Life cycle assessment of solar energy use in metro systems within the scope of environmental sustainability

In terms of energy transition policies that are being implemented on a global scale, interest in rooftop photovoltaic (PV) applications is on the rise, sites for these applications are being evaluated, and incentive programs are being put in place, especially in cities. Therefore, it is important to comprehensively evaluate new PV technologies and applications as they evolve in order to understand their environmental performance and make meaningful comparisons with other energy sources. The focus of this study is the assessment of the life cycle environmental impacts of a rooftop PV system with an installed capacity of 1956.15 kilowatt peak (kWp) on the roof of a workshop building at a railway system campus in Istanbul, Türkiye. This is the first time that a PV module system using one of the latest technologies, the high-efficiency Passivated Emitter and Rear Cell (PERC), has been integrated into transportation building components and its environmental impacts have been comprehensively assessed. The study also takes into account the Türkiye location of the installation. The scope of the life cycle assessment (LCA) conducted with SimaPro v9.2 software was determined as "cradle to grave" and the functional unit was considered as 1 kilowatt hour (kWh) electricity generation. Over the life cycle of the rooftop PV system, the total greenhouse gas (GHG) emissions per 1 kWh of electricity produced are calculated to be 55.1 grams of carbon dioxide equivalent (g CO2 eq.). Compared to grid electricity, the analysed system is estimated to avoid a total of ~1146 tons/year of CO2 over its lifetime. Other significant environmental impacts are calculated as human toxicity non-carcinogenic; 1.9E-06 CTUh/kWh, human toxicity carcinogenic; 7.6E-08 CTUh/kWh, freshwater eutrophication; 0.03 g P eq./kWh, freshwater ecotoxicity; 2649.9 CTUe/kWh, fossil resource use; 570.2 MJ/kWh and mineral and metal use; 0.0050 g Sb eq./kWh. The biggest source of environmental impact is the production phase of the PV module (approximately 82.2-99.0 %). The sensitivity analysis of the manufacturing location showed that module assembly and energy-intensive processes (poly-Si and Cz-Si) in a country with a more environmentally friendly energy mix could result in an improvement of the environmental performance. The best performance in terms of environmental impact was achieved by glass-glass PERC. The results support that on-site solar electricity generation is more advantageous than fossil fuel electricity generation in most of the impacts. Rooftop PV installations on components of rail systems, one of the main modes of transportation, can provide environmental benefits, especially in large cities such as Istanbul. However, it should be noted that these systems may also have environmental impacts (such as resource use, minerals and metals) that may arise from other stages of these systems, in particular from manufacturing the modules. The inclusion of life cycle assessment in pre-project analyses in terms of sustainability of the applications will be beneficial in identifying hotspots that cause environmental impacts and taking measures accordingly.