Dual-Tilt Mounting Systems for Low-Slope Roofs
Inside this Article
More and more vendors are offering dual-tilt mounting systems, which orient modules in undulating east-west or even north-south rows. Is this the wave of the future?
Module-mounting strategies have evolved over time. In the early to mid 2000s, for example, it was common for designers to tilt modules at latitude. This approach optimized specific yield (kWh/kW), ensuring that the modules were as productive as possible. This design approach made sense when the modules constituted the most expensive part of a PV system. As module costs have declined, however, so have module tilt angles.
The industry movement toward lower tilt angles was first evident on commercial rooftops. To optimize energy production in this space-constrained setting, designers have long opted to reduce array tilt and tighten array spacing to fit more modules per square foot. For example, PowerLight (now SunPower) began volume production on its flat-tilt PowerGuard solution for commercial rooftops in September 1999. Although this high-power–density design approach predates the era of low-cost PV modules, it is especially well suited to optimizing financial performance at the system level rather than specific yield at the module level.
Dual-tilt mounting systems are a continuation of this trend in low-slope rooftop applications. Lower-cost PV modules incentivize designs that maximize roof coverage ratios and installed PV capacity while minimizing shade and soiling effects. This design philosophy gave birth to dual-tilt mounting systems that allow for high-power densities on rooftops with minimal self-shading as well as arrays that are self-cleaning during rain events. However, since south-facing PV arrays have been the de facto industry standard on commercial rooftops in North America for many years, system designers may initially be confused by dual-tilt mounting approaches or even suspicious of vendor performance claims.
In this article, we analyze the pros and cons of dual-tilt mounting systems for low-slope roofs, providing quantitative examples of how this design strategy differs from traditional approaches. We illustrate how to use an economic model to evaluate the financial performance of dual-tilt versus south-facing designs. Based on these results, we describe how and where designers can deploy dual-tilt systems most effectively. In the event that this design approach is ideal for projects you are developing, we provide a brief overview of vendors offering dual-tilt mounting systems in North America.
Evaluating the Dual-Tilt Value Proposition
The first step in evaluating dual-tilt mounting is to understand the trade-offs associated with this design approach, some of which Table 1 details. Some of the potential benefits that vendors tout (such as increased power density) are self-evident, whereas designers need to model and analyze others (such as time of delivery [TOD] gains). Most important, a decrease in specific yield relative to south-facing arrays tempers the potential benefits of dual-tilt arrays.
Power density. Traditional south-facing fixed-tilt mounting systems for low-slope roofs require a gap between rows of modules to prevent interrow shading. The width of this gap represents a trade-off between power density and module productivity. Interrow spacing typically ranges between 1 and 3 feet on rooftop systems. It is much larger on ground-mounted systems, which tend to have tall array tables.
By contrast, dual-tilt mounting systems orient modules in a “wave” pattern that inherently mitigates self-shading effects. Since there is no need for additional interrow shading allowances, most dual-tilt systems simply have rows at regular intervals to accommodate system maintenance. In most cases, dual-tilt mounts have a narrow gap only at the peak of each ridge to facilitate airflow around the modules and equalize pressure differentials associated with wind loads.
As a result of this fundamental design difference, dual-tilt arrays typically have a core ground coverage ratio (not counting obstructions or walkways) of approximately 0.9. By comparison, south-facing fixed-tilt arrays typically have core ground coverage ratios in the 0.5–0.8 range. This means that designers can increase system capacity 15%–35% by using a dual-tilt rather than a traditional south-facing design approach.
Specific yield. While power is an important variable in terms of a PV system’s economic performance, reduced energy yield per unit of power offsets capacity gains with a dual-tilt array. Though dual-tilt arrays are less sensitive to azimuth than traditional fixed-tilt arrays, vendors typically advertise these products as east-west mounting solutions. Not surprisingly, an array that has half of its modules facing east and half facing west will generate fewer kilowatt-hours per kilowatt than an array that has all of its modules facing south. This reduction in specific yield is a simple function of the lower average annual irradiance in the plane of the dual-tilt array.