‘Zero-Energy’ Windows Near?

Windows in the United States consume 30 percent of building heating and cooling energy, representing an annual impact of 4.1 quadrillion Btu (quads) of primary energy, but it may come as a surprise that they have the potential to be “net energy gainers” or “zero-energy” products.

That is the vision of the future advanced by a trio of experts from Lawrence Berkeley National Laboratory. Their findings were reported in “Zero Energy Windows,” a paper authored by Dariush Arasteck, Steve Selkowit and Josh Apte, and presented at an American Council for an Energy-Efficient Economy (ACEEE) seminar. The presentation notes that while standard windows have become considerably more efficient, even if all installed window stock were replaced with more efficient products, window energy consumption would still be approximately 2 quads.

However, the authors say, “highly insulating products in heating applications can admit more useful solar gain than the conductive energy lost through them. Dynamic glazings can modulate solar gains to minimize cooling energy needs and, in commercial buildings, allow daylighting to offset lighting requirements.

Among technologies that the authors see on the horizon are:

  • Highly insulating systems with U-factors of 0.1 Btu/hr-ft²-°F
  • Dynamic windows: glazings that change from clear to tinted and/or reflective) in response to climate conditions
  • Integrated facades for commercial buildings to control/ redirect daylight

Development of the technologies, however, will not be sufficient without “market transformation policies” to promote them, the authors say. It is important to look at the effects of emerging window technologies in the context of existing stock since over 50 percent of window sales are to the replacement/renovation market, they add.

The task will not be insignificant. While today’s efficient windows are much more efficient than windows from prior decades, they are still significant energy liabilities. Achieving zero-energy windows means improving performance of current efficient windows by 60 to 80 percent, according to the paper.

The authors note that “technologies and systems are not inherently self-optimizing or self-assembling: manufacturers, architects, engineers, home builders and homeowners need data and tools to guide their decision making.” Windows are intended to last 20 to 50 years, they note, and initial decisions can be changed later only at great cost.

It’s not that progress hasn’t been made in window efficiency. Developed during the 1970s and 1980s, low-emissivity (Low-E) coatings dramatically reduce radiative heat transfer through double-glazed windows, thereby lowering window U-factors (increasing R-values). Some Low-E coatings also reduce overall solar heat gain by reflecting incident solar infrared radiation, a tremendous benefit in cooling-dominated climates. Their market share has gone from a few percent during the 1980s to 50 percent or more in 2005, according to the paper.

The National Research Council (NRC) estimates that Low-E windows were responsible for 6.1 quads of cumulative savings in residential heating energy consumption from 1983 to 2005, valued at $37 billion (2003 dollars) in direct energy cost savings. This figure excludes both savings in the commercial sector and additional savings from reduced cooling energy demand.

Window technologies of the future

The authors indentify several promising technologies currently undergoing research and development:

Highly Insulating Windows: To reduce heating energy losses through windows, they advocate development of windows with U-factors of 0.1 Btu/(hr-ft²-°F). These products are intended for use in Northern (heating-dominated) climates, mostly in residences. Commercial buildings and residential applications in other climates will also benefit from highly insulating products, but the U-factor requirements for these applications are not as stringent.

Improving the insulating value of window glazing has been the subject of research since the 1980s, the authors recount. They identify three key research paths:

(1) Aerogel, a micro-porous insulating material currently under R&D worldwide. Minimizing haze and manufacturing cost remain major challenges.

(2) Vacuum glazings offer theoretically high performance by using a vacuum to eliminate all conduction/convection between the two layers of glass. (Most windows contain air or a gas to limit heat transfer, which is not as effective as a complete vacuum.) Performance is compromised by structural spacers that keep the glass layers apart, edge “short circuiting,” and the need to develop Low-E coatings that can sustain high temperatures during the edge-welding process. Structural issues (glazing implosion) are also a concern. Manufacturing processes are being developed internationally.

(3) Gas-filled Low-E windows, which have three or more glazing layers. These products are available today and can meet the 0.1 Btu/(hr-ft²-°F) (center-of-glass but not total unit) performance goal. They use center layer(s) of either thin Low-E coated polyester or conventional glass.

Challenges, the paper notes, include increased labor costs, the use of dual spacer systems (which also raise concerns about gas loss leakage), increased weight (with glass), increased overall insulating glass widths which preclude their use in many existing window frame cross sections, and manufacturing processes which are not optimized for such products.

Current research at LBNL is focused on a new option: lightweight, thin, nonstructural center glazing layers. To achieve a total window U-factor of 0.1 Btu/(hr-ft²-°F) will require development of highly insulating frames and spacers. Current research has focused on understanding frame heat transfer, which is essential for developing new designs. Two promising approaches are the use of hollow cavities to increase frame insulation and the use of insulated solids (i.e., foams) with durable skins.

Dynamic Glazings: Optimizing residential and commercial window energy performance requires dynamic solar control that responds to daily, seasonal and climatic differences. A residential window that admits sunlight to reduce winter heating must also reject sunlight during the summer peak-cooling season. A commercial window that admits diffused light from an overcast sky must control daylight from a bright sky. The two approaches to dynamic control are glazings with intrinsic optical control and add-on shading systems to supplement glazing properties.

Technologies include passive dynamic glazings (photochromics, thermochromics), which change optical properties in response to environmental changes, e.g. presence/absence of sunlight; active dynamic glazings, which change properties in response to applied voltage, current or certain gases; and dynamic façade controls, a.k.a. automated shading systems.

The authors offer the following observations on energy savings potentials in the residential sector:

  • The “Energy Star” scenario offers relatively modest energy savings beyond the business-as usual case (0.3 quads). This is due to the large fraction of Energy Star windows which make up current sales.
  • Triple pane Low-E windows, today’s highest-performers, offer 0.8 quads of savings beyond the business-as-usual case, focused mainly in heating dominated climates.
  • Next-generation “High-R Superwindows” offer energy savings significantly beyond sales (1.0 quads), with savings again mostly in heating applications.
  • Even deeper energy savings can be achieved by coupling dynamic solar heat gain control with highly insulating windows. High-R Dynamic windows offer ~1.4 quads of energy savings beyond sales. Here, the entire U.S. window stock would result in zero net heating energy consumption on a national basis, while cooling energy consumption would be reduced by 80 percent from current values.

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