Renewable Energy Sources II: Alternatives Part II. Lecture #11 HNRS 228 Energy and the Environment

Transcription

1 Renewable Energy Sources II: Alternatives Part II Lecture #11 HNRS 228 Energy and the Environment 1

2 Hydroelectric Power Wind Power Chapter 5 Summary Ocean Thermal Energy Conversion Biomass as Energy Geothermal Energy Tidal Energy Wave Energy Today s Focus Biomass Others 2

3 Recall Renewable Resources Renewable means anything that won t be destroyed 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) 3

4 Renewable Energy Consumption Energy Source QBtu (1994) Percent (1994) QBtu (2003) Percent (2003) Hydroelectric Geothermal Biomass Solar Energy Wind Total

5 Another look at available energy flow The flow of radiation (solar and thermal) was covered previously 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 5

6 The Renewable Budget 6

7 Outstanding Points from Fig. 5.1 Incident radiation is W this is 1370 W/m 2 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 but wait! there s more 7

8 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 W; half in ocean (plankton) humans are 6 billion times 100 W = W this is 1.5% of bio-energy; % 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 8

9 iclicker Question With respects to energy, hydroelectric power represents A remnant electric power from storms B remnant water energy from chemical bonds C remnant energy of chemical bonding D remnant gravitational potential energy of precipitation E a form of fictitious energy 9

10 iclicker Question What is true about hydroelectric power generation since 1950? A It has always increased in MW produced B It has always decreased in MW produced C It has increased and decreased in total MW produced, but is now at a peak D It has both increased and decreased in total MW produced E The percentage of electric power produced by hydroelectric plants has generally increased over time 10

11 iclicker Question What is about the maximum efficiency of energy generation using the wind? A 20% B 40% C 60% D 80% E 100% 11

12 iclicker Question Which state generates the most amount of electricity derived from wind power? A Virginia B Alaska C Montana D California E Texas 12

13 Biomass Biomass is any living organism, plant, animal, etc W out of the 174, 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 W, or to W of human biological activity Fossil fuels represent stored biomass energy 13

14 Photosynthesis Typical carbohydrate (sugar) has molecular structure like: [CH 2 O] x, where x is some integer refer to this as unit block : C 6 H 12 O 6 (glucose) has x=6 Photosynthetic reaction: xco 2 + xh 2 O + light [CH 2 O] x + xo g 0.6 g 16 kj 1 g 1.07 g Carbohydrate reaction (food consumption) is 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) 14

15 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% 15

16 Realistic photosynthetic efficiency Location Plant Production (g/m 2 per day) Solar Energy Conversion Efficiency Potential Maximum 71 5% Polluted stream (?!) 55 4% Iowa cornfield % Pine Forest 6 0.5% Wyoming Prairie % Nevada Desert % 16

17 iclicker Question The photosynthesis reaction A takes in sugar and water and produces carbon dioxide and energy B takes in sugar and sunlight and produces sugar and energy C takes in sunlight and water to produce sugar and oxygen D takes in sunlight, carbon dioxide and water to produce sugar and oxygen E takes in sunlight, oxygen and water to produce sugar and energy 17

18 iclicker Question Metabolic consumption of food is like photosynthesis in reverse in that A you use oxygen and water to produce energy and carbon dioxide B you use carbon dioxide and water to produce energy and oxygen C you use sugar and oxygen to produce energy and carbon dioxide D you use sugar and oxygen to produce carbon dioxide and water E you use sugar and carbon dioxide to produce energy and oxygen 18

19 How much biomass is available? Two estimates of plant production in book come up with comparable answers: grams per year 320 grams per m 2 averaged over earth s surface consistent with 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 19

20 Ethanol from Corn One can make ethanol (C 2 H 5 OH: 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 20

21 Does Ethanol as a Fuel Make Sense? We put more energy into agriculture than we get out (in terms of Caloric content) by about a factor of two 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 means a net gain, but 83% of your total budget goes into production; only 17% of crop is exported as energy 21

22 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 much of our foods high fructose corn syrup cow feed corn oil for cooking powerful presence in the halls of Congress the corn lobby is partially responsible for pervasiveness of corn in our diet (soft drinks) 22

23 iclicker Question Fructose is bad for your health. A True B False Sucrose is better for you than fructose. A True B False 23

24 Food For Thought Differences between glucose, fructose, and sucrose And then there is ethanol 24

25 Ethanol Issues, 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 CO 2 -absorbing jungle 25

26 iclicker Question The basic chemical formula for both glucose and fructose is C 6 H A True B False 26

27 iclicker Question Sucrose is a complex sugar made of glucose and fructose. A True B False 27

28 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 J per year = J/yr to replace petroleum this is W: thus need W input (at 0.13%) at 200 W/m 2 insolation, need m 2, or (2,200 km) 2 of land that s a square 2,200 km on a side 28

29 What does this amount of land look like? We don t have this much arable land! And where do we grow our food? 29

30 Take Home Points 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 30

31 Other Renewable Resources Consult text and other books for more on the other renewable resources Note that there are few likely major players Restricted by location and development costs When considering most abundant renewable resources consider the approximate value of QBtu available annually compare to our consumption of 100 QBtu per year 31

32 Renewable Resources Review 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. 32

33 Geothermal Energy 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 Binary-cycle Binary-cycle power plants use moderatetemperature water (225 ºF 360 ºF, or 107 ºC 182 ºC) from the geothermal reservoir. In binary systems, hot geothermal fluids are passed through one side of a heat exchanger to heat a working fluid in a separate adjacent pipe. The working fluid, usually an organic compound with a low boiling point such as iso-butane or isopentane, is vaporized and passed through a turbine to generate electricity. 33

34 Geothermal Energy Dry steam Use very hot (>455 F, or >235 C) steam and little water from the geothermal reservoir. Steam goes directly through a pipe to a turbine to spin a generator that produces electricity. This type of geothermal power plant is the oldest, first being used at Lardarello, Italy, in Flash steam Flash steam power plants use hot water (>360 ºF, or >182 ºC) from the geothermal reservoir. When the water is pumped to the generator, it is released from the pressure of the deep reservoir. The sudden drop in pressure causes some of the water to vaporize to steam, which spins a turbine to generate electricity. Both dry steam and flash steam power plants emit small amounts of carbon dioxide, nitric oxide, and sulfur Generally 50 times less than traditional fossil-fuel power plants. Hot water not flashed into steam is returned to the geothermal reservoir through injection wells. 34

35 Tidal Energy Tidal: oscillating hydroelectric dams a few rare sites are conducive to this (Bay of Fundy, for example) can only generate when the tide is flowing in or out only for about 10 hours each day up to 1 QBtu/yr practical worldwide Tidal Energy System in France 35

36 Ocean Thermal Energy Conversion (OTEC) Ocean Thermal Energy Conversion (OTEC) use thermal gradient to drive heat engine complex, at sea, small power outputs 36

37 iclicker Question Are there any other alternative renewable energy resources? A Yes B No Don t forget that there is more to energy than meets the Earth 37