*2.1. Determination of Spectral Properties*

The International Glazing DataBase (IGDB) is a collection of optical data for glazing products. Spectral transmittance and reflectance are measured in a spectrophotometer and contributed to the IGDB by the manufacturer of the glazing product, subject to a careful review. The IGDB currently only allows the inclusion of specular glazing materials without patterns, such as monolithic glass, plastic, laminates, applied films on glass, or thin-film coated glass. The products included in the WFG catalog are low-emissivity (Low-E) glass, high selective glass, and solar polyvinyl butyral (PVB) interlayers to increase sun energy absorbance. The solar energy spectrum can be divided into the ultraviolet (UV) light, visible light, and infrared (IR) light, depending on the wavelength. The wavelengths of the ultraviolet light range from 310 to 380 nanometers. The visible light ranges from 380 to 780 nanometers. The infrared light spectrum is transmitted as heat into a building and begins at wavelengths of 780 nanometers. Solar heat radiation has shortwave energy, and it is known as near-infrared (NIR), whereas the heat radiating off warm objects has higher wavelengths and is known as far-infrared (FIR). Three different WFGs were tested:


Figure 1 shows the front and back reflectance (*R*), transmittance (*T*), and absorptance (*A*), as a function of the angle of incidence. It illustrates the glass panes, coatings, and the position of the air and water chambers. Case 1 showed the highest infrared absorptance (*A*), the lowest front reflectance (*R*), and the lowest infrared transmittance (*T*). It seemed the best option to heat up water. Case 2 showed a

high near-infrared (NIR) absorptance, low far-infrared (FIR) absorptance, and high front far-infrared (FIR) reflectance. The absorptance was not as high as in Case 1, but its ability to reflect heat made it the right solution for large glass areas in warm climates. Case 3 showed very low infrared absorptance and very high infrared front reflectance. This case would have been the best option to reject energy and prevent heat from entering the indoor space, but it would not have been appropriate to heat up water.

**Figure 1.** Spectral properties as a function of the solar wavelength. Description of the layers and coatings. (**a**) Case 1: 8 + 8/24 w /8 + 8/16 a /6 + 6. (**b**) Case 2: 10/16 a/Low-E8 + 8/24 w/8 + 8. (**c**) Case 3: 10XNII/16 a/8 + 8/24 w/8 + 8.
