Performing daylight assessments for formulating and evaluating daylighting design strategies
The methods for assessing daylight quantity and quality in buildings can categorized and illustrated in the following chart.
Both computer-based and physical-experiment-based simulation capabilities have been developed by the research group at NC State for performing each of the qualitative and quantitative assessments illustrated above. Although computer simulation is increasingly used for daylighting research and design, conducting physical experiments is still crucial for the following reasons:
- Manipulation of real objects (the models) still engages the designers and clients more readily than screen or paper representations.
- Light quality and spatial perception can be difficult to assess by computer simulations. A large enough scale model allows people to assess light quality by observing through the view ports provided on the walls, so that the observer can be immersed in the luminous surround.
- There are still many materials, devices and products that cannot be easily characterized by computer algorithms.
- Fast turnaround: results can be immediate, and although a model needs to be constructed, they can often be very simple and still provide good results.
1. “Static” daylight quantity assessments for obtaining LEED Daylighting Credit: Developed in the UK over 50 years ago, the Daylight Factor is simply the ratio of indoor horizontal illuminance (e.g. at the task plane) to unobstructed outdoor horizontal illuminance under overcast sky conditions. Because the luminance distribution of an overcast sky is symmetrical about the vertical axis going through the zenith and the sun component is excluded from the scenario, this method is simple and easy to use. The Daylight Factor concept has been widely accepted and become the quantitative performance indicator adopted by the Leadership in Energy and Environmental Design (LEED) for assessing its Daylight Credit.
The freely available RADIANCE software is one of only a few lighting analysis tools able to accurately calculate illuminance levels on surfaces within a building model. Ecotect is a design tool that includes a wide range of simulation and modeling functions required to understand and predict how a building will operate and perform. Daylight factors can be calculated by using RADIANCE as the simulation engine and by using Ecotect as the modeling tool. The process involves building a digital model of the space and assigning material properties in Ecotect, and calling RADIANCE simulation algorithms to develop Daylight Factors.
For developing Daylight Factors, an alternative method besides computer simulation is to test physical models under real overcast skies or in a mirror-box artificial sky. Such a device is intended to simulate overcast sky condition and provide a stable sky luminance for assessing models of different design options and comparing their performances. An 8’x8′ mirror box artificial sky was constructed at NC State University College of Design. This device has been validated against real overcast sky. Photocell sensors will be placed inside the model and outside facing the unobstructed sky. Illuminance readings from the indoor and outdoor sensors are used to calculated daylight factors. In addition to the quantitative data, the visual quality of the distribution of light within the modeled space may be assessed, and recorded by conventional or digital photography.
2. “Dynamic” daylight quantity assessments for predicting year-round daylighting performance and estimating energy savings: The above Daylight Factor method is based on overcast sky conditions, which present two disadvantages: 1. It is insensitive to the building orientation due to the symmetrical sky luminance distribution; 2. It is insensitive to the location, hence the climate, of the building. Based on this approach, the Daylight Factor of a daylighting design would be predicted to have the same performance whether the building faces north in rainy Seattle or faces south in sunny Phoenix. An alternative approach for daylighting research and application is Climate-based Daylight Modeling (CBDM). CBDM is the prediction of various luminous quantities using sun and sky conditions that are derived from meteorological datasets and therefore is dependent upon both locale and orientation, in addition to building configuration and composition.
As one of the most commonly used Climate-Based Daylight Modeling software, DAYSIM was developed at the National Research Council Canada and the Fraunhofer Institute for Solar Energy Systems in Germany. DAYSIM is capable of carrying out CBDM by using meteorological datasets. By the comparisons with actual measurements, it has been proven to be reasonably accurate for performing annual daylight predictions. DAYSIM uses the RADIANCE as simulation algorithms in conjunction with a Daylight Coefficient approach to calculate illuminance distributions under all sky conditions in a year.
Dr. Wayne Place from the research group has also developed an experiment-based method for dynamic daylight assessments – Coefficient of Utilization (CU) Method. CU is the ratio of interior illuminance to exterior vertical illuminance. It is often developed to establish the correlations between interior illuminances and exterior daylight resources. If exterior daylight recourse data (e.g. hourly sky illuminances on vertical planes in a full year) are available at the building site, annual system performances can be assessed by multiplying the exterior daylight levels by the CU. This method has been validated by a study conducted in the context of Raleigh, North Carolina.
3. Daylight quality assessments for studying human perceptions on luminous environment: One of the major concerns with lighting quality is the issue of glare. Glare is a common problem in daylighting environments, especially in office buildings, since major activities in office buildings, such as reading, writing, and working with computers, require a glare free environment. Similar to the light quantity assessments above, both computer simulation and physical experiment have been made available at NC State for studying luminous environment for quality issues.
Ecotect in conjunction with RADIANCE algorithms is one of the methods for simulating and analyzing daylight quality issues, such as direct glare, interior luminance distributions, shading strategies, etc. Various physical testing tools are also effective for performing such assessments. Physical models at different scales can be studied for daylight qualities by using devices such as artificial sky, Sun Angle Simulator, Scanning Sky Simulator, Heliodon, etc. ______________________________________________________________________________________
Engaging building industry for developing, testing, and commercializing integrated building systems and products
There has been a large gap among the disciplines in building design. Major components of the building systems – architectural, mechanical, electrical, and structural – tend to be dealt with separately in design education and practice, and the coordination between these disciplines are often limited to systems error checking (e.g. beam and ductwork conflicts). One of the important goals of the research is the development and commercialization of building systems that are optimized by systems integration. An expected outcome is that industrial partners will be engaged and/or that companies will be spun off to produce and market these systems. Another way of engagement is to make data and findings of the research available to the industries for them to develop better products and make more informed decisions. Currently the following concepts are being developed.
1. Integrated roofing system with dynamically shaded horizontal aperture: Horizontal apertures have the advantage of facing the most luminous part of the sky during those times when the daylight resource is weakest, i.e., under overcast sky conditions. They also have the advantage that there are many situations where the only available natural light is from overhead. This will commonly occur in urban environments, with tall surrounding structures, or building sites surrounded by tall trees. Tracking shading elements are installed to protect the glazing.
2. Integrated Precast Concrete Flooring system for reducing building floor-to-floor height: This system is integrated with the Under-Floor Air Distribution for reducing floor-to-floor height in multi-story commercial buildings”. The project will focus on developing the flooring system so that it can meet various span and load requirements and that it can be efficiently manufactured, cost effective, thermally efficient, easily erected, and accommodate all the intended utility functions. The proposed system has multiple application areas in the building industry, including precast concrete, composite fiber reinforced polymer, heating ventilation & air-conditioning, architecture/engineering design professions, etc.
3. Multi-tier dynamic light shelf system integrated with curtain wall, PV and LED lighting: The system features a 4-ft deep fixed light shelf and a dynamically controlled upper light shelf. During winter months when solar angles are lower, the upper light shelf will be set in place to help intercept the sunbeams, whereas for the rest of the year when solar angles are higher, it will be “removed” by lowering to the top of the fixed shelf so that the daylight and the view through the daylight glazing can both be maximized.The system is intended to provide three major functions: 1. Reflect light deeper into the building, thereby providing more light in the interior; 2. Block excessive light from entering spaces close to the perimeter wall; 3. Maintain the view through the daylight glazing, which is especially desirable for users sitting away from windows.