Validation of skylight performance assessment software

ASHRAE Transactions, July, 2006 by A. Laouadi, C. Arsenault

ABSTRACT

This paper presents a comparison study between the simulations of SkyVision and measurements made in a scale model. SkyVision is a computer tool used to calculate the overall optical characteristics, indoor daylight availability, and energy-saving potential of projecting and tubular skylights. The measurements included the overall visible transmittance and indoor daylight illuminance of the skylight. A rectangular wooden box was used as a scale model of a simple commercial building. The top surface of the box was fitted with a curbed opening to accommodate the skylights to be tested. Seven skylight shapes were tested: two circular dome models, one with clear and one with white acrylic glazing; two rectangular bubble dome models, one with clear and one with white acrylic glazing; one clear acrylic hexagonal pyramid model; one clear polycarbonate barrel vault model; and one tubular skylight model. The measurements were conducted for a whole day, thereby covering different sky conditions: overcast, partly cloudy, and clear sunny skies. SkyVision's simulations for the skylight transmittance compared reasonably well with the actual measurements, except for the hexagonal pyramidal skylight. The hexagonal pyramid surface exhibited some lens effects around the surface vertices, which were not possible to model in SkyVision. As for the skylight indoor illuminance comparison, the SkyVision's simulations were in good agreement with the actual measurements for all occurrences of sky conditions.

INTRODUCTION

Skylights are important architectural design elements of many building types, such as commercial, institutional, and industrial buildings and houses. They connect building occupants to the changing outdoor environment and provide the architectural spaces with natural illumination and solar heat. Properly designed skylights may save a substantial amount of energy for lighting, heating, and cooling (Laouadi 2004; McHugh et al. 2004) and help avoid peak energy demands and, therefore, reduce utility costs. Recent research has shown that the skylight's shape and glazing type can significantly alter the skylight's energy performance (Laouadi 2004; Laouadi and Atif 2002). The air space underneath the skylight may reduce the solar heat gains by up to 25% (Klems 2003), the annual cooling energy by up to 6%, and heating/cooling peak loads by up to 15% (Laouadi et al. 2002). Furthermore, some studies around the world showed that skylights may improve non-energy aspects of buildings such as retail sales (HMG 2003a, 1999).

However, the potential energy benefits and amenities of skylights have not been fully exploited in today's building designs due to some theoretical and technical challenges. The lack of design tools is one of the major hurdles building designers face in adopting such products and quantifying their energy benefits. Currently available fenestration simulation software deals with only planar and transparent geometry, such as windows and flat skylights (CANMET 2004; LBL 2004a). Sophisticated lighting simulation software (LBL 2004b; LT 2003; IEA 2000) is not only cumbersome to use but also does not provide any output related to the skylight optical characteristics required for product rating and selection. Specialized skylight software is very rare and limited. The SkyCalc program (HMG 2003b) is limited to some USA climatic regions and handles only flat translucent skylights.

Recognizing the limitations of the current fenestration computer tools, we developed SkyVision, a computer program to assess skylight performance. SkyVision aims to assist skylight manufacturers and building designers in developing appropriate skylight designs for given building types and daylighting applications. SkyVision may also help fenestration rating councils to rate skylight products. The current version of SkyVision calculates the overall optical characteristics of projecting and tubular skylights, performance indicators of skylight/room interfaces, indoor daylight availability, and monthly/annual lighting energy savings. To maximize the energy-saving potential of skylights, SkyVision accounts for the skylight shape, size, glazing type, curb/well geometry, building location and orientation, lighting and shading controls, and prevailing climate. Two beta versions of the program were released for evaluation by end-users. This release was followed by extensive measurements and comparison studies to validate the software's simulations. The users' feedback and comparison studies were extremely useful in shaping the final look of the software and fine-tuning its simulation models. The final version of SkyVision is now available free of charge on the Web (http://irc.nrc-cnrc.gc.ca/ie/light/skyvision/). More details on the program can also be found in the distribution Web site.

Objectives

The purpose of this paper is to conduct a validation study between SkyVision's simulations and measurements carried out under real sky conditions. The measurements included the visible transmittance of the skylight and the indoor daylight illuminance. More details on the comparison study may be found in the report by Laouadi and Arsenault (2004). The report also includes additional comparison studies that used the experimental results of the California Public Interest Energy Research (PIER) program (McHugh et al. 2004). Other comparison studies (model to model comparisons) are found in Laouadi et al. (2003a, 2003b). The specific objectives of this paper are to outline the measurement methodology and analyze the comparison results.