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re: solar wood kilns
22 aug 1996
richs@freenet.edmonton.ab.ca wrote:

>subject: re: solar wood kilns

i helped cabinetmaker bob harrington build a solar wood kiln a few years ago.
he says this topic was recently covered in _fine woodworking_.
 
bob says "dry wood stickered outdoors under cover, with no heat, for 1 year
for each 1/4" thickness, from its "fiber saturation point" to about 20%
moisture. this is what makes wood expensive, vs the cost of the wood itself,
ie, taking the time to dry it outdoors. for instance, it takes four years to
air-dry 16/4 lumber outdoors." can that process be hurried, with careful
temperature and humidity and wood moisture control? if the wood is surrounded
by warm moist air (150 f at 50% relative humidity?) it might dry quicker
without checking.

bob says initial drying time varies with moisture content, being lowest for
wood cut when it is fairly dry, eg in january, vs early spring, when it has
taken up a lot of moisture.

the next step is to dry the wood from 20% to 6 or 7% moisture content over 
about 2 weeks, perhaps during july or august, eg inside a greenhouse with open
endwall vents. the final drying time depends on the wood species. bob says 
black walnut takes about 2 weeks, poplar and cherry a bit less time, at an
average temperature of 80 f. higher temps are ok, up to 150 f, he says.

he cautions against case-hardening, ie letting the wood dry too fast, so
the fibers outside dry and shrink over the damp fibers inside, trapping the
moisture inside and creating great pressure. he says it's dangerous to cut
case-hardened wood, and this condition can last for years. others say that
rarely happens, because the outside wood splits or checks first, if it is
dried too quickly.

how about storing the wood inside a commercial plastic film greenhouse?
bob says that's all that is needed... but can this be speeded up? 

where i live, near philadelphia, with an average december temp of 36 f and
1000 btu/ft^2/day of sun falling on a south wall, a 20' wide x 10' tall x 32'
long (ew) greenhouse with a 1300 ft^2 double polyethylene film cover and a
20' wide reflecting pool along the south side (about $500 worth of parts and
a day's labor) might collect about 425k btu/day of sun and lose about
6hr(t-36)1300ft^2/r1.2 = 6500(t-36) btu of heat during a 6 hour winter day,
so if it had little thermal mass the average daytime temperature t might be
36 + 425k/6.5k = 101 f... the greenhouse would be the "sunspace." 

the "solar closet" might be around the stack of wood, say 24' long x 8' tall
x 8' wide, so the south wall could be hinged at the base and folded down into
the 20' wide greenhouse for access. the wood stack might have some straw as
insulation on the north and top sides, with an r20 fiberglass south side and
endwalls. the south side could have some polycarbonate glazing with an air
gap behind that, then dark-colored insulation to make a solar air heater.
a motorized damper or some fans would move warm heated air through the wood
stack when the sun is shining. if the wood had a temperature t, it might
receive 192k btu/day of solar heat through its 192 ft^2 south side in december
in philadelphia, and lose about

      6 hr(t-101)192ft^2/r1  = 1152t - 116352 south wall, day
    +18 hr(t-36)192ft^2/r20  =  173t -   6221 south wall, night
    +24 hr(tc-36)512ft^2/r20 =  614t -  22118 other walls, 24 hr
                             = 1939t - 144691 = 192k btu, so 

       t = (192k + 144,691)/1939 = 174 f :-) ?

warm enough to remove moisture, with a fan controlled by a humidistat, that
circulates stack air through the greenhouse, where the water vapor condenses
inside (blowing water vapor out of the greenhouse wastes heat in the winter.)

if the wood stack were white oak with a specific heat of 0.57 btu/f-lb and
a density of 47 lb/ft^3, the stack would weigh 24x8x8x47 = 72k pounds dry,
and it would have an a thermal capacitance c = 72k x 0.57 = 41k btu/f. the
surrounding thermal resistance would be r = 20/704 ft^2, with no airflow,
so the rc time constant would be 1164 hours. after a cloudy week in december,
the dry wood stack temperature might be 36+(174-36)exp(-168/1164) = 155 f. 
a slightly less accurate way to calculate this is to find the amount of heat
that leaves the stack in a week at the starting temperature, with no airflow,
ie 168hr(174-36)704ft^2/r20 = 816k btu, then find the decrease in temperature
at the end of the week, 816k btu/41k = 19.9 f, using the wood capacitance,
and find the temperature at the end of the week, 174-19.9 = 154.1 f. this
is a lower temperature because the wood actually loses less heat every day
as it cools.

it takes about 1000 btu to evaporate a pound of water, so starting with wood
with a 50% moisture content, ie one pound of water per pound of dry wood, one 
lower limit for stack drying time would be 72kx1000/425k btu/day = 169 days,
yielding about 5 gallons of water per day.
 
nick



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