Efficiency assessment of indoor environmental policy for air-conditioned offices in Hong Kong

To reduce carbon dioxide (CO₂) emissions through thermal energy conservation, air-conditioned offices in the subtropics are recommended to operate within specified ranges of indoor temperature, relative humidity and air velocity. As thermal discomfort leads to productivity loss, some indoor environmental policies for air-conditioned offices in Hong Kong are investigated in this study with relation to thermal energy consumption, CO₂ emissions from electricity use, and productivity loss due to thermal discomfort. Occupant thermal response is specifically considered as an adaptive factor in evaluating the energy consumption and productivity loss. The energy efficiency of an office is determined by the productivity which corresponds to the CO₂ generated. The results found that a policy with little impact on occupant thermal comfort and worker productivity would improve the office efficiency while the one with excessive energy consumption reduction would result in a substantial productivity loss. This study is a useful reference source for evaluating an indoor thermal environmental policy regarding the energy consumption, CO₂ emissions reduction, thermal comfort and productivity loss in air-conditioned offices in subtropical areas.     

An energy benchmarking model for ventilation systems of air-conditioned offices in subtropical climates

Since the energy crisis in 1973, engineers have endeavoured to implement energy conservation in buildings. Unfortunately, the effort resulted in energy savings without the fundamental delivery of indoor satisfaction in many cases. In this study, a benchmarking model for the energy consumption of ventilation systems in air-conditioned offices was proposed. This model was developed from the fundamental psychrometric analysis under probable office design conditions in Hong Kong. The results showed that the annual energy-consumption of a ventilation system per unit floor area would be correlated closely with the carbon dioxide (CO₂) concentration in the space, but its correlation with the air temperature set-point would be less significant. In some offices, significant energy-savings potential was demonstrated to provide satisfactory indoor air quality (IAQ) without any comfort penalty to the occupants. This model would be useful for the energy performance evaluation and benchmarking of ventilation systems in air-conditioned offices.     

Daylighting and energy analysis for air-conditioned office buildings

We propose a simple method for estimating the likely energy savings in electric lighting due to daylighting and the possible cooling penalty. Fractions of the working year and cooling season when daylighting alone is adequate to provide the indoor design illuminance are presented for on-off and top-up controls. Based on the simple average daylight factor method, energy savings in electric lighting have been estimated for a generic office building using measured outdoor illuminance data in Hong Kong. The daylight-induced cooling penalty is estimated using average solar heat gain factors. Our case study suggests that daylighting schemes can result in substantial energy savings in air-conditioned office buildings in Hong Kong.     

A transient ventilation demand model for air-conditioned offices

Ventilation to supply fresh air in an air-conditioned office consumes a considerable portion of energy in an air-conditioning system and affects the indoor-air quality (IAQ). The ventilation demand is primarily related to the occupant load. In this study, the ventilation demands due to occupant load variations were examined against certain IAQ objectives using the mass balance of carbon dioxide (CO₂) volume fractions in an air-conditioned office. In particular, this study proposed a transient ventilation demand model for occupant load, with the parameters determined from a year-round occupant load survey in Hong Kong. This model was applied to evaluate the performance of energy saving in different operating schedules of ventilation systems for typical office buildings in Hong Kong. The results showed that the energy consumption of a ventilation system would be correlated with the transient occupant load and its variations in the air-conditioned office. The ventilation system, with schedules taking account of the transient occupant loads, would offer a reduction in energy consumption up to 19% as compared with an operating schedule that assumed a steady occupant-load in the office during working hours. In both cases, the same IAQ objective was achieved.     

Building energy research in Hong Kong: A review

Hong Kong is located in a typical subtropical region. The buildings in Hong Kong are subjected to high cooling demands for their air-conditioning systems throughout most of the year, and their contribution toward the total energy consumption is about 40%. Therefore, energy efficiency in buildings is essential to reduce the global energy use and improve the local environmental sustainability. This paper provides an overall review of the building energy research and efforts in Hong Kong over the last decade. Various aspects and energy saving measures, including energy policy, energy audit, design, control, diagnosis, building performance evaluation and renewable energy systems, studied or used to enhance the energy efficiency in buildings are reviewed. A brief introduction of the Hong Kong Building Energy Codes (BEC) and Hong Kong Building Environmental Assessment Method (HK-BEAM) are also provided to present the efforts of the local government and community on energy efficiency in buildings. This review aims at providing building researchers and practitioners a better understanding of buildings energy saving opportunities and approaches in cities particularly in subtropical regions and taking further proper actions to promote buildings energy efficiency and conservation.     

Assessment of climate change impact on building energy use and mitigation measures in subtropical climates

Likely increase in energy use in air-conditioned office buildings due to climate change in subtropical Hong Kong was estimated for two emissions scenarios. Towards the end of the 21st century (i.e. 2091-2100), the average annual building energy use would be 6.6% and 8.1% more than that in 1979-2008 for low and medium forcing, respectively. Potential mitigation measures concerning the building envelope, internal condition, lighting load density (LLD) and chiller plant were considered. Thermal insulation to the external wall would not be effective to mitigate the expected increase in building energy use due to climate change. Controlling the amount of solar heat gain through the window would be a better option. Lowering the current LLD of 15 W/m² to about 13 W/m² would result in substantial energy savings because of the reduction in electricity consumption for both electric lighting and air-conditioning. As for the chiller plant, the coefficient of performance (COP) should be improved from the current minimum requirement of 4.7 to at least 5.5 to alleviate the impact of climate change. Raising the summer set point temperature (SST) to 25.5 °C or higher would have high energy saving and hence mitigation potential, which could be readily applied to both new and existing buildings.     

Assessment of Thailand indoor set-point impact on energy consumption and environment

The paper presents an investigation of indoor set-point standard of air-conditioned spaces as a tool to control electrical energy consumption of air-conditioners in Thailand office buildings and to reduce air pollutants. One hundred and forty-seven air-conditioned rooms in 13 buildings nationwide were used as models to analyze the electricity consumption of air-conditioning systems according to their set indoor temperatures, which were below the standard set-point and were accounted into a large scale. Then, the electrical energy and environmental saving potentials in the country were assessed by the assumption that adaptation of indoor set-point temperature is increased up to the standard set-point of 26 °C. It was concluded that the impacts of indoor set-point of air-conditioned rooms, set at 26 °C, on energy saving and on environment are as follows: The overall electricity consumption saving would be 804.60 GWh/year, which would reduce the corresponding GHGs emissions (mainly CO₂) from power plant by 579.31×10³ tons/year.     

An evaluation of the appropriateness of using overall thermal transfer value (OTTV) to regulate envelope energy performance of air-conditioned buildings

This paper inquires into whether overall thermal transfer value (OTTV) is an appropriate building envelope energy performance index for use in regulatory control. First, a historical review of the use of OTTV in American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) Standard 90 is presented, followed by a review of more recent work on its further development and application. The major deficiencies of OTTV are then discussed, and simulation study results meant to highlight the impacts of such deficiencies are presented. The study embraced air-conditioned office buildings and air-conditioned high-rise residential buildings in Hong Kong. Results of this study clearly show that the OTTV calculated with the use of pre-calculated coefficients may not truly reflect the thermal performance of a building envelope. Therefore, a second thought should be given to the use of OTTV in building energy codes.     

Analysis of energy and carbon dioxide emission caused by power consumption

The main objective of this on‐site study is to use a full‐scale Heating, Ventilating, and Air‐Conditioning (HVAC) system installed in an office building in Taiwan for comparing the power consumption, energy‐saving, and carbon dioxide (CO₂) reduction of two different strategies for controlling the HVAC. These strategies are the Constant Volume (CV) system [Constant Air Volume+Constant‐flow], and the Variable Volume (VV) system [Variable Air Volume +Variable‐flow]. The on‐site experimental results indicate that average power consumptions are 164 kW for the CV system, and 88 kW for the VV system; the average electric current drops from 469 A for the CV system to 258 A for the VV system. Approximately 46% of the average energy‐saving can be achieved if the HVAC system is operated as a VV system. Additionally, the reduced quantity of accumulated CO₂ emission varies from 67 to 3687 kg with 0.637 kg CO₂ kwh^?1 emission factor during the office hours of 08:30 (a.m.)–17:00 (p.m.). The results demonstrate that switching the operation of an office building HVAC system from CV to VV will significantly enhance energy‐savings and CO₂ reduction. This studywill offer useful information for evaluating an indoor environmental policy with respect to energy‐savings and CO₂ emission reduction for office HVACs used in subtropical regions. Copyright © 2010 John Wiley & Sons, Ltd.     

Lighting and cooling energy consumption in an open-plan office using solar film coating

In subtropical Hong Kong, solar heat gain via glazing contributes to a significant proportion of the building envelope cooling load. The principal fenestration design includes eliminating direct sunlight and reducing cooling requirements. Daylighting is an effective approach to allow a flexible building façade design strategy, and to enhance an energy-efficient and green building development. This paper studies the lighting and cooling energy performances for a fully air-conditioned open-plan office when solar control films together with daylight-linked lighting controls are being used. Measurements were undertaken at two stages including the electricity expenditures for the office using photoelectric dimming controls only (first stage) and together with the solar control film coatings on the windows (second stage). Electric lighting and cooling energy consumption, transmitted daylight illuminance and solar radiation were systematically recorded and analysed. The measured data were also used for conducting and validating the building energy simulations. The findings showed that the solar film coatings coupled with lighting dimming controls cut down 21.2% electric lighting and 6.9% cooling energy consumption for the open-plan office.     

Lighting and energy performance of solar film coating in air-conditioned cellular offices

In subtropical Hong Kong, the principal objectives of fenestration design include eliminating direct sunlight and decreasing cooling loads. To avoid the problems of glare, excessive brightness and thermal discomfort, occupants may block the windows with internal shading devices, resulting in poor daylighting performance and very small amount of electric lighting energy savings. Recently, the advances in thin film coatings for window glass products provide a means of substantially reducing heat gain without proportionally reducing daylight transmittance. It has been suggested that film coatings together with photoelectric lighting control systems could minimise the electric lighting and cooling requirements without causing undue visual and thermal discomfort to the occupants. This paper presents field measurements on solar control film coatings in fully air-conditioned offices in Hong Kong. Solar heat gains, indoor illuminance levels and the electricity consumption by the fluorescent luminaires were systematically recorded and analysed. Measurements were made for two cellular offices, one with solar control film coating on the window glass and the other without. The findings showed that the solar film coating could cut down energy expenditures for air-conditioned buildings, especially for spaces with large glazing areas subject to substantial amount of direct sunlight. Results are presented and the design implications discussed.     

Overall energy performance of semi-transparent single-glazed photovoltaic (PV) window for a typical office in Hong Kong

This paper develops an overall methodology for investigating the thermal and power behaviors of semi-transparent single-glazed photovoltaic window for office buildings in Hong Kong. In order to estimate its overall energy performance, this study is conducted in terms of total heat gain, output power and daylight illuminance. Three simulation models are established, including one-dimensional transient heat transfer model, power generation model and indoor daylight illuminance model. A typical office room reference is chosen as case study, and the weather data from 2003 to 2007 from the Hong Kong Observatory are used as the simulation inputs. By incorporating the simulation results, the overall energy performance can be evaluated in terms of electricity benefits corresponding to five orientations of the studied typical office. The priority of office orientation considering overall energy performance is: south-east, south, east, south-west and west. The findings show that thermal performance is the primary consideration of energy saving in the entire system whereas electricity consumption of artificial lighting is the secondary one. The overall annual electricity benefits are about 900 kWh and 1300 kWh for water-cooled and air-cooled air-conditioning systems respectively. The application of semi-transparent PV glazed window can not only produce clean energy, but also reduce building energy use by reducing the cooling load and electrical lighting requirements, which definitely benefits our environmental and economic aspects.     

Towards a low-carbon future in China's building sector—A review of energy and climate models forecast

This article investigates the potentials of energy saving and greenhouse gases emission mitigation offered by implementation of building energy efficiency policies in China. An overview of existing literature regarding long-term energy-demand and carbon dioxide (CO₂) emission forecast scenarios is presented. Energy consumption in buildings could be reduced by 100–300 million tons of oil equivalent (mtoe) in 2030 compared with the business-as-usual (BAU) scenario, which means that 600–700 million metric tons of CO₂ emissions could be saved by implementing appropriate energy policies within an adapted institutional framework. The main energy-saving potentials in buildings can be achieved by improving a building's thermal performance and district heating system efficiency. The analyses also reveal that the energy interchange systems are effective especially in the early stage of penetration. Our analysis on the reviewed models suggests that more ambitious efficiency improvement policies in both supply- and demand-side as well as the carbon price should be taken into account in the policy scenarios to address drastic reduction of CO₂ emission in the building sector to ensure climate security over the next decades.     

An analysis of future building energy use in subtropical Hong Kong

Principal component analysis of prevailing weather conditions in subtropical Hong Kong was conducted, and a new climatic index Z (as a function of the dry-bulb temperature, wet-bulb temperature and global solar radiation) determined for past (1979–2008, measurements made at local meteorological station) and future (2009–2100, predictions from general circulation models) years. Multi-year (1979–2008) building energy simulations were carried out for a generic office building. It was found that Z exhibited monthly and seasonal variations similar to the simulated cooling/heating loads and building energy use. Regression models were developed to correlate the simulated monthly building cooling loads and total energy use with the corresponding Z. Error analysis indicated that annual building energy use from the regression models were very close to the simulated values; the difference was about 1%. Difference in individual monthly cooling load and energy use, however, could be up to 4%. It was also found that both the DOE-simulated results during 1979–2008 and the regression-predicted data during 2009–2100 indicated an increasing trend in annual cooling load and energy use and a gradual reduction in the already insignificant heating requirement in cooling-dominated office buildings in subtropical climates.     

Applying water cooled air conditioners in residential buildings in Hong Kong

The objective of this study is to conduct a realistic prediction of the potential energy saving for using water cooled air conditioners in residential buildings in Hong Kong. A split type air conditioner with air cooled (AAC) and water cooled (WAC) options was set up for experimental study at different indoor and outdoor conditions. The cooling output, power consumption and coefficient of performance (COP) of the two options were measured and calculated for comparison. The experimental results showed that the COP of the WAC is, on average, 17.4% higher than that of the AAC. The results were used to validate the mathematical models formulated for predicting the performance of WACs and AACs at different operating conditions and load characteristics. While the development of the mathematical models for WACs was reported in an earlier paper, this paper focuses on the experimental works for the AAC. The mathematical models were further used to predict the potential energy saving for application of WACs in residential buildings in Hong Kong. The predictions were based on actual building developments and realistic operating characteristics. The overall energy savings were estimated to be around 8.7% of the total electricity consumption for residential buildings in Hong Kong. Wider use of WACs in subtropical cities is, therefore, recommended.     

Can envelope codes reduce electricity and CO2 emissions in different types of buildings in the hot climate of Bahrain?

The depletion of non-renewable resources and the environmental impact of energy consumption, particularly energy use in buildings, have awakened considerable interest in energy efficiency. Building energy codes have recently become effective techniques to achieve efficiency targets. The Electricity and Water Authority in Bahrain has set a target of 40% reduction of building electricity consumption and CO₂ emissions to be achieved by using envelope thermal insulation codes. This paper investigates the ability of the current codes to achieve such a benchmark and evaluates their impact on building energy consumption. The results of a simulation study are employed to investigate the impact of the Bahraini codes on the energy and environmental performance of buildings. The study focuses on air-conditioned commercial buildings and concludes that envelope codes, at best, are likely to reduce the energy use of the commercial sector by 25% if the building envelope is well-insulated and efficient glazing is used. Bahraini net CO₂ emissions could drop to around 7.1%. The simulation results show that the current energy codes alone are not sufficient to achieve a 40% reduction benchmark, and therefore, more effort should be spent on moving towards a more comprehensive approach.     

Evaluation of plume potential and plume abatement of evaporative cooling towers in a subtropical region

Hong Kong is a typical subtropical region with frequently high humidity in late spring and summer seasons. Plume from evaporative cooling towers, which service air-conditioning systems of civil buildings, has aroused public concerns since 2000 when the fresh water evaporative cooling towers were allowed to be used for high energy efficiency and environmental issues. This paper presents the evaluation of the plume potential and its effect on the sizing of the plume abatement system in a large commercial office building in Hong Kong for practical application. This evaluation was conducted based on a dynamic simulation platform using the typical meteorological year of Hong Kong since the occurrence of the plume heavily depends on the state conditions of the exhaust air from cooling towers and the ambient air, while the state condition of the exhaust air is determined by the total building cooling load and the control strategies of cooling towers employed mainly for improving energy efficiency. The results show that the control strategies have a significant effect on the plume potential and further affect the system design and sizing of the plume abatement system.     

Regulatory and voluntary approaches for enhancing energy efficiencies of buildings in Hong Kong

Regulatory control forms the bedrock of the environmental policies of many countries. Whilst this interventionist approach can ensure a certain minimum standard would be achieved, far greater results can be achieved if it is augmented by a voluntary assessment-scheme. The benefits of having both approaches are explored in the paper with reference to the building energy codes and a voluntary building environmental-performance assessment scheme HK-BEAM, both of which are being implemented in Hong Kong. Quantification of the possible range of energy saving was based on the design characteristics of 22 commercial buildings in Hong Kong. The need for and the benefits of using the energy assessments within HK-BEAM as an alternative compliance route to the building energy codes are confirmed and the compliance criteria are established. The potential energy-saving through the use of the regulatory and the voluntary-based approaches were estimated to be from about 8% to more than 30%.     

Simulation–optimization of solar-assisted desiccant cooling system for subtropical Hong Kong

Solar cooling is a novel approach, which primarily makes use of solar energy, instead of electricity, to drive the air-conditioning systems. In this study, solar-assisted desiccant cooling system (SADCS) was designed to handle the cooling load of typical office in the subtropical Hong Kong, in which half of the building energy is consumed by the air-conditioning systems. The SADCS mainly consisted of desiccant wheel, thermal wheel, evaporative coolers, solar air collectors and gas-fired auxiliary heater, it could directly tackle both the space load and ventilation load. Since the supply air flow is same as the outdoor air flow, the SADCS has a feature of sufficient ventilation that enhances the indoor air quality. Although it is inevitable to involve the auxiliary heater for regeneration of desiccant wheel, it is possible to minimize its usage by the optimal design and control scheme of the SADCS. Through simulation–optimization approach, the SADCS can provide a satisfactory performance in the subtropical Hong Kong.     

Analysis of embodied energy use in the residential building of Hong Kong

Energy use in buildings accounts for nearly half of the total primary energy use in Hong Kong. Until now, studies have primarily focused on energy conservation in building operation, even though recent research has indicated that the embodied energy used in residential buildings could account for up to 40% of the life-cycle energy used in residential buildings. Accordingly, this paper presents a study on the energy embodied in the residential building envelope of Hong Kong. A model for estimating the intensities of the embodied and demolition energy for buildings has been developed. Two typical high-rise residential buildings, the Housing Authority Harmony 1 and the New Cruciform blocks, are analysed based on the developed model. The results of the analysis provide an insight into the embodied energy usage profile in residential buildings in Hong Kong. Energy embodied in steel and aluminium ranks as the first and second largest energy demand and may account for more than three-quarters of the total embodied energy use in a residential building envelope in Hong Kong. This reveals those building components with significant potential for reduction in embodied energy demand.