Coated with advanced, light-regulating materials, smart windows adjust their transparency and insulating properties in response to light and temperature changes, limiting heating in summer and maximizing solar gain in winter. For commercial buildings, where glazed facades are often extensive and energy costs significant, this makes them a compelling efficiency tool.
Understanding Solar Radiation
Only a portion of the sun’s light, or solar radiation, is in the form of visible light. This is the light that, by passing through the glass, allows windows to be transparent. The rest of the solar radiation that reaches Earth is non-visible light: infrared and UV. Since infrared is primarily responsible for solar heating, this form of light is the focus of smart window technologies.
When temperature and light levels change, the properties of the smart coatings also need to change to allow the right amount of infrared radiation (and visible light) to pass through. During the colder months, smart windows maximize solar gain, especially on sunnier days, to provide heating effects. In summer, the opposite is true. Smart windows limit amount of infrared light passing through the windows to prevent excessive heating.
During the development of next-generation smart windows, the coating materials must be tested with tunable light, which isolates infrared wavelengths to confirm how much of this light is transmitted through the coating under specific conditions. There are two types of smart window technology, passive and active, and they differ in the mechanism that tunes the material properties to control light transmission.
Passive Technologies
In passive technologies, the change in the optical properties of the coating material is triggered by external conditions. For example, thermochromic glass adjusts its optical properties depending on the temperature of the window surface, while photochromic glass adjusts according to UV exposure, allowing both types to move between two states.
Thermochromic glass is most popular in passive smart window technology as heat is a more direct factor of energy efficiency than UV exposure. By responding directly to temperature, thermochromic windows require no electrical infrastructure or building management system integration, making them a straightforward option to improve efficiency with minimal operational complexity. However, as they are limited to these temperature-specific state changes, any customizability is locked into the development stage and unavailable to end users.
Active Technologies
Active technology, specifically electrochromic glass, requires the input of an electrical current to trigger a state change. These windows exploit the tunable properties of advanced materials to vary transmission, reflection, and absorption of solar radiation. The electrochromic material must remain stable across a wide range of temperature and light conditions, only switching state when exposure to an electrical current.
In advanced systems, electrochromic glass can move into three or more states, providing greater customizability of the window efficiency, even for end users. This is possible because, by altering the electrical input, the optical properties of the smart materials can be split into multiple phases which achieve specific states of infrared and visible light transmission. In large buildings, this is particularly useful as different areas could be configured independently to meet specific comfort and efficiency requirements.
Integrating Sensor Technology
To maximize energy efficiency, electrochromic technologies can be integrated with temperature and light sensors so that they switch between phases automatically. This means they can respond appropriately to the cold, dark days in winter and the warm, sunny days in summer without manual activation.
In manual systems, the user is able to control the state of the window using a switch or remote which activates or deactivates the electrical current. When automated, the electrical current is controlled by specific input received by the sensor. For advanced systems with multiple state changes, the sensor is able to respond to multiple environmental conditions and trigger the coordinating phase change. This allows smart windows to slot into broader building efficiency strategies alongside HVAC and lighting operation.
Why are Smart Windows Important?
The IEA reports buildings account for around 30% of global energy demand and have contributed around 20% of the growth in total demand since 2019. Space cooling is a much smaller portion of total demand but is expected to grow as global temperatures rise. As a result, overcoming inefficient heating technologies and poor insulation is key to accelerating building energy efficiency and moving toward a more sustainable future. Smart windows can maximize their own energy efficiency when environmental conditions change, reducing reliance on heating and cooling systems and lowering year-round operating costs.
For commercial building owners and facilities managers, smart windows represent a practical upgrade worth prioritizing. Both passive and active technologies can be retrofitted to reduce HVAC load and cut energy costs, while sensor-integrated controls eliminate the need for manual oversight across large or multi-zone buildings.
