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re: generator/battery/refrigerator?
21 mar 2001
scott willing wrote:
>you don't need "an electric potty" or, as someone else has suggested, "a
>propane toilet" or even an outhouse.
"incinerating toilets" with sterilized poop and no liquid discharge
make sewage enforcement officers happy, but they need lots of energy,
something like 3 kwh/day to evaporate 10 pounds of water.
>get a copy of the humanure handbook by joe jenkins...
the time/temp graph on page 133 of the first edition says 122 f will kill
pathogens in a day...
>...there's no reason to waste water, pollute the planet, etc. if you can
>afford a couple of buckets and you can whack together a compost bin...
code people have trouble with possible liquid discharge and incomplete
sterilization. why not build a heater and heat exchanger into a sawdust
toilet? can a still with enough heat transfer surface and insulation that
operates sufficiently slowly be arbitrarily close to 100% efficient?
according to ashrae, the rate of evaporation from an a ft^2 surface with
water vapor pressure pw ("hg) in the air near the surface and pa in the
surrounding air is 0.1a(pw-pa) pounds per hour. if a is large, as with
slowly tumbling damp sawdust, pw can be almost the same as pa...
at 122 f, pw = exp(17.863-9621/(460+122)) = 3.79 "hg, approximately, which
makes the humidity ratio 0.62198/(29.921/pw-1) = 0.0902 lb h20/lb dry air,
so we need to heat c cfm of dry outdoor air to 122 f and exhaust it at 100%
rh to evaporate 10 lb water/day, where 60m/hx24h/dxcx0.075lb/ft^3x0.0902
= 10, so c = 1.027 cfm, on a continuous basis. not much...
a 1' cubical heat exchanger made with 1/8" coroplast (a corrugated plastic
material used for temporary outdoor signs) might have 1'x12/1/8" = 96
1 ft^2 layers with about 200 ft^2 of heat exchange surface. incoming air
would travel through corrugations which act as heat exchange fins with
no condensation while outgoing air would exchange heat with the flat
surfaces outside the corrugations. this makes ntu = au/cmin = 200x1.5/1
= 300 for slowly-moving air, which makes the counterflow heat exchange
efficiency e = ntu/(ntu+1) = 0.9967.
if outdoor air enters the toilet stack at 30 f, outgoing air would leave
at 122-e(122-30) = 30.306 f, so the heat loss from the airflow temp diff
would only be about 24hx1cfm(30.306-30) = 7.3 btu or 2.1 wh/day :-)
with 6 ft^2 of 6" styrofoam (r30) walls, the heat loss through the walls
of the box might be about 24h(122-70)6ft^2/r30 = 250 btu or 73 wh/day.
a $23.42 18 cfm 1.6w 12v fan from grainger (4wt34) would add 24hx1.6 =
38.4 wh/day, making the total 114 wh/day or 3.4 kwh/month.
the box might contain a 4 watt bulb and a $9.95 cooling thermostat
(tm93hvc2416) and a $2.50 heating thermostat (tm02hvc2293) and a $4.95
humidistat (tm89hvc5203) and a small sludge-turning motor from herbach
and rademan (800) 848-8001, http://www.herbach.com.
we might wire up the thermal components like this (in courier font):
< 50% < 122 f < 100 f
-----------x------------|---------------------|-----------------
humidistat heating | cooling |
thermostat < thermostat -----
12v < bulb | fan |
< -----
| |
----------------------------------------------------------------
when you poop, the humidity rises, which turns on the bulb. if the
temperature reaches 100 f and the humidity is still more than 50%,
the fan turns on. when the poop is dry, the box stays warm enough
to keep the rh less than 50%, with very little power consumption.
nick
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