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Unformatted text preview: UCSD Physics 12 Renewable Energy I
Hydroelectricity Wind Energy UCSD Physics 12 Renewable Resources Renewable means anything that won't be depleted by using it sunlight (the sun will rise again tomorrow) biomass (grows again) hydrological cycle (will rain again) wind (sunlight on earth makes more) ocean currents (driven by sun) tidal motion (moon keeps on producing it) geothermal (heat sources inside earth not used up fast) Spring 2010 2 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 Slide copied from Lecture 9 3 UCSD Physics 12 Another look at available energy flow The flow of radiation (solar and thermal) was covered in Lecture 9 earth is in an energy balance: energy in = energy out 30% reflected, 70% thermally re-radiated Some of the incident energy is absorbed, but what exactly does this do? much goes into heating the air/land much goes into driving weather (rain, wind) some goes into ocean currents some goes into photosynthesis 4 Spring 2010 UCSD Physics 12 The Renewable Budget Spring 2010 5 UCSD Physics 12 Outstanding Points from Fig. 5.1 Incident radiation is 174 1015 W this is 1370 W/m2 times area facing sun (R2) 30% directly reflected back to space off clouds, air, land 47% goes into heating air, land, water 23% goes into evaporating water, precipitation, etc. (part of weather) Adds to 100%, so we're done but wait! there's more... Spring 2010 6 UCSD Physics 12 Energy Flow, continued 0.21% goes into wind, waves, convection, currents note this is 100 times less than driving the water cycle but this is the "other" aspect of weather 0.023% is stored as chemical energy in plants via photosynthesis total is 40 1012 W; half in ocean (plankton) humans are 6 billion times 100 W = 0.6 1012 W this is 1.5% of bio-energy; 0.00034% of incident power All of this (bio-activity, wind, weather, etc.) ends up creating heat and re-radiating to space except some small amount of storage in fossil fuels Spring 2010
Q 2 7 UCSD Physics 12 The Hydrologic Cycle Lots of energy associated with evaporation: both mgh (4% for 10 km lift) and latent heat (96%) of water Spring 2010 8 UCSD Physics 12 Energetics of the hydrologic cycle It takes energy to evaporate water: 2,250 J per gram this is why "swamp coolers" work: evaporation pulls heat out of environment, making it feel cooler 23% of sun's incident energy goes into evaporation By contrast, raising one gram of water to the top of the troposphere (10,000 m, or 33,000 ft) takes
mgh = (0.001 kg) (10 m/s2) (10,000 m) = 100 J So > 96% of the energy associated with forming clouds is the evaporation; < 4% in lifting against gravity
Spring 2010 9 UCSD Physics 12 Let it Rain When water condenses in clouds, it re-releases this "latent heat" but this is re-radiated and is of no consequence to hydro-power When it rains, the gravitational potential energy is released, mostly as kinetic energy and ultimately heat Some tiny bit of gravitational potential energy remains, IF the rain falls on terrain (e.g., higher than sea level where it originated) hydroelectric plants use this tiny left-over energy: it's the energy that drives the flow of streams and rivers damming up a river concentrates the potential energy in one location for easy exploitation Spring 2010 10 UCSD Physics 12 How much of the process do we get to keep? According to Figure 5.1, 40 1015 W of solar power goes into evaporation this corresponds to 1.6 1010 kg per second of evaporated water! this is 3.5 mm per day off the ocean surface (replenished by rain) The gravitational potential energy given to water vapor (mostly in clouds) in the atmosphere (per second) is then:
mgh = (1.6 1010 kg) (10 m/s2) (2000 m) = 3.2 1014 J One can calculate that we gain access to only 2.5% of the total amount (and use only 1.25%) 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 Spring 2010 11 UCSD Physics 12 Power of a hydroelectric dam 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 12 GW 11 other dams in U.S. in 12 GW range 74 GW total hydroelectric capacity, presently
Q 2 Spring 2010 12 UCSD Physics 12 Importance of Hydroelectricity Spring 2010 13 UCSD Physics 12 Hydroelectric potential by region, in GW
Region New England Middle Atlantic East North Central West North Central South Atlantic East South Central West South Central Mountain Pacific Total Potential 6.3 9.8 2.9 6.2 13.9 8.3 7.3 28.6 64.4 147.7 Developed 1.9 4.9 1.2 3.1 6.7 5.9 2.7 9.5 38.2 74.1 Undeveloped 4.4 4.9 1.7 3.1 7.2 2.4 4.6 19.1 26.2 73.6 % Developed 30.1 50.0 41.3 50.0 48.2 71.1 36.9 33.2 59.3 50.2 Spring 2010 14 UCSD Physics 12 Hydroelectricity in the future? We're almost tapped-out: 50% of potential is developed remaining potential in large number of small-scale units Problems with dams: silt limits lifetime to 50200 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 UT) Disasters waiting to happen: 1680 deaths in U.S. alone from 19181958; often upstream from major population centers Spring 2010
Q 15 UCSD Physics 12 Sorry: try again... So hydroelectricity is a nice "freebee" handed to us by nature, but it's not enough to cover our appetite for energy Though very efficient and seemingly environmentally friendly, dams do have their problems This isn't the answer to all our energy problems, though it is likely to maintain a role well into our future Spring 2010 16 UCSD Physics 12 Wind Energy Spring 2010 17 UCSD Physics 12 The Power of Wind We've talked about the kinetic energy in wind before: 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 m3 density of air is = 1.3 kg/m3 at sea level mass is v kg K.E. = (v)v2 = v3 (per square meter) 0.65v3 at sea level Spring 2010 18 UCSD Physics 12 Wind Energy proportional to cube of velocity The book (p. 134) says power per square meter is 0.61v3, which is a more-or-less identical result might account for average density in continental U.S. (above sea level, so air slightly less dense) So if the wind speed doubles, the power available in the wind increases by 23 = 2 2 2 = 8 times 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
Q 19 UCSD Physics 12 Can't get it all A windmill can't extract all of the kinetic energy available in the wind, because this would mean stopping the wind entirely 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, you won't get much energy Theoretical maximum performance is 59% of energy extracted corresponds to reducing velocity by 36% Spring 2010 20 UCSD Physics 12 Practical Efficiencies Modern windmills attain maybe 5070% of the theoretical maximum 0.50.7 times 0.59 is 0.300.41, or about 3040% this figure is the mechanical energy extracted from the wind Conversion from mechanical to electrical is 90% efficient 0.9 times 0.300.41 is 2737% Spring 2010 21 UCSD Physics 12 Achievable efficiencies Spring 2010 22 UCSD Physics 12 Typical Windmills A typical windmill might be 15 m in diameter 176 m2 At 10 m/s wind, 40% efficiency, this delivers about 100 kW of power this would be 800 kW at 20 m/s typical windmills are rated at 50 to 600 kW How much energy per year? 10 m/s 610 W/m2 40% 240 W/m2 8760 hours per year 2,000 kWh per year per square meter but wind is intermittent: real range from 100500 kWh/m 2 corresponds to 1157 W/m2 average available power density Note the really high tip speeds: bird killers Spring 2010 23 UCSD Physics 12 Average available wind power
recall that average solar insolation is about 150250 W/m2 Spring 2010 24 UCSD Physics 12 Comparable to solar? These numbers are similar to solar, if not a little bigger! Let's go to wind! BUT: the "per square meter" is not land area--it's rotor area Doesn't pay to space windmills too closely--one robs the other Typical arrangements have rotors 10 diameters apart in direction of prevailing wind, 5 diameters apart in the cross-wind direction works out to 1.6% "fill factor" Spring 2010
Q 25 UCSD Physics 12 Current implementations Rapidly developing resource 1400 MW in 1989; up to 6400 MW in 2003 but still insignificant total (compare to large dams) cost (at 57 per kWh) is competitive growing at 25% per year expect to triple over next ten years Current capacity: 11.6 GW (April 2007) Texas: 2,768 MW (recently took lead over California!!) California: 2,361 MW Iowa: 936 MW Minnesota: 895 MW Washington: 818 MW http://www.awea.org/newsroom/releases/Annual_US_Wind_Power_Ranki Spring 2010 26 UCSD Physics 12 Flies in the Ointment Find that only 20% of rated capacity is achieved design for high wind, but seldom get it Only 1.2% of electrical capacity in U.S. is now wind total electrical capacity in U.S. is 948 GW tripling in ten years means 3.6% but achieving only 20% of capacity reduces substantially If fully developed, we could generate an average power almost equal to our current electrical capacity (764 GW) but highly variable resource, and problematic if more than 20% comes from the intermittent wind Spring 2010 Q 27 UCSD Physics 12 Announcements/Assignments Read Chapter 5, sections 1, 2, 3, 5, 7 HW 6 to be posted today (5/10) Spring 2010 28 ...
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