Smart window controlled by electrical signals

Korean researchers have developed a glare-free, heat-blocking ‘smart window’ that can be used in buildings or vehicles.

‘Active smart window’ technology, which enables the free adjustment of light and heat based on user operation, has been attracting a lot of attention recently in the research community. Unlike conventional windows that passively react to changes in temperature or light, these next-generation window systems can be controlled in real time via electrical signals.

The technology has significant potential for the building sector, which currently accounts for approximately 40% of global energy consumption, according to the Korea Advanced Institute of Science and Technology (KAIST).

A KAIST research team have now presented ‘pedestrian-friendly smart window’ technology that is designed to not only reduce heating and cooling energy in urban buildings, but also to resolve the persistent issue of light pollution in urban living.

Led by Professor Hong Chul Moon from KAIST’s Department of Chemical and Biomolecular Engineering, the team said the technology allows users to control the light and heat entering through windows according to their intent, and effectively neutralise glare from external sources.

Dubbed RECM (Reversible Electrodeposition and Electrochromic Mirror), the smart window system is based on a single-structured electrochromic device that can actively control the transmittance of visible light and near-infrared (heat). An electrochromic device is a device whose optical properties change in response to an electrical signal.

A particular benefit of RECM is that it suppresses the glare phenomenon caused by external reflected light, leading to a ‘pedestrian-friendly smart window’ which is suitable for building facades. This sets the RECM system apart from traditional metal deposition smart windows, which tend to suffer from glare. (Deposition is a process using an electrochemical reaction to coat metal ions onto an electrode surface in solid form.)

The RECM system developed in the KAIST study operates in three modes depending on voltage control:

• Mode I (Transparent Mode) is advantageous for allowing sunlight to enter the indoor space during winter, as it transmits both light and heat like ordinary glass.
• In Mode II (Coloured Mode), Prussian blue (PB) and DHV+• chemical species are formed through a redox (oxidation-reduction) reaction, causing the window to turn a deep blue colour. In this state, light is absorbed, and only a portion of the heat is transmitted, allowing for privacy while enabling appropriate indoor temperature control. (Prussian blue is an electrochromic material that transitions between colourless and blue upon electrical stimulation. DHV+• is a radical-state coloured molecule generated upon electrical stimulation.)
• Mode III (Coloured and Deposition Mode) involves the reduction and deposition of silver (Ag+) ions on the electrode surface, reflecting both light and heat. Concurrently, the coloured material absorbs the reflected light, effectively blocking glare for external pedestrians.
   

The research team validated the practical indoor temperature reduction effect of the RECM technology through experiments utilising a miniature model house. When a conventional glass window was installed, the indoor temperature rose to 58.7°C within 45 minutes. However, when RECM was operated in Mode III, the temperature reached 31.5°C, demonstrating a temperature reduction effect of approximately 27.2°C.

Additionally, since each state transition is achievable solely by electrical signals, it is regarded as an active smart technology capable of instantaneous response according to season, time and intended use.

“This research goes beyond existing smart window technologies limited to visible light control, presenting a truly smart window platform that comprehensively considers not only active indoor thermal control but also the visual safety of pedestrians,” Moon said.

“Various applications are anticipated, from urban buildings to vehicles and trains.”

The team’s findings have been published in Volume 10, Issue 6 of ACS Energy Letters [DOI: 10.1021/acsenergylett.5c00637]. The authors are Hoy Jung Jo, Yeon Jae Jang, Hyeon-Don Kim, Kwang-Seop Kim and Hong Chul Moon.

Image credit: iStock.com/BSPollard