After years of research, researchers at the National Renewable Energy Laboratory (NREL) collected a dataset of 25,000 PV inverters from nearly 2,500 utility-scale companies located in 37 U.S. states and territories. This is the PV Plant Performance Data Program, a more than $5 million project to get a clearer picture of the condition of PV plants in the U.S. and determine how external environmental factors affect the performance and aging of solar panels.
The PV Fleet project collected a large amount of data from PV systems in different states in North America to identify possible causes of production losses. Photo: Werner Slocum, NREL.
Photovoltaic (PV) systems are designed to last 20, 30, or even 50 years. However, small losses in energy production can make a significant difference over its lifetime. These differences can even determine whether the system is profitable or loss-making. However, small changes in energy production are difficult to measure.
While many studies have examined the on-site degradation rate of PV modules, the program brings together data from many large PV installations to create a benchmark for the cumulative performance and degradation rate of PV units in the United States.
The performance of all solar panels is expected to degrade over time due to exposure to the elements. However, a number of factors have led to the degradation and average rate of PV power generation.
Scientists have quantified the effect of temperature on the performance and deterioration of photovoltaic systems. Map: NREL.
In 2022, the PV Fleet team found in its first significant finding that the national average yield loss was 0.75% per annum, confirming similar values previously reported in studies analysing smaller data sets. In addition, the new analysis found that system yield losses in warmer regions were about twice as high as in colder regions.
The researchers noted that this average performance loss is a crucial figure, as it shows that the PV system in the U.S. is not failing critically overall, but is degrading at a modest rate within the expected range. However, the researchers note that it is important to quantify this ratio as accurately as possible, as this figure is used in almost all financing agreements for solar projects and provides guidance to the industry.
Effect of meteorological phenomena on solar panels
Extreme weather events (floods, high winds, hail, wildfires, and lightning) can cause damage to PV systems, which can undoubtedly lead to long-term performance losses.
In a scientific paper published in the IEEE Journal of Photovoltaics, the researchers quantified the impact of extreme weather on solar panels, comparing the performance of PV Fleet's PV system dataset with the National Oceanic and Atmospheric Administration's (NOAA) map of extreme weather events. and an interactive map of the Federal Emergency Management Agency's (FEMA) Country Risk Index, which allows you to see hail risk in different states. The researchers looked at how the performance of each solar installation was affected when extreme weather events occurred within ten kilometers of their location.
The researchers recorded severe weather phenomena less than ten kilometers away from the photovoltaic installation, but in most cases, the damage was minimal. Figure: NREL.
They determined that the short-term impact of extreme weather events would be minimal for most systems. In general, short-term power outages caused by extreme weather conditions, such as modules disturbed by high winds or inverters damaged by flooding, have little impact on most solar systems. Between 2008-2022, the average duration of disruption after extreme weather events was 2 to 4 days, resulting in an average loss of only 1% of annual production. Out of 6,400 systems, only 12 experienced longer outages, i.e., two weeks or more.
Most outages are due to flooding and rainfall, followed by wind-related weather conditions. Most PV systems have experienced only one weather-related outage.
The climate is accelerating the long-term degradation of photovoltaic systems
Short-term disruptions and loss of production are not the only effects of extreme weather. Wind, hail, and snow can cause cracks in PV cells and other forms of PV system degradation.
PV Fleet has found a clear trend in the long-term performance of PV installations after exposure to extreme weather events. These systems show greater annual production losses after weather events that exceed a certain threshold, hail with a diameter greater than 25 mm, wind speeds greater than 90 kilometers per hour, or snow depths greater than 1 meter. Below these thresholds, the performance loss of these systems is similar to that of a normal PV system.
The yield loss rate of the PV system studied is less than 2% per year. Figure: NREL.
Even systems consisting of modules certified to International Electrotechnical Certification (IEC) 61215, including a 25 mm diameter hail impact test, have a higher rate of loss of performance when hail of the same size is exposed to the elements. This indicates the need for more rigorous hail testing.
Best practices for tackling climate impacts
PV Fleet's researchers believe that the analysis of the data collected does not indicate that PV systems are unreliable or particularly vulnerable to extreme weather events. However, they point to a number of measures that can be taken to improve equipment quality, particularly installation best practices, to improve resilience to weather events.
The Federal Emergency Management Agency's (FEMA) Country Risk Index provides an interactive map for looking at the risk of future hail in the United States, a tool used by PV fleet experts. Photo: Federal Emergency Management Agency (FEMA).
First, they point out that in order to strengthen PV systems against extreme weather, module manufacturers and PV testing organizations need to understand the thresholds at which damage can occur. Once thresholds for potential damage have been identified, the industry can begin designing for those conditions and, more importantly, creating tests that expose solar panels to real-world stress. While the recently developed new hail test is a big step in that direction, researchers say more stringent testing standards for wind and snow loads should also be considered.
Secondly, experts from the PV fleet project point out that high-quality installations are also key to improving weather adaptability. They point out that standardized installation practices, such as the use of straight-through bolts and brackets far enough from the edge of the roof in windy areas, can help mitigate the impact on system performance.
PV Fleet's researchers point out that PV module manufacturers need to test their panels more rigorously to prevent potential weather damage. Photo: U.S. General Services Administration.
In addition, PV system operators should also know the threshold at which PV panels can withstand extreme weather conditions so that they know when to further analyze the affected system. Compiled by Chen Jiaoyun
Finally, the researchers emphasize that the industry needs to be aware that recent trends such as larger modules, thinner cells, and thinner front glass can increase the vulnerability of systems if not properly designed and tested. Coordinating operations and maintenance records, as well as monitoring of PV assets, will be able to proactively detect potential degradation caused by new module designs, experts say.
For NREL researchers, the next step is to develop a device that uses photoluminescence to analyze damage to solar panels. The device, known as PLatypus, illuminates the solar cell and then emits light back to the device's camera. A damaged battery emits a less bright and fast light, which indicates the condition of the solar panel.