F.W.H. Yik

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.

Influence of thermal insulation position in building envelope on the space cooling of high-rise residential buildings in Hong Kong

At the present time, thermal insulation is almost not used in fabric of tall residential buildings in Hong Kong, as their fabric slabs usually comprise concrete layer covered on each side by plaster layers. This study investigates into the influence of an existence of the thermal insulation layer in the outside walls on the yearly cooling load and yearly maximum cooling demand in two typical residential flats in a high-rise residential building by employing HTB2, detailed building heat transfer simulation software. During the investigations, the thermal insulation layer up to 15 cm thick was placed either at the inside, or at the outside, or at the middle part of the outside wall structure. Then, the concrete layer was up to 40 cm thick. The simulation predictions indicate that the highest decrease in the yearly cooling load of up to 6.8% is obtained when a 5 cm thick thermal insulation layer faces the inside of the residential flat. The highest decrease in the yearly maximum cooling demand of 7.3% is recorded when a 5 cm thermal insulation layer faces either the outside or the inside of the flat; this depends on the flat orientation. However, much weaker reductions in the yearly cooling load and yearly maximum cooling demand are found when the thickness of thermal insulation is increased above 5 cm and the thickness of concrete is increased above 10 cm.

On the study of the credit-weighting scale in a building environmental assessment scheme

A major strength of the voluntary building environmental assessment method is to encourage good environmental practice beyond the current industry practice. For a successful voluntary scheme, the credit scale must be attractive to investors and performance-based criteria should be used. However, most of the current schemes use a combination of prescriptive and performance-based criteria, and credits are allocated using subjective judgement, irrespective of cost and practical implications of attaining improved performances. This is pointed to the need to establish an incentive-crediting scheme, which is oriented towards performance-based scales having credits linked to cost and performance. In formulating the incentive-crediting scheme, the cost effectiveness of different energy-saving level in a base building is investigated under the premise that the investors would pursue a strategy in the most cost-effective manner. This paper presents a conceptual framework for policy makers to formulate an incentive-crediting scale for accrediting criteria within the greenhouse gas objectives of a building environmental assessment scheme in Hong Kong, aiming to attract participation of the profit-maximising investors. Establishment of an incentive-crediting scale starts from developing a carbon dioxide abatement curve for greenhouse gas emission. This is subsequently followed by dividing the maximum energy-saving potential into ten equal energy reduction intervals. The incentive-crediting scheme is formulated on a basis that proportionally more credits would be rewarded for diminishing investment return of consecutive reduction levels in a base building.

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.

Energy performance of windows in high-rise residential buildings in Hong Kong

At the present time in Hong Kong in its hot and humid climate, a single pane, clear glazing is most often used in windows of tall residential buildings. During this study, we employed HTB2, detailed building heat transfer simulation software, to evaluate a decrease in the yearly cooling load (Q) and in the peak cooling-load (D) in two residential flats due to different glazing single pane types and different flat orientations. The investigated glazing types were clear glazing, tinted glazing, reflective glazing, and tinted and reflective glazing.

Influence of envelope and partition characteristics on the space cooling of high-rise residential buildings in Hong Kong

Abstract Nowadays, many apartments in residential buildings in Hong Kong are equipped with air-conditioners to provide comfort cooling for the living, dining and bedrooms. Air-conditioners serving the living and dining rooms would be operated mostly in the evenings and sometimes during daytime as well, but those serving the bedrooms could remain operating overnight. Other rooms, such as bathrooms and kitchens, will not normally be air-conditioned. The paper describes a study into the influence of the building envelope construction on the space cooling loads in residential buildings. The yearly cooling loads of apartments were predicted by using HTB2, a detailed building heat transfer simulation program. The study includes modifications to the base-line characteristics of walls and doors of two typical apartments. The modified walls have an additional 50 mm thick thermal insulation and/or a thickness of up to 400 mm of concrete. Modified doors have an additional 38 mm thick thermal insulation. The simulation predictions indicate that the highest reduction in the yearly cooling load could be obtained when the modified walls and doors were used at suitable locations. In addition, the study results show that the yearly cooling load would increase when the thermal insulation was applied to external walls without increasing the thickness of the concrete layer, and when the concrete layer was thickened but without addition of insulation.

Locating air-conditioners and furniture inside residential flats to obtain good thermal comfort

Majority of residential apartments in Hong Kong may be equipped with up to three window-type air-conditioners that operate during 7 months of hot and humid weather. The units are placed in the living rooms and bedrooms. The positions of these units and furniture inside these small rooms are directed in some extent by the architectural design, however, residents have possibility to decide about their actual position. The occupants should locate their air-conditioner with respect to the room furniture to avoid the cold air re-circulation and draft, and to obtain good thermal comfort for residents. To accomplish this, flat manufacturers should give some recommendations to the occupants, and architects should include such concerns in their design. For three positions of an air-conditioner unit and one position of a bed and a cupboard in a typical Hong Kong residential bedroom, the air velocities, temperatures, and air diffusion performance index are determined by using the computational fluid dynamics (CFD) software FLOVENT. The cases with the maximum benefit and maximum consequences in terms of thermal comfort in the bedroom are identified.

Flow and temperatures outside a high-rise residential building due to heat rejection by its air-conditioners

Abstract Summary: Recessed spaces outside high-rise residential buildings of public-type in Hong Kong often contain a large number of condenser units of window-type air-conditioners that during summer reject heat into this space at the same time. For four condenser units per story, a computational fluid dynamics (CFD) code is used to predict temperature and flow fields inside the recessed spaces that differ in heights and condenser-unit locations. The results show that a powerful rising hot air stream and one vortex pair are formed in the space near to its corner wall. In addition, the results show that condenser units will suffer more when they are placed inside the taller recessed space, and at locations higher at this space and nearer to its corner wall. Then, these units would draw less air at an elevated temperature, and therefore become less energy efficient and de-rated in output capacity and sometime have interrupted operation. However, the condenser units at lower levels may benefit because they will draw more air at nearly environmental temperature.

Problems with specialist subcontracting in the construction industry

It is envisaged that more and more specialist works will be employed in modern buildings for enhancing performance and sustainability. Subcontracting is an economical way of procuring specialist works for modern buildings but it may also give rise to various kinds of problems. Such problems may originate from the characteristics of the specialist works and from subcontracting as a procurement method. In the paper, the key characteristics of specialist works are identified and their implications are analysed. Through discussing how new technologies penetrate the market, the pricing strategies that vendors of new technologies may adopt together with their implications are unveiled. The problems with procuring specialist works through subcontracting are then reviewed, and the issues that building developers should take note of in deciding to equip a building with a specialist work and in procurement and management of the specialist subcontract are highlighted and discussed. Practical application: By providing an in-depth exposition on subcontracting within the construction industry, the paper can act as a source of information and guide for those initiating and managing projects that involve substantial amounts of subcontracting, especially to project clients when they procure specialist works through subcontracting. The potential problems discussed include not only those that may arise during the construction stage, readers’ attention is also drawn to the impacts on operation and maintenance and the aftermarket monopoly power of vendors of proprietary products and services.

Influence of a depth of a recessed space to flow due to air-conditioner heat rejection

Abstract During summer, a large number of air-conditioners at the same time could be rejecting condenser heat into a recessed space formed by the external walls outside many high-rise residential buildings in Hong Kong. Prediction of the temperature and flow field of a powerful rising hot air stream formed in a 30-story high recessed space is performed by using FLUENT 5.0, a computational fluid dynamics (CFD) code. For a 30-story high recessed space of “I” type with four window-type air-conditioner units per story, this paper describes the findings for two depths of the recessed space. For the buoyant air stream, the results show that the mass-flow rate and temperature inside the recessed space are not significantly influenced by the recessed space depth. For air streams used by the condenser units, deeper recessed space yields an increase in the enthalpy in the air stream used by the outer units, and a decrease in the enthalpy in the air stream used by the inner units, which draw the hottest air from the recessed space.