Rack Hacking John Drake copyright 1991 John Drake This is about support PV structures, fasteners and assembly. You see little in print about these subjects. This is a shame because when support structures fail they can, at best, cause physical damage to the system. At the worst, it may result in bodily injury. Support Structures Considering the investment in time and money involved in setting up a photovoltaic array, attention should be given to "the little things" that determine long term success or failure. The most common metals available in descending order of effectiveness are: stainless steel, aluminum, galvanized steel, structural, and mild steel. Stainless Steel Stainless steel is the king of materials in most environmental situations. On exposure to the air it forms a tenacious chromium oxide layer which gives it its stain-less qualities. Please remember, it is called stainless steel, not stainproof steel. The easiest way to corrode stainless is to clean it with a regular wire brush or steel wool. By doing so, you introduce iron into the surface. Many stainless steels have little or no iron content. For most people stainless steel is difficult to work with and join properly. For drilling into stainless steel it is best to use low speeds, a cobalt bit and plenty of cutting fluid. On the plus side, stainless (e.g. type 304) will last as long as you need. It may even aid in the resale value of an AE system. This is because most people equate stainless steel with permanent non-maintenance. Aluminum Aluminum is most commonly used in commercially available structures. These are usually anodized, which is an intensive conversion at the surface to aluminum oxide (same material as in most grinding wheels) in an electrolytic process. Unfortunately they are often much too light (member thickness and shape) for the intended use. Aluminum, as a rule, is stronger than steel by weight, but not by volume. It is easy to cut and drill non- hardened aluminum, but any break in the coating should be sealed. There are user-applied coatings available from industrial coating and paint suppliers. Except in a pristine environment, don't use bare aluminum. I have handled 1/4" and 5/16" thick aluminum sections which have been exposed to the elements in Southern California for one to three years. These samples had the strength of a soda cracker and crumbled (flaked) just as easily. If you choose to use un-anodized aluminum, especially near salt water, I suggest a marine grade of the 5000 or 6000 series alloys. High levels of air pollution or acid rain can also degrade the integrity of aluminum structural members. As a rule most anodized coatings are effective within an environmental ph range of 4 through 9. Galvanized Steel Galvanized steel is a good choice if you are not near salt water or in a high pollution area. The coating, usually a zinc alloy, sacrifices itself to the steel member. In a mild environment it can last for years. Many people use slotted galvanized angle for frames. These are available from many local sources. This is fine if you are in a fairly benign environment and avoid contact with active soil. Structural or Mild Steel Structural or mild steel would be my last choice because, in most cases, they offer little corrosion resistance as received from the mill. The presence of mill scale can accelerate the degradation in use. You can paint them, but this will be an on-going project. Steels are very susceptible to corrosion damage because the oxides that form on the surface do not bond to the metal underneath. They continually flake off and expose fresh metal to the elements. When a structure is designed using exposed structural steel members, the yearly rate of material loss is taken into account to determine the life span and maintenance schedule of the finished project. If you choose these metals, clean the members as much as possible without polishing. Use a good quality metal primer before using a high quality paint formulated for steel. Corrosion Protection When handling any coated metals: Set the components together, do not (when possible) slide them together. This can scratch the coating, thereby opening the door to corrosion. Corrosion can be caused by oxidation - metal mixing with the oxygen in the air. It can also be galvanic in nature - moisture infiltration between faying (touching) surfaces sets up an electrical (like a cell) potential which transfer metallic ions from one to the other. This is why pipelines use a cathodic protection by burying a sacrificial anode nearby. The anode transfers to the pipe instead of the pipe transferring to minerals in the soil. Boats also use similar techniques for protection. Most corrosion that we are familiar with causes a de-alloying which removes alloys from the member (like removing water from a sponge) which can not be seen. Stress corrosion cracking may also occur when cracks appear at sites attacked by corrosion. Galvanic Series These metals are arranged by their tendency to corrode galvanically; Corroded end (anodic) Magnesium Magnesium alloys Zinc Aluminum 1100 Cadmium Aluminum 2017 Steel or iron Cash iron Chromium-iron (active) Ni-Resist 18-8 Chromium-nickel-iron (active) 18-8-3 Chromium-nickel-molybdenum-iron (active) Lead-tin solders Lead Tin Nickel (active) Inconel (active) Hastelloy C (active) Brass Copper Bronzes Copper-nickel alloys Monel Silver solder Nickel (passive) Inconel (passive) Chromium-iron (passive) 18-8 chromium-nickel-iron (passive) 18-8-3 chromium-nickel-molybdenum-iron (passive) Hastelloy C (passive) Silver Graphite Gold Platinum Protected end (cathodic) A less noble metal will sacrifice itself to a more noble one. Zinc plating (galvanizing) will sacrifice itself to steel. The heavier the coating, either dip or electroplating, the longer the steel will last. Eventually the coating will fail and substrate (in this case the protected metal) will follow. One should note that these coatings go into the environment. Cadmium, a very popular plating material, is also a nasty heavy metal. Bones readily absorb this and other heavy metals. Many steels contain lead to give the alloy its desirable properties. In the near future you should see many coatings and surface alloying methods which are less harmful to our planet. The assembly of aluminum to conventional steels should be avoided. The aluminum will sacrifice itself to uncoated and some coated steels. If you must put aluminum in contact with steel it is a good idea to use an inert insulator. Fasteners My advice is, if it is not stainless steel, do not use it. Do not mix stainless and non-stainless steel fasteners together. When purchasing stainless fasteners your best bet is to buy from a fastener distributor. Hardware stores and marine supply centers will usually charge more (sometimes they have to), not have the item in stock or know nothing about it. I have been told that stainless steel lag screws do not exist. Our arrays are mounted with these non-existent lag screws. If in doubt as to whether a fastener is stainless, check it with a magnet. Except for some 400 series stainless, a magnet is not attracted to stainless steel. When purchasing stainless fasteners for some of my fabrication work and save up to 60%, by purchasing them by box lots, over the single unit price. Assembly The most common structural shape you will encounter is the angle. You should always place the horizontal leg on top. If you do not, the member will lose over half of its load carrying potential. INSERFIG1 To me, the best way to join something is to weld it. This is not always possible. In cases where it is, please follow some guidelines. If you are not positive you will do it right - find someone who will. I have seen welding on mild and structural steel, let alone stainless steel and aluminum, that is an accident waiting to happen. If you are mounting an array onto an existing structure (like a tower) you can damage the structure itself if not careful. If in doubt, use graded (hardened) fasteners instead. When drilling any hole for a fastener, drill the smallest hole possible. This will minimize corrosion sites, reduce material loss degradation and member movement. When welding, remove all rusts or oxides. Keep gaps small and make sure all welds are full penetration and are thoroughly cleaned when finished. On stainless steel in salt environments I would recommend filler metals with the letter "L" (low carbon) following the alloy number. Roof Mounting For roof mounting, one option is to mount the frame pads (feet) to 2"x6" treated lumber. This will spread the weight over as many rafters as possible. You may choose to thorough bolt to a back-up board, using oversize washers, to prevent pull through. If you lag into the rafters themselves, unless they are oversize, you may appreciably weaken them. When using lag screws, drill the proper pilot and size holes to maintain grip and retain the lumber's strength. Ground Installations For ground installations it is best to use a concrete pad or pillars. When pouring concrete you should keep in mind that you want water to run away from the ground mounting pads. The most readily available mounts for wet concrete would be "el" shaped anchor bolts. For existing concrete you may use either lead shielded anchors (with lag screws) or the "red devil" type. With the "red devil" types you can drill and insert the anchors through the mounting pads in place. INSERFIG2 Whenever possible, plug any unused holes, especially in vertical sides, with snug fitting fasteners. This will reduce the possibility of failure at these weakened sites. It is common practice to do this on the frame rails of large trucks. Structure Stress You need to be concerned with two types of stresses your structure will face. The first is static load: The constant pull of gravity on the frame, modules and wiring. The second is dynamic load: Momentary forces applied to the structure; wind, storms, earthquakes and the neighbor's kids. If you ask your self how nature would defeat your structure then you will have better insight into preventing this. Site location can also aid in the array's mechanical longevity. Use a good base and, if possible, locate it out of harm's way (people as well as nature can be indiscriminate). In Conclusion If you give as much attention to the mechanical facets of your system as the electrical ones, you may prevent failures down the road. If a system fails mechanically, the uninformed public may view it as a failure of solar energy in general. When we step over a dollar to pick up a dime - none of us can complain about being short changed. Access John Drake, Long Beach, CA 90805 ù (213) 423-4879 List of Galvanic series is from the Metals Handbook - Desk Editor, by the American Society for Metals (now the ASM International)