Energy and visual comfort performance of electrochromic windows with overhangs
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 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 EC 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 EC 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 EC 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/m2) 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 EC 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 occupant's eye and desk surface and to reduce the window luminance level to below 5000 cd/m2. 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/m2 for old cathode ray tube computer monitors versus Lw-max=4000–5000 cd/m2 for the modern day flat-screen low-reflectance monitors. Summer energy savings for the GC (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-area windows (WWR=0.60) compared to a conventional window (Tv=0.75, SHGC=0.62) with the same Venetian blind control for the Rome, Stockholm, and Brussels climates. Heating energy use was in the same range as the reference case in all climates. Karlsson and LBNL simulation studies have modeled more advanced reference case windows so their estimates of energy savings were more conservative for a given climate. Platzer [7] conducted a TRNSYS-Radiance simulation study and found that switching according to room temperature yielded the least primary energy consumption but it was difficult to assess these results given the brevity of the article.
This study was conducted with several objectives. First, with the conclusion of the above simulation studies and field studies that investigated visual comfort issues associated with EC windows [8], [9], practical solutions are needed that couple the EC technology with sun-blocking technologies to meet visual comfort requirements yet still maintain the energy-efficiency advantage of using EC windows over conventional windows. Wienold [6] studied this issue using sophisticated simulation tools and algorithms but the main control function of the EC or GC windows in his study was to avoid summer overheating as described above. In the US, mechanical air-conditioning is a foregone conclusion even in the northern climates so the control schemes must tradeoff cooling versus lighting energy use savings while addressing visual comfort requirements. This study begins to look into such solutions. Second, a more detailed study was warranted after completing a broad parametric DOE-2 simulation study [10]. This prior LBNL study quantified annual energy use (ATE) and peak demand reductions resulting from unshaded EC windows controlled to optimize daylight compared to a variety of reference windows with and without manually deployed interior shades, fixed exterior overhangs and/or fins (depending on orientation), or exterior horizon obstructions as found in built-up urban environments. This study investigates alternate exterior-shaded EC strategies which may compete with conventional exterior-shaded windows. Third, it is likely that architects will wish to combine EC windows with attached exterior elements. This study provides some indication of how best to control EC windows given typical architectural solutions and the level of performance that is likely to result.
In this study, we use the DOE-2 simulation program to estimate possible energy savings when EC windows are configured and controlled with the goal of achieving energy-efficiency and visual comfort. A south-facing window wall has been divided into an upper and lower aperture. This configuration has been identified in prior research [11] as being beneficial for energy-efficiency and comfort—the upper aperture can be used to admit or even redirect controlled daylight while the lower aperture can be used to satisfy the view and visual comfort requirements of the occupant sitting close to the window. Control strategies for the upper and lower EC apertures were parametrically varied in order to identify the best strategies that would yield the least total energy use and yet provide the best control over discomfort glare. The window was also combined with an overhang to control direct sun. The overhang was placed either at the top of the overall window wall or between the two apertures. An interior shade was not modeled with the EC window. The rationale for combining the EC with an overhang is as follows. The overhang provides some measure of direct sun protection, although it reduces the need for the EC's variable solar heat gain rejection properties during the summer period since incident irradiation levels are significantly reduced. The overhang also reduces one's need and reliance on optimal interior shade control. Prior studies indicate that manually deployed interior shades are lowered typically because of direct sun or intolerable glare conditions then rarely readjusted [12]. Use of automatically controlled interior shades increases the cost of the overall EC window wall system. However, the overhang is unable to block low-angle direct sun in the early morning or late afternoon throughout the year nor low-angle winter sun throughout the day unless the overhang is impractically deep. For these periods depending on where the occupant is sitting and the latitude, an interior shade will be required to keep the sun orb out of view of the occupant. Total primary ATE, peak electric demand (PED), average annual daylight illuminance (DI), and the average annual daylight glare index (DGI) were computed using the DOE-2 simulation program for perimeter zone private offices in Houston and Chicago. Savings are compared to the state-of-the-art static spectrally selective low-e window with the same aperture size and zoning, overhang configuration, and daylighting control system. This reference window was combined with an interior shade which was deployed hourly to control direct sun and discomfort glare. Since the EC was not combined with an interior shade, DGI data will reflect periods of discomfort due to direct sun.
Section snippets
Building module
A 4459 m2 commercial office building prototype identical to that used in prior LBNL simulation studies [10] was modeled using the DOE-2.1E building energy simulation program. The prototype is a synthetic, hypothetical building, not a physically real building, with size, envelope construction, HVAC system type, operating schedules, etc. based on the mean prevailing condition among statistical samples and engineering judgment. The three-story prototype consists of a ground, intermediate and
Results
For each climate, performance data are given in Fig. 2, Fig. 3 for the two window sizes, six EC control algorithms (see Table 4 for key), overhang position, and overhang depth. The lines connecting the points (e.g., control scheme F and SS-a case) are meant to ease grouping of the datasets and are not meant to imply a relationship between distinct cases. Total primary ATE, PED, energy use savings, and demand savings are shown on the plots. Energy use and demand data are given per unit floor
Discussion
The key objective of this work was to identify practical solutions that both meet comfort requirements and deliver energy savings. There are several issues that require discussion. First, as mentioned earlier, horizontal projections such as overhangs do not block the orb of the sun during the critical periods when the sun is low on the horizon. For example, on clear sunny days between October 21 through February 21 in Chicago, even with a 1.5-m deep overhang on the south facade, direct sun is
Conclusions
DOE-2 building energy simulations were conducted to determine if there were practical architectural and control strategy solutions that would enable 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
Acknowledgments
This work was supported by the California Energy Commission through its Public Interest Energy Research Program, by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, State and Community Programs, Office of Building Research and Standards of the US Department of Energy under Contract no. DE-AC02-05CH11231, and by the Scientific and Research Council of Turkey (TUBITAK) through NATO-B2 Fellowship Program.
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