Technical Masking in Solar PV — What Is Really Behind the Curtain?

One of the most vexing issues in the solar PV industry today is that of technical masking. Masking is all about accurately identifying technical issues, finding what is behind the curtain, the unseen, what is real but not being measured, and what is taking place that is not being accounted for.

When L. Frank Baum, wrote The Wonderful Wizard of Oz, so fetchingly portrayed in the 1939 film, he could have been writing about the PV industry in the 21st century. He so richly addressed the importance directly connected to the challenges of not having most if not all of the facts. Sometimes those facts and details are right in front of us but we just can’t see them.

So what is PV masking and why is it so important?

PV system masking occurs when single and/or multiple system defects, anomalies, faults or failures are introduced or evolve. They negatively impact the condition and health of a plant to produce energy and/or reduce the capability of the system to produce energy, and are not readily or transparently observed. The issues do not usually present themselves initially in performance data.

They result from a series of specification selections determined during design from choices of over building like clipping and shouldering among many others. They obscured problems, including the actual or unanticipated deterioration of the health or condition of the system and therefore may be generally or specifically unobserved. Masking tends to eventually result in a PV system or plant not being able to meet its contractual requirements over time, thereby risking project default.  Masking is often initially suspected resulting in asking the question, “Where did the missing energy to go?”

Although masking is most commonly associated with overbuilding of plants, a.k.a., high DC to AC ratios, this is not a discussion on how much overbuilding is appropriate. It does focus however, on understanding masking in addressing the issues of how to overcome its impact. It focuses on the criticality of addressing masking issues at specification prior to EPC bidding and design.

The standard today can be paraphrased from the wizard when he boomed, “Pay no attention to the man behind the curtain!” Today masking is generally ignored, although the consequences cannot be ignored for long. In time, the faults, failures, defects and anomalies that are common in plants today become numerous enough to eat up the overbuild that was intended, in many instances, to offset what is assumed to be and considered normal degradation and derating.

With the lack of quality, consistency and granularity in data collection, these issues are undetected until they become catastrophic or near catastrophic. This is a fact of PV life! Today’s modeling, sensing, data collection and awareness are not designed to specifically focus on these anomalies. Generally unseen, they’re usually ignored until they become very uncomfortable expenses.

Partial detection may be seen in the use of EM technologies such as IR and electroluminescence imaging. The challenge, however, is greater than that. It’s about the effective analysis that’s missing.

The expectation today is that with existing O&M practices and performance data, technicians can identify sufficient numbers of issues as they make inspections of all types of plants.

The result is “you can’t get there from here” based on the use of existing commonly used technology and processes alone. Therefore, there is a technological need to focus on new more advanced existing technology that provides that essential granularity and analytical results. More advanced processes and procedures are necessary to identify, characterize, analyze and make substantially greater and more detailed determinations regarding those masked anomalies.

An example of how today’s technology falls a little short is that with land-based or aerial IR camera imagery, hotspots can be identified. However, there is no consistent root cause analysis that comes out of this process and analysis so therefore, for example, identifying a module hotspot does not tell us enough. Nor is there of value given to the impact of those hotspots on a string or the rest of the system BOS. The advanced technology must consistently identify what kind of hot spot it is, what its cause is and which of the 86 potential module defects it can be characterized as. This information is crucial for determining how it will affect present and future performance, O&M and revenue.

It’s not enough to look for four or five different types of defects and miss the other 80 plus, which is the common practice today. This is critical when it comes to making warranty claims where manufacturers require specific detailed and accurate data. That data should come directly and accurately from the field and not be required to be tested in a lab component by component.

Of those 86 module defects, 25 are safety oriented which require that the module be replaced immediately while a number of others are reliability based, and also existing, potential or future warranty claims. Unfortunately, most are never identified as they sit there to deteriorate, grow and fester over time. The same can be said for many other plants defects, all too often ignored simply because they are masked. For all intents and purposes, they may be suspected yet are invisible.

The granularity required must be sufficiently detailed to identify specific modules, cells and defects whether they are of an electrical or structural nature. The process must be automated to become cost effective. This work is taking place in American universities in cooperation with private organizations, yet few companies are taking advantage of this level of technology and savings.

Relying on human technicians to provide this level of analysis is not cost-effective while appearing to be somewhat futile.  Nevertheless, technologies are available today to do that level of analysis. The results are in reducing O&M costs, dramatically raising accuracy, and supplying the level of detail necessary for rapid lower cost warranty claims. When begun as an appropriate benchmarking process at commissioning, these advanced applications dramatically improve plant reliability, availability, maintainability, testability and performance throughout the plant lifecycle.

The impact to the industry will be in dramatically lower expected and unexpected operations and maintenance costs and far more consistently operating and reliable plants that are profitable through their expected lifetimes and beyond. Therefore, don’t ignore what’s behind the curtain.

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John R. Balfour, MEP, PhD, is President and CTO of AstroPower Corp. Dr. Balfour has spent 32 of his 40 years of PV experience as an EPC and has been a PV energy consultant and author since 1977. AstroPower is a consortium of specialized, experienced PV- and technology-related organizations dedicated to making PV projects more efficient and profitable throughout their lifecycle. Contact:

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