A prior article discussed functional and economic benefits of using aluminum extrusions for PV and CSP mounting structures. This article outlines the “how” of designing those extrusions.
By: Craig Werner
Extrusions are available in a variety of alloys and tempers. While other alloys may provide greater strength, electrical conductivity or other functionality for CSP and PV applications, 6000 series is generally the most suitable.
Alloys to consider include 6063, 6061 or 6005A. Designers should work with extrusion suppliers and other experts when choosing alloys, but here is a quick overview.
Alloy Selection
6063 (and variants): This alloy the lowest strength and slightly lower costs than other choices, but with better extrudability and surface finish. It is appropriate for applications such as rooftop PV mountings where properties such as yield and ultimate strength are not controlling factors, but where elastic buckling or deflection is paramount. Most aluminum has substantially equal modulus of elasticity, so a higher strength alloy may provide no deflection benefits. Applications requiring an anodized finish for aesthetic reasons may best use 6063.
6061 (and variants): This alloy has high strength and higher cost, due to lower extrudability, but poorer surface finish. It is often specified due to familiarity, but generally 6005A is better suited to CSP and PV applications.
6005A: This is a newer alloy with high strength, moderate cost, good extrudability and surface finish. Because it is much less “quench sensitive” than 6061, extrusion process parameters are more reliably achieved without distortion from water quenching. It is typically recommended for structural solar applications.
Profile Design
Working with extrusion suppliers can be tremendously beneficial for those not well experienced in designing extruded solutions.
Designers should keep a few guidelines in mind as they put metal where it is needed:
- The larger the circle size, particularly above 10 inches, the higher the tooling and extrusion cost.
Where possible:
- Avoid dramatic differences in wall thicknesses.
- Use smaller “tongue ratios” (e.g.: less than 3) (the area of the “void” that the die must create / the length of the base dimension of the “tongue” that creates this void²).
- Avoid sharp corners and radii.
- Use symmetrical shapes.
- Simplify shapes while incorporating necessary features.
It’s important to working with suppliers early in the process to ensure that the designs meet application requirements while being cost-effective to extrude.
Structural Considerations and Design
Think broadly about the form and function of the part and designs, without being blinded by standard shapes. Creating optimal structural solutions is often an iterative process, starting with the function required and then working through various forms (geometries, joining means, etc.).
A high-level overview of Werner Extrusion Solutions’ design process:
- Determine environmental conditions (wind and snow loads, for example)
- Understand the function required of the structural system (parabolic troughs, for example, must rotate to follow the sun and must adequately support the parabolic mirrors in varying wind conditions while maintaining optical alignment to ensure that solar radiation is collected efficiently).
- Detail known constraints (maximum and minimum heights, exact shape of the parabola for CSP, etc.).
- Develop alternate geometric approaches to evaluate.
- Use design software (e.g. Bentley Software’s Ram Elements) and “idealized” members (simple tubular members, not yet focusing on connections, etc.).
- Vary the designs, geometry etc. and use the software to estimate the weights and number of parts of steel and extruded components.
- Estimate assembly costs.
- Choose the most likely alternatives.
- Use more detailed design analyses, included in Aluminum Design Manual–2010 Edition , the AISC Steel Construction Manual (available at www.aluminum.org), Finite Element Analysis (FEA) and design software such as Autodesk Inventor to develop actual part designs and extruded shapes necessary for each alternative.
- Iteratively fine tune these alternatives to achieve the optimal solution for each, taking material, fastener and assembly costs into account.
- Select one or two designs worthy of complete/final design and produce appropriate extrusion, subassembly and assembly drawings.
- Develop rapid prototyped part and assembly examples.
- Obtain quotations and make a final decision.
Don’t hesitate to ask for help from extruders!
Thermal Expansion
It’s important to keep thermal expansion in mind when designing with aluminum extrusions–especially long elements–because aluminum’s thermal expansion is greater than steel’s. In the desert, structures can see a 100° F change in temperature from night to daytime peaks. For instance, a 12-m stringer will expand 0.6 inches with such a change.
To mitigate the impact of thermal expansion:
- Provide for expansion/contraction in connections– particularly when connecting to elements with substantially different coefficients of expansion (e.g. steel).
- Pay particular attention to tolerance build-ups.
- Specify a reference temperature for dimensions between critical elements–like mounting holes–to ensure that fabrication and subsequent quality checks take place at a given temperature.
Fabricating And Joining
Nevada Solar One in Boulder City, Nevada is a wonderful example of using aluminum extrusions to provide an effective solution for parabolic trough concentrated solar power application. The installation used more than 7 million pounds of extrusions to produce approximately 9,200 8-m long parabolic trough frames. No welding or bolting was used, because the combination of the extruded design, precise fabrication and simple interference pin fastening yielded a terrific solution.
The extrusion, fabrication, transportation, assembly and ultimate performance of the extruded approach proved that extrusions were not only a possible alternative to previous steel alternatives used, but the preferred solution. The system continues to perform well, requires virtually no maintenance of the extruded framing system and has yielded almost no mirror breakage due to thermal expansion or other factors.
Finishing
As noted previously, steel components require coatings such as galvanizing or painting to prevent corrosion, both on the base material and after fabrication and welding. Aluminum structures, however, do not. But often extruded aluminum solar structures are specified with anodic finishes.
The reason for this is simple–anodizing aluminum extrusions is a terrific way to provide an enhanced surface, from an aesthetic–and performance–perspective. Anodized components have been used successfully. We tend to stay with what we know and carry technical specifications forward from prior designs without asking “Is this the best decision”?
Mill finish extrusions, without anodizing or painting, were used successfully at Nevada Solar One and in myriad other structural applications. Anodizing or painting is only beneficial from a corrosion standpoint in installations where the ambient environment is especially challenging (perhaps subject to frequent sandstorms or is highly caustic or acidic). Most solar installations do not require anodizing or painting on the extruded finish. This only adds cost.
Resources
The Aluminum Extruders Council (AEC): AEC.org provides information about aluminum extrusions and their use. The site hosts a newly-released Aluminum Extrusion Manual (free) and links to other information such as IBIS economic studies on solar mounting systems. AEC.org also provides an electronic “Buyers’ Guide” listing member extrusion, fabrication and finishing capabilities, as well as locations and contacts.
The Aluminum Association: The Aluminum Association (aluminum.org) has a wealth of information about aluminum and its uses. Of particular importance to those considering extrusion is the Aluminum Design Manual, for design requirements and material properties.
Read more about functional and economic benefits of using aluminum extrusions for PV and CSP mounting structures in a previous article by the Aluminum Extruders Council, “The Case For Aluminum Solar Racking Systems.”
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