Advantages of Extruded Aluminum
in Solar Power Generation Systems Processes
Stig Tjotta
Hydro Aluminum
North America
St. Augustine, Fla.
Aluminum is
becoming the
material of
choice in the
development
of utility- and
large-scale
solar powergeneration
projects
because of its
cost and
performance
advantages
over other
materials.
A
lternative power sources are gaining
momentum. According to reports published by the United Nations Environment Programme and the Renewable Energy
Policy Network for the Twenty-First Century
(REN21), in 2009 (for the second year in a row)
both the U.S. and Europe added more power
capacity from renewable sources like wind and
solar than from conventional sources like coal,
gas, and nuclear. In the same year, renewables
accounted for 60% of newly installed capacity
in Europe and more than 50% in the U.S. In
2011, experts predict the world will add more
capacity to the electricity supply from renewables than from nonrenewables.
Focusing on solar in North America, Solar
Energy Industries Association (SEIA) reports
that in 2009, six utility-scale solar photovoltaic
(PV) plants (plants that generate energy directly
from sunlight) were put online in the U.S. The
market research firm iSuppli Corp., El Segundo,
Calif., cited aggressive government incentives as
the catalyst for booming growth in solar installations in Ontario, Canada, in 2010. All of this
means that design engineers are being tasked
with developing renewable systems more often
and more quickly than ever before.
As with any industry, the most effective, efficient technologies, materials, and practices
will rise to the top and become the core design
elements going forward. In solar power generation, extruded aluminum is a core material
coming into focus. It is fast becoming the material of choice in the development of utilityand large-scale solar power-generation projects
View of 64-MW Nevada Solar One facility.
28
ADVANCED MATERIALS & PROCESSES • JANUARY 2011
because of its cost and performance advantages
over steel, composite materials, and other aluminum product forms. The design of, materials
for, and assembly of frames and support structures for a concentrating solar power (CSP)
trough can account for 25 to 30% of the solar
field cost, which typically accounts for 11% of
the total project cost. The numbers are slightly
different for photovoltaics (PV), but framing
and supports still are a significant portion of
the project. As cutting development costs has a
significant impact on the bottom line, selection
of aluminum offers cost benefits in the design
of solar power-generation systems.
Why today’s solar facilities
incorporate aluminum
Two of the most significant utility-scale
CSP facilities in America opted for extruded
aluminum support structures and collector
frames. In 2007, the 64-MW Nevada Solar One
facility helped launch the U.S. into the modern
solar era. Its solar field has 760 total solar collector assemblies comprising 9,120 trough
frames each measuring 8 m, 182,400 mirror
segments, and 18,240 receiver tubes. The parts
were extruded from 6061 T6 aluminum alloy,
chosen for its strength and machinability. The
metal contains more than 70% recycled material, and yet retains the performance characteristics of the original material. The facility boasts
an output that exceeds specification by up to
38%, thanks to precision engineering that was
possible only with extruded aluminum.
A hybrid solar plant in Florida completed
early in 2010 produces 75 MW using a collection system that uses 6,816 12-m frames made
of extruded aluminum. The solar field passed
testing to ensure that the structures could withstand Florida hurricanes. Extruded aluminum
was the only material that provided the required torsional strength at a price-point that
made the project cost feasible.
Lightweight, extrudable aluminum
offers cost savings
Aluminum offers similar performance to
that of steel, but weighs about one third as much
as steel. It costs less to transport than other materials, and transport vehicles have higher load
capacities and significantly lower fuel costs, cre-
Extruded aluminum allows
designing and producing
complex shapes such as this
gitter.
Solar frame constructed from extruded aluminum.
ating lower emissions and less packaging waste.
In the construction of solar fields, lightweight aluminum installations are easier and faster to erect. For example, an 8 m (26 ft) extruded aluminum solar frame (the
most common recently installed size trough) can be installed in less than five man-hours. Installation does not
require any welding or cranes to hoist parts into place. In
addition, after they are assembled, aluminum frames typically do not require any focus adjustments in the field,
which cuts labor costs and assembly time dramatically.
Aluminum frames are generally more accurate.
Unlike steel and other materials, extruded aluminum
enables engineers to design and produce complex pieces
without the need for secondary operations. Designs that
typically would require multiple pieces can easily be extruded into a single piece without sacrificing part strength.
Extrusion requires fewer steps than roll forming or
casting, and extruded parts can include joining structures
as part of the design, which reduces production costs. For
example, integrated joints and hinges can provide 90-degree movement without machining. A geared-hinge assembly, where two curved gear-like extrusions interweave
within a protective third extruded housing to form a
unique hinge, connects parts without screws or bolts.
Aluminum’s flexibility also allows for snap joints, which
effectively click together to join two or more extrusions
when it is impractical to extrude a single large part.
When screws are required, designers can engineer
screw grooves for self-tapping screws or plastic screws into
the extrusion die. Screw grooves require insignificant
amounts of material and can be produced at lower costs
than conventional methods of drilling and threading screw
holes.
Corrosion resistance for ongoing solar cost savings
Aluminum has intrinsic production and maintenance
advantages over steel, which must be galvanized for corrosion protection before being used outdoors. Aluminum
is inherently corrosion resistant, so it is well suited to years
of maintenance-free service in the solar field. Although
aluminum is chemically active, its behavior is stabilized by
a protective oxide film that naturally forms on the surface.
Generally, the film is stable in aqueous solutions having a
pH of 4.5 to 8.5. In the outdoor environment, aluminum is
very corrosion resistant. For example, aluminum was cho-
Geared-hinge assembly without
screws or bolts.
Aluminum’s flexibility allows for
snap joints.
sen as the material for the cap of the Washington Monument. Analysis of the cap after 73 years in service shows
only 0.005 in. (0.13 mm) corrosion.
Corrosion resistance in soil differs according to the
soil’s moisture and pH level. Aluminum surfaces that may
come into contact with soil are best treated with a thick
layer of bitumen or a powder coating. However, groundmounted solar collectors are typically mounted on concrete pads, so the soil conditions are not a factor.
Solar power systems are subject to galvanic corrosion
at points where they are connected to other types of metals, typically anchor brackets, mounts, and connections
with other equipment. Galvanic corrosion can be avoided
or minimized by avoiding materials having large galvanic
potential differences in a particular environment (stainless
steel not included). If that is not practical, electrically insulating different materials with a metallic coating (Al-Zn),
organic coating (lacquer, paint, plastic, and rubber), or a
special coating for screws and bolts will stop or slow reactions. Surface treatment must not be done only on the anodic material. It is important to check conductivity using a
resistance-measurement instrument, such as a multimeter. Sacrificial anodes are also a good, cost-effective way to
protect aluminum structures.
As solar power generation competition heats up, designs will become more forward-thinking and incorporate
the most effective, efficient materials possible. Whoever
develops a solar power system that achieves maximum
output and requires the least amount of maintenance will
have a competitive advantage. Extruded aluminum is a material that should be in every design engineer’s toolbox to
help them create a next-generation solar power system that
will provide long life and generate power in a more environmentally friendly way.
For more information: Stig Tjotta is vice president for Technology and Manufacturing for Hydro Aluminum North America; tel: 904/794-1500; fax: 904/794-1508; email: stig.tjotta@
hydro.com; Web site: www.solararraymaterials.com.
ADVANCED MATERIALS & PROCESSES • JANUARY 2011
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