“Liquid Window” Significantly Reduces Energy Consumption in Buildings

A team of researchers from the Nanyang Technological University, Singapore (NTU Singapore) has designed a liquid window panel that not only blocks the Sun to control the transmission of solar radiation but also traps the thermal heat that can be discharged through the day and night, leading to reduced consumption of energy in buildings.

A composite photo showing the window in the before (cool) and after (hot) state. Image Credit: Nanyang Technological University.

The NTU team designed their energy-saving “smart window” by introducing hydrogel-based liquid inside the glass panels, and they later discovered through simulations, that this window can decrease up to 45% of heating, ventilation, and air-conditioning energy used in buildings when compared to conventional glass windows.

The “smart window” is also 30% more energy efficient when compared to low-emissivity (energy-efficient) glass available in the market, and at the same time, it can be developed more cost-effectively.

The “'smart window” is the first example reported in a scientific journal of energy-saving smart windows developed using a liquid and supports the vision of the NTU Smart Campus which is aiming to create technologically sophisticated solutions for a sustainable future.

Despite being a major part of the design in a building, windows are also the least energy-efficient component. Since heat can easily transfer via glass, windows have a major impact on both the heating and cooling costs of a building.

The United Nations released a report in 2009 that stated that buildings are responsible for 40% of global energy consumption, and windows account for 50% of that energy usage.

Traditional energy-saving low-emissivity windows are designed with costly coatings that reduce infrared light traveling out and into a building, thereby helping to decrease the heating and cooling demands. But such windows do not control visible light, which is a crucial part of sunlight that is responsible for causing the buildings to heat up.

Hence, to design a window that can resolve these restrictions, the NTU team used water, which is capable of absorbing a large quantity of heat before it starts to get hot; this phenomenon is referred to as high specific heat capacity.

The team produced a combination of micro-hydrogel, a stabilizer, and water, and discovered through simulations and experiments that this combination can effectively decrease energy usage in a wide range of climates. This can be attributed to its potential to react to temperature changes.

Because of the hydrogel, the liquid mixture changes into opaque upon exposure to heat. This, therefore, blocks sunlight, and, when cool, comes back turns to its “clear” original state.

“Liquid Window” Most Suitable for Office Buildings

Moreover, thanks to the high heat capacity of water, a huge amount of thermal energy can be preserved instead of being transmitted into the building through the glass during the hot daytime. This heat will then be slowly cooled and discharged at night-time.

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.

Dr Long Yi, Study Lead Author and Senior Lecturer, School of Materials Science & Engineering, Nanyang Technological University

The study was recently published in the Joule journal.

Due to these aspects, the NTU researchers assumed that their latest innovation is ideally suitable for use in office buildings, wherein operating hours are mostly in the daytime. As proof of concept, the team carried out outdoor tests in cold (Beijing) and hot (Singapore, Guangzhou) environments.

The test conducted in Singapore showed that the smart liquid window exhibited a lower temperature of 50 °C during noon, which is the hottest time of the day, when compared to a regular glass window (84 °C).

The tests conducted in Beijing revealed that the room utilizing the smart liquid window used 11% less energy to sustain the same level of temperature when compared to the room that has a regular glass window.

Smart Window Shifts Electricity Load Peak, Blocks Noise

Besides, the team also determined when the highest value of preserved thermal energy of the day took place. In the regular glass window, this “temperature peak” was 12 pm, but was shifted to 2 pm in the smart liquid window.

If this shift in temperature peak is translated to a shift in the time that a building needs to consume electricity to warm or cool the building, it should lead to lower energy tariff charges for users.

Simulations utilizing a real building model as well as weather data of four cities—Singapore, Riyadh, Las Vegas, and Shanghai—demonstrated that the smart liquid window had the most optimal energy-saving performance in all the four cities in comparison to low emissivity windows and standard glass windows.

Furthermore, soundproof tests indicated that the smart liquid window decreases noise 15% more effectively when compared to double-glazed windows.

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.

Wang Shancheng, Study First Author and Project Officer, School of Materials Science & Engineering, Nanyang Technological University

Dr. Zhou Yang is the other first author of the study and was a Ph.D. student in NTU Singapore.

He is presently an Associate Professor at China University of Petroleum-Beijing (CUPB).

Offering an independent opinion, Professor Ronggui Yang from the Huazhong University of Science and Technology, China, and the recipient of the 2020 Nukiyama Memorial Award in Thermal Science and Engineering, stated, “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.”

Professor Yang is also an expert in thermal and energy systems.

The researchers are currently looking for collaborations with industry partners to market the smart window.

The study is funded by the National Research Foundation, Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program, and the Sino-Singapore International Joint Research Institute.

Energy-saving ‘liquid window’ created by NTU Singapore scientists

Video Credit: Nanyang Technological University.

Journal Reference:

Zhou, Y., et al. (2020) Liquid Thermo-Responsive Smart Window Derived from Hydrogel. Joule. doi.org/10.1016/j.joule.2020.09.001.

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