New Tools for Analyzing Thermal and Daylighting Performance of Fenestration in Multistory Buildings

Accurately predicting the energy-related impact of fenestration is essential to the design of energy-efficient buildings. For complex nonresidential buildings, a complete understanding of fenestration performance requires not only thermal modeling but daylighting prediction as well. Multistory buildings tend to have higher skin-to-floor ratios than shorter, more compact structures of equal floor area and, thus, their performance is influenced to a greater extent by design decisions that affect the thermal and solar optical properties of the building skin. Despite the computational power of modeling programs, there are tradeoffs and limitations among accuracy, the cost of running the model, and the flexibility to model the large range of architectural solutions for high-rise buildings.

We recently completed the first phases of a project to add a daylight-modeling capability and related thermal algorithms to the DOE-2.1B energy analysis program. In order to provide accuracy and computational efficiency along with the ability to model geometrically complex buildings, we developed a family of supporting computational tools and experimental techniques. The next version of the DOE-2 program will have a daylighting model driven by a library of stored coefficients of utilization, which are either developed from scale model measurements in our sky simulator or calculated by a new daylighting illumination program, SUPERLITE. In addition, coefficients for unique designs can be determined from model measurements and entered into the program. SUPERLITE provides detailed data on illuminance distribution in an interior space, but is too complex for use directly within the hour-by-hour model. Because the solar gains through sophisticated daylighting aperatures are not adequately calculated in current models, our procedure will also use a library of coefficients stored in DOE-2. These coefficients will be determined from sun and sky simulator measurements of the solar optical properties of devices.

We describe our major experimental procedures and analytical models and present validation studies of DOE-2.lB and SUPERLITE. We illustrate the applicability of these tools by showing results from a study of optimal fenestration performance as a function of climate and orientation.