best thermal mass materials?

I'm getting ready to build my first Rocket Mass Heater. I am wondering what the best material or mixture is to use to make the "cob" portion of the bench. I am already planning on using less cob than normal basicly making a cob bench wrapped in clay bricks, so you don't see any of the cob, you just see bricks.

I was wondering if cob, with planer shavings would work as well as cob with straw. I have alot of planer shavings and I have no straw. I also have tons of clay and sand. In my experiments so far I have found that compared to the clay and sand only mix the clay, sand, and sawdust mix seems to be much lighter in weight but just as strong. Of course I have only experimented with mixing small amounts.

I've read that sawdust makes it more insulative. Is that going to cause a problem with the heat transfer in the bench?

I also have alot of bank run gravel, could I just fill a brick "box" with the gravel for a thermal mass? If I do that am I going to have problems with my thin metal duct deteriorating and the gravel shutting off the exhaust? That idea scares me, I don't want carbon monoxide leaking through the thermal mass.

Shuold I cover the duct with a few inches of cob with sawdust then fill wit the gravel bits? Again if I do that is enough heat going to transfer through the cob?

Oh I also have a bunch of broken bits of concrete blocks as well as clay bricks.

Sorry about all the questions. I took one of the workshop classes when Ernie and Erica where on tour this summer, I guess I should have asked then but I didn't think of it.

One more thing. I would really like to use the cob/ sawdust mixture to save on weight. I have a suspended wood joist floor and don't like the idea of putting five tons of cob on it.

Forget about sawdust, straw perlite, vermiculite or other insulants.

Your mass must be monolithic. Ie, no air gaps, as air is insulative.

Your gravel could be a good base, as usualy stones hold heat well. In Ianto's book, he talks about urbanite, concrete or tarmac rubble. Granite seems to be good too. I know concrete is not good ecologicaly, and is not as cheap as clay, but it is a better heat tranfer material than clay or cob.

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

Also, there's one thing, your mass should be able to act as à damper, i mean, exhaust temps range in the 60 to 200c°, but you want it to release more like 20c° for a longer period of time than it took to accumulate it. It is not always easy to get it right. Between mass needed top store the energy for a certain period of time, in à certain burn time etc.

I'm by no means expert. And i have a lot of trouble getting things right.

Satamax Antone wrote:
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

Solids - Specific Heats
http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html

Great re-post, Karol.
This chart makes earth and/or dense brick-work look like pretty good options... unless you have a handy source of molybdenum in your neighborhood!

*edit for spelling (guess what word).

Chris Sturgeon wrote:Great re-post, Karol.
This chart makes earth and/or dense brick-work look like pretty good options... unless you have a handy source of molybdenum in your neighborhood!

*edit for spelling (guess what word).

?

Molybdenum cannot store much heat, like any metal.

By looking at both charts it is quite obvious, that high thermal conductivity and high specific heat capacity are mutually exclusive.

If by "store" you mean heat units held and then released over long periods of time, then yes you are correct. But mostly I was just being silly.

If your purpose was to extract as much heat out of your exhaust as possible and to radiate it (over a short period of time) into your living/work space, then there are all sorts of materials on that list one could build different and differing systems out of.

Sorry for the thread drift. Back to a related topic: at a certain point the interior of your mass (let's say cob) will warm to a similar temperature as your exhaust. Diminishing return. Has any one played with conductive (let's say molybdenum, for laughs) fins, spikes, etc surrounding their exhaust pipe to move the heat more uniformly into their mass?

We all have a tendency to take Ianto Evans Book 'Rocket Mass Heaters' as bible verse, some of us more unquestioning than others, that having been said, the book says that any thing that causes turbulence at the j-bend of the combustion tube is good, that which causes turbulence in the piping in the thermal mass is bad ! We need more research on this ! A.L.

Maybe I should make a new thread about this subject.

Aaron, have you found any way to re-enforce the floor? It may be the simplest solution as it frees you from needing to reduce the weight of your thermal mass. Then you could use all that gravel and clay/sand as a kind of cob conglomerate (coblomerate?).

Chris Sturgeon wrote:
If your purpose was to extract as much heat out of your exhaust as possible and to radiate it (over a short period of time) into your living/work space, then....

Thats the purpose of the barrel.

Has any one played with conductive (let's say molybdenum, for laughs) fins, spikes, etc surrounding their exhaust pipe to move the heat more uniformly into their mass?

Increasing the contact surface with fins, spikes, etc will certainly work.
Implementing some bells in the mass to increase the contact surface would be even simpler, more durable and more effective too.

For very fast moving of heat inside the mass one could use heat pipes.
Everyone able of soldering or welding can create heat pipes.
Close one end of a tube and fill it with a small amount of water,
then close the other end while the water in the tube is boiling and steam is escaping.
A welded heat pipe can operate at several hundred degrees,
a soldered up to the softening point of the solder.

Yes I think I will just reinforce the floor. There is a basement underneath the area That I want to install the RMH. ALthough after seeing the chart on thermal materials, I'm thinking it might be a good idea to build the RMH in the basement since my entire basement is made of a gigantic thermal mass of concrete.

The only drawback to that is that I wanted to keep the basement cool to store vegies and other food. ALthough I do have some logs, maybe a Sepp Holtzer cellar would be a good idea.

Aaron Esch wrote:
I'm thinking it might be a good idea to build the RMH in the basement since my entire basement is made of a gigantic thermal mass of concrete.

It is also a gigantic heat transfer surface to the ground,
which is usually not desirable.

Certainly there could be a lot of difficulties but water stores more heat per volume than just about anything else. If you can find
a way to keep things water tight, running chimney exhaust pipe through water would store the most heat in the smallest volume.

That means if you had enough more water, it would take more energy and longer to heat it up but it would give out more heat
over a longer period of time. A decade or two ago, I remember a guy that had thousands of two liter bottles in a space under the floor
that he pushed hot air through and there were his thermal storage battery.

Scott Perkins wrote:Certainly there could be a lot of difficulties but water stores more heat per volume than just about anything else. If you can find
a way to keep things water tight, running chimney exhaust pipe through water would store the most heat in the smallest volume.

That means if you had enough more water, it would take more energy and longer to heat it up but it would give out more heat
over a longer period of time. A decade or two ago, I remember a guy that had thousands of two liter bottles in a space under the floor
that he pushed hot air through and there were his thermal storage battery.

And in more recent times we have a lot of puffer tanks that do the job. I also thinking about installing a RMH directly inside a self made puffer tank. (well mostly inside anyway)

But to the topic:
If we need a lot of heat capacity and willing to make water tight containers we can also make a wax container.
Using wax we can take advantage of the big latent heat capacity involved in the phase change.
Wax changes to a liquid at around 55 - 60 °C. (130 - 140 °F)
Thus we can store most heat at a convenient temperature, ( both easily charged and easily extracted) since the wax does not cool under 55 C until it has molten parts.
Wax have less density than water, and boils at a higher temp, making it a lesser concern for closed containers.
However it is flammable, which makes a puncture very dangerous.

The end result is a small and light heat battery.
I heard there are fancy plasters that have small encapsulated wax pieces for covering walls, creating thermal mass for buildings that need it.

I was thinking of the differences in different materials so I am glad you guys posted tht link. I deleted the ones that would have no place in building an RMH (still plenty on here that also dont belong but many of them could). Here is the thermal conductivity aranged by least conductive to most, note that the low values were used when there was a range given. I think the idea isn't "is one material suitable or not" but more so how would the system react using one material as opposed to another. This is the reason certain materials are used througout the system to get the desired effect at each spot. I saw someone on donkey forums using ~1" thick vermiculite board screwed together for a riser. Looked simple and reproducible, wondering if it has been tested to work well or not?




0.00137 Perlite, vacuum
0.021 Urethane foam
0.024 Air, athmosphere (gas)
0.029 Cotton Wool insulation
0.03 Plastics, foamed (insulation materials)
0.03 Polyurethane foam
0.031 Perlite, atmospheric pressure
0.033 Styrofoam
.035 - 0.16 Insulation materials
0.039 Sheeps wool
0.04 Felt insulation
0.04 Glass, wool Insulation
0.04 Mineral wool insulation materials, wool blankets ..
0.045 Rock Wool insulation
0.048 Balsa wood
0.05 Paper
0.055 Wood across the grain, balsa
0.058 Vermiculite
0.06 Diatomaceous earth (Sil-o-cel)
0.07 Wool, felt
0.08 Sawdust
0.09 Straw slab insulation, compressed
0.1 - 0.3 Concrete, lightweight
0.12 Softwoods (fir, pine ..)
0.12 Wood across the grain, white pine
0.147 Wood across the grain, yellow pine, timber
0.15 - 1.8 Clay, dry to moist
0.15 Engine Oil
0.15 Hardboard high density
0.15 Oil, machine lubricating SAE 50
0.15 - 0.25 Sand, dry
0.15 - 2 Soil, with organic matter
0.16 Hardwoods (oak, maple..)
0.17 Wood, oak
0.19 PVC
0.2 Plaster light
0.25 - 2 Sand, moist
0.28 Plaster, wood lath
0.29 Cement, portland
0.4 - 0.7 Concrete, medium
0.47 Plaster, metal lath
0.5 - 2.5 Rock, porous volcanic (Tuff)
0.54 Cod (83% moisture content)
0.58 Water
0.6 - 4 Soil, saturated
0.6 - 2.5 Clay, saturated
0.7 Gravel
0.71 Plaster, sand
0.75 Asphalt
1.0 - 1.8 Concrete, dense
1.005 Pyrex glass
1.05 Glass
1.26 - 1.33 Limestone
1.31 Brick dense
1.4 Fireclay brick 500oC
1.5 Earth, dry
1.7 Concrete, stone
1.7 Sandstone
1.7 - 4.0 Granite
1.73 Cement, mortar
2.0-4.0 Sand, saturated
2.0-7.0 Rock, solid
2.08 - 2.94 Marble
16 Stainless Steel
43 Steel, Carbon 1%
54 Carbon Steel
55 Iron, cast
59 Iron, wrought
69 Cobalt
80 Iron
401 Copper

Thanks for the effort to load this Nice list but I think it is a list of "heat conductivity" and NOT heat thermal mass capability.

BTW, Aluminum is a very useful material left off your list and not quite as good as copper in conductivity.

Pure silver, while too expensive, has an even higher conductivity than copper.

Let's look at this as en engineering problem. If you were a customer of mine I would summarize your design constraints as:

-Heat a main floor with a RMH
-Store thermal mass on a floor joist structure
-Maintain basement structure as a non-conditioned space

I would need more information to design a proper system. Is your floor insulated? Where will the main structure of the RMH go? Where are the load-bearing structures located? Do you have a fireplace and chimney that can be retrofitted?

Now... If the thermal-capacitance-to-mass ratio is the primary concern, nothing beats water. You can even super-simplify a RMH using water jackets. Instead of radiative heating from a steel barrel to create the draft, creating all sorts of bends and turns, just use an insulated chimney with a water pipe wrapped between the chinmey and insulation, and the water going to a large water tank. The heat can then be drawn out using an under-floor hydronic heating system.

The system would require careful design, and unfortunately it would require at least one pump, so it is more likely to fail than a standard RMH with a cob bench.

This brings up concrete. I really like how quickly and easily cement can be mixed and poured by hand. It's especially encouraging to have the new EMC cement mix powder manufactured with much less fossile energy than traditional portland. I have just rebuilt the thermal mass out of concrete for my chimney, a bench on the floor of my first RMH using concrete I mixed by hand in a wheel barrow into blocks that conform and contact very well with the chimney: 8" sch. 40 steel pipe (about 5/16" steel). It works enormously better than the clay/sand mixture I initially plastered around it.

I am now building my third RMH for a two story house 30 X 52. It has a 6" schedule 80 steel chimney under the slab on the floor, 30' horizontally under the floor (contacting the concrete), and 19' up through the roof. I'm happy with steel chimneys and concrete thermal mass for my RMH design, but am jealous of the ambience of cob.

I'm still searching for discussion on concrete thermal mass and steel chimneys for RMH's.

It seems that wax in reality is only about 1/3 better for heat storage than water. While it is true it has a good amount of heat of fusion (200 j/(g*K) ) , but it has a lover specific capacity (2,4 j/(g*K) ) than water (4,2 j/(g*K) ).
I know that it has a higher boiling temp than water, but we should not take advantage of that.
So we should just use water, its a lot cheaper.

karol kerl wrote:

Chris Sturgeon wrote:Great re-post, Karol.
This chart makes earth and/or dense brick-work look like pretty good options... unless you have a handy source of molybdenum in your neighborhood!

*edit for spelling (guess what word).

?

Molybdenum cannot store much heat, like any metal.

By looking at both charts it is quite obvious, that high thermal conductivity and high specific heat capacity are mutually exclusive.

The two are not mutually exclusive, just two different things that aren't related in a linear fashion. Heat capacity measures how much heat energy a unit weight (lb or kg) can absorb per degree increase in temperature - all pure asbestos has a specific heat capacity of .2 Btu/lbm oF. Thermal conductivity measures how quickly heat conducts through a material. In solid rock form (dense), that asbestos has a thermal conductivity of ~2 W/(m.K) yet if we take that same material and arrange in a different form, mill board, we can get the thermal conductivity down to .14 W/(m.K)

You can change the thermal conductivity of a material by how you arrange it, but you can't change the specific heat capacity. You can only change the overall heat capacity by adding/subtracting mass.

Since we're throwing around ETB links, here's a good one for realistic heat storage in materials: http://www.engineeringtoolbox.com/sensible-heat-storage-d_1217.html

edited to add heat storage link

speaking from experience here: I used crushed gravel and compacted it, just like i do when building roads or laying base for a patio or concrete. despite being professionally compacted, the air gaps in there are still insulative. There is a youtube video where one of those expert RMH builders tried to use fine sand as the mass. Conclusion is that there is just too much air space for good thermal conduction. So I had to scoop out the whole mass and mix it with a clay slip. Tamp down any air pockets. actually for mark II, I laid in a layer of clay slip and gravel, then laid in slabs of granite. It's like a lasagna. The results were immediate and pronounced (once it all dried).

The mass must be __monolithic__.

a guy over on donkey's board reinforced his wood joist floor last year. His was pretty bombproof. there are a hundred ways to skin that cat though.

allen lumley wrote:We all have a tendency to take Ianto Evans Book 'Rocket Mass Heaters' as bible verse, some of us more unquestioning than others, that having been said, the book says that any thing that causes turbulence at the j-bend of the combustion tube is good, that which causes turbulence in the piping in the thermal mass is bad ! We need more research on this ! A.L.

This might make a separate thread, Allen.
But that piece of the book is pretty straightforward.101
- Mixing the fuel and air during combustion is good = clean, complete combustion.
- Turbulence = drag = slower flow in the bench. If you want to move the sluggish, warm exhaust through more pipe and extract more heat, you want smooth flow everywhere downstream.
The only pieces of the book I've seen get people into trouble so far are the vagueness around the manifold and gaps; and the insistence on the low-height exhaust option. If you've also used Ianto's other book and built yourself a magical passive-solar cob cottage with pond liner over the roof (instead of vents 20 feet overhead), you can get away with a magical no-chimney (or short-chimney) stove. Conventional buildings call for conventional chimneys. But Ianto hates to sacrifice even the last 30 degrees of warmth that make the difference between exhaust that rises, and exhaust that condenses and falls.

Be cautious about replacing any 'optimized' material with a 'maximum.'
Corrugated materials designed to 'maximize' heat transfer do so at the expense of the flow - they are designed to be used with powerful fans or pumps circulating the fluids, to overcome any problems retrofitting them into an existing building - not for passive convection. Fans imply a fair amount of energy being pumped into the system. If you use corrugated pipe in a passive-flow rocket mass heater, the effective drag is about 10x that of smooth pipe, limiting your passive bench length to about 6 feet of corrugated ducting! It better be darn magical if I'm going to replace 40 feet of straight pipe with 6 feet of something 'more efficient.'

Effective trumps efficient.

Likewise, the benefits of using cob are not just relative thermal density, but all the other functions one needs in a heated bench you can live with.
Cob can be surprisingly comfortable to sit on (more so than concrete); relatively easy to re-work if surface damage or patching is needed (e.g. for cracking due to failure to anticipate thermal expansion); it tempers indoor humidity too, after the initial steam-drying phase. And it's relatively well-tested in this application as regards appropriate thickness and volume. You can also refinish with lime or gypsum plasters, tile or wood trim, relatively easily without needing toxic gick to stick these things onto the surface - you can dig in and anchor the trim. Yet it's hard enough to handle being stood on, kicked, etc.
If you switch to concrete, you need to get the thickness (and cleanout locations, and expansion joints, etc.) right the first time, and you might have too great a conductivity and hardness for comfort. Plus more density means more weight on that floor. Using concrete rubble with cob mortar seems to offer the best of both worlds, which is why so many builders do it.

I'd use either cob or concrete before I'd do the gravel alone, for the reasons you mention (safety: both sealing and supporting the thin-walled metal pipes; and heat transfer). I've heard reports on five different loose-fill installations now, and only two of them are performing to their owners' satisfaction. One of the successes was loose mineral soil, not gravel; after the initial test phase the owner re-built the surround and tamped the soil in damp to make a kind of lazy man's cob. He's planning on bridging above it to prevent damage to the pipes. Sand didn't work well; mixed gravel worked kinda OK; big rocks alone didn't work well. Smallish gravel (1"- minus) worked OK in one case, but those guys haven't seen actual cob to compare it with. None of these installations have been tested in place for a full year, let alone with a CO meter as the pipes age. If any readers do decide to try a loose fill, please double-seal all joints with some kind of heat-tolerant material.

As far as making cob: I wouldn't bother to include sawdust unless it's really long curls, and then only in the top layer of material (to help resist cracking and wear). "Thermal cob" with no straw is what we typically use inside the bench around the pipes. Like a clay-sand mortar, about 3 parts mixed-size masonry sand to 1 part pure clay (e.g. ceramic clay scraps or refractory fireclay), or whatever mixture of local materials works out to a similar consistency. It's like brownie batter or shortbread dough, mostly sand with just enough clay to make it sticky. Make some test bricks - you can bake them dry in the kitchen oven if you can't spare the time - and improve your mix until it neither cracks nor crumbles as it dries. (Cracks = too much clay, crumbles = too much sand or silt, both = not enough mixing). Soaking the clay ahead of time, and/or grating it through a screen, improves workability, as it's the clay lumps that will take up most of your time and effort on the mixing tarp.

One other option to reduce weight, while maintaining good heating performance, is to make some room-air channels underneath the bench and up the wall behind it. This turns your radiant heat source into a combined radiant/convective heat source. Masonry spacers with masonry board or pavers above are my first choice; skinny cinderblock might work OK too.

Yours,
-Erica W

One other option to reduce weight, while maintaining good heating performance, is to make some room-air channels underneath the bench and up the wall behind it. This turns your radiant heat source into a combined radiant/convective heat source. Masonry spacers with masonry board or pavers above are my first choice; skinny cinderblock might work OK too.

Thank you Erica! that is exactly the sooution I was looking for!

Scott Perkins wrote:Thanks for the effort to load this Nice list but I think it is a list of "heat conductivity" and NOT heat thermal mass capability.

Oops, good catch, I should sit down and make the list from the table you mention, would make a better reference.

How about driveway 'chat'? We typically use a finely ground limestone rock for driveways under 1/2" with the dust included. When it compacts it is almost like concrete. I was thinking of using large rocks as much as possible then filling the gaps with this stuff and tamping it down. It won't get as compressed as when driven on but shouldn't be too bad. Wouldn't this be preferable to pea gravel?

Greg

I had a question to answer, almost 2 years ago and did this cob vs water comparison. It comes up occasionally, so here it is again.
-------------------------------------------------------------------------------------------
Here are some very useful figures, for anyone who is trying to choose whether or not to include a water tank within an RMH. Cob weighs 95 pounds per cubic foot. Water weighs 62 pounds per cubic foot. The heat capacity of cob is .2 which is 1/5 that of water which has a capacity of 1.00 So supposing we want to build a RMH which occupies 100 ft.³ of space.

First the cob - 100x95 equals 9500 pounds. 100 ft.³ of cob will weigh 9500 pounds. 9500x.2 equals 1900. So our cob bench has the same heat capacity as 1900 pounds of water.

Water weighs 62 pounds per cubic foot, therefore the tank containing 100 ft.³ of water weighs 6200 pounds

6200 divided by 1900 equals 3.26

A given volume of water can store 3.26 times as much heat as the same volume of cob.

It is true that the cob bench could be heated to temperatures far beyond the boiling point of water. But in order to store the same amount of energy as water at 200°F, a cob bench would have to be heated to more than 650°F. This is not common practice and if it were it would result in lowered efficiency with higher exhaust temperatures and badly burned bums Water stores much more heat at temperatures which are practical and safe. And because heat transfers through a body of water through convection, heat being absorbed by the thermal mass will be available in short order. If some lag time is desired the tank could be cobbed over.

Some may be worried about the danger of steam explosion. A water tank which has an open vent to the exterior, is no more dangerous than a rattlesnake on TV . It's easy enough to monitor the temperature and allow the fire to burn out before the boiling point is reached.

It would be a shame to have a giant water tank like this for thermal storage only. Water could be drawn off to heat a hot tub and for regular domestic uses. For those who don't want to do any fiddling a tank could be placed into a cob bench which would be a pre-heater on the way to the hot water tank. During the heating season your hot water tank would receive preheated water. During the summer when the heater is not in use cold well water would absorb heat from the thermal mass. This would have a mild air conditioning effect and the water would enter the hot water tank at a higher temperature.

Greg Harvey wrote:How about driveway 'chat'? We typically use a finely ground limestone rock for driveways under 1/2" with the dust included. When it compacts it is almost like concrete. I was thinking of using large rocks as much as possible then filling the gaps with this stuff and tamping it down. It won't get as compressed as when driven on but shouldn't be too bad. Wouldn't this be preferable to pea gravel?

Greg

Hello Greg,
We have used "road grade" or "pit run" - basically mixed, fine-grade rocks - both for making cob and for infill.
It makes awesome cob; sometimes if there's a little clay in the mix, you have ready-mix where you can do poured or tamped infill and it basically becomes just like cob.
I suspect even without the wetting/mixing process, it would compact better than pea gravel and give you something in between the two for performance. If you include large rocks, I'd layer them so you get good filling (fewer air pockets) between the large rocks.

Pea gravel (in our recent Montana experiments) lets a lot of air through, resulting in warm bums faster, but much shorter duration of warmth after the fire is out. Probably would still extend the warmth beyond what you'd expect from a woodstove, but not as long as I've come to expect from solid cob or masonry heaters.

Another option if you are building a bench and doing tamped infill is just to go with the local subsoil, whatever it is. Leave out the roots, creepy-crawlies, and surface topsoil, and use the layers below that. Sprinkled with water and tamped, with or without large rocks. One of our neighbors did this -built a brick box and infilled with our local silty subsoil, just sprinkled it with a watering can and tamped it down between the pipes. It works pretty well; maybe not quite as miserly as our cob/fieldstone heater but it was way less fiddly to build.

Yours,
Erica

Great reply, Erica. I didn't want to build a cob filled bench but your description of just tamped sub-soil sounds great and a lot cheaper than buying a load of driveway gravel.

Erica Wisner wrote:Pea gravel (in our recent Montana experiments) lets a lot of air through, resulting in warm bums faster, but much shorter duration of warmth after the fire is out. Probably would still extend the warmth beyond what you'd expect from a woodstove, but not as long as I've come to expect from solid cob or masonry heaters.

Was that with Pea Gravel alone or mixed in with some kind of wonky clay mix?

I was considering pea gravel for my RMH, but wondered about heat release time.

Willie Shannon wrote:

Erica Wisner wrote:Pea gravel (in our recent Montana experiments) lets a lot of air through, resulting in warm bums faster, but much shorter duration of warmth after the fire is out. Probably would still extend the warmth beyond what you'd expect from a woodstove, but not as long as I've come to expect from solid cob or masonry heaters.

Was that with Pea Gravel alone or mixed in with some kind of wonky clay mix?

I was considering pea gravel for my RMH, but wondered about heat release time.

We only have a few examples so far, but from Paul's experiments with pea gravel (and river rock, layered in like a casserole), it looks like it will hold some heat overnight. It loses heat about twice as fast as my solid rock/cob version: mine drops from 80 F to 70 F overnight, and his pea gravel version drops from 75 F to about 55 F.
The solid heaters can usually absorb more heat, and can keep a room warm longer between firings.
But either one will keep a space from freezing overnight, if used each day.

Re: materials comparison above:
I would LOVE to see a solid silver mass heater! But I suspect short of piping exhaust channels through Fort Knox, it will never happen. Some of the old fancy woodstoves did use a lot of massive nickel decorations, though, which presumably had a similar effect.

Water is one of the highest-heat-capacity materials available that are cheap and non-toxic. But in addition to exploding, water has problems of corrosion, leakage, and a tendency to evaporate (thus cooling itself quickly, and sometimes depositing crud).
Water also tends to cultivate life, and lukewarm water open to air (thus safed from exploding) can become an algae, bacterial, or mosquito soup.
I like the use of simple cookpots with ordinary lids - keeps the dust out, reduces crud buildup and evaporation. These can be used as additional thermal mass on any stove designed for cooking - just watch the weights, and consider boiling or changing the water every few days.
I'd like to see more examples of tanked or container-embedded mass heaters before I make up my mind to recommend how to do them.

-Erica