PCM (Industrial Paraffin) for the Mass in a RMH?

Only discovered these a day ago. Still learning about them. Wondering if anyone has tried (or experimented) on using a PCM (phase change material) for the "mass" part of a RMH? Possibly paraffin, as being experimented with for solar heat batteries? (Or paraffin with graphite). As a way of using a lighter mass to store the same amount of heat energy. (I think I have read paraffin will store the same amount of heat as concrete at 1/13 of the mass. Not sure how correct that is). I believe the melting point can be formulated as low as 115 deg. F. What temp. does the outlet exhaust from the burn tube exit? I have read a couple different numbers, but I believe generally higher than 115 deg. F.? So, possibly enough for the phase change in the paraffin.  

Hi Rob;     Welcome to Permies!

So this is interesting.  
To start I will say that temps at the transition area are several hundred degrees maybe 300F-400F
They cool to hopefully 150F or so as they exit the mass /bell. In this scenario, you could not use a bell.

Is this a transition from a solid to a semi-liquid?
If so it would need to be contained.
The pipes thru the "mass' would need to be liquid-tight.
How does this wax compare with a much denser material than concrete?  
Heavy large stone is the preferred mass material to use, with just enough cob to fill any air gaps.

Tell us more.

If this could be made to work effectively I can see places it might expand the use of a rocket heater.

However, would it just "collect the energy of the heat output" or would it replace the big benefit of "mass" that slowly releases heat over time which is one of the big reasons people find RMH heat comfortable for living with.

A friend and I looked into a Phase Change Material for house temperature management years ago, and even though it appeared to help on paper, we found it couldn't compete in real life because to *really* hold energy, you have to get that phase change to happen, and most inexpensive materials we could access, have a very narrow range of temperature where that phase change happens. Out of that narrow range, you're really just back relying on it's mass.

That doesn't mean that someone with a really good workshop and the time to put into developing a system that works shouldn't give it a go. Even if you don't succeed, the knowledge gained could be valuable in some other application or some other use than house heating.

I really like the phase change idea for a lot of things and RMHs could be one of them.  The challenge as I understand it is to pick a material that freeze/thaws at a proper temp for your RMH.   Let's say 150F.  So it's easy to get it above that and that's a comfy temp to give off heat.  Maybe there's a better number but I'll go with that for now.

Once you get your RMH above 150F, the phase change material would start to melt and absorb the excess heat.  Eventually (more PCM means it would take longer) all the PCM would be melted and then its temp could go above 150.  That would be an indicator that you've stored the lion's share of the heat and you can let the fire die out.  Then the PCM would slowly cool off and "freeze" solid.  In doing so it would hold the 150F until it's fully solid, at which time it would then cool off to room temp.

I think water crossing the phase change point (211 to 212F) takes around 100x the energy of raising it from 93 to 94 degrees.  Glycerin is around 60x.  So you could replace all the traditional mass with, I'd guess, under 100 lbs of parafin.

Sorting out a design where you contain the PCM and have good heat exchange surface area with the exhaust gasses would be the trick.  Maybe an exhaust pipe inside a larger pipe full of parafin?

This idea comes up from time to time. I think one of the main things preventing people from trying it is the mess (and potential flammability of a paraffin leak).

But... I was looking at a greenhouse book recently where the people poured their PCM (I think they were using Glauber's Salt in their case) into a length of black plastic drain pipe and capped the ends to make it a self-contained unit!

I think a person could creatively lay several such pipes in their RMH bench parallel to the heat tubes and increase its heat storage capacity considerably.

It absolutely could work as the mass, but it is expensive compared to cob. It might be worth it if that means you can install it somewhere that can’t handle the weight.  Like second stories and RVs and tiny houses and trailers and…

Thanks for the input. I had read quite a while ago that some European wood stove manufacturers have been experimenting with this concept. (https://www.sintef.no/en/projects/2021/pcm-stove/)  But, maybe cost prohibitive? Industrial paraffin is presently about $1355/ ton. It would take no where near a ton though. The heat exchanger would need to be liquid tight as the wax does melt (hence, the phase change). I believe the paraffin can be formulated to work at the temp. needed for the exhaust gases. I am thinking of greater "acceptance" into the market if someone like Liberator Rocket stove produced a "accessory" for mass heat storage for their stoves with an "add-on heat battery". Something that could be adapted more easily into existing homes. The concept of these heaters is so exceptional, I don't know why they are not used more. But I would guess lack of knowledge that they exist, and the fact that they are site-built which some people will just not take on ... and is an impediment to acceptance with building codes and insurance companies. The paraffin PCM does work the same as a masonry mass, in that it releases heat long after the fire is out. It does it more effectively because of the latent heat energy. Was wondering if someone possibly experimented with the concept.  

How much would you need for a 6in RMH? 19gallon barrel? 30 gallon? 55 gallon?

I am thinking hard about using this in the greenhouse. Not sure how long it will take to make 55 gallons? lol