# Heat Storage Tank



## Mike in Maine (May 8, 2008)

It seems one of several common threads to discussions and answers on good levels of efficiency, involve adequately sized Hot Water Storage Tanks.
I would like to know if there any out there that aren't 8'  or more around as I must place it in an 8' wide breezeway area(no Basement)  Are there any elongated or vertical versions to set up with a Greenfire, Benjamin, Econoburn, or EKO (confused and undecided as of yet)??  Will a more traditional technology boiler come close to being as effective, as long as a large enough storage tank is put in also? what size tank with a 1800 sq ft hot water  baseboard house to handle heat and hot water or just hot domestic water?      
Wood Boiler Novice, trying to comprehend in Maine


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## WRVERMONT (May 9, 2008)

Hello mike,
There are tanks that have narrower diameters.   Without knowing the details of your heating requirements, I would say to use at least 500 gallon storage.  A little larger 600 or so would be even nicer.  There are some people using propane tanks for storage or a custom welded tank.  You can buy a pre-manufactured thermal storage tank that measures 58" diameter by 60" high (533gals.) Couple this with 80,000 to 100,000btu gasification boiler and you should be pretty good.  Good Luck with your system.


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## jebatty (May 9, 2008)

Might be a little large for you. I have a 3'D x 19'L propane tank, 1000 gal, for storage. Not easy to move anywhere, weight > 2000 lbs.


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## Nofossil (May 9, 2008)

General rule of thumb for storage: bigger is better. At a minimum, you want enough to carry you from one fire to the next. I build my fires in the evening, so I need to cover my heating needs from about 3:00 AM until 6:00 the next evening. To figure it out, you need to do some level of heat loss analysis on your house and figure out how many BTUs you would expect to need over that period (300,000 in my case). Then, figure out the peak planned tank temperature (170 in my case) and the lowest usable water temperature (120 in my case). A BTU heats one pound of water by one degree, and there's 8.3 pounds in a gallon. Divide your heat storage requirement by the temperature difference and again by 8.3. That will give you the minimum tank size that you want: 300,000/50/8.3 = 723 gallons in my case. Again, bigger is better.

Another way to look at it is how long it will take your boiler to heat the tank. My boiler can raise the tank temperature by about ten degrees per hour, so getting the 50 degree temperature rise takes about five hours of flat-out burning.

Storage will probably make any boiler more efficient, since it allows it to operate at higher temperatures. However, even the best conventional boiler operated carefully under ideal conditions does not approach the efficiency of a gasifier.


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## Mike in Maine (May 9, 2008)

Found a collapsible HW heat storage/exchanger manufacturer, STSS, with sizes to fit the limited space with a taller version.

http://www.stsscoinc.com/specification_TankSizes.aspx

Have had folks finally responding and pushing  Benjamin $4500 (simple old school), 

Greenfire ~$5000, (mentions refractory and 100% combustion)

 Biasi 3 pass $3400... mentions secondary gas burning (not quite "gasification" I gather unless air is forced into a chamber?)

The Econoburn has been noted as having a shorter burn cycle and problems heating storage water here in postings...

The area Tarm dealer was outrageously high$ for the total package $22K...

Not familiar with the EKO cost$ can you help me out with a ballpark figure, benefits ...  ????? 

Closing in on answers on whether burn time or high temp is king with WELCOME help 
Thanks 
dodging deadly Black Flies...


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## free75degrees (May 9, 2008)

That sounds high for the Tarm.  I think they are quoting you a lot of extras plus instalation and it still sounds high.  To be fair, if the other quotes you had are just for the boiler purchase, the Tarm boiler runs somewhere in the 7k ballpark (depends on model and sale(s)).


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## chuck172 (May 9, 2008)

The most recent price quote from tarm:
List pricing SoloPlus 40
boiler package $8,150.00
Tank package    6,089.00
Total    $14,239.00

That includes all the what they consider accessories.


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## jebatty (May 10, 2008)

And there are tank options, although the collapsible has its advantage in getting into a limited access place. My 1000 gal was $850, and others have quoted even lower prices, although some quote higher. Just look around for options.


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## MrEd (May 10, 2008)

Jebatty - a question for you. What type of storage tank are you using with your Tarm...I have been reading thru 100's and 100's of posts trying to get up to speed on all this, but I am pretty sure you said you have 3 used oil tanks and a plate exchanger setup...but elsewhere I think you mentioned a pressurized 1000 gallon propane tank? If you did have 3 used oil tanks, and then switched to the pressurized tank - can you explain why you changed? Did the three oil tanks not work adequately?


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## AndrewChurchill (May 10, 2008)

I'm installing a Harman PB105 pellet boiler would it make any sense to install a heat storage tank?


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## koch (May 13, 2008)

I've been reading lots of the threads within this forum and find them very helpful. I have just ordered a Greenfire 130 that I will use to heat both the house and swimming pool. In all of the threads no one has mentioned dry thermal storage. I have been thinking that I could use compacted gravel to store heat. Based on some calculations I did last night I should be able to store ~350K btu in 5’ by 8’ by 4’ space with a 40 degree temperature change, about the same as 1000 gallons of water. My idea for the Hx is 250’ of  3/4 pex looping within the mass. Has anyone heard of someone using gravel as a thermal mass?


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## free75degrees (May 13, 2008)

That reminds me of the project up in Canada where they are pumping an entire summer's worth of solar energy down into the earth then pumping it back out for heat in the winter.

How compacted will the gravel be?  Will there be air spaces?  Are you planning on insulating the volume of gravel?

One of the nice things about water is that heat can spread around by convection, which you will lose with a solid.  That doesn't mean it won't work though.  It would be great if it did due to the simplicity.


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## jebatty (May 13, 2008)

MrEd said:
			
		

> Jebatty - a question for you. What type of storage tank are you using with your Tarm...I am pretty sure you said you have 3 used oil tanks and a plate exchanger setup...but elsewhere I think you mentioned a pressurized 1000 gallon propane tank? If you did have 3 used oil tanks, and then switched to the pressurized tank - can you explain why you changed? Did the three oil tanks not work adequately?



I bought the Tarm to heat my shop in replacing a worn out OWB. I was a newbie from the get-go on this Tarm project, never done anything like it before. It's installed in the shop. I read on this forum about the high desirability for storage, but with all the money into the Tarm I did not want to put more into the STSS tank setup, not knowing how well (or not) the Tarm would work. Yet I was pretty sure that storage was the right way to go. I got a deal on 3 - 275 gal used heating oil tanks (all three for $125) which I also installed in the shop and plumbed them in series. 

Let me say right away that the Tarm throws out lots of btu's, and keeping it in high burn for a full load burn and more with a way to store the btu's is important. Once you use a gasifier you will realize the great efficiency gained from full burns. The gasifier will work OK without doing this, but unless you have a way to store or use the extra heat, you will want to get storage as soon as you can, and generally, the more the better.

For open storage, the 3 tanks worked fine, but after leaving air space and dealing with some plumbing issues with a series setup, and more reading on this forum, I decided that closed loop, pressurized storage, had real advantages. By this time I was ready to pay more to get that storage, and I began the search for a used LP tank, finally finding one in good condition which I bought from a local LP dealer for $850 delivered. 

Why the change? 1) I realized that I could use more than the approx 725 gal of storage with the steel oil tanks, so I aimed for a 1000 gal tank; 2) I never completely solved, although I'm sure it could be solved, the plumbing issues with the 3 tanks in series, and daily attention to balancing the water volume was needed (tanks 1 and 2 would build-up water/air vapor, forcing more and more water into tank 3, which ultimately would overflow; every day I would have to vent tanks 1 and 2 and run the circ pump to re-fill these thanks to prevent this, got to be a hassle); 3) through lack of knowledge never balanced the pH in the steel storage tanks and I got lots of rust; 4) really wanted a storage system I could largely ignore, maintenance-free, once it was in operation.

I'm very happy with the change. The storage now is maintenance-free. I added boiler chemical to raise the pH, scavenge O2 and provided some buffer capacity, and now there is no rust at all. I draw a tiny bit of water occasionally, clear as can be, and test the pH just to make sure. I used a plate hx with the old tanks and continue to use it with the new, in part because the plumbing was easiest without pulling it out and in part because I have an antifreeze solution in the Tarm and for now I want the added safety of the antifreeze. We like to go for a week or two in winter and I do not want to take any chance of the Tarm freezing up. I will have to deal with this, because now I can't let the LP tank freeze up either. That will be a project for next fall.

Don't try to run any system with steel without getting the pH into the 7.5 and 9 range. Acid hits steel very hard. With a plate hx and a used tank, get a high temp water filter or screen to catch any crud that might plug the hx. Plump any parts that may need maintenance with isolation valves so the parts can be removed easily.

This project was challenging for a newbie, but my nature tells me to accept any challenge. It was really a fun project, and the results were incredibly satisfying. I have become a passionate advocate of wood gasification technology for home, shop and small business heating needs, and have made several presentations at local business group meetings.


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## koch (May 14, 2008)

free75degrees said:
			
		

> That reminds me of the project up in Canada where they are pumping an entire summer's worth of solar energy down into the earth then pumping it back out for heat in the winter.
> 
> How compacted will the gravel be?  Will there be air spaces?  Are you planning on insulating the volume of gravel?
> 
> One of the nice things about water is that heat can spread around by convection, which you will lose with a solid.  That doesn't mean it won't work though.  It would be great if it did due to the simplicity.




I am going to compact the gravel with a hand tamper, and use 2 inch foam insulation on all sides. I hope that if there is enough pex runs the loss of convection will not be a problem. I am still just thinking it through as was hoping that someone might have already tried this methiod.


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## free75degrees (May 14, 2008)

I just looked up some specific heat capacity values:
water: 4.18 MJ/m3K
quartz:  2.13 MJ/m3K
concrete:  1.93 MJ/m3K

Specific heat capacity is the amount of energy to raise a given amount of material by 1 degree.  This is important because it tells us how much material we need in a tank to hold a given amount of energy.  I couldn't find the capacity of gravel, but assuming zero air space it should be close to quartz and concrete.  Basically water is about twice as good as gravel with no air spaces.  However, it would be impossible to have no air spaces.  Air spaces are not only bad for heat capacity, but also for heat transfer, so getting rid of air will be very important.  If you could totally eliminate the air and get enough pex spread around the volume of concrete to account for the lack of convection, then you would need roughly twice as much volume.  If you want to eliminate water, you might be better of using solid concrete.  It would be a lot of concrete but i think it would work (it would still need twice the volume as the water).


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## koch (May 16, 2008)

free75degrees said:
			
		

> I just looked up some specific heat capacity values:
> water: 4.18 MJ/m3K
> quartz:  2.13 MJ/m3K
> concrete:  1.93 MJ/m3K
> ...




I found three properties for "Earth, Coarse Gravelly" *Engineering Heat Transfer*, Karlekar, West Publishing 1977 that I think are important;

Specific Heat   - 0.44 Btu/lb F
Thermal Conductivity - .30 Btu/hr ft F
Density -128 lb/Cubic Ft

As compared to water at 140 F;

Specific Heat   - 0.9994 Btu/lb F
Thermal Conductivity - .376 Btu/hr ft F
Density -61.5 lb/Cubic Ft

Gravel has less then half the Specific Heat of water .44 vs .9994 but is two times dense 128 vs 61.5 so for the same volume of gravel you get just about the same heat storage per degree of temperature change, .44 * 128 = 56.3 BTU/Cubic Ft F vs .9994 * 61.5 = 61.4 Btu/ Cubic ft F.  The two materials have similar Conductivity but you will not get the convection that water gives you....

I think that for the price on eight yards of gravel a few sheets of 2 inch foam and 250 feet of pex, I should be able to store just about the same amount of heat as a 1000 gallon system.


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## Willman (May 16, 2008)

> I think that for the price on eight yards of gravel


What size do you think would work best ? I am assuming you mean a crushed stone graded to size and not pit run. Are you from the Sabattus area ? There is an Oak Hill there.


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## free75degrees (May 16, 2008)

This would be really cool if it works.  If so I will be mad that I didn't do it too.  You may have already considered this, but I would also make sure the foam is suitable for long periods in the earth with moisture exposure.  Also, I am guessing that the reason the heat capacity quoted in that book is so high is because they figure the earth has some water content.  Do they mention anything about that?


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## koch (May 16, 2008)

The chart I found the values for gravel did not reference a moisture content but it did have specific heat for a wide range of material from 
Asbestos to Clay to Ice to Plate Glass and Silk to name a few. I was thinking I'd get gravel with 3/4 aggregate, the type used for driveways and road beds because I need some for my driveway anyway.

I'd use 2 inch insulation that is rated for underground, the type you put on the outside of a foundation. I have an old farm house that has a brick arch way that holds up the fireplace, the arch is 5' by 8' and about 6' high, thus the size of my pile of gravel.

I do live close to Sabattus, Topsham in a part of town that was once known as Oak Hill. When we bought the farm it came with a old sign "OAK HILL FARM" thus the name. I'll post a picture sometime as the sign is just over the door to my wood shed where I hope to install my Greenfire very soon


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## leaddog (May 16, 2008)

Oakhill Farmer said:
			
		

> The chart I found the values for gravel did not referance a mosture content but it did have specific heat for a wide range of material from
> Asbestos to Clay to Ice to Plate Glass and Silk to name a few. I was thinking I'd get gravel with 3/4 aggreget, the type used for driveways and road beds because I need some for my driveway anyway.
> 
> I'd use 2 inch insolation that is rated for underground, the type you put on the outside of a foundation. I have an old farm house that has a brick acrh way that holds up the fireplace, the arch is 5' by 8' and about 6' high, thus the size of my pile of gravel.
> ...


i'd think about adding more than 2in of insulation. I would at least double that and triple would be better. It would be hard to add latter and you don't want to loose all those btu's from your storage. The reason for STORAGE is to store the heat and use it when you want. The more insulation the better and you will still lose some.
leaddog


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## Vtgent49 (May 16, 2008)

Well, I have to admit to building a gravel heat storage system in about 1978 or so. It was intended to store heat removed from a solar room by pumping the hot ceiling air down and thru the rocks. The glass portion was only 16x7 feet, and the rock storage was about 8' by 12 by 20 feet. Insulation was 2" of foam.

Basically, it didn't work at all. The fans ran all summer, but hardly budged the rock temps. Too much air space, too great heat loss, noisy blowers, and not really enough of a delta T using hot room air. We basically turned it off and forgot about it. It did however qualify the whole addition for a very generous tax credit, 5 or 6000.$ if I recall correctly, which in the 70's was enough to pay for the entire addition.

There are some people using pex to heat packed sand below their slabs. I'd look in the radiant heat websites, and you might find these designs. Heat transfer efficiency, both in and out, seems to be the key to the usefulness of a storage system.


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## free75degrees (May 16, 2008)

Vtgent49 said:
			
		

> Well, I have to admit to building a gravel heat storage system in about 1978 or so. ...
> Basically, it didn't work at all.



From your description it sounds like you were using air to transfer the heat.  Air has both very low heat capacity and very poor heat conductivity which may explain why you were not able to heat the earth.  Using water might work much better.


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## tom in maine (May 17, 2008)

I think the concept is great to use gravel or even sand as a storage medium. Where there is a problem is in the time lag for inputing and extracting heat.
Water convects around the heat exchanger coils and moves a fair bit of heat. In a solid material like gravel, there is no convection and the heat exchangers would have to be larger. Much larger. One possible way around this is to bury a big tank in the gravel, which negates the concept somewhat.
If you think about a radiant floor, where there is very intimate contact between the PEX tubing (or copper from years ago!) and the concrete, the heat exchange rate is about 30 btu's per lineal foot. By that measure, a 180' copper HX might move 5400 btu's with a 20 degree delta T. It would be more, but not the 100K btus that you might want in a boiler input.

Will be fun to try, but I suspect the laws of physics are going to nip you.

Water is a great medium for heat storage, not perfect, but quite good.


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## koch (May 18, 2008)

Tom G said:
			
		

> I think the concept is great to use gravel or even sand as a storage medium. Where there is a problem is in the time lag for inputing and extracting heat.
> Water convects around the heat exchanger coils and moves a fair bit of heat. In a solid material like gravel, there is no convection and the heat exchangers would have to be larger. Much larger. One possible way around this is to bury a big tank in the gravel, which negates the concept somewhat.
> If you think about a radiant floor, where there is very intimate contact between the PEX tubing (or copper from years ago!) and the concrete, the heat exchange rate is about 30 btu's per lineal foot. By that measure, a 180' copper HX might move 5400 btu's with a 20 degree delta T. It would be more, but not the 100K btus that you might want in a boiler input.
> 
> ...



I am thinking that I might be able to get around lower conduction and hx area problems by adding the aluminium plates/fines that are designed for radiant floor retro fits. It will conduct the heat way from and then back to the pex, thus making the hx work much better. 

Still on the fence about trying this but might try a prototype test before I add 8 yards of gravel in my basement...


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## hkazemi (May 19, 2008)

The Canadian project mentioned in a previous post is the Drake Landing Solar Community, using BTES, http://www.dlsc.ca/borehole.htm .  This is one version of a seasonal thermal store (see http://en.wikipedia.org/wiki/Seasonal_thermal_store ), and a similar, lower temperature concept is an annualized geo solar storage, http://en.wikipedia.org/wiki/Annualized_geothermal_solar , http://www.greenershelter.org/index.php?pg=2

From the ideas I seen so far, I think the best solution is to dedicate a basement or sub-basement space to thermal storage using wet sand.  The space should be waterproofed from outside water infiltration (possibly Platon, http://www.systemplaton.com/ and/or Drylok and/or RadonSeal and/or exterior foundation tiling/drains), and insulated as well (polyisocyanurate and a radiant barrier?).  In a new house, it would be a sub-basement, possibly made using ICFs, pre-fabricated concrete, or tilt-up construction, with 8 to 12 foot walls.  For an existing house, a new 2-3 car garage with a waterproofed, insulated basement under it could work.  Appropriately spaced PEX or pex-al-pex (PAP) would probably be the most cost effective solution for heat exchanging.  A portion might be dedicated to cold storage, by circulating an antifreeze solution to the outside air in the winter.  The reason for wet sand is to increase the thermal performance (better heat conductivity and use of storage volume) (.19 BTU/lb.°F for dry sand at ~ 110 lbs/cu.ft.) (http://2the4.net/heat1.htm ).  Water itself would be 1 BTU/lb.°F and 62.4 lbs/cu.ft., but that would need an actual tank, tank liner and a roof/cover capable of spanning the full width of the basement and supporting everything above, while wet sand could have a poured floor above it.  If one did want to make the subbasement not have a poured slab ceiling and instead have a real span, pre-fab/pre-stressed concrete spans could be used, or products like Insul-Deck, http://www.insul-deck.org/ , or Quad-deck, http://www.quadlock.com/products/quad-deck.htm .

I do not know whether it makes more sense to simply have wet sand (damp), or saturated sand.  The main points of using wet/damp sand are it can't easily all leak away, should need less maintenance than an actual water tank, and could use material already existing at the construction site that might otherwise need to be hauled away.

In areas where the soil is sand or gravel at sub-basement depths (i.e. between 7 to 20 feet below the surface), the area could be excavated, basement floor and walls created, waterproofed, and insulated, and then the sand/gravel backfilled into this space in layers along with PEX/PAP coils for heat exchangers.  Areas with deep clay might do better by bringing sand from elsewhere, although bringing in sand might cost around $10/ton (prices I saw online).

In order to control the moisture level in the sand, a water refill/topping off mechanism would be appropriate.  This could be achieved by simply running a loop of perforated PEX at the top of the thermal store.  The other half of moisture control would be a way to drain excess moisture from the store, near the bottom.  I think it would also make sense to leave a corner of the sub-basement, say 6x6 feet not filled in, as an access point to the lowest level of the basement and to a sump pump that might be necessary to drain away any infiltrating ground water from high water table events and to handle water collected by exterior perimeter drains.  One would want to make sure that the ceiling of this sub-basement was also well sealed and insulated to prevent heat and water vapor and moisture from escaping into the house above when it wasn't wanted.

Note: while at this stage of construction, one could plan for a rain water cistern that could collect roof-runoff for non-potable uses like yard watering and toilet flushing.  Properly placed, this same cistern could also be refilled from the perimeter drains.

Thermal sensors positioned throughout the thermal store would be a asset to knowing how much it has stored.  I think waterproofed versions of the DS18S20 sensors discussed at https://www.hearth.com/econtent/index.php/forums/viewthread/17569/ might be a good choice.
DS18S20 specifications: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2815
DS18S20 with Linux: http://www.digitemp.com/building.shtml
DS18S20+-ND pricing: http://www.digikey.com/scripts/us/dksus.dll?Detail?name=DS18S20+-ND

This page has a good description of sub-basement thermal storage, and is one of the sources for some of the ideas above (I had thought about large-scale water storage, however the large-scale solid thermal mass ideas here are also good).  While this page primarily focuses on an air tube based system, it would be readily adapted to a fluid circulation system.  Fluid circulation avoids potential air contamination problems, but adds complexity by needing additional heat exchangers.
http://mb-soft.com/solar/subbase.html

More information related to the thermal storage - Underground A/C Alternate System - http://mb-soft.com/solar/alternwa.htm
More information related to the thermal storage: http://mb-soft.com/solar/intake.html

One recently published book where DIY homeowner Kenneth Clive created a small ~6 foot cube rock storage system for his solar collector is 'Build Your Solar Heating System', http://www.amazon.com/Build-Your-Solar-Heating-System/dp/0975423622 .

This article published in 1978 by Steve Eckhoff and Martin Okos, Department of Agricultural Engineering, Purdue University also covers thermal storage ideas: http://www.ces.purdue.edu/extmedia/ae/ae-89.html


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## tom in maine (May 19, 2008)

Isn't this getting a little complicated and costly??
And taking up a lot of real estate?
For something that might not work?

Use a DIY tank with a plate hx and get on with life. This is a fun exersize, but even I, a chronic tinkerer,
see that creating a vessel to hold wet, moist, or saturated sand is a lot like a water tank, but only a lot bigger
and more difficult to move heat into or remove heat from.

A solar system inputs much lower amounts of energy than a wood boiler. Follow the btu's!!


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## hkazemi (May 19, 2008)

Tom G said:
			
		

> Isn't this getting a little complicated and costly??
> And taking up a lot of real estate?
> For something that might not work?
> 
> ...



Rock and sand thermal storage has been successfully used by others, and buried under a building as I described it isn't taking up any extra real estate.  A common reason for any underground storage tank to not work as intended is insufficient insulation and also ground water washing away the stored heat.  If you can keep a storage well insulated, and keep water from moving in and out of the storage area, this shouldn't be a problem.

Again, reasons for the sand is to have a thermal mass (using existing on site materials) that cannot simply drain away, and it could easily have a slab poured over it, unlike a water tank.  Massive hot water tanks would be quite dangerous in case their covers collapsed, so one would need to make sure that wasn't even a remote possibility.  (A poured slab over sand doesn't have the same risk.)

For a solar thermal storage, the inputs can be impressive on a seasonal basis.  For example, a small 30 evacuated tube array from Thermomax will put out about 43 kbtu/day in July, and 20 kbtu/day in December, or 12 mbtu/year.  Increase that to 120-150 tubes, and you now have sufficient collection capacity to gather 48-60 mbtu/year, which if stored should be more than enough to handle all the heating loads of a house, even without a backup heat source.  (If you consider a home that uses 120 therms of natural gas in January, that's 12 mbtu/month, or 387 kbtu/day on average.  The home's peak thermal demand loads (design load) will be higher than the average load, however the difference between heat collected per day and heat demanded can be made up using the thermal store and/or a backup heat source.

Cost wise, an array of 30 evacuated tubes can be had for $1100 from Sunmaxx Solar or less if you find one of their sales; x4 = $4400, x5 = $5500.  Of course, you could still use a wood stove as a backup, or as a heat supplement during winter peak demand times, but it wouldn't strictly be necessary.  Evacuated tubes significantly outperform flat plate collectors under cold weather and cloudy conditions...the test data from the SRCC and real-life observations verify it.  They're readily capable of producing high temperature water.  I know that some forum members are using solar thermal in addition to their wood burners or gasifiers, e.g. Nofossil, http://www.nofossil.org/solar.html .

At present there is a residential federal solar tax credit, separately applicable to solar thermal and PV, capped at $2000.  It is calculated as 30% of the installed system cost.


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## tom in maine (May 20, 2008)

I have done a sand storage system underneath a slab. It was about 2 feet deep and insulated with 2" of foam. It was tied to a solar system and a gas fired water heater.
It also had about 1500 lf of 1/2" PEX in the sand. It was manifolded so no loop was more than 200'.
There was a big time lag on heat in and heat out. The boiler, which was the bigger heat input, about 100k, never fired continuously when directly feeding
the sand. If the system could handle big heat inputs, it should've fired continuously.
It was a great storage system for slow heat movement. It worked pretty well with the solar.
When the backup system went down and we never knew it for several days because the occupants never paid attention to the firing of the backup system.
It held heat pretty well, but I suspect it might've struggled with a wood system trying to dump in 100K+ into the sand. The burner does not cycle like it does on an oil or gas system.

My 2 cents worth. I would like to see someone try it. Just be prepared to write it off, given the laws of physics.
Might work very well for a small super-insulated house.


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## hkazemi (May 20, 2008)

Tom G said:
			
		

> I have done a sand storage system underneath a slab. It was about 2 feet deep and insulated with 2" of foam. It was tied to a solar system and a gas fired water heater.
> It also had about 1500 lf of 1/2" PEX in the sand. It was manifolded so no loop was more than 200'.
> There was a big time lag on heat in and heat out. The boiler, which was the bigger heat input, about 100k, never fired continuously when directly feeding
> the sand. If the system could handle big heat inputs, it should've fired continuously.
> ...


It's great to hear of your experience with this on a small scale!  Was your design a dry sand or wet sand storage?  By under slab, was this actually enclosed as a 'room' and protected from ground water infiltration?  Can you share the approximate length/width dimensions of this storage?

To me it sounds like the sand thermal storage system couldn't take the heat as fast as the boiler/burner could supply, hence the cycling.  The Drake Landing system using borehole storage (BTES) uses a system of two very large stratified hot water tanks (120 cu. m. or 31700 gallons each) as a buffer between the solar collectors and the in-ground borehole storage.  This is described at http://www.dlsc.ca/how.htm where they say "The collectors will heat up the STTS about twice as fast as the BTES can remove heat from the STTS. Consequently the collector pump will shut off when the sun goes down while the BTES pump will run most of the night."

Your experience, along with the design of the Drake Landing system, suggests that either a better heat exchange mechanism is needed between the solid thermal storage system and the heat source, or a buffer that can aborb a lot of heat quickly (water tanks) and retransmit it to the long term storage.  Maybe this could be accomplised using heat distributers on the PEX (e.g. flat aluminum panels like used for radiant heat) in the thermal store?  Or maybe a water saturated sand storage would conduct heat better?


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## hkazemi (May 21, 2008)

Here is a post I found that discusses large scale thermal storage which also mentions a relevant book.  The book is 'Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems' by Bob Ramlow, http://www.newsociety.com/bookid/3916 ISBN 0865715610

Source: http://listserv.repp.org/pipermail/greenbuilding_listserv.repp.org/2008-January/005891.html


> Niko Horster, niko at oharagercke.com, Tue Jan 29 09:39:42 CST 2008, said:
> 
> Bob Ramlow from Wisconsin has built these sand storage systems for over 10 years and has written a book about them (well among other things) with New Society "solar water heating". I have a copy in my office if you would like to peruse it. The sand needs to be wet in my opinion and if you look at capacity factors, because your storage would have to be insanely huge to have seasonal storage capacity otherwise.
> 
> ...


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