绿色住宅=豪宅？BEM 应用及香港绿色住宅设计探讨

Green Apartment = Luxurious Apartment? Application of BEM in design of green

public-housing in Hong Kong

R. Yin, E. Leung, M.H. Chan

Ove Arup & Partners Hong Kong Ltd, Hong kong, China

Abstract

The number of public-housing has continue to increase since the end of World War

Two, which in the year 1996, there are a total of 92,000 public-housing units available

and accommodating approximately 3.2 million people in Hong Kong. As public-

housing is designed for the low-income families, these buildings are seldom

considered as green buildings as there is a general misconception that green building

is luxurious. However, on the contrary, public-housing should be green as to lower the

daily operating cost imposing onto the low-income families, thus lowering energy

consumption.

Building Environment Modelling (BEM) is adopted throughout the design of public-

housing, which the incorporation of advance computation tool has provide

opportunities to implement several enhancement measures to improve ventilation

performance, daylight accessibility and lower the solar heat gain.

In order to justify the accuracy of the BEM, computation simulation results are

validated with the measurement results that have been undertaken in the public-

housing upon completion. In contrast, suitability of BEM in public housing design is

justified through face-to-face interviews, which shows a high satisfactory rate in the

overall condition of the public-housing, use of natural ventilation and daylight

accessibility within indoor habitable environment. Therefore, BEM is highly suitable

for green public-housing design while lowering operating cost for the low-income

families.

KEYWORDS: Public-Housing, Building Environment Modelling, Green Buildings

Introduction

After World War Two, construction of public-housing with small-unit-size housing

but low rental became inevitable as a mean to maintain low labour cost and low

inflation. However, as the economy of Hong Kong continue to grow; public-housing

becomes an approach to attract population to boost the new towns, which include

Shatin, Tai Po and Tin Shui Wai. Figure 1 illustrates the development of public-

housing in Hong Kong, which by the year 1996, there were a total of 920,000 public-

housing units, which accommodated approximately 3.2 million people [1].

Figure 1 Development of public-housing [1]

Although public-housing can provide accommodation for the low-income families,

the high urban densities have created several critical environmental issues in Hong

Kong, which includes poor daylight access and wind availability, noise and air

pollutant accumulation. Therefore, in order to maintain a comfortable indoor

environment for the occupants in public-housing, systems such as air-conditioning

and lightings are adopted. In 2008, the public-housing sector has consumed 15,366

Terajoules of energy [2], which accounts for 28.6% of the entire residential sector in

Hong Kong.

In order to reduce energy consumption while maintaining a comfortable environment

for the public-housing residents, public-housing should be designed “green”.

However, there is always a misconception that green buildings are more expensive

than typical designs, and thus assume that public-housings are not designed green.

However, this is mostly due to the lack of careful planning to understand the

microclimatic condition near the site for the public-housing. Secondly, public-housing

have more reason to be greener than other types of buildings as to lower the cost

imposing on the low-income families.

In this paper, an application – Building Environmental Modelling (BEM) is discussed

to identify the benefits in energy reduction and cost effectiveness design for green

public-housing design.

Carbon emission in Hong Kong

Climate Change is a major concern in the globe, which researches have shown that it

is due to the carbon emission of human activity. Climate Change is of more serious

concern to the coastal cities due to the rising ocean and increasing strength of typhoon.

This is of particular importance with Hong Kong as the sea levels have risen by an

average rate of 2.3mm per year around the coast of China [3]; especially when it is

one of the cities that have the world highest urban density, which many would be

affected if a catastrophic climatic condition occurs.

According to the Environmental Protection Department (EPD), carbon emission has

increased from 33.3 million tonnes in 1990 to 43.4 million tonnes in 2008 [4], which

show an average annual increase of 2%.

If carbon emission continues to grow, it is expected that the Hong Kong climatic

condition would change with a rise of annual mean temperature by 3.5oC by the end

of the century based on the Inter-governmental Panel on Climate Change (IPCC)

prediction [5].

Figure 2 Prediction of temperature rise in Hong Kong [3]

Furthermore, the urban density of Hong Kong also affects the health of many that

could be fatal [6, 7]. A study has shown that there is an estimate of 1.8% increase of

mortality with an increase of 1oC in daily mean temperature above 28

oC [8]. In

addition, the rise in temperature would result with a higher operating cost in air-

conditioning and more equipment failure (high-cut for many of the equipments).

Therefore, it is essential to reduce carbon emission. IPCC suggests that the allowable

temperature rise should not be more than 2oC as it would results with irreversible

consequences in the climatic system. As a result, the G8 has pledged for an average

global reduction in carbon emission by 50% during the 35th

G8 Summit in July 2009

in L’Aquila, Italy.

Based on this prediction, if Hong Kong has to reduce carbon emission to achieve the

2oC limit by IPCC, a minimum reduction of carbon emission by 2020 would have to

be 26 million tonnes against the Business As Usual (BAU) case as shown in Figure 3.

Figure 3 Required reduction to achieve IPCC 2

oC target

Green Building Design

If carbon emission has to be reduced in public-housing, there are various types of

energy saving strategies that could be implemented. However, strategies that are

expensive should be avoided to maintain low cost for the public housing. Therefore,

passive designs are commonly used in public-housing to reduce energy demand and

hence to reduce energy consumption.

Figure 4 Passive Design Strategy as the foundation of all strategies

Passive Design Strategy is defined as a “series of architectural design strategies used

by the design” to allow buildings to be able to respond to the changes in climate and

reduce energy consumption [9]. This would means that buildings would need to be

designed with care to achieve harmonization with the environment, which then allow

the maximise use of natural resources, such as daylighting and wind, and maintaining

an acceptable microclimate climatic condition to the surrounding buildings. While

this is true, due to the nature of the public-housing, the passive design should have a

very low cost implication during building design.

Although passive design is the most cost effective approach amongst the three types

of design strategies to achieve energy savings for public-housing; however, the

process for passive design for public-housing is even more difficult than active design

and renewable design as the most optimum passive design must incorporate several

environmental conditions, such as sun, light and wind. This would mean a large

amount of time would be used on assessing the microclimatic condition on site prior

of the existence of the public-housing, and with no indication on whether the

prediction is accurate or not until the construction is upon completion. Therefore,

sustainability design for public-housing will be difficult to achieve with the typical

approach. These factors may affect the effectiveness of the design and contradicts the

original design intent.

However, with the advancing computation technology, a design approach – Building

Environment Modelling (BEM) is developed. This approach would allow for green

building design with consideration for air ventilation, thermal comfort, visual comfort,

solar heat gain and dynamic thermal modelling as shown in Figure 5.

Figure 5 Building Environment Modelling for Green Building Design

BEM can accurately predict the environmental condition at design stage, the cost in

design would be reduced by reducing man-power and working period. As BEM

shows great potential in practice for green building design, it is widely used in public-

housing design projects from the Planning & Conceptual Stage to the Post Occupied

Stage, which the deliverables under different stage of construction is as shown in

Figure 6.

Figure 6 Involvement of BEM in different design stages of green public-housing design

Building Environment Modelling

In 2002, an extensive microclimate study has been undertaken for a design of a green

public-housing. Prior of the design stage, information of the site and the building

geometry are required:

Site Topography – use of Geotechnical Information System (GIS) to build the

3D computational model. Figure 7 compares the 3D model with the actual site.

Wind Availability – results from testing of a topographical wind tunnel to

obtain information on wind directions and velocities.

Sky Condition – to obtain the sky condition, which includes clear sky

condition, partly cloudy sky and overcast sky conditions, to identify the source

of daylight, either be direct sunlight or diffuse sunlight.

Solar Information – information comprises of direct and indirect solar

intensity on site

Figure 7 Comparison between the 3D model (left) and the actual site (right)

3D model Actual Site (At Present)

Planning & Conceptual Stage

During the Planning & Conceptual Stage, three different designs of the public-

housing are proposed that satisfy the required plot ratio and other planning

requirements, which are the Linear Block Design, Single Point Block Design and the

Twin Point Block Design. These building designs are evaluated with respect to

several performance factors, such as ventilation performance, daylight provision, solar

heat gain, view and microclimatic wind velocity.

In order to identify which design is the most optimum design under the local climatic

condition, a performance indicator is created to assess the overall building

performance, which the indicator is weighted in accordance to the relative

significance of the performance factors on the proposed building design. In order to

ensure the indicator is applicable for public-housing design in Hong Kong, the

weighting system is according to the number of credits in HK-BEAM [10] for

ventilation (6 credits), daylight (3 credits) and solar heat gain (2 credits), which gives

the weighting of 0.462, 0.231 and 0.154 respectively.

Based on this theory, view and microclimatic wind velocity are not meant to be

evaluated as there is no credit in HK-BEAM for these performance factors. However,

due to the complex built form of Hong Kong, these two performance factors are of

extreme importance to human comfort. Therefore, performance indicators for these

two factors are considered to have the same weighting as the overall environmental

performance with a value of 1 each.

With this in mind, a 3-point scoring scheme is adopted to assess the performance of

the public-housing design, which the higher the score, the more green the design of

the public-housing. Table 1 shows the scoring scheme for the performance indicator

that is adopted for the analysis. It should also be noted that this scoring scheme still

remains valid for green building design even with the use of the internationalized

version of HK-BEAM, which is known as the BEAM Plus.

Table 1 Scoring and weighting performance indicator

Score 0 1 2 3

Scale 0 Ventilation performance

Pressure difference (Pa)

9

Scale 0 Day lighting (Lux) 18750

Scale 800 Solar heat gain (W/m2) 0

Scale 0 Microclimate assessment - Air velocity (m/s) 5

Scale 0 Vertical View angle (degree) 90

With the use of this performance indicator, it is found that the Single Point Block

Design has the highest score amongst the three proposed design; therefore, Single

Point Block is recommended during the Planning & Conceptual Stage. Figure 8

shows the results on ventilation performance and the daylight performance for the

three proposed designs.

Figure 8 Simulation results for the Linear Block Design (Left), Single Point Block (Mid) and the

Duplex Point Block (Right)

Apart from the three proposed design, several enhancement measures including sky

garden, gradual height transition and mixed cluster are also modelled. It is found that

the designs of sky garden and gradual height transition could have an even higher

score than the Single Point Block Design. However, these enhancement measures

have a larger cost implication than the environmental benefits, thus these measures

are not considered. Therefore, optimization could be achieved with respect to

sustainability and construction cost of public-housing in the Planning & Conceptual

Stage.

Scheme Design Stage

During the scheme design stage, BEM is adopted to assess the microclimate condition

in the public area with the selected Single Point Block Design to identify the best

locations to allocate both the passive (resting area) and the active (Children Play Area,

badminton court) areas.

In Hong Kong, the environmental factors that identify whether a public area is good

for outdoor activities in public-housing is through the reviewing of the local wind

velocity and the local daylight accessibility. Although rough estimation could be done

via the typical design approach; yet with BEM, a more accurate prediction can be

undertaken to identify public area with good wind availability and daylight access for

outdoor activities. This would reduce the time for assessment for the most suitable

location for outdoor activities, and thus reduces cost at the Scheme Stage during the

design for a green and cost effective public-housing.

Two different programs are used at this stage to identify the microclimatic condition

of the site, which are Computation Fluid Dynamic (CFD) for wind environment

analysis and Radiance for daylight accessibility.

CFD is adopted to simulate the wind environment at the public area within site based

on the selected Single Point Block Design during the Planning & Conceptual Stage.

As shown in Figure 9, wind pattern on site at pedestrian level (2m above ground) are

simulated and found that the wind velocities on site is within the range of 1 – 3 m/s,

which is acceptable for the provision of comfortable outdoor environment. As a result,

several locations for outdoor activities are identified as shown in Figure 9. It is also

found that there is no location where wind velocity exceeds 5 m/s, which means that

there is no gust wind that could have an adverse impact on pedestrian wind comfort

under the easterly prevailing wind.

Figure 9 Wind flow diagram at the podium level with prevailing wind from the East

Sun-shading patterns by Radiance simulation [10] helps to plan for a comfortable

outdoor space to the occupant. As illustrated in Figure 10, public areas such as the

ball-courts, CPA and pedestrian routes connecting the two bridges in the Eastern and

Western sides of the site in summer and winter seasons are investigated. The two ball-

courts and CPA can have direct sunlight in summer morning and in shade during

summer afternoon and during winter day, which means that these areas are

comfortable for people to conduct physical exercises. The pedestrian route is also

found partially having access to direct sunlight and partially in shade during summer

and winter.

Summer time under clear sky condition Winter time under clear sky condition

9:00am

9:00am

5:00pm

5:00pm

Figure 10 Selected hourly diagrams showing sunlight patterns on a typical day in summer (left) and

winter (right) under clear sky conditions

Detailed Design Stage

In order to further optimize building disposition and orientation, as well as the extent

of building permeability for wind environment enhancement of the design for the

public-housing, the BEM studies helps to achieve the following in respect to better

wind environment and adoption of natural ventilation:

Suitable distance between building heights and appropriate wind corridors are

recommended based on the wind environment simulation. The aim is to

achieve an average pedestrian wind velocity at main circulation and passive

open space ranging from 0.8 m/s to 2 m/s and that for active open space to

range from 1 m/s to 2 m/s. As for the area exposed to un-obstructed prevailing

winds, the target wind velocity is aimed at 3 m/s;

Wing walls as shown in Figure 11 are added to buildings in several locations

to enhance air flow in stagnant zones and/or wind catches on site to “catch the

wind”. Performance of wing walls is found to be significant to improve air

movement in public corridors as the provision of natural ventilation at public

corridors can be increased by 7% with the incorporation of wing walls at the

ends of certain corridors;

Ball-court 1: In sunshine in

whole summer

morning

Pedestrian route:

Partially in shade for

whole day in winter Design

CPA: In shade for

whole winter

day

CPA: Unshaded in the

whole summer

afternoon

Figure 11 Implementation of wing walls to enhance cross-wind natural ventilation

Cross ventilated corridor design by providing openings on each floor is

adopted to enhance the permeability of the public-housing towards prevailing

wind direction. This would allow wind to flow through the tall buildings more

easily. The benefit of this design is to reduce wind blockage as to reduce the

impact the public-housing have on the nearby environment, as well as

increasing the indoor ventilation rate of the public-housing. In this analysis, it

is found that the domestic flats have an Air Change per Hour (ACH) of 8.7,

which is well above the 1.5 ACH as stated in the local standard for healthy

living environment [11]. Some stagnant areas on two sides of the lift lobby at

certain building blocks are identified, which mechanical ventilation is

recommended to enhance air circulation, in particular during summer season;

and

To ensure effective pollutant dispersion, the Refuse Collection Points (RCPs)

are strategically located so that any residual odour even remaining after bio-

chemical treatment can readily disperse without causing nuisance to the

residents and people nearby. Pollutant dispersion analyses are also used for

carpark that adopts natural ventilation and commercial kitchen exhaust point

as to confirm the dispersion of the pollutants from these locations does not

affect daily activities of people nearby.

Construction Stage

BEM is also adapted to domestic flat design of the public-housing to ensure that

during the Construction Stage, flats are designed to utilize as much natural resources

as possible. Involvement of BEM in the Construction Stage would include simulations

for indoor ventilation, daylighting, thermal comfort and energy calculation to compute

temperature profile for the internal environment of domestic flats of different

orientations, which include:

Several natural ventilation initiatives are studied to facilitate cross-ventilation

within each flat. This can encourage residents to utilize natural ventilation

more often, thus reduces the energy consumption and cost in air-conditioning

operation. As a result, enhancement in air flow performance can be achieved.

Furthermore, staggered facades within flats and side windows can be provided

where possible; and

Reduce energy consumption on air conditioning and mechanical ventilation,

environmental façade design was utilized to reduce solar heat gain both on the

building façade and in individual flat. The façade features that affect cooling

load and achievable ventilation rates and daylight luminance include (but not

limited to) wall to roof construction, window to wall area ratio, glazing type,

building orientation, configuration and separation, floor level, external wall

finishes and colour, shading device etc. Certain area of the site is to use the

balcony to act as shading devices to reduce the construction cost of the public-

housing.

Wall finishing colours are strategically selected based on the wall orientation, level of

flats and gaps between adjacent buildings. In Hong Kong, eastern and western walls

are exposed to high intensity of solar radiation in summer and southern façade

absorbs more solar heat in winter. Therefore, light colour is recommended at the east

and the west facades of the public-housing to reduce solar heat gain in the morning

and the afternoon respectively. On the contrary, south facades may incorporate dark

colour to enhance solar heat absorption to reduce heating demand in winter.

In respect to daylight access at flats of low levels, light colour is recommended to

increase light reflectance as the lower levels are poorer in daylight performance as

there is less direct sunlight.

The recommended colour pattern is to maintain the Vertical Daylight Factor (VDF) at

8%, which is the minimum VDF requirement for habitable room as stated in

PNAP278, and to enhance daylight performance of the flats at low levels and areas at

the narrow gap between adjacent building blocks. In addition, this colour design can

reduce the solar heat gain at the east and the west facades of the building by

approximately 20% in summer and increase the solar heat absorbed at the south

facades by approximately 17% in winter, as compared to the case if the colour of the

external walls is assumed to be light grey.

Post Occupied Stage

In order to improve the accuracy and further justify the adaptability and cost

effectiveness of BEM in public-housing design, on site measurement are undertaken

during Construction Stage and in Post Occupied Stage. Measurement data for both

stages are used to compare with the simulation design parameters adopted during the

Planning & Conceptual Stage and the Detailed Design Stage. By collecting the data

after flats are occupied, database is established as a referenced benchmark for future

designs. As for this project, the following parameters are measured.

Table 2 Measurement parameters for the public-housing

Wind

Environment Outdoor air velocity

Air temperature and humidity

External wall surface temperature

Natural

Ventilation Indoor air velocity (lift lobby & corridor)

Ventilation rate

Wind distribution near Wing Wall

Daylight Outdoor illuminance level

Indoor Lux Level (lift lobby & corridor)

Daylight factor (by calculation)

Indoor

thermal

condition

Indoor air temperature (lift lobby & corridor)

Indoor relative humidity (lift lobby & corridor)

Internal wall surface temperature (lift lobby & corridor)

Two key environmental factors are validated for this public-housing project, which

are the wind condition in the outdoor environment and the daylighting level in the

indoor environment. Figure 12 shows the comparison in wind direction and wind

velocity between the results of CFD simulation and measurement. It is found that

CFD can effectively predict the wind condition as the simulation matches closely with

the measurement results. As for the daylighting accessibility in the indoor

environment, Figure 13 has shown that the public corridors in real application are the

same as the simulation results undertaken during the Detailed Design Stage. Based on

the close match between simulation and measurement results, BEM is proved to be

highly accurate for public-housing design.

Figure 12 Comparison between simulation and measurement results

Figure 13 daylighting level in the public corridors

Although BEM has proven to be accurate to predict the microclimatic condition on

site with the proposed design, the suitability of BEM in public-housing design

projects should be evaluated through the feedbacks of residents.

In order to collect reliable feedbacks from residents, face-to-face interviews are

undertaken in the household visits one year after occupation. The aim of the

interviews is to collect resident feedbacks on the overall environment satisfaction, the

use of natural ventilation and daylight accessibility within indoor habitable

environment. The result of the interviews have shown that the residents satisfaction

level on overall condition, natural ventilation and daylight accessibility are 90%, 91%

and 80% respectively. By collecting the survey results, an environmental performance

indicator for public-housing tenant’s life style can be established. These results can

also help to develop a benchmark for ventilation, lighting and thermal comfort of

various types of building design.

Justification of BEM for a cost effective public-housing design

Given that the results of the interview are highly satisfactory, BEM is very suitable

for green and cost effective public-housing design. By adopting BEM at early stage of

the design, accuracy of the local climatic condition can be predicted with the proposed

building design. This would allow lowering in design and construction cost by

minimizing design time and reduce chances of making mistake for the most optimum

design option during design stages. Furthermore, as natural ventilation and

daylighting accessibility is highly satisfactory, the public-housing can be proved to be

“green” while reducing the daily cost of the low-income families, thus proving that

green public housing does not apply only to high-income families, but to all types of

families at different ranges of income.

Conclusions

Development of public-housing begins after World War Two, which 92,000 public-

housing units are available and accommodates approximately 3.2 million of the

population in Hong Kong in 1996. Public-housing is seldom seen as green as there

has always been a misconception that green buildings are luxurious. However, on the

contrary, public-housing should be designed green in order to lower the operating cost

imposing on the low-income families.

In order to design a green and cost effective public-housing, Building Environment

Modelling (BEM) is adopted to assist in several stages of the construction for a public

housing project, which includes the Planning & Conceptual Stage, Schematic Design

Stage, Detailed Design Stage, Construction Stage and Post Occupied Stage.

The BEM model adopts several advance computation tools, which include

Computational Fluid Dynamic (CFD) simulation for wind condition on site and

Radiance for daylight access and dynamic thermal modelling on solar heat gain to

assess the microclimate condition on site. This would allow for a reduction in

construction cost. Furthermore, several enhancement measures are incorporated in the

design based on the simulation results, which reduces operation cost with

improvement in ventilation performance, daylight accessibility and lowering of solar

heat gain. In order to justify the accuracy of the simulation, the results are validated

with measurement results when the public-housing is upon completion.

In order to justify the use of BEM for public-housing design, face-to-face interviews

have been undertaken and show a high satisfactory rate in the overall condition, use of

natural ventilation and daylight accessibility within indoor habitable environment.

This shows that BEM is highly suitable for a green and cost effective public-housing

design.

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