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Comment by lolinder | Hacker News Reader

Less energy would definitely be a huge plus but unless this violates our understanding of thermodynamics there will still be unwanted heat put out into the air. The heat from a dehumidifier comes primarily from the latent heat in the water being released so that the water can become liquid. This heat must be released somehow in this process unless they actually did find something that breaks our understanding of physics.

Obligatory Technology Connections video on the topic: https://www.youtube.com/watch?v=j_QfX0SYCE8

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That's true, but I was under the impression that most of the heat generated by current compressor dehumidifiers is just waste heat from the mechanical operation of the compressor itself. The phase change heat is there too, but it's significantly less. So there should still be a lot of room for improvement, theoretically.

Yeah, there's certainly some of that, but going off of Alec's numbers and trial above you end up with a larger portion coming from the latent heat than from the mechanism, for the same reason that a heat pump is more efficient than an electric heater: it's condensing more water than it is running mechanisms.

That said, his demo is not under typical operating conditions in that a dehumidifier is normally expected to actually be able to catch up and reduce humidity in the room, while his demo ensures that the humidity levels stay high throughout the hour. So it's likely that under normal operating conditions the mechanism's proportion of the waste heat is higher than it is in his demo.

> That's true, but I was under the impression that most of the heat generated by current compressor dehumidifiers is just waste heat from the mechanical operation of the compressor itself.

Nope. It's almost all (>80%) latent heat. I believe, the theoretical limit is around 90% for typical room temperatures.

typical room temperatures is doing a lot of work there.

>The heat from a dehumidifier comes primarily from the latent heat in the water being released so that the water can become liquid.

A dehumidifier movies heat from one side to another using electricity to do the work. One side gets cold so the water can condense on it, while the other side gets hot from extracting the heat from the cold side. Heat is still generated from this process even if there are 0 water molecules in the air and no water is collected. The water does not create the heat, the electricity does.

I don't think there has to be any heat involved with collecting water molecules in the air into a larger volume of water, depending on the process used.

It’s not just the electricity creating the perceived heat https://en.wikipedia.org/wiki/Enthalpy_of_vaporization

Both processes create/release heat, and in Alec's tests in the linked video the bulk of the heat from the dehumidifier running in a humid space is coming from latent heat released from the water. That may not be true in regular operating conditions, but there will always be a substantial amount of heat released from the water when you trigger a phase shift.

> I don't think there has to be any heat involved with collecting water molecules in the air into a larger volume of water, depending on the process used.

The only other option is to increase the pressure in the room or in a space within the room, which this material pretty clearly isn't doing.

> I don't think there has to be any heat involved with collecting water molecules in the air into a larger volume of water, depending on the process used.

For the water to condense, there must be heat given off, unless I’m fundamentally misremembering my high school physics class.

state change takes 63 calories per cubic centimeter of H2O unless I am misremembering my high school science class.

at least for water solid to liquid

If water changes phase from a gas to a liquid, it releases a large amount of energy (enough heat to heat five times as much water from 0 to 100°C). That's likely far more heat than the electricity generates - though, of course, the heat released by electricity is very real as well, and as you say will even happen if the air is completely dry.

Possibly this doesn't happen if the condensation happens in a capillary (there is some funkyness related to energy levels), but then it must stay trapped there.

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