Due Diligence 101-Part 1: Assessing the site of a solar array
By Nick Hudson
This is the first in a series of blogs outlining important considerations when performing technical due diligence on a solar asset.
Any asset acquisition requires due diligence, and solar PV is no exception.
A PV system’s lack of moving parts may offer a false sense of security that only a cursory overview is needed. But a systematic review—literally from the ground up—can help identify issues that may affect system performance in the long term.
Here, we consider site assessment issues for both ground- and roof-mounted PV systems.
At a macro level, a site assessment should consider factors such as:
Foundation type and attachment
Shading from nearby buildings and trees
Environmental soiling
Weather damage and mitigation
Foundation type and attachment
We first need to assess the foundation—what was the system installed on and how is it attached?
Let’s start with ground-mount systems. In general, the farther north you go, the more that local freeze-and-thaw cycles come into play as the ground heaves and shifts between winter and summer. Farther south, clay soil can expand and contract depending on moisture conditions. In western desert conditions, corrosive soils can deteriorate foundation piles over time.
Rooftop-mounted solar arrays have a similar number of concerns, but we look at different factors. The issues often start with the roof itself. Typical asphalt roofing lasts as few as 15 years; most commercial rooftop membrane materials are good for no more than 20 to 25 years. That means even a new roof’s lifespan might fall short of the 25-30 years that the solar array’s productive life.
Removing a rooftop solar system and then putting it back on to replace a roof can be messy, to say the least. Enclosures and wiring components may have become brittle over time and can break in the process, causing both safety and performance issues. This is why it’s important to align roof material life with the solar asset’s life to avoid having to remove and recommission the site.
How the rooftop solar array attaches also matters, as winds and thermal shifting can affect mounting systems.
In recent years, rooftop racking components have shed a lot of weight as manufacturers move toward lighter materials that are easier to handle and less costly to ship. But racking systems can become almost too light, and require additional anchor points and ballast blocks to secure the array to a roof.
An unanchored array, even when properly ballasted, can “walk” over time, moving across the roof’s surface. Over the course of several years, even small shifts can create conflicts with other rooftop equipment. This shifting can also stretch and pull electrical conduits and cables from their attachment points, creating potential points of failure.
Builders often use ballast blocks to anchor an array to the roof, but these come with their own risks. These concrete blocks almost never attach to the rack itself; instead, they usually sit on the racking system’s rails. Exposed to the weather, the blocks sometimes crack and fall off the rails. Or, a contractor may move a ballast block and then fail to put it back in the right place.
In contrast, mechanical anchors provide a positive attachment: the anchor is permanent and immovable, and resists sliding forces from high winds. The array still expands and contracts over the course of a year due to thermal heating and cooling. But using mechanical anchors keeps the system where it was originally designed to sit.
Shading from nearby buildings and trees
Environmental factors like shading, soiling and weather damage should also be considered. In a dynamic urban setting, a previously unobstructed location could receive shade from a new building. Reviewing the zoning of adjacent properties can help you determine whether potential obstructions might be built in the future.
Trees can also cause issues. If the solar array is sited in an area with young trees, then future shading impacts should also be considered.
Environmental soiling
Soiling poses another risk factor, as dust and dirt reduce the amount of solar energy that can be absorbed by the module. Here, it’s important to consider not only how much dirt accumulates, but also natural precipitation and clearing rates.
In a dusty, arid part of the country, your array may require periodic mechanical washing to clear off excessive dust and dirt.
And in an urban environment, soot from smokestacks and tailpipes can stick to panels in a way that natural materials would not. Rainfall alone may not clear these dense, hard or hydrophobic deposits.
In more rural areas, large soiling events can occur during certain times of the year. For example, a site might have low soiling during most of the year, but see a dip in production as fields are plowed or crops are harvested. In those cases, a mechanical washing schedule aligned to the growing season might be in order.
Weather damage and mitigation
We also assess the likelihood and impacts from weather damage, particularly hail. If the solar array has a single-axis tracker, then you can physically turn the panels away from an approaching hailstorm if the system is actively monitored through a remote operations center.
But if the system has a fixed tilt, or if it's a rooftop system, then such stow options are not available. The risk of hail damage may increase, but that risk may be better addressed through insurance than engineering.
Projecting a project's useful lifetime and expected loss of performance involves assessing these and other site risks. Particularly when it comes to the weather, different data sources can yield different results. An independent consultant’s report can help validate a project’s underlying energy model.
In future posts, we’ll walk you through the other elements of our due diligence analysis, including technology selection, performance model validation, and overall system design.
Nick Hudson is a Principal Engineer / Senior Project Manager at CEA.
