Environmental Problems, Their Causes, and Sustainability 1

surface. Researchers in several countries are exploringwhether these zones can provide affordable geothermalenergy.Currently, about 22 countries (most of them in thedeveloping world) are extracting energy from geothermalsites to produce about 1% of the world’s electricity.Geothermal energy is used to heat about 85% ofIceland’s buildings, produce electricity, and provideheat to grow most of its fruits and vegetables in greenhousesheated by geothermal energy. The world’slargest operating geothermal system, called The Geysers,extracts energy from a dry steam reservoir northof San Francisco, California. It provides electricity forabout 1.7 million homes. In 1999, Santa Monica, California,became the first city in the world to get all itselectricity from geothermal energy.Figure 18-29 lists the advantages and disadvantagesof using geothermal energy. It generally has amuch lower environmental impact than fossil fuel energyresources.But geothermal energy has two main problems.One is that the cost of tapping large-scale reservoirs ofgeothermal energy is too high for all but the most concentratedand accessible sources. New technologiesmay bring these costs down.The other is that some dry- or wet-steam geothermalreservoirs can be depleted if heat is removed fasterthan natural processes renew it. Thus geothermal resourcescan be nonrenewable on a human time scale,but the potential supply is so vast that it is usually classifiedas a renewable energy resource. Recirculating allof the hot water back into the underground reservoircan also slow heat depletion from such reservoirs.18-8 HYDROGENCan Hydrogen Replace Oil? Good-byeOil, Smog, and CO 2 Emissions, HelloHydrogenSome energy analysts view hydrogen gas asthe best fuel to replace oil during the last halfof this century.When oil is gone or what is left costs too much to use,how will we fuel vehicles, industry, and buildings?Many scientists and executives of major oil companiesand automobile companies say the fuel of the future ishydrogen gas (H 2 )—envisioned in 1874 by sciencefiction writer Jules Verne in his book The MysteriousIsland.Figure 18-30 (p. 402) lists the advantages and disadvantagesof using hydrogen as an energy resource.Electricity (electrolysis) or high temperatures (thermolysis)can be used to split water molecules into gaseoushydrogen and oxygen (2 H 2 O 2 H 2 O 2 ). AndAdvantagesVery high efficiencyModerate netenergy ataccessible sitesLower CO 2emissions thanfossil fuelsLow cost atfavorable sitesLow land useLow land disturbanceModerateenvironmental impactT rade-OffsGeothermal EnergyDisadvantagesScarcity ofsuitable sitesDepleted if usedtoo rapidlyCO 2 emissionsModerate to highlocal air pollutionNoise and odor(H 2 S)Cost too highexcept at the mostconcentrated andaccessible sourcesFigure 18-29 Trade-offs: advantages and disadvantages ofusing geothermal energy for space heating and to produceelectricity or high-temperature heat for industrial processes.Pick the single advantage and the single disadvantage that youthink are the most important.when the hydrogen gas is used as a fuel it combineswith oxygen gas in the air and produces nonpollutingwater vapor (2 H 2 O 29 2H 2 O).*Proponents envision using hydrogen in energyefficientand nonpolluting fuel cells to provide electricityfor running buses, cars (Figures 18-10 and 18-11),houses, and other buildings. Widespread use of hydrogencould provide most of the energy needed to run aneconomy (Figure 18-31, p. 403). Proponents believethat such systems can be available by 2020–2030 andthen be phased in during this century.So what is the catch? There are three problems inturning the vision of widespread use of hydrogen as afuel into reality. First, hydrogen is chemically lockedup in water and organic compounds such as methaneand gasoline. Second, it takes energy and money toproduce hydrogen from water and organic compounds.In other words, hydrogen is not a source of energy.It is a fuel produced by using energy—lots of it. Third,fuel cells are the best way to use hydrogen to produceelectricity, but current versions are expensive.*Water vapor is a potent greenhouse gas. However, becausethere is already so much of it in the atmosphere, human additionsof this gas are insignificant.http://biology.brookscole.com/miller14401