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.

I am hoping one of the many that support this idea can give the rest of us a report.

Simply take a steel container, a 20-30 gal oil drum seems like it will work for this test.  if your in north east Iowa, I would give you a barrel or three.

Please pick out a location for your resistance wires/rod/element, what ever you want to call it. And please do tell where you put them  in your mass,  And then fill the barrel up with sand.

At this point, plug into your solar panel and let it start charging, (passing voltage to the element)  or if you want, you buy a 110 volt water heating resistant element as many of you stated these are only 10-15 dollars.  Plug it in and let it rip. Make sure you record temp of sand at bottom, middle and top,  And if you like, let us know of which method you used. What size you used and what the dry sand weighs, that way we can easily figure out how many BTUs your bank is storing

Let us know the results after one day, then two days and so on.  Make it scientific, your test could help create world peace.. So this will require nice dry sand as wet sand might be not safe and possibly a bit variable due to temp variations,  sand grade or size would also be nice, so others can duplicate when it works well for you.

And at the end of next week, you should be able to post the results.  I would think sand weight, sand height, and volume will also help.  Keeping it simple as stated on this forume,   would be is very important. No need for switchs, just let the mass storage begin and then report NOT what is thought should happen  but what it really did happen.  

We will wait for the results. and please let us know.

Electrical storage heaters  heat ceramic bricks during times of day when electricity is cheapest:
https://en.wikipedia.org/wiki/Storage_heater

This is old tech.
Coupling it with PV solar is old tech
The difference is a rocket stove heats through radiation and a storage heater will often have ducts and fans as well.

So yes, it would work, but I doubt it will work as easily as we might hope.
For example, most electric water heaters in the US use 120vac.
Most ovens in the US use 240vac.
The water heater thermostatic switch will probably not work well with coils from an oven.
Of course ovens come with their own, so we could salvage that.
Also you could use probably two of them(I think water heaters generally have two) and switch each leg of the 240V circuit.
We can install the coil in the base of our bell and the thermostatic switch into the wall of the bell.
Hmm, the inside if a rocket mass heater bell is way hotter than
The inside of an oven or the skin of a water heater.
No matter, we will either embed the coil and thermostatic switches in the walls to protect them , or get higher rated components off of the Internet.

Now to power the thing.
I would assume we would wire it into mains power, since solar produces DC voltage and we need to rectify that over to AC voltage for the sake of the components we are using.
There will be loss, but we are talking about storing otherwise wasted energy, so it's not really a big deal.
PV solar often uses water heaters as dump loads, so I imagine there are off the shelf solutions we can use to ensure we only fire up the up the coil when we have excess solar pv.

Great ideas from great minds, I offer kudos to all involved.

Could a person use 12v  instead of including an inverter for heating?  For example, this truck stop special is tiny but at 4.99......could a few of these  perhaps give equal heat?

I should have quoted from B, his recommended reading turned up this

Many commercial heating elements have built-in thermostats, which can complicate temperature regulation inside the oven. They also require a voltage input that does not align with the voltage output of most solar panels, which introduces the need for an extra electronic component (a buck converter). Securely fixing commercial heating elements proved to be difficult as well, and we had trouble keeping moisture away from the electrical system, which at one point resulted in an electrical fire. By embedding a self-made heating element in a mortar base, we solved all these problems.

Mea culpa, I should have added details of the "5 below" coffee cup heater, it draws 130 to 140watts while boiling water.  It can boil a pint of room temp water "pdq" , for lack of a precise data point.
I only offer this for a generic example of a DC heat coil, not as a reliable substitute for 120V heaters.  Any experience with this idea, among the faithful?

William,
AC resistance heaters will heat up with DC voltage. A water heater switch affixed to the mass of the RMH in a similar fashion to how they are affixed to the water tank, the back temp sensing portion pressed against the mass wouldn't care if it were AC or DC either. Sensing temperature on the bench portion of the RMH and shutting it down at a safe temp..
We wouldn't be within the bell we would be imbedded within the mass part of a RMH.
We could loop into a solar array that is dumping after the battery bank is full, or be using a direct lone panel or two
Moisture concerns within the RMH shouldn't be an issue after the heater has run a few cycles and cured.
Imbedded in the mass the element through conduction heating the mass allowing the mass to radiate after it is charged.

I know it has only been a few days, but did anyone decided to heat up their sandbank with excess wattage from there solar panels?

Is your typical panel around 350 Watt with a  typical excess planned for about 3 hours per day?   or about 1050 watts per panel if you have that much excess.

So this is about 3500 btu's gained per panel if you have that much excess or about 3/4 of a pound of wood for heat production per panel

 My engergy coop does not allow for us to install EXCESS production, unless we are off grid.   So we have NO excess that could be harvested per say.   But it sounded like a lot of you, had excess wattage, and figured it would be easyily accebbible,   So I was wondering how the sand was warming, and how warm the sand was getting.  

Would like to see the results.

I don't have a RMH, but I do want to try a sand battery heater in the green house.
My test will be creating a sand battery 2ft x 2ft x 12 ft corrugated sides, to simulate the mass of a RMH Two courses of bricks at the base, the remainder will be filled with sand.
I picked up the bed materials today from Home Depot. Spent some time with Jeff Besos ordering heating elements from Amazon today.
My experiment will be placing the different elements within the central 4 feet of the bed.
The heating elements I ordered consist of a broiler element, a water heating element, an industrial immersion heater, a clothes dryer heating element, an eight inch stove top element, 6 PTC elements. So about 150.00 dollars worth of different heat sources that will be placed within that central 4 feet of the 12 ft bed.
I will be using an EL-USB-1-PRO temperature logger set to read at 15 min increments.  I will use three candy thermometers  pushed into the sand to monitor visually. I have a laser thermometer to check side temps from top to bottom. I only have one day off a week. That one day I will run un-switched to see how hot it will get. 130-140 seems to be the upper limit of a comfortable heated seat so that will be my temp setting when running through the switch.
I will try two different switch types, a water heater switch and a stove bourbon tube style. I will run direct from the solar panel to each element, switching to a different element after a seven day run.
Should I create a void down the center to simulate the RMH chimney?
If nothing else I hope to use the sand battery as a heated seed starting table during my trial run.
Any suggested additions to the experiment?
I do have a 500 watt wind turbine that I considered hooking up to wring some energy out at night but think I'll hold off for the initial trials.

Robert Ray wrote:I don't have a RMH, but I do want to try a sand battery heater in the green house.
My test will be creating a sand battery 2ft x 2ft x 12 ft corrugated sides, to simulate the mass of a RMH Two courses of bricks at the base, the remainder will be filled with sand.
I picked up the bed materials today from Home Depot. Spent some time with Jeff Besos ordering heating elements from Amazon today.
My experiment will be placing the different elements within the central 4 feet of the bed.
The heating elements I ordered consist of a broiler element, a water heating element, an industrial immersion heater, a clothes dryer heating element, an eight inch stove top element, 6 PTC elements. So about 150.00 dollars worth of different heat sources that will be placed within that central 4 feet of the 12 ft bed. .

That is fantastic and sounds like you will have a excellent systematic way of testing the results.  I am sure there are many that will await your results. Please do keep us informed.  Best of success!

If you have a wind turbine, that is an excellent dump load for the winter

Robert Ray,

it has been about a week, but don't know if you started any part of the experiment, if so, any hot sand showing up? Enough to heat a room?

Scott

The final heating elements (PTC) were delivered today. So I will begin this weekend on the assembly.

So project moving slower than I had hoped.  Here is the components I will be trying, left to right. 8 in stove element, bottom left 4 PTC heating elements 110-volt, thermostatic switch, 240-volt water heater element 18 inches long, industrial 240-volt immersion heater, broiler element, top far right silicone wire for inside the sand battery to take the heat, bottom far right 36-inch 240-volt dryer element coil. material for sand battery underneath.