Transcript Chapter 2 Solar Radiation Powerpoint

The electromagnetic spectrum
The blackbody spectrum
BB formula
I ( , T ) 
2hc
1
3 e hc / kT  1
Applet
(all called ‘short wave’)
Visible (400 – 700 nm) is 43 % of total irradiance
Note funny
units
UV =
5%
IR (> 700 nm) is 52% of total irradiance
All EM radiation may also be considered to be
made up of photons with energies (in eV):
E photon  hf 
hc


1240nm.eV

Wavelength in meters
Latitude and longitude
Where is the latitude zero?
Where is the longitude zero?
0
0
Hour angle and solar noon
Solar noon: When the Sun lies in the
meridian plane at your location
(when the Sun is most nearly overhead)
Hour angle ω: angle the Earth has rotated
since the last solar noon
Earth rotates 15o per hour, so therefore


15
h
(
t
1
h
)
 


The “equation of time”
0

1
sola
What’s that?
0

1
zone
zo
e

15
h
(
t

12
h
)

(

)

The “equation of time” – how much the “mean sun” is too slow
see wikipedia for mathematical details
Mean
Sun
“late”
Mean
Sun
“early”
ωeq
Earth’s orbit around the sun
Earth’s spin axis is 23.450 tilted with respect to normal to orbital plane
Vernal equinox 3/20-21
Summer
solstice
Winter
solstice
(NH)
12/21-22
(NH) 6/20-21
Autumnal
equinox 9/22-23
A really nice animation here
Apparent
daily motion
of the Sun
Days near the summer solstice
(NH) – longest day
Days near the vernal &
autumnal equinoxes
Days near the winter
solstice (NH) – shortest day
Apparent motion of Sun across the sky
Sun shown at solar noon on
3 special days of year
0
Solar declination
= angle between
sun’s rays & the
plane of the
Earth’s equator.
For the 3 dates
shown, what is ?
0
90

0
North star


23
.
45
0


What is her latitude?
Number of hours of daylight
Cool applet
Above what latitude
does the sun never
set?
When is the length of
the day changing
fastest?
At what latitude are
the days & nights
always 12 hrs long?
Example: How many hours of daylight are
there in Fairfax on March 29?
Our latitude:   38.85 deg
n = 89th day of year (leap year)
 3600 (284  n) 
 3600 (373) 
0
   0 sin 

23
.
45
sin

3
.
2



365
365




2
N  cos 1 ( tan  tan  )
15
2
2
2
1
1
N  cos ( tan 38.85 tan 3.2)  cos (0.045)  (91.5)  12.2
15
15
15
What is an easy check on this answer?
Monthly insolation versus time of year at 5 latitudes
I
What is assumed
about collector
orientation?
Which nation leads
the world in solar
electricity
production? Is this
the sunniest?
World insolation map
Germany is the leader in solar
electricity capacity with 55% of the
world total.
x
Is the insolation
symmetrical
above & below
the equator?
Why is the max
insolation not at
the equator?
Max insolation over deserts (not rain forests), where
moisture tends to generate clouds
Why the big
difference?
“window” in
“window” out
Width changes
with concentration
GHG’s
Infrared
absorption from
molecules with 3
or more atoms