Heat Storage Tank

[tom in maine]

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!!

 

[hkazemi]

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!!

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.

 

[tom in maine]

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.

 

[hkazemi]

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.

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?

 

[hkazemi]

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, (broken link removed to 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.

The houses he has built with this system are all, as far as I know, direct usage, solar active goes in, but heat dissipated passively through the slab, which sits over a 4-5 foot sand bed with pex at the bottom and insulated with 4 -6" of blue board around all sides except the top. There you have a vapor barrier and I would advise for a 4000 psi concrete mix with a good parking garage type water sealer in it. So when the plastic breaks down you still have concrete that is watertight and vapor impermeable. We have used some of the newer Radon sealers and they do not smell and cut the vapor transmissivity measured with a calcium chloride test in half.

Still, depending on your heatloss the storage is quite impressive with sand. Thermal conductivity of sand dry is .37 BTU/h/ft/F, wet sand at 10% AND 13% resp are 0.8 and 1.13 so even a little moisture goes a long way to improve conductivity. (Kusuda et al, ASTM pub 04-9220000-10 google book search available) other give numbers of water 0.346 / wet sand 0.95 / dry sand 0.157
The other important factor is specific heat capacity: (btu/lb F)
Water is 1, Sand is 0.19

There are two primary materials that through which heat is transported in an aquifer, the mineral matrix and the entrained groundwater. Density of the mineral matrix and water, porosity, and heat capacity combine to govern the temperature response of an aquifer to heat flow. The specific heat Cp (as BTU/lb0F), dry density (rho, as lbs/ft3) and percent porosity (phi) of some soils and minerals are as follows:

Specific Heat - Cp
Water 1
Clay 0.27
Sand 0.19
Granite 0.20

Density (rho) lbs/ft3
Water 62.4
Clay 65
Sand 110
Granite 165

% Porosity (phi)
Water
Clay 75
Sand 35
Granite 1

The thermal energy capacity per unit volume per degree of temperature change is termed the aquifer thermal capacity (q). With the above information it is possible to calculate the thermal capacity (q) of an aquifer:

q = (Cp x rho)rock(1 - phi) + (Cp x rho)water x phi

Assuming a sandy aquifer with a specific heat of 0.19 for the mineral matrix, a density of 110 lbs/ft3, and a porosity of 0.35 the specific thermal capacity per cubic foot of aquifer is:

* q = (0.19 x 110)rock x (1 - 0.35) + (1.0 x 62.4)water x 0.35
* q = 13.59rock + 21.84water = 35.4 BTU per cubic foot per 1 degree Fahrenheit

Heat flow through a mass (qx) is calculated by the equation:

qx = Ak(dT/dX)

* A is the cross sectional area normal to the heat flow,
* k is the thermal conductivity,
* T is temperature in Fahrenheit,
* X is the distance over which the heat must flow.

The solution to the above function requires substitution of an equation describing the system geometry for the dX term followed by integration. That is beyond the scope of this column.

Thermal diffusivity (in units of square feet per hour) is equal to the thermal conductivity (k) divided by the thermal capacity (q) of the aquifer,
it measures the rate at which temperature changes occur in the soil mass. Higher values of thermal diffusivity result in more rapid changes in
temperature and deeper penetration of heat into the soil.

You probably have looked into al that and as Keith said earlier is need to be pretty big storage to do the trick. Ramlow has told me he uses 4 - 5 feet under the basement slab and then just lets is bleed into the house. Not sure if you would want to do that with a slab being directly under your prime living space. The more heat exchangers you have the more inefficient the system becomes (delta T and higher storage temps are necessary then)

As always it points to a modestly sized and very well insulated structure to be heated. Nothing new there either. I still like the idea you floated a while ago of installing the solar within a green house to get higher efficiency in our cold northern latitudes.

Best,

Niko