Inefficient windows in buildings represent one of the biggest energy problems in the military today. Facilities consume 30% of all U.S. Department of Defense (DoD) energy demand (DoD 2016). This massive energy footprint costs taxpayers billions of dollars each year and impacts DoD mission assurance by straining fragile public electricity grids.
In the United States, >50% of building energy is used for cooling, heating, and lighting (DOE 2015), all of which are directly impacted by windows. The thermal envelope impacts about 56% of total commercial energy consumption. Windows are considered to be the “Achilles’ heel” of the building envelope, as they allow unwanted solar heat to enter during the summer via radiation and conduction, increasing cooling energy requirements and peak loads, and allowing internal heat to escape during the winter, increasing season heating. Beyond negative energy impacts, current windows allow glare, which reduces occupant comfort; over-use of window blinds; and the over-use of artificial lighting energy.
Electrochromic, or ‘dynamic’ glass, represents a promising technology for the reduction of energy use in DoD buildings. Dynamic glass windows are manufactured with an electrochromic coating enabling them to electronically change their visual and solar heat gain characteristics. A previous Environmental Security Technology Certification Program (ESTCP) project demonstrated the capability of dynamic glass for meaningful whole-building energy savings that included heating, ventilation, and air conditioning (HVAC) savings of 29% and potential artificial lighting savings of up to 62% (NREL 2015, ESTCP 2014). However, the installation of dynamic glass required full demolition and replacement of the existing windows. This level of construction activity involved high construction expenses and the displacement of occupants, adding to the final financial cost of the project.
An alternate installation method, termed ‘Dynamic In-Fill,’ may offer a method for reducing deployment costs while retaining the energy and non-energy benefits of dynamic glass. Dynamic in-fill is a method of installing an additional, non-structural window unit interior to the existing window. It adds insulative glass and air layers plus it enables the glass to actively change its performance in response to the environment or user preferences. It does not require demolition of the original window or displacement of occupants in operational facilities. In this way, Dynamic In-Fill has the potential to remove these primary barriers to adoption of dynamic windows for DoD installations. The objective of this project was to demonstrate the energy and capital savings enabled by View, Inc.’s dynamic windows by performing a limited-scope retrofit at the National War College in Washington, D.C.
Electrochromic glass is a window material that darkens when a voltage is applied to the glass. This darkening of the glass reduces the transmission of visible and infrared sunlight. When this glass is used in a typical double-paned window, it helps to reduce the heat load in a building that comes from solar irradiance.
By controlling the voltage, a dynamic glass glazing assembly can vary its solar heat gain coefficient (SHGC) from 0.46 to 0.09. This indicates that the transmitted radiant solar heat varies between approximately 46% to about 9% of the incident solar radiation. Likewise, the electrochromic device can vary its visible light transmission from 58% transmission to just 3% total light transmission. In addition, intermediate tint states can be selected to optimize performance of the windows throughout the day.
The purpose of this project was to demonstrate the energy and capital savings enabled by View, Inc.’s dynamic windows by performing a limited-scope retrofit at the National War College in Washington, D.C. Over the course of the project, the team developed a detailed energy-model for the demonstration site, with and without dynamic windows. The team then installed monitoring equipment in the demonstration site and performed baseline energy measurements. This data was then used to calibrate the baseline energy models and extrapolate energy consumption over an annual period. That extrapolated energy model was verified against measured building energy consumption.
The team then retrofitted 58 existing windows with Dynamic In-Fill glass units while the building remained occupied and operational. The installation took less than one month and was followed by months of energy and occupant monitoring. These results were used to further refine the energy models and the modeled results were annualized to determine the total annual impact of Dynamic In-Fill windows. From this data, total lifecycle cost, energy savings, and greenhouse gas (GHG) reductions relative to upgrading to state-of-the-art low emissivity (low-e) windows at the host site were quantified.
The project demonstrated a reduction in HVAC energy consumption of 6,900 kilowatt hours (kWh) or 6% compared to the existing windows baseline. Peak HVAC loads were reduced by 23% allowing for significant HVAC system downsizing and cost savings during their next maintenance cycle. Additionally, the daily peak cooling requirement shifted from 1 p.m. (typical of existing DoD buildings) to approximately 5 p.m., allowing off-occupancy, tapered conditioning. The reduced peak loads and load shift allowed for better system balancing (hot versus cold offices) for the entire building. Office temperatures were cooler when under direct sunlight. Visual analysis showed an elimination or reduction in the use of blinds and shades and more natural daylight within the offices.
Overall, the installation was completed in a timely manner and on budget. All performance objectives were met, and the host site staff and visitors have been enthusiastic and pleased with the impact on comfort in the building.
Most implementation and data collection issues were related to the renovation of the demonstration site. Due to the limitation of the existing panel layout in the building, the measured lighting energy and HVAC energy were combined in the collected data, which made it impractical to calibrate the lighting and HVAC energy separately. Second, the existing HVAC zoning and distribution system did not correlate 1:1 with the demonstration areas. Some energy exchange between treated and untreated spaces was likely, but unaddressed in the calculations.
One issue unrelated to the project site was occupant training and expectation management. While the demonstration project team conducted a basic training and operational overview of dynamic glass for the occupants, initially many of the test subjects expressed frustration with the transition time or confusion regarding transition logic. Better training protocols are recommended for future installations.
This project has created awareness and, most importantly, confidence with installing dynamic glass across many DoD installations.