Optimal energy performance of dynamic sliding and insulated shades for residential buildings

Highlights

• New and low-cost dynamic and insulated shading systems are introduced.

• Energy benefits of the dynamic shades are assessed for residential buildings.

• Optimal controls for the dynamic shades are determined for US climates.

• Dynamic shades achieve over 50% reduction in heating and cooling energy use.

Abstract

The paper evaluates the potential energy efficiency benefits for dynamic sliding exterior shades suitable for windows of both new and existing buildings. In particular, the analysis outlined in the paper summarizes the energy performance of dynamic sliding shades when applied to windows for apartment units located in various US climates. A series of sensitivity analyses is performed using a detailed whole-building simulation coupled with modeling techniques of dynamic envelope systems to determine the optimal positions for the dynamic sliding shades to minimize the annual demand for US housing units. Several operation strategies are evaluated using continuous and stepped settings of the positions for the dynamic sliding shades using monthly, daily, and hourly adjustment frequencies. The results of the analyses indicate that the dynamic shades have significant energy efficiency potential for all US climates and design conditions with annual savings in HVAC energy end-use of over 50% especially in hot and mild climates. The performance of the dynamic shades is even higher for large and single-pane windows indicating that these systems can serve as retrofit alternatives to window replacements.

Introduction

In the US, buildings consumed 39% of total primary energy use with heating and cooling equipment accounting for a significant portion of this energy needed to maintain indoor thermal comfort [1]. US Department of Energy (DOE) estimated that windows are responsible directly for 10% of energy use in buildings and affect indirectly other end-uses such heating, cooling lighting, fans, and pumps which account for 40% of total US buildings' energy consumption [2]. Specifically, windows affect thermal performance of buildings through several mechanisms heat transmission (i.e., conduction and convection through the glazing layers), air infiltration (i.e., uncontrolled airflow through cracks around the windows’ frames), and solar heat gains (i.e., solar radiation transmitted through the transparent glazing). In addition, windows affect natural light aperture and consequently the use of electrical lighting based on daylighting controls. In the last decades, significant efforts have been made to improve the energy performance of windows using thin triple or vacuum-insulated glazing [3], high-performing inert gas fills [4], insulating frames [5], switchable or dynamic glazing [6]. While high performance windows can be implemented in new constructions, the replacement of existing windows are generally expensive and not cost-effective. Therefore, low-cost technologies to improve the energy efficiency of windows suitable for existing buildings are still required especially for US housing units [7]. Indeed, recent DOE predictions indicate that only third of residential units will be replaced by 2050 compared to more than half for the commercial buildings [7].

As alternatives to high-performance windows, static and dynamic attachments can be effective to reduce energy use of buildings. Indeed, external and internal shading systems can reduce air conditioning needs for buildings by managing solar heat gains from windows including overhangs [8], external roller shades [9], and venetian blinds [10,11]. Automated controls for interior shades have been evaluated extensively in the literature. In particular, several studies have shown that interior shading systems with automatic controls have the potential to reduce significantly energy use of buildings compared to static or manually operated systems [[12], [13], [14], [15]]. Through a controlled laboratory experimental analysis, dynamic roller shades found to reduce heating and cooling energy use by 26% compared to no-shading option [12]. The effectiveness of simplified control schemes in balancing visual and thermal performance for automatic roller shades has been evaluated both through modeling analysis and experimental testing [13] for office spaces [14]. Moreover, Tan et al. [15] modeled the impacts of dynamic roller shades on energy demand of residential buildings using equivalent shading schedules. Their analysis indicated that the shading performance vary significantly and can result in a decrease of up to 14.2% and an increase of up to 22.2% in the annual energy heating and cooling demand [15].

However, some simulation and experimental based analyses have indicated that exterior shades provide more energy benefits than interior blinds [16] including better management of solar gains [17] and higher reduction in air conditioning needs [18]. Limited analyses have been reported for dynamic external shading systems for buildings [[19], [20], [21], [22]]. For instance, Vlachokostas and Madamopoulos [21] evaluated the energy efficiency potential of using dynamic shading devices by adjusting the direct and diffuse solar radiation data in the weather file. Using a simulation-based analysis for an office space in New York City, NY, they found that dynamic shading devices reduce annual energy use by 33%–36% if controlled on an hourly basis [21]. Rotating overhangs with the ability to be adjusted continuously are estimated to save 9.5% of annual heating and cooling needs for a home located in Chicago, IL, when compared to the baseline case with no-overhangs [22]. On the other hand, static overhangs applied to the same home would increase the heating and cooling energy use by 6.8% relative to the baseline [22].

However, automatically operated dynamic shading systems have limited market adoption due to several factors including high costs and complexity of operation [7]. Moreover, the lack of optimized and yet simple controls is not readily available and proven [7]. In this paper, low-cost dynamic and insulating shading systems suitable to be controlled automatically to manage solar heat gains as well as to enhance the thermal performance of windows are described and evaluated. The dynamic shading systems can be deployed for new and existing residential and commercial buildings to minimize cooling and heating thermal loads. First, the basic design features and operation strategies of the dynamic shading systems are introduced. Then, the modeling approach is outlined including the characteristics of the housing unit considered in the analysis. Finally, the results of a series of sensitivity analyses are discussed to assess the performance of the dynamic shading systems for various operation scenarios, design features, and climate conditions.