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re: portable cabin?
22 nov 2002
annie  wrote:

>i was obsessed with the idea of portable cabins for years. here's a
>website showing portable cabins designed by a group trying to help
>homeless people:

interesting site. i offered them some advice... 


hi frank,

i like your huts and cogen system. i suppose you don't have much need
for heat in atlanta, but solar heating the huts would be interesting.
you might store the heat in water, and insulate the huts with soap
bubble foam inside poly film walls, like a plastic film greenhouse.
a hut like this might only weigh 10 pounds. where would it go?
where do the owners take showers or go to the bathroom? how would
you protect a plastic film hut from burglars and vandals?

in pa, cars that need lots of body work to pass state inspection go
for peanuts, often less than $100, even if the engines work fine.
as an engineer, i can suggest a few improvements for your cogen system...

1. a car burning 1 gallon of  gasoline per hour might make about
100,000 btu per hour, comparable to a home furnace. your 15'x5/8"
copper tube has a = 2.45 ft^2 of surface, and it might have u = 30
btu/h-f-ft^2 of water-water film conductance. with, say 2 gpm of
hot water flowing through it, the heat capacity flow rate cmin = 2x8x60 
= 960 btu/h-f, which makes the number of heat transfer units ntu = au/cmin
= 2.45x30/960 = 0.077 for a heat exchanger effectiveness e = 1-e^(-ntu)
= 0.074, ie this heat exchanger is only 7.4% efficient.

if 180 f water enters the coil and the drumwater is 80 f, we can only
move ecmin(180-80) = 7076 btu/h of heat into the drumwater, a lot less 
han the engine can make. it's no wonder that "engine life was greatly
reduced" when you disconnected the air cooling fan on the radiator.
you need a lot more tubing. or just run the radiator water into the
heat store, and use a heat exchanger (eg a couple of 4"x5' pvc pipes
near the top) for the unpressurized potable hot water. 

2. you might measure the temperature of the water returning from the
radiator to the engine as the car normally operates, then disconnect
the radiator hoses and run all the radiator water through the heat
exchanger. as long as the water returning to the engine is still less
than the normal temp you measured, everything should be ok. you might
put a thermostat on the supply line to shut off the engine if the water
becomes too hot.

3. you say the drum water heat store for the huts stays hot "for hours."
seems to me you could easily make this "days," with a larger heat store
and more insulation. a 4'x8'x8' plywood box with r30 walls (r19 fiberglass
insulation between 2x6 studs on the outside, and 2" styrofoam over that,
(with latex paint on the foamboard) full of 180 f water would have 256ft^2
of surface with a thermal conductance g = 256/30 = 8.5 btu/h-f, and
c = 4x8x8x64 = 16384 btu/f (pounds of water), so rc = c/g = 1927 hours,
or 80 days(!). starting with 180 f water, surrounded by 50 f air, it might
cool to 120 f in -1927ln((120-50)/(180-50)) = 1193 hours, or 50 days,
if nobody's using the water... you might line the box with a single 16x20'
piece of epdm rubber roofing material folded up like a chinese take-out
box, and float some foamboard on top. if the car ends up making more heat
than the electricity needed, you might also heat the huts with hot water.

4. another alternative would be to solar heat the tank water with a box
with a transparent south side (polycarbonate plastic behind 2"x4" welded
wire fencing) and an 8'x8' reflector that folds down on sunny days and
folds up tight at night, with a few solar panels on top of the box, and
batteries and an inverter beneath. atlanta's a great climate for solar
house heating, with 1120 btu/ft^2 of sun that falls on a south wall on
an average 41 f january day, with an average daily max of 50.4. you might
collect 1.3x8x8x0.9x1120 = 83.9k btu of solar heat every day and lose
about (t-41)(6hx64ft^2/r1+18hx64ft^2/r30+24hx192ft^2/r30) at water temp
t = 185 f. this could be a fine cloudy-day heat store for multiple huts. 

good luck,


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