SNU researchers develop multifunctional smart windows that lower indoor temperature without power consumption and generate electricity from raindrops

■ Research Necessity

o Recently, with the significant increase in cooling demand due to global warming, a vast amount of energy is being consumed for heat management inside buildings. Existing windows, which have a high solar absorption rate and low reflectance, lead to considerable energy loss. Therefore, energy-saving windows are emerging as a practical solution to global challenges such as responding to climate change and ensuring energy sustainability. These windows not only provide optimal thermal comfort to occupants but also contribute to economic development by reducing dependence on conventional cooling systems.

o For windows to effectively save energy in buildings, it is necessary to adopt energy-efficient cooling technology (Zero Energy) and further ensure energy harvesting methods (Plus Energy) that guarantee sustainable power supply. Additionally, windows must maintain high transparency, which is their fundamental function, even on cold or foggy days.

■ Research Achievements / Expected Effects

o The multifunctional smart windows developed in this research demonstrate their effectiveness as next-generation energy-saving devices by implementing three main functions.

o First, they provide radiative cooling that lowers indoor temperature on sunny days without energy input. Second, they generate electricity using raindrops on rainy days. Third, they implement a transparent heater function to quickly remove frost from the windows on cold days.

■ Research Details 

□ Research Content Overview

o The research team led by Professor Seung Hwan Ko from the Department of Mechanical Engineering at Seoul National University has developed "multifunctional smart window technology" that lowers indoor temperatures without electricity consumption and generates power using the frictional electricity from raindrops. This research is significant in that it pioneers new possibilities for Plus Energy technology, surpassing Zero Energy to contribute to improving energy self-sufficiency in response to global warming.

□ Background

o Recently, implementing Plus Energy Buildings (PEBs) that surpass Zero Energy has become a key task for achieving energy self-sufficiency in buildings. Next-generation PEBs are buildings that go beyond minimizing energy loads and can autonomously produce energy. Buildings inherently consume a massive amount of energy for heat management, and with the rise in cooling demand due to global warming, energy usage has surged dramatically. Furthermore, existing windows with high solar absorption and low reflectivity result in substantial energy losses during cooling. Therefore, to realize economically efficient next-generation Plus Energy Buildings, it is necessary to develop multifunctional smart windows equipped with transparent cooling technology (Zero Energy-based) and further energy-harvesting technology (Plus Energy-based) that ensures sustainable power supply.
o To address these issues, researchers worldwide are focusing on the development of smart windows that maximize energy savings. Smart windows are often thought to adjust internal temperatures by changing color to control sunlight. However, this method has limitations since the windows become opaque during the cooling process, thus failing to maintain high transparency, which is the window's primary function.

□ Key Research Methods

o The research team is actively working on developing new technologies that improve energy efficiency while preserving the transparency of windows. As part of this effort, Professor Ko's research team developed a Zero Energy-based "transparent radiative cooling technology" that maintains transparency while enabling cooling without using electricity. Additionally, they developed energy-harvesting technology that produces electricity through the friction generated when raindrops contact the window surface, introducing a Plus Energy-based smart window technology that surpasses Zero Energy. The team also developed a transparent heater technology that quickly clears frost from windows on cold or foggy days, thereby implementing three functions—radiative cooling, power generation, and frost removal—simultaneously in a single device for the first time in the world.
o The research team achieved these three functionalities in a single device by fabricating windows with a layered structure of silver and ITO (Indium Tin Oxide), materials with excellent electrical conductivity and unique optical properties. First, the "transparent radiative cooling technology" minimizes the absorption of sunlight entering indoors while emitting radiant heat outdoors to lower the temperature. Unlike conventional air conditioning systems that use refrigerants, this radiative cooling technology offers cooling performance without consuming electrical energy. The research team focused on allowing only the visible light spectrum from sunlight to pass through the window while selectively reflecting near-infrared sunlight to lower indoor temperatures and maximize cooling. Second, the "frictional electricity-based power generation technology" generates electricity when raindrops contact the window surface on rainy days. For this purpose, an electrode material covering the window surface is necessary, and thanks to the excellent electrical conductivity of the layered silver and ITO structure, the smart window can generate electricity through frictional electricity. Lastly, through "Joule heating," the transparent electrodes also serve as a heater that quickly removes frost or ice from the window, ensuring clear visibility on cold days. The multifunctional smart windows developed by the research team can provide transparent radiative cooling on sunny days, generate power on rainy days, and remove frost or ice on cold days.

□ Results

o The research team led by Professor Seung Hwan Ko confirmed that the smart windows they developed maintained a temperature approximately 7 degrees lower than regular windows in hot environments under direct sunlight. In an experiment simulating rainy conditions, the smart windows generated 8.3 W m-2 of power with just a single raindrop, while also clearing frost from the window twice as fast as regular windows through Joule heating, demonstrating both high performance and multifunctionality.

□ Expected Effects

o Professor Seung Hwan Ko stated, “This achievement of presenting next-generation smart window technology optimized for responding to the depletion of fossil fuels and global warming offers valuable insights into the technological advancements for Plus Energy buildings and the eco-friendly electric vehicle industry. Smart windows are expected to be applied across various industries because they address environmental pollution, reduce cooling energy, and overcome the limitations of conventional battery technologies through self-power generation.”

□ Achievements

o This research was supported by the Basic Science Research Program through the National Research Foundation of Korea, and it has gained global attention, being published in the October 2024 issue of the prestigious journal Nano Energy (Impact factor: 16.8, Top 5.3%) under the title: "Energy-saving window for versatile multimode of radiative cooling, energy harvesting, and defrosting functionalities."

o Meanwhile, Dr. Yeongju Jung, the lead author of this study, is currently conducting follow-up research at Professor Ko's laboratory in the Department of Mechanical Engineering at Seoul National University and is preparing for a postdoctoral research fellowship abroad.

□ Introduction to the SNU College of Engineering

Seoul National University (SNU) founded in 1946 is the first national university in South Korea. The College of Engineering at SNU has worked tirelessly to achieve its goal of ‘fostering leaders for global industry and society.’ In 12 departments, 323 internationally recognized full-time professors lead the development of cutting-edge technology in South Korea and serving as a driving force for international development.