Environmental Engineering Reference
In-Depth Information
TABLE 7.2
Average Energy Flux in Renewable Energy Systems
Heat
Work
(W/m
2
)
(W/m
2
)
Source
Area
Solar
Collector
150
20
Photovoltaic
Cell
30
Hydropower
Drainage basin
0.01
Wind
Turbine disk
40
Geothermal
Field
0.1
0.02
Biomass
Field
0.5
0.1
Ocean tidal
Tidal pond
1
Ocean thermal
Surface area
Ocean wave
Frontal area
10,000
power output is higher for renewable than for conventional power plants, but the cost of fuel
for the conventional plant may more than offset its lower capital cost, making renewable energy
cheaper.
Energy from sunlight is only available about half of the time, and it reaches its higher intensities
in the several hours on either side of local noon. On cloudy and partly cloudy days, there is a
considerable reduction in solar insolation compared with clear days. Electric power generated from
sunlight must be stored or integrated into a distribution network with other power sources to be useful
in an industrial society. Similar considerations apply to wind, wave, and tidal power systems, which
are also intermittent sources. On the other hand, hydropower, biomass, and geothermal systems
have storage capabilities that permit them to deliver power when it is needed, as do conventional
systems.
In this chapter we explain the physical basis for each of the renewable energy sources listed
above and also discuss the technology used to collect and utilize that energy. In some cases it
is possible to identify costs and performance for systems in use. We also discuss environmental
effects which, while greatly reduced compared to traditional energy sources, can be significant.
7.2
HYDROPOWER
Before steam engines were developed, mechanical power generated by river water flowing past
water wheels was the major source of power for industrial mills. These mills had to be located near
river falls so that water could be diverted from an impoundment upstream of the falls and fed to a
water wheel or turbine discharging to a lower level downstream of the falls. The need for mechanical
power and sites for industrial facilities soon outgrew the availability of hydropower, leading to the
introduction of steam engines and, eventually, steam electric power plants that distribute their
power via electric lines to consumers far removed from the power plant site. Nevertheless, today
hydropower continues to be an important source of electric power generation, supplying 10.7% of
U.S. and 19% of world electricity production.
In the United States in 1980, the installed hydropower capacity totaled about 63 GW generated
in 1384 plants distributed throughout the country, as listed in Table 7.3, operating at an average
capacity factor of 51%. It is estimated that the total potential capacity in the United States is about
