- Email: [email protected]
Evaluation of photovoltaic-green and other roofing systems by means of ReCiPe and multiple life cycle–based environmental indicators
Building and Environment 93 (2015) 376e384
Contents lists available at ScienceDirect
Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Evaluation of photovoltaic-green and other roofing systems by means of ReCiPe and multiple life cycleebased environmental indicators Chr. Lamnatou*, D. Chemisana Applied Physics Section of the Environmental Science Department, University of Lleida, c/Pere Cabrera s/n, 25001 Lleida, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2015 Received in revised form 11 June 2015 Accepted 27 June 2015 Available online 2 July 2015
The present study evaluates the environmental profile of a PV-green roof (PV panels over a soil/plant layer) and other roofing systems (PV-bitumen, PV-gravel, gravel, extensive green and intensive green). The analysis is based on multiple life-cycle impact assessment methodologies (ReCiPe, etc), several scenarios (for example with and without recycling) and it provides a deeper analysis as well as additional results to authors' previous investigation about PV-green roofs. The evaluation of the PV-green system (in terms of material manufacturing phase) shows that PV laminates (multi-Si) and steel components (joist, decking, balance of system) are responsible for the greatest part of the total footprint, based on GWP (global warming potential) and ReCiPe. Among the roofs which do not produce electricity, material manufacturing phase reveals that intensive green configuration has considerably higher impact in comparison with gravel and extensive green systems. Concerning PV roofs, PV-green configuration on a long-term basis (by considering material manufacturing, use phase, transportation and disposal), after a critical point, pays back its additional environmental impact (related with the “green layer”) and it becomes more eco-friendly than the other two PV roofs. Certainly, this is due to the benefits (cooling effect of evapotranspiration, etc) of the soil/plant layer which result in PV output increase. The above mentioned critical point is determined by means of ReCiPe payback time and greenhouse-gas payback time. Several environmental indicators are calculated and presented along with results from the literature. A critical discussion is also provided. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Life cycle analysis (LCA) Photovoltaic (PV)-green roofs and buildings IPCC GWP ReCiPe USEtox Ecological footprint Payback time
1. Introduction Photovoltaic (PV)-green roofs combine PVs with simple green roofs (with soil/plant layer) and they have been studied by the authors of the present work from experimental [1] as well as from theoretical point of view [2,3]. At this point it should be noted that for PV-green applications are more appropriate shallow-substrate green systems (extensive green roofs) and not deep-substrate green systems (intensive green roofs). This is related with factors such as the high height of the soil/plant layer of the intensive green configurations, their high weight as well as with aesthetic/building integration issues [1,3]. A critical analysis of factors affecting PVgreen roof performance has been presented by Lamnatou and Chemisana [3]. A principal advantage of the plant/PV combination is the increase of PV output because of evapotranspiration cooling effect and in general because of plant/PV synergy, along with the other
* Corresponding author. E-mail address: [email protected] (Chr. Lamnatou). http://dx.doi.org/10.1016/j.buildenv.2015.06.031 0360-1323/© 2015 Elsevier Ltd. All rights reserved.
advantages which are also provided by the simple (without PVs) green roofs [3,4]. In the literature, there are few investigations about PV-green roofing systems: experimental studies [1,5e8], theoretical/modeling studies [2,6,9,10], critical review [3]. These investigations examine plant/PV interaction under several conditions (in terms of plant species, etc). For example for the Mediterranean climate summer conditions, two PV-green roofs (Gazania rigens and Sedum clavatum) and a PV-gravel roof (reference system) were examined [1]. The results for a sunny, five-day time period demonstrated an average increase of the maximum power output of the PV panels (ranging from 1.29% to 3.33% depending on the plant), verifying the positive interaction between PVs and plants [1]. By focusing on the techniques for the evaluation of the environmental impact of a system, Life Cycle Analysis (LCA) is a useful tool. However, in the literature there are few LCA studies about green roofs. In the following paragraphs, some of these studies are presented. Saiz et al. [11] investigated an eight-story, residential building, in Madrid. By replacing a common flat roof with a green one (extensive), the environmental impact showed a reduction ranging
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
from 1.0 to 5.3%. LCA was conducted for the whole building and by changing each roof option (assuming a 50-years building lifespan). A “bottom-up” approach was adopted. The stages of the life-cycle which were studied included material production and transportation, building operation and building maintenance (the construction phase and the end-of-life phase were not included) [11]. Kosareo and Ries [12] conducted a comparative LCA of intensive and extensive green roofing systems versus conventional roofs. The extensive green roof was based on an actual green roof project (retail store in Pittsburgh, PA, USA). The phases of fabrication, transportation, installation, operation, maintenance and disposal were considered. Ozone layer depletion, acidification, eutrophication, global warming and IMPACT 2002þ were adopted for the analysis. The results showed that the green roofs performed better than the control roof. In addition, the intensive green roof showed better performance than the extensive one. It should be noted that the goal of the LCA of [12] was to compare the environmental aspect and potential impact associated with constructing, maintaining and disposing of a roof (1115 m2) and to determine the option with the lowest negative impact. The aim was the identification of the environmentally preferable choice between an extensive green roofing system, an intensive green roof and a conventional stone ballasted roof. Several aspects were considered in terms of the creation, operation and demolition of the roof area. Multiple environmental factors were monitored, such as thermal transmittance and water run-off. In addition, the energy consumption of the building, depending on the type of roof, was taken into account [12]. Rivela et al. [13] conducted an LCA, based on CML 2000, about green roofs of Carex Testacea and Nassella tenuissima (stipa) in Spain. The functional unit was “one square meter of a reverted flat roof with tile floating floor for private pedestrian use”. The results revealed that the structural support had the highest contribution in all the studied impact categories, with the exception of “ozone layer depletion” category in which insulation showed 95% contribution. Lamnatou and Chemisana [2] evaluated a PV-green roof along with other roof configurations: PV-gravel, green (extensive and intensive) and gravel, by means of different Life Cycle Impact Assessment (LCIA) methodologies: EI99, IMPACT 2002þ and Cumulative Energy Demand (CED). Stages of the phases of material manufacturing, material transportation, use and disposal were considered. The functional unit was the whole roofing system (300 m2). The results revealed that material manufacturing is the most energy-demand phase for all the studied configurations. Emphasis was given on the PV-green roof and its comparison with the PV-gravel one, based on different scenarios. The results showed that although the PV-green configuration has an additional environmental impact in comparison with the PV-gravel one (because of the green-layer components), this additional impact on a log-term basis can be compensated. Other studies in the field of green-roof LCA are those of: Hong et al. [14] (about life-cycle cost and life-cycle CO2 analysis of green roofs in elementary schools with energy saving measures); Bianchini and Hewage [15] (about LCA of green-roof materials: it was demonstrated that the green-roof materials need to be replaced by n-Palma et al. [16] more environmentally friendly products); Cero (several green-space strategies for the building sector were examined); Bozorg Chenani et al. [17] (LCA of green-roof layers was conducted). The literature review reveals that most of the LCA studies: 1) concern simple (without PV panels) green roofs, 2) focus on the benefits of the soil/plant layer for the building (e.g. energy savings during building use phase), 3) do not include multiple and newlydeveloped LCIA methodologies. In continuation to authors' previous LCA about PV-green and other roofing systems [2], the present
377
article aims to provide new results by investigating the environmental performance of PV-green and other roofs by means of different LCIA methodologies, including newly-developed ones such as ReCiPe. An additional roofing system which includes PV modules over grey bitumen layer is introduced and compared with the other PV systems, strengthening the environmental benefits of the PV-green configuration. The analysis is based on multiple scenarios and several environmental indicators are evaluated, especially for the PV-green roof in comparison with other PV roofing systems. Critical points, after which the PV-green configuration becomes more eco-friendly than the other PV roofing systems, are identified. In this way, the present work along with authors' previous LCA study [2] provide a comprehensive profile of the environmental performance of the proposed PV-green configuration, based on multiple approaches. 2. Materials and methods For the implementation of the LCA, according to ISO 14040:2006 [18] and ISO 14044:2006 [19], the following phases are adopted: 1) goal and scope definition, 2) life-cycle inventory, 3) life-cycle impact assessment and 4) interpretation. 2.1. Functional unit and system boundaries The whole roofing system (300 m2) is used as functional unit. The system boundaries include the roof in terms of material manufacturing phase. However, for the comparison of the PV roofs on a life-cycle basis, the boundaries except of material manufacturing include also use phase, transportation and disposal. Details about the components/materials and the adopted methods follow below. 2.2. System definition 2.2.1. Technical characteristics of the studied configurations The roofing systems which are examined are: 1) gravel, 2) PVgravel, 3) extensive green, 4) PV-green (extensive), 5) intensive green, 6) PV-bitumen (grey bitumen). The available roof area is assumed to be 300 m2, considering a typical building with 30 m façade and 10 m width. The PV roofs refer to grid-connected PVs. The tilt angle of the PV modules, optimized in terms of the annual production for the case of Lleida (Catalonia, Spain), is 33 . Two rows of PVs are placed in parallel with around 5 m distance between the rows. Each row has 30 poly-crystalline Silicon (Si) PV modules. Each panel has the following characteristics: 230 Wp, Imp ¼ 7.98 A, Vmp ¼ 29.2 V, 60 cells, 1.66 0.99 m2 dimensions, electrical efficiency ¼ 13.9%, weight ¼ 18 kg (Source: [20]). Each row achieves 6.9 kWp; thereby, the total peak production of electricity is 13.8 kWp. The BOS (balance of system) includes: aluminum frame, support structure (steel), copper and plastic materials for cables and contact boxes. In terms of the roof components, all the roofing systems have structural support member (steel joist), decking (corrugated steel), insulation (polystyrene), underlayment (fiberboard) and asphalt adhesive. The above mentioned elements are common for all the roofs and they have the same amount of material, except for the intensive green roof where robust construction is needed and thereby, more steel is used for the support member. In addition, all the studied roofs include waterproofing membrane: 3-ply SBS (styrene-butadiene-styrene) for the non-green roofs and StressPly EUV (extreme ultraviolet) for the green roofs. Moreover, the green configurations have high-density polyethylene (HDPE) as drainage layer and filter fabric. Regarding the top layer, the green roofs have growing medium (height: 10 cm for the extensive and 100 cm for
378
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
Fig. 1. a) Sedum clavatum (left) and Gazania rigens (right) PV-green roofs, b) PV-gravel roof, developed by the authors at the University of Lleida, in Spain. Source: authors' archive of pictures.
the intensive green roof) while the non-green configurations have gravel or bitumen (instead of soil/plant layer). More details about the considered roofing systems for the LCA can be found in Ref. [2] while information about the PV-green and PV-gravel roofs developed by the authors (Fig. 1) can be found in Ref. [1].
green and green roofs [2], are presented. The reference materials and the construction of the roof basic configuration are based on reference [12].
2.4. Life-cycle impact assessment methodologies 2.2.2. Assumptions For the intensive green roof, shrubs and small trees are incorporated into the LCA model as small plants. In terms of the extensive green roofs, the impact of seeds is considered. The soil substrate of the green systems is based on clay. Concerning the PV roofs, the analysis is performed for multi-Si PV laminates and BOS impact refers to a typical grid-connected rooftop installation [21]. Regarding SBS and EUV membranes (modified bitumen), the impact of bitumen is taken into account. Moreover, it should be noted that each roofing system is considered as a “single” system (and not as a subsystem of a larger system (building)); thereby, the results are not presented in terms of the total building performance. For the use phase, the lifespan of the PV roofs is assumed to be 30 years. The PV output of the PV-gravel roof has been calculated for the case of Lleida [2]. Concerning the inputs during use phase, are taken into account: 1) for all the roofing systems: general maintenance (as 10% of material manufacturing impact) [2], 2) for the green roofs: fertilizing, irrigation and adding of soil substrate [2], 3) for the PV systems: replacement of the BOS (once over roof lifespan) [2], 4) for the PV-bitumen configuration: replacement of the bitumen layer (once over roof lifespan). The transportation distances are from/to the building (Lleida) by a truck. All the components except of the PV modules are assumed to be purchased from the local market (thus, transportation includes a distance of 5 km) while the PV panels are assumed to be purchased from Barcelona1 (round-trip distance: 266 km). Regarding disposal, landfill appropriate for each material is considered. For the green roofs, it is assumed that the end-of-life plants will be composted while the end-of-life soil will be reused in agriculture (for these two last cases, zero impact is considered).
IPCC 2013 GWP 20a V1.00, IPCC 2013 GWP 100a V1.00 and IPCC 2013 GWP 500a V1.00 are utilized in order to calculate GWP (global warming potential) of the studied roofs.3 Furthermore, ReCiPe Endpoint (H) V1.10/Europe ReCiPe H/A is also adopted (singlescore, endpoint approach), providing results for the damage categories Human Health, Ecosystems and Resources. Moreover, Ecological footprint V1.01/Ecological footprint (with characterization; results in m2a (¼ m2 per year)) and USEtox (default) V1.03/ Europe 2004 (with characterization; results in CTU (¼ comparative toxic unit)) are also utilized (Source: [22]). In terms of the adopted methodologies, IPCC 2013 is an update of IPCC 2007 developed by the International Panel on Climate Change. This method is listing the climate change factors (of IPCC) with a timeframe of 20, 100 and 500 years [23]. ReCiPe is the successor of EI99 and CML-IA. The purpose at the start of the development was to integrate the ‘problem oriented approach’ of methodology CML-IA and the ‘damage oriented approach’ of methodology EI99 [23]. The ecological footprint is the biologically productive land and water a population requires to produce the resources it consumes as well as to absorb part of the waste generated by fossil and nuclear fuel consumption [23]. In terms of the characterization, in the frame of LCA, the ecological footprint of a product is the sum of time integrated direct and indirect land occupation, related to nuclear energy use and to CO2 emissions from fossil energy use [23]. On the other hand, the USEtox model is an environmental model for the characterization of human and eco-toxicological impacts in the context of life-cycle impact assessment and comparative risk assessment [23]. In accordance to CED impact which was used for the evaluation of the energy Payback Time (PBT) of the PV roofs [2], the concept of ReCiPe PBT is introduced:
2.3. Life cycle inventory 2
SimaPro 8 and ecoinvent 3 database are utilized (Source: [22]). In Tables 1 and 2, the materials/components of the studied non-
1 For the transportation of the PV panels, only the purchasing point is considered. It is assumed that the PV panels are not imported from elsewhere. 2 For few cases, ELCD, LCA Food DK and EU & DK Input Output Database are also adopted.
ReCiPe PBT ¼
Imat þ Itransp þ Idisp Iout:a IO&M:a
ðyearsÞ
(1)
where I is the total endpoint ReCiPe score (Pts) regarding: material manufacturing (Imat); transportation (Itransp); disposal (Idisp); annual avoided impact due to the use of PV electricity instead of using
3
GWP is associated with carbon footprint.
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
379
Table 1 Life cycle inventory for the materials/components of the non-green roofs. Materials/components
Gravel roof: mass of the materials (kg)
PV-gravel roof: mass of the materials (kg)
PV-bitumen roof: mass of the materials (kg)
Steel joist Corrugated steel decking Polystyrene (insulation) Fiberboard (underlayment) 3-ply SBS (waterproofing membrane) Asphalt (adhensive) Gravel Bitumen PV laminates (multi-Si) BOS Aluminium (frame) Steel (support structure) Copper (cables and contact boxes) Plastics (cables and contact boxes)
1614.35 672.65 538.12 941.70 538.12 538.12 24000 e e
1614.35 672.65 538.12 941.70 538.12 538.12 24000 e 99 m2 (13.8 kWp)
1614.35 672.65 538.12 941.70 538.12 538.12 e 1200 99 m2 (13.8 kWp)
e e e e
188.10 2475.02 3.96 3.96
188.10 2475.02 3.96 3.96
Table 2 Life cycle inventory for the materials/components of the green roofs. Materials/components
Green roof (extensive): mass of the materials (kg)
PV-green roof (extensive): mass of the materials (kg)
Green roof (intensive): mass of the materials (kg)
Steel joist Corrugated steel decking Polystyrene (insulation) Fiberboard (underlayment) StressPly EUV (waterproofing membrane) Asphalt (adhensive) HDPE (drainage layer) HDPE (filter fabric) Soil substrate Plants (or seeds) PV laminates (multi-Si) BOS Aluminium (frame) Steel (support structure) Copper (cables and contact boxes) Plastics (cables and contact boxes)
1614.35 672.65 538.12 941.70 1210.76 538.12 53.81 53.81 28500 0.9 kg (seeds) e
1614.35 672.65 538.12 941.70 1210.76 538.12 53.81 53.81 28500 0.9 kg (seeds) 99 m2 (13.8 kWp)
2152.47 672.65 538.12 941.70 1210.76 538.12 53.81 53.81 285000 3000 (number of small plants) e
e e e e
188.10 2475.02 3.96 3.96
e e e e
Spain's electricity mix (Iout.a); annual impact during use phase (IO&M.a). In the same way with Eq. (1), Greenhouse Gas (GHG) PBT is evaluated by means of the following formula:
GHG PBT ¼
GHGmat þ GHGtransp þ GHGdisp GHGout:a GHGO&M:a
ðyearsÞ
(2)
where GHG is the total GWP100a impact (in tones CO2.eq) regarding: material manufacturing (GHGmat); transportation (GHGtransp); disposal (GHGdisp); annual avoided impact because of the use of PV electricity instead of using Spain's electricity mix (GHGout.a); annual impact during use phase (GHGO&M.a). It should be noted that for the evaluation of ReCiPe and GHG PBT, the electricity of Spain is considered as reference. On the other hand, the ratio Rx of the Ecological Footprint (EF) EFx (in m2a) and EI99x (in ecopoints) is evaluated based on the following equation [24]:
Rx ¼
EFx EI99x
(3)
The ratio Rx quantifies the relationship between EF (as a relative simple environmental indicator) and EI99 (as a relative complex environmental indicator) [24]. Rx ratio is a potential conversion factor between EI99 results (Pts) and EF results (m2a). If the ratio is approximately equal for all the evaluated products, this implies that the two methods do not differ in their gross ranking of the products
[24]. It should be noted that for the calculation of Rx (Eq. (3)), EI99 results from authors' previous LCA [2] are utilized. In the frame of the present study, Rx is used as an additional result in order to compare with the findings of Huijbregts et al. [24]. 2.5. Sensitivity analysis For all the roofing systems, three different scenarios in terms of GWP time horizon are examined: 20, 100 and 500 years. Regarding the PV roofs, an additional scenario “Recycling” vs. “No Recycling” is adopted in order to examine the effect of BOS aluminium and copper recycling. Furthermore, for the PV-green roof, eight scenarios in terms of PV output increase due to plant/PV synergy are adopted, based on literature studies (experimental and theoretical) about PV-green roofs (Table 3). Several scenarios are examined: PV output increase ranging from 0.08% (pessimistic scenario: [9]) to 8.3% (optimistic scenario: [6]). By evaluating these different cases, a wide view of the studied issues is provided while some of these scenarios can be considered as realistic for the Mediterranean climatic conditions of Spain. 3. Results and discussion 3.1. PV-green roof: material manufacturing In this section the environmental profile of the PV-green roof is
380
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
Table 3 Scenarios in terms of PV output increase due to plant/PV interaction, based on references from the literature. Scenarios
References from the literature
PV output increase
1 2 3 4 5 6 7 8
Witmer [9] Nagengast et al. [8] Witmer [9] Chemisana and Lamnatou [1] Perez et al. [7] Chemisana and Lamnatou [1] Hui and Chan [6] Hui and Chan [6]
0.08% 0.50% 0.55% 1.29% 2.56% 3.33% 4.30% 8.30%
a
a Based on authors' experimental study [1], scenario 4 refers to PV-gazania and scenario 6 concerns PV-sedum roof.
Fig. 3. ReCiPe scores (Pts) (for Human Health, Ecosystems and Resources) of the PVgreen roof for certain roof components. Scenario “No Recycling”. Phase of material manufacturing.
Fig. 2. GWP100a (t CO2.eq) of the PV-green roof for certain roof components. Scenario “No Recycling”. Phase of material manufacturing. Fig. 4. GWP20a, GWP100a and GWP500a (t CO2.eq) for the gravel, extensive green and intensive green roofs. Phase of material manufacturing.
presented. In Fig. 2, the GWP100a footprint of each roof component (scenario: No Recycling) is illustrated.4 PV laminates are responsible for the greatest part of the total impact of the PV-green roof and steel (including steel joist, decking and BOS) is the material with the second highest contribution. The participation of the PV laminates and steel components to the total GWP100a footprint of the PV-green roof is around 49% and 32%, respectively. All the other components of the PV-green configuration (Table 2) show percentages less than 8% (in terms of their contribution to the total GWP100a impact). The effect of recycling (based on GWP20a, GWP100a and GWP500a) has been also examined. The results demonstrate that recycling leads to an impact reduction of 3e3.3 t CO2.eq. Furthermore, in Fig. 3 ReCiPe scores (Pts) for Human Health, Ecosystems and Resources are presented. By taking into account the total scores of the three endpoint categories, the results demonstrate that Human Health and Resources have higher ReCiPe footprint in comparison with Ecosystems. Regarding the contribution of each part of the PV-green roof to the total ReCiPe impact of each category, the findings reveal that: 1) PV laminates and steel (joist, decking and BOS) show the highest contribution, 2) the other components of the PV-green system (Table 2) show percentages less than 8%. On the other hand, the adoption of recycling results in ReCiPe impact reduction around 4e8%, depending on the impact category.
4 From Fig. 2 (as well as from Fig. 3) have been excluded the components which have considerably lower impact than those presented in the graphs. However, the calculations have been conducted by taking into account all the components.
3.2. All the roofs: a comparison based on material manufacturing 3.2.1. Roofing systems which do not produce electricity In Fig. 4, GWP20a, GWP100a and GWP500a for the gravel, extensive green and intensive green roofs are illustrated. It can be seen that the extensive green roof has around 4e5% higher GWP impact in comparison with the gravel configuration. Moreover, the intensive green roof presents remarkably higher GWP impact (about 33e41%) comparing to the extensive green and gravel roofing systems. By considering the effect of the time horizon, it can be observed (as it was expected) that by adopting a longer time horizon there is an impact reduction. With respect to ReCiPe results (Fig. 5), the extensive green roof has 20, 15 and 69 Pts (for each damage category, respectively) higher footprint comparing to the gravel configuration. On the other hand, the intensive green roof has remarkably higher (about 29e40%) ReCiPe impact than gravel and extensive green roofing systems. The considerably higher (GWP and ReCiPe) footprint of the intensive green roof is mainly associated with the high amounts of steel (for steel joist) and soil (Table 2) which are required for the green intensive configuration. An additional impact of the green intensive roof (in comparison with the green extensive and gravel) is related with the fact that the life-cycle impact for plant production is taken into account. Furthermore, from Fig. 5 it can be noticed that for all the studied cases the lowest footprint is for Ecosystems (less than 400 Pts). 3.2.2. Roofing systems which produce electricity In Fig. 6, GWP20a, GWP100a and GWP500a results for the PV roofs (scenario “No Recycling”) are illustrated. It can be seen that
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
Fig. 5. ReCiPe scores (Pts) (for Human Health, Ecosystems and Resources) of the gravel, extensive green and intensive green roofs. Phase of material manufacturing.
381
Fig. 7. ReCiPe scores in Pts (Human health, Ecosystems and Resources) for the PV roofs. Scenario “No Recycling”. Phase of material manufacturing.
Fig. 6. GWP20a, GWP100a and GWP500a for the PV roofs. Scenario “No Recycling”. Phase of material manufacturing.
PV-green roof has higher footprint than PV-bitumen and PV-gravel while PV-bitumen shows higher footprint than PV-gravel. Nevertheless, for all the cases of Fig. 6 the differences in CO2.eq emissions are less than 1.5%. In terms of GWP time horizon (as for the case of Fig. 4), the adoption of longer time horizon results in impact reduction. The effect of recycling has been also examined and the findings show that for all the studied PV roofs recycling results in GWP reduction of around 6e8%. Regarding ReCiPe results (Fig. 7, scenario “No Recycling”), the impact for the PV roofs ranges from around 900 to 2200 Pts with the PV-green system showing 1e69 Pts higher impact (for most of the damage categories) than the other two PV configurations. With respect to recycling, there is a ReCiPe impact reduction of about 4e8% for all the studied PV roofs. 3.3. The impact related to the specific PV-roof layers: material manufacturing Each PV roofing system has some particular components.5 These special parts are: 1) gravel and 3-ply SBS for the PV-gravel, 2) bitumen layer and 3-ply SBS for the PV-bitumen, 3) plants, soil, drainage layer, filter fabric and StressPly EUV for the PV-green roof. In this section the impact associated to material manufacturing of the above mentioned specific layers is presented. In Fig. 8, USEtox findings for Human toxicity (cancer and non-cancer) (Fig. 8a) and Ecotoxicity (Fig. 8b) are illustrated. It can be observed that there is a considerable difference in the toxicity of the studied cases. More analytically, PV-bitumen specific layers show higher impact than
5 Moreover, the upper roofing layers interact with the PV panels and thus, influence PV output.
Fig. 8. USEtox results for the specific layers of the PV roofs: a) Human toxicity (cancer and non-cancer) (in CTUh) and b) Ecotoxicity (in CTUe). Phase of material manufacturing.
PV-green and PV-gravel particular components. On the other hand, PV-green specific layers have approximately double footprint in comparison to PV-gravel particular components. Nevertheless, by considering the life-cycle of each PV roof (Section 3.5), it is verified that the PV-green roof, on a long-term basis, compensates the above mentioned initial impact (due to the “green layers”) and after a certain point PV-green becomes more eco-friendly than the other two PV configurations. Certainly, this is related with PV-green increased electricity production because of plant/PV interaction. 3.4. The impact related to use phase of the PV roofs The impact associated with use phase is considerably different for the PV-green roof than for the other two PV systems. Details were previously presented in the assumptions (Section 2.2.2). Certainly, soil/plant layer has significantly higher maintenance needs than gravel or bitumen layer. Based on GWP100a findings
382
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
Fig. 9. GWP100a (t CO2.eq) of the PV roofs during use phase. Results on a lifespan basis of 30 years. Scenarios “No Recycling” vs. “Recycling”.
(Fig. 9), PV-bitumen and PV-gravel systems have similar impact (approximately 15 t CO2.eq for the scenario without recycling) while PV-green roof has around 7 t CO2.eq higher emissions than the other two PV configurations. In terms of recycling, for all the PV roofs there is a footprint reduction of 3.4 t CO2.eq. The impact during use phase has been also calculated by means of ReCiPe. The results show total ReCiPe scores (scenario without recycling) of 2561, 1925 and 1802 Pts for the PV-green, PV-bitumen and PV-gravel, respectively. Moreover, according to ReCiPe findings, recycling results in an impact reduction of around 14e19% for all the studied PV roofs. At this point it should be noted that in Section 3.5, the results about the payback times of the PV roofs verify the fact that on a long-term reference frame the PV-green roof compensates this higher impact related with its use phase (and in general, the higher footprint because of the green layer) and after a certain point PVgreen becomes more eco-friendly than the other two PV roofing systems.
3.5. ReCiPe payback time and GHG payback time of the PV roofs The evaluation of the PV roofs over their life-cycle is conducted based on ReCiPe and GWP100a. ReCiPe PBT and GHG PBT are calculated by means of Eq. (1) and Eq. (2), respectively. In Fig. 10, the findings (for the case without recycling) are presented. For the PVgreen roof several scenarios (in terms of PV output increase because of plant/PV interaction: Table 3) are examined while the results for the PV-gravel and PV-bitumen are presented as straight lines. From Fig. 10a it can be observed that: 1) there is a critical point at 6% PV output increase which refers to the comparison of the PV-green with the PV-gravel system, 2) there is another critical point at 4.2% PV output increase which concerns the comparison of the PVgreen with the PV-bitumen configuration. In the same way, based on GHG PBT results (scenarios of Table 3; case without recycling), from Fig. 10b two additional critical points are identified at 4.7% and 5.5% PV output increase (regarding the comparison of the PV-green with the PV-bitumen and PV-gravel, respectively). The above mentioned critical points determine the critical PV output increase after which the PV-green roof pays back its additional environmental impact (because of the “green” layer: Sections 3.3 and 3.4) and becomes more eco-friendly than the other two PV roofing systems. The range of 4e6% critical PV output increase (identified by Fig. 10), based on the literature (Table 3) can be considered as reasonable for certain climatic conditions. Finally, it should be noted that ReCiPe PBT and GHG PBT of the PV roofs have been also calculated for the scenario with recycling. The findings demonstrate that for all the studied cases recycling results in ReCiPe PBT and GHG PBT reduction of approximately 0.4e0.6 years.
Fig. 10. PV roofs (scenario “No Recycling”): a) ReCiPe PBT and b) GHG PBT. For the PVgreen roof the scenarios regarding PV output increase are based on the literature (Table 3).
3.6. Comparison with the literature A direct comparison with the literature is not possible since there are no available results regarding the same cases in terms of the roofing systems (components, assumptions, etc) as well as in terms of the adopted LCIA methodologies. Nevertheless, in the following paragraphs some literature studies concerning LCA of green roofs, PVs, etc, are presented along with results of the present study. In the literature, there is an LCA about green roofs, embedding substrate in the environmental assessment [25]. The functional unit of 1 m2 of an extensive green roof was adopted. The GWP100a results for the production phase showed an impact of 21.1 kg CO2.eq. Based on the results of the present work for material manufacturing, the extensive green roof has a GWP100a footprint of: 1) 17.34 kg CO2.eq per m2 (by taking into account all the roof components except of steel joist and decking), 2) 23.96 kg CO2.eq per m2 (by considering all the roof components except of steel joist). Thus, the findings of the present study are quite close to [25], taking into account the differences regarding the considered roofing layers between the present investigation and reference [25]. Concerning ReCiPe LCIA methodology, Mohr et al. [26] found an overall damage score (ReCiPe, endpoint) of 0.01 ecopoints/kWh for a multi-Si PV system. In the present investigation, the PV-gravel roof shows a life-cycle (ReCiPe, endpoint) footprint of: 1) 0.014 Pts/kWh for the scenario without recycling, 2) 0.013 Pts/kWh for the case with recycling. Thereby, it can be seen that the results of the present work are quite close with those of [26], taking into consideration the differences between the two studies. On the other hand, Huijbregts et al. [24] calculated Rx for 19 homogeneous product/process subgroups (in total 1549 processes) and it was found that the majority of the products have Rx around
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
30 m2a/Pts ± a factor of 5. The present PV-green roof, for the scenario without recycling and by taking into account the impact for material manufacturing, shows Rx equal to 28.55 m2a/Pts; thus, within the range of reference [24]. 3.7. Future prospects The incorporation into the LCA model of building energy consumption/energy savings would be of great interest. This is because PV-green configuration can offer the advantages of a simple (without PV panels) green roof while at the same time produces energy with higher PV output than for example a PV-gravel system [1]. Certainly, this energy can be utilized to cover building energy needs during its use phase. The benefits of a PV-green roof for the building have been presented in the review study of the authors about critical factors which influence PV-green roof performance [3]. Building structure could be also incorporated into the LCA model as an extension of the present study. Another interesting future prospect could be an uncertainty analysis which could provide a wider view of the studied issues. On the other hand, the inclusion of the carbon sequestration benefits from the soil/plant layer could also offer interesting results since this is an additional advantage for the green roofing systems in comparison with the non-green roofs, depending on the selected plant species [3]. 4. Conclusions An LCA for different roofing systems: PV-green (extensive), extensive green, intensive green, PV-gravel, gravel and PV-bitumen (grey bitumen) is conducted by means of multiple LCIA methodologies (ReCiPe, etc) and different scenarios. The results for the PV-green roof (material manufacturing phase) show that PV laminates (multi-Si) and steel components (joist, decking and BOS) are responsible for the greatest part of the impact, based on GWP and ReCiPe. Regarding the roofs which do not produce electricity, intensive green system (based on GWP and ReCiPe findings) has remarkably higher impact comparing to the extensive green and gravel roofing systems. Concerning material manufacturing of the specific layers6 of each PV roof, PV-bitumen special components show higher USEtox footprint than PV-green and PV-gravel specific layers. Moreover, PV-green specific components have approximately double USEtox impact in comparison to PV-gravel special layers. With respect to use phase, PV-bitumen and PV-gravel systems have similar impact (around 15 t CO2.eq for the scenario without recycling) while PV-green roof has around 7 t CO2.eq higher emissions than the other two PV systems (because of the maintenance needs of the soil/plant layer). Nevertheless, by studying the life-cycle of each PV roofing system, it is demonstrated that PV-green configuration, on a long-term basis, compensates the above mentioned initial impact (due the “green” layer) and after a critical point it becomes more ecofriendly than the other two PV configurations. Certainly, this is related with the fact that PV output increases because of plant/PV interaction (evapotranspiration cooling effect, etc). This critical point is determined by means of ReCiPe PBT and GHG PBT and refers to a PV output increase around 4e6% (which can be considered reasonable for certain climatic conditions based on the literature: Table 3).
6 These specific layers are: 1) gravel and 3-ply SBS for the PV-gravel, 2) bitumen layer and 3-ply SBS for the PV-bitumen, 3) plants, soil, drainage layer, filter fabric and StressPly EUV for the PV-green roof.
383
The effect of recycling is also examined. For the scenario without recycling, ReCiPe PBT shows values: 1) from 6.9 to 7.5 years for the PV-green roof, depending on PV-output scenario, 2) 7.03 and 7.17 years for the PV-gravel and PV-bitumen system, respectively. On the other hand, GHG PBT (without recycling) is 5.3e5.8 years for the PV-green, 5.46 years for the PV-gravel and 5.50 years for the PVbitumen system. For all the studied PV roofs, the results reveal that recycling leads to ReCiPe PBT and GHG PBT reduction of about 0.4e0.6 years, depending on the studied case. At this point it should be noted that for the results/conclusions of the present LCA study, it should be taken into account the fact that the LCA has been conducted for certain boundaries and based on certain assumptions. With reference to the comparison of the present findings with the literature, several environmental indicators show quite good agreement with literature data, even if a direct comparison is not possible (because there are differences between the present study and those of the literature: in terms of the considered systems, etc). Conclusively, the results of the present investigation along with the findings from authors' previous LCA [2] provide a complete picture of the PV-green roofs in comparison to other roofing systems, based on multiple approaches and LCIA methodologies. Acknowledgements The authors would like to thank “Ministerio de Economía y Competitividad” of Spain for the funding (grant reference ENE201348325-R). References [1] D. Chemisana, Chr. Lamnatou, Photovoltaic-green roofs: an experimental evaluation of system performance, Appl. Energy 119 (2014) 246e256. [2] Chr. Lamnatou, D. Chemisana, Photovoltaic-green roofs: a life cycle assessment approach with emphasis on warm months of Mediterranean climate, J. Clean. Prod. 72 (2014) 57e75. [3] Chr. Lamnatou, D. Chemisana, A critical analysis of factors affecting photovoltaic-green roof performance, Renew. Sustain Energy Rev. 43 (2015) 264e280. [4] H.F. Castleton, V. Stovin, S.B.M. Beck, J.B. Davison, Green roofs; building energy savings and the potential for retrofit, Energy Build. 42 (2010) 1582e1591. € hler, W. Wiartalla, R. Feige, Interaction between PV-systems and [5] M. Ko Extensive Green Roofs, Greening Roofs for Sustainable Communities, Minneapolis, April 29 e May 1, 2007. [6] S.C.M. Hui, S.C. Chan, Integration of green roof and solar photovoltaic systems, in: Joint Symposium 2011: Integrated Building Design in the New Era of Sustainability, Nov. 22 2011. Hong Kong. [7] M.J.R. Perez, V.M. Fthenakis, N.T. Wight, C. Ho, Green-roof integrated PV canopies e an empirical study and teaching tool for low income students in the South Bronx, in: Proc. WREF World Renewable Energy Forum, May 13e17 2012. Colorado. [8] A. Nagengast, C. Hendrickson, H.S. Matthews, Variations in photovoltaic performance due to climate and low-slope roof choice, Energy Build. 64 (2013) 493e502. [9] L. Witmer, Quantification of the Passive Cooling of Photovoltaics Using a Green Roof, MSc thesis, The Pennsylvania State University, 2010. [10] A. Scherba, D.J. Sailor, T.N. Rosenstiel, C.C. Wamser, Modeling impacts of roof reflectivity, integrated photovoltaic panels and green roof systems on sensible heat flux into the urban environment, Build. Environ. 46 (2011) 2542e2551. [11] S. Saiz, C. Kennedy, B. Bass, K. Pressnail, Comparative life cycle assessment of standard and green roofs, Environ. Sci. Technol. 40 (2006) 4312e4316. [12] L. Kosareo, R. Ries, Comparative environmental life cycle assessment of green roofs, Build. Environ. 42 (2007) 2606e2613. lisis de Ciclo de Vida para [13] B. Rivela, I. Cuerda, F. Olivieri, C. Bedoya, J. Neila, Ana ~ o del sistema Intemper TF de cubierta ecolo gica aljibe, Mater. el ecodisen n 63 (2013) 131e145. Construccio [14] T.H. Hong, J.M. Kim, C.W. Koo, LCC and LCCO2 analysis of green roofs in elementary schools with energy saving measures, Energy Build. 45 (2012) 229e239. [15] F. Bianchini, K. Hewage, How “green” are the green roofs? Lifecycle analysis of green roof materials, Build. Environ. 48 (2012) 57e65. n-Palma, E. Sanye -Mengual, J. Oliver-Sola , J.I. Montero, C. Ponce-Ca[16] I. Cero ballero, J. Rieradevall, Towards a green sustainable strategy for social neighbourhoods in Latin America: case from social housing in Merida, Yucatan, Mexico, Habit. Int. 38 (2013) 47e56.
384
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
€virta, T. Ha €kkinen, Life cycle assessment of layers of [17] S. Bozorg Chenani, S. Lehva green roofs, J. Clean. Prod. 90 (2015) 153e162. [18] ISO 14040, Environmental Management e Life Cycle Assessment - Principles and Framework, ISO, Geneva, 2006. [19] ISO 14044, Environmental Management e Life Cycle Assessment - Requirements and Guidelines, ISO, Geneva, 2006. [20] www.recgroup.com (PV panels: REC Peak Energy Series: REC230PE). [21] M. Raugei, S. Bargigli, S. Ulgiati, Life cycle assessment and energy pay-back time of advanced photovoltaic modules: CdTe and CIS compared to poly-Si, Energy 32 (2007) 1310e1318. [22] SimaPro 8, www.pre-sustainability.com/simapro.
, various authors, SimaPro Database Manual, Methods Library, Report [23] Pre version 2.6, May 2014. [24] M.A.J. Huijbregts, S. Hellweg, R. Frischknecht, K. Hungerbühler, A.J. Hendriks, Ecological footprint accounting in the life cycle assessment of products, Ecol. Econ. 64 (2008) 798e807. [25] G. Peri, M. Traverso, M. Finkbeiner, G. Rizzo, Embedding “substrate” in environmental assessment of green roofs life cycle: evidences from an application to the whole chain in a Mediterranean site, J. Clean. Prod. 35 (2012) 274e287. [26] N.J. Mohr, A. Meijer, M.A.J. Huijbregts, L. Reijnders, Environmental life cycle assessment of roof-integrated flexible amorphous silicon/nanocrystalline silicon solar cell laminate, Prog. Photovolt. Res. Appl. 21 (2013) 802e815.
Contents lists available at ScienceDirect
Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Evaluation of photovoltaic-green and other roofing systems by means of ReCiPe and multiple life cycleebased environmental indicators Chr. Lamnatou*, D. Chemisana Applied Physics Section of the Environmental Science Department, University of Lleida, c/Pere Cabrera s/n, 25001 Lleida, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2015 Received in revised form 11 June 2015 Accepted 27 June 2015 Available online 2 July 2015
The present study evaluates the environmental profile of a PV-green roof (PV panels over a soil/plant layer) and other roofing systems (PV-bitumen, PV-gravel, gravel, extensive green and intensive green). The analysis is based on multiple life-cycle impact assessment methodologies (ReCiPe, etc), several scenarios (for example with and without recycling) and it provides a deeper analysis as well as additional results to authors' previous investigation about PV-green roofs. The evaluation of the PV-green system (in terms of material manufacturing phase) shows that PV laminates (multi-Si) and steel components (joist, decking, balance of system) are responsible for the greatest part of the total footprint, based on GWP (global warming potential) and ReCiPe. Among the roofs which do not produce electricity, material manufacturing phase reveals that intensive green configuration has considerably higher impact in comparison with gravel and extensive green systems. Concerning PV roofs, PV-green configuration on a long-term basis (by considering material manufacturing, use phase, transportation and disposal), after a critical point, pays back its additional environmental impact (related with the “green layer”) and it becomes more eco-friendly than the other two PV roofs. Certainly, this is due to the benefits (cooling effect of evapotranspiration, etc) of the soil/plant layer which result in PV output increase. The above mentioned critical point is determined by means of ReCiPe payback time and greenhouse-gas payback time. Several environmental indicators are calculated and presented along with results from the literature. A critical discussion is also provided. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Life cycle analysis (LCA) Photovoltaic (PV)-green roofs and buildings IPCC GWP ReCiPe USEtox Ecological footprint Payback time
1. Introduction Photovoltaic (PV)-green roofs combine PVs with simple green roofs (with soil/plant layer) and they have been studied by the authors of the present work from experimental [1] as well as from theoretical point of view [2,3]. At this point it should be noted that for PV-green applications are more appropriate shallow-substrate green systems (extensive green roofs) and not deep-substrate green systems (intensive green roofs). This is related with factors such as the high height of the soil/plant layer of the intensive green configurations, their high weight as well as with aesthetic/building integration issues [1,3]. A critical analysis of factors affecting PVgreen roof performance has been presented by Lamnatou and Chemisana [3]. A principal advantage of the plant/PV combination is the increase of PV output because of evapotranspiration cooling effect and in general because of plant/PV synergy, along with the other
* Corresponding author. E-mail address: [email protected] (Chr. Lamnatou). http://dx.doi.org/10.1016/j.buildenv.2015.06.031 0360-1323/© 2015 Elsevier Ltd. All rights reserved.
advantages which are also provided by the simple (without PVs) green roofs [3,4]. In the literature, there are few investigations about PV-green roofing systems: experimental studies [1,5e8], theoretical/modeling studies [2,6,9,10], critical review [3]. These investigations examine plant/PV interaction under several conditions (in terms of plant species, etc). For example for the Mediterranean climate summer conditions, two PV-green roofs (Gazania rigens and Sedum clavatum) and a PV-gravel roof (reference system) were examined [1]. The results for a sunny, five-day time period demonstrated an average increase of the maximum power output of the PV panels (ranging from 1.29% to 3.33% depending on the plant), verifying the positive interaction between PVs and plants [1]. By focusing on the techniques for the evaluation of the environmental impact of a system, Life Cycle Analysis (LCA) is a useful tool. However, in the literature there are few LCA studies about green roofs. In the following paragraphs, some of these studies are presented. Saiz et al. [11] investigated an eight-story, residential building, in Madrid. By replacing a common flat roof with a green one (extensive), the environmental impact showed a reduction ranging
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
from 1.0 to 5.3%. LCA was conducted for the whole building and by changing each roof option (assuming a 50-years building lifespan). A “bottom-up” approach was adopted. The stages of the life-cycle which were studied included material production and transportation, building operation and building maintenance (the construction phase and the end-of-life phase were not included) [11]. Kosareo and Ries [12] conducted a comparative LCA of intensive and extensive green roofing systems versus conventional roofs. The extensive green roof was based on an actual green roof project (retail store in Pittsburgh, PA, USA). The phases of fabrication, transportation, installation, operation, maintenance and disposal were considered. Ozone layer depletion, acidification, eutrophication, global warming and IMPACT 2002þ were adopted for the analysis. The results showed that the green roofs performed better than the control roof. In addition, the intensive green roof showed better performance than the extensive one. It should be noted that the goal of the LCA of [12] was to compare the environmental aspect and potential impact associated with constructing, maintaining and disposing of a roof (1115 m2) and to determine the option with the lowest negative impact. The aim was the identification of the environmentally preferable choice between an extensive green roofing system, an intensive green roof and a conventional stone ballasted roof. Several aspects were considered in terms of the creation, operation and demolition of the roof area. Multiple environmental factors were monitored, such as thermal transmittance and water run-off. In addition, the energy consumption of the building, depending on the type of roof, was taken into account [12]. Rivela et al. [13] conducted an LCA, based on CML 2000, about green roofs of Carex Testacea and Nassella tenuissima (stipa) in Spain. The functional unit was “one square meter of a reverted flat roof with tile floating floor for private pedestrian use”. The results revealed that the structural support had the highest contribution in all the studied impact categories, with the exception of “ozone layer depletion” category in which insulation showed 95% contribution. Lamnatou and Chemisana [2] evaluated a PV-green roof along with other roof configurations: PV-gravel, green (extensive and intensive) and gravel, by means of different Life Cycle Impact Assessment (LCIA) methodologies: EI99, IMPACT 2002þ and Cumulative Energy Demand (CED). Stages of the phases of material manufacturing, material transportation, use and disposal were considered. The functional unit was the whole roofing system (300 m2). The results revealed that material manufacturing is the most energy-demand phase for all the studied configurations. Emphasis was given on the PV-green roof and its comparison with the PV-gravel one, based on different scenarios. The results showed that although the PV-green configuration has an additional environmental impact in comparison with the PV-gravel one (because of the green-layer components), this additional impact on a log-term basis can be compensated. Other studies in the field of green-roof LCA are those of: Hong et al. [14] (about life-cycle cost and life-cycle CO2 analysis of green roofs in elementary schools with energy saving measures); Bianchini and Hewage [15] (about LCA of green-roof materials: it was demonstrated that the green-roof materials need to be replaced by n-Palma et al. [16] more environmentally friendly products); Cero (several green-space strategies for the building sector were examined); Bozorg Chenani et al. [17] (LCA of green-roof layers was conducted). The literature review reveals that most of the LCA studies: 1) concern simple (without PV panels) green roofs, 2) focus on the benefits of the soil/plant layer for the building (e.g. energy savings during building use phase), 3) do not include multiple and newlydeveloped LCIA methodologies. In continuation to authors' previous LCA about PV-green and other roofing systems [2], the present
377
article aims to provide new results by investigating the environmental performance of PV-green and other roofs by means of different LCIA methodologies, including newly-developed ones such as ReCiPe. An additional roofing system which includes PV modules over grey bitumen layer is introduced and compared with the other PV systems, strengthening the environmental benefits of the PV-green configuration. The analysis is based on multiple scenarios and several environmental indicators are evaluated, especially for the PV-green roof in comparison with other PV roofing systems. Critical points, after which the PV-green configuration becomes more eco-friendly than the other PV roofing systems, are identified. In this way, the present work along with authors' previous LCA study [2] provide a comprehensive profile of the environmental performance of the proposed PV-green configuration, based on multiple approaches. 2. Materials and methods For the implementation of the LCA, according to ISO 14040:2006 [18] and ISO 14044:2006 [19], the following phases are adopted: 1) goal and scope definition, 2) life-cycle inventory, 3) life-cycle impact assessment and 4) interpretation. 2.1. Functional unit and system boundaries The whole roofing system (300 m2) is used as functional unit. The system boundaries include the roof in terms of material manufacturing phase. However, for the comparison of the PV roofs on a life-cycle basis, the boundaries except of material manufacturing include also use phase, transportation and disposal. Details about the components/materials and the adopted methods follow below. 2.2. System definition 2.2.1. Technical characteristics of the studied configurations The roofing systems which are examined are: 1) gravel, 2) PVgravel, 3) extensive green, 4) PV-green (extensive), 5) intensive green, 6) PV-bitumen (grey bitumen). The available roof area is assumed to be 300 m2, considering a typical building with 30 m façade and 10 m width. The PV roofs refer to grid-connected PVs. The tilt angle of the PV modules, optimized in terms of the annual production for the case of Lleida (Catalonia, Spain), is 33 . Two rows of PVs are placed in parallel with around 5 m distance between the rows. Each row has 30 poly-crystalline Silicon (Si) PV modules. Each panel has the following characteristics: 230 Wp, Imp ¼ 7.98 A, Vmp ¼ 29.2 V, 60 cells, 1.66 0.99 m2 dimensions, electrical efficiency ¼ 13.9%, weight ¼ 18 kg (Source: [20]). Each row achieves 6.9 kWp; thereby, the total peak production of electricity is 13.8 kWp. The BOS (balance of system) includes: aluminum frame, support structure (steel), copper and plastic materials for cables and contact boxes. In terms of the roof components, all the roofing systems have structural support member (steel joist), decking (corrugated steel), insulation (polystyrene), underlayment (fiberboard) and asphalt adhesive. The above mentioned elements are common for all the roofs and they have the same amount of material, except for the intensive green roof where robust construction is needed and thereby, more steel is used for the support member. In addition, all the studied roofs include waterproofing membrane: 3-ply SBS (styrene-butadiene-styrene) for the non-green roofs and StressPly EUV (extreme ultraviolet) for the green roofs. Moreover, the green configurations have high-density polyethylene (HDPE) as drainage layer and filter fabric. Regarding the top layer, the green roofs have growing medium (height: 10 cm for the extensive and 100 cm for
378
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
Fig. 1. a) Sedum clavatum (left) and Gazania rigens (right) PV-green roofs, b) PV-gravel roof, developed by the authors at the University of Lleida, in Spain. Source: authors' archive of pictures.
the intensive green roof) while the non-green configurations have gravel or bitumen (instead of soil/plant layer). More details about the considered roofing systems for the LCA can be found in Ref. [2] while information about the PV-green and PV-gravel roofs developed by the authors (Fig. 1) can be found in Ref. [1].
green and green roofs [2], are presented. The reference materials and the construction of the roof basic configuration are based on reference [12].
2.4. Life-cycle impact assessment methodologies 2.2.2. Assumptions For the intensive green roof, shrubs and small trees are incorporated into the LCA model as small plants. In terms of the extensive green roofs, the impact of seeds is considered. The soil substrate of the green systems is based on clay. Concerning the PV roofs, the analysis is performed for multi-Si PV laminates and BOS impact refers to a typical grid-connected rooftop installation [21]. Regarding SBS and EUV membranes (modified bitumen), the impact of bitumen is taken into account. Moreover, it should be noted that each roofing system is considered as a “single” system (and not as a subsystem of a larger system (building)); thereby, the results are not presented in terms of the total building performance. For the use phase, the lifespan of the PV roofs is assumed to be 30 years. The PV output of the PV-gravel roof has been calculated for the case of Lleida [2]. Concerning the inputs during use phase, are taken into account: 1) for all the roofing systems: general maintenance (as 10% of material manufacturing impact) [2], 2) for the green roofs: fertilizing, irrigation and adding of soil substrate [2], 3) for the PV systems: replacement of the BOS (once over roof lifespan) [2], 4) for the PV-bitumen configuration: replacement of the bitumen layer (once over roof lifespan). The transportation distances are from/to the building (Lleida) by a truck. All the components except of the PV modules are assumed to be purchased from the local market (thus, transportation includes a distance of 5 km) while the PV panels are assumed to be purchased from Barcelona1 (round-trip distance: 266 km). Regarding disposal, landfill appropriate for each material is considered. For the green roofs, it is assumed that the end-of-life plants will be composted while the end-of-life soil will be reused in agriculture (for these two last cases, zero impact is considered).
IPCC 2013 GWP 20a V1.00, IPCC 2013 GWP 100a V1.00 and IPCC 2013 GWP 500a V1.00 are utilized in order to calculate GWP (global warming potential) of the studied roofs.3 Furthermore, ReCiPe Endpoint (H) V1.10/Europe ReCiPe H/A is also adopted (singlescore, endpoint approach), providing results for the damage categories Human Health, Ecosystems and Resources. Moreover, Ecological footprint V1.01/Ecological footprint (with characterization; results in m2a (¼ m2 per year)) and USEtox (default) V1.03/ Europe 2004 (with characterization; results in CTU (¼ comparative toxic unit)) are also utilized (Source: [22]). In terms of the adopted methodologies, IPCC 2013 is an update of IPCC 2007 developed by the International Panel on Climate Change. This method is listing the climate change factors (of IPCC) with a timeframe of 20, 100 and 500 years [23]. ReCiPe is the successor of EI99 and CML-IA. The purpose at the start of the development was to integrate the ‘problem oriented approach’ of methodology CML-IA and the ‘damage oriented approach’ of methodology EI99 [23]. The ecological footprint is the biologically productive land and water a population requires to produce the resources it consumes as well as to absorb part of the waste generated by fossil and nuclear fuel consumption [23]. In terms of the characterization, in the frame of LCA, the ecological footprint of a product is the sum of time integrated direct and indirect land occupation, related to nuclear energy use and to CO2 emissions from fossil energy use [23]. On the other hand, the USEtox model is an environmental model for the characterization of human and eco-toxicological impacts in the context of life-cycle impact assessment and comparative risk assessment [23]. In accordance to CED impact which was used for the evaluation of the energy Payback Time (PBT) of the PV roofs [2], the concept of ReCiPe PBT is introduced:
2.3. Life cycle inventory 2
SimaPro 8 and ecoinvent 3 database are utilized (Source: [22]). In Tables 1 and 2, the materials/components of the studied non-
1 For the transportation of the PV panels, only the purchasing point is considered. It is assumed that the PV panels are not imported from elsewhere. 2 For few cases, ELCD, LCA Food DK and EU & DK Input Output Database are also adopted.
ReCiPe PBT ¼
Imat þ Itransp þ Idisp Iout:a IO&M:a
ðyearsÞ
(1)
where I is the total endpoint ReCiPe score (Pts) regarding: material manufacturing (Imat); transportation (Itransp); disposal (Idisp); annual avoided impact due to the use of PV electricity instead of using
3
GWP is associated with carbon footprint.
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
379
Table 1 Life cycle inventory for the materials/components of the non-green roofs. Materials/components
Gravel roof: mass of the materials (kg)
PV-gravel roof: mass of the materials (kg)
PV-bitumen roof: mass of the materials (kg)
Steel joist Corrugated steel decking Polystyrene (insulation) Fiberboard (underlayment) 3-ply SBS (waterproofing membrane) Asphalt (adhensive) Gravel Bitumen PV laminates (multi-Si) BOS Aluminium (frame) Steel (support structure) Copper (cables and contact boxes) Plastics (cables and contact boxes)
1614.35 672.65 538.12 941.70 538.12 538.12 24000 e e
1614.35 672.65 538.12 941.70 538.12 538.12 24000 e 99 m2 (13.8 kWp)
1614.35 672.65 538.12 941.70 538.12 538.12 e 1200 99 m2 (13.8 kWp)
e e e e
188.10 2475.02 3.96 3.96
188.10 2475.02 3.96 3.96
Table 2 Life cycle inventory for the materials/components of the green roofs. Materials/components
Green roof (extensive): mass of the materials (kg)
PV-green roof (extensive): mass of the materials (kg)
Green roof (intensive): mass of the materials (kg)
Steel joist Corrugated steel decking Polystyrene (insulation) Fiberboard (underlayment) StressPly EUV (waterproofing membrane) Asphalt (adhensive) HDPE (drainage layer) HDPE (filter fabric) Soil substrate Plants (or seeds) PV laminates (multi-Si) BOS Aluminium (frame) Steel (support structure) Copper (cables and contact boxes) Plastics (cables and contact boxes)
1614.35 672.65 538.12 941.70 1210.76 538.12 53.81 53.81 28500 0.9 kg (seeds) e
1614.35 672.65 538.12 941.70 1210.76 538.12 53.81 53.81 28500 0.9 kg (seeds) 99 m2 (13.8 kWp)
2152.47 672.65 538.12 941.70 1210.76 538.12 53.81 53.81 285000 3000 (number of small plants) e
e e e e
188.10 2475.02 3.96 3.96
e e e e
Spain's electricity mix (Iout.a); annual impact during use phase (IO&M.a). In the same way with Eq. (1), Greenhouse Gas (GHG) PBT is evaluated by means of the following formula:
GHG PBT ¼
GHGmat þ GHGtransp þ GHGdisp GHGout:a GHGO&M:a
ðyearsÞ
(2)
where GHG is the total GWP100a impact (in tones CO2.eq) regarding: material manufacturing (GHGmat); transportation (GHGtransp); disposal (GHGdisp); annual avoided impact because of the use of PV electricity instead of using Spain's electricity mix (GHGout.a); annual impact during use phase (GHGO&M.a). It should be noted that for the evaluation of ReCiPe and GHG PBT, the electricity of Spain is considered as reference. On the other hand, the ratio Rx of the Ecological Footprint (EF) EFx (in m2a) and EI99x (in ecopoints) is evaluated based on the following equation [24]:
Rx ¼
EFx EI99x
(3)
The ratio Rx quantifies the relationship between EF (as a relative simple environmental indicator) and EI99 (as a relative complex environmental indicator) [24]. Rx ratio is a potential conversion factor between EI99 results (Pts) and EF results (m2a). If the ratio is approximately equal for all the evaluated products, this implies that the two methods do not differ in their gross ranking of the products
[24]. It should be noted that for the calculation of Rx (Eq. (3)), EI99 results from authors' previous LCA [2] are utilized. In the frame of the present study, Rx is used as an additional result in order to compare with the findings of Huijbregts et al. [24]. 2.5. Sensitivity analysis For all the roofing systems, three different scenarios in terms of GWP time horizon are examined: 20, 100 and 500 years. Regarding the PV roofs, an additional scenario “Recycling” vs. “No Recycling” is adopted in order to examine the effect of BOS aluminium and copper recycling. Furthermore, for the PV-green roof, eight scenarios in terms of PV output increase due to plant/PV synergy are adopted, based on literature studies (experimental and theoretical) about PV-green roofs (Table 3). Several scenarios are examined: PV output increase ranging from 0.08% (pessimistic scenario: [9]) to 8.3% (optimistic scenario: [6]). By evaluating these different cases, a wide view of the studied issues is provided while some of these scenarios can be considered as realistic for the Mediterranean climatic conditions of Spain. 3. Results and discussion 3.1. PV-green roof: material manufacturing In this section the environmental profile of the PV-green roof is
380
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
Table 3 Scenarios in terms of PV output increase due to plant/PV interaction, based on references from the literature. Scenarios
References from the literature
PV output increase
1 2 3 4 5 6 7 8
Witmer [9] Nagengast et al. [8] Witmer [9] Chemisana and Lamnatou [1] Perez et al. [7] Chemisana and Lamnatou [1] Hui and Chan [6] Hui and Chan [6]
0.08% 0.50% 0.55% 1.29% 2.56% 3.33% 4.30% 8.30%
a
a Based on authors' experimental study [1], scenario 4 refers to PV-gazania and scenario 6 concerns PV-sedum roof.
Fig. 3. ReCiPe scores (Pts) (for Human Health, Ecosystems and Resources) of the PVgreen roof for certain roof components. Scenario “No Recycling”. Phase of material manufacturing.
Fig. 2. GWP100a (t CO2.eq) of the PV-green roof for certain roof components. Scenario “No Recycling”. Phase of material manufacturing. Fig. 4. GWP20a, GWP100a and GWP500a (t CO2.eq) for the gravel, extensive green and intensive green roofs. Phase of material manufacturing.
presented. In Fig. 2, the GWP100a footprint of each roof component (scenario: No Recycling) is illustrated.4 PV laminates are responsible for the greatest part of the total impact of the PV-green roof and steel (including steel joist, decking and BOS) is the material with the second highest contribution. The participation of the PV laminates and steel components to the total GWP100a footprint of the PV-green roof is around 49% and 32%, respectively. All the other components of the PV-green configuration (Table 2) show percentages less than 8% (in terms of their contribution to the total GWP100a impact). The effect of recycling (based on GWP20a, GWP100a and GWP500a) has been also examined. The results demonstrate that recycling leads to an impact reduction of 3e3.3 t CO2.eq. Furthermore, in Fig. 3 ReCiPe scores (Pts) for Human Health, Ecosystems and Resources are presented. By taking into account the total scores of the three endpoint categories, the results demonstrate that Human Health and Resources have higher ReCiPe footprint in comparison with Ecosystems. Regarding the contribution of each part of the PV-green roof to the total ReCiPe impact of each category, the findings reveal that: 1) PV laminates and steel (joist, decking and BOS) show the highest contribution, 2) the other components of the PV-green system (Table 2) show percentages less than 8%. On the other hand, the adoption of recycling results in ReCiPe impact reduction around 4e8%, depending on the impact category.
4 From Fig. 2 (as well as from Fig. 3) have been excluded the components which have considerably lower impact than those presented in the graphs. However, the calculations have been conducted by taking into account all the components.
3.2. All the roofs: a comparison based on material manufacturing 3.2.1. Roofing systems which do not produce electricity In Fig. 4, GWP20a, GWP100a and GWP500a for the gravel, extensive green and intensive green roofs are illustrated. It can be seen that the extensive green roof has around 4e5% higher GWP impact in comparison with the gravel configuration. Moreover, the intensive green roof presents remarkably higher GWP impact (about 33e41%) comparing to the extensive green and gravel roofing systems. By considering the effect of the time horizon, it can be observed (as it was expected) that by adopting a longer time horizon there is an impact reduction. With respect to ReCiPe results (Fig. 5), the extensive green roof has 20, 15 and 69 Pts (for each damage category, respectively) higher footprint comparing to the gravel configuration. On the other hand, the intensive green roof has remarkably higher (about 29e40%) ReCiPe impact than gravel and extensive green roofing systems. The considerably higher (GWP and ReCiPe) footprint of the intensive green roof is mainly associated with the high amounts of steel (for steel joist) and soil (Table 2) which are required for the green intensive configuration. An additional impact of the green intensive roof (in comparison with the green extensive and gravel) is related with the fact that the life-cycle impact for plant production is taken into account. Furthermore, from Fig. 5 it can be noticed that for all the studied cases the lowest footprint is for Ecosystems (less than 400 Pts). 3.2.2. Roofing systems which produce electricity In Fig. 6, GWP20a, GWP100a and GWP500a results for the PV roofs (scenario “No Recycling”) are illustrated. It can be seen that
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
Fig. 5. ReCiPe scores (Pts) (for Human Health, Ecosystems and Resources) of the gravel, extensive green and intensive green roofs. Phase of material manufacturing.
381
Fig. 7. ReCiPe scores in Pts (Human health, Ecosystems and Resources) for the PV roofs. Scenario “No Recycling”. Phase of material manufacturing.
Fig. 6. GWP20a, GWP100a and GWP500a for the PV roofs. Scenario “No Recycling”. Phase of material manufacturing.
PV-green roof has higher footprint than PV-bitumen and PV-gravel while PV-bitumen shows higher footprint than PV-gravel. Nevertheless, for all the cases of Fig. 6 the differences in CO2.eq emissions are less than 1.5%. In terms of GWP time horizon (as for the case of Fig. 4), the adoption of longer time horizon results in impact reduction. The effect of recycling has been also examined and the findings show that for all the studied PV roofs recycling results in GWP reduction of around 6e8%. Regarding ReCiPe results (Fig. 7, scenario “No Recycling”), the impact for the PV roofs ranges from around 900 to 2200 Pts with the PV-green system showing 1e69 Pts higher impact (for most of the damage categories) than the other two PV configurations. With respect to recycling, there is a ReCiPe impact reduction of about 4e8% for all the studied PV roofs. 3.3. The impact related to the specific PV-roof layers: material manufacturing Each PV roofing system has some particular components.5 These special parts are: 1) gravel and 3-ply SBS for the PV-gravel, 2) bitumen layer and 3-ply SBS for the PV-bitumen, 3) plants, soil, drainage layer, filter fabric and StressPly EUV for the PV-green roof. In this section the impact associated to material manufacturing of the above mentioned specific layers is presented. In Fig. 8, USEtox findings for Human toxicity (cancer and non-cancer) (Fig. 8a) and Ecotoxicity (Fig. 8b) are illustrated. It can be observed that there is a considerable difference in the toxicity of the studied cases. More analytically, PV-bitumen specific layers show higher impact than
5 Moreover, the upper roofing layers interact with the PV panels and thus, influence PV output.
Fig. 8. USEtox results for the specific layers of the PV roofs: a) Human toxicity (cancer and non-cancer) (in CTUh) and b) Ecotoxicity (in CTUe). Phase of material manufacturing.
PV-green and PV-gravel particular components. On the other hand, PV-green specific layers have approximately double footprint in comparison to PV-gravel particular components. Nevertheless, by considering the life-cycle of each PV roof (Section 3.5), it is verified that the PV-green roof, on a long-term basis, compensates the above mentioned initial impact (due to the “green layers”) and after a certain point PV-green becomes more eco-friendly than the other two PV configurations. Certainly, this is related with PV-green increased electricity production because of plant/PV interaction. 3.4. The impact related to use phase of the PV roofs The impact associated with use phase is considerably different for the PV-green roof than for the other two PV systems. Details were previously presented in the assumptions (Section 2.2.2). Certainly, soil/plant layer has significantly higher maintenance needs than gravel or bitumen layer. Based on GWP100a findings
382
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
Fig. 9. GWP100a (t CO2.eq) of the PV roofs during use phase. Results on a lifespan basis of 30 years. Scenarios “No Recycling” vs. “Recycling”.
(Fig. 9), PV-bitumen and PV-gravel systems have similar impact (approximately 15 t CO2.eq for the scenario without recycling) while PV-green roof has around 7 t CO2.eq higher emissions than the other two PV configurations. In terms of recycling, for all the PV roofs there is a footprint reduction of 3.4 t CO2.eq. The impact during use phase has been also calculated by means of ReCiPe. The results show total ReCiPe scores (scenario without recycling) of 2561, 1925 and 1802 Pts for the PV-green, PV-bitumen and PV-gravel, respectively. Moreover, according to ReCiPe findings, recycling results in an impact reduction of around 14e19% for all the studied PV roofs. At this point it should be noted that in Section 3.5, the results about the payback times of the PV roofs verify the fact that on a long-term reference frame the PV-green roof compensates this higher impact related with its use phase (and in general, the higher footprint because of the green layer) and after a certain point PVgreen becomes more eco-friendly than the other two PV roofing systems.
3.5. ReCiPe payback time and GHG payback time of the PV roofs The evaluation of the PV roofs over their life-cycle is conducted based on ReCiPe and GWP100a. ReCiPe PBT and GHG PBT are calculated by means of Eq. (1) and Eq. (2), respectively. In Fig. 10, the findings (for the case without recycling) are presented. For the PVgreen roof several scenarios (in terms of PV output increase because of plant/PV interaction: Table 3) are examined while the results for the PV-gravel and PV-bitumen are presented as straight lines. From Fig. 10a it can be observed that: 1) there is a critical point at 6% PV output increase which refers to the comparison of the PV-green with the PV-gravel system, 2) there is another critical point at 4.2% PV output increase which concerns the comparison of the PVgreen with the PV-bitumen configuration. In the same way, based on GHG PBT results (scenarios of Table 3; case without recycling), from Fig. 10b two additional critical points are identified at 4.7% and 5.5% PV output increase (regarding the comparison of the PV-green with the PV-bitumen and PV-gravel, respectively). The above mentioned critical points determine the critical PV output increase after which the PV-green roof pays back its additional environmental impact (because of the “green” layer: Sections 3.3 and 3.4) and becomes more eco-friendly than the other two PV roofing systems. The range of 4e6% critical PV output increase (identified by Fig. 10), based on the literature (Table 3) can be considered as reasonable for certain climatic conditions. Finally, it should be noted that ReCiPe PBT and GHG PBT of the PV roofs have been also calculated for the scenario with recycling. The findings demonstrate that for all the studied cases recycling results in ReCiPe PBT and GHG PBT reduction of approximately 0.4e0.6 years.
Fig. 10. PV roofs (scenario “No Recycling”): a) ReCiPe PBT and b) GHG PBT. For the PVgreen roof the scenarios regarding PV output increase are based on the literature (Table 3).
3.6. Comparison with the literature A direct comparison with the literature is not possible since there are no available results regarding the same cases in terms of the roofing systems (components, assumptions, etc) as well as in terms of the adopted LCIA methodologies. Nevertheless, in the following paragraphs some literature studies concerning LCA of green roofs, PVs, etc, are presented along with results of the present study. In the literature, there is an LCA about green roofs, embedding substrate in the environmental assessment [25]. The functional unit of 1 m2 of an extensive green roof was adopted. The GWP100a results for the production phase showed an impact of 21.1 kg CO2.eq. Based on the results of the present work for material manufacturing, the extensive green roof has a GWP100a footprint of: 1) 17.34 kg CO2.eq per m2 (by taking into account all the roof components except of steel joist and decking), 2) 23.96 kg CO2.eq per m2 (by considering all the roof components except of steel joist). Thus, the findings of the present study are quite close to [25], taking into account the differences regarding the considered roofing layers between the present investigation and reference [25]. Concerning ReCiPe LCIA methodology, Mohr et al. [26] found an overall damage score (ReCiPe, endpoint) of 0.01 ecopoints/kWh for a multi-Si PV system. In the present investigation, the PV-gravel roof shows a life-cycle (ReCiPe, endpoint) footprint of: 1) 0.014 Pts/kWh for the scenario without recycling, 2) 0.013 Pts/kWh for the case with recycling. Thereby, it can be seen that the results of the present work are quite close with those of [26], taking into consideration the differences between the two studies. On the other hand, Huijbregts et al. [24] calculated Rx for 19 homogeneous product/process subgroups (in total 1549 processes) and it was found that the majority of the products have Rx around
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
30 m2a/Pts ± a factor of 5. The present PV-green roof, for the scenario without recycling and by taking into account the impact for material manufacturing, shows Rx equal to 28.55 m2a/Pts; thus, within the range of reference [24]. 3.7. Future prospects The incorporation into the LCA model of building energy consumption/energy savings would be of great interest. This is because PV-green configuration can offer the advantages of a simple (without PV panels) green roof while at the same time produces energy with higher PV output than for example a PV-gravel system [1]. Certainly, this energy can be utilized to cover building energy needs during its use phase. The benefits of a PV-green roof for the building have been presented in the review study of the authors about critical factors which influence PV-green roof performance [3]. Building structure could be also incorporated into the LCA model as an extension of the present study. Another interesting future prospect could be an uncertainty analysis which could provide a wider view of the studied issues. On the other hand, the inclusion of the carbon sequestration benefits from the soil/plant layer could also offer interesting results since this is an additional advantage for the green roofing systems in comparison with the non-green roofs, depending on the selected plant species [3]. 4. Conclusions An LCA for different roofing systems: PV-green (extensive), extensive green, intensive green, PV-gravel, gravel and PV-bitumen (grey bitumen) is conducted by means of multiple LCIA methodologies (ReCiPe, etc) and different scenarios. The results for the PV-green roof (material manufacturing phase) show that PV laminates (multi-Si) and steel components (joist, decking and BOS) are responsible for the greatest part of the impact, based on GWP and ReCiPe. Regarding the roofs which do not produce electricity, intensive green system (based on GWP and ReCiPe findings) has remarkably higher impact comparing to the extensive green and gravel roofing systems. Concerning material manufacturing of the specific layers6 of each PV roof, PV-bitumen special components show higher USEtox footprint than PV-green and PV-gravel specific layers. Moreover, PV-green specific components have approximately double USEtox impact in comparison to PV-gravel special layers. With respect to use phase, PV-bitumen and PV-gravel systems have similar impact (around 15 t CO2.eq for the scenario without recycling) while PV-green roof has around 7 t CO2.eq higher emissions than the other two PV systems (because of the maintenance needs of the soil/plant layer). Nevertheless, by studying the life-cycle of each PV roofing system, it is demonstrated that PV-green configuration, on a long-term basis, compensates the above mentioned initial impact (due the “green” layer) and after a critical point it becomes more ecofriendly than the other two PV configurations. Certainly, this is related with the fact that PV output increases because of plant/PV interaction (evapotranspiration cooling effect, etc). This critical point is determined by means of ReCiPe PBT and GHG PBT and refers to a PV output increase around 4e6% (which can be considered reasonable for certain climatic conditions based on the literature: Table 3).
6 These specific layers are: 1) gravel and 3-ply SBS for the PV-gravel, 2) bitumen layer and 3-ply SBS for the PV-bitumen, 3) plants, soil, drainage layer, filter fabric and StressPly EUV for the PV-green roof.
383
The effect of recycling is also examined. For the scenario without recycling, ReCiPe PBT shows values: 1) from 6.9 to 7.5 years for the PV-green roof, depending on PV-output scenario, 2) 7.03 and 7.17 years for the PV-gravel and PV-bitumen system, respectively. On the other hand, GHG PBT (without recycling) is 5.3e5.8 years for the PV-green, 5.46 years for the PV-gravel and 5.50 years for the PVbitumen system. For all the studied PV roofs, the results reveal that recycling leads to ReCiPe PBT and GHG PBT reduction of about 0.4e0.6 years, depending on the studied case. At this point it should be noted that for the results/conclusions of the present LCA study, it should be taken into account the fact that the LCA has been conducted for certain boundaries and based on certain assumptions. With reference to the comparison of the present findings with the literature, several environmental indicators show quite good agreement with literature data, even if a direct comparison is not possible (because there are differences between the present study and those of the literature: in terms of the considered systems, etc). Conclusively, the results of the present investigation along with the findings from authors' previous LCA [2] provide a complete picture of the PV-green roofs in comparison to other roofing systems, based on multiple approaches and LCIA methodologies. Acknowledgements The authors would like to thank “Ministerio de Economía y Competitividad” of Spain for the funding (grant reference ENE201348325-R). References [1] D. Chemisana, Chr. Lamnatou, Photovoltaic-green roofs: an experimental evaluation of system performance, Appl. Energy 119 (2014) 246e256. [2] Chr. Lamnatou, D. Chemisana, Photovoltaic-green roofs: a life cycle assessment approach with emphasis on warm months of Mediterranean climate, J. Clean. Prod. 72 (2014) 57e75. [3] Chr. Lamnatou, D. Chemisana, A critical analysis of factors affecting photovoltaic-green roof performance, Renew. Sustain Energy Rev. 43 (2015) 264e280. [4] H.F. Castleton, V. Stovin, S.B.M. Beck, J.B. Davison, Green roofs; building energy savings and the potential for retrofit, Energy Build. 42 (2010) 1582e1591. € hler, W. Wiartalla, R. Feige, Interaction between PV-systems and [5] M. Ko Extensive Green Roofs, Greening Roofs for Sustainable Communities, Minneapolis, April 29 e May 1, 2007. [6] S.C.M. Hui, S.C. Chan, Integration of green roof and solar photovoltaic systems, in: Joint Symposium 2011: Integrated Building Design in the New Era of Sustainability, Nov. 22 2011. Hong Kong. [7] M.J.R. Perez, V.M. Fthenakis, N.T. Wight, C. Ho, Green-roof integrated PV canopies e an empirical study and teaching tool for low income students in the South Bronx, in: Proc. WREF World Renewable Energy Forum, May 13e17 2012. Colorado. [8] A. Nagengast, C. Hendrickson, H.S. Matthews, Variations in photovoltaic performance due to climate and low-slope roof choice, Energy Build. 64 (2013) 493e502. [9] L. Witmer, Quantification of the Passive Cooling of Photovoltaics Using a Green Roof, MSc thesis, The Pennsylvania State University, 2010. [10] A. Scherba, D.J. Sailor, T.N. Rosenstiel, C.C. Wamser, Modeling impacts of roof reflectivity, integrated photovoltaic panels and green roof systems on sensible heat flux into the urban environment, Build. Environ. 46 (2011) 2542e2551. [11] S. Saiz, C. Kennedy, B. Bass, K. Pressnail, Comparative life cycle assessment of standard and green roofs, Environ. Sci. Technol. 40 (2006) 4312e4316. [12] L. Kosareo, R. Ries, Comparative environmental life cycle assessment of green roofs, Build. Environ. 42 (2007) 2606e2613. lisis de Ciclo de Vida para [13] B. Rivela, I. Cuerda, F. Olivieri, C. Bedoya, J. Neila, Ana ~ o del sistema Intemper TF de cubierta ecolo gica aljibe, Mater. el ecodisen n 63 (2013) 131e145. Construccio [14] T.H. Hong, J.M. Kim, C.W. Koo, LCC and LCCO2 analysis of green roofs in elementary schools with energy saving measures, Energy Build. 45 (2012) 229e239. [15] F. Bianchini, K. Hewage, How “green” are the green roofs? Lifecycle analysis of green roof materials, Build. Environ. 48 (2012) 57e65. n-Palma, E. Sanye -Mengual, J. Oliver-Sola , J.I. Montero, C. Ponce-Ca[16] I. Cero ballero, J. Rieradevall, Towards a green sustainable strategy for social neighbourhoods in Latin America: case from social housing in Merida, Yucatan, Mexico, Habit. Int. 38 (2013) 47e56.
384
C. Lamnatou, D. Chemisana / Building and Environment 93 (2015) 376e384
€virta, T. Ha €kkinen, Life cycle assessment of layers of [17] S. Bozorg Chenani, S. Lehva green roofs, J. Clean. Prod. 90 (2015) 153e162. [18] ISO 14040, Environmental Management e Life Cycle Assessment - Principles and Framework, ISO, Geneva, 2006. [19] ISO 14044, Environmental Management e Life Cycle Assessment - Requirements and Guidelines, ISO, Geneva, 2006. [20] www.recgroup.com (PV panels: REC Peak Energy Series: REC230PE). [21] M. Raugei, S. Bargigli, S. Ulgiati, Life cycle assessment and energy pay-back time of advanced photovoltaic modules: CdTe and CIS compared to poly-Si, Energy 32 (2007) 1310e1318. [22] SimaPro 8, www.pre-sustainability.com/simapro.
, various authors, SimaPro Database Manual, Methods Library, Report [23] Pre version 2.6, May 2014. [24] M.A.J. Huijbregts, S. Hellweg, R. Frischknecht, K. Hungerbühler, A.J. Hendriks, Ecological footprint accounting in the life cycle assessment of products, Ecol. Econ. 64 (2008) 798e807. [25] G. Peri, M. Traverso, M. Finkbeiner, G. Rizzo, Embedding “substrate” in environmental assessment of green roofs life cycle: evidences from an application to the whole chain in a Mediterranean site, J. Clean. Prod. 35 (2012) 274e287. [26] N.J. Mohr, A. Meijer, M.A.J. Huijbregts, L. Reijnders, Environmental life cycle assessment of roof-integrated flexible amorphous silicon/nanocrystalline silicon solar cell laminate, Prog. Photovolt. Res. Appl. 21 (2013) 802e815.