Lecture Outlines Chapter 20 Environment: The Science behind the Stories 4th Edition Withgott/Brennan © 2011 Pearson Education, Inc. This lecture will help you understand: • Reasons for seeking alternative fuels • Contributions to world energy by alternative fuels • Nuclear energy • The social debate over nuclear power • Bioenergy • Hydroelectric power © 2011 Pearson Education, Inc. Central Case: Sweden’s search for alternative energy • In 1980, Sweden’s people voted to phase out nuclear energy • The government has promoted hydroelectric, biomass, and wind power • Sweden will still use nuclear power instead of fossil fuels • Public support for nuclear power has increased © 2011 Pearson Education, Inc. Alternatives to fossil fuels • Our global economy is powered by fossil fuels - These fuels also power ⅔ of electricity generation • Fossil fuels are limited and pollute - We need to shift to resources that are less easily depleted and environmentally gentler © 2011 Pearson Education, Inc. Conventional alternatives • We have alternatives to fossil fuels - They are renewable and less polluting and harmful • But they are more expensive in the short term when external costs are not included in market prices • The most widely used “conventional alternatives” to fossil fuels: - Nuclear, hydroelectric, and biomass energy • They exert less environmental impact - These are intermediates along a continuum of renewability © 2011 Pearson Education, Inc. The U.S. relies on fossil fuels • The U.S. relies more on fossil fuels and nuclear power than other countries • Conventional alternatives play minor, yet substantial, roles • The use of conventional alternatives has been growing more slowly than fossil fuels © 2011 Pearson Education, Inc. Nuclear power • Nuclear energy occupies an odd and conflicted position in our debate over energy • It is free of air pollution produced by fossil fuels - Yet it has been clouded by weaponry, waste disposal, and accidents • Public safety concerns have led to limited development • The U.S. generates the most electricity from nuclear power - But only 20% of U.S. electricity comes from nuclear - France gets 76% of its electricity from nuclear power © 2011 Pearson Education, Inc. Fission releases nuclear energy Nuclei of large atoms are bombarded with neutrons, releasing energy and neutrons • Nuclear energy = the energy that holds together protons and neutrons within the nucleus of an atom • Nuclear fission = the splitting apart of atomic nuclei - The reaction that drives the release of nuclear energy in power plants • This chain reaction keeps a constant output of energy © 2011 Pearson Education, Inc. Nuclear energy comes from uranium • Nuclear reactors = facilities within nuclear power plants • Nuclear fuel cycle = the process that begins when uranium is mined • Radioisotopes = emit subatomic particles and high-energy radiation as they decay into lighter radioisotopes - They become stable isotopes Uranium is used for • Uranium-235 decays into lead- nuclear power because it is radioactive 207 © 2011 Pearson Education, Inc. Nuclear reactors use uranium-235 • Over 99% of uranium occurs as uranium-238 (238U) - It does not emit enough neutrons for a chain reaction - So we use 235U, with a half-life of 700 million years • 235U is enriched to 3% and formed into pellets (UO2) - Which are incorporated into fuel rods used in nuclear reactors • After several years in a reactor, uranium is depleted - The fuel no longer generates enough energy • Spent fuel can be reprocessed, but it is expensive - So it is disposed of as radioactive waste © 2011 Pearson Education, Inc. Fission in reactors generates electricity • A moderator = a substance (water or graphite) that slows the neutrons bombarding uranium - Allows fission to begin in a nuclear reactor - Excess neutrons must be soaked up • Control rods = a metallic alloy that absorbs neutrons - They are placed into the reactor among the waterbathed fuel rods - They are moved into and out of the water to control the rate of the reaction © 2011 Pearson Education, Inc. A nuclear power plant • The reactor core is housed in a reactor vessel - The vessel, steam generator, and plumbing are located in a containment building • Containment buildings are constructed to prevent leaks of radioactivity due to accidents or natural catastrophes - Not all nations require containment buildings © 2011 Pearson Education, Inc. A typical light water reactor © 2011 Pearson Education, Inc. Breeder reactors make better use of fuel • Breeder reactors use 238U (normally a waste product) - A neutron is added to 238U to form 239Pu (plutonium) • They make better use of fuel, generate more power, and produce less waste • But breeder reactors are more dangerous than conventional reactors - Its highly reactive coolant raises the risk of explosions - Plutonium can be used in nuclear weapons - They also are more expensive • Most of the world’s breeder reactors have been closed © 2011 Pearson Education, Inc. Fusion remains a dream Tremendous energy is released when deuterium and tritium are fused to form helium • Nuclear fusion = forces together small nuclei of lightweight elements under extremely high temperature and pressure • Drives the sun’s output of energy and hydrogen (thermonuclear) bombs • If we could control fusion, we could produce vast amounts of energy from water © 2011 Pearson Education, Inc. Nuclear power delivers energy cleanly • Nuclear power helps us avoid emitting 600 million metric tons of carbon each year • Power plants pose fewer health risks from pollution - They are safer for workers than coal-fired plants • Uranium mining damages less land than coal mining • Drawbacks of nuclear power: - Nuclear waste is radioactive - If an accident or sabotage occurs, the consequences can be catastrophic • The world has 436 operating nuclear plants in 30 nations © 2011 Pearson Education, Inc. Coal versus nuclear power © 2011 Pearson Education, Inc. Nuclear power poses small risks, but… • It poses the possibility of catastrophic accidents • The most serious accident in the U.S. = Three Mile Island in Pennsylvania in 1979 • Meltdown = coolant water drained from the reactor • Temperatures rose inside the reactor core … • Melting the metal surrounding the fuel rods … • Releasing radiation The emergency could have been far worse © 2011 Pearson Education, Inc. Chernobyl was the worst accident yet • 1986 explosion at the Chernobyl plant in Ukraine - The most severe nuclear plant accident ever seen - It was due to human error and unsafe design • For 10 days, radiation escaped while crews tried to put out the fire - More than 100,000 residents were evacuated • The landscape for 19 miles still remains contaminated • The accident killed 31 people directly - Thousands more became sick or developed cancer © 2011 Pearson Education, Inc. The Chernobyl accident The destroyed reactor was encased in a massive concrete sarcophagus, which is still leaking radioactive material © 2011 Pearson Education, Inc. Radioactivity from Chernobyl spread widely Atmospheric currents carried radioactive fallout from Chernobyl across much of the Northern Hemisphere © 2011 Pearson Education, Inc. Smaller-scale accidents have occurred • Western reactors are safer than Chernobyl - But smaller accidents have occurred - A 1999 accident in Japan killed two workers and exposed 400 others to radiation • Aging plants require more maintenance and are less safe - Recent terrorist attacks raised fears that similar attacks could be carried out against nuclear plants - Or stolen radioactive material could be used in attacks • The U.S. “megatons to megawatts” program buys radioactive material from the former Soviet Union - Using it in power plants © 2011 Pearson Education, Inc. Waste disposal remains a problem • Spent fuel rods and all other waste must be put in a safe location - Where leaking radioactivity will not harm future generations • Waste is held in temporary storage - Spent rods are stored in water • U.S. plants are running out of room - Waste is now stored in thick barrels of steel, lead, and concrete © 2011 Pearson Education, Inc. U.S. storage of high-level radioactive waste • Waste is held at 125 sites in 39 states • 161 million citizens live within 75 miles of nuclear waste © 2011 Pearson Education, Inc. Waste storage at Yucca Mountain, Nevada • It is safer to store all waste in a central repository - It can be heavily guarded • Yucca Mountain, Nevada was chosen for this site - President Obama’s administration does not support it • So waste will remain at its current locations © 2011 Pearson Education, Inc. Yucca Mountain, Nevada © 2011 Pearson Education, Inc. Benefits of storing waste at Yucca Mountain • It is remote and unpopulated • It has minimal risk of earthquakes that could damage the tunnels and release radioactivity • Its dry climate reduces chances of groundwater contamination • The water table is deep underground, making groundwater contamination less likely • It is on federal land that can be protected from sabotage © 2011 Pearson Education, Inc. Concerns with Yucca Mountain as a site • Some argue that earthquakes and volcanoes could destabilize the site’s geology • Fissures in the rock could allow rainwater to seep into the caverns • Nuclear waste will need to be transported there - From current storage areas, and from future nuclear plants and military installations - Shipments by rail and truck over thousands of miles could cause a high risk of accident or sabotage © 2011 Pearson Education, Inc. Dilemmas slow nuclear power’s growth • Concerns over waste disposal, safety, and costs have affected nuclear power’s growth • It is enormously expensive to build, maintain, operate, and ensure the safety of nuclear facilities - Decommissioning plants can be more expensive than construction • Power plants serve less than half their expected lifetimes • Electricity costs more than from coal and other sources - Governments must subsidize nuclear power © 2011 Pearson Education, Inc. The future of nuclear energy • 75% of nuclear power plants in Western Europe will be retired by 2030 - But some nations are rethinking this because of concerns over climate change • Asian nations are increasing nuclear capacity - 56 plants are under construction • The U.S. nuclear industry has stopped building plants - Expanding nuclear capacity would decrease reliance on fossil fuels and cut greenhouse gas emissions - Engineers are planning ways to make nuclear power plants safer and less expensive © 2011 Pearson Education, Inc. Bioenergy • Bioenergy (biomass energy) = energy obtained from organic material that makes up organisms - Wood, charcoal, agricultural crops, manure • Bioenergy has great potential for addressing our energy challenges Over 1 billion people use wood for heat, cooking, and light © 2011 Pearson Education, Inc. Overharvesting and developing new sources • Biomass is only renewable if it is not overharvested - Overharvesting causes deforestation, erosion, and desertification - Heavily populated arid regions are most vulnerable - Cooking produces indoor air pollution • New biomass sources are being developed • Biopower = biomass sources are burned in power plants - Generating heat and electricity • Biofuels = liquid fuels used to power automobiles © 2011 Pearson Education, Inc. Biopower generates electricity Many types of biomass are combusted to generate electricity • Waste products of industries or processes - Woody debris, crop residues • Specifically grown crops (fastgrowing willow trees, bamboo) • Co-firing combines biomass and coal • Gasification turn biomass to steam • Pyrolysis produces a liquid fuel © 2011 Pearson Education, Inc. Scales of production • Farmers, ranchers, or villages use manure, wood waste, or biogas from digestion to generate electricity - Small household biodigesters work in remote areas • The U.S. has dozens of biomass-fueled power plants • Biomass power increases efficiency and recycling - It reduces CO2 emissions and dependence on imported fossil fuels - It is better for health and supports rural economies • But burning crops deprives the soil of nutrients - Relying only on bioenergy is not a sustainable option © 2011 Pearson Education, Inc. Ethanol can power automobiles • Ethanol = a biofuel made by fermenting carbohydrate-rich crops - Ethanol is added to U.S. gasoline to reduce emissions • In 2009, 10 billion gallons were made in the U.S. from corn • Congressional mandates will increase ethanol production © 2011 Pearson Education, Inc. Cars can run on ethanol • Flexible-fuel vehicles run on E-85 - 85% ethanol, 15% gasoline - 8 million cars are in the U.S. - Most gas stations do not yet offer this fuel • Bagasse = crushed sugarcane residue used to make ethanol - 50% of new Brazilian cars are flexible-fuel vehicles © 2011 Pearson Education, Inc. Ethanol may not be sustainable • Environmental scientists don’t like corn-based ethanol • Growing corn impacts ecosystems - Pesticides, fertilizers, irrigation - Takes up land that could be left unfarmed • Ethanol competes with food and drives up food prices - As farmers shifted to ethanol, corn for food dropped - Mexicans could not afford tortillas, and so they rioted • Growing corn requires energy for equipment, pesticides, and fertilizers • Its EROI ratio is about 1.5:1, so it is inefficient © 2011 Pearson Education, Inc. Biodiesel powers engines • Biodiesel = produced from vegetable oil, cooking grease, or animal fats • Vehicles can run on 100% biodiesel - B20 = 20% biodiesel • Biodiesel reduces emissions • Its fuel economy is good • It costs a bit more than gasoline • Crops are specially grown - Using land, deforestation © 2011 Pearson Education, Inc. Novel biofuels are being developed • Algae produce lipids that can be converted to biodiesel - Their carbohydrates can be fermented to make ethanol • It can be grown in ponds, tanks, or photobioreactors • Algae grows fast and can be harvested every few days - It can use wastewater, ocean or saline water - It can capture CO2 emissions to speed its growth • Biofuels from algae are currently expensive • Cellulosic ethanol = produced from structural plant material (e.g., corn stalks) that has no food value - Switchgrass provides ethanol, habitat, and high EROI © 2011 Pearson Education, Inc. Two novel biofuels Algae is a candidate for a next-generation biofuel Switchgrass provides fuel now and may provide cellulosic ethanol © 2011 Pearson Education, Inc. Is bioenergy carbon-neutral? • In principle, biomass energy releases no net carbon - Photosynthesis removes carbon that is released when biomass is burned • Burning biomass is not carbon-neutral: - If forests are destroyed to plant bioenergy crops - If we use fossil fuel energy (tractors, fertilizers, etc.) • The Kyoto Protocol gives incentives to destroy forests for biofuel crops - Only emissions from energy use (not land-use changes) are “counted” toward controlling emissions © 2011 Pearson Education, Inc. Hydroelectric power (hydropower) • Hydropower = uses the kinetic energy of moving water to turn turbines to generate electricity • Storage technique = water stored in reservoirs behind dams passes through the dam and turns turbines • Run-of-river approach generates electricity without disrupting the river’s flow - Flow water over a small dam that does not impede fish passage - Useful in areas away from electric grids © 2011 Pearson Education, Inc. A typical dam © 2011 Pearson Education, Inc. A run-of-river system © 2011 Pearson Education, Inc. Hydroelectric power is widely used • Hydropower accounts for 2.2% of the world’s energy supply - And 15.6% of the world’s electricity production • Nations with large rivers and economic resources have used dams • However, many countries have dammed their large rivers - People want some rivers left undammed • The U.S. government built dams to employ people and help end the economic depression of the 1930s - Engineers exported their dam-building techniques © 2011 Pearson Education, Inc. Hydropower is clean and renewable • Hydropower has two clear advantages over fossil fuels for producing electricity: - It is renewable: as long as precipitation fills rivers we can use water to turn turbines - It is clean: no carbon dioxide is emitted • Hydropower is efficient - It has an EROI of 10:1 - As high as any modern-day energy source © 2011 Pearson Education, Inc. Hydropower has negative impacts • Damming rivers destroys wildlife habitats - Upstream areas are submerged - Downstream areas are starved of water • Natural flooding cycles are disrupted - Downstream floodplains don’t get nutrients • Downstream water is shallower and warmer • Periodic flushes of cold reservoir water can kill fish • Dams block passage of fish, fragmenting the river and reducing biodiversity • Large dams can cause earthquakes or collapse © 2011 Pearson Education, Inc. Hydropower may not expand much more • China’s Three Gorges Dam is the world’s largest dam - It displaced 1 million people - Generates as much electricity as dozens of coal-fired or nuclear plants • Most of the world’s large rivers have already been dammed • People have grown aware of the ecological impact of dams and resist more construction • Developing nations with rivers will increase hydropower © 2011 Pearson Education, Inc. Conclusion • With limited fossil fuel supplies, nations are trying to diversify their energy portfolios • Nuclear power showed promise - But high costs and public fears stalled its growth • Biomass energy sources include wood and newer biofuels - They can be carbon-neutral but are not strictly renewable • Hydropower is a renewable, pollution-free alternative - But it is nearing maximal use and can involve substantial ecological impacts © 2011 Pearson Education, Inc. QUESTION: Review Conventional alternative fuels: a) Exert less environmental impact than fossil fuels but are currently not feasible b) Are intermediate sources of fuel that can help us on our path towards sustainability c) Are final sources of fuel that will give us energy independence d) Are no longer available for widespread use © 2011 Pearson Education, Inc. QUESTION: Review The reaction that drives the release of energy in today’s nuclear power plants is: a) b) c) d) Nuclear fission Nuclear fusion Control rods Nuclear emergencies © 2011 Pearson Education, Inc. QUESTION: Review Why have nuclear power plants not been popular in the United States? a) b) c) d) Fears about accidents or sabotage Storage of nuclear waste is still not solved High costs of building and maintaining plants All are issues regarding nuclear energy © 2011 Pearson Education, Inc. QUESTION: Review Ethanol in the United States is made mainly from ______, and is used to ______. a) b) c) d) Soybeans, heat homes Sugarcane, drive cars Corn, drive cars Willow trees, make electricity © 2011 Pearson Education, Inc. QUESTION: Review Which of the following is an interesting future biofuel? a) b) c) d) Corn Algae Nuclear Shale oil © 2011 Pearson Education, Inc. QUESTION: Review Which of the following forms of hydropower is least environmentally destructive? a) b) c) d) The storage approach Run-of-river approach Bend-of-river approach All of these are destructive forms and none should be used. © 2011 Pearson Education, Inc. QUESTION: Weighing the Issues Given the choice of living next to a coal-burning power plant or nuclear plant, which would you choose? a) The nuclear plant, because it’s cleaner. b) The coal plant, because it won’t be as likely a target for terrorists. c) Neither one; I’d move to another place. d) Either one; I don’t care. © 2011 Pearson Education, Inc. QUESTION: Interpreting Graphs and Data If ethanol in the United States continues to be produced from corn, a drawback suggested from this graph could be: a) More corn is available for ethanol. b) More competition between food and fuel. c) Less land planted in corn. d) None of these. © 2011 Pearson Education, Inc.