This is somewhat of a reality check request. I saw something the other day that reminded me of a Solar farm that used mirrors to focus on a tower that melted and stored Sodium and used that to generate steam for hours after the sun was down. it was a sale on swimming pool salt. package said "high purity" and only listed sodium chloride as the contents. $6 for 40 lbs.
that made me wonder if I could use something as basic and abundant as NaCl, regular old salt. not Iodized. im not sure what that would do under heat, but from what i've read there is no harmful fumes or products from melting salt.
melting temp is just short of 1500F. boiling point is 2670F with a rocket heater, I don't see the boiling point being reached, but i've seen numbers being thrown around that suggest a common operating temperature could generate enough heat to melt a pot of salt. of course the melting pot would have to be big enough to hold a sizeable mass of salt, and it would ultimately be replaced as it would be consumed by that kind of heat. I was thinking salvaged water heater tanks, or propane bottle halves. of course, that would require a barrel with a removable top, and quite possibly a replacement for the barrel much sooner than with a more standard RMH.
the idea of using a rocket heater to heat water comes up often, but the problem with that is that it only holds its 200F heat down to a "non heating" temperature for so long. but with salt, it liquefies at 1500F and so would release much more heat for longer as it cooled down after a burn.
say it took two hours to melt the pot, then it released its stored heat down to ambient temperature over the course of another two hours. im not sure how long it would take to do that melting, but that's why we test.
Yes, holding onto 1500F liquid for any length of time could be dangerous, but then so is fire. with all of the cob and brick that RMH are usually made of, surely with a little sealing between bricks they could contain a spill or leak.
so, correct me if I'm wrong here, but if a rocket stove was used to liquefiy salt, wouldn't that mean that it was removing that heat from the rest of the system and so would require a shorter system, perhaps just a burn tube, riser, a partial brick or cob insulated barrel covering the salt and directing heat toward the exhaust. im sure the only way to know for certain is to actually do it, but I can think a little faster than I can come up with time and money to build and test.
That is a life "truism" Thinking faster, than coming up with the time or money!
There is a discussion about this in another thread. I will attempt to locate it.
The gist of that chat, was that indeed this could be done .
The problem's that we came up with were the size of the salt tank, the weight of the salt tank , the placement of the tank could only be directly above the riser outside the barrel.
But most of all NOBODY thought having a sizable tank of molten salt in your living room was such a smart thing to do...
Melting salt and storing that heat is really an industrial application.
Rocket mass heaters are for your personal comfort.
Brownie points to you for thinking outside the box!
Gain enough brownie points and you get a rousing Atta boy!
Ha. another life "truism"... there's always a post somewhere else on any given topic.
I guess it's better to do alot of thinking first.
I think it could be designed to offer sufficient containment in the event of a spill. everything that might happen would be behiind brick, cob, metal barrel or insulation.
its funny that so many things are considered risky or dangerous, but we don't bat an eye at the regular RMH burn process that produces same amount of heat with which the salt can be melted. because we feel it is contained.
salt directly above the riser is what I had come up with sketching. acting as a sort of diverter around its vessel then acting as a normal RMH from there. i am currently only thinking of a system that would run outdoors to warm animals in the winter. during the recent super cold snap we had, we had roughly 20 rescue pups / dogs housed in electrically heated storage sheds. if the power had gone out in our area it would have been all over. if any molten salt spill occured, it would be well away from the kennels with lots of room for containment, even if it was just sand and gravel under the burn and riser areas. I am hoping to prove to my wife that a RMH is safe enough and cheap enough to use for the house to replace our ailiing pellet stove. maybe not with the salt thing until that can be shown to be effective.
I guess I just thought that the salt mass would act as a more compact mass for smaller spaces. it's surely not necessary.
i'd like to see that other thread if you can find it. I tried to search to see if one was there, but got bogged down by all the irrelevant results.
from the other article: "solar salt, 60% NaNO3, 40% KNO3"
sodium nitrate "Sodium nitrate is a white deliquescent solid very soluble in water. It is a readily available source of the nitrate anion (NO3−), which is useful in several reactions carried out on industrial scales for the production of fertilizers, pyrotechnics and smoke bombs, glass and pottery enamels, food preservatives (esp. meats), and solid rocket propellant. It has been mined extensively for these purposes. "
and potassium nitrate "Major uses of potassium nitrate are in fertilizers, tree stump removal, rocket propellants and fireworks. It is one of the major constituents of gunpowder (black powder)."
buying these in any quantity sounds like a good way to strike up a friendly conversation with the FBI and the ATF ...
The thing that comes to mind is: With vessel holding 1500 degree liquid is how is the transfer going to occur into the room? With the masonry mass it is a a slow release into the room. If I have a vessel heated up to 1500 degrees and it releasing that 1500 degree heat into the room what would that look like in real life? Sure it is a reservoir of heat but will it be an effective comfortable release or too much? Is the reservoir going to be surrounded by a masonry mass? If it is encapsulated in the masonry mass, maintenance on the reservoir? If the vessel is encapsulated in the masonry mass the projected surface temperature of the traditional bench? I would want to insure the RMH mass to be completely seasoned and dry. The expansion of steam that may occur should a molten leak occur could be spectacular.
What are your projections/ideas on those questions?
Our inability to change everything should not stop us from changing what we can.
I had envisioned a typial L or J tube into a Barrel that is all or partially surrounded by cob or brick. some area would be left metal to act as instant heat.
I can't imagine that heat release from a pot of 1500deg liquid salt is much different than the heat from the even hotter burn elsewhere in the system. the rate of release would depend on the thickness of the cob or brick cladding.
I was hoping to use a barrel with a top removable lid so that the melt vessel could be replaced, and the system could be cleaned at that point. I wanted to have the core riser insulated and surrounded by brick that would also support the vessel.
in the event of a spill or leak, the liquid would be contained inside the barrel cladding. I might need to have a reservoir at the bottom of the riser that could double as an ash cleanout that could contain a spill should it go down the riser. operating so close to its melting point, the salt should begin to re-solidify soon after a spill.
it will all require experimentation, as most of these RMH systems do. it turns out that Rocket Science is not an exact science...
I have yet to find time to try and melt a small dish of salt on the core I have just to see if it does or not. Im looking at investing in a multi probe thermocouple that will read these high temps. that part isn't necessary for most systems but this one is trying to reach a certain temperature or above.
in the event that there is too much heat, then there could be more mass utilized in a longer system.
I once worked in a nuclear power plant where molten sodium was used as the reactor coolant. So I can appreciate what you are saying. Like others have said, probably not practical for home use. Besides, sodium metal is very hazardous in the presence of water. But here is a thought. We do know that sodium is highly efficient at storing heat. So what if one were to use salt water as a means of storing and transporting the heat from their RMH or BBRMH? Would the addition of the salt to the water improve it's heat carrying capacity?
I realize the larger industrial applilcations relied on actual Sodium metal, but I was considering using Sodium Chloride. NaCl. common salt. there have been repeated reminders of risks of working with high temperatures, but the heart of every rocket mass heater is a very hot combustion process contained in the same way with the same materials that i am considering. outside of any heat involved, there is no risk of contact with water. the Rocket Heater most likely cannot make enough heat to vaporize the Sodium Chloride. the sodium chloride cannot get any hotter than the fire, it can only possibly last longer than the burn itself.
the introduction of moisture into an RMH system is an interesting thought. I wonder, once a system cools, could humidity in the air condense inside in enough quantity to cause problems, no matter what mass is involved? I guess just a short burn on a daily basis would prevent too much moisture inside without heating everything up during half the year when heat indoors would not be wanted. (depending on whether or not your area has actual seasons. mine seems to be random)
im just thinkiing outside the box about ways to store heat in a mass with less actual bulk. for small places, or limited weight applications. I would love a RMH in my home, but it is old and frail and I very seriously doubt that the floor could support the kinds of weights that I have seen mentioned in here. I might be able to support something that was alot less weighty and bulky.
I think most of the bases have been covered here pretty well. In my opinion, Molten Salt, or metallic sodium, is a good thermodynamic choice if you have a really high temperature operation that would otherwise simply radiate, conduct or convert that heat away in a non-useable manner (waste). I think you are wise to rule out molten metallic sodium for safety reasons. Solar salt melts at a much lower temperature than NaCl. But my personal thoughts are that if you already have all the mass of the RMH soaking up the heat, I am not certain of the advantages of absorbing more in a pot of molten salt.
Please don’t misunderstand me, I actually love the idea and have toyed with the concept myself. Maybe this would be useful if you don’t want the RMH to heat up a room so much, but wanted that heat to last longer? Just a thought. But at any rate, I would think solar salt (especially the KNO3) would be a better choice than NaCl which might not melt on an RMH in the first place, but again, I could be wrong there. But either salt would be chemically stable as opposed to liquid metallic sodium which might be prone to doing some very interesting and dangerous things.
This is really a cool idea and I am really interested in where you go with it!
Some places need to be wild
Rocket Mass Heater Plans: Annex 6" L-shaped Bench by Ernie and Erica