doug
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Post by doug on Jan 16, 2011 19:36:59 GMT -8
To put a RMH on a framed floor generally requires substantial reinforcement under the joists to hold the weight. Are there any potential materials that hold a lot of heat but are light weight? A crude example: I have a titanium framing hammer. It has the same mass as a steel hammer, thus it imparts the same force, but it weighs a fraction of the steel hammer, and is kinder to my wrist and elbow. Are their materials that have the same heat storage capacity as cob, but weigh a lot less?
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Post by shahborn on Jan 17, 2011 16:48:51 GMT -8
I dont know it it will work like cob, but if you mixed some perlite in with the cob mix it should reduce the weight some. Just and idea, i have not tried it myself.
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Post by Donkey on Jan 17, 2011 18:16:53 GMT -8
Umm... Weight usually implies mass.
I think that water has one of (if not) the best weight to heat storage ratios..
I would think that adding pearlite would make the bench more insulative, which isn't what you want.
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joe
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Post by joe on Jan 18, 2011 9:07:09 GMT -8
You could also use wax as a heat storage battery. It has a fairly high specific heat capacity, and as it melts, it will store heat energy in the from of "latent heat of fusion". During this stage, the wax will not increase in temp...likewise, as it solidifies, it will remain at a constant temp (about 140-160 deg F.), while shedding lots of heat energy to its surroundings.
Joe
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Post by nedreck on Jan 25, 2011 8:38:53 GMT -8
You have mistaken "mass" for volume.
Mass is indeed weight excluding the gravitational constant, in other words mass is the density of a material at a given volume.
Total heat content is measured by mass not volume.
Thermal mass storage is simply capturing heat that is produced quickly and placing it into a large mass that will release it at a preferred rate at a later time. It is also used to store "reduced heat" as well, meaning you take a lot of heat out of a mass so you may use it later to absorb heat from something else later.
A building might have a huge heat load during the day, but at night that load might only be 30%, instead of running the cooling equipment at 30% run it at 100% and remove heat from a thermal mass and then have that thermal mass absorb heat from the building tomorrow during the high heat load.
Instead of blasting the area heated by a RMH, the heat energies produced are put into a large thermal mass that releases that heat over a long period of time at a reduced rate to provide many hours of warmth from a few hours of firing time. There is not "increase" of enrgy, it is still X btu's but instead of super hot releasing 20,000 btus in an hour, the 20k is stored and released at 5k for five hours, same 20k but longer release.
Materials store heat by "mass" not volume, the density of a material dictate the volume, conductivity dictates the rate heat will be absorbed and released, surface area dictates how that rate will be distributed.
In other words, from a practical view point, the weight of the material will always dictate how much heat you can store, a combination of the rate of conductivity and exposed surface area (sort of volume) will dictate the rate heat will be absorbed into and extracted out of said material, finding the balance for the application is indeed key.
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Post by nedreck on Jan 25, 2011 8:41:08 GMT -8
no edit key, should read 5k for FOUR hours, brain fart only excuse  |
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bryan
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Post by bryan on Feb 8, 2011 10:51:02 GMT -8
Nedreck has corrected the terminology but has not gotten to the root of what doug was asking. The answer is yes, there are different materials which store different amounts of energy for each unit change in temperature. As Joe stated this is the specific heat capacity and as noted water is quite high (about 5 times typical stone). So to raise 1000 lbs of water 25 degC will take 5x the energy of raising dirt 25degC. And so it then will also store and release back to the room 5x better.
Wax is about 3x better than soil in terms of specific heat capacity. Heating it enough that it changes from solid to liquid is a whole different thing though. The energy required to change phase with most materials is very substantial. For example if you put water in a pot on the stove you can bring it up to a boil in 5 minutes but to boil the whole pot may take hours. Melting the wax is the same thing, though it is a bit scary to use wax since it is pretty flammable! There has been a lot of research on salts which can be melted and have large latent heat values (solid to liquid energy requirement).
For example for melting water (solid to liquid) the latent heat of fusion is 330 kJ/kg. So to transition 1 kg of ice to liquid takes 330 kJ (about 310 btu's). In comparison the specific heat capacity of liquid water is only 4.2 kJ/(kg degC). So if you add another 330 kJ to the 1kg of water it will heat up 78degC!
So with the fancy 'salts' that have been studied they can adjust the chemistry to set the melting temp and say put it at 150 degF. Then throw lots of heat into it to melt it and then stop heating it and it will release back to the room while it resolidifies. For a RMH though it will be a little tricky since it will be similar to trying to heat a water tank with an RMH. The catch is that it is hard to get the surface area needed, whereas 30ft of air duct into a bench provides lots of surface area.
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Post by nedreck on Feb 10, 2011 8:32:54 GMT -8
Nedreck has corrected the terminology but has not gotten to the root of what doug was asking. The answer is yes, there are different materials which store different amounts of energy for each unit change in temperature. As Joe stated this is the specific heat capacity and as noted water is quite high (about 5 times typical stone). So to raise 1000 lbs of water 25 degC will take 5x the energy of raising dirt 25degC. And so it then will also store and release back to the room 5x better. 1. One at over 62 pounds per cubic foot, water is by no means what so ever low weight. 2. when you start entering the latent heat associated with the chance of state in the materials, you develop a lot more energy due to the potential energy release, however even salted water expands huge once vaporized and significant pressures can develop in seconds. Not really a task one should undertake without significant understanding of what happens internally IMHO. 3 water is popular BECAUSE of its high mass and fluidity, that does not mean it is a better choice for energy storage and release than solids, it just means you can move it with ease. 4 the purpose of the cob is not to absorb heat fast, it is the opposite, the slower something absorbs heat, the slower it will release it. The RMH wants the insulation to drive the internal temp up and folks want the slow release of even heat in contrast to high and fast heat which leads to other losses as well. There also is nothing light at all about a vast majority of salts. I would also offer that very few folks on the face of the planet have experience with molten salt although it is a cleaning technology used primarily in industrial applications. I can not possibly imagine the layman being capable of a molten salt application without a year or betters research. When working with LiBr you have to maintain such a careful balance at all times. I would actually wager there are not 500 persons in the US capable of trimming a lithium bromide system today. For those of you wondering what I am talking about, when you deal with salts the mix has to be near perfect at all times because as soon as it gets out of whack, it crystallizes and there you are, whammo, pipes and exchangers filled with solid salt! I know I can be kind of a prick by personality, but the root of what he was asking was for a large light weight material and since he said it incorrectly I tried to be clear, but suggesting water fits that bill is far from correct and then to explore salts which is a very technical field with few whom even grasp the basics when we are talking about a heater that is specifically designed to be basic, well that does not make sense to old Ned, none at all. I mean really dude, an ionic fluid where the vast majority of folks are going to be working with soft metals and certainly not high nickle stainless steel? No offense Brian, but remember to keep it simple when you have your thinking cap on and remember to snug it up when you opt to correct others because little of what you said makes application specific sense. Peace, Ned |
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Post by artificer on Feb 10, 2011 10:18:23 GMT -8
I've been looking into phase change heat storage for several years. Since I have a source of waste motor oil, I was intending to build a boiler that burned that fuel. Like a rocket heater, it needs someone there to watch it while its on. Since water tanks typically cost around $1/gal, I wanted to minimize the tank size. In the long run, its just cheaper/easier to get a bigger tank, or build something yourself like a feroconcrete one. Another idea is to just use more thermal mass, even if it doesn't have as high BTU/lb as water for thermal storage. My woodshop still doesn't have a concrete floor yet. I've been thinking of excavating the floor down a foot deeper. Put a thin layer of sand in, then the floor insulation. Cover that with 12" of sand and the radiant heating coils. Concrete on top, and you have 57 TONS of thermal mass. Cost: $500 And the best part is that it is out of sight with no maintenance. Since the OP has problems already with weight, I would suggest water storage. You don't get into the long term problems of the phase change materials, but you still get 5x the heat storage, or have 1/5 the weight. If done properly, you can even remotely locate the storage tank. If the tank is lower than the heater, you can use a pump to actively move the water during the heating stage, and a passive thermosiphon stage to reheat the cob, or just a radiator in the room. With water storage, the tank must be vented to avoid pressure buildup. Watch the Mythbusters exploding water heater video to see why... dsc.discovery.com/videos/mythbusters-exploding-water-heater.htmlMichael |
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bryan
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Post by bryan on Feb 10, 2011 21:10:44 GMT -8
Well I seem to have somehow invited Nedreck into a flame war all because I said you were correct but that you didn't answer the original question. Yes then I rambled a bit off topic because while I was not suggesting using a salt system as a homebuilt RMH, it is an interesting topic and people on this forum generally seem to like investigating things like that.
To be straight to the point: Mass does not equal heat storage. 1 lb of stone does not store nearly as much heat as 1 pound of water (and I am not talking about phase changes). The specific heat of water is 5x higher than average stone.
Also the best setup is not specifically to have something that absorbs heat slowly and releases it slowly. The best would be that the inside of the mass absorbs heat as readily as possible from the exhaust gases and then outside of the mass was insulated to the right level so that it releases the heat into the room at the desired rate.
So as artificer is saying a possible design (that would certainly have issues to work out) would be to heat a mass of water with an RMH. Water's density (mass per volume) is 30 or 40% less than clay but since its specific heat is 5x higher then the water would store 3x more heat in the same volume and 5x more for the same mass. I would thus say that using water instead of clay could indeed be called light. But no it is no particularly low density.
Bryan
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Post by nedreck on Feb 10, 2011 23:02:44 GMT -8
Well I seem to have somehow invited Nedreck into a flame war all because I said you were correct but that you didn't answer the original question. Yes then I rambled a bit off topic because while I was not suggesting using a salt system as a homebuilt RMH, it is an interesting topic and people on this forum generally seem to like investigating things like that. To be straight to the point: Mass does not equal heat storage. 1 lb of stone does not store nearly as much heat as 1 pound of water (and I am not talking about phase changes). The specific heat of water is 5x higher than average stone. No invite to a flame war, I am just kind of a dick, not all your fault and not all mine either, but now that you know it you get the option of dealing with it as you see fit, its some what variable and unless it is a deal breaker for you, I tend to ignore the past. |
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Post by nedreck on Feb 11, 2011 0:04:12 GMT -8
To be straight to the point: Mass does not equal heat storage. 1 lb of stone does not store nearly as much heat as 1 pound of water (and I am not talking about phase changes). The specific heat of water is 5x higher than average stone. Bryan Oops, you forgot something significant sir. If we do in fact assume average stone the the specific heat is indeed .2, however and it is a big however in this case, you forgot the huge difference in intensity required to reach the phase change. In English units instead of SI since that is what we use on this side of the pond, your statement becomes inaccurate because you forgot you changed mediums. Those reading along, Bryan is speaking to specific heat in a single substance which would be: Q=c*m*delta T Where: Q=btu c=specific heat factor m=mass delta T=change in temp within the same medium Water 1=1*1*1f Stone .23=.23*1*1f Now if we said we had 100 Btu 100/1*1=delta or 100f for water 100/.23*1=delta or 435f The problem with the point Bryan tried to make is the boiling point aka phase change. for water that is 212 aka 180 over the phase change to solid aka ice. Stone does not share that same limit. Average stone phase changes at 2,000 degrees F so lets look at the capacity. 2k-32=1968 Q=.23*1*1968 Q=452.64 BTU's More than double the capacity of water, not because Bryan was "wrong" but because we get to heat rocks up much hotter than we can water without causing a phase change. I am still very unclear as to why he said "Mass does not equal heat storage" as it is a functional part of the specific heat equation and a primary point in heat storage. No matter the material selected as you lower the mass you will lower the capacity of the battery. |
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Post by artificer on Feb 11, 2011 9:38:17 GMT -8
... To be straight to the point: Mass does not equal heat storage. 1 lb of stone does not store nearly as much heat as 1 pound of water (and I am not talking about phase changes). The specific heat of water is 5x higher than average stone. ... I have to disagree... sort of. If you take the statement as mathmatical, your units are wrong. lbs x temp = (lb)(degrees), which isn't heat. If you add the proper physical property (specific heat capacity) then the units work out. lbs x temp x (Btu/lb temp) = BTU of heat The simple statement is correct... sorta. I can see what you're talking about, however. I would suggest to not use density, unless you are also talking about volume. Heat storage is going to be a function of mass x specific heat value. nedreck: your argument about higher temps and lower specific heat values having more storage capacity than water is a true, but a bit extreme. It makes your point, but I don't think I want to sit on a yellow hot bench. ;D One of the reasons I like water for heat storage, other than its high specific heat value, is its flexibility. With a solid mass, about the only way to regulate the heat output is with insulation or management of heat input. With water, you can move the heat around, have one way heat transfer (solar powered water heaters in cold climates), easily expand the capacity by adding storage, remotely store the heat, control the heat flow automaticly... In the long run, as long as the RMH isn't on the second floor, it may just be easier to reinforce the wood framing |
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Post by Donkey on Feb 11, 2011 10:11:28 GMT -8
1) The bench CAN be impossibly hot on the inside, yet still comfy to sit on.
2) For me, The MAJOR drawback of water as heat storage is it's fussyness. It want's to get out of whatever you put it in. If it CAN get out it will and if it can't YET, it will eventually. With water you need to regulate heat INPUT and/or resort to safety measures or invite the thumb of removal. Also, if the plan is to use water in radiators or subject it to fairly intense heating, it needs to be softened, otherwise hard-water scale will build up and clog the works.
It is a bonus (indeed the only good reason FOR it's use) that it can be actively (or passively) moved around, though that takes us into another layer of maintenance and gizmo failure.
As I've said on other threads, for me the necessity of pumps and moving parts to provide BASIC comfort in a home means you're fighting an eventual loosing battle, stacking the entropy blocks ever higher, making up for earlier moments of poor design by overextending the credit (so to speak). If you gotta do it 'cause yer past the point of making the earlier decisions and you have little other choice, then fine. BUT, if yer designing complex motorized gizmo-ness into the plan from the start, IMHO yer up the wrong tree.
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Post by artificer on Feb 11, 2011 11:28:49 GMT -8
Donkey, I'm not advocating the use of water instead of solid thermal mass. Its just that sometimes its the only practical solution, and the hassles of containment/maintenance/softening are worth it.
In the OP's situation, if you can't put sufficient traditional thermal mass in the heater, it offers an alternative.
Did I mention I build a masonry heater as well? In my case the water heater would use WMO, wood chips, corn stover, sawdust, and I need a way to get the heat into the workshops thermal mass, which is the floor. This is a secondary heat source on a different floor. (1st floor workshop, 2nd floor masonry heater)
Michael
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