Robert Ray

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

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.

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.

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.

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.

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

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.

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 have a peeler that will cut very fine julienne strips and ferment those. It is an excellent garnish pickle for sandwiches.

.