passive solar windows SOLAR.KnowledgePublications.com

passive solar windows
29 nov 2000
at the june, 1999 ases conference in portland, me, physicist tim ellison
gave a paper on his "ten-day thermal time constant" house (a solar house
with a hydronic slab used for heat distribution and collection, with storage
in "an outdoor strawbale structure containing 3 3,000 gallon polyethylene
tanks" :-) he mentioned that "low-e hardcoat" (a technology that's apparently
becoming obsolete in the us, vs canada) is more efficient than more recent
low-e coatings, and he presented some data from various window manufacturers.

he suggested a weighted "figure of merit" based on the heat gain minus the
u-value for a simple passive direct gain house (vs other dramatically more
efficient configurations, eg houses with low-thermal-mass sunspaces.) on
an average january day in pennsylvania, 1000 btu/ft^2 of sun falls on a
south wall, so a square foot of window gains 1000shgc btu of sun per day
and loses 24h(70-30)u-value = 960(u-value) over an average day, so the
net heat gain is approximately proportional to the shgc minus the u-value,
with a weighting of about 1:1. colder cloudier places would weight heat
loss more and heat gain less, and so on... 

to generalize for windows facing different directions over a heating season,
with solar heat gain coefficient (solar transmittance/100) s and u-value u:

                                             heat   heat   figure of merit
              nov  dec  jan  feb  mar  apr   gain   loss 

degree days   558  905 1073  896  701  378          108ku

north         210  170  190  260  340  430    48ks btu/ft^2   s-2.25u
east          450  370  420  570  740  900   104ks per        s-1.04u
south         990  900 1000 1080 1070  950   180ks winter     s-0.60u
west          450  370  430  560  730  880   103ks            s-1.05u

the figure of merit for each direction is proportional to solar gain
minus heat loss. for instance, the seasonal net heat gain for a north-
facing window is 48ks-108ku, so the north facing figure of merit is 
s - 108k/48ku = s - 2.25u. 

a * marks the most energy-efficient window (the one with the largest
directional figure of merit) for each direction below...

glazing type                            shgc  uv      n    e    s    w 

clear single pane                       0.85  1.11  -1.65 -.30  .18 -.32
cardinal low e^2-145                    0.32  0.27   -.29  .04  .16  .04
marvin low e^2                          0.41  0.32   -.31  .08  .22  .07
cardinal low e^2-171                    0.41  0.24  -1.26  .16  .27  .16
marvin high r tripane                   0.34  0.14    .03  .19  .26  .19
clear hm tc88 1/4" g 1.5" ig            0.47  0.18   *.07  .28  .36  .28
ppg clr sungate e-500 (3) 1.5/0.5 w ar  0.47  0.18   *.07  .28  .36  .28
cardinal low e^2-178                    0.58  0.26    .01  .31  .42  .31
clear hm tc88 1/8" g 1.5" ig            0.50  0.18    .10  .31  .39  .31
pilkington e-adv 1/4" ar ib             0.66  0.28    .03  .37  .49  .37
3/4" ig hardcoat low e w ar             0.71  0.29    .06 *.41 *.54 *.41
3/4" ib low-e hardcoat w ar (marvin)    0.71  0.29    .06 *.41 *.54 *.41

with low enough u-values and high enough shgcs, even north-facing windows
can help heat a passive solar house with a small heating fraction (< 50%?),
ie they can gain more net energy in spring and fall (and contribute more
net heat to a house than a perfectly insulated wall) than they lose in the
dead of winter, ie in colder, cloudier weather. 

window prices vary, of course, and manufacturers are not always aware
of the best combinations. reps and salespeople often don't realize that
particular combinations are available.

i've yet to find anyone at marvin (800-346-5044) who can identify
or verify the performance of the last window on the list...

nick