Angular selective window systems: Assessment of technical potential for energy savings

• Simulation study of 3 angular selective windows in commercial buildings.
• Energy savings in perimeter zones can reach 30–50% relative to ASHRAE 90.1-2004.
• Peak window heat gains can be reduced to under 43 W/m2-floor.
• These systems can admit useful daylight, control glare and allow exterior view.
• Critical performance parameters were identified.

Static angular selective shading systems block direct sunlight and admit daylight within a specific range of incident solar angles. The objective of this study is to quantify their potential to reduce energy use and peak demand in commercial buildings using state-of-the art whole-building computer simulation software that allows accurate modeling of the behavior of optically-complex fenestration systems such as angular selective systems. Three commercial systems were evaluated: a micro-perforated screen, a tubular shading structure, and an expanded metal mesh. This evaluation was performed through computer simulation for multiple climates (Chicago, Illinois and Houston, Texas), window-to-wall ratios (0.15–0.60), building codes (ASHRAE 90.1-2004 and 2010) and lighting control configurations (with and without). The modeling of the optical complexity of the systems took advantage of the development of state-of-the-art versions of the EnergyPlus, Radiance and Window simulation tools. Results show significant reductions in perimeter zone energy use; the best system reached 28% and 47% savings, respectively, without and with daylighting controls (ASHRAE 90.1-2004, south facade, Chicago, WWR = 0.45). Angular selectivity and thermal conductance of the angle-selective layer, as well as spectral selectivity of low-emissivity coatings, were identified as factors with significant impact on performance.