Field monitoring of solar installations enables operators to maximize revenue generation and reduce operation and maintenance costs. This application note includes a solar monitoring starter kit that includes LabVIEW software code that you can use as a starting point for building a solar monitoring system using National Instruments Wireless Sensor Network (WSN) and/or NI CompactRIO hardware. In this example application, one or more NI WSN nodes monitor solar irradiance, panel temperature and weather conditions. NI CompactRIO monitors and analyzes AC and DC electrical power and acts as a data aggregator for the NI WSN nodes. A PXI server running LabVIEW DSC acts as the historical data server for the solar monitoring system performing alarming and trending for analysis of system performance and the intelligent dispatching of maintenance crews.

Background and Motivation

Monitoring can help solar operators make sure they are getting the best return on investment from solar power plants. For many operators, the day to day operation and maintenance of a solar farm is relatively new.

Whether you are operating a small installation or a utility scale solar farm, monitoring systems can provide a wealth of information to help you keep your solar energy generation running at peak performance while enabling performance issues to be detected and diagnosed. For example, a common challenge for solar farm operations is the proper scheduling of solar panel cleaning. A solar monitoring system can detect when soiling of the panels— caused by dust, dirt or debris— is reducing electric power generation output. This enables operators to more intelligently schedule cleaning operations.

Since the output power of a solar array is depends on environmental factors such as solar radiation and temperature, an understanding of how system performance correlates with environmental data is needed. With a multi-year set of monitoring data, users can understand trends that affect solar output and efficiency, and they can use this data to optimize solar farm operations and maintenance, such as the panel cleaning schedule.  Historical data can provide information on power output trends.  In summary, monitoring systems for solar power plants can provide a wealth of valuable information that enables operators to better understand and optimize their solar system.

Typical Solar Monitoring Goals

• Diagnose performance issues in the array or inverter (i.e. soiling, incorrect alignment)
• Optimize solar farm operations and maintenance (i.e. panel cleaning schedule)
• Evaluate selection of equipment and installation (i.e. performance compared to name plate ratings)
• Evaluate long term system reliability and MTBF (i.e. long term reliability and performance, inverter failure rate)
• Evaluate solar power plant design (i.e. Actual solar farm performance compared to expected)
• Audit energy production (independent of utility meter)

Sensors and Instrumentation

The sensing and instrumentation components of a typical solar monitoring system are shown below. Solar irradiance sensors enable the monitoring system to measure the amount of sunlight reaching the panels. Monitoring the temperature of the solar array is important since power generation performance also depends on temperature. By comparing the expected power output of the system based to the actually power output, performance metrics such as “array utilization” are calculated. An unexpected drop in array utilization typically indicates a problem that should be evaluated by maintenance crews. Inverter monitoring is another key part of a solar monitoring system to monitor the efficiency, performance and reliability of the solar inverters. All of the measurement data should be logged to a historical data server so the data is logged and records of solar farm operations are available over the course of years so long term performance trends can be evaluated. Solar monitoring systems typically include a thin-client interface available to web browser and smart phone users that provide easy access to current and historical data for the solar farm.

Performance Analysis

Some of the most influential factors that affect solar performance include solar availability, array temperature, array soiling, solar spectral variation, and solar angle of incidence. A solar monitoring system can provide historical data to track these factors and determine the root cause of performance degradations. The data can also be utilized for economic analysis, such as cost per peak watt ($/Wp), final system yield, and performance ratio. Some typical performance analysis measures are listed below.

• System performance metrics: Capacity factor, system efficiency, availability, performance ratio, performance index, array utilization, cost per peak watt ($/Wp), ‘final system yield’ or ‘specific yield’ (kWhac/kWp), reference yield, ‘performance ratio’ (kWhac/kWp) x (1 kWsun/m2) / (kWhsun/m2), ‘performance index’ (kWhac/kWhac-model)
• Typical factors impacting performance: solar availability, array temperature, soiling/shading, solar spectral variation, solar angle-of-incidence effects, inverter efficiency, mounting orientation, tracking accuracy

Demonstration System

The demonstration system shown below was used to develop the attached solar monitoring starter kit. The system includes a solar panel, battery bank, solar charge controller, inverter, load control relays and instrumentation hardware. The system is capable of operating completely off grid using the power from the panels and batteries.

As shown in the image below, National Instruments Wireless Sensor Network (WSN) nodes are used to monitor the solar irradiance, panel and ambient temperatures, and wind speed/direction. The NI CompactRIO embedded control and acquisition system performs high speed waveform monitoring and analysis of the voltage and current from the panel, batteries, loads and the AC microgrid, and includes a relay module for load control. The NI CompactRIO system, running LabVIEW Real-Time and LabVIEW FPGA , aggregates data from the wireless sensor network, performs power metering and quality analysis, and sends the data to an NI PXI chassis. The NI PXI system, running the LabVIEW Datalogging and Supervisory Control Module (LabVIEW DSC), acts as the historical data logger. The LabVIEW DSC Module includes tools for logging data to a networked historical database, real-time and historical trending, managing alarms and events, networking LabVIEW Real-Time targets and OPC devices into one complete system, and adding security to user interfaces.

System Diagram

A complete solar monitoring system will measure the environmental factors necessary for calculating expected solar energy production and monitoring the actual power output of the array.  Below is a block diagram of a typical solar monitoring system, including the sensors, instrumentation nodes, battery charge controller, and inverter.

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The electrical power analyzer system will measure AC and DC voltages and currents and calculate instantaneous power (kW) and total energy production (kW.h). In addition, the system can calculate the power factor and power quality of the AC electricity produced by the inverter. It is also possible to monitoring the inverter health and temperature, characterize inverter efficiency, array utilization, power factor, harmonics, distortion, and the transient response to abnormal grid conditions and utility commands.  In a system that includes batteries, the system can also monitor battery voltage, current, temperature and charge levels. The NI CompactRIO control and acquisition system provides an ideal platform for taking these measurements and performing on-board analysis before sending the data to the historical datalogging server.

Table 1.  Electrical Power Monitoring Measurements

Environmental measurements are also important in a solar monitoring system, as they enable you to determine the expected performance of the system. Key environmental factors include solar irradiance, ambient and panel temperature. The environmental monitoring system may also include wind speed, direction and humidity. The battery powered, mesh networking NI Wireless Sensor Network (WSN) nodes provide an ideal platform for solar environmental monitoring.

Table 2. Solar Environmental Monitoring Measurements

Requirements

Filename: solarmonitoring.zip

Software Requirements

Application Software: LabVIEW Full Development System 2010
Toolkits and Add-Ons: LabVIEW Real-Time Module 2010, LabVIEW Datalogging and Supervisory Control Module 2010, LabVIEW FPGA Module 2010
Language(s): LabVIEW

Hardware Requirements

Hardware Group: CompactRIO
Hardware Model: cRIO-9024
Driver: NI WSN 1.2, NI-RIO 3.5.1