Estimating heat flux transmission of vertical greenery ecosystem

Ecological Engineering 37 (2011) 1112–1122

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Estimating heat flux transmission of vertical greenery ecosystem C.Y. Jim ∗ , Hongming He Department of Geography, The University of Hong Kong, Pokfulam Road, Hong Kong

a r t i c l e

i n f o

Article history: Received 26 July 2010 Received in revised form 25 January 2011 Accepted 15 February 2011 Available online 17 March 2011 Keywords: Vertical greening Green wall Global solar radiation Direct solar radiation Diffuse solar radiation Thermodynamics transmission model (TTM) Heat flux Thermal shielding coefficient

a b s t r a c t Nurturing vegetation on building envelopes provides an innovative and eco-friendly alternative to urban greening especially in compact cities. Whereas the thermal and other benefits of green roofs have been studied intensively, green walls have received scanty attention. This study evaluates the thermodynamic transmission process of the vertical greenery ecosystem. We designed a field experiment to monitor solar radiation and weather conditions, and developed a thermodynamics transmission model to simulate heat flux and temperature variations. The model was calibrated, tested, and proved to be highly efficient. The results show that seasonal global and direct solar radiation drops to minimum in winter in January and February, and reaches maximum in summer in July and August (1168 W m−2 for global solar radiation and 889 W m−2 for direct solar radiation). Diffuse solar radiation attains maximum in summer (586 W m−2 ) with moderate rainfall in July and August, and minimum in winter with no rainfall in January and February. Radiation transmission of the green wall strongly correlates with canopy transmittance and reflectance (R2 = 0.83). Thermal shielding effectiveness varies with orientation, with the south wall achieving a higher coefficient (0.31) than the north wall. The south wall has lower heat flux absorbance and heat flux loss than the north wall. The south wall can transfer much more heat flux through the vertical greenery ecosystem due to more intensive canopy evapotranspiration effect. The model matches the transmission properties of green wall radiation, and the model simulation fits empirical transmission results. © 2011 Elsevier B.V. All rights reserved.

1. Introduction A comfortable and healthy environment is increasingly demanded to support urban sustainability. Vegetation plays an important role in this endeavour. In compact city areas with inadequate street-level spaces for greening, building envelopes offer feasible alternatives. Whereas green roofs have been actively promoted in many cities, green walls (referring to walls covered by vegetation in the sense of building engineering and building physics) have received less attention. To understand the green city benefits, researchers have focused on the ameliorative effect of regulating the energy balance of urban areas, the quality of air flowing through the urban canopy and its acoustic property (Akbari et al., 1997; Bartzanas et al., 2005; Harmanto and Salokhe, 2006; Emilsson, 2008; Grigoletti et al., 2008; Halwatura and Jayasinghe, 2009; Teemusk and Mander, 2010). Vegetation covering a building could induce cooling of indoor space by reflecting and absorbing solar radiation, cooling by evapotranspiration, enhancing insulation, and acting as

∗ Corresponding author. Tel.: +852 2859 7020; fax: +852 2559 8994. E-mail address: [email protected] (C.Y. Jim). 0925-8574/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2011.02.005

a thermal barrier. The vegetative shield provides an additional barrier between the building interior and the hot (or cold) external environment. The green plant protection mechanism helps to maintain the temperature differential between the interior and exterior of the building (Gomez et al., 1998; Miyawaki, 1998; Rosenfeld et al., 2001; Picot, 2004; Bartzanas et al., 2005; Kurpaska et al., 2005; Pearlmutter and Rosenfelda, 2005; Harmanto and Salokhe, 2006). The amount of cooling provided by a vegetated building depends greatly on weather conditions and on greenery design and management (Wilmers, 1990; Panferov et al., 2001). It is important to explore quantitatively vegetation–atmosphere interactions in addressing solar energy transmission through the atmosphere to the vegetation canopy (Panferov et al., 2001; Chen et al., 2007). Various factors influence energy exchange and transmission of a vertical greenery (referring to walls covered by vegetation in the sense of ecological functions in an ecological system) ecosystem, such as weather condition, plant physiological function, and building structure. The amount of energy absorbed by an ecosystem is governed by the biophysical properties of the vegetation canopy (referring to the upper surface of vegetation), and the driving potentials established by temperature and humidity gradients between the surface and the atmosphere (Raupach,

C.Y. Jim, H. He / Ecological Engineering 37 (2011) 1112–1122

1995; Baldocchi and Vogel, 1997; Wang et al., 2004). Partitioning of the incoming radiation into vegetation canopy absorption, transmission, reflection and transformation of absorbed energy help to understand the interactions between solar energy and plants. Many studies investigated such interactions using canopy radiation models (Emilsson, 2008; Grigoletti et al., 2008). However, they tend to emphasize the scattering behaviour of different vegetation types, which is correlated with vegetation–atmosphere processes. Few studies evaluated the radiation regime within vegetation canopy by characterizing its state based on the law of energy conservation (Emilsson, 2008; Grigoletti et al., 2008; Halwatura and Jayasinghe, 2009). Various methods have been developed to study the thermodynamics of vertical greenery ecosystem. Some papers address fundamental research and important emerging applications (Holm, 1989; Bastianoni and Marchettini, 1997; Dufrene et al., 2005; Alexandria and Jones, 2008; Sailor, 2008; Ip et al., 2010). For example, the enhanced DOE-2.2-derived user-interface of eQUEST for building energy use simulation provides a perspective of simulation in the energy-efficient design process. The eQUEST software is designed to perform detailed analysis of building design technologies using sophisticated building energy use simulation techniques (James J. Hirsch & Associates, 2009). A complete solution of the energy transmission problem requires knowledge of the volume field of temperature and physical properties of the medium at each point of a system (Goldstein et al., 2003). Quantitative assessment of the interactions between solar radiation and vegetation canopy demands specification of variables that determine radiation transport through the vegetation canopy. These variables include intrinsic canopy properties such as absorption, reflection and transmission from the atmosphere into the canopy. The canopy radiation regime is a function of the optical properties of individual leaves and ground surface under the canopy and canopy structure. This feature of the shortwave energy conservation in vegetation canopy provides powerful means for accurate specification of changes in canopy structure. Solar radiation is the primary energy source for physical, biological and chemical processes of plant photosynthesis, respiration and associated transpiration (Nieto-Vesperinas, 1986; Palomo, 1998). Through combined consideration of landform and weather conditions, analysis of heat flux variations at different time scales such as diurnal, seasonal and annual cycles have been widely conducted (Theodosiou, 2003). The availability of accurate solar radiation data at the study location is essential. The separation of solar irradiance into diffuse and beam components is necessary for a wide range of solar engineering tasks and is now a key feature of several canopy scale models of photosynthesis. It is necessary to calculate accurately solar radiation especially when in situ observation is difficult. Time series analysis of solar radiation transmission between the atmosphere and vegetation canopy has been emphasized in some studies (Roderick, 1999). However, the solar radiation transmission model for vertical greening has not been attempted. Greenery on buildings is a combination of natural vegetation and artificial structures, incurring marked differences from natural conditions. A dedicated modelling study of the structure and parameters of solar radiation transmission should be considered. The evaluation of shading performance of vertical plant canopy is crucial to improve greenery system design and management. Shading performance is affected by the biological–horticultural characteristics of the plant species and environmental growth conditions. Some investigations involve thermal performance analysis by establishing efficient solutions to determine the heat and mass transfer processes (Pearlmutter and Rosenfelda, 2005; Halwatura and Jayasinghe, 2009; Ip et al., 2010). They focus on

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identifying the heat transfer parameters with influence on the shading performance of vertical climbing plant canopies (Wong et al., 2003, 2009, 2010). However, few studies address the delayed response of radiation transmission due to attenuation effect. The primary objective of our study is to understand the thermodynamic transmission process of the vertical greenery ecosystem, and to optimize its design to contribute to an eco-friendly urban environment. We developed a field experiment to monitor the solar radiation and weather conditions. We then proposed a thermodynamics transmission model (TTM) for vertical greenery ecosystem to simulate the dynamic heat flux transmission and temperature variations. The model is calibrated and tested by observations of an empirical experimental setup. It addresses specifically the process of thermodynamic transmission through the vertical green wall ecosystem, and its solar radiation shielding effects.

2. Materials and methods 2.1. Experimental design and data acquisition The primary procedure for data acquisition is to validate the proposed thermodynamics transmission model with high-quality in situ observations. Observation of heat flux and temperature is carried out on the roof of the four-storied Runme Shaw Building in the University of Hong Kong to establish a thermal reduction map of different scenarios. The experiment (Fig. 1) consisted of four treatments, including a south green wall, a north green wall (both with length 150 cm × height 85 cm × width 35 cm), an unshielded concrete south wall, and an unshielded concrete north wall. Each green wall is composed of a plastic framework with interlocked receptacles to receive 70 herbaceous plants, in seven rows with 10 plants per row. A gap of 15 cm is kept between the concrete wall and the green wall. A drip irrigation system provides supplementary water supply. An evergreen perennial tropical plant, Euphorbia x lomi ‘Salmon’ (Supergrandiflora Salmon; Euphorbiaceae; originated from Madagascar), which is a drought-tolerant succulent para-shrub, has been used for the experiment. Data sources including solar radiation, microclimatic and soil conditions were automatically recorded at 15 min intervals with the help of data loggers (8160, Lufft, Fellbach, Germany). Four components of solar radiation were recorded, including incoming and outgoing shortwave, and incoming and outgoing longwave. They were detected by a four-in-one combination type of radiometer (CNR1, Kipp & Zonen, the Netherlands) mounted horizontally at 150 cm above the soil surface. Soil heat flux was monitored by a dedicated thermal transmission sensor (HFP01SC, Hukseflux, the Netherlands). Soil moisture and temperature sensors (respectively S-SMC and S-TMB, Onset Hobo, USA) were buried in different layers of the substrate. Surface temperature was assessed by an infrared radiometer made for outdoor use (SI-111, Apogee, USA). Six surfaces were monitored continually: the vegetated surface of the north and south green walls serving as experimental treatments, the concrete wall shielded by the north and south green walls serving as experimental treatments, and the unshielded concrete north and south walls serving as controls. The infrared sensors are all pointing at the centre of the target to obtain the most representative data and to minimize the edge effect (Fig. 1). The thermister air temperature probes (8160.TFF, Lufft, Germany) with radiation shields were installed at 20 cm, 60 cm, and 200 cm above the ground surface at a nearby site. Meteorological factors were observed using a weather station setup (Onset Hobo) located near the green wall site, including air relative humidity (RH), air temperature, and dew point.

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Fig. 1. The design of the green wall experiment on the roof of the Runme Shaw Building at the University of Hong Kong. (a) The modular plastic framework; (b) the framework filled by 70 herbaceous plants; and (c) the layout plan and dimensions of the two green walls (experimental treatments) and adjacent bare concrete walls (controls).

2.2. Numerical simulation of heat flux shielding of vertical greenery ecosystem

The relationship among solar elevation angle (˛, degree), the local geographical latitude of observer (ϕ, degree), solar declination (ı, degree) and solar hour angle (ω, degree) is defined by:

2.2.1. Solar radiation model of green wall The solar energy reaching the periphery of the earth’s atmosphere is considered to be constant. The solar constant value is estimated on the basis of the solar radiation received on a unit area exposed perpendicularly to the rays of the sun at an average distance between the sun and the earth. Only a portion of the solar radiation reaches the earth surfaces due to passage control and attenuation by the atmosphere. The total radiation received at ground level is greatly reduced by cloud cover of varied thickness (Fig. 2A). Direct solar radiation is the part of the sunlight that reaches directly the surface of the earth through the atmosphere. The remaining part of the sunlight, known as diffuse solar radiation, takes a more indirect route as it becomes scattered or reflected by air molecules, water vapour and dust particles. Global solar radiation encompasses all the sunlight that reaches the earth’s surface, which is the sum of the direct and diffuse solar radiation (Akhmanov and Nikitin, 1999).

sin ˛ = sin ϕ · sin ı + cos ϕ · cos ı · cos ω

(1)

Solar radiation of a slope object is controlled by incident solar radiation angle ( i , degree), object slope (ˇ, which is 90◦ for a vertical wall); object aspect or direction (, clockwise from north is 0–360◦ ); solar declination (ı, degree), and solar hour angle (ω, degree):

⎧ cos i = A1 + A2 + A3 ⎪ ⎪ ⎪ A1 = sin ı · (sin ϕ cos ˇ − cos ϕ sin ˇ cos ) ⎪ ⎪ ⎪ A2 = cos ı cos ϕ · (cos ϕ cos ˇ + sin ϕ sin ˇ cos ) ⎪ ⎪ ⎪ ⎪  sin ω ⎪  ⎨ A3 = cos ı sin ˇ sin 360◦ · (10 + N)   ı = 23.45 · cos , 365 ⎪ ⎪ ⎪ ı > 0, when sun position is in the north ⎪ ⎪ ⎪ ı < 0, when sun position is in the south ⎪ ⎪ ⎪ ⎪ N, the number of days from January 1 of a year ⎪ ⎩ ω = 15 × (t − 12),

t is local time

(2)

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Fig. 2. Parameters for the systematic characterization of solar radiation transmission through a green wall ecosystem.  i ,  r and  t are angles that the incident, reflected and refracted rays make respectively to the normal of the interface. A0 B0 C0 D0 , solar constant value (S0 , 1367 W m−2 ); A1 B1 C1 D1 , solar radiation over the earth’s atmosphere (I0 , W m−2 ); A2 B2 C1 D1 , global solar radiation (IG0 , W m−2 ); I , radiation intensity after moving out of the greenery layer (W m−2 ); QG0 , solar radiation received before moving through the green wall; QT , solar radiation received after moving through the green wall; Q solar radiation received after reflecting through the green wall; the total radiation (QG ) equals to the sum of transmission (Q ), absorption (Q˛ ) and reflection (Q ) of canopy through the green wall ecosystem. L, the distance of radiation transmission which is a function of green wall thickness (mm).

As green wall is vertical (ˇ = 90), solar radiation of the north and south aspect of green wall can be calculated by:



cos i = cos ı · sin ω cos i = sin ı · cos ϕ + cos ı · cos ω · sin ϕ

North wall : South wall :

(3)

The sky clearness coefficient ( Dir ) is function of atmosphere mass (Mh of elevation h, a.s.l., and M0 of the average sea level is 5.3 × 1018 kg), which is closely related to atmospheric pressure mass (Ph of elevation h, a.s.l., and P0 of the average sea level is 101.325 kPa):

⎧  = 0.56 · [exp(−0.56 · M ) + exp(−0.095 · M )] dir h h ⎪ ⎪ ⎪ ⎪ Mh = M0 · Ph ⎨ P 0

2 1/2

M0 = [1229 + (614 · sin ˛) ] − 614 · sin ˛ ⎪ ⎪  5.256 ⎪ ⎪ ⎩ ph = (288 − 0.0065 · h) P0

(4)

288

Direct solar radiation (Idir , W m−2 ) of slope object under consideration of sky clearness coefficient ( dir ) is calculated by:



Idir = I0 · dir · cos i I0 = S0 · (1 + 0.0344 · cos

 N · 360◦

(5)

365

where I0 is the solar radiation over the earth’s atmosphere (A1 B1 C1 D1 , W m−2 ); S0 is the solar constant value (A0 B0 C0 D0 , 1367 W m−2 ). Diffuse solar radiation includes atmospheric diffusive solar radiation (Idif , W m−2 ) and ground reflected solar radiation (Iref , W m−2 ), where, atmospheric diffuse solar radiation (Idif , W m−2 ) is determined by diffusion coefficients ( dif ):



dif

2

(sin ˇ) 2 sin ˛ = 0.271 + 0.706 · dir

Iref = r0 · I0 · ref · ref

(7)

(8)

2.2.2. Canopy transmittance (), absorbance (˛) and reflectance () of green wall Patterns of thermodynamic transfer of vertical greenery ecosystem are determined by heat flux transmission, reflectance and absorption. Meteorological conditions and vegetation structure of greenery ecosystem determine the behaviors of internal disturbance of the thermodynamic transfer process. According to energy conservation, the total radiation (QG ) equals to the sum of transmission (Q ), absorption (Q˛ ) and reflection (Q ) of canopy through the green wall ecosystem (Fig. 2B). Therefore, canopy transmittance (), absorbance (˛) and reflectance () can be expressed as:

⎧ QG = Q + Q˛ + Q ⎪ ⎪ ⎪ Q Q˛ Q ⎪ ⎨ + + =1 QG

QG

QG Q˛ = ˛; QG +˛+ =1

Q ⎪ = ; ⎪ ⎪ Q ⎪ ⎩ G

(9)

Q = QG

Suppose that green wall is a partly transparent object that permits some sunlight to move through the greenery layer. Reflection and refraction of the light may occur when light moves from the first medium into the second. Refractive index (n) is mathematically described as velocity of light in a vacuum divided by velocity of light in a medium (Fig. 2B). The reflection coefficient (r) is given by the Fresnel equations (Akhmanov and Nikitin, 1999):

(6)

Ground reflected solar radiation (Iref , W m−2 ) is determined by diffusion coefficients ( b ) and surface ground reflectance (r0 , the reference value is 0.31):



IG0 = Idir + Idif + Iref

r=

2

(cos ˇ) 2 sin ˛ = 0.271 − 0.294 · dir

Idif = I0 · dif ·

Global solar radiation (IG0 , A2 B2 C1 D1 , W m−2 ) is the sum of direct solar radiation (Idir ) diffusive solar radiation (Idif ) and ground reflected solar radiation (Iref ):

I = IG0

 =

n1 cos i − n2 cos t n1 cos i + n2 cos t

n1 cos i − n2 n1 cos i + n2

2



2

1 − ((n1 /n2 ) sin i )

2



1 − ((n1 /n2 ) sin i )

2 (10)

where I (W m−2 ) is solar radiation intensity as reflection by green wall; n1 and n2 , are refractive indices of radiation from the first medium into the second;  i ,  r and  t are angles that the incident, reflected and refracted rays respectively make to the normal of the interface.

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Solar radiation, attenuated after absorption by the partly transparent green wall, can be expressed by (Trenberth and Kevin, 1992; Gao, 1996):

⎧ I = IG0 · exp(−K · L cos i) ⎪  TPAR ⎪ ⎨ K = ln

PAR

· LAI

(11)

⎪ ⎪ ⎩ a = 1 − I = 1 − exp(−K · L) IG0

where I (W m−2 ) is radiation intensity after moving out of the greenery layer; L (mm) is the distance of radiation transmission which is a function of green wall thickness (mm) and refraction angle; K, is attenuation coefficient which describes the extent to which the intensity of an energy beam is reduced as it passes through a specific material; PAR (W m−2 ) is photosynthetically active radiation over the greenery surface; TPAR (W m−2 ) is transmitted photosynthetically active radiation behind the greenery layer; LAI is canopy leaf area index. From the above relationship, canopy absorbance (˛) reflectance () and transmittance () can be further expressed as (Lu et al., 1990; Gao, 1996):

⎧ a · (1 − r) 2 ⎪ ˛ = a · (1 − r) · [1 + r · (1 − a) + r 2 · (1 − a) + . . .] = ⎪ ⎪ 1 − r · (1 − a) ⎪ 2 2 2 ⎪ ⎪ ⎨  = r · {1 + (1 − a) · (1 − r)  · [1 + r · (1 − a) + . . .]} = 2 2 (1 − a) · (1 − r)

r· 1+ 2 ⎪ 1 − r 2 · (1 − a) ⎪ ⎪ ⎪ ⎪ ⎪ ⎩  = (1 − a) · (1 − r)2 · [1 + r 2 · (1 − a)2 + . . .] =

(1 − a) · (1 − r) 1−

r2

· (1 − a)

(12)

2

2

2.2.3. Shading performance of green wall on solar radiation To evaluate shading effect of green wall, we introduce a shading coefficient to represent the shading performance. Shading coefficient is defined as the proportion of solar radiation received before (QG0 ) and after (QT ) moving through the green wall (Ip et al., 2010). Shading effectiveness changes at different times, and it is capable of representing the shading performance time period cycle. Shading coefficient (SC) is represented as (Lu et al., 1990; Trenberth and Kevin, 1992; Gao, 1996): QG0 SC = QT

(13)

Delay response of radiation transmission is further employed to investigate the attenuation effect of radiation transmission. We define transmission delay coefficient ( ) as:



1 Ai = × 40.5 × (Rgw · Sgw + Rair · Sair ) − arc tg √ 15 Ai + Yi 2 +arc tg

Ae



√ Ae + Ye 2

(14)

where Rgw and Rair are respectively thermal resistance of green wall and air; Sgw are Sair are storage coefficients of green wall and air; Ai and Ae are thermal exchange coefficients of outer and inner surfaces of green wall; Yi and Ye are storage coefficients of inner and outer surfaces of green wall. 2.3. Evaluation of model performance Model performance is evaluated by the Root Mean Square Error (RMSE) and Nash–Sutcliffe Efficiency Coefficient (NSEC, see Eq. (15), Nash and Sutcliffe, 1970). Table 1 summarizes the statistical test results RMSE and NSEC for both models:

n

NSEC = 1 −

2 t (T t − Tsimulated ) t=1 observed 2 n (T t − T¯ observed ) t=1 observed



(15)

Fig. 3. Hourly solar radiation of horizontal roof: comparison between observed and simulated scenarios (November 08, 2008 to August 21, 2009); Difference (O − S) refers to the difference between observed and simulated results. t t where Tobserved is the observed value at time t; Tsimulated is the sim¯ ulated value at time t; Tobserved is the average value of the observed.

Nash–Sutcliffe efficiencies can range from ∞ to 1. An efficiency of 1 (NSEC = 1) corresponds to a perfect match of modelled discharge to the observed data. An efficiency of 0 (NSEC = 0) indicates that the model predictions are as accurate as the mean of the observed data, whereas an efficiency less than zero (NSEC < 0) occurs when the observed mean is a better predictor than the model or, in other words, when the residual variance (described by the nominator in the expression above), is larger than the data variance (described by the denominator). Essentially, the closer the model efficiency is to 1, the more accurate the model is. It should be noted that Nash–Sutcliffe efficiencies can be used to describe the predicative accuracy of other models as long as there is observed data to compare the model results. 3. Results and discussion 3.1. Validation of model performance Performance of the proposed solar radiation model of green wall is evaluated by the RMSE and NSEC. The models are calibrated and evaluated, and proved to be highly efficient for our study objectives (Fig. 3, and Table 1). The estimated values of the two models are in favorable agreement with the observed values. The simulation results of temperature have lower RMSE values (2.79–9.71) than heat flux (19.17–90.78). For NSEC values of the heat flux model, temperature and heat flux get relatively high NSEC values (0.54–0.87) and only 9% falls within 0.56–0.69. These results denote excellent indicators that the radiation damping oscillation model can give precise estimation of radiation transmission with acceptable errors. 3.2. Period characteristics of canopy solar radiation The observations for solar radiation near the ground provide general insight on the surface energy balance and seasonal variation near the ground. Period characteristics of solar radiation near the ground can be achieved by comparison between the observed and simulated values. To illustrate the solar radiation field in the neighborhood of the ground, the hourly and daily sequence (November 8, 2008–August 22, 2009) of solar radiation is shown in Figs. 3–5. For the diurnal cycle, a total of 12 days representing three typical weather days (sunny, cloudy and rainy) in four seasons (winter – February, spring – May, summer – August and autumn – November), were selected

C.Y. Jim, H. He / Ecological Engineering 37 (2011) 1112–1122

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Table 1 Evaluation of model performance (RMSE and Nash–Sutcliffe Efficiency Coefficient). Thermodynamics

Temperature

Solar radiation

*

Experiment treatment

South green wall North green wall South control wall North control wall South green wall North green wall South control wall North control wall

February

May

August

NSEC

RMSE

NSEC

RMSE

0.87 0.81 0.84 0.78 0.71 0.74 0.82 0.77

3.01 7.92 5.92 9.60 53.56 61.34 44.17 59.92

0.80 0.78 0.83 0.79 0.70 0.74 0.82 0.77

5.64 6.91 3.88 6.54 44.10 33.98 19.17 30.43

November

NSEC 0.83 0.77 0.87 0.81 0.85 0.69* 0.79 0.74

RMSE

NSEC

RMSE

5.80 7.89 2.79 7.11 20.40 66.09 39.40 48.89

0.75 0.81 0.84 0.78 *0.67 0.73 * 0.54 0.76

9.71 7.60 5.88 8.47 68.46 57.19 90.78 63.71

Value <7.0.

for detailed analysis (Figs. 4–6). They depict the commonality of energy fluxes in the temporal–seasonal and major weather patterns. The global solar radiation increases from 0 at 0600–0800 h in the morning to peak values at 1200–1300 h, and then decrease to 0 at around 1800–2000 h. The beginning of diurnal heating is clearly seen at 0600 h by the flow of heat into the ground, the maximum temperature at 1200 h is associated with the maximum solar radiation, and the green wall is later cooled in response to reduced solar radiation. This can be proved by the nocturnal distribution of temperature. The influence of radiation predominates and the minimum is found at north- and south-facing green wall. The seasonal heat flux on green wall varies with the fluctuations of the underlying meteorological driving forces. It follows clearly the seasonal solar radiation regime, with deviations brought by the monsoonal rainfall rhythm. The seasonal trends are affected by rainfall and cloud cover (Figs. 4–6). Seasonal global solar radiation and direct solar radiation drop to a minimum value in winter, especially in January and February, and reach a maximum in summer in July and August (1168 W m−2 of global solar radiation and 889 W m−2 of direct solar radiation). In terms of daily weather condition, the maximum values of global solar radiation and direct solar radiation occur on the summer sunny day, and the minimum on the winter rainy day. Diffuse solar radiation attains a maximum value in summer with moderate rainfall in July and August, and a minimum in winter with no rainfall in January and February (Figs. 4–7). The daily weather pattern shows quite notable differences between direct solar radiation and diffuse solar radiation. The maximum direct solar radiation occurs on the summer sunny day and the minimum on the winter rainy day. The highest flux occurs during high temperature, coinciding with an adequate supply of water to enhance evaporation. The diurnal range of direct solar radiation reaches the widest in winter and narrowest in summer, with medium values for spring and autumn. For all seasons, the sunny day gets the widest diurnal range, with suppression on cloudy day, and diminishing to almost 0 on rainy day. As to diffuse solar radiation, it reaches the maximum on cloudy summer day followed by rainy and sunny days. The extreme values of diffuse solar radiation correlate well with cloud cover (Figs. 6 and 7). In summer, moderate cloud amount in July and August reduce both global and direct solar radiation, but not diffuse solar radiation. In contrast, the limited rainfall associated with the prolonged cloudy-humid weather in spring in April admits less diffuse solar radiation. The dynamics of heat flux magnitude differ notably by experimental treatment (control and vegetation types) and temporal groups (diurnal and seasonal changes) (Figs. 4–7). The hourly average has a value around 455–610 W m−2 through the four seasons. The fluctuations of direct solar radiation for both groups are greater

than diffuse and global solar radiation. The difference of hourly heat flux between north and south wall ranges from 13 to 39 W m−2 . The south wall control treatment gets the highest value of hourly global solar radiation (1168.23 W m−2 ) in summer, followed by north wall control (1026.48 W m−2 ), south green wall (889.38 W m−2 ) and north green wall (731.12 W m−2 ). 3.3. Impacts of shielding structures on thermodynamic transfer of green wall The simulation of reflectance and transmittance of heat flux by vegetation shielding structure (referring to the radiation shielding effect afforded by green plants that cover a wall) is based on the radiation shielding effectiveness model. From the heat flow perspective, the performance of green roofs as insulators is associated with shading, heat absorption, and latent heat transfer. They depend greatly on a number of variables, including the moisture content and temperature regime. The results show that the shading effect of vegetation is dramatic, especially during high temperature periods in daytime. Green walls evidently reduce heat flux and temperature more effectively than control walls. For example, when global solar radiation and temperature of the south control wall reaches maximum values (1168 W m−2 , 48.48 ◦ C), the south green walls register much lower values (586.89 W m−2 , 39.65 ◦ C). The differences between back and front sides of the green wall demonstrate obvious shading effects. For example, the hourly global average solar radiation has been reduced by 31.54 W m−2 in the south and 11.36 W m−2 in the north. Radiation transmission strongly correlates with vegetation structure and canopy transmittance and reflectance. The model simulation results of reflectance and transmittance of the canopy are presented in Figs. 8 and 9. The canopy reflectance ranges from 0.1 to 0.58, and canopy transmittance values lie between 0.06 and 0.38 within the major heat flux wavelength spectrum. Statistical analysis indicates that the canopy reflectance is correlated well with the canopy transmittance (R2 = 0.83). Shielding effectiveness varies, and the south green walls have higher values of reflectance and transmittance. The south green wall has the highest shielding effectiveness (0.31), followed by the north green wall. Thus the green wall absorbs more heat flux than the control wall (Figs. 7–9). Shielding effectiveness by green-wall vegetation absorbs radiant energy and prevents it from reaching the building surface. A very important function of the vegetation layer is to create a quiescent layer of air immediately in front of the green wall surface. Green wall vegetation absorbs and store large amounts of heat to form a thermal subsidence effect. The effect is to create a buffer against the daily macro-climatic fluctuation in temperature (Figs. 4–7). It shows that a vegetation-shielded wall can drastically suppress the air temperature at 1200 h by more than 10.3 ◦ C in

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Fig. 4. Simulated hourly solar radiation of green wall ecosystem (November 8, 2008–August 21, 2009); T N Ctrl, T N Front, T N Back: north wall temperature of control plot, north green wall temperature of front side and back side; T S Ctrl, T S Front, T S Back: south wall temperature of control plot, south green wall temperature of front side and back side; H N Ctrl, H N Front, H N Back: north wall temperature of control plot, north green wall temperature of front side, and back side; H S Ctrl, H S Front, H S Back: south wall temperature of control plot, south green wall temperature of front side and back side.

comparison with the unshielded wall (control treatment). With a green wall in place, the extremes of temperature are eliminated. The minimum temperature of a green wall (7.6 ◦ C of the south) is lower than the control treatment (7.9 ◦ C). The maximum temperature of the vegetated wall (44.5 ◦ C of the south) is lower than the control (48.5 ◦ C of the south). It suggests that the overall heat transfer through the green wall has been reduced. Another benefit of foliage is that it absorbs radiant energy, utilizing it to fuel photosynthetic processes. This effect contributes to increasing the

effective albedo of the vegetated wall. The benefit of green wall as an insulator depends upon vegetation structure, with both the insulation and thermal mass effects contributing to dampening of temperature changes. The above simulation results are deemed to be notably accurate. The thermodynamics transmission model is proved to match well the transmission properties of green wall radiation, and the model simulation results are commensurate with empirical transmission records.

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Fig. 5. Simulated hourly solar radiation under different weather conditions (sunny, cloudy and rainy) of green wall ecosystem (November 8, 2008–August 21, 2009); T N Ctrl, T N Front, T N Back: north wall temperature of control plot, north green wall temperature of front side and back side; T S Ctrl, T S Front, T S Back: south wall temperature of control plot, south green wall temperature of front side and back side.

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Fig. 6. Simulated hourly wall temperature of different weather conditions (sunny, cloudy and rainy) of vertical green wall ecosystem (November 8, 2008–August 21, 2009); T N Ctrl, T N Front, T N Back: north wall temperature of control plot, north green wall temperature of front side and back side; T S Ctrl, T S Front, T S Back: south wall temperature of control plot, south green wall temperature of front side and back side.

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3.4. Shading performance of green wall In the diurnal cycle, the simulation results show that the south green wall can transfer much more heat flux than the north wall (Figs. 4–6 and 8). For the same season, the north green wall has narrower amplitudes in heat flux due to smaller differences between the maximum and the minimum, followed by the south green wall, unshielded north and unshielded south walls. The green wall has a higher heat flux due to canopy evapotranspiration effect (He and Jim, 2010; Jim and He, 2010), and this can be proved by the notably higher heat flux value of south green wall. Thus the south green wall has the lowest heat flux absorbance and heat flux loss, followed by the north green wall. On the contrary, the unshielded walls have the lowest heat flux absorbance in daytime and highest heat flux loss in nighttime. The results echo the ability of vertical greenery ecosystem to mitigate the solar radiation. Regarding seasonal difference, the air temperature and heat flux of unshielded walls show greater differences between winter and summer than green walls according to thermodynamic variation at daytime (0600–1800) and nighttime (1800–0600). Such results suggest that the vegetation-shielded wall can bring important cooling effect in summer. The green wall contributes to the moderation of air temperature. In summer, the external building surfaces with green wall are heated notably less than the unshielded surfaces.

Fig. 7. Decoupling global solar radiation and direct and diffuse solar radiation (GSR, global solar radiation; clear, cloud amount <20%, cloud, cloud amount >50%).

Fig. 8. Impact of shielding structures on thermodynamic transfer of green wall (canopy transmittance, absorptance and reflectance).

Fig. 9. Shading performance of green wall (November 8, 2008–August 21, 2009).

4. Conclusion and discussion The study investigates the ecological dynamics of heat flux transmission and energy balance of the vertical greenery ecosystem as to develop a scientific basis for its design and management. We designed a field experiment to monitor the total solar radiation and net radiation, and proposed a numerical model of solar radiation on vertical greenery ecosystem to simulate the dynamics of temperature transfer in vertical greenery ecosystem. The thermodynamics transmission model is determined by global solar radiation and ground absorption of solar radiation, effective atmosphere and ground long wave radiation, convective heat flux exchange between ground and atmosphere, and heat flux of vegetation evapotranspiration. We also explore the effect of vegetation on radiation energy absorption and thermal energy transmission in relation to the field microclimatic regime. The study has been able to generate meaningful findings. The results show that the seasonal heat fluxes on green wall vary with the fluctuations of the underlying meteorological driving forces. Seasonal global and direct solar radiation drop to a minimum value in winter especially in January and February, and reach a maximum in summer peaking in July and August. Diffuse solar radiation attains a maximum value in summer with moderate rainfall in July and August, and a minimum in winter with no rainfall in January and February. Radiation transmission strongly correlates with canopy transmittance and reflectance. Shielding effectiveness by green-wall vegetation absorbs radiant energy and prevents it from reaching the building surface. For heat flux transfer in the diurnal cycle, the south green wall can transfer much more heat flux through the vertical greenery due to canopy evapotranspiration effect. The south green wall has the lowest heat flux absorbance and heat flux loss in comparison with the north green wall. Locality and climatic elements are important factors that need to be addressed in using the model. Hong Kong has a subtropical climate, with hot-humid summer and cool-dry winter. The canopy solar radiation presents distinct characteristics as mentioned above. Our proposed model has been able to generate accurate simulation results and proved to be efficient in computation. The thermodynamics transmission model and the shield

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coefficient model are commensurate with the transmission properties of green wall radiation, and they fit well with empirical transmission. As an abstraction of reality, modelling will always have some shortcomings. Nevertheless, we have employed simulations to anticipate future implications of current decisions. From the present results, it is clear that our model could be further refined. For example, to facilitate the development of our model, we have simplified the calculation of diffuse solar radiation which otherwise would involve complex calculations. In addition, to analyze the shading effectiveness changes at different time scales, we need to develop more elaborate algorithms for more accurate computations. We intend to improve the model by acquiring more in-depth data in future studies. Despite the model’s deficiencies, we have confidence in the basic results of our simulation because the predictions are generally valid and reasonable. Acknowledgements We acknowledge with gratitude the research Grants kindly provided by the Midland Charitable Foundation, and Stanley Ho Alumni Challenge Fund. We gratefully appreciate Pegasus Company for donating the vertical greening setup of our experimental site. References Akbari, H., Kurn, D.M., Bretz, S.E., Hanford, J.W., 1997. Peak power and cooling energy savings of shade trees. Energy Buildings 25, 139–148. Akhmanov, S.A., Nikitin, Y.S., 1999. Physical Optics, vol. 405. Clarendon Press, Oxford, p. 420. Alexandria, E., Jones, P., 2008. Temperature decreases in an urban canyon due to green walls and green roofs in diverse climates. Building Environ. 43, 480–493. Baldocchi, D.D., Vogel, C.A., 1997. Seasonal variation of energy and water vapor exchange rates above and below a boreal jack pine forest canopy. J. Geophys. Res. 102, 28939–28951. Bartzanas, T., Tchamitchian, M., Kittas, C., 2005. Influence of the heating method on greenhouse microclimate and energy consumption. Biosyst. Eng., 487–499. Bastianoni, S., Marchettini, N., 1997. Emergy/exergy ratio as a measure of the level of organization of systems. Ecol. Model. 99, 33–40. Chen, R., Kang, E., Ji, X., Yang, J., Wang, J., 2007. An hourly solar radiation model under actual weather and terrain conditions: a case study in Heihe river basin. Energy 32, 1148–1157. Dufrene, E., Davi, H., Francis, C., Le Maire, G., Le Dantec, V., Granier, A., 2005. Modelling carbon and water cycles in a beech forest. Part I. Model description and uncertainty analysis on modelled NEE. Ecol. Model. 185, 407–436. Emilsson, T., 2008. Vegetation development on extensive vegetated green roofs: influence of substrate composition, establishment method and species mix. Ecol. Eng. 33 (3–4), 265–277. Gao, G.D., 1996. Climatology Tutorial. Meteorology Press, Beijing, pp. 395– 397. Goldstein, R.J., Eckert, E.R.G., Ibele, W.E., Patankar, S.V., Simon, T.W., Kuehn, T.H., Strykowski, P.J., Tamma, K.K., Heberlein, J.V.R., Davidson, J.H., Bischof, J., Kulacki, F.A., Kortshagen, U., Garrick, S., 2003. Heat transfer: a review of 2001 literature. Int. J. Heat Mass Transfer 46, 1887–1992. Gomez, F., Gaja, E., Reig, A., 1998. Vegetation and climatic changes in a city. Ecol. Eng. 10 (4), 355–360. Grigoletti, G., Sattler, M.A., Morello, A., 2008. Analysis of the thermal behaviour of a low cost, single-family, more sustainable house in Porto Alegre, Brazil. Energy Buildings 40, 1961–1971.

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