A successful solar racking solution must meet some minimum requirements in order to make it useful to the installer and customer. Racking is an important component to the overall functionality of the solar system. A well-designed racking solution will provide many years of functionality. In this blog I will cover important and critical aspects of racking.
A well-designed solar racking offers an efficient means of transferring load from the solar panel to the racking and then to the earth. All of this must occur through the attachment points of the solar panel to the framing and then from the framing to the underlaying structure.
The solar racking industry has evolved over time especially since solar came of age about 10 years ago. Before the recent solar age, and before solar racking standards were established by the industry, custom engineered and fabricated systems were used. Today, standards such as UL2703 govern a large portion of the racking industry. And it is widely accepted that UL Listing is a minimum requirement.
We will not cover the UL Listing requirements here since those are typically well covered in the respective certifications. The UL 2703 test is for electrical bonding, mechanical strength and fire class. The requirement is to test for each different module manufacturer and their frame class.
The electrical bonding test comprises of testing the bonding ampacity rating. Since most solar module has a 15A fuse rating, electrical bonding is done to 30A as part of the UL2703 test. This test is a good test and does ensure the electrical bonding is adequate given the torque and fastening of the solar module to the frame.
However, I would like to say that there is some level of unrequired redundancy in UL2703 Listing. For example, it is standard practice that aluminum is anodized to 0.005” to 0.010” thickness. All solar module frames have the same anodizing thicknesses. It appears very redundant and expensive to require electrical bonding tests for each solar module to racking pair. As all manufacturers use the same anodizing thickness the requirement by UL 2703 to do bonding tests for each manufacturer to rack solution pair appears unnecessary. Fortunately, most AHJs acknowledge this unnecessary requirement and accept as long as a UL electrical bonding test was performed, and it was certified.
There has been a lot of talk about how solar racking has reached maturity, and we set out here to explore aspects of racking never discussed before in this paper. Most racking solutions target only permit approval requirements such as UL Listing, wind and seismic requirements. A good racking solution will not only cover these but will also incorporate aspects such as applicability, install-ability, permit-ability, reliability, serviceability and availability.
Racking is specific to application. The types of racking available are for sloped roof, flat roof, ground mount and carports. Each of these have evolved into industry segments with almost no overlap as the solar industry has grown.
Ground Mount Solar Racking
The most used racking is the ground mounted racking since it serves the commercial and utility scale solar markets which are probably 50 to 60% of the solar market. A vast majority of the ground mounted solar market is far away (>30 miles) from load. Hence, these systems also require some level of additional balance of systems requirements such as transformers to get solar generation voltages to the distribution line voltages.
Ground mounted racking is of three kinds, driven pile solution, concrete peer and concrete slab foundation. Once the load bearing structures are determined the solar module framing is a network of rails placed on the foundation structure. The foundation structure provides the load bearing underlayment over which the solar module frameworks are attached.
The primary engineering challenge for ground mounted solar racking are, wind resistance, seismic resistance, snow accumulation, water flow, and environmental considerations such as flood protection and organic matter accumulation.
Monetary costs for ground mounted solar are typically higher as more materials and labor are required than for roof mounted solar. However, since these costs are amortized over a larger array, the overall cost impact is reduced compared to a residential system where each small installation requires independent assessments for each category of concern.
Environmental impact costs for ground mount include additional land use which could be used for other purposes including or not limited to farming and wildlife. Typically ground mounted solar requires property taxes, land management (shrub growth) and habitat management (animal migration).
Flat Roof Solar Racking
According to the Yale School of Environment there is 145 GW of rooftop solar space available in the USA. This provides a large opportunity for near load solar generation.
Conventional flat roof racking utilizes thin metal sheet and light materials as containers where the solar panels are mounted. These containers are equipped with baskets or containers where concrete ballasts are placed so individual solar modules are weighted down during high wind or seismic impact events. The design engineering thrust for flat roof solar racking are wind resistance, seismic resistance, fireman access, materials shipping and installation ease and speed.
The accumulation of on-site trash from concrete ballast packaging and the labor time and disposal costs are higher in these which ultimately are part of the overall racking cost. These costs such as ballasts cost, large amount of trash accumulation on site, labor required to dispose the trash and cost to dispose the packaging trash are typically not considered by the racking manufacturer and are passed to the installer.
The overall environmental impact due to use of concrete which is one of the largest green house gas emitters is largely ignored by the flat roof industry. Similarly, the large of trash created due to the use of large amount of ballasts is also largely ignored.
Z Lite is a self-ballasted solar racking solution addresses each aspect of these considerations such as wind resistance, seismic resistance, fireman access, materials shipping, installation ease and speed, 95% lower concrete use, 95% lower trash accumulation and reduced labor time in handling and disposal of trash.
Sloped Roof Solar Racking
Solar Racking for sloped roofs is specialized for shingle roofs or metal roof or tile roof. The most popular and readily available sloped roof solutions are for shingle roofs, tile roofs and standing seam metal roofs. And there are many roof materials for which traditional solar racking solutions are absent. Further, these specific use solar racking have specific components such as rails, brackets, flashings and hardware. It is impossible to find these brackets, rails and hardware in a big box home improvement store.
Z Rack is a solar rack solution that has found application in all different roof materials. The solution for shingle, tile, metal, wood and even many others that conventional racking solutions do not handle can be installed using Z Rack. The simplicity in installation, versatility in application and easy installation makes Z Rack a compelling use by installers and homeowners who have become aware of the Z Rack solution.
Conventional sloped roof racking solution relies on anchors or attachment points that transfer load (uplift and downward force) to the underlaying structure (rafter, trusses, walls and ground). There are typically 6 to 8 attachments per kW of solar. Or there are 2 to 3 attachments per solar module.
These attachments use lag bolts that are typically 5/16” diameter and 3 to 4” long. There is an assumption that the attachment is done to a rafter or a structural member of the truss or joist. And that the attachment has at least 2” of attachment depth into the rafter. It is assumed the load transfer is about 200 lbs per inch hence an average load of 400 lbs for the 2” of penetration.
If the attachments are not made to the rafter and are made only to the decking board on the roof, the pull out force resistances will be significantly lower.
The rafters are 1.5” wide and the lag bolt are 5/16” diameter. According to Structural Builders Components Association (SBCA ), “Variance from the design dimensions is also allowed: 3/4 in. in span and 1/2 in”. Which means, a 24” on center can be 23 5/8” to 24 3/8” in span.
The attachments also assumes that pilot holes initially drilled to locate the rafter are appropriately closed with a sealant and flashings placed so water flow is diverted away from the purported location.
The requirement for tile roofs is also similar that there are specialized brackets designed for the type of tile roof wherein these brackets are attached to the rafters and provide rail attachments to transfer load from the solar modules to the roof truss members.
The responsibility of anchoring the solar racking to the roof structure and its proper depth and dimensions are the responsibility of the installer. The variations in truss spacing due to fabrication needs to be considered by the installer in their installation procedures. If the lag bolts are not square to the rafter or penetrate the rafter at an angle reducing the pull-out force or load transfer capability, places the burden of responsibility on the installer. As solar installers see pricing pressures from customer accusation, and if the solar installer is no longer in business, the responsibility shifts to the homeowner. As these are 25+ year functioning systems, these factors must be considered with appropriate justification.
The checklist for permit-ability are whether the racking system is UL Listed, has structural integrity to meet structural engineering codes and a factor ignored by most racking providers is Authority Having Jurisdiction (Building Official and Electrical Inspectors) having familiarity with the racking.
UL Listing is a process by which a nationally recognized laboratory will perform testing of the said racking to the standards. And if it passes, the racking will be given the listing.
Structural Engineering is done by a licensed professional structural engineer who understands the code (ASCE and SEAOC ) and is willing to provide his/her stamp that the racking solution will withstand code requirements for the location. All professional structural engineers will require additional testing or reports from wind design experts and seismic experts as third party to authenticate the tests performed by the racking company.
AHJ familiarity is mostly with penetrated systems in the last 5 to 7 years. This is because the penetrated systems gained popularity initially and efforts by the companies continued in marketing, sales and distribution of their products. There is little to no difference between the penetrated systems as far as code compliance are concerned. They are providing safety in that it has been listed and has the wind and seismic resistance for the location.
Recently there has been consolidation in the residential solar racking industry. The consolidation appears to be mostly for improving sales and distribution channels rather than for technology and customer service.
Z Rack and Z Lite although have the UL Listing and Structural Engineering requirements, is less familiar with the AHJs as it is a new way of racking. Most AHJs are not structural engineers but interpret the ASCE code book sometime verbatim. Hence for Z Rack and Z Lite to pass through AHJ acceptance will require some initial hand holding. It is with confidence; I can say that LADWP and Contra Costa County in CA which are couple highly reviewed districts for plan reviews have provided acceptance for Z Rack. Hence this is a mark of success for the racking solution. Further over 200 residential systems have been installed in the Stockton, Tracy, Modesto, Fountain Valley, Bakersfield and LA County.
The less known fact about Z Rack is the fact that we use Unistrut as the rail and the associated hardware with it. Unistrut has been used in electrical hardware for over 6 decades and it has its own UL Listing. However, we also obtained a Z Rack and Z Lite specific UL Listing as our proprietary method of making the racking system network.
Since Unistrut is made of galvanized steel, it is denser than aluminum and provides overall weight that helps to provide wind resistance. Besides the stiffness of steel (Young’s Modulus) is three times that of aluminum. Hence a network of steel rails acts to transfer loads when one end of the array experiences lift the other end that is not experience lift prevents lift to occur. This is illustrated in the figure below in a test done by SolarPod, located in Minnesota, to prove the stiffness of the Z Rack and Z Lite racking solution.
The characteristic of a good racking system is its ability to stay strong through local weather conditions. Most penetrated solar racking uses aluminum as its main material for roof top solar. And steel for ground mount and carports. The choice of using aluminum or steel is an economic decision based on material cost and transportability.
Both aluminum and steel are particularly good structural materials. Steel is one of the most recycled materials in the world due to its magnetic properties it can be separated easily. Further, it can be remelted any number of times and will not loose its properties. Besides the energy to remelt steel is about 625 kWh per ton as opposed to 600 kWh per ton. So not that large a difference in remelting cost between them. A factor that tipped the scale for steel for Z Rack and Z Lite is the fact that steel will not burn while aluminum will burn in the presence of fire and oxygen. And once the aluminum burning process begins it is very exothermic and will create a larger intense heat. As we were designing the Z Rack and Z Lite for roof tops, we felt using steel will help in the fire resistance and fire classification.
Further, steel being a denser material adds weight to the framing. This added weight reduces the need for ballasts which is an unnecessary material. And most importantly, the ballasts used adds a lot of on-site trash by way of packaging. These ballasts have to be boomed up to the roof, hence each pallet weight has to be less than 1500 lbs. This creates a lot of pallets to be used and these pallets go to landfills and costs the installer for trash removal and disposal.