Journal of Cleaner Production 367 (2022) 133037 Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro Life cycle cost analysis of tomato production in innovative urban agriculture systems Alexandra Peña a, *, M. Rosa Rovira-Val a, b, Joan Manuel F. Mendoza c, d a Sostenipra (SRG 01412), Institut de Ciència i Tecnologia Ambientals (ICTA-UAB), Z Building, Universitat Autònoma de Barcelona (UAB), Campus Bellaterra, 08193, Cerdanyola del Vallès, Barcelona, Spain b Business Department, B Building, Universitat Autònoma de Barcelona (UAB), Campus Bellaterra, 08193, Cerdanyola del Vallès, Barcelona, Spain c Mondragon Unibertsitatea, Faculty of Engineering, Mechanics and Industrial Production, Loramendi 4, 20500, Mondragon, Gipuzkoa, Spain d IKERBASQUE, Basque Foundation for Science, Bilbao, Spain A R T I C L E I N F O A B S T R A C T Handling Editor: Mingzhou Jin The construction of innovative urban agriculture systems in cities has increased due to food and environmental concerns. While the environmental performance of urban agriculture has been extensively studied, research on Keywords: the life cycle costs of urban agriculture systems is still limited, which constraints sustainability-oriented decision- Economic viability making processes. This paper analyses the economic viability of tomato production cycle in an innovative Food security building with an integrated urban agriculture system in rooftop by applying the life cycle cost methodology. The LCC data was collected from direct measurements and internal and external sources. To calculate labour costs, a Sustainable cities Urban agriculture customised data collection sheet was created. The results are presented by life cycle stage, cost category and type Urban food production of cost (fixed & variable). Results indicate that the main cost drivers for tomato production are labour (24.7%), the rooftop greenhouse structure (15%), external pest control (12.6%), and rainwater consumption (9.5%), accounting altogether for 61.8% of the total costs. Accordingly, cost reduction solutions are evaluated through the development of sensitivity scenarios (rooftop greenhouse structure design, tap water use and rainwater tank size), including the consideration of another relevant aspect, such as the role of the production level output, as it can greatly influence the economic viability and profitability. Finally, the main environmental and social aspects of these urban production systems are also included. 1. Introduction Additionally, agriculture is responsible for 70% of global freshwater use and deforestation, making the agri-food industry the world’s second The world population is estimated to increase from 7.7 billion in largest emitter of greenhouse gas (GHG) emissions, accounting for 25% 2019 to 9.7 billion (+26%) in 2050 (UN, 2019). Currently, more than of all human-caused emissions (FAO, 2017; Ellen MacArthur Founda­ half of the world population lives in urban areas and this tendency is tion, 2019). Interestingly, the Ellen MacArthur Foundation (EMF) has projected to reach 68% in 2050 (UN, 2018). Therefore, population estimated that from the 7.1 billion tons of food produced globally, growth and rapid urbanization is putting pressure on global food secu­ approximately 40% is eaten in cities, where 2.8 billion tons of organic rity due to the increased demand on food (UNCCD, 2017). For instance, waste is produced (Ellen MacArthur Foundation, 2019). Likewise, it is it was estimated that 9.2% of the world population (over 700 million estimated that 80% of all food will be destined for consumption in the people) experienced serious problems regarding food security in 2018 cities by 2050 and therefore they have huge potential to influence the (Egal, 2019). Nowadays, a third of all edible food (1.8 billion tonnes) is way in which food is produced and eaten, and how food waste is wasted (Ellen MacArthur Foundation, 2019), even though there are managed. currently more than 820 million hungry and malnourished people (FAO, In the context of sustainable city development, the implementation IFAD, UNICEF, WFP and WHO, 2019). This is due to existing differences of urban agriculture (UA) can provide relevant opportunities for effi­ in land and water availability between developed and underdeveloped cient food production (Pearson et al., 2010; Thomaier et al., 2015), countries (Ibarrola Rivas and Nonhebel, 2016). helping achieve the 2030 sustainable development goals (SDGs) (UN, D. * Corresponding author. E-mail address: alexandra.pena@uab.cat (A. Peña). https://doi.org/10.1016/j.jclepro.2022.133037 Received 8 October 2021; Received in revised form 27 May 2022; Accepted 2 July 2022 Available online 8 July 2022 0959-6526/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by- nc/4.0/). A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 E.S.A., 2016). UA can likewise increase biodiversity in cities (Van Tuijl applied at same level since the principal analysis was always LCA, while et al., 2018) and help to reduce pollutant emissions, facilitating the LCC played just a very secondary role. This is an important constraint adaptation and mitigation of the impact of climate change (SDG 13) since although LCA is a relevant tool to analyse environmental impacts, (Bendt et al., 2013; Lwasa et al., 2014). UA may contribute in at least LCA findings are limited for effective decision-making without the three ways on a social level: i) it is an important element of food security integration of complete economic data with LCC (Norris, 2001). strategies to ‘feed citizens’, and to fight chronic hunger (SDG 2) in Hence, the overall objective of this paper is to analyse the economic developing countries, ii) it an also contribute to community develop­ viability of artisan tomato production to provide recommendations to pro­ ment, e.g., through activities to increase social cohesion (SDG 10) be­ mote the UA in rooftop greenhouses in cities. To achieve the main objective, tween different groups in society to provide work opportunities for the following specific objectives were established: unemployed workers, and iii) it can be used for educational purposes (SDG 4) to increase awareness among citizens regarding food produc­ (i) To identify main cost drivers and propose reduction alternatives. tion, e.g. by organizing workshops, courses and tours. Finally, it can as (ii) To analyse the potential variability in the results based on well contribute to improving economic sustainability in cities (SDG 8) changes in main cost drivers. by (i) generating new income (SDG8) since there are several companies (iii) To analyse the role of production level output as important var­ that use UA for commercial purposes (e.g. Lufa Farms in Montreal iable affecting the economic viability and profitability. (Canada), Panasonic Factory Solutions Asia Pacific (Singapore), The Plant in Chicago (USA), (ii) promoting innovation, research, and The tomato crop was selected for analysis since (i) tomatoes are the knowledge (SDG9) through the creation of R&D labs on-site in the urban most consumed vegetable worldwide after potatoes (20.8 kg/capita in farms or collaboration with educational institutes (e.g., Science Barge, 2017) (FAO, 2020) and (ii) they are typically used in greenhouse pro­ Sky Green, UrbanFarmers AG); and finally, (iii) offering recreational and duction (Hochmuth and Hochmuth, 2018) because it is relatively easy to tourist activities (e.g. Brooklyn Grange in New York (USA), Xiedao grow in comparison to cucumbers and lettuce, and yields can be high. Green Resort in Beijing (China) (Van Tuijl et al., 2018). Likewise, tomatoes were the second most sold vegetable in Mercabarna UA can be defined as “an industry located within (intra-urban) or on (the food distribution centre of Barcelona) at 87,100 tonnes sold or 14, the fringe (peri-urban) of a town, city or metropolis, which grows or 31% of the total of sold vegetables in 2019 (MercaBarna, 2019). raises, processes and distributes a diversity of food and non-food prod­ As far as the authors are aware, this study is the first to analyse the ucts” (Mougeot, 2000, p.11) and it comprises of a huge variety of LCC of tomato production in i-RTGs well made for better decision- different forms such as community gardens, vertical farming, or urban making by (i) including infrastructure and labour costs, (ii) classifying farms among many (Van Tuijl et al., 2018). However, there is a growing fixed and variable costs, and (iii) applying additional break-even point interest in the development of Vertical farming (VF) or ZFarming forms (BEP) analysis to find the optimal level of production to be sold and of UA such as indoor farms, rooftop greenhouses (RTGs), or rooftop determinate the maximum level of fixed costs at different selling prices. gardens due to the insufficient space available in cities to support The study is also complementary to the previous research of traditional ground agriculture and the lack of resources needed for Sanjuan-Delmás et al., (2018a) where the LCA was performed to quan­ agricultural production, such as water and energy (Specht et al., 2014; tify the environmental impacts of tomato production in the same case Thomaier et al., 2015). Among the VF forms of UA, RTGs are gaining study. popularity due to the increasing interest in the development of inno­ vative food production spaces and promotion of food self-sufficiency in 2. Methodology urban areas (Sanyé-Mengual et al., 2015). RTGs can have notable environmental, economic and social benefits, This section explains how the LCC was performed to determine the such as reduced food miles, improved community food security, and economic viability of artisan i-RTG tomato production. First, the case community outputs (Cerón-Palma et al., 2012; Specht et al., 2014). study is presented. Secondly, the application of the LCC methodology is Going one step further, additional benefits could be expected through explained. Primary data was gathered through novel data collection the construction of integrated roofto greenhouses (i-RTGs). This inno­ protocols developed for this purpose (e.g., registry of the hours worked vative option has the particularity that resource flow such as rainwater, on the crop and consumption of materials) and secondary data from CO₂ and heat, can be integrated in a bidirectional way building-rooftop internal and external sources. greenhouse. This integration will contribute to the reduction of the environmental impact of the building and the food production system 2.1. Description of the case study overall (Sanyé-Mengual et al., 2015). The life cycle environmental performance of UA has been extensively The case study is the integrated rooftop greenhouse of the LEED-Gold analysed in the literature by means of life cycle assessment (LCA), certified building that hosts the Institute of Environmental Science and however the use of life cycle cost (LCC) for the evaluation of the eco­ Technology (ICTA) in the main campus of the Universitat Autònoma de nomic viability is still very limited (Sanye-Mengual et al., 2017). This Barcelona (UAB) in Barcelona province (Figure B1 in Appendix B). Both, was also supported by a previous literature review study based on 20 the building, and its greenhouse, are innovative systems. On one side, the references analysing the use of LCC in UA over the last two decades building’s LEED-Gold certificate recognizes its high level of Leadership (Peña and Rovira-Val, 2020). The results of that study identified prob­ in Energy and Environmental Design. On the other, the greenhouse in lems in the application of LCC methodology in UA such as the frequent the rooftop is also innovative because of the integration of several flows not inclusion of essential costs such as operational labour and infra­ between the building and the greenhouse (rainwater, CO2 and energy), structure intro the cost calculation. According to Lu et al. (2017), the which optimise the environmental performance of the system (Pou et al., exclusion of the labour identified as the most significant operation cost 2015). Complementary information can be seen in Appendix A- A1. (Sanyé-Mengual et al., 2015) and an important production factor Description of the case study. (Baumgartner and Belevi, 2001) was the main reason for the incomplete The flow integration of the studied case is limited to the use of LCC analysis in many UA studies (Lu et al., 2017). Regarding, the rainwater. Two consecutive tomato crops with the same characteristics infrastructure cost (e.g., greenhouse structure), its inclusion in the LCC is and production cycles, were studied: years 2018 and 2019. However, crucial for future development of UA in cities, especially for boost the only the LCC results for 2019 crop are presented and discussed. This is implementation of innovative environmentally friendly UA systems on a due to the problem that there was not a device for measuring labour large scale. Moreover, the results of that literature review also releveled hours, unlike other crop parameters quantified by electronic measure­ that the LCC was frequently integrated with LCA, but they were not ment tools (e.g., water consumption, solar radiation, etc.). This problem 2 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 was observed in the crop 2018 and consequently in the crop 2019 a economic viability of any business (Walther and Skousen, 2009). Fixed standard time for carry out each defined crop production task was costs were calculated as a proportional part of the economic amortiza­ established to have an accurate measure of labour time for crop pro­ tion of assets during the analysed period (7 months), while variable costs duction. Hence, the 2018 crop was considered as a trial version that were quantified based on the unitary cost of consumable resources. allowed the development of refined data collection tools with the pur­ The scope of the study was from cradle to gate, covering two main pose of get more accurate results for the crop 2019. stages: (i) infrastructure and (ii) production. The infrastructure stage in­ The tomato variety cultivated was Coeur-de-boeuf (Lycopersicon cludes initial investment costs of assets (tangible and intangible) needed esculentum var. Arawak) which stands out for the size of its pieces, for production, all of them are fixed cost. The production stage includes which can reach up to 500g and it is mainly used for fresh salads. This input item costs and waste costs (classified as outputs), these costs are variety is highly appreciated for its size and flavour, with an average mainly variable and those specific items that are not variable by unit price of 2,92 €/kg (OCU, 2018). were calculated as a proportional part during the analysis period. Fig. 1 The tomato was cultivated in a hydroponic system, a soilless system illustrates the scope of the study, whereas Table 1 provides detailed that uses perlite volcanic stones as a substrate (Fig B2 from Appendix B), information of all considered costs. containing 171 plants in total. The productive area (substrate area) was Costs related to the maintenance activities stage were not considered 84.3 m2 from the total extension of the i-RTG (122.1 m2) and the period due to the following reasons: (i) no reparation or replacement activities of study was the crop cycle: 7 months, from January to July. took place during the analysed period and (ii) if maintenance operations were to be required, the costs are negligible (e.g., change of small spare parts such as ball valves, PVC elbow, etc.). Thus, the exclusion of 2.2. Life cycle cost (LCC) maintenance costs is not considered to affect the results. Finally, costs at the end-of-life (EoL) stage, the costs related to decommissioning of the LCC is an economic evaluation technique that aims to quantify all greenhouse structure, the production system and the recycling of ma­ costs and cash flows that emerge during the entire life cycle of a product, terials were not included due to the uncertainty in waste management service, and project (Ammar et al., 2013). The LCC of artisan tomato practices after the long lifespan of the infrastructure, which for buildings production followed the guidelines provided by Hunkeler et al., (2008), is typically considered 50 years. Swarr et al., (2011) and ISO (2008), as described below. The functional unit (FU) used in the calculations was defined as “1 kg of tomatoes grown and harvested in a i-RTG over a 7-month production 2.2.1. Goals, scope, and functional unit cycle in the Metropolitan Area of Barcelona (Spain)”. This is the typical The aim of the applied LCC was to quantify the total cost of artisan i- FU (1 kg of product) considered in most UA studies (Peña and RTG tomato production, with the following specific objectives: Rovira-Val, 2020). (i) To present the costs of tomato production by life cycle stage, by 2.2.2. Life cycle cost calculation cost category and by fixed and variables. The total LCC cost of the 2019 crop was calculated as described in (ii) To identify the main cost drivers and propose reduction Equation (1) alternatives (iii) To analyse the costs variation considering different sensitivity LCC (€/kg) = CI+CP (1) scenarios Where CI = infrastructure costs and CP = production costs. For the infrastructure costs, initial capital investments needed for Regarding total costs, they are presented in terms of variable and production (greenhouse structure and other asset categories (i.e., the fixed costs because both are relevant to make decisions regarding the Fig. 1. Scope of the study. Acronyms: RTG structure = Rooftop greenhouse structure; SS=System of sensors; IS=Irrigation system; CPS= Curtains and partitions system; PSS= Production supporting system; ITE: Information technology equipment; EPCS = External pest control specialist; PBWC= Pruning biomass waste collection; FBWC=Final biomass waste collection. 3 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 Table 1 Life cycle cost inventory of i-RTG tomato production in 2019. Life cycle Costs group Cost category Fixed or Cost item Lifespan Quantity Cost (€) b Stage Variable (years) a Cycle 1 kg Cycle a 1 kg Cost (1,068 kg) (1,068 kg) INFRASTRUCTURE INTANGIBLE Computer software Fixed Computer software 10 5.83E-02 5.46E-05 35.9 0.03 ASSETS for sensors data Subtotal Computer software 35.9 0.03 TANGIBLE Buildings Fixed Rooftop greenhouse 50 1.17E-02 1.09E-05 813.3 0.76 ASSETS structure (122.14 m 2) Subtotal Buildings 813.3 0.76 Technical Fixed System of sensors 10 5.83E-02 5.46E-05 260.7 0.24 installations Irrigation system 10 5.83E-02 5.46E-05 236.6 0.22 (Auxiliary facilities) Curtains and 10 5.83E-02 5.46E-05 139.9 0.131 partitions system Production 3 1.94E-01 1.82E-04 45.1 0.04 supporting system Subtotal Technical 682.3 0.6 installations Machinery Fixed Balance; maximum: 5 1.17E-01 1.09E- 04 28.6 0.027 6.5 kg Balance; maximum: 5 1.17E-01 1.09E- 04 77.8 0.073 60 kg Conductivity tester 5 1.17E-01 1.09E- 04 8.8 0.008 Ph tester 5 1.17E-01 1.09E- 04 8.8 0.008 Backpack Sprayer, 5 1.17E-01 1.09E- 04 5.5 0.005 capacity 12L Backpack Sprayer, 5 1.17E-01 1.09E- 04 1.3 0.001 capacity 1L High pressure cleaner 5 1.17E-01 1.09E- 04 9.6 0.009 Hand pallet truck up 5 1.17E-01 1.09E- 04 47.5 0.044 to 300 kg Security camera 5 1.17E-01 1.09E- 04 135.2 0.127 Subtotal Machinery 323.1 0.3 Equipment Fixed Hose,25 m 5 1.17E-01 1.09E-04 5.12 € 0.0048 Hose holder 5 1.17E-01 1.09E-04 6.45 € 0.0060 Broom 5 2.33E-01 2.18E-04 0.39 € 0.0004 Dustpan 5 1.17E-01 1.09E-04 1.47 € 0.0014 Nylon working gloves 5 4.67E-01 4.37E-04 0.40 € 0.0004 Goatskin working 5 1.17E-01 1.09E-04 0.92 € 0.0009 gloves Pruning scissors 5 3.50E-01 3.28E-04 6.37 € 0.0060 Belt (pruning scissors) 5 1.17E-01 1.09E-04 0.73 € 0.0007 Cover for pruning 5 2.33E-01 2.18E-04 0.62 € 0.0006 scissor Drill, 710W 5 1.17E-01 1.09E-04 6.56 € 0.0061 Protective glasses 5 1.17E-01 1.09E-04 1.39 € 0.0013 Protective mask 5 1.17E-01 1.09E-04 3.93 € 0.0037 Tool case 5 1.17E-01 1.09E-04 4.25 € 0.0040 Pliers 5 3.50E-01 3.28E-04 9.06 € 0.0085 Blade cutter 5 1.17E-01 1.09E-04 0.57 € 0.0005 Screwdriver 5 4.67E-01 4.37E-04 1.62 € 0.0015 Flexometer, 5m 5 1.17E-01 1.09E-04 0.57 € 0.0005 Wrenches 5 2.33E-01 2.18E-04 4.33 € 0.0041 Hammer 5 1.17E-01 1.09E-04 1.40 € 0.0013 Handsaw 5 1.17E-01 1.09E-04 0.75 € 0.0007 Flange tension gun 5 1.17E-01 1.09E-04 1.75 € 0.0016 Polyethylene shovels, 5 4.67E-01 4.37E-04 0.66 € 0.0006 Electrician scissors 5 1.17E-01 1.09E-04 0.92 € 0.0009 Subtotal Equipment 60.2 0.056 Furniture Fixed Wooden wardrobe 5 1.17E-01 1.09E-04 4.8 0.0045 Wooden table 5 1.17E-01 1.09E-04 4.8 0.0045 Aluminium ladder, 5 1.17E-01 1.09E-04 8.7 0.0081 PVC rolling stool 5 1.17E-01 1.09E-04 2.8 0.0026 Plastic bin 5 1.17E-01 1.09E-04 3.6 0.0034 Subtotal Furniture 24.6 0.023 Information Fixed Desktop computer 10 5.83E-02 5.46E-05 20.9 0.020 technology equipment Subtotal Infor. technology equipment 20.9 0.020 Total Infrastructure stage (I) 1,960.3 1.8 PRODUCTION INPUTS & Direct labour Variable Labour (hrs) 239 2,21E-01 1,339.7 1.3 PRODUCTION OUTPUTS Subtotal Direct labour 1,339.7 1.3 External services Fixed External pest control cycle proportion c 682.2 0.6 specialist (€) Pruning biomass 1 9.36E-04 142.8 0.13 waste collection (unit) Variable Electrical energy 1,903 1.78E+00 189.3 0.18 (kWh) (continued on next page) 4 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 Table 1 (continued ) Life cycle Costs group Cost category Fixed or Cost item Lifespan Quantity Cost (€) b Stage Variable (years) Cycle a 1 kg Cycle a 1 kg Cost (1,068 kg) (1,068 kg) Final biomass waste 1 9.36E-04 169.4 0.20 management (unit) Subtotal External services 1,183.7 1.1 Raw materials Variable Rainwater d 59.5 5.57E-02 517.4 0.5 Biological control 0.7 6.6E-04 167.3 0.16 Fertilizers e 89.5 8.4E-02 92.2 0.09 Tomatoes plants 171 1.60E-01 68.2 0.064 Pesticides f 0.932 4.7E-04 27.2 0.0254 Subtotal Raw materials 872.3 0.84 Sundry materials Variable Clips for tomato 2,565 2.40E+00 27.2 0.025 plants Protection clothing 16 1.50E-02 23.2 0.022 Disposable gloves 50 4.68E-02 3.2 0.003 Biomass collection 30 2.81E-02 11.2 0.010 bags Recyclable plastic 4 3.75E-03 1.0 0.001 bags Subtotal Sundry materials 65.8 0.062 Total Production stage (II) 3,461.5 3.3 TOTAL FIXED COST (TFC) 2,785.3 2.6 TOTAL VARIABLE COST (TVC) 2,636.5 2.5 TOTAL COST (IþII) 5,421.8 5.1 a Based on 7-month crop consumption in physical units. b VAT % excluded. c Calculated as 2/3 of the total invoice of annual service contract for pest control monitoring. d Rainwater cost was calculated as 7-month amortization of the available Rainwater harvest system. e KH2PO4, KNO3, CaCl2, Mg (NO3)2, K2SO4, Ca (NO3)2, Sequestrene (Fe), Hortrilon (Fe, Mn, Zn, Cu, B, Mo). f Pesticides included: Sulfur (S), Heliosufre (sulfur 72%), Insecticidal soap and Neemazal (natural insecticide). assets that last more than one crop cycle), the economic depreciation Initial cost 7 month cost (also named amortization) of such assets was calculated applying Amortisation cost = × (2) Lifespan (years) 12 the 2nd Accounting Standard Property, plant, and equipment of the Spanish general accounting plan (ICAC, 2007). Specifically, section 2. Regarding the production cost, consumed items (consumables), in Subsequent measurement, 2.1. Depreciation, which provides the the 7-month production cycle, these were calculated as in Equation (3) depreciation definition: Cost (consumable item) = Consumption × unit cost € ) (3) Property, plant, and equipment shall be depreciated on a systematic and rational basis over the useful life of the assets, considering their residual value and based on impairment normally incurred due to 2.2.3. Life cycle inventory operational wear and tear, and considering potential technical or com­ Table 1 presents the life cycle inventory of all considered costs of mercial obsolescence. artisan i-RTG tomato production. The costs are presented by (i) Life cycle No residual value was considered feasible for any of the assets. stage; (ii) Cost group. The Spanish general accounting plan (ICAC, 2007) Regarding their useful life, this was defined as years of operational use was used to classify the infrastructure items into (a) intangible assets: and the depreciation period associated to each asset element (see com­ computer software and (b) tangible assets: buildings, technical in­ plete list in Table 1) was estimated according to the greenhouse tech­ stallations, machinery, equipment, furniture, and information technol­ nicians’ opinion: ogy equipment; (iii) Cost category. In this regard, four technical installations were identified: (i) system of sensors; (ii) irrigation system; • Computer software (for sensors data): 10 years. (iii) curtains and partitions system and (iv) production supporting sys­ • Building or construction cost is separated from Land cost. According tem. Detailed information about the composing elements of each tech­ to financial accounting standards, depreciation is only applied to the nical installation can be found in the Appendix B (Table B1) and (iv) Cost construction cost. For building, the usual criterion was applied: 50 item. years. It is worth mentioning that labour and transport costs were included • Technical installations: 10 years, except for the Production sup­ in all costs at the infrastructure stage. This was because the elements of porting system (bags of substrate: perlite volcanic stones) that need this stage, such as rooftop greenhouse structure and building in­ to be renewed every 3 years. stallations, were part of the ICTA-UAB building constructed in 2014 and • Machinery: 5 years. detailed information about the number of working hours spent on con­ • Equipment: 5 years. This estimation could be 3 or 5 years. The last struction and transportation was not available. As was the case with the was selected because we estimated that these small tools could be cost of machinery and tools. At the production stage, transport costs used during more time than just one single research project (in Spain were also integrated in the cost of all items since the transportation cost 3 years). was unknown because they were not specified in the invoices of raw and consumable materials. After estimating the years of lifespan, the proportional amortization cost for one tomato crop period (7 months) was calculated using the 2.2.4. Data collection and monetarization following equation: This section explains how consumption and monetarization data was collected and calculated. The information is presented following the two stages included in the case study: infrastructure and production. The 5 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 percentage of the VAT was not included in costs due to the research- rodents, and other pests. It was estimated that the service for the tomato oriented nature of the building of the case study. production was 2/3 of the total invoice. For electrical energy cost, units consumed were taken from a previously created register (Excel file) and 2.2.4.1. Infrastructure costs. The costs of the infrastructure stage are the energy price was provided by an expert involved in the project. presented in Table 1 and Fig. 2 (in the Results section). Secondary data Regarding PBWC cost, the service of urban waste collection was used such as invoices and other similar documents provided mainly by the (municipality of Cerdanyola del Vallès) and its cost was gathered from internal accounting system was used. When the data was not complete, the Barcelona Metropolitan area’s website. Finally, the cost of the external sources were consulted such as suppliers, online shops, and FBWM was estimated based on a carrier budget and included the experts. In the case that a cost element was no longer available on the recollection of the final biomass waste and its transportation to the market, similar products were used to obtain the approximated cost. treatment plant in the nearest municipality. It was not possible to separate the specific structure cost of the Raw material costs included five cost items: rainwater, biological rooftop greenhouse from the total building cost. For that reason the control, fertilizers, tomato plants and pesticides. The rainwater structure cost was calculated as un average cost based on (i) the real consumed came from the rainwater harvest system (RWHS) which is construction cost for 1 m2 of the building, which is high since it includes part of the ICTA-UAB building and supplies rainwater to the toilets of the all technical installations, materials and elements used for the different building, ornamental plants, and all crops. Hence, only the proportional activities of all floors; (ii) the cost for 1 m2 of rooftop based on a budget, amount of the amortization cost of the RWHS parts was included. Data excluding electrical installations; and (iii) the cost for 1 m2 of the rooftop about consumed biological control, fertilizers, tomato plants and pesti­ greenhouse structure estimated in Sanyé-Mengual et al., (2015) without cides were obtained from the daily register of the research group Sos­ considering electrical installations as well. tenipra running the Fertilecity project, while their unitary prices were Finally, to estimate the lifespan needed for the calculation of eco­ collected from invoices or delivery notes. nomic amortization of the intangible and tangible assets, experts (ar­ Finally, data about consumed sundry consumable materials (e.g., chitects, civil engineers, and agricultural engineers) and references in clips for tomato plants, protection clothing, disposable gloves) was the literature (e.g., Sanjuan-Delmás et al., 2018a) were consulted. gathered from the afore mentioned Excel file, while unitary prices were collected from invoices and websites (online shops, products databases). 2.2.4.2. Production costs. The cost of the production stage (see Table 1 and Fig. 2) includes the costs of all inputs items as well as two outputs 3. Results and discussion (biomass waste). They were classified in four cost categories: direct la­ bour, external services, raw materials, and sundry materials. In this section the results and discussion are presented in four parts. Regarding direct labour, this cost category involved the labour of The first part presents the LCC results of artisan i-RTG tomato produc­ people who directly participated in the tomato production process. tion as follows: (i) contribution by life cycle stage and cost category; (ii) Unlike other crop parameters measured using technical devices (e.g., variable and fixed costs; (iii) the four main cost items (cost drivers) water consumption, solar radiation, etc.), a device to quantify for responsible for 61.8% of the total cost are discussed: labour, rooftop working hours was not available. A daily register of working hours of greenhouse structure, EPCS, and rainwater. The second part presents the cultivation tasks was designed and implemented (e.g., plant monitoring, results of the sensitivity assessment to determine the potential vari­ water monitoring, nutrient solution preparation, pruning, staking, har­ ability in the results according to changes in: rooftop greenhouse vesting). The template used for this purpose can be seen in the Appendix structure and rainwater. The sensitivity assessment was omitted from B (Table B2). A tested standard time for each task was established to labour cost since no significant difference between the working hours secure an accurate measurement of labour time consumed. The unitary spent on tomato production in the studied case and conventional cost per working hour for a basic agriculture worker was obtained from greenhouses was found. Sensitivity scenarios were not established for the database of the Ministry of Agriculture, Livestock, Fisheries and the EPCS either due to uncertainty about the time spent (hrs) on tomato Food of the Government of Catalonia (Goverment of Catalonia, 2016). production. In the third part, the role of the production level output as The next group, external services, included the following four costs an important element affecting the economic viability and profitability items: external pest control specialist (EPCS), electrical energy, prun­ is presented. Moreover, environmental, and social aspects of the i-RTGs ning biomass waste collection (PBWC) and final biomass waste man­ are addressed. agement (FBWM). The cost of the EPCS was calculated based on the annual service contract for monitoring the crops for signs of insects, Fig. 2. Five main cost categories responsible for 90.2% of the total cost. Presented in €/cycle and €/kg (in parentheses). 6 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 3.1. LCC of artisan i-RTG tomato production amount of the annual fee for the urban waste collection service in Cer­ danyola del Vallès municipality. Nevertheless, these costs would be 3.1.1. Contribution by life cycle stage and cost category avoided in the future if other options were available, that is: (i) if their Total tomato production in 2019 was 1,068 kg at a total cost of own staff had pest control expertise and (ii) if the pruning waste were 5,421.8 € (VAT % excluded), which is equivalent to 5.1 €/kg (see used for compost. Table 1). The production stage had the largest contribution with 63.8%, Regarding the fixed costs, the following six cost items are identified followed by the infrastructure stage with 36.2%. as having important contributions (81.7%) to the TFC: (i) i-RTG struc­ By cost category, the following five are responsible for 90.2% of the ture with 29.2%; (ii) EPCS with 24.5%; (iii) system of sensors (SS) with total cost as follows: (i) direct labour with 24.7%, (ii) external services 9.4%; (iv) irrigation system (IS) with 8.5%, (v) PBWC with 5.1%, (vi) with 21.8%, (iii) raw materials with 16.1%, (iv) buildings with 15.0% curtains and partitions system (CPS) with 5% (Fig. 3). and (v) technical installations with 12.6% (Fig. 2). Concerning variable costs, five cost items contribute to 90.4% of the From these five cost categories, the four main cost items (cost TVC: (i) labour with 50.8%, (ii) rainwater with 19.6%, (iii) electrical drivers), responsible for 61.8% of the total cost, are discussed later: la­ energy with 7.2%, (iv) FBWM with 6.4% and (v) biological control with bour from direct labour, i-RTG structure from buildings, EPCS from 6.3%. The amount as €/cycle and €/kg can be seen in Fig. 3. external services, and rainwater from raw materials. 3.1.3. Characterization of main cost drivers 3.1.2. Variable and fixed costs The following four cost items have a key role in the total cost (TC): (i) As mentioned in section 2.2.1., by life cycle stage all infrastructure labour; (ii) rooftop greenhouse structure; (iii) EPCS; and (iv) rainwater. stage costs are fixed and in the production stage almost all costs are Each one contributes over 9% to the TC, while their sum contribution variable with exception of two specific items: (i) the EPCS, and (ii) the accounts for 61.8% (Fig. 4). Therefore, the reduction of these costs is PBWC. In this regard, the total fixed cost (TFC) accounts for 51.4% essential in achieving economic viability since they are the main drivers (2,785.3 €/2.6 €/kg) and the total variable cost (TVC) for 48.6% of the TC. The next sections analyse each of them in-depth and propose (2,636.5 €; 2.5 €/kg). alternatives for cost reduction. From the production stage the EPCS is a fixed cost because it is a fixed annual amount for the service contract for pest control monitoring 3.1.3.1. Labour cost. Labour was the core cost driver, accounting for on several crops and the proportion for tomato crop was estimated at 2/ 50.8% of the TVC and 24.7% of the total cost. Previous research on the 3 of the total invoice. The PBWC cost is a similar case since it is the topic demonstrated that labour contributed to 30–45% of the total Fig. 3. Main variable (above) and fixed (below) cost items presented in €/cycle and €/kg (in parentheses). Acronyms: RTG (rooftop greenhouse), SS (system of sensors), IS (irrigation system), PBWC (pruning biomass waste collection), CPS (curtains and partitions system), EPCS (external pest control specialist). 7 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 Fig. 4. Contribution of the four cost drivers to 61.8% of the total cost. tomato production cost. Estimating the usual 50-year lifespan for economic amortization of In the case studied, it was difficult to calculate the working hours as buildings, its cost was calculated to be 11.4 €/m2/year, which was it was necessary to separate the time spent on production from the time 22.6% higher than the average cost for a high-tech RTG of 9.3 €/m2/year devoted to other experimental tasks with the same tomato crop, such as (calculated with the average of 329 €/m2 and 600 €/m2) (Ackerman, nutrients recovering in Rufí-Salís et al. (2020 a,b). 2012 Milford et al., 2019). The higher cost in the case studied is because The results, are consistent with other studies in the literature high­ information on the specific rooftop greenhouse structure cost was not lighting that labour is the main cost driver in tomato production (Çetin available, and its cost was calculated as an average cost (see 2.2.4.1). and Vardar, 2008; Keskin et al., 2010; Barrett et al., 2012; Taki et al., However, this cost could have been reduced if a study for optimization of 2013; Testa et al., 2014; Sanyé-Mengual et al., 2015; Albaladejo-García construction materials had been carried out during the building design et al., 2018; Cáceres Hernández et al., 2018). The reason is that tomatoes phase (Sanjuan-Delmás et al., 2018b). Moreover, the size of the studied are one of the highest labour demanding crops since it is mainly har­ case structure is small (artisan) with a production area of only 84.3 m2, vested by hand, probably due to the availability of cheap labour (Çetin therefore there is a need for designing medium and large size i-RTGs to and Vardar, 2008). This happens, for instance, in Turkey, China, and facilitate the RTG expansion in cities in the future. India who are among the top 10-tomato producers worldwide (Çetin and Based on these considerations, future research should optimise the Vardar, 2008) and also applies in Spain and The Netherland, the biggest rooftop greenhouse structure (prototype, materials, and cost) by European tomato producers (Ibarrola-Rivas et al., 2020). considering different sizes (e.g., small, medium, and large) helping to On the other hand, the labour cost strongly depends on the working make decisions for implementing these innovative UA systems on a hours required. In this regard, the working hours spent per m2 for to­ larger scale. Sanyé-Mengual et al., (2015) discussed that this is a crucial mato cultivation in conventional greenhouses for industrial production condition since the rooftop greenhouse structure cost could be a possible in Almeria is 2.83 h/m2 per crop (based on 1840 working hours and 650 barrier for future development. m2) (Cámara-Zapata et al., 2019), which is the same efficiency ratio as the artisan tomato production in the i-RTG (based on 239 working hours 3.1.3.3. External pest control specialist. The EPCS was the third main and 84.3 m2). This is an important finding that makes a valuable cost driver, contributing to 24.5% of the TFC and 12.6% of the TC. It was contribution of this study to the literature on innovative rooftop the service provided from an external specialist for monitoring the crops greenhouse tomato production. It demonstrates that the efficiency level for signs of insects, rodents, and other pests. As mentioned in 2.2.4.2., is the same (i) in two different tomato production systems (industrial this is an annual service contract with a closed price, classified as a fixed conventional greenhouses versus innovative i-RTG) and (ii) in two cost, and the cost assigned to the tomato crop was calculated as 2/3 of different sized productions (large versus small). the total invoice. There are several examples in the literature about labour reduction This significant cost could be reduced or avoided in the future by costs by using non-paid working hours. For instance, the use of volunteer providing specialized training on sustainable pest prevention and con­ work is one of the most commonly applied in UA projects (Lui, 2015). trol to the personnel responsible for the tomato production. Another way is through the “self-pick” strategy. This strategy also called “you-pick”/“pick-your-own” is a direct marketing approach where cus­ 3.1.3.4. Rainwater cost. Rainwater was the fourth largest cost, ac­ tomers do the harvest task themselves and this is a way to decrease the counting for 19.6% of the TVC and 9.5% of the total cost of the crop in labour cost since harvesting consumes many working hours. (Ernst and 2019, where 59.5 m3 of rainwater was consumed, that is 0.056 m3/kg Woods, 2014; Liu, 2015). (56 litres/kg). As mentioned in 2.2.4.2., this cost was calculated as the economic amortization of the RWHS (pipes, water tank, materials). For 3.1.3.2. Rooftop greenhouse structure. The rooftop greenhouse structure the crop period of 7 months, it was 517.4 €, i.e., 1 m3 of rainwater costs was the second most important cost driver for artisan tomato produc­ 8.7€. This cost is more than three-times higher than the cost of tap water, tion, contributing to 29.2% of the TFC and 15.0% of the TC. In the case estimated at 2.5 €/m3 based on 150.9€/cycle (Aigües de Barcelona, studied, the i-RTG structure is an integral part of the building 2020). This large cost is due to the great capacity of the RWHS (water (Sanjuan-Delmás et al., 2018b) which is high-tech and composed of steel tank, materials used) which was designed to supply rainwater to the (0.836 kg/m2), concrete (0.212 kg/m2), polycarbonate (0.032 kg/m2), toilets, ornamental plants of the building and all crops in the rooftop low-density polyethylene (0.006 kg/m2), polyester (0.0008 kg/m2) and greenhouses (Sanjuan-Delmás et al., 2018b). aluminium (0.0008 kg/m2) (Sanyé-Mengual et al., 2015). In this regard, previous studies that analysed the economic 8 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 performance of the rainwater harvesting installations concluded to be double polyethene plastic (PE) or rigid plastic. While the high-tech financially non-viable (Christian Amos et al., 2016; Gao et al., 2017; support structure has a steel or aluminium frame and the covering ma­ Ishida et al., 2011; Roebuck et al., 2011, 2012). However negative terials are more durable such glass and polycarbonate, which is the case financial viability does not necessarily mean negative economic viability of the studied case. since the LCC results give economic measures, not economic decisions The cost of a conventional medium-tech greenhouse varies from 26 (Amos et al., 2018). Therefore, the economic evaluation should include to 88 €/m2 ($30–100) depending on the used materials and the cost of wider considerations such as the definition of need, and indirect benefits high-tech greenhouses, both on the ground, are from 126 to 252 shown in improved health through water, sanitation, and hygiene ($150–300) depending on materials, climate control, ventilation (Alexander et al., 2014), which have a socio-economic impact that is (Proksch, 2017). But placed on the roof, their costs can increase up to often complex to measure in financial terms. Benefits for the whole so­ three times (Milford et al., 2019). Hence, the average cost of building ciety may have more value than simply economic costs (Domènech and medium-tech RTGs varies from 171€/m2 to 378€/m2, while for Saurí, 2011; Beatty and McLindin, 2012). For instance, these technical high-tech RTGs from 329 to 426 €/m2 ($375–485). However, in 2019, it installations have a great potential to alleviate the increased water de­ was estimated that a high-tech RTG covered by glass could reach 600 mand caused by urbanization (Barthwal et al., 2014) and improve the €/m2. The lifespan was considered to be 50 years by Sanyé-Mengual water security in urban areas (Amos et al., 2018). Hence, the rainwater et al., (2015) and Benis et al., (2018) due to the concrete structure. Since capturing, and use may help to reduce both tap water consumption and the cost of greenhouse structures can vary across countries (Harada and the energy for water treatment and pumping, which contribute to Whitlow, 2020; Proksch, 2017), in the sensitivity analysis the average sustainability. cost for a high-tech structure of 465 euro/m2 or 9.3 €/m2/year from a Unlike tap water with which scarcity is one of the main environ­ cost rank of 329 €/m2 to 600 €/m2 was used. In comparison, the cost of mental concerns (European Commission, 2010), rainwater is a relatively the studied i-RTG structure was 570 €/m2. The cost of each mentioned clean and abundant renewable resource, especially in the Mediterranean rooftop farming structure can be seen in Table B3 from the Appendix B area. For instance, it has been forecasted that on average in 2051, tap and in Table 2 (here in €/m2/year). water availability for the region of Catalonia will decrease by 17.8% and Table 2 presents the results of the sensitivity analysis from replacing in Southern Catalonia, the decrease could be higher, 70–75% (Duran the studied case structure cost (Scenario 0) with the cost of (i) an et al., 2017). Furthermore, there is an uncertainty with the price of the intensive roof for open-air cultivation (Scenario 1); (ii) a medium-tech tap water since a report from the Catalan Water Agency indicated that in RTG (Scenario 2) and (iii) an average high-tech RTG (Scenario 3). For the period 2005–2015, the price of water increased by 50% (5% annual) Scenario 1 and Scenario 2, a lifespan of 40 years was considered since and this tendency will continue in the following years (Vargas-Parra this structure is made of less resistant materials than the high-tech RTG. et al., 2019). The results revealed that the most notable cost reductions in the total A previous case study explained that in a Mediterranean climate with tomato cost and the tomato cost per kg were very similar for the two low and variable precipitations, the use of RWHSs covered most of the simplest structures with 11.9% in Scenario 1 (reduction of 578.2 water need for flushing toilets and 60% of the demand for landscape €/cycle; 0.54 €/kg) and 10.9% (reduction of 506.9 €/cycle; 0.47 €/kg) in irrigation (Domènech and Saurí, 2011). Similarly, Fragkou et al., (2016) Scenario 2. While by comparing the case studied structure cost (Scenario demonstrated the high potential of the Mediterranean region to supply 0) with the average cost of the same high-tech structure in Scenario 3, its water needs from rainwater runoff, taking into account all urbanized only a small decrease of 2.9% (150.7€/cycle; 0.14 €/kg) in the total cost areas as collectors, where the water self-sufficiency potential varies from and the cost per kg was noticed. Hence, the sensitivity analysis sup­ 8% to 500% with an overall average above 100% for the regional ported that the structure of the case studied is high-tech RTG and sup­ system. ports the recommendation of Sanjuan-Delmás et al., (2018b) that the Based on the expected contribution of rainwater to improve the structure cost could be reduced in future research if a optimization study water security in urban areas, the RWHS should be optimized in future of construction materials were carried out during the building design research in terms of design, materials, and cost. This recommendation is phase. also supported by Sanjuan-Delmás et al., (2018b) where the authors explained that both the rooftop greenhouse structure and the RWHS 3.2.2. Rainwater were exaggerated in size, and the material used for its construction could As has been mentioned in 3.1.3.4., this cost was calculated as the have been reduced if a study of its optimization had been completed economic amortization of the RWHS (ICTA-UAB). The rainwater tank during the building design. had a considerable capacity of 100 m3, used to supply toilets, orna­ mental plants and all rooftop crops in the building (Sanjuan-Delmás 3.2. Sensitivity assessment et al., 2018b). The rainwater tank cost was 6.800€ which dispropor­ tionately increased the RWHS (ICTA-UAB) cost. Nevertheless, it was 3.2.1. Rooftop greenhouse structure estimated that a 20 m3 tank (this is a fifth part) would be enough for 90% The potential variability in the results was analysed by comparing of rainwater needed for the crop irrigation (Sanjuan-Delmás et al., the structure of the case studied to three structural systems suitable to be 2018b). placed on the roof (RF structure hereafter): (i) intensive green roof for Therefore, here the sensibility assessment (Table 3) was carried out open-air farming (Benis et al., 2018), (ii) medium-tech RTG (Proksch, to estimate the costs variations (total cost and cost per kg) by replacing: 2017), and (ii) high-tech RTG (Proksch, 2017; Milford et al., 2019). (i) the use of rainwater with tap water and (ii) the rainwater tank ca­ The intensive green roof is an uncovered structure for open-air pacity with a smaller one, i.e., 20 m3 instead of 100 m3 as agricultural production. The building cost per m2 is around 130 euro/ Sanjuan-Delmás et al., (2018b) proposed. m2 over a lifespan of 40 years or 3.3 euro/m2/year, including (i) a Cost data from the Barcelona water supplier website (Aigües de waterproofing membrane and a root barrier that divide the wet layers Barcelona, 2020) and online supplier were used to estimate the tap from the underlying building rooftop; (ii) a drainage layer that facili­ water cost and the cost of the 20 m3 water tank. tates the removal of excess water; (iii) a filter fabric that avoids the If tap water was used for irrigation, the cost would be 150.9 € or drainage layer from clogging; and (iv) other layers: water retention, 70.8% less than using rainwater (517.4 €), while the reduction in the substrate and vegetation (Benis et al., 2018). The main difference be­ total tomato production cost is 366.5 € or 6.8% (Scenario 1). However, tween the medium-tech and high-tech RTGs is in the construction ma­ although the cost of consuming rainwater is higher in comparison to tap terials (Proksch, 2017). For instance, the medium-tech greenhouse water, the use of rainwater can bring significant environmental, eco­ support structure has a steel frame, and the covering materials are nomic, and social benefits. Environmentally, rainwater harvesting on 9 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 Table 2 Life cycle cost variation by using alternative types of RF structure. Scenario Lifespan RF structure cost Tomato total cost Tomato cost per kg Variation (Scenario 1,2,3 - Scenario 0) (€/m2/year) (€/cycle) (€/kg) Tomato total cost Tomato cost per kg % (€/cycle) (€/kg) Scenario 1: Intensive roof for open- 40 3.3 4,843.6 € 4.5 − 578.2 − 0.54 − 11.9 air cultivation Scenario 2: Medium-tech RTG 40 4.3 4,914.8 € 4.6 − 506.9 − 0.47 − 10.9 Scenario 3: High-tech RTG 50 9.3 5,271.1 € 4.9 − 150.7 − 0.14 − 2.9 Scenario 0: Baseline 50 11.4 5,421.8 € 5.1 Table 3 where SP = selling price per unit, VC = variable cost per unit. Life cycle cost variation by using tap water and 20 m3 water tank. The current average market price is between 3.0 €-4.0 € for 1 kg of tomatoes Coeur de boeuf with VAT included (4% in Spain). The analysis Variation (Scenario 1, 2 - Scenario 0) assumes that 1 kg consists of 5 tomatoes (number of physical units) Tomato Tomato Tomato Tomato % which was the average for two consecutive crops (2018 and 2019) and total cost cost per total cost cost per (€/cycle) Kg (€/kg) (€/cycle) Kg (€/kg) that all produced tomatoes would be commercialized without discrim­ inating their size. Scenario 1: Tap 5,055.3 4.7 − 366.5 − 0.3 − 6.8 The complete table of results of the BEP analysis can be seen in water Scenario 2: 5,372.0 5.0 − 49.8 − 0.1 − 0.9 Table B4 from the Appendix B. Prices between 3.0 € and 5.0 € are not Small suitable for the studied case because the BEP is above the i-RTG pro­ rainwater ductive capacity (the production average was 5,415 units in 2018 and tank (20 m3) 2019 crops). Hence, price range to be used it 5.1 € to 5.5 € for the Scenario 0: 5,421.8 5.1 Baseline productive area of 84.3 m2 with a total fixed cost of 2,785.3 € and rainwater & variable cost per unit of 0.49 €. But in the authors’ opinion, this range of big water tank prices would be too high for the local market. The reasons for the high (100 m3) selling prices of 1 kg of tomatoes are the elevated fixed costs (rooftop greenhouse structure) and the small cultivation area (84.3 m2) which limits the productive capacity. the roof can reduce the impact of storm water runoff in the area, which can otherwise damage creeks and other diversity of species. Addition­ The BEP equation can be applied here to determine the maximum level of fixed cost for a specific production area size, which includes the ally, it was demonstrated that the construction of RWHS combined with food production is associated with low environmental impacts (Tobo­ production output, and specific unitary variable cost. In this regard, the maximum level of fixed cost is calculated at selling between 3.0 €-5.0 € so-Chavero et al., 2019). Economically, rainwater use can contribute to to find how much the fixed costs have to decrease in order to establish saving money on water bills by storing water in an economic way. For selling prices below 5.0 €. For the studied case: a productive area of 84.3 instance, in some areas, local councils have introduced cash-back refund m2 with an average production output of 5,415 units (for two consec­ plans for those who install a rainwater tank. In this regard, rainwater use utive years), and variable unitary cost of 0.49 €. for irrigation could be beneficial in the following years due to the un­ The complete table of results can be seen in Table B5 from the Ap­ certainty of the future price of supply water (Amos et al., 2018). Finally, there are expected social benefits related to health issues and personal pendix B. Selling prices between 3.00 € and 3.6 € are discarded since their average fixed costs would be 920.6 € meaning that they must preferences. For example, some people prefer to consume rainwater since there are no added chemicals that are used to treat mains water decrease by 1,864.7 € or 66.9% comparing with the current fixed cost of 2,785.3 €, which is hard to achieve. At selling prices between 3.7 € - 4.3 supply. Moreover, rainwater is a suitable option in some areas where water is salty and scarce, contains heavy metals or has an unpleasant €, the average fixed cost would be 1,678.7 €, hence a large reduction of 39.7% on average must be made to achieve it. Finally, at the selling price odour (Rain Harvesting, 2021). Regarding the size of the rainwater tank, if 20 m3 were used, the cost range from 4.4 € to 5.0 €, the average fixed costs would be 2,436.8 € and must decrease by 12.5% on average, which seems feasible with the of the rainwater tank would be reduced from 6,800 € (100 m3) to 2,530 € (20 m3) or 62.8% less which supposes a decrease of 49.8 € in the total optimization of the rooftop greenhouse structure and the technical in­ stallations in future research. For instance, this can be possible by using tomato production cost or 0.9%. Therefore, the substitution with a smaller rainwater tank could be a viable option. reduced, recycled, or less costly materials. Lastly, previous studies that analysed the economic potential of the 3.3. Production level output RTGs through LCC demonstrated that for some agriculture practices such as hydroponics, the unitary economic cost strongly depended on The production level output is an important aspect affecting the the yield size (Sanyé-Mengual et al., 2015; Benis et al., 2018; Weidner et al., 2019). For instance, Sanyé-Mengual et al., (2015) found that local economic viability and profitability of any economic activity. For this reason, has been calculated for the tomatoes production of the studied tomatoes grown in small yield RTGs have higher unitary economic costs than those produced via conventional large-scale production. In case by adding an additional BEP analysis. The BEP is the level of pro­ duction to be sold that completely covers the TFC. At this level the contrast, tomatoes grown in local RTGs with high crop yield size >25 kg/m2 have not only a lower unitary economic cost but also at the same company has no losses. From this level, every additional unit sold con­ tributes to generate profit. The BEP is very useful in knowing the number time have better environmental characteristics. In this regard, it was estimated that to achieve higher economic and environmental perfor­ of units to be sold so as not to have losses from the production activity (Gutierrez and Dalsted, 2012). A BEP analysis is performed in this sec­ mance, the i-RTGs require an annual tomato crop yield of 55 kg/m2 (Sanyé-Mengual et al., 2015). While in Benis et al., (2018), the recom­ tion (Equation (4)). mended yield size for this purpose was calculated to be approximately (Fixed costs) 70 kg/m2. In the case of the 2019 i-RTG tomato crop, the yield was Break even point (in units) = (4) (SP − VC) 12.66 kg/m2, considerably lower than required. 10 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 For a specific production size, based on the BEP analysis and the perceptions (Ercilla-Montserrat et al., 2019; Grebitus et al., 2020) but previous research, it could be concluded that it is crucial to optimise the high price is a big barrier for posterior purchase intentions (Grebitus fixed costs, otherwise it would be necessary to sell the products at a high et al., 2017). Nonetheless, customers tend to pay a premium price for price that allows to cover all these costs. For instance, it might be locally grown products (Willis et al., 2016; Boys et al., 2014) since they possible by using reduced, recycled, or less costly materials of the assume that they are fresher, of higher quality and better tasting. rooftop greenhouse structure. Additionally, local production can also benefit de local community enhancing the local economy and benefit the environment at the same 3.4. Environmental and social aspect of i-RTGs time (McGarry-Wolf et al., 2005; Zepeda and Leviten-Reid, 2004) (See A3. Customer preferences from Appendix A). Unlike the non-integrated RTGs and conventional greenhouses on the ground, i-RTGs have demonstrated better environmental 4. Conclusions (Sanyé-Mengual et al., 2018; Sanjuan-Delmás et al., 2018a). For instance, Sanyé-Mengual et al., (2018) demonstrated that i-RTGs envi­ This paper analysed the economic viability of an artisan tomato ronmental savings were 2.1 times for avoided CO2 emissions and 1.8 for production in the rooftop of an innovative building with an integrated energy consumption by comparing the differences between urban agriculture system. LCC was applied to quantify its total cost by non-integrated RTGs and i-RTGs in retail parks. In comparison with life cycle stage, by cost category and by fixed and variable cost, identi­ conventional greenhouses, i-RTGs have between a 50 and 75% lower fying the main cost drivers, proposing cost reduction alternatives, using impact on five of six impact categories. Specifically, the environmental sensitivity scenarios, and including the production level output calcu­ savings of i-RTG artisan tomato production were 0.58 kg of CO2 lations, as a relevant factor for the economic viability and profitability. equivalent per kg versus 1.7 kg of CO2 from conventional greenhouses As far as the authors are aware, this is the first study analysing the life (Sanjuan-Delmás et al., 2018a). cycle economic viability of tomato production in i-RTGs including Moreover, the role of i-RTGs to improve energy efficiency in build­ essential costs such as labour and infrastructure, which tend to be ings was analysed in Nadal et al., (2017). They found that i-RTGs could missing in research on UA. The results are valuable for public admin­ recycle 43.78 MWh of thermal energy (or 341.93 kWh/m2/yr) from istrations or investors with ability to promote policies or funding for the buildings and that compared to the conventional greenhouse, heated implementation of economically and environmentally sustainable food with oil, gas, or biomass systems, i-RTGs can also achieve greater annual production in cities. It also contributes to the UA literature by improving carbon and economic savings as follows: (i) 113.8 kg CO2(eq)/m2/yr academic knowledge on the economic performance of alternative pro­ and 19.63 €/m2/yr compared to oil heated; (ii) 82.4 kg CO2(eq)/m2/yr duction systems for further sustainability-oriented research on the topic. and 15.88 €/m2/yr compared to gas heated, and (iii) 5.5 kg CO2(eq)/­ The results indicated that the production stage had a major contri­ m2/yr and 17.33 €/m2/yr compared to biomass heated. bution to the TC and five cost categories (direct labour, external services, Later, Muñoz-Liesa et al., (2020) quantified the bi-directional energy raw materials, buildings, and technical installations) account for 90.2% exchange between greenhouses and buildings. Together with Nadal of it. The main cost drivers are labour, rooftop greenhouse structure, et al., (2017), they demonstrated that 98 kWh/m2/year of heating en­ EPCS and rainwater, determining nearly 62% of the TC. Thus, the ergy is passively recovered (84% during night-time) by i-RTGs from reduction of these costs is an essential requirement to achieve economic building waste heat. As well as that, the energy savings of the building viability. are 35 kWh/m2/year (equal to 4% of the building’s annual electricity An important finding that makes a valuable contribution to the needs) thanks to the insulating capacity of i-RTGs. This results in an literature on innovative rooftop greenhouse tomato production is that overall 128 kWh/m2 of annual net energy savings equivalent to 45.6 kg there was the same efficiency in the main cost driver, labour (hours CO2 eq/m2, considering 5 kWh/m2/year are required to operate the spent per m2) both (i) in two different tomato production systems building climate system that enables the bi-directional (green­ (conventional greenhouses versus innovative i-RTG) and (ii) in two house-building) thermal exchange. different size productions (large versus small). Regarding social sustainability, UA activity has been demonstrated The managerial implications derived from findings to facilitate the to have positive contributions in different aspects: (i) better food and economic viability and contribute to the implementation of i-RTG pro­ nutrition security, (ii) health improvement, (iii) establishment of jobs duction are derived from (i) the reduction strategies of cost drivers and for the urban poor; and (iv) inclusion of disadvantaged people or social (ii) to stablish the adequate production level output. Respecting the (Orsini et al., 2013). However, the construction of building-based UA reduction of cost drivers (labour, rooftop greenhouse structure, EPCS forms such as rooftop farms (open-air) and rooftop greenhouses can be and rainwater cost), labour and rooftop greenhouse structure are associated with additional social benefits such as improved customer crucial. As the core cost driver (50.8% of TVC and 24.7% of TC) awareness about the origin of the food (Specht et al., 2016; considered strategies for labour cost reduction were: (i) use of volunteer Sanyé-Mengual et al., 2016), improved community building work and (ii) customers’ participation in harvest task. Regarding the (Sanyé-Mengual et al., 2016), educational benefits (Kortright and second cost driver, the rooftop greenhouse structure, that could be a Wakefield, 2010; Specht et al., 2016), transparency and creation of new possible barrier for implementing these innovative UA systems on a experimental spaces (Specht et al., 2016). large scale (Sanyé-Mengual et al., 2015) it is a key condition to reduce its If integrating greenhouses into buildings for UA would have positive cost optimising prototypes, materials, and sizes (e.g., small, medium and impacts as foment food self-sufficiency policies or energy/water-saving large) to allow making decisions for this initial investment. Regarding policies in the short term (Cerón-Palma et al., 2012), special attention the third and fourth cost drivers, the EPCS could be reduced or avoided if needs to be devoted to the great opportunity for environmental educa­ staff training was provided and the rainwater cost could be decreased by tion of building-based UA because of its possible effects in the longer run. optimising the rainwater tank size according to the productive area. Nowadays the interest in analysing social aspects of UA in rooftop Last, the size of production area is relevant for the role of production greenhouses, as a necessary component of this activity is growing. In this level output. As break-even point demonstrated, high fixed costs and low regard, it is worth to mention the ongoing project GROOF-Greenhouses yields is a combination that impede the economic viability and profit­ to reduce CO2 on rooftops, aimed to define the state of the art of the ability of i-RTG artisan production. building integrated greenhouse for a more resilient built environment, This study has several constraints: (i) the costs at EoL stage were not which includes the analysis of their social performance, but results are included; (ii) it was carried out in a Mediterranean climatic zone with a not available yet (GROOF, 2022). mild and hot climate and no abundant rains; and (iii) the economic costs Finally, UA products are mostly associated with positive customer of additional innovative technical installations (e.g., water recycling 11 A. Peña et al. Journal of Cleaner Production 367 (2022) 133037 system), used to reduce environmental impacts, which could increase Appendix A. 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Economic analysis and feasibility of rainwater harvesting systems in urban and peri-urban environments: a review of the global situation with a special focus on Australia and Kenya. Water 8 This work is part of a project entitled Fertilecity II: Integrated rooftop (4), 149. greenhouses: a symbiosis of energy, water and CO2 emissions with the de Barcelona, Aigües, 2020. Precios Y Tarifas. http://www.aiguesdebarcelona.cat/factur building – Towards urban food security in a circular economy supported adelaigua/es/precios-tarifas/. (Accessed 15 July 2020). by the Spanish Ministry of Economy and Competitiveness (CTM2016- Domènech, L., Saurí, D., 2011. A comparative appraisal of the use of rainwater harvesting in single and multi-family buildings of the Metropolitan Area of 75772-C3-1-R, AEI/FEDER, UE). This research has received financial Barcelona (Spain): social experience, drinking water savings and economic costs. support from the European Union’s Horizon 2020 research and inno­ J. 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