Answer to Question 1
Few sites remain that are suitable for hydroelectric facilities in North America or elsewhere. Only 2 of the nation's rivers remain free flowing, and many of these are now protected by the Wild and Scenic Rivers Act of 1968, a law that effectively gives certain scenic rivers the status of national parks. Proposals for new dams outside of the United States are embroiled in controversy over whether the projected benefits justify the ecological and social trade-offs.
Answer to Question 2
The solar-trough collector system has collectors that are long, trough-shaped reflectors tilted toward the Sun. The curvature of the trough is such that all the sunlight hitting the collector is reflected onto a pipe running down the center of the system. Oil or some other heat-absorbing fluid circulating through the pipe is thus heated to very high temperatures. The heated fluid is passed through a heat exchanger to boil water and produce steam for driving a turbogenerator. The potential for the solar-trough system is quite good because the cost of producing a kilowatt-hour is barely more than the cost from coal-fired facilities. These facilities need to be located where there is plenty of sunlight.
Another system is the power tower. A power tower is an array of Sun-tracking mirrors that focuses the sunlight falling on several acres of land onto a receiver mounted on a tower in the center of the area. The receiver transfers the heat energy collected to a molten-salt liquid, which then flows either to a heat exchanger to drive a conventional turbogenerator or to a tank . . . to store the heat for later use.
Map of Mars from Hubble Space Telescope as seen near the 1999 opposition. (North top)
Major trunk lines carrying long-distance traffic crisscrossed the area east of the Mississippi. The
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