Energy Solutions to Air Pollution and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 320

The main advantage of both large hydro and run-of-the
river hydro with a modest storage pond behind it is that
they can provide electricity within 15 to 30 seconds of
a need as long as other water needs are met. Hydroelec-
tricity is thus ideal for providing peaking power (i.e.,
electric power at times of maximum and unexpected
demand during the day) and filling in gaps when winds
are not blowing or the sun is not shining. It is also used
to provide baseload power (constant power all day less
than or equal to minimum demand during the day).

13.2.5. Tidal
Tides are oscillating currents in the ocean caused by
the rise and fall of the ocean surface due to the gravita-
tional attraction among the Earth, moon, and sun. The
rising and sinking motion of the ocean surface forces
water below the surface to move horizontally as a cur-
rent. A tidal turbine captures the kinetic energy of
the ebbing and flowing current just as a wind turbine
captures the kinetic energy of the wind. Like a wind tur-
bine, a tidal turbine consists of a rotor, which provides
rotating kinetic energy to a generator and converts it to
electrical energy that is transmitted to shore. The turbine
is generally mounted on the sea floor (Figure 13.7) and
may or may not extend to the surface. The tidal turbine

Figure 13.8. Solar photovoltaic farm, March 2011.
C

View7/Dreamstime.com.

rotor, which lies under water, may be fully exposed to
the water or placed within a narrowing duct that directs
water toward it. Because tides run about 6 hours in one
direction before switching direction for another 6 hours,
they are fairly predictable, so tidal turbines may be used
to supply baseload energy.

13.2.6. Solar Photovoltaics
Solar photovoltaics (PVs) are arrays of cells (Figure
13.8) containing a material that converts solar radiation
into direct current (DC) electricity. An inverter is then
used to convert DC electricity into alternating cur-
rent (AC) electricity . Photovoltaics can be mounted
on roofs or combined in large-scale power plants. In
power plants, panels are sometimes mounted on track-
ers that rotate to follow the sun. The presence of clouds,
shadow-casting buildings or trees, dust that accumu-
lates on solar panels, and extremely high temperatures
can reduce the efficiency of a solar PV panel. However,
when the sun is behind a tree or at an angle relative
to the panels, the presence of thin clouds can signifi-
cantly enhance solar radiation incident upon the panels
by scattering light from multiple locations in the cloud
more directly onto the panels.

13.2.7. Concentrated Solar Power
Concentrated solar power (CSP) is a technology by
which sunlight is focused (concentrated) by mirrors or

Figure 13.7. Illustration of tidal turbine. Courtesy
Hammerfest-Strøm. www.hammerfeststrom.com/
products/tidal-turbines/.