Developing an Outdoor Research Facility for Photovoltaics

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"The performance of the PVORF depends critically on its software control and data acquisition systems, which were developed by EnvisEng using the real-time NI PXI and CompactRIO platforms."

- Karina Taylor, EnvisEng

The Challenge:
To improve on existing models for predicting the energy output of a photovoltaic (PV) solar power plant to better forecast future revenue and provide data that improves the bankability of PV, particularly in Australia.

The Solution:
CSIRO constructed a PV outdoor research facility (PVORF) at its Energy Centre in Newcastle, New South Wales. The facility hosts up to 110 (currently 60) solar PV modules supplying power to the site and allowing current-voltage (I-V) testing on demand, with concurrent high-end monitoring of weather and solar conditions, including the solar spectrum. All results are stored to a database for advanced statistical extraction. The performance and value of the PVORF depends critically on its software control and data acquisition systems, which were developed by EnvisEng using real-time NI PXI and CompactRIO platforms.

Author(s):
Karina Taylor - EnvisEng
Chris Fell - CSIRO

A major barrier to the wider implementation of PV is that, compared to other electricity sources, the investment is incurred primarily up front. This means the return on investment occurs over time, which makes understanding the performance and durability of the system critical. Like many renewable energy sources, both the performance and durability of a PV system depend in a non‑trivial way on the environmental conditions at the point of use, hence, for financing purposes, it is important that these dependencies are well understood.

In a project funded by the Australian Renewable Energy Agency (ARENA), CSIRO and its partners are studying the way solar PV modules respond to typical variations in solar conditions such as temperature, irradiance, solar angle and the solar spectrum. The results will feed into models that link the manufacturer’s power rating to the energy the panels generate over time, and will also allow precise measurement of any degradation in the modules. Research outcomes will form the basis for more accurate revenue projections to industry and finance institutions; the aim is to encourage investment and widespread deployment of large-scale PV power stations. The project will also help develop Australian and international standards for predicting the performance of solar PV power systems.

CSIRO approached EnvisEng, a NI Alliance Partner with a proven track record in scientific monitoring applications for CSIRO, to design and develop a LabVIEW-based control and data acquisition system for the PVORF. Requirements included:

  • Providing reliable switching of up to 110 PV modules individually between an AC subsystem for extraction power and a DC subsystem for measuring I-V curves
  • Using an advanced algorithm for scheduling I-V testing on an arbitrary timebase for each PV module in the field
  • Controlling I-V testing hardware
  • Logging of the back-surface temperature for every PV module in the field
  • Allowing the system to “sleep” between certain configurable hours, in which weather data is still being acquired and displayed but not logged, and panels are not being measured at all
  • Adding a “test now” facility, that allows one module to be tested in rapid repeat, for performing specialised measurements such as to determine temperature coefficients
  • Acquiring and analysing data from approximately 11 instruments for continual monitoring of solar and weather conditions
  • Logging all measurement data to an SQL database
  • Providing the ability for scientists to configure all individual sample rates, scaling factors, data limits, and channel connections, and to add and remove PV modules to the schedule without stopping the flow of existing experiments
  • Offering remote access to PVORF system configuration screens through password-protected web pages, and remote access to PVORF real-time data results through web pages aesthetically suitable for publication and marketing
  • Ensuring time-synchronisation of all system hardware
  • Sending emails to configured users upon error or configuration changes;
  • Operating 24 hours per day, seven days per week with minimal down time for maintenance and updates.

Figure 1. CSIRO’s PVORF Showing the Location of PV Module Racks and the Solar Measurement Station


We used LabVIEW running on a NI PXIe-8101 RT embedded controller to control the scheduling, switching, and measurement of PV panels (via the GPIB I-V curve tracer), based on a web configuration page that scientists use to enter individual PV panel experiment parameters. Scientists enter a PV panel field location, serial number and model information, experiment frequency, and sweep parameters (measurement type: sweep, short circuit current or open circuit voltage, number of points, start voltage, and stop voltage) for each panel on the field, and the LabVIEW software and PXI system take care of the scheduling, switching, and all measurements.

Figure 2. Experiment Configuration Web Interface

For each PV module in the list, the PXI system determines when it next needs to be measured to meet the configured measurement frequency, and adds it to the measurement queue. For each measurement, the PXI switches the module from the grid to the I-V curve tracer, performs the configured I-V measurements, takes a temperature reading from a sensor located on the rear of the PV panel, and sends measurement and time data to the Windows-based server for display and logging purposes.

Figure 3. Snapshot of live I-V Curve Screen From PVORF Showing Recent and Upcoming Modules for Testing

The PVORF also includes a high-end solar ground station located on the Southern edge of the field. The ground station continually monitors and records solar and weather conditions including:

    • Solar irradiance (x4 in plane-of-array and x3 horizontal
    • Solar spectrum (350 nm to 1700 nm) in plane-of-array
    • Air temperature
    • Humidity
    • Wind direction and speed
    • Rainfall
    • Camera for sky imaging (1 horizontal and 1 in plane‑of‑array)

We chose the NI 9148 expansion chassis for its Ethernet connectivity, ruggedness, and high operating temperature range. We mounted it on the solar ground station to acquire data from all instruments and relay them back to the PXI in the control room.

We took the solar irradiance measurements from pyranometers. They required low-voltage yet high-accuracy inputs, as well as thermistor and ventilator rotation speed inputs for each instrument. We used the NI 9214 high-accuracy thermocouple module as a raw voltage input module for voltages up to 80 mV, in conjunction with the NI 9205 analogue input module and NI 9265 analogue output module for thermistor inputs, and the NI 9401 high-speed bidirectional digital I/O module for ventilator rotation speed. We also used the NI 9476 digital output module to control power to the heaters and fans in the pyranometers to keep the instruments measuring accurately.

Unlike most ground stations associated with PV test fields, the CSIRO PVORF ground station includes a second spectroradiometer, allowing measurement of the solar spectrum up to a wavelength of 1700 nm. To do this, the solar spectrum needed to be acquired from two serial instruments and stitched together. We used an NI 9870 serial module in the NI 9148 CompactRIO chassis to acquire data from the two solar spectrum measurement devices.

The air temperature, humidity, wind direction, wind speed, and rainfall data are measured using a Vaisala WXT-520 weather station with SDI-12 connectivity. We used the Waterlog H-4191 RS232 to SDI-12 Converter and the LabVIEW SDI-12 API from the NI Instrument Driver Network, as described in the Building an Environmental Monitoring System for SDI-12 Devices white paper, to acquire environmental data through the NI 9148. Finally, we connected the sky cam to the NI 9148 via Ethernet, and used HTTP commands to acquire string images that were sent to the PXI for conversion to .jpg images.

All PV module experiment data and ground station measurement results are collated on the real-time PXI, then sent to a Windows server running a LabVIEW executable that captures incoming data and displays it on various web pages for live data viewing, as well as historical data viewing of environmental conditions. The Windows server also saves all data to a local MySQL database for experimental post-processing. 

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Figure 4. Solar Ground Station Live Data Web Interface

The PVORF was commissioned at the CSIRO Energy Centre in December 2013. Currently, the system is measuring 60 PV modules from 17 manufacturers and captures 12 different types of PV device. Over the next 20 years the system is expected to underpin research that will significantly improve the understanding of performance and durability for both commercial and research PV devices, particularly with respect to Australian climate conditions.

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Figure 5: Images of the PVORF Field

Author Information:
Karina Taylor
EnvisEng
karina@enviseng.com.au

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