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Economic and Environmental Evaluation Model for Selecting the Optimum Design of Green Roof Systems in Elementary Schools JiMin Kim,† TaeHoon Hong,*,‡ and Choong-Wan Koo† †

Graduate Research Assistant, Department of Architectural Engineering, Yonsei University, Seoul, 120-749, Korea Associate Professor, Department of Architectural Engineering, Yonsei University, Seoul, 120-749, Korea

‡

S Supporting Information *

ABSTRACT: Green-roof systems offer various benefits to man and nature, such as establishing ecological environments, improving landscape and air quality, and offering pleasant living environments. This study aimed to develop an optimal-scenario selection model that considers both the economic and the environmental effect in applying GRSs to educational facilities. The following process was carried out: (i) 15 GRSs scenarios were established by combining three soil and five plant types and (ii) the results of the life cycle CO2 analyses with the GRSs scenarios were converted to an economic value using certified emission reductions (CERs) carbon credits. Life cycle cost (LCC) analyses were performed based on these results. The results showed that when considering only the currently realized economic value, the conventional roof system is superior to the GRSs. However, the LCC analysis that included the environmental value, revealed that compared to the conventional roof system, the following six GRSs scenarios are superior (cost reduction; reduction ratio; in descending order): scenarios 13 ($195,229; 11.0%), 3 ($188,178; 10.6%), 8 ($181,558; 10.3%), 12 ($130,464; 7.4%), 2 ($124,566; 7.0%), and 7 ($113,931; 6.4%). Although the effect is relatively small in terms of cost reduction, environmental value attributes cannot be ignored in terms of the reduction ratio.

1. INTRODUCTION The recent increase in the amount of greenhouse gases (GHGs), including CO2, has led to the emergence of global warming as a serious environmental issue. Accordingly, many countries have made an effort to proactively cope with the climate changes, and to reduce their GHGs emission.1,2 The South Korean government is promoting a policy that encourages its people to voluntarily participate in reducing their GHGs emissions through the carbon mileage system; Korea Environment Corporation, part of the Ministry of Environment, gives carbon points to people depending on the reduction amount of GHGs,3 and provides technical support for the introduction of new renewable energy and energy-saving measures (ESMs). With the increase in the urban dwellers’ demand for a pleasant life, much effort has been actively made in establishing a green environment in the urban landscape.4 However, most large cities have a high density of buildings and lack green spaces, which contributes to the urban heat island phenomenon5−7 and subsequently inconvenience urban dwellers. In particular, it is almost impossible to secure land for creating parks and green areas in downtown Seoul, and over 75% of such areas are concentrated outside of the city center. Thus, creating parks and green areas where people work and live is difficult.8 In terms of the importance of green spaces, according to a report by the National Institute of Metrological Research, the © 2012 American Chemical Society

CO2 emissions from fossil fuels in the U.S. are 5,794 T-CO2/ year, which is higher than that in China (5,749 T-CO2/year). However, the net CO2 emissions in the U.S. is 4,264 T-CO2/ year due to the significant CO2 absorption by forests; thus, the net CO2 emissions in the U.S. are less than those in China (5,075 T-CO2/year) (Supporting Information (SI) in Table S1).9 This relationship shows that the amount of green spaces can affect the net CO2 emissions rankings. Furthermore, there has been a growing interest in green-roof systems (GRSs), an eco-friendly technology that allows for green spaces inside cities, and improves the sustainability of buildings. South Korea has been promoting Green-School Project as part of the Green New-Deal Program, which aims to combine green growth and job creation from 2009 to 2012. This project conducts full-scale repair and replacement work in existing elementary and middle schools that are highly decrepit, by implementing eco-friendly technologies, including GRSs. Additionally, since 2002, Seoul City has been conducting a series of support programs that encourage the introduction of GRSs to buildings within the city. The Green-Roof Project was recently started and included on 29,827 m2 of roofs on 39 large Received: Revised: Accepted: Published: 8475

December 8, 2011 April 16, 2012 July 9, 2012 July 9, 2012 dx.doi.org/10.1021/es2043855 | Environ. Sci. Technol. 2012, 46, 8475−8483

Environmental Science & Technology

Article

Figure 1. System boundary conditions for the economic and environmental evaluation model.

conventional roof systems and GRSs showed that the environmental impact decreased by 1.0−5.3%.12 Second, the previous studies also focused on the LCC analysis of GRSs.13,14 The studies reflected the cost-saving effect generated in the operation and maintenance phase together with the initial cost. For example, the studies included the rainwater storage effect,15−17 the energy-saving effect,18−20 and the CO2 emission reduction effect, among others. As a result, compared to the conventional roof systems, GRSs reduced the net present value (NPV) by 30−40%.21,22 Niu,21 however, did not consider the maintenance and repair costs. Third, the previous studies analyzed the air purification effect of GRSs.23−26 To evaluate the level of air pollution removal using GRSs in Chicago, the dry deposition model was implemented. By applying 19.8-ha GRSs, a total of 1.675 kg of air pollutants can be removed in a year.27 Meanwhile, plants absorb CO2 during the day and reduce the CO2 concentration in the air. When the weather is good, such effect increases, thus reducing the CO2 concentration by 2% more.28 Another study was conducted to quantify the carbon storage potential of extensive green roofs. The entire extensive green roof system sequestered 375 g C·m−2 in above and below ground biomass and substrate organic matter.29 Fourth, the previous studies performed research on thermalperformance improvement. A study on the GRSs in Singapore showed that up to 60% of heat gain could be blocked through the GRSs.30 When the temperature differences incurred by the use of the conventional roof systems and the GRSs in nine

buildings near Mt. Namsan, located at the heart of Seoul. Seoul City has also announced that it plans to support 100% of the construction projects for public buildings, and 70% of the cost for private buildings.10 As such, GRSs are drawing a significant amount interest not only because they provide more green spaces, which is lacking in the city, but also because they provide ecosystem and landscape improvements, which can cope with the demand for GHGs reduction, and with climate change. Many studies have been conducted both in South Korea and abroad to verify these effects of GRSs. Compared to the conventional roof system, the effects of GRSs were evaluated in the previous studies, largely from four viewpoints: (i) the study of the life cycle assessment (LCA) of the materials used in conventional roof systems and GRSs; (ii) the study of life cycle CO2 (LCCO2) and life cycle cost (LCC), which convert the energy and GHGs emission reduction effects of the introduction of GRSs into economic values; (iii) the study of the air purification effect of plants due to the introduction of GRSs; and (iv) the study of the thermalperformance improvement due to the introduction of GRSs. First, the previous studies focused on LCA analyses of GRSs. Compared to conventional roof systems, GRSs can reduce the environmental impact of roof systems by changing the type of roof materials, reducing the amount of materials, reducing the amount of energy used by buildings and extending the life cycles of buildings.11 The results of the LCA assessment of the 8476

dx.doi.org/10.1021/es2043855 | Environ. Sci. Technol. 2012, 46, 8475−8483


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Table 1. Properties of the Selected Green Roof Systems green roof systems (GRSs) vegetation type type

soil type

sedum

grass

low shrub Ardisia crenata

shrub

tree

sprunum

Kamtsch aticum

azalea

Zelkova

CO2 absorption amount (kgCO2/m2/year) height of plant (m)

3.1

5.7

4.1

4.9

8.7

0.1

0.15

1

2

4

leaf area index

2.1

2.5

2.64

3.5

5.12

vegetation

type

natural soil

improved soil

artificial soil

mixtures

natural soil

natural soil + perlite or peat moss

vermiculite, perlite and peat moss

conductivity of dry soil (W/m-k) density of dry soil (kg/m3) specific heat of dry soil (J/kg-k)

0.5

0.41

0.2

1350

755

300

1674

1000

800

following characteristics to accommodate the plant’s sunlightblocking effect and the soil’s insulation effect. First, different foliage densities and leaf-area indices (LAI) resulted from different plant types.33 As the LAI values of the plants changed, the sunlight-blocking effects changed accordingly. Moreover, the thickness of the soil changed depending on the plant type, which resulted in a difference in the insulation effect.34,35 On the basis of the suggestion of the Seoul Metropolitan Government (2008), the plant types were categorized into sedum, grass, low shrub, shrub and tree in this study. Second, the different soil types caused a difference in the insulation effect and methods of construction.36,37 As suggested by Marico (2003), in this study, soil was categorized into three types: natural, improved, and artificial soil.38 Table 1 shows the properties of three types of soil. By combining these three soil and five plant types, 15 GRSs types were established, whose soil depths are shown in Supporting Information Table S2. On the basis of the 20 cm soil depth, intensive and extensive systems were categorized.39 For each of the 15 established GRSs types, an appropriate plant was selected. The following selection criteria were used: (i) plants that could be vegetated in South Korea; (ii) plants with excellent CO2 emission reduction effects within the identical type; and (iii) plants with low initial-investment and operation and maintenance costs within the identical type. A preliminary survey of GRSs cases in South Korea and abroad was conducted, and five representative plants that can vegetate in Gyeonggi, South Korea were selected. First, the amount of CO2 absorption for the trees and shrubs were expressed in “kg CO2/tree.” According to the standards proposed by the Landscape Ordinance of the Korean Building Code, 0.2 trees per 1 m2 and 1 shrub per 1 m2 were set. As such, a representative tree was the Zelkova (8.7 kg CO2/tree), and a representative shrub was the Azalea (4.9 kg CO2/tree).40,41 Meanwhile, representative low shrub, grass, and sedum were selected based on the following two criteria: (i) out of the 80 GRSs cases in South Korea, the CO2 absorption for the 16 plants with the highest frequency were analyzed;42 and (ii) research results from Japan, which has climate conditions similar to those of South Korea, were analyzed.36 Through these analysis processes, Ardisia crenata (4.1 kg CO2/m2/year), Kamtschaticum (5.7 kg CO2/m2/year), and Sprunum (3.1 kg CO2/m2/year) were selected as the representative low shrub, grass, and sedum, respectively. Table 1 shows properties of three soil types and five plant types included in the selected green roof systems. To calculate the amount of CO2 absorption for each GRS plant, the CO2 absorption values shown in Table 1 were

cities were compared, the roof temperature during the day in Moscow was lowered by 9.1 K on average, and the largest temperature decrease was 12.8 K.31 Previous studies targeted either economic evaluation or environmental impact for the introduction of GRSs, that is, it was shown that a comprehensive, integrated approach considering a variety of perspectives did not include sufficient LCC and LCCO2 analysis due to the following omissions:4,21,23 (i) CO2 emissions in the LCA analysis (i.e., the embedded CO2 associated with materials); (ii) the CO2 absorption capacity of the plants; and (iii) certified emission reductions (CERs) with which CO2 emissions can be converted into economic values.. Meanwhile, Hong et al. 32 performed LCC and LCCO2 analyses of GRSs (natural soil + grass) with 4 different ESMs (i.e., external insulation, exterior blinds, double low-e glazing, and light emitting diodes (LED)) in an elementary school. In this study, the LCCO2 analysis included the effects from the CO 2 emission reductions on energy savings and CO 2 absorption by the plants from the GRSs. According to the results of the LCC and LCCO2 analyses, the scenarios that include GRSs, LED, and double low-e glazing, were shown to be cost-effective, especially the scenario with GRSs and LED, which was selected as the optimal scenario. In this study, more comprehensive research, focused on GRSs, will be conducted as a follow-up to the research of Hong et al.32 This study aims to develop an economic- and environmental-evaluation model to select an optimal GRS design alternative based on the characteristics of the elementary school facilities. To assess the effects of the introduction of GRSs, the LCA, LCCO2, and LCC were comprehensively analyzed in the entire process, from the initial phase of the GRSs use to the GRSs disposal phase. Furthermore, the optimal GRS for the elementary school facilities was selected by reviewing the GRSs scenarios in various combinations based on the various plant and soil types. Figure 1 shows the system boundary conditions for analysis of this paper.

2. MATERIALS AND METHODS In introducing GRSs, this study aimed to develop a model that is capable of selecting an optimum GRSs design alternative that considers both the economic and environmental effects. 2.1. Type of Green Roof Systems and Its Characteristics. GRSs mainly use three soil types (natural, improved, and artificial soil) and five plant types (sedum, grass, low shrub, shrub, and tree). By combining these soils and plants, 15 types of GRSs were established (Supporting Information Figure S1). Additionally, the amount of CO2 absorption for the five plant types was reviewed by unit area. Each GRSs type showed the 8477



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applied to the 1,466 m2 the roof area of “Y” Elementary School, as a target facility that was used in Hong et al.,32 resulting in 4.5t CO2 equivalent per year(CO2 e/y) for sedum, 8.3t CO2 e/ y for grass, 6.0t CO2 e/y for low shrub, 7.2t CO2 e/y for shrub, and 12.7t CO2 e/y for tree. These CO2 absorption values were converted into economic values based on the profit that would result from the sale of equivalent carbon credits called certif ied emission reductions (CERs, $4.80/t CO2 e) and were used for the LCC analysis.43 2.2. Life Cycle Assessment Analysis. By applying LCA techniques to 16 types of GRSs (one conventional roof and 15 types of GRSs), the CO2 emissions for the entire process, ranging from the production to disposal phases, were analyzed. According to the Implementing Regulations in Housing Act in Korea (Appendix 5), the replacement cycle of the roof waterproofing system should occur every 15 years.44 Thus, the service life of a conventional roof was assumed to be 15 years. Meanwhile, the service life of a GRS is generally 1.5 to 3 times that of a conventional roof and is thus assumed to be 40 years 45”. The LCI database (DB) that was used in this study was developed by the Ministry of Environment (ME), Ministry of Knowledge Economy (MKE), and Ministry of Land, Transport, and Maritime Affairs (MLTM).46−48 The LCI DB is a categorized database of the environmental impacts associated with all of the stages of a product’s life as “a cradle to grave approach” (i.e., the impacts of the product are tracked from raw material extraction through materials processing, manufacturing, distribution, use, repair and maintenance, and disposal or recycling). This paper considered CO2 emissions as the primary cause of global warming. To perform LCA, it is necessary to set the materials for the roof. First, in the production and construction phases of the conventional roof, the materials used in the target facility, “Y” Elementary School, were selected. Also selected were expended polystyrene for insulation, EPDM(ethylene propylene diene Mclass) rubber waterproofing sheet for waterproofing, and press concrete. Second, as “Y” Elementary School does not have GRSs, materials used in the general GRSs were assumed, and identical waterproofing and roof barrier materials were applied in the production and construction phases, regardless of the GRSs scenario. For waterproofing, EPDM rubber waterproofing sheet was used, and for the roof barrier, polyethylene plastic, which can prevent cracks on the waterproofing layer as roots of the plants penetrate into it, was used. Moreover, the thickness and CO2 absorption of soil were reviewed based on the plant and soil types. The LCAs for all of the soil types were conducted with the same LCI data due to limitations of the LCI database. However, depending on the amount of soil for a specific type of GRS, the CO2 emissions were calculated differently. Third, in the operation and maintenance phases of the GRSs, minimum maintenance, including regular checkups and repairs, is required for the growth and development of the plants. The plant and soil types differed depending on the GRS types, resulting in different operation and maintenance activities. However, in conducting the LCA, the CO2 emissions for each material were evaluated. Thus, common activities were examined (i.e., fertilizers, irrigation systems, and weeders) in the operation and maintenance phases. Based on the above standards, the materials by the conventional roof system and GRSs were selected, and the mass and CO2 emission amount of each material was reviewed. The quantity of materials was calculated based on the roof area

(1,466 m2) of “Y” Elementary School (refer to Supporting Information Table S3). 2.3. System Boundary Conditions for Analyzing LCC and LCCO2. For LCC and LCCO2 analyses, various elements should be set according to the characteristics of the analysis. The six key elements that must be set before LCC analysis are as follows: (i) the analysis approach; (ii) a realistic discount rate; (iii) the inflation and increase rate; (iv) the analysis period; (v) the starting point of analysis; and (vi) the significant cost of ownership.49 As the basic assumption for the LCC and LCCO2 analysis, the realistic discount rate was calculated by considering the nominal interest rate and the inflation rate. On the basis of the data provided by the Bank of Korea Economic Statistics System (ECOS) and the Korean Statistical Information Service (KOSIS), the real discount rate on the inflation rate (3.30%), the electricity price growth rate (0.66%), the gas price growth rate (0.11%), and the CO2 emission trading price growth rate (2.66%) were calculated.50,51 For the LCC and LCCO2 analysis, the initial-investment, operation and maintenance, and demolition costs should be considered.52 However, this study assumed that the waste disposal costs and salvage value, constituents of the demolition cost, offset each other; thus, the demolition cost was excluded from the analysis. Thus, only the initial investment and operation and maintenance costs were used to perform the LCC analysis. Considering that the service life of steelreinforced concrete, proposed by the standard service life and service life scope chart under the Enforcement Regulations on Corporate Income Tax Act, is 40 years, the LCC analysis period was set at 40 years.53 In this study, the initial-investment and operation and maintenance costs of 15 GRSs scenarios were examined based on the combination of different plant and soil types. For the analysis of the initial-investment cost, General Information for Construction and Monthly Construction Market Price was used.54,55 Further, the detailed data, including the operation and maintenance cost, were surveyed through interviews with specialists connected with GRSs service providers. Furthermore, the CO2 emission and CO2 absorption quantities were converted into the economic value using the profit from the sale of carbon credits, (CERs, $4.80/t CO2 e).43,56−58

3. RESULTS AND DISCUSSION 3.1. Analysis of Energy Consumption. Hong et al. 32 conducted LCC and LCCO2 analyses on 16 improvement scenarios based on the combination of GRSs (here, the plant and soil types for GRSs were sedum and natural soil, respectively) and ESMs (external insulation, exterior blinds, double low-e glazing, and LED). The results showed that all the top four scenarios used LED. Moreover, the 11th Report (June 8, 2011) of the Presidential Committee on Green Growth in Korea proposed the “LED 2060 Plan”,59 according to which 60% of building lights will be substituted with LED by 2020, and the government plans to offer financial subsidies of 50− 70% of the initial construction cost. In fact, “Y” Elementary School plans to introduce LED to its facility, and it was thus assumed in this study that LED was the basic specification of the existing facility. Meanwhile, “Y” Elementary School exchanged its fan coil unit with a ceiling-type package EHP in the summer of 2008, and is currently using electric energy for all its heating and cooling loads. Thus, such characteristics were reflected in the 8478



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Table 3. Annual Energy Consumption by GRSs Scenariosa

energy simulation. Furthermore, the equipment schedule, heating and cooling schedules, etc., based on the facility information presented in the design documents, the academicschedule data, and the facility maintenance manager reviews were made full use of in this study. The classrooms and school offices were set as air-conditioned rooms, whereas stairways, corridors, and bathrooms were set as non-air-conditioned rooms. The properties were used in this study is shown in Table 1 and Table 2.

soil type

conventional roof system (existing facility with LED) natural soil sedum grass low shrub shrub tree improved soil sedum grass low shrub shrub tree artificial soil sedum grass low shrub shrub tree

Table 2. Basic Conditions for Energy Simulation class

detailed information

weather data

climate data in Incheon, which are similar to those in Seoul 01/01/2010−12/31/2010 Korea, 2010 school schedule 5 stories, 4,265 m2 (2,723 m2)

run period holiday schedule total building area (net conditioned area) roof area conditioning system

1,466 m2 packaged EHP (electric heating pump) system global thermal characteristics of each thickness conductivity (W/ wall (mm) m·K) the lowest floor floor ceiling walls fronting on outside walls thickness (mm) glazing (double low-e)

lighting load density 23 w/m2 equipment load density winter setting temperature simulation time step

scenario no.

annual energy consumption

rank

S1

266,862

16

S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16

265,376 265,165 265,175 264,835 264,730 265,324 265,063 264,964 264,582 264,465 264,891 264,930 264,688 264,242 263,539

15 12 13 7 6 14 11 10 4 3 8 9 5 2 1

a

Note: Energy consumption unit = kWh, S = scenario (i.e., S1 = scenario 1.)

consumption. In other words, the thermal conductivity based on the soil type was greatest in natural soil, followed by the improved and artificial soil (in descending order), and accordingly, the annual energy consumption was greatest with natural soil, followed by improved and artificial soil (in descending order). (The greater the thermal conductivity of the soil was, the greater the external effect became, and therefore, the annual energy consumption increased.) Meanwhile, the heating energy consumption pattern was different from the cooling energy consumption pattern. Figure 2 shows the heating energy consumption and soil depth by scenario. For scenario 1, the conventional roof system that used the existing facility with LED, had the highest value (77,824 kWh), and scenario 16 (artificial soil + tree), one of the GRSs, had the lowest value (74,401 kWh), resulting in a difference of 3,423 kWh. Figure 3 shows the cooling energy consumption and soil depth by scenario. Scenario 14 (artificial soil + low shrub) had the highest value (46,808 kWh), and scenario 13 (artificial soil + grass) had the lowest value (46,514 kWh). The difference (294 kWh) was negligible. 3.2. Analysis of LCC and LCCO2. By considering the CO2 emissions over the life cycle of the GRSs materials, Supporting Information Figure S2 shows the LCC analysis results by GRSs scenario, including the CO2 conversion price in a time series graph. The X-axis indicates the service life of the facility while the Y-axis indicates the accumulated LCC with CO2 conversion price from scenario 1(conventional roof, existing facility + LED) through scenario 16. At time 0, the costs only included the initial investment cost and initial investment CO2 conversion price. As time progresses, energy consumption and maintenance costs incurred, and then the CO2 emissions were generated from the energy consumption. Additionally, CO2 absorption occurred in the plants. The total CO2 emissions from the facilities were calculated. By applying the profit from the sale of carbon credits (CERs, $4.80/t CO2e) to the result, the environmental value was converted to the economic value. If all of the graphs cross the S 1 line, then the implementation of GRSs will not have an LCC reduction effect. When the GRSs were implemented (scenario 2−16), the

105 0.029 30 0.029 160 0.030 85 0.032 60 0.032 thermal transmittance (W/ m2·K)

12.5(gas) air-conditioning space

vegetation type

2.8 non-air-conditioning space 6 w/m2 16 w/m2 summer

18 °C 23 °C every five minutes (for green roof systems)

Energy plus, version 6.0 (U.S. Department of Energy, DOE), was used in the energy simulation as a building energy simulator, and the plug-in function of Google Sketch Up (version 8) was used for 3D modeling.60,61 In this study, energy modeling of 15 GRSs types was conducted. As has been mentioned, the LED-applied scenario, selected as the optimum scenario by Hong et al.,32 was included as the basic ESM of the conventional roof system. Table 3 shows the annual energy consumption and rank of the conventional roof system (the existing system with LED) and the 15 GRSs. Table 3 shows three marked characteristics. First, in comparison with the conventional roof system (scenario 1), the 15 GRSs (scenarios 2−16) showed annual-energyconsumption reduction. Second, while the soil types are identical, the soil thickness was proportional to the annualenergy-consumption reduction rate. For example, in natural soil, the soil thickness increased from scenario 2 (natural soil + sedum) to scenario 6 (natural soil + tree); accordingly, the annual energy consumption decreased. The complex sunblocking effect of the LAI of the plants caused some scenarios to move away from the overall trend. Third, in case that the soil thicknesses were identical, the higher thermal conductivity based on the soil type increased the annual energy 8479



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Figure 2. Heating energy consumption and soil depth by scenario. The heating energy consumption by GRSs scenario showed very similar patterns to the three characteristics of the annual energy consumption that were examined. Namely, the heating energy consumption tends to decrease when (1) GRSs are implemented (i.e., group 1−3); (2) the soil thickness increases (i.e., from S2 to S6 in group 1); and (3) the thermal conductivity by soil type decreases (i.e., from group 1 to group 3). As a result, as the insulation effect becomes superior, the heating energy consumption decreases because the better insulation effect prevents the heat generated inside the building from dissipating outside.

Figure 3. Cooling energy consumption and soil depth by scenario. GRSs implementation tends to increase the cooling energy consumption in some scenarios. Additionally, while some differences were also shown in the pattern of the cooling energy consumption by GRSs scenario, such pattern is equally affected by the soil depth and thermal conductivity based on the soil type. In addition, the diagram shows that the cooling energy consumption is also affected by LAI, based on the plant type. In other words, the cooling energy consumption tends to increase when: (1) the soil depth increases (i.e., from S2 to S6 in group 1); (2) the thermal conductivity by soil type is low (i.e., from group 1 to group 3); and (3) the LAI by plant type is small (i.e., from S6 to S2 in group 1). Consequently, as the insulation effect is superior and as the sun-blocking effect is small, the cooling energy consumption tends to increase because the superior insulation effect prevents the heat generated inside the building from dissipating outside, and also because the low sun-blocking effect means much more heat is delivered from outside to inside.

cross with the line from scenario 1 throughout the entire life cycle for the 40-year service life. Thus, these scenarios were superior to the conventional roof system (scenario 1, existing facility + LED) in terms of LCC and LCCO2. In contrast, GRSs with trees (scenarios 6, 11, and 16) intersected the line from scenario 1 within the 40-year service life. Although the initial investment cost and initial investment

graphs were initially below the line from scenario 1. Thus, compared to the conventional roof system (scenario 1, existing facility + LED), the cases with GRS implementation had lower initial investment costs (including the CO2 conversion price) for the production and construction phases. As shown in Supporting Information Figure S2, the six GRSs with sedum and grass (scenarios 2, 3, 7, 8, 12, and 13) did not 8480



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Figure 4. LCC and LCCO2 analysis results by scenario. Each bar chart includes the CO2 conversion price for the CO2 generated from each phase, along with the key items for the LCC (i.e., initial investment, replacement, maintenance, and energy consumption costs). The results of the implementation of scenario 1 (existing facility + LED), which includes the conventional roof system, are as follows: when only economic value (i.e., initial investment and maintenance costs) was considered, scenario 1 was superior to all of the GRS scenarios, but when environmental value (i.e., initial investment and maintenance CO2 conversion prices) was considered along with the economic value, six GRS scenarios (2, 3, 7, 8, 12, and 13) were superior to scenario 1. These results suggest that the CO2 emissions for the materials used in the initial construction phase of scenario 1 (existing facility + LED) were considerable.

phosphoric acid was also used in this study. If this fertilizer will be changed to an eco-friendly one, it is expected that the CO2 emission quantities of materials will be considerably reduced. In conclusion, while the economic aspects (e.g., initialinvestment and maintenance costs) should be considered in the implementation of GRSs, it was shown in this study that the environmental aspects (e.g., initial-investment and maintenance CO2 conversion prices) should be also considered. For the selection of the optimal GRSs scenario, a model that considered both the environmental and economic values was suggested in this study. This model can be used in determining the introduction of various other ESMs besides the use of GRSs. In addition, if the proposed model will be developed into a Web-based system, the user convenience can be improved, and the assessment results from various ESMs will be continuously included in the database. The recent increase in precipitation in South Korea and abroad due to climate change has resulted in many natural disasters. Consequently, urban dwellers are showing much interest in GRSs. As such, there is an increasing demand for a model with which to evaluate the effect of GRSs on preventing flood, and the economic effect of GRSs lease businesses. This research team is currently conducting a series of studies to address these issues.

CO2 conversion price for these scenarios are lower than that of the conventional roof system, the maintenance cost and maintenance CO2 conversion price are more expensive than that of the conventional roof system. Consequently, these scenarios were inferior to the conventional roof system from years 9−14 in terms of LCC and LCCO2. Meanwhile, as shown in Figure 4, the application of GRSs (scenarios 2−16) resulted in different patterns based on the initial investment and maintenance costs by scenario. First, the six scenarios (2, 3, 7, 8, 12, and 13) with sedum and grass applied as plants showed relatively low total costs in terms of LCC and LCCO2. The results showed that when considering the currently realized economic value, the conventional roof system is superior to the GRSs. However, when considering the environmental value, the LCC analysis revealed that the opposite trend when compared to the conventional roof system (scenario 1, existing facility + LED). In particular, scenario 13 (artificial soil + grass) showed the lowest initial investment and operation and maintenance phase costs. Thus, it was determined that the optimal scenario in terms of LCC and LCCO2 is scenario 13. Second, the scenarios that used trees for plants, such as scenarios 6 (natural soil + tree), 11 (improved soil + tree), and 16 (artificial soil + tree), had high initial investment and maintenance costs. Thus, these scenarios are thought to be inappropriate. In implementing GRSs based on the above results, it was shown that eco-friendly approaches should be employed not only in the production and construction phases but also in the operation and maintenance phases. For example, if urethane waterproofing is used as waterproofing material, the CO2 emission of the material is 366,220 kg CO2/kg, whereas EPDM rubber waterproofing sheet generates only 4.3 kg CO2/ kg, about 85,000 times smaller than the first. It was thus determined that this study implemented an eco-friendly approach to the production and construction phases. On the other hand, the fertilizer that often uses mixed nitric and

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ASSOCIATED CONTENT

S Supporting Information *

Detail data on the deployment of green roofs at the elementary school. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Address: Yonsei University, 262 Seongsanno, Seodamun-gu, Seoul, 120-749, Korea. Phone: 82-2-2123-5788. Fax: 82-2-3654668. E-mail: [email protected]. 8481



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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by a grant from High-Tech Urban Development Program (10CHUD-C03) funded by the Ministry of Land, Transport and Maritime affairs, South Korea. This research was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Education, Science and Technology, MEST) (No. 2011-0018360).

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