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Unformatted text preview: UCSD Physics 12 Renewable Energy II
Biomass Other Renewables UCSD Physics 12 Biomass Biomass is any living organism, plant, animal, etc. 40 1012 W out of the 174,000 1012 W incident on the earth from the sun goes into photosynthesis 0.023% this is the fuel for virtually all biological activity half occurs in oceans Compare this to global human power generation of 12 1012 W, or to 0.6 1012 W of human biological activity Fossil fuels represent stored biomass energy
Spring 2010 2 UCSD Physics 12 Photosynthesis Typical carbohydrate (sugar) has molecular structure like: [CH2O]x, where x is some integer refer to this as "unit block": C6H12O6 (glucose) has x=6 Photosynthetic net reaction: xCO2 + xH2O + light [CH2O]x + xO2 1.47 g 0.6 g 16 kJ 1g 1.07 g Carbohydrate reaction (food consumption) is essentially photosynthesis run backwards 16 kJ per gram is about 4 Calories per gram Basically a "battery" for storing solar energy usage just runs reaction backward (but energy instead of light) Spring 2010 Q 2 3 UCSD Physics 12 Photosynthetic efficiency Only 25% of the solar spectrum is useful to the photosynthetic process uses both red and blue light (reflects green), doesn't use IR or UV 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 the rest is heat akin to photovoltaic incomplete usage of photon energy Net result is about 6%
Spring 2010 4 UCSD Physics 12 Realistic photosynthetic efficiency
Location Potential Maximum Polluted stream (?!) Iowa cornfield Pine Forest Wyoming Prairie Nevada Desert Plant Production (g/m2 per day) 71 55 20 6 0.3 0.2 Solar Energy Conversion Efficiency 5% 4% 1.5% 0.5% 0.02% 0.015% Spring 2010 Q 5 UCSD Physics 12 How much biomass is available? 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 U.S. annual harvested mass corresponds to 80 QBtu comparable to 100 QBtu total consumption U.S. actually has wood-fired plants: 6,650 MW-worth in 2002, burned equivalent of 200,000 barrels of oil per day Spring 2010 Q 6 UCSD Physics 12 Ethanol from Corn One can make ethanol (C2H5OH: a common alcohol) from corn chop; mix with water cook to convert starches to sugars ferment into alcohol distill to separate alcohol from the rest 7 Spring 2010 UCSD Physics 12 Why are we even talking about Ethanol?! We put more energy into agriculture than we get out (in terms of Caloric content) by about a factor of 210 at least in our modern, petrol-based mechano-farming sure, we can do better by improving efficiencies Estimates on energy return controversial: some say you get out 0.7 times the energy out that you put in (a net loss); others claim it's 1.4 times; often see numbers like 1.2 1.2 means a net gain, but 83% of your total budget goes into production; only 17% of crop is exported as energy Spring 2010 8 UCSD Physics 12 Ethanol, continued Right now, using tons of fossil fuels to get ethanol and not clear we're operating at a net gain Why on earth are we trying? corn has worked its way into many of our foods high fructose corn syrup cow feed corn oil for cooking powerful presence in the Halls of Power the corn lobby is responsible for pervasiveness of corn in our diet (soft drinks) are they then implicated in U.S. health/diet problems? Spring 2010 9 UCSD Physics 12 Ethanol problems, continued Energy is a high-payoff business, especially when the government helps out with subsidies thus the attraction for corn ethanol (which does get subsidies) Can supplant actual food production, driving up price of food there have been tortilla shortages in Mexico because corn ethanol is squeezing the market after all, we only have a finite agricultural capacity both land, and water are limited, especially water Ethanol from sugar cane can be 8:1 favorable Brazil doing very well this way: but corn is the wrong answer! but lookout rain forests: can actually increase CO 2 by removing CO2absorbing jungle Spring 2010 10 UCSD Physics 12 Quantitative Ethanol Let's calculate how much land we need to replace oil an Iowa cornfield is 1.5% efficient at turning incident sunlight into stored chemical energy the conversion to ethanol is 17% efficient assuming 1.2:1 ratio, and using corn ethanol to power farm equipment and ethanol production itself growing season is only part of year (say 50%) net is 0.13% efficient (1.5% 17% 50%) need 40% of 1020 J per year = 4 1019 J/yr to replace petroleum this is 1.3 1012 W: thus need 1015 W input (at 0.13%) at 200 W/m2 insolation, need 5 1012 m2, or (2,200 km)2 of land that's a square 2,200 km on a side Spring 2010 11 UCSD Physics 12 What does this amount of land look like? We don't have this much arable land! And where do we grow our food? Spring 2010 12 UCSD Physics 12 The lesson here Hopefully this illustrates the power of quantitative analysis lots of ideas are floated/touted, but many don't pass the quantitative test a plan has to do a heck of a lot more than sound good!!! by being quantitative in this course, I am hoping to instill some of this discriminatory capability in you Spring 2010 Q 13 UCSD Physics 12 Other renewables We won't spend time talking about every conceivable option for renewable energy (consult text and other books for more on these) Lots of imagination, few likely major players As a way of listing renewable alternatives, we will proceed by most abundant for each, I'll put the approximate value of QBtu available annually compare to our consumption of 100 QBtu per year Spring 2010 14 UCSD Physics 12 Renewables list Solar (photovoltaic, solar thermal) get 100 QBtu/yr with < 2% coverage of U.S. land area Wind maybe 180 QBtu/yr worldwide, maybe 25 QBtu in U.S. Biomass if we divert 10% of the 40 TW global budget into energy, would net 4 TW, or 120 QBtu worldwide; maybe 7 QBtu in U.S., given about 6% of land area Hydroelectric 70 QBtu/yr feasible worldwide: twice current development 5 QBtu/yr max potential in U.S. Spring 2010 15 UCSD Physics 12 Renewables, continued Geothermal: run heat engines off earth's internal heat could be as much as 1.5 QBtu/yr worldwide in 50 years limited to a few rare sites Tidal: oscillating hydroelectric "dams" a few rare sites are conducive to this (Bay of Fundy, for example) up to 1 QBtu/yr practical worldwide Ocean Thermal Energy Conversion (OTEC) use thermal gradient to drive heat engine complex, at sea, small power outputs Spring 2010 16 UCSD Physics 12 Assignments Read Chapter 6 on nuclear energy for Monday 5/17 Homework #6: due Friday, 5/14 Power Plant tours: think about a 1.5 hour block of time you would be able to do this (we'll pick two blocks in class) Spring 2010 17 ...
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