Thermochromic Windows
Window Glazing: Super-insulated Thermochromic vs. Conventional
Windows with coatings that automatically tint glazing during hot, sunny weather to reduce cooling load.
Synopsis:
Thermochromic glazing materials tint reversibly from a clear to dark tinted shade either passively in response to environmental conditions or actively in response to a command from a building automation system. Some designs change from clear to tinted at a design temperature, others may gradually tint as temperature is increased. Addition of a Low-E coating significantly improves the performance of thermochromic windows. Field tests (in Colorado) indicate that thermochromic windows do not provide additional benefits in internal load dominated buildings with single-pane wood-framed windows (such as offices) above those achieved with conventional Low-E windows. Occupant satisfaction with the windows may be negative if the tinting makes it appear "dark and stormy" outside during sunny days, when outside views are distorted, and when the windows present a mottled appearance---this can occur when temperature gradients occur i.e. if portions of the window do not react to solar gain due to localized shading due to overhangs while other areas are heavily tinted.
The evolution of windows has moved from single pane to double pane, and now to triple pane. “Super-efficient” triple pane windows can have R-values of R-5 or better, with a solar heat gain coefficient (SHGC) of less than 0.2. Code-minimum windows have an R value between 0.8 to R-2.86. Scores of manufacturers have now tested super-efficient windows per the National Fenestration Rating Council (NFRC).Ina typical medium-sized office building [as defined by the Pacific Northwest National Laboratory (PNNL)], with 33% glazing, the total energy bill could be shaved by 8% by replacing windows that meet code minimum standards (as allowed in 2006) with windows that are highly insulating (U-0.19) and good at shading the building’s interior from solar heat gain (SHGC-0.20).
Smart Windows from the National Renewable Energy Laboratory (NREL) go several steps beyond triple pane windows; these thermochromic windows can block as much as 98 percent of direct sunlight, which can save on cooling costs. Tests in Colorado indicate that replacing single-pane clear with thermochromic windows can provide HVAC cooling and heating savings of about 21% through modulating the amount of light coming through the glass in response to changing environmental conditions such as sunlight intensity or air temperature.
Energy Savings: 21%
Energy Savings Rating: Not rated.
What's this?
| Level | Status | Description |
| 1 | Concept not validated | Claims of energy savings may not be credible due to lack of documentation or validation by unbiased experts. |
| 2 | Concept validated: | An unbiased expert has validated efficiency concepts through technical review and calculations based on engineering principles. |
| 3 | Limited assessment | An unbiased expert has measured technology characteristics and factors of energy use through one or more tests in typical applications with a clear baseline. |
| 4 | Extensive assessment | Additional testing in relevant applications and environments has increased knowledge of performance across a broad range of products, applications, and system conditions. |
| 5 | Comprehensive analysis | Results of lab and field tests have been used to develop methods for reliable prediction of performance across the range of intended applications. |
| 6 | Approved measure | Protocols for technology application are established and approved. |