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re: frugal and free power for an emergency
25 jun 1999
news wrote:
>> my dad's neighbor, who built his own new house, pumped his own
>> solar heated water through his concrete foundation.... and he pumped
>> the heated water with a small, dc pump, which also used a small
>> photo voltaic panel to power the pump.... the beauty of 'heating the
>> concrete slab' system is that only a small pump (vs a 15 amp blower
>> motor) is needed to distribute the heated water through the house...
>
>the same can be done much cheaper and simpler by using concrete ducts in the
>foundation and using air - no leaks.
is that cheaper and simpler? a tiny pump and some 1" wirsbro plastic pipe
on 2' centers in a slab, vs a fan with 4" pvc pipes in a thicker slab? some
solar air heating systems have a cop of 5. might as well use a heat pump.
water has a much higher heatflow capacity and thermal conductance than air.
another advantage of moving water is that the slab can be a sun/air-water
heat exchanger for a larger tank full of water, like physicist tim ellison's
"10-day time constant passive pyramid house" with 3 3000 gallon polyethylene
tanks in an outdoor strawbale sculpture, as described at last week's portland,
or ases conference.
>keep it simple, keep it passive solar not active solar.
who cares these days about "active and passive," vs simplicity, cop,
payback time, and comfort? a tiny pump may be "more passive," when it
comes to power consumption.
>have a duct with a small permanent magnet motor fan fixed in the duct (heat
>from motor is used) to take air from the highest point of the house to the
>lowest - simple and cheap ans switch on by a simple thormostat at high level.
bigger and slower fans are more efficient. why not use a closet for an
airshaft (and clothes dryer :-), with a 4' diameter 110v 100w 25k cfm
ceiling fan to force air down through a low-temp "warm room" full of
thermal mass in the basement and back out through a sunspace?
collect 100 f warm air behind a dark mesh near the top of the sunspace,
and use it to heat 20k pounds of water from 80 to 90 f on an average day.
at 33k btu/h with a 10 f delta t and 40x25 = 1000 ft^2 of surface for 40
55 gallon drums in an 8'x10'x8' tall uninsulated box with a minimum thermal
conductance 2 + v/2 = 33kbtu/h/(10fx1000ft^2) = 3.3 btu/h-f-ft^2, we need
a minimum air velocity v = 2.8 mph or 230 feet per minute. the air volume
might be 4k cfm flowing through a 4'x4' cross-sectional area, which
determines the closet duct size and drum spacing.
nick
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