Key Takeaways & Executive Findings
- •• A Janus smart window integrating polymer-stabilized liquid crystal and thermochromic materials achieves dual electro-thermal control, enabling dynamic adjustment of solar transmittance and infrared emissivity. • Outdoor tests demonstrate significant temperature regulation: indoor temperature drops by 8 °C during daytime and 5 °C at night, with simulated cooling powers of 93 W m−2 (day) and 142 W m−2 (night). • The window also provides electromagnetic interference shielding in the X-band (8.2–12.4 GHz), adding multifunctionality for smart building applications. • Energy simulations show superior energy savings across various climate zones compared to conventional smart windows and standard glass, highlighting its potential for green buildings.
Abstract
The advancement of sophisticated smart windows exhibiting superior thermoregulation capabilities in both solar spectrum and long-wave infrared range maintains a prominent objective for researchers in this field. In this study, a Janus window is proposed and prepared based on polymer-stabilized liquid–crystal films/thermochromic materials. It can achieve switchable front long-wave infrared emissivity (εFront) and solar modulation ability (ΔTsol) through dynamic flipping, making it suitable for different seasonal energy-saving requirements. Outdoor experiments show that under daytime illumination, the indoor temperature decreases by 8 °C, and the nighttime temperature drops by 5 °C. MATLAB simulation calculations indicate that the daytime cooling power is 93 W m−2, while the nighttime cooling power reaches 142 W m−2. Interestingly, by modifying the conductive layer, it can effectively shield electromagnetic radiation (within the X-band frequency range (8.2–12.4) GHz). Energy simulation reveals the substantial superiority of this device in energy savings compared with single-layer polymer-stabilized liquid crystal, poly(N-isopropyl acrylamide), and normal glass when applied in different climate zones. This research presents a compelling opportunity for the development of sophisticated smart windows characterized by exceptional thermoregulation capabilities.
1. Introduction
Approximately 40% of the world's energy consumption is attributed to energy use in buildings and the main culprits are heating, ventilation and air-conditioning (HVAC) [1, 2]. The research report of China building energy consumption and carbon emissions highlighted that carbon dioxide emissions from buildings reached 22.22 tons in 2022, representing approximately 20% of the country's total carbon emissions. Among these, energy losses through glazing, windows, and doors account for 5% of the total building energy consumption. In today's “carbon–neutral” context, the trend of green and energy-efficient buildings has become a necessity for environmental protection and market development.
Windows play a crucial role as the primary channel for heat exchange between the indoor environment and the outside world in green buildings. They are the primary light-gathering components of a building, but they are also the weakest link in the thermal insulation of the building envelope, resulting in a low energy utilization rate [3–5]. In winter, the heat lost through glass windows accounts for 30%–50% of the heating load, and in summer, the refrigeration consumption caused by the passage of radiative heat from sunlight through glass windows accounts for 20%–30% of the air-conditioning load [6, 7]. Hence, it is crucial to minimize heat loss from windows and enhance the efficiency of solar energy utilization. Smart windows, which can adjust the amount of solar radiation passively or actively, showcase significant potential for application in green buildings.
A perfect smart window should be able to control three wave bands of sunlight, namely visible (0.38–0.78 μm), near-infrared (NIR, 0.78–2.5 μm), and the long-wave infrared (LWIR, 2.5–25 µm) bands. Visible and NIR transmittance (0.38–2.5 µm) determines the indoor solar heat gain, whereas LWIR thermal emittance (εLWIR) dominates the radiative cooling (RC) to the outer spaces. An ideal energy-saving smart window should have a low solar transmittance (Tsol) and a high εLWIR in summer to minimize the heat gain of the building through the window, as shown in Fig. 1a. Conversely, low εLWIR and high Tsol are beneficial to provide heat gain and suppress RC in winter.
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Zuowei Zhang, Meina Yu, Cong Ma, Longxiang He, Xian He, Baohua Yuan, Luoning Zhang, Cheng Zou, Yanzi Gao, Huai Yang (2025). A Janus Smart Window for Temperature-Adaptive Radiative Cooling and Adjustable Solar Transmittance. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01740-1
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Frequently Asked Questions
What is a Janus smart window?
A Janus smart window is a dual-functional window that can dynamically adjust its solar transmittance and infrared emissivity to adapt to different seasons and weather conditions, thereby optimizing energy efficiency in buildings.
How does the Janus smart window achieve temperature-adaptive radiative cooling?
It combines polymer-stabilized liquid crystal (active control) and thermochromic materials (passive control) to modulate heat flow. By flipping the window, it can switch between high and low emissivity states, enabling radiative cooling in summer and heat retention in winter.
What are the key performance metrics of the Janus smart window?
Outdoor experiments show indoor temperature reductions of 8°C during daytime and 5°C at night. Simulated cooling powers are 93 W/m² (day) and 142 W/m² (night). It also provides electromagnetic shielding in the X-band (8.2–12.4 GHz).
How does the Janus smart window compare to traditional smart windows?
Energy simulations indicate that the Janus smart window achieves superior energy savings compared to single-layer polymer-stabilized liquid crystal, poly(N-isopropyl acrylamide), and normal glass across various climate zones, making it a more efficient solution for green buildings.
What are the potential applications of this technology?
The Janus smart window is ideal for green buildings, energy-efficient retrofits, and smart infrastructure, offering both thermal regulation and electromagnetic shielding, which is beneficial for modern urban environments.
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