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Filling water heat storage tank with rocks
By mbeland, Section Heat Posted on Fri May 9th, 2008 at 12:58:06 PM MST
Could it help reduce size of tank?

In a combined solar dhw and space heating application where I would use a large storage tank to store heat for cloudy days, the large size of heat storage tank is one of the problems to solve. Since rock has approx 3 times the density of water, adding it inside the tank could increase its storage capacity. However, it would also limit heat exchange through any heat exchanger but there may be some configuration that might be practical.
Another problem is limiting the dissolution of minerals in the water. It could be glass instead of rock. Could be another way to recycle old beer bottles...
Any thoughts?
Martin

Filling water heat storage tank with rocks | 16 comments (16 topical, 0 editorial)

Re: Filling water heat storage tank with rocks (3.00 / 0) (#1)
by TomW on Fri May 9th, 2008 at 07:28:39 AM MST
(User Info)

First. I think you are misinformed on the thermal capacity of rocks vs. water.

Not positive but I would check that "fact" first because, personally, I just don't believe that statement.

Tom

If everyone is thinking alike, then somebody isn't thinking.

--George S. Patton

Re: Filling water heat storage tank with rocks (3.00 / 0) (#2)
by Chagrin on Fri May 9th, 2008 at 07:51:06 AM MST
(User Info)

You need to look at the "specific heat" of the substance which is a measure of how much energy it takes to raise the temperature of the substance 1 degree. Water is 1 kcal/kg. Compare to the list here:

http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html

Specific heat (3.00 / 0) (#3)
by wdyasq on Fri May 9th, 2008 at 07:58:40 AM MST
(User Info)

I don't know why Tom and Chagrin want to ruin an idea with facts..........

Ron
Adventure is just bad planning." -- Roald Amundsen

Re: Filling water heat storage tank with rocks (3.00 / 0) (#4)
by Capt Slog (Capt.Slog(at this)gmail.com) on Fri May 9th, 2008 at 08:50:00 AM MST
(User Info)

I can see your reasoning, but it will only work if you can find a 'rock' with high density AND high specific heat capacity. A quick look through the tables found me this...

Glass has a density of 2.4- 2.8 (where water is 1) but it's specific heat capacity is 0.84 (where water is 4.19 in the same units) so no net gain there.

Similar stories with sandstone, density 2.1 - 2.4, sp.heat 0.8
and brick density 1.9, spheat 1.9.

As far as I can see, only when you multiply the two numbers and get a result greater than 4.19 will there be an advantage over water.

.

"Slowly changing the world, one watt at a time!"

Re: Filling water heat storage tank with rocks (3.00 / 0) (#5)
by mbeland on Fri May 9th, 2008 at 09:58:48 AM MST
(User Info)

Thanks,

I should have checked before. That rules out the idea.

Martin
Eau, soleil, le vent
[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#6)
by Norm (peppysue@suite224.net) on Fri May 9th, 2008 at 03:53:14 PM MST
(User Info)

I always liked the idea of just filling up the basement with
large rocks and blowing hot air thru them....course I don't
have a basement and if
I did it would be full of junk anyway...Hmmm..
does iron have high specific heat and density?
( :>) Norm
[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#7)
by fcfcfc on Fri May 9th, 2008 at 04:45:50 PM MST
(User Info)

Hi:

I believe you will find that per equal volume, Iron will do better than water but of course the weight becomes quite large. But though, water is free and everything else isn't, at least in the metals area... That's why water is so tough to beat...
.....Bill

[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#9)
by scottsAI (user name at eml dot cc) on Sat May 10th, 2008 at 08:08:24 AM MST
(User Info)

fcfcfc,

Iron is .866 of water. May be 7.8 times more dense, CP only 0.11
Copper is even more dense at 8.96, yet CP is 0.09 = 0.806.
Not too many things better than water, yet if price is no object it can be found.

Have fun,
Scott.

[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#10)
by fcfcfc on Sun May 11th, 2008 at 08:35:01 AM MST
(User Info)

Hi:

Those numbers are probably right... I thought Iron had tipped the scale to the other side but perhaps not. Most of the research and products that will do better than water are all of a phase change nature. You can get a large heat release at a particular temperature and some sensible heat capacity above and below the transition point as well. But of course, the cost is a different matter.....
.....Bill

[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#11)
by scottsAI (user name at eml dot cc) on Sun May 11th, 2008 at 08:10:21 PM MST
(User Info)

Yep, phase change is a smaller tank, for a price.
Common phase change materials are 1/3 the size of a water tank.
Need to store 500,000BTU below 200 deg F. Big tank.
Then found how to make a cheap tank, so gave up on the phase change.
Price of water is hard to beat!

Have fun,
Scott.

[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#12)
by TomW on Sun May 11th, 2008 at 10:00:23 PM MST
(User Info)

Scott;

Care to share the cheap tank construction method? Since you mentioned it, I am curious how you did it.

Tom

If everyone is thinking alike, then somebody isn't thinking.

--George S. Patton
[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#13)
by scottsAI (user name at eml dot cc) on Mon May 12th, 2008 at 10:49:13 AM MST
(User Info)

Hello TomW,

The steel tanks were a show stopper to my cost targets, spend a long time finding:
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/Tank/SizingTank.htm
Tank Cost:
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/Tank/Cost.htm

Plan to put tank under back porch (over sized), in ground with insulation.

EPDM liner
http://en.wikipedia.org/wiki/EPDM_rubber
Good to 150 deg C, great for solar heating projects!

Unfortunately I did not get to build the tank, was laid off before I got the property to build my Net Zero Energy Home design.

What do you think?

Have fun,
Scott.

[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#14)
by TomW on Mon May 12th, 2008 at 12:00:58 PM MST
(User Info)

Scott;

I kind of figured it was an in ground insulated tank. The rubber sheet is the key there. Must be cheaper than steel and never rots. Been looking at poly tanks for rainwater capture for backup and garden / tree watering. They are pretty dear and not suitable for anything hot. I want gravity feed on the outflow so in ground not an option for me for the rainwater gathering.

Thanks for the feedback.

Tom

If everyone is thinking alike, then somebody isn't thinking.

--George S. Patton
[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#15)
by scottsAI (user name at eml dot cc) on Tue May 13th, 2008 at 12:25:53 AM MST
(User Info)

TomW,

This is where I come to learn, and help when I can.

Why the gravity feed?
How about two tanks, upper is small for daily flow.
Lower for the mas storage. Pump to upper tank when you have the power.
Ebay plastic potable water tank 2500 gal for $1000 w/ship.
I will use one or two for potable water. Rain water.
EPDM Liner for the heating tanks. What did you want to pay?
In ground was cheaper, to make an above ground tank, surely it will cost more than the pump?

Consider an above ground pool as a storage tank, used pools are cheap.
Liners are not too costly or use EPDM liner for heating.

Pump, I am sure you have looked at RV pumps?
Ebay: 4.7gpm RV pump for $50 w/ship. Works great on my well.
Not sure how long it will last, couple friends have been using them all summer long for years, still working.

Have fun,
Scott.

[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#16)
by TomW on Tue May 13th, 2008 at 07:26:26 AM MST
(User Info)

Scott;

Gravity feed was based on simplicity. the location will be away from power sources and mostly for garden / tree watering. If it requires much more than a faucet the better half will consider it "too complicated". If you are married you will understand these type limits.

Nothing is ever as simple as it seems.

The cost is not such a limiting factor but keeping it down leaves more for other stuff.

Right now this is a wish list project so I have not done any in depth research.

When it gets closer to an actual project I will look closer at options

Thanks for the feedback.

Tom.

If everyone is thinking alike, then somebody isn't thinking.

--George S. Patton
[ Parent ]

Re: Filling water heat storage tank with rocks (3.00 / 0) (#8)
by jmk on Fri May 9th, 2008 at 07:53:18 PM MST
(User Info)

I'll tell yo what I did with my rocks is put them in the swimming pool to hold down my steps. They for the third year were polluted my pool with all kinds of growing green stuff. Rocks have things embeded in them that you wouldn't expect. plus who knows what they will contribute to water as far as chemicals?
jmk

Filling water heat storage tank with rocks | 16 comments (16 topical, 0 editorial)

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