Environmental Problems, Their Causes, and Sustainability 1

AdvantagesCan be producedfrom plentifulwaterLowenvironmentalimpactRenewable ifproduced fromrenewable energyresourcesNo CO 2 emissions ifproduced fromwaterGood substitutefor oilCompetitive priceif environmentaland social costsare included in costcomparisonsEasier to store thanelectricitySafer thangasoline andnatural gasNontoxicHigh efficiency(45–65%) in fuel cellsTrade-OffsHydrogenAnode (-)CatalystCathode (+)DisadvantagesNot found in natureEnergy is neededto produce fuelNegative net energyCO 2 emissions ifproduced fromcarbon-containingcompoundsNonrenewable ifgenerated byfossil fuelsor nuclear powerHigh costs (but mayeventually comedown)Will take 25 to 50years to phase inShort drivingrange for currentfuel cell carsNo fuel distributionsystem in placeExcessive H 2 leaksmay deplete ozoneFigure 18-30 Trade-offs: advantages and disadvantages ofusing hydrogen as a fuel for vehicles and for providing heat andelectricity. Pick the single advantage and the single disadvantagethat you think are the most important.We could use electricity from coal-burning andconventional nuclear power plants to electrolyze water.But doing this is expensive and subjects us to theharmful environmental effects associated with usingthese fuels (Figure 17-21, p. 365, and Figure 17-26,p. 370). We can also use a reforming process thatinvolves using high temperatures and chemical processesto separate hydrogen from carbon atoms inorganic compounds found in conventional fuels suchas natural gas, methanol, ethanol, or gasoline. A problemis that getting hydrogen from organic compoundssuch as methane (CH 4 ) produces carbon dioxide(CH 4 2 H 2 O 4 H 2 CO 2 ). And according to a2002 study by physicist Marin Hoffer and a team ofother scientists, these reforming processes add moreCO 2 to the atmosphere per unit of heat generated thandoes burning these carbon-containing fuels directly.Thus using this approach could accelerate projectedglobal warming unless we can develop affordableways to store (sequester) the CO 2 underground or inthe deep ocean. We can also gasify coal or biomass toproduce hydrogen, but this is more expensive than usingnatural gas and also releases CO 2 .Most proponents of hydrogen believe that if weare to get its very low pollution and low CO 2 emissionbenefits, the energy used to produce H 2 by decomposingwater must come from low-polluting, renewablesources that emit little or no CO 2 . The most likelysources are electricity generated by wind farms, hydropower,geothermal energy, solar cells (when theirprices come down), or biological processes in bacteriaand algae (Spotlight, p. 404).In 1999, DaimlerChrysler, Royal Dutch Shell,Norsk Hydro, and Icelandic New Energy announcedgovernment-approved plans to turn the tiny countryof Iceland into the world’s first “hydrogen economy”by 2040—the brainchild of chemist Bragi Árnason,known as “Professor Hydrogen.” The country’s abundantrenewable geothermal energy, hydropower, andoffshore winds will be used to produce hydrogenfrom seawater and the H 2 will be used to run itsbuses, passenger cars, fishing vessels, and factories.Iceland’s first hydrogen service station opened in2003.Once hydrogen is produced we must have a wayto store it for use as needed. Here are some of the waysthat scientists and engineers are investigating for hydrogenstorage.Store it in compressed gas tanks either above orbelow the ground or aboard motor vehicles. In2002, General Motors developed a lightweight highpressurehydrogen storage tank that can be used oncars and can store enough hydrogen to provide a rangeof nearly 480 kilometers (300 miles) before refueling.Store it as more dense liquid hydrogen. But the liquidhydrogen must be stored in tanks kept at very lowtemperatures. This is costly, takes almost a third of thehydrogen’s original fuel energy, and requires a largeamount of insulation.Store it in solid metal hydride compounds. Certainmetals can absorb and chemically bond hydrogen intheir latticework of atoms. Heating such metal hydridecompounds releases the hydrogen gas asneeded. DaimlerChrysler has found a way to store hydrogenas sodium borohydride in a nontoxic and nonflammablesolution that can be pumped in and out ofthe vehicle safely and cleanly and without leaks of hydrogengas.Absorb hydrogen gas on activated charcoal or graphitenanofibers. Like hydrides, this is a safe and efficientway to store hydrogen, but an input of energy isneeded to release the hydrogen. Trap and store hydrogen402 CHAPTER 18 Energy Efficiency and Renewable Energy