fruit from the outside in, and presumably that causes the "case hardening" the article mentions.  Maybe a convection drier would work better if could somehow be designed so that the air flow only went over one side of the fruit.  

I can think of another source of radiant heat in cool season...  The barrel from a rocket mass heater!  It should work fine I think....It's worth a try anyway.  
Although if you're drying more than a peck of apples at a time you might need to vent the water vapor somewhere else besides inside your house.  

I've found it true that "selective surface" paint is expensive.  I'm not sure why.  The primary material for it, the black oxides of chromium or nickel or copper are often available as pigments for ceramics.  You'd just have to come up with a way to adhere it to the surface using some sort of paint or adhesive.  These oxides can also be made.  Black copper oxide forms at a reasonably low temperature.  Although you probably wouldn't want this to flake off and go in your food, so maybe it's better to skip it.  

I think your math is a little misleading in that, if you were going to be doing that amount of dehydrating, you'd probably opt for larger units.  I imagine running three nine-tray dehydrators would cost considerably less than running five five-tray dehydrators (and would give you another two trays to boot).

And to "save" $150 per year, you probably ought to take into consideration cost of building a solar dehydrator and cost of purchasing electric ones, plus upkeep on both options.  Of course there are many variables (Did you buy electric ones new? On sale?  Were they given to you?  Did you purchase all new materials for a solar one?  Use scraps already lying around?), but that might at least ensure you're comparing apples to apples.

Wes Hunter wrote:I think your math is a little misleading in that, if you were going to be doing that amount of dehydrating, you'd probably opt for larger units.  I imagine running three nine-tray dehydrators would cost considerably less than running five five-tray dehydrators (and would give you another two trays to boot).

And to "save" $150 per year, you probably ought to take into consideration cost of building a solar dehydrator and cost of purchasing electric ones, plus upkeep on both options.  Of course there are many variables (Did you buy electric ones new? On sale?  Were they given to you?  Did you purchase all new materials for a solar one?  Use scraps already lying around?), but that might at least ensure you're comparing apples to apples.

Hi Wes, thanks for your take on this. You are right to point out that I would likely not buy 4 more of the same unit I already have. Furthermore it is possible that here are electric dehydrators that are more efficient than mine. That said, I think that as you add trays you also generally add wattage. The first search result on Google gave the following result:

https://www.amazon.ca/Excalibur-2900ECB-9-Tray-Economy-Dehydrator/dp/B001NZPP6U

This 9 tray dehydrator uses 600 watts, which is more than twice the power of my 5 tray one that pulls 250W. It is totally possible that the 9 tray dehydrator is able to dry the fruit faster than mine but with these things in consideration I don't think that there is a huge difference in power consumption from my original estimate. Maybe there are 9 tray dehydrators that can get the job done with similar power to mine and if so I agree my numbers need some adjustment.

I think bringing up that you need to build a dehydrator is fair but I think it's still quite a bit cheaper to build one with new materials than buying multiple electric ones at full price that may have some planned obsolescence built into them. Build a good solar dehydrator, take good care of it, and I think that puppy will cost very little when amortized over its lifetime.

These numbers may not work for everyone. Others may want to dehydrate way less or live where it's wet and cloudy during drying season. I was just surprised at the significance of the numbers in my own scenario and thought I'd share.

If you can shorten the time your food is in the dehydrater, it will save considerably on power.

We dry our fruit on stainless racks outside in the sun without any special dehydrater or anything else.  And while this may not be possible in your climate, I would imagine that just putting stuff out in the sun for one day would significantly bring the moisture content down, thus making it much quicker to dry them in an electric machine.

Mike Phillipps wrote:Fascinating!  I guess the photo showing the "hood up" is just for display...   So if this works better than convection, maybe that is because it only heats one side of the fruit, so it would dry from the top down, whereas with convection I suppose it would try to dry the fruit from the outside in, and presumably that causes the "case hardening" the article mentions.  Maybe a convection drier would work better if could somehow be designed so that the air flow only went over one side of the fruit.  

I can think of another source of radiant heat in cool season...  The barrel from a rocket mass heater!  It should work fine I think....It's worth a try anyway.  
Although if you're drying more than a peck of apples at a time you might need to vent the water vapor somewhere else besides inside your house.  

I've found it true that "selective surface" paint is expensive.  I'm not sure why.  The primary material for it, the black oxides of chromium or nickel or copper are often available as pigments for ceramics.  You'd just have to come up with a way to adhere it to the surface using some sort of paint or adhesive.  These oxides can also be made.  Black copper oxide forms at a reasonably low temperature.  Although you probably wouldn't want this to flake off and go in your food, so maybe it's better to skip it.  

Yes, the "hood" is up for the photo. The heat is generated by the cover collector and the reflective surface underneath "bounces" heat back upward, all the while air is being passively pulled into the airspace under the screens and exiting out the upper side. It is possible to get "case hardening" in this dryer with certain crops, but there are techniques to avoid it.
Our backup plan is using our wood-fired sauna when we've got crops that won't wait for sunny weather, and yes, you do have to vent it. This year we had so many apples at once that we were using the solar dryer every day AND the wood-fired sauna to try to catch up. Most years the backup plan is never implemented.
We've never bothered with any collector paint more high tech than B-B-Q black, high temperature paint.

Marco Banks wrote:If you can shorten the time your food is in the dehydrater, it will save considerably on power.

We dry our fruit on stainless racks outside in the sun without any special dehydrater or anything else.  And while this may not be possible in your climate, I would imagine that just putting stuff out in the sun for one day would significantly bring the moisture content down, thus making it much quicker to dry them in an electric machine.

Placing any food, other than mushrooms, in direct sunlight causes loss of nutrients and "bleaching" of color from the food being dried. In LA you could probably get by with putting the screens out in the sun and merely draping a piece of black cloth over the food to protect it from direct sun and it will keep flies off as well. We've actually used this method in Minnesota to dry apple pomace leftover from cider making to put up as chicken feed. We set up a couple of sawhorses in the driveway with 2 sections of extension ladder on them and rolled out some 4' wide window screen, spread out the pomace, and covered it with a navy-blue bed sheet clothes-pinned in place. Without the glazing covers, this setup dries much more slowly and needs to be covered at night (or brought indoors) to protect it from dew.
By the way, check out Paul Stamets work on vitamin-D production in mushrooms by exposing the gill side to UV light.

Shawn Klassen-Koop wrote:

Wes Hunter wrote:
And to "save" $150 per year, you probably ought to take into consideration cost of building a solar dehydrator and cost of purchasing electric ones, plus upkeep on both options.  Of course there are many variables (Did you buy electric ones new? On sale?  Were they given to you?  Did you purchase all new materials for a solar one?  Use scraps already lying around?), but that might at least ensure you're comparing apples to apples.

I think bringing up that you need to build a dehydrator is fair but I think it's still quite a bit cheaper to build one with new materials than buying multiple electric ones at full price that may have some planned obsolescence built into them. Build a good solar dehydrator, take good care of it, and I think that puppy will cost very little when amortized over its lifetime.

Interesting points being discussed here. Our dryer design can accommodate used stuff, although we do advocate for taking the plunge and investing in stainless steel screen for food safety reasons. The cost of materials per square foot of drying space, even if all new, is less than any commercial electric model we've seen. Since there are no operating costs over its lifetime (use the free nuke in the sky;>)), every year it's used the savings just pile up. Put it in a shed over the winter and it will last even longer with very little maintenance.

Sorry as this is a little bit off topic, but has anyone tried drying clothes in their solar dehydrator?  Compared to a clothes line I'm wondering if it would be better at drying in the winter, be faster, work better when humidity is high or else also be good at keeping the clothes from fading by keeping the clothes out of the light?  (Although the UV is good at killing bacteria in the clothing)

Shawn Klassen-Koop wrote:Hi Wes, thanks for your take on this. You are right to point out that I would likely not buy 4 more of the same unit I already have. Furthermore it is possible that here are electric dehydrators that are more efficient than mine. That said, I think that as you add trays you also generally add wattage. The first search result on Google gave the following result:

https://www.amazon.ca/Excalibur-2900ECB-9-Tray-Economy-Dehydrator/dp/B001NZPP6U

This 9 tray dehydrator uses 600 watts, which is more than twice the power of my 5 tray one that pulls 250W. It is totally possible that the 9 tray dehydrator is able to dry the fruit faster than mine but with these things in consideration I don't think that there is a huge difference in power consumption from my original estimate. Maybe there are 9 tray dehydrators that can get the job done with similar power to mine and if so I agree my numbers need some adjustment.

Some back of the envelope math, for kicks.  I have no agenda here; indeed, I assume that over time a solar dehydrator is the better and more economical option.  But...

As for comparing the 5-tray and 9-tray models, we should be consistent.  You showed 600w for the 9-tray, from the Amazon listing.  That's fair, but the 5-tray model shows 440w, not the 250w you stated.  Perhaps in the real world yours draws less power (or maybe you just have a different model), but then maybe the 9-tray one would too.

Trying to accurately compare models, it looks like a 9-tray model can be had for US$210, while the 5-tray runs US$170.  Three of the former would cost $630 and provide a total of 45 square feet.  Five of the latter would cost $850 and provide 40 square feet.  Spend $220 less and gain 5 square feet of drying space.

Three 9-tray models would use 43.2 kw in 24 hours, which at what I pay from my local electric co-op would cost $2.98.  Five 5-tray models would use 52.8 kw, costing $3.64 per day.

I suppose my conclusion there is that, if you're going to be running multiple electric dehydrators, it certainly pays to get the larger models.

Another consideration, regarding comparisons between electric and solar models, is that electric models can be run 365 days per year.  So if you have a glut of fruit and veggie goodness followed by a week of thunderstorms, you're good to go (as long, of course, as the thunderstorms don't knock out the power lines!).

I'd love to see some figures on cost of producing various models of solar dehydrators, compared to square footage of drying space they contain.

I'm surprised solar ovens aren't used more; they are ideally simple, low cost and high efficiency.  They can be cardboard or wood construction with aluminum foil or mirror reflectors, and crumpled newspaper insulation, a glass window and an optional oven thermometer.  The maximum temperature can be adjusted by changing the solar concentration ratio.  

I'm wondering if a solar dehydrator might be simpler/less-expensive if it was all in one box instead of two.  

I haven't done fruit drying in years but from what I remember the fruit can stick to the screens and can be a pain to clean.
How do you keep the fruit from sticking to the surface?  Did you know that parchment paper is made of silicone?!  If you're using infrared/radiant heat instead of convection, do you even need the screen?  Maybe some well-placed vents would provide sufficient air flow.  Maybe there's less heat build-up on the screen which might reduce case-hardening, on the other hand if the screen tends to dry out the unheated side of the fruit then it could increase case-hardening.  I'm wondering if air-cooling on the back any cooking surface might do as well, and maybe this would make it easier to clean.  

Another possibility is to use desiccants like silica gel to reduce humidity.  After the fruit is taken out, they can be regenerated with solar heat.

I don't know, just thinking out loud.

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"I like any kinda chicken 'cept *dried-out* chicken!"