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Forum Index : Solar : Phase Change Materials (PCM) as heat storage in floor

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orangezero
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Joined: 12/04/2024
Location: United States
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Posted: 02:41am 04 Jul 2024

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I'm curious if anyone has experience with phase change materials. I ended up getting a 40lb bag of sodium sulfate, which is a salt (glauber's salt) often discussed as a phase change material. Pretty cool stuff, literally. I mixed up 1liter of water, 240ml of sodium sulfate, and 60ml of sodium chloride, as well as some xantham gum. Added heat and stirred a while. What you get is a gelled salt solution that melts at around 65f.

This also means as it is melting the temperature of the solution stays right at 65f. I have a bag of it that was on my concrete basement floor "recharging" for free and now I have a bag cooling down my knee. I've seen vests of the stuff used in industry but is often rather expensive. I have found sodium sulfate for less than $1 a pound, maybe less than that in larger amounts. Most of the cost is in shipping.

In any case, if I would add less sodium chloride the melting point would be higher. I could mix up a batch so the melting point is around 80f, which appears to be more ideal and safe long term for my hardwood floor than going higher. My plan would be to mount bags of this stuff (mylar, vacuum seal rolls, etc) underneath my floorboards in the ceiling of the basement, then add insulation underneath.

During each day my solar panels send heat to these bags and heat them up to 80f, but as they absorb more and more energy they stay at 80f. I'd have a cutoff around 83f or so, because it would start to rise after all of the salt had melted and the sensible heat storage is not nearly as impressive as the latent heat.

I've also been reading that borax may help deal with any supercooling issues. The xantham gum helps keep it more in a gel form, which is supposed to keep it more uniform throughout and prevent degradation of heat capacity over time.

I'm not entirely sure if it ever goes bad. It appears more that it would need a good shaking up now and again, rather than throw away and replace. Or maybe I'd need to heat it up and remix.

Downsides... well, it is still hard to get my head around this. Most studies seem to want to pump water through a pcm and supply heat that way, which gives you some ability to adjust the temperature. My goal would be more to decrease the heat I need to add to my house each night. This pcm seems to act as a time release mechanism. I can foresee this getting my floorboards up to 80f and staying there, as the heat is slowly leaving the floor up into the cooler air. The only thing moving in my case would be electrons flowing through a cable. No fans or pumps or anything else.

I think from what I've read, the end result of X amount of pcm with X kwh of solar input would result in a more steady 80f temperature for a longer amount of time into the evening compared to a tank of water that is limited to 80f. One study I read said their storage volume was around 2.5x less than a similar amount of energy storage in water. This weight/volume savings seems ideal for my potential application.

I know another salt used is calcium chloride, which is also used to de-ice roads and isn't as expensive during the right time of year.

Thoughts?

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