Taking the Measure of Photovoltaic System Output

Led by solar photovoltaics, the market for harvesting energy from renewable sources is booming. At the simplest level, a single residence is equipped with a PV system, generating electricity for its own use, and feeding the excess back to the national grid. At the highest level are utility-scale PV plants and solar farms, contributing to improved grid resiliency and helping governments meet global targets to reduce CO₂ emissions.

Meanwhile, the smart grid concept is emerging as the way forward to provide a much-needed upgrade to national electrical infrastructures. The deployment of smart meters and off-peak tariffs will help utilities meet peak demands without excessively increasing overall capacity. Techniques such as ‘demand response’ and ‘transactive energy’ will help us exploit renewable energy generation effectively.

The keys to the success of the smart grid are visibility, controllability, and two-way communications. Underlying these critical aspects is the need for accurate monitoring of both energy production and consumption, and the gathering of data for analysis.

At the heart of measuring and monitoring, the performance of energy generation and consumption is the current sensor. This article will consider a select number of typical devices to show how they can be used in smart meters and energy-monitoring systems connected to photovoltaic panels.

Allegro offers a range of linear Hall-Effect sensors and integrated current sensors covering a number of applications, including the output side of the PV cell inverter, and for non-intrusive measurements and monitoring alongside smart meters.

Analog Devices, a pioneer in electronic energy metering, offers its CN0241 high-side current-sensing circuit with input overvoltage protection plus an evaluation package.

Finally, the 78M6613, 78M6618, and 78M6631 AC power-monitoring SoC devices from Maxim Integrated provide an alternative approach for single- and three-phase systems, also based on current-sensing technology.

Market growth

According to analyst company Lux Research, the global solar-photovoltaic market is poised to reach $155 billion by 2018 with a CAGR of 10.5%.¹ Supply and demand is coming back into balance following a period of oversupply in 2011, which drove down prices and profits. Demand is increasing steadily, and is particularly strong in developing countries seeking to minimize their reliance on fossil fuels while creating efficient industries.

Growth in the US is dramatic too, with the current solar-system installation rate estimated at one every four minutes, according to another market research company GTM Research.² In addition, with 1 million cumulative installations forecast by 2016, the market will have grown tenfold since 2010.

Meanwhile, GTM Research predicts the global smart grid market exceeding $400 billion by 2020, growing at 8% CAGR. Smart-grid data analytics has been identified as a key and fast growing sector within this market, expected to reach $9.7 billion by 2020. It is recognized as a particularly ‘hot’ technology in Europe. Photovoltaic monitoring providers are subdivided into four categories, independent suppliers of stand-alone products, solar inverter manufacturers, integrated solar system vendors, and industrial and building management system companies.

It has been estimated by a leading European monitoring system vendor, Skytron Energy, that photovoltaic system owners who fail to monitor performance typically lose three to six percent of their potential performance efficiency. Yet the savings made in the rapid detection (and correction) of a failure is said to pay for a monitoring system in less than 2 years.

Any size of installation in the short term will benefit from a monitoring system, if only to detect cell or string failures or loss of efficiency swiftly. Longer term, the gathering and analyzing of data on performance over time and weather variations can provide valuable information on technology choices, maintenance, and equipment degradation, as well as, at the utility level, output predictions that can help planning for future growth.

Monitoring the Farm

The ability to monitor energy being produced as well as being consumed during the day is important to maximize efficiency and minimize costs, whether at the household or solar farm scale. Some level of monitoring has always been incorporated in large-scale photovoltaic installations. However, it is only recently that their potential value has been recognized. Gathering, storing, and analyzing data over a longer timescale are core to identifying true optimized performance figures. Data on failed photovoltaic modules is just as useful as data on efficient systems. Further, photovoltaic panel technology is changing rapidly, making operational data more important to measure efficiency improvements or cost savings.

Monitoring systems for solar farms today are generally highly sophisticated, and typically integrated with SCADA-like plant supervision software platforms and asset management systems. The software is typically updated regularly with new features. Environmental sensors and weather stations are incorporated to provide a record of factors such as irradiation levels, temperature, and hours of sunlight. In fact, weather data has been identified as an important input for performance-ratio planning.

Detailed data from alarm monitoring and error logs can be cross-referenced to subsystem topology and inverter model. Analytical software can make sense of the data to evaluate performance, identify routine and unplanned maintenance issues, and provide models for the future.