Integration: Lighting and HVAC systems

04/29/2015


Shading and siting

Is there any time that we don't need to shade and protect a building from the solar environment?

Go back to the earlier statement that "for the best potential results, daylighting must be considered very early in the design process for any building." The best way to allow daylight with the fewest chances of problematic sun side-effects is to create the best shape of the building and the best location for windows.

Windows facing east or west will get glare from the sun at low angles in the sky every day(morning sunrise on the east, morning sunset on the west), and they also will have the high angle and high heat sun at midday for at least part of the time before or after noon.

Windows facing south (or facing north in the Southern Hemisphere) will get the high-angle, high-heat sun throughout most of the day all year, but will not have the sunset or sunrise directly ahead of them. In favorable locations, this can simplify shading requirements for the high angle sun, and alleviate the need for shading of lower angle sun positions. In locations farther from the equator, however, the low-angle sun in winter could cause glare at more times of day. Because winter sunrise and sunset are more southerly, they have a notable impact on the southern exposure.

Windows facing north (or facing south in the Southern Hemisphere) are the ideal for avoiding solar heat gain while admitting useful diffused daylight. When the sun rises or sets in the northerly sky during the summer, it's typically very early or very late in the day and outside of normal work hours, with no need for glare protection. When the sun is high in the sky and the outside temperatures are high, the sun is never on the side of the building that faces away from the earth's equator.

Figure 2: In this south façade section at the Duke University Environment Hall, circulation corridors located nearest the south façade are shaded in pink, with external shading to limit heat gains, but temperature fluctuations are allowed in the corridorIf every building was constructed in an anecdotal "vacuum," then every building could be a long floor plate, with circulation spaces tolerant of temperature variations along the south façade, firestairs on the east and west ends of the building with no need for daylight, and a long façade of windows for occupied spaces facing away from the equator. These theoretical buildings would also need to have all the beautiful landscape features and distant city skyline views in the same direction that the occupied spaces are getting their diffuse daylight from-if we want to avoid disrupting this idealized scenario.

In the real world, buildings are limited to particular sites, with street frontage and size of lots impacting building shape; desirable views to natural or manmade landmarks in various directions(or avoiding undesirable views); and resulting in windows facing east, south, west, and north. Still,early design considerations prevail over engineering solutions. For example, can cafes, break rooms, lounges, lobbies, and other transient spaces with easier acceptance of glare and temperature variation be located where the sun is most problematic? Can the spaces that require the least light-that can have the smallest windows and use the least energy when blinds are closed-be located to the east and west where blinds will be closed daily at sunrise and sunset?

After we assume the building has at least partially non-ideal orientation of occupied spaces, how can we accommodate daylight in a way that considers the best outcome for both lighting and thermal concerns? It becomes necessary to be selective about what portions of the sun's radiation are admitted to the building. A building can select what wavelengths of sun to admit, at what times sun is admitted to each window, and what angles of sun to admit.

Figure 3: The façade system photographed by Zhou Ruogu Architecture Photography integrates photovoltaics with glazing to provide partial shading, plus energy generation during the day, and is transformed with LED color-changing media effects at night poweGlazing and shading

Selecting wavelengths could be considered a part of the status quo approach. It is typical that in climates requiring multi-pane insulated glazing, the glazing is likely to have a low-emissivity coating. In milder climates, a film or interlayer with particular reflectance and transmittance properties may take the place of low-e coating to limit infrared transmission into a building, while still allowing proportionally more visible light indoors. Even when additional solar design techniques are employed, the benefits of coatings to tailor wavelength transmission can be valuable. Filtering wavelengths of light can improve the HVAC situation by limiting sun, but it can't improve the issues of glare and requirements for window blinds.

Selecting times of day also could be considered part of the status quo in many cases, although there is quite a range of methods by which solar impact on buildings can be filtered temporally-running the gamut from user-operated interior window blinds to computer-controlled and motor-operated exterior louver systems. The fallback lowest risk option is manually operated window blinds: people can always lower the blinds when needed, the cost and complexity of installation is basic, and if the shades are left down by occupants, the mechanical system doesn't suffer,though electric lighting stays energized to compensate and counteracts much of the intention for the glass in the first place.

If interior window blinds can be motorized and automated, it is a marginal improvement that the blinds are opened at least once a day, and admit useful visible radiation (daylight) for most of the day until a person or an automated sensor closes the blinds-at the times that blinds must be down to prevent glare for workplace comfort and productivity.

When simple blinds are internal, they might reflect a bit of infrared energy back through the glass outward to the exterior, but some of the heat has already been absorbed into the fabric or venetian blinds and transmitted through them to the building interior. If an automated motorized shading system can be moved to the building exterior, it can block as much infrared heat from the building at some times as it can block the visible glare at other times, and serve a more useful dual purpose. A more advanced and complex step is to combine automation for various times of day and seasons, with angular solar selectivity.

Selecting angles of radiation to admit can be done even without automation, by using fixed louvers, cellular panelized modules, overhangs, and so on. In traditional architecture, this could take the form of window shutters and jalousie systems, eaves and overhangs, colonnades and porticos, mashrabiya, and more. While architectural styles evolve over time, and these traditional shading forms may be reserved for historical revivals, the current necessity for cautious use of energy brings notably visible shading systems back to the forefront of architecture. It is only with notable shading systems that any degree of expansive glazing design can be responsibly enjoyed.

Figure 4: At the Sacramento International Airport Terminal B, photographed by John Swain Photography, an external horizontal shading system reduces solar heat gains while also tempering direct sunlight for visual comfort in the ticketing hall.The most productive angular selection tends to be a horizontally biased louver or shading grid to cut out the highest and hottest sun angles. On the east and west, one often reiterated but problematic thought might be to implement vertical louver-like systems. The low-angle sun can probably only be shaded for limited times of the year by such a system, while a horizontal shading system can block some mid-day sun throughout the year. However, if an angularly selective set of louvers can be adjustable and motorized, they might be able to adjust at various times of day to block both the high-angle sun for heat concerns, and the low-angle sun for glare concerns at other times, while rarely blocking as much daylight as a non-angular shading system would block.

Aside from selective shading of solar radiation, non-selective general shading such as fritting and mesh shade screens is another option. While blanket-coverage shading on the outside of a façade can reduce solar heat gain, it likely reduces potentially useful visible light to the same degree, and limits this useful light at all hours, including when there is no glare or heat gain concern. In some cases, such as frit or diffusing interlayers, these approaches may even reduce visible radiation more so than infrared, which may help glare or excess illumination exposure but may not have as notable a double-duty benefit as infrared protection at the same time.

There is one more key consideration in optimal and smart design and planning of glazing: striving toward glass of the size and specification for useful coverage and transmission without excess.While admitting radiation in the form of useful light and infrared heat gain, glass is also commonly a chink in the armor of the building envelope's insulation. So excess glass might allow excess heat into the building during the day beyond useful illumination and views, while also allowing excess heat to leave the building at night and possibly adding to heating energy expenditure. Glazing panels are always limited in their insulating ability compared to the potential insulation of other portions of the building envelope, making excess glass at least a three-fold problem in glare, infrared heat, and insulation impacts.



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