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Unformatted text preview: UCSD Physics 12 Solar Energy
Introduction to renewable energy Energy from the sun
3 Q UCSD Physics 12 Renewable Energy Consumption
Energy Source Hydroelectric Geothermal Biomass Solar Energy Wind Total QBtu (1994) 3.037 0.357 2.852 0.069 0.036 6.351 Percent (1994) 3.43 0.40 3.22 0.077 0.040 7.18 QBtu (2003) 2.779 0.314 2.884 0.063 0.108 6.15 Percent (2003) 2.83 0.32 2.94 0.06 0.11 6.3 much room for improvement/growth, but going backwards! Spring 2010 2 UCSD Physics 12 The Solar Spectrum
above the atmosphere at ground level O2 H2O Atmospheric absorption H2O H2O,CO2 H2O, CO2 Spring 2010 3 UCSD Physics 12 How much energy is available? Above the atmosphere, we get 1368 W/m2 of radiated power from the sun, across all wavelengths This number varies by 3% as our distance to the sun increases or decreases (elliptical orbit) The book uses 2 calories per minute per cm2 (weird units!!) At the ground, this number is smaller due to scattering and absorption in the atmosphere about 63%, or ~850 W/m2 with no clouds, perpendicular surface probably higher in dry desert air Spring 2010 Q 4 UCSD Physics 12 Input flux (average properties) Spring 2010 5 UCSD Physics 12 Making sense of the data We can infer a number of things from the previous figure: 52% of the incoming light hits clouds, 48% does not 25% + 10% + 17% in cloudless conditions, half (24/48) is direct, 63% (30/48) reaches the ground in cloudy conditions, 17/52 = 33% reaches the ground: about half of the light of a cloudless day averaging all conditions, about half of the sunlight incident on the earth reaches the ground the above analysis is simplified: assumes atmospheric scattering/absorption is not relevant when cloudy Spring 2010
2 Q 6 UCSD Physics 12 A naturally balanced budget Spring 2010 7 UCSD Physics 12 Comparable numbers Both versions indicate about half the light reaching (being absorbed by) the ground 47% vs. 51% Both versions have about 1/3 reflected back to space 34% vs. 30% Both versions have about 1/5 absorbed in the atmosphere/clouds 19% vs. 19% Spring 2010 8 UCSD Physics 12 Energy Balance Note that every bit of the energy received by the sun is reflected or radiated back to space If this were not true, earth's temperature would change until the radiation out balanced the radiation in In this way, we can compute surface temperatures of other planets (and they compare well with measurements) Spring 2010 9 UCSD Physics 12 Average Insolation The amount of light received by a horizontal surface (in W/m2) averaged over the year (day & night) is called the insolation We can make a guess based on the facts that on average: half the incident light reaches the ground half the time it is day the sun isn't always overhead, so that the effective area of a horizontal surface is half it's actual area half the sphere (2R2) projects into just R2 for the sun twice as much area as the sun "sees" So 1/8 of the incident sunlight is typically available at the ground 171 W/m2 on average Spring 2010 10 UCSD Physics 12 Insolation variation While the average insolation is 171 W/m2, variations in cloud cover and latitude can produce a large variation in this number A spot in the Sahara (always sunny, near the equator) may have 270 W/m2 on average Alaska, often covered in clouds and at high latitude may get only 75 W/m2 on average Is it any wonder that one is cold while one is hot? Spring 2010 Q 11 UCSD Physics 12 Average daily radiation received ranges in W/m2: < 138 138162 162185 185208 208231 > 231 divide by 24 hr to get average kW/m2 Spring 2010 12 UCSD Physics 12 Higher Resolution Insolation Map Spring 2010 Q 13 UCSD Physics 12 Tilted Surfaces Can effectively remove the latitude effect by tilting panels raises incident power on the panel, but doesn't let you get more power per unit area of (flat) real estate flat arrangement tilted arrangement Spring 2010 14 UCSD Physics 12 Which is best? To tilt, or not to tilt? If the materials for solar panels were cheap, then it would make little difference (on flat land) If you have a limited number of panels (rather than limited flat space) then tilting is better If you have a slope (hillside or roof), then you have a built-in gain Best solution of all (though complex) is to steer and track the sun
Spring 2010 15 UCSD Physics 12 Orientation Comparison Spring 2010 16 UCSD Physics 12 Numerical Comparison: winter at 40 latitude
based on clear, sunny days Date Perpendicular (steered, W/m2) Horizontal (W/m2) Vertical S (W/m2) 60 South (W/m2) Oct 21 Nov 21 Dec 21 Jan 21 Feb 21 Mar 21 Spring 2010 322 280 260 287 347 383
overall winner 177 124 103 125 186 243
better in summer 217 222 216 227 227 195
good in winter 272 251 236 256 286 286 17
2nd place UCSD Physics 12 Total available solar energy Looking at average insolation map (which includes day/night, weather, etc.), I estimate average of 4.25 kWh/day/m2 = 177 W/m2 The area of the U.S. is 3.615 106 square miles this is 9.36 1012 m2 Multiplying gives 1.66 1015 Watts average available power Multiply by 3.1557 107 seconds/year gives 5.23 1022 Joules every year This is 50 1018 Btu, or 50,000 QBtu Compare to annual budget of about 100 QBtu 500 times more sun than current energy budget Spring 2010 18 UCSD Physics 12 So why don't we go solar? What would it take? To convert 1/500th of available energy to useful forms, would need 1/500th of land at 100% efficiency about the size of New Jersey But 100% efficiency is unrealistic: try 15% now need 1/75th of land Pennsylvania-sized (100% covered) Can reduce area somewhat by placing in S.W. Spring 2010 19 UCSD Physics 12 Making sense of these big numbers How much area is this per person? U.S. is 9.36 1012 m2 1/75th of this is 1.25 1011 m2 300 million people in the U.S. 416 m2 per person 4,500 square feet this is a square 20.4 meters (67 ft) on a side one football field serves only about 10 people! much larger than a typical person's house area rooftops can't be the whole answer, especially in cities Spring 2010 Q 20 UCSD Physics 12 Ways of using solar energy Direct heating of flat panel (fluids, space heating) Passive heating of well-designed buildings Thermal power generation (heat engine) via concentration of sunlight Direct conversion to electrical energy Spring 2010 21 UCSD Physics 12 Assignments Read Chapter 4 if you haven't already HW4 available on web Midterm will be Monday, May 3, Peterson 103 will need red half-sheet scan-tron with place for Student ID study guide posted on web site problems com from this study guide! will plan review session, at a time TBD Extra Credit instructions can be found on website Spring 2010 22 UCSD Physics 12 My Plans for Your Brain
this is your brain... real world stuff ...this is your brain after physics 12 real phys 12 world stuff stuff you learn in school stuff you learn in school Spring 2010 23 ...
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