Page 22 - Geothermal Energy Renewable Energy and The Environment

4 Geothermal Energy: Renewable Energy and the Environment

GeoThermal enerGy as a renewable enerGy soUrce
no fuel, few emissions
As noted above, the historical use of energy has relied on fuel to make heat. The sources of fuel
have traditionally been wood, coal, oil, and natural gas. The changes in the use of these fuels over
time within the United States, as well as the contribution of hydroelectric and nuclear generation,
are shown in Figure 1.3. Coal, natural gas, and oil account for more than 85% of the energy used
in the United States. Although these fuels may, in combination, provide decades to several hundred
years of supply, the combination of greenhouse gas emissions and competition for the petroleum-
based fuels that are implicit in Figures 1.1 and 1.2 have made problematic reliance on these energy
sources. In addition, these fuels are derived from a resource base that is not being replenished.
Instead, these resources are being extracted from geologically derived materials that took millions
of years to form. As a commodity, these fossil fuels are not renewable and will become more and
more difficult, and hence more costly, to extract.
It is for these reasons that there is growing interest in finding sources of energy that reduce reli-
ance on fossil fuels to generate heat to produce work, and which can reduce or eliminate production
of greenhouse gases. Criteria that are generally used to establish the viability of an energy source
that would supplant or displace reliance on fossil fuels are the following:
• It is sufficiently abundant to meet a significant percentage of the market demand.
• It can be obtained at a cost competitive with existing energy sources.
• Its use will reduce or eliminate greenhouse gas emissions.
• It is self-replenishing (i.e., renewable).

These criteria can be best employed in selection of alternative energy sources by understanding how
fossil-fueled energy is used and how it contributes to greenhouse gas emissions.
Figure 1.4 shows the energy-consuming sectors of the United States economy and their respec-
tive greenhouse gas emissions in 2006. Currently, the transportation sector is the single largest

4.0

3.5
Oil
United States energy consumption,
3.0 by source
Natural
Joules (×10 19 ) 2.5 Coal
gas
2.0

1.5
Nuclear
1.0 Hydroelectric

0.5 Wood

0
1650 1750 1850 1950 2050
Year

FIGUre 1.3 (See color insert following page 17.0..) Sources of energy in the United States since 1650. U.S.
Energy Information Administration (http://www.eia.doe.gov/emeu/aer/eh/intro.html).