SECTION 08 8836.21 – ELECTROCHROMIC TINTABLE GLASS
In the U.S. and much of Europe, the building sector accounts for nearly 40 percent of the energy consumption and over 70 percent of the use of electricity. The use of energy for expanding building construction is on the rise in other places throughout the world. Windows are generally considered one of the least energy efficient components in a building. They may be responsible for up to 40 percent of the building’s heating, cooling and lighting energy consumption.
Smart glass or switchable glass is considered the next generation of window technology. There are different types of smart glass technologies; electrochromic (EC), thermochromic, photochromic, polymer dispersed liquid crystal (PDLC) and suspended particle devices (SPD). The thermochromic and photochromic are passive types and respond to non-electrical stimuli such as heat and ultra-violet light respectively and cannot be manually controlled.
The others, PDLC, SPD and EC are active smart glass types and respond to electrical voltage stimuli and may be controlled manually or automatically. PDLC consists of micro droplets of liquid crystals encapsulated in a polymer matrix. SPD consists of a thin film laminate of rod-like nano-scale particles suspended in a liquid and placed between two pieces of glass or plastic, or attached to one layer. For both of these smart glass types the particles are aligned allowing light to pass through only when a 110 volt AC electrical charge is activated. When the power is turned off the PDLC and SPD panels go dark.
EC consists of a microscopically thin, clear conductive coating on the glass surface. It is activated by passing an electrical charge across the coating layers that then change color from clear to dark. The electrical charge does not need to be continuous to maintain a particular shade; it is only required to change the opacity or tint of the glass. The switching of the tint speed is slow with EC glass, and varies depending upon the size of the panel. The consistency of the tint also varies, with larger panels sometimes exhibiting changes in tint that begin at the glazing’s outer edges and then move inward; also known as the “iris effect.”
System Control There are a variety of ways for the EC tintable insulating glass units (IGU) to be controlled. A fully-automated control system may be based on various types of input, such as; daylight sensors, time of day, and solar angle, or the controls may be integrated with the building management system. There may also be manual overrides for local control of designated fenestration zones, or custom controls for special events as required for the building in coordination with the Owner’s representative as necessary. Coordinate with the system manufacturer for what user interface is appropriate for the EC system. There are a variety of options available, such as:
- LonWorks (Local Operating Network).
- BACnet (Building Automation and Control Networks).
- Mobile apps.
- Web browser.
- Universal Serial Bus (USB) Interface.
- TIA 232/RS-232 Serial Communication.
- TIA-485/RS-485 Balanced Digital Midpoint Systems.
Framing and Other Glazing Systems EC tintable IGU is generally installed in the framing of a curtain wall, storefront or skylight manufactured and installed by another entity. It is essential that all the parties affected by this work are in close communication with each other and coordinating the timeline and use of compatible framing and glazing accessories as required for the long term successful installation of these systems.
There are specific requirements being called out for the framing and IGU, such as; edge, bite and face clearances. The glass that is used to produce the IGU panels also needs to be of a type and thickness to withstand the dead and live loads caused by positive and negative wind pressure acting on the planes of glass. Verify the appropriate standard to be referenced to comply with local building codes. The glazing and framing systems shall withstand normal thermal movement resulting from ambient temperature changes and the resultant higher temperature range within the IGU and framing members. The cabling required for connecting power and control system to the IGU’s needs to be routed through the framing system. Any holes required to route the cabling or for pigtails attached to the IGU shall also be provided within the framing. In the process of that work, the framing system must still weep any moisture collected to the outside face.
EC tintable IGU panels may be produced up to 5 feet by 10 feet in size. These units can have various shapes as well. Comply with the manufacturer’s guideline requirements and limitations for the size and shape of the IGU. Keep in mind that wind loads on the panels, such as along coastal areas, may limit the overall dimensions of IGU panels to less than these manufactured sizes in order to withstand wind loading on the glass without the use of thicker glass or laminated glass.
Refer to Supplemental Information for Section 08 8000 – Glazing for additional information related to Heat-Treated Glass (KIND HS and KIND FT), Laminated Glass, Insulating Glass Units (IGU), Glazing Methods, and Accessories.
EC Glass Coating On its inside surface (facing inside building), an electrochromic (EC) window has a double-sandwich of five ultra-thin layers; Separator in the middle, two Electrodes (thin electrical contacts) on either side of the Separator, and then two transparent electrical layers or Conductors on either side of the Electrodes. The basic working principle involves lithium ions (positively charged lithium atoms - with missing electrons) that migrate back and forth between the two Electrodes through the Separator. Normally, when the glass is clear, the lithium ions reside in the inner-most Electrode. When a small voltage is applied to the Electrodes, the ions migrate through the Separator to the outermost Electrode, where they scatter away most of the incoming light and turn the glass opaque. They remain there all by themselves until the voltage is reversed, causing them to move back so the glass turns transparent once again. No power is needed to maintain the windows in their clear or dark state — only to change them from one state to the other. The total EC coating is less than 1/50th the thickness of a human hair.
The advantages of using EC glass coatings involve significant environmental benefits. In a darkened state, or full tint mode, they reflect virtually all (or 98 percent) of the natural light shining on them. This can drastically reduce the air conditioning required, both the size of the installation along with the costs of running it. According to scientists at the U.S. Dept. of Energy’s National Renewable Energy Laboratory (NREL), these types of windows could save up to 8 percent of a building’s total energy consumption. These windows use very small amounts of electricity to switch from dark to light; 100 EC windows use as much energy as a single incandescent lamp.
The drawback to the use of an EC coating is the expense of the elaborate metal coating on the glass and the expense to install them. The recent market cost to install EC tintable IGU is about $200 per square foot, compared to the installation of typical IGU with static coatings of about $75 per square foot. There may also be questions about how durable the materials are. EC is currently the only smart glass technology that has developed and passed the accelerated environmental durability standard that is considered equivalent to greater than 50 years. (ASTM E2141 – Standard Test Method for Accelerated Aging of Electrochromic Devices in Sealed Insulating Glass Units; 2014)
Another drawback of current EC windows is the time they take for tint transition from clear to opaque and back again. This transition speed is related to the ambient temperature and size of the IGU panel, and it may take a few minutes for a large sized window.
Capillary Tubes Transporting IGU’s through or to high elevation areas (in excess of 2500 feet) typically requires breather tubes from the air space to allow the unit to adjust to these pressure changes. North American fabricators generally provide either capillary tubes or breather tubes that are to be sealed upon arrival at the project or that remain open after glazing. Consult with glazing fabricators for the capillary/breather tube project requirements. Insulating glass warranty may be void upon failure to properly handle this issue.
Obscuration Band The various glass coatings applied to the glass (EC, Low-E, etc.) are removed from the perimeter edge of the IGU panel to provide for an excellent spacer and edge seal of the IGU. It is critical that the edge of the glass upon installation does not allow any sunlight to pass through. An obscuration band is one way to help prevent this by adding a black coating from the outer edge of the glass inward just enough to cover the outermost edge of the EC coating on the glass. This obscuration band may not be necessary for all applications and may not be available from all manufacturers.
Busbar A busbar is a thin narrow strip of metal applied to the EC glass that conducts electricity, and is used to apply voltage across the EC surface of the IGU. This busbar is typically located along each of the longer edges of the IGU. When the shorter dimension exceeds 60 inches, a busbar is located along each of the longer edges and a third busbar is added to the center of the glass. This is done to maintain tint transition speed. The maximum size of the EC tintable IGU is currently indicated in the specification as 60 inches by 120 inches. If the narrow width of the IGU panel exceeds 60 inches for certain IGU it may be necessary to provide a center busbar. Review the potential options with the manufacturer regarding your project specific conditions.
Pigtails A pigtail is the cable attached to the IGU and extending out the edge into the glazing pocket of the framing system. The pigtail provides a unique connection with EC tintable IGU panel and the control system. It has a unique location, size, and type of frame cable connection depending on the manufacturer of the EC tintable IGU system. Specific sized holes are also required in the framing system to allow for passage of pigtail and connection with frame cable. These connections may also require accessibility for future maintenance requirements.
Type of EC Tintable IGU The section includes a “Type” designation that can be copied to create different types of EC tintable IGU panels if necessary on your project. The section text includes various options, such as choice fill-ins for maximum size, application, fill for cavity space (air, argon, krypton or choice fill-in), EC coating location, the types of inboard and outboard lites, STC rating, and overall thickness. The outboard lite may be monolithic and consisting of annealed, fully tempered, heat-strengthened, or choice fill-in glass type. The thickness and color is also variable. If it is necessary to provide a laminated outboard lite, that option is also available with variations for outer ply, inner ply, and interlayer. If the window is triple glazed, that option is available with various glass types, as well as thickness and color of glass and possible Low-E or other type of coating to be applied if necessary. The inboard lite may also be monolithic or laminated and has choices for glass type, thickness, glass color, interlayer, and possible Low-E or other type of coating to be applied if necessary.
Characteristics of EC Tintable IGU The section also includes a “Type” designation for the available tinting that can be copied to create different types of tinting characteristics for EC tintable IGU panels if necessary on your project. There are four (4) tint levels provided, with each calling out Visible Light Transmittance (VLT), Visible Light Reflectance, for both “Inside” and “Outside”, Thermal Transmittance (U-Value), and Solar Heat Gain Coefficient (SHGC).
Visible Light Transmittance (VLT) This is a measurement of the amount of visible light that passes through the glazing assembly. The visible spectrum of sunlight weighted for the sensitivity of the human eye are the wavelengths of 380 to 750 nanometers, or the visible colors ranging from violet, blue, green, yellow, orange and red. The eyes sensitivity to light is referred to as Visual Acuity. A common misconception is that a window treated with a low VLT film will hamper or block outside viewing. In reality, darker glazing systems have no such effect. This is due to Visual Acuity where the human eye basically adjusts to its light surroundings and stabilizes sight enabling an unrestricted view of the surroundings, even when the light transmission has been reduced by up to 70 percent. VLT is expressed as a number or percentage between 0 and 1; with the higher the percentage, the greater visible light is being transferred through the glazing assembly.
Visible Light Reflectance The natural reflectivity of glass is dependent on the type of glazing material, the quality of the glass surface, the presence of coatings, and the angle of incidence of the light. Most glass is float glass in the U.S., which reflects 4 percent of visible light at each glass-air interface or 8 percent total for a single pane of clear, uncoated glass. The sharper the angle of incidence the light strikes, however, the more the light is reflected rather than transmitted or absorbed. Even clear glass reflects 50 percent or more of the sunlight striking it at incident angles greater than about 80 degrees. (The incident angle is formed with respect to a line perpendicular to the glass surface.)
The reflectivity of various glass types becomes especially apparent during low light conditions. The surface on the brighter side acts like a mirror because the amount of light passing through the window from the darker side is less than the amount of light being reflected from the lighter side. This effect can be noticed from the outside during the day and from the inside during the night.
The Visible Light Reflectance is listed in the section as “Inside” and also “Outside." Provide the information available from the manufacturer for the various tint levels as necessary for the project.
Thermal Transmittance (U-Value) The principle energy concern of windows is their ability to control heat loss. Heat flows from warmer to cooler bodies, thus from the inside face of a window to the outside in winter, reversing direction in summer. Overall heat flow from the warmer to cooler side of a window unit is a complex interaction of the three basic heat transfer mechanisms; conduction, convection, and thermal radiation. A window assembly's capacity to resist this heat transfer is referred to as its Thermal Transmittance or U-Value; also referred to as insulating value, or U-Factor.
Conduction occurs directly through glass, and the air cavity within double-glazed IGU, as well as through a window's spacers and frames. Some frame materials, like wood, have relatively low conduction rates. The higher conduction rates of other materials, like metals, have to be mitigated with discontinuities, or thermal breaks, in the frame to avoid energy loss.
Convection within a window unit occurs in three places: the interior and exterior glazing surfaces, and within the air cavity between glazing layers. On the interior, a cold interior glazing surface chills the adjacent air. This denser cold air then falls, starting convection current. On the exterior, the air film against the glazing contributes to the window's insulating value. As wind blows (convection), the effectiveness of this air film is diminished, contributing to a higher heat rate loss. Within the air cavity, temperature-induced convection currents facilitate heat transfer. By adjusting the cavity width, adding more cavities, or choosing a gas fill that insulates better than air, (argon and krypton) windows can be designed to reduce this effect.
Thermal Radiation is emitted by each object, with warmer objects emitting more than colder ones. Through radiant exchange, the objects in the room, and especially the people (who are often the warmest objects), radiate their heat to the colder window. People often feel the chill from this radiant heat loss, especially on the exposed skin of their hands and faces, but they attribute the chill to cool room air rather than to a cold window surface. Similarly, if the glass temperature is higher than skin temperature, which occurs when the sun shines on heat-absorbing glass, heat will be radiated from the glass to the body, potentially producing thermal discomfort.
U-Value is a standardized way to quantify overall heat flow. For windows, it expresses the total heat transfer coefficient of the system (in Btu/hr-sf-degrees F), and includes conductive, convective, and thermal radiation transfer. It represents the heat flow per hour (in Btu’s per hour or watts) through each square foot of window for a 1 degree F temperature difference between the indoor and outdoor air temperature. The insulating value or R-Value (resistance to heat transfer) is the reciprocal of the total U-Value (R=1/U). So the higher the R-Value of a material, the higher the insulating value, and the lower the U-Value, the lower the rate of heat flow.
Given there are various materials within a window unit and each of them have unique thermal properties, the U-Value is commonly expressed in two (2) ways:
- U-Value is of the “Total Window Assembly” and combines the heat flow of the glazing assembly, the effects of the edging materials used in the IGU, and the window frame and/or sash.
- U-Value is of the “Center-of-Glass” and assumes that the heat flow is perpendicular to the window plane, and does not include the thermal effects of the edging materials used in the IGU, and the window frame and/or sash material.
The various tinting characteristics for the EC tintable IGU in the section is called out as “Center of Glass” given the numerous variations for types of IGU edge spacers and framing.
The U-Value of the glazing portion of the IGU is affected primarily by the total number of glazing layers or panes of glass, their dimension, the type of gas within their cavity, and the characteristic of coatings on the various glazing surfaces. As windows are complex three-dimensional assemblies, with materials and cross sections changing in a relatively short distance, it is limiting, however, to only consider the glazing. For example, metal spacers at the edge of an IGU have a much higher heat flow than the center of the insulating glass, which causes increased heat loss along the outer edge of the glass. The use of the newer warm-edge spacer technology can have a positive impact on the overall rate of heat flow.
Solar Heat Gain Coefficient (SHGC) Another major energy-performance characteristic of windows is the ability to control solar heat gain through the glazing. Solar heat gain through windows is a significant factor in determining the cooling load of many commercial buildings. The origin of solar heat gain is the direct and diffuse radiation coming from the sun and the sky (or reflected from the ground and other surfaces). Some radiation is directly transmitted through the glazing to the building interior, and some may be absorbed in the glazing and indirectly admitted to the inside. Some radiation absorbed by the frame will also contribute to the overall window solar heat gain factor. Other thermal (non-solar) rates of heat flow effects are included in the U-Value of the window.
The SHGC is also affected by shading from the frame as well as the ratio of glazing and frame. The SHGC is expressed as a dimensionless number from 0 to 1. A high coefficient signifies high heat gain, while a low coefficient means low heat gain.
SHGC is influenced by the glazing type, the number of panes, and any glass coatings. SHGC of glazing ranges from above 0.80 for uncoated clear glass to less than 0.20 for highly reflective coatings on tinted glass. A typical double-pane IGU has a SHGC of around 0.70. This value decreases somewhat by adding a Low-E coating and decreased substantially when adding a tint. Since the area of a frame has a very low SHGC, the overall window SHGC is lower than the center-of-glass value.
Thermal and Optical Properties Coated IGU should be specified by selecting performance characteristics that are based on the manufacturer’s data. These performance characteristics should be established in accordance with Lawrence Berkeley National Laboratory’s (LBNL) WINDOW 5.2/6.3 computer program, that was developed by the LBNL Windows and Daylighting Group with contract support by the Department of Commerce (DOC), and the National Fenestration Rating Council (NFRC) procedures as indicated in the section. Coordinate the glazing IGU performance criteria with the buildings’ mechanical system design. It is critical that these values for performance of the IGU be included in the section text.
Multi-Zoning of Single EC Tintable IGU This is an option that may be available for specific IGU glazing panels. It is provided by applying a horizontal busbar at the vertical 1/2 way point of the EC tintable IGU to provide a two zone unit; or two busbars, one at the vertical 1/3 way point and another at the 2/3 way point of the EC tintable IGU to provide a three zone unit. These tint zones may be controlled separately from each other within the single IGU. Verify the availability of this option with local EC tintable IGU manufacturer.
Editing of Section 08 8836.21 – Electrochromic Tintable Glass is designed to start in PART 2 - PRODUCTS, progress to PART 3 - EXECUTION, and finish with PART 1 - GENERAL. The various performance requirements, glass materials, and system controls comprise the main articles in PART 2. Since this is a very particular type of glazing, there is a single listing under SECTION INCLUDES article in PART 1. Related articles and optional text in PART 1 and PART 3 will be activated as necessary.
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