Unformatted text preview: ation curves λ max = const./T
Energy
emitted
per
unit
area Hot
Object Total energy emitted = kT4
k = 5.67 x 10 8 J s 1 m 2 K 4 Cold
Object Wavelength
Sun = hot > visible radiation
Earth = cold > IR radiation Solar radiation reaching Earth, and its fate How much energy comes from the sun?
We need to know the Sun s temperature!
Wien s Law à༎ wavelength of emitted radiation is shorter for
bodies at higher temperature
Measure sunlight
wavelengths
~ 500 nm
So sun is about
55006000 K How much energy comes from the sun?
Sun is about 55006000 K
We know E = kT4
And we know k:
k = 5.67 x 108 J s1 m2 K4
Esun = 5.67 x 108 J s1 m2 K4 * (5780 K)4
Esun = 63 x 106 J s1 m2
Whoa!!!!!
How does this then give
Esunlight ~ 340 W/m2 How much energy actually reaches the Earth? Energy from Sun arriving at Earth
= (Solar output)(4πr2)/(4πR2)
= (Solar output)(r2/R2)
= 1360 W/m2
NOT ACCOUNTING FOR REFLECTION
r = Sun’s radius = 6.96 x 105 m
R = Earth sun dist. = 1.50 x 1011 m Solar radiation reaching Earth, and its fate Solar radiation reaching Earth
watts/m2
1365 Arriving top of atmosphere, equator, noon (“solar constant”)
435 Average over day at equator (divide by π)
341 Average over Earth (divide by 4)
164 52% reflected or absorbed by clouds, atmosphere
(depends on latitude, season, atmospheric conditions;
latitud...
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 Fall '13
 Energy, Photovoltaics

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