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re: concrete solar collector feasibility in pnw?
27 may 1998
laren333 wrote:
>with the average solar noon altitude during this mar.-oct.
>period being less than 55 deg. and considering 8 hour(?)
>days of considerably lower altitudes. your absorbtion
>efficiency factor of 0.9 appear to be a bit optomist
arrgh, such english anguish.
i think what makes all this seem optimistic is that solar heated pools
normally have such poorly insulating covers, if they have covers at all.
would you try bailing a bottomless boat? :-) would you try heating a
house with all the doors and windows wide open? would you listen to
salescreatures saying "you need a bigger furnace"? solar diplomats
vs statesmen... okokok, statespersons...
nrel says 700 btu/ft^2 of global (diffuse+beam, vs. beam only) sun falls
on a horizontal surface on an average october day in seattle. i guess we
don't have any reason to doubt their data. an average october seattle day
has about 10.5 hours of daylight. using a 10.5 vs 8 hour solar collection
day for this calculation doesn't lower the pool temperature much.
but we'd probably want the masonry part of the cover up out of the water,
leaving only the foamboard touching the water near dusk and dawn, when
temperatures and solar intensities are lower, and the pool heat loss in
collecting mode is more than the solar gain. later, we'd let some
floatation air out to lower the cover slightly during the day, in order
to drain the heat from the dark masonry top when gains exceed losses.
we'd unhook the safety ropes around the edge and let the cover sink slowly
all the way to the bottom for swimming. then pump in a little more air to
float it again. the cover initially needs about 4 psi to rise from under
an 8' deep pool, but if it's near bouyancy, it doesn't require much air
volume, nor does it need a continuous air supply, just static pressure.
perhaps laren is saying "the solar transmission of polyethylene film (the
top cover) at angles less than 90 degrees is less than 90%," which is true,
but that may not matter much, if much of the solar radiation is diffuse,
and comes from the entire sky, vs direct beam radiation, which is often
the case in the pnw. the average "clearness index" kt = 0.41 in seattle
in october. the erbs correlation in figure 2.11.2 on page 84 of duffie
and beckman's 1991 "solar engineering of thermal processes" book says that
makes 1+0.2832kt-2.5557kt^2+0.8448kt^3, ie 74% of the solar radiation
diffuse vs beam. page 227 says "...beam radiation incident at an angle of
60 degrees has the same transmittance as isotropic diffuse radiation."
the transmission of glass decreases more quickly than poly film's with
off-normal incidence, since the film has a lower index. d & b don't give
its refractive index, but their example 5.3.3 talks about pvf with an
index of 1.45 (vs. 1.526 for glass), saying "the plastic film is thin
enough so that absorption within the plastic can be neglected." they
calculate a transmittance of 86% vs 81% for glass at an incidence angle
of 60 degrees (0 degrees being normal.)
the 1994 revision of the nraes-33 greenhouse engineering book gives a
normal solar transmittance for poly film of 92%, ie a reflection of 4%
per surface, so 0.04 = ((n-1)/(n+1))^2, and n = 1.5 by that estimation,
or less, counting absorption.
the angular dependence is a consideration, but it doesn't look like
it will make make much of a numerical difference in this case.
nick, "solar since '68"
nicholson l. pine system design and consulting
pine associates, ltd. (610) 489-0545
821 collegeville road fax: (610) 489-7057
collegeville, pa 19426 email: nick@ece.vill.eedeeyou
computer simulation and modeling. high performance, low cost, solar heating and
cogeneration system design. bsee, msee. senior member, ieee. registered us
patent agent. hi/dvc board member. web site: http://www.ece.vill.edu/~nick
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