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Sand beds for thermal storage

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Posted by phillip on March 26, 2007, 8:57 am
A while back, there was a boisterous debate about the efficacy of
thermal storage using a sand bed.  Alan at daycreek.com has been
building a home in SE Minnesota, which he heats via active solar
collection, with the resulting heat stored in a 100-ton sand bed
beneath his house. He updates his website continouously with
temperature measurements throughout his house.  Even in winter, his
home is heated to an impressive 72 degrees from the sand bed.

The ensuing discussion concluded that since the sand was wet when it
was installed underneath the home, the water in the sand is storing
much of the heat, and there were predictions that the heat storage
capability of this system would degrade after the sand dried up.  Even
if the water in the sand is providing most of the heat storage, I'd
like to ask, what's wrong with employing this system?  Couldn't a
soaker hose or pipe be embedded in the sand bed so that one can
periodically replenish the water content of the sand bed?

Isn't this system the most practical and cost-effective means of
heating a house?  There's no need for a large water tank. One need
only dig a large pit, insulate it, fill it with wet sand, install
radiant coils and embed a soaker hose, place a cement foundation over
it, and build a well-insulated house on top.  After that, heated water
from solar collectors can transfer the heat into the wet sand for
storage and later retrieval.  What am I missing? If Alan is getting
the heating performance that he's demonstrating, why aren't others
following suit?  It seems as though he's solved his solar heating
problem.  Hasn't he?


Posted by Jeff on March 26, 2007, 1:36 pm
phillip@doramail.com wrote:

   That is a lot of sand (over 2,000 cubic feet). Sand has a specific
heat of .19 BTU/lb F (water is 1), So that store will have nearly .4
therms (40,000 BTUs) per degree F. At 40 F temp change that is 16
therms. Sand is about 50% denser than water so the sand thermal store
would need to be 3.3 times as large as the water store.

   It's all a tradeoff. The sand volume is larger and would need more
and thicker (because of the greater area) insulation than water if you
were worried about losses, some of the losses may be into the house though.

   It can be easier to add and remove BTUs from a water store just by
pumping in and out water. No need for a heat exchanger.

   But then, sand never leaks and needs the care that water might.


  After that, heated water

Posted by daestrom on March 26, 2007, 9:58 pm

I'd agree. I just add that it is easier to build over a sand bed then a
water tank.  So losses do migrate into the home.

As far as wetting it down, that will certainly increase it's storage ability
and the conductivity to get heat in/out of it.  But there may be some risks
if you put *too* much water in it.  If it starts to liquify (i.e.
'quicksand') you could have some sudden shift/settling.


Posted by Solar Flare on March 27, 2007, 12:09 am
 The heat may be hard to control. Effectively you could have no more
heat storage than you can use in house temperature for that day or
your house would overheat. The wetting system may be a control for
overheating, absorbing more heat into the cold water when too much
heat is available. The gorund and foundation mass would make a great
thermall storage mass for regulating temperatures over the period of a
few days, though, and floor heat is very comfortable way to heat a

Typically, water under your foundation is not desirable and people put
in sump pits to attempt to carry it away from the foundations.

Posted by phillip on March 28, 2007, 6:13 pm
 Thaks to be everybody who replied.

While possible, but I think such risks can be mitigated.  In addition
to embedding heat sensors in the sand bed, one can also embed moisture
sensors to determine if too much water is present, an to preent


Wetting the sand bed is intended to replenish its heat storage
capacity, not serve as a temperature regulator. Alan at DayCreek
designed his system so that no controls would be required:


The advantage of this system is its simplicity and sustainability. The
system is entirely powered by the sun and for the most part is
maintenance free. There are no electronic control systems or tanks of
water that need to be maintained. The system is totally independent of
the grid and will continue to operate during power outages.

The disadvantage to this system is lack of control. Once the system is
charged and heat begins radiating from the floor, there's no way to
immediately control the heat and it may require opening a few windows
in the house to cool things off on a warm, late winter day.

While on, his system merely dumps heat into the sand bed continually.

It struck me, though, that this is essentially what the people at
Drake Landing solar community are doing as well, using active solar to
heat up the ground for thermal storage.  They do not seem to use sand,
but plain earth, but the idea is still the same.  A similar project
uses mud, instead of earth, sand or wet sand, for its even greater
heat retention capability:



Why wet dirt instead of the more traditional water or rock storage?
Joe cites three reasons: "First, because dirt is handy . . . it's
already on the building site. Second, wet earth has surprisingly good
heat-holding characteristics. (It has a capacity of 30 to 44 Btu's per
cubic-foot-degree, versus about 20 for rocks and 62 for water.)

"What really convinced me to go the mud route, though," says Joe, "is
its low cost compared to other storage mediums. On a cost-per-Btu-of-
storage-capacity basis, you just can't beat mud for heat storage."


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