Selecting materials for solar structures is all about shape
By: Craig Werner
For solar power to become less dependent on political will, technologies must continue to improve. This article compares steel and aluminum — commonly used in photovoltaic (PV) and concentrated solar power (CSP) racking systems. It highlights where and how each material should be used to optimize installation and operating costs.
Steel Design: Limited But Well-Known Choices
Steel structures are widely understood by engineers, and designers know how to design, evaluate and specify steel structures based on limited but familiar shapes and common design software. They know how to fabricate these choices into structures that meet their needs. The finite selection of shapes results from rolling and manufacturing to produce steel members.
Aluminum Design: Creating Forms To Meet Function
Unfortunately, designers receive limited, if any, exposure to aluminum and the extrusion process. Their often-narrow view of the use of extrusions limits them to materials and shapes that may not optimize their designs.
Aluminum alloys have a high strength-to-weight ratio, corrosion resistance, formability and other advantages that engineers can exploit. Alloys can be chosen to deliver the finish, strength and shapes required for structural applications. However, simply converting a steel design to a similar aluminum shape rarely leads to an ideal solution.
Extrusion allows creating a shape with metal placed exactly where required for structural or functional purposes. Solid or hollow shapes can be customized to best achieve their intended purpose.
Designing structures with extrusions involves the same design analysis and software as steel, with one critical difference. While steel shapes are limited to a finite variety handled by software tables, the variety of extruded shapes — because of the extrusion process — is virtually infinite. This is a boon and a bane to the designer because design software can not yet automatically choose the optimal extruded shape.
Where Steel Excels
While properly alloyed aluminum can equal the yield and strength of steel, steel possesses approximately three times the modulus of elasticity (E) of aluminum. Thus, under load, identically-sized steel members deflect only a third as much as aluminum. For certain applications, this stiffness is important.
Designers skilled in working with both aluminum and steel often find that vertical mounting posts for CSP and PV can most efficiently and cost-effectively be designed and manufactured from steel I-beams, wide-flanged beams or simple rectangular or hollow tubes. Often the required mounting heights and unsupported lengths, coupled with wind-induced bending moments, are best met with steel components, properly selected and protected from corrosion.
Where Aluminum Excels
As noted, extrusion places material where needed in a shape. Extrusions can incorporate infinite variations in dimensions and shape (e.g., flange widths and web heights for I-beams). Besides modifications to basic structural shapes, extrusions can easily incorporate functional features such as stiffening ribs, mounting devices or heat dissipation fins.
For CSP and PV mounting systems, extrusions often are the optimal choice for axially loaded members such as struts, or chords and connectors. Extrusions are often also the best solution for members like mounting beams for CSP parabolic mirrors and PV panels.
For struts and chords, cross-sectional dimensions can be varied to best handle loads and connections. Extruded nodes can often replace steel parts requiring extensive fabrication. Extrusion allows placing metal where desired, which enables alternative-fastening solutions such as struts pinned at nodes to lower costs and enhance performance.
For elements that move, such as frames in tracking systems, the weight reduction with extruded components can yield significant downsizing and cost reduction in drive systems and other components.
Overcoming Aluminum’s Lower Modulus Of Elasticity
The design flexibility offered by extrusions provides alternatives to handle bending moments easily handled by steel. Custom shapes or extrusion-based trusses are readily designed, fabricated, transported and assembled to provide required stiffness.
Steel vs. Aluminum Mounting And Framing Systems
Design: As noted, steel designs are currently easier to develop because of limited shapes and their incorporation into design software. Extruded aluminum designs require more design effort.
Even with existing software, designers willing to invest the additional effort in referencing the Aluminum Design Manual (the Aluminum Association, www.aluminum.org) can harvest substantial advantages from extrusions.
Prototype Development And Tooling Costs: It is critical that designers considering extrusions recognize the low tooling cost and speed of production possible for extruded shapes. Using only standard shapes generally results in sub-optimal designs. Thinking creatively about built-in features for the extruded shapes allows designers to meet fastening, structural and other requirements. Extruded designs excel when the ability to place metal where needed for structure of function is exploited.
Tooling and extrusion costs depend on the shape’s weight-per-foot and circle size (the smallest circle that fits around the shape). There are many 10-inch circle-size and smaller extrusion presses in North America (sufficient for all but the most demanding solar applications). Extrusion tooling for these presses often costs between $300 and $1,500 for solid shapes, and $1,200 to $4,000 for hollows, with typical lead times of 2 to 3 weeks. The larger the press, the fewer available, with presses at 16-inch and larger quite limited. Tooling for these larger presses is more expensive, because of the quantity of tool steel required. Designers, particularly considering shapes larger than 9- or 10-inch circle size, should consult with extruders early in their design process.
Because extruded designs have low tooling costs and short lead times, prototyping often requires less time, facilitating sequential prototypes. In one recent project, such iterative design reduced extrusion weights by 40%.
Differences In Fabrication And Assembly
Steel designs typically require extensive fabrication. While pre-galvanized (or otherwise protected) shapes are available, drilling, sawing, welding or other operations compromise these coatings. Once complete, parts must be recoated or otherwise protected to prevent rusting.
Aluminum instantly forms a tenacious oxide film, protecting against corrosion. When using extrusions, there is often no need to provide any coating, except for severe service conditions or for aesthetics.
Custom-extruded shapes can minimize extensive fabrication because required features can be built into the shape. Extrusions, however, can be fabricated with saws, mills, punch presses and many other types of equipment. Joining can be accomplished with mechanical fasteners (pins, rivets, bolts) or via swaging, welding or adhesive bonding.
Sustainability
Aluminum’s properties do not degrade when recycled. Once produced, it can be recycled repeatedly without any loss in quality.
Extrusion feedstock (billet) often contains 70% or more recycled content, providing a small carbon footprint compared to steel (aluminum 0.7 CO2/MT versus steel 1.4-1.6 CO2/MT).
The light weight of extruded components, combined with extrusion production in most local markets, allows for efficient delivery with sustainability benefits.
Life-Cycle Economics
While the weight, design flexibility, sustainability and durability attributes of aluminum extrusion are certainly attractive, it is true that aluminum-per-pound is more costly than steel. Yet evidence shows that aluminum-extrusion-based balance of systems designs are more cost effective.
IBIS Associates, a technology strategy consultant, conducted an extensive cost assessment of steel and aluminum mounting systems, concluding that the total cost of materials, fabrication, corrosion protection, transportation and assembly of aluminum CSP and PV designs is typically 20 to 30% less than steel, and the aluminum structures have three times the residual value upon decommissioning. SPW
For more thoughts on how to design solar structures with aluminum extrusions see a follow-up article by the Aluminum Extruders Council, “From Concept To Reality: Designing Solar Structures With Aluminum Extrusions“.
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