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re: dirigible wind turbines
4 dec 1997
mark j. lapierre wrote:
>what about fermenting pig shit or something similar?
i don't own a pig, but maybe a cesspool with a floating cover would work.
the phoenixville, pa sewage treatment plant has 70' diameter x 28' high
anaerobic digesters with floating covers that move up and down 7', which
are kept at about 100 f by boilers burning their methane. metcalf and eddy's
wastewater engineering book says secondary sewage treatment plants produce
about one cubic foot per capita per day.
on a smaller scale, the nraes-20 on-farm biogas production manual says one
"sow unit" (an average animal weight of 1300 pounds of pigs and sow producing
an average of 16 pigs per year) makes 1.75 ft^3 of manure per day, yielding
56 ft^3 over a retention time of about 15 days. manure from one cow produces
about the same.
the chinese biogas manual (trans. michael crook, int'l tech pubs, 1979,
isbn 0 903031 65 5) says a metric tonne of dried "general stable manure"
makes 270 m^3 of biogas, dried pig manure makes 561, horses, 250, fresh
grass, 630, straw, 342, leaves, 250, and sludge, 640. they also give c:n
ratios to help in making an optimum 25:1 mix. "fresh human manure" is
about 3:1, horse and cow manure are about 25:1, leaves are 40:1; dry straw
is about 90:1. they say a family of 5 in the countryside requires about
1 m^3/day of biogas for cooking and lighting, and each m^3 of pit digester
produces about 0.2 m^3/day in the summer and 0.1 in the winter. making
20 ft^3 (0.5 m^3) of biogas per day might require a 3 m^3 digester fed with
one person's manure and something like an average of 5 pounds of dry leaves
per day. using 10 pounds of leaves or 5 pounds of straw might make 1 m^3/day
with a better c:n ratio.
>it gives off heat and gases, mostly methane. i think methane is
>lighter than air...
ch4, right? the nraes manual says biogas is 55-70% methane, and a lot of the
rest is co2. a mole of methane weighs about 12+4 = 16 grams vs air at 30,
0.716 kg/m^3, vs 1.293, so it's heavier than hydrogen, but maybe it won't
diffuse as fast through balloon walls. those 0.0025" polyurethane-vinyl
advertising balloons lose about 1%/day of helium volume, maybe the same
for hydrogen. it takes at least 0.1 kwh of electrical energy to make 1 ft^3
of h2, i think (thanks to dr. ray schultz for help in finding this number),
so 7 8' weather balloons containing 1800 ft^3 of h2 might require 20 ft^3
h2/day, ie about 2 of the 10 kwh (?) made by 4 375 watt air-303s windmills
aloft, ie about 20% of the power output.
>this could be done in a digester on the ground and connected to the
>balloon by a small tube.
the digester might be the ballast foundation and methane store. a 16' sphere
in an 80 mph wind might have a pull on the order of 4000 pounds. a teardrop-
shaped balloon might have a lot less pull. a $600 1500 gallon concrete septic
tank has a volume of about 5 m^3 and weighs about 14,000 pounds empty and
26,000 pounds full. if completely surrounded with 2" of styrofoam (glued to
the outside, with a coat of latex paint) it would have a time constant
rc = r10/320ft^2x14k btu/f = 447 hours or 18 days. should the digester
have an electrolyzer inside? would a mixture of methane and hydrogen be
useful for energy storage and use?
>of course the thing would have to be purged of air before inflating with
>digester gas.
balloons have a way of doing that :-)
nick
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