# 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
A summary of measured window performance data obtained from another production batch of similarly-designed solar windows of total area 0.85 m\({}^{2}\) is shown in Table 1.  
Each window within this batch had a glass panel area of 0.75 m\({}^{2}\) and external CIS modules area of 0.1 m\({}^{2}\). Due to cutting of original Avancis circuits, their re-encapsulation processes and overall factors affecting the window-module performance, the FF reduced from nominal 0.66 to \(\approx\)0.5, which effectively manifested in running all individual CIS cell modules at an average reduced efficiency of about 9.3%. The electrical mismatch losses within the window circuit partially resulted from a requirement to fit the external dimensions of windows to the required values, which affected the design features of the electric circuitry through limitations imposed on the CIS cut-out lengths. The electric power output per 1 m\({}^{2}\) of the total framed window area reached about 25 W\({}_{\text{p}}\)/m\({}^{2}\) and, therefore, approached about 26% of the nominal output available from 1 m\({}^{2}\) of the standard Avancis CIS module. Theoretical performance limits for high-transparency window-type luminescent concentrators have been reported recently in Reference [20]. An alternative way of calculating the same performance limit for an arbitrary level of window transparency presumes the idealized (complete) optical power collection of all available incident light by solar cells of known efficiency. Both calculations point to the same region of data for the maximum theoretical electric power output between 46 W\({}_{\text{p}}\)/m\({}^{2}\) to 57 W\({}_{\text{p}}\)/m\({}^{2}\) at T\({}_{\text{vis}}\) = 70% when using CIS cells of 12.3% nominal efficiency (and also by considering the Shockley-Queisser limit for CIGS cells if following the methodology of Reference [20]). This theory-limit performance is calculated for the peak geometric orientation and tilt of concentrator panels at AM1.5G, 1000 W/m\({}^{2}\). The practically-achieved solar window performance in factory-assembled framed window samples is near 50% of its theoretical limit. It is interesting to \begin{table}
\begin{tabular}{c c c} \hline \hline
**Electric Output Parameters at Peak** & & **Measurement Conditions/Notes** \\ \hline \multirow{7}{*}{\(\bullet\)} & \(\bullet\) & Measurements performed in field conditions. \\  & \(\bullet\) & Measurements made at \textless{}1000 W/m\({}^{2}\) irradiation intensity at the optimized window orientation and tilt. \\  & \(\bullet\) & The surface temperature of all CIS modules was \textgreater{}40 \({}^{\circ}\)C. \\  & \(\bullet\) & Nominal CIS module efficiency 12.3\%. Nominal CIS module \\  & \(\bullet\) & FF = 0.663. \\ \(\bullet\) & \(\bullet\) & Framed window area A\({}_{\text{tot}}\) = 0.85 m\({}^{2}\). The total area of CIS cells \\  & \(\bullet\) & installed ACIS = 0.28 m\({}^{2}\). \\ \(\bullet\) & \(\bullet\) & Avancis PowerMax 3.5 130 CIS modules output nominally \\  & \(\bullet\) & 97 W/m\({}^{2}\) at the Nominal Operation Cell Temperature (NOCT) \\  & & and standardized solar irradiation spectrum (AM 1.5 G). \\  & \(\bullet\) & External cells facing direct full-spectrum sunlight occupied 37\% of the total ACIS and contributed about 43\% of total P\({}_{\text{max}}\). \\  & & The edge-mounted cells occupied 29\% of ACIS and contributed \\  & & 25.3\% of P\({}_{\text{max}}\). \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electric output summary and parameters of solar windows used in prototype bus-stop installation. The measurement conditions are described in detail in order to provide an outlook on the potential usefulness of solar window products for use in different applications versus using conventional PV modules. compare this result with the record-performing CIGS cells (\(\eta\approx 20\%\)), which themselves operate at nearly 65% of the Shockley-Queisser limit, which was reported in Reference [25].