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Lect17 -- Solar

Lect17 -- Solar - GEOL 241 Fall 2013 Lecture 17 Solar...

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GEOL 241 Fall 2013 Lecture 17: Solar Energy …or are we…?
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Hydrogen fusion: Why the Sun shines
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Balance sheet of nuclear fusion Solar fusion: 4 x 1 H à 4 He Helium nucleus ( 4 He) is lighter than the four protons! Mass difference is 4.029 – 4.0015 = 0.0276 a.m.u. Lost mass must be converted to energy E = mc 2 1 a.m.u. = 1.6605 × 10 -27 kg 0.0276 a.m.u = 4.58 × 10 -29 kg multiply by c 2 to get 4.12 × 10 -12 J/kg Or 150 million kilocalories per gram compare to 16 million kcal/g uranium 10 kcal/g gasoline
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Confining the fusion reaction (1) Gravity : requires a huge mass, like the core of the Sun, so not practical on Earth as an energy source (except indirectly, by harvesting solar energy ) (2) Magnetic (3) Inertial
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Energy Source QBtu / % (1994) QBtu / % (2003) QBtu / % (2011) Hydroelectric 3.037 / 3.43 2.779 / 2.83 3.171 / 3.26 Geothermal 0.357 / 0.40 0.314 / 0.32 0.226 / 0.23 Biomass 2.852 / 3.22 2.884 / 2.94 4.511 / 4.64 Solar Energy 0.069 / 0.077 0.063 / 0.06 0.158 / 0.16 Wind 0.036 / 0.040 0.108 / 0.11 1.168 / 1.20 Total 6.351 /7.18 6.15 / 6.3 9.135 / 9.39 much room for improvement/growth, but went backwards from 1994 to 2003! Renewable energy in the US Table from Tom Murphy, UCSD
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But what is light? Energy from the Sun reaches Earth as light Empedocles, 5 th Century BC in Greece Light is a fire within the human eye Lucretius, 55BC in Rome Light is minute atoms Faraday and Maxwell (late 19 th Century): light as a electromagnetic wave Planck (1900): quantum mechanical view of “photons” as both particle and wave (“wave packets”)
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Photons are “wave packets” of electromagnetic energy travel at the speed of light in a vacuum behave like particles and like waves we focus on wave-like properties. Light as photons
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Electromagnetic radiation: oscillating electrical and magnetic fields that propagate through space figure from wikipedia.org Wave properties of electromagnetic energy
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λ = wavelength f = frequency c = speed of light = (frequency)*(wavelength)
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Radio Waves Micro Waves Infrared Visible Ultra Violet X-Rays Gamma Rays 100 1 10 -2 10 -4 10 -6 10 -8 10 -10 Wavelength (cm) 700 nm 650 nm 600 nm 550 nm 500 nm 450 nm 400 nm Red Orange Yellow Green Blue Violet The electromagnetic spectrum: variability in photon energy The electromagnetic spectrum: Photon energy increases with increasing frequency Photon Energy = (Plank's constant)*(frequency) = h*f
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Hot Object Cold Object Energy emitted per unit area Wavelength λ max = const./T Total energy emitted = kT 4 k = 5.67 x 10 -­‐8 J s -­‐1 m -­‐2 K -­‐4 Sun = hot -> visible radiation Earth = cold -> IR radiation Black body (idealized) radiation curves
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Solar radiation reaching Earth, and its fate
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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 5500-6000 K How much energy comes from the sun?
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Sun is about 5500-6000 K We know E = kT 4 And we know k: k = 5.67 x 10 -8 J s -1 m -2 K -4 E sun = 5.67 x 10 -8 J s -1 m
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