Examination of the technical potential of near-infrared switching thermochromic windows for commercial building applications

• Scientists are working to develop near-infrared thermochromics with a low switching temperature.
• A parametric study was conducted to evaluate if this goal was appropriate for commercial buildings.
• A near-IR thermochromic was modeled with different ranges of switching temperature.
• Energy savings were greatest for the thermochromic with the lowest switching temperature range.
• NIR thermochromics with a low switching range are appropriate for commercial buildings.

Current thermochromic windows modulate solar transmission primarily within the visible range, resulting in reduced space-conditioning energy use but also reduced daylight, thereby increasing lighting energy use compared to conventional static, near-infrared selective, low-emittance windows. To better understand the energy savings potential of improved thermochromic devices, a hypothetical near-infrared switching thermochromic glazing was defined based on guidelines provided by the material science community. EnergyPlus simulations were conducted on a prototypical large office building and a detailed analysis was performed showing the progression from switching characteristics to net window heat flow and perimeter zone loads and then to perimeter zone heating, ventilation, and air-conditioning (HVAC) and lighting energy use for a mixed hot/cold climate and a hot, humid climate in the US. When a relatively high daylight transmission is maintained when switched (Tsol=0.10–0.50 and Tvis=0.30–0.60) and if coupled with a low-e inboard glazing layer (e=0.04), the hypothetical thermochromic window with a low critical switching temperature range (14–20 °C) achieved reductions in total site annual energy use of 14.0–21.1 kW h/m2-floor-yr or 12–14%1 for moderate- to large-area windows (WWR≥0.30) in Chicago and 9.8–18.6 kW h/m2-floor-yr or 10–17%2 for WWR≥0.45 in Houston compared to an unshaded spectrally-selective, low-e window (window E1) in south-, east-, and west-facing perimeter zones. If this hypothetical thermochromic window can be offered at costs that are competitive to conventional low-e windows and meet esthetic requirements defined by the building industry and end users, then the technology is likely to be a viable energy-efficiency option for internal load dominated commercial buildings.