Commercial PV System Data Monitoring, Part One

Project owners, developers, contractors and financers all have a significant monetary stake in PV system performance. Without accurate and reliable data monitoring, evaluating and maintaining optimal system performance is just a guessing game.

For years, PV system data monitoring was mainly an afterthought, used primarily as an educational tool or for publicity value. However, data monitoring is now required for utility-scale PV systems, where it is used to track performance and comply with regulatory reporting requirements, and increasingly used in commercial applications. Despite happening in fits and starts, the research and development of commercial PV monitoring systems is resulting in more creative and ingenious solutions that streamline system integration and installation. At the same time, the variety of solutions and providers is increasing.

These coincident trends offer an opportunity and a challenge. PV system integrators and installers are used to developing and implementing code-compliant electrical and mechanical designs. However, mastering the myriad variables that an IT component adds—with its new terminology, hardware, architectures and configurations—can prove challenging. In addition, while the cost for specific PV monitoring services is declining, the total price to monitor large PV systems may actually be increasing as more functionality is expected from data monitoring systems as a whole.

Working in PV project development and construction, we have seen firsthand the difficulties developers and integrators encounter when data monitoring systems are not specified until near project completion. To control the associated component and installation costs and optimize system reliability, data monitoring must be given a seat at the design table.

In this two-part article, we describe the value proposition of monitoring commercial PV systems. In Part One, we consider data monitoring options, components and selection criteria, as well as the pros and cons of various levels of monitoring granularity. We explain how monitoring networks transfer information from one place to another. In Part Two, we will examine site-specific considerations, potential design and hardware responses, and provide a cost analysis case study. Throughout, we reference best practices and common mistakes.

Value Proposition

Because monitoring systems provide a remote visual representation of PV system performance, they are valuable tools for system owners, investors, installers and operators. One way to ensure that PV systems are operating optimally is to physically go on-site to take instantaneous irradiance, cell temperature and inverter output power measurements. You can then calculate whether the actual system output power acceptably approximates the expected output power. (See “PV System Commissioning,” October/November, 2009, SolarPro magazine.) Alternatively, you can specify a monitoring system to continuously provide the desired level of performance assurance.

Over the past few years, a major shift has taken place in the way PV systems in North America are evaluated. The standard of evaluation used to be based on capacity (kW or MW), meaning either the installed dc nameplate-rated power (kWSTC) or the capacity at PVUSA test conditions (kWPTC). Now the emphasis has moved to ac energy production (kWh or MWh) or specific yield (kWh/kW or MWh/MW). According to Bill Reaugh, VP of project development at Draker Laboratories, a data monitoring hardware and services provider, “The PV industry in the US and Canada has moved from simply trying to install the most capacity possible to trying to get the best energy harvest, because we are catching up to Europe both in operating practice and incentive structure.”

The goal of plant production estimates is to calculate expected energy production or specific yield for a PV system as accurately as possible by modeling component performance across a range of operating and environmental conditions. However, system downtime is potentially the greatest loss factor for total system performance. Even if you have the highest efficiency modules mounted on the most sophisticated tracking system available, money is lost every minute that the sun is shining and the system is out of service or operating suboptimally, for whatever reason.

In the experience of Thomas Tansy, VP of business development at Fat Spaniel Technologies, a monitoring company recently acquired by inverter manufacturer Power-One, PV plant monitoring is a given in utility- and industrial-scale applications. “In this context, data monitoring has the potential to generate the highest return on investment and is required 100% of the time,” he explains.

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