Introducing Electrochromic (EC) Glass

Electrochromic (EC) glass is electronically tintable glass that can be used for windows, skylights, and curtain walls. EC glass actively controls the amount of sunlight and heat that is allowed to pass through the glazing and into the interior of the building, without compromising the view to the outdoors, and without requiring the addition of shades or blinds to the facade.

What separates EC glass from the standard IGU is the EC coating applied to the inside, or cavity-facing, surface of the exterior pane of glass. The coating itself consists of five layers of ceramic material. Applying a low-voltage direct current to the coating causes it to gradually tint, providing an increasingly more effective barrier to light penetration and solar heat gain, while preserving the outdoor views. The effect is also easily reversed, untinting the glass and returning it to its highest transmittance state.

EC glass can provide different degrees of tint in a single pane, allowing this glass to exist in multiple stages across the range of clear to fully tinted. These tint options enable EC glass to tailor the level of light and heat control it provides to best match the conditions of the time of day or support the unique visual needs of the task at hand.

For optimum system performance, control is automated using light sensors mounted on the exterior of the building causing the glass to tint appropriately in response to environmental conditions. When automated, the control system can be programmed to maintain a constant and optimal light level on the interior throughout the day. In the presence of direct and intense sunlight, the glass can be made to tint to its lowest transmittance level, maximizing the light control at the pane, and then untints automatically when the glare condition has passed to optimize daylight admission.

Dynamic Glare Control

EC glass provides dynamic glare control by modifying the amount of visible light that it allows to penetrate the interior. Technically speaking, the visible light transmittance (VT) of a pane of glass describes the percentage of visible light that strikes the exterior surface of the window, which is then transmitted through the glass and into the interior. Higher VT values indicate that a greater percentage of the available visible light is transmitted into the building, while lower VT values indicate that the glazing is more effectively reflecting or absorbing the visible light.

At its most protective, EC glass can provide a VT value of one percent, meaning that only one percent of the available visible light is allowed to pass through the glass. At its most transparent, EC glass allows 60 percent of the available visible light into the interior space, similar to regular low-e coated clear glass products. This range of light control enables this one solution to accommodate the sun control needs of facades when receiving intense, direct sunlight, as well when they are receiving primarily low-intensity diffuse light and need to let in as much daylight as possible.

Minimum One Percent VT

The one percent minimum VT is an important and differentiating feature in the ability of dynamic glass to truly manage glare from direct sunlight.

Product solution minima for different types of dynamic glazing can range from one to over 10 percent. Studies have shown that three percent VT, and even a two percent VT, are not sufficient to control direct beam sunlight glare in office environments. A study conducted by LBNL (Bob Clear et. al. LBNL report 57125) found that spaces using EC glass with a minimum three percent VT still had more than 25 percent of occupants pulling the blinds to achieve visual comfort. This has led LBNL to state that EC glass achieving a one percent VT or less is required in order to control sunlight glare effectively, without needing additional mechanical shading.

As an example of the needed attenuation for glare control, consider the intensity that the rays from direct sunlight can possess. At its most intense, this light can exceed 100,000 lux. If the EC glass is fully tinted to allow one percent of the visible light into the building, when the pane receives an exposure of 100,000 lux, the light level at the window would remain a visually manageable 1,000 lux. At higher transmission levels, the interior becomes quickly over-lit and uncomfortable. With EC glass, the interior remains comfortable and productive, although brutally glaring conditions may exist outside.

Dynamic Solar Heat Gain Control

EC glass modulates the visible light transmission and also modulates the transmission of near infrared (NIR) radiation. This provides a large modulation of solar heat gain coefficient (SHGC) of the pane. The SHGC represents the fraction of solar radiation admitted through a window. This coefficient takes into consideration the portion of solar energy that is transmitted directly through the glass and into the interior and the portion that is absorbed by the glass and then re-radiated into the space. A lower SHGC indicates that the window pane provides a greater degree of solar heat gain control and allows less of the available solar radiation into the building. As the SHGC value rises, more solar energy is allowed into the building.

The SHGC of a typical EC glass varies from 0.41 down to 0.09, meaning that in its most tinted state, only nine percent of the solar radiation that contacts the glass pane is transmitted inside. In its most clear state, 41 percent of the available solar radiation will be allowed into the interior, similar to double silver low-e products. The dynamic nature of this control creates a year-round energy benefit. Effectively blocking intense solar radiation during the summer months reduces the cooling load the HVAC system must manage. During the winter months, EC glass can serve as a mechanism for achieving passive solar heat gain that can reduce the strain on the heating system as it tries to keep the interior warm.

The energy impact of EC glass can create interesting opportunities in the design of a project. Designers can use more glass in the envelope of a building without incurring an energy penalty and, with the ability to manage direct sunlight, designers can even incorporate glass onto east and west-facing facades and in complex geometries that are otherwise difficult to shade mechanically. EC glass can reduce peak demand so significantly that more innovative heating and cooling technologies, such as chilled beams and radiative heating and cooling systems, can be used.