Water versus cob – thermal storage

    Here are some very useful figures for anyone who is trying to choose whether or not to include a water tank within an RMH.    Cob weighs 95 pounds per cubic foot. Water weighs 62 pounds per cubic foot.    The heat capacity of  cob is .2 which is 1/5 that of water which has a capacity of 1.00 So supposing we want to build a RMH which occupies 100 ft.³ of space. 

        First the cob -  100x95 equals 9500 pounds.    100 ft.³ of cob will weigh 9500 pounds.    9500x.2 equals 1900.  So our cob bench has the same heat capacity as 1900 pounds of water.

  Water weighs 62 pounds per cubic foot,  therefore the tank containing 100 ft.³ of water weighs 6200 pounds

    6200 divided by 1900 equals  3.26   

    A given volume of water can store 3.26 times as much heat as the same volume of cob.

  It is true that the cob bench could be heated to temperatures far beyond the boiling point of water. But in order to store the same amount of energy as water at 200°F, a cob bench would have to be heated to more than 650°F. This is not common practice and if it were it would result in lowered efficiency with higher exhaust temperatures and badly burned bums     Water stores much more heat at temperatures which are practical and safe. And because heat transfers through a body of water through convection, heat being absorbed by the thermal mass will be available in short order. If some lag time is desired the tank could be cobbed over.

    Some may be worried about the danger of steam explosion. A water tank which has an open vent to the exterior is no more dangerous than a rattlesnake on TV . It's easy enough to monitor the temperature and allow the fire to burn out before the boiling point is reached.

    It would be a shame to have a giant water tank like this for thermal storage only. Water could be drawn off to heat a hot tub and for regular domestic uses. For those who don't want to do any fiddling a tank could be placed into a cob bench which would be a pre-heater on the way to the hot water tank. During the heating season your hot water tank would receive preheated water. During the summer when the heater is not in use cold well water would absorb heat from the thermal mass. This would have a mild air conditioning effect and the water would enter the hot water tank at a higher temperature.

   

Hello Seth Pogue,

I do agree water is a good heat storage medium, however bitumen or pitch is far better. Sorry I not have any figures only experience.  Peter

dale hodgins wrote:
    Here are some very useful figures for anyone who is trying to choose whether or not to include a water tank within an RMH.    Cob weighs 95 pounds per cubic foot. Water weighs 62 pounds per cubic foot.    The heat capacity of  cob is .2 which is 1/5 that of water which has a capacity of 1.00 So supposing we want to build a RMH which occupies 100 ft.³ of space. 

According to the boat building world.... fresh water is 60lbs per cubic foot, sea water is 63 lbs per cubic foot (doing a web search just now gives 62 and 64lbs/cf at the temp boats use, so who knows, when building a boat I guess it is best to be conservative... though at 200F fresh water is only 60lb/cf). The heat capacity of that extra 2 or 3 pounds (of salt) is 0.22. However, it may keep the water tank from freezing if the dwelling is left to cool in the winter for a few weeks.... Ja, with water storage one has to look at both ends of the temperature range.

On YouTube [I know the disdain in which much on there is held, so don't beat me with it!] there is a chap, Van Powell that makes a great deal of noise about how Rocket stoves are dead [from a technology relevance perspective] because water stores more heat than cob. 1st off what he is referring to is a RMH not and Rocket stove. 2nd he seems to think that everyone wants to figure out how to build a water jacket RMH that won't explode. I am waiting to her the bang from his nieghborhood.

Dale Hodgins wrote: It is true that the cob bench could be heated to temperatures far beyond the boiling point of water. But in order to store the same amount of energy as water at 200°F, a cob bench would have to be heated to more than 650°F. This is not common practice and if it were it would result in lowered efficiency with higher exhaust temperatures and badly burned bums

Just a minor correction. The water or cob temperature is not likely to drop below 100F or so. Therefore, a mass of cob heated from 100F to 450F represents the same energy gain as compared to the same mass of water heated from 100F to 200F. This is still too hot, but perhaps the average temperature of the cob might be 450F, but the exterior remain much cooler (with interior at an even higher temperature). I think water is the way to go, I'm posting only for sake of clarity.

Instead of a bench, a WATERBED!!! All you need is the disco lights and Barry White music, please....

Zeolite and other desiccants can be used to store energy for heating applications. Zeolite achieves twice the energy density of water (by volume), and about four times (by mass). So, a volume of zeolite provides twice the heat as an equal volume of heated water. The procedure for space heating is to blow humidified air through the zeolite. Water vapor is adsorbed by the zeolite causing its temperature to rise, and this heats the air. Hot air with increased humidity leaves the zeolite for space heating. I've seen systems described that bring in outside air (cold and dry), preheat the air with the warm and more humid air exhausted from the home, humidify the incoming preheated dry air with a wet pad (essentially an evaporative cooler), then send the cool moist air through the zeolite to generate hot air that's not too dry. Of course, the zeolite has to be regenerated with heat, but in principle it's possible to use the hot moist air exhausted from the zeolite during regeneration for water heating.

Desiccants can also be used for space cooling by drying the air enough to allow evaporative cooling to be effective.

I think there's promise in making use of desiccants like zeolite for HVAC for modest off-grid homes. I had considered a system to leverage a limited quantity of biomass fuel in the off grid setting. Imagine a small wood gas engine system operated a few hours daily for bulk battery charging. The engine exhaust and hot air from the cylinder cooling fan is directed through a bed of zeolite. The hot and humid air expelled from the zeolite is used for water heating. The zeolite bed is flushed and cooled, then used for space cooling. The system can be reconfigured for space heating if desired. Note that I don't necessarily consider this to be practical (might as well just get a large photovoltaic system and operate an efficient but more or less conventional system), but I do find the prospect appealing for its low tech approach. Also, it does a good job in stretching a limited quantity of wood fuel.

The idea of water for heat storage is intriguing. It complicates the heat exchange, since running ducting through a water tank will require that it all be water tight. Corrosion would be a more serious concern too.

I'm thinking of a hybrid approach. Please understand that this is just arm chair speculation. I have not yet built a RMH or rocket stove of any kind.

How about embedding many water-filled, sealed, 2-litre soft drink bottles into the (otherwise) cob bench. You get to benefit from water's higher specific heat and lower specific gravity without the complications of introducing a water tank. This would only be safe if temperatures within the bench are guaranteed to stay below the boiling point. I don't happen to know if that is realistic. What is the typical range of temperatures in the bench during a super-long, super-hot burn?

Here's a possible modification to deal with this issue where operating temperatures in the bench are expected, but not guaranteed, to remain below boiling. Drill a small (1/64") hole in each bottle cap and orient the embedded bottles vertically. There should be minimal loss of water if temperatures remain below boiling. If the bench goes above the boiling point you end up with a moisture problem but nobody's going to be in danger.

Further refinement: use salt water for it's higher specific heat.

Thoughts?

sealed water heated is referred to as BOOM SQUISH. Although in this case it is more like boom sizzle. You don't want to seal the water, you want it open so any steam safely vents off.

I would build a small bench with concrete blocks along the sides to hold the water tanks directly over the exhaust tubes with just enough cob to cover them and make solid heat transfer. Or long tubular tanks directly adjacent to the exhaust and both covered with minimal cob. Or the heat transfer coil in the first part of the bench like Ernie has described connected to a holding tank as a thermosiphon.

Micky Ewing wrote:The idea of water for heat storage is intriguing. It complicates the heat exchange, since running ducting through a water tank will require that it all be water tight. Corrosion would be a more serious concern too.

Thoughts?

A better way to heat the water is to contain the water in tubing for heating. The heated water can then be stored in a large insulated vessel. Use a fan coil unit for air heating. This is a viable DIY project. The water might be heated with a thermosiphon set up (see example here: http://www.youtube.com/results?search_query=endless+hot+water), but there are advantages to using a small pump. A system might use an efficient furnace to heat a large store of water in an insulated tank. The water in the tank might then be distributed to a fan coil unit in the home. I recommend a small dc magnetic drive pump for this application (manufactured in China in large quantities for solar water heated purposes) as these are inexpensive, reliable, and efficient. Keep the tank vented to prevent possibility of pressurizing the system.

If your going to store heat in a tank use, Bitumen, Oil or Salt.

Let's examine what could go wrong.

A vented tank of water is heated too much. --- Some steam escapes and we have to top it up.

Bitumen, oil or salt are overheated. --- Fire, explosion and
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And what if the tank leaks and everything runs down the drain. (I wouldn't have a big tank of water in the house without a floor drain)

A tank of water leaks and runs down the drain. --- Oh no, the water is gone.

A tank of bitumen or oil runs down the drain. --- Could cost hundreds of thousands to do environmental clean up.

A tank of salt runs down the drain. --- We could have a salty well for many years.
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A fire breaks out. It's not related to our heat storage.

The water absorbs some of the heat and eventually boils over.

The bitumen and oil produce toxic fumes that kill everybody or they just contribute to the inferno.

I'm not sure what would happen to the salt.

Please..everyone...if you use water for thermal storage in any system, and it has the potential to get to boiling...please put a stack pipe to atmosphere.

And... connect all the high points in the system to the stack pipe.

one cup of water = 16,000 cups of steam = bada big boom, if trapped or not relieved.

jeff ramage wrote:Please..everyone...if you use water for thermal storage in any system, and it has the potential to get to boiling...please put a stack pipe to atmosphere.

And... connect all the high points in the system to the stack pipe.

one cup of water = 16,000 cups of steam = bada big boom, if trapped or not relieved.

DISCLAIMER: I did not pick this particular message for itself, but because of the general number of messages like this that pop up (I've written a few too) whenever heating water is mentioned.

Yup all this is true. However, this is a RMH we are talking about. The top of the barrel is hot and the flue gas is maximum there. By the time those gases get to the bottom of the barrel they have cooled a lot and it seems the bench part takes quite a long time to get to comfortable sitting (100F). This is with a cob bench, the water would take more than 3 times as long to get to the same place if it were the same size. The time required to go from comfortable to uncomfortably hot would be a while (120 to 130F for most people) The time to get from there to explosive would be twice as long as it took to get uncomfortable. In the mean time, the direct heat from the barrel top end is warming the room, and once the water is over 100F it is also warming the room. Generally, the room will overheat before the water. I would think also the RMH is a continuous feed, that is, a feed full of wood does not burn for long, it requires frequent feeding making the operator aware of the room temperature. So a full load of fuel is not likely (I know this doesn't mean impossible) while the operator is out chopping more wood or whatever to take a water tank from nice to over the top.

Check your steam tables too, That steam has to get to 328F before the pressure is even 100psi (see here and/or here ) Any TP valve for water heating goes off a long time before that (just over 14psi typically) and 100 PSI is not very high, shop air is often higher and the internal pressure in the compressor itself may be as much as double that. The heat required to get a large tank of water/steam from 212F to 300F is still a long time in a room that is getting more uncomfortable all the time and a very noisy TP valve that is emptying the water out pretty quick. Assuming the flue goes through centre of the tank and that the tank is on it's side... standard gas hot water tank... when the water level gets to be lower than the flue, you are finally hitting your first danger point.... provided the flue gas is actually hotter than 328F (I have not yet measured one that is, but I have only played with one) The flue is now heating the steam directly and the water is no longer helping cool the flue gas down (though it will still take heat away from the steam). Did you get that? Up till this time the flue gas maximum temperature has been regulated by the water. (The water temperature is also regulated by the room temperature to some extent as well)

I think we have all seen a water tank blown up on youtube using only the electric elements in the tank to heat it. It blows up with an amazing amount of ferocity. But that is a purposeful setup. All of the safety features of that HW tank have been removed or bypassed. A production HW tank has to pass having the thermostat stuck on too. The safeties have to work so they can sell it (it may be that the occupants are in the house but can't hear the TP valve and feel the heat steaminess because the basement door is closed). People have been heating water from their cook stove and masonry heaters (where the water coil is within flue gases that are much higher than the RMH bench ever gets) for years and it can be done safely. I have seen what a propane tank going off can do to a house (unlivable), yet we all have one sitting around too.

It may not be a DIY project, or at least it takes some caution, but using water to store heat (as is the norm for solar heating which can also get things over boiling BTW) is certainly a valid way of doing things. Most DIY steam blowups have been people purposely trying to create pressure in a boiler to get steam. There are much safer ways to generate steam, leave the boilers to the engineers.

The same thing applies here. it is worth the money to have a professional well versed in heating water and it's dangers install or at least go over your design and final install before it is covered with mud or whatever. Someone who has installed wood heater water heaters would be best. A good sense of how often the safety equipment should checked and replaced (average water tank gets replaced every ten years or so) is also in order.

Thumbs up and 2 apples for Len. If you read through a thread and safety measures have been duly covered, another post warning of the dangers is redundant fear mongering. The person who hasn't researched these things is unlikely to get off the couch and pick up a soldering gun and therefore couldn't possibly endanger himself with a steam explosion.

I favor gravity fed open systems that could never blow up, because the tank has a pipe with no pressure valve leading from the top of the tank to a drain. I wonder if I drew a picture of a large vat with an open top like a bathtub, if it would still draw warnings about the awesome power of a steam explosion.

Whenever something with potential danger is discussed, the original poster should bring it up. If not, someone should. Then let it go. The greatest danger for most people when it comes to home heating, is that they will never get off that couch and build something that makes them completely impervious to the problems of supply and price of fossil fuels.

It seems like running a water jacketed RMH with a small pump through a radiant floor system would serve a similar effect as the cob bench, but over your entire floor(s), thereby making everything fall closer to that just right level of comfort. PEX tubing will hold together up to at least 250F so as long as the system was appropriately sized you could get your Rocket Slab Heater to more evenly distribute heat through your place. If you ran a coil of copper or stainless tubing inside the water jacket(to the pump) and insulated the exterior, you could control when you wanted to tap off of the bulk heat storage and distribute it off for different uses or times as needed. You'd pretty much be making a hybrid hot water heater/radiant floor/zoned central heating system that you could still cook on. You could even tap into it for deicing/defrosting applications. Hot water heaters are one of the biggest(~17%) drains on household power. That's 17% less solar/battery power needed for most people. Plus its one stop shopping. Just plumb it in to your homes existing hot water system along with some thermal protection of course.

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