Thermodynamics

mechanical energy, and thus electric cars are often touted as “zero emission”vehicles and their widespread use is seen by some as the ultimatesolution to the air pollution problem. It should be remembered, however,that the electricity used by the electric cars is generated somewhere elsemostly by burning fuel and thus emitting pollution. Therefore, each time anelectric car consumes 1 kWh of electricity, it bears the responsibility for thepollutions emitted as 1 kWh of electricity (plus the conversion and transmissionlosses) is generated elsewhere. The electric cars can be claimed to bezero emission vehicles only when the electricity they consume is generatedby emission-free renewable resources such as hydroelectric, solar, wind, andgeothermal energy (Fig. 2–68). Therefore, the use of renewable energyshould be encouraged worldwide, with incentives, as necessary, to make theearth a better place to live in. The advancements in thermodynamics havecontributed greatly in recent decades to improve conversion efficiencies (insome cases doubling them) and thus to reduce pollution. As individuals, wecan also help by practicing energy conservation measures and by makingenergy efficiency a high priority in our purchases.Chapter 2 | 91FIGURE 2–68Renewable energies such as wind arecalled “green energy” since they emitno pollutants or greenhouse gases.© Corbis Royalty FreeEXAMPLE 2–18Reducing Air Pollution by Geothermal HeatingA geothermal power plant in Nevada is generating electricity using geothermalwater extracted at 180°C, and reinjected back to the ground at 85°C. Itis proposed to utilize the reinjected brine for heating the residential andcommercial buildings in the area, and calculations show that the geothermalheating system can save 18 million therms of natural gas a year. Determinethe amount of NO x and CO 2 emissions the geothermal system will save ayear. Take the average NO x and CO 2 emissions of gas furnaces to be 0.0047kg/therm and 6.4 kg/therm, respectively.Solution The gas heating systems in an area are being replaced by a geothermaldistrict heating system. The amounts of NO x and CO 2 emissionssaved per year are to be determined.Analysis The amounts of emissions saved per year are equivalent to theamounts emitted by furnaces when 18 million therms of natural gas areburned,NO x savings 1NO x emission per therm2 1No. of therms per year2 10.0047 kg>therm2 118 10 6 therm>year2 8.5 10 4 kg>yearCO 2 savings 1CO 2 emission per therm2 1No. of therms per year2 16.4 kg>therm2 118 10 6 therm>year2 1.2 10 8 kg>yearDiscussion A typical car on the road generates about 8.5 kg of NO x and6000 kg of CO 2 a year. Therefore the environmental impact of replacing thegas heating systems in the area by the geothermal heating system is equivalentto taking 10,000 cars off the road for NO x emission and taking 20,000cars off the road for CO 2 emission. The proposed system should have a significanteffect on reducing smog in the area.