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 Email: a.n.baldwin@lboro.ac.uk

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

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 125-250 mm concrete wall with no external insulation ~ significant heat gain in summer

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)

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

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 40 - 80% (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

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)

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

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

Typical Layout of High-rise Residential Buildings Symmetrical design ~ nos. of domestic units in the 4 orientations are the same 125-250mm external wall with no insulation Most households install air-conditioning for cooling in summer

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

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%

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 = 4.16 + 1.35 = 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.

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%

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

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 133.1 kg of carbon dioxide per annum (US Department of Agriculture, 1999) DSGF can save planting 26 x 106 trees

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

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

Reference Alexandria E, Jones P. Temperature decreases in an urban canyon due to green walls and green roofs in diverse climates. Build Environ. 2008; 43:480-493. Bass B. Baskaran B. Evaluating rooftop and vertical gardens as an adaptation strategy for urban area. Institute for Research and Construction. NRCC-46737. 2003. Borik M. Yi F. Cooling energy evaluation for high-rise residential buildings in Hong Kong. Energy and Buildings 2004; 36: 1259-1263. China Light and Power Group. Sustainability Report (2009). Dunnet N. Kingsbury N. Planting green roofs and living walls. Oregon:Timber Press; 2004. Electrical and Mechanical Services Department H. Hong Kong Energy End-use Data 2012. 2012. Green Rooftop. HK Observatory. Hoyano A. Climatological Uses of Plants for Solar Control and the Effects on the Thermal Environment of a Building. Energy and Buildings. 1988; 11:181-199. Ip K, Lam M, Miller A. Shading performance of a vertical deciduous climbing plant canopy. Build Environ. 2010; 45:81-88. McPherson EG. Cooling urban heat islands with sustainable landscapes. 1994. Miller A, Shaw K, Lam M. Vegetation on building facades:” Bioshader”. Case study report. 2007. Larsen SF, Filippín C, Lesino G. Thermal Simulation of a Double Skin Façade with Plants. Energy Procedia. 2014; 57:1763-1772. Olette’ M. The green building envelope: Vertical greening. SiecaRepro, the Netherland.2011. Pérez G, Rincón L, Vila A, González JM, Cabeza LF. Green vertical systems for buildings as passive systems for energy savings. Applied energy. 2011;88:4854-4859. Perini K, Ottele M, Fraaij ALA, Haas EM, Raiteri R. Vertical greening systems and the effect on air flow and temperature on the building envelope. Build Environ. 2011; 46:2287-2294. Sheweka SM, Mohamed NM. Green Facades as a New Sustainable Approach Towards Climate Change. Enrgy Proced. 2012; 18:507-520. Stec WJ, Van Paassen AHC, Maziarz A. Modelling the double skin façade with plants. Energy and Buildings. 2005; 37:419-427. US Department of Agriculture. USDA Forest Service General Technicl Report PSW-GTR-171, 1999. Wong NH, Tan AYK, Chen Y, Sekar K, Tan PY, Chan D, et al. Thermal evaluation of vertical greenery systems for building walls. Build Environ. 2010; 45: 663-672. Wong I. Yang HX. Study on remote source solar lighting system application in high-rise residential buildings in Hong Kong. Energy and Buildings 2012; 60: 225-231.

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