Ideas needed, Put your thinking cap on

[Richardin52]

I am looking for a practical solution to a problem having to do with long term heat storage.

It is a known fact that when clay is dried it absorbs considerable heat energy, and on wetting again it releases the same amount of heat. This is called heat of wetting.

Using this method it is conceivable that clay could store all kinds of heat energy, solar, wood, etc. and store it until such time that it is needed. Water storage needs to be insulated and still looses heat over time. Clay of the right particle consistency may be able to absorb and release heat fairly quickly when needed without the problems associated with water storage.

What I am looking for is practical ideas of using clay as a medium for both long and short term heat storage and ways that could be used to release that heat when needed to heat a home or for other uses. So put your thinking caps on and please give me some suggestions.

For instance, an idea that I have thought about would be a series of plates not unlike a car battery made up a clay in a sealed tank. Dry heat via hot water pipes could be pumped into the tank drying the clay which absorbs the heat. Then when needed the tank could have water put into it and the clay would release the heat.

 

[steam man]

Devil's advocate here-if it works so well, why hasn't anyone developed a practical application up to now? What exactly is the heat storage capacity compared to water? My first thought is that the drying part of the cycle to remove the moisture after extracting the heat may be the problem.

Mike

Keep it going.

 

[Richardin52]

Mike

I think the reason no one has developed a practicle application up to now is "cheap energy". In the past it was not cost effective to store heat after it was made. They just stored the oil instead, but it's a whole new ball game now.

What volume of clay needed to hold say 1 million BTU's of heat energy is one of the things I would like to find out. I do know that there are several types of clay made up a differing particles that can also differ in size and that this could have an effect on the amount of clay needed to store a given volume of heat energy.

 

[steam man]

I would have at least thought the concept would have been explored. I just haven't come acrossed it in research for storage mediums so you may be on to something. I'll be watching.

Mike

 

[BrownianHeatingTech]

Look up the numbers, and compare them to water.

Joe

 

[slowzuki]

Yeah, I haven't seen the numbers but I'd say its impractical compared to water, even the just the swelling of clay is hard to deal with. Eutectic salts had a lot of promise back in the day, haven't heard much lately.

 

[Richardin52]

Here’s the numbers

1 gram of clay can retain about 6.8 calories. (This is an average fine clay)
1 BTU = 251.9957 calories
1 lb of clay = 453.59 grams
so 1 lb. of clay can store 3,084.4 calories
this is equal to 12.23 BTU’s per lb. of clay

2000 gallons of water would weigh about 16,000 lbs.
16,000 lbs of clay could store 195,680. BTU’s of heat energy (not enough to make it cost effective)

My question is how much heat energy could 16,000 lbs of water store?

 

[Corey]

1 btu per pound per degree F

1btu x 16,000lbs x (180F - 80F) = 1,600,00(0) btu

I think the big problem with your clay would be drying it back down. If you start with dry clay, then you'd get your btu's. But once it's wet (and cold), then you have to heat all the clay and water up to 212F+ or higher to get out the water. So now you've got very hot clay (which is loosing exponentially more heat than warm water of typical storage) Plus, how hot do you have to go to get water out of actual hydrated clay (not only wet, but with the water actually incorporated into the molecular structure?) This is not a trivial task as now you need a pressurized steam boiler to go 212F+...then once you boil all the water out of the clay, you'd either have to condense it or loose all the energy put into heating and boiling that water. You could try to condense it, but then you'd need a place to dump that energy, and the fact that you're drying clay means you don't really need any heating energy - otherwise you'd be way better off to just use that energy directly where you need it. So it would seem that the whole exercise becomes a bit pointless...unless by 'long term storage' you're talking about solar drying the clay during the summer and having a 'one shot' heat blast during the winter.

There are some materials that would have a phase change and could probably store more energy than water, but of course expense goes up exponentially. Plus, with any of these schemes, you really aren't 'gaining' any heat...bitter laws of thermodynamics prevent that. The best you can do is increase the storage density a bit over water.

 

[BrownianHeatingTech]

2000 gallons of water would weigh about 16,000 lbs.
16,000 lbs of clay could store 195,680. BTU’s of heat energy (not enough to make it cost effective)

My question is how much heat energy could 16,000 lbs of water store?

Depends upon the temperature range you're working with. How hot is the tank when fully charged, and what's the minimum temperature that can still delivery heat?

Baseboard can operate down to 140, and a fully-charged tank might be in the 180-200 range. Radiant can operate down to 90-110, depending upon the design. So, you can discharge the tank by somewhere from 40-110 degrees before it gets too low to use.

16,000 pounds of water, with only the 40-degree delta-T would store 640,000 btu's. If you could get the 110-degree delta-T, you could store 1,760,000 btu's.

So, the same weight of water would store something between three and nine times as much usable energy.

Joe

 

[BrownianHeatingTech]

1 btu per pound per degree F

1btu x 16,000lbs x (180F - 80F) = 160,000 btu

You're missing a zero... it's 1.6M, not 160k.

Joe

 

[Corey]

You're right, Joe. No more drinkin' and postin' for me! Original edited for correctness.

 

[pybyr]

if I were not so far behind the curve on my basic install compared to the arrival of winter, the alternative "phase change" idea is still very interesting

I actually found a source a while back of not too exotic/ expensive paraffin that melts at about 160F, and was thinking it'd be cool to put a sealed 35 gallon stainless drum of that inside my big unpressurized tank.

But I haven't quite gotten my head around whether the storage gain from phase change makes it worth the bother.

any merit there for me to think about before I bolt up the final side of the tank (actually, right now, given the time of year, I am likely to do my Econoburn without storage first, and then finish the tank later ) (primary/ secondary is going to make that install and later transition easier)?

 

[Richardin52]

I’m really thankful for all your impute and ideas. It does not look like clay is the answer but the concept of long term heat energy storage using a medium that can store heat without the problem of heat loss is very intriguing.

Slowzuki mentioned Eutectic salts as another medium that might be used. I guess the first thing to do would be to make a list of what material that has the capacity to store heat in the manner we are talking about and then to evaluate the different materials on the list.

I do not doubt for a moment that this type of storage is possible. We just need to figure out how to do it.

 

[Richardin52]

Regarding the use of phase change materials to store and transfer heat I found the following;

The temperature range offered by the PCM technology provides a new horizon for the building services and refrigeration engineers regarding medium and high temperature energy storage applications. The scope of this thermal energy application is wide ranging of solar heating, hot water, heating rejection, i.e. cooling tower and dry cooler circuitry thermal energy storage applications.
Since PCMs transform between solid-liquid in thermal cycling, encapsulation[7] naturally become the obvious storage choice.

Microencapsulation: Microencapsulation allows the PCMs to be incorporated into construction materials, such as concrete, easily and economically. Microencapsulated PCMs also provide a portable heat storage system. By coating a microscopic sized PCM with a protective coating, the particles can be suspended within a continuous phase such as water. This system can be considered a phase change slurry

As phase change materials perform best in small containers, therefore they are usually divided in cells. The cells are shallow to reduce static head - based on the principle of shallow container geometry. The packaging material should conduct heat well; and it should be durable enough to withstand frequent changes in the storage material's volume as phase changes occur. It should also restrict the passage of water through the walls, so the materials will not dry out (or water-out, if the material is hygroscopic). Packaging must also resist leakage and corrosion. Common packaging materials showing chemical compatibility with room temperature PCMs include stainless steel, polypropylene and polyolefin.



Thermo-physical properties of selected PCMs
Materials Melting point (oC) Heat of fusion (kJ/kg) Specific Heat solid/liquid(kJ/kgoC) Density solid/liquid(kg/m3)
Water 0 333.6 2.05 / 4.18 999 / 1000
Organic PCMs[3][4]
Lauric acid 41 - 43 211.6 1.76 / 2.27 1007 / 862
Trimethylolethane (63 wt%) + water (37 wt%) 29.8 218.0 2.75 / 3.58 1120 / 1090
Inorganic PCMs[5][6]
Mn(NO3) 6H2O+ MnCl 4H2O (4 wt%) 15 - 25 125.9 2.34 / 2.78 1795 / 1728
Na2SiO 5H2O 48 267.0 3.83 / 4.57 1450 / 1280