itself. Because the mirrors are small and
great precision is needed, they tend to
use electric actuators.
The more familiar type of solar installation is photovoltaics (Figure 6). Any
solar collector works best if it’s kept
aimed at the sun as it moves across the
sky, and the actuators used tend to be
electric, often powered by direct current
with battery backup. These systems—
like with hydraulic systems—minimize
overall energy use; each individual actuator runs for just a moment at intervals
of about 10 minutes. Operating the actuators one at a time minimizes the
required power.
Challenges and the Future
Solar systems are expected to last about
30 years, and they tend to be installed in
deserts, frozen areas and other inhospitable places. Because of this, they’re
challenged by sand and dirt, tremendous
rainstorms, and high humidity and salt.
“If you’re using cylinders, you have to
deal with ingression of contaminants
coming in through the rod seal,”
explains Nagel. Rotary positioners suffer
less because “everything is redundant
sealed, so everything is self contained,”
he explains.
Other problems include the number of
connections and the loss of pressure in
accumulators. Even normal maintenance can be a challenge, both because
the number of units to be serviced is so
large—a 250 MW field may have close
to 4,000 drives—and because someone
has to check the seals, monitor the
hydraulic fluid and fix cylinders. Many
of these systems are installed in places
such as rural Spain where it can be difficult to find trained personnel.
Still, solar will continue to grow in
size as better, stronger systems are put
into place.
ACTUATORS CONTROLLING
WIND TURBINES
Most people are familiar with modern
wind turbines. These units are steadily
increasing in size, with blades between
100 and 150 feet long, weighing
between 7 and 14 tons, according to
Dheeraj Choudhary, business unit man-
ager, Global Wind Energy, Parker Han-
nifin. Some of the newer units have
blades more than 200 feet long and elec-
trical outputs greater than 5 MW per
turbine.
PHOTO: LINAK-US
emergency or during servicing. These
brakes are generally spring-applied and
either electrically or hydraulically
released, because it’s better not to use
power while the turbine has no output
and because a spring-applied brake is
inherently failsafe.
Both electric and hydraulic systems
have advantages and disadvantages for
wind turbines. Hydraulic systems have to
include rotary unions, and it’s difficult to
prevent these from leaking. Electric systems don’t leak, which means not only a
cleaner installation but also nothing slippery will drip onto the brake pads or
cause environmental problems.
On the other hand, hydraulic systems
don’t use gears, says Choudhary, while
electric ones do. The turbines and the
wind itself are not static “so as these
blades are vibrating, they are causing
fretting issues. The bigger/heavier the
blade, the bigger the fretting issues,” he
points out. In areas where low temperatures or salt air are a problem, the actuators are more subject to corrosion and
poor battery performance. Hydraulics
also have greater power density than
electric so Choudhary predicts newer
generations of larger turbines will
increasingly use hydraulics.
GEOTHERMAL:
CORROSION/EROSION
CHALLENGES
When compared to tapping wind energy,
which goes back to prehistoric times,
geothermal sources may seem like newcomers; the first recorded geothermal
electric generating plant was established
at Larderello in Italy’s Piedmont region
in 1904. Today, geothermal provides significant amounts of energy in locations
where it’s available, and its use is
increasing. In Iceland, for example, geothermal provides 24% of the nation’s
electric power and 87% of its domestic
heat and hot water.
Still, according to the Geothermal
Energy Association, the United States
is the world leader in use with a total
installed capacity of 3152.72 MW as
of October 2009. In that same month,
Nevada Geothermal Power Inc. dedicated its 49.5 MW Blue Mountain
“Faulkner 1” Geothermal Power Plant
in Humboldt County, NV, which will
