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

Abstract

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, 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 kWh/m 2 -floor-yr or 12-14% 2 for moderate-to large-area windows (WWR≥0.30) in Chicago and 9.8-18.6 kWh/m 2 -floor-yr or 10-17% 3 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 aesthetic 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.

Thermochromic Oxide-Based Thin Films and Nanoparticle Composites for Energy-Efficient Glazings

“…This type of variation in T(λ,τ) clearly is the desired one, and a glazing with a VO 2 -based TC thin film transmits more energy below τ c than above τ c . Similar properties have been recorded many times [31][32][33][34][35][36][37][38][39][40][52][53][54]. Figure 3 also demonstrates that corresponding data on R(λ) increase monotonically towards long wavelengths for the metallic-like state above τ c , which is the expected behavior.…”

Thin Films for Advanced Glazing Applications

“…A number of transition metal compounds, including Fe 3 O 4 , FeSi 2 , NbO 2 , NiS, Ti 2 O 3 , Ti 4 O 7 , Ti 5 O, V 2 O 3 and VO 2 , have been found to have thermochromic properties. Even though they offer good transmittance modulation in the visible region, they generally do not offer proper switching in the near IR range of the spectrum or have transitions at suitable temperatures [149]. Among them, vanadium dioxide (VO 2 ) based thin films have been emerged as suitable candidates for TC intelligent glazing applications since they exhibit a sharp transition temperature (T c ) close to room temperature (68 • C) [150].…”

“…Warwick et al [171] used electric field The production of VO 2 films with high visible transmittance, efficient optical NIR transition and reflection modulation properties, lower T c values, along with narrower and sharper hysteresis loop widths, remains a worthy pursuit as a significant energy benefit can be derived from such materials [149].…”

Thermochromic light scattering from particulate VO2 layers

“…10,11 Thin films and nanoparticle composites of VO 2 are of considerable interest for "smart" glazings in buildings as well as for numerous other technologies. 3,[12][13][14][15][16][17] Most optical applications of VO 2 require essentially non-scattering properties, and both films and nanoparticle composites have been subject to detailed studies recently. 13,15 Only a few prior investigations 9,[18][19][20][21][22][23][24][25] have considered light scattering, and we particularly note recent work by Jin et al 20,21 on diffuse transmittance through VO 2 -based functional fiber mats and by Moot et al 22 on composites of VO 2 and elastomers.…”