The What and Why of ASTM Solar Capacity Testing
Why is ASTM Capacity Testing Necessary?
In the 1990s and 2000s, as the solar industry started building MW scale utility power plants in earnest, the industry began a search for more reliable ways to determine if sites were performing efficiently. Testing the efficiency of traditional power plants, like coal and natural gas, is relatively simple - you can simply measure how much coal or natural gas must be consumed to produce your desired output of electricity. The relationship between fuel consumption and power output is your efficiency.
But solar power plants depend on variable weather as an input. You can’t simply crank up the sun to your desired output level, and even if you could, solar irradiance isn’t the only factor affecting the plant - solar modules are very sensitive to temperature as well. Modules are less efficient in high ambient temperature, and because high wind speed can reduce the temperature of a module, (making it more efficient), wind is a factor as well. This reliance on the weather complicates performance commissioning.
Over the years, several methods were developed for taking weather into account during the commissioning process to assess whether solar power plants were performing to spec.
Today, ASTM has emerged as the gold standard for capacity and performance testing of solar plants.
What does Capacity/Performance Testing actually test?
Capacity testing provides a big picture overview of the plant’s performance. While modern DAS systems often have better methods available for obvious issues, like faulty inverters and even disconnected DC strings, they often cannot provide a complete assessment of plant efficiency.
ASTM Capacity Testing answers a simple question : Is the plant performing well compared to what we’d expect given the weather over the last few weeks/months?
Asking this question can catch problems that traditional O&M troubleshooting can’t catch. Let’s consider an example. Imagine your solar plant got a bad batch of solar modules. Maybe they were labeled with the incorrect wattage, or micro-fractured during transport. Across the site, efficiency is down 5%. Your commissioning team likely wouldn’t catch the issue, as they’d usually look for issues by comparing the output of one part of the plant to another part of the plant. If production is down equally everywhere, that wouldn’t yield results. Similar issues could occur if a contractor consistently used a smaller than specced conductor on part of the site (both a performance and potentially a safety/reliability issue), or if inverters were not as efficient as their spec.
This is where the only reliable way to assess performance is by considering output in relation to the actual weather.
Additionally, capacity testing often provides a “catch all” for performance issues not otherwise specified. Most capacity tests will be failed if a site has inverter faults, inactive DC strings, monitoring connectivity problems, or other common quality issues. While these issues could be evaluated more simply through a DAS in most cases, including capacity testing in EPC contracts provides insurance that if issues fall through the gaps during commissioning, they will be fixed before the project is considered complete.
How does ASTM Capacity testing work?
The ASTM Capacity Test is actually composed of two different protocols.
The first protocol, ASTM E2939, starts by examining the expected production project production model (PV Syst). PV Syst and other comparable programs incorporate historical weather data for the location of the plant and the actual plant equipment, so by applying a protocol to the PV Syst expected production data, E2939 allows performance engineers to determine what “normal” weather and “normal” output/efficiency is for the given time of year in this particular place with this particular equipment and design.
The second protocol ASTM E2848, begins by examining actual production and weather data from your DAS system. It takes a filtered slice of that actual production data and generates an assessment of
This two part protocol is what makes the ASTM protocols flexible and reliable. Part One (2939) develops a baseline that is reasonable for the project given its equipment and climate. Part Two (2848) accounts for the actual weather during the testing period (which may be better or worse than the predicted weather).
Together they compare the reality of weather and production to a realistic baseline that is unique to the project, given its climate conditions. As we’ll discuss below, using methods developed for projects in sunny California can create complications if commissioning a plant in the winter in cloudy Rhode Island.
Background of Solar Plant Performance Testing - How ASTM Became a Requirement for Most EPC Contracts
Early solar EPC contracts often relied on comparing modeled output (usually with PVSyst) to actual output of the plant over a period of time. However, even over longer time scales, weather can vary significantly in the same location. In some areas of the country, a sunny cool year vs a cloudy warm year can swing PV plant performance 10-20%. As a result, relying on plant output to determine the efficiency of the plant led to frequent disagreements and occasional litigation between asset owners, developers, vendors, and EPCs.
Several new metrics were introduced to attempt to create a fairer system. Performance ratio - a measure of the efficiency of conversion of light to DC power became common (and still is used for O&M troubleshooting), but failed to take into account AC performance factors, including inverter clipping (which became much more common as the price of solar modules fell over the course of the subsequent decades). Availability metrics (percent of the site that is operational over time) were adopted, and are still the backbone of asset management. However, they failed to address more nuanced situations or provide an overall picture of performance.
In the 1990s the National Renewable Energy Laboratory developed a method called “PVUSA” that addressed the elephant in the room - the challenge of incorporating weather into plant assessments. The PVUSA method used a statistical method called a “multivariate regression” to take multiple weather factors into account when determining whether a solar plant was performing well or not.
The PVUSA method, and a related concept used for commissioning known as “performance ratio,” were major innovations for performance testing, but as solar grew, it was found to suffer from a limitation. Most early solar plants in the U.S. were built in the arid southwest and California, where one could expect very clear weather most of the time. As solar spread to the rest of the country, the PVUSA method was found to be less reliable. When applied to more complex climates (snowy, cloudy, etc.), and a new wave of innovations in modules, inverters, and site design, the PVUSA method was found to rely on too many assumptions to apply predictably to the full solar diaspora. Contract disagreements once again became thorny issues for EPCs and asset owners attempting to agree on whether a plant had been built to spec.
Enter ASTM Performance Testing
To plug the gap, the American Society for Testing and Materials published ASTM E2939 and E2848, to create a more flexible and reliable performance testing methodology for solar plants.
The protocol was first published in 2013 after years of testing and development.
While ASTM is not perfect (it unfortunately still depends on accumulating quite a bit of data from reasonably stable weather, is impacted by the accuracy of project weather sensors, requires reasonable implementation of filtering protocols, and can be challenging to understand for people without a performance engineering background), it has become the gold standard for performance engineering of solar sites. If performed correctly by independent third parties, the ASTM methodologies can serve the essential purpose of providing reassurance for solar developers, asset owners, and EPCs. ASTM provides a method for coming to an objective measure whether a plant will be capable of producing the intended and promised results.
If you have a project that needs an ASTM performance assessment conducted soon, check out our “Instant Capacity” same-day capacity test or contact us for questions or bulk/subscription pricing.
