Development of analysis software for the optical characteristics and daylighting performance of conventional and tubular skylights

This paper presents an overview of the development of analysis software for the optical characteristics and daylighting performance of conventional and tubular skylights. The software is useful to skylight manufacturers, building designers and fenestration rating councils. The software development process spans over a four-year period, and includes development of prediction models for optical and daylighting performance of different skylight shapes, experimental and analytical validation of the prediction models, and software design. This paper focuses on the software structure, inputs, outputs and analysis modules. Preliminary results from the prediction models are also shown for the beam transmittance of barrel vault skylights, diffuse transmittance of domed skylights under different sky conditions, and average Daylight Factor on internal room surfaces. BACKGROUND The skylight market has been booming in recent years. Skylights are found in many modern or retrofitted buildings. In commercial and institutional buildings, skylights are used to simulate the outdoors and to bring natural light and solar heat into the indoor space. In residential buildings and houses, skylights are used mainly for illumination. Recently, tubular skylights have emerged as a new technology in commercial buildings and houses, potentially eliminating some of the drawbacks of the excessive solar heat gains of conventional skylights and extending the application of skylights in areas not reached by conventional skylights and windows (Bajda and Carter 2000; Salih et al. 2000; Shao et al. 1998). Skylights have the inherent potential to save electrical lighting, cooling and heating energy, beside their psychologically positive effects on building occupant satisfaction (HMG 2001, 1999; Heschong and McHugh 2000; Allen 1997; AAMA 1987, 1981; Cassidy 1986; Treado et al. 1983). However, despite these amenities, skylights integrated in building design may result in high-energy consumption if not properly designed. Skylight manufacturers lack design tools to assess the optical characteristics and daylighting performance of skylight products owing to complex skylight shapes that change with design requirements, and large sizes that impede fitting skylight products into measurement facilities. Furthermore, while measurements on some skylight products are possible, they can not be generalised for other products. In addition, fenestration simulation software such as VISION4 (CANMET 1995) and WINDOW (LBL 1992) deal with only planar geometry, such as windows and flat skylights. OBJECTIVES The Institute for Research in Construction, PERD (Panel for Energy Research and Development), and NRCan have started a four-year project to develop software to analyse the optical characteristics (transmittance, absorptance and reflectance) and daylighting performance of conventional and tubular skylights. The project is accomplished through three main tasks. The first task is devoted to developing prediction models for skylight optical characteristics and indoor daylight availability. The second task is the experimental and analytical validation of the prediction models. The third task focuses on the design of the software. The software will enable skylight manufacturers to prototype and characterise skylight designs before building them, and will aid building designers in skylight selection to meet design criteria for energy savings. The software will also help fenestration rating councils to rate the optical and daylighting performance of skylight products. The specific objective of this paper is to provide an overview of the software structure, analysis modules, and input and output parameters with some preliminary results from the first task of the project. SOFTWARE STRUCTURE Figure 1 shows a flow chart of the software structure. The main components of the software are the inputs, databases, analysis modules and outputs.