Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 2 | 91

mechanical energy, and thus electric cars are often touted as “zero emis-
sion” vehicles and their widespread use is seen by some as the ultimate
solution to the air pollution problem. It should be remembered, however,
that the electricity used by the electric cars is generated somewhere else
mostly by burning fuel and thus emitting pollution. Therefore, each time an
electric car consumes 1 kWh of electricity, it bears the responsibility for the
pollutions emitted as 1 kWh of electricity (plus the conversion and transmis-
sion losses) is generated elsewhere. The electric cars can be claimed to be
zero emission vehicles only when the electricity they consume is generated
by emission-free renewable resources such as hydroelectric, solar, wind, and
geothermal energy (Fig. 2–68). Therefore, the use of renewable energy
should be encouraged worldwide, with incentives, as necessary, to make the
earth a better place to live in. The advancements in thermodynamics have
contributed greatly in recent decades to improve conversion efficiencies (in
some cases doubling them) and thus to reduce pollution. As individuals, we
can also help by practicing energy conservation measures and by making
energy efficiency a high priority in our purchases.

EXAMPLE 2–18 Reducing Air Pollution by Geothermal Heating

A geothermal power plant in Nevada is generating electricity using geother-

mal water extracted at 180°C, and reinjected back to the ground at 85°C. It

is proposed to utilize the reinjected brine for heating the residential and

commercial buildings in the area, and calculations show that the geothermal

heating system can save 18 million therms of natural gas a year. Determine

the amount of NOxand CO 2 emissions the geothermal system will save a

year. Take the average NOxand CO 2 emissions of gas furnaces to be 0.0047

kg/therm and 6.4 kg/therm, respectively.

Solution The gas heating systems in an area are being replaced by a geo-

thermal district heating system. The amounts of NOxand CO 2 emissions

saved per year are to be determined.

Analysis The amounts of emissions saved per year are equivalent to the

amounts emitted by furnaces when 18 million therms of natural gas are

burned,

Discussion A typical car on the road generates about 8.5 kg of NOxand

6000 kg of CO 2 a year. Therefore the environmental impact of replacing the

gas heating systems in the area by the geothermal heating system is equiva-

lent to taking 10,000 cars off the road for NOxemission and taking 20,000

cars off the road for CO 2 emission. The proposed system should have a sig-

nificant effect on reducing smog in the area.

1.2 108 kg>year

 1 6.4 kg>therm 2118  106 therm>year 2

CO 2 savings
 1 CO 2 emission per therm 21 No. of therms per year 2

8.5 104 kg>year

 1 0.0047 kg>therm 2118  106 therm>year 2

NOx savings
 1 NOx emission per therm 21 No. of therms per year 2

FIGURE 2–68

Renewable energies such as wind are

called “green energy” since they emit

no pollutants or greenhouse gases.

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