Project Title : Development of a Simple Graphical Approach based on Daylight Coefficient Concept and the CIE 15 Standard Skies for Indoor Daylight Luminance Prediction

Applicant : City University of Hong Kong (Dr Danny Li)

Total Approved Grant : $127,945.92

Duration : 1/9/2004 to 30/9/2005

Project Status/Remarks : Completed

Scope : The project aims to develop simple graphical charts to determine the indoor daylight illuminance based on daylight coefficient concept and the CIE 15 standard skies, to evaluate the proposed approach in comparison with results obtained by the traditional daylight factor method and measured data, and to compute the prevailing indoor daylight illuminance and the likely electric lighting savings for some typical fenestration designs using the proposed approach.

Summary of findings :

A simple graphical approach in the form of nomograph using daylight coefficient (DC) concept for determining daylight illuminance under unobstructed CIE standard skies was proposed. The determined daylight illuminances for 49 reference points of a typical room were used to demonstrate the performance of the model. Overall speaking, the percent differences for more than half of the reference points between the estimated values and simulated results from a lighting software, RADIANCE, were within ±5%. Study was also conducted for the average daylight factor (ADF) under the 15 CIE standard skies. Based on split-flux theory, the sky-diffuse (C) and ground reflected (D) components were computed. Again, the performance was compared with the results given by the RADIANCE. The coefficients C and D computed by the two methods are in very good agreement and the peak discrepancy in ADF for the typical room was less than 0.5%. Scale model and real building measurements were considered for validating the approach. The scale model with internal surfaces covered individually by black paper and white paper located at the rooftop of the University main building were analysed. For real building measurement, a hostel study-bed room block in the campus were adopted. It was found that the daylight data obtained by the approach were in reasonably good agreement with the measured readings. The calculations of the indoor daylight at different times of a year under various CIE standard skies were also investigated. The findings supported the contention that daylighting designs based on traditional CIE overcast pattern only would considerably underestimate the interior daylight illuminance and hence the electric lighting energy savings.

The proposed simple and reliable approach enables building professionals in particular for architects and building engineers to determine the indoor daylight illuminance more accurately and easily. It can promote daylighting designs including innovative daylighting systems with complex optical properties, achieving energy savings and visual comfort building development. The simple graphical approach which incorporates daylight coefficient concept to estimate internal daylight illuminance based on the newly adopted CIE standard skies would be of significant academic as well as practical values, and are of interest to local and overseas peers. The work would also contribute to teaching and the findings would be made available to architecture and building engineering students. With more information becoming available, it can encourage the adoption of daylighting coefficient concept and the CIE standard skies in various building regulations and codes relating to daylighting performance calculations (e.g. code of practice for OTTV, Performance-based Building Energy Code (PB-BEC), Comprehensive Environmental Performance Assessment Scheme (CEPAS) for lighting, and lighting and ventilation of Building Regulations CAP123). All planned research studies including literature review, simple charts and approach development and evaluation, model and full scale measurements and verifications, computer simulation and case studies were conducted and completed. The major findings in the form of articles have been submitted to various international referred journals and presented in an international conference.