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RET Optics Research Workshop Workshop #2 Solar Energy Solar Cells and Solar Ovens Dr. Mike Nofziger Professor College of Optical Sciences University of Arizona Dr. Mike Nofziger 2013 Workshop #2 Outline: ● Solar Energy - Basics of Energy - Our Sun - The solar spectrum - The greenhouse effect ● Solar Cells ● Solar Ovens Dr. Mike Nofziger 2013 Workshop 2-1 Basics of Energy: ● Two fundamental types of energy: - Potential Energy : “stored” energy (work could be done with this available energy) - Kinetic Energy : “working” energy (work is being done with this energy) ● Forms of Energy: - Light (radiant) Heat (thermal) Motion (kinetic) Electrical Chemical Nuclear Gravitational Dr. Mike Nofziger 2013 Workshop 2-2 Basics of Energy: ● Renewable Energy Sources: - Solar energy → electricity or heat - Wind - Geothermal energy from heat inside the Earth - Biomass from plants - firewood, wood waste - ethanol from corn - biodiesel from vegetable oil - Hydropower from hydro-turbines at a dam ● Non-Renewable Energy Sources: - Fossil fuels - oil - natural gas - coal Dr. Mike Nofziger 2013 Workshop 2-3 Basics of Energy: ● Energy is conserved - Scientifically speaking, “Conservation of energy” does not mean “saving energy” “Law of Conservation of Energy” - The total amount of energy in a closed system remains constant. - Energy does not disappear, or “get used up.” - Energy is changed from one form to another when it is used. www.eia.gov Dr. Mike Nofziger 2013 Workshop 2-4 Energy vs. Power: ● Energy is defined as the capacity for doing work. - Fundamental units of energy: - Joule, Calorie, British Thermal Unit (BTU) 1 J = 0.23889 calories 1 J = 0.947816x10-3 BTU ● Power is defined as the rate of using energy. - Fundamental units of power: - Watt, Horsepower 1 Watt ≡ 1 Joule/sec. 1 hp = 746 watts - Therefore, energy ≡ power x time - An equivalent unit of energy is: Watt·hour (Wh), kilo-Watt·hour (kWh) 1Wh 1W 1hour 1J s 1hour 60min/h 60sec /min 3600J Units! (“love ‘em” or “hate ‘em”…..teach your students to “love ‘em”!) Dr. Mike Nofziger 2013 Workshop 2-5 Example of Power, Energy, and Photons/sec: Each photon of light carries a specific amount of energy: Energy per photon of light: where h is Planck’s constant h = 6.626x10-34 J-s E h h c Energy and Power in a Laser Beam: “A typical red laser pointer emits 2-3 mW of power, at a wavelength of 650 nm. (For simplicity, assume the power is 1 mW)” • How much energy is delivered by this laser beam, in 1 sec? E 1mW W 1J s 1s 103 J 1mJ 10 mW W 3 • How many photons per second are in this laser beam? Photons/sec = energy sec photon energy = photons sec Photons/sec = power energy photon -1 = photons/sec 34 8 1 J 6.626 10 Js 3 10 m / s 109 nm 1W J / s hc 15 = 1mW 3 3.26 10 photons sec .001 10 mW W s 650 nm 1 m 1 Dr. Mike Nofziger 2013 Workshop 2-6 Our Sun: ● The “ROY G BIV” solar spectrum: Wavelength (in nanometers) 700 600 500 400 Visible spectrum Wavelength (m) 108 107 106 105 104 103 102 10 10- 1 10- 2 10- 3 10 - 4 10- 5 10- 6 10- 7 10- 8 10 - 9 10- 10 10- 11 10- 12 10- 13 10- 14 10- 15 10- 16 1 X rays Radio waves 2 10 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 10 11 10 12 10 13 10 14 10 15 10 16 10 17 10 18 10 Gamma rays 19 10 20 10 21 10 1022 1023 1024 Frequency (Hz) ● The Ideal Blackbody (solar) spectrum: Dr. Mike Nofziger 2013 Workshop 2-7 Our Sun: ● The “Real” solar spectrum: ● The sun delivers ≈ 1000W/m2 to the surface of the Earth! ● The Earth receives more energy from the Sun in just one hour than the world uses in a whole year. http://org.ntnu.no/solarcells/pages/Chap.2.php Dr. Mike Nofziger 2013 Workshop 2-8 Our Sun: ● The Ability to harness solar energy by concentrating it: Dr. Mike Nofziger 2013 Workshop 2-9 Our Sun: ● The Ability to harness solar energy by using solar cells: Dr. Mike Nofziger 2013 Workshop 2-10 Basis for the Heating in a Solar Oven (a.k.a. a “Greenhouse Effect”): • The wavelength of peak output from a blackbody (an ideal emitter, much like our sun) is given by: 3000 m K max T where T(K) = T(°C) + 273° max 3000 m K T • The surface temperature of our sun is ≈ 6000K • The wavelength where our sun emits most energy is, therefore: max 3000 m K 1000nm 0.5 m 500nm 6000 K 1 m • 500 nm is in the green portion of the visible spectrum. • The peak sensitivity of human (daylight) vision is at 550nm………?! Dr. Mike Nofziger 2013 Workshop 2-11 Basis for the Heating in a Solar Oven (a.k.a. a “Greenhouse Effect”): • The wavelength of peak output from a solar oven cavity, at T≈ 400°F: 5 T [ K ] 400 32 C 273C 477 K 9 3000 m K max 6 m 477 K Dr. Mike Nofziger 2013 Workshop 2-12 Basis for the Heating in a Solar Oven (a.k.a. a “Greenhouse Effect”): • Typical “window” material for a student solar oven is a single sheet of Mylar (Xerox Overhead Transparency): Xerox Overhead Transparency Transmission Spectra (8 cm -1) 0.9 0.8 Transmission (%) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 2 4 6 8 10 Wavelength (m) 12 14 16 18 Fig. A.3. Infrared transmission spectrum of one layer of Mylar for wavelengths from 2 to 18 micrometers • High transmission in the visible/near IR spectrum • Low transmission in the thermal IR spectrum (i.e. at 6 μm) - The cavity absorbs visible light but has trouble emitting (radiating) thermal energy at 6 μm, therefore the cavity heats up. • Basic “Greenhouse Effect” (car interiors, the Earth, etc.): Dr. Mike Nofziger 2013 Workshop 2-13 Basics of Solar Cells: ● A solar cell is a Photovoltaic (“PV”) detector: - is made of Silicon (not silicone!!) - absorbs light from ≈ 350nm – 1100nm - the absorption of light “frees up” electrons - This creates a voltage at the terminals of the cell (the “Open-Circuit” voltage) - If a load resistor is connected to the cell, a current will flow (the “Photocurrent”) - If the cell’s terminals are shorted, the maximum current will flow (the “Short-Circuit” current) Dr. Mike Nofziger 2013 Workshop 2-14 Basics of Solar Cells: ● The IV Curve of a PV detector is given by: qVkT I I o e 1 I ph ● The Photocurrent of a PV detector is given by: I ph q ph q hc Dr. Mike Nofziger 2013 Workshop 2-15 Basics of Solar Cells: ● The Power (Watts) that the cell can produce is given by: P V I ● Because of internal resistance in the cell, the maximum power you can generate is across a load resistance equal to the internal resistance. www.keithley.com Dr. Mike Nofziger 2013 Workshop 2-16 Basics of a Solar Oven: Dr. Mike Nofziger 2013 Workshop 2-17