An analysis of daylighting performance for office buildings in Hong Kong

Daylighting has often been recognized as a useful source of energy savings and visual comforts in buildings. Occupants expect good daylight in their working spaces. The quality and quantity of natural light entering a building depend on both internal and external factors. In Hong Kong, commercial building accounts for the major building energy use and electric lighting is one of the major electricity-consuming items. This paper studies the daylighting performance and energy implications for office buildings. A total of 35 commercial buildings have been selected in the survey. Key building parameters affecting daylighting designs are presented. Two typical office blocks were further analysed based on a lighting simulation program. The daylighting performance was evaluated in terms of daylight factor, room depth and glare index. It has been found that the daylighting performance for office buildings is quite effective. About one-third of the office areas that are near the perimeter regions have an average daylight factor of 5%. For inner region of deep plan offices, some innovative daylighting systems such as light redirecting panels and light pipe could be used to improve the daylighting performance. In general, the office building envelop designs are conducive to effective daylighting and proper daylight linked lighting controls could save over 25% of the total electric lighting use.     

A study of the daylighting performance and energy use in heavily obstructed residential buildings via computer simulation techniques

The quality and quantity of natural light entering a building depends on both internal and external factors. In Hong Kong, many buildings are high-rise blocks constructed close to each other and hence the external factor plays a significant role in daylighting designs. This paper studies the daylighting performance and energy use for residential flats facing large sky obstructions via computer simulations. Key building parameters affecting daylighting designs are presented. The daylighting performance for typical interior rooms was investigated in terms of illuminance level and daylight factor. The daylight levels of residential flats can be severely reduced by neighboring buildings and hence the externally reflected component would be the main source of natural light. The indoor daylight levels for kitchen and living/dining faced large neighboring building were found always less than the standard maintenance illuminance during daytime period. These imply that many residential flats in Hong Kong would have to rely on supplementary electric lighting.     

Regression analysis of high-rise fully air-conditioned office buildings

The building energy simulation computer program DOE-2 was used to carry out a parametric study of a generic high-rise air-conditioned office building in Hong Kong. A total of 28 design parameters related to the building load, the heating, ventilating and air-conditioning (HVAC) system and the HVAC refrigeration plant were found to correlate well with the predicted annual electricity consumption. Both linear and non-linear multiple regression techniques were used to develop regression models and energy equations for the prediction of annual electricity use. Twelve input design parameters (six from building load, four from HVAC system and two from HVAC refrigeration plant) were considered to be the most significant design variables and were used in the energy prediction equations. It is hoped that the resulting models and energy equations can be used as a simplified design tool for comparing the relative thermal and energy performance of different design schemes during the early design stage. This paper presents the methodology and the development work, and discusses the findings.     

Energy performance criteria in the Hong Kong building environmental assessment method

The Hong Kong Building Environmental Assessment Method (HK-BEAM), an environmental assessment method for air-conditioned office buildings, has been developed by the Real Estate Developers Association (REDA) for use in Hong Kong. Two versions of the scheme have been developed; one for new office premises and the other for existing office premises. HK-BEAM embraces a variety of issues related to the global, local and indoor environments. The efficient use of electricity forms a major part of the assessment. The basis for the energy performance assessment method, and the criteria for obtaining the relevant credits are described and discussed.     

Risks of unsatisfactory airborne bacteria level in air-conditioned offices of subtropical climates

Adjustment of the indoor air temperature and relative humidity set points for energy conservation is adopted in many air-conditioned offices in the subtropics. This study examines the environmental risks in an air-conditioned office with a ‘readjusted’ thermal environment from the perspective of the probable airborne bacteria level exceeding certain limits. In particular, a cross-sectional study of bacteria concentrations in 422 air-conditioned offices in Hong Kong was conducted, and a mathematical expression was proposed to correlate the indoor bacteria level with the indoor air temperature and relative humidity in an air-conditioned office. With the proposed correlation, the probability of an air-conditioned office having an unsatisfactory airborne bacteria level could be determined. The model results were verified with the measurement results from some other studies. This model would be a useful tool for preliminary assessment of the environmental risks, regarding the airborne bacteria level while balancing some energy conservation measures, in an air-conditioned office in the subtropics. The study also provides a template for developing an environmental risk assessment model in air-conditioned spaces elsewhere.     

Neutral temperature in subtropical climates—A field survey in air-conditioned offices

The thermal environment for air-conditioned offices in subtropical climates is examined from the prospect of maintaining an optimum operative temperature for the occupants. In this study, the optimum neutral temperature is evaluated from 422 occupants’ responses towards the perceiving thermal environment in 61 air-conditioned offices and 186 complaints of thermal discomfort in an air-conditioned office building on an electronic questionnaire, using a semantic differential evaluation scale and a dichotomous assessment scale. In particular, physical parameters for the thermal comfort study were measured by an indoor environmental quality (IEQ) logger, and the operative temperature was correlated with the occupants’ thermal responses. The probability of accepting an operative temperature for the thermal comfort of the occupants was correlated with logistic regression curves; the optimum operative temperature was derived in order to maximize the probability of thermal comfort expressed by the occupants. The results showed that the thermal neutral temperatures for air-conditioned offices in subtropical climates were 23.6 and 21.4 °C in summer and winter, respectively. The preferred thermal environment in Hong Kong should be slightly cool, corresponding to about 1 °C below the neutral temperature, in order to satisfy most of the occupants in the office space.     

昆明地区某全幕墙办公建筑外遮阳优化设计实例分析

以昆明地区某全玻幕墙办公建筑外遮阳精细化设计过程为例,重点分析建筑外遮阳构件对建筑节能、室内自然采光、室内视野、室内自然通风等带来的影响。借助模拟工具对不同尺寸的外遮阳构件设计方案进行对比分析,并通过因素评价矩阵量化评估,选取兼顾建筑节能和室内舒适性要求的最优方案,达到外遮阳优化设计的目的。     

Future trends of building heating and cooling loads and energy consumption in different climates

Principal component analysis of dry-bulb temperature, wet-bulb temperature and global solar radiation was considered, and a new climatic index (principal component Z) determined for two emissions scenarios – low and medium forcing. Multi-year building energy simulations were conducted for generic air-conditioned office buildings in Harbin, Beijing, Shanghai, Kunming and Hong Kong, representing the five major architectural climates in China. Regression models were developed to correlate the simulated monthly heating and cooling loads and building energy use with the corresponding Z. The coefficient of determination (R²) was largely within 0.78–0.99, indicating strong correlation. A decreasing trend of heating load and an increasing trend of cooling load due to climate change in future years were observed. For low forcing, the overall impact on the total building energy use would vary from 4.2% reduction in severe cold Harbin (heating-dominated) in the north to 4.3% increase in subtropical Hong Kong (cooling-dominated) in the south. In Beijing and Shanghai where heating and cooling are both important, the average annual building energy use in 2001–2100 would only be about 0.8% and 0.7% higher than that in 1971–2000, respectively.     

Simulation of ventilated facades in hot and humid climates

The Hong Kong climate is sub-tropical with hot and humid weather from May to September and temperate climate for the remaining 7 months period. A mechanical ventilation and air-conditioning (MVAC) system is usually operated to avoid the high temperatures resulting in high peak cooling loads. The facade design has a significant influence on the energy performance of office buildings. This work evaluates different ventilated facade designs in respect to energy savings.Thermal building simulations (TRNSYS) were linked to nodal airflow network simulations (COMIS) for detailed ventilated double-skin facade performance. In order to validate the model, simulations were carried out for an office building in Lisboa; the results were compared with measured data from the same building. The simulation results of surface and air temperatures show good agreement with the measurements. The results of the study can be used to reduce surface temperatures by using different materials for the roller blind that is positioned in the cavity of the double-skin facade. The results can further be used to reduce the high peak cooling loads during the summer period. This may result in significant energy savings and a reduction in the system's cooling capacity. It proved that a careful facade design can play an important role in highly glazed buildings and provides potential for energy efficiency.     

Predicting air-conditioning energy consumption of a group of buildings using different heat rejection methods

Water-cooled air-conditioning systems (WACS) are, in general, more energy efficient than air-cooled air-conditioning systems (AACS). The Laws of Hong Kong prohibit the use of fresh water from town mains for comfort air-conditioning, which rules out the use of cooling towers to most air-conditioned buildings. In the absence of lakes or major rivers in Hong Kong, only those buildings that are situated near the harbor can adopt WACS. As a measure to cut down greenhouse gases emissions, the Hong Kong Government is currently exploring the feasibility and viability of widening the use of WACS by the provision of centralized, district-wide water supply systems, which include seawater supply systems for once-through condenser cooling or for making up of water losses at cooling towers, and district cooling systems. Evaluation of the likely energy, economic and environmental benefits of such capital-intensive infrastructure developments requires estimation of the simultaneous cooling demand of a large group of buildings on a district scale, and the energy use for air-conditioning such buildings. This paper describes the method used in these estimations. Comparisons of the results estimated by this method with results obtained by detailed simulation, and with building energy use data obtained from surveys and audits are presented.     

Estimation of lighting energy savings from daylighting

This paper refines and validates the predictions of a simplified analysis method for evaluating the potential of daylighting to save energy use associated with electrical lighting. Specifically, impacts on daylighting performance are investigated for several combinations of building geometry, window size, and glazing type for several US and international locations. The impact of both dimming and stepped daylighting controls and their settings are also investigated. Predictions from the simplified method are validated using measurements obtained from field-testing of a daylighting control system utilized to operate lighting fixture illuminating an office space.     

Bayesian adaptive comfort temperature (BACT) of air-conditioning system in subtropical climate

Indoor thermal climate is an important issue affecting the health and productivity of building occupants. In the designing of commercial air-conditioning systems, it is believed that the conventional fixed temperature set point concept is limited because indoor comfort temperature depends on the business culture, such as the nature of activities and dress code of occupants, etc. Researchers have been interested in investigating adaptive temperature control for a realistic in-situ control of comfort. Unfortunately, those studies put great emphasis on energy saving opportunities and sometimes might result in thermal discomfort to individuals. This study argues that complaints of thermal discomfort from individuals, despite representing only a small portion of the population, should not be ignored and can be used to determine the temperature setting for a population in air-conditioned environment. In particular, findings of a new notion of Bayesian adaptive comfort temperature (BACT) in air-conditioned buildings in a humid and subtropical climate like Hong Kong are reported, and the adaptive interface relationship between occupants’ complaints of thermal discomfort and indoor air temperature is determined. This BACT algorithm is intended to optimise the acceptance of thermal comfort, as determined by physical measurements and subjective surveys.     

Defining the environmental performance of neighbourhoods in high-density cities

ABSTRACT

The regenerative design framework aims to ‘engage a broader range of possibilities by moving beyond the immediate building and site boundaries’. It implies that the environmental performance of buildings requires a revised definition so that it considers not only the building itself, but also its contribution beyond its own boundary, i.e. neighbourhoods. In high-density cities, outdoor spaces are culturally considered as the extension of one’s living spaces. The environmental performance of neighbourhoods is particularly important to the health and wellbeing of urban inhabitants. This paper aims to define the environmental performance of neighbourhoods in high-density urban context based on the experience acquired in previous studies in Hong Kong over the last 15 years. These studies cover a wide range of environmental issues including urban climate, outdoor thermal comfort, and daylighting design in high-density cities. Subsequent development of the assessment tools for environmental performance of neighbourhoods in Hong Kong is also presented. The framework of stakeholder engagement, as an integral part of the neighbourhood assessment tool, is discussed. This paper highlights the distinctive features of the environmental performance of neighbourhoods in high-density urban context and how it influences the professional practices in Hong Kong.     

Impact of electric lighting efficiency on the energy saving potential of daylighting from roof monitors

A developmental version of the building energy analysis computer program BLAST was used to perform simulations of a prototypical, single-story office building. Total annual energy consumption was computed using Typical Meteorological Year (TMY) [1] weather data from three locations in the United States. For each location, two electric lighting designs were tested on the baseline building (no roof monitors) to compare the energy requirements of current-practice and more efficient electric lighting designs. Then roof monitors were added to evaluate their energy saving potential for each of the electric lighting designs. The roof monitors had highly diffusing, vertical glazings facing southeast and southwest.The results show that improving electric lighting system efficiency and adding roof monitors for daylighting both have the potential for substantially reducing lighting electricity and the energy cost of operating the building. The potential benefits of daylighting are substantially lower for a building outfitted with a more efficient electric lighting system, although still significant. To determine the limits of validity of the simulations, a number of sensitives studies were performed. Among the issues investigated were: dirt deposits, snow accumulation, glazing optical properties, interior design, luminous efficacy of admitted sunlight, and thermostatic controls.     

A study of energy efficiency of private office buildings in Hong Kong

A benchmark study of the energy efficiency of private office buildings in Hong Kong was conducted in 2002 because energy efficiency was declining. In the study, private office buildings were divided into five user groups. For each group, a multiple regression model was developed to find the relationship between Energy Use Intensities (EUIs) and other factors, such as operating hours, for normalization and benchmarking purposes. In this paper we make use of the regression results to study the energy efficiency of private office buildings by different grades. In Hong Kong, office buildings are divided into three grades (A, B, and C) based on the quality of the facility, which is reflected in rental values; a Grade A office building denotes expensive luxury. We found that the EUI of Grade A office buildings is the highest, consuming over 50% of the total energy used in office buildings. Recently, the annual EUI of office buildings has improved even though Grade A floor space is increasing. This may be due to the promotion of the energy efficiency program launched in the last decade.     

Ventilation of air-conditioned residential buildings: A case study in Hong Kong

More and more studies reported that there were insufficient ventilation and excessive CO_2 concentration in air-conditioned residential buildings, but few solutions were provided. This study investigates the overnight evolution of CO_2 concentration in air-conditioned residential buildings and then focuses mainly on the evaluation of three ventilation strategies, including overnight natural ventilation, short-term mechanical ventilation and short-term natural ventilation. On-site measurements were conducted in a typical residential bedroom in Hong Kong in September. The indoor and outdoor CO_2 concentration, air temperature and relative humidity as well as the outdoor wind speed during the measurements were analysed. Ventilation rates were calculated based on the time series of CO_2 concentration. This study confirms that additional ventilation is usually needed in air-conditioned residential buildings. Overnight natural ventilation with even a small opening is associated with excessive energy consumption and deteriorated indoor thermal environment. Short-term natural ventilation strategies are inefficient and uncontrollable. Compared to the best short-term natural ventilation strategy, a reasonably designed short-term mechanical ventilation strategy requires only a 41% of ventilation period to complete one full replacement of indoor air and to reach a lower indoor CO_2 concentration. Nighttime case studies and a theoretical analysis suggest that a few several-minute mechanical ventilation periods could potentially maintain an acceptable indoor air quality for a normal sleeping period of 8 h.     

Energy impact assessment for the reduction of carbon dioxide and formaldehyde exposure risk in air-conditioned offices

Treatment of fresh air in ventilation systems for air-conditioning consumes a considerable amount of energy and affects the indoor air quality (IAQ). In this study, energy impact on ventilation systems was examined against certain IAQ objectives for indoor formaldehyde exposure risk in air-conditioned offices of Hong Kong. Thermal energy consumptions for ventilation systems and indoor formaldehyde exposure concentrations based on some regional surveys of typical offices in Hong Kong were reviewed. The thermal energy consumptions of ventilation systems operating for CO₂ exposure concentrations between 800 ppmv and 1200 ppmv for typical office buildings and the corresponding formaldehyde exposure risks were evaluated. The results showed that, for a reference indoor environment at a CO₂ exposure concentration of 1000 ppmv, the average thermal energy saving of ventilation system for a unit increment of the acceptable formaldehyde exposure limit of 1 h (loss of life expectancy of 0.0417 day) was 280 MJ m⁻² yr⁻¹; and for a unit decrement of the exposure limit of 1 h, an additional average thermal energy consumption of 480 MJ m⁻² yr⁻¹ was expected. This study would be a useful source of reference in evaluation of the energy performance of ventilation strategies in air-conditioned offices at a quantified exposure risk of formaldehyde.     

Evaluation on four sampling schemes for assessing indoor air quality

Continuous sampling is one of the common approaches for assessing indoor pollutant level. It is believed that the longer the measurement time, the higher the accuracy and confidence level of the measurement can be achieved. In 2003, the Hong Kong Environmental Protection Department (HKEPD) launched an Indoor Air Quality (IAQ) certification scheme to promote an acceptable IAQ in workplaces. However, measurement efforts and uncertainties associated with the sampling method have not been addressed. Alternative sampling schemes taking shorter measurements in the sampling period were proposed in some circumstances. In this study, the average carbon dioxide (CO₂) concentration of a workplace is selected as an indicator of the indoor air quality to investigate the probable errors and measurement efforts in four sampling schemes, regarding the sampling period: in Scheme A, it is from a continuous sampling throughout the measurement; in Scheme B, it is from two sampling periods of two equal sessions of the measurement; in Scheme C, it is from two structural sampling periods of the two sessions; and in Scheme D, the average concentration is from four sampling periods in four equal sessions of the measurement. In particular, a year-round indoor CO₂ concentration at 17 locations in a typical office in Hong Kong was used to evaluate the probable errors using these four sampling schemes. At certain confidence levels, the required measurement times of the alternative sampling schemes (Schemes B, C, D) were evaluated and compared with that of an 8-hour continuous one (Scheme A). It was found that Scheme C would offer a reduction of measurement effort up to 30%. It is recommended to specify the uncertainties and efforts of measurement in future codes, and to consider these sampling schemes in determining practical strategies for IAQ measurement.     

A fire safety study of Hong Kong refuge floor building wall layout design

Since 1996, refuge floors have been an indispensable passive building element of the fire protection plan for Hong Kong high-rise buildings. These floors must be designed to comply with the relevant requirements of the Building Codes of Hong Kong. Hence, 50% of the floor area must be deployed for refuge purposes. And they must have at least two fully open building side walls so that a desired wind effect is assumed so that any smoke that enters will be purged from the safe area. Therefore wind-induced natural cross-ventilation is of prime importance for the protection of the refuge floor in fire safety. If this wind effect is not achieved, entering smoke will accumulate and cause the safety system to fail. Therefore fundamental studies of this flow phenomenon are very important and needed. This paper studies the wind-induced flow behaviour of Hong Kong's refuge floors. Different building wall layout designs are addressed. By way of this study, the author suggests that a refuge floor shall be designed to have two opposite building side walls open instead of the present Building Codes that require at least any two of them.     

The impact of daylighting on peak electrical demand

A complete analysis of the cost-effectiveness of daylighting strategies should include the impact of daylighting on peak electrical demand as well as on energy consumption. We utilized an hour-by-hour building energy analysis program to study the thermal and daylighting impacts of fenestration on peak demand. Fenestration properties and lighting system characteristics were varied parametrically for office buildings in Madison WI and Lake Charles LA. Peak electrical demand was disaggregated by component and by zone, monthly patterns of peak demand were examined, and impacts of fenestration performance on chiller size were studied.The results suggest that for daylighted office buildings, the peak electrical demand results from a complex trade-off between cooling load due to fenestration parameters, lighting load reductions due to glazing and lighting system characteristics. Lowest peak demands generally occur with small to moderate size apertures. With daylighting, peak electrical demand is reduced by 10 to 20% for the building configuration studied (37% perimeter zone, 63% core zone). This work indicates that solar gain through fenestration must be effectively controlled in order to realize the potential of daylighting to significantly reduce peak electrical demand.