Energetica India Magazine: september 2020

SOLAR POWER Purlin profile height Getting into detail in the magnitude of the torque tube shading, we see that the height of the purlin may vary its value. The purlin is the tracker part that connects the photovoltaic mod- ules to the torque tube and creates a space between them. An in-depth analysis of the purlin height reveals that an in- crease of 10mm implies a reduction on the shading factor val- ue of around just only 0.1%. On the other hand, higher purlin profiles mean a greater non-balance on the tracker, as the modules mass is located further from the rotation axis. This requires a higher structural demand on the torque tube and the motor resistances. Torque tube-induced shading practically vanishes in the ab- sorbing surface in a similar order of magnitude, regardless of the distance to the capturing surface, always considering a realistic purlin height of 40-80mm. The torque tube shade can be assessed as a sum of the differ- ent shades coming from different rays, each of them with dif- ferent solar irradiance intensity (the largest coming obviously from the sunlight area of the ground): Figure 4. Torque tube shade differential calculus In order to avoid the major part of the torque tube shade, and to produce a substantial aid, the purling should be around 15,350mm height, an ideal concept that cannot be implement- ed in real projects. Figure 5. Torque tube shade intensity As for shading distribution, it is observed that as the separation between the shade-generating element and shade-absorbing element increases, shading dissipates more uniformly on the intermediate or central cells, spreading progressively to the adjacent ones. Lower height of the purlin produces a higher concentrated peak on captured irradiance between the different cells of the module. This difference eventually causes an electrical mismatch that produces a reduction in solar production. As a first approach in mismatch magnitude, we see a natural value because of the edge effect of around 1.4% (blue line in the graph be- low) for common albedo values. Adding the torque tube as the shade-creating element, that value is increased to around 2-3% (orange line). Figure 6. Electrical mismatch vs purlin height The existing design of STI Norland includes a purlin at a height of 60 mm, thus allowing common bifacial panels with a 30-mm frame to have a free space of 90 mm between the torque tube and the rear side of the panel. The mismatch value here is about 2.8%. As seen in the distribution, the reduction affects not only the intermediate cells 6 and 7 but also to the adjacent ones 4, 5, 8, and 9.This means that the torque tube shade is spread throughout a large part of the rear side of the module. Leaving a gap in between the two modules in the 2P configura - tion does not avoid that shade. Besides, it implies lowering the view factor of the tracker as the PV surface increases in width but not in height. The shade of the torque tube impacts on the rear side of the module, no matter the gap in between the modules: 28 energetica INDIA- September_2020