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Browsing the internet yesterday I came across this very esoteric paper which purports to describe the construction of a device which uses the electrolysis of water into hydrogen and oxygen (a highly endothermic process) with KOH as a catalyst to cool down a room as a replacement for an Air Conditioner: https://www.researchpublish.com/upload/book/Electrolysis%20Air%20Cooler-3057.pdf

The enthalpy of the reaction is +285.83 kJ/mol, and while most of the energy input comes from the electricity being supplied, a fair portion (around 20%) comes from heat in the surroundings at standard temperatures so the basic chemistry/physics is there:

As stated, splitting a mole of liquid water to produce a mole of hydrogen at 25°C requires 285.8 kJ of energy—237.2 kJ as electricity and 48.6 kJ as heat.

Reference for this: https://www.nrel.gov/docs/fy10osti/47302.pdf

The authors of the paper describe the construction of such a device as well as a test showing that it is indeed capable of reducing temperatures inside a room (the paper itself is not well written so a lot of things are unclear, but they definitely did get the idea to work )

I found the idea quite intriguing, especially given that it generates hydrogen which can be stored to be used later as a fuel. I was not able to find any other references to this idea on the internet, which makes me suspect there is something else going on which makes the idea impractical for why other people haven't independently come up with the same thing and developed it further but I can't think of anything except for the hydrogen gas generated being an explosive hazard (but that can be stored/vented away so shouldn't be a big issue).

So, is this idea viable, or is there something big I'm missing?

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2 Answers 2

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You have referenced a process with a Co-efficient Of Performance of 0.25. According to Wikipedia, "Most air conditioners have a COP of 2.3 to 3.5"

You suggest that, if the energy could be recovered from the separated hydrogen and oxygen, you could get an improved COP.

If what you want to do is "cool a room", electrolysis of water is an extremely ineffective way of doing so, even with the referenced catalyst.

There are other similar cooling systems that work on chemical association/dissociation rather than phase change. It's not a crazy idea. And, as you suggest, hydrogen generation is currently an area of high interest. But using this process for room cooling is not something that, at present, would interest anybody who actually wanted to cool a room.

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There's a reason no one has heard of this

  1. Electrolysis doesn't absorb heat from its environment. The change in energy state between water and hydrogen/oxygen gas is equal to the electrical energy you put into them. If we could create hydrogen gas via cooling our surroundings we would have discovered essentially free unlimited energy. We can't, and we haven't.

  2. Putting hydrogen and oxygen into your home would be a terrible idea. This would be the equivalent of using an open methane pipe to cool your home. Even if it worked you're introducing a highly flammable gas to your home. The nickname of the oxygen generator (which used electrolysis) on my old submarine was nicknamed The Bomb for exactly this reason.

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  • $\begingroup$ 1. is not correct, electrolysis does indeed absorb heat from the environment as a portion of the energy it uses to break up the molecule, and it actually absorbs a greater fraction of the total energy as heat the higher you raise the temperature, see: en.wikipedia.org/wiki/High-temperature_electrolysis . At room temperatures roughly 20% of the energy comes from heat in the environment rather than electricity. I agree 2. is a good reason for why this is a bad idea and I was wondering if there were any others. $\endgroup$
    – Hadi Khan
    Sep 27, 2023 at 19:06
  • $\begingroup$ Quote from the link: High temperature electrolysis is more efficient economically than traditional room-temperature electrolysis because some of the energy is supplied as heat, which is cheaper than electricity, and also because the electrolysis reaction is more efficient at higher temperatures. In fact, at 2500 °C, electrical input is unnecessary because water breaks down to hydrogen and oxygen through thermolysis. $\endgroup$
    – Hadi Khan
    Sep 27, 2023 at 19:06
  • $\begingroup$ @HadiKhan "electrolysis does indeed absorb heat from the environment as a portion of the energy it uses to break up the molecule," I can find no source to back up this claim, and to absorb heat from a room it needs to be at a lower temp than the room so high temp electrolysis is not applicable. $\endgroup$
    – Tiger Guy
    Sep 27, 2023 at 20:01
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    $\begingroup$ I literally gave you two sources, one in the question: nrel.gov/docs/fy10osti/47302.pdf and one in my comment above: As stated, splitting a mole of liquid water to produce a mole of hydrogen at 25°C requires 285.8 kJ of energy—237.2 kJ as electricity and 48.6 kJ as heat. You can do this experiment too, and I have, it does indeed absorb measurable heat from the environment to the point of lowering temperatures nearby (very slightly in my case as it was a very small scale operation, and you need to make sure you don't give more than 1.23 volts of P.D.) $\endgroup$
    – Hadi Khan
    Sep 27, 2023 at 20:12
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    $\begingroup$ See also: pubs.rsc.org/en/content/articlehtml/2022/ma/d2ma00185c : This translates into a thermoneutral voltage of 1.48 V. If an electrolyser works at 100% efficiency (i.e. at 1.48 V under the standard conditions), the heat generated from the electrolyser equals to the heat needed for the electrolysis to proceed, and therefore, a thermoneutral situation is achieved. If the voltage is below a cell voltage of 1.48 V (but above 1.23 V), the electrolysis cell acts as a refrigerator continuously absorbing heat from the surroundings. If the voltage is above 1.48 V, excess heat is generated $\endgroup$
    – Hadi Khan
    Sep 27, 2023 at 20:19

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