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.     

Energy impact of indoor environmental policy for air-conditioned offices of Hong Kong

Air-conditioned office buildings are one of the biggest energy consumers of electricity in developed cities in the subtropical climate regions. A good energy policy for the indoor environment should respond to both the needs of energy conservation and the needs for a desirable indoor healthy environment with a reduction in carbon dioxide (CO₂) generation. This study evaluates energy implications and the corresponding CO₂ generation of some indoor environmental policies for air-conditioned office buildings in the subtropical climate. In particular, the thermal energy consumption in an air-conditioned office building was evaluated by the heat gains through the building fabric, the transport of outdoor fresh air for ventilation, and the heat generated by the occupant and equipment in the space. With the Monte-Carlo sampling technique and the parameters from the existing office building stocks of Hong Kong, the energy consumption profiles of air-conditioned office buildings in Hong Kong were evaluated. Energy consumption profiles were simulated for certain indoor environmental quality (IEQ) policies on indoor air temperature and CO₂ concentration settings in the offices, with other building parameters remaining unchanged. The impact assessment and the regression models described in this study may be useful for evaluation of energy performances of IEQ policies. They will also be useful for the promotion of energy-saving measures in air-conditioned office buildings in Hong Kong. This study presented a useful source of references for policymakers, building professionals and end users to quantify the energy and environmental impacts due to an IEQ policy for air-conditioned office buildings.     

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.     

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.     

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.     

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.     

Potential of renewable hydrogen production for energy supply in Hong Kong

Hong Kong is highly vulnerable to energy and economic security due to the heavy dependence on imported fossil fuels. The combustion of fossil fuels also causes serious environmental pollution. Therefore, it is important to explore the opportunities for clean renewable energy for long-term energy supply. Hong Kong has the potential to develop clean renewable hydrogen energy to improve the environmental performance. This paper reviews the recent development of hydrogen production technologies, followed by an overview of the renewable energy sources and a discussion about potential applications for renewable hydrogen production in Hong Kong. The results show that although renewable energy resources cannot entirely satisfy the energy demand in Hong Kong, solar energy, wind power, and biomass are available renewable sources for significant hydrogen production. A system consisting of wind turbines and photovoltaic (PV) panels coupled with electrolyzers is a promising design to produce hydrogen. Biomass, especially organic waste, offers an economical, environmental-friendly way for renewable hydrogen production. The achievable hydrogen energy output would be as much as 40% of the total energy consumption in transportation.     

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.     

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.     

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.     

Potential application of “see-through” solar cells in ventilated glazing in Hong Kong

This paper reports the findings on the energy performance of “see-through” PV glazing as applied to a typical open-plan office environment of Hong Kong. An experimental system was first set up and the measurements were used to verify the theoretical models developed via the ESP-r simulation platform. The validated models were subsequently used to evaluate the annual variations in thermal loads and electricity generation for two PV glazing systems as compared to the common absorptive glazing provisions. The results indicate that the innovative natural-ventilated PV double-glazing technology could significantly cut down the air-conditioning power consumption by 28%, as compared to the conventional single absorptive glazing system.     

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.     

A micro tri-generation system based on direct flame fuel cells for residential applications

A novel micro tri-generation system which combines a direct flame fuel cell, a boiler and a double-effect absorption chiller is proposed and analyzed for residential applications. Parametric analyses are conducted to investigate the effects of operating parameters (i.e. the equivalence ratio and the fuel utilization factor of the fuel cell) on the system efficiency and the thermal-to-electric ratio. Then optimum operating parameters are determined based on the typical energy demand of Hong Kong in the summer and the typical energy demand of Beijing in the winter, respectively. It is found that very high efficiency (over 90%) can be achieved by this novel tri-generation system for both Hong Kong and Beijing. Besides, the system is modified for combined heat and power cogeneration, combined cooling and power cogeneration and their efficiencies are compared with the tri-generation system to evaluate the effect of each single unit on the whole system.     

Cooling-energy reduction in air-conditioned offices by using night ventilation

Night ventilation has been applied successfully to many passively-cooled or low-energy office buildings. This paper investigates the applicability of night ventilation in air-conditioned office buildings. A thermal and ventilation simulation model, together with suitable weather data were used to examine the potential for energy savings and/or improved internal comfort conditions by applying night ventilation cooling. It was found that natural ventilation strategies could save cooling energy in typical air-conditioned offices. However, the use of mechanical ventilation could lead to increased energy-consumption. If typical offices are modified to incorporate features assisting the application of night ventilation, then cooling energy could be saved when mechanical ventilation is used and further reduced in the case of natural ventilation. Such features would include exposed thermal mass or offices designed to ‘best practice’ guidelines, such as airtight construction and minimisation of internal and solar heat gains.     

Energy saving by realistic design data for commercial buildings in Hong Kong

Oversized equipment is one of the key factors for poor energy performance of commercial buildings in Hong Kong. Similar situations exist also in many buildings in different parts of the world. The use of realistic design criteria has been identified as an effective method to reduce the equipment oversizing problem. A set of realistic design criteria for lighting power density, occupation density, and appliances' load-intensity for various types of premises have been established based on data obtained in walk-through surveys of energy end-uses in 31 Hong Kong office buildings. Potential electricity savings through the use of realistic design criteria were estimated to be 6–22% of the electricity consumption of Hong Kong, which corresponds to an annual cost of HK$12.2–44.7 billion.     

Solar hybrid cooling system for high-tech offices in subtropical climate – Radiant cooling by absorption refrigeration and desiccant dehumidification

A solar hybrid cooling design is proposed for high cooling load demand in hot and humid climate. For the typical building cooling load, the system can handle the zone cooling load (mainly sensible) by radiant cooling with the chilled water from absorption refrigeration, while the ventilation load (largely latent) by desiccant dehumidification. This hybrid system utilizes solar energy for driving the absorption chiller and regenerating the desiccant wheel. Since a high chilled water temperature generated from the absorption chiller is not effective to handle the required latent load, desiccant dehumidification is therefore involved. It is an integration of radiant cooling, absorption refrigeration and desiccant dehumidification, which are powered up by solar energy. In this study, the application potential of the solar hybrid cooling system was evaluated for the high-tech offices in the subtropical climate through dynamic simulation. The high-tech offices are featured with relatively high internal sensible heat gains due to the intensive office electric equipment. The key performance indicators included the solar fraction and the primary energy consumption. Comparative study was also carried out for the solar hybrid cooling system using two common types of chilled ceilings, the passive chilled beams and active chilled beams. It was found that the solar hybrid cooling system was technically feasible for the applications of relatively higher cooling load demand. The annual primary energy consumption of the solar hybrid cooling system was lower than that of the conventional vapour compression refrigeration system up to 36.5%. Between the two options of chilled ceilings, the passive chilled beams were more energy-efficient to work with the solar hybrid cooling system in the hot and humid climate. Harnessing solar energy for driving air-conditioning would help in reducing the carbon emission, hence alleviating the climate change.     

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.     

The effect of the London urban heat island on building summer cooling demand and night ventilation strategies

This paper investigates the effect that increased air temperature due to the London heat island has on the effectiveness of stack night ventilation strategies for office buildings. Stack ventilation was investigated as the most suitable night ventilation strategy because this is largely independent of wind variations affected by local urban morphology. The paper presents a summary of the results of air temperature measurements carried out in London in 1999/2000 which were used to quantify the London Urban Heat Island Intensity. It then presents data for two representative weeks, one with extreme hot weather and one with typical hot weather in the centre of the London heat island and a rural reference site. These data are used to carry out a parametric analysis by using a thermal and air flow simulation tool specifically designed for offices in SE England. A reference and optimised office module are described. A comparison of the building types based in the same location suggests that during the typical hot week the rural reference office has 84% energy demand for cooling compared to a similar urban office. A rural optimised office would not need any artificial cooling and would be able to maintain temperatures below 24 °C. An urban optimised office would not be able to achieve this. A rural optimised office would need 42% of the cooling required for an optimised urban office. A comparison of the optimised to the reference office module suggests that an urban optimised office reduces the cooling demand to 10% of the urban reference office.     

Annual performance of building-integrated photovoltaic/water-heating system for warm climate application

A building-integrated photovoltaic/water-heating (BiPVW) system is able to generate higher energy output per unit collector area than the conventional solar systems. Through computer simulation with energy models developed for this integrative solar system in Hong Kong, the results showed that the photovoltaic/water-heating (PVW) system is having much economical advantages over the conventional photovoltaic (PV) installation. The system thermal performance under natural water circulation was found better than the pump-circulation mode. For a specific BiPVW system at a vertical wall of a fully air-conditioned building and with collectors equipped with flat-box-type thermal absorber and polycrystalline silicon cells, the year-round thermal and cell conversion efficiencies were found respectively 37.5% and 9.39% under typical Hong Kong weather conditions. The overall heat transmission through the PVW wall is reduced to 38% of the normal building facade. When serving as a water pre-heating system, the economical payback period was estimated around 14 years. This greatly enhances the PV market opportunities.     

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.