Posted by Nick Pine on August 16, 2003, 10:44 am
No. That's the amount of sun on a typical day.
As I recall, 311 Btu/h-ft^2 is the max in the Sahara at noon.
We are more likely to see 250.
I don't see that in NREL's Solar Radiation Data Manual for Buildings.
It is? :-) "Penelope dear, let's go bask in the rate of energy transfer..."
Or Btu per square foot per day.
Posted by Nick Pine on August 16, 2003, 1:17 pm
No. That's the amount of sun on an average October day. It's a useful way
to specify solar heat in calculations that use Ohm's law for heatflow over
24 hours. If a thing gathers 1020 Btu per day of heat which warms it to
a constant temp T and it loses heat through a thermal resistance R to an
ambient temp Ta over 24 hours, T = Ta + 1020/(24R).
For example, a pool with an R1 cover with 80% solar transmission
would gain 0.8x1020 = 816 Btu/ft^2 on an average October day and
lose 24h(T-56.4)1ft^2/R1, which makes T = 56.4+816/(24x1) = 90.4 F.
In October, where I live near Phila, with an average humidity ratio w = 0.007,
the vapor pressure of water in air Pa = 29.921/(0.62198/w+1) = 0.333 "Hg, so
at pool temp T (F), with no wind, we might lose Qc = 2(T-56.4) Btu/h-ft^2 by
convection. Bowen's equation says we'd lose Qe = 200(Pw-Pa) by evaporation,
where Pw = e^(17.863-9621/(460+T))... 24h(Qc+Qe)+1020 = 0 makes T = 59.3 F
for a well-stirred uncovered pool.
Posted by Nick Pine on August 16, 2003, 5:57 pm
Then again, you mighta read this:
But this is a peak rate. Perhaps you failed to read this:
NREL says 450, 360 and 190 Btu/ft^2 fall on a north wall on an average day
in Nov, Dec and Jan in 40 N Phila. East and west walls get 450, 370, and
430... 680, 530 and 620 fall on a horizontal surface. Barrow, AK (N 71)
walls and roofs receive 0 Btu/ft^2-day in Dec and Jan. You've stated your
point (whatever it was) imprecisely.
Not if you care about the amount of sun on an average day.
That's why an average day number is useful.
Posted by Joe Fischer on August 17, 2003, 12:01 am
: NREL says 450, 360 and 190 Btu/ft^2 fall on a north wall
: on an average day in Nov, Dec and Jan in 40 N Phila.
I can't disagree, but the poor Jan number is due
to statistical cloud cover for that city, and may not
apply to all of the 40th parallel.
This seems more relevant to photovoltaics than
to outdoor pool heating.
I don't think statistical cloud cover should
carry a lot of weight in designing a system.
: East and west walls get 450, 370, and
: 430... 680, 530 and 620 fall on a horizontal surface. Barrow, AK (N 71)
: walls and roofs receive 0 Btu/ft^2-day in Dec and Jan. You've stated your
: point (whatever it was) imprecisely.
If you are going to bring up Alaska, obviously. :-)
It seems that people living where there is less
sun are the ones always asking about solar energy.
I was going by some graphs in a 1955 book
of the ASHAE on sun angle, I will look at it again
and see if they consider cloud cover.