@article {55433, title = {Lighting energy savings potential of split-pane electrochromic windows controlled for daylighting with visual comfort}, journal = {Energy and Buildings}, volume = {61}, year = {2013}, month = {06/2013}, pages = {8-20}, abstract = {

A simulation study was conducted to evaluate lighting energy savings of split-pane electrochromic (EC) windows controlled to satisfy key visual comfort parameters. Using the Radiance lighting simulation software, interior illuminance and luminance levels were computed for a south-facing private office illuminated by a window split into two independently-controlled EC panes. The transmittance of these was optimized hourly for a workplane illuminance target while meeting visual comfort constraints, using a least-squares algorithm with linear inequality constraints. Blinds were successively deployed until visual comfort criteria were satisfied. The energy performance of electrochromics proved to be highly dependent on how blinds were controlled. With hourly blind position adjustments, electrochromics showed significantly higher (62\% and 53\%, respectively without and with overhang)lighting energy consumption than clear glass. With a control algorithm designed to better approximate realistic manual control by an occupant, electrochromics achieved significant savings (48\% and 37\%, respectively without and with overhang). In all cases, energy consumption decreased when the workplace illuminance target was increased. In addition, the fraction of time during which the occupant had an unobstructed view of the outside was significantly greater with electrochromics: 10 months out of the year versus a handful of days for the reference case.

}, keywords = {Building energy-efficiency, Control optimization, daylighting, Electrochromic windows}, doi = {10.1016/j.enbuild.2012.10.057}, author = {Luis L. Fernandes and Eleanor S. Lee and Gregory J. Ward} } @article {58920, title = {Modeling of optical and energy performance of tungsten-oxide-based electrochromic windows including their intermediate states}, journal = {Solar Energy Materials and Solar Cells}, volume = {108}, year = {2013}, month = {1/2013}, pages = {129 - 135}, abstract = {

Tungsten-oxide-based electrochromic (EC) windows are currently the most robust and matured dynamic windows where the transmittance of visual light and near-infrared radiation can be controlled by a small applied voltage. In its standard application, the window is commonly either in its clear or colored state. In this contribution, we study the optical and energy performance of such window in the fully bleached and fully colored state as well as when it is kept in intermediate states. Different configurations in terms of placement of the EC layer stack and possible additional low-emissivity (low-E) coating within the insulated glass unit are considered. Using optical data and software tools we find that even a small coloration has a significant effect on the energy performance because the solar heat gain coefficient is readily reduced by the absorption of the EC layer stack. We compare the performance of the EC windows to commercially available solar-control (spectrally selective) low-E windows.

}, keywords = {Electrochromic windows, Energy efficient window modeling, Energy simulation, Smart windows}, issn = {09270248}, doi = {10.1016/j.solmat.2012.09.010}, author = {Sunnie H.N. Lim and Jan Isidorsson and Lizhong Sun and B. Leo Kwak and Andr{\'e} Anders} } @article {55431, title = {End User Impacts of Automated Electrochromic Windows in a Pilot Retrofit Application}, journal = {Energy and Buildings}, volume = {47}, year = {2012}, month = {04/2012}, pages = {267-284}, chapter = {267}, abstract = {

Automated electrochromic (EC) windows, advanced thermally-improved window frames, and a dimmable lighting system were installed in a single, west-facing conference room in Washington DC. The EC windows were commercially-available, tungsten-oxide switchable devices, modulated automatically between either fully clear or fully tinted transparent states to control solar gains, daylight, and discomfort glare. Occupants were permitted to manually override the automated EC controls. The system was monitored over a 15-month period under normal occupied conditions. The last six months were used in the analysis. Manual override data were analyzed to assess the EC control system design and user satisfaction with EC operations. Energy and comfort were evaluated using both monitored data and simulations.

Of the 328 meetings that occurred over the six month period, the automatic system was manually overridden on 14 or 4\% of the meetings for reasons other than demonstration purposes. When overridden, occupants appeared to have switched the individual zones with deliberation, using a combination of clear and tinted zones and the interior Venetian blinds to produce the desired interior environment. Monitored weekday lighting energy savings were 91\% compared to the existing lighting system, which was less efficient, had a higher illuminance setpoint, and no controls. Annual performance was estimated using EnergyPlus, where the existing condition met the ASHRAE 90.1-2007 prescriptive requirements except for a higher window U-value. Annual energy savings were 48\% while peak demand savings were 35\%.

}, keywords = {buildings energy efficiency, control systems, daylighting, Electrochromic windows, Integrated systems, Intelligent buildings}, doi = {10.1016/j.enbuild.2011.12.003}, author = {Eleanor S. Lee and Erin S. Claybaugh and Marc LaFrance} } @article {11811, title = {Energy and visual comfort performance of electrochromic windows with overhangs}, journal = {Building and Environment}, volume = {42}, year = {2007}, month = {06/2007}, pages = {2439-2449}, chapter = {2439}, abstract = {

DOE-2 building energy simulations were conducted to determine if there were practical architectural and control strategy solutions that would enable electrochromic (EC) windows to significantly improve visual comfort without eroding energy-efficiency benefits. EC windows were combined with overhangs since opaque overhangs provide protection from direct sun which EC windows are unable to do alone. The window wall was divided into an upper and lower aperture so that various combinations of overhang position and control strategies could be considered. The overhang was positioned either at the top of the upper window aperture or between the upper and lower apertures. Overhang depth was varied. EC control strategies were fully bleached at all times, modulated based on incident vertical solar radiation limits, or modulated to meet the design work plane illuminance with daylight. The EC performance was compared to a state-of-the-art spectrally selective low-e window with the same divided window wall, window size, and overhang as the EC configuration. The reference window was also combined with an interior shade which was manually deployed to control glare and direct sun. Both systems had the same daylighting control system to dim the electric lighting. Results were given for south-facing private offices in a typical commercial building.

In hot and cold climates such as Houston and Chicago, EC windows with overhangs can significantly reduce the average annual daylight glare index (DGI) and deliver significant annual energy use savings if the window area is large. Total primary annual energy use was increased by 2-5\% for moderate-area windows in either climate but decreased by 10\% in Chicago and 5\% in Houston for large-area windows. Peak electric demand can be reduced by 7-8\% for moderate-area windows and by 14-16\% for large-area windows in either climate. Energy and peak demand reductions can be significantly greater if the reference case does not have exterior shading or state-of-the-art glass.

}, keywords = {building simulation, Control algorithms, daylighting, Electrochromic windows, energy efficiency}, doi = {10.1016/j.buildenv.2006.04.016}, author = {Eleanor S. Lee and Aslihan Tavil} } @techreport {1077, title = {Advancement of Electrochromic Windows}, year = {2006}, month = {04/2006}, abstract = {

This guide provides consumer-oriented information about switchable electrochromic (EC) windows. Electrochromic windows change tint with a small applied voltage, providing building owners and occupants with the option to have clear or tinted windows at any time, irrespective of whether it{\textquoteright}s sunny or cloudy. EC windows can be manually or automatically controlled based on daylight, solar heat gain, glare, view, energy-efficiency, peak electricity demand response, or other criteria. Window controls can be integrated with other building systems, such as lighting and heating/cooling mechanical systems, to optimize interior environmental conditions, occupant comfort, and energy-efficiency.

}, keywords = {commercial buildings, daylight, daylighting controls, Electrochromic windows, energy efficiency, human factors, peak demand, switchable windows, visual comfort}, author = {Eleanor S. Lee and Stephen E. Selkowitz and Robert D. Clear and Dennis L. DiBartolomeo and Joseph H. Klems and Luis L. Fernandes and Gregory J. Ward and Vorapat Inkarojrit and Mehry Yazdanian} } @article {1861, title = {Subject Response to Electrochromic Windows}, journal = {Energy and Buildings}, volume = {38}, number = {7}, year = {2006}, month = {07/2006}, pages = {758-779}, chapter = {758}, abstract = {

Forty-three subjects worked in a private office with switchable electrochromic windows, manually-operated Venetian blinds, and dimmable fluorescent lights. The electrochromic window had a visible transmittance range of approximately 3-60\%. Analysis of subject responses and physical data collected during the work sessions showed that the electrochromic windows reduced the incidence of glare compared to working under a fixed transmittance (60\%) condition. Subjects used the Venetian blinds less often and preferred the variable transmittance condition, but used slightly more electric lighting with it than they did when window transmittance was fixed.

}, keywords = {Electrochromic windows, energy use, Subjective response, Venetian blind use}, doi = {10.1016/j.enbuild.2006.03.011}, author = {Robert D. Clear and Vorapat Inkarojrit and Eleanor S. Lee} }