re: double pane windows/housing
3 oct 1997
geoffrey l savage wrote:
>i am seeking the best, easiest to understand source of info that will
>help my client understand that double pane windows, per ashrae specs re:
>insulation, will be a definate advantage in ac unit sizing...
the window companies might have the best propaganda.
>...the hvac folks (at least locally) say that double pane windows
>generally play no real effect on energy savings. please help...
your local hvac criminals may be right. it may not make much difference
in energy savings, although it could affect sizing for maximum capacity.
you might expose your client to "ohm's law for heatflow": a 30x40x8' tall
airtight house with r20 walls and ceiling, and 10% of the floorspace as
double-pane windows has 2,400 ft^2 of exterior surface with a thermal
conductance of 2400ft^2/r20 = 120 btu/h-f and 120ft^2/r2 = 60 btu/h-f for the
windows, a total of 180 btu/h-f. making the windows single-pane raises the
total to 240 btu/h-f. the 24 hour average temperature in miami in july is
83 f, so keeping this theoretical double-pane house 70 f inside during july
requires about 30d(24h)(83-70)180 = 1.7 million btu, ie 1.7m/3/3410 = 165 kwh
with an ac with a cop of 3, $16 at 10 cents/kwh. the single-pane version
might use 240/180x$16 = $21 worth of electrical energy.
real houses need more, because of air infiltration and internal energy use
and green plants and cooking and showers and damp basements and sun shining
in windows, but it seems to me you would still only have this fairly small
$5 single-/double-pane difference on top of that larger number.
and the difference might be even less, with an usine a gaz like the "unglazed
collector/regenerator performance for a solar assisted open cycle absorption
cooling system" described 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 [lithium chloride] absorbent... experimental results [using a 36' x 36'
roof] show a regeneration efficiency varying between 38 and 67%, and the
orresponding cooling capacities ranged from 31 to 72 kw (8.8 to 20 tons)",
ie about 1 ton per 100 square feet of roof area, 1 ton per square :-)
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 l (425 gallon) fiberglass tank... in
the event of of a rain, fluid flowing off the collector could be manually
diverted to a 946 l (250 gallon) wash tank or to a roof drain. during the
initial phase of the rain, residual salt would be washed from the roof
and collected in the wash tank to be stored for later regeneration. after
sufficient rainfall, the rainwater is diverted to the roof drain."
one might avoid this entertainment and licl loss and contamination by adding
one or two layers of plastic film spaced up over the roof shingles. the open
system described in the paper worked best in low winds, about 2 mph. warm
humid air rising might provide such a wind, and draw in fresh air through a
counterflow plastic film air-air heat exchanger, eg the outer glazing cavity,
to make this thermally more efficient, especially with outgoing condensation.