Concepts: Getting to Zero-Energy Façades
Concept 1. Find the optimum between daylight and solar heat gains admissions.
Let's begin with a bit of history on how designers have been quietly and indirectly pushed by codes and standards to optimum solutions. In the early 1980s, ASHRAE commissioned a DOE-2 building energy simulation study [1] involving tens of thousands of parametric runs of commercial buildings situated in various climates. The project applied regression analysis on these data and quantified trade-off relationships between the façade, lighting, and HVAC systems which were later incorporated into the ASHRAE 90 Standard. Equations resulting from this study are still seen in the ASHRAE 90.1-2004 performance approach method's Appendix C. Due to its complex presentation, it is unlikely that most who see this Appendix will understand the simple synergistic relationship it is trying to convey.
This trade-off synergistic relationship is simple and applies to the perimeter zones of typical commercial buildings in both hot and cold climates throughout the US with high internal loads:
- Decrease window area or its solar transmission and cooling energy use is decreased.
- Increase window area or its daylight transmission and lighting energy use and associated heat gains are decreased.
Minimum source or primary energy use is achieved through a balance between these two competing design objectives.
Cooling (left) energy use increases with increased window area or solar transmittance of the glass. Lighting (right) energy use decreases then levels out with increased window area or visible transmittance of the glass.
Use early schematic design simulation tools like COMFEN to find the optimum combination of window orientation, area, glass, and attached shading for specific façade designs.
Cold Climates: Chicago, Illinois
A. With clear double glazing, a very small south-facing window with daylighting controls yields the least annual energy use — 10% below ASHRAE 90.1-2004, irrespective of interior or exterior shading.
B. With triple-pane, low-e clear glazing, an overhang, and daylighting controls, you can use larger windows and reduce annual energy use by 24% below code.
Hot Climates: Houston, Texas
C. With bronze tinted double glazing, a very small south-facing window with daylighting controls yields least annual energy use if no shading is used.
D. With triple-pane, low-e clear glazing, an overhang and fins, and daylighting controls, you can use larger windows and reduce annual energy use by 24% below code.
Note: Annual (primary) energy use includes heating, cooling, lighting, and all other energy end uses in a typical commercial office building. A site-to-source efficiency factor of 3:1 was used for electricity and 1:1 for natural gas.
[1] Johnson, R., R. Sullivan, S. Nozaki, S. Selkowitz, C. Conner, D. Arasteh. Final Report: Building Envelope Thermal and Daylighting Analysis in Support of Recommendations to Upgrade ASHRAE/IES Standard 90. LBNL-16770. [PDF]
Concept 2. Maximize daylight without introducing discomfort glare.
The classic problem that plagues sidelit perimeter spaces is that occupants sitting nearest the window will lower the shades to avoid thermal discomfort from direct sun or visual discomfort from glare. When conventional top-down shades are lowered, it tends to eliminate much of the useful daylight and view, causing occupants farther from the window to rely more on the electric lighting system. Often, shades are left lowered for days or weeks at a time, irrespective of sunny or cloudy conditions.
The concept of useful or efficient daylighting is to be able to balance the luminance conditions in the overall space, lowering dark and light contrasts, so that daylit conditions are comfortable. High-performance façades in combination with thoughtful interior design are able to maintain comfort conditions and daylight efficiency so that electric lighting system use is minimized.
As a general rule of thumb, direct sun should be blocked from falling on task surfaces and the occupant. Analyze when direct sun will affect comfort, then select shading solutions to block sun yet maximize daylight and view out.
Discomfort glare is much more difficult to quantify and design for since it is highly dependent on the occupant's direction of view and task. Very few studies have been conducted to derive models to predict discomfort glare. For reading and writing tasks involving paper, discomfort glare from windows is less of an issue than for tasks involving computer displays. So if computers are involved, take particular care to keep interior luminance (brightness) levels well controlled.
In the LBNL Windows Testbed, we see that zoned shading systems can bring daylight in through the top while controlling glare at the bottom. Brightness of the ceiling and the upper clerestory can also cause discomfort glare and so should be carefully controlled.
Red clocks and blue lines (left) show when the sun is obstructed by attached shading and nearby buildings (right). In the New York Times Building, automated roller shades were retracted when there was no direct sun or glare, increasing daylight to the interior.
Concept 3. Eliminate perimeter cooling and heating.
Neutralize the façades so that additional perimeter heating or cooling is not required. This can often be done using simple and straightforward measures such as limiting window area and decreasing the U-factor of the windows and frames. High-R windows and frames raise the interior surface temperature of the glazing and frame, lessening thermal discomfort due to differences in mean radiant temperature between the occupant and surrounding room surfaces (e.g., radiation from a hot oven causes warm thermal sensation, not the air temperature).
The window on the left is a double glazing with low-E and an insulating spacer. The window on right also uses low-E and a partially insulating spacer. The difference is that on the right the window uses three different low-E coatings in a quadruple layer design and the air inside the panes has been replaced with more insulating krypton gas. Such a high performance window is called a "superwindow". These windows are being cooled on the back side with wind at -17.8°C (0°F).
Concept 4. Consider opportunities for low-energy cooling.
Low-energy cooling strategies provide significant opportunities to reduce or even eliminate the need for conventional refrigerent-based chiller systems. Underfloor air distribution (UFAD) systems, radiant cooling, and natural ventilation strategies all require very careful control of façade thermal loads to ensure comfort conditions are maintained. Remember daylight-cooling trade-offs (Concept 1) in the quest to reduce façade thermal loads: lighting and heating energy use could increase if daylight is severely restricted and if measures block solar radiation during the heating season.
Low-energy strategies may also involve the façade directly. Natural ventilation during the daytime can be used to offset cooling requirements when the outdoor temperature is within an acceptable range. Nighttime natural ventilation using automated windows can be used to pre-cool the building or charge strategically-located thermal mass for daytime radiant cooling.
Progress Towards Zero-Energy Building (ZEB) Solutions
How to get to low-energy or even zero-energy building solutions? The following example shows how the concepts above can be applied to reach low-energy goals.
0% Savings: Code Baseline
Traditional code-compliant façade designs sacrifice daylight to reduce HVAC energy use due to solar and conductive loads. The daylit zone is typically at most 15 feet deep and is compromised by manually-operated interior shades that significantly reduce daylight and view.
20% Savings: Integrated Daylighting Solutions
The New York Times Headquarters: 100% lighting controls, automated shades, and underfloor air distribution (UFAD) systems
Intelligent, dynamic and/or light-redirecting façades combined with automated lighting controls can extend the daylit zone up to 20-30 feet deep by actively balancing daylight and thermal loads on a real-time basis while mitigating sunlight and glare. This approach, however, requires careful space planning.
30-50% Savings: Integrated Façades with Low-Energy Cooling
Nord LB Building in Hannover: Natural ventilation, radiant cooling, and daylighting brings annual energy use down to 61 kBtu/sf-yr, as reported by designers of the building.
Photo courtesy of Roland Halbe
Combine high-performance façades, daylighting, and low-energy cooling strategies such as natural ventilation and radiant cooling to eliminate the HVAC system entirely in some climates. High-R windows and dynamic façades can significantly reduce thermal loads during critical peak periods while maintaining high daylight efficiency. Use building integrated photovoltaics for energy supply.
