# Spectrally-Selective Energy-Harvesting Solar Windows for Public Infrastructure Applications
## 2 Design Features and Performance Characteristics of Transparent Solar Windows
### Summary of Solar Windows Performance and Comparisons with Theory Limits  
Two similar factory-assembled models of large-area (\(\approx\)0.85 m\({}^{2}\)) installation-ready framed solar windows were fabricated at Qingdao Rocky Ltd. (Qingdao, China) in small test-run production batches between 2016 and 2017. Their performance characteristics varied slightly between window types and from sample to sample. However, the electric power outputs per window started to approach about  
Figure 4: Measurements of the excitation (**a**) and emission (**b**) spectra of epoxy-based lamination interlayers containing suspended particles of ZnS:(Ag, Tm) luminophore. A Perkin-Elmer LS-50B luminescence spectrometer was used to collect and analyze the optical radiation flux propagating out of an edge area of 50 \(\times\) 50 \(\times\) 3 mm\({}^{3}\) concentrator samples. 20 W in early samples and was produced industrially. Figure 5 shows solar windows during outdoor testing experiments (conducted in Perth, Australia, in May 2017) and the I-V curve measurement results. The measurements were conducted using PROVA 200A Solar Module Analyzer. The windows demonstrated their maximum electric power output when placed into a sun-facing near-vertical position (with the glass plane normally aligned with the Sun azimuth direction) with a small (\(\approx\)25\({}^{\circ}\)) tilt backwards away from the vertical plane. The measurement results obtained were consistent with the expected weather-dependent autumn performance and the observed orientation-angle dependency of output power (which is yet to be characterized in detail and reported elsewhere) was not significant. Rather large angular orientation changes applied to windows (e.g., when rotated in the horizontal plane, up to 30\({}^{\circ}\) to 40\({}^{\circ}\) in either direction away from the optimum orientation) resulted in less than 20% of I\({}_{\text{sc}}\) variations. Similarly, small I\({}_{\text{sc}}\) variations were observed when tilting the windows either towards the vertical sun-facing orientation or further backwards by applying an additional 5\({}^{\circ}\) to 10\({}^{\circ}\) downward tilt. The measurements used to generate the data of Figure 5b were performed on a hazy autumn day in Perth with low UV index and solar irradiation intensity not exceeding 800 W/m\({}^{2}\) with sun-facing samples slightly tilted backwards from a vertical position. The window design features three parallel-connected solar-module (CIS) subsystems with each using stripe-shaped Avancis PowerMax 3.5 CIS circuit module cut-outs. The three subsystems include the edge-mounted modules, the backside perimeter-mounted modules, and external frame-mounted modules, which are used to stabilize the overall window-module operation to achieve maximized energy-harvesting performance over the course of the day. As is evident from the data of Figure 5, the  
Figure 5: Solar windows constructed using glass panel dimensions of 1200 mm \(\times\) 600 mm and their electric output performance measured outdoors in autumn conditions. (**a**) Factory-assembled solar window samples and (**b**) Measured I-V curve data. overall module fill factor (MFF) of a solar window employing multiple interconnected CIS modules is significantly lower than the nominal FF (of near 0.66) of Avancis CIS products. This is largely due to the fact that each individual CIS module installed into a window receives substantially different amounts of illumination (and also shading) due to the variations in their geometric orientation with respect to the incoming sunlight and also due to some shading caused by the framing system. Many individual CIS modules also differ in length (and, therefore, in the number of series-connected CIS cells) and in V\({}_{\text{oc}}\). These variations lead to changes in the I-V curve shape features and affect the overall system FF, which, in turn, reduces the effective CIS-module efficiency compared to its nominal value.