Environmental Engineering Reference
In-Depth Information
TABLE 7.1
1997 U.S. Renewable Energy Production
a
Electrical
Thermal
Electrical Capacity
Capacity Factor
(10
9
kWh/y)
(10
9
kWh/y)
Type
(GW)
(%)
Hydroelectric
359
—
79.8
51.4
Biomass
57.8
614
10.7
61.7
Geothermal
14.6
47.2
2.85
15.6
Wind
3.38
—
1.62
23.8
Solar
0.89
18.8
0.33
30.4
Total
435.7
680
95.3
52.2
a
Data from USDOE, 1998. DOE/EIA-0603(98) and DOE/EIA-0484(98).
duction by type of source. Both electric and thermal contributions, where appropriate, are listed.
Of the electrical power component, hydropower accounted for 83%, biomass 13%, geothermal
3%, wind 0.8%, and solar 0.2%. Together, renewable electricity accounted for 13% of U.S. 1997
electricity production. (On a worldwide basis, renewable electricity is a greater proportion, 21.6%.)
Also listed in Table 7.1 for each type is the installed electrical capacity and capacity factor (ratio
of the actual annual electrical energy produced to that which would be possible if the source ran at
its installed capacity continuously for a full year).
2
Renewable energy is characterized by several important traits:
•
Ubiquity of renewable energy sources.
•
Low intensity of energy fluxes being captured, compared to conventional systems.
•
Random, intermittent nature of renewable energy fluxes.
Some form of renewable energy is available everywhere on earth. Averaged over a year,
the entire earth's surface (including the polar regions) intercepts some sunlight, making solar
insolation universally available; although low-latitude, dry (clear) climates have the most, whereas
the polar regions have the least, solar flux at ground level. Solar insolation induces circulation in
the earth's atmosphere, giving rise to wind and ocean waves, along with precipitation into elevated
drainage basins, providing secondary sources of solar energy. Biomass crops and forests grow
in tropical and temperate lands having sufficient precipitation. Ocean tides are noticeable along
continental margins. Temperature gradients beneath the earth's surface exist everywhere, providing
the possibility of geothermal energy. But it is generally true that there is considerable geographic
variability of each form of energy, so that some locations are more favorable for its development
than others.
Fossil- and nuclear-fueled power systems operate under conditions of intense energy trans-
fer within their components, typically of the order of 1E(5) W/m
2
, so that they produce large
amounts of power per unit volume. In contrast, renewable energy fluxes are much smaller, al-
though quite different among the various sources, as listed in Table 7.2, requiring larger struc-
tures to reap the same amount of power. As a consequence, the capital cost per unit of average
2
For the United States in 1997, the electrical power produced by both utilities and industrial sources amounted
to 3340E(9) kWh/y, the installed electrical capacity was 683.2 GW, and the average capacity factor was 55.8 %.
