Solar sand/thermal mass battery in combination with RMH

Has anyone combined a solar sand/mass battery with a RMH?  During construction the addition of a heating element in the mass of the RMH would be an easy addition.

You can try here https://permies.com/t/206700/Sand-battery-thermal-mass-proto

Dry sand has proven to be a pretty good insulator, and pretty poor at heat storage,(getting it uniformally heated up) vs brick/stone and other solid material.  Although It can be transportaed and placed fairly easily. that does not make it that versistal for heat storage.  And of wet sand works great up to the boiling point of the "wetness" But that is a whole different can of worms.

Pebble banks, for much of the same reasons. Both have similar claim in that they are easy to build with.

Finland's, "Polar Night Energy", has been online since 2022 and is heating a neighborhood. Their system is far larger than a home RMH but the use of the radiant heat from sand storage  seems to be a viable and proven heat storage source. One wouldn't be generating electricity in a RMH but just heat storage to bleed off during the night. In an insulated vessel high temperatures have been able to keep sand at 600 degrees for over a month.

I do think Scott has a point here.
Sand is cheap, available and easy to work with.
But it’s also somewhat of an insulator. So apart from the heat capacity (1200 kJ/(m3*K) in sand vs 4200 kJ/(m3*K) in water)the question of how to heat it up is an important one.
I don’t know how the fins took care of that. Maybe pressing air through the sand?
There’s a reason a lot of heat storage happens with water. Not the best conduction either, but when you heat some it rises so your heater can continue heating cooler water. And it has a big capacity.
Is water an option for your case?

Or is your question rather whether anyone ever installed an electric heat source in their RMH mass?

Benjamin Dinkel wrote:I do think Scott has a point here.
Sand is cheap, available and easy to work with.
But it’s also somewhat of an insulator. So apart from the heat capacity (1200 kJ/(m3*K) in sand vs 4200 kJ/(m3*K) in water)the question of how to heat it up is an important one.
I don’t know how the fins took care of that. Maybe pressing air through the sand?
There’s a reason a lot of heat storage happens with water. Not the best conduction either, but when you heat some it rises so your heater can continue heating cooler water. And it has a big capacity.
Is water an option for your case?

Or is your question rather whether anyone ever installed an electric heat source in their RMH mass?

Thanks Benjamin, if I may, I will explain a bit more, where I feel projects go from basics to complex, and maybe I am missing something, but in general, this is often how it goes.

if we consider our base heating system being used  in of the latest stove designs, which heats our mass in as effecient method as we possibly can, and we have proven all over the world that we can do this with NO mechanical means.

From this, we as rocket designers-users-even dreamers, start looking at additional storage methods, thus the name of this thread. Very quickly we note IT CAN BE DONE, but just as quickly find in most cases that it involves, a vast addition of mechanical means.  I bring this up, for one reason only, that being, I was not trying to say it can't be done, but rather if it is attempted, beaware that it can easily approach a level of no return on investment.

All great ideas start somewhere,  I just wish some of these ideas go down the practical path first, then get cleaned up,  Rather than proposing a dream, and spending years to convince others that it just maybe, quite possibly, with greatest of hopes, can work.

Sorta like spending $10K to save $1K, we often see in many simple ideas that have grown into complex ones. Another proof of this is the very often "showed on the internet" technolgies that promise great things, just as soon as they get
1) funding
2) figure out a way to make it work for the masses,
3) more funding,
4) and get technology to let them do what they proposed.  
     This of course is not true in all cases, but is generally par for the course.

Best of success!

With a RMH one would have the mass to utilize as a sand battery. The installation of a heating element would be a simple step.  
The reason sand is used because it can accept the high temperatures that makes it dangerous to use water. There is no degradation of the sand. There is no need for a pressurized vessel. There is no need to make this complex. The addition of a thermostatic switch could regulate the input if there is a danger of the battery getting the mass to hot for the RMH in home use. The cost would be minimal, an industrial immersion heater 25.00, thermostatic switch could be as simple as a water heater switch 10.00.  Scavenged items could be used. The heating element from an oven could be placed between a course of bricks.
Heating sand as a battery has a proof of concept with the Finn's.
Now stepping back and looking at a RMH the simple addition of a heating element, switch and solar panel wouldn't be an over complicated or expensive addition in making use of mass already being put in place. I keep saying sand battery but the cob, brick, masonry construction of the RMH would fall into my sand battery description, perhaps "thermal mass battery" would be a better descriptor. There is no question the material of a RMH is already proven to be an effective radiant heater.
Less than 50.00 dollars in parts and a solar panel would be an additional method of storing heat in a RMH that would require no monitoring or feeding of the heater when one was absent.
The use of cob or masonry in the construction is also an insulator I don't want to dismiss the idea of my using sand battery in my query just as it is an insulator it is also an effective storage medium.
Using a solar panel or wind turbine to heat up a resistance heater in a mass that is going to be there anyway is not a complex, difficult or an overly expensive thought.
 The Finnish studies show that a sand battery can retain 95% of the heat for several days proving that not only is it an insulator but an effective storage medium. One wouldn't expect the uninsulated heater's mass to retain heat for that long.
The ability to use solar or wind to charge an existing mass for heat that would be minimally invasive and not a difficult addition during construction is my premise.
We know the mass is an effective radiant heater.
During construction the installation of a radiant heating element would be an inexpensive and easily achieved addition.
We know that sand/cobb is not only an insulator but a proven heat sink.



https://www.bing.com/videos/riverview/relatedvideo?q=Are+sand+batteries+effective&mid=EACAD0EE4F9E69C5BCB8EACAD0EE4F9E69C5BCB8&FORM=VIRE

Robert Ray wrote:Has anyone combined a solar sand/mass battery with a RMH?  During construction the addition of a heating element in the mass of the RMH would be an easy addition.

 I'm missing something here which strikes me strange since I've been off grid 18 years and interested in other power for near 40 years.

 What purpose does a heating element in a RMH serve? Using the RMH to to heat the sand and using it as a heat battery works and TEG's in the RMH to produce power from the heat might make sense.

One would have a storage vessel in the mass of the RMH. A redundant way to heat that mass that requires no physical presence or labor with a small financial cost. It would be off grid. It would not interfere with the operation of the RMH. Stacking functions, an additional method of heating the thermal mass. I am just looking at storing the heat not using the heat to produce anything other than charging the thermal mass for heat.

The use case is this:

On a sunny winter day your batteries are charged by noon, all the afternoon’s power is wasted. You can set up the heat battery to only run when the batteries are full (another $50 in parts) and you will have free heat.  Less wood to burn and a backup that keeps your house from freezing while you are gone for a couple days.

R. Scott exactly. If your already running solar the RMH heating element would be a place to dump excess charge. If you're not on solar a single panel would be able to heat the element when not on site. A thermal switch would isolate the element when not needed.

Ok, I understand the use case.

How big is that heating element? I'm trying to figure out how much mass you could actually heat up with this setup. The ones I have in mind (for heating water) are like 8" long and 2" wide.

I imagine it would depend on the construction of the RMH. An oven element placed between a course of bricks would cover the most area and are usually over 3000 watts. PTC's are a low draw and quite small. Commercial immersion heaters vary in length and diameter; water heater elements are the same. The greater the area of contact would be ideal, but the immersion heater or water heater element would still heat the mass.
We can surmise that a water heater element can heat 50 gallons of water to over 100 degrees and it takes more energy and time to initially heat 50 gallons of water than 50 gallons of sand. Let's assume that the mass of a RMH is in that 50 gallon size.
If one is running off of an established solar charging system and are just dumping excess from when that battery bank is full, we'd have a figure that we could draw from. If we are just running off of a single solar panel it might be a bit harder to get a number to plug in a formula.

Robert Ray wrote:Finland's, "Polar Night Energy", has been online since 2022 and is heating a neighborhood. Their system is far larger than a home RMH but the use of the radiant heat from sand storage  seems to be a viable and proven heat storage source.

While heat storage in sand can be done, it's just not as simple as most people imagine.
I looked at the "Polar Night Energy". And their system of getting the heat into the sand is definitely more sophisticated than putting a resistance in a pile of sand.

Quote from their website:
"We use electricity from the grid or local renewable sources like wind and solar. The system charges when clean, low-cost electricity is available. Electrical energy is transferred to the storage via a closed-loop air-pipe system where air is heated by electrical resistors and circulated through heat transfer pipes."

Benjamin, we have the RMH mass. what is the difference in charging that mass with fire and a heating element. The addition of a free stacking method to heat that mass that requires no labor input or fuel after install. Let's forget sand battery since that seems to be an issue and call it thermal mass. A RMH's mass is a proven heating system the hiccup seems to be how we are charging the mass. The Polar Night system I reference was to show that a sand battery is in use, but it is using an entirely different process to use that heat. Charging the RMH mass is an elementary process.

The biggest difference I see is that the fire provides it's own method of "charging" the mass. It leads hot gases with some movement through the mass and has a lot of surface area to pass the heat energy on to the mass.
I don't see that when we're talking about a resistance heater. Unless we're talking a resistance and a fan, pushing hot air through the mass.

I'm not saying it's impossible. But it seems like no one here has tried it, which is what you were first asking for.
Please share your results once you have tested a system.

This idea is explored pretty thoroughly in the series of Low-Tech Magazine articles as in here:

https://solar.lowtechmagazine.com/2025/11/winter-is-coming-build-a-solar-powered-foot-stove/

Incorporating the DIY heating element as shown in this article:

https://solar.lowtechmagazine.com/2025/10/how-to-build-an-electric-heating-element-from-scratch/

Benjamin, the heat transfer of a resistance heater in the battery mass would be through conduction, no need for the addition of a fan.