Moist-Electric Wallpaper for Indoor Energy Harvesting and Humidity Management | Hacker News Reader

Moist-Electric Wallpaper for Indoor Energy Harvesting and Humidity Management

advanced.onlinelibrary.wiley.com

69 pointscroes37 comments

Cool concept. As they mention, more work needs to be done on long term durability, although they saw no degradation in their (very short) testing period. To me, the humidity control seems more worthwhile/useful than the power generation, although that could be a useful solution for small IoT devices like door and window sensors that don't need much power.

I like it as a proof of concept, would love to see a more serious engineering effort to demonstrate what real world costs, durability, and aesthetics might look like (if significant portions of indoor walls are going to be covered in this stuff, it needs to be able to look decent).

I've thought for a while that it would be interesting to have micro dehumidifiers on my roof that supplement my water supply from humidity.

Turn myself into a moisture farmer if you will.

But then I wonder, would it be a problem if everyone did it? Would it cause humidity to drop in a bad way?

It would probably depend on how you use the water. Watering plants (including via gray water systems) would mean a lot of the water would evaporate or transpirate. Flushing it down the drain means it exits the system.

In humid regions, there is no practical limit that would cause any noticeable effects on the humidity of outdoor air. There is just so much water in the air that even if you were running dehumidifiers across all available land area, the air is still going to feel humid.

True but in those areas it also tends to rain a lot so it's less necessary. I'm thinking about Mediterranean and desert climates.

As long as the water is being locally used and locally discharged (i.e. via a drainage field) it shouldn't seriously have an impact. The water would return to the water table and either be used by plants or get wicked back into the air via capillary action from evaporation on the surface.

I guess technically you’d be adding water to the water table since the water was in the air before you pulled it out.

In places with low ambient humidity, harvesting water from the air is an atrocious process. The physics have not changed since people came up with this idea the last time, and the time before that.

For reasons of thermodynamics, pulling water out of the air uses more energy than desalination, and that's before the fact that you have to have a large fan to move a large volume of air to get a few drops of water.

It is on the order of using 60X as much energy to dehumidify air if you are trying to produce bulk water, rather than just desalinating water through a method like reverse osmosis

When these systems are designed for low ambient humidity areas, they use a desiccant based first stage, where desiccant wheel or loop captures water from the low humidity air, and then is heated in a chamber to "regenerate" it and drive off the water into a 100% humidity chamber where the condensation can happen.

The applications of condensate water sources typically aren't in areas where desalination is the competing technology -- usually the competition is piping water in from far away via a utility system.

The other side of that is that a big problem we're going to have moving forward is dealing with increased humidity - the climate change angle isn't just heat, heat + high humidity (the wet bulb temperature) is the actual health risk and the thing that takes a heat wave from inconvenient to a genuine crisis. Turning high humidity into drinkable water is two birds with one stone, if you do it right.

(To your point, it's almost impossible to imagine we'd be able to operate it at a scale that makes a genuine impact, but these are all chaotic systems and a couple percent here and there can compound to keep things on the right side of the phase shift.)

For non-potable use, probably. Potable use might require some more treatment checks on it. It's one thing to use it for flushing the toilet... showering might be ok. Not sure about washing dishes. Washing clothes, maybe.

And for the "the amount of times I change out 2 gallons of water from the dehumidifier in the basement in the summer"... there's a lot of moisture out there.

This is constrained by season. In the winter, its much easier to get to 0% humidity than it is in the summer.

https://tropic.ssec.wisc.edu/real-time/mtpw2/product.php?col... for the total precipitable water over North America. Through the midwest, it's in the 40-50mm range.

Consider XKCD's giant raindrop - https://what-if.xkcd.com/12/

> We’ll imagine our storm measures 100 kilometers on each side and has a high TPW content of 6 centimeters. This means the water in our rainstorm would have a volume of: 100km x 100km x 6cm = 0.6km^3

> That water would weigh 600 million tons (which happens to be about the current weight of our species). Normally, a portion of this water would fall, scattered, as rain—at most, 6 centimeters of it.

Change that to 4cm for current conditions... but that's a lot of water.

https://en.wikipedia.org/wiki/Humidity#Relationship_between_...

That has a table of how much water at different humidity and temperature combinations. It's currently about 20 °C and that gives us 15.6 g/m^3.

... but there's an awful lot of air around.

This can also be complicated if you have trees or other vegetation around. The humidity at my parents place (fairly deep in the woods in farmland) is always much higher than it is if you go to the road (or in town).

https://extension.osu.edu/about/resources/corn-sweat-and-hum...

> Some research conducted by USDA Agricultural Research Service suggests that corn can contribute between 3,500 and 5,000 gallons of water per acre to the atmosphere over the course of one to two days. A typical pool contains 18,000 - 20,000 gallons.

...

So, it's complicated. Maybe, a little bit, depends on where you are.

oo, fermi estimate time!

Typical 50% RH at ground level is ~7g/kg air, air is roughly 1kg/m^3, so between the surface and the planetary boundary layer (let's say 1km) is about 7L of water per m^2 of surface. Moving along at ~3m/s means 21L/s passes by each meter of fence line.

PBL probably mixes after about 50km, so it takes ~5 hours to normalize after you pull out water at ~5 knot wind speed. If you want to not make a dent, means you can pull less than 1L per m^2 per hour, or about 8m^3 pear, which is about 8x the yearly requirement for farmland.

So, you can irrigate a farm and your neighbors would mostly be fine.

(math not checked)

Absolutely can pull the water you need for e.g. household purposes out of the air - somewhere north of $0.10 a gallon or so, spendy, but close to trucked water - maybe $1k a month, give or take. People pay that much for trucked water, though a well is rapidly cheaper if it's available.

For a farm, say growing almonds (notably water intensive, but a reasonable first pass), you need about 4 acre-feet per acre per year, which would cost... hmm, somewhere in the very rough ballpark of $200k / yr.

That might put the economics of almond farming substantially in doubt, given that an acre of almonds might yield a value (revenue, not profit) of, say, $10k on a good year.

There are commercially available systems that do that now, one that seems popular in my area is https://www.aquaria.world/ .

As to your question, would it be a problem if everyone did it -- it depends obviously on your definition of "problem", but consider that most people doing this have no way to dispose of the water they take from the air except by putting it back in the air. It might increase regional power consumption, and smooth out nightly shifts in atmospheric humidity, but unless there is a new river running to the sea, the average atmospheric humidity will remain the same.

This is fun, thank you! I wonder why home HVAC units couldn't just capture the drain water to accomplish something similar for "free".

The condensate water is laden with bacteria, mold spores, and pollution concentrated from the air. It can definitely be filtered and sterilized, but in water scarce environments you usually can't pull enough water from the air to be worth it.

Side-tangent, but this reminded me of the health nightmare that the incredibly common “kinetic mist” style humidifiers/vaporizers are.

I think there may be a general lack of understanding to how pollution and bacteria concentrate in our daily environments.

Condensate is not suitable for drinking and 99.9% of homes don’t have a separate gray water plumbing system or storage tank. Even the evaporator coil in the furnace/fan coil unit is going to be dirty, and the condenser coil outside is definitely filthy.

If it's for watering plants it works fine, if you want to store it and use it otherwise you need a filter and UV sterilization, many of these devices also mineralize the water for taste. The condensate will usually have a lot of mold and dust.

> I've thought for a while that it would be interesting to have micro dehumidifiers on my roof that supplement my water supply from humidity.

Hi! This is friendly neighborhood energy conservation law speaking!

You can't just get moisture out of the atmosphere without investing energy. This device works backwards, it exploits the difference in humidity between the indoors and the outdoors. The source of this difference is ultimately the stored energy in food. Humans burn the food and release water vapor as a result, raising humidity inside the building.

How to make this work with infrared wallpaper?

"Ask HN: Are there infrared wallpaper products in US markets?" (2025) https://news.ycombinator.com/item?id=45444514 :

> /? infrared wallpaper heating : https://www.google.com/search?q=infrared+wallpaper+heating

For it to be "infrared" I would expect the panel to be insulated from heat loss to the surrounding air. This is how infrared cooling panels work, but the principle should apply (actually more!) for heating panels.

It's unusual thinking of heating the air as "loss," because normally we would say any heat loss still heats the room, but infrared heating is used precisely because heating air is relatively inefficient: less heat reaches the skin, which is all that really matters!

For low cost and ease of use, chicken heater bulbs continue to be quite efficient IR sources.

For it to be "infrared" I would expect the panel to be insulated from heat loss to the surrounding air. This is how infrared cooling works, but the principle should still apply (actually more!) for heating panels.

It's unusual thinking of heating the air as "loss," because normally we would say that any lost heat will still heat the room, but infrared heating is used precisely because heating the air is less efficient (ie less of the heat reaches human skin, which is all that really matters!)

> A 1596-unit array powers a wireless keyboard and simultaneously reduces indoor humidity from 38% to 32%,

isn't below 40% "bad"?

Bad by what metric?

Bad as in your lips will be prone to chapping, you'll get frequent random static zaps, and your guitar top will split?

Depends on your region, house, and preferences:

https://midwesthvacpro.com/what-should-indoor-humidity-be-a-...

That’s impressive humidity drop for a small panel area. Curious how big the room was.

Meanwhile I'm struggling to increase humidity as it is unhealthily dry at my place... (and ultrasonic evaporators are a no-go)

I wonder if this would get moldy.

The condensate (water vapor that gets condensed out of the air in the process of generating the electricity) has to go somewhere; if it's not properly handled, yes, I would expect mold.

Some heat pumps have built-in UV-C sanitization to prevent mold.

You can install an aftermarket UVC light to sanitize to prevent mold in heat pumps, humidifiers, etc

Mold isn't really your main concern tbh. Instead it would be Legionnaires' Disease.

https://en.wikipedia.org/wiki/Legionnaires%27_disease

No, I'm pretty sure I'm far more concerned about mold. Thanks for the link. I knew I was vague on what Legionnaire's was, but now I know why.

vaporware

Secret services love this one weird trick ...

very solarpunk

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