Post on 19-Jul-2016
description
ARTICLE IN PRESS
0360-1323/$ - se
doi:10.1016/j.bu
�Correspondfax: +9111 265
E-mail addr
sckaushik@ces.
Building and Environment 40 (2005) 1505–1511
www.elsevier.com/locate/buildenv
Performance evaluation of green roof and shading for thermalprotection of buildings
Rakesh Kumar�, S.C. Kaushik
Centre for Energy Studies, Indian Institute of Technology, New Delhi-110016, India
Received 29 July 2003; received in revised form 5 July 2004; accepted 22 November 2004
Abstract
The present paper describes a mathematical model for evaluating cooling potential of green roof and solar thermal shading in
buildings. A control volume approach based on finite difference methods is used to analyze the components of green roof, viz. green
canopy, soil and support layer. Further, these individual decoupled models are integrated using Newton’s iterative algorithm until
the convergence for continuity of interface state variables is achieved. The green roof model is incorporated in the building
simulation code using fast Fourier transform (FFT) techniques in Matlab. The model is validated against the experimental data
from a similar green roof-top garden in Yamuna Nagar (India), and is then used to predict variations in canopy air temperature,
entering heat flux through roof and indoor air temperature. The model is found to be very accurate in predicting green canopy-air
temperature and indoor-air temperature variations (error range 73.3%, 76.1%, respectively). These results are further used to
study thermal performance of green roof combined with solar shading. Cooling potential of green roof is found adequate (3.02 kWh
per day for LAI of 4.5) to maintain an average room air temperature of 25.7 1C. The present model can be easily coupled to different
greenhouse and building simulation codes.
r 2005 Published by Elsevier Ltd.
Keywords: Foliage; Canopy; Green rooftop; Leaf area index; Shading
1. Introduction
Enormous use of ground for various purposes has leadto disappearance of green planted surfaces. In order toprevent dangerous and uncomfortable urban heat islandeffects the indispensable need of planted surfaces is quietinevitable as is confirmed by many researchers viz. [1–5].Space constraints have further reduced the applicability ofgreen surfaces in various areas surrounding the buildingenvelope. Consequently, planted roofs become the onlypromising and stabilizing choice in the present scenario.Good thermal protection can greatly reduce the high
thermal loads that badly affect the comfort conditioning
e front matter r 2005 Published by Elsevier Ltd.
ildenv.2004.11.015
ing author. Tel.: +9111 2659 1253;
9 1266.
esses: krakesh1999@hotmail.com (R. Kumar),
ernet.iitd.in (S.C. Kaushik).
of building during summers. Eumorfopoulou andAravantinos [2] reported that planted roofs contributenot only in reducing the thermal loads on the building’sshell but also in reducing urban heat island effects indensely built areas having a little natural environment.Akbari et al. [6] have described the cooling energypotential of shade trees by reduction of the localambient temperature. For their biological functionssuch as photosynthesis, respiration, transpiration andevaporation, the foliage materials absorb a significantproportion of the solar radiation. Thermal protectiontechniques of green roof can provide a great degree ofreduction in the local air temperature near canopy, thusreducing the incoming heat flux into the building. Astudy done by Onmura [4,7] revealed that in closedspaces with the planted roofs, the air temperaturebeneath the plants is lower than that of the air above,by nearly 4–51C. Thermal performance of green roof
ARTICLE IN PRESS
Nomenclature
d average leaf thicknessDVT vapor diffusivityhaN heat transfer coefficient to airhg convective coefficient of vapor transportHc height of green canopyHs height of soil layerHR thickness of roof supporti nodal pointsL canopy layer thicknesspl leaf vapor pressurepa air vapor pressurepg vapor pressure at soil surface
re mean canopy resistance to sensible heattransfer
T c temperature of canopyTa temperature of airT sky sky temperatureTg ground temperatureT so soil TemperatureTN air temperature
Greek letters
ts ¼ shortwave transmittancett ¼ transmittance of the leaf tissueya air specific humiditycs solar radiationr densitylsu thermal conductivity of support material
Fig. 1. Schematic of green roof model.
R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–15111506
have been studied in great detail recently by severalresearchers. It is worth mentioning the works onprediction of thermal performance of green roofs byDel Barrio [3] and Good [8]; and on implementation ofgreen roof in the buildings by Dominguez and Lozano[9]; Eumorfopoulou and Aravantinos [2] and Takakuraet al. [10]. Capelli et al. [11] predicted thermal behaviorand effectiveness of vegetation covers with differentaverage absorptance for solar radiation and diffusiveproperties, which shields roof-covering structures ofdifferent masses. In a study conducted by Niachou et al.[12], investigation of green roof is done in two phases; inthe first phase, extensive data measurements fortemperature, both indoor and outdoor are considered,and in the second phase, thermal properties of greenroof are studied using a mathematical approach.Hoyano [13,14] conducted an experimental study oneffect of rooftop lawn planting on thermal environmentand also described for climatological uses of plantsfor solar control and the efforts on the thermalenvironment.Many of the studies predict thermal performance of
green roof localized to experimental site or employseveral numerical techniques to evaluate thermal per-formance. This restricts the applicability of the greenroof to particular buildings and hence thermal spaceconditioning of different building cannot be predicted,as green roof model is to be coupled to the buildingsimulation code. Effect of parametric variations inthermal components of green roof on cooling potentialis also not described. This model improves upon theseaspects by incorporating thermal modeling of green roofcomponents, parametric variations in the green roofcomponents and coupling the model to the buildingsimulation code. The process of heat transfer into theplanted roof is very different from bare roof, both
qualitatively and quantitatively. The analysis of greenroof can be classified into three sub-regions, viz. greencanopy, soil, and the roof support. Dynamic perfor-mance of each of the sub-region can be evaluated andfurther coupled with each other using the boundaryconditions.The objective of studying green roof is multifold: to
determine the effect of variations in foliage character-istics, viz. leaf area index (LAI) and foliage heightthickness on thermal performance of green canopy,estimation of thermal load reduction in the building andevaluation with thermal shading on building spaceconditioning. Results obtained from the simulation ofgreen roof model are validated with experimental datafrom an existing green roof with similar foliagecharacteristics and building parameters in YamunaNagar, Haryana state of India.
2. Mathematical formulation
The green roof top modeling described is based onDel Barrio [3]. Fig. 1 shows the model of green roof
ARTICLE IN PRESS
Fig. 2. Average calculation of LAI for foliage.
R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–1511 1507
constituting three elements of green canopy, soil and theroof support. A very important parameter for greencanopy is denoted by LAI. It is the ratio of area ofleaves to the area of the base occupied. Fig. 2 shows theaverage LAI for soyabean crop.The governing equation of energy balance for the
green canopy model is
ðrCpÞd � LAIdT c
dt¼ ½1� ts � ð1� tsÞr1�ð1þ tsrgÞcs
þ ð1� tlÞ½sT4sky þ sT4
g � 2sT4c �
� 2LAIrCp
re
ðTc � TaÞ
� 2LAIrCp
gðre þ riÞðpc � paÞ; ð1Þ
where pa is the vapor pressure of the canopy air, pc is thevapor pressure at the soil surface and pN is the outdoorair vapor pressure. Here, the first term denotes net solarradiation absorbed by the canopy. tsrgcs represents netsolar radiation reflected by the ground. The second termdenotes net thermal radiation in long wave as absorbedby the canopy. Tg is the ground temperature, T sky is thesky temperature, re indicates the mean canopyresistance. The third term signifies net convectiveheat transfer from green canopy to air. Finally, thefourth term denotes net transpiration losses from plantto air.Canopy air temperature can be determined from the
following equation:
ðrcÞaLdTa
dt¼ 2LAI
rCp
re
ðT c � TaÞ
þ hgðTg � TaÞ þ ha1ðT1 � TaÞ; ð2Þ
where hg is the convective coefficient of vapor transportand haN is the heat transfer coefficient between canopyair and outdoor air, re is the mean canopy resistance tothe sensible heat transfer, and TN is the outdoor airtemperature.
Energy balance for air specific humidity is given by
raLqyaqt
¼ hgðpg � paÞ � ha1ðpa � p1Þ: (3)
Soil model is governed by the following:
rCpðx; tÞqT soðy; tÞ
qt
¼qqy
lþ LDvT ðo;T soÞ½ �qT soðy; tÞ
qy
� �; ð4Þ
where l is the soil thermal conductivity, x is the localvolumetric moisture content in the porous mediumdomain, L is the latent heat of vaporization and DnT isthe non-isothermal vapor diffusivity.This equation is reduced into algebraic form using
finite difference method. Backward differencing in spaceand time has been used. The roof support is consideredas a homogenous material. The overall one-dimensionalheat conduction equation is taken as
rCp
qT suðy; tÞ
qt¼ lsu
q2T suðy; tÞ
qt2; (5)
where T suðy; tÞrepresents the temperature field, r is thedensity, and lsu is the thermal conductivity of thesupport material.Finite volume method [15] is used to discretize the
governing equation in to algebraic equation. Backwarddifferencing and explicit time-marching scheme is used.Mixed or Robin boundary condition is applied on thebottom of the roof support layer, whereas, specifiedtemperature or Dirichlet boundary condition is appliedat the top of the roof support layer to preservecontinuity.The three decoupled models are then coupled by the
interfaces where the relations between the connectedmodels are described. Newton iteration technique isemployed to determine the interface state variables thatsatisfy the constraints of flux continuity, together withcontinuity of state variables.The room temperature determined from building
simulation code is governed by one-dimensional heatconduction equation. The resultant heat flux from greenroof model is incorporated in the building simulationcode [16] in Matlab using FFT. Solar thermal shading isincorporated in the building simulation code byascertaining the fraction of solar radiation received byvarious building components based on building loca-tion, orientation and shaded portions.A room size of 6� 5� 4 with occupancy of 4 people is
considered for modeling. An air change rate of 5 h�1 istaken for a window area of 2m2. The predicted results ofgreen roof model coupled to building simulation codeare obtained for parametric values shown in Table 1.However, the model is adaptive to include any value
of the parameter specified by the user as per buildingspecification and selected green roof parameters.
ARTICLE IN PRESS
Table 1
Planted roof parameters and properties
Element Effective conductivity (lw) Thickness Specific heat (J/kgK)
Green canopy Varying (on foliage characteristics) 0.4m Variable
Soil 1.21W/mK 840
Sieve layer 0.4W/mK 0.002 1340
Wash off layer 0.035W/mK 0.03 Varying
Reinforced Concrete 1.58W/mK 0.006 880
Roof plaster 0.72 W/m-K 0.018 820
Fig. 3. Schematic of experimental site.
R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–15111508
3. Results and discussions
The parametric values used in this research forvarious constants are:
Soil thermal diffusivity
2.2e(�4)m2/s Vapor diffusivity 2.e( 6)m2/s Latent heat of evaporation 2257 kJ/kgThe accuracy of the presented model is verified on thebasis of a comprehensive set of experimental data. Fivetemperature and humidity sensors (8160TFF10) sup-plied by Luftopus-200/200I were placed in each room atthe corners and the center of green canopy layer exactlyat the central height of the foliage (0.20, 0.40, 0.60 and1.0m above the soil surface). Five platinum resistancethermometers (PT-500) were used to measure the soiltemperature at a height of 0.10, and 0.20m above theroof surface. Three temperature humidity sensors wereplaced at 0.50, 1.5 and 2.5m above the floor in side theroom. Fig. 3 shows the photo of the experimental sitewith marked circles to indicate the testing points.The performed simulations extend over validation
and parametric study of green roof with cooling energypotential. Fig. 4 shows the roof top garden to which the
modeling simulation results are validated. Validation ofsimulated results with experimental observations isshown in Fig. 5 for canopy air temperature. And it isfound that the results matched experimental data withinan accuracy of73.3%. Further, validation of indoor airtemperature obtained from building simulation code(after incorporating green roof model) is shown in Fig.6. Results matched within an accuracy of 76.1% withexperimental data obtained from 8 check points tomeasure the room temperature.Effect on indoor air temperature is also shown in Fig.
6. It is found that the indoor air temperature is reducedby an average of 7.2 1C. However, the reduction oftemperature follows a pattern with a maximum reduc-tion observed in peak heating period (12:00–15:00 h) anda minimum in off sunshine period.LAI is one of the important parameter affecting the
micro-climate of the green canopy and hence theinteriors of the building. Fig. 7 shows the effect ofparametric variations in LAI on canopy air temperature.A peak reduction of 9.3 1C in canopy air temperature isobserved for a cycle of 8 days from June 1–8. Peakcanopy air temperature and temperature width are bothreduced with increased in LAI as summarized in Fig. 7.Hence, it is observed from the figure that fluctuations ofthe order of 11.6 1C are reduced to only 3.6 1C with anincrease in LAI from 0.5 to 3.5.
ARTICLE IN PRESS
24
26
27
28
29
30
31
48Time (Hrs)
Can
opy
air
tem
pera
ture
(˚C
)
72 96 120
Predicted Canopy air temperature data
10 - 14 June, 2001LAI: 1.5
Experimental measurements data
Fig. 5. Validation for green canopy air temperature.
Fig. 4. Green roof top garden (picture view from side).
100 200 300 400 500 600 700
20
25
30
35
40
Ambient air temperature
Station : Yamuna Nagar, HaryanaTime : 1May to 30 May, 2001
Indoor air temperature (Theoretical)
MeasurementsIn
door
air
tem
pera
ture
(˚C
)
Time (Hrs)
Fig. 6. Prediction and validation of indoor air temperatures for
monthly cycle.
R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–1511 1509
The long-term dynamic performance of green roof forshielding heat radiations is presented with parametricvariations in foliage thickness and LAI in Figs. 8 and 9.Heating flux entering green roof with a foliage height of0.6m is 1.94Whm�2 (6984 J) and this increasedfurther by nearly 4 times (14,400 J)Whm�2 for the bareroof as seen from Fig. 8. Also, the fluctuations in theentering heat flux are very less for green roof ascompared to bare roof.It is important to determine the heat flux penetrating
the building affected by green roof for various leaf areaindices denoting the extent of coverage of the roof.Internal gains for the room as shown in Fig. 9 are anaverage 3.02 kWh (10872 kJ) (5.18 kWh for bare roof &2.16 kWh for LAI of 4.5). The results show an increasein the entering heat flux with decreased in LAI. Increase
in LAI in general leads to a reduction in coverage areaand hence the insulating effect. However, it also relatesto lessening of biological activities by foliage materials,affecting the micro-climate of the building.Finally, building space conditioning is shown in Fig.
10. Average indoor air temperature of a (6� 5� 4)room is 28.47 1C without green roof and this is reducedby 3.3 1C, leading to an average indoor air temperatureof 25.7 1C. For shading combined with green roof,temperature reduces by 2.1 1C leading to an averageindoor air temperature of 23.6 1C.Diurnal fluctuations in indoor air temperature
show reduced temperature width with green roof.Hence, it is seen from the figure that the temperaturewidth of 10.2 1C in indoor temperature withoutgreen roof is reduced to only 5.1 1C with green roof
ARTICLE IN PRESS
Time (Hrs)
Can
opy
air
tem
pera
ture
(0 C)
24 48 72 96 120 144 168 192
20
25
30
35
1-8 June, 200140 Ambient air temperature
LAI =1.5LAI = 2.5LAI = 3.5
Fig. 7. Effect of various LAI on canopy air temperature.
50 100 150
7
6.5
6
5.5
5
4.5
4
3.5
3
2.5
2
Time (Hrs)
Hea
t flu
x (W
/ m
2 )
bare roofTime : 1 to 8 June, 2001Roof Area : 30 Sq.m.
fht = 0.6fht = 0.5fht = 0.4fht = 0.3fht = 0.2
Fig. 8. Heat flux through roof for various foliage height thickness.
Time (Hrs)
Hea
tflu
x(W
/m2 )
0 24 48 72 96 120 144 168 192-2
0
2
4
6Bare roofLAI = 0.5LAI = 2.5LAI = 3.5
17 - 25 May, 2001Yamuna Nagar
Fig. 9. Total heat flux through planted roof for various leaf area
indices.
50 100 150Time (˚C)
Indo
or a
ir te
mpe
ratu
re (
˚C)
20
22
24
26
28
30
32
34
36
38
40
Ambient air temperatureStation : Yamuna Nagar, HaryanaTime : 1 to 8 June, 2001
Indoor air temperature With bare roof
Green roof
Green roof + Shading
Fig. 10. Indoor air temperature variations (from building simulation
code).
R. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–15111510
which has further dropped to only 2.1 1C while shadingis included.
4. Conclusions
1.
Thermal performance of green roof model is pre-sented with extensive set of validation with experi-mental data for canopy air temperature and indoorair temperature.2.
Parametric variations in LAI and foliage heightthickness are carried out to determine the modulationof canopy air temperature, and reduction in tem-perature width and to estimate the penetrating heatflux. Results determine that larger LAI reduces thecanopy air temperature, stabilized the fluctuatingvalues and reduced the penetrating flux by nearly4W/m2.3.
Indoor air conditioning of the building is predicted.Green roof combined with solar thermalshading reduced averaged indoor air temperature by5.1 1C, from the average indoor air temperature forthe bare roof. Green roof provided a coolingpotential of 3.02 kWh per day which is found to beadequate.Earth covering methods for thermal protection likegreen roofs and solar thermal shading provide a veryeffective solution in building habituation for presentstate of affairs being very energy efficient and ecoresponsive. Results of this investigation are applicable tothe entire building geometrical realm.
ARTICLE IN PRESSR. Kumar, S.C. Kaushik / Building and Environment 40 (2005) 1505–1511 1511
Acknowledgements
The authors are thankful to Ministry of Non-Conventional Energy Sources (MNES) New Delhi, forits financial help in carrying out this research work. Weare also thankful to Indian Meteorological Department(IMD) New Delhi, for providing the Weather Data.
References
[1] Wong NH, Chen Y, Ong CL, Sia A. Investigation of thermal
benefits of rooftop garden in the tropical environment. Building
and Environment 2003;38:261–70.
[2] Eumorfopoulou E, Aravantinos D. The contribution of a planted
roof to the thermal protection of buildings in Greece. Energy and
Buildings 2003;27:29–36.
[3] Del Barrio EP. Analysis of the green roofs cooling potential in
buildings. Energy and Buildings 1998;27:179–93.
[4] Onmura S, Matsumoto M, Hokoi S. A study on evaporative
cooling effect by roof lawn garden. Proceedings of the European
conference on energy performance and indoor climate in buildings
1994:634–9.
[5] Haefeli P, Lachal B, Weber W. Experiences with green roofs in
Switzerland. Proceedings of the PLEA’98, Portugal 1998:365–8.
[6] Akbari H, Kurn DM, Bretz SE, Hanford JW. Peak power and
cooling energy savings of shade trees. Energy and Buildings
1997;25:139–48.
[7] Onmura S, Matsumoto M, Hokoi S. Study on evaporative
cooling effect of roof lawn gardens. Energy and Buildings
2001;33:653–66.
[8] Good W. Factors in planted roof design. Constructions Specifica-
tion 1990;43(11):132.
[9] Dominguez J, Lozano A. Green roof systems. Comput. Mech.
Publication 1998;1:615–24.
[10] Takakura T, Kitade S, Goto E. Cooling effect of greenery cover
over a building. Energy and Buildings 2000;31:1–6.
[11] Cappelli M, Cianfrini C, Corcicone M. Effects of vegetation
roof on indoor temperatures. Heat Environment 1998;16(2):
85–90.
[12] Niachou A. Analysis of the green roof thermal properties and
investigation of its energy performance. Energy and Buildings
2001;33:719–29.
[13] Hoyano A, He J, Horiguchi T, Wang G. Experimental study on
heat budget for foliage layer on lawn planting. Part 1. Effect of
rooftop lawn planting on thermal environment. Journal of
architecture Planning and Environmental Engineering, Transac-
tions of AIJ 1994;462:31–9.
[14] Hoyano A. Climatological uses of plants for solar control and the
efforts on the thermal environmental of a building. Energy and
Buildings 1988;11:29–36.
[15] Patankar SV. Numerical heat transfer and fluid flow. New York:
Hemisphere Publishing Corporation; 1980.
[16] Kaushik SC, Sodha MS, Bansal PK, Bhardwaj SC. Solar thermal
modeling of a non-air conditioned building: evaluation of overall
heat flux. Energy Research 1982;6:143–60.