Windows present an opportunity to reduce electrical consumption in buildings by producing power with building-integrated photovoltaics (BIPV) or by reducing air-conditioning loads with solar heat gain control. Taking advantage of windows is especially important in high-rise buildings due to their large window area and limited roof area for photovoltaic (PV) generation.
BIPV windows typically take one of two forms: 1) a mosaic of crystalline silicon PV cells spaced apart or 2) thin-film PV windows with laser-scribed cell separation lines and/or relatively weak light absorption (to allow a degree of vision through the window). There is an inherent trade-off in both constructions between vision through the window and photovoltaic efficiency, with higher efficiency resulting in less vision and vice-versa. BIPV windows can be made with cells from almost any PV manufacturer; representative suppliers for BIPV windows are Onyx Solar (amorphous silicon thin-film) and Canadian Solar (crystalline silicon).
Window blinds, louvers, and smart window technologies can all be used to control solar heat gain through windows. Smart windows are mainly of two technologies: electrochromic and thermochromic. Electrochromic windows use electricity to change the tint of the window, thereby affecting the window’s solar heat gain coefficient (SHGC) and visible light transmission (VLT). Representative suppliers are View and Saint Gobain’s SageGlass. Because electrochromic windows are actively controlled, any degree of SHGC or VLT within their operating range can be selected, and the control can be done manually or by a building automation system. Moreover, transparency is maintained over the entire range of window tints.
Thermochromic windows depend on outside air temperature (OAT) and incident sunlight to change tint. Because of the passive control, SHGC and VLT are solely a function of the temperature of the glass. Like its counterpart, thermochromic windows maintain transparency over the range of tints. Representative manufacturers are RavenWindow and Pleotint’s Suntuitive.
A third, hybrid category has recently emerged that combines the benefits of BIPV and smart windows. The National Renewable Energy Laboratory (NREL) is developing a thermochromic window that produces PV power. Their SwitchGlaze technology switches from transparent to opaque with photovoltaic generation when an OAT threshold is reached. Control is passive like its related thermochromic technologies. This promising technology is currently in laboratory stage.
Another hybrid technology in development switches optically between nearly full transparency with partial PV power to limited transparency with full PV power. Unlike its BIPV counterparts, Stellaris’s ClearPower’s energy density in full power mode is comparable to that of conventional PV modules, as there is no separation between the cells. In this mode, transparency is limited to angles of sight somewhat above the horizon and lower. In transparent mode, some PV power is still generated depending on the angle of sunlight. Like electrochromics, it either can be controlled manually or by a building automation system.
Perhaps the most important criterion for the adoption of a window technology is subjective. While improving a building’s energy performance is very important, a building’s aesthetics is almost always an overriding factor. Windows are meant to be looked out of, and a building’s appearance is paramount. Any window technology that significantly degrades views or detracts from a building’s attractiveness may not be accepted even if it greatly improves energy performance.