16_energy

40 1012 w out of the 174000 1012 w incident on the

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Unformatted text preview: 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 – but wind is intermittent: real range from 100–500 kWh/m2 – corresponds to 11–57 W/m2 average available power density • Note the really high tip speeds: bird killers Winter 2007 Lecture 16 75 Winter 2007 76 19 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Comparable to solar? Average available wind power recall that average solar insolation is about 150–250 W/m2 • 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 meter” area—it’ rotor area • Doesn’t pay to space windmills too closely—one robs Doesn’ closely— 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” Winter 2007 77 Winter 2007 78 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 Current implementations Biomass • California is biggest participant, with 1,745 MW capacity • Biomass is any living organism, plant, animal, etc. • 40 1012 W out of the 174,000 1012 W incident on the out 10 incident 10 earth from the sun goes into photosynthesis – cost is 5–7¢ per kWh (1993) getting to be competitive – but still insignificant total (one large hydro plant) – 0.023% – this is the fuel for virtually all biological activity – half occurs in oceans • Find that only 20% of rated capacity is achieved – design for high wind, but seldom get it • If fully developed, we could generate an average could generate power comparable to our current electricity demands (764 GW) – but highly variable resource, and problematic if more than 20% comes from the intermittent wind Winter 2007 Lecture 16 79 • Compare this to global human power generation of 13 1012 W, or to 0.6 1012 W of human biological 10 of 10 W, activity • Fossil fuels represent stored biomass energy stored biomass Winter 2007 80 20 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 Photosynthetic efficiency UCSD: Physics 121; 2007 Realistic photosynthetic efficiency • Only 25% of the solar spectrum is useful to the photosynthetic process Location – uses both red and blue light (reflects green), doesn’t use IR or UV Plant Production (g/m2 per day) per Solar Energy Conversion Efficiency 81 1.5% 6 0.5% 0.3 0.02% Nevada Desert Winter 2007 20 Wyoming Prairie • Net result is about 6% 5% 4% Iowa cornfield – the rest is heat – akin to photovoltaic incomplete usage of photon energy 71 55 Pine Forest • 70% of this light is actually absorbed by leaf • Only 35% of the absorbed light energy (in the useful wavelength bands) is stored as chemical energy Potential Maximum Polluted stream (?!) 0.2 0.015% Winter 2007 82 UCSD: Physics 121; 2007 UCSD: Physics 121; 2007 How much biomass is available? Notes on Ethanol • Two estimates of plant production in book come up with comparable answers: 1017 – grams per year – 320 grams per m2 averaged over earth’s surface – consistent with 40 1012 W photosynthesis • Extracting ethanol from corn is probably not a good idea – may be an energy wash: put in as much as we get out • most optimistic estimates: get 1.4 units of energy out for every 1.0 units you put in – probably don’t have agricultural capacity to replace gasoline with corn-ethanol – probably don’t have the (ever diminishing) water resources – not enough farmers: and do we really want to go this way? • U.S. annual harvested mass corresp...
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