Investigating the effective factors on the reduction of energy consumption in residential buildings with green roofs

Investigating the effective factors on the reduction of energy consumption in residential buildings with green roofs

Renewable Energy 80 (2015) 595e603 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Inve...

913KB Sizes 7 Downloads 79 Views

Renewable Energy 80 (2015) 595e603

Contents lists available at ScienceDirect

Renewable Energy journal homepage: www.elsevier.com/locate/renene

Investigating the effective factors on the reduction of energy consumption in residential buildings with green roofs Amir Hossein Refahi, Hossein Talkhabi* Department of Mechanical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 October 2013 Accepted 12 February 2015 Available online 13 March 2015

Heating and cooling of residential and commercial buildings account for approximately 40 percent of world's total energy consumption [1]. This considerable amount of energy consumption have made scientist to search for every possible way to reduce it. Among the most effective approaches is the use of green roof for buildings. Although the positive effects of using such roofs are well proven, the amount of energy reduction which can be achieved by this method is another issue that this paper is trying to investigate. In this study, three different climates of Iran are chosen for the analysis. The results demonstrate that using green roof in Very Hot Dry (Bandar Abbas), Warm Dry (Isfahan), and Mixed Dry (Tabriz) climates causes energy consumption to decrease by approximately 8.5, 9.2 and 6.6 percent, respectively. Considering the reduction of energy consumption as the only desirable benefit of green roof, the payback period would be 25e57 years (depending on the climate). © 2015 Elsevier Ltd. All rights reserved.

Keywords: Green roof Energy consumption decrease LAI Soil depth Payback period

1. Introduction The ever-increasing human population of the world requires more construction and more energy supply. Furthermore, considering the fact that the resources of fossil fuels, which are considered to be primary sources of energy production in the world, are in limited supply; men have started to look for other new efficient methods of reducing energy demand, especially in the area of buildings energy consumption. One of these innovative methods is use of green roof. Green roof is defined as the use of vegetation covering on the roof of a building. Using green roof has many advantages such as decreasing Urban Heat Island Effects [2e6], improving stormwater management [7e9], better usage of space [10], decreasing the amount of dust particles in the air [11,12], decreasing noise pollution [13], providing natural habitat for animals and birds [14e16], and decreasing costs of repair and renovation [17e19]. However, the main advantages of green roof are decreasing a building energy consumption by increasing insulation thickness of the roof, providing a natural shade against direct rays of the sun, thus decreasing temperature of inner and outer surfaces of the roof as well as decreasing inside

* Corresponding author. Tel.: þ98 2173932686. E-mail addresses: [email protected] (A.H. Refahi), [email protected] (H. Talkhabi). http://dx.doi.org/10.1016/j.renene.2015.02.030 0960-1481/© 2015 Elsevier Ltd. All rights reserved.

temperature of the building, and therefore optimizing rate of energy consumption in the building [8,10,20e22]. Although using green roof is equally advantageous in all parts of the world, Europe and North America are the regions where green roofs are more advantageous [17,19]. As a general classification, green roofs are classified as Intensive and Extensive types. Extensive green roofs have thinner layer and less vegetation density (soil thickness is generally less than 15 cm) compared to intensive green roofs (soil thickness is generally more than 15 cm) [8,17,22e25]. Regarding the selection of green roof type for the building, it must be noted that extensive green roofs are better option to use in the buildings with roofs which cannot tolerate unusual (unexpected) loads [17]. Also, use of vegetation with high capacity for maintaining water is more desirable in these types of buildings. Ease of installation and plantation are also important [10]. Also, choosing the best type of vegetation and proper layers of soil depends on many variables including climate [26]. Positive effect of green roof on building inside temperature and consequently on the rate of energy consumption depends on many factors including Leaf Area Index (LAI) and soil layer depth; which are studied in the present paper. Furthermore, this article which studies three different climates in Iran, investigates the effect of the number of building floors on green roof benefit. Eventually, economic analysis and return on investment issues regarding the application of green roofs are investigated.

596

A.H. Refahi, H. Talkhabi / Renewable Energy 80 (2015) 595e603

Nomenclature g

Ce Chg Cpa C Cenergy e0 Ep F H i Is IsY Iir IirY L l LAI N p q q00sg

bulk transfer coefficient regarding the latent heat bulk transfer coefficient regarding the sensible heat air specific heat at constant pressure (J kg1 K1) bulk transfer coefficient energy rate (US$ kWh1) windless sensible heat correction factor (2.0 Wm2) total initial costs sum of energy terms (Wm2) sensible heat transferred (Wm2) interest rate shortwave radiation (Wm-2) incoming solar radiation (Wm-2) long wave radiation (Wm-2) incoming infrared radiation (Wm-2) latent heat (Wm-2) latent heat of evaporation (J kg1) leaf area index hours of system operation in a year payback period mixing ratio of air Conductive heat transfer in soil

2. Modeling 2.1. Energy modeling Modeling of green roof and analysis of building energy consumption can be carried out using the simplest and most common method e which is choosing the proper heat transfer coefficients and performing energy balance [20,27,28]e however, other more accurate methods, which consider more details, also exist [2,23,29e33]. The following items are among the details that must be considered for more accurate analysis [23]:  Short and long wave radiation into canopy and soil  Conductive heat transfer in soil  Convective heat transfer between the foliage and the air and between the air and soil in the canopy  Heat transfer by evapotranspiration in the foliage

qaf qf.sat 00 r Taf Tr Waf _ net W qf.sat 00 r Taf

ratio of air mixture adjacent to vegetation surface ratio of saturation mixture at leaf temperature foliage surface wetness factor temperature of air surrounding the leaf (Kelvin) temperature at the top level of the roof slab (Kelvin) wind velocity between air and leaf (in ms-1) performed work in kW ratio of saturation mixture at leaf temperature foliage surface wetness factor temperature of air surrounding the leaf (Kelvin)

Greek symbols Stefan-Boltzmann coefficient (5.69910-8 Wm2 K-4) ε emissivity factor raf density of air surrounding the leaf (kg m3) a shortwave albedo

s

Subscripts n Time step f foliage g ground

The best and most comprehensive method is the method used by Sailor [23], it is based on the calculations introduced by Frankenstein and Koenig (FASST method) [32]. This method is also the computational basis for commercial software EnergyPlus [34]. EnergyPlus is computational software for energy analysis and thermal load simulation in a building. This software, which has a non-user-friendly environment, uses an independent computational engine to run the thermal simulation. Therefore, DesignBuilder used in this study also functions on the same computational basis as EnergyPlus, but with a more user-friendly environment. A brief description of the method is presented here. Variables and symbols used in the formulas, all were given in nomenclature and their description was avoided within text. According to the following figure, thermal balance for canopy is as follows:

h   i s ε εg s   f f Tg4 Tf4 þHf þLf Ff ¼ sf Is 1af þεf Iir εf sTf4 þ ε1 (1) s f εf εg s ðTg4 ε1

where ½Is ð1  af Þ þ εf Iir  εf sTf4 ,  Tf4 Þ, Hf and Lf are shortwave solar radiation absorption by foliage, long wave radiation exchange between the sky and foliage, convective heat transfer between the air and foliage as sensible heat flux, and evapotranspiration in foliage surface as latent heat flux, respectively. ε1 can be written as:

ε1 ¼ εf þ εg þ εf εg

(2)

Sensible heat flux (convective heat) (Hf) is calculated by using the equation introduced by Deardoff [35]:

   Hf ¼ e0 þ 1:1LAIraf Cpa Cf Waf Taf  Tf

Fig. 1. Thermal balance for a green roof.

(3)

Latent heat flux between vegetation and the air adjacent to the vegetation surface (surface perspiration) is obtained from the following relation:

A.H. Refahi, H. Talkhabi / Renewable Energy 80 (2015) 595e603

  Lf ¼ LAIraf Cf lWaf r 00 qaf  qf :sat

(4)

Thermal balance of the soil can be divided into 5 sections:  Shortwave solar radiation absorption by the soil  Long wave radiation exchange between the sky and soil  Sensible heat flux by air inside the canopy (Convective heat flux)  Latent heat flux (evapotranspiration)  Conductive heat transfer in soil

h  i s ε εg s     f f Y Tg4  Tf4 Fg ¼ 1  sf IsY 1  ag þ εg Iir  εg sTg4 þ ε1 þ Hg þ Lg þ q00sg (5) Sensible heat flux (convective heat), Hg is obtained by the following equation:

   Hg ¼ e0 þ rag Cpa Chg Waf Taf  Tg

(6)

Latent heat flux (Lg) is also equal to the following relation:

  g Lg ¼ rag Ce lWaf qaf  qg

(7)

Boundary conditions for the canopy and soil areas are as follows:



Ts ð0Þ ¼ Tg Ts ðLÞ ¼ Tr

Ts(0) and Ts(L) are the temperatures of soil upper and lower sections. Equations (1) and (5) are 4th degree equations with unknown temperatures. These two equations can be linearized by the following equations:

h h

ðnþ1Þ Tf

ðnþ1Þ

Tg

i4 i4

¼

h

ðnÞ Tf

h

ðnÞ

¼ Tg

i4

i h i h ðnÞ 3 ðnþ1Þ ðnÞ Tf þ 4 Tf  Tf

i4

i h i h ðnÞ 3 ðnþ1Þ ðnÞ Tg þ 4 Tg  Tg

f

f

p X

_ net  N  Cenergy ð1 þ iÞpm W

(12)

m¼1

Interest rate is considered to be 0.25 percent yearly [41].

(9)

The superscript “n þ 1” refers to temperature for a time step after Tn. Eventually, the following system of two-equations in twounknowns is achieved: f

Installation cost of green roof depends on many factors including green roof type (extensive green roofs are less expensive than intensive green roofs), roof surface area (the more the roof surface area, the less would be the cost of green roof installation per square meter), roof height (cost for high-rise buildings increases due to the fact that transporting construction material and higher level equipment would be more costly), insulation and sealing systems, construction method, roofing contractor company, etc. [36]. Therefore, a constant value cannot be reported for the cost of green roof installation. It is considered that installation cost for extensive and intensive green roofs varies between 60 and 100 £/ m2 (90-150 US$/m2) and 100e140 £/m2 (150e210 US$/m2), respectively [37]. Beside other cost evaluations for green roof installation [16], EPA considers the values of 10 US$/ft2 (107.6 US$/ m2) and 25 US$/ft2 (269 US$/m2) for installation costs of extensive and intensive green roofs, respectively [38]. EPA also states that the maintenance cost of green roof for both intensive and extensive types is within the range of 0.75e1 US$/ft2 (8.07e10.76 US$/m2) [38]. Another important parameter for economic evaluation of green roof is fuel cost. The values reported by US Energy Information Administration are 0.121 US$/kWh and 4.21 US$/1000000BTU for electricity and natural gas, respectively [39]. One of the most important issues analyzed in this study is the payback period. However, having a green roof is initially more costly than the conventional roof; green roof has some economic advantages that result in better return of investment. In order to accurately calculate the payback period, total initial costs as well as all other benefits of green roof installation should be considered in full-scale or only part of a city. However, the aim of this study is not to provide a comprehensive economic evaluation on green roof; therefore, only return on investment resulted from energy consumption (electricity and natural gas) is investigated. The relation between the payback period and the initial costs is as follows [40]:

Ep ¼ (8)

597

C1 þ C2 Tg þ C3 Tf ¼ 0

(10)

C1g þ C2g Tg þ C3g Tf ¼ 0

(11)

NewtoneRaphson method is used to calculate the related equations. For more details about mathematical calculations including obtaining coefficients, refer to the reference [32]. Tg and Tf are obtained by solving these equations, and these two temperatures can be used to calculate heat transfer inside the building. Therefore, by having heat transfer inside the building, the effect of green roofs on building energy consumption can be investigated. 2.2. Economic analysis Economic analysis performed in this article investigates the costs required for installation and maintenance of green roof, the effect of green roof on costs regarding cooling and heating of the building, and also return on investment issues.

3. Case study The case study in this article is a three-story residential building. Building and roof surface areas are 637.2 and 212.4 square meters, respectively. The ratios of window-to-the wall for north, east, west and south walls are 0.5, 0, 0 and 0.8, respectively. This sample building is studied under three different climates in Iran including Bandar Abbas (very hot dry), Isfahan (warm dry) and Tabriz (mixed dry) denoting as 1B, 3B and 4B climates, respectively [42]. Wall material, type of activity and set temperatures are distinctively defined for each space using software fully comprehensive data base. Different systems are chosen for each climate knowing that HVAC systems vary with the climate. Split þ separate mechanical ventilation system is considered for Bandar Abbas; hot water radiator, mechanical supply þ extract ventilation system are considered for Tabriz and hot radiator heating, mixed mode with natural ventilation and local cooling system chosen for Isfahan. All the results obtained for sample building are compared to the results for a building having a conventional roof composed of 20 cm aerated concrete slab as well as 5 cm asphalt. Also, the roof of the case study consists of 20 cm of aerated concrete slab plus the related green roof installation. All designs and calculations are carried out using DesignBuilder software which is based on EnergyPlus computational basis. The calculations are performed for one

598

A.H. Refahi, H. Talkhabi / Renewable Energy 80 (2015) 595e603

year period and the three cities using standard ITMY (Iranian Typical Meteorological Year) meteorological files. Installation cost of green roof is considered to be 110 US$/m2. First, the best LAI factor and soil layer thickness are chosen for each city comparing the green roof with a conventional roof. Then, in order to choose the best climate, results for the three cities are compared to each other. Afterwards, the effect of the number of stories in a building on benefits of green roof is investigated for the best studied climate; and finally, a brief economic evaluation regarding the payback period presented. 4. Results and discussion First, maximum and mean inside temperature within one year and yearly energy consumption, including cooling and heating, for each climate are presented in odd-numbered tables. Then, the above mentioned results are obtained for a green roof building; first by modifying LAI factor and then by modifying soil layer thickness. Afterwards, the optimum results are presented in even-numbered tables as the optimum values for yearly temperature and building energy consumption. These results are presented in a comparative form between the studied roof and a conventional roof. In fact, the values shown in the figures are differential values between the studied green roof and a conventional roof. Positive values indicate the decrease in energy consumption, which is desirable, and negative values, which is undesirable, indicate increase in energy consumption.

Fig. 4. The effect of soil depth on indoor air temperature for a green roof in Bandar Abbas.

4.1. Very hot-dry climate (Bandar Abbas) According to Fig. 2, maximum and minimum temperature decrease occur at LAI ¼ 5 and LAI ¼ 0.5, respectively; therefore,

Fig. 2. The effect of LAI factor on indoor air temperature for a green roof in Bandar Abbas.

Fig. 5. The effect of soil depth on yearly energy consumption for a green roof in Bandar Abbas.

LAI ¼ 5 is clearly a better choice. And the same situation exists for analysis of building energy consumption (Fig. 3). For the cooling, the rate of decrease in energy consumption at LAI ¼ 5 is significantly high; however, the variation of LAI factor does not have a considerable effect on heating energy consumption. Hence, based on the above discussion, LAI ¼ 5 is chosen for green roof in Bandar Abbas climate Fig. 1. Soil depth does not have a significant effect on mean and maximum values of inside temperature (Fig. 4); but, the case for energy consumption is quite different. The best result for the cooling is obtained at thickness equal to 10 cm; however, the thickness has no considerable effect on heating energy consumption (Fig. 5). Hence, the thickness of 10 cm is chosen as the optimum thickness. Therefore, the best choices of LAI factor and soil thickness for a green roof installation in Bandar Abbas are 5 and 10 cm, respectively (Table 2) Table 1. 4.2. Warm-dry climate (Isfahan)

Fig. 3. The effect of LAI factor on yearly energy consumption for a green roof in Bandar Abbas.

The results for Isfahan climate is presented in the following table and figures: General results of LAI factor for Isfahan are the same as the results for Bandar Abbas. Regarding inside temperature, when LAI ¼ 5, the decrease in mean and maximum values is more significant than other values of LAI (Fig. 6). For the heating and cooling, the best results are obtained at LAI ¼ 0.5 and LAI ¼ 5, respectively. Between these two distinct results, LAI ¼ 5 is chosen due to its superiority over LAI ¼ 0.5 (Fig. 7) Table 3. Regarding soil depth, thickness value of 10 cm has a better result on desired temperature variation in Isfahan climate (Fig. 8). The results for the cooling and heating are quite the opposite of each other; the optimum thickness for the cooling is 10 cm, while for the

A.H. Refahi, H. Talkhabi / Renewable Energy 80 (2015) 595e603

599

Table 1 A building with a conventional roof in Bandar Abbas. Roof type

Conventional

Inside temperature ( C)

Energy consumption (kWh/m2)

Mean

Maximum

Heating

Cooling

Yearly

Heating

Cooling

Yearly

26.5

28.5

7.6

135.5

143.1

0.1

16.4

16.5

Energy cost (US$/m2)

Table 2 A building with a green roof in Bandar Abbas. Roof type

Inside temperature ( C)

Energy consumption (kWh/m2)

Mean

Maximum

Heating

Cooling

Yearly

Heating

Cooling

Yearly

Green

26.3

28.2

7.6

123.4

131

0.1

14.9

15

Fig. 6. The effect of LAI factor on indoor air temperature for a green roof in Isfahan.

Energy cost (US$/m2)

Fig. 9. The effect of soil depth on yearly energy consumption for a green roof in Isfahan.

heating is 30 cm. Thickness value of 10 cm is chosen; since, it has a better effect on yearly total energy consumption (Fig. 9). Therefore, the best choices of LAI factor and soil depth for a green roof installation in Isfahan are 5 and 10 cm, respectively (Table 4). 4.3. Mixed-dry climate (Tabriz)

Fig. 7. The effect of LAI factor on yearly energy consumption for a green roof in Isfahan.

Fig. 8. The effect of soil depth on indoor air temperature for a green roof in Isfahan.

In this section, the results for Tabriz climate are discussed Table 5. It can be seen that LAI factor of 5 has the best effect on inside temperature (Fig. 10). Similar to the results for Isfahan, the results regarding cooling and heating are the opposite of each other; hence, by considering total yearly energy consumption, LAI ¼ 5 is chosen (Fig. 11). As for the thickness, the values of 10 cm and 15 cm have the best effect on inside temperature (Fig. 12). Similar to the results for Isfahan climate, the results for Tabriz climate in the area of cooling and heating are the opposite of each other; i.e. the best result for the heating is obtained at thickness value equal to 30 cm, while it occurs at 10 cm for the cooling. And regarding total energy consumption, the optimum thickness is 25 cm (Fig. 13). In fact, Fig. 12 suggests using the lower thickness (10 and 15 cm), while Fig. 13 suggests using the higher thickness (20 and 25 cm); and this difference makes the optimum decision a challenging task. Since the installation costs of intensive and extensive green roofs are considerably different, therefore in order to make a precise decision, issues regarding decrease in energy consumption must be carefully examined. As can be seen in Fig. 14, maximum decrease rate in energy consumption is achieved at thickness equal to 10 cm and is mostly related to reduction in electricity usage. This result is different from the result of energy consumption shown in Fig. 13, and this is due to

600

A.H. Refahi, H. Talkhabi / Renewable Energy 80 (2015) 595e603

Table 3 A building with a conventional roof in Isfahan. Roof type

Conventional

Inside temperature ( C)

Energy consumption (kWh/m2)

Mean

Maximum

Heating

Cooling

Yearly

Heating

Cooling

Yearly

22.5

27.1

27

40.05

67.05

0.4

4.8

5.2

Energy cost (US$/m2)

Table 4 . A building with a green roof in Isfahan. Roof type

Inside temperature ( C)

Energy consumption (kWh/m2)

Mean

Maximum

Heating

Cooling

Yearly

Heating

Cooling

Yearly

Green

22

26.65

28.6

32.3

60.9

0.4

3.9

4.3

Fig. 10. The effect of LAI factor on indoor air temperature for a green roof in Tabriz.

Energy cost (US$/m2)

Fig. 13. The effect of soil depth on yearly energy consumption for a green roof in Tabriz.

difference of energy costs between gas and electricity and their different roles in decreasing the rate of yearly building energy consumption. Considering the discussion above, the best choices of LAI factor and soil depth for a green roof installation in Tabriz climate are 5 and 15 cm, respectively Table 6. The optimum characteristics of green roof for Isfahan and Bandar Abbas climates are similar to each other; however, Tabriz needs more soil thickness due to its cold climate during about half of the year. 4.4. Comparing the results

Fig. 11. The effect of LAI factor on yearly energy consumption for a green roof in Tabriz.

Fig. 12. The effect of soil depth on indoor air temperature for a green roof in Tabriz.

According to Fig. 15, Isfahan, Tabriz and Bandar Abbas hold the best results regarding mean inside temperature, respectively. The same conclusion goes for the results obtained for maximum inside temperature. Tabriz and Isfahan climates have the optimum benefit for using green roof in the area of heating and cooling, respectively; for Bandar Abbas climate, no considerable advantage has been seen in this regard. It should also be noted that, although there is 5.6 percent increase (equal to 1.6 kWh/m2) in the area of heating for Isfahan climate, this negative effect is counterbalanced by good result for its cooling; hence, the overall effect of green roof for Isfahan climate is positive. Concerning the cooling, Tabriz with 4.4 kWh/m2, Isfahan with 7.5 kWh/m2 and Bandar Abbas with 12.1 kWh/m2 hold the optimum results, respectively. Firstly, however the absolute positive effect of green roof in the area of cooling is on the inside temperature and consequently on energy consumption, the results for the heating are not consistent and mostly depend on the climate. Secondly, the results in terms of “rate of reduction in energy consumption” and “percentage rate of reduction in energy consumption” are different; namely, despite the fact that 9.2% energy consumption reduction in Isfahan, 8.5% in Bandar Abbas and 6.6% in Tabriz have been observed but we should not

A.H. Refahi, H. Talkhabi / Renewable Energy 80 (2015) 595e603

601

Table 5 A building with a conventional roof in Tabriz. Roof type

Conventional

Inside temperature ( C)

Energy consumption (kWh/m2)

Mean

Maximum

Heating

Cooling

Yearly

Heating

Cooling

Yearly

20.8

26.1

63.4

21.4

84.8

0.9

2.6

3.5

Energy cost (US$/m2)

Table 6 A building with a green roof in Tabriz. Roof type

Inside temperature ( C)

Energy consumption (kWh/m2)

Mean

Maximum

Heating

Cooling

Yearly

Heating

Cooling

Yearly

Green

20.6

25.7

62.7

16.5

79.2

0.9

2

2.9

Energy cost (US$/m2)

4.5. The effect of the number of floor

Fig. 14. The effect of soil depth on cost of yearly energy consumption in a green roof in Tabriz.

forget that 9.2% in Isfahan means 6.15 kWh, 8.5 in Bandar Abbas means 12.1 kWh and 6.6% in Tabriz means 5.6 kWh. So, if basis for selecting climate to efficient use of green roof is only “percentage rate of reduction in energy consumption”; warm-dry climate, very hot-dry climate and mixed-dry climate are ranked, respectively. But if “rate of reduction in energy consumption” is basis for selection; very hot-dry climate, warm-dry climate and mixed-dry climate are respectively more important in selecting proper climate to conduct green roof. Of course, the latter seems more logical. The results for yearly energy consumption are quite the same and decrease of 6e10 percent is seen for all three studied climates.

In this section, the relation between the number of stories in a building and the effects of green roof on energy and temperature parameters are studied by comparing the results between a single floor green roof building and a conventional roof building. This comparison is also performed for two and three-story building, then the percentages of these advantages are compared with each other. All the previous parameters including temperature, energy consumption and energy cost are obtained for single-storey and twostorey buildings and then these results compared with three-storey building. In this section, the analysis is only performed for Tabriz climate (4B). As seen before, using green roof instead of conventional roof decreases almost all of the parameters in a desirable manner. According to the figure, the positive effect of using green roof is more in two-storey building and less in single-storey building (Fig. 16). For instance, when using a green roof in a three-storey building, there is 22.9 percent decrease in cooling energy consumption; under the same situation, this value is 29 and 46.3 percent for two-storey and single-storey buildings, respectively. As another example, decrease in yearly energy cost for three-storey, two-storey and single-storey buildings is 17.1%, 21.9% and 34%, respectively. 4.6. Return on investment The results for the payback period, which is only limited to building yearly energy cost spent for the cooling and heating, are presented in Table 7. Installation cost of green roof for the studied building is considered to be 23,364 US$. Although values for the payback periods are somewhat large and unreasonable and might make the green roof looks economically unjustifiable, the reality is

Fig. 15. The effect of green roof on indoor air temperature, energy consumption and energy cost for Bandar Abbas, Isfahan and Tabriz.

602

A.H. Refahi, H. Talkhabi / Renewable Energy 80 (2015) 595e603

Fig. 16. The effect of green roof on indoor air temperature, energy consumption and energy costs for single-storey, two-storey and three-storey buildings in Tabriz.

Table 7 Desirable benefits gained because of using green roof. City

Decrease in yearly energy consumption (kWh)

Isfahan 3918.8 Bandar 7710.1 Abbas Tabriz 3568.2

Decrease in yearly energy cost (US$)

Payback period (years)

582.54 932.92

39 25

384.35

57

that this study only considers decrease in costs of energy consumption; when all the benefits and desirable effects of green roof installation are taken into consideration, the value of the payback periods decreases and makes the green roof more economically justifiable. On the other hand, economic analysis regarding other advantages of green roofs requires a study on a number of buildings (for example, all the buildings in a city or only buildings located within a specific district in a city); therefore, it can be argued that application of the green roofs is economically justifiable only when its use become widespread. For an instance, proper return on investment cannot be expected when applying green roof in just a few buildings for an area of 10 ha. Production cost, fuel cost, and interest rate are three factors affecting the payback period. In other words, decreasing production cost and increasing both fuel cost and interest rate cause a decrease in the payback period. Clark [43,44] has done an investigation on Chicago and Detroit and reported that Net Present Value for a green roof is less than a conventional roof at end of the 20th year and all other costs are compensated after 11 years. Groundwork Sheffield [45] also claims that, only with green roof even without buying air conditioning equipment, a building in Frankfurt, Germany, is able to respond to all its air conditioning needs; and by savings done from areas of fuel consumption, equipment purchase and maintenance, the payback period decreases to 2e3 years. 5. Conclusions The following results can be directly extracted from the present study:  Proper selection of green roof type depends on climate;  Using green roof causes a sufficient decrease in energy consumption and consequently in the related energy costs;

 The decrease in energy consumption is more for higher values of LAI factor;  Although decrease in energy consumption can be seen in all thicknesses, the decrease in lower thickness values is more;  Increasing the thickness of soil layer results in increase in the required energy for the cooling and decrease in the required energy for the heating;  The desirable and positive effects of green roof on energy consumption are better for the buildings with fewer floors;  When performing an economic analysis of green roof, all the advantages and disadvantages must be included;  Decreasing energy consumption cannot be the only factor that economically justify the use of green roofs. References [1] Manzoni D, Bollati F. Smart grid applications for energy efficiency. Altren Ital Technol Rev 2011;7:5e14. [2] Wong NH, Chen Y, Ong C, Sia A. Investigation of thermal benefits of rooftop garden in the tropical environment. Build Environ 2003;38:261e70. [3] Takebayashi H, Moriyama M. Surface heat budget on green roof and high reflection roof for mitigation of urban heat island. Build Environ 2007;42: 2971e9. [4] Gomez F, Gaja E, Reig A. Vegetation and climatic changes in a city. Ecol Eng 1998;10:355e60. [5] Alexandri E, Jones P. Temperature decreases in an urban canyon due to green walls and green roofs in diverse climates. Build Environ 2008;43:480e93. [6] Banting D, et al. Report on the environmental benefits and costs of green roof technology for the City of Toronto. 2005. [7] Mentens J, Raes D, Hermy M. Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landsc Urban Plan 2006;77: 217e26. [8] Fioretti R, Palla A, Lanza LG, Principi P. Green roof energy and water related performance in the Mediterranean climate. Build Environ 2010;45: 1890e904. [9] Stovin V, Dunnett N, Hallam A. Green roofs-getting sustainable drainage off the ground. In: 6th International Conference of Sustainable Techniques and Strategies in urban water management (Novatech 2007), Lyon, France; 2007. p. 11e8. [10] Keshtkar Ghalati A, Ansari M. Green roof study and design in accordance with eco-condition. Department of Architecture Faculty of Art and Architecture Tarbiat Modares University; 2009. [11] Yang J, Yu Q, Gong P. Quantifying air pollution removal by green roofs in Chicago. Atmos Environ 2008;42:7266e73. [12] Li J-F, Wai OWH, Li YS, Zhan J-M, Ho YA, Li J, et al. Effect of green roof on ambient CO2 concentration. Build Environ 2010;45:2644e51. [13] Van Renterghem T, Botteldooren D. Insitu measurements of sound propagating over extensive green roofs. Build Environ 2011;46:729e38. € ning M. Soil formation on green roofs and its contribution to [14] Schrader S, Bo urban biodiversity with emphasis on Collembolans. Pedobiologia 2006;50: 347e56.

A.H. Refahi, H. Talkhabi / Renewable Energy 80 (2015) 595e603 [15] Brenneisen S. Space for urban wildlife: designing green roofs as Habitats in Switzerland. Urban Habitat 2006;4:27e36. [16] Koehler M. Plant survival research and biodiversity: lessons from Europe. In: Greening Rooftops for Sustainable Communities, Chicago; 2003. p. 313e22. [17] Castleton HF, Stovin V, Beck SBM, Davison JB. Green roofs; building energy savings and the potential for retrofit. Energy Build 2010;42:1582e91. [18] Wilkinson S, Reed R. Green roof retrofit potential in the central business district. Prop Manag 2009;27(5):284e301. [19] Drivers Jonas Deloitte/Gary Grant. Greater manchester green roof programme e guidance document. 2009. [20] Santamouris M, Pavlou C, Doukas P, Mihalakakou G, Synnefa A, Hatzibiros A, et al. Investigating and analysing the energy and environmental performance of an experimental green roof system installed in a nursery school building in Athens, Greece. Energy 2007;32:1781e8. [21] Eumorfopoulou E, Aravantinos D. The contribution of a planted roof to the thermal protection of buildings in Greece. Energy Build 1998;27:29e36. [22] Lazzarin RM, Castellotti F, Busato F. Experimental measurements and numerical modelling of a green roof. Energy Build 2005;37:1260e7. [23] Sailor Dj. A green roof model for building energy simulation programs. Energy Build 2008;40:1466e78. [24] Spala A, Bagiorgas HS, Assimakopoulos MN, Kalavrouziotis J, Matthopoulos D, Mihalakakou G. On the green roof system. Selection, state of the art and energy potential investigation of a system installed in an office building in Athens, Greece. Renew Energy 2008;33:173e7. [25] Kosareo L, Ries R. Comparative environmental life cycle assessment of green roofs. Build Environ 2007;42:2606e13. [26] Jaffal I, Ouldboukhitine SE, Belarabi R. A comprehensive study of the impact of green roofs on building energy performance. Renew Energy 2012;43: 157e64. [27] Wong NH, Cheong DKW, Yan H, Soh J, Ong CL, Sia A. The effects of rooftop garden on energy consumption of a commercial building in Singapore. Energy Build 2003;35:353e64. [28] Niachou A, Papakonstantinou K, Santamouris M, Tsangrassoulis A, Mihalakakou G. Analysis of the green roof thermal properties and investigation of its energy performance. Energy Build 2001;33:719e29.

603

[29] Del Barrio EP. Analysis of the green roofs cooling potential in buildings. Energy Build 1998;27:179e93. [30] Takakura T, Kitade S, Goto E. Cooling effect of greenery cover over a building. Energy Build 2000;31:1e6. [31] Kumar R, Kaushik SC. Performance evaluation of green roof and shading for thermal protection of buildings. Build Environ 2005;40:1505e11. [32] Frankenstein S, Koenig G. FASST vegetation models. Technical Report TR04e25. Hanover, NH: US Army Engineer Research and Development Center, Cold Regions Research and Engineering Laboratory; 2004. [33] Theodosiou TG. Summer period analysis of the performance of a planted roof as a passive cooling technique. Energy Build 2003;35:909e17. [34] US Department of Energy. EnergyPlus documentation. 2012. p. 122e32. [35] Deardoff JW. Efficient prediction of ground surface temperature and moisture, with inclusion of a layer of vegetation. J Geophys Res 1978;83:1889e903. [36] Green roofs: benefits and cost implications. LivingRoofs.org; 2004. [37] The Green Roof Centre. Cost of green roof. 26.02.2013. Available from: http:// www.thegreenroofcentre.co.uk/pages/faq.html. [38] United States Environmental Protection Agency. 23.03.2013, Available from: http://www.epa.gov/heatisld/mitigation/greenroofs.htm. [39] U.S.Energy Information Administration. 26.02.2013], Available from: http:// www.eia.gov/forecasts/steo/report/prices.cfm. [40] Shirazi A, Aminyavari M, Najafi B, Rinaldi F, Razaghi M. Thermal-economicenvironmental analysis and multi-objective optimization of an internalreforming solid oxide fuel cell-gas turbine hybrid system. Hydrogen Energy 2012;37:19111e24. [41] Trading Economics. 26.02.2013, Available from:: http://www. tradingeconomics.com/united-states/interest-rate. [42] ANSI/ASHRAE/IESNA. Building Envelope Climate Criteria. In: 90.1-2007 Normative Appendix B. [43] Clark C, Adriaens P, Brian Talbot F. Green roof valuation: a probabilistic economic analysis of environmental benefits. University of Michigan; 2006. [44] Carter T, Keeler A. Life-cycle cost-benefit analysis of extensive vegetated roof systems. Environ Manag 2008;87:350e63. [45] Green roof developer's guide. Groundwork Sheffield: Lifeþ, Green Roof Centre; 2011.