Time-resolved life cycle assessment of district energy systems: a methodological approach for multi-vector and multi-level environmental evaluation

District energy systems are commonly assessed using annual energy balances, renewable-energy shares, self-consumption ratios, and reductions in imported energy, although these indicators may overlook temporal operation, electricity–heat coupling, and life-cycle burdens. This study introduces a time-resolved Life Cycle Assessment (LCA) approach that integrates operational energy balances with Life Cycle Inventory modelling in a multi-vector, multi-level structure. It is demonstrated for a district in Turku, Finland, within the RESPONSE project, using two scenarios. The Real scenario represents monitored 2024 open-system operation, including imported electricity and district heating, on-site photovoltaic production, and building-level heat pumps. The Hypothetical scenario retains the same district boundary, consumption profiles, and temporal resolution, but modifies the supply side to test a complex multi-source and cross-vector configuration. Over 50 years, it reduced imported electricity and district heating by 59.8% and 86.1%, respectively, but increased lifetime primary energy consumption by 19.6% and greenhouse-gas emissions by 176%. These increases were driven mainly by the added local supply structure and electricity demand for heat-pump-based district-heating production. Building-cluster decomposition showed that contributions depend on electricity demand, district-heating demand, local systems, and heated area. Lower imports and greater local production therefore do not necessarily reduce life-cycle impacts, supporting time-resolved, multi-vector, and multi-level assessment.