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phase change materials for heat sinks  RSS feed

 
                        
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For people who (like me until yesterday), haven't heard about these materials:
Phase change is stuff that changes from liquid to solid  (e.g. water to ice) and back again but with stable higher and lower temps...water will change to ice and stay that way no matter how cold it gets and under normal conditions won't change from liquid to vapour unless it gets WAY over the temp at which it turns back into a liquid, but instead just absorbs the heat that is warmer than it is and then releases it again when the ambient temp is colder than it is. The major problem with water is that it changes volume and if heated beyond the liquid stage can explode its container with rather dramatic effect.

Apparently a company in Germany has developed a miniscule bead of paraffin enclosed in (plastic of some sort I think) which acts to absorb the heat of the room and then release it over time. The beads have been embedded in gyprock panels in the States and are undergoing testing but it appears as though various versions of this are already in fairly widespread use in Europe.  Paraffin is one of several phase change materials but there are some problems with most of them ..caustic or poisonous or lose their changeability over time..this company appears to have solved the problems associated with wax.

Anyone know anything about these materials? I am looking for something  to use when cob/clay/brick/gravel/rock etc is not an option.
 
Dale Hodgins
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        Phase change salts were all the rage in 1970s and 80s solar home construction. Corrosion and failure of containers where the main problems. Initial cost was also a huge factor. Eventually many builders settled on Trombe walls, concrete floors and other building styles high in thermal mass.    But I'm still hoping for some sort of magic phase change material which will eliminate the need for all that heavy mixing and lifting.

    There's a handy online resource called the engineering toolbox. If you search heat capacities you'll find just about every solid youth ever heard of listed. This will give you an idea of how much heat can be stored in any given material.  The best bang for the buck probably comes from gypsum since it is available for free in the form of used drywall.
 
Frank Rasmussen
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Maybe this is a stupid question, but could paraffin wax be embedded into the cob thermal battery? e.g. as you mix the cob, put in beads of paraffin wax in the mixture. Paraffin wax vaporizes at 370 degrees Celcius, so I doubt it is going to cause an explosion like water would unless it was right near the "hot end" and next to the flue. I'm not sure whether it would be thin enough to migrate through the cob, like water does. If that happens, it may or may not be appropriate to use.

However, one might be able to add paraffin wax in some form of container, either in something like jam jars (might shatter), plastic containers perhaps (would worry about high temperature tolerance), or metal jars (higher cost, would cope with temperature and heat stresses possibly). These could be embedded throughout the cob, or in the case of a masonry heater, encased in regions within the brickwork, with cob perhaps in between. If the containers are small enough, the thermal insulating properties of the paraffin wax would be negated.

I've run the calculations last night, and I suspect that paraffin is probably superior to water for the purpose of a thermal battery. Of course, you can't put it anywhere that it might react with oxygen and a flame, but other than that, it has both a remarkably high specific heat (compare 2.9J/kg/K with water at 4.2), and then you have the enthalpy of fusion that is going to want to keep the temperature at the range of whatever your paraffin wax likes to melt at, which is very significant at 200kJ/kg (water is 334kJ/kg). You could probably get a very good result out of maybe 500kg of paraffin wax or less, which is 555 litres (the density is 0.9kg/l).

One benefit would be that once your thermal battery is charged up, it maintains its temperature (like an electric battery maintains voltage) rather than dropping the temperature out of comfortable range. Another benefit is reduced space, so you could either have a heater with a smaller footprint, or less frequent charges with a larger thermal battery (you might get several days out of the one, large, charge). Another benefit is the lack of moving parts and corrosives. Once built, it would theoretically stay built and stay functional. Thoughts?

Another idea would be to encase the heat exchange barrel in a larger barrel of paraffin wax, which would be sealed. This would obviously need to be tested outside for safety away from people or surrounding structures and with good temperature monitoring, but I would think that because of the point of this is in part to create a draw, theoretically the cooler you can cool this part of the system, the more draw you will create. Any liquid is going to outperform air in terms of transferring heat I would think. The paraffin wax would soon melt in this area, and you would get a convection loop happening pretty quickly. It would obviously need to be sealed. Would be interesting to try.
 
Frank Rasmussen
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I've been thinking about this some more and note that paraffin expands when it melts and shrinks when it solidifies. Perhaps it could be put into empty coffee cans, maybe 85% full so that there is some room to expand. These could then be placed in the cob. A metal coffee can style container would conduct heat well, probably not be too big that heat exchange becomes an issue, and would be impermeable for the life of the heater.

You might even get away with empty soda cans, as long as they are orientated mouth up. Should be able to get these free and in quantity. Once fixed in place, the paraffin wax should not evaporate, and if the can is less than 90% full the paraffin won't contact the cob. These are also a small enough size that I would expect the heat would transfer well enough for the purpose.

The wax itself costs about $4/kg or less for quantities of 20kg. I'm sure if you bought 500kg of it, the cost would go down significantly. Possibly soy wax might also be used. You could quite easily tell which wax is superior with a simple stovetop test. Set a temperature above both melting points, and whichever wax reached the temperature last (starting from a cold plate) is the one you want to use.

As to the ideal melting point of the wax you use, this might depend on how hot you want the room (higher melting point means a hotter room), and the lower melting point probably means lower exhaust temperature and hence more efficiency perhaps, but also a lower boiling point so you would not want that to be too close to the hot area's flue. You might use two or more different melting points for different parts of the thermal battery. Higher melting point for the hot area, and lower melting point near the exit so that you can still scavenge the remaining heat.
 
allen lumley
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Frank R. : Good review, I especially like the idea of using wax of two different melting temperatures, in two different places in the Thermal Mass.
If this turns into a personal project, please keep us informed ! For the Craft ! PYRO AL
 
Frank Rasmussen
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allen lumley wrote:Frank R. : Good review, I especially like the idea of using wax of two different melting temperatures, in two different places in the Thermal Mass.
If this turns into a personal project, please keep us informed ! For the Craft ! PYRO AL


Thanks Al. I will keep the forum updated. I rent, so unless this can get built somehow in the basement (or find a friend that wants to make one) it's not going to happen. If I was going to build it myself, I would need to figure out how to do in a much smaller volume than a conventional RMH. But that can be surmounted.

A couple things I want to add to this thread. Creation costs of energy consuming devices such as heaters and cars are usually far less important than the efficiencies of those devices over a lifetime. 400kg of wax in energy terms is like 10 fillings of a car's gasoline tank, or equivalent in energy to maybe a quarter year's worth of RMH fuel. over the lifetime of a RMH that's nothing.

I want to caution other experimenters out there thinking of incorporating paraffin that one has to be wary of the smoke point as well, which is about 199°C. However, this can be dealt with if you can ensure that there is no air in contact with the wax, ever, and that the vessel can cope with either dealing with a vacuum or increases in size of 10% or so. Then you only have to worry about the boiling point. Alternatively, one might test and design to ensure that the smoke point is never reached.

It would be very interesting to see logs of temperature probes placed along the exhaust line in a typical RMH so that one knows what to expect.

Yet another idea that could be successfully incorporated in RMH is the use of basalt. I have yet to hear much about its use on this forum. It is not far from soapstone in terms of high thermal mass, but far cheaper. And of course, there are no flammability or explosion issues.
 
R Scott
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There is lots of work being done with phase change in "useful for human" temperature ranges for protective gear (think firefighters and foundry workers) and recreation (motorcycle riders and race car drivers). There are a few non-toxic versions out there that change in the 60-80 degree range. They would be a good place to start research, as they can also be used for summer cooling mass.

You could build it into a giant double-boiler setup encased in water to make sure you never hit the smoke point.
 
allen lumley
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R. Scott : Could you give me some examples of personal wear equipment w P-C Materials ?

I was thinking that if a phase-changing paraffin wax could be sealed in silicone balls similar to Bucky Balls, and constituted as a 'putty' that it might, being flexible-ish,
and slowly changing volume as a result/side effect of 'going thru the change' , might improve the long term sealing of the barrel to the rocket stove or last thru
a few removals/re-instalments! I know its reaching but so was lithium grease ! As always - all comments are solicited, appreciated, and welcome !

For the Good of the Craft ! - Be safe, keep warm ! - PYRO AL
 
R Scott
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http://www.glaciertek.com/RPCM_Cooling_Vest/default.aspx?AspxAutoDetectCookieSupport=1

http://www.grainger.com/Grainger/ALLEGRO-Phase-Change-Cooling-Vest-Inserts-5WYD6?gclid=CJP-3vnmmLcCFa5DMgodzD4AvA&cm_mmc=PPC:GooglePLA-_-Safety-_-Workwear-_-5WYD6&ci_src=17588969&ci_sku=5WYD6&ef_id=UXGVlQAABe8McCCp:20130515190534:s
 
allen lumley
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R. Scott : Great, Thank you ! big AL
 
Marc West
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Hi all,

Firstly, to answer the original poster: there is an encapsulated PCM material called "Micronal" made by the German company BASF:
http://www.micronal.de/portal/basf/ien/dt.jsp?setCursor=1_290798
but I appreciate that that post dates from a couple of years ago...

I have a slightly different enquiry, and I wonder if anyone can help.

I co-own some land with a small community, and we wish to build a cobb wall on it, partly to provide support and warmth for trees like apricots and peaches. (We are in Cardiff, Wales, UK). Now, it has long seemed to me that incorporating PCM in the cobb structure would be a great way to extend the warm period at the end of a hot day. I've just read this thread and realise that paraffin wax in tins embedded in the wall may be just the thing. (Thanks to Frank R for his clear posts!)
The issue for me, then, is to find a paraffin wax that melts at the right temperature, which I guess should be in the region of 22 degrees C or so, for it to be most use for extending the warmth of the day for plants. (Anyone have a better guess?). Now, as I understand it, paraffin wax has a melting point of between 46 and 68 degrees C (variable because paraffin wax is not a single substance but a mixture of compounds [called 'alkanes'] of the form CnH(n+2), for example C25H52). So using 'ordinary' paraffin wax wouldn't be any use, because its melting point would be too high.

So how to get a lower melting point paraffin wax-like substance?

2 possibilities occur to me:
1) mix in or use some shorter chain alkanes. Ought to work fine, but looks like it will be a lot more costly than standard paraffin wax is;
2) Dissolve something into standard [inexpensive!] paraffin wax to lower the melting point, in rather the way that dissolving salt in water lowers its melting point (which is what we do to ice to make it melt when on driveways etc). Does anyone know what substances can be dissolved in paraffin wax to lower the melting point? Salt? Some other "more organic" substances that will dissolve better? I'd be happy to experiment with a thermometer and a saucepan of the stuff in a water bath, but I don't really understand how to calculate the likely effect on melting point of dissolving a certain quantity of material X into paraffin wax.

Not being a chemist, I don't really know where next to go with this. I'd appreciate any thoughts you guy may have on this. Or if someone knows of affordable off-the-shelf paraffin wax substances with melting point in the right range perhaps they could let me know.

Thanks in anticipation...

Marcus
 
Frank Rasmussen
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Marc West wrote:Hi all,

Firstly, to answer the original poster: there is an encapsulated PCM material called "Micronal" made by the German company BASF:
http://www.micronal.de/portal/basf/ien/dt.jsp?setCursor=1_290798
but I appreciate that that post dates from a couple of years ago...

I have a slightly different enquiry, and I wonder if anyone can help.

I co-own some land with a small community, and we wish to build a cobb wall on it, partly to provide support and warmth for trees like apricots and peaches. (We are in Cardiff, Wales, UK). Now, it has long seemed to me that incorporating PCM in the cobb structure would be a great way to extend the warm period at the end of a hot day. I've just read this thread and realise that paraffin wax in tins embedded in the wall may be just the thing. (Thanks to Frank R for his clear posts!)
The issue for me, then, is to find a paraffin wax that melts at the right temperature, which I guess should be in the region of 22 degrees C or so, for it to be most use for extending the warmth of the day for plants. (Anyone have a better guess?). Now, as I understand it, paraffin wax has a melting point of between 46 and 68 degrees C (variable because paraffin wax is not a single substance but a mixture of compounds [called 'alkanes'] of the form CnH(n+2), for example C25H52). So using 'ordinary' paraffin wax wouldn't be any use, because its melting point would be too high.

So how to get a lower melting point paraffin wax-like substance?

2 possibilities occur to me:
1) mix in or use some shorter chain alkanes. Ought to work fine, but looks like it will be a lot more costly than standard paraffin wax is;
2) Dissolve something into standard [inexpensive!] paraffin wax to lower the melting point, in rather the way that dissolving salt in water lowers its melting point (which is what we do to ice to make it melt when on driveways etc). Does anyone know what substances can be dissolved in paraffin wax to lower the melting point? Salt? Some other "more organic" substances that will dissolve better? I'd be happy to experiment with a thermometer and a saucepan of the stuff in a water bath, but I don't really understand how to calculate the likely effect on melting point of dissolving a certain quantity of material X into paraffin wax.

Not being a chemist, I don't really know where next to go with this. I'd appreciate any thoughts you guy may have on this. Or if someone knows of affordable off-the-shelf paraffin wax substances with melting point in the right range perhaps they could let me know.

Thanks in anticipation...

Marcus

Hi, thanks for the compliment. I haven't yet got to playing around with paraffin wax for this purpose yet. However, a quick googling suggests that petrolatum (petroleum jelly) may be added to reduce melting point of paraffin wax. Since that melts at 37 deg C, it might pay to add something with shorter chains, perhaps a mineral oil. Good luck.
 
allen lumley
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Mark W. : Welcome to Permies, I'm sure that we will see you often in these forums!

I am struck over and over again where Some Clever person discovers a piece of research, that an original discoverer had published out to the
world - a '' High guys did you know that this would do that ? " Followed by YEARS of ?apathy? And then a SECOND original thought occurs and
a new, or very old idea is finally exploited ! Here is where I usually say "You will long remember your First Post here!'', I think that our com -
bined futures is found in inquisitive minds like yours, and we will long remember your first post here, with a 'I remember him when' nostalgia !
Keep on bringing in your contributions and hopefully together we will find the little gems without loosing any HOPE Diamonds because we 'Dont
like the color of that one' ! !

For the Good of the Craft! Be safe, keep warm! PYRO Magically Big AL - As always, your questions/comments are solicited and are welcome! A. L.
 
Marc West
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Hi Allen Lumley,

What an extraordinary and wonderful welcome! Thank you so much for that.

I'll let you know how I get on with my search for the right PCM material. In the meantime, if anyone has any thoughts about altering the melting temperature of paraffin waxes through mixing or dissolving or ...... then do please post!!
 
Frank Rasmussen
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I had some more thoughts, Marc. I was thinking of straight paraffin wax for a thermal mass in use with wood heat, so that it is at a temperature that will be able to bring down the temperature of the exhaust gas to a point where most of the energy is sucked out of the exhaust stream (for efficiency), while still being able to radiate/convect the heat effectively to a room. This is not what you want for walls. If you just want to capture the energy from sunlight on a warm day, I would look for something that changes phase at room temperature, possibly at the minimum (20C). That way it is absorbing heat from the moment the wall gets hotter than is necessary, and the low temperature inclines it to suck in more heat from the environment.

Paraffin wax on its own is too high melting point for this application. I would look further down in melting points, such as petrolatum (petroleum jelly) and mineral oil (baby oil). Mineral oil is pretty cheap. Maybe they can be used straight or in combination? I don't know. I note that DuPont Energain (TM) has a melting point of 22C and is composed of a paraffin wax (not sure exact chemical composition, but certainly petroleum based).

Some experimenting is probably required, let us know how you go.
 
Marc West
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Hi Frank,

Your post puts me in mind of the Kachelofen (='Tile oven' - really a wood burning stove), where there's a combustion chamber and a very long exhaust path which winds its way through a lot of thermal mass and gives up most of its heat on the way. There's a rapid, high-temperature, very high-efficiency burn which puts a lot of heat into the thermal mass, and the heat in the thermal mass diffuses out comfortably for the next 10 hours or so. Anyway, one of the virtues of this kind of heater is the high temperature of the burn, which produces very efficient combustion (which continues along the length of the exhaust path) in which almost all that is produced is waste gases [and heat] - very little tars or other gooey substances. (You can get tars condensing in chimneys in systems with less efficient combustion, and they can cause trouble - I remember reading an account of tar dripping onto the congregation in churches with less efficient heaters, and they can catch fire - the tars, not the congregation, that is). The point of this paragraph? - I wonder whether extracting heat from the exhaust gases might cause more tar production etc. Just a thought...

You say: "If you just want to capture the energy from sunlight on a warm day, I would look for something that changes phase at room temperature, possibly at the minimum (20C). That way it is absorbing heat from the moment the wall gets hotter than is necessary,..."

Well, exactly, hence my comments about trying to devise a paraffin wax mixture that melts in the region of 22 degrees.

HOWEVER, I have since been wondering whether my approach is wrong. Do I really need a phase change material at all? If we are able to control the thickness of the wall, then can we calculate and choose just the right wall thickness so that it will absorb the sun's heat that falls on it and will undergo a slight temperature increase as it absorbs that heat and conducts some of it to its interior, and then as the sun goes, that stored heat is available to radiate out of the front again at a nice slow rate. (We will need to insulate the reverse side of the wall to prevent it 'leaking' out in that direction). By carefully choosing the wall thickness, can we contrive to get that 'slight temperature increase' so that the stored heat doesn't radiate out at too fast a rate, so we get that slow release of heat to the plants that we want. I suppose that this matter is at least partly governed by the thermal conductivity of the wall material - if that were really low then the surface of the wall would heat up more rapidly than we want, irrespective of the thickness and thermal mass of the wall. Any thoughts, anyone - do I really need phase change material after all? I've already said I'm not a chemist. I'm not a thermodynamicist either...

 
Tom Rutledge
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Rather that using paraffin, use oils ( coconut, lard, bacon grease, margarine, coco butter, vegetable shortening, palm oil ), maybe sugars for higher temperatures, possibly crystalized honey.

Funny story time. So there I was in the kitchen. I looked over and saw a nice jar of clear coconut oil. I expected to see the normally opaque version of the solidified stuff. I picked it up and shook it around. It was very melted. Yup, that's how you know it's a hot day in the kitchen.

2+2 = 5

As for containment and testing. Set samples in canning jars wherever you'd like to test it.

(very much guessing is bellow)
To get the oxygen out of the container It might be possible to run the jars through a canning cycle (add some water to displace the normal air, then boil off).

Maybe a better choice would be to fill and seal the jars in a liquid bath of the stuff. If the jars are inverted and a little nitrogen is injected into the jar for some expansion room ( http://www.winepreservation.com.au/Gas.html ). Seal the jar under the bath and no oxygen should be there.

To scale up the system 50 gallon drums or 275 ibs totes.

Failure modes?
I wonder about the long term stability of such systems. As was pointed out, rusting and such might become a problem. Fire safety in a fire is of course horrible and it may be horribly against code to have a few thousand gallons of about to burn liquid fuel laying around...

I'm totally going to be doing this at some point.


Notes :
(stay far below this temperature ) http://en.wikipedia.org/wiki/Smoke_point
(science ) http://en.wikipedia.org/wiki/Enthalpy_of_fusion
( interesting, but toxic giky possibly ) http://bem0665.cafe24.com/korean/UserFiles/File/SuminKim_paper/SCI%2068(1).pdf
( as Allen Lumley hypothesized , research with enough references to keep people busy for quite some time ) http://web.missouri.edu/~suppesg/paper31.pdf

Pyrolysis products condensed? (pine tar, pitch, etc)

Even with large tankage costs I'm guessing that large water tanks are cheaper safer solutions if they work for the use case.
 
Darren Hatina
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Perhaps it's possible to make a greasy high performance cob

Rather than using water to dissolve the clay in order to bind the cob, and then having to dry/drive the water out again - cob could be made by mixing earth with some melted coconut oil, vegetable shortening, or even BACON GREASE!!!(emphasis for Paul) in a sauce pan. It works for cookies and they're delicious. Throw in some iron metal filings(from a gas pipe fitter) and you'd end up with a cob mix with improved density, heat conductivity, specific and latent heat, and possibly stronger. Most importantly it would smell like bacon and coconuts!

I am really looking forward to the comments on this one.

Limitations of course..
-The ratio of greasiness would have to be limited so it didn't drop out of suspension when melted or become too plastic
-It would also have to stay far enough away from the heat to not start smoking

My wife won't let me touch her coconut oil otherwise I'd be baking some cob cookies right now.
 
Tom Rutledge
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Small batches of greasy cob should be doable. Try some out. By varrying the amount and type of pcm material used one might get a fair idea of the practicality.

 
Len Ovens
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This is an interesting thread. The idea of using multi temp materials is good... However, I think that the materials being suggested for use in a RMH mass are wrong... at least for all but the very last part. The thing to remember is that you do want heat to move. For that to happen the mass needs to be hotter than the room to heat. Probably the minimum thaw point should be close to 200F. Many of the suggested materials (bacon fat for example) melt below 100F which is too close to the target room temperature. I'm thinking 68F or less... but that is a different topic... think 65 to 72 in any case, but that is a small range.

So materials with higher melting points that are:
- cheap
- do not need to be mixed with some other liquid (the way salts do and they don't stay suspended very well over time)
- are not corrosive (like maybe not sulfur ... melts at 240F which would be ideal for heating... but has problems with reacting with metal containers)

I have actually been thinking about using Tin which melts at 450F. I am thinking that something the size of a pot belly stove with a 3 or 4 inch rocket might work very well. Tin has actually been used in solar cooking projects where the Tin is heated to melting and then kept hot in an insulated container for cooking after the sun sets. The key here is "insulated container". We know what temperature we want the room to be and so we have the temperature differential from the Tin to the room. The Tin is at 450F and the room at 68F. We want the outside of the insulation case (the case would be black sheet metal) to be 150F to 200F depending on the size of the room, how well the walls reflect or absorb heat and other factors which we have in general not bothered with when thinking about heat, but should have We should be able with this information to calculate the "R" value needed for our insulation.... electric range manufactures do this all the time when deciding how much insulation they need between the inside of the oven and the wood cabinet an inch away or less. I have called this heater a RPCH (rocket phase change heater ) and in fact did start a thread about it some time back. However life is still in the way... it is on my list. This would not be the standard contraflow as the RMH is, but rather more like the rocket stove used for cooking where the flue path is always rising. My idea is that the Tin container would be removable and that when put in another insulator could be moved to another part of the house to be used for heat there.... for example the heater heats the main room/kitchen and at bed time the Tin container could be taken to the bedroom to prewarm the bed and then keep the room warm for the night. It could even hang inside a canopied bed. This would be small enough to use in a boat or a very small cabin. The hole where the Tin pot was removed from could be used as an oven for cooking in if a lid/door was affixed. More than one Tin container could be kept on hand so that while one was being used as a stove hob to cook on another could be in the heater be heated. The insulating case could be "over insulated" and then vented with a variable port to regulate how fast the stored heat was transfered to the room. Ideal to put under the table with a long table cloth while the family eats as has been done in the past with a charcoal heater in Japan and other places.

To answer Frank Rasmussen's comment:

If I was going to build it myself, I would need to figure out how to do in a much smaller volume than a conventional RMH. But that can be surmounted.

For the same performance:
A RMH that used water as mass would be 1/4 the size of a masonry RMH and perhaps less than a 1/4 of the size of a cob RMH. Note that this is not just size but weight as well.

Now, we add the idea of phase change materials and the size/weight drops again. So the use of phase change materials instead of straight mass is all about space and weight saving. Otherwise there is no purpose to choosing a more expensive option than free cob.

 
Len Ovens
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Just as an added note... one of the reasons masonry heaters and cob RMH work so well is the partly insulating properties of these materials that slow the release of heat over a longer time.
 
Topher Belknap
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My business partner worked on PCMs for the department of energy back in the seventies. He said that most of the hydrated salts worked great for a while and then they started separating into water and non-hydrated salts. Wax had problems in large volumes (thus the current small capsules).

The main problem is that it is hard to beat recycled brick in terms of cost. BTU/Degree by dollar is vastly in favor of cheap high mass stuff. The only reason to use PCMs would be if you had severe space restrictions.

Or, of course, the state of the art might move on.

Thank You Kindly,

Topher
 
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