Feasibility of lead as phase change material for a high temp thermal battery 2

[Furious.George]

I'm looking into making a thermal battery. I'd like to be able to heat a large block of mass, probably concrete based, to 500 C above ambient. Unfortunately, I don't know much about materials science. I'd like to know more, so to that end I've been reading peer reviewed research on the performance of various mediums for thermal storage. (Citations upon request

Water, paraffin, glycols, and concrete are commonly used, but they all have their shortcomings. For instance, concrete holds a lot of heat, but is a poor conductor of heat. Paraffin holds less heat, but is more efficient at lower temperatures due to the phase change. It also conducts heat poorly. One way to make concrete more thermally conductive is to add paraffin, in order to take advantage of the phase change. One way to make paraffin more conductive is to add aluminum (in the form of aluminum wool). Both of these scenarios have been tested and seem fairly easy to replicate. so I thought I'd give it a shot.

Then it occurred to me: why wouldn't lead work in lieu of paraffin and/or aluminum, for higher temperatures? I know it has a larger coefficient of expansion than most materials, but I thought it might be possible to slowly heat the block while it is drying. Hot enough and the lead would melt, presumably boring out a larger cavity for itself to accommodate future expansion, and without cracking the concrete and injecting air into the system.

Does that sound right?

 

[MintJulep]

There is the whole "it's toxic" thing to think about.

 

[Furious.George]

Thanks, MintJulep

I forgot to bring up that not far from where I live currently a ball field was installed, and somehow the artificial turf had enough lead in it that the field was never used and people in the neighborhood said that lead dust would accumulate on their window sills. If I'm not mistaken, a couple of them got sick and sued.

In the scenario I was curious about, the lead would be embedded in concrete in the form of pellets or flakes, which would in turn be in a vessel, which would be insulated and buried. That said, I wouldn't want to mess with it if there were a chance that the vessel would rupture and pollute everything. If it would rupture and the lead would just stay in the concrete that would not be so bad, but I don't presume there is a way to be reasonably sure of that, given how it expands with heat, and how concrete tends to crack with heat.

Perhaps a dumb question on my part. It's difficult to Google, because even when your trying not to, you get a lot of results about lead-acid electric batteries.

 

[IRstuff]

I think you missed the totality of MJ's point; it's not just about the immediate usage danger of toxics. There are accidents and natural disasters like Fukushima, where a massive flood resulted in a complete loss and meltdown of a nuclear reactor; there are also disposal questions, at some point in time someone wants to demo the installation and is left to dispose of hundreds of pounds of lead. Sustainable design is not a single point in a lifecycle of a product; the entirety of the lifecycle needs to be sustainable. Blending the lead directly into the concrete exacerbates the issue.

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[Furious.George]

Thanks, that makes sense a lot of sense. Even without much context, his post actually did get me thinking of a small earthquake we felt in the area a few years back, and Fukushima.

Even paraffin embedded in concrete will give off some toxic fumes if heated enough, and so might not work for similar reasons. It would be efficient if it could work safely and reversibly, as that would be two phase changes. This is probably why molten salts are popular.

UPDATE: Found a good article on molten salts, but having said I've also been convinced that the first iteration of anything I try should be much simpler e.g. just gravel.

 

[EdStainless]

Molten salts are the go to material for solar thermal plants.
Na and K nitrate salts are the most common (and least corrosive).

https://www.solarreserve.com/en/technology/molten-salt-energy-storage

You want a material with its phase change near the bottom end of your operating range so that as it cools the latent heat helps extend your use, but it still needs to be stable at the top end.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MortenA]

lead is also pretty expensive

 

[Furious.George]

Thanks for the input, guys.

Google's AIs might be in my head, because an article came through my feed about a thermal battery using molten silicon, which was recently commercially released.

 

[georgeverghese]

That article on silicon based thermal energy storage requires melting silicon at approx 1500degC. That would be useful when you've got excess electrical power to store. Higher operating temp = greater energy recovery eff.
A search on google tells me a lead - tin mix, one recipe for which is used in solder melts at much lower temps,which may be more practical for recovery from lower grade waste energy streams. See also this E-tips thread:

https://www.eng-tips.com/viewthread.cfm?qid=74700

Lowest melting point is at 62% Sn - m.p. 183degC.

But if you are not interested in high density energy storage ( solid - liquid phase change)at these lower temps, any one of the easily available industrial inert heating oils would do for low pressure storage, for example any one of the Therminol HTF's from Monsanto.