energy and housing scale SOLAR.KnowledgePublications.com

energy and housing scale
29 feb 2000
it's hard to build small solar-heated houses because the volume
available for heat storage grows as the cube of the dimension, vs
the heat-losing, air-leaking exterior surface, which only grows
as the square.

and if larger buildings contain more people who use electricity,
the ratio of internal heat gain to exterior surface grows. otoh, 
maybe that means more energy is needed for cooling... 

is there an optimum size for a building with multiple dwelling units
which depends on the climate? consider a 64 foot cube containing 16
2,048 ft^2 units. with 6" r25 sip walls and ceiling and 8% of the
wallspace as r4 windows, its thermal conductance is approximately
1300ft^2/r4 = 325 btu/h-f for the windows plus 767 for the walls and
ceiling, a total of about 1100 btu/h-f. where i live, near phila, pa,
it needs about 24h(70f-30f)1100btu/h-f = 1056k btu/day to stay warm
on an average december day.

forty warm people might contribute 40x300btu/h, 50% of the time,
ie 144k btu/day. if each unit uses 300 kwh/month of electricity, that
adds another 546k btu. if the windows have 50% solar transmission,
and 50% of them face south, and 25, 15, and 10% face east, west and
north, that adds 450k btu, for a total of 1140k btu, more heat than
the building needs on an average december day, with no special
"solar features."

it needs about 2.25 million btu of heat for 5 cloudy december days
in a row, which might come from 2250k/(150f-100f) = 45k pounds or
5,600 gallons of rainwater cooling from 150 to 100f in 4 $400 1500
gallon polyethylene tanks on the ground, after it's heated in a
trough along the north wall of a concentrating solar attic, which
might be a large open common area with a parabolic north roof
that's reflective underneath. 

for air-conditioning, the attic might might concentrate a lithium
chloride solution more efficiently than the roof described in
"unglazed collector/regenerator performance for a solar-assisted
open cycle absorption cooling system" by m.n.a. hawlader, k.s. novak,
and b.d. wood of the center for energy system research, college of
engineering and applied sciences, arizona state university, tempe,
az 85287-5806 usa, in solar energy, vol. 50, pp 59-73, 1993...

"an ordinary black shingled roof... was used as a collector/regenerator
for the evaporation of water to obtain a strong solution of [licl] 
absorbent... experimental results [using a 36'x36' roof] show a
regeneration efficiency varying between 38 and 67%. cooling capacities
ranged from 31 to 72 kw (8.8 to 20 tons)", ie about 1 ton per 100
square feet of roof area.

in the house "water [the refrigerant] is sprayed into an evaporator,
evacuated to about 5 mmhg of pressure, where it immediately flashes
into vapor... cold water, pumped from the bottom of the evaporator, 
flows through a fan coil... that blows cool air into the conditioned
space. the absorber acts as a vapor compressor and condenser for
the system. water vapor from the evaporator flows over the absorber
where it is absorbed by the concentrated absorbent. the continuous
absorption of water vapor maintains a low pressure in the system
and permits flashing of water in the evaporator... the product of the 
absorption process, a weak absorbent solution, collects at the bottom
of the absorber to be pumped [up over the roof] for concentration."

"the dilute licl solution was delivered to the collector surface
through a spray header spanning the top of the roof and made from
50.8 mm (2 in) diameter cpvc pipe fitted with 35 evenly spaced
brass nozzles. the concentrated solution collected at the bottom...
in a pvc rain gutter, and returned via gravity feed to a 1608 liter
(425 gallon) fiberglass tank...

nick