High-Precision Control System Used for Optical Efficiency Measurements of Parabolic Trough Collectors at NREL

The testing facility called the Outdoor Collector Test Loop (OCTL), which is located at the Solar Industrial Mesa Top Area (SIMTA) of National Renewable Energy Lab (NREL), measures the optical efficiency of parabolic trough collectors. It uses a dual-axis, large-payload solar tracker to hold a parabolic trough collector module and track the sun. Due to the growing need for measurement accuracy and efficiency, a new tracking control system for the tracker has been acquired and successfully commissioned as of February 2012. As part of the customization needed to address the unique testing requirements at the OCTL, new tracking modes have been designed and embedded into the new controller. In particular, the incidence angle modifier (IAM) and fixed-azimuth modes allows the OCTL to readily measure the IAM values for a trough collector, significantly improving the speed and efficiency of IAM data collection compared to the previous controller (with test times of days versus weeks). The Siemens S7 1200 PLC integrates various hardware components (such as the hydraulic pump, encoders, sun sensor and wind sensor) through corresponding communication channels, and a Simatic HMI panel provides a powerful user-friendly interface for operation, monitoring, and diagnostics. In addition, NREL integrated the Siemens tracking control program with the existing LabVIEW program that serves as a user interface of the thermal fluid loop, and calibrated the tracking platform as a whole to characterize its tracking accuracy. At last, the challenges and opportunities for the control system in the area of concentrating solar power (CSP) are briefly discussed.