Danny H.W. Li

Evaluation of lighting performance in office buildings with daylighting controls

Lighting control integrated with daylighting is recognised as an important and useful strategy in energy-efficient building designs and operations. It is believed that proper daylighting schemes can help reduce the electrical demand and contribute to achieving environmentally sustainable building development. This paper presents field measurements on daylighting for a fully air-conditioned office building in Hong Kong. Electricity consumption by the fluorescent luminaires, indoor illuminance levels and the room parameters affecting daylighting designs were recorded and analysed. The measurements covered several cellular offices facing opposite orientations with and without daylighting controls. The findings suggest that daylighting schemes can result in substantial energy savings in air-conditioned office buildings in Hong Kong. Results are presented and the design implications are discussed.

CORRELATION BETWEEN GLOBAL SOLAR RADIATION AND ITS DIRECT AND DIFFUSE COMPONENTS

Abstract Work on typical correlations between global solar radiation and its direct and diffuse components is reviewed. Data for diffuse, global horizontal radiation measured at the City University of Hong Kong for the period 1991–1994 are analysed. Three correlations are considered — the diffuse coefficient (K d ) and the clearness index (K t ), the diffuse fraction (K) and K t , and the direct normal (I dn ) and K t . A hybrid correlation model based on hourly measured data is proposed for the prediction of hourly direct and diffuse components from the global solar radiation for Hong Kong.

An analysis of daylighting and solar heat for cooling-dominated office buildings

Computer simulation techniques were used to assess the energy performance of a generic commercial office building in Hong Kong. The simulation tool was DOE-2.1E. The thermal and energy performance of daylighting schemes were analysed in terms of the reduction in electric lighting requirement and the cooling penalty due to solar heat. Regression analysis was conducted to correlate the peak electricity demand and annual incremental electricity use with two fenestration variables, namely the solar aperture and the daylighting aperture. Contours of equal annual incremental electricity use were shown to be a function of the solar and daylighting apertures. It is envisaged that these simple charts can be a useful design tool for architects and engineers to assess the relative energy performance of different fenestration designs, particularly during the initial design stage when different building design schemes and concepts are being considered and developed.

An analysis of climatic parameters and sky condition classification

Abstract In modelling solar radiation, of the required data daylight illuminance, luminous efficacy, radiance and luminance sky distributions, are always analysed under various sky types. Sky conditions are frequently categorised into overcast, partly cloudy and clear using some common climatic data including cloud cover ( CLD ), sunshine hour ( SH ) and solar radiation. This paper presents the study of these climatic parameters and their applications in the classification of sky condition. Frequency of occurrence and cumulative frequency distribution of each sky indicator have been established to interpret the prevailing sky conditions in Hong Kong. Special features on defining the sky clearness are highlighted and the implications for energy-efficient building designs are discussed.

Determining the Optimum Tilt Angle and Orientation for Solar Energy Collection Based on Measured Solar Radiance Data

A prior requirement to the design of any solar-based conversion systems is the knowledge of optimum orientation and tilt surface at which peak solar energy can be collected. In many parts of the world, however, the solar radiation data for the surfaces of interest are not always available. This paper presents a numerical approach to calculate the solar radiation on sloped planes by integrating the measured sky radiance distributions. The annual total solar yield at different sloped surfaces facing various orientations and monthly solar radiations at the optimal tilt surface and three vertical planes facing east, south, and west were determined. The energy outputs and efficiencies were simulated using a computer package. The environmental benefits in terms of greenhouse gases reductions and cost implications were also considered. The findings provide technical information for engineers to design and evaluate photovoltaic (PV) systems which could contribute to the environmental, energy, and economic aspects.

Solar heat gain factors and the implications to building designs in subtropical regions

In subtropical Hong Kong, solar heat through fenestration, particularly on vertical surfaces, plays a major role in determining the thermal performance of a building. For sizing air-conditioning equipment, maximum solar heat gain factors (SHGFs) are used for estimating the design peak load. Recently, the SHGF data representing the prevailing weather conditions have become important and more practical for evaluating daylighting designs and the determination overall thermal transfer value (OTTV). This paper describes a procedure for calculating hourly SHGFs for horizontal and vertical surfaces based on the 8-year (1991 to 1998) horizontal global and diffuse radiation and the 3-year (1996 to 1998) vertical global solar radiation data measured at the City University of Hong Kong. A simple approach for estimation of solar heat gain is developed. Average and peak SHGFs and sky factors for OTTV are computed and compared. An analysis of the SHGF data has shown that except for the north-facing surface, the ASHRAE clear sky approach tends to overestimate the solar heat gains, particularly at large significant levels. The findings provide information for the determination of the total air-conditioning plant capacity at peak load operation and multiple equipment sizing under part load condition.

An analysis of electricity end-use in air-conditioned office buildings in Hong Kong

Abstract There has been a growing interest in the electrical end-use patterns in air-conditioned commercial buildings in Hong Kong. A total of four high-rise fully air-conditioned office buildings were selected as case studies. The total building electrical load was broken down into four major electricity end-users, namely heating, ventilation and air-conditioning (HVAC), lighting, small power and lift/escalator. The hourly load profiles of these electricity end-users were monitored during the hot summer months of July and August. It was found that HVAC was the single largest electricity end-user, accounting for 30–60% of the total building electrical demand during office hours. Electric lighting was second with 20–35% and small power third with 15–25%. Lift/escalator accounted for a few percentage of the total building load. An analysis of the peak load revealed that chiller load was a major component. It was found that chiller load shifting using thermal chilled store could reduce the peak building load by about 20%.

Measurements of solar radiation and illuminance on vertical surfaces and daylighting implications

There is a growing concern about the rapid development of infrastructure and building projects and their likely impacts on the environment. Particular concerns have been raised about office building developments and energy consumption issues. In recent years, there has been increasing interest in using daylight to save energy in buildings. Lighting control integrated with daylighting is recognised as an important and useful strategy in terms of energy-efficient building design. It is believed that proper daylighting schemes can help reduce the electrical demand and contribute to achieving environmentally sustainable building developments. This paper presents a simple method for estimating the likely energy savings in electric lighting due to daylighting and the possible cooling penalty. Vertical solar radiation and illuminance data measurements are described. Cumulative frequency distributions of daylight availability are reported. The likely energy savings in office buildings are determined based on on–off and top-up controls, and the energy and environmental implications are discussed.

Daylighting and its implications to overall thermal transfer value (OTTV) determinations

The overall thermal transfer value (OTTV) and daylighting are two approaches controlling building energy use. In Hong Kong, although OTTV calculations are mandatory in the submission of building plans for approval, daylighting credits are not included in building envelope designs. To promote energy-efficient building designs we use the computer simulation tool, DOE-2, to illustrate the energy performance of a generic commercial building due to various daylighting schemes and OTTV designs. The year-round energy expenditures and loads are determined from the simulation results. Analysis is carried out in terms of the reduction in electric lighting requirement and the cooling penalty due to solar heat. Regression techniques are conducted to correlate the annual incremental electricity use with OTTV and daylighting aperture (DA) (product of window-to-wall ratio (WWR) and light transmittance (LT)). Contours of equal annual incremental electricity use for different building envelope parameters are developed. Important features for daylighting schemes are highlighted and implications for OTTV designs are discussed.

A Study of 15 Sky Luminance Patterns against Hong Kong Data

A good knowledge of the sky luminance distribution is essential for efficient day lighting designs and developments. This paper studies a set of 15 standard skies established by Kittler, Perez and Darula using the measured sky luminance data from subtropical Hong Kong. A statistical analysis of the results has shown that the standard sky set is adequate to cover the various sky conditions. A reduction of the standard set to five skies is sufficient to model the luminance distributions. The ratio of zenith luminance to horizontal diffuse illuminance (L/zD) which can be used to define homogeneous sky standards was analysed based on these five selected skies. It has been shown that the trend of L/zDv ratios under different sky types followed the ranges described by Kittler et al.