# Spectrally-Selective Energy-Harvesting Solar Windows for Public Infrastructure Applications
## 1 Introduction  
In recent years, there has been a significant and growing research interest dedicated to the engineering and characterization of unconventional photovoltaic (PV) devices and systems. In particular, large-area transparent luminescent solar concentrators (LSC) [1; 2; 3; 4] and solar windows [5; 6; 7; 8] are currently receiving more attention. Of special importance is the emergence of newly-commercialized, semi-transparent photovoltaic technologies and building-integrated photovoltaic (BIPV) systems, which have been demonstrated in practical architectural deployment applications [9; 10; 11; 12]. At present, most commercial energy-generating solar glass and solar window technologies suitable for BIPV applications feature semi-transparent (up to about 40% of spectrally-averaged visible-range transmission) appearance and are most often not color-neutral. This is due to typically relying on the use of either amorphous silicon-based or dye-sensitized solar cell materials integrated into glass panel structures in BIPV applications. In building-applied PV (BAPV) applications, the energy conversion is typically accomplished by either silicon or Copper Indium diselenide based (CuInSe\({}_{2}\), or CIS) solar cell modules embedded into, or onto, glass panels, building roofs, or facade walls. While such BAPV systems generate adequate electricity, their transparent area fraction is limited, which often makes them unattractive in architectural installations. A number of recent studies have been dedicated to uncovering the potential of renewable energy technologies including photovoltaics for reaching the important goal of net-zero energy consumption in buildings and infrastructural installations [13; 14; 15; 16; 17; 18]. Figure 1 summarizes three well-known commercialized and recent examples of the relevant technologies and also provides the data on the currently-available electric power generation capacities per unit wall area. Somewhat naturally, the electric outputs of solar window-type systems decrease with increasing visible-range transparency regardless of the materials or technologies used. This is true for either the light concentrating or light-trapping (e.g., LSC)technologies or the systems relying on solar cell-based direct wide-area PV conversion. The figure of 38 \(W_{\rm p}/\)m\({}^{2}\) (standardized peak-rated electric power output per unit module area) from the Hanergy product manual [10] reported for a window panel of 40% averaged visible-range transparency, which appears to describe the top performance among all (or at least most) direct wide-area semitransparent glass-based energy converters available to date. This reported figure of performance is also well ahead (to the best of our knowledge) of all transparent concentrator-based window technologies or systems reported so far. A notable example of semitransparent photovoltaic facade installation engineered by Solaronix, Inc. and Swiss Federal Institute of Technology (EPFL, Lausanne, Switzerland) [19] is yet to undergo long-term performance characterization and testing. However, we can derive a figure of performance of about 28 \(W_{\rm p}/\)m\({}^{2}\) of electric output by using the reported data on the predicted annual energy generation, the energy-converting area installed within the facade, and by approximating the other parameters (e.g., using the reported 300 m\({}^{2}\) figure for the wall area and by estimating 5 peak-equivalent sunshine-hours per day and about 190 sunny days per year). Future BIPV technologies are widely expected to feature a combination of energy-saving functionality due to superior thermal insulation properties provided by advanced glazing systems and low-emissivity coatings and smart-window functionality offering active control over window transparency along with a possibility of significant energy-harvesting performance available in increasingly high-transparency glazings [8; 9; 20].