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High Performance Façades

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Dynamic automated façade systems control the synergies of the sun on a real time basis to enable reduction of lighting and HVAC use of the building delivering increased comfort and amenity to the occupants. Architect Rajinder M Srivastava explains, how a façade system delivers the greatest performance when it becomes an essential part of a fully integrated building design.

Globally, there is a growing interest in designing, analyzing and operating a façade as part of a system. Technological solutions are being used to produce high-performance building façades that are based on fundamental concepts of daylighting, solar heat gain control and ventilation. These façade systems also respect the limits of latitude, location, solar orientation & acoustics and have the capability to respond and adapt to dynamic variables of climate and occupant needs. A typically, high-performance façade is characterized by:

• Enhanced sun protection & increased thermal comfort

• Reduced operating costs via daylighting-thermal tradeoff

• Natural ventilation strategy by employing the façade as an active air-control element

Exterior Solar Control

The general concept of exterior solar control is to stop direct sunlight from entering the building. Exterior solar control can be provided by overhang, fin or full window-screen geometries. Exterior solar control should be designed to intercept direct sun when cooling-load control is desired. Some examples include louvers and blinds composed of horizontal or vertical slats. Exterior blinds are more durable and made of galvanized steel, anodized or painted aluminum or PVC for low maintenance. With different shapes and reflectivity, louvers and blinds are used not only for solar shading, but also for redirecting daylight. While fixed systems are designed mainly for solar shading, operable systems can be used to control thermal gain, reduce glare, and redirect sunlight. Operable systems (whether manual or automatic) provide more flexibility, responding to outdoor conditions. These systems perform well in all climates. For buildings in hot climates, the system may be more energy efficient if placed on the exterior of the building while blocking solar radiation. For buildings in cold climates, the system can be used to provide more daylight and absorb solar radiation. The sun louvre system can be projected to the façade, parallel to the façade or otherwise designed in relation to the angle of the sun to blend in style and functionality with building’s exterior

Sunlight Redirection

Sunlight-redirection systems rely on reflection, refraction, diffraction or non-imaging optics to modify or enhance the distribution of incoming daylight. One example of a sunlight-redirection system is light shelves. The horizontal, exterior projections use a high-reflectance, diffuse, or semi-specular (shiny) upper surface to reflect sunlight to a certain interior depth from the window wall. Redirection systems for direct sunlight are most effective on the south façade and are designed based on seasonal variations in solar altitude. In summer, light shelves block direct sun at both the upper and lower windows. In winter, low sun can penetrate to the back through the clerestory, preheating occupied space in the morning and providing light. For moderate and hot climates, daylighting strategies are integrated with solar-gain control. Tinted glazing can be used on lower-view windows, while clear glazing can be used at the clerestory level to increase daylight admission.

Skylight Redirection

These systems designed to diffuse skylight are intended to increase overall interior daylight levels with less emphasis on the depth of light redirection. The systems are most useful in climates with predominantly cloudy conditions. Typical characteristics of anidolic systems include sharp cut-off angles for admitting and delivering light and very few reflections of light rays when passing light through the system. Holographic optical elements (HOEs) can also be applied to redirect skylight. Tilted-glass HOE overhangs can be placed over north-facing windows so that diffuse daylight is redirected into the building interior. (HOE glazing is still under development).

 

Heat Extraction Double-Skin Façades

These systems consist of a exterior layer of heat-strengthened or laminated safety glass with exterior air inlet and outlet openings manually or automatically controlled. The interior façade consists of fixed or operable, double- or single-pane windows. In between these two façades are retractable or fixed blinds, or roller shades again manual operated or automated. For cooling conditions, the blinds/shades cover the full height of the façade and are tilted to block direct sun. The concept is similar to exterior shading systems but the heat absorbed by the between-pane shading system is released within the intermediate space and drawn off through the exterior skin by natural or mechanical ventilation means. Other benefit includes heat-recovery opportunities in the winter and heat-extraction opportunities in the summer due to the second layer of glass.

Night time Ventilation

The nighttime ventilation system minimizes air-flow obstructions. System implementation involves motor-operated flaps and windows that are controlled by a centralized building automation system. During the summer and in climates with adequate variation in temperature and good prevailing winds, nighttime ventilation can be used to cool down the building reducing air-conditioning loads. Heat gains generated during the day due to absorption by furnishings, walls, floors and other surfaces are removed through cross-ventilation methods that rely on wind-induced flow, stack effect, and/or mechanical ventilation.

Facades with such intelligent shading systems have optical and thermal properties that can be dynamically changed in response to climate, occupant preferences and building energy management control system (EMCS) requirements. These include motorized shades, switchable electro-chromic or gaso-chromic window coatings and double-envelope macroscopic window-wall systems. “Smart windows” could reduce peak electric loads by 20-30% in many commercial buildings and increase daylighting benefits as well as improve comfort and potentially enhance productivity in homes and offices. While, these technologies provide maximum flexibility in managing demand and energy use in buildings, it also offers building owners options to dynamically control envelope-driven HVAC and lighting loads.

When considering high performance façade, discussion between all involved parties is the most effective way to make choices. Computer-based tools can also aid in the decision-making process. In addition, advanced software can simulate the energy performance of a building over periods of time to emulate the effects of a particular high-performance façade system on energy consumption.

References: Lawrence Berkeley National Laboratory’s
High-Performance Commercial Building Façades review.

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