This is an Accepted Manuscript of the article Corcelli, F., Fiorentino, G., Petit-Boix, A., Rieradevall, J., & Gabarrell, X. (2019). Transforming rooftops into productive urban spaces in the Mediterranean. An LCA comparison of agri-urban production and photovoltaic energy generation. Resources, Conservation and Recycling, 144, 321-336 available online at https://doi.org/10.1016/j.resconrec.2019.01.040. ©2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license. 1 Transforming rooftops into productive urban spaces in the Mediterranean. An 2 LCA comparison of agri-urban production and photovoltaic energy generation 3 4 Corcelli, F.a,*, Fiorentino, G.a, Petit-Boix A.b,c, Rieradevall, J.b,d,e, Gabarrell, X. b,d,e 5 a Department of Science and Technology, Parthenope University of Naples, Centro Direzionale – Isola C4, 80143 Naples, 6 Italy 7 b Sostenipra Research Group (2017SGR1683), Institute of Environmental Sciences and Technology (ICTA), Unidad de 8 excelencia «María de Maeztu» (MDM-2015-0552), Autonomous University of Barcelona (UAB), Campus UAB, 08193, 9 Bellaterra, Barcelona, Spain 10 c Chair of Societal Transition and Circular Economy, University of Freiburg, Tennenbacher Str. 4, 79106 Freiburg i. Br., 11 Germany 12 d Department of Chemical, Biological and Environmental Engineering, Catalan Biotechnology Reference Network -XRB 13 Autonomous University of Barcelona (UAB), Campus UAB, 08193, Bellaterra, Barcelona, Spain 14 e Department of Chemical, Biological and Environmental Engineering, School of Engineering, Autonomous University 15 of Barcelona (UAB), Campus UAB, 08193, Bellaterra, Barcelona, Spain 16 17 Abstract 18 A key strategy towards sustainable urban development is designing cities for increased circular metabolism. The 19 transformation of areas underused, such as urban rooftops, into productive spaces is being increasingly implemented as a 20 result of associated multiple benefits. Rooftop greenhouses (RTGs) are an interesting option for exploiting urban rooftops 21 with direct exposure to sunlight, reducing food miles and creating new agricultural spaces, while building-applied solar 22 photovoltaic (BAPV) panels provide clean energy and reduce greenhouse gas emissions. However, a proper assessment 23 of environmental costs and benefits related to both systems is vital for a successful implementation. By means of life 24 cycle assessment method, modelled in the professional software SimaPro, this paper aims at comparing the environmental 25 performance of different productive uses of rooftops under Mediterranean climatic conditions. The results showed that 26 both systems are favourable and contribute to decreasing the environmental impacts thanks to the production of resources 27 on-site. BAPV system shows the highest avoided burdens in comparison with RTG: for instance, the impacts generated 28 by BAPV on climate change and fossil depletion categories, corresponding to - 430 kg CO2 eq/m2 and -110 kg oil eq/m2 29 respectively (versus -22 kg CO2 eq/m2 and -4.7 kg oil eq/m2 in the RTG system), are around 20 times lower than RTG. 30 Furthermore, a sensitivity analysis was performed through different scenarios, based on reductions or substitution of the 31 most sensitive input flows, thus providing some useful tools for improved environmental performances. Attention to 32 additional energy and material efficiency, in favour of the more environmentally sustainable choice, should remain a main 33 point of investigation. 34 35 *Corresponding author: fabiana.corcelli@uniparthenope.it; Tel: +390815476666 36 Keywords: cities, LCA, agri-urban, photovoltaic energy, rooftop, circular economy 37 38 39 Acronyms BAPV Building Applied Photovoltaic LCIA Life Cycle Impact Assessment BOS Balance of System MD Metal Depletion CC Climate Change OD Ozone Depletion c-Si crystallin Silicon POF Photochemical Oxidant Formation EoL End-of-Life RTG Rooftop Greenhouse FD Fossil Depletion TA Terrestrial Acidification FE Freshwater Eutrophication TE Terrestrial Ecotoxicity FU Functional Unit UA Urban Agriculture GHG Greenhouse Gas UAB Autonomous University of Barcelona GR Green Roof WD Water Depletion LCA Life Cycle Assessment Wp Peak Watt LCI Life Cycle Inventory WEEE Waste Electrical and Electronic equipment 40 41 1. Introduction 42 43 Urban areas play a significant role in addressing the 21st century’s challenges of sustainably realizing climate, energy and 44 economic objectives. Currently, urban environments have proved to be not self-sustaining, as they heavily rely on 45 imported resources and their environmental footprint exceeds their natural bio-capacity (Doughty and Hammond 2004). 46 Cities host more than 50% of the global population (United Nation, 2014), consuming 60-80% of global primary energy 47 and generating 70% of the world’s total greenhouse gas (GHG) emissions, primarily through the consumption of fossil 48 fuels for energy supply, transportation and food production (UN-Habitat, 2016). To transform modern cities into more 49 sustainable environments, cities need to develop a more circular metabolism where more resources are recycled, reused 50 or produced on-site whilst cleaner forms of energy are produced and consumed (Doughty and Hammond 2004). 51 Additionally, there is a need to optimize land use in over-populated cities where land competition becomes a problem. 52 In this context, sustainable solutions for food, water, energy, and transport of food or waste are needed as integrated 53 components of a city’s climate change adaptation (World Bank, 2010). Sustainable urbanization practices offer many 54 opportunities for optimizing resource use efficiency and developing mitigation measures to deal with such problems, 55 especially through urban planning (e.g., exploitation of unused areas for local resources production, efficient waste 56 management) and design (e.g., green construction, use of recycled materials, upgraded technologies for material/energy 57 recovery) (UN-Habitat, 2016). Urban deployment of such strategies is often amalgamated under concepts such as the 58 ‘eco-city’, ‘circular city’ or ‘sustainable city’, and have found wide spread local, regional, and national acceptance (Castán 59 Broto and Bulkeley, 2013; Petit-Boix and Leipold, 2018). In this sense, converting vacant rooftops into productive spaces 60 is a recognised strategy among researchers, city planners and developers (Carter and Keeler 2008; Elzeyadi et al., 2009, 61 Nadal et al., 2017a). Rooftops have an unprecedented exploitation potential, as they cover up to 32% of cities and built- 62 up areas (Frazer, 2005) and can improve the urban metabolism by producing resources such as energy, greening, food 63 and water (Mahmoud et al., 2014; Specht et al., 2014; Goldstein et al., 2016, Yang and Zou, 2016; Petit-Boix et al., 2018). 64 The transformation of these urban rooftops into productive spaces is becoming standard practice in many cities worldwide 65 (Proksch, 2011). For instance, rooftop greenhouses (RTGs) for food production are gaining popularity in large cities such 66 as New York, Singapore and Montreal (Ackerman et al., 2012; Deng and Quigley, 2012; Haberman et al., 2014). 67 Similarly, as recently noted by Sanyé-Mengual et al. (2015a), RTGs are finding deployment in experimental projects also 68 in the Mediterranean context because of the favourable climate conditions. At the same time, in order to mitigate the 69 environmental impacts of urbanization, researchers worldwide have been recently looking at ways of turning buildings 70 into net energy producers. Solar energy is an infinite and clean resource, and scientists have been assessing systems such 71 as Building-Applied Photovoltaics (BAPV), which consist of generating considerable fractions of urban electricity 72 without the need of dedicating exclusive surface areas for solar photovoltaic (PV) plant installations (Santos and Rüther, 73 2012; Benis et al., 2018). BAPV systems are typically used in retrofits, with off-the-shelf PV panels mounted on a separate 74 metal support structure, superimposed on an existing building’s roof or façades (Santos and Rüther, 2012). Among 75 existing applications for BAPV, rooftops are considered the ideal option, since pitched roofs with a proper angle and 76 orientation provide the highest energy harvesting (Heinstein et al., 2013; Benis et al., 2018). Food production and energy 77 generation on urban rooftops are also an important source of environmental benefits. Table 1 summarizes the main 78 environmental benefits relevant for the RTGs and BAPV systems under investigation in this study. 79 In the recent scientific literature, sustainable urban solutions addressing food supply, on the one hand, and energy supply, 80 on the other, have been increasingly assessed. A range of studies have focused on the role played by urban agriculture 81 (UA) in urban food self-reliance and environmental impact mitigation (Grewal and Grewal, 2012; Haberman et al., 2014; 82 Orsini et al., 2014; Benis and Ferrão, 2016; Wielemaker et al., 2018, among others). Further studies have estimated the 83 potential of renewable energy such as solar power systems to satisfy the high-energy demands of growing urban areas 84 (Hofierka and Kanuk, 2009; Amado and Poggi, 2014; Byrne et al., 2015) and investigated the environmental costs and 85 benefits of this technology (Fthenakis et al., 2009; Peng et al., 2013; Corcelli et al., 2017, 2018a; Eskew et al., 2018; 86 among others). Green roofs (GR) are the main rooftop interventions considered, but comparison with food production 87 remains unexplored. For instance, Elzeyadi et al. (2009) investigated the effectiveness of GRs and ‘cool’ roofs in 88 mitigating the Urban Heat Island (UHI) comparing temperatures on a single building and finding that GRs are cooler in 89 both north and south roof orientation. The importance of maximizing spatial efficiency and the need to design spatial 90 decision support systems that integrate all potential renewable resources in a given area were highlighted by Calvert et 91 al. (2015). As for land use, the generation of multifold renewable resources can also be addressed by rooftops, that are 92 suitable for the integrated application of multiple technologies in a given site. Some recent studies investigated the 93 integration of solar energy systems into rooftop farming activities. Hui and Chan (2011) and Nagengast et al. (2013) 94 found that, in places where temperature is normally higher, the benefits of an integrated PV-GR will be more visible. 95 Perez et al. (2012) and Lamnatou and Chemisana (2014, 2015) explored the environmental performance of different roof 96 uses (PV-GR, PV-gravel, GR, standard built-up roof). In all above-mentioned works, the results showed that the 97 technology combining PV-GR offers multiple benefits, in particular in warmer climates and in the long term. 98 Moreover, a wide variety of studies have used the life cycle assessment (LCA) method to compare different types of 99 rooftop uses. For example, Saiz et al. (2006) conducted a comparative LCA study between an extensive GR, a white roof 100 and a gravel roof. A similar study was conducted by Kosareo and Ries (2007), which compared extensive and intensive 101 GRs with a conventional roof. Carter and Keeler (2008) conducted two experimental studies in order to examine three 102 environmental local benefits, i.e. stormwater retention, temperature mitigation and habitat creation of GRs compared to 103 impervious surfaces. In all cases, the results verified the benefits of green roofs (e.g. for the energy savings of a building) 104 in comparison with conventional roofs (Lamnatou and Chemisana, 2014). Sanyé-Mengual et al. (2015a) and Sanjuan- 105 Delmás et al.’s (2018a) studies on UA in Barcelona (Spain) compared the environmental performance of growing 106 tomatoes in RTGs against conventional supply chains, finding that the former can have lower life-cycle GHG emissions 107 and toxicity impacts. A recent review by Goldstein et al. (2016) found that UA is posited to have numerous environmental 108 advantages over conventional agriculture. Benis et al. (2018) compared four different rooftop systems (three greenhouse 109 systems and one PV system) in the Mediterranean context using a cost-benefit analysis. 110 To date, LCA studies have yet to compare alternative uses of building rooftops for food or energy production, accounting 111 for a variety of environmental indicators and using a systematic framework with common assumptions and boundaries 112 for the assessment of both systems. Our study aims at filling this gap by analysing the strategic use of rooftops in urban 113 areas in order to provide a basis to local stakeholders and policy makers for comparing the environmental advantages and 114 disadvantages of implementing these productive uses of rooftops under Mediterranean climatic conditions, such as in the 115 city of Barcelona (Spain). The objective was to answer the following question: “If a given surface of urban roof is 116 available, which is the best option in terms of environmental impacts for solar energy exploitation: food or energy 117 production?” Indeed, Barcelona is endowed with abundance of solar energy, receiving about 1,660 kWh/m2/year of solar 118 radiation per year (Perpiña Castillo et al., 2016). When an RTG is placed in the available surface a given amount of food 119 is produced on-site and the conventional production is avoided. Alternatively, if a BAPV system is installed, electricity 120 is produced, but also in this case, it is possible to account for the savings in primary energy, according to the selected 121 electric mix (Carnevale et al., 2014). In particular, this work aims to compare the environmental performances of both 122 pilot rooftop systems located at the Autonomous University of Barcelona Campus (Barcelona, Spain). The life cycle for 123 each system was evaluated using LCA, with a special attention on those phases and hotspots that display the highest 124 environmental impacts and proposing enhancement scenarios for decreasing such impacts. The originality of this study 125 lies in the first comparison of two different rooftop systems by thoroughly assessing the environmental burdens and 126 benefits of both production processes. A large set of environmental indicators and real empirical data were used to 127 evaluate the environmental loads of local food and energy production. Additionally, the potential benefits deriving from 128 energy and material efficiency strategies were considered in order to optimize the environmental performance of both 129 analysed systems. 130 131 132 133 134 135 136 137 138 139 140 Table 1. Summary table of the main environmental benefits of using photovoltaic panels for energy generation and 141 rooftop greenhouses for food production, under investigation in this study. Sustainable Theme Benefits Rooftop Systems BAPV RTG Energy Produce energya ✓ - Save direct energy consumptiona ✓ - Save indirect energy consumptionb - ✓ Mitigate Urban Heat Islandb - ● Food Increase urban food securityb - ✓ Environmental Benefits Reduce product lossesa,b,c ✓ ✓ Water Prevent aquatic pollution from urban - ✓ runoffb Mitigate storm waterb - ✓ Rainwater harvesting b - ✓ Land Prevent soil erosion d - ✓ Optimize urban space b,c ✓ ✓ Reduce waste through recovery e,f ✓ ✓ Air Reduce GHG emissions g ✓ ✓ Improve air qualityg ✓ ✓ Ecology Enhance biodiversityb - ● Landscape Improve aestheticsh,i ● ● Other Improve rooftop’s performancel ● ✓ Reduce noisel - ✓ 142 143 Legend of symbols: Yes (✓), No (-), Possibly (●) 144 a) Eskew et al., 2018, b) Cerón-Palma et al., 2012, c) Sanyé-Mengual et al., 2015a; d) Specht et al., 2013, e) Sanyé-Mengual et al., 145 2013, f) Sica et al., 2018, g) Goldstein et al., 2016, Sanjuan-Delmás et al., 2018a; h) Proksch, 2016, i) Prasad, 2014, l) Proksch, 2011 146 147 2. Materials and methods 148 The assessment has been performed by applying LCA methodological framework based on ISO standards (ISO 2006 a, 149 b) and International Reference Life Cycle Data System (ILCD) Handbook guidelines (EC 2010, 2011). LCA is a common 150 and standardised tool for quantitatively assessing the potential environmental burdens during all stages of a product 151 system, from raw material extraction to the disposal of the product at the end of its life, including manufacturing, 152 transportation, installation, use and disposal or recycling (i.e., cradle-to-grave). LCA identifies the most relevant 153 environmental impacts and key processes (‘hotspots’) and can support decision-making strategies for environmental 154 improvements from a life cycle perspective (Baumann and Tillmann, 2004). For the implementation of the LCA the 155 following steps are adopted in this study: i) goal and scope definition; ii) life-cycle inventory analysis (LCI); iii) life-cycle 156 impact assessment (LCIA) and iv) interpretation of results. 157 158 2.1. Goal and scope definition 159 The goal of this work was to quantify and compare the environmental impacts related to two types of rooftop systems, 160 namely RTG and BAPV in the Metropolitan Area of Barcelona (second largest city in Spain). It is worth clarifying that 161 the investigated roofs are pre-existent, easily accessible and did not undergo any structural modification for implementing 162 both systems. Furthermore, each roof was studied as a ‘single’ system (and not as subsystem of the building) to understand 163 its individual impacts; thereby, the results are not presented in terms of the total building performance. In order to guide 164 decision-making and help select the most suitable system, the functional unit was 1 m2 of flat rooftop using either RTG 165 or BAPV. 166 The system boundaries of the LCA, shown in Fig. 1, include the following life cycle phases: raw material extraction, 167 manufacturing processes, installation, operation/maintenance and end-of-life (EoL) (dismantling, recycling and final 168 disposal). Therefore, a ‘cradle-to-grave’ approach was adopted. In line with Sanyé-Mengual et al. (2015a), the 169 consumption phase is excluded from the assessment due to its dependence on tomato preparation methods (e.g. from raw 170 consumption to oven-grilled). Most of the past studies did not include the EoL of PV technologies, mainly because of the 171 low number of panels that reached their end of useful life and the lack of data (Latunussa et al., 2016). Nevertheless, a 172 comprehensive analysis should consider the contributions of each phase of the life cycle (Fthenakis et al., 2009). During 173 the last years, the recycling processes were investigated and developed and the EoL management of PV is gaining more 174 interest (Corcelli et al., 2018a; Xu et al., 2018). Additionally, in Europe, a drive towards responsible EoL management 175 for PV panels has taken form in the Directive on Waste Electrical and Electronic Equipment (WEEE; Directive 176 2012/19/UE of the European Parliament and the Council), according to which decommissioned PV panels are included 177 as domestic and professional types of WEEE (Sica et al., 2018). For this reason, the EoL step of such technology was 178 included in this study as an important step which needs to be investigated. 179 This study is intended to provide policy makers with potentially useful suggestions for local resources production 180 planning, without however accounting neither for large-scale consequences on the background system (e.g., large-scale 181 food and energy sectors, marginal changes of resource costs due to recovery, policy options etc) nor for the effects of the 182 investigated systems on the market (Ripa et al., 2017). According to the ILCD Handbook (EC, 2010) the analyzed context 183 can be identified as a micro-level decision support (so called situation A in ILCD) and an attributional LCI modeling 184 framework (e.g. the potential environmental impacts are attributed to a system or a product over its life cycle) is therefore 185 applied. As a consequence, a system expansion based on average data (i.e. market mix) was carried out for crediting 186 energy recovery in the case of BAPV system and tomatoes production in the case of RTG. Within the attributional LCI 187 modelling, the average technology was selected (rather than the marginal technology), according to Finnveden et al. 188 (2009) and to Nemecek et al. (2015), the latter highlighting that in crop production, average practice should be understood 189 as conventional agriculture as practiced by a majority of producers. 190 191 192 193 Figure 1. System boundaries and process chain under study. 194 195 2.2. System description 196 Main assumptions and life cycle phases accounted for in the inventory of the RTG and BAPV systems under analysis are 197 presented below. 198 2.2.1. Case study: food production from rooftop greenhouse 199 The ICTA-ICP building houses the headquarters of the Institute of Environmental Science and Technology (ICTA) and 200 the Catalan Institute of Paleontology (ICP) (Nadal et al., 2017b). The building is situated in the Autonomous University 201 of Barcelona (UAB) campus, 25 km away from the Barcelona city centre. It can be considered representative of office 202 buildings in cities, because it holds four floors with offices and is similar in terms of size (7,500 m2) and users to other 203 standard buildings (Schloss, 1984; Sanjuan-Delmás et al., 2018a). Additionally, its design is based on building-integrated 204 agriculture philosophy, multifunctionality and passive systems that promote energy efficiency (Nadal et al., 2017b; 205 Sanjuan-Delmás et al., 2018b). The external structure of the ICTA-ICP building is composed of a metal frame with 206 corrugated polycarbonate sheets that can be opened or closed to provide ventilation. This structure covers the walls and 207 the roof of the building and the RTG. The pilot RTG under study, implemented by the Fertilecity project (funded by the 208 Spanish Ministry of Economy and Competitiveness - MINECO), is placed on the building roof and utilises residual heat 209 from the building, CO2 concentrations in this residual air and rainwater collected from the rooftop (Sanyé-Mengual et al., 210 2015a). This study considers RTG crops based on hydroponic systems (i.e. soil-less crops that use mainly inert substrates), 211 as the agro-urban system applied. Rainwater collection is included in this study, although it does not represent a standard 212 for RTG systems, particularly in a retrofit situation. Residual heat and CO2 integration are expected to increase crop 213 yields, whilst untreated rainwater is used in the RTG to irrigate the crops and water ornamental plants in the building, 214 decreasing the demand for drinkable water from the conventional water supply system (Sanjuan-Delmás et al., 2018a). 215 Despite the potential benefits of the RTG on the building, our study focuses on the greenhouse structure and predicts 216 potential crop outputs but, except for the rainwater collection, does not include an assessment of flow exchanges in the 217 building due to lack of data. 218 219 Characteristics of the crop 220 The total area of RTG is 122.8 m2, 84.34 m2 of which are cropped (Fig. 2). The crops were beef tomato varieties 221 (Lycopersicon esculentum, Arawak for spring crops and Tomawak for winter crops), grown from February 2015 to July 222 2016 (Sanjuan-Delmás et al., 2018a). A hydroponic system was employed for irrigation to distribute a nutrient solution 223 (water plus fertilisers, also called ‘fertigation’) to plants on an inert substrate consisting of perlite bags. The system 224 produced 30.1 kg of tomatoes per square meter over 15.5 months, furnishing a total of 2,540 kg of food and covering the 225 requirements for tomatoes of nearly 60% of the building (Sanjuan-Delmás et al., 2018a). Further technical information 226 about the crop and the greenhouse structure can be found in Sanjuan-Delmás et al. (2018a). 227 Infrastructure & Installation. The RTG’s structure mainly consists of steel, aluminium, polycarbonate covers, low-density 228 polyethylene film curtains and concrete block anchors. In addition, the installation is equipped with thermal screens, 229 backup lighting, rainwater harvesting systems, and climate control systems. The installation stage accounts for energy 230 consumption requirements of the machinery used to construct the RTG. According to earlier studies, a 50 years lifespan 231 was considered for the rainwater harvesting system (Vargas-Parra et al., 2013; Sanjuan-Delmás et al., 2018a) and the 232 greenhouse structure (Sanye-Mengual et al., 2015a), and a 10 years lifespan was assumed for the auxiliary equipment 233 (Hoffman et al., 2007). Transportation of materials from the market to the RTG was also included. The averaged distance 234 travelled was 35 km for fertilisers, pesticides and auxiliary equipment, 60 km for rainwater harvesting construction 235 materials and 850 km for substrate bags imported from Almeria (South of Spain). 236 237 Operation & Maintenance. The RTG operation consisted of inputs required for hydroponic cultivation (fertilizers, 238 pesticides, compost, etc.), water (consumed for crop irrigation) and energy needs (i.e. electricity used for opening and 239 closing the walls and roof of the greenhouse and for lighting and irrigation pumps). In particular, the assessment of 240 fertilisers and pesticides included local emissions to air generated during their application and the treatment of leachates 241 in a wastewater treatment plant. Furthermore, the waste biomass from the crop plants was composted in the greenhouse, 242 thus avoiding transport and landfilling, although emissions generated during the composting process were accounted for. 243 According to Sanjuan-Delmás et al. (2018a), a lifespan of 3 and 5 years was assumed for the perlite bags and HDPE 244 materials, respectively. 245 246 End-of-life. For the EoL assessment, the impacts of landfilled materials (substrate) were included. Infrastructure and 247 auxiliary equipment (pumps, rainwater tanks) were assumed to be recycled. A distance of 30 km was assumed from the 248 RTG to the landfill or the recycling facility. 249 250 251 252 253 254 255 256 257 258 259 260 261 Figure 2. Top view of rooftop greenhouse (on the left, source: 2017 © Google LLC) and tomatoes produced (on the right, the investigated RTG source: Sanjuan- 262 Delmás et al., 2018a). 263 264 2.2.2. Case study: energy generation from Building-Applied Photovoltaics 265 Building-Applied Photovoltaics (or ‘BAPV’), in Fig. 3, is a form of on-site electricity generation that can balance the 266 emissions from more environmentally intensive sources of electricity and reduce electricity transmission losses (Cubi et 267 al., 2016). The BAPV examined has a total nominal power of 50.49 kWp and is placed in UAB campus, on the restaurant 268 and library building’s rooftops. The PV system was implemented within the framework of univERsol, a European project 269 developed between 2002-2004, whose objective was the installation of PV panels in 26 universities, schools, technology 270 centres and city councils in four European Union countries (Spain, France, England, Holland). The total roof area used is 271 1,600 m2, whereas the roof area covered by PV panels is 380 m2 (UAB’s personnel. Personal communication, 2017). 272 Infrastructure & Installation. The BAPV installation includes 297 single crystalline silicon (c-Si) photovoltaic panels and 273 the balance of system (BOS). Each panel has the following characteristics: 170 Wp, 72 cells, 159 x 79 cm2 dimensions, 274 tilt angle=35°, electrical efficiency=14%, weight=15.4 kg. In particular, the PV cells are enclosed in an aluminium frame 275 and sealed between two plastic sheets in ethylene vinyl acetate (EVA) foil and glued between glass and polyvinyl fluoride 276 (PVF). In accordance with Fu et al. (2015), the system boundary of the research included upstream processes, ranging 277 from silica extraction to the crystalline silicon bar and ingot growth, and midstream processes, which involved cell and 278 panel fabrication as well as aluminium frame and BOS production. 279 Regarding the installation phase, it was modelled by considering the electricity consumption for PV infrastructure 280 installation work. The BOS components included the mounting structure (aluminium and steel), 17 inverters (necessary 281 for transforming the direct current to alternating current and for connecting to the normal electricity grid), copper and 282 plastic materials for cables and contact boxes. In line with Eskew et al. (2018), components excluded from the system are 283 the surge protector, pyranometer, digital indicating controller, uninterruptible power supply device, and computer 284 monitoring system. The life expectancy of the PV panels and metal support structures were assumed to be 30 and 60 285 years, respectively (Peng et al., 2013; Sherwani et al., 2010). Inverters and transformers were considered to last for 20 286 years, but parts must be replaced every 10 years, according to well-established data from the power industry on 287 transformers and electronic components (Fthenakis and Kim, 2011). 288 The transportation distances were covered by a heavy truck. All the components, except for the PV panels, were assumed 289 to be purchased from factories 100 km away from Barcelona, while the PV panels were purchased from Madrid (UAB’s 290 personnel, Personal communication, 2017). 291 Operation & Maintenance. Usually, PV systems do not show any emission to air or water during operation (Alsema and 292 de Wild-Scholten, 2006; Raugei and Fthenakis, 2010; Tao and Yu, 2014, Eskew et al., 2018). Some panels might be 293 washed by the user on an annual basis. In this study, the use of 20 litres of water per year and square meter for washing 294 the panels was assumed (Frischknecht et al. 1996). Moreover, the inverters were assumed to have a 10-years lifetime, 295 thus requiring to be replaced during the 30-years lifetime of the system. The electricity produced by the BAPV system 296 amounts to 62.089 MWh/yr (UAB’s personnel, Personal communication, 2017). 297 298 End-of-life. Most materials in PV systems are reusable, including aluminium, glass, silicon or copper (IRENA and IEA- 299 PVPS, 2016; Xu et al., 2018). Therefore, a recycling scenario of all recyclable materials was supposed for the EoL phase, 300 assuming a distance of 590 km for the transportation of BAPV system components to the recycling facility (located near 301 Madrid). 302 303 304 305 306 307 308 309 310 311 312 Figure 3. Top view of roof-mounted photovoltaic (on the left, source: 2017 © 313 Google LLC) and the investigated PV sytems (on the right). 314 315 316 317 318 2.3 Life cycle inventory and main assumptions 319 Inventory data for RTG and BAPV case studies are given in Tables 2 and 3, referred to the selected functional unit (1 m 2 320 of flat rooftop). In order to make possible a comparison between tomatoes production and PV energy production, 321 inventory data for tomato production, which referred to 15.5 months, were averaged over 12 months, taking into account 322 the variability of climate conditions along the whole year for both systems. For the inventory analysis both systems were 323 structured in several stages in order to facilitate the interpretation of the results obtained. Regarding the RTG system, 324 specific literature was used as data sources for the LCA. In particular, the inventory for the infrastructure manufacturing, 325 installation, operation/maintenance and transport of waste to the treatment site was deduced from Sanyé-Mengual et al. 326 (2015a) and Sanjuan-Delmás et al. (2018a). The inventory data about BAPV systems, including the material consumption 327 and environmental emissions involved in the production of solar-grade silicon, wafers, cells, and panels and their EoL 328 were mainly obtained from ecoinvent 3.1 database (Jungbluth et al., 2012, Fu et al., 2015) and literature (Corcelli et al., 329 2018a). Additionally, for the installation and operation/maintenance phases foreground data were provided by expert 330 personnel in UAB. Other background data, related to energy use, auxiliary materials and impacts of the waste management 331 (e.g. wastewater treatment, composting, EoL treatments of infrastructure materials) have been derived from the ecoinvent 332 3.1 database (Wernet et al., 2016). It should be pinpointed that the present study is representative of technologies installed 333 in the Spanish territory. As a consequence, this analysis assumes that all the production, installation, operation processes 334 and also the recycling treatments for both roofing systems are developed in Spain, thus the Spanish power mix (2015) 335 was used as a reference. The energy outputs from the BAPV system were considered to be used as alternative to energy 336 produced by means of traditional fossil fuels combustion, in particular, to the electricity from the Spanish grid, where 337 over 60% of electricity come from nuclear and fossil fuels (coal) (Red eléctrica de España, 2015), thus drawing a potential 338 evaluation of environmental benefits. Additionally, the tomatoes production from the RTG system was considered as 339 substitute for tomatoes produced in Spain (Almeria) by means of conventional farming in a standard greenhouse 340 (Nemecek et al., 2015). The multi-tunnel greenhouse model was used as a representative conventional greenhouse 341 commonly used in Spain (Torrellas et al., 2012). The recycling costs and benefits related to materials recovery were 342 accounted for in the analysis. In this sense, the substitution ratio takes into account the quality of the products from waste 343 in comparison with that of the corresponding avoided product. In particular, the avoided production of primary steel, 344 aluminium, silicon, iron and copper were assumed for crediting metals recovery, with a substitution ratio of 0.9:1, due to 345 processing losses (Ramachandra Rao, 2006). Likewise, for crediting lithoid based materials such as concrete and glass 346 recovery, the avoided production of the corresponding primary material was assumed, with a substitution ratio of 0.9:1, 347 due to degradation in the recycling process (Blengini and Di Carlo, 2010.). A substitution ratio of 1:1 was assumed for 348 plastics scraps, meaning that 1 unit of secondary material replaces 1 unit of the corresponding primary material (Thoden 349 van Velzen et al., 2017). In the case of RTG system, the credits deriving from the irrigation water recovery was also 350 considered, thus reducing the demand for potable water from the conventional distribution network. It should be 351 highlighted that most of the water consumed in the RTG was rainwater collected from the building (80-90%), while the 352 remaining 10-20% was tap water (Sanjuan-Delmás et al. 2018a). Moreover, for crediting tomato biomass composting, 353 the avoided production of inorganic fertilizers was included. In this latter case, the considered substitution ratio of 0.4:1 354 means that 1 ton of biomass waste corresponds to 0.4 ton of compost/dry waste (as average value). In the case of 355 composting, 1 ton of compost was assumed to substitute: 23 kg of N-fertilizer, 9.5 kg of P fertilizer and 9 kg of K-fertilizer 356 (Blengini et al., 2008; Ripa et al., 2017). 357 358 Table 2. Life cycle inventory data for tomatoes production in RTG, referred to 1 m2 of flat rooftop (FU) (timeframe 1 359 year). Materials/Energy Unit/ FU Amount Data Sources Infrastructure manufacturing RTG structure: Steel kg 8.36E-01 Sanyé-Mengual et al. (2015a) Concrete kg 2.12E-01 // Low Density Polyethylene (LDPE) kg 7.80E-02 // Polycarbonate kg 1.60E-01 // Polyester kg 7.80E-03 // Aluminium kg 7.80E-03 // Auxiliary equipment: Low Density Polyethylene (LDPE) kg 2.30E-02 // Polystyrene kg 2.60E-02 // High Density Polyethylene (HDPE) kg 9.40E-03 // Polyvinylchloride (PVC) kg 4.40E-03 // Steel kg 5.00E-04 // Installation* Electricity kWh 4.00E-04 Sanyé-Mengual et al. (2015a) Transport, lorry tkm 3.24E-01 // Transport, transoceanic freight ship tkm 1.61E-01 // Transport, van tkm 2.00E-04 // Operation and Maintenance Substrate: Expanded perlite kg 1.87E+00 Modified from Sanjuan-Delmás et al. (2018a) High Density Polyethylene (HDPE) kg 4.88E-02 // Fertilizers: KNO3 kg 3.95E-01 // KPO4H2 kg 2.00E-01 // K2SO4 kg 4.41E-01 // Ca(NO3)2 kg 5.57E-01 // CaCl2 kg 1.82E-01 // Mg(NO3)2 kg 3.72E-01 // Hortrilon/Tradecorp kg 1.49E-02 // Sequestrene kg 1.49E-02 // Pesticides kg 6.89E-02 // Water (rainwater) m3 1.30E+00 // Tap water m3 2.26E-01 // Electricity kWh 4.73E-01 // Local emissions to water: Cl- kg 8.11E-02 // NO3- kg 3.29E-01 // PO43- kg 3.64E-02 // SO42- kg 1.70E-01 // K+ kg 2.06E-01 // Mg2+ kg 2.32E-02 // Ca2+ kg 1.05E-01 // Local emissions to air: NH3 kg 6.40E-03 Estimated from Montero et al. (2009) N20 kg 2.67E-03 // NOx kg 2.13E-02 // Produced tomatoes on-site kg 2.33E+01 Modified from Sanjuan-Delmás et al. (2018a) End-of-life Steel scraps (to recycling) kg 8.37E-01 Ecoinvent 3.1 database (Wernet al., 2016) Plastics scraps (to recycling) kg 3.57E-01 // Aluminium scraps (to recycling) kg 7.80E-03 // Concrete scraps (to recycling) kg 2.12E-01 // Perlite substrate waste (to landfill) kg 1.87E+00 // Tomatoes biomass waste (to composting) kg 9.45E+00 // Wastewater treatment m3 1.53E+00 // Transport* to landfill, lorry (perlite tkm 5.62E-02 Modified from Sanjuan-Delmás et al. substrate) (2018a) Transport* to recycling facility, lorry tkm 4.24E-02 Sanyé-Mengual et al. (2015a) (aluminium, steel, plastics, concrete scraps) 360 361 * Process of transport included vehicle, road manufacture and maintenance, as well as diesel consumption and relative emissions. 362 363 Table 3. Life cycle inventory data for energy generation from BAPV, referred to 1 m2 of flat rooftop (FU) (timeframe 1 364 year). Materials/Energy Unit/ FU Amount Data Sources Infrastructure manufacturing PV panel components: m2 2.30E-01 Ecoinvent 3.1 database (Jungbluth et al., 2012) Solar cell (c-Si) m2 2.14E-01 // Aluminium alloy kg 6.05E-01 // Polyvinyl fluoride kg 2.54E-02 // Polyethylene terephthalate kg 8.58E-02 // Glass sheet, tempered kg 2.32E+00 // Ethylene vinyl acetate kg 2.30E-01 // Copper kg 2.59E-02 // Nickel kg 3.74E-05 // Soldering flux kg 2.02E-03 // Methanol kg 4.96E-04 // Silicone kg 2.80E-02 // Corrugated board box kg 2.52E-01 // Tap water kg 4.90E+00 // Electricity kWh 1.39E+00 // Installation Balance of System: Steel kg 1.21E+00 Field data supplied by UAB’s engineers Aluminium kg 8.76E-01 // Concrete kg 1.23E+01 // Copper kg 1.14E-01 // Polyvinylchloride (PVC) kg 6.50E-02 // Inverters p 1.06E-02 // Electricity kWh 4.76E-02 // Transport*, van tkm 1.41E+00 // Transport*, lorry tkm 3.37E+00 // Operation and Maintenance# Tap water kg 1.00E+00 Field data supplied by UAB’s engineers Produced electricity on-site from PV kWh 3.54E+01 // End-of-life Aluminium scraps (to recycling) kg 1.74E+00 Modified from Corcelli et al. (2018a); Ecoinvent 3.1 database (Wernet al., 2016) Glass scraps (to recycling) kg 1.87E+00 // Silicon scraps (to recycling) kg 2.25E-01 // Copper scraps (to recycling) kg 9.96E-02 // Iron scraps (to recycling) kg 5.22E-06 // Steel scraps (to recycling) kg 1.21E+00 // Concrete scraps (to recycling) kg 1.23E+01 // Plastics scraps (to recycling) kg 6.53E-02 // Wastewater treatment kg 1.00E+00 // Transport*, lorry tkm 1.01E+01 Field data supplied by UAB’s engineers 365 * Process of transport included vehicle, road manufacture and maintenance, as well as diesel consumption and relative emissions. 366 # The emissions during the operation phase were considered negligible according to Alsema and de Wild-Scholten (2006) and Peng et al. (2013). 367 368 2.4. Life Cycle Impact Assessment (LCIA) 369 The environmental impact assessment was performed using the ReCiPe Midpoint method (hierarchist perspective), one 370 of the most recent and up-to-date LCA methods (Goedkoop et al., 2009; Vezzoli, 2018). The LCA software SimaPro 371 version 8.0.5 (Pre-Consultants, 2014), integrated with ecoinvent v3.1 database (Wernet et al., 2016), was used to construct 372 the LCA model and implement the impact assessment calculations. In this study, the following impact categories were 373 explored: Climate Change (CC, in kg CO2 eq), Ozone Depletion (OD, in kg CFC-11 eq), Terrestrial Acidification (TA, 374 in kg 1,4-DB eq), Freshwater Eutrophication (FE, in kg P eq), Photochemical Oxidation Formation (POF, in kg NMVOC), 375 Terrestrial Ecotoxicity (TE, in kg 1,4-DB eq), Water Depletion (WD, in m3), Metal Depletion (MD, in kg Fe eq) and 376 Fossil Depletion (FD, in kg oil eq). Furthermore, a sensitivity analysis was conducted to find out how sensitive the model 377 was to changes in the input parameters. To this end, alternative scenarios were proposed, based on progressive reductions 378 or substitution of the most sensitive input flows. 379 380 3. Results and Discussion 381 As a first approach to the evaluation of the systems performance, the “avoided impacts” were not included in the 382 assessment, neglecting the potential credits or benefits for secondary materials production (recycling) or recovered energy 383 (incineration and sometimes landfilling). In such a way the crucial phases still needing improvement are not hidden 384 (Section 3.1). However, additional information about potential benefits may also be needed. Therefore, a second 385 assessment was carried out including a system expansion in order to consider the environmental burdens that are avoided 386 due to the recovery of goods and energy (Section 3.2). The observed negative values mean that the credits associated to 387 the recovery of goods (i.e. metals, biomass) and energy (i.e. electricity, heat) are greater than the environmental burdens 388 generated during the whole life cycle (Gala et., 2015; Corcelli et al., 2018b). Finally, a comparison between RTG and 389 BAPV systems was performed in order to see which system shows the best environmental performance (Section 3.3). 390 3.1. Environmental costs 391 3.1.1. RTG system 392 Table 4 shows the characterized impacts of the RTG system, with reference to the functional unit of 1 m2 of flat rooftop, 393 broken down into the different life cycle steps with the percentage contribution to the total environmental impacts. The 394 results show that most of the environmental impacts are generated during the Operation & Maintenance phase in almost 395 all analysed impact categories, especially in ozone depletion (OD), freshwater and terrestrial eutrophication (FE, TE), 396 where the contribution from Operation & Maintenance phase ranges from 73% to 80%. In the remaining impact 397 categories, the impacts linked to Operation & Maintenance overcome 58%, except for water depletion (WD) and metal 398 depletion (MD) where the main impacts are generated by EoL (91%) and infrastructure (63%), respectively. 399 Within the Operation & Maintenance of the RTG, the use of fertilizers generates the highest impacts in five out of nine 400 impact categories (from 21% in POF to 62% in TE), whereas the use of pesticides contributes to 47% of the impacts to 401 ozone depletion (OD). Among fertilizers, calcium nitrate resulted the most impactful (data not shown) due to the large 402 amounts of chemicals (such as nitric acid) and energy (heat and electricity) required for its production. Local emissions 403 (namely, emissions from the foreground system) affect only terrestrial acidification (23%), freshwater eutrophication 404 (65%) and photochemical oxidant formation (31%) categories. To a lesser extent, the substrate bags also generate 405 substantial impacts, accounting for 12-19% of climate change (CC), ozone depletion (OD), terrestrial acidification (TA), 406 photochemical oxidant formation (POF) and fossil depletion (FD) and <3% of the other four remaining impact categories. 407 Most of these environmental impacts are generated during the production of the substrate. 408 Infrastructure manufacturing is another environmental hotspot of RTG. The RTG structure generates between 16-63% of 409 the impacts on photochemical oxidant formation (POF), ozone depletion (OD), climate change (CC), fossil depletion 410 (FD) and metal depletion (MD) and 2-10% of the impacts on the remaining categories. In particular, steel is the material 411 that generates the largest environmental load (from 48% in CC to 100% in MD), followed by polycarbonate (2-39%) 412 especially in those categories where thermoplastics tend to have the most significant impact (OD, FD). Concrete only 413 marginally affects the different indicators (<2 %). 414 Concerning the EoL phase, wastewater treatment entails the highest contribution to water depletion (corresponding to 415 90%), while the transport, metals recycling, composting and substrate landfilling steps display minor impacts (always 416 less < 11%) in all analyzed impact categories. 417 418 419 Table 4. Characterized impacts calculated for RTG system, broken down into different process steps, referred to a 420 functional unit of 1 m2 of flat rooftop. CC OD TA FE POF TE WD MD FD 3 kg CO2 eq kg CFC 11 kg SO2 kg P eq kg kg 1,4 - m kg Fe eq kg oil eq eq eq NMVOC DB eq Infrastructure manufacturing RTG structure 3.2E+00 5.1E-07 1.3E-02 1.3E-03 1.1E-02 2.9E-04 3.2E-02 2.5E+00 8.6E-01 [%] 19.5 18.4 10.5 7.2 15.9 6.3 2.1 63.0 25.4 Auxiliary equipment 1.7E-01 2.6E-09 5.4E-04 5.9E-06 6.9E-04 4.1E-06 3.3E-03 2.1E-03 1.1E-01 [%] 1.0 0.1 0.4 0.03 1.0 0.1 0.2 0.1 3.2 Installation Transport 2.9E-02 5.6E-09 1.3E-04 2.2E-06 1.5E-04 1.5E-05 1.0E-04 1.1E-03 1.1E-02 [%] 0.2 0.2 0.1 0.01 0.2 0.3 0.01 0.03 0.3 Electricity 1.6E-04 1.9E-11 9.9E-07 4.1E-08 5.3E-07 5.1E-09 5.5E-07 4.3E-06 3.9E-05 [%] <<1 <<1 <<1 <<1 <<1 <<1 <<1 <<1 <<1 Operation & Maintenance (tomatoes production on-site) Substrate 2.5E+00 3.6E-07 1.5E-02 5.9E-04 8.5E-03 1.4E-04 1.5E-02 5.2E-02 6.5E-01 [%] 14.9 12.8 12.5 3.2 11.8 3.0 1.0 1.3 19.4 Water 9.2E-05 1.0E-11 4.1E-07 5.2E-08 2.8E-07 1.0E-08 2.3E-04 1.4E-05 2.4E-05 [%] <<1 <<1 <<1 <<1 <<1 <<1 0.02 <<1 <<1 Fertilizer 5.6E+00 3.6E-07 3.1E-02 1.6E-03 1.5E-02 2.8E-03 7.5E-02 6.2E-01 9.8E-01 [%] 34.0 13.1 25.6 8.5 21.2 61.7 4.9 15.9 29.1 Pesticides 7.6E-01 1.3E-06 6.9E-03 3.7E-04 3.1E-03 4.8E-04 5.7E-03 8.1E-02 2.8E-01 [%] 4.6 46.7 5.6 2.0 4.4 10.6 0.4 2.1 8.2 Electricity 1.8E-01 2.3E-08 1.2E-03 4.9E-05 6.3E-04 6.0E-06 6.5E-04 5.1E-03 4.7E-02 [%] 1.1 0.8 1.0 0.3 0.9 0.1 0.04 0.1 1.4 Local emissions 8.1E-01 << 1 2.8E-02 1.2E-02 2.2E-02 << 1 << 1 << 1 << 1 [%] 4.9 << 1 23.1 65.5 30.6 << 1 << 1 << 1 << 1 End-of-life Transport 3.7E-02 6.4E-09 1.2E-04 3.3E-06 1.4E-04 1.3E-05 1.1E-04 1.9E-03 1.3E-02 [%] 0.2 0.2 0.1 0.02 0.2 0.3 0.01 0.05 0.4 Recycling metals 6.1E-01 1.0E-07 4.1E-03 7.1E-04 2.4E-03 3.7E-04 1.9E-02 3.3E-01 1.9E-01 [%] 3.7 3.7 3.3 3.9 3.3 8.2 1.3 8.6 5.7 Recycling plastic 1.47E-01 1.98E-08 8.36E-04 5.06E-05 5.08E-04 1.56E-05 1.13E- 2.24E- 4.56E- 03 03 02 [%] 0.9 0.7 0.6 0.3 0.7 0.3 0.1 0.1 0.4 Recycling concrete 1.2E-03 2.2E-10 9.7E-06 4.7E-08 1.7E-05 4.3E-08 2.7E-06 4.5E-05 4.2E-04 [%] 0.01 0.01 0.01 <<1 0.02 <<1 <<1 <<1 0.01 Tomatoes biomass 1.6E+00 2.1E-10 1.4E-02 4.5E-06 2.9E-03 6.7E-07 3.8E-03 5.7E-04 3.2E-04 waste composting [%] 9.8 0.01 11.1 0.02 4.0 0.01 0.3 0.01 0.01 Perlite substrate to 2.0E-02 5.2E-09 1.4E-04 5.7E-06 1.9E-04 1.9E-06 5.2E-04 1.5E-03 1.1E-02 landfill [%] 0.1 0.2 0.1 0.03 0.3 0.04 0.03 0.04 0.3 Wastewater treatment 8.3E-01 8.4E-08 7.1E-03 1.7E-03 4.1E-03 4.1E-04 1.4E+00 3.4E-01 1.8E-01 [%] 5.0 3.0 5.8 9.0 5.7 9.0 89.7 8.7 5.3 TOTAL 1.7E+01 2.8E-06 1.2E-01 1.8E-02 7.2E-02 4.6E-03 1.5E+00 3.9E+00 3.4E+00 [%] 100 100 100 100 100 100 100 100 100 421 422 3.1.2. BAPV system 423 Table 5 shows the characterized impacts of the BAPV system, with reference to the functional unit of 1 m2 of flat rooftop, 424 split over the different life cycle phases with the percentage contribution to the total environmental impacts. The results 425 display that the Manufacturing phase is dominant in almost all impact categories. The portion of total potential 426 environmental impact associated with the Manufacturing phase ranges from 52% in freshwater eutrophication (FE) to 427 93% in terrestrial ecotoxicity (TE), except for a share of 30% in metal depletion (MD). The cell production process 428 especially impacts on ozone depletion (OD) and terrestrial eutrophication (TE) categories, with loads corresponding to 429 41% and 89%, respectively, whilst processes of wafer and panel production contribute to a minor extent (<18%). 430 In contrast, the Installation phase only displays a significant impact on metal depletion (MD), with a share of 56%. In 431 deeper detail, each element of the balance of system (BOS) contributes differently to the Installation stage. Aluminium is 432 the material that shows the largest environmental impacts (33–57%, data not shown) in almost all investigated categories, 433 whilst freshwater eutrophication (FE) and metal depletion (MD) are mostly affected by the inverter (since it contains 434 silver, gold, and zinc). To a lesser extent, the Operation & Maintenance and EoL phases generate impacts lower than 435 11%. 436 437 Table 5. Characterized impacts calculated for BAPV system, broken down into different process steps, referred to a 438 functional unit of 1 m2 of flat rooftop. CC OD TA FE POF TE WD MD FD 3 kg CO2 eq kg kg SO2 eq kg P eq kg kg 1,4 - m kg Fe eq kg oil eq CFC 11 eq NMVOC DB eq Infrastructure manufacturing Solar grade 2.7E+01 2.9E-06 1.5E-01 9.1E-03 7.9E-02 1.6E-03 1.9E+00 5.5E-01 6.9E+00 silicon [%] 35.6 17.2 32.3 21.4 24.3 1.4 71.2 2.0 33.5 Single-Si wafer 7.9E+00 6.5E-07 4.1E-02 2.9E-03 2.4E-02 6.9E-04 1.1E-01 1.3E+00 2.4E+00 [%] 10.6 3.9 9.1 6.7 7.5 0.6 4.1 4.9 11.7 Single-Si PV cell 5.3E+00 6.8E-06 3.1E-02 4.5E-03 6.1E-02 1.1E-01 2.1E-01 3.1E+00 1.1E+00 [%] 7.2 40.6 6.9 10.6 18.8 89.2 7.4 11.3 5.4 Single-Si PV 1.1E+01 1.8E-06 8.2E-02 5.6E-03 5.2E-02 1.7E-03 1.3E-01 3.1E+00 3.5E+00 panel [%] 14.5 10.8 18.2 13.3 15.9 1.4 4.5 11.5 16.7 Installation Steel 2.6E+00 1.5E-07 1.2E-02 1.9E-03 1.1E-02 3.5E-04 4.1E-02 3.6E+00 5.7E-01 [%] 3.5 0.9 2.6 4.4 3.4 0.3 1.5 13.2 2.7 Aluminium 5.7E+00 1.6E-06 3.9E-02 3.2E-03 3.1E-02 1.2E-03 6.5E-02 1.8E+00 1.9E+00 [%] 7.7 9.5 8.8 7.4 9.6 1.0 2.4 6.5 9.1 Concrete 2.2E+00 1.2E-07 6.5E-03 3.0E-04 6.5E-03 1.6E-04 2.1E-02 1.8E-01 3.1E-01 [%] 3.0 0.7 1.5 0.7 2.0 0.1 0.8 0.6 1.5 Copper 2.2E-01 2.6E-08 6.6E-03 2.7E-03 2.8E-03 2.3E-04 1.1E-02 5.0E+00 6.2E-02 [%] 0.3 0.2 1.5 6.3 0.9 0.2 0.4 18.3 0.3 PVC 1.3E-01 9.8E-10 3.6E-04 4.5E-06 6.7E-04 5.9E-06 1.2E-02 4.9E-04 7.1E-02 [%] 0.2 0.01 0.1 0.01 0.2 0.01 0.4 <<1 0.3 Transport 2.4E+00 4.2E-07 9.1E-03 1.9E-04 1.2E-02 5.5E-04 7.1E-03 1.2E-01 8.3E-01 [%] 3.2 2.5 2.0 0.5 3.6 0.5 0.3 0.4 4.0 Electricity 1.9E-01 2.3E-09 1.2E-04 4.9E-06 6.4E-05 6.1E-07 6.6E-05 5.1E-04 4.7E-03 [%] 0.3 0.01 0.03 0.01 0.02 <<1 <<1 <<1 0.23 Inverter 2.2E+00 4.5E-07 2.9E-02 7.5E-03 1.4E-02 1.2E-03 3.6E-02 4.7E+00 5.9E-01 [%] 3.0 2.7 6.6 17.6 4.1 1.0 1.3 17.2 2.8 Operation & Maintenance (electricity generation on-site) Tap water 2.4E+00 4.5E-07 1.7E-02 1.8E-03 1.0E-02 3.7E-03 8.6E-02 8.6E-01 6.5E-01 [%] 3.2 2.7 3.7 4.3 3.0 3.1 3.1 3.1 3.0 End-of-life Transport 2.2E+00 3.9E-07 7.1E-03 1.7E-04 8.5E-03 4.7E-04 6.5E-03 9.9E-02 7.8E-01 [%] 3.0 2.4 1.6 0.4 2.6 0.4 0.2 0.4 3.7 Recycling metals 3.01E+00 8.63E-07 1.77E-02 2.60E-03 1.02E-02 9.28E-04 6.71E-02 2.91E+00 8.19E-01 [%] 4.0 5.2 4.0 6.1 3.1 0.8 2.4 10.6 3.9 Recycling glass 6.4E-01 1.1E-07 4.0E-03 7.8E-05 1.8E-03 5.8E-05 1.5E-03 9.9E-03 2.2E-01 [%] 0.9 0.7 0.9 0.2 0.6 0.1 0.2 0.04 1.0 Recycling 7.1E-02 1.3E-08 5.6E-04 2.7E-06 9.6E-04 2.5E-06 1.6E-04 2.6E-03 2.4E-02 concrete [%] 0.1 0.1 0.1 0.01 0.3 <<1 <<1 0.01 0.1 Recycling plastics 4.7E-02 6.3E-09 2.7E-04 1.6E-05 1.6E-04 4.9E-06 3.6E-04 7.2E-04 1.5E-02 [%] 0.1 0.04 0.7 0.04 0.1 <<1 0.01 <<1 0.1 Energy recovery 3.4E-02 4.9E-09 2.1E-04 9.4E-06 1.2E-04 1.2E-06 1.4E-04 1.0E-03 8.6E-03 [%] 0.1 0.03 0.1 0.02 0.04 <<1 <<1 <<1 0.04 TOTAL 7.4E+01 1.7E-05 4.5E-01 4.2E-02 3.3E-01 1.2E-01 2.8E+00 2.7E+01 2.1E+01 [%] 100 100 100 100 100 100 100 100 100 439 440 3.2. Environmental benefits of circular use of resources 441 From a circular economy standpoint, the implementation of measures for energy and material efficiency in the assessed 442 systems leads to environmental benefits that can be quantified by accounting for the avoided impacts of conventional 443 production of electricity, tomatoes, tap water, fertilizers and virgin metals. The characterized results of the assessment for 444 the RTG and BAPV systems, referred to the selected functional unit (1 m2 of flat rooftop in a timeframe of 1 year), are 445 shown in Table 6 and 7, respectively. 446 In the case of RTG systems (Table 6), the environmental benefits – i.e. negative values (in bold)– deriving from material 447 efficiency are much higher than the environmental loads attributable to the structure, local emissions during operation 448 and wastewater treatment in six out of nine impact categories. In particular, the most relevant benefits are achieved thanks 449 to the tomato production on-site (e.g., 25 kg CO2 eq/m2 are saved in climate change category), whilst a smaller benefit is 450 provided by the avoided production of fertilizers (N, P, K) thanks to biomass waste composting. The use of rainwater to 451 supply water requirements of the crop provides constrained benefits apart from the water depletion (WD) category, 452 amounting to –1.59 m3 of water/m2 of rooftop. Metal recycling (aluminium and steel) shows a valuable contribution only 453 in the metal depletion (MD) category, with a saving of 0.89 kg Fe eq/m2 of rooftop. 454 The percentage contribution to the total environmental impacts of each step in the RTG is shown in Fig. 4. 455 Table 6. Characterized impacts calculated for the RTG system (broken down into different process steps), referred to a 456 functional unit of 1 m2 of flat rooftop. Negative values (in bold) correspond to avoided impacts thanks to energy and 457 material recovery. CC OD TA FE POF TE WD MD FD kg CO2 eq kg CFC 11 kg SO2 eq kg P eq kg kg 1,4 - m3 kg Fe eq kg oil eq eq NMVOC DB eq Infrastructure 3.4E+00 5.2E-07 1.3E-02 1.3E-03 1.2E-02 2.9E-04 3.5E-02 2.5E+00 9.6E-01 manufacturing Installation 2.9E-02 5.6E-09 1.3E-04 2.2E-06 1.5E-04 1.5E-05 1.0E-04 1.1E-03 1.1E-02 Operation & -2.5E+01 6.4E-07 -8.5E-02 3.2E-03 -1.2E-01 -8.4E-03 1.1E-02 1.9E-01 -5.6E+00 Maintenance End-of-life Transport 3.7E-02 6.4E-09 1.2E-04 3.3E-06 1.4E-04 1.3E-05 1.1E-04 1.9E-03 1.3E-02 Recycling metals -1.2E+00 -2.7E-08 -3.8E-03 -3.4E-04 -4.9E-03 2.5E-04 -5.4E-03 -8.9E-01 -1.6E-01 Recycling plastic 1.5E-01 1.9E-08 8.4E-04 5.1E-05 5.1E-04 1.6E-05 1.1E-03 2.2E-03 4.6E-02 Recycling concrete 1.2E-03 2.2E-10 9.7E-06 4.7E-08 1.7E-05 4.3E-08 2.7E-06 4.5E-05 4.2E-04 Tomatoes biomass 5.3E-01 -6.7E-08 8.1E-03 -2.1E-04 2.3E-04 -3.7E-04 -1.8E-02 -8.4E-02 -1.7E-01 waste composting Perlite substrate to 2.0E-02 5.2E-09 1.4E-04 5.7E-06 1.9E-04 1.9E-06 5.2E-04 1.5E-03 1.1E-02 landfill Wastewater 7.3E-01 7.4E-08 6.7E-03 1.6E-03 3.8E-03 4.0E-04 -1.6E+00 3.3E-01 1.5E-01 treatment 458 459 CC [kg CO2 eq] Infrastructure manufacturing OD [kg CFC-11 eq] Installation Operation & Maintenance (avoided TA [kg SO2 eq] conventional tomatoes production) Transport FE [kg P eq] Recycling metals (aluminium and POF [kg NMVOC] steel) Recycling plastics TE [kg 1,4-DB eq] Recycling concrete WD [m3] Tomatoes biomass waste composting MD [kg Fe eq] Perlite substrate to landfill FD [kg oil eq] Wastewater treatment (water recovery) -100% -80% -60% -40% -20% 0% 20% 40% 60% 80% 100% Avoided impacts Impacts End-of-life 460 461 Figure 4. Percentage contribution of each phase to the overall environmental impacts of RTG system, referred to a 462 functional unit of 1 m2 of flat rooftop. Results include avoided impacts (negative values) due to recovery of energy and 463 material flows. 464 465 In the case of the BAPV system (Table 7), overall negative scores (in bold) can be observed for almost all the 466 environmental impact categories. This means that, except for TE, WD and MD categories, the environmental benefits 467 from electricity production on-site from renewable source (Operation & Maintenance phase) (e.g., 475 kg CO2 eq/m2 are 468 saved in climate change category) prevail on the environmental loads produced during the Manufacturing, Installation 469 and EoL phases. It should be noted that the energy outputs from the BAPV system were considered to be used as 470 alternative to the Spanish electricity grid, mainly based on nuclear (21.8%), coal (20.3%) and oil (10.1%) (Red eléctrica 471 de España, 2015), thus entailing large environmental benefits. Metal recycling (aluminium, silicon, steel, copper) plays a 472 minor role in lowering the environmental loads as well, with a relevant share on metal depletion (MD) category only. 473 The percentage contribution to the total environmental impacts of each step in the BAPV is shown in Fig. 5. 474 475 Table 7. Characterized impacts calculated for the BAPV system (broken down into different process steps), referred to a 476 functional unit of 1 m2 of flat rooftop. Negative values correspond to avoided impacts thanks to energy and material 477 recovery. CC OD TA FE POF TE WD MD FD kg CO2 eq kg CFC 11 kg SO2 eq kg P eq kg kg 1,4 - m3 kg Fe eq kg oil eq eq NMVOC DB eq Infrastructure 5.1E+01 1.2E-05 2.9E-01 3.5E+01 2.2E-01 1.1E-01 2.4E+00 8.1E+00 1.4E+01 manufacturing Installation 1.5E+01 2.7E-06 1.0E-01 2.8E+01 7.7E-02 3.7E-03 1.9E-01 1.5E+01 4.3E+00 Operation & -4.8E+02 -5.7E-05 -3.0E+00 -1.0E+02 -1.7E+00 -1.3E-02 -1.7E+00 -9.4E+00 -1.2E+02 Maintenance End-of-life Transport 2.2E+00 3.9E-07 7.1E-03 4.3E-01 8.5E-03 4.7E-04 6.5E-03 9.9E-02 7.8E-01 Recycling metals -1.4E+01 -2.5E-06 -9.2E-02 -6.7E+00 -7.5E-02 -1.9E-03 -1.2E-01 -6.5E+00 -4.0E+00 Recycling glass -9.7E-01 -1.1E-07 -8.2E-03 -3.7E-01 -4.9E-03 -3.6E-04 -9.9E-03 -3.9E-02 -2.9E-01 Recycling concrete -2.2E+00 -1.0E-07 -5.9E-03 -4.5E-01 -5.5E-03 -1.6E-04 -2.1E-02 -1.7E-01 -2.8E-01 Recycling plastics -2.1E-01 2.5E-09 -5.7E-04 -5.2E-03 -8.3E-04 -6.2E-07 -3.6E-03 -4.1E-03 -1.7E-01 Energy recovery -6.1E-01 -3.3E-08 -1.7E-03 -4.6E-02 -6.7E-04 -3.3E-05 -1.3E-04 -6.8E-03 -2.1E-01 Solar grade silicon CC [kg CO2 eq] Single-Si wafer OD [kg CFC-11 eq] Single-Si PV cell TA [kg SO2 eq] Single- Si PV panel Installation FE [kg P eq] Operation & Maintenance (avoided POF [kg NMVOC] conventional electricity generation) Transport TE [kg 1,4-DB eq] Recycling metals (aluminium, silicon, steel, iron, copper) Recycling glass WD [m3] Recycling concrete MD [kg Fe eq] Recycling plastics FD [kg oil eq] Energy recovery -100% -80% -60% -40% -20% 0% 20% 40% 60% 80% 100% Infrastructure manufacturing Avoided impacts Impacts End-of-life 478 479 480 Figure 5. Percentage contribution of each phase to the overall environmental impacts of BAPV system, referred to a 481 functional unit of 1 m2 of flat rooftop. Results include avoided impacts (negative values) due to recovery of energy and 482 material flows. 483 3.3. Comparison RTG – BAPV systems 484 Table 8 compares the total characterized results of RTG and BAPV systems, with reference to the selected functional 485 unit. Almost all values are negative, meaning that both systems turn out to be favourable (i.e. they contribute to decreasing 486 the impacts) thanks to the production of resources on-site. Material and water recycling provide a relevant environmental 487 benefit for both roofing systems, thus confirming the efficiency of circular economy patterns. Such advantages are 488 negligible if compared with the avoided emissions produced by the substitution of fossil energy in the case of BAPV and 489 the traditional tomato supply chain in the case of RTG. 490 Except for TE, WD and MD, BAPV system shows the highest avoided impacts in comparison with RTG: for instance, 491 the impacts generated by BAPV on climate change (CC) and fossil depletion (FD) categories, corresponding to -4.3E+02 492 kg CO2 eq/m2 and -1.1+02 kg oil eq/m2, respectively (versus -2.2E+01 and -4.7E+00 in the RTG system), are around 20 493 times lower. Conversely, in the case of TE and WD, BAPV delivers environmental loads while RTG generates benefits 494 (i.e. negative impacts). Moreover, in MD, the impacts due to BAPV are higher than RTG. 495 496 Table 8. Characterized impacts calculated for RTG and BAPV systems, referred to a functional unit of 1 m 2 flat rooftop. 497 Negative values correspond to avoided impacts thanks to energy and materials efficiency. Impact category Unit/FU RTG BAPV CC kg CO2 eq -2.2E+01 -4.3E+02 OD kg CFC-11 eq 1.2E-06 -4.4E-05 TA kg SO2 eq -5.9E-02 -2.7E+00 FE kg P eq 5.7E-03 -4.9E+01 POF kg NMVOC -1.0E-01 -1.4E+00 TE kg 1,4-DB eq -7.8E-03 9.9E-02 WD m3 -1.6E+00 8.1E-01 MD kg Fe eq 2.0E+00 7.6E+00 FD kg oil eq -4.7E+00 -1.1E+02 498 499 When seeking to identify the most environmentally friendly way of using 1 m 2 of flat rooftop, the competition between 500 rooftop greenhouse farming and electricity production by means of PV technology seems to be an unsolved question. 501 Modern urban systems often import food, energy, water and other resources to fulfill essential needs, which results in the 502 emission of harmful greenhouse gases (Grewal and Grewal, 2012). In this study, throughout the production of local 503 resources, RTG and PV systems enhance the practical realization of the circular economy in cities, which might increase 504 the efficiency of the system. The choice of the Mediterranean context is very appropriate, since it is considered one of the 505 world’s best locations for solar energy use with a large exploitation potential (Girard et al., 2016). Both agri-urban and 506 PV energy systems positively affect the current metabolism of Mediterranean cities and increase their multifunctionality, 507 implementing a new symbiotic model between urban and natural systems from a circular perspective. In particular, RTG 508 systems can be easily realized in Mediterranean cities without additional heating because of their warm climate and their 509 high level of solar radiation, in contrast to northern Europe where additional heat is required thus resulting in an increased 510 energy demand (Sanyé-Mengual et al., 2013). The implementation of RTG systems may also represent a means to reduce 511 the food losses during the transport and retail stages and to eliminate transport requirements, namely the main contributor 512 to GHG emissions (Smith et al., 2005). Additionally, organic waste from RTGs can be reused as fertilizer, resulting in 513 less waste collection costs for the city and reduced expenditures on synthetic fertilizers (Grewal and Grewal, 2012). 514 Likewise, recirculating rainwater for food production (here, tomatoes) reduces the demand for water and the costs of 515 stormwater management. According to existing literature, among the benefits coming from local food production, the 516 satisfaction of basic food needs can be promoted only in cities located in developing countries or neighbourhoods with 517 limited food retail. In developed countries, additional social benefits prevail, such as learning and education facilities for 518 children and adult city-dwellers or bridging the gap between consumers and producers (Specht et al., 2014). Moreover, 519 the use of rooftops farming can minimize the cost of land use (in contrast to rental contracts for agricultural soil) (Specht 520 et al., 2014). 521 In the case of PV installations, flat rooftops are generally acknowledged to be good spots for a solar energy system, given 522 that solar panels can be adjusted to the correct angle and the most appropriate orientation (Specht et al., 2014). Many 523 advantages of PV applications on rooftop can be obtained: they reduce dependence on fossil fuel use for electricity 524 generation, reduce the energy losses associated with transmission and distribution and do not require land for installation. 525 Nevertheless, different environmental, social, economic and technical constraints have been highlighted in association 526 with both retrofitting uses of rooftops. In the case of RTG, some concerns were expressed within the social dimension, 527 since consumers may be reluctant to use soil-less growing techniques. Furthermore, health risks might possibly derive 528 from air pollution or irrigation with improperly treated wastewater (Specht et al., 2014), although Ercilla-Montserrat et 529 al. (2018) concluded that the heavy metal content in the air of Barcelona is not a source of contamination for urban crops 530 including high traffic areas. On the other hand, the environmental impacts of PV electricity are mainly determined by the 531 upstream emissions associated with the mining and purification of raw materials and by the emissions associated with the 532 electricity needed for the manufacturing of panels (Meijer et al., 2003; Fthenakis and Kim, 2011; Chatzisideris et al., 533 2016). In particular, within Si-based panels life cycle, the energy requirement for the purification step from the 534 metallurgical-grade silicon that is extracted from quartz to solar-grade silicon by either a silane (SiH4) or trichlorosilane 535 (SiHCl3)-based process resulted to be the main criticality (Fthenakis et al., 2008). The wide spread of PV technology 536 market shows that PV is attractive as a long-term sustainable economic option (EPIA and Greenpeace, 2011; Girard et 537 al., 2016; IRENA and IEA-PVPS, 2016) whereas as for the RTG technology, Cerón-Palma et al. (2012) and Sanyé- 538 Mengual et al. (2015a,b) highlighted that the costs of RTG (especially for the greenhouse structure and operation) are 539 greater than conventional or open-field agriculture. However, the investigated RTG is a pilot system at a small scale 540 whose main purpose is to conduct research. According to Sanjuan-Delmás et al. (2018b), the application of the RTG 541 technology at an industrial scale would contribute to reduce the economic costs of production and make this technology 542 economically attractive. On the other hand, it should be noted that the implementation of RTGs is not always feasible due 543 to the technical characteristics of the selected rooftop (Sanyé-Mengual et al., 2015b). Concerning the slope, the flat roofs 544 are definitely the most indicated for greenhouses development and, at the same time, the material and structural resistance 545 must be appropriate to match the load requirements of the greenhouse. For instance, RTGs can be implemented on 546 reinforced concrete rooftops. In contrast, roofs made of metal (which can be both steel and aluminium structures) would 547 have to be reinforced in a rehabilitation process. In addition, a further barrier for RTG systems can depend on local 548 regulations concerning the maximum building height, surface area, and volume that can be built in a certain place (Cerón- 549 Palma et al., 2012). 550 It is worth noting that the outcomes of both analysed systems are very site-specific, depending on yields which vary 551 according to local conditions, such as solar radiation that determines the efficiency of the system. Moreover, the 552 magnitude of produced environmental costs and benefits can be affected by several uncertainty sources, such as the quality 553 of inventory data, system boundaries definition, impact assessment methods and the modelled sources of electricity and 554 heat (Cellura et al., 2011).Great care and attention should be devoted when comparing the results of different LCA studies: 555 even if the FU selected is most often 1 m2 of rooftop (Benis et al., 2018), food and PV energy production technologies 556 can widely vary (PV in combination with GR or gravel, GR, building applied or building integrated PV, etc), as well as 557 the operation and EoL phases assessed. As a consequence, generalizations are hardly possible. 558 559 3.4. Sensitivity 560 The assessment carried out in this study elucidated both pros and cons of RTGs and BAPV systems separately. In 561 particular, in accordance with previous studies, the use of fertilizers resulted to be a very impacting input flow for RTGs 562 (Sanyé-Mengual et al., 2015a; Sanjuan-Delmás et al., 2018a), whereas the electricity used in the production of solar panels 563 was the main hotspot in the BAPV systems (Pacca et al., 2006; Bekkelund, 2013; Eskew et al., 2018). A sensitivity 564 analysis was performed in order to test the robustness of results when changing (by reducing or substituting) the input 565 flows responsible for the highest environmental impacts (i.e. electricity and chemicals). Hence, two independent 566 sensitivity analyses have been performed for the investigated RTG and BAPV systems. 567 Regarding the RTG system, the first step of the sensitivity analysis consisted in a careful sensitivity check related to the 568 variability of major chemical inputs in the Operation & Maintenance phase. Indeed, as described in section 3.1.1, 569 fertilizers were the most important flow (from 5% to 62%) for all the considered impact categories, especially due to the 570 use of nitrogen fertilizers. Taking this hotspot into consideration, a sensitivity analysis was carried out on the basis of 571 three alternative scenarios for tomatoes grown in greenhouses, compared with the original tomato cultivation analysed in 572 this study (S0, base scenario): 573 1) S1– reduction (−10%) of the amounts of each fertilizer input (Torrellas et al., 2012); 574 2) S2– reduction (–20%) of the amounts of each fertilizer input (Torrellas et al., 2012); 575 3) S3 – horn meal (an organic fertilizer) was assumed to substitute calcium nitrate at a ratio 1:1, due to the relatively high 576 content of N. 577 These scenarios were developed assuming that the reduction or change of used fertilizers does not affect the crop yield, 578 thanks to more efficient technological application per area (e.g. amount of fertiliser per area of crop) rather than marginal 579 consumption per amount of production. 580 As shown in Figure 6, the decreased use of fertilizers generates a negligible decrease in the impacts on the investigated 581 categories of the Operation & Maintenance phase (WD category is not shown since it is not affected by this hotspot at 582 all). When the use of fertilizers is reduced by 20% of the original amount (S2 versus S0), impacts decrease in a range of 583 2-15% approximately, while a smaller decrease of impacts in the range 1-9% is achieved when fertilizer use decreases by 584 10% (S1 versus S0). Additional reductions of impacts could be reached by means of further optimized fertilization 585 techniques, such as drip irrigation and fertigation, that are commonly recognized to increase fertilizing efficiency (Worrell 586 et al., 1995; Kennedy et al., 2013; Solis et al., 2013) or by improving the efficiency of the background processes for 587 industrial fertilizer production (Fiorentino et al., 2014). 588 100% S0 (Base scenario) 80% S1 (10% Reduction of fertilizers) 60% S2 (20% Reduction of fertilizers) 40% S3 (- Calcium nitrate + Horn meal 20% (organic fertiliser)) 0% S4 (RTG structure: - primary steel/aluminium + secondary steel/aluminium; - PCB + PET) S5 (S3 + S4 ) 589 590 Figure 6. Sensitivity analysis for changes to fertilizers and infrastructure materials input flows, referred to RTG system 591 (PCB= polycarbonate, PET= polyethylene terephthalate). 592 Since the RTG structure manufacturing was noted as a potential limitation to the implementation of RTGs due to the high 593 environmental impact (see Section 3.1.1.), the second step of the sensitivity analysis was focused on the infrastructure 594 materials. Thus, scenario S4 was designed considering both the substitution of virgin metals (aluminium and steel) with 595 secondary (i.e. recycled) metals and the substitution of polycarbonate (PC) with polyethylene terephthalate (PET) (with 596 a better environmental profile according to Franklin Associates, 2010). 597 The S4 scenario analysis showed that almost all impacts categories were sensitive to the material substitutions. A 598 reduction of impacts can be observed with respect to the reference system (S0), ranging from 5% in freshwater 599 eutrophication (FE) to 60% in metal depletion (MD). The terrestrial eutrophication (TE) is the only category that remained 600 unchanged. 601 Finally, scenarios S3 and S4 were combined in the scenario S5. The results indicate that using organic fertilizer for 602 cultivation and recycled materials for RTG infrastructure may improve the sustainability of supplying locally produced 603 food. Indeed, the adoption of both strategies at the same time significantly reduces the impacts in all investigated 604 categories, especially in climate change (CC) (32% reduction) and metal depletion (MD) (66%). 605 Concerning the BAPV system, one of the main aims of renewables is to contain the greenhouse effect. Accordingly, a 606 sensitivity analysis was carried out only for the key impact category, i.e. climate change (CC). As observed in Section 607 3.1.2., during the life cycle of PV, emissions to the environment mainly occur from using electricity when producing 608 materials for solar panels (manufacturing phase). These emissions are strongly linked to the electricity mix used. Since 609 the world’s PV market is mainly dominated by China, USA, Germany and Italy (Solar Power Europe, 2018), a production 610 chain of crystalline silicon-based panels in these countries was modelled and compared with the base scenario (Table 4). 611 In addition, a comparison with the PV production chain in Sweden – leader country on renewable energy among the EU 612 Member States (Eurostat, 2018) – was included, in order to see how the use of an electricity mix with a high rate of 613 renewable sources affects the results. The life cycle inventory data of silicon production and crystalline silicon ingots, 614 wafers, cells and panels were extrapolated from ecoinvent database and modelled by changing the Spanish electricity mix 615 (base scenario) with the electricity mixes of the other six countries (referred to 2015) and by taking into account the 616 transport of the manufactured panels to Barcelona, by transoceanic freight ships in the case of China, USA and Japan, 617 and by truck in the case of European countries (Germany, Italy and Sweden). 618 Figure 7 shows the results of the comparison between the different electricity mixes applied during the manufacturing 619 phase and variation of transport distances (the Operation & Maintenance and EoL phases of the base scenario are not 620 shown in the figure since they remain unchanged). As shown in Figure 7, the c-Si panels produced in China cause about 621 40% higher GHG emissions compared with Spanish panels. The highest emissions result from the electricity mix used in 622 the Chinese industry with a high share of fossil fuel power (72%). If the panel production in Japan, USA, Germany and 623 Italy is considered, it is possible to observe GHG emissions about 15.8%, 13.6%, 14.3% and 12.9%, respectively, higher 624 compared to panels produced in Spain. In contrast, the use of electricity from renewable energy sources (e.g. in the case 625 of Sweden the share of renewable power is 63%) in the production chain reduces the GHG emissions by about 14% (- 7 626 kg CO2-eq per m2 of rooftop). The contribution from the transport step does not exceed 3.4%, except for the case of 627 Sweden (9%) due to the longer distance travelled by truck. 80 PV Manufacturing Transport 73.29 59.77 60.55 59.04 59.44 60 52.31 kg CO2 eq/ FU 44.94 40 20 0 SWEDEN SPAIN (Base ITALY GERMANY USA JAPAN CHINA scenario) 628 629 Figure 7. Sensitivity analysis for changes to electricity-mix and transport input flows, referred to BAPV system (only 630 manufacturing phase is included). 631 632 4. Conclusion 633 Sustainable urban planning is essential to effectively address the needs of a growing population by changing the 634 consumption patterns towards a better management of resources. In this sense, the transformation of urban underused 635 rooftops into productive spaces can improve the urban metabolism by producing or collecting locally resources such as 636 energy, greening, food or water. The innovative contribution provided by this study is the comparison of two different 637 rooftop systems for resources production (i.e. food and energy) with the aim of supplying additional helpful elements for 638 promoting the circular economy at the urban scale. The adoption of the avoided burden approach allows to highlight the 639 environmental benefits deriving from the implementation of energy and material efficiency measures, especially in warm 640 Mediterranean climates, and from the concomitant avoided costs of conventional production of tomatoes and electricity 641 (with savings of 22 kg CO2 eq and 430 kg CO2 eq for RTG and BAPV respectively). The BAPV system is more 642 environmentally sound in all impact categories, except for TE, WD and MD, the latter being the only category that is not 643 advantaged by neither RTGs nor BAPVs. Even if the solar PV technology impacts produced during the material 644 manufacturing phase are high (from 52% to 93%), they are copiously balanced by the avoided impacts associated to 645 energy output during the operation phase. Nevertheless, similar advantages are achieved by means of rooftop farming. 646 The options suggested in the sensitivity analysis may provide some useful tools for improved environmental 647 performances, but cannot definitely overcome the limits of each system. 648 In the future, potentially improved performances may be gained by means of synergy effects achieved when combining 649 both food and energy production (Marucci and Cappuccini, 2016; Trypanagnostopoulos et al., 2017; Loik et al., 2017). 650 Preliminary tests have shown that PV panels work more efficiently over a green roof that cools down the cells through 651 evapotranspiration (Köhler et al., 2007). Meanwhile, the panels shade the plants, thus reducing sun exposure and 652 favouring heat-sensitive crops. Therefore, sustainable rooftop planning for urban buildings might lead to multifunctional 653 uses of the same roof throughout the integration or the coexistence of both investigated systems. 654 In conclusion, a leading point of investigation should always be a careful focus on additional energy and material 655 efficiency and more environmentally sustainable choice of materials. For instance, in the case of RTGs, both the 656 greenhouse structure and the rainwater harvesting system can be reduced in size, and in the design phase the amount as 657 well as the quality of materials used for their manufacture can be optimised. Likewise, a decrease in the electricity demand 658 for manufacturing BAPV systems together with a well-designed recovery treatment would contribute to a more 659 sustainable supply chain. Additionally, economic and social assessments should be performed in further research in order 660 to provide a more nuanced contextualisation (either at country or city level) of RTG and BAPV systems within these 661 sectors. 662 663 Conflicts of interest 664 The authors declare no conflict of interests. 665 666 Acknowledgements 667 The authors Corcelli and Fiorentino gratefully acknowledge the financial support received from the European Union’s 668 Horizon 2020 research and innovation program under grant agreement No. 649342 (EUFORIE). G. Fiorentino also 669 acknowledges the research grant received from Parthenope University, project DSTE332 - Material and Energy Efficiency 670 in Chemical Processes for Industry and Environment. A. Petit-Boix thanks the Spanish Ministry of Education, Culture 671 and Sports for the grant awarded (FPU13/01273) while employed at UAB, and the German Federal Ministry of Education 672 and Research for funding the research group “Circulus - Opportunities and challenges of transition to a sustainable circular 673 bio-economy”, grant agreement No. 031B0018. Furthermore, two anonymous reviewers are gratefully acknowledged for 674 their useful suggestions. 675 676 677 References 678 Ackerman, K., Plunz, R., Katz, R., Dahlgren, E., and Culligan, P., 2012. Potential for urban agriculture New York City. 679 (New York). 680 Alsema, E., de Wild-Scholten, M., 2005. Environmental impact of crystalline silicon photovoltaic module production. In: 681 Material Research Society Fall Meeting, Symposium G: Life Cycle Analysis Tools for ‘‘Green” Materials and Process 682 Selection, Boston, MA. 683 Amado, M. and Poggi, F., 2014. Solar urban planning: A parametric approach. Energy Procedia, 48, 1539–1548. 684 http://dx.doi.org/10.1016/j.egypro.2014.02.174. 685 Baumann, H. and Tillman, A.M., 2004. The hitch hiker's guide to LCA: an orientation in life cycle assessment 686 methodology and application. Studentlitteratur, Lund, Sweden 687 Benis, K. and Ferrao, P., 2016. Potential mitigation of the environmental impacts of food systems through Urban and 688 Peri-Urban Agriculture (UPA) – A Life Cycle Assessment approach. Journal of Cleaner Production, 140, 784–795. 689 http://dx.doi.org/10.1016/j. jclepro.2016.05.176. 690 Benis, K., Turan, I., Reinhart, C., Ferrão, P., 2018. Putting rooftops to use – A Cost-Benefit Analysis of food production 691 vs. energy generation under Mediterranean climates. Cities 78, 166–179. doi:10.1016/j.cities.2018.02.011 692 Bekkelund K., 2013 A Comparative Life Cycle Assessment of PV Solar Systems, Norwegian University of Science and 693 Technology. https://daim.idi.ntnu.no/masteroppgaver/010/10240/masteroppgave.pdf. 694 Blengini, G.A., Genon, G., Fantoni, M., 2008. LCA del sistema integrato di gestione dei rifiuti nella provincia di Torino 695 (Research programme financed by Servizio Pianificazione Sviluppo Sostenibile e Ciclo Integrato di rifiuti della 696 Provincia di Torino). Politecnico di Torino, Turin, Italy. 697 Blengini, G.A., Di Carlo, T., 2010. The changing role of life cycle phases, subsystems and materials in the LCA of low 698 energy buildings. Energy Building 42 (6): 869-880. 699 Byrne, J., Taminiau, J., Kurdgelashvili, L. and Nam, K., 2015. A review of the solar city concept and methods to assess 700 rooftop solar electric potential, with an illustrative to the city of Seoul. Renewable and Sustainable Energy Reviews, 701 41, 830–844. http://dx.doi.org/10.1016/j.rser.2014.08.023. 702 Carnevale E, Lombardi L, Zanchi L., 2014. Life cycle assessment of solar energy systems: comparison of photovoltaic 703 and water thermal heater at domestic scale. Energy; 77:434–46. http://dx.doi.org/10.1016/j.energy.2014.09.028 704 Carter, T. and Keeler A.., 2008. Life-Cycle Cost-Benefit Analysis of Extensive Vegetated Roof Systems. Journal of 705 Environmental Management 87: 350–63. doi:10.1016/j.jenvman.2007.01.024. 706 Castán Broto, V. and Bulkeley, H., 2013. A survey of urban climate change experiments in 100 cities. Global 707 environmental change : human and policy dimensions, 23(1), 92-102. 708 Cellura M., Longo S., Mistretta M., 2011. Sensitivity analysis to quantify uncertainty in life cycle assessment: the case 709 study of an Italian tile, Renew. Sustain. Energy Rev. 15 (9), 4697–4705. doi:10.1016/j.rser.2011.07.082 710 Cerón-Palma, I., Sanyé-Mengual, E., Oliver-Solà, J., Montero, J.I., Rieradevall, J., 2012. Barriers and Opportunities 711 Regarding the Implementation of Rooftop Eco. Greenhouses (RTEG) in Mediterranean Cities of Europe. J. Urban 712 Technol. 19, 87–103. doi:10.1080/10630732.2012.717685 713 Corcelli, F., Ripa, M., Ulgiati, S., 2017. End-of-life treatment of crystalline silicon photovoltaic panels. An emergy-based 714 case study. J. Clean. Prod. 161. doi:10.1016/j.jclepro.2017.05.031 715 Corcelli F., Ripa M., Leccisi E., Cigolotti V., Fiandra V., Graditi G., Sannino L., Tammaro M., Ulgiati S., 2018a. 716 Sustainable urban electricity supply chain – Indicators of material recovery and energy savings from crystalline silicon 717 photovoltaic panels end-of-life. Ecol. Indic, 94, 37-51. https://doi.org/10.1016/j.ecolind.2016.03.028 718 Corcelli F., Fiorentino G., Vehmas J., Ulgiati S., 2018b. Energy efficiency and environmental assessment of papermaking 719 from chemical pulp - A Finland case study. J. Clean. Prod. 198, pp. 96–111. doi:10.1016/j.jclepro.2018.07.018 720 Cubi, E., Zibin, N.F., Thompson, S.J., Bergerson, J., 2016. Sustainability of rooftop technologies in cold climates: 721 comparative life cycle assessment of white roofs, green roofs, and photovoltaic panels. J. Ind. Ecol. 20, 249–262. 722 https://doi.org/10.1111/jiec.12269 723 Doughty, M.R.C. and Hammond, G.P., 2004. Sustainability and the Built Environment at and beyond the City Scale. 724 Building and Environment 39 (10): 1223–33. doi:10.1016/j.buildenv.2004.03.008. 725 Deng, Y., Li, Z., and Quigley, J. M., 2012. Economic returns to energy-efficient investments in the housing market: 726 Evidence from Singapore. Regional Science and Urban Economics, 42(3), 506–515. 727 http://dx.doi.org/10.1016/j.regsciurbeco.2011.04.004. 728 Elzeyadi, I.M.K., Harrison, M., Palmer, T., Jones, T., Elizabeth, S. T. H., 2009. Cool roof/green roof: benefits and 729 biophilia – a comparative study. Proceedings of the Solar Conference, pp. 2087–2108. 730 European Photovoltaic Industry Association (EPIA) and Greenpeace, 2011. Solar photovoltaic electricity empowering 731 the world. In: Solar generation 6. Available online at: https://www.greenpeace.org/archive- 732 international/Global/international/publications/climate/2011/Final%20SolarGeneration%20VI%20full%20report%20l 733 r.pdf (last accessed December 2018) 734 Ercilla-Montserrat, M., Muñoz, P., Montero, J.I., Gabarrell, X., Rieradevall, J., 2018. A study on air quality and heavy 735 metals content of urban food produced in a Mediterranean city (Barcelona). J. Clean. Prod. 195, 385–395. 736 doi:10.1016/j.jclepro.2018.05.183 737 Eskew, J., Ratledge, M., Wallace, M., Gheewala, S.H., Rakkwamsuk, P., 2018. An environmental Life Cycle Assessment 738 of rooftop solar in Bangkok, Thailand. Renew. Energy 123, 781–792. doi:10.1016/j.renene.2018.02.045 739 Eurostat, 2018. Share of energy from renewable sources. Available on-line at: http://ec.europa.eu/eurostat/statistics- 740 explained/index.php/Renewable_energy_statistics#Renewable_energy_produced_in_the_EU_increased_by_two_third 741 s_in_2006-2016 (last accessed June 2018). 742 European Commission (EC) - Joint Research Center - Institute of Environment and Sustainability, 2010. International 743 Reference Life Cycle Data System (ILCD) Handbook - General Guide for Life Cycle Assessment - Detailed Guidance. 744 First edition March 2010. EUR 24708 EN. Publications Office of the European Union, Luxembourg, LU. 745 European Commission (EC) - Joint Research Center - Institute of Environment and Sustainability, 2011. Supporting 746 Environmentally Sound Decisions for Waste Management – A technical guide to Life Cycle Thinking (LCT) and Life 747 Cycle Assessment (LCA) for waste experts and LCA practitioners. EUR 24916 EN. Publications Office of the European 748 Union, Luxembourg, LU. 749 European Photovoltaic Industry Association (EPIA), 2012. Solar generation 6. Solar photovoltaic electricity empowering 750 the world. 2011. Available at: https://www.greenpeace.org/archive- 751 international/Global/international/publications/climate/2011/Final%20SolarGeneration%20VI%20full%20report%20l 752 r.pdf (last accessed March 2018) 753 Finnveden, G., Hauschild, M. Z., Ekvall, T., Guinée, J., Heijungs, R., Hellweg, S., Koehler, A., Pennington, D., Suh, S, 754 2009. Recent developments in life cycle assessment. J. Environ. Manage., 91 (1), 1–21. 755 Fiorentino, G., Ripa, M., Mellino, S., Fahd, S., 2014. Life cycle assessment of Brassica carinata biomass conversion to 756 bioenergy and platform chemicals. J. Clean.Prod. 66, 174–187. https://doi.org/10.1016/j.jclepro.2013.11.043 757 Fthenakis, V.M., Kim, H.C., Alsema, E., 2008. Emissions from Photovoltaic Life Cycles Emissions from Photovoltaic 758 Life Cycles, vol. 42, pp. 2168-2174. https://doi.org/10.1021/es071763q. 759 Fthenakis, V.M., Kim, H.C., Held, M., Raugei, M., Krones, J., 2009. Update of PV energy payback times and life-cycle 760 greenhouse emissions. 24th Eur. Photovolt. Sol. Energy Conf. 4412. doi:10.4229/24thEUPVSEC2009-6DO.10.5 761 Fthenakis, V.M., Kim, H.C., 2011. Photovoltaics: Life-cycle analyses. Sol. Energy 85, 1609–1628. 762 doi:10.1016/j.solener.2009.10.002 763 Frischknecht R., Bollens U., Bosshart S., Ciot M., Ciseri L., Doka G., Dones R., Gantner U., Hischier R. and Martin A., 764 1996. Ökoinventare von Energiesystemen: Grundlagen für den ökologischen Vergleich von Energiesystemen und den 765 Einbezug von Energiesystemen in Ökobilanzen für die Schweiz. Bundesamt für Energie (Hrsg.), Bern, CH. 766 Frazer, L. 2005. Paving Paradise. Environmental Health Perspectives. 113: 457-462 767 Franklin Associates, 2010. Final Report—Life Cycle inventory of 100% Postconsumer HDPE and PET recycled from 768 postconsumer containers and packaging. Prepared for The Plastics Division of the American Chemistry Council Inc., 769 the Association of Postconsumer Plastic Recyclers (APR), the National Association for PET Container Resources 770 (NAPCOR) and the PET Resin Association (PETRA). 771 Fu, Y., Liu, X., Yuan, Z., 2015. Life-cycle assessment of multi-crystalline photovoltaic (PV) systems in China. J. Clean. 772 Prod. 86, 180-190. https://doi.org/10.1016/j.jclepro.2014.07.057 773 Gala, A., Raugei, M., Fullana-i-Palmer, P., 2015. Introducing a new method for calculating the environmental credits of 774 end-of-life material recovery in attributional LCA. Int. J. Life Cycle Assess. http://dx.doi.org/10.1007/s11367-015- 775 0861-3. 776 Girard, A., Gago, E.J., Ordonez, J., Muneer, T., 2016. Spain’s energy outlook: a review of potential and energy export. 777 Renew. Energy 86, 703-715. doi.org/10.1016/j.renene.2015.08.074 778 Goedkoop, M., Heijungs, R., Huijbregts, M., Schryver, A., De Struijs, J., Zelm, R.V., 2009. Report I: Characterisation. 779 ReCiPe A life cycle impact Assess method which comprises Harmon. Categ. Indic. Midpoint Endpoint Lev. 132. 780 Goldstein, B., Hauschild, M., Fernandez, J., Birkved, M., 2016. Urban versus conventional agriculture, taxonomy of 781 resource profiles: a review. Agron. Sustain. Dev. 36 (1), 1-19. http://dx.doi.org/10.1007/s13593-015-0348-4. 782 Grewal, S. S., and Grewal, P. S., 2012. Can cities become self-reliant in food? Cities, 29(1), 1–11. 783 http://dx.doi.org/10.1016/j.cities.2011.06.003. 784 Haberman, D., Gillies, L., Canter, A., Rinner, V., Pancrazi, L., and Martellozzo, F., 2014. The potential of urban 785 agriculture in Montréal: A quantitative assessment. ISPRS International Journal of Geo-Information, 3, 1101–1117. 786 http://dx.doi.org/10.3390/ ijgi3031101. 787 Haberman, D., Gillies, L., Canter, A., Rinner, V., Pancrazi, L., and Martellozzo, F., 2014. The potential of urban 788 agriculture in Montréal: A quantitative assessment. ISPRS International Journal of Geo-Information, 3, 1101–1117. 789 http://dx.doi.org/10.3390/ ijgi3031101. 790 Heinstein, P., Ballif, C. and Perret-Aebi, L.-E., 2013. Building Integrated Photovoltaics (BIPV): Review, potentials, 791 barriers and myths. Green. http://dx.doi.org/10.1515/ green-2013-0020. 792 Hofierka, J. and Kanuk, J., 2009. Assessment of photovoltaic potential in urban areas using open-source solar radiation 793 tools. 34, 2206–2214. http://dx.doi.org/10.1016/j.renene. 2009.02.021. 794 Hoffman, G.J., Evans, R.G., Jensen, M.E., et al., 2007. Design and Operation of Farm Irrigation Systems, second ed. 795 American Society of Agricultural Engineers 796 Hui, S.C.M., Chan, S.C., 2011. Integration of Green Roof and Solar Photovoltaic Systems. Joint Symposium 2011: 797 Integrated Building Design in the New Era of Sustainability. Nov. 22, Hong Kong. 798 IRENA (International Renewable Energy Agency) and IEA-PVPS (International Energy Agency), 2016. End-of-Life 799 Management: Solar Photovoltaic Panels, International Renewable. Available online at: 800 http://www.irena.org/publications/2016/Jun/End-of-life-management-Solar-Photovoltaic-Panels (accessed February 801 2018) 802 ISO (International Organization for Standardization), 2006a. Environmental management — life cycle assessment — 803 principles and framework. Standard ISO 14040. Geneva, Switzerland. 804 ISO (International Organization for Standardization), 2006b. Environmental management — life cycle assessment — 805 requirements and guidelines. Standard ISO 14044. Geneva, Switzerland. 806 Jungbluth, N., Stucki, M., Flury, K., Frischknecht, R., Büsser, S., 2012. Life cycle Inventories of Photovoltaics. ESU- 807 Services Ltd. 808 Kennedy, T.L., Suddick, E.C., Six, J., 2013. Reduced nitrous oxide emissions and increased yields in California tomato 809 cropping systems under drip irrigation and fertigation. Agri. Ecosys. Environ. 170, 16-27. 810 Köhler, M., Wiartalla, W. and Feige, R., 2007. Interaction between PV-systems and extensive green roofs, In Proceedings 811 of the Fifth Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show, 812 Minneapolis, April 29-May 1, 2007. 813 Kosareo, L. and Ries, R., 2007. Comparative environmental life cycle assessment of green roofs. Building and 814 Environment, 42, 2606–2613. doi:10.1016/j.buildenv.2006.06.019 815 Lamnatou, Chr. and Chemisana D., 2014. Photovoltaic-green roofs: a life cycle assessment approach with emphasis on 816 warm months of Mediterranean climate, J. Clean. Prod. 72, 57-75. doi: 10.1016/j.jclepro.2014.03.006 817 Loik, M. E., Carter, S. A., Alers, G., Wade, C. E., Shugar, D., Corrado, C., Jokerst, D., & Kitayama, C., 2017. Wavelength- 818 Selective Solar Photovoltaic Systems: Powering Greenhouses for Plant Growth at the Food-Energy-Water Nexus. 819 Earth’s Future, 5, 1044–1053, https://doi.org/10.1002/2016EF000531 820 Mahmoud, W.H., Elagib, N.A., Gaese, H., Heinrich, J., 2014. Rainfall conditions and rainwater harvesting potential in 821 the urban area of Khartoum. Resour. Conserv. Recycl. 91, 89–99. doi:10.1016/j.resconrec.2014.07.014 822 Marucci, A., Cappuccini, A., 2016. Dynamic photovoltaic greenhouse: Energy efficiency in clear sky conditions. Appl. 823 Energy 170, 362–376. doi:10.1016/j.apenergy.2016.02.138 824 Meijer, A., Huijbregts, M.A.J., Schermer, J.J., Reijnders, L., 2003. Life-cycle assessment of photovoltaic modules: 825 comparison of mc-Si, InGaP and InGaP/mc-Si solar modules. Prog. Photovoltaics Res. Appl. 11, 275-287. 826 https://doi.org/10.1002/pip.489. 827 Montero, J.I., Anton, A., Torrellas, M., et al., 2009. EUPHOROS deliverable 5. Report on environmental and economic 828 profile of present greenhouse production systems in Europe. In: European Comssion FP7 RDT Project Euphoros 829 (Reducing the Need for External Inputs in High Value Protected Horticultural and Ornament). 830 Nadal, A., Alamús, R., Pipia, L., Ruiz, A., Corbera, J., Cuerva, E., Rieradevall, J., Josa, A., 2017a. Urban planning and 831 agriculture. Methodology for assessing rooftop greenhouse potential of non-residential areas using airborne sensors. 832 Sci. Total Environ., 601–602, 493–507. doi: 10.1016/j.scitotenv.2017.03.214 833 Nadal, A., Llorach-Massana, P., Cuerva, E., Lopez-Capel, E., Montero, J.I., Josa, A., Rieradevall, J., Royapoor, M., 834 2017b. Building-integrated rooftop greenhouses: An energy and environmental assessment in the mediterranean 835 context. Appl. Energy 187, 338–351. doi:10.1016/j.apenergy.2016.11.051 836 Nemecek T., Bengoa X., Lansche J., Mouron P., Riedener E., Rossi V. & Humbert S., 2015. Methodological Guidelines 837 for the Life Cycle Inventory of Agricultural Products. Version 3.0. World Food LCA Database (WFLDB). Quantis and 838 Agroscope, Lausanne and Zurich, Switzerland. 839 Orsini, F., Gasperi, D., Marchetti, L., Piovene, C., Draghetti, S., Ramazzotti, S., Gianquinto, G., 2014. Exploring the 840 production capacity of rooftop gardens (RTGs) in urban agriculture: The potential impact on food and nutrition security, 841 biodiversity and other ecosystem services in the city of Bologna. Food Security, 6(6), 781–792. 842 http://dx.doi.org/10.1007/s12571-014-0389-6. 843 Pacca S., Sivaraman D., Keoleian G.A, 2006. Life Cycle Assessment of the 33 kW Photovoltaic System on the Dana 844 Building at the University of Michigan: Thin Film Laminates, Multi-crystalline Modules, and Balance of System 845 Components, Center for Sustainable Systems, Report No. CSS05e09, University of Michigan. 846 Peng J, Lu L, Yang H. Review on life cycle assessment of energy payback and greenhouse gas emission of solar 847 photovoltaic systems. Renewable and Sustainable Energy Reviews 2013;19:255–74. 848 Perez, M.J.R., Wight, N.T., Fthenakis, V.M., Ho, C., 2012. Green-Roof Integrated PV Canopies – an Empirical Study 849 and Teaching Tool for Low Income Students in the South Bronx. May 13-18, Colorado. 850 Perpiña Castillo, C., Batista e Silva, F., Lavalle, C., 2016. An assessment of the regional potential for solar power 851 generation in EU-28. Energy Policy 88, 86–99. doi:10.1016/j.enpol.2015.10.004 852 Petit-Boix, A., Leipold, S., 2018. Circular economy in cities: Reviewing how environmental research aligns with local 853 practices. J. Clean. Prod. 195, 1270–1281. doi:10.1016/j.jclepro.2018.05.281 854 Petit-Boix, A., Devkota, J., Phillips, R., Vargas-Parra, M.V., Josa, A., Gabarrell, X., Rieradevall, J., Apul, D., 2018. Life 855 cycle and hydrologic modeling of rainwater harvesting in urban neighborhoods: Implications of urban form and water 856 demand patterns in the US and Spain. Sci. Total Environ. 621, 434–443. doi:10.1016/j.scitotenv.2017.11.206 857 Prasad, D., 2014. Snow, M. Designing with Solar Power: A Source Book for Building Integrated Photovoltaics (BiPV); 858 Routledge: Abingdon, UK 859 Proksch, G., 2011. Urban Rooftops as Productive Resources. Rooftop Farming versus Conventional Green Roofs. ARCC 860 Considering Res. Reflecting upon Curr. themes Archit. Res. 497–509. 861 Proksch G., 2016. Creating Urban Agricultural Systems: An Integrated Approach to Design Published by Taylor Francis 862 Ltd, United Kingdom 863 Ramachandra Rao S.R., 2006. Resource Recovery and Recycling from Metallurgical Wastes, Volume 7 (Waste 864 Management), Elsevier Science, ISBN 0080463207. 865 Raugei, M., Fthenakis, V., 2010. Cadmium flows and emissions from CdTe PV: Future expectations. Energy Policy 38, 866 5223–5228. doi:10.1016/j.enpol.2010.05.007 867 RED Eléctrica de España, Informe del Sistema Eléctrico Español, 2015. Spanish Electricity System 2015 Report. 868 Available online at: https://www.ree.es/en (last accessed November 2017). 869 Ripa, M., Fiorentino, G., Vacca, V., Ulgiati, S., 2017. The relevance of site-specific data in Life Cycle Assessment (LCA). 870 The case of the municipal solid waste management in the metropolitan city of Naples (Italy). J. Clean. Prod. 142, 445– 871 460. https://doi.org/10.1016/j.jclepro.2016.09.149 872 Saiz, S., Kennedy, C., Bass, B. and Pressnail, K., 2006. Comparative Life Cycle Assessment of Standard and Green Roofs. 873 Environmental Science and Technology, 40(13), 4312–4316. 874 Sanjuan-Delmás, D., Llorach-Massana, P., Nadal, A., Ercilla-Montserrat, M., Muñoz, P., Montero, J.I., Josa, A., 875 Gabarrell, X., Rieradevall, J., 2018a. Environmental assessment of an integrated rooftop greenhouse for food production 876 in cities. J. Clean. Prod. 177, 326–337. doi:10.1016/j.jclepro.2017.12.147 877 Sanjuan-Delmás, D., Llorach-Massana, P.,Nadal, A., Sanyé-Mengual, E., Petit-Boix, A., Ercilla-Montserrat, M., Cuerva, 878 E., Rovira, M.R., Josa, A., Muñoz, P., Montero, J.I., Gabarrell, X.,Rieradevall, J., Pons, O., 2018b. Improving the 879 metabolism and sustainability of buildings and cities through integrated rooftop greenhouses (i-RTG). In: Nandwani D. 880 (eds) Urban Horticulture. Springer, Cham, pp. 53–72. https://doi.org/10.1007/978-3-319-67017-1_3 881 Santos, ÍP Dos and Rüther, R, 2012. The potential of building-integrated (BIPV) and building-applied photovoltaics 882 (BAPV) in single-family, urban residences at low latitudes in Brazil. Energy Build; 50: 290–7. 883 doi:10.1016/j.enbuild.2012.03.052 884 Sanyé-Mengual, E., Cerón-Palma, I., Oliver-Solá, J., Montero, J.I., Rieradevall, J., 2013. Environmental analysis of the 885 logistics of agricultural products from roof top greenhouses in mediterranean urban areas. J. Sci. Food Agric. 93, 100– 886 109. doi:10.1002/jsfa.5736 887 Sanyé-Mengual, E., Oliver-Solà, J., Montero, J.I., Rieradevall, J., 2015a. An environmental and economic life cycle 888 assessment of rooftop greenhouse (RTG) implementation in Barcelona, Spain. Assessing new forms of urban agriculture 889 from the greenhouse structure to the final product level. Int. J. Life Cycle Assess. 20, 350–366. doi:10.1007/s11367- 890 014-0836-9 891 Sanyé-Mengual, E., Cerón-Palma, I., Oliver-Solà, J., Montero, J., Rieradevall, J., 2015b. Integrating horticulture into 892 cities: A guide for assessing the implementation potential of Rooftop Greenhouses (RTGs) in industrial and logistics 893 parks. J. Urban Technol. 22(1):87-111. 894 Schloss, N., 1984. Technical note: use of employment data to estimate office demand, Monthly Labor Review, Vol. 107 895 No. 12, pp. 40-44. 896 Sica, D., Malandrino, O., Supino, S., Testa, M., Lucchetti, M.C., 2018. Management of end-of-life photovoltaic panels 897 as a step towards a circular economy. Renew. Sustain. Energy Rev. 82, 2934–2945. doi:10.1016/j.rser.2017.10.039 898 Sherwani, F., Usmani, J., Varun, 2010. Life cycle assessment of solar PV based electricity generation systems: A review. 899 Renew. Sustain. Energy Rev. 14, 540–544. doi:10.1016/j.rser.2009.08.003 900 Smith, A., Watkiss, P., Tweddle, G., McKinnon, A., Browne, M., Hunt, A., et al., 2005. The validity of food miles as an 901 indicator of sustainable development. Oxon, UK: Defra. ED50254, -103. 902 Solar Power Europe, 2018. Global Market Outlook for Solar Power / 2018-2022. Available on-line at: 903 http://www.solarpowereurope.org/global-market-outlook-2018-2022/ (last accessed June 2018). 904 Solis, A., Vidal, I., Paulino, L., Johnson, B.L., Berti, M.T., 2013. Camelina seed yield response to nitrogen, sulfur, and 905 phosphorus fertilizer in South Central Chile. Ind. Crop. Prod. 44, 132-138. 906 Specht, K., Siebert, R., Hartmann, I., Freisinger, U.B., Sawicka, M., Werner, A., Thomaier, S., Henckel, D., Walk, H., 907 Dierich, A., 2014. Urban agriculture of the future: An overview of sustainability aspects of food production in and on 908 buildings. Agric. Human Values 31, 33–51. doi:10.1007/s10460-013-9448-4 909 Tao, J., Yu, S.R., 2014. Review on feasible recycling pathways and technologies of solar photovoltaic modules. Sol. 910 Energy Mater. Sol. Cells 141, 108–124. 911 Thoden van Velzen, E. U., Jansen, M., Brouwer, M. T., Feil, A., Molenveld, K., Pretz, Th., 2017. Efficiency of recycling 912 post-consumer plastic packages. In AIP Conference Proceedings 1914, 170002 (2017). 913 https://doi.org/10.1063/1.5016785 914 Torrellas, M., Antón, A., López, J.C., Baeza, E.J., Montero, J.I., 2012. LCA of a tomato crop in a multi-tunnel greenhouse 915 in Almeria. Int J Life Cycle Assess 17, 863–875. doi:10.1007/s11367-012-0409-8 916 Trypanagnostopoulos, G., Kavga, A., Souliotis, Tripanagnostopoulos, Y., 2017. Greenhouse performance results for 917 roof installed photovoltaics. Renew. Energy 111, 724–731. doi:10.1016/j.renene.2017.04.066 918 United Nations, Department of Economic and Social Affairs, Population Division, 2014. World Urbanization Prospects: 919 The 2014 Revision, Highlights (ST/ESA/SER.A/352) Available on-line at: 920 https://www.compassion.com/multimedia/world-urbanization-prospects.pdf (last accessed June 2018). 921 UN-Habitat, 2016. World Cities Report 2016: Urbanization and Development – Emerging Futures. Available on-line at: 922 https://unhabitat.org/books/world-cities-report/ (last accessed July 2018). 923 Vargas-Parra, M.V., Villalba, G., Gabarrell, X., 2013. Applying exergy analysis to rainwater harvesting systems to assess 924 resource. Resour. Conserv. Recycl. 72,50-59 925 Wernet, G., Bauer, C., Steubing, B., Reinhard, J., Moreno-Ruiz, E., Weidema, B., 2016. The ecoinvent database version 926 3 (part I): overview and methodology. Int. J. Life Cycle Assess. 21, 1218-1230. 927 Wielemaker, R.C., Weijma, J., Zeeman, G., 2018. Harvest to harvest: Recovering nutrients with New Sanitation systems 928 for reuse in Urban Agriculture. Resour. Conserv. Recycl. 128, 426–437. doi:10.1016/j.resconrec.2016.09.015 929 Worrell, E., Meuleman, B., Blok, K., 1995. Energy savings by efficient application of fertilizer. Resour. Conserv. Recycl. 930 13, 233-250. 931 World Bank, 2010. World development report 2010: development and climate change. Available online at: 932 http://web.worldbank.org (last accessed June 2018). 933 Xu, Y., Li, J., Tan, Q., Peters, A.L., Yang, C., 2018. Global status of recycling waste solar panels: A review. Waste 934 Manag. 75, 450–458. doi:10.1016/j.wasman.2018.01.036 935 Yang, R.J., Zou, P.X.W., 2016. Building integrated photovoltaics (BIPV): Costs, benefits, risks, barriers and improvement 936 strategy. Int. J. Constr. Manag. 16, 39–53. doi:10.1080/15623599.2015.1117709