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Heat storage proposal for tiny homes  RSS feed

 
Mike Jay
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After a weekend of binge watching tiny home shows and seeing Erica's post on Tiny House Heating and working on greenhouse designs, I have the following proposal for someone out there.  Mind you, I'm not building a tiny home but I may experiment with this idea for my greenhouse.

The idea is phase change heat storage built into the walls of a tiny home.

Phase Change sounds fancy but it's just the act of something freezing or boiling.  When water decreases in temperature from 34 to 33 degrees it gives off 1 BTU/lb.  When it drops 1 degree AND freezes, it gives off 143 BTU/lb.  When it heats up that same one degree is absorbs 143 BTU/lb as well.  So getting a material to freeze or melt takes a TON of energy.  If any chemists would like to improve on my generalizations, please do

The issue is that for this property to be useful you need that phase change to happen at a comfortable temperature.  Enter glycerin.  It melts/thaws at 64 degrees.  And its latent heat factor is 75.6 BTU per lb (half as awesome as water).  So for every pound of glycerin you can hide in your tiny home, it would hold and then later release 75 BTUs as it crosses the 64 degree mark.  Glycerin weighs about 10 lbs per gallon so each gallon would store/release around 756 BTUs.  So if you can work 20 gallons of glycerin into the structure of your tiny home, it would act as a 15,000 BTU heat battery.

Keep in mind you need a lot of surface area on the glycerin to allow the ambient temperature to act upon it.  One big tank wouldn't heat up or cool off fast enough to help you.  I'm picturing 2" pvc pipes, 8' long hidden in the walls.  Or something like that.  Allowances for expansion/contraction would also have to be made.  I'm thinking a cap at each end of the pipe with 6-12" of air at the top of the chamber.

So if this system would work, you could run your heater longer and "store" some of the heat in these batteries. Once the home got warm enough that you shut off the heater, it would cool down as usual.  But when it hits 64 degrees the batteries start to freeze and give off their heat.  It would hold the home temperature around 64 until they are done freezing, at which time the temperature would resume dropping in the home.  Then you kick on the heat when it gets too chilly.

So that's my brainstorm.  Crazy?  Best thing ever?  Tried it and it worked/didn't work?
 
Rae Legend
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certainly a clever concept
 
Travis Johnson
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I am not following you on this I guess, and don't take this as a negative reply as much as questions because I am not sure I understand what you are proposing. I am guessing from the lack of responses, neither are other people, so I will go out on a limb and ask (and potentially look dumb).

Now I did some research on glycerin and it seems it is very similar to glycerol which is used in modern heating systems, however I refuse to use it in my own home because it actually makes any heating system that uses it 10% less efficient. That is a huge factor right there, but honestly its only use is to keep systems from freezing up and there are cheaper alternatives to doing that. Glycerol is very expensive, and a back up generator is a whole lot cheaper then the amount of glycerol that it would take to adequately protect my pipes from freezing. Be that as it may though, I cannot figure out how you deduced it could be more efficient when heating systems that uses its sister-compound says its less efficient?

Another question I have is why use PVC pipe when that product s very brittle and breaks upon freezing? Wouldn't you prefer to use PEX tubing that can expand 300% times its own diameter before bursting? I am not sure of what the expansion rate of glycerin is, but water expands by 15% so it will not burst Pex which is why it is used in hydronic heating situations.

Now if there was an alternative to get that glycerin to freeze that was inexpensive, then it may be worth doing. Here in Maine our constant ground temperature below frost line (4 feet) is 57 degrees, so if glycerin does freeze at that temperature it is possible that the cost of running geothermal pumps as a chiller is possibly efficient, and the technology exists in the form of chillers for ice arenas. However that technology removes heat from a surface and does not heat it. It also uses ammonia (old school) or brine because you cannot pump ice. In the case of trying to heat a structure, you would have to try and freeze the glycerin throughout the network of pipes efficiently first. The only way to do that is bring the temperature down over every square inch of the pipe. But therein lies the problem. You could not pump geothermally cooled water first and chill the pipe, then send in the glycerin because the first few feet of pipes would freeze and the rest of your pipes would be filled with air and be empty. Nope, that is no good, and you could not run pipes over each other because it would chill the space while trying to freeze the glycerin. The only way to accomplish a freezing of the glycerin and not chill the space too, would be to run a pipe within a pipe. That would allow the geothermally chilled water to freeze the glycerin without cooling the room directly. But after that, the inner pipe would have to have the temperature raised from 57 degrees to 65 to release thaw the glycerin and release the heat. That would take a heat source other then a conventional boiler because it would short-cycle too much and destroy itself with condensation, but solar or wind heat would work. And all this being hydronic regulated, existing technology like PLC's, circulators and mixing valves are all available and very inexpensive.

The biggest downside of this would be the lack of volume as you would need more double walled pipe since the volume is reduced by the inner pipe containing the chilled/heated water. But as I think through this, by having a PLC inject geothermally chilled water into the inner tube to freeze the glycerin, then after complete freeze up checked by sensors, it sends in heated water (although be it 65 degree water) to allow the frozen glycerin to release heat into the space...I think in theory it would work. This would be done in quick succession and continue until the space reached desired temperature. However accomplishing all this with a return on investment that is comparable to traditional hydronic heat may be problematic.

I would put the pipes in the flooring though and not the walls so you get more heating surface. It could not be embedded in concrete however as it would bust up the concrete, but in my home, where I was retrofitting an old concrete floor to radiant heat, I just embedded the pex tubing in a layer of live sand. It even shocked me that the amount of heat transfer was just as high as concrete. In that way the sand would expand and contract along with the constantly freezing/thawing pex tubing.

But all these thoughts are based upon the notion that glycerin freezes/thaws at 64 degrees; I never found anything about 64 degrees, rather I saw -36 degrees being the freeze point. I am not challenging you on this, I just never found it, but obviously you got 64 degrees from somewhere could you say where exactly? I find this intriguing...
 
Travis Johnson
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Please excuse the above post somewhat. I tried my best to organize and word my thoughts on a very interesting but complex heating potential. Forgive me if it was confusing.
 
Mike Jay
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Sorry Travis, I'm guessing I wasn't clear enough on my idea.  Here's a Link with the melting point of Glycerine (also called glycerol or glycerin).  291K equals 64F. 

The idea isn't to pump the glycerine anywhere or use geothermal.  The basic idea is to put some jugs/capped pipes/barrels of glycerine inside the heated envelope of the house.  Then heat/cool the house as you normally would.  Due to the large amount of energy required to phase change the glycerine from liquid to solid and verse visa, you'd be able to run your heater/stove longer and store more heat in the building without cooking yourself out.  And then when the heater or wood stove turns off, the glycerine battery would bleed off that heat and keep you warm for a corresponding longer time.

So it's a way to pour heat into and get heat out of a tiny space without having huge temperature swings and sweaty or cold occupants.

One suggestion to hold the glycerine is pvc pipes with caps at each end.  Another could be gallon jugs.  The list goes on and on.  But the idea would be to allow for thermal expansion and give enough surface area so the heat of the building can quickly melt/freeze the glycerine.

Hopefully that makes more sense?
 
Cristo Balete
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The weight of the liquid could be an issue.  A tiny home is built on a frame with wheels that can only hold so much weight, whether you intend to pull it or leave it sit.    Filling the walls with liquid would be a lot of weight.   A pint's a pound, the world around.... 

If the home is mobile, if they were solid pipes they would run the risk of be jostled to the point where joints would leak.  A flexible pipe would be easily chewed by a rodent.  Rats have gnawed through PVC pipe.  And oddly enough, rodents tend to like the flavor of radiator fluid.  If they can smell it at all, and they are awfully good at smelling (a leaky joint) they will come back until they can get to it.

Possibly trying this experiment on a small shed with a solid foundation might be interesting.

As far as a greenhouse, the earth is a constant 50 degrees where it isn't frozen, and that's the advantage that a greenhouse has.  The frame of my quonset hut-style greenhouse probably couldn't support pipes full of heavy liquid, either.  but large compost piles inside put off even more heat.   In a 20-foot greenhouse in January/winter I maintain 12 inches of lasagna layering of straw, mowed weeds and manure putting off a lot of heat.  When I plant tomato transplants in March the soil is ready to go, and often a few of last summer's tomatoes are resprouting, particularly the cherry tomatoes that have overwintered.  But I would call this a mild winter area, frost on the ground, no frozen ground.  But enough frost to kill tender vegetables, some plants have to be covered. 




 
Todd Parr
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I think it sounds like a great idea if you could overcome the weight issue.  I would be concerned about the expansion issue however.  I'm not sure it is as simple as leaving some air space in the pipes, but I'm also not qualified to make an assumption about it.  How much can air be compressed before it can't be compressed anymore and explodes?
 
Glenn Herbert
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Air can be compressed until the gases in it condense to liquid... but that can take maybe thousands of pounds per square inch, far more than any home setup can reasonably achieve. If glycerin doesn't want to evaporate into the air, you could just have a tiny vent hole at the top. If it does evaporate, add a rubber diaphragm to the cap.

A majority of compounds contract when they freeze, water is an exception. But keeping critters away from the glycerin might be a real issue and require metal containers. Metal conducts heat much better than pvc anyway.
 
Mike Jay
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Yup, it would add some weight.  In my example above to get 15,000 BTU of storage, you'd have to hold 200 lbs of glycerine.  So it's not insignificant but I'd imagine that compared to the weight of an entire tiny house it could be workable.

Regarding expansion we'd have to do some math.  And it's been waaaaay too long since I took whatever classes would cover thermal expansion.  I did just look up the coefficient of volumetric expansion of glycerine and it's .0005 (1/degrees C). Water is about .00021.   So it expands a bit more than twice what water does.  Air compresses a lot in comparison.  For instance, when you pump up your bike tire, imagine the volume of air in the bike pump.  Then remember how far down you press the handle before the air reaches 40 psi and starts to go into the tire.  Schedule 40 pvc pipe is good to 450 psi (Link).  I feel like a pipe with 3/4 glycerine and 1/4 air in it by volume wouldn't be an explosion risk.  A guy could thread a pressure gauge on the end and test it out. 

Critters could be an issue but hopefully the house it built tight enough to keep them out.  Otherwise they'll also be in your food/bed/etc.
 
Troy Rhodes
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As a phase change material, glycerin would behave exactly as you describe, stabilizing the temperature around 64F.

But keep in mind, storing and releasing a lot of BTU's does nothing to prevent the heat loss through the walls.

If you had to choose, I would much rather you put those money/space/volume/weight resources into more and/or better insulation, so you need less total heat energy to start with.


Stated differently, thermal mass can make temperature changes happen more slowly, but they do not necessarily make for lower heat bills.

Thermal mass is a very useful tool if you have a lot of heat input at a certain time (like say from big solar collectors during the sunny part of the day, or a hot wood stove while the fire is going) and release that at night when you need it more.

But it is not a substitute for insulation.  They do very different things.


Finest regards,

troy
 
Mike Jay
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Yes, I generally agree.  I figure the main opportunity for this in tiny houses is that heating systems may be oversized for the size of the house.  So this would allow them to run longer and less often while maintaining the temps around 64F.  If you have a heater that is sized appropriately then more insulation and leak sealing would probably be a better investment.
 
John Weiland
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In addition to the thermal banking idea, it seems like for certain locations in the northern hemisphere 'hybridizing' a tiny house (south side) with a shop (north side) might also assist with this goal.  The photo below shows a general idea clipped from a marketing brochure, but the tiny house portion could be possibly incorporated as shown with the shop providing a large, dead-air insulator and area for the storage of desired climate banks.
ShopHomeHybrid.JPG
[Thumbnail for ShopHomeHybrid.JPG]
 
Travis Johnson
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John is absolutely right: I used to have a lambing shed that was insulated, but not heated. It was a pain because as the sheep lambed out, their water buckets would freeze. But then two years ago I added a barn to the north side of the structure. Because sheep are sheep and need ventilation, it is completely wide open. Yet despite this openness, with no other changes to the lambing shed, just the fact that the wind no longer blows against the building is enough to keep the water buckets from freezing.

Of course I am not sure how much value this has in tiny houses because of potential restrictions to adding on.

My current home used to be a tiny house, and my ex-wife and I got by. I hope to build another tiny house, this time out in the woods with micro-hydro and let one of the kids have this one. It is so big now that it pains me to see my wife just try and keep in picked up.
 
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