DESIGN FOR DAYLIGHT HARVESTING: Concepts, Strategies and Rules of Thumb
As described neatly by Dr. Reinhart in his Daylighting Handbook I the daylight harvesting for architectural spaces can take different meanings depending on your perspective (architectural, engineering or an integrated one). Daylight Harvesting is:
The controlled use of “natural light” in and around buildings.
The act of lighting the interior and/or exterior of a building with “natural daylight”.
The interplay of “natural light” and building form to provide a visually stimulating, healthful and productive interior environment.
The replacement of indoor electric illumination needs by “daylight”, resulting in reduced annual energy consumption for lighting.
The use of fenestration systems and responsive electric lighting controls to reduce overall building energy requirements (heating, cooling, lighting).
In all these diverse perspectives, we can also see a range of motivators such as designing spaces that are aesthetically pleasing, visually stimulating, providing visual access to nature, offering good color rendering, healthy and productive indoors, as well energy savings through peak load reductions and shifting. Hitting all the rights spots in each design study natural would require a high level of systems thinking and a truly embraced multi-disciplinary approach in architectural and engineering design which we can refer as “integrated design”. Like all design approaches that are aiming to incorporate natural processes (of light, heat and sound) into the early phases of an integrated process, daylight harvesting can also be conceptualized with some focus on the fundamental physical processes involving “sunlight” and “daylight” (and views) in relation to architectural materials, elements, technologies, systems and operations as well. Here is a list of physical process in relation to daylight (and sunlight) harvesting:
<TRANSMIT>
Direct intake for illumination and passive solar heating. Direct intake refers to accepting the energy in the photons of light without a time lag which is possible with the transparent building assemblies (vertical glazing and skylights). Attention to Visible Transmittance (VT) and Solar Heat Gain Coefficient (SHGC) indices.
<BLOCK>
Solar shading (external), opaque façade elements. While exterior shading would be the most effective means of solar radiation control, shading from daylight to avoid daylight-sources glare is best when applied from the interior (with manual user controls). Geometric design of external, integral, and internal shading devices needs to be supplemented with the optimization of the surface properties. Solar and Visible Absorptance and Visible Reflectance (RVIS) are key indices.
<FILTER>
Direct light to diffuse light transformation. This can be best realized by “translucent” building elements (in the form of shading or glazing). Translucency is reduced transparency with increased reflectance or absorptance. Attention to VT, and RVIS both of which will also impact the SHGC. Translucency can be static in time (as a permanent material property), or it can be dynamic as in the case of switchable/electrochromic glazing elements.
<REFLECT>
Specular-Diffuse reflections, light bounces off the surfaces. The key point is to be able to predict where the bounced light is going to (indoors or outdoors to your neighbors?). Visible Reflectance (RVIS) is the key index. However, the visible reflectance can be specular or diffuse which are usually individually listed as distinct optical properties of a building surface material.
<CONSUME>
Daylight harvesting, direct gain heating, delayed release. The entire range of design strategies for daylight harvesting is to consume this natural and ambient resource to replace it with electricity (that can be consumed by the electric lights for space illumination). Direct gain systems (of southern oriented glazing) can help heating of interiors with solar radiation which is a similar concept of replacing it with fossil fuel-based space heating.
<ABSORB>
Solar radiation to thermal radiation (a change of wavelength). Absorption can take place both in opaque and transparent building elements. Attention to Visible and Solar Absorptance indices. The energy of the absorbed photons will be converted into long-wave thermal radiation to be release sometime later to the indoors or to the outdoor. The impact will be delayed by time lag depending on how dense is the building elements that is absorbing the (sun)light.
<STORE>
Thermal mass, greenhouse effect, battery systems. Storage is the process that is following the absorption of the photons. The goal with the storing the energy is control the timing of its subsequent usage. Instantaneous impacts can be saved for later use as thermal power (with building thermal mass) or electrical power (with solar battery systems).
<CONVERT>
Sunlight to DC current/power (photovoltaic panels). Solar cells can a part of the daylight harvesting strategies since semi-transparent cells can be integrated to the transparent glazing systems (creating a translucent building element which can generate electricity at the same time).
All these fundamental physical processes can be used as fertilizers for novel design ideas which should embrace environmental, contextual, and architectural boundaries of a design study with daylight harvesting as the main theme. Below is a list of key design considerations in this regard:
Building Morphology/Tectonics
Space geometry (height, width, depth, surface angles)
Size of daylight apertures (windows, skylights, clerestories, etc.)
Location of daylight apertures (side-lighting vs top-lighting)
Access to daylight (site context, building context, room context)
Location of the “point of interest” relative to apertures
Three-section facades (Bottom: Spandrel, Middle: Vision Glass, Top: Daylight Glass)
Three-section facades with "split blind" systems (for Vision Glass and Daylight Glass)
Glazing Properties (sunlight: SHGC, daylight: Visible transmittance - VT, Visible Reflectance - RVIS)
Opaque - Transparent - Translucent (Diffusers) Surface Ratios
Frame & Divider Properties (depth and reflectance)
Interior Surface Reflectance & Textures (Brightness - Color Scheme)
Exterior Surface Reflectance & Textures (affecting daylight entering the aperture)
Daylight Enhancement/Redirecting Devices (e.g., Light Shelves, Light Scoops, Light Tubes)
External - Integral - Internal shading Devices (imprinted frits-static-dynamic)
Landscape Features: Softscape and hardscape Elements as site shading devices
Supplemental Artificial Lighting Sources (Electric lights but not a replacement for daylight)
Controls (photosensor-linked dimming, occupancy-based, vacancy-based automated controls w/ manual overrides)
I’d like to emphasize here that irrespective of the selected daylight harvesting strategy, every design study should be initialized with a comprehensive understanding of a building’s global/geographical location and the prevailing climatic conditions in relation to solar radiation and ambient illumination (temporal dimensions, and varying sky conditions should be matters of design interest).
Finally, I’d like to give a short list of my go to rule of thumb to get started with a daylight harvesting design study. Note that these are just starters (which can be expanded), and every design project needs to be fine tuned and optimized for its unique environmental and architectural constraints using scope-specific analytical tools and/or simulation programs.
100% of ambient light from daylight whenever is available (minimum is 40%, sDA >= 40%)
40% < WWR < 60%
View/Vision Glass Visible Transmittance > 60%
SHGC > 50% for winter (cold climates)
SHGC < 25% for summer (hot climates)
Large Glazing Areas: S, SW, SE (+- 25o)
Daylight glazing, sky lights, clerestory glazing can be translucent
Skylight to Roof Ratio (SRR) <= 5%
Light-to-Solar Gain Ratio (LSG): VT / SHGC
The greater the LSG, the more suitable a glazing is for daylighting in hot climates (or wherever cooling is the dominant for thermal condition)
LSG >= 2.7 (Goal daylighting performance goal)
Daylight Penetration Depth = 2.5 x the head height of the window ABOVE the work plane (2.5H RULE) (Assumptions: Clear glazing, no major obstructions, and WWR is about 50% or more).
The 15/30 RULE: 15-ft (4.6m) zone from a window wall can be daylit sufficiently for office tasks. The next 15-ft zone can be partially daylit and supplemented with electric lights. Zones > 30ft would receive very little daylight.
Light Reflectance Values (LRV) (that is the visible reflectance value - Rvis)
Ceiling Surfaces = 85%
Walls = 60%
Floor = 25%
Furniture & Partitions = 45-50%
Window Blinds = 70%
Luminance Ratios (10:3:1 RULE)
Task-to-Remote Background = 10:1
Task-to-Adjacent Background = 3:1
Within the Task = 3:1
WWR: Window-to-Wall Ratio
SRR: Skylight-to-Roof Ratio
VT: Visible Transmittance
SHGC: Solar Heat Gain Ratio
LRV: Light Reflectance Value
LSG: Light to Solar Gain Ratio
sDA: Spatial Daylight Autonomy
Omer T. Karaguzel, PhD