As global warming intensifies, window technologies that can automatically regulate indoor light and humidity in response to environmental changes could help reduce building energy consumption and support the development of a low-carbon city. A research team at the International Centre of Urban Energy Nexus (UEX) of The Hong Kong Polytechnic University (PolyU) has developed an innovative smart window film that leverages the unique properties of various materials to lower indoor temperatures during the day while absorbing moisture and reducing its transparency at night, thereby enhancing energy efficiency as well as improving indoor comfort and privacy. Antibacterial, durable and cost-effective, this innovative window film is particularly suitable for use in humid regions such as Hong Kong, offering a promising solution for green building development.
Known as the "Moisture-responsive and Light-regulating" (MRLR) film, development of the PolyU innovation was led by Prof. YAN Jinyue Jerry, Chair Professor of Energy and Buildings, and Dr LIU Junwei, Research Assistant Professor of the Department of Building Environment and Energy Engineering at PolyU. The team have revolutionised existing window film designs by adopting a dual-layer structure composed of a hydrophilic polyacrylonitrile (PAN) nanofiber layer and a transparent polyacrylamide (PAAM) hydrogel with excellent water retention. By leveraging the materials' water absorption and evaporation characteristics, the film is able to regulate light, heat and humidity simultaneously.
Prof. Yan said, "Currently, the industry widely uses 'thermochromic' or 'photochromic' materials in smart windows to improve thermal management in buildings, but these films often face challenges such as high cost and insufficient durability. The PolyU team has innovated the film design using eco-friendly and low-cost materials to achieve simultaneous regulation of indoor light, heat and humidity, thereby reducing buildings' reliance on cooling systems. Our film features a simple structure and has strong potential for large-scale production, making it highly promising for sustainable green building applications in the future."
The MRLR film performs different functions in response to day-night environmental changes. During the day, sunlight causes the water stored within the film to evaporate, reducing the amount of solar radiation entering the room and lowering the indoor temperature. At night, when relative humidity rises, the hydrophilic materials in the film absorb moisture from the air, which also lowers the film's transparency, ultimately reducing indoor humidity with enhanced privacy. Research results showed that the film effectively reduced humidity in a test chamber from 91.73% to 53.76% within six hours at night, while lowering the indoor air temperature by up to 21.1°C during the day, demonstrating remarkable environmental regulation performance.
To further evaluate the MRLR film's energy-saving potential and carbon reduction efficacy, the team established a global model using typical meteorological year data from several major cities. The results indicated that the film could effectively regulate indoor temperatures and reduce seasonal energy demand, offering significant benefits for global energy conservation and emissions reduction. Based on the data from the studied cities, the team estimated that the average annual energy consumption could drop by over 20%, cutting global annual carbon emissions by more than 18kg per square metre.
Dr Liu added, "Low-latitude regions usually have a higher demand for air conditioning, making conventional cooling systems a major source of building energy consumption. By providing all-day regulation of light, heat and humidity, the MRLR film is particularly effective for cities in tropical and subtropical regions like Hong Kong."
The team also conducted 300 cycles of moisture adsorption and desorption tests, confirming the film's excellent durability and stability. Due to the film's low manufacturing cost, the investment payback period is only one to two months, offering significant economic benefits. Furthermore, experimental observations have confirmed that the film effectively resists bacterial and fungal contamination and maintains dust-repellent properties even after five days of continuous exposure to severely polluted air.
The study, titled "Scalable moisture-responsive and light-regulating films for building energy saving and privacy protection", has been published in the academic journal, Advanced Energy Materials.
