The largest systems currently are
solar thermal systems, which concentrate sunlight using mirrors to generate
steam for turbines (Figure 2). The
largest currently in operation is the
Solar Electric Generating System
(SEGS) installation, a group of nine
plants spread over 1,000 acres in the
Mojave desert, which have an installed
capacity of 354 MW. SEGS uses
400,000 trough collectors, long parabolic mirrors that focus sunlight onto tubes;
the tubes are filled with a heat-transfer
fluid (a synthetic oil) that is pumped to
steam generators that feed turbines. Sin-gle-axis actuators keep the collectors
pointed at the sun.
In the other main type of solar thermal plant, a field of movable mirrors
called heliostats focus sunlight onto a
collector atop a central tower containing
a boiler or heat collector. California-based PG&E is currently under contract
with BrightSource Energy to install such
a system in California’s Mojave Desert
that will have an aggregate capacity of
1310 MW. One such installation,
planned for Ivanpah, CA, will occupy 6
square miles and produce 400 MW.
Resistance from environmental groups
has led to the cancellation of another
BrightSource project elsewhere in the
desert, and the Ivanpah site itself may
run into similar difficulties.
Besides the actuators that move the
collectors in solar systems are the valves
and actuators that help to control the hot
oil pumped in solar processes. An example is the Andasol 1 plant in the Spanish
province of Granada, which has 510,000
square meters of collector surface and
an output of 50 MW. The plant uses
intelligent electric actuators (from
Rotork) with digital control in all areas
of the generating process—on-off valve
control using multi-turn and quarter-turn electric actuators, and for the 10
control valves on each of the networks,
modulating actuators with proportional
controllers and current position transmitters operating from a 4-20 mA control signal.
PHOTO: SANDIA NATIONAL LABS
sun, 30 degrees to move to the stowed
position, and sometimes an additional 30
degrees to reach a maintenance position.
Hydraulically, this can be done in two
ways: a rack-and-pinion actuator (
Figure 4) or a push/pull system (Figure 5).
The rack-and-pinion is the quickest to
install. “You have a drive pylon that’s
sunk in the ground and connected to a
concrete piling and that actuator bolts
with 4, 8, 16 bolts. You attach your
torque arms, which are the plates that
secure the mirror to the actuator, and
you’re up and running in a couple of
minutes,” Nagel explains.
The other method—push/pull—
involves two long-stroke cylinders; one
pushes part of the way; the other takes
over for the rest of the rotation. “You do
have to have a fairly long stroke actuator, and you have to push the cylinder
half way on the torque arm. Once you get
half way, the other side has to take
over,” says Nagel.
Hydraulics are not noted for energy
efficiency, but that’s not a problem in
solar applications because the duty cycle
over the course of a day is less than 5%.
It takes all day for the troughs to swivel
from one horizon to the other. “A pulse
can be anywhere from 200 to 500
microseconds, and you can increment
anywhere from 30 seconds [of angle] to
2 minutes, depending on wind load [and]
internal leakage in the hydraulic system,” says Nagel.
Heliostats are made up of thousands
of individual mirrors, each moving in
two axes and each pointing in a slightly
different direction with enough precision
to keep the sun’s reflection centered on a
space not much larger than the mirror
Figure 3. The Kramer Junction solar trough
plant uses a variety of actuator systems.
ness unit manager, Parker Hannifin, says
the Kramer Junction solar trough plant
(Figure 3), which is part of the SEGS
installation and was built in the mid- to
late-1980s, uses a variety of actuator
systems: cable drums, gear boxes and
hydraulic push/pull. Hydraulics offer the
advantage that they can be built with a
fair degree of compliance to withstand
impact loads using a crossover relief circuit and sometimes an accumulator,
Nagel explains. Meanwhile, an electric
drive involves gears with metal-to-metal
contact where high impacts can break
teeth, he says.
The other advantage of a hydraulic
system is the power it can deliver.
Troughs are getting longer and wider, so
there is a lot of area affected by wind,
Nagel explains. Normal tracking torque,
which might assume 40 miles per hour
headwind, would mean about 300,000
and 500,000 inch-pounds (in-lbs); the
torque to get to a - 30 degree stowed
position would be similar. However, the
torque required in a high wind can reach
3 million in-lbs, which is generally provided by a mechanical clamping system
to keep from overloading the actuator.
The range of motion is also significant: It takes 180 degrees to track the
Electric vs. Hydraulic
Heliostats and photovoltaic installations
tend to be electric, while many trough
systems are hydraulic. Rich Nagel, busi-
PHOTO: PARKER HANNIFIN
PHOTO: PARKER HANNIFIN
Figure 4. A hydraulic rack and pinion actuator
is shown atop a pylon at the Nevada One solar
plant.
Figure 5. This image shows a push/pull
hydraulic system for a solar tracker.
