As the PV solar industry matures, greater emphasis will be placed on improving the energy production and reliability of photovoltaic (PV) arrays, improving ROI and reducing financial risk. This trend is already apparent as panel and inverter manufacturers tout expanded warranties, operations and maintenance (O&M) teams sign tighter contracts assuring minimum power production guarantees and banks insist on improved system monitoring as a condition of financing.

A key hurdle to improving array lifetime performance is the lack of real understanding of how arrays deteriorate over time and the lack of detailed and accurate insight into array impairments. Energy production starts with the PV panel, but until recently, it has been difficult to detect all but the most obvious panel-level failures. However, as a result of distributed electronics, such as microinverters and optimizers, making their way into the industry, new data is available that suggests that on average, 0.6 percent of panels fail each year. The compounding effect of these failures results in a system that loses more than four percent of its energy production capability throughout its life. Contrast this with claims from major site financial backers that a one percent gain in annual output translates to a 10 percent gain in total project ROI.

Panel-Level Insight

Panel-level monitoring offers insight into array performance with unprecedented granularity. This article aims to show that, although this comes with increased upfront costs, the increased energy production can significantly offset these additional costs due to the identification and replacement of failed panels.

A panel-level monitoring system measures both current and voltage for every panel in an array with great precision (within one half percent) to determine overall system, string and individual panel performance. The resulting data enables intelligent software tools to identify array impairments and recommend remedial action, such as panel replacement. The unique benefit of such a system is the ability to distinguish failed panels from other impairments, such as soiling or wiring faults. By comparison, string and inverter monitoring systems lack accuracy (typically to within five percent) and have no insight to panel performance and so are typically only able to inform the O&M team that energy production is slightly lower than expected. Without panel-level data, these systems are unable to diagnose the root cause or suggest a resolution plan.

By deploying a panel-level monitoring system, owners and maintainers can identify any panel impairments remotely. In the example outlined below, by replacing panels that had impairments during annual maintenance, a 100kW array in San Diego, CA would generate an additional $14,900 worth of energy, a value greater than the combined cost of the panel-level monitoring system and the labor to replace the panels. Thus, the panel-level monitoring solution would completely pay for itself from this one benefit alone. Its ability to monitor cleaning requirements, wiring and fuse faults, damage, theft, zonal shade encroachment, and long term panel degradation are all financial upside.

Typical Maintenance Schedules

Most PV systems have regularly scheduled maintenance that is conducted on an annual basis. These yearly checkups vary based on the array, but typically include cleaning, preventative inverter maintenance, and a variety of minor repairs. However, panel replacements are rarely completed during these routine maintenance events because of the high associated costs to locate and identify failures.

For example, a 100kW array with a central inverter system would require more than $700 worth of labor to perform the measurements to identify the locations of any failed panels. These would have to be removed and sent for testing to verify any suspected fault, taking any affected strings out of the array during that time, unless site owners purchased spare panels.  If not, a further trip to the site would be required to install the replacement panels after the manufacturer ships them. In these circumstances, the additional energy generated by the replaced panels rarely outweighs the upfront cost of labor and any lost power due to downtime.

Panel-Level Advantages

By contrast, a panel-level monitoring system significantly changes the economics of this situation. With the ability to determine panel-level performance remotely, O&M teams are able to identify failed panels with accuracy and certainty, then request replacements, potentially under warranty, prior to conducting a maintenance visit. The marginal cost to replace failed panels while the O&M team is already on site drops to $60-$75 per panel (approximately one-tenth the cost of the method previously described).

Figure 1 illustrates the effect of panel-level failures. In a 100kW system, the energy losses amount to a value of $14,900 over the life of the system. By comparison, the yearly expected value of energy output from such a system is approximately $35,000 when the array is fully functional.

Figure 1: Cumulative Value of Energy Lost Due to Panel Failures

Figure 2 shows that the energy that can be recovered by resolving panel-level failures is more than 40 percent of an entire year’s worth of energy production, and is greater than the combined cost of a monitoring system and labor associated with panel replacements.

Figure 2: Associated Costs and Benefits

Panel Degradation

In addition to detecting catastrophic failures (such as permanently snapped diodes), it should be noted that other more subtle issues can also be identified by using the accuracy and granularity that comes with panel-level monitoring.

As solar panels are quickly becoming commoditized, panel manufacturers are attempting to use panel degradation warranties to differentiate themselves from the competition. For example, several manufacturers now warrant a linear degradation of less than 0.7 percent per year that can be enforced in any year of life.

The problem for owners and O&M teams is that these degradation warranties are virtually impossible to enforce without deconstructing the entire array and conducting extensive lab tests on every panel. Despite these limitations, Figure 3 suggests that while on average degradation rates are lower than 1 percent, a significant percentage of panels have substantially higher degradation rates and can have a disproportionate impact on array performance.

Figure 3: Histogram of published degradation rates ¹

As stated earlier, panel monitors can measure panel voltage, current and power output to within 0.5 percent constantly throughout the life of the array. By using this accuracy of measured data for every panel in a large scale array, intelligent Cloud-based analysis tools can isolate and track individual panel performance, normalized for irradiance and temperature, and adjusted for known soiling and shading conditions. This enables a very accurate real-time panel degradation monitor, with the ability to produce substantial qualifying data, for presentation and review in any warranty claim negotiation. Individual panel IV curves can be created for suspect panels remotely, without having to visit the site, disassemble panels, or disrupt energy production.

Thus, widespread panel-level monitoring will create a fundamental change in the industry by allowing owners to enforce both early life manufacturing and degradation warranties and give high quality panel manufacturers that embrace this new technology,  the opportunity to substantiate their claims and differentiate themselves from less capable competitors.

Conclusion

Panel-level monitoring is a new and valuable weapon for array owners and O&M companies to improve financial performance and reduce risk in owning and operating large-scale solar arrays. The accuracy and insight from this new technology, coupled with intelligent Cloud-based analysis and diagnostics, bring many opportunities to improve energy output and improve efficiency in O&M activities. Furthermore, from a brief analysis of just one of the benefits of deploying such technology, that of identifying and replacing defective panels, these systems pay for themselves very rapidly and provide many additional operational benefits that can improve ROI.

Assumptions

The examples above were based on a model with the following assumptions.

Pane-Level Failures

The percentage of panel-level failures used in this model is 0.6 percent per year.² The majority of panel-level failures are the result of a permanently snapped diode. In this model, a snapped diode reduced the energy output of a panel to 66.7 percent. Panel-level failures that were not snapped diodes were assumed to affect the entire panel and accounted for 20 percent of all panel-level failures. In addition, the effect of mismatches was also accounted for in this model.

Labor Rate

A rate of $80.00 per hour was used based on average electrician rates in the United States.³

Time for Circuit Check

The typical amount of time necessary to detect panel-level failures, by conducting an IV trace curve measurement, is between 5-10 minutes. A value of 7.5 minutes was used in this example.

Monitoring Unit Costs

A value of $0.10 per Watt was used based on typical pricing for Solar Power Technologies Inc.'s Clarity^TM Panel Monitoring system with a monitoring unit installed on each panel. This value pertains specifically to arrays that use crystalline type solar panels. This example also assumes a new installation would qualify for a 34 percent rebate from the combined federal and state incentives.

Electricity Prices

For this model a value of $0.18 per kWh was used for small - medium commercial electricity prices for San Diego.⁴

About the Author

Ray Burgess joined the Solar Power Technologies team as President and CEO in July 2009. He has over 30 years of leadership experience in the technology industry, spanning semiconductors, software and micro-mechanical systems.  Ray can be reached at: connecting@spowertech.com.

About Solar Power Technologies, Inc.

Based in Austin, Texas, Solar Power Technologies, Inc. was founded in 2009 and is focused on helping owners and developers maximize the financial performance of large-scale solar assets. The company provides a range of compatible monitoring and optimization hardware products, supported by a suite of web-based data management and analysis tools. The unprecedented level of insight provided by the Clarity™ hardware technology, coupled with the sophisticated data analysis and diagnostics of the Intelligent Array™ software system, enables next generation solutions for the operations and maintenance of large scale solar arrays. For more information on our solutions, visit http://www.spowertech.com.

The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag

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