1. The 7th International Conference on Sustainable Development in Building and Environment
Investigation of the Potential of Saving Building Energy by Applying a Vertical Green Wall System to High-rise Residential Buildings in a Sub-tropical Region
Irene Wong (The Hong Kong Polytechnic University)
Andrew N. Baldwin* (The Chongqing University)
*Corresponding author

2. Major Building Energy Consumption in Sub-tropical Regions
CLIMATE
Hot & humid summer
> 30ᵒC
Mild winter
Cooling in Summer
Central heating system is rare
Air-conditioning

3. Typical Construction of High-rise Residential Buildings in Hong Kong
Central core design ~ units facing 4 directions, cannot benefit from orientation
Concrete infra-structure ~ low insulation
mm concrete wall with no external insulation ~ significant heat gain in summer

4. Vertical Green Facades
Vertical green façade is a façade system in which evergreen or deciduous climbing plants grow on walls or are plotted in hanging containers in a special supporting strictures to over the external walls (Periniet al 2011; P’erez et al, 2011).
Acts as a living cladding system
(Ottele’, 2011)

5. Categories of Green Façade
Traditional: climber plants directly grow on facades
Double-skin: double-skin framework separated from wall#
Perimeter Flower Pots
# suitable or high-rise buildings

6. 4 Fundamental Mechanisms (1)
Shadowing effect
Solar transmittance: 0.43 ~ 0.14 for single and 5 layers of creeping plants e.g. Virginia Creeper (Sheweka and Mohamed, 2012)
Reduce solar radiation by % (Ip et al, 2010; Miller et al, 2007; Wong et al, 2012)
5 - 30%of remaining solar radiation penetrates to affect internal temperature (Perini et al, 2011)
Filter UV and IR ~ reduce maintenance cost

7. 4 Fundamental Mechanisms (2)
Thermal insulation
External insulation is more effective than internal (Wong et al, 2010)
Air space reduces 0.24kWh/m2 (Green Rooftop; Hoyano, 1998)
60% of absorbed solar radiation is turned into latent heat (Stec et al, 2005)
Reduce energy consumption by air-conditioning by 20% (McPherson, 1994; Stec et al, 2005)
8% saving in annual energy consumption by cooling (Bass and Baskaran, 2003)

8. 4 Fundamental Mechanism (3 & 4)
Evaporative cooling
Acts as passive air-conditioning system (Pe’rez et al, 2011)
Consumes 680 kWh for evaporating every m3 of water (Skeweka and Mohamed, 2012)
Blockage of wind
Acts as wind barrier to prevent cooling
Not necessary for mild winter

9. Case Study Climate of Hong Kong
Long summer ~ space cooling for residential buildings is required for 6 months (Borik and Yik, 2004)
Mean temperatures in summer season (HK Observatory)
May
June
July
August
September
October
Mean Temperature (degree C)
27.0
28.1
28.8
29.5
28.0
25.6
Mean Max Temperature
(degree C)
29.4
30.5
31.6
32.2
30.9
28.3

10. Typical Layout of High-rise Residential Buildings
Symmetrical design ~ nos. of domestic units in the 4 orientations are the same
mm external wall with no insulation
Most households install air-conditioning for cooling in summer

11. Schematic Design Assumptions: Only direct irradiance is considered
Numbers of domestic units facing 4 orientations are the same
Mean temperature in summer is 28 ᵒC
Interior temperature is set to 24ᵒC
Double-skin Green Façade (DSGF):
1 layer of deciduous climbing plant with 0.03 m air space [UDSGF = 0.2] (Larsen et al, 2013)
0.2 m concrete external wall [Uwall = 0.74 ]
U is solar transmittance

12. Energy Consumption of High-rise Residential Buildings in Hong Kong
3 types: 1) Public Rental Housing (PRH), 2) Home Ownership Scheme (HOS) and Private Housing (PH)
Table 2 Energy end-use by housing sectors per annum in 2012 (EMSD, 2012)
Housing Types
Space conditioning (KWh x 106)
Total (KWh x106) energy use
Percentage
PRH
841
17 045 5%
HOS
601
9 892 6% PH
2 046
29 779 7%
Total
3 488
56 716
3488/56726 = 6%

13. Preliminary Study of Energy Saving (1)
R = 1/U; R is the Solar Resistance
Rtotal = R1 + R2 + …. + Rn
where: RDSGF = 4.16; Rwall = 1.35;
RT = = 5.51; is the Solar Resistance of DSGF
UDSGF = 1/5.51 = 0.18
Implies that only 18% of solar radiation affects the internal climate of the building, which is within the range of reduction in solar radiation as demonstrated by Perini et al.

14. Preliminary Study of Energy Saving (2)
According to Alexandri and Jones (2008), for a simplified steady-state analysis, heat gain or loss (qE) from a building fabric with U, indoor temperature (Tin) and outdoor temperature (Tout) is:
qE = U (Tout -Tin ) W/m2
Energy consumption to maintain an interior of 24ᵒC in summer season of mean temperature of 28ᵒC for the same solar irradiance through concrete wall and composite façade are 5.4 and 0.64 W/m2, respectively
Agrees with Dunnet and Kingsbury (2004)’s research that a decrease of 4ᵒC in internal temperature can reduce energy consumption by 64%

15. Preliminary Study of Energy Saving (3)
Saving in cooling energy by DSGF c.f. to concrete wall = (3 – 0.72)/3 x 100% = 76% say 80%
Installation of DSGF can reduce 3,488 x 106 x 80% = 2,790 x 106 kWh per annum
Demonstrates the potential of applying DSGF to high-rise residential buildings for energy conservation

16. Environmental Benefits
The estimated amount of carbon dioxide emitted from generating electricity is 0.83 kg/kWh (China Light and Power, 2009)
DSGF can reduce 3,361 x 106 kg of carbon dioxide
1 medium size deciduous tree can sequestrate kg of carbon dioxide per annum (US Department of Agriculture, 1999)
DSGF can save planting 26 x 106 trees

17. Discussions Other aspects to be considered in future study:
Horticultural – local climatic condition, plant selection
Building design – orientation, close proximity of neighbouring building
Technical – support system, weight of fully grown plants, weight of rainwater, dew & watering, wind load
Economic – payback period, maintenance
Social – aesthetic, government policy, building regulations

18. Concluson
Vertical greenery is visual appealing and ability to create a comfortable indoor micro-climate
Reduce 2,790 kWh x 106 kWh of electric power
Reduce 3,361 x 106 kg of carbon dioxide
The application of green wall systems can help to conserve our environment
Promote both government and private developers as responsible and environmentally aware organization

19. Reference
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20. Thank You