HIgh Mass & Low Weight

[nedreck]

Feb 11, 2011 9:38:17 GMT -8 artificer said:

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

The heat disperses as it travels through the cob as it is conductive heat. My point was not really having anything to do with the 2k degrees of the cob and everything to do with the limit of 212 of the water. If you start out with 62 degree water, you only have 50 degrees left so that would amount to only 50 btu’s per pound. That same scenario with stone adding 50 btu’s would bring the temp to 250F.

With water you are somewhat limited in your surface area in most configurations that make sense and with the cob it is spread out over a significant area. You could configure a water heat exchanger to cover as much area, but the RMH has very high heat intensity aka temp and while stone is capable of absorbing that much higher heat intensity and dispersing it through conduction, because water has its higher specific heat, it does not transfer throughout the total mass as well. This results in the hottest areas going into the phase change were significant pressures begin to build and the hazards go up tenfold.

Again, we can argue this all day long and it is not going to change, water is a great heat transport system but in liquid form it is a pretty shitty heat storage battery unless you are going to tap the potential energies and push it through a phase change.

[bryan]

Michael,

Yep I was being very clumsy with units. Sort of intentionally though since the original question had mass and weight sort of mixed up. So was avoiding using engineering units and just went with rough layman's terms. Plus english units are so goofed when it comes to weight and mass (lbf vs lbm). Maybe we should do it old fashioned style and talk how many 'stones' worth of stone to use ;D

In metric though it is pretty clean:
Water: 4.2 kJoules / (kgram deg Kelvin) for specific heat and a density of 1kg/liter
Clay : 0.8 kJoules / (kgram deg Kelvin) and 1.4 kg/liter

Lets say the exhaust gasses going into the bench are 400degK (275 degF) and somehow we manage to get the bench to reach that temp (it obviously won't) and the room is at 300degK (80degF). Then deltaT is 100degK. Then lets say we have a small bench that is 300kg (661 lbs).

So with water the amount of energy you would have stored is:
4.2 x 300kg x 100degK = 126000 kJ = 119000 btu.
Vs clay:
0.8 x 300kg x 100degK = 24000 kj

If you instead want to compare it on equal space usage then water gets penalize by a factor of 1.4 because of the lower density.

Note I made many simplifications to make that comparison such as successfully getting the bench to warm all the way up to the exhaust gas temp (it of course won't even be close). Of even temp distribution through the mass (in this case water does not have nearly as good thermal conductivity but it does have the huge advantage of being a liquid and so can utilize mixing).

So while I whole heartedly agree with Donkey that water makes the contraption way more complicated, it also poses a really interesting opportunity. There have been many many installations of wood stoves with water jackets and a pump with radiant heating. Yes the water needs to be soft and will likely also need additives in to both raise the boiling point and also reduce corrosion (glycol might work but its also flammable if you boil off the water). The main flaw of those designs was that the fire hit the cold walls of the stove (cold from the water) and would not burn fully and coat it with creosote.

The RMH burn tunnel approach though could completely solve that. So could be some neat installs or retrofits, such as if there is radiant floor heating to us a RMH instead of gas to heat the water and then the cement floor is already there as a giant mass. Also if you are using wood for water heating too then combining the system into one is an advantage. No doubt though it adds complexity/maintenance/problems but some cases may be worth it. Plus its fun!

Burn on!

[nedreck]

Feb 11, 2011 20:47:15 GMT -8 bryan said:

Lets say the exhaust gasses going into the bench are 400degK (275 degF) and somehow we manage to get the bench to reach that temp (it obviously won't) and the room is at 300degK (80degF). Then deltaT is 100degK. Then lets say we have a small bench that is 300kg (661 lbs).

So with water the amount of energy you would have stored is:
4.2 x 300kg x 100degK = 126000 kJ = 119000 btu.
Vs clay:
0.8 x 300kg x 100degK = 24000 kj

If you instead want to compare it on equal space usage then water gets penalize by a factor of 1.4 because of the lower density.


Yes the water needs to be soft and will likely also need additives in to both raise the boiling point and also reduce corrosion (glycol might work but its also flammable if you boil off the water). The main flaw of those designs was that the fire hit the cold walls of the stove (cold from the water) and would not burn fully and coat it with creosote.


Burn on!

Delta T is not the temperature difference in two mediums, it is the temperature change within a singular medium, there is a huge difference.

There are some apples and oranges involved in this discussion that is for sure.

No matter how you try and slice it, using warmed water to warm the bench will NEVER out perform direct fired, the entropy losses alone dictate this before you even start.

A RMH derives it heating capacity from direct fire of the riser at over 1,000 degree temps, exhaust gases in direct contact with the cob warm it. Putting water and tubing in between that exhaust gas and the cob will never begin to approach the direct contact levels.

Glycol's raise the boiling point, but not really very high relatively speaking, they also significantly change the specific heat some what defeating your claimed purpose. They also are an extremely high TDS fluid and when overheated the solids fall out and the separation is not field recoverable. Points of flame impingement would be very serious problems for the fluid.

Once you get your process figured out, your going to find your thoughts on water usage to change dramatically and significantly under perform your claims, it simply does not work the way you are saying sir.

your thoughts involve:

Exhaust to tubing to water to tubing to clay

At each "to" you are going to have to account for the natural heat transfer algorithm and use your logarithmic mean temperature difference between each medium not your delta.

If your going to add glycol to the mix, your going to have to at the very least figure for an 11 to 20% reduction at each change of medium (the to's) and you can't really get away from that unless your going to stay below 205F in temp.

You do realize you are attempting to say using a heat transfer fluid will out perform direct fired now I hope.

Again, water makes a great heat mover but it is not a great mass storage while staying in phase, it is actually pretty poor.

[bryan]

Nedreck,

I agree if you are building a bench then it would make no sense to involve water. There would have to be significant other design factors, such as already having radiant floor heating, or having gas fired radiant heat, or needing to also get a lot of domestic hot water from wood heat, or even having super sandy soil and no clay to work with. Or even having water jacket wood stove heat setup but sick of the stench of creosote (I lived in such a house and it is a fairly reasonable setup from a heating perspective--control the pump to control how much of the heat goes into the water vs into the wood stove room, whole house heating, but STINKY).

Also the gas temps are as you say well over 1,000 degrees (probably close to 1500degF at the top of the heat riser), but the exit temps from the barrel are more likely under 600deg (galvanized ducting is a common material inside the bench and so can't be too much hotter than that). Lots of people stating they get gas exit temps under 150degF. Surface temps if using a bench can't be much more than 120degF. So the average temp of the whole bench must be somewhere around 200-300 degF. Per your point that you can't heat water over 200 without some trouble, then the bench approach clearly has an advantage. But since it would be easy to mix the water then the whole mass of water could be brought up to 200 degF and so compared to the average temp of a bench then water again looks attractive.

But water definitely lacks the simplicity and beauty of a cob bench.

[nedreck]

Feb 14, 2011 10:18:26 GMT -8 bryan said:

Nedreck,

I agree if you are building a bench then it would make no sense to involve water.

Sounds to me like a major portion of the confusion was the goal and therefore the methods.

I also have a hard time with the less than long life expectancies as many of the issues surrounding water a chronic not instant. Weeping, metal fatigue, reduced capacity through contact loss, corrosion, scale and a host of others enter my mind as I am thinking on the 20-30 year life cycle.

Some of those can get in the way real fast aka 2 to 3 years, others build over time.

While you may indeed feel water can be used without too much trouble, I still disagree. Hydronic heating is not without hazards and a typical system will have at least 3 levels of redundancy to prevent over pressure simply because of how catastrophic that over pressure situation can become in very short periods of time.

The attached photos had the required safeties in place, my inspection for cause came post explosion, I am kinda glad too cause that was sure loud and had a lot of high velocity metal flying about!

[s2man]

As mentioned, changing a material's phase involves a lot of energy. Phase-change salts are available in sealed tubes, with the phase change occurring at at your desired temperature (within reason). They are often used in passive solar heating applications where there is not enough mass to absorb the energy from the sun, and not enough space or aesthetic reasons not to add more mass. A compact solution, but they are definitely not cheap.

I left my lunch in the truck and discovered that cheese is a good room-temp phase change material. Not sure about the longevity of it, though ;-)

[wallybear]

I definitely found the right place. Thank you Nedreck, I thought I was going to explode for a minute.

Here are some questions I have about "heat batteries".
#1 Is the absorption and release of heat strictly a linear equation based on mass and density?
Base your answer on efficiency rather than change points.

#2 If it is not a linear equation then what would be the most efficient material for a battery? Meaning which material would absorb heat the fastest while releasing it the slowest? I am asking this in the hopes that I am seeing this wrong and that maybe there is a material out there that works differently then I am aware of.

#3 Based on a linear equation answer. What about a mechanical solution. What if we were to insulate the battery while it was heating and then opened it as heat was needed?
Also in a related question; What about insulating the battery from heat sinks, such as building directly on the ground, exterior walls or other areas that could rob heat?

#4 What about surface area? Surface area being a direct link to density of the mass would seem to play a integral part to how fast heat was released.

Last thought/question: What would happen if we took material such as steel plates separated by space so that they heated faster, then pressed them together so that the heat was released back out of them at a slower rate?