EE580 – Solar Cells Todd J. Kaiser Lecture 03 Nature of Sunlight 1Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Wave-Particle Duality Light acts as –Waves: electromagnetic radiation Refraction - bending of light through a prism Diffraction - bending of light around an edge Interference – adding of light waves –Particles: photons – individual packets of energy Photoelectric Effect Blackbody Radiation 2Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Properties of Light Sunlight contains photons of many different frequencies ( we see as different colors) Visible Light –0.38 microns – 0.77 microns (10 -6 meters) –Blue - Red Frequency (f) and Wavelength ( ) are inversely related: f = 1/  –High Energy  High Frequency  Short Wavelength –Low Energy  Low Frequency  Long Wavelength 3Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Electromagnetic Spectrum Gamma Rays X Rays Ultraviolet Infrared Microwaves Radio Waves Visible Frequency (Hz) Wavelength (meters) Red Orange Yellow Green Blue Indigo Violet Roy G. BIV 0.6 microns 0.4 microns 0.5 microns 0.38 microns 0.77 microns Color, wavelength and frequency are interrelated 4 Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Photon Energy High energy photon for blue light Low energy photon for red light Very low energy photon for infrared (invisible) 5Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Photon Flux & Energy Density For the same intensity of light shorter wavelengths require fewer photons, since the energy content of each individual photon is greater 6Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Radiant Power Density The total power density emitted from a light source The spectral irradiance is multiplied by the wavelength range for which is was measured and summed over the measurement range 11 22 33 44 55 66 77 F(R)F(O)F(Y)F(G) F(B) F(I) F(V) 7Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Spectral Irradiance Xenon – (left axis)Sun - (right axis) Halogen – (left axis) Mercury – (left axis) Power Density at a particular wavelength Visible 8Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Solar Radiation 9Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Blackbody Radiation A body in thermal equilibrium emits and absorbs radiation at the same rate A body that absorbs all the radiation incident on it is an ideal blackbody The power per area radiated is given by the Stefan-Boltzmann Law This function has a maximum given by Wien’s Displacement Law The spectral irradiance of a blackbody radiator is given by Planck’s Law 10Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Plot of Planck’s Law 11Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Sun’s maximum at visible spectrum Visible 12Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Sun Fusion reaction at 20,000,000 degrees Converts H to He Fusion reaction at 20,000,000 degrees Converts H to He Convection heat transfer Photosphere (sun’s surface) at 5760±50 K 13Montana State University: Solar Cells Lecture 3: Nature of Sunlight

The energy is emitted from the surface of the sun as a sphere of light The size of the sphere grows as the light moves farther from the sun causing the power density to reduce Sunlight in Space 14Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Sunlight at Earth’s Orbit 15Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Sunlight at Earth’s Surface Atmospheric Effects –Absorption –Scattering –Local Variations Clouds Pollution Weather Latitude Season of Year Time of Day 16Montana State University: Solar Cells Lecture 3: Nature of Sunlight

100 % input Clear Sky Absorption & Scattering BOR0606C Ozone km 2% Absorbed Upper Dust Layer km 1% Absorbed 0.5% Scattered to Space 1% Scattered to Earth Air Molecules 0-30 km 8% Absorbed 1% Scattered to Space 4% Scattered to Earth Water Vapor 0-3 km 6% Absorbed 0.5% Scattered to Space 1% Scattered to Earth Lower Dust 0-3 km 1% Absorbed 1% Scattered to Earth 0.5% Scattered to Space 18% Absorbed 70% Direct to Earth 3% Scattered To Space 7% Scattered To Earth 17 Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Air Mass The path length that light takes through the atmosphere normalized to the shortest possible path length x h  18Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Measuring the Air Mass   Object of height (L) Shadow from object (S) h Neglects the curvature of the earth’s atmosphere 19Montana State University: Solar Cells Lecture 3: Nature of Sunlight

EAST SOUTH WEST NORTH What time of year is this image true? March 22: Spring Equinox Sept 23: Fall Equinox 20Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Declination Angle Sep 23 Declination (  ) = 0º Mar 23 Declination (  ) = 0º Jun 23 Declination (  ) = 23.45º Dec 23 Declination (  ) = º 21Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Declination Angle of Sun North Pole Summer Solstice  = 23.45º  Fall Equinox  = 0º  Winter Solstice  = º 22Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Elevation Angle The elevation angle is the angular height the sun makes with the horizontal 23Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Polar Plot of Sun Position for Bozeman Summer Spring/Fall Winter 24Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Passive Solar Heating 25 Winter Sun Summer Sun Overhang designed to remove direct sunlight from window during the heat of summer and allow the sunlight in during cooler winters Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Insolation 26Montana State University: Solar Cells Lecture 3: Nature of Sunlight

miles by 100 miles in Nevada would provide the equivalent of the entire US electrical demand Distributed (to sites with less sun) it would take less than 25% of the area covered by US roads. Solar Cell Land Area Requirements for the USA’s Energy with Solar PV Montana State University: Solar Cells Lecture 3: Nature of Sunlight

Solar Cell Land Area Requirements for the World’s Energy with Solar PV 28 6 Boxes at 3.3 TW Each Montana State University: Solar Cells Lecture 3: Nature of Sunlight