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re: solar power efficiency pv vs heat collector
3 jul 1997
wrote:
>nick pine wrote:
>> >>...if we really need seasonal storage, how much would it cost to build a
>> >>5 meter cube full of water drums, sitting outside on the ground, with a
>> >>flat rubber roof, surrounded by strawbales?... perhaps it would help if
>> >>the walls were made of used tires filled with compost :-)
the local gas station will pay me $3 each to take away used tires...
>> [or] "smaller," eg 32 vs 256 55 gallon drums. or maybe just a couple of
>> uninsulated drums, if the heat is stored by concentrating a lithium
>> chloride solution in a solar still, and recovered by condensing water
>> out of ground-heated house air at 1000 btu per pound of water.
>i've often wondered what it would take to make some sort of "reservoir"
>to hold "heat"...
you might make a large insulated box filled with sealed containers of water,
heated via solar-warmed air that flows into the box during the day, from some
sort of low-thermal mass isolatable plastic film sunspace that gets cold at
night when dampers or windows or doors close and airflow stops.
>how big it would have to be and how long it would hold "useful heat."
that depends on the heat loss of the house, the heat capacity of the thermal
mass, and its initial and final temperatures. for instance, an 8' cubical
"house" with r20 walls and no air leaks needs (70f-30f)6x64ft^2/r20 = 768
btu/hour to stay 70 f inside when it's 30 f outdoors. a single 55 gallon drum
full of 130 f water cooling to 80 f stores about 55gx8lb/g(130-80)1btu/lb-f
= 22,000 btu, enough to keep the "house" at 70 f for 22k/768 = 29 hours when
it's 30 f outdoors.
>what's the calculation you have used here to come up with these figures?
that's the simple version above. if you live inside the heat store, eg in a
house with masonry walls and insulation outside the walls, the house temp is
less controllable, the temp swing of the thermal mass is smaller, so it stores
less useful heat, and the math is slightly different. suppose an 8' cube has
a = 6x64 = 384 ft^2 of 6" concrete walls with volume v = 0.5a = 192 ft^3 and
thermal capacitance c = 25v = 4800 btu/f, and r20 insulation outside, with
thermal resistance r = r20/a = 0.0052 f-h/btu, and time constant rc = 250
hours, or about 10 days. then if the initial concrete temperature is 70 f,
after 29 hours it would be 30+(70-30)exp(-29/250) = 65.6 f. after a cloudy
week, the house temperature would be 30+(70-30)exp(-168/250) = 50.4 f.
replacing the concrete with 1/4 the volume of 130 f water inside an insulated
box in the house would allow storing about 192/4(130-80)64 = 154k btu, which
could keep the house at exactly 70 f for 154k/768 = 200 hours or 8 days.
>it would be interesting to see if this sort of system could be planned
>prior to building a house.
most of it can be planned :-)
in our home solar heating system we used water as the thermal storage
medium for an air-transfer unit, the water being contained in 1000
one-gallon polyethylene bottles stacked so that air could flow between
them. they worked satisfactorily until some desert pack rats invaded
the storage bin, making nests of the insulation and chewing holes
in the water bottles.
p 468, _applied solar energy_, by
aden b. meinel and marjorie p. meinel
addison-wesley, 1976
nick
nicholson l. pine system design and consulting
pine associates, ltd. (610) 489-0545
821 collegeville road fax: (610) 489-7057
collegeville, pa 19426 email: nick@ece.vill.edu
computer simulation and modeling. high performance, low cost, solar heating and
cogeneration system design. bsee, msee. senior member, ieee. registered us
patent agent. solar closet paper: http://leia.ursinus.edu/~physics/solar.html
web site: http://www.ece.vill.edu/~nick. today's special: send me $3, your
postal address and a us patent number, and receive a copy of the patent as
well as post-issuance actions in about 4 days...
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