## 2 Materials and Methods
### PV Test Facility at Technical University of Denmark (DTU)  
We compared the outputs from eight bifacial PV modeling tools to 12 months of field measurements (April 2019 to March 2020) made on kilowatt-scale bifacial and monofacial systems located in Roskilde, Denmark (55.6\({}^{\circ}\) N, 12.1\({}^{\circ}\) E). The site includes eight HSATs and eight south-facing fixed-tilt 45-m-long substructures as shown in Figure 1.  
All 16 substructures (including the south facing units) are SF7s (Soltec, Madrid, Spain) with 2 modules in portrait. The bifacial substructures feature a torque tube and mounting rails that do not directly cover the backside of the PV modules (Figure 2a). The inclination of the south-facing structures is adjustable from 0\({}^{\circ}\) to 60\({}^{\circ}\), but the tilt was programmed to remain at 25\({}^{\circ}\) during the 12-month period studied here. On each substructure, 88 PV modules are mounted, wired into four series strings (i.e., 22 panels per string). The panels on each substructure are either 60 cell monofacial with a 305-W\({}_{\rm p}\) rating, or 60 cell bifacial with a 295-W\({}_{\rm p}\) front side rating. The cell technology in both module types is passivated emitter and rear contact (PERC). The four strings on each substructure are  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**PV Simulation** & \begin{tabular}{c} **Sun** \\ **Position** \\ **Algorithm** \\ \end{tabular} & **Irradiance** & \begin{tabular}{c} **G\({}_{\text{POA,Rear}}\)** \\ **Method** \\ \end{tabular} & \begin{tabular}{c} **IAM** \\ **Model** \\ \end{tabular} & \begin{tabular}{c} **1-Diode** \\ **Electrical** \\ **Model** \\ \end{tabular} & **Thermal** & **Ref.** \\ \hline bifacialvf (0.1.7) & Michalsky & DNI, DHI & 2-D VF & Air-glass 1 & N.A. & N.A. & [8] \\ MoBiDiG VF & NREL SPA & DNI, DHI & 2-D VF & Air-glass 1 & 5 param. & \begin{tabular}{c} 5 param. \\ [PERSON] \\ \end{tabular} & \begin{tabular}{c} Faiman \\ \end{tabular} & [14] \\ MoBiDiG & Michalsky & DNI, DHI & 3-D RT & N.A. & \begin{tabular}{c} 5 param. \\ De Soto \\ \end{tabular} & \begin{tabular}{c} Faiman \\ \end{tabular} & [29,30] \\ PlantPredict & & & & & & \\ (8.7.0) & NREL SPA & GHI, DHI & 2-D VF & ASHRAE & PVsyst & Faiman & [8,31] \\ pvfactors (1.4.1) & NREL SPA & DNI, DHI & 2-D VF & Sandia & N.A. & N.A. & [9] \\ PVsyst (7.0.5) & US Navy & GHI, DHI & 2-D VF & Air-glass 1 & PVsyst & Faiman & [32] \\ System Advisor & & & & & & & \\ Model & Michalsky & DNI, DHI & 2-D VF & Air-glass 1 & 6 param. & \begin{tabular}{c} 6 param. \\ De Soto \\ \end{tabular} & \
\begin{tabular}{c} NOCT \\ \end{tabular} & [33] \\ SolarFarmer & (1.0.187.0) & NREL SPA & GHI, DHI & 2-D VF &
\begin{tabular}{c} Martin \& \\ [PERSON] \\ \end{tabular} & PVsyst & Faiman & [34,35] \\ \hline \hline \end{tabular}  
* IAM losses calculated using a “slab” approach similar or identical to that described in [36].  
\end{table}
Table 1: Descriptions of the bifacial performance tools compared in this study. All tools implement the Perez transposition model for calculating global irradiance on the frontside plane of array.  
kept at their maximum power point (P\({}_{\text{MP}}\)) by a single maximum power point tracker. The ratio of direct current (DC) to alternating current (AC) capacity is 1.04:1, which is substantially lower than what is found in typical utility-scale PV installations. The advantage of this comparatively low DC:AC ratio is that no inverter clipping is observed and that the DC performance at full solar irradiance can be studied. The ground cover ratio (GCR) is 0.28 for the HSAT field and 0.40 for the field of fixed-tilt field systems, both of which are slightly lower than in typical utility-scale systems with limited land area. The irradiance sensors are cleaned approximately on a weekly basis. Although the PV arrays are never cleaned--other than by rainfall events--our soiling measurements made at a 50 MW utility-scale PV site in Hanstholm, Denmark show very low (<0.25%) soiling ratios. Therefore, 0.25% soiling losses are applied in the simulations.