Massy floors for solar heat storage

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I am looking at a new build and considering building a heat sink into
the material under the basement floor slab.

I'd call that a heat store. To me, heat sinks just dissipate heat from
transistors and other things.

This would involve excavating down an additional depth in a 2000 sf ft
slab... The walls up to the eventual basement floor grade would
possibly be an ICF... to provide an insulated below grade wall that
will be completely buried when completed. Within this structure I would
first put 1 ft of silty till... and then lay loops of heat exchange
coil(at a spacing of about 3 ft?). Next cover with 3 ft of silty till
(and then lay another course and then perhaps another 3 ft of silty
till?) and a final set of heat exchange loops finally covered by the
final 1 ft of silty till and then gravel... and then poured concrete
floor (no insulation under the concrete pad). The basement walls would
then be poured concrete with R-40 on the exterior below grade to
provide a good insulated thermal mass surrounding the full basement.
(This heat sink would thus have heat exchange loops cubing about each
cu m of till - is this overkill given the thermal conductivity of a
silty till - whatever that may be?)

That depends on how fast you need to withdraw the heat, which increases
with the house thermal conductance and indoor-outdoor temperature
difference.

The question is how many BTUs' can I store in dry in this till...

Dry soil has about 30 Btu/F-ft^3 of thermal capacitance and about
R1 per foot of resistance,

The climate is southern Ontario and normal deep ground temperature
approx 8 deg C. I am assuming that... I can get the temp up to near 25
deg C by the end of fall when I need to start to withdraw heat and drop
it down to near 5 deg C (in late spring)for a 20 deg C seasonal swing.

That's 36Fx2000ft^2x8'd = 17.3 million Btu. Is that enough? Too much?

I am premising this on the fact that this type of system would be
easier (and less expensive) to install and control than the typical
vertical or horizontal heat exchange loop system for a conventional
ground source heat pump (and in the event of any prolonged power
failure keep the building well above freezing for an extended period
of time).

A 48'x48'x8' house with 96 ft^2 of R4 windows with 96/4 = 24 Btu/h-F of
thermal conductance and 3744 ft^2 of R40 walls and ceiling with 3744/40
= 94 and 30 cfm of air leaks with about 30 Btu/h-F totaling 148 would
only need 1.8(0-(-3))148 = 800 Btu/h to stay 0 C indoors on an average
-3C January day in Toronto, so the basement might keep it from freezing
for 17.3M/(24hx800) = 901 days

... I may need to do a heat loss calculation first and then work
backward to calculate heating requirements

That's a good idea. If cloudy days are like coin flips, storing heat for
5 days can make a house 97% solar-heated. If it's 70 F for 12 hours and
60 for the other 12, it needs 24h(65F-27F)148 = 135K Btu/day, or 675K
Btu for 5 days. With lots of surface, a 48'x48'x8" deep floor with 12
$25 42"x48'x6" deep water-filled polyethylene film greenhouse air ducts
laid flat on the ground among 144 hollow concrete blocks under 4'x4'
plywood floor slabs with 1008 ft^3 of water and C = 1008x62.33 = 68.2K
Btu/F can provide that with no heat pump if the floor is 70+675K/68.2K
= 81 F on an average day.

My 1981 NRCC Solarium Workbook (one copy that was not burned by the
Canadian government says 2785 Wh/m^2 (883 Btu/ft^2) of sun falls on
a south wall and 1321 (419) falls on east and west walls on an average
January day in Toronto. If equal windows on 3 sides transmit 50%, the
house gains 0.5x32ft^2(883+2x419) = 27.5K Btu/day. We can get the
remaining 135K-27.5K = 107K from A ft^2 of R1 $1/ft^2 corrugated
polycarbonate Dynaglas south wall "solar siding" with 90% solar
transmission over a 100 F air gap if 0.9x883A -6h(100-27)A/R1 = 107.5K,
ie A = 301 ft^2. With $2 R2 Therma-Glas Plus twinwall polycarbonate with
80% transmission, A = 220 ft^2. A solar attic could also work.

A $35 1000 cfm car radiator with its 2 fans in series (20-watts total)
could move solar warmed air down under the floor through a duct near the
slab center with a separate siding cavity air return duct. We could heat
the house by allowing floor air to flow up naturally through the same duct
and back into the floor near the perimeter, using a 2-watt motorized damper.
The car radiator could also heat water for showers with a $60 1"x300'
pressurized black plastic pipe coil in a 4'x8'x3'-tall 140 F box with
a folded 10'x14' EPDM rubber roofing liner.

... are you confident in the "1000 CFM @ 20w" figure you quote at the end?
From what I've seen of blowers, that seems off by an order of magnitude.

Nathan Hurst measured 1000 Btu/h-F in water heat gain in Melbourne (see
http://www.builditsolar.com/Projects/Sunspace/LowCostHtStorageNathan.pdf.)
That requires about 1000 cfm of matching air movement. He ran his fans
at 16 W total with PWM controllers. Nathan and I ran my car radiator and
2 fans in series from a 20 W PV panel at the PA Renewable Energy Fest
last September. They use 36 W from a 12 V battery.

For example: http://www.waveplumbing.com/store/index.php?main_page=product_info&cPath=55_459&products_id=3455
rate about 8 CFM per watt.

Longer blades tend to be more efficient. I like Lasko's $50 2155A 20"
window fan, which can move 2470 cfm with 90 watts, ie 27 cfm/W..

Though the study on the gossamer ceiling fans indicate flow rates of
150-200 cfm/watt for "typical ceiling fans". I suppose that's with
essentially zero static pressure.

http://www.fsec.ucf.edu/en/publications/html/FSEC-CR-1059-99/

With large ducts and low airspeeds (<400 lfm) and few turns, we can get
close to zero pressure. Grainger's $149 3C690 48" industrial ceiling fan
moves 21K cfm with 86 watts, ie 244 cfm/W. Their $221 4C761 60" fan moves
46K cfm with 105 watts, ie 438 cfm/W.

With lots of surface, a 48'x48'x8" deep floor with 12 $25 42"x48'x6"
deep water-filled polyethylene film greenhouse air ducts laid flat on
the ground among 144 hollow concrete blocks under 4'x4' plywood floor
slabs with 1008 ft^3 of water and C = 1008x62.33 = 68.2K Btu/F can
provide that with no heat pump if the floor is 70+675K/68.2K = 81 F on
an average day.

PhD Rich Komp (author of "Practical Photovoltaics") solar-heats his
Maine house with a similar "hypocaust" hollow mass floor (a concrete
slab over lots of hollow blocks) and a small PV-powered fan.

Nick