Elsevier

Energy and Buildings

Volume 112, 15 January 2016, Pages 279-298
Energy and Buildings

Balancing daylight, glare, and energy-efficiency goals: An evaluation of exterior coplanar shading systems using complex fenestration modeling tools

https://doi.org/10.1016/j.enbuild.2015.12.009Get rights and content

Highlights

  • New Radiance and EnergyPlus simulation tools can model shades more accurately.

  • Exterior shades were characterized with bidirectional scattering distribution data.

  • Shade geometry, material finish, and solar cut-off angle affect system performance.

  • For some systems, additional indoor shades are needed to control discomfort glare.

  • With good design, exterior shades can significantly lower annual energy use.

Abstract

Exterior shades are the most effective way to control solar load in buildings. Twelve different coplanar shades with different geometry, material properties, and cut-off angles were investigated for two California climates: the moderate San Francisco Bay Area climate and a hot and dry Southern California climate. The presented results distinguish themselves from other simulation studies by a newly developed method that combines three research-grade software programs (Radiance, EnergyPlus, and Window 7) to calculate heat transfer, daylight, and glare resulting from optically-complex fenestration systems more accurately. Simulations were run for a case with constant electric lighting and a case with daylighting controls for a prototypical, internal load dominated office building.

In the case of daylighting controls, the choice of slat angle and solar cut-off angle of a fixed exterior slat shading system is non trivial. An optimum slat angle was identified for the considered cases. Material properties (e.g., solar and visible reflectance) did not affect energy use if constant electric lighting was assumed, but they did have a significant influence on energy use intensity (EUI) when daylighting controls were assumed. Energy use increased substantially when an additional interior shade was used for glare control.

Introduction

In the attempt to save energy in commercial buildings, exterior shades are more and more frequently seen in new construction projects in California, the US, and worldwide, and entire architectural trends are formed around the need to reduce solar load in the perimeter zones of high-rise buildings. Exterior shades, compared to other types of shading devices such as interior shades, between-pane shades or compared to electro-chromic glazing, are the most efficient way to reduce solar load because they block a large percentage of direct radiation before it hits the building envelope.

Solar radiation can be very beneficial, for example, at times when heating energy is needed. In office buildings, a significant benefit can be achieved by the visible light from the sun if there is a controlled lighting system installed which dims the electric lighting whenever the required illuminance level in the room can be fully or partially met by daylight (“daylighting control”). Therefore, blocking the sun means a trade-off between heating, cooling, fan, and lighting energy which has to be considered when making the decision for an exterior shade.

While there are established standards (e.g., NFRC [1]) and metrics (e.g., SHGC1) to determine the performance of glazing systems with various substrates and coatings, there are only few simple methods available to classify external shading systems such as EN 13363 [2] which calculates an overall solar heat gain coefficient for slat shading systems including the glazing system. Classifying exterior shades is challenging because of their two- or three-dimensional geometry. Depending on the sun angle, the transmitted radiation and the heat transfer through a complex fenestration system (CFS) varies significantly over the course of the day and the course of the year.

The section for non-residential buildings in Title 24 [3], the mandatory energy-efficiency standard for new constructions in California, requires in its prescriptive approach a RSHGC2 value for the whole window (including frame and potential window reveals) of 0.25 for fixed windows, 0.22 for operable windows, and 0.26 for curtain walls. The prescriptive approach of Title 24 defines a glazing system to be code compliant if it is combined with “an exterior operable shading louver or other exterior shading device that meets the required SHGC” or “a combination of Items A (glazing system) and B (exterior shading) to achieve the same performance” [3]. However, Title 24 does not provide a calculation method of RSHGC for windows with external shading devices other than overhangs and fins.

The prescriptive method is only applicable to buildings with a window-to-wall ratio (WWR) ≤40%. For WWRs greater than 40% the alternative calculation method (ACM) with a compliant building simulation software program has to be used. Similar to the prescriptive approach, Title 24 compliant software for proof of code compliance of the building does not necessarily provide the capability of modeling exterior shades other than overhangs and fins.

The accurate modeling of exterior shading devices and their impact on energy performance of a building requires a detailed representation of the geometric model in the simulation, the knowledge of the material surface properties (short-wave and long-wave reflectance, and transmittance, absorptance/emittance), and the necessary simulation tools to calculate angle-dependent properties of the system and their impact on energy performance of the building. The aim of this work is therefore manifold: (1) evaluate the potential of energy savings through exterior shades for large office buildings in California, (2) assess the variance of performance by modeling a variety of exterior shading systems which represent current practice in new construction and retrofit, and (3) gain insight as to which class of exterior shading system is preferable for a certain window-to-wall ratio and for specific climates.

While most building energy simulation programs have some capability of modeling blinds or simple shade geometries, the presented results distinguish themselves from previous studies by the selection of the modeling tools used and the subsequent accuracy of the results. Furthermore it considers a long neglected aspect in the trade-offs related to exterior shades which is especially important in office buildings and which has been excluded from building energy analysis for a long time: the occurrence of glare and the necessity to deploy an additional interior shade for glare control under certain conditions.

If office employees at work stations are disrupted in their work by high luminance levels from the façade, they are likely to lower the interior shade to reduce discomfort glare at their desks. Interior shades are only moderately efficient in reducing solar load because they block radiation only after it has entered the indoor space, but they reduce the availability of daylight significantly. The reduction of glare is the primary purpose when using interior shades. Unfortunately, however, interior shades often remain deployed after the glare condition has been resolved, and the deployed shades keep reducing daylight availability and the benefit of daylighting controls.

Section snippets

Characterization of the complex fenestration systems

The 12 exterior shading systems assessed in this study were chosen to represent a wide range of exterior shades and materials used in modern architecture as well as in retrofit projects. The combination of shading system and glazing system is called complex fenestration system (CFS) in the following sections.

Influence of slat angle and cut-off angle on energy use intensity

In order to assess the impact of slat angle and cut-off angle for an exterior blind system with flat slats and a moderate reflectance (Rvis = Rsol = 0.5), the geometry of the commercially available product shd 2 was modified (see Table 3). The cut-off angle is defined as the highest profile angle at which direct radiation is transmitted into the space.

This section presents annual energy use intensity (EUI) in kW h/m2 a of a 4.5 m deep perimeter zone. A different trend for site vs. source energy can be

Influence of glare on energy use intensity

While the previous Section 3.2 showed a benefit of highly reflective surfaces on exterior blinds when using daylighting controls, it did not consider the possibility of discomfort glare.

Not only direct radiation can lead to discomfort glare but light that is reflected from the shade into the room may also contribute to a situation where visual comfort at a workstation cannot be maintained without the use of an interior shade. The probability of glare conditions in the perimeter zone of an

Energy use intensity depending on window-to-wall ratio with daylighting control and glare control

Section 4 has shown that the impact of glare control on the overall source energy use intensity is minor when no daylighting controls are installed. However, when electric lighting is dimmed according to daylight availability, the deployment of an interior shade to avoid discomfort glare reduces significantly the energy savings that can be achieved with exterior shades on facades with a WWR of 60%.

To assess this impact further, this section contains results for all exterior shading systems with

Discussion

The paper presents results of a simulation study where the geometry and material properties of 12 different exterior shading systems were investigated. For lighting energy, the prototypical large office building was modeled with and without daylighting controls. In Sections 4 Influence of glare on energy use intensity, 5 Energy use intensity depending on window-to-wall ratio with daylighting control and glare control, an additional interior shade was modeled and deployed when a discomfort glare

Conclusions

The study showed that the paradigm of moderate window-to-wall ratios to reduce cooling load is questionable given the possible energy savings that can be achieved with a combination of exterior shades and daylighting controls. While the presented results assumed fixed exterior shades that were not controlled in any way, one can presume that adding shade automation will improve solar control and lead to even better performing buildings. One of the major tasks in the future will be to develop

Acknowledgments

This work was supported by the California Energy Commission through its Public Interest Energy Research (PIER) Program on behalf of the citizens of California.

Completion of the paper was supported through the research center High Performance Composite Constructions, HiPerCon, at University of Kaiserslautern.

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