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thermal storage for heating a van
21 dec 1996
meanwhilst, for those with more portable lifestyles...
morse (?) wrote:
>harvey white (madyn@ix.netcom.com) wrote:
>: look into thermal mass...
good idea.
>: ...most vehicles are insulated as well as a can of catfood.
heh :-) but an 8' x 8' x 16' van-thingy with r20 insulation (2" of styrofoam)
and a thermal conductance of 2x24'x8'/r20 (walls) + 2x128ft^2/r20 (roof+floor)
= 32 btu/hr-f would need about 24hr(68-30)32 = 30k btu/day to stay 68 f inside
when it's 30 f outside, say 100k btu for 3 days without any sun.
>: ...heat up... water.
how much? if it's 130 f to start with, and it can provide useful heat until
it reaches 80 f, we need about 100k/(130-80)=2000 pounds of water, about 250
gallons, 31 cubic feet, an 8'x8'x6" or 8'x16'x3" water wall...
or a waterbed on the floor, with a sunspace on the south side of the van and
2' x 16' of vertical plastic film glazing on the south side underneath? in
philadelphia, with a 30 f average outdoor temp and 1000 btu/ft^2/day of sun
falling on a south wall in january, a single-glazed sunspace collects about
750 btu/day of sun during the day, so we'd need about 30k/700 = 43 ft^2 of
sunspace to keep the van warm on an average jan day, say a 6'wide x 8'tall x
3" thick air heater patch on the south wall or an 8'deep x 16'wide x 8'tall
unfolding sunspace along the south wall.
a sunspace at 68 f during the day with an r8 space blanket skirt around the
enw sides of the van would let the van bottom collect about 0.92^2x1000x32ft^2
= 27k btu/day of heat, and lose about 6hr((t-68)32ft^2/r1+(t-30)64ft^2/r8)
= 240t - 14k of heat on an average day, for an average waterbed temperature of
41k/240 = 171 f :-) neglecting radiation loss, which would limit the temp to
something closer to 130 f. the waterbed needs a quilt on top, and some sort
of grillwork underneath to act as fins and mechanical support. this might
increase the heat exchange area to about 320 ft^2 for thermal charging and
discharging. a fan moving 300 cfm of air at 8 mph might decrease the required
grill area to about 100 ft^2, eg 18 fins 1' apart, each 8' long x 8" tall, as
well as assure more precise air temperature control in the van, and decrease
the minimum useful waterbed temperature to 70 f for 68 f space heating, thus
increasing the useful waterbed thermal storage capacity.
unrolling a white reflector on the ground in the sunspace would add about 30%
to the waterbed solar input, but that doesn't seem needed here.
>...the amount of water needed to store the heat after the sun goes down
>is heavy enough to add to fuel costs if you drive the van.
this would work better if the van didn't move very often, and the waterbed
were emptied before moving. filling the waterbed might take a half-hour at
5 gpm with a garden hose. the waterbed might warm up at a rate of about
27k/(250x8) = 13.5 f per day, or some 5 times faster and surer (127k btu/hr)
if the grillwork underneath were welded onto some interconnected pipes with
hot radiator antifreeze running through them as the engine charged the
battery, perhaps to make 110 vac with a small inverter...
>plus it just seems bulky.
maybe not, if it needs a bed anyway, or if the area under the bed can also
be used for storage...
>...can anyone visualize rigging the inside of the van or car with,
>for example, a black container for water to heat up water through
>the van window for coffee, dehydrated meals, etc.?
sure... but it seems simpler to preheat some water from a gravity roof supply
with a heat-exchanger via the waterbed, and then further heat it electrically
in a very small insulated tank with a thermostat, if required.
now how can we do this with a small boat, to inexpensively and compactly
combine thermal storage with tracking solar collection (rotate the boat),
wind power, photovoltaics, water storage, rain collection, desalination,
water recycling, wastewater treatment, engine cooling, ballast and keel?
nick (back from st. martin sailing, 17 f this am, vs 85 f yesterday morning)
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