Evaluation of energy performance of dynamic overhang systems for US residential buildings
Graphical abstract
Introduction
For buildings, windows provide several benefits to occupants including access to natural light, outdoor views, and even fresh air. However, windows are also the sources of unwanted heat losses and gains and possibly condensation problems even when high performance glazing is used [1], [2]. During 2018, it is estimated that windows are responsible for 4.6 quadrillion of Btu (i.e., 11,342 TWh) of primary energy use corresponding to 12% of the total energy consumed by the existing US residential and commercial building stocks [3]. Uncontrolled solar heat gains represent 37% of the US primary energy use attributed to windows [3].
As architectural features for solar radiation controls, shading devices have been widely considered for both residential and commercial buildings. Common shading devices evaluated in the literature include overhangs [4], external roller shades [5], and venetian blinds [6], [7]. Well-designed shading devices can significantly reduce building heating and cooling energy demand and electrical peak heat gain as well as and improve natural lighting quality and visual comfort. The effectiveness of these devices depends on their geometric features, their optical properties as well as the climatic conditions and seasonal variations [8], [9].
The analysis of fixed exterior shading devices has been widely evaluated for various building types and climatic conditions [10], [11], [12], [13], [14], [15], [16]. For instance, Cho et al. [13] have analyzed the energy and daylighting performance of over 48 exterior shading devices for high-rise residential buildings in Seoul, Korea. They found that horizontal overhangs can save up to 20% of cooling energy demand. Using a life cycle assessment (LCA) analysis, Babaizadeh et al. [14] compared the environmental and economic performance of various exterior shading devices made of different materials (wood, PVC, and aluminum) when applied to a one-story home located in five US climates. They found that overhangs (horizontal or slightly tilted down) provide the best annual energy savings especially for hot climates. Moreover, wood fabricated shades were deemed to have the best LCA scores with higher environmental benefits compared to those made up of aluminum and then PVC. Hoffman et al. [15] showed through a detailed simulation analysis that the energy savings of exterior shading systems when applied to office buildings located in Burbank, CA, depend significantly on the window size as well as glare controls. While the energy savings from applying static exterior shades can exceed 40% when the window-to-wall ratio is 60%, these savings do not exceed 10% for WWR below 30% considering glare controls.
As alternative to static systems, dynamic building envelope components and façades have been shown to reduce energy use as well as adapt better to occupant needs [17], [18]. In particular, the potential benefits of movable and adaptive solar shading devices have been reported in the literature [19], [20], [21]. For instance, Nielsen et al. [19] have shown that a theoretical dynamic exterior shading system can reduce total energy demand compared to a no-shading option for an office room by 16% when applied to southern exposed window in a heating dominated climate. The dynamic shading system is estimated to have less energy efficiency potential when the windows were exposed east, west, and especially north for which no energy saving was obtained compared no-shading case. The analysis did not consider other climates as well as spaces including entire buildings with windows exposed to more than one orientation. Moreover, the benefits of using movable exterior shading systems for windows have been evaluated for an office space in Abu-Dhabi characterized by a very hot climate [20]. The analysis includes the energy and daylighting performance of movable louver systems as exterior shading systems for windows. In particular, it is found that the movable louvers applied to south oriented windows can save up to 34% in energy use mostly due to daylighting effect when compared to no-shading device and dimming control. Fixed louvers set an optimal angle of 20° were found to save 31% of the total energy demand of the same office space with south façade. By adjusting directly the solar radiation levels within the weather file, Vlachokostas and Madamopoulos [21] estimated the benefits of solar control through dynamic shading devices. Specifically, they reduced both the direct and diffuse solar radiation components by applying a reduction factor ranging from 100% to 10% using 1% increment. Through a combinatory simulation analysis for an office space located in New York City, NY, they estimated the potential energy savings for dynamic control strategies. They found that dynamic shading devices can be beneficial for windows oriented South, West, and East. Moreover, they have indicated that the annual energy savings depend largely on the time scale and range from 15% to 18% for monthly controls, 20%–22% for daily controls, and 33%-36% for hourly controls.
The vast majority of the existing whole-building energy simulation tools are not capable of modeling dynamic building envelope systems [18], [22]. However, some tools have some flexibility to allow evaluation of dynamic systems such as the case for EnergyPlus [23] and DOE-2 [24] with the ability to model moveable shading devices even though with limited control options using shading and solar transmittance schedules [22]. Moreover, alternative modeling techniques for adaptive building envelope systems have been proposed and testing including distinct building energy models (DBEMs) encompassing different models for various static shading states or the recently proposed parametric behavior maps (PBMs) approach suitable for dynamic systems with several settings [22].
In this paper, a novel system is evaluated to maximize the energy efficiency benefits of dynamic overhangs when applied to US residential buildings. The system consists of overhang that can rotate around one axis to allow its shading effect to be adjusted depending on the season and even the hour of the day. The rotating overhangs can be automatically controlled to optimize overall heat gain though the windows to ultimately minimize heating and cooling thermal loads throughout the year. Specifically, the controls operate the overhang shades to minimize cooling energy use during the summer as well as heating energy use during the winter. The basic design features and operation of the dynamic overhang is first described. Then, the analysis approach is outlined including the building energy models considered in the analysis. The performance of the dynamic overhang systems is assessed for various operation scenarios, design features, and climate conditions.
Section snippets
Analysis approach
The analysis in this study is carried out using hourly building energy modeling of typical US housing units with various window sizes and glazing types as well as dynamic overhang designs and operating controls. First, the basic design of the dynamic overhang is described. Then, the control strategies as well as the modeling approach of the dynamic overhangs are outlined. Finally, the characteristics of the housing units considered in the analysis are summarized.
Discussion of results
In this section, some of the analysis results for individual room are first presented when the housing unit is located in Boulder, CO, to assess the main factors that affect the energy performance of the dynamic overhangs. Then, the main findings specific to the application of the dynamic overhang shades to a prototypical range house in various US locations are summarized considering various operating scenarios and design features.
Summary and conclusions
In this paper, the energy performance of dynamic overhangs is evaluated for US residential buildings. The analysis has indicated the proposed dynamic overhangs have significant energy efficiency potential for both new and existing buildings compared to static overhangs and no-overhang options especially for warm US climates even when simplified controls are considered. Specifically, three operating scenarios are considered depending on the angle positions allowed for the dynamic overhangs to
CRediT authorship contribution statement
Moncef Krarti: Conceptualization, Investigation, Data curation, Software, Methodology, Validation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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2022, Sustainable Cities and SocietyCitation Excerpt :According to different geometric features, operation modes, and evaluation methods, dynamic shading can be classified into conventional dynamic shading and complex dynamic shading (see Fig. 1). Conventional dynamic shading systems, such as overhangs (Krarti, 2021), Venetian blinds (Tabadkani, Tsangrassoulis, Roetzel, and Li, 2020), roll shades (Do and Chan, 2021), and green façades (Bakhshoodeh, Ocampo, and Oldham, 2022), improve daylighting and thermal performance by eliminating excessive sunlight and blocking unnecessary solar radiation. Complex dynamic shading systems, such as origami-based kinetic facades (Le-Thanh et al., 2021), biomimetic adaptive skins (Kuru, Oldfield, Bonser, and Fiorito, 2022) or bio-inspired skins (Hosseini, Mohammadi, Schröder, and Guerra-Santin, 2021), and productive skins (Zhang, Zhang, and Li, 2022) (Akbari Paydar, 2020), involve advanced control mechanisms with extra architectural aesthetic features and identities.