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RMH: Specific heat of cobb vs water  RSS feed

 
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While I've seen some very ingenious hot water heaters heated by rocket mass stoves (i.e. Nick Ritar's), I haven't seen many people use water as a heat mass.  More people seem to tend to gravitate towards cobb as a heat mass.  I think waters high specific heat and ability to efficiently conduct heat would make it an excellent  heat mass for a rmh, but I'd be a little concerned about it's weight.

Since I'm quite a noob when it comes to RMH's, I have a few questions:

1. What's the specific heat of cobb?  Water has a very high specific heat of 4.18 J/g.  I know this will greatly depend on the clay source and ratio of straw to clay, but does anyone know a general estimate?

2. What's the density (gram per cu cm) of cobb?  I've heard Mr. Wheaton mention a few places that it's quite heavy. 


Thanks!
 
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Location: Washington
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According to one website, the heat capacity of "sandy clay" is 1381 J/kgC, while "wet mud" is 2512. I doubt anyone has done a formal test for cob. Either way, it's a lot less than water. I can't imagine water would weigh much more than cob, but I wouldn't be the best one to ask. Water would be a superior thermal mass when looking at the numbers, but I imagine dealing with a liquid for a RMH has some heavy disadvantages too.

 
pollinator
Posts: 163
Location: Point Arena, Ca
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Actually, water ain't that great as thermal mass.
It does have a high specific heat, which looks good at first..
The biggest problem with water is that the phase change temperature of water is comparatively VERY low.. You can't get much heat into it before it turns to steam, which is quite dangerous. Also, it tries to get out of whatever you put it in and given enough time, it WILL find a way.
'Course, the advantage is that it can be pumped around.

I don't know what the specific heat of cob is, probably not very high compared with water. The advantages of cob though completely outweigh this one disadvantage. Those being that it can be made into ANY shape, it's SUPER cheap, non flammable, provides decent thermal mass, is very easy to work with and more.
 
pollinator
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Location: Victoria British Columbia-Canada
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I'm in the demolition business and large vessels capable of containing water are a dime a dozen. Big propane tanks, oil tanks and other vessels are commonly cut up for scrap. One of these items could be set in place at the end of a cob bench and  molded into the structure. Heat from the cob would transfer to the vessel without coils or other piping. An open vent to the exterior would deal with the unlikely event of reaching the boiling point. If a clean vessel is used water could be drawn off for bathing and other household uses. If an oil contaminated vessel is used a heat exchanger could warm up well water before it hits the hot water heater. Used oil tanks have little if any market value and would be ideal for this purpose.
 
Dale Hodgins
pollinator
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    The heat capacity of cob is .2 and this is one fifth the heat capacity of water. Heat capacity is concerned strictly with the weight of a given material and not with its density. Any mix containing, common clays, sands, bricks, rocks and pebbles will come very close to this figure.

    In the discussion called"rocket stove efficiency/certification"I give the details of how this was worked out including the engineering tables. I've also worked out a very clear plan on how to arrive at a reliable efficiency rating based on burning a known weight of fuel inside a rocket stove of known weight. If you have a rocket stove and live on southern Vancouver Island or in Vancouver I would be interested in conducting this efficiency test and publishing the results. A properly conducted a test like this is what building authorities are going to want to see rather than huge amounts of anecdote.
 
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COB specific heat capacity, for an average moisture content of 3%, was -891 J/kgK. Nomenclature C Specific heat capacity of the cob material (J/kgK).

https://www.witpress.com/elibrary/wit-transactions-on-the-built-environment/16/10643
Composition, Effective Thermal Conductivity And Specific Heat Of Cob Earth-walling
Author(s)
S.M.R. Goodhew, P.C. Grindley & S.D. Probeif

Samples of cob earth-walling of the type used in vernacular buildings in the West Country region of the United Kingdom and thought to be of significance from both structural and historic points of view, have been assessed. The compositions of the samples were as expected. A transient thermal-probe technique was used to determine the thermal conductivities (~0.6W/mK) of the samples in their normally-moist state.Their specific heat capacity, for an average moisture content of 3%, was -891 J/kgK. Nomenclature C Specific heat capacity of the cob material (J/kgK). k Apparent thermal conductivity of the cob material, (W/mK). PVC Polyvinylcloride
 
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Containing water, and having enough of it with efficient enough heat exchange, to store the amount of heat the stove delivers without turning much of the water to steam, are the real challenges of using water.

 Masonry/cob on parts of the interior surfaces of ducting or a bell might be 3-600F by the end of a burn, but the diffisuon of that heat into connected mass, toward the outer oayer of cob, make it so our butts never see 300-600F...

 Water can convect heat away, and allow it to stratify toward the top of a container being heated...  and can be pumped through insulated piping, to deliver that heat to remote areas...  and with apparently about 5x the heat capacity by weight of cob, has it’s advantages for certain types of installations.

Reina LLC has some interesting videos about their solar heated tanks that along with super insulation, and use of solar heat gain in the sunny season, allow them to make net zero homes in Alaska.   The key is that they install solar thermal panels, and charge 5-15 thousand gallon insulated stratification tanks built into the home.

In my mild climate, I could probably do well with 300-500 gallons of water as solar thermal storage.

Their video “Mythbusting Solar Thermal” on youtube is quite excellent.
 
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