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Unformatted text preview: SD: Physics 121; 2007 How much of the process do we get to keep? • When water condenses in clouds, it re-releases this “latent heat” heat” Winter 2007 66 – based on the 1.8% land area of the U.S. and the maximum potential of 147.7 GW as presented in Table 5.2 67 Winter 2007 68 17 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Power of a hydroelectric dam Importance of Hydroelectricity • Most impressive is Grand Coulee, in Washington, on Columbia River – 350 feet = 107 m of “head” – > 6,000 m3/s flow rate! (Pacific Northwest gets rain!) – each cubic meter of water (1000 kg) has potential energy: mgh = (1000 kg) (10 m/s2) (110 m) = 1.1 MJ – At 6,000 m3/s, get over 6 GW of power • Large nuclear plants are usually 1–2 GW 1– • 11 other dams in U.S. in 1–2 GW range 1– • 74 GW total hydroelectric capacity, presently Winter 2007 69 Winter 2007 70 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Hydroelectric potential by region, in GW Hydroelectricity in the future? Region Potential Developed Undeveloped % Developed New England 6.3 1.9 4.4 30.1 Middle Atlantic 9.8 4.9 4.9 50.0 East North Central 2.9 1.2 1.7 41.3 West North Central 6.2 3.1 3.1 50.0 South Atlantic 13.9 6.7 7.2 48.2 East South Central 8.3 5.9 2.4 71.1 West South Central 7.3 2.7 4.6 36.9 Mountain 28.6 9.5 19.1 33.2 Pacific 64.4 38.2 26.2 59.3 Total 147.7 74.1 73.6 50.2 Winter 2007 Lecture 16 • We’re almost tapped-out: We’ – 50% of potential is developed – remaining potential in large number of small-scale units • Problems with dams: – silt limits lifetime to 50–200 years, after which dam is useless and in fact a potential disaster and nagging maintenance site – habitat loss for fish (salmon!), etc.; wrecks otherwise stunning landscapes (Glenn Canyon in AZ/UT) – Disasters waiting to happen: 1680 deaths in U.S. alone from 1918–1958; dams often upstream from major population centers 71 Winter 2007 72 18 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 The Power of Wind Can’t get it all Can’ • The kinetic energy in wind: – a wind traveling at speed v covers v meters every second (if v is expressed in m/s) – the kinetic energy hitting a square meter is then the kinetic energy the mass of air defined by a rectangular tube – tube is one square meter by v meters, or v meters cubed – density of air is = 1.3 kg/m3 – mass is v kg – K.E. = ( v )·v2 = v3 (per square meter) • Thus power per square meter is ~0.65v 3, • A windmill can’t extract all of the kinetic energy available in the can’ a ll of wind, because this would mean stopping the wind entirely stopping the • Stopped wind would divert oncoming wind around it, and the windmill would stop spinning • On the other hand, if you don’t slow the wind down much at all, don’ you won’t get much energy won’ • Theoretical maximum performance is 59% of energy extracted – corresponds to reducing velocity by 36% • Modern windmills attain maybe 50–70% of the theoretical 50– theoretical maximum – so if the wind speed doubles, the power available in the wind increases by 2 3 = 2 2 2 = 8 times – 0.5–0.7 times 0.59 is 0.30–0.41, or about 30–40% – this figure is the mechanical energy extracted from the wind – a wind of 10 m/s (22 mph) has a power density of 610 W/m2 – a wind of 20 m/s (44 mph) has a power density of 4,880 W/m2 • Conversion from mechanical to electrical is 90% efficient – 0.9 times 0.30–0.41 is 27–37% Winter 2007 73 Winter 2007 74 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Achievable efficiencies Typical Windmills • A typical windmill might be 15 m in diameter – 176 m2 • At 10 m/s wind, 40% efficiency, this delivers...
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