So crazy it might work?

Hi all, and thanks in advance for reading this--this is an out-there question.

I saw on another forum a crazy idea for potentially massive heat retention from a RMH.  The idea was to incorporate--somewhere, I am not exactly certain where--a container of solar salt that would melt under the operation of a RMH during the burn phase and release it back as it froze again.  The salt in question would be a type called Solar Salt and is in fact used in large thermal-solar plants as a way of storing energy over the nighttime hours.  Molten salts have a MASSIVE potential for storing heat, thus potentially increasing the time in which a RMH could just sit idle, radiating out the heat created during combustion.  As I said, I saw this on another forum and it got just a bit of attention, but with none of the expertise that exists here.  I should probably say right now that while I find this a very interesting concept I am not about to go out and do it myself.

So what do you all think?  The melting point for Solar Salt is something like 500F.  Would this even be technically feasible?  If so, where would it go on an RMH?  There are a couple of other salt mixtures that have even lower melting points, perhaps they have potential?

Again, I am not trying to advocate for this directly, but I thought it an interesting thought experiment--a way to really extend the "idle" time in which a RMH passively radiates heat created during combustion.  


Any thoughts?

Eric

Hi Eric;
Great question!   I had to look up information about exactly what solar salt was... figured it wasn't the water softener sold at walmart.
Here is what I found.    
Today, molten salt (solar salt, 60% NaNO3, 40% KNO3) is recognized as the most successful material for solar heat transport. This eutectic mixture melts at 220 °C and remains a liquid at 290 °C in an insulated storage tank, called a 'cold tank'.

Ok cool stuff!   I did not read the whole mind numbing article, so I'm sure I do not have the complete picture.
Here is my take on this.

The only place in any RMH where you will reach  550 F  is directly over the riser.    
So ss starts melting at 428F  and is a liquid at 554F . Easy to reach and maintain those temps.
It remains a liquid in an insulated tank.   I wonder how long it would retain its heat in an uninsulated container?

So with just this tiny bit of information I see this as not very practical in a RMH setup.
I am envisioning a tank sitting on your barrel or bell directly over the riser.
How big a tank?  Well a second 55 gal comes to mind.  
So if we use that we would need to know expansion rates and also how slowly or quickly that 50 gallons of ss will share its accumulated heat?
Next question is weight?   How much are you sitting on top of your barrel/riser?  
I'm also thinking that in a dyi home situation having a 55 gallon barrel of  500F liquid could be hazardous situation.

So professor;
My synopsis is,   In an industrial application this is a fascinating product. As a RMH addition its dangers outweigh the benefits.

I'd be concerned that the "warm butt" portion of the rocket mass heater would become a less tenable proposition.

But...
A bank of solar salts in the basement with an option to turn a fan on through a salt to air heat exchanger has potential,

From what I've read about them Solar Salts are more than a little corrosive...so a suitable vessel is probably not an economical proposition.
But a long term payoff may be possible, if you build it in your youth, or it's a generational proposition.

As far as mechanical execution, Perhaps a rocket heater built into the base of a concrete (perhaps with additives to slow the corrosion) containment, a stainless barrel above the riser in intimate contact with the salts proper. A tube type heat exchanger built into the top of the container. Probably some venting to address expansion and off gassing. A t stat to control the fan...

Thomas,

Glad you could see the potential, even if not practical.  I was also thinking about putting it on top of the riser.  And I don’t know what expansion/contraction factors look like and that right there might actually be the deal killer.  Possibly it would lower the temperature of the bench (maybe) as a lot of heat will go into melting the salt.  I suppose if the container were made of some flexible, corrosion resistant material—maybe a titanium barrel—we could get an operating device, but I bet it just gets a whole lot cheaper to throw another log in the burn chamber than go through all of this.

At any rate, I thought it an interesting thought experiment.

Eric

Bill,

I actually like the way you are going with this idea, but unfortunately it stops becoming passive really quickly.  I think this is an inevitable downside of using a circulating fluid as a heat battery, but man is the idea ever cool!

Eric

This is a very interesting concept. I'm not quite sure where and how to apply it, but a meltable and flowable thermal mass has many possibilities. Not necessarily including molten salt nuclear reactors, which also have possibilities, IMO, cautiously.

That sounds like a phase change material.  There are lots of them (water being a common one).  All you need to do is find one that melts/freezes at a temperature that is convenient to your equipment.  

Mike, everyone,

Actually my first idea for improving a RMH was to somehow incorporate a tank of water, not to undergo a phase change, but perhaps to sit on the bench somewhere and soak up 100F+ temperatures to hold on for later.  I was thinking that with water’s amazing specific heat capacity, a large (another 50 gallon tank of water?) tank of water would be a great way to hold on to heat and release it over time.

I got the solar salt idea by wondering just how far I could push the concept.

In my hypothetical, I would only want the system to be passive—no pumping hot fluids, though going down that rabbit hole brings up some really fascinating ideas, but I wanted to stay as conceptually simple as possible.  I also realize that keeping 50 gallons of molten salt on top of the RMH riser would be a bad deal should it ever leak or tip over.

Bottom line:  I was wondering what would absorb heat better than cob/stone/masonry.  Solar Salt seemed like the ultimate example.  But please, if anyone has a better idea, I would love to hear it.

Eric

Tons of materials hold heat better than masonry, the question is if it's cost effective/simple/safe enough to do.  There's a long list of PCMs (phase change materials) that do their change at a wide range of temps.  Seems like many of them may work for your idea.

Phase Change Materials - Wikipedia

I don't have much to add here, but I wanted to say that I think this is a massively cool idea, and posts like this are what keep me interested in these forums.

I agree with Tom that this seems more dangerous and a lot of work for the relatively small benefit in heat storage capacity you get over masonry or water, but that's just like, my opinion man.

I've been listening to a podcast called the energy transition show that has mentioned concentrated solar power several times, this might be something that others here find interesting. You have to pay 5 $/month for full episodes, which I find well worth while:

https://xenetwork.org/ets/episodes/episode-132-the-future-of-solar/