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Fusion has been accomplished in labs in big plasma

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Unformatted text preview: this is a small effect, especially compared to fission • D-D reaction requires higher temperature, but could be sustained for many millennia many millennia Winter 2007 Lecture 16 39 Winter 2007 40 10 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Why don’t we embrace fusion, then? don’ Fusion Successes? • Believe me, we would if we could would if could • It’s a huge technological challenge, always 50 years It’ from fruition • Fusion has been accomplished in labs, in big plasma has been machines called Tokamaks Tokamaks – got ~6 MW out of Princeton Tokamak in 1993 – but put ~12 MW in to sustain reaction – must confine plasma at 50 million degrees!!! – all the while providing fuel flow, heat extraction, tritium supply, etc. – hurdles in plasma dynamics: turbulence, etc. • Hydrogen bomb also employs fusion – fission bomb (e.g., 239Pu) used to generate extreme temperatures and pressures necessary for fusion – LiD (lithium-deuteride) placed in bomb – fission neutrons convert lithium to tritium – tritium fuses with deuterium • Still pursued, but with decreased enthusiasm, increased skepticism – but man, the payoff is huge: clean, unlimited energy Winter 2007 41 Winter 2007 UCSD: Physics 121; 2007 1994 Renewable Energy Consumption Energy Source QBtu 3.037 2.852 3.22 Geothermal 0.357 0.40 Solar Energy 0.069 0.077 Wind 0.036 0.040 Total 6.351 • Above the atmosphere, we get 1368 W/m2 of radiated of power from the sun, across all wavelengths 3.43 Biomass UCSD: Physics 121; 2007 How much solar energy is available? Percent Hydroelectric 42 7.18 – This number varies by ±3% as our distance to the sun increases or decreases (elliptical orbit) • 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 much room for improvement/growth Winter 2007 Lecture 16 43 Winter 2007 44 11 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Making sense of the data Input flux (average properties) • We can infer a number of things from the previous figure: – 52% of the incoming light hits clouds, 48% does not – 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 Winter 2007 45 Winter 2007 46 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Comparable numbers A naturally balanced budget • 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% Winter 2007 Lecture 16 47 Winter 2007 48 12 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Average Insolation Insolation variation • The amount of light received by a horizontal surface (in W/m2) averaged over the year (day & night) is called the insolation insolation • We can make a guess based on the facts that on average: • While the average insolation is 171 W/m2, variations in cloud cover and latitude can produce a large variation in this number – 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 – A spot in the Sahara (always sunny, near the equator) may have 270 W/m2 on average –...
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