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Old 12-29-2019, 01:16 PM   #1
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Solar powered dehumidifier project

I am starting a little developmental project to put together a solar powered dehumidifier for my boat and would like some thoughts/input from the solar geeks on this board (and there are quite a few here!) . Here are the parameters of this project:

My boat is quite small, just a cuddy cabin for sleeping. It has about 100 cu ft. of volume. I want to keep the mildew under control in the winter while it is at our slip. That is pretty easy as the slip has power. Just plug in a dehumidifier and run the condensate down the sink.

I just bought an incredibly cheap ($20) Peltier effect dehumidifier, see- https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1. FWIW it is also incredibly well made with an electronic humidistat. It extracts a pint a day (not much but I shouldn't need much) and draws 2.8 amps at 13 V DC from an AC converter/brick.

Why Peltier? Well it draws 2.8 amps DC or about 35 watts and other compressor dehumidifiers I could find specs on were all AC and draw a couple of 100 watts or so.

I am a little disappointed at the current draw. I saw other Peltier dehumidifiers that extract 5 times more water at only a little more current. But at $20 who cares. But if this one doesn't cut it I could buy a larger one for about $60.

I hooked it up in the living room with about 60% ambient relative humidity and after ten minutes it was putting out 40% dehumidified air. The air flow was small, guessing much less than 10 cu ft cfm, but if the air leakage is small into my cuddy cabin it should keep up. I will report tomorrow what I see after 24 hours inside the boat.

But 6 months out of the year I keep the boat at a storage yard on its trailer. I am not around to monitor it and there is no AC power available at the yard. So I am thinking solar.

Peltier is simple. No moving parts and I suspect it is somewhat voltage tolerant. Probably works at 10-15 volts. The little fan motor is probably more voltage sensitive than the Peltier module but hopefully will run ok within that range.

So what I plan to do is hook it up to a 100 watt panel through a cheap PWM controller. I will try to find a controller that only starts putting out power when the voltage rises to 10V or so and limits it to less than 15V (acceptance cutoff). I think they are common.

The unit draws 2.8 amps and the 100 watt solar panel should be able to supply that for about 6 hours each day. I don't plan to use a battery but more about that as follows:

I have used sophisticated mppt controllers such as Blue Seas in boat solar systems but they had a problem with this scheme- they wouldn't work without a battery as the internal electronics were powered by the battery input/output terminals and wouldn't start working until the battery terminal voltage got up to about 10V.

Is this true in general? Not sure how to find out as I don't think that the Blue Sea manual ever said much about this. And I don't want to spend a bunch on my system. I will have $20 invested in the dehumidifier, $80 for the 100W panel and would like to use a cheap Renogy controller or similar.

And finally is there any battery (cheap of course) that can stand discharging every night to about 10 volts where the controller cuts off and back up again in the middle of the day.

More to come tomorrow when I report how the little dehumidifier dealt with the humidity on my boat.

David
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Old 12-29-2019, 01:41 PM   #2
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Why have a battery? There are enough hours in the day to let it go off overnight. Get a bigger panel and drive it harder.
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Old 12-29-2019, 02:13 PM   #3
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Why have a battery? There are enough hours in the day to let it go off overnight. Get a bigger panel and drive it harder.
That is exactly what my first plan is to do. Get 6 hours or so of dehumidification during the day and coast for the next 18 hours. Any bigger panel might pick up a few more hours but not much more and all of its extra power is wasted without a battery to store it for overnight use.

The battery question was to utilize this wasted power as well as perhaps deal with any controller issues with no battery.

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Old 12-29-2019, 03:33 PM   #4
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A solar panel puts out a nominal 17 volts. You will probably need at least a small battery for surge dampening or the controller will just cycle on and off continuously. Even a motorcycle battery will do for damping.
The battery will not last long if you drag it down to 10 volts day after day. Perhaps a timer on the dehumidifier that only allows it to run during daylight hours?
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Old 12-30-2019, 08:25 AM   #5
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Well, the experiment with this Peltier effect dehumidifier is a bust. It just wouldn't remove enough water to bring down the humidity in my boat's cuddy cabin.

It's spec is 1 pint per day at 85 deg/80% RH and it removed about 3/4 of that albeit at slightly lower conditions 75 deg/75% RH. That was not near enough to bring down the humidity in the cabin. After 20 hours of use the humidity hardly budged.

So it will take a compressor dehumidifier to have enough capacity to do any good. And even Energy Star rated ones use 300 watts of AC to remove 30 pints per day. That means maybe 500 watts of panels, an inverter and some kind of battery that won't be harmed by discharging fully each day. That kind of system will cost at least $500 assuming I can find a battery.

It just isn't going to work. Oh well, I only invested $20 in this project.

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Old 12-30-2019, 09:05 AM   #6
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I'm wondering if you gave it long enough. I would assume that there is stuff inside the boat that absorbs and wicks moisture. It may take a week or more for the boat to show a significant reduction in humidity. When using the dehumidifier (compressor style) in my boat, water production diminishes over time as the boat completely dries out. I would guess that the compressor duty percentage went from 90% the first day to maybe 20% now.

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Old 12-30-2019, 09:14 AM   #7
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So it will take a compressor dehumidifier to have enough capacity to do any good. And even Energy Star rated ones use 300 watts of AC to remove 30 pints per day. That means maybe 500 watts of panels, an inverter and some kind of battery that won't be harmed by discharging fully each day. That kind of system will cost at least $500 assuming I can find a battery.

What about one of the bigger peltier units you found? Given a solar setup, battery and some kind of low voltage disconnect to keep it from drawing the battery down too far if it's running during non-solar hours, it should have a decent shot. And in really humid weather, you could supplement with one of those big buckets of Damprid or similar, as that would give you more capacity for no extra power use.
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Old 12-30-2019, 09:20 AM   #8
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David I used a Peltier Effect dehumidifier on Possum. It dropped the humidity from about 80% to less than 65%. This was good enough to stop the bad mildew problem I had. I paid about fifty bucks for it and it was rated at 1 pint per day. I was plugged into shore power and it ran 24/7.
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Old 12-30-2019, 10:23 AM   #9
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OK, I will give the dehumidifier more time. But it seems if it couldn't drop the humidity significantly over 24 hours then on solar power only it would have a hard time keeping up only running 6-7 hours each day.

I checked other Peltier dehumidifiers and contrary to what my first post said I could only find units that went up to 2 pints a day, not much of an improvement. It seems that there are two types: Peltier effect dehumidifiers that remove 1-2 pints each day and draw 35-40 watts and compressor dehumidifiers that remove 30 or more pints per day but draw 300 watts or more.

What made this one interesting is that it actually uses 13V DC power from a power adapter which made it reasonably easy to power from a solar panel.


I will report the results after a week of use.


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Old 12-30-2019, 10:44 AM   #10
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Dave,

I used two Eva-dry Edv-1100 Electric Petite Dehumidifiers in my Mainship 34T for summer storage in FL. One in each of the sinks with a hole drilled in the tank and I had zero mildew anywhere in the boat. I would think one of these would work in the space you have. As we had shore power I left them on non-stop for the 5 months in storage (in water under cover).

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Old 12-30-2019, 11:52 AM   #11
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I don't have AC power at my storage yard. That is why I was looking for a solar powered solution.


Your two Eva-Dry dehumidifiers plugged in full time probably remove 8 times as much water as one running 6-7 hours on solar power. So to accomplish the same thing I would need 8 dehumidifiers running on solar or one compressor dehumidifier. That would take at least 500 watts of solar.


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Old 12-30-2019, 05:31 PM   #12
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"OK, I will give the dehumidifier more time. But it seems if it couldn't drop the humidity significantly over 24 hours then on solar power only it would have a hard time keeping up only running 6-7 hours each day."


Give it at least a week. You will be stunned about just how much water a boat can hold. Even with a 6 pint dehumidifier it took a week to dry out our boat. Now just a few hours every day takes care of it.
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Old 12-30-2019, 06:20 PM   #13
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"OK, I will give the dehumidifier more time. But it seems if it couldn't drop the humidity significantly over 24 hours then on solar power only it would have a hard time keeping up only running 6-7 hours each day."

Give it at least a week. You will be stunned about just how much water a boat can hold. Even with a 6 pint dehumidifier it took a week to dry out our boat. Now just a few hours every day takes care of it.
You may be right. The bilge does communicate to the cuddy cabin but only through a not so tight hatch cover. So any water and there will be some that the bilge pump doesn't get out could evaporate and keep the humidity high in the cabin until it is gone.

So I will let it run for a while but at 3/4 pints a day it could take longer than a week. I will start it in a few days and report after a week and if it isn't lower report again in another week.

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Old 12-31-2019, 05:25 PM   #14
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Not exactly your setup and I do run mine from the AC system but I use two of the Eva Dry Petite units and the little tray is always filled to the drain tube and the sinks are wet. They made a noticeable difference in my boat. Of course mine are used for winter humidity control but the problem is similar, humidity of 70-80+ % even with the colder area. It is raining like the dickens today so humidity should be around 100%.

I wonder if you simply didn't start out too small. Maybe a second unit of the same size or one larger unit.

Can you rig 120Vac for a test for a day or two and/or scrounge another unit from some one else.?

If two can handle it then maybe a single larger unit would actually not use twice the power.

Of course I realize the cost is mounting but you may not be far off but just enough that the single cannot do the job.
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Old 01-04-2020, 11:47 AM   #15
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Though it is more complicated, a compressor dehumidifier, solar panel, battery, and small inverter might work. 300W yes, but only when running. In that small space, once all the moisture is out of the cushions and woodwork, the duty cycle will be low. On my boat (I installed a built in 70 pint dehumidifier) once things are dried out to keep it at 55% the duty cycle is about 10-20%. That is the whole boat (an AT 34). If set on a timer so it only runs during the day, 100W of solar should easily keep up with that.
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Old 01-04-2020, 12:27 PM   #16
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Though it is more complicated, a compressor dehumidifier, solar panel, battery, and small inverter might work. 300W yes, but only when running. In that small space, once all the moisture is out of the cushions and woodwork, the duty cycle will be low. On my boat (I installed a built in 70 pint dehumidifier) once things are dried out to keep it at 55% the duty cycle is about 10-20%. That is the whole boat (an AT 34). If set on a timer so it only runs during the day, 100W of solar should easily keep up with that.

My boat has a very simple electrical system: 20A charger, two Group 24 batteries, nothing else. I take the batteries home and put them on a trickle charger while the boat is in yard storage for about 6 months, unattended with no shore power available.


So in addition to the solar panel, I would have to add an inverter, a battery and some way of protecting the battery from full discharge on a string of cloudy days plus a $100+ compressor dehumidifier. Not worth it.


I have in mind a simple system: 100 watt (6 amps at most) panel connected to a simple PWM controller with a battery input and load output. I would connect the load output to the Peltier dehumidifier and the controller would drop out when the voltage gets low- 10V or so. No battery to get ruined quickly in this environment. But it is not obvious that the controller will function with no battery. Here is one that is cheap and has separate output and battery terminals: https://www.amazon.com/gp/product/B0...82XA8GYFJQJ1VJ



The Peltier system is simple and shouldn't be ruined by operating at low voltage before it cuts off. The fan is probably the biggest risk but we will have to see if it will be harmed by running near 10V.



I have the Peltier effect dehumidifier running now and will check it daily to see if it drops the humidity over time. But at 3/4 pints a day and that at 80% humidity, I have my doubts. After 24 hours it is still about 80%. Stay tuned.


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Old 01-10-2020, 06:00 PM   #17
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Desiccants?

An alternative to an electric dehumidifier may be a desiccant, like calcium chloride.


A 50 lb bag of calcium chloride is inexpensive and I suspect would absorb a considerable amount of water. If mesh bags, or maybe a colander, containing a desiccant were hung or placed over the boat's sinks, the absorbed water vapor would become liquid and could exit the boat.


I haven't tried this, but am curious if someone has.
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Old 01-12-2020, 02:44 PM   #18
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Well as suggested I have been running the dehumidifier 24/7 for about a week. The simple conclusion to this design development effort is a bust. Here is why:

Until 4 days ago we were having very cool, low humidity (35 deg dew point) days here in SW Florida. So I opened the boat up, let it dry out in the hot sun, low humidity to get all of the residual bilge water out and as dry as possible before the hot/humid weather came back. I then started up the dehumidifier 4 days ago and the temp and humidity climbed. The humidity inside the boat rose to a 70 deg dew point and stayed there, approximately equal to the outside ambient value.

The dehumidifier was just too small to keep up with air leakage. During this 4 days it probably made 3 pints of water but that wasn't enough to keep the humidity down.

It would take a much bigger, almost certainly a compressor based dehumidifier, to keep up. Unfortunately the smallest I could find makes 20 pints a day and draws 280 watts. That would take a 500 watt solar system, 30 amp controller, a battery and an inverter. That is assuming I could figure out how to start and stop the electrics once the battery got down to 12.0 volts. Otherwise when the solar panel voltage dropped at night and the battery ran down, the compressor would stall, possibly burn out and definitely draw down the battery to zero. No battery I am aware of can deal with that. And that system would cost about $1,000.

I did learn a few things about Peltier vs compressor dehumidifiers. Peltier dehumidifiers produce between 3/4 and 1-1/2 pints a day and take 50 watts to remove a pint of water in a day. Compressor dehumidifiers produce between 20 and 70 pints a day and take 14 watts to remove a pint of water a day. Compressor dehumidifiers are therefore about 3 times more efficient.

A Danfoss based compressor dehumidifier would be even more efficient. The 12 V (input) Danfoss compressors running slowly get down to 3 amps DC and would probably do the job and I bet would take maybe 10 watts to produce a pint of water each day. That would probably mean 7 pints each day for a small system which would probably work for me. Alas I couldn't find any Danfoss systems and if I did they would probably be $500 or more.

So the project was a bust. Others have suggested using desiccants but at maybe as much as 5 pints a day humidity leakage I don't think even a 50 lb bucket would last 6 months unattended.

I am going to do one more thing. I have taped up all of the seams to the companionway to seal it off the best I can and let it run for 3 more days. I have no illusions about the outcome, but we can hope.

David
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Old 01-12-2020, 05:01 PM   #19
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The Danfoss idea is a good one... You could probably build one yourself readily enough. And at the higher condenser temperature for a dehumidifier compared to a fridge, those compressors are even more efficient than we're used to in a fridge application.
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Old 01-12-2020, 05:23 PM   #20
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The Danfoss idea is a good one... You could probably build one yourself readily enough. And at the higher condenser temperature for a dehumidifier compared to a fridge, those compressors are even more efficient than we're used to in a fridge application.
Too expensive at least for inividual parts bought from Rparts.com. $225 for the compressor, $185 for the power module plus evaporator and condenser coils.

It only works if I can get a throw away, working Alder Barbour or similar Danfoss based system for little money.

David
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