- Email: [email protected]
Thermal performance evaluation of a new type of green roof system
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
ScienceDirect ScienceDirect Available onlineatatwww.sciencedirect.com www.sciencedirect.com Available online Energy Procedia 00 (2018) 000–000 Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy (2018) 000–000 384–389 EnergyProcedia Procedia152 00 (2017) www.elsevier.com/locate/procedia
CUE2018-Applied Energy andLow Forum 2018: Low carbon andsystems, Applied Energy Symposium andSymposium Forum 2018: carbon cities and urbancities energy Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems, CUE2018, 5–7 June 2018, Shanghai, China urban energy systems, 5–7 June 2018, Shanghai, China CUE2018, 5–7 June 2018, Shanghai, China
The 15th International Symposium District Heating and Cooling Thermal performance evaluation of aonnew type of green roof system Thermal performance evaluation of a new type of green roof system a a Hea, Hang Yuof *, using Pengda the Chenheat , Meidemand-outdoor Zhaoa AssessingYang the feasibility Yang Hea, Hang Yua*, Pengda Chena, Mei Zhaoa Institute of HVAC function Engineering, Tongjifor University, No. 4800 Caoan Highway, Jiading District, 201804, China temperature a long-term district heatShanghai, demand forecast Institute of HVAC Engineering, Tongji University, No. 4800 Caoan Highway, Jiading District, Shanghai, 201804, China a,b,c
a
a
b
c
c
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre Abstract I. Andrić Abstract a Center thermal for Innovation, Technology Policy Research - Instituto Superior Pais 1,conditions 1049-001 Lisbon, Portugal TheIN+ increase capacity of greenand roof compared with common roof Técnico, may leadAv.toRovisco bad indoor sometimes. This b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France The increase thermal capacity of green roof compared with common roof may lead to bad indoor conditions sometimes. This paper proposes a new type of green roof system with ventilation layer. And the new green roof could lower internal surface c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France paper proposes a new and typeincrease of greeninternal roof system ventilation layer.inAnd newclimate green zones roof could lower internal surface temperature in summer surfacewith temperature in winter five the typical of China. And the modelling temperature summer and increase surfacereduction temperature winter surface in five typical of China. And the modelling results showinthat maximum averageinternal temperature of in internal reachesclimate 0.1℃zones on typical summer day, and the results show that maximum average reduction of internal surface reaches 0.1℃ on typical the maximum average temperature rise oftemperature internal surface temperature reaches 0.4℃ on typical winter day.summer Finally,day, the and thermal maximum temperature riseunder of internal temperature 0.4℃and oninitial typical winter day. ratio Finally, thermal performance of green roof system differentsurface substrate depth, leafreaches area index water content are the analyzed in Abstract average performance of green roof system under different substrate depth, leaf area index and initial water content ratio are analyzed in details. details. District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the Copyright © 2018 Elsevier Ltd. All rights reserved. greenhouse gas fromAll therights building sector. These systems require high investments which are returned through the heat Copyright © 2018 2018emissions Elsevier Ltd. Ltd. reserved. Copyright © Elsevier All rights reserved. Selection and peer-review under responsibility of the scientific committee ofpolicies, Applied heat Energy Symposium and Forum 2018: Low sales. Due to the changed climate conditions demand in the future could decrease, Selection and peer-review under responsibility ofand thebuilding scientificrenovation committee of the CUE2018-Applied Energy Symposium and Selection andthe peer-review under responsibility of the scientific committee of Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems, CUE2018. prolonging investment return Forum 2018: Low carbon cities andperiod. urban energy systems. carbon cities and urban systems, CUE2018. The main scope of thisenergy paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand
Keywords: roofof system; Ventilation layer; Heatingwas effect.used as a case study. The district is consisted of 665 forecast. New The green district Alvalade, located inCooling Lisboneffect; (Portugal), Keywords: New green roof system; Ventilation layer; Cooling effect; Heating effect.
buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were 1.compared Introduction with results from a dynamic heat demand model, previously developed and validated by the authors. 1.The Introduction results showed that when only weather change is considered, the margin of error could be acceptable for some applications Green system has been around the all world about the thermal and energy saving And its (the errorroof in annual demand was studied lower than 20% for weather scenarios considered). However, afterperformance. introducing renovation Green roof system hasincreased been around the world about the thermal and energy saving performance. And its scenarios, thedepends error value to 59.5% (depending on the weather and renovation scenarios combination considered). performance greatly on studied theuplocal climate and the hygrothermal properties. Through a comparison test between [1] range of 3.8% performance greatly the local and the hygrothermal Through test roof between The value ofdepends slope increased onclimate average within the up to 8% per decade, that corresponds towas the found that properties. the cooling effect ofa comparison earth-sheltered earth-sheltered roofcoefficient and greenon roof in Israel, Schweitzer [1] heating season (depending on the combination of weather and decrease in the number of heating hours of 22-139h during the earth-sheltered roof and Israel, Schweitzer foundplays that an theimportant cooling effect rooflayer, was significantly smaller thangreen greenroof roof,insuggesting that plant layer role. of Forearth-sheltered the effect of soil renovation On the other hand,of function intercept increased for 7.8-12.7% per decade (depending on the [2] considered). significantly smaller than green roof, suggesting that layergreen plays an important role. the effect of soil layer, showed that soil temperature anplant intensive roof remained ratherFor stable at 10, 50 and 90cm the study byscenarios Jim [2] The values suggested could be used to modify the function parameters for the scenarios considered, and coupled scenarios). the study by Jim showed soil temperature intensive greenbe roof remained at 10,With 50 and 90cm depth, which suggested that that thermal performanceof ofan green roof could obtained withrather only stable 10cm soil. regard to improve the accuracy of heat depth, which suggested that demand thermalestimations. performance of green roof could be obtained with only 10cm soil. With regard to © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
* Corresponding author. Tel.: +86 18817308597. * E-mail Corresponding Tel.: +86 18817308597. address:author. [email protected] Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the Applied Energy Symposium and Forum 2018: Low carbon cities Selection and peer-review under responsibility the scientific Selection peer-review responsibility of the scientific committee of the Applied Energy Symposium and Forum 2018: Low carbon cities and urbanand energy systems, under CUE2018. and urban energy systems, CUE2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the CUE2018-Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems. 10.1016/j.egypro.2018.09.161
2
Yang He et al. / Energy Procedia 152 (2018) 384–389 Author name / Energy Procedia 00 (2018) 000–000
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the effect of evapotranspiration, Sun [3] further noted that the contribution depended on local climate and was mainly controlled by energy availability and water availability. In addition, many researchers found that the increase of the thermal capacity of green roofs compared to traditional roofs, if not controlled with insulation, may lead to higher cooling and heating loads [4,5]. Consequently, this paper proposes a new type of green roof system to improve thermal performance of green roof based on free cooling and heating effect of outdoor air in summer and winter respectively. Firstly, a coupled heat and moisture transfer model of common green roof (CGR) is briefly introduced according to our previous study. Secondly, the principle and structure of the new type of green roof (NGR) system is introduced. Then the thermal performance between CGR and NGR is compared in different climate zones of China, and the effect of substrate depth, leaf area index and initial water ratio on thermal performance of NGR is discussed. 2. Coupled heat and moisture transfer model of common green roof system The model developed in this paper is based on our previous study[6] which considered the following aspects: (1) multiple longwave and solar radiation in canopy layer of plant; (2) the effect of wind profile on turbulent transfer coefficient below and above canopy surface based on Soil-Vegetation-Atmosphere-Transfer (SVAT) theory by B.J. Choudhury[7]; (3) coupled heat and moisture transfer in soil based on the theory of Philip & De Vries[8]; The schematic of this model is illustrated in Fig. 1, and the detailed development and validation of this model could be seen in our previous paper[6]. Meteorological Data
Radiation distribution of plant canopy layer Long wave radiation
Energy balance of plant canopy layer Long wave radiation
Moisture transfer
Energy balance of substrate surface Heat conduction & Phase change
Coupled hygrothermal transfer process of substrate layer Heat conduction
Heat transfer process of structure layer Heat convection and longwave radiation
Indoor Space
Fig. 1. Schematic of hygrothermal transfer model of common green roof system
3. Principle and structure of new green roof In order to clarify the principle of new green roof, a common building model (Fig.2.) is developed in Trnsys, and above model is coupled with Type 56. The detailed thermal properties of the building model in different climate zones is shown in table 1, and the hygrothermal properties of green roof system is shown in table 2. The schedules of people, lighting and equipment are set according to the design standard for energy efficiency of public buildings in China (GB50189-2015). Table 1. Heat transfer coefficients of building envelope in different climate zones. Building envelope Harbin Beijing Shanghai （W/m2K） （W/m2K） （W/m2K） External wall 0.35 0.45 0.6 Roof 0.25 0.4 0.40 Window 2.3 2.5 3.0 Ceiling 0.35 0.45 0.7 Window-wall ratio 0.3 0.3 0.3
Kunming （W/m2K） 0.8 0.5 4.0 1.5 0.3
Guangzhou （W/m2K） 0.8 0.5 4.0 1.5 0.3
Yang He et al. / Energy Procedia 152 (2018) 384–389 Author name / Energy Procedia 00 (2018) 000–000
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Fig. 2. Schematics of building model Table 2. Parameters of green roof Parameters Plant height(cm) Minimum stomatal resistance(s/m)
Value 15(Summer), 2(Winter)
Value 0.3
5.5(Summer), 0.4(Winter)
Parameters Leaf albedo Thickness of structure layer(cm) Albedo of structure layer
600*1000
Emissivity of plant layer
0.9
Emissivity of structure layer
0.9
200
Leaf area index Substrate capacity(J/m3K) Substrate depth(cm)
4
Heat conductivity of substrate layer(W/m·K)
Emissivity of substrate layer
0.374 ∗ (𝜃𝜃)0.403
Water capacity of substrate layer(pa-1)
1.575 ∗ (𝜃𝜃 −4.35 )
Water permeability of substrate layer (m/s)
2.37 ∗ 10−5 ∗ (𝜃𝜃)24.42
24 0.2
0.95
Saturated water content ratio
0.63
Albedo of substrate layer
0.2
Common roof Green roof Outdoor air temperature
42 40 38 36 34
Tgr>Ta
32 30 28 26 24 22 20 00:00
Tgr>Ta 06:00
Tgr>Ta 12:00
Summer (1)
18:00
00:00
Outer surface temperature of structure layer (oC)
Outer surfacetemperature of structure layer (oC)
Taking the typical meteorological day of Shanghai as an example, based on above model, the temperature distributions of common green roof and common roof could be seen in Fig. 3. It indicates that, green roof system lowers the outer surface temperature of structure layer in the daytime for its shading and evapotranspiration effect. And green roof increases this temperature at night for its greater heat storage effect. In order to make full use of the thermal benefit of green roof, a new type of green roof with ventilation system is proposed. Just as illustrated in Fig. 4. Common roof Green roof Outdoor air temperature
20
15
Tgr>Ta
10
Tgr>Ta 5
Tgr>Ta
0
00:00
06:00
12:00
Winter (2)
Tgr>Ta 18:00
00:00
Fig. 3. Temperature comparison between green roof and common roof on the typical meteorological day of Shanghai: (1) Summer case; (2) Winter case.
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(1) (2) Fig. 4. Schematic of Common green roof and new green roof: (1) Common green roof; (2) New green roof
The controlling strategy of the fan in ventilation layer is as follows. In summer, when the temperature in ventilation layer is greater than outdoor air temperature at night, the fan is turned on, and cools the substrate and structure layer. When the temperature in ventilation layer is smaller than outdoor air temperature in the daytime, the fan is turned off, and the ventilation layer is closed. In winter, when the temperature in ventilation layer is smaller than outdoor air temperature, the fan is turned on and heats the substrate and structure layer. When the temperature in ventilation layer is greater than outdoor air temperature, the fan is turned off and ventilation layer is closed. Through free cooling and heating of outdoor air temperature, it is possible to avoid the negative effect of green roof. 4. Thermal performance comparison between common green roof and new green roof 2
Common Green Roof New Green Roof
26.8
Internal surface heat flux (W/m2)
Internal surface temperature (oC)
27.2
26.4
26.0
25.6
25.2
24.8
Harbin
Beijing
Shanghai
Kunming
0
-1
-2
-3
-4
Guanzhou
Common Green Roof New Green Roof
1
Harbin
Beijing
(1)
Common Green Roof New Green Roof
30 25 20 15 10 5 0
Kunming
Guanzhou
Kunming
Guanzhou
(2)
2
Internal surface heat flux (W/m2)
Internal surface temperatue (oC)
35
Shanghai
Summer
Summer
0
Common Green Roof New Green Roof
-2
-4
-6
-8 Harbin
Beijing
Shanghai
Winter
Kunming
Guanzhou
Harbin
Beijing
Shanghai
Winter
(3) (4) Fig. 5. Thermal performance comparison between CGR and NGR: (1) Average temperature in summer; (2) Average heat flux in summer; (3) Average temperature in winter; (4) Average heat flux in winter.
Based on the principle of NGR, thermal performance between CGR and NGR is compared on typical meteorological day of five climate zones in China, including severe cold region (Harbin), cold region (Beijing), hot
Yang He et al. / Energy Procedia 152 (2018) 384–389 Author name / Energy Procedia 00 (2018) 000–000
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summer and cold winter region (Shanghai), mild region(Kunming), hot summer and warm winter region (Guangzhou). And the average internal surface temperature and heat flux of CGR and NGR is presented, as is shown in Fig. 5. It can be found that NGR shows better thermal performance in all the five climate zones. In summer, NGR lowers the internal surface temperature and the heat flux into the room, while increases the internal surface temperature and lowers the heat flux out of the room. The maximum average temperature reduction of internal surface reaches 0.1℃ in summer, and the maximum average temperature rise of internal surface temperature reaches 0.4℃ in winter. 5. Thermal performance of NGR under different conditions
-0.065
-0.35
-0.070 -0.075
-0.40
Temperature difference (oC)
-0.30
-0.060
Heat flux difference (W/m2)
Temperature difference (oC)
-0.25
-0.055
-0.080 -0.085
4cm
6cm
8cm
10cm
Temperature difference Heat flux difference
0.29
Temperature difference Heat flux difference
-0.050
0.56
0.30
-0.20
-0.045
0.54 0.52
0.27
0.50
0.26
0.48
0.25
0.46
0.24
0.44
0.23
0.42
0.22
0.40
0.21
-0.45
12cm
0.28
4cm
6cm
8cm
10cm
Heat flux difference (W/m2)
-0.040
0.38
12cm
Soil thickness Winter of Shanghai
Soil thickness Summer of Shanghai
（1） （2） Fig. 6. The effect of soil thickness on relative cooling and heating effect of new green roof system compared with common green roof system: (1) Summer case; (2) Winter case.
-0.210
-0.040
-0.215
-0.041
-0.220
-0.042
-0.225
-0.043
-0.230
-0.044
1
2
3
4
5
6
Temperature difference Heat flux difference
0.27
Temperature difference (oC)
-0.039
Heat flux difference (W/m2)
Temperature difference (oC)
Temperature difference Heat flux difference
0.26
0.48
0.25
0.46
0.24
0.44
0.23
0.42
0.22
0.40
0.21
-0.235
0.50
0
1
2
3
4
5
Heat flux difference (W/m2)
-0.205
-0.038
0.38
Leaf area index Winter of Shanghai
Leaf area index Summer of Shanghai
（1） （2） Fig. 7. The effect of leaf area index on relative cooling and heating effect of new green roof system compared with common green roof system: (1) Summer case; (2) Winter case. 0.219
-0.236 -0.238
-0.0445 -0.240 -0.0450
-0.242
0.2
0.3
0.4
0.5
Initial water ratio Summer of Shanghai
0.6
0.7
-0.244
0.392
0.213 0.384 0.210 0.376
0.207 0.204
0.368
0.201
Heat flux difference (W/m2)
-0.234
-0.0440
-0.0455
Temperature difference Heat flux difference
0.216
Temperature difference (oC)
-0.0435
0.400
-0.232
Temperature difference Heat flux difference
Heat flux difference (W/m2)
Temperature difference (oC)
-0.0430
0.360 0.2
0.3
0.4
0.5
0.6
0.7
Initial water ratio Winter of Shanghai
（1） （2） Fig. 8. The effect of initial water ratio of substrate layer on relative cooling and heating effect of new green roof system compared with common green roof system: (1) Summer case; (2) Winter case.
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Yang He et al. / Energy Procedia 152 (2018) 384–389 Author name / Energy Procedia 00 (2018) 000–000
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Thermal performance of green roof not only depends on the local climates, but also related to its hygrothermal properties. Taking the typical meteorological day data of Shanghai as an example, this study simulated thermal performance of NGR under different substrate layer depth, leaf area index and initial water content ratio. Increasing the soil layer thickness not only could improve the thermal capacity of green roof; but also enhance the thermal resistance. According to the simulation result (Fig. 6.), it can be found that, as soil thickness increases, the cooling effect of ventilation layer on green roof thermal performances becomes greater in summer, and the increase ratio gradually decreases. In winter, the heating effect of ventilation layer is not monotonous, and it firstly increases and then decreases. Thus, it can be deduced that, an optimal soil thickness exists in different climate conditions. A larger leaf area index means a greater shading and cooling effect from plant layer in the daytime, and also means less longwave radiation reaching on the substrate surface. According to Fig. 7., we can find that, the cooling and heating effect of ventilation layer both increase in summer and winter. A larger water content ratio in substrate layer benefits to improve the evapotranspiration intensity of green roof, and also reduces the thermal resistance of substrate layer. From Fig. 8, it’s shown that, the effect of initial water ratio on cooling effect of ventilation layer is not monotonous in summer, and it firstly decreases and then increases as initial water content ratio rises. And the heating effect of ventilation layer in winter improves initial water content ratio rises. 6. Conclusion Based on the analysis of temperature distribution of common green roof system, a new green roof system with ventilation layer is proposed making full use of free cooling and heating effect of outdoor air temperature. And the simulation result demonstrates that the new green roof system has a better cooling effect in summer and a better insulation effect in winter in five typical climate zones of China. The modelling results show that maximum average temperature reduction of internal surface reaches 0.1℃ on typical summer day, and the maximum average temperature rise of internal surface temperature reaches 0.4℃ on typical winter day. Parametric analysis has been carried out to identify the effect of three parameters on thermal performance of new green roof system. For thermal performance of new green roof system in Shanghai, the effect of soil thickness and initial water content ratio differs in summer and winter. For soil thickness, it has a positive effect in summer, while a nonmonotonic effect in winter. And for initial water content, its effect is opposite to soil thickness. And leaf area index has a positive effect both in summer and winter. Acknowledgements The study has been supported by the China National Key R&D Program 'Solutions to heating and cooling of buildings in the Yangtze River region' (Grant No. 2016YFC0700305-02) and the Chinese Scholarship Council through the "National high-level University Graduate" program. References [1] Schweitzer O, Erell E. Evaluation of the energy performance and irrigation requirements of extensive green roofs in a water-scarce Mediterranean climate. Energy and Buildings. 2014; 68:25-32. [2] Jim CY, Tsang SW. Biophysical properties and thermal performance of an intensive green roof. Building and Environment. 2011;46(6):12631274 [3] Sun T, Bou-Zeid E, Ni G-H. To irrigate or not to irrigate: analysis of green roof performance via a vertically-resolved hygrothermal model. Building and Environment. 2014; 73:127-137. [4] U. Berardi, A. GhaffarianHoseini, A. GhaffarianHoseini. State-of-the-art analysis of the environmental benefits of green roofs, Applied Energy 2014, 115: 411-428. [5] O. Saadatian, K. Sopian, E. Salleh, C.H. Lim, S. Riffat, E. Saadatian, A. Toudeshki, M.Y. Sulaiman. A review of energy aspects of green roofs, Renewable and Sus-tainable Energy Reviews. 2013, 23:155–168. [6] He Y, Yu H, Ozaki A, Dong N, Zheng S. Influence of plant and soil layer on energy balance and thermal performance of green roof system. Energy. 2018; 141:1285-99. [7] Choudhury B, Monteith J. A four‐layer model for the heat budget of homogeneous land surfaces. Q J Roy Meteor Soc. 1988;114(480):37398. [8] Philip J, De Vries D. Moisture movement in porous materials under temperature gradients. Eos, Transactions American Geophysical Union. 1957;38(2):222-32.
ScienceDirect ScienceDirect Available onlineatatwww.sciencedirect.com www.sciencedirect.com Available online Energy Procedia 00 (2018) 000–000 Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy (2018) 000–000 384–389 EnergyProcedia Procedia152 00 (2017) www.elsevier.com/locate/procedia
CUE2018-Applied Energy andLow Forum 2018: Low carbon andsystems, Applied Energy Symposium andSymposium Forum 2018: carbon cities and urbancities energy Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems, CUE2018, 5–7 June 2018, Shanghai, China urban energy systems, 5–7 June 2018, Shanghai, China CUE2018, 5–7 June 2018, Shanghai, China
The 15th International Symposium District Heating and Cooling Thermal performance evaluation of aonnew type of green roof system Thermal performance evaluation of a new type of green roof system a a Hea, Hang Yuof *, using Pengda the Chenheat , Meidemand-outdoor Zhaoa AssessingYang the feasibility Yang Hea, Hang Yua*, Pengda Chena, Mei Zhaoa Institute of HVAC function Engineering, Tongjifor University, No. 4800 Caoan Highway, Jiading District, 201804, China temperature a long-term district heatShanghai, demand forecast Institute of HVAC Engineering, Tongji University, No. 4800 Caoan Highway, Jiading District, Shanghai, 201804, China a,b,c
a
a
b
c
c
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre Abstract I. Andrić Abstract a Center thermal for Innovation, Technology Policy Research - Instituto Superior Pais 1,conditions 1049-001 Lisbon, Portugal TheIN+ increase capacity of greenand roof compared with common roof Técnico, may leadAv.toRovisco bad indoor sometimes. This b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France The increase thermal capacity of green roof compared with common roof may lead to bad indoor conditions sometimes. This paper proposes a new type of green roof system with ventilation layer. And the new green roof could lower internal surface c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France paper proposes a new and typeincrease of greeninternal roof system ventilation layer.inAnd newclimate green zones roof could lower internal surface temperature in summer surfacewith temperature in winter five the typical of China. And the modelling temperature summer and increase surfacereduction temperature winter surface in five typical of China. And the modelling results showinthat maximum averageinternal temperature of in internal reachesclimate 0.1℃zones on typical summer day, and the results show that maximum average reduction of internal surface reaches 0.1℃ on typical the maximum average temperature rise oftemperature internal surface temperature reaches 0.4℃ on typical winter day.summer Finally,day, the and thermal maximum temperature riseunder of internal temperature 0.4℃and oninitial typical winter day. ratio Finally, thermal performance of green roof system differentsurface substrate depth, leafreaches area index water content are the analyzed in Abstract average performance of green roof system under different substrate depth, leaf area index and initial water content ratio are analyzed in details. details. District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the Copyright © 2018 Elsevier Ltd. All rights reserved. greenhouse gas fromAll therights building sector. These systems require high investments which are returned through the heat Copyright © 2018 2018emissions Elsevier Ltd. Ltd. reserved. Copyright © Elsevier All rights reserved. Selection and peer-review under responsibility of the scientific committee ofpolicies, Applied heat Energy Symposium and Forum 2018: Low sales. Due to the changed climate conditions demand in the future could decrease, Selection and peer-review under responsibility ofand thebuilding scientificrenovation committee of the CUE2018-Applied Energy Symposium and Selection andthe peer-review under responsibility of the scientific committee of Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems, CUE2018. prolonging investment return Forum 2018: Low carbon cities andperiod. urban energy systems. carbon cities and urban systems, CUE2018. The main scope of thisenergy paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand
Keywords: roofof system; Ventilation layer; Heatingwas effect.used as a case study. The district is consisted of 665 forecast. New The green district Alvalade, located inCooling Lisboneffect; (Portugal), Keywords: New green roof system; Ventilation layer; Cooling effect; Heating effect.
buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were 1.compared Introduction with results from a dynamic heat demand model, previously developed and validated by the authors. 1.The Introduction results showed that when only weather change is considered, the margin of error could be acceptable for some applications Green system has been around the all world about the thermal and energy saving And its (the errorroof in annual demand was studied lower than 20% for weather scenarios considered). However, afterperformance. introducing renovation Green roof system hasincreased been around the world about the thermal and energy saving performance. And its scenarios, thedepends error value to 59.5% (depending on the weather and renovation scenarios combination considered). performance greatly on studied theuplocal climate and the hygrothermal properties. Through a comparison test between [1] range of 3.8% performance greatly the local and the hygrothermal Through test roof between The value ofdepends slope increased onclimate average within the up to 8% per decade, that corresponds towas the found that properties. the cooling effect ofa comparison earth-sheltered earth-sheltered roofcoefficient and greenon roof in Israel, Schweitzer [1] heating season (depending on the combination of weather and decrease in the number of heating hours of 22-139h during the earth-sheltered roof and Israel, Schweitzer foundplays that an theimportant cooling effect rooflayer, was significantly smaller thangreen greenroof roof,insuggesting that plant layer role. of Forearth-sheltered the effect of soil renovation On the other hand,of function intercept increased for 7.8-12.7% per decade (depending on the [2] considered). significantly smaller than green roof, suggesting that layergreen plays an important role. the effect of soil layer, showed that soil temperature anplant intensive roof remained ratherFor stable at 10, 50 and 90cm the study byscenarios Jim [2] The values suggested could be used to modify the function parameters for the scenarios considered, and coupled scenarios). the study by Jim showed soil temperature intensive greenbe roof remained at 10,With 50 and 90cm depth, which suggested that that thermal performanceof ofan green roof could obtained withrather only stable 10cm soil. regard to improve the accuracy of heat depth, which suggested that demand thermalestimations. performance of green roof could be obtained with only 10cm soil. With regard to © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
* Corresponding author. Tel.: +86 18817308597. * E-mail Corresponding Tel.: +86 18817308597. address:author. [email protected] Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the Applied Energy Symposium and Forum 2018: Low carbon cities Selection and peer-review under responsibility the scientific Selection peer-review responsibility of the scientific committee of the Applied Energy Symposium and Forum 2018: Low carbon cities and urbanand energy systems, under CUE2018. and urban energy systems, CUE2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the CUE2018-Applied Energy Symposium and Forum 2018: Low carbon cities and urban energy systems. 10.1016/j.egypro.2018.09.161
2
Yang He et al. / Energy Procedia 152 (2018) 384–389 Author name / Energy Procedia 00 (2018) 000–000
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the effect of evapotranspiration, Sun [3] further noted that the contribution depended on local climate and was mainly controlled by energy availability and water availability. In addition, many researchers found that the increase of the thermal capacity of green roofs compared to traditional roofs, if not controlled with insulation, may lead to higher cooling and heating loads [4,5]. Consequently, this paper proposes a new type of green roof system to improve thermal performance of green roof based on free cooling and heating effect of outdoor air in summer and winter respectively. Firstly, a coupled heat and moisture transfer model of common green roof (CGR) is briefly introduced according to our previous study. Secondly, the principle and structure of the new type of green roof (NGR) system is introduced. Then the thermal performance between CGR and NGR is compared in different climate zones of China, and the effect of substrate depth, leaf area index and initial water ratio on thermal performance of NGR is discussed. 2. Coupled heat and moisture transfer model of common green roof system The model developed in this paper is based on our previous study[6] which considered the following aspects: (1) multiple longwave and solar radiation in canopy layer of plant; (2) the effect of wind profile on turbulent transfer coefficient below and above canopy surface based on Soil-Vegetation-Atmosphere-Transfer (SVAT) theory by B.J. Choudhury[7]; (3) coupled heat and moisture transfer in soil based on the theory of Philip & De Vries[8]; The schematic of this model is illustrated in Fig. 1, and the detailed development and validation of this model could be seen in our previous paper[6]. Meteorological Data
Radiation distribution of plant canopy layer Long wave radiation
Energy balance of plant canopy layer Long wave radiation
Moisture transfer
Energy balance of substrate surface Heat conduction & Phase change
Coupled hygrothermal transfer process of substrate layer Heat conduction
Heat transfer process of structure layer Heat convection and longwave radiation
Indoor Space
Fig. 1. Schematic of hygrothermal transfer model of common green roof system
3. Principle and structure of new green roof In order to clarify the principle of new green roof, a common building model (Fig.2.) is developed in Trnsys, and above model is coupled with Type 56. The detailed thermal properties of the building model in different climate zones is shown in table 1, and the hygrothermal properties of green roof system is shown in table 2. The schedules of people, lighting and equipment are set according to the design standard for energy efficiency of public buildings in China (GB50189-2015). Table 1. Heat transfer coefficients of building envelope in different climate zones. Building envelope Harbin Beijing Shanghai （W/m2K） （W/m2K） （W/m2K） External wall 0.35 0.45 0.6 Roof 0.25 0.4 0.40 Window 2.3 2.5 3.0 Ceiling 0.35 0.45 0.7 Window-wall ratio 0.3 0.3 0.3
Kunming （W/m2K） 0.8 0.5 4.0 1.5 0.3
Guangzhou （W/m2K） 0.8 0.5 4.0 1.5 0.3
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Fig. 2. Schematics of building model Table 2. Parameters of green roof Parameters Plant height(cm) Minimum stomatal resistance(s/m)
Value 15(Summer), 2(Winter)
Value 0.3
5.5(Summer), 0.4(Winter)
Parameters Leaf albedo Thickness of structure layer(cm) Albedo of structure layer
600*1000
Emissivity of plant layer
0.9
Emissivity of structure layer
0.9
200
Leaf area index Substrate capacity(J/m3K) Substrate depth(cm)
4
Heat conductivity of substrate layer(W/m·K)
Emissivity of substrate layer
0.374 ∗ (𝜃𝜃)0.403
Water capacity of substrate layer(pa-1)
1.575 ∗ (𝜃𝜃 −4.35 )
Water permeability of substrate layer (m/s)
2.37 ∗ 10−5 ∗ (𝜃𝜃)24.42
24 0.2
0.95
Saturated water content ratio
0.63
Albedo of substrate layer
0.2
Common roof Green roof Outdoor air temperature
42 40 38 36 34
Tgr>Ta
32 30 28 26 24 22 20 00:00
Tgr>Ta 06:00
Tgr>Ta 12:00
Summer (1)
18:00
00:00
Outer surface temperature of structure layer (oC)
Outer surfacetemperature of structure layer (oC)
Taking the typical meteorological day of Shanghai as an example, based on above model, the temperature distributions of common green roof and common roof could be seen in Fig. 3. It indicates that, green roof system lowers the outer surface temperature of structure layer in the daytime for its shading and evapotranspiration effect. And green roof increases this temperature at night for its greater heat storage effect. In order to make full use of the thermal benefit of green roof, a new type of green roof with ventilation system is proposed. Just as illustrated in Fig. 4. Common roof Green roof Outdoor air temperature
20
15
Tgr>Ta
10
Tgr>Ta 5
Tgr>Ta
0
00:00
06:00
12:00
Winter (2)
Tgr>Ta 18:00
00:00
Fig. 3. Temperature comparison between green roof and common roof on the typical meteorological day of Shanghai: (1) Summer case; (2) Winter case.
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(1) (2) Fig. 4. Schematic of Common green roof and new green roof: (1) Common green roof; (2) New green roof
The controlling strategy of the fan in ventilation layer is as follows. In summer, when the temperature in ventilation layer is greater than outdoor air temperature at night, the fan is turned on, and cools the substrate and structure layer. When the temperature in ventilation layer is smaller than outdoor air temperature in the daytime, the fan is turned off, and the ventilation layer is closed. In winter, when the temperature in ventilation layer is smaller than outdoor air temperature, the fan is turned on and heats the substrate and structure layer. When the temperature in ventilation layer is greater than outdoor air temperature, the fan is turned off and ventilation layer is closed. Through free cooling and heating of outdoor air temperature, it is possible to avoid the negative effect of green roof. 4. Thermal performance comparison between common green roof and new green roof 2
Common Green Roof New Green Roof
26.8
Internal surface heat flux (W/m2)
Internal surface temperature (oC)
27.2
26.4
26.0
25.6
25.2
24.8
Harbin
Beijing
Shanghai
Kunming
0
-1
-2
-3
-4
Guanzhou
Common Green Roof New Green Roof
1
Harbin
Beijing
(1)
Common Green Roof New Green Roof
30 25 20 15 10 5 0
Kunming
Guanzhou
Kunming
Guanzhou
(2)
2
Internal surface heat flux (W/m2)
Internal surface temperatue (oC)
35
Shanghai
Summer
Summer
0
Common Green Roof New Green Roof
-2
-4
-6
-8 Harbin
Beijing
Shanghai
Winter
Kunming
Guanzhou
Harbin
Beijing
Shanghai
Winter
(3) (4) Fig. 5. Thermal performance comparison between CGR and NGR: (1) Average temperature in summer; (2) Average heat flux in summer; (3) Average temperature in winter; (4) Average heat flux in winter.
Based on the principle of NGR, thermal performance between CGR and NGR is compared on typical meteorological day of five climate zones in China, including severe cold region (Harbin), cold region (Beijing), hot
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summer and cold winter region (Shanghai), mild region(Kunming), hot summer and warm winter region (Guangzhou). And the average internal surface temperature and heat flux of CGR and NGR is presented, as is shown in Fig. 5. It can be found that NGR shows better thermal performance in all the five climate zones. In summer, NGR lowers the internal surface temperature and the heat flux into the room, while increases the internal surface temperature and lowers the heat flux out of the room. The maximum average temperature reduction of internal surface reaches 0.1℃ in summer, and the maximum average temperature rise of internal surface temperature reaches 0.4℃ in winter. 5. Thermal performance of NGR under different conditions
-0.065
-0.35
-0.070 -0.075
-0.40
Temperature difference (oC)
-0.30
-0.060
Heat flux difference (W/m2)
Temperature difference (oC)
-0.25
-0.055
-0.080 -0.085
4cm
6cm
8cm
10cm
Temperature difference Heat flux difference
0.29
Temperature difference Heat flux difference
-0.050
0.56
0.30
-0.20
-0.045
0.54 0.52
0.27
0.50
0.26
0.48
0.25
0.46
0.24
0.44
0.23
0.42
0.22
0.40
0.21
-0.45
12cm
0.28
4cm
6cm
8cm
10cm
Heat flux difference (W/m2)
-0.040
0.38
12cm
Soil thickness Winter of Shanghai
Soil thickness Summer of Shanghai
（1） （2） Fig. 6. The effect of soil thickness on relative cooling and heating effect of new green roof system compared with common green roof system: (1) Summer case; (2) Winter case.
-0.210
-0.040
-0.215
-0.041
-0.220
-0.042
-0.225
-0.043
-0.230
-0.044
1
2
3
4
5
6
Temperature difference Heat flux difference
0.27
Temperature difference (oC)
-0.039
Heat flux difference (W/m2)
Temperature difference (oC)
Temperature difference Heat flux difference
0.26
0.48
0.25
0.46
0.24
0.44
0.23
0.42
0.22
0.40
0.21
-0.235
0.50
0
1
2
3
4
5
Heat flux difference (W/m2)
-0.205
-0.038
0.38
Leaf area index Winter of Shanghai
Leaf area index Summer of Shanghai
（1） （2） Fig. 7. The effect of leaf area index on relative cooling and heating effect of new green roof system compared with common green roof system: (1) Summer case; (2) Winter case. 0.219
-0.236 -0.238
-0.0445 -0.240 -0.0450
-0.242
0.2
0.3
0.4
0.5
Initial water ratio Summer of Shanghai
0.6
0.7
-0.244
0.392
0.213 0.384 0.210 0.376
0.207 0.204
0.368
0.201
Heat flux difference (W/m2)
-0.234
-0.0440
-0.0455
Temperature difference Heat flux difference
0.216
Temperature difference (oC)
-0.0435
0.400
-0.232
Temperature difference Heat flux difference
Heat flux difference (W/m2)
Temperature difference (oC)
-0.0430
0.360 0.2
0.3
0.4
0.5
0.6
0.7
Initial water ratio Winter of Shanghai
（1） （2） Fig. 8. The effect of initial water ratio of substrate layer on relative cooling and heating effect of new green roof system compared with common green roof system: (1) Summer case; (2) Winter case.
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Thermal performance of green roof not only depends on the local climates, but also related to its hygrothermal properties. Taking the typical meteorological day data of Shanghai as an example, this study simulated thermal performance of NGR under different substrate layer depth, leaf area index and initial water content ratio. Increasing the soil layer thickness not only could improve the thermal capacity of green roof; but also enhance the thermal resistance. According to the simulation result (Fig. 6.), it can be found that, as soil thickness increases, the cooling effect of ventilation layer on green roof thermal performances becomes greater in summer, and the increase ratio gradually decreases. In winter, the heating effect of ventilation layer is not monotonous, and it firstly increases and then decreases. Thus, it can be deduced that, an optimal soil thickness exists in different climate conditions. A larger leaf area index means a greater shading and cooling effect from plant layer in the daytime, and also means less longwave radiation reaching on the substrate surface. According to Fig. 7., we can find that, the cooling and heating effect of ventilation layer both increase in summer and winter. A larger water content ratio in substrate layer benefits to improve the evapotranspiration intensity of green roof, and also reduces the thermal resistance of substrate layer. From Fig. 8, it’s shown that, the effect of initial water ratio on cooling effect of ventilation layer is not monotonous in summer, and it firstly decreases and then increases as initial water content ratio rises. And the heating effect of ventilation layer in winter improves initial water content ratio rises. 6. Conclusion Based on the analysis of temperature distribution of common green roof system, a new green roof system with ventilation layer is proposed making full use of free cooling and heating effect of outdoor air temperature. And the simulation result demonstrates that the new green roof system has a better cooling effect in summer and a better insulation effect in winter in five typical climate zones of China. The modelling results show that maximum average temperature reduction of internal surface reaches 0.1℃ on typical summer day, and the maximum average temperature rise of internal surface temperature reaches 0.4℃ on typical winter day. Parametric analysis has been carried out to identify the effect of three parameters on thermal performance of new green roof system. For thermal performance of new green roof system in Shanghai, the effect of soil thickness and initial water content ratio differs in summer and winter. For soil thickness, it has a positive effect in summer, while a nonmonotonic effect in winter. And for initial water content, its effect is opposite to soil thickness. And leaf area index has a positive effect both in summer and winter. Acknowledgements The study has been supported by the China National Key R&D Program 'Solutions to heating and cooling of buildings in the Yangtze River region' (Grant No. 2016YFC0700305-02) and the Chinese Scholarship Council through the "National high-level University Graduate" program. References [1] Schweitzer O, Erell E. Evaluation of the energy performance and irrigation requirements of extensive green roofs in a water-scarce Mediterranean climate. Energy and Buildings. 2014; 68:25-32. [2] Jim CY, Tsang SW. Biophysical properties and thermal performance of an intensive green roof. Building and Environment. 2011;46(6):12631274 [3] Sun T, Bou-Zeid E, Ni G-H. To irrigate or not to irrigate: analysis of green roof performance via a vertically-resolved hygrothermal model. Building and Environment. 2014; 73:127-137. [4] U. Berardi, A. GhaffarianHoseini, A. GhaffarianHoseini. State-of-the-art analysis of the environmental benefits of green roofs, Applied Energy 2014, 115: 411-428. [5] O. Saadatian, K. Sopian, E. Salleh, C.H. Lim, S. Riffat, E. Saadatian, A. Toudeshki, M.Y. Sulaiman. A review of energy aspects of green roofs, Renewable and Sus-tainable Energy Reviews. 2013, 23:155–168. [6] He Y, Yu H, Ozaki A, Dong N, Zheng S. Influence of plant and soil layer on energy balance and thermal performance of green roof system. Energy. 2018; 141:1285-99. [7] Choudhury B, Monteith J. A four‐layer model for the heat budget of homogeneous land surfaces. Q J Roy Meteor Soc. 1988;114(480):37398. [8] Philip J, De Vries D. Moisture movement in porous materials under temperature gradients. Eos, Transactions American Geophysical Union. 1957;38(2):222-32.