RMH with Sand Battery Hybrid?

Hi friends, first post here and I imagine I'll be around for a longer time since all of these topics hit on in permies are hard to come by in social media platforms.

I had a question as it relates to Rocket Mass Heaters after coming across it and another concept called Sand Batteries while thinking of the RMH build.

I saw RMH builds that have a sofa-like structure that comes off the RMH. Does heat radiate from this and does this whole structure intend to serve as the heat distribution center? Is there ever a "too hot" moment that needs to be considered?

Another thought is could this RMH sofa be filled with Sand and operate dual purpose to radiate heat from the cob structured lining of sofa while having electrodes to pull heat away from it to a real battery? Is it now less efficient to use this heat stored into a battery back through electrical heating gadgets? How to ensure heat distributes an entire barndominium from an RMH starting point?

Is this a good idea, bad idea, or just not feasible? I'm new to both concepts but the potentials are intriguing. I'd someone could point me to a more modern fundamental design of RMH that I should learn from I would appreciate it and any thoughts to this idea or how to heat the house. Thank you!

Hi Ryan,

Welcome to Permies.

The "sofa-like structure" is referred to as a bench, and is a primary part of the heat storage and distribution. It could be possible to get the surface of a mass bench too hot for comfort in spots, which is why there are guidelines for how much cob thickness there should be over the ducts - more near the combustion core and less at the end of the duct run.

You would not want to use sand in this, because sand has millions of tiny air spaces and slows down heat absorption and transmission. Think of a beach on a sunny afternoon - the top is burning hot, but dig down a few inches and it is cool. Cob, stone, brick, anything noncombustible and dense will work for the thermal mass.

The J-tube combustion core with ducts running through a mass to capture heat is the original RMH. There are a few important alternatives developed since, mainly the batch box which allows faster burning and more concentrated heat delivery to the mass, and the "bell" or stratification chamber, essentially a hollow masonry box which absorbs heat efficiently with very little friction allowing easier draft. Many other details are possible if you want to get into the fine points.

I doubt that it would be worthwhile to try to extract electrical energy from the RMH mass. I expect there are more efficient and effective ways to charge batteries.

The only real place for heat collection into an earthen solid state battery bank would be the bell section of the rmh. Everything running through the bench section would be temperature deficient for such use. I agree with the above poster about the sand acting as an insulator, however I have seen a brick energy bank in use and it's pretty impressive.

Scientific American
https://www.scientificamerican.com › ...
Bricks Can Be Turned into Batteries

Bricks or cobbles which allow free airflow through the bed could indeed be a viable thermal battery. I was contemplating a cobble bed for heat storage when I started planning my own house out of architecture school in the 1980s, as there was information available even then on designs for that.


Either a bench or a taller bell could be arranged as a cobble bed for direct thermal extraction and storage. A bell is not inherently at a higher temperature than the channel in a ducted bench, though that may often be the case. The pebble bed as used at Wheaton Labs is an indirect medium, as the heat radiates, convects and conducts from the ductwork and then is absorbed by the pebbles.

I'm not sure I understand how it would not be possible to combine the several concepts that I will list below.

I'm no hvac nor energy conservation guru but the concepts seem like they should be possible to combine for maximum harvest.

1. Rocket Mass Heater design with stove top.
2. Sand Battery for stove top and energy preservation https://youtu.be/ejTsdtZHRPw
3. TEG (thermal energy based) fan to distribute heating while being on top of the stove top sand battery. Best materials for sand storage and insulation of sand storage TBD. May also be able to create leads to store in a different battery if/as desired. Portable sand batteries possible.
4. Extraction of creosote and more refined fuel in iterations for later biodiesel usage. https://youtu.be/5dbvJ3oXYoc

Am I overthinking or not thinking clearly about this? Appreciate your input.

Why the focus on a sand battery? Yes they can work, but sand is a mediocre thermal mass and a mediocre insulator - not the best at either job. A solid material that is at least as dense as sand can probably hold more heat than the same volume of sand, absorb it faster and more effectively, and if partially or variably insulated on the outside, hold it as long as sand.  The main advantage I can see to sand is that it is easy to place, and can easily be removed if desired.

You can have some sort of a stove top on the roof of a batch box, or on the barrel over a heat riser if you go that route rather than a masonry bell. A Walker masonry stove with glass top is made to be good at cooking while also storing heat if desired. Yes, you can put a Stirling engine fan on top of a RMH radiator.

Extraction of creosote will not be a thing. There is no combustible material left after a proper RMH burn.

I think the industrial-scale sand batteries in Finland are viable partly because sand of the grade used is very cheap, and when you are using hundreds of tons, material expense is critical. With some sort of distributed charging mechanism laced through the mass, they can effectively use it.

On a home scale, the material expense is minimal compared to the surrounding infrastructure. To get to the red-hot temperatures referenced in the big projects, you would need sophisticated mechanics and it would likely not work as effectively. Scale matters in many cases, and this would be one. At household scales, I believe other techniques would work better.