A copper-bismuth electrochromic coating and rotatable infrared-emitting structure give smart windows new control over sunlight and thermal radiation.
Paper: Solar and thermal dual-band electrochromic windows for energy-efficient buildings. Image credit: AI-generated image created using ChatGPT/OpenAI
A recent study published in the journal Nature Communications introduces a solar and thermal dual-band electrochromic (STDEC) window for energy-efficient buildings.
The researchers combined a copper-bismuth (Cu–Bi) reversible metal electrodeposition system with a rotatable window architecture to regulate solar transmission and long-wave infrared emissivity. Outdoor chamber tests and building energy simulations suggest that the dual-band design could reduce building energy use while lowering associated carbon emissions.
Smart Windows for All-Season Thermal Control
Improving the energy efficiency of buildings has become a major priority as cities work to reduce energy consumption and greenhouse gas emissions. Buildings account for about one-third of global energy use, and conventional windows can increase heating, ventilation, and air-conditioning (HVAC) demand. Windows are among the largest contributors, as they allow solar heat to enter buildings during summer while also allowing heat to escape during winter. Developing windows that can adapt to changing environmental conditions is therefore critical for improving building performance.
Electrochromic smart windows have emerged as an attractive solution because they can reversibly adjust their transparency using a small electrical voltage. Most existing systems regulate visible and near-infrared light while providing little control over long-wave infrared (LWIR) thermal radiation. This limitation reduces their ability to balance summer cooling requirements with winter heat retention.
Researchers developed a design that combines a copper-bismuth (Cu–Bi) reversible metal electrodeposition system with a simple rotational architecture. This allows the window to switch between heating and cooling modes by controlling both incoming solar radiation and outgoing thermal radiation.
The study reports that this dual-band strategy can support year-round thermal management, with low operating voltage, 36-hour bistability, and stable optical performance during the reported cycling and environmental tests.
Engineering Dual-Band Smart Windows for Year-Round Thermal Control
The researchers designed the STDEC window by combining reversible metal electrodeposition (RME) with a simple rotatable architecture. The device consists of a double-sided indium tin oxide (ITO) working electrode, a copper frame counter electrode, a transparent gel electrolyte containing copper and bismuth ions, and a polyethylene (PE) membrane. Mechanically assisted rotation of the window changes the orientation of the low- and high-emissivity surfaces, enabling season-specific independent control of solar transmission and thermal radiation.
The team optimized several design parameters to maximize optical and thermal performance. Platinum nanoparticles on the ITO surface promoted uniform Cu–Bi deposition and reduced uneven coloration. The researchers also compared ITO layers of different thicknesses to balance electrical conductivity, visible transparency, and infrared reflectivity.
The researchers characterized the window using various optical, electrochemical, and thermal characterization techniques. Scanning electron microscopy (SEM) was used to examine the deposited films, while ultraviolet-visible-near-infrared spectroscopy measured solar transmission. Fourier-transform infrared spectroscopy (FTIR) measured LWIR reflectance, from which emissivity was determined. Finally, outdoor test-chamber experiments examined the window's seasonal performance, while EnergyPlus simulations across 19 representative cities spanning climate zones quantified its potential to reduce building energy use and carbon emissions.
Dual-Band Regulation Improves Energy Efficiency Across Seasons
The STDEC window achieved substantial control over both solar transmission and thermal emissivity. Reversible Cu–Bi electrodeposition delivered a solar transmission modulation (ΔTsol) of 52.2%, while the rotatable architecture produced an emissivity modulation (ΔεLWIR) of 0.87. The dual-band design enables the window to adapt to changing seasons. In winter, the transparent, low-emissivity state maximizes solar heat gain while limiting heat loss. In summer, the dark, high-emissivity state reduces solar heating and enhances radiative cooling.
The device also combines low-voltage operation (±1 V) with long bistability, retaining its optical state for more than 36 hours and retaining a large solar-transmittance contrast after 2,000 switching cycles, with about 1% degradation. Outdoor testing in test chambers confirmed the practical advantages of the design. During summer, the STDEC window reduced the temperature inside the chamber by an average of 14.1°C compared with conventional glass, achieving a maximum cooling effect of 22.2°C under direct sunlight.
In winter, the heating mode raised the daytime test-chamber temperature 12.3°C above ambient conditions. At night, the window retained heat more effectively than reference windows, keeping test-chamber temperatures 1.3 to 1.5°C higher during insulation tests. This seasonal adaptability demonstrates the advantage of independently controlling solar transmission and thermal emissivity.
Building energy simulations demonstrated the potential of the STDEC window to improve energy efficiency. The model predicted average HVAC energy savings of 19.5% for a standardized medium-sized office building across 19 representative cities spanning global climate zones. The greatest savings occurred in regions with extreme hot or cold climates, although the window also delivered notable improvements in temperate and subtropical regions. The simulation estimated an average annual reduction of 39.3 tonnes of carbon dioxide emissions for the reference office building, further highlighting its potential to support more sustainable building design.
A Step Toward Smarter, Low-Carbon Buildings
This study demonstrates the potential of dual-band electrochromic windows to improve building energy efficiency. By combining reversible Cu–Bi metal electrodeposition with a rotatable architecture, the researchers developed a smart window that can be configured for cooling in summer and heat retention in winter. Its broad optical modulation, low operating voltage, long bistability, and reported cycling performance address several limitations of existing electrochromic window technologies.
The researchers also outlined several opportunities to advance the technology. Improving long-term durability under ultraviolet exposure, humidity, and extreme temperatures will be important for real-world applications. Scaling the design to architectural-sized windows and optimizing transparent electrodes and electrolyte formulations could further enhance performance while enabling scale-up.
The STDEC design could help reduce building energy consumption, manage indoor temperatures, and lower carbon emissions. Its low-voltage operation and possible integration with intelligent building management systems could further support the development of energy-efficient buildings. As the demand for sustainable construction grows, dual-band electrochromic windows could play an important role in shaping the next generation of low-carbon buildings.
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