Autonomous Solar Tracking on a PLC for CPV in the Built Environment

Abstract

Results and Discussion Solar tracking with 0.1° accuracy under all weather conditions during all seasons poses a challenge for CPV systems in the built environment. Our investigations in a model greenhouse indicate that reflection of windows or outdoor white surfaces make optical sun tracking devices unreliable. These devices also react unpredictably on sudden changes in weather conditions. Our solution for this tracking challenge is using an algorithm which predicts the position of the sun with high accuracy, without using excessive computation time. It is an improvement of the PSA algorithm which allows accurate tracking even when the sun is at low altitude. Validation with VSOP87 [3] shows an average zenith angle error of less than 0.004°, an improvement factor of about 19. Input variables for the improved PSA-algorithm are collected by a GPS device. These variables are: Longitude, latitude and GPS time, which is corrected to Universal Time (UT1). Changing weather conditions are detected by a Class 1 pyranometer, which measures direct and diffuse sunlight. If direct sunlight exceeds 120 W/m 2 , the sun’s position is tracked. In the absence of direct sun light, the CPV system automatically assumes a favorable position for overcast skies [2] is automatically realized (see flow chart in fig. 4). In the flow chart, the criterion for sunrise and sunset is set at a zenith angle of 90.5°, assuming an angular extent of the solar disk of about 0.5°. Finally, we have been able to implement all functionality into a modern PLC system, equipped with stepper motor drivers for variable (low) speed motion. Which runs autonomously, once the software has been deployed. Further investigations include testing our method in practice with a system of Fresnel lenses and triplejunction CPV cells in the model greenhouse at HAN University.