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Energy and visual comfort performance of electrochromic windows with overhangs 1
Submitted to Building and Environment, November 3, 2005,
and accepted for publication April 11, 2006.
LBNL-59064
Energy and visual comfort performance of electrochromic
windows with overhangs
E.S. Lee
*1
, A. Tavil
2

1
Building Technologies Program, Environmental Energy Technologies Division, Lawrence Berkeley
National Laboratory, Mailstop 90-3111, 1 Cyclotron Road, Berkeley, CA 94720, USA
2
Faculty of Architecture, Istanbul Technical University, Takla, Taksim, 34437, Istanbul, Turkey


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 static glass.

Keywords: Building simulation; Energy efficiency; Electrochromic windows; Daylighting; Control
algorithms

1. Introduction
Switchable electrochromic (EC) windows rely on a nanometer-thick switchable coating on glass to
reversibly change tint (clear to Prussian blue) without loss of view. For near-term products, the multi-layer
tungsten-oxide coating switches in the broadband range of visible and near-IR solar radiation, is absorptive,
and is best used on the inside surface of the exterior pane in a dual-pane unit so as to provide efficient solar
heat gain rejection when required. This coating placement also ensures that the EC glazing layer (in
combination with a low-emittance coating) does not become an interior radiator with high impinging solar
radiation levels, thereby addressing thermal comfort concerns. Switching ranges for near-term products are

* Corresponding author. Tel.: +1-510-486-4997; fax: +1-510-486-4089. Email address: eslee@lbl.gov (E.S.Lee). 2
fairly broad the contrast ratio (ratio of maximum-to-minimum visible transmittance) of one known product
is 12:1 with a corresponding thermal contrast ratio (ratio of maximum-to-minimum solar heat gain
coefficient) of 4:1 to 5:1. A comprehensive review of the progress toward viable EC commercial products
is given in [1]. EC products (which switch with a small applied dc voltage of 1-5 V) have been introduced
into the market (starting with Flabeg GmbH in Germany in 1997) but cost and concerns over durability are
still major impediments for the industry. Gasochromic windows (similar to electrochromic but are switched
using an inert hydrogen gas) are also under development. Suspended particle devices are a different class of
switchable devices that are similar in outward appearance to the electrochromic devices limited data
indicates that these devices may not possess the solar heat gain rejection properties nor longevity needed for
long-term energy-efficient commercial building applications.
Over the past decade, many simulation studies have been conducted to estimate the energy-savings
potential of electrochromic windows for various climates. Simulation studies have also been used to
identify control strategies that yielded the lowest energy use. The Lawrence Berkeley National Laboratory
(LBNL) conducted numerous DOE-2 commercial building energy simulation studies in the mid-1990s [e.g.,
2-3], concluding that significant annual total energy savings can be obtained compared to spectrally-
selective low-emittance (low-e) windows in moderate to hot climates if large-area EC windows are
controlled to maintain the interior illuminance setpoint level and are combined with daylighting controls. In
northern EU where commercial buildings are often heating-dominated and passive cooling is encouraged,
researchers have investigated alternate strategies with and without daylighting controls where the EC is
switched to provide passive heating during the winter and to reduce cooling requirements and overheating
during the summer. Karlsson [4] quantified heating and cooling annual energy savings for EC windows
controlled by incident vertical solar radiation limits (50-300 W/m
2
) in combination with occupancy-
controlled lighting and ventilation systems and a heat recovery mode regulated by interior temperatures.
This mode of control yielded small savings in the northern Stockholm climate and slightly greater savings in
the warmer climates of Denver and Miami given the moderate window-to-wall ratio (WWR=0.30).
Gugliermetti and Bisegna [5] conducted a parametric study to identify optimum incident solar radiation
limits that would yield the least total primary energy. A second set of simulations was conducted to address
discomfort glare due to bright sky luminance then compared to these optimal savings. For these
simulations, the daylight glare index was related to incident vertical solar radiation levels, then these limits
were used to switch the EC windows for visual comfort. Total primary energy use was increased by a small
margin (4-10%) on the east, south, and west facades and significantly (19%) on the north façade with the
visual comfort strategy compared to the best incident solar radiation strategies. These results were given for
a moderate sized window (WWR=0.33) in a typical office for three climates in Italy. The electric lights
were dimmed in response to daylight with photoelectric controls. When the space was unoccupied, the EC
was bleached (which unfortunately increases cooling loads) and the lights were turned off.
Simulations conducted as part of the Switchable Facades Technology (SWIFT) EU collaborative R&D
project directly addressed the concerns of visual comfort. Wienold [6] conducted a Radiance-ESP-r
simulation study to estimate lighting and cooling energy use savings for an electrochromic or gasochromic
(GC) system combined with a Venetian blind. The switchable glazings were fully colored to reduce solar
heat gains during the summer based on a incident vertical solar radiation threshold and fully bleached to
admit solar radiation (passive heating) during the winter. The Venetian blind was modeled to emulate
manual control the blind was lowered then the slat angle was tilted to block direct sun incident on the
occupants eye and desk surface and to reduce the window luminance level to below 5000 cd/m
2
. The
lighting control strategy included daylight-responsive dimming controls with occupancy-based switching.
Optimum vertical irradiance switching thresholds were identified through parametric runs for various EU
climates. Energy savings were found to be highly dependent (factors of 2-4) on the maximum acceptable
window luminance threshold, Lw-max, which unfortunately varies amongst various standards, occupant
views, and applications: e.g., Lw-max=400 cd/m
2
for old cathode ray tube computer monitors versus Lw-
max=4000-5000 cd/m
2
for the modern day flat-screen low-reflectance monitors. Summer energy savings for
the gasochromic (Tv=0.60-0.15, SHGC=0.47-0.14) were found to be 18-28% for moderately-sized windows
(WWR=0.30) and 48-55% for large-