## 3 Results
### Bifacial Gain  
Figure 11 shows the monthly bifacial gain from five software that are capable of simulating electrical performance. The black lines in each plot show the monthly results from the DC string measurements. The error bars show the inner quartile range of daily measured bifacial gains observed in each month. The daily bifacial gains in winter fluctuate greatly because the daily energy production in winter can be an order of magnitude less than in summer months: This variability is illustrated with the wider error bars in winter. Recall that the measured results are normalized with the I-V measurements made at DTU per Equation (1). If the normalization was instead made using the manufacturer's nameplate rating, the measured bifacial gain according to Equation (1) would be 1.5% higher than what is shown. This difference can significantly affect the economics of project decisions that are made based on an expected bifacial energy gain. The measured monthly bifacial gain on the fixed-tilt system is between 4.3% and 7.3% from March to October. Meanwhile, the bifacial gain on the tracker is consistently higher, between 6.6% and 8.5% during the same months. The higher bifacial gains observed on the HSAT are likely due to the wider 12-m spacing between rows (GCR = 0.28) versus the narrower 7.6-m spacing on the fixed-tilt system (GCR = 0.4), which creates more self-shading within the inner rows of the fixed-tilt field. From November to February, the measured bifacial gain on the two system types shows opposing trends, wherein the HSAT system shows an increase and the fixed-tilt system shows a decrease. These months were characterized consistently as cloudy skies with mean monthly diffuse fractions between 88% and 94%. The works of other authors have demonstrated that bifacial gain will increase as the fraction of diffuse light increases [64], which can explain the higher bifacial gain on the HSAT system in winter. It also been shown that bifacial PV systems require higher tilt angles to capture the benefit of such diffuse conditions [65]. This can explain why the simulations and the measurements in winter show lower bifacial gains on the fixed-tilt system (25\({}^{\circ}\)) than the HSAT system (\(\pm\)60\({}^{\circ}\)). Figure 10: (**a**) Variability plot showing the mean absolute error of DC power for the four PV system types as simulated by six software. The chart displays unweighted error and error weighted by the inverse of the uncertainty of the solar radiation measurements. (**b**) Variability plot showing the mean bias error of DC power. Only the unweighted error is shown in these plots. The dashed orange lines show the mean error of the bifacial and monofacial arrays. The simulated bifacial gain values largely follow the trends of the measurements, but the most notable exception is the fixed-tilt system in winter (November through February). This discrepancy is likely due to the significant amount of inter-row shading on the fixed-tilt system from November to February. The fixed-tilt system has a shade angle (at solar noon) of 16\({}^{\circ}\), but on the winter solstice, the sun elevation peaks at 12\({}^{\circ}\). Therefore, the fixed-tilt rows are partially shaded at practically all times in December and January. In fact, negative bifacial gains were measured in November and December on the fixed-tilt systems. Such an incongruuous result points toward issues in the measurements rather than the shade-loss models. A visual inspection on a clear winter day near solar noon confirmed that there was more inter-row shading on the bifacial arrays than on the monofacial arrays. This is attributed primarily to the torque tube gap on the bifacial arrays, and the lack of such a gap on the monofacial arrays. The torque tube gap on the bifacial arrays (Figure 2a) places the bifacial modules approximately 5 cm higher than modules on the monofacial arrays that completely cover the torque  
Figure 11: Monthly bifacial gain on fixed-tilt and HSAT systems calculated with Equation (1).