The Optimal Tilt Angle of Monofacial and Bifacial Modules on Single-Axis Trackers

The optimal tilt
β
_opt
of the modules in a single-axis tracker is often determined by assuming all sunlight is direct and ground reflectance is zero. Prior works, however, have demonstrated that
β
_opt
is smaller when diffuse light is significant. In this article, we determine how
β
_opt
decreases as the conditions change from clear sky to overcast, accounting for many complicated effects like row-to-row shading of diffuse light, ground reflection, structural shading, cell-to-cell mismatch, and angular and spectral dependencies. We find that when compared to monofacial systems, bifacial systems tend to have a higher
β
_opt
when it is cloudier and a lower
β
_opt
when it is sunnier. We also quantify the increase in annual yield that arises when accounting for indirect light at three example sites with different climates, finding it to be ∼30% lower for a bifacial system than a monofacial system; the gain was 0.8%–1.5% for bifacial systems and 1.1%–2.1% for monofacial systems, where the highest gain was attained in the cloudiest climate.