16_energy

51 incident radiation is 174 1015 w this is 1370 wm2

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Unformatted text preview: 03/15/2007 UCSD: Physics 121; 2007 What about energy “pay-back”? pay-back” UCSD: Physics 121; 2007 Another look at available energy flow • I’ve heard the rumor that more energy goes into making a PV cell than it will deliver in its lifetime • The flow of radiation (solar and thermal) was covered earlier – if this were true, PVs would be sunk • A number of estimates calculate a current payback of 3–4 years, expecting future payback of 1–2 years 1– – Given 30 year lifetime, it’s still a winning proposition – 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? – – – – Winter 2007 61 much goes into heating the air/land much goes into driving weather (rain, wind) some goes into ocean currents some goes into photosynthesis Winter 2007 UCSD: Physics 121; 2007 The Renewable Budget 62 UCSD: Physics 121; 2007 Outstanding Points from Fig. 5.1 • Incident radiation is 174 1015 W 10 – this is 1370 W/m2 times area facing sun ( R 2) • 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 we’ – but wait! there’s more… Winter 2007 Lecture 16 63 Winter 2007 64 16 Energy, Sustainability 03/15/2007 UCSD: Physics 121; 2007 Energy Flow, continued UCSD: Physics 121; 2007 Energetics of the hydrologic cycle • 0.21% goes into wind, waves, convection, currents • It takes energy to evaporate water: 2,444 J per gram – note this is 100 times less than driving the water cycle – but this is the “other” aspect of weather – 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 • 0.023% is stored as chemical energy in plants via photosynthesis – total is 40 1 012 W (1021 J per year: 2.5 our industrial usage) – 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 (bio-activity, creating heat and re-radiating to space • 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 – except some small amount of storage in fossil fuels Winter 2007 65 Winter 2007 UCSD: Physics 121; 2007 Let it Rain – but this is re-radiated and is of no consequence to hydro-power • According to Figure 5.1, 40 1015 W of solar power goes into 10 of evaporation – this corresponds to 1.6 1 010 kg per second of evaporated water! – this is 3.5 mm per day off the ocean surface (replenished by rain) • 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 t iny bit rain falls on terrain (e.g., higher than sea level where it originated) • The gravitational potential energy given to water vapor (mostly in clouds) in the atmosphere (per second) is then: mgh = (1.6 1 010 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%) – 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 Lecture 16 UC...
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This note was uploaded on 01/30/2014 for the course PHYS 121 taught by Professor Staff during the Winter '08 term at UCSD.

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