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Is anyone using a peltier dehumidifier in a drybox?
(lemmy.world)
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this post was submitted on 09 Jan 2024
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Peltier units are solid state heat pumps with awful efficiency. Where power consumption is not an issue, It is significantly cheaper to simply get a normal resistive heating dehumidifier, as the additional heating efficiency of a Peltier operating at 0.6 COP is not worth the upfront investment.
Small heating box dehumidifiers are a bit of a misnomer too. All they do is heat the air so it's relative humidity is lower since hot air can hold more water. The amount of water does not change. However for most environments this works OK. Only units with full refrigerant pumps inside that can cool the air to its dew point, condense the water out, then warm it back up are true dehumidifiers.
Small correction: Did you mean 0.06 COP? 60% efficiency (0.6 COP) at room temperature is impossible. 6% would be about right. Some of the Pelletier coolers are operating at just 1-2%.
You're thinking of carnot efficiency power cycles where you are removing energy from a fluid, maybe? I meant exactly what I said with their COP.
Refrigeration/heat pump cycle COP's can be all over the place. Often they are much higher than 1.0. Where a refrigeration cycle's COP is calculated as Q(cold) / Work(electrical), many heat pumps can move more watts than they take to run. TEC's are no exception.
TEC's in particular are especially sensitive to the total temperature differential between hot and cold plates when it comes to COP. They hate making temperature differentials much higher than 30-40 kelvin. Most efficient when the temperature delta is like 10K or less. COP 0.6 is about typical for a TEC running across the fairly large temperature differential that you would need to get air down to dew point and condense water, at a high current required to make the TEC's cost effective. Example graph of COP vs temperature differential
Huh? Running it backward as a thermoelectric generator is low efficiency (Carnot limit) but the efficiency pumping heat from e.g. freezing to room temperature isn’t that awful. Picking some completely random model from DigiKey, if the hot side was at 27c/80f, and you wanted the cold side at 0c, 2.4 amps at about 7V (16.8W input power) would let you remove about 15W from the cold side with a deltaT of 30c (i.e. the cold side at -3c/27f) per the graphs on page 5. That’s a COP of about 0.9 for cooling. You’d need to remove about 30W from the hot side.
In practice, cooling the hot side is difficult so you might instead use the Th=50c graph, at which point 2.4A @ 8v (19.2W input power) would give 3-4W cooling power with the cold side at 0c, for a COP of about 0.18. But nowhere near your numbers.
If you put some decent thermal mass on the hot side and operated intermittently, as long as you drained the condensation, you could get closer to the Th=27c performance.
I have some TE modules that run at 10W or so and can cool 250mW at I think -50°C to -70°C so they can be awful.
I see. You have a huge delta-T there, which is unnecessary for keeping a filament box dry.
A couple sources say PA6 (a random moisture-sensitive filament) should be stored below 20% , or at 15% relative humidity. In a 66f/19c room temperature, that 15% RH would be a dew point of 17f/-8c according to dpcalc.org. In a 74f/23c room, 15% is a dew point of 23f/-5c; as you warm up the temperature further the dew point to achieve a particular RH increases (which is why we heat air to dry things).
I don’t think the efficiency is a big deal; you’d only have to run the peltier intermittently (e.g. each time you open the box) as long as you can drain the condensation somehow and it’s otherwise well-sealed. I’ve idly pondered OPs idea myself and I think it’s not a terrible way to dry a box. You avoid the consumables or need to recharge your dessicant materials.
Yeah, intermittent operation based on RH% would probably be okay as far as power consumption goes. But at the same time, do you really gain anything in terms of performance and cost vs just resistance-heating the box up warm enough that the RH% drops below 30, and then cycling air in and out once in a while?
You’re right, 50%@70f (let’s say Texas indoor conditions) is 19%@100f, so you’re right that just heating is probably perfectly good.
You’d need a very sensitive filament to bother doing more. That said, if you did have a peltier with the cold side at 32f, you’d get 25%@70f or 9%@100f…which actually isn’t amazing since you still need to warm it up to 100f to get that 9%! Peltiers are inexpensive and I might still experiment with it one day, but…it doesn’t seem like a huge win.
I think it should be theoretically possible to make a Peltier based machine that will extract moisture the same as a refrigerant pump machine. Ask any overclocker who's tried to use a Peltier directly on a CPU or motherboard; it's perfectly possible to get the cold side of a Peltier below the dew (or frost!) point when it has little or no thermal load on it.
Now as to whether or not any commercial product exists that actually works that way is another story. I see a ton of them e.g. on Amazon, but I have not tried any out or taken one apart or anything. But with a suitably shaped heat sink you could get condensation to form on the cold side and then drip or shake or blow it off or whatever into a catch bucket, just like your basement dehumidifier does.
oh, anything is technically possible. It's totally possible to get a peltier to drive a sufficient temperature delta to condense water at a low dew point. The issue is the exponentially worsening efficiency curve as delta-T increases that would make it's power requirements - and waste heat generation - quickly become untenable for what you're accomplishing.