And a search:
Edit: Paul, Ernie & Erica talk about it in this podcast too: http://www.richsoil.com/permaculture/2925-podcast-241-rocket-stoves-efficienc/
Warning: I've read some about RMS, but not built one yet. This is worth what you paid for it.
Your problem in Montana (I'm in Alberta, so I feel for you. OUR minimum is -45) is that the sun can make it too warm in the day, and still freeze stuff at night.
And some days can be -25, and three days later -10. A heat store for a week doesn't work. (People with heavy radiant slab heating have this problem too. The system does not respond well to weather changes.
Fill material: Around the piping itself you need cob. There is a good chance that the hot gasses will corrode the pipe into scrap over the course of several years. You want a to have a chamber to continue to move the gasses the right way. Outside of 2" of cob, I can see merit in using gravel. The air spaces will allow for much more rapid heat transfer through it. I'd estimate something like 4-6 inches per hour instead of 1" per hour. There is no reason to not use whatever rock you can get -- broken concrete, river run cobble. Whatever is cheap and easy to handle. You also want cob on the outside as a jacket. 8"?
Suppose you made a 2.5 foot high 4 foot wide 'bench' this way, running the exhaust as low down as you can fit it in. About half the mass would be cob, the other half rock. Figure on an average mass of 150 lb/cubic foot. 60 foot long bench would have 600 cubic feet of mass, weighing 90,000 pounds. dirt and rock have a specific heat of about 0.2, so this mass stores about 18,000 BTU per degree F that the whole mass changes. Assume (bad idea...) that you get the center of the mass up to 200 F, and the skin to 90 at the base, and 75 at the top. So you are heating the mass to say an average of 120F -- 50 degrees warmer than the nominal greenhouse temp. In fact you do a bit better than this, because some heat will travel into the ground too.
That same greenhouse is averaging R1 (double poly or twinwall is actually R2, but we'll call it R1, and use the floor space for figuring out the leakage. So, on a night that is -30 outside, there is a 100 degree differential between inside and outside. If the greenhouse is 30 feet wide, you have 1800 square feet of floor space, so you are using 180000 BTU/hour to keep warm. So, in this case you get 1 hour's night temperature storage for each 10 degrees you raise the mass.
Not clear if you can get that heat in and out fast enough, but at least we are talking about the right order of magnitude.
Adjusting heat: So what do you do on a night that is only getting down to 20 degrees. You will only need half the heat. Build a smaller fire.
Another way to adjust heat would be to have curtains or foam panels that you can place on the sides of the bench, slowing down the heat transfer between the bench and the room.
One of the downsides is that the bench will be a LOT warmer at one end than the other. One possibility is to have the exhaust pipe rise through the length of the bench, so as the gasses cool they are closer to the top of the bench. This means that less of the mass at that end is effectively used. Another way may be to run the pipe twice through the bench, initially at fairly small diameter heating furiously at the bottom of the mass, then a more leisurely trip back in larger pipe, exiting near the stove. (If you have a shed attached to the green house for firewood, you may be able to use that last residual heat (50 C is claimed) to dry your firewood.)
Still you will have varying temps along the length of the bed. It's not a bug, it's a feature. Put the kales at the cold end, and eggplants at the warm end. Indeed, you may want to curtain the greenhouse so that one end is always 15 degrees colder.
So we have enough heat at night. What about during the day? In midwinter we are intercepting height x length square feet of sunlight. (Remember I am in Alberta, The sun is 12 degrees over the horizon at NOON. 15 feet x 60 feet = 900 square feet. if it's 200 BTU/square foot/hour then the greenhouse would stabilize (in theory) at close to 200 degrees above outside ambient. You will need to ventilate. Indeed, you need to get rid of 18,000 BTU/hour less heating needs at peak.
Lets look at fall and spring when the outside temp is about freezing. 18000/BTU/hour.
So call it 36 degree temperature differential between inside and outside. Each kg of air will take about a BTU per degree. 18,000/36 = 500 kg of air to bring in mix and expel per hour. That will be about 400 cubic meters of air. If you had a meter square hole in each end, you would have to run air through that opening at 400 meters per hour. That's about 1/2 km/hour or about 1/3 mile/hour.
Not sure how to mix it. If you don't mix it, then you risk a cold spot making your eggplants unhappy.
To give you and idea how strong this effect can be:
Last winter my minigreenhouse 6' x 8' was left all winter with the door propped open. Despite this, the 3 45 gallon barrels that support one bench ddin't freeze solid enough to burst.
wide for the screen, usually I just refuse to read these, but its in an area i am interested in! S.G. Botsford I need a little more information !
You say, that the use of gravel in place of cob will increase the speed of penetration of heat to the surface of the Bench, you seem to believe
that the air trapped between the individual stones will speed heat movement through the gravel. With the barrel radiating heat off of its skin
we have the immediate need for prompt heat dealt with, We want the thermal mass to accept the maximum amount of heat and are pleased
with its slow release,in this case because 'air' is such a poor material to store heat it keeps the total heat stored in our thermal mass down,
and delays the transfer of heat through itself. Compare the amount of heat that 'dry air' can hold and compare it to a hot potato !
Over the years, many types of heat storage have been used. Before the creation of the rocket stove most of the storage was related to the
use of a masonry heater,in many places in northern china the indigenous people use a combined cook stove, masonry heater, and a thermal
mass 3'-4' high,5'-6' long, and as wide as needed for the size of the family say 5' wide for a family of three! This is a storage and sleeping
platform to take advantage of warmer air trapped up by the ceiling. I mention this because a thermal bench 30'' high was proposed for heat
storage in a greenhouse, I know that generally most greenhouse operators build their work tables/benches between 18'' and 24'', I would
propose a lower bench covered on top, but not sealed by plastic with a shallow pan of water, to help heat transfer and catch runoff water from
the seedling bed immediately above it !
This would be in contrast to the thermal bench in the home designed to be used as built in furniture, needing a height approximately, 16''-18''
high and about 15'' deep. Generally speaking whether in a home or in a greenhouse a 45'-50' straight run of stovepipe buried within a thermal
would be near the limit of a Good, 8'' Rocket Stove !
I can't argue the math, dealing with heat loss, except the total area mentioned did not allow for the parabola of the skin of the greenhouse or
the greenhouse's ends. But I would be in my house keeping warm, and waiting for the temps to moderate before i started up my Green-
house for the season ! Y.M.M.V.
Be Safe, keep Warm, PYRO magically Big Al
Back Of The Envelope -- calculation. The idea is to get within 25% of the right answer to see if we're in the right ballpark.
To get closer than this requires a closer approximation to the shape, including the ends, considering of condutive heat transfer
under the edges of the greenhouse, air leakage, radiant tranfer in addition to conduction through the envelope, convenction
inside the greenhouse envelope, heat movment into and out of the soil, heat transfer by evaporation/condensation, and likely
a bunch of other things previously only thought to affect the number of sunspots, and the flavour of pizza.
Half of those are beyond my meager abilities. And in the end you can calcuate to death, but at some point you need to
just build it and see what happens. I'd sure hate to move 45 tons of stuff and find out later that I only needed to move 10, or
worse, that I actually needed 200.
Gravel: Convection becomes a significant heat transfer mechanism when air spaces are larger than about 1/2" So fill made from
large gravel/river cobble when heated from the bottom should distribute the heat by convection faster than by conduction.
My understanding of masonry heaters is a figure of 1 inch per hour for effective heat transfer. In a green house we usually won't
need heat during the day, but will at night. So we need a system with a heat/cool time of around 12 hours.
Gravel also is less work than cob.
The pictures of benches I've seen have had heights of around 20 inches, and widths of about 2 feet, with the exhaust running
through the centre of this. In my proposal, I'm going for twice the width, and over twice the depth,(I want the exhaust pipe at
the bottom of the mass.) Using the 1"/hour figure this system would have a cooling time on the order of 48 hours.
A tight house with small windows uses heat at a very constant rate. Some less during the day, becuase it's warmer outside.
Some less late at night because everyone is toasty in their beds. Add more windows on the south, and a house needs
less energy during the day. (At present we have a conventional wood stove, and with our current temps of -10 C at night
and +3 C during the day (14-38F) and we fire it up with a single load around supper time. That keeps the house warm
enough for the evening, and it's still reasonable in the morning.) A greenhouse gets way too hot in daytime, and cools off
rapidly at night. We want to get most of that heat out before dawn.
My design goal is 16 hours. Fire the stove an hour before sunset -- 4 p.m. and have enough heat to carry through until 8 the next morning.
A gravel core should also make the heat tranfer graph. 'flatter' it should ramp up/down faster at the either end, and not
peak as much in the middle. The cost of this is a small decrease in the mass, due to the voids. However I think that bulk rock with
air spaces will turn out to have a density close to that of cob. Cob is fairly porous from voids left by water that dries out, as well as the
hollow fibers of straw.
One way to test this could be done as follows.
* Pick up some identical crates off of kijiji. Here there are free crates every few weeks.
* On box is solid cob, except for a large tin can set into the top.
* One box has a 4-6" cob liner and is filled with rocks, with a cob cap, except for a large tin can.)
* Wire a light and put it in the can.
* Invert the box so the bulb is now at the bottom.
* Drill a few holes into the side. These should be of varying depths, and should be sized to
put a temperature probe from a multi meter into the crate.
* Plug in the lamps.
* Measure the temperatures every two hours.
What I expect to happen:
* The surface temperature should eventually settle a few degrees above ambient.
If the crates are the same size, the tempertures reached should be about the same.
* The rock crate will start to warm sooner than the all cob crate.
* Interior temps next to the can will be lower in the rock crate than in the cob crate.
* When the power is cut, the crate with the rocks will maintain that surface temperature longer,
then cool faster.
Ideally I'd do this with a more intense heat source than a 100 w. bulb, but even a 100 w bulb inside a
tin can is going to get *hot*. (Warning: Do this on a GFI circuit, and a ceramic socket. Arrange the wires so that if all
the insulation burns off there is no shock hazard.
In some ways I don't want the immediate heat at all, but that hot barrel skin doing the initial cooling of the exhaust is
what drives the draft.
One possible answer is to wrap that barrel skin with copper pipe, and use the initial heat to warm up a large tank of water,
which in turn can be kept for running radiators late at night when the mass of the bench is nearly cold.
But it takes a very large tank -- on the order of 5-10,000 gallons -- to store a day's heat for this size of green house.
(Water stores more heat per pound, but cob/rock can store at higher temperatuares.) But this adds a lot of complexity,
plumbing, pumps, timers, thermostats. and certain parts cannot be allowed to freeze, unless they are drained.
this does not include much in the way of thermal mass as generally it takes several hrs to 'load' the mass as it also does to bleed off that heat
through radiation, convection, and conduction.
The use of a rocket stove mass heater in the home setting requires a major modification of Life Style. Even with the 8'' system the operator
expects to burn small wood efficiently over several hrs. This while performing other duties in close proximity to their rocket to give the dragon
that lives inside the frequent snacks it requires. Many Off-griders and homeschooling people can easily make the life style adaption of being
in close atendence on their dragon, with frequent feedings up to every 20 minute over a 6-8 hour period to get a charged Thermal Mass that can
re-radiate out the heat over 20-25 hours ! Because we are NOW looking for away to efficiently use this high temperature gas stream a dense
Thermal Battery is an ideal medium for heat storage. People who are not ready for this kind of commitment, or who make the mistake of putting
their Rocket Stove in a remote location, requiring them to stop what they are doing and travel back and forth to their Rocket Stove for Dragon
feeding, find that they are unhappy with their heating source.
Using a Rocket Stove In a greenhouse requires major modifications in the way one heats/tempers the greenhouse environment. including every
thing mentioned above, there is usually a requirement to store their Rocket Stove's wood supply in an adjacent location the greenhouse having
an environment too humid for long term wood storage! As we don't want to be constantly carrying cold air in with every load of wood we will
need an adjacent wood box with sealed trap doors that will be loaded daily ! Any cob used in a greenhouse needs to be protected from direct
contact with water, Especially in the high humidity areas of N.E. And N.W. Coastal area of America. (Generally worse at the start of the growing-
Your testing bench proposal sounds valid,though I am expecting different results. I am proposing a simple test of making up three buckets, one
with Cob using clay and limestone 'sharp sand' sold locally to Famers as 'sure-foot', matching that bucket against a 2nd identical bucket containing
Limestone 'chips' or landscaping gravel, approximately the size of the last joint on a mans thumbs. I would wash and drain the limestone gravel
setting aside until the cob is dry enough to turn out of the bucket this will speed further drying of the cob and will eventually give us a benchmark
benchmark for dry cob. The third bucket wound contain a very clay slip rich bucket of the limestone landscaping chips, all buckets filled to an equal
volume. This should give repeatable results. Obviously, if you had local access to 'crusher-run', iron rich, hematite ore, the denser mass would be
your first choice !
Perhaps a forth bucket would be in order, as any bench the size you are promoting would definitely be helped by the addition of chopped straw for
When I get near the end of one of these Permies threads, I get a little nervous that I will loose the whole thing,so I'm not going to pause this while
I go look for a specific 'Rocket Stove in greenhouse' thread, but you could checkout web4deb in You Tube, which will refer you back to this forum !
For the good of the craft, be safe, keep warm, PYRO-magicly Big Al
you may want to fast forward the play list to rocket mass heater on Steriods !
Also : www.bigelowbrook.com
Back at Permies rocket stoves forum check out the threads 1) Burning pellets in a rocket mass heater
You can do a Permies search for Rob Tocellini and check out 2) RMH in the Geodesic Dome Greenhouse !
For the good of the craft, be safe, keep warm, and keep them coming ! PYRO magic Big Al !
weather. Typically they were run very hot and damped as soon as the fire was out.
We have a wood stove in the kitchen -- an actual range. When we fire it up in the winter, it requires two
fore-arm sized logs or one larger one every half hour to 45 minutes. I know about living as a slave to the
iron maiden. I don't like it.
The living room stove, in the old part of the house in winter takes feeding about every 2-3 hours to keep it
running. On a cloudy day in mid winter we may have both stoves going all day. I figure that half the wood I cut (about 4
cords a year) is used up in 3 weeks. Just a couple good cold snaps. House's total size is around 2500 square feet.
Our house has a band of trees on the north side -- 50-60' high mix of spruce, and black poplar 50-300 feet wide. I can often work
in the sun in shirtsleaves in my front yard on days that in the main field I'd be wearing a quilted jacket and toque.
The gas boiler provides heat for the DHW, and also
heats the two bathrooms. The rest of the house has radiant baseboard heaters, but they only come when we are on holiday
in winter. Our electric bill is pretty high, since both my wife and I are heavy computer
users. (We do NOT shut them down.) All the excess turns to heat. The new end of the house is almost half glass on the south side.
This makes this side warm any day the sun is out. Since this side is 2 floors high, it means that in
summer if we open the patio doors on the main floor, and the windows on the second floor, even in summer
it takes several days of hot weather to make the upstairs uncomfortable. Mind you for us 75 F is a warm
The entire house is 6" walls. It's not R-2000 construction, but it's close.
For the greenhouse, I was thinking of having the feed hole outside. I don't really want to enter the greenhouse to fuel it
nor do I want to use interior air for combustion air. I knew I didn't want to keep wood in the green house.
One of the things I was considering was to give it multiple feed chutes so that if you loaded them with ordinary logs, they would
have the right separating distance to burn well in the firebox. Separate chutes would decrease the chance of logs wedging.
I don't want to use brush for this. My wood cutting experience is that the optimum size tree is one 6" across. You will have to
split some of the bottom 8 feet, most of the rest can be used unsplit. Larger one's aren't much more work up to about 20" There is more
splitting, but less work with limbing, and dealing with brush.
Smaller wood than this doubles the work per cord for each time the diameter halves. Thus 1" wood takes at least 6 times as much work per ton
than 6" wood.
The fine grain versions of the proposed experiment I would expect to to have identical results to cob. I don't think there is signficant convection
in a body of rock until the air spaces are at least 1/2" Your thumb sized rock I think would have air passages that were marginal, unless
your thumbs are a lot bigger than mine. I would want golfball size to tennis ball size rocks.
Alas there is no hematite around here. Our local rock is mostly sandstone and limestone as bedrock, and quartzite and granite in the glacial
till in the eskers. Specific gravity around 2 - 3.
Another alternative to making the stove out of cob would be to make most of it out of 2 foot square patio stones
for the faces of it, and fill with rocks. This would make the parts that normally would get wet indifferent to water.
I would still use cob around the actual pipe. I'm not confident that the pipe by itself is good for 20-40 years use
without replacement. Some water would perk thorough, but the core gets hot enough to redry it. Putting a double layer
of 6 mill plastic under the cap stones would direct water away from the cob portion of the construction.
Patio stones however even in bulk are about $7 each. Wonder how much they are by the pallet load?
To side and cap them would take ~120 2x2 foot slabs. Might be worth it just to not have to mix up that much cob
as long as its temperature is above Absolute Zero, Radiant Energy travels through any medium, even a vacuum. Conduction can be considered by itself,and requires
contact between the two materials, conduction will continue until equilibrium occurs, In the absence of direct contact, Convection BECOMES the dominate
form of heat transfer.
When I go to www.engineering toolbox.com I find that the 'overall heat coefficient of air' is given a baseline number of 1, other mediums / materials being rated
higher. Am I guilty of looking in the wrong place,or misunderstanding what I am reading?
While there exist several sources reporting sub 6'' Rocket Stoves, including a Company (Dragon Heaters) that will ship you a 4 piece core in 4'' for a D.I.Y. project,
I take all these claims with a grain of salt! It is generally considered an absolute that the Interior Cross Sectional Area of the principle parts of a Rocket Stove, the
Feed Tube, the Burn Tunnel, the Heat Riser, and the horizontal pipe discharging into the Thermal Mass have identical area.
In practice the last part is shaped like a funnel as it necks down to the same Cross Sectional Area (while passing over a generous ash pit ) of the Rocket Stoves
I mention this because you can run a length of 3'' fire hose several hundred feet before the softening of the fire hose and force of the stream leaving the fire nozzle
tell us we have too much friction loss. The easiest way to increase pressure and flow is to reduce down to say 1 1/2 '' fire hose. while there is a reduction in flow it is
smooth and constant . If on the other hand you attempt to flow water through a 1 1/2 ''hose, and then into a 3'' hose, what then comes out the end of the hose is a
series of spurts of water much in the way a man with an enlarged prostate pisses. This and a few other characteristics of fluid flow convince me that I will have more
luck maintaining a high volume of flow reducing stove pipe flow only after the decreased temperature convinces me that the pressure and rate of flow have diminished!
For the good of the Craft, be Safe, keep Warm, and keep it coming, PYROmagicly Big Al !
The physics gets messy, because finding a good example of one heat transfer mechanism in isolation is uncommon.
In a nutshell: radiation -- movement by the heat equivalent of light. Present only when the space between is transparent to the
frequency involved. Everything is radiating, and absorbing all the time. An object at room temperature (300Kelvin - 23 C) radiates at a
constant 75 watts per square meter * it's emissivity. And object absorbs the same way. In a closed room, every wall is radiating and
absorbing to every other wall. If one wall is white, it reflects more, but radiates less. Net effect: All the walls stay the same temp.
Let there be Window.
The world outside is colder. Outside radiates less. So radiant energy that can go through the glass is not balanced by the radiation off all that snow and ice outside. Net loss.
The stove on the otherhand is hotter and radiates LOTS (radiation is proportional to Kelvin temperature to the 4th power.) The room radiating back doesn't amont to much.
Conduction is the result of atoms bumping into each other. Much like a fast cue ball hitting a nest of still ones, a fast atom (temperature is a measure of the average kinetic energy of atoms) hits a slow one, they come off with the fast atom moving slower, and the slow one moving faster. The exception to this is metals. In a metal, the HUGE dominance of heat transfer is done by electrons. The electrons are free to move from atom to atom, (which is why it conducts electricity) and since they are light weight (1837 e = 1 proton) they move like stink. Hence metals conduct heat FAST. And, by the way, the ability to conduct heat and conduct electricity are linked. Stainless steels that are comparitively bad heat conductors are also bad electrical conductors.
Convection is the movement of different temperature parcels of stuff due to their different density. Hot air rises. Cold air sinks. That's convection.
Convection never happens in isolation. There is always conduction through a surface, conduction between a surface and a fluid (gas or liquid) then once you get enough of the fluid hot, it can separate and travel on its own in responce to bouyancy forces.
Level two. Bouyancy forces? As the fluid gets warm, it expands. a given volume isn't as heavy. A heavier volume someplace else moves down, pushing it up.
Fluid movement is critically dependent on boundary layers. The layer of air right next to a solid doesn't move. As you get further, it can move some. Boundary layer effects depend on the characteristics of the fluid, (including temperature) the force involved, and the distance from the solid.
At small spaces, the boundary layers dominate. At very large spaces, boundary effects can be almost ignored. (Except that almost any boundary adds R0.5 to the heat transfer, due to the still fluid on either side of the boundary.)
The transition is roughly 1/4" . This is why window panes are typically 1/4" apart. A larger distance makes a twin pane window LESS efficient. Now the heat doen't have to jostle itself from molecule to molecule, it can instead ride over in a tiny parcel of air moving from the hot side of the window to the cold side.
Convection will still occur in smaller spaces. Just not as fast. If you ever take apart a house, look at the fiberglass around the base of walls and around electrical outlets. Look at the dirt. That's years of convection -- hot air moving up through the fiberglass sucking cold dusty air whereever it can get in. The dust moves only so far through the fibers before being caught.
As the spaces get smaller convection gets slower. Don't remember the numbers off hand but it's a power law -- 1/X^2 or 1/X^3 where X is the size of the space. So dry sand has an utterly ignorable convection rate. In a pile of watermelons, convection will dominate.
Ok, ok, Sherwood is in teacher mode. Gimme some real examples where it matters: Grain. Stack grain that isn't quite dry enough, and respiration in the grain generates heat. The spaces beween the grains aren't big enough for convection. So the inside of the pile heats up, and you get spoiled canola. (The same thing happens with green hay, and with piles of walnuts. But walnuts don't catch fire. But this is why you age walnuts in sacks, hung from the rafters. Convection even in a walnut stack is limited enough that they can't cool off fast enough.
Now, back to your email: Convection is the only space dependent heat transfer mechanism.
Radiation happens when there is NO contact. Conduction happens when there is contact.
Convection only happens when there is room enough.
And it's a slippery transition.
Transfer of fluids thorugh a pipe depends on:
* diameter of the pipe.
* Pressure difference between the ends.
* Viscosity of the fluid being moved.
* Velocity of the fluid.
A 3" firehose typically runs at 150 PSI or so. But it's moving a more viscous fluid. But it's moving it a hell of a lot
faster. (But see kinematic velocity and Reynolds number.)
Flow resistance decreases with the square of the diameter, increases linearly with length, increases with the cube of speed up to
Reynold's number, then it goes wonky, is partially dependent on the roughness of the surface. It's one of those things where step 1: find a
table or a suitable equation.
If we look at a rocket stove, and suppose that the air in the riser averages 810 K (540C) and that coming down the other side it's 540K (270 C)
It's density has changed from something like 0.4 kg/m3 to 0.8 kg/m3. So if your riser is a meter high, you are talking about pressures around 0.4 kg/m2 = pascals. Standard air pressure is about 100,000 pascals. Tiny pressures. Furnace guys talk about "inches water pressure" This works out to be about 40 mmH20 = 1.6" H20. You can see why the move to taller chimney stacks, and why conventional stoves need to have hot exhaust.
This is also why you work really hard to not restrict the flow to less than that critical cross section area.
Ingmars Klyv !
So - You keep on adding up all the boundary layers as you travel through the gravel and the net effect is that the air spaces end up slowing down the
transfer of heat through your mixed medium ! I will concede that after the whole Thermal Bench is nearly up to temperature those individual air pockets
are hot, their atoms are 'In an excited state' and the boundary layer will have been scrubbed down/diminished, but even then the boundary layer does
For the good of the craft, be safe and warm, Pyro Al
Anyway, You know how to go to Permies' search engine where you log in and out to retrieve stuff, right !
Check out ; 'RocketMass Floor Heater - - and it works', It shows an outside feed tube, an interesting exhaust
manifold/stovepipe, thermal bed, Annnnd - -"amish porn " !
Also; 'rocket mass heater - exhaust can be simpler than chimney' Erica W. discusses exhaust and wraps up
with a discussion of a specific build where the exhaust was reduced near the end !
For the good of the Craft, b.s.k.w., PYRO AL
allen lumley wrote: Sherwood B. : found a very interesting video on the 'Best wood splitter ever' tucked away in the woodlands forum go to youtube.com and type in
Ingmars Klyv !
So - You keep on adding up all the boundary layers as you travel through the gravel and the net effect is that the air spaces end up slowing down the
transfer of heat through your mixed medium
Small air spaces slow it down. Dry sand is a better insulator than concrete. Making the step from a sand particle to the air space in between.
Air generally is a worse heat conductor. Fewer atoms. So air filled sand:
* Stores less heat becuase it has fewer atoms.
* Conducts less heat because the air is less dense and most of the heat is transfered where the sand grains touch.
Large spaces allow for conduction. If we use softball sized rocks, the air spaces are around an inch.
* The spaces still lower the average density in the mass, so it holds less heat. (About 20-30% less)
* But convection moves heat through that mass more quickly.
By putting a cob blanket on the rock pile, this faster transfer doesn't go all the way to the surface.
In principle we can tune the heating behaviour of the pile to our climate.
Actual numbers would depend on modeling it in some fancy heat transfer program. Any thermodynamics enginneers here?
This also opens up the possibility of using forced air with a mass of large rocks to extract heat from the rocks faster.
You might do this in a greenhouse for example, by heating with radiation thorugh most of the night, but as the night got colder
and so did the rocks, that at 3:00 a.m. fan comes on sucking air through the rock pile pulling heat out of the rocks faster.
If you do bedding plants, having the green house warm at sunrise is significant. Being cold at first light (redder light) encourages longer
internode distances. They get leggy. If you can warm the green house from just before dawn for 2 hours, you get more compact growth.
Large rock heat masses may be problematic in humid summer climates. I've heard tales, can't ifnd them right now, where they were heated by air pumped from an attic. The problem was condensation and dust. The forced air brought dust in with it. When the rocks were cold, humid air condensed on the rocks. Not much. Combined with the dirt, fly crap (flies go everywhere.) you had a mold colony. Mold isn't good for you.
I don't think that would apply to a thermal mass heated by a rocket stove. You are heating the pile to a temperature that would make most molds unhappy, and I don't see much condensation happening.
The world's cheapest jedi mind trick: "Aw c'mon, why not read this tiny ad?"
five days of natural building (wofati and cob) and rocket cooktop oct 8-12, 2018https://permies.com/t/92034/permaculture-projects/days-natural-building-wofati-cob