Quantifying Shading’s Economic Impact

While it seems prudent to avoid array shading, an overconservative design approach may cost system owners and developers money. So how much shading can designers tolerate before the energy losses become a problem? The answer may surprise you.

Project developers and engineering firms typically approach array shading according to rules of thumb that are based on low tolerance for shade or that seek to avoid it altogether. For example, the system designer might eliminate modules if they are shaded during a specific window of time, or if they are located within a certain proximity to an obstruction. These conservative design approaches are largely based on assumptions that have lost their relevance.

First, traditional design approaches for dealing with shading developed at a time when modules were the most expensive components of a PV system. Therefore, it made sense for system designers to prioritize production efficiency. Second, for many years no software tools on the market were capable of calculating the actual energy production and mismatch effects of shaded modules. As a result, designers could not determine which modules to include or exclude based on an evaluation of economic performance at the system level.

Today, module prices make up a smaller percentage of total project costs, having fallen by approximately 60% over the last 4 years, and system designers have access to new software tools that can calculate the production of shaded modules. In light of these changes, it is worth reevaluating traditional design approaches to array shading, and considering instead a cost-benefit approach that looks at component costs in relation to potential revenue.

Here we explore the system-level effects of shade to better understand optimal design approaches to array shading. We first consider common shade types and traditional design approaches for dealing with the system-level effects of shade. We then discuss energy losses associated with shading and consider the results of detailed shading analyses performed using simulation software tools. Finally, we present the results of a cost-benefit case study, identifying the optimal amount of shade tolerance for a space-constrained PV system based on specific shade profiles.

Shade Types and Frequency

Shade impacts PV systems of all sizes, and a wide variety of obstructions can create shade. While many system designers think of shading as a problem confined to residential systems, obstructions are often present on and around commercial rooftops, as well as within and around ground-mounted arrays. To understand shading losses, we can classify shade into three categories: self-shading, near-object shading and far-horizon shading.

Self-shading. The most common example of self-shading is row-to-row shading, which results when a tilted row of modules shades an adjacent row of modules. Because adjacent rows of tilted modules are typically located close to one another, row-to-row shading can affect the system throughout the entire year. If the system designer does not anticipate and account for self-shading, it can have a large impact on system production.

Near-object shading. This type of shading results when objects directly shade the array, causing shadows to move across it. In some cases, the obstructions responsible for near-object shading, such as rooftop units, vents or parapet walls, are relatively close to the array (within 10 feet). However, obstructions located in the middle distance (up to 100 feet), such as trees, utility poles, water towers or nearby buildings, can also cause near-object shading. Obstructions that are farther away typically affect the array only during certain times of day, whereas shade from nearby objects is more persistent.

Far-horizon shading. This type of shading results from obstructions on the far-horizon line and impacts the entire array. Mountain ranges or city skylines are typical examples of far-horizon shading. Because these obstructions are so far away, they are generally assumed to shade all modules at once whenever the sun is below the horizon line. As a result, there is no specific design or engineering response to far-horizon shading. However, designers do need to calculate its impact on total system performance.

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