BIPV

Photo courtesy of Ubiquitous Energy

Ubiquitous Energy transparent solar window project at Michigan State University in East Lansing, Michigan.

Basics: Building-integrated photovoltaics are solar energy-generating systems incorporated into a building envelope. They can be used in windows, facades, canopies, skylights, and more, providing opportunities for energy generation on all surfaces of a building. Glass is an essential component in most BIPV products, protecting solar cells and modules while maximizing solar and light transmission.

Performance: BIPV meets the requirements for sustainability and integration of alternative sources of energy. Growing the use of on-site renewable energy is critical to achieving net-zero energy and carbon targets and will also help increase grid resiliency and U.S. energy independence. Additionally, new versions of the energy codes, including ASHRAE 90.1 2022 and the 2024 IECC, include requirements for on-site renewables, which would include BIPVs.

History: Photovoltaic technology was invented in 1954 when Daryl Chapin, Calvin Fuller, and Gerald Pearson developed the silicon PV cell at Bell Telephone Laboratories, according to the U.S. Department of Energy’s Energy Efficiency and Renewable Energy office. BIPVs came to fruition in the 1990s. The evolution of the technology in recent years has helped it to become more mainstream, says Jemssy Alvarez, product manager, Vitro Architectural Glass.

Development:Alvarez says he expects to see BIPV in more places in and around buildings as owners and architects look to more sustainable energy sources. “I think you’re going to start seeing park benches that are going to have solar panels installed so people utilizing those facilities will have the ability to charge their phones, and more atriums and canopies and skylight applications with PV integrated because it’s a natural fit,” he says.

Additionally, industry manufacturers have developed BIPV products that offer a range of aesthetic options for designers, including transparent or near-transparent BIPVs that offer the benefits of traditional glass systems while generating energy.

“Transparent solar window products are aesthetically pleasing and energy -efficient. They look and function like traditional windows but also double as renewable energy generation sources that help provide clean electricity to the home or building,” says Veeral Hardev, vice president of strategy, Ubiquitous Energy.

Ubiquitous Energy is in the early commercialization phase with transparent solar window products, says Hardev. “We are currently building our first high-volume transparent solar window manufacturing facility here in the U.S. We expect that this facility will start producing window units as large as 5 feet by 10 feet in 2025,” he says. “These window units will be sold into the residential and commercial markets directly through our partners, including [Andersen Windows & Doors], as well as commercial building developers and contractors.”

Challenges: A challenge of BIPV is cost. Alvarez says it’s hard for BIPV to compete with the pricing of what someone will pay in electricity. “However, in time, the cost of energy isn’t going to get any cheaper, and at some point, there’s going to be a parity between what you can do with these renewable sources and what it will cost you from the standard,” he says.

Resources: The NGA developed the Glass Properties Pertaining to Photovoltaic Applications as a primer on the role of glass in PV, including the types of glass used and the roles of glass as both a cover or backing for PV. The document was originally developed in 2014 (updated in 2019). Download the complete glass technical paper.

VIG

Photo courtesy of VIG Glass Technologies

A residential project in Shaanxi, China, with VIG in windows and skylights. The glass is laminated to provide additional protection.

Basics: In VIG, two glass panes are hermetically sealed together around the edges, separated by micro spacers, and the air between the glass panes is extracted. The vacuum is very effective at minimizing conduction and convection heat losses, lowering the U-factor at a very thin cavity depth.

Performance: The overall insulating performance of VIG rivals that of traditional windows with an R-value of 10, approaching a properly insulated wall rated at 12 or higher, says Daniel Sutton, product manager, Versatex Building Products, and former product manager for Vitro.

“Historically speaking, the insulating value of windows has been dramatically less performing than a wall with insulation,” he says. “With VIG, now you’re leveling the playing field in terms of the insulating performance that is roughly on par with wall insulation.”

History: The concept of VIG was first described in a 1913 German patent by Zoller. The first commercial VIG product, Spacia, was launched by Nippon Sheet Glass in 1996.

Development: “Where VIG is going, it’s now getting into bigger sizes with architects and designers. The bigger the size, the bigger the windows, the better we can get more natural light in,” Sutton explains. Additionally, manufacturers will begin moving toward temperature/heat-strengthened glass because of its versatility, Sutton says.

Challenges: VIG comes with some fabrication challenges. According to NGA’s technical paper on VIG, the units are limited in size. Units must have at least two 90-degree corners, and shapes are limited to fairly simple parallelograms or simple arcs. VIG is also not available as bent glass.

Sutton says another limitation of VIG is its dependence on what low-e coating is used within the vacuum space; hopefully, as the technology evolves there will be more freedom to use different high-performing low-e coatings to meet different energy restrictions or energy code mandates.

A limitation of VIG is that there is limited North American-based domestic sourcing for VIG, so access and cost can be a challenge. Sutton says Vitro hopes to mitigate this in the future and begin VIG manufacturing in the U.S.

Resources: The NGA developed the Vacuum Insulating Glazing glass technical paper to offer guidance on VIG market applications, performance, and size applications. Energy performance, acoustic improvements, size and shape options, as well as testing standards, and a thorough list of terminology specific to VIG are also outlined in the technical paper. Download the document.

Thin Glass

Photo courtesy of NGA

Thin glass, when used in a multi-cavity IGU, allows for large thermal performance improvements without adding much weight.

Basics: The glass industry is looking to thin glass—generally considered any glass less than 1.6 millimeters thick—in two key applications to greatly improve window system thermal performance. First is the use of thin glass as interior lite or lites in multi-cavity IGUs. The second, more recent, application is thin glass in window retrofit systems.

Thin glass is produced using two methods: horizontal float soda line (a process used by NSG Group) and vertical fusion drawn boro-aluminosilicate (a process used by Corning).

Performance: Thin glass, when used in a multi-cavity IGU, allows for large thermal performance improvements without adding much weight. Additionally, thin-glass IGUs are much thinner than traditional multi-cavity IGUs and can more easily be accommodated by existing framing systems.

“With the thin triple, you get about an R-8 center of glass,” says Selkowitz. “And if you have a slightly larger and wider glass package, you can add two pieces of thin triple glass (making a quad IGU) and get R-14 center of glass. … This is great news as it offers new opportunities for glass to provide improved thermal comfort, reduced HVAC size, and deeper energy savings.”

Development: Thin-glass triple glazing was invented by Selkowitz in the late 1980s, achieving an invention registration in 1991. However, “no one wanted triples back then and thin glass didn’t exist,” Selkowitz says. Then came the fast emergence and growth of smartphones and flat-screen televisions in the 2000s, which created a new market for thin glass. Today, thin glass is readily available and affordable. “There is a whole industry out there that knows how to make, cut, and transport thin glass. It’s a pretty fast learning curve to bring this to industry,” Selkowitz says.

Thin-glass multi-cavity IGUs have been produced in North America for several years by Alpen High Performance Products, says company president Brad Begin. The company began working with the U.S. DOE on developing thin-glass triples for commercial use in 2018. By 2019, it had launched as a product offering, following extensive testing and field validation. By mid-2023, the company expects to pass a million square feet of thin-glass products sold into the market for both commercial and residential applications. “Thin triples are at niche volume now, but [the technology] has the potential to be more mainstream,” says Selkowitz. “From the technology side, there is nothing to hold it back. The price is fine. The handling is fine. What’s needed is the market pull side.”

Thin-glass retrofit products are newer but are already at the installation stage. Alpen received a patent for its thin-glass high-performance secondary window in 2021. Begin says that retrofit products help to address the “real elephant in the room, which is trying to address how we make existing buildings better. … On the commercial side, even though half of the U.S. building stock [has] single or low-performing double [windows], only a very small percentage of existing stock does anything with replacement or upgrade.”

Challenges: “The one thing you can’t do with thin glass is temper it,” says Selkowitz. However, there is ongoing research in the industry to address tempering issues, and Begin notes the industry has “six different market-ready solutions that solve the issue.”

The thin-glass industry has also been limited by size. However, Begin says Alpen sources thin-glass sheets that are typically 50 to 60 square feet, with access to sizes of 70 square feet or larger.

Resources: The January NGA Glass Conference: Miramar Beach included two presentations on thin triples:

• Thin Triple Glazings: Potentials, Status, Futures Concept Products, by Selkowitz
• Thin Glass from a Fabricator Front Line Perspective, by Begin

The presentations are available as webinars from the NGA.

Dynamic Glass

Basics: Dynamic glass, sometimes referred to as smart glass, switches between clear and tinted states on demand, providing glare control, addressing solar heat gain considerations by reducing a building’s energy load for heating and cooling, and offering access to natural daylight and unobstructed views. Dynamic glass technologies make it possible to change the light transmission characteristics of the glass at required times. These features make it an efficient and environmentally friendly tool for filtering heat and light for more efficient climate control in enclosed spaces. While the glass industry produces smart glass products intended to provide privacy solutions, this section focuses on switchable products designed for solar control.

Performance: There are currently three primary technologies for dynamic glass that are used in the built environment: electrochromic, photochromic and thermochromic. The principle common to them all is the use of materials whose exposure to electrical voltage, heat or sunlight changes their composition and consequently their color and texture.

Electrochromic glass technology uses a small DC voltage to change a thin coating from clear to tinted and darken the interior of a piece of glass, all within only a few minutes. This color-changing technology can block 99% of sunlight and is considered one of the most energy-efficient solutions, contributing greatly to LEED scores in architecture. Photochromic glass materials change their color composition when exposed to ultraviolet wave radiation, usually from sunlight, and darken while absorbing the light and heat of the sun. And thermochromic glass materials change color and darken when exposed to higher temperatures.

The electrochromic technology segment dominated the global smart glass market, according to Grand View Research, with a share of 83.6% in 2021 due to low driving voltage, high blockage ratio of ultraviolet rays and capability to integrate with large glass panels.

History: Dynamic glass products have been around for over 100 years and have only recently been implemented, and some are based on modern discoveries. The science behind the electrochromic process goes back to patents in 1843 from Scottish engineer Alexander Bain, while thermochromic glass was under development in laboratories as early as the 1960s, and the first photochromic technology was offered in 1966 by Corning Glass Works Inc.

Development: Over the past few decades, advancements in smart window research have moved the technology well beyond the lab. The market for smart glass is expected to reach $7.5 billion by 2028, according to Grand View Research. Additionally, according to Culp, the adoption of smart windows is expected to grow quickly with the Inflation Reduction Act adding dynamic glass to the Investment Tax Credit, covering up to 30% of the costs associated with dynamic glass for eligible projects and moving it towards cost parity with traditional glazing and shading solutions.

Next-generation thermochromic windows may use different materials that absorb light instead of reflecting it, creating a continuous tint as temperatures rise, similar to electrochromic windows. One thermochromic material that’s long been studied is vanadium dioxide, or VO2, which can transition at higher temperatures (about 154°F), increasing its ability to reflect infrared light. This allows visible light to continue to stream in, brightening the room, while lowering the amount of incoming heat, keeping the room cooler.

Challenges: Electrochromic glass, the most common technology for switchable glass, does have limitations when it comes to switching speed and shows characteristics of blue glass appearance in the tinted state. There are also challenges for widespread use, such as expense and an installation process that requires some additional training.

Resources: The NGA published the glass technical paper Dynamic Glazing for High Performance Buildings, which discusses the characteristics of dynamic glazing that can mitigate against the influence of the sun. The paper was updated in 2018. Download the document.

Katy Devlin is the content director for the National Glass Association and the editor in chief for the NGA’s leading architectural glass publication, Glass Magazine. Norah Dick is Glass Magazine’s senior editor; Tara Lukasik, managing editor; and Rachel Vitello, assistant editor.