Tuf HyhrnnFN FfONOMY
In this chapter, the following introductory chemistry topics are used;
a Ideal gas law
■ Thermochemistry calculations
■ Electronic structure of atoms
■ Electrochemistry: oxidation numbers; redox half-reactions; batteries; electrolysis
M Crystalline versus amorphous solids
■ Basic organic chemistry structures: alcohols, ethers, carboxylic acids, esters, sugars, carbohydrates
■ Vapor pressure of liquids
Background from previous chapters used in this chapter:
■ Greenhouse gases (carbon dioxide, methane, nitrous oxide) (Chapter 6)
■ Anaerobic decomposition; clathrates (Chapter 6)
■ Light absorption as energy; photons (Chapter 1)
■ Air pollution: photochemical smog, particulates, gaseous pollutants (Chapters 3 and 5)
■ Catalytic converters; thermal NO (Chapter 3)
In Chapters 3-7, we have seen how the atmosphere has been affected by emissions into it of pollutant gases such as sulfur and nitrogen oxides and greenhouse gases such as carbon dioxide and methane. The emphasis in this chapter is on alternative technologies under development that could reduce the anthropogenic production of such gases in the future while still allowing economic growth to occur. We begin by considering some possible solutions to the further buildup of atmospheric C02 by a partial switchover from fossil fuels to renewable energy, especially solar power. We then make an extensive survey of the various alternative fuels, including biofuels and hydrogen, that may be more "greenhouse friendly" than those used at present and that also would be effective in reducing air pollution. The generation of energy by nuclear power is discussed in Chapter 9.
The Sun sends enough energy to the Earth to supply all of our conceivable energy requirements, about 10,000 times more than we use now and will in the future, if only we could trap it efficiently. In addition to being plentiful and reliable, it is renewable energy—energy that will not run out and whose capture and use do not result in the direct emission of greenhouse gases or other pollutants.
The world currently uses about 12 terawatts (TW, 1012 watts) of power, about 85% of which is generated by the burning of fossil fuels. Since 1 watt is 1 juule per second, and since there are 3.2 X 107 seconds in a year, out annual power consumption is about 3.8 X 1020J, 380 EJ. Given that an average light bulb is rated at 60 W, we are using the equivalent of 200 billion light bulbs at a time, an average of about 35 per person, nonstop. Of course, this figure is an average for people in developed and developing countries; if we redo the calculation for North Americans, we are using about 200 60-W light bulbs for every man, woman, and child!
The pie chart in Figure 8-1 illustrates the sources of the world's commercial energy in 2004; the percentages for energy used to generate electricity are shown in parentheses. Clearly, most renewable energy currently is generated by burning biomass and by hydroelectricity, with the latter used to generate electricity. An assessment in 2003 by the European Union (EU) for energy use
Traditional bjomass •
in the year 2030 predicts that renewable energy, including wind, geothermal, and the direct forms of solar energy, will not keep pace with rising energy demand. Because people from rural regions of Asia and Africa will burn less firewood, due both to their migration to cities and to disappearing forests, renewable energy collectively will drop from the current 13% (only 2% of which is not from biomass) to only 8% of the global supply.
Of all the forms of renewable energy, hydroelectric power is by far the most important. Worldwide, it constitutes over 80% of renewable energy (other than that based upon biomass) and 2% of global commercial energy.
Hydroelectric power is an indirect form of solar energy. In the hydrological cycle, the Sun s energy evaporates water from oceans, lakes, rivers, and the soil and transports the H 20 molecules upward in the atmosphere via winds. After the water molecules condense to raindrops, they still possess considerable potential energy owing to their elevation, only a portion of which is dissipated if they fall onto land or a water body that lies above sea level. We can harness some of its remaining potential energy by forcing the downward-flowing water to turn turbines and thereby generate electricity.
Although there are small hydroelectric installations that use the flow of a river, most large-scale facilities use dams and waterfalls, where the water pressure—and hence the power yield—is much greater. In particular, the energy imparted to a turbine by falling water is directly proportional not only to the volume of the water but also to the height from which it falls. For this reason, new hydroelectric projects usually involve the construction of a high dam along the path of a flowing river. Water then collects behind the dam and its level rises to a considerable height. The water that is allowed to flow over the top of the dam falls a considerable distance before encountering the turbines positioned near the bottom. Unfortunately, the collection of water behind the dam floods considerable areas of land, creating a lake with environmental problems such as those discussed below.
Small hydro 0.41% Solar 0.53% Wind 0.32%
New renewables 3.40%
FIGURE 8-1 Sources of primary world energy supply in 2004. ISource: J. Goldemberg, "Ethanol for a Sustainable Future/' Science 315 (2007): 808-810.)
If all sites worldwide were exploited, the total amount of energy obtainable from hydroelectric sources would be about 100 EJ per year; about 20% of this total is obtained currently. Most of the sites that require little modification to use, and that are located within a reasonable distance of centers that use considerable electric power, have already been exploited; to use a common expression, most of the "low-hanging fruit" has already been picked. However, there are many river systems in developing countries, especially Africa, where considerable new hydroelectric power is currently being developed by the construction of dams.
Although hydropower is often thought of as pollution-free, there are environmental and social costs associated with it, especially ones resulting from the creation of the reservoirs behind dams. The most important of these costs include
• the displacement of human populations from lands flooded to create reservoirs;
• the eutrophication of water in reservoirs;
• the release of greenhouse gases, especially methane, from flooded areas;
• the release of mercury into reservoir water and consequently into the fish that swim in the water and human populations that eat the fish (this topic is discussed in more detail in the online Case Study Mercury Pollution arid die James Bay Hydroelectric Project (Canada) and in Chapter 15);
• the devastation to fish populations, such as salmon, from the blockage of their migratory routes by dams; and
• the buildup of silt behind dams, with the result that less silt is carried to locations farther along the waterway.
Unfortunately, the construction of new hydroelectric projects involving the damming of river systems, especially in developing countries, often proceeds without adequate environmental assessment and planning ahead of time. The World Bank and several other large financiers of such hydroelectric facilities do insist on an independent assessment of the project's impacts before they provide financial assistance.
The largest hydroelectric project in the world is the 26-turbine Three Gorges Dam in China, which, when completed in 2009, will provide 18 MW of power—equivalent to five large coal-fired power plants—and will have cost $25 billion to construct. Although about a million people had to be relocated to avoid being flooded by the artificial lake, the dam also controls flooding on the Yangtze River and thereby saves thousand of lives.
The expansion of wetlands that occurs by the deliberate flooding of land to produce a large, deep reservoir of water generally creates a long lake covering hundreds or thousands of square kilometers. The Three Gorges Dam will result in a lake that is 600 km long! The deep water in such lakes is largely anaerobic, especially if the flooded land was not first cleared of vegetation. The anaerobic decomposition of the original trees, bushes, etc. on the land produces carbon dioxide and methane in almost equal volumes, both of which escape from the surface and enter the atmosphere. The emissions from such reservoirs are significant. This is particularly true for methane, since it is such a powerful greenhouse gas (Chapter 6). Deep, small reservoirs produce and emit much less methane than do shallow ones that contain large areas of flooded biomass, such as those in the Brazilian Amazon. Indeed, the combined global warming effect of the methane and carbon dioxide produced by a large, shallow reservoir created to generate hydroelectric power can, for many years, exceed that of the carbon dioxide that would be emitted if a coal-fired power plant were used instead to generate the same amount of electrical power! Even after the original vegetation has decayed, new plants that have grown on the shores of the lakes during the dry season, when water levels recede, are later engulfed by rising water in the wet season; they eventually decompose, releasing more methane.
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