Liquid Window Saves Energy

The smart window turns opaque when exposed to heat, thus blocking sunlight, and, when cool, returns to its original clear state. (Source: NTU)

Scientists at the Nanyang Techno­logical University, Singapore (NTU Singapore) have developed a liquid window panel that can simul­taneously block the sun to regulate solar transmission, while trapping thermal heat that can be released through the day and night, helping to reduce energy consumption in buildings. The NTU researchers developed their smart window by placing hydrogel-based liquid within glass panels and found that it can reduce up to 45 percent of heating, venti­lation, and air-condi­tioning energy consumption in buildings in simulations, compared to tradi­tional glass windows. It is also around 30 per cent more energy efficient than commer­cially available low-emissi­vity (energy-efficient) glass, while being cheaper to make.

The smart window is the first reported instance in a scientific journal of energy-saving smart windows made using liquid, and supports the NTU Smart Campus vision which aims to develop techno­logically advanced solutions for a sustainable future. Windows are a key component in a building’s design, but they are also the least energy-effi­cient part. Due to the ease with which heat can transfer through glass, windows have a significant impact on heating and cooling costs of a building. According to a 2009 report by the United Nations, buildings account for 40 percent of global energy usage, and windows are responsible for half of that energy consumption. Conven­tional energy-saving low-emissi­vity windows are made with expensive coatings that cut down infrared light passing into or out of a building, thus helping to reduce demand for heating and cooling. However, they do not regulate visible light, which is a major component of sunlight that causes buildings to heat up.

To develop a window to overcome these limitations, the researchers turned to water, which absorbs a high amount of heat before it begins to get hot. They created a mixture of micro-hydrogel, water and a stabilizer, and found through experi­ments and simulations that it can effectively reduce energy consumption in a variety of climates, due to its ability to respond to a change in temperature. Thanks to the hydrogel, the liquid mixture turns opaque when exposed to heat, thus blocking sunlight, and, when cool, returns to its original clear state. At the same time, the high heat capacity of water allows a large amount of thermal energy to be stored instead of getting trans­ferred through the glass and into the building during the hot daytime. The heat will then be gradually cooled and released at night.

Long Yi, Senior Lecturer at the School of Materials Science & Engineering said: “Our inno­vation combines the unique properties of both types of materials – hydrogel and water. By using a hydrogel-based liquid we simplify the fabrication process to pouring the mixture between two glass panels. This gives the window a unique advantage of high uniformity, which means the window can be created in any shape and size.” As a result of these features, the team believes that their inno­vation is best suited for use in office buildings, where operating hours are mostly in the day. As a proof of concept, the scientists conducted outdoor tests in hot (Singa­pore, Guangzhou) and cold (Beijing) environ­ments. The Singapore test revealed that the smart liquid window had a lower tempera­ture (50 °C) during the hottest time of the day (noon) compared to a normal glass window (84 °C). The Beijing tests showed that the room using the smart liquid window consumed 11 percent less energy to maintain the same temperature compared to the room with a normal glass window.

Long Yi and Wang Shancheng have developed a smart window with hydrogels. (Source: NTU)

The scientists also measured when the highest value of stored thermal energy of the day occurred. This tempera­ture peak in the normal glass window was 12 pm, and in the smart liquid window was shifted to 2 pm. If this tempera­ture peak shift is translated to a shift in the time that a building needs to draw on electrical power to cool or warm the building, it should result in lower energy tariff charges for users. Simu­lations using an actual building model and weather data of four cities (Shanghai, Las Vegas, Riyadh, and Singapore) showed that the smart liquid window had the best energy-saving per­formance in all four cities when compared to regular glass windows and low emissi­vity windows.

Soundproof tests also suggested that the smart liquid window reduces noise 15 per cent more effec­tively than double-glazed windows. Wang Shancheng, project officer at the School of Materials Science & Engi­neering said: “Sound-blocking double glazed windows are made with two pieces of glass which are separated by an air gap. Our window is designed similarly, but in place of air, we fill the gap with the hydrogel-based liquid, which increases the sound insu­lation between the glass panels, thereby offering additional benefit not commonly found in current energy-saving windows.” Providing an inde­pendent view, Ronggui Yang of the Huazhong University of Science and Tech­nology, China, said: “This is the first instance of a hydrogel-based liquid smart window, and it takes us far from a conven­tional glass design. The disruptive inno­vation leads to solar regulation and heat storage, which together render out­standing energy-saving performance.” The research team is now looking to colla­borate with industry partners to commercia­lize the smart window. (Source: NTU)

Reference: Y. Zhou et al.: Liquid Thermo-Responsive Smart Window Derived from Hydrogel, Joule, online 23 September 2020; DOI: 10.1016/j.joule.2020.09.001

Link: School of Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore

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