Environmental Problems, Their Causes, and Sustainability 1

Some analysts believe that liquid methanol producedfrom biomass could replace gasoline and dieselfuel when oil becomes too scarce or expensive.Methanol is made mostly from natural gas but can alsobe produced at a higher cost from coal and biomasssuch as wood, wood wastes, agricultural wastes,sewage sludge, and garbage.Figure 18-28 lists the advantages and disadvantagesof using methanol as a vehicle fuel comparedto gasoline. According to a 1997 analysis by DavidPimentel and two other researchers, “Large-scale biofuelproduction is not an alternative to the current useof oil and is not even an advisable option to cover asignificant fraction of it.”However, chemist George A. Olah believes thatestablishing a methanol economy is preferable to thehighly publicized hydrogen economy. He points outthat methanol can be produced chemically fromcarbon dioxide in the atmosphere, which could alsohelp slow projected global warming. In addition,methanol can be converted to other hydrocarbon compoundsthat can be used to produce a variety of usefulchemicals like those made from petroleum and naturalgas.xHOW WOULD YOU VOTE? Do the advantages of usingliquid methanol as a fuel outweigh the disadvantages?Cast your vote online at http://biology.brookscole.com/miller14.AdvantagesHigh octaneSome reduction inCO 2 emissionsLower total airpollution (30–40%)Can be made fromnatural gas, agriculturalwastes,sewage sludge,and garbageCan be used toproduce H 2 forfuel cellsT rade-OffsMethanol FuelDisadvantagesLarge fuel tankneededHalf the drivingrangeCorrodes metal,rubber, plasticHigh CO 2 emissionsif made from coalExpensive toproduceHard to start in coldweatherFigure 18-28 Trade-offs: general advantages and disadvantagesof using methanol as a vehicle fuel compared to gasoline.Pick the single advantage and the single disadvantage that youthink are the most important.18-7 GEOTHERMAL ENERGYWhat Is Geothermal Energy? Tappingthe Earth’s Internal HeatWe can use geothermal energy stored in the earth’smantle to heat and cool buildings and to produceelectricity.Geothermal energy consists of heat stored in soil, undergroundrocks, and fluids in the earth’s mantle.Examples are volcanic rock, geysers, and hot springs.Scientists have developed several ways to tap into thisstored energy to heat and cool buildings and to produceelectricity.Throughout most of the world (except tundra areaswith permafrost) the temperature of the earth at a depthof about 3 meters (10 feet) is 10–16°C(50–60°F). Geothermalheat pumps can tap into this difference between undergroundand surface temperatures in most places anduse a system of pipes and ducts to heat or cool a building.These devices use the earth as a heat source in winterand as a heat sink during summer. They are a very efficientand cost-effective way to heat or cool a space.Arelated way to heat or cool a building is geothermalexchange or geoexchange. It involves using buriedpipes filled with a fluid to move heat in or out of theground depending on the season and the heating orcooling requirements. In the winter, for example, heatis removed from fluid in pipes buried in the groundand blown through house ducts. In the summer thisprocess is reversed. According to the U.S. EnvironmentalProtection Agency, geothermal exchange is themost energy-efficient, cost-effective, and environmentallyclean way to heat or cool a building.We have also learned to tap into deeper and moreconcentrated underground reservoirs of geothermalenergy. One type of reservoir contains dry steam withwater vapor but no water droplets. Another consists ofwet steam, a mixture of water vapor and waterdroplets. The third is hot water trapped in fractured orporous rock at various places in the earth’s crust.If such geothermal reservoirs are close to the surface,wells can be drilled to extract the dry steam, wetsteam, or hot water (Figure 17-2, p. 351), which can beused to heat homes and buildings or to spin turbinesand produce electricity.There are three other nearly nondepletable sourcesof geothermal energy. One is molten rock (magma). Anotheris hot dry-rock zones, where molten rock that haspenetrated the earth’s crust heats subsurface rock tohigh temperatures. A third source is low- to moderatetemperaturewarm-rock reservoir deposits. Heat fromsuch deposits could be used to preheat water and runheat pumps for space heating and air conditioning.Hot dry-rock zones can be found almost anywhereabout 8–10 kilometers (5–6 miles) below the earth’s400 CHAPTER 18 Energy Efficiency and Renewable Energy