3.3 Multilayer Thin-film Based Nanophotonic Passive Windows
3.3.7 Figure of Merit of Our Passive Window Glasses
To compare the performance of our MIM thin-film based passive window glasses with previously reported and existing commercial windows, here, we introduce fig- ure of merit (FOM) used in the industry [7, 157, 158]. The three main parameters in- clude visible transmittance (VT), IR transmittance (IRT), and solar heat gain coefficient (SHGC), which basically gives the fraction of visible, infrared, and total solar radia- tion transmitted through a window glass over a specific wavelength window, respec- tively [37]. For our window glass design, the spectral range of VT, IRT, and SHGC lies within 400–750 nm, 750–1800 nm, and 400–1800 nm, respectively.
3.3 Multilayer Thin-film Based Nanophotonic Passive Windows
dow glasses, illumination inside the room is equally an important aspect of a window design. For the case when maximum illumination inside the room is desirable, an ideal window designed for hot climate condition should have visible transmittance as high as possible, and IR transmittance as low as possible (VT→1, IRT → 0), which corre- sponds to our BC mode. Whereas for cold climate, both VT and IRT should be as high as possible (VT, IRT →1), which corresponds to our BW mode. On the contrary, for the case when minimum illumination is required for cold climate, ideally, VT should be as low as possible, and IRT should be as high as possible (VT→0, IRT→1), which corresponds to our DW mode. For calculating SHGC value of an ideal window in BC mode, we considerTvisible(λ)= 1, andTIR(λ)= 0, which results in SHGC = 0.55, within 400 – 1800 nm spectral range. Similarly, for the case of BW and DW modes, the ideal SHGC value is 1 and 0.45, respectively, for the same spectral range. However, it is im- portant to note that SHGC value for practical purposes may vary depending upon the geographical location.
Figure 3.16 shows a comparison among the figure of merit data obtained for var- ious window glasses in BC, BW, and DW modes. The figure of merit obtained using our relatively inexpensive metals (Ag, Cu, ITO, AZO, and ALON) show overall better performance compared to those of industry-standard commercial windows and pre- viously reported infrared-blocking plasmonic glasses, in terms of VT, IRT, and SHGC values obtained. Most of the previously reported works focus solely on blocking IR ra- diation and allowing visible radiation, which eventually falls under BC mode. Figure 3.16(a) shows a comparison of figure of merit for BC mode among our MIM thin-film based glasses and three double pane argon low-emissivity coating commercial win- dows (CW I, CW II, and CW III) [7, 8, 154]. The figure of merit obtained using our relatively inexpensive metals—specifically silver outperforms industry-standard com- mercial window glasses.
Figure 3.16: Figure of merit showing VT, IRT, and SHGC values for (a) BC mode, comparing our MIM thin-films (where, SiO2 is used a dielectric layer for Au, Ag, and Cu based design, and Si is used as dielectric layer for ITO, AZO, and ALON based design) with three commercial double-pane argon low-emissivity coating glasses (CW I, CW II, and CW III) [7,154,157], (b) BC mode, comparing our Ag–SiO2–Ag based glasses with Ag nanoshell based plasmonic glasses [37], (c) BW mode, comparing our MIM thin-films (where, TiO2 is used as a dielectric layer for Au, Ag, and Cu based design, andSiO2 is used as a dielectric layer for ITO, AZO, and ALON based design) with commercial double-glazed, high-solar-gain low-emissivity glass (CW IV), single- and double-pane clear glasses (CW V and CW VI) [7, 157], and (d) new DW mode, comparison among noble metals used in our MIM thin-films (Si is used as a dielectric layer).
Besteiroet al.reported the design of IR blocking plasmonic glasses using nanoshell, nanorod, and nanocup over 200 – 1700 nm spectral regime [37]. Out of the three cases, silver nanoshell based IR blocking plasmonic glasses produced the best result in terms of VT, IRT, and SHGC values obtained. Hence for comparison, we consider silver nanoshell based plasmonic glasses of Ref. [37] by taking the mean values of manually
3.3 Multilayer Thin-film Based Nanophotonic Passive Windows
design parameters of our silver-based MIM thin-film over the same spectral rangei.e.
200 – 1700 nm, considering top and bottom Ag layer, each 8 nm thick; with 80 nm thick SiO2layer in between. The calculated ideal SHGC value over this spectral range is 0.43.
Figure 3.16(b) shows that our silver based MIM thin-films show better performance in all the three parameters (VT, IRT, and SHGC) compared to silver nanoshell based plasmonic glasses of Ref. [37]. Especially, the SHGC values obtained by our design are very close to the ideal SHGC value.
Figure 3.16(c) shows figure of merit for BW mode, comparing our MIM thin-film based glasses with commercial double-glazed, high-solar-gain low-emissivity glass (CW IV), single and double pane clear glass windows (CW V and CW VI) [7, 157].
For this case, our alternative inexpensive materials (ITO, AZO, and AZO) outperform the commercial glasses in terms of figure of merit obtained. At last, for our new DW mode, silver shows overall better performance than gold and copper considering ideal VT, IRT, and SHGC values [see Fig. 3.16(d)]. Hence, the figure of merit obtained using our MIM thin-film based passive glasses is promising and these category of glasses can be used as a viable solution for different climate conditions.