Energy use in buildings contributes over 40% of total global energy consumption, of which lighting and space cooling represent a significant portion. Traditional glass windows are the least energy efficient components of buildings. In summer, near-infrared sunlight transmitted through windows produces unwanted heating. In turn, the high mid-infrared reflectance limits heat rejection from the building. This “greenhouse effect” increases the consumption of cooling energy.
To counter this problem, researchers from Wuhan University in China have come up with a new hydrogel glass design that consists of a hydrogel layer and a normal glass layer that can selectively block the Sun’s heat without block its light.
The new hydrogel glass has a higher level of visible light transmission, stronger near-infrared light blocking, and higher mid-infrared thermal emission than traditional glass. Thanks to all these properties, the researchers demonstrate that hydrogel glass windows can improve interior lighting and reduce interior temperature.
To further demonstrate the potential of hydrogel glass for building windows, the researchers placed the hydrogel glass on a small house model with a size of 20 cm × 20 cm × 20 cm and measured the irradiance and temperature at different places. The results showed that the indoor illuminance of the house with hydrogel glass is slightly higher than that of the house with normal glass in sunny weather.
Improved illuminance helps reduce lighting electricity consumption. More importantly, the indoor temperature of the house with the hydrogel glass is always lower than that of the house with normal glass. The greatest temperature reduction reaches 3.5°C at noon with the highest solar intensity of 58.7 mW/square centimeter.
In testing, the hydrogel glass achieved a high thermal emittance of 96%, which is higher than other transparent radiative cooling windows. The glass exhibited a reflection band at about 9 μm due to strong refraction, which led to a low thermal emittance of about 84%.
The average transmittance of the new glass was 92.8%, with a 0.15 mm thick hydrogel layer. When further increasing the thickness of the hydrogel to 1.7 and 3.4 mm, it decreased to 92.6% and 92.5%, respectively. The simulations suggest that energy savings ranging from 2.37 to 10.45 MJ/square meter per year can be achieved for typical school buildings located in different cities around the world.
With broadband light management covering the visible and thermal infrared regions of the spectrum, hydrogel glass has great potential for application in energy saving windows.