11.3 Life cycle assessment of propulsion systems

Life cycle assessment (LCA) is crucial for evaluating the environmental impact of propulsion systems. It examines every stage, from raw material extraction to disposal, helping identify areas for improvement and guiding sustainable design choices.

LCA for propulsion systems involves defining boundaries, collecting data, and assessing impacts. By analyzing hotspots and trade-offs, engineers can optimize designs, reduce emissions, and make informed decisions about materials and technologies for greener aerospace solutions.

Life Cycle Assessment Principles

Systematic Approach for Environmental Impact Evaluation

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• Life cycle assessment (LCA) is a systematic approach for evaluating the environmental impacts of a product or system throughout its entire life cycle, from raw material extraction to end-of-life disposal or recycling
• LCA methodologies for propulsion systems should consider the specific characteristics and complexities of these systems, such as the diverse range of technologies (gas turbines, electric motors), fuel types (jet fuel, hydrogen), and operational profiles (flight missions, duty cycles)

Main Stages of LCA

• The main stages of an LCA include goal and scope definition, life cycle inventory analysis (LCI), life cycle impact assessment (LCIA), and interpretation
  • Goal and scope definition involves specifying the purpose (comparative analysis, eco-design), system boundaries (cradle-to-grave, gate-to-gate), functional unit (thrust output, passenger-kilometer), and assumptions (technology readiness level, geographic scope) of the LCA study
  • LCI involves quantifying the inputs (energy consumption, material usage) and outputs (emissions, waste generation) associated with each life cycle stage
  • LCIA involves characterizing and assessing the potential environmental impacts (global warming potential, acidification potential) based on the LCI results
• Key principles of LCA include life cycle thinking, comprehensiveness (considering all relevant environmental aspects), transparency (clear documentation of methods and assumptions), and iterative nature of the assessment process (refinement based on new data or insights)

LCA for Propulsion Systems

Defining System Boundaries and Functional Unit

• Applying LCA to propulsion systems involves defining the system boundaries to include all relevant life cycle stages, such as raw material extraction (mining of metals, fossil fuel extraction), manufacturing (component production, assembly), operation (fuel consumption, maintenance), and end-of-life (recycling, disposal)
• The functional unit should be carefully selected to ensure fair comparison between different propulsion systems (per unit of thrust output, per passenger-kilometer transported)
  • For example, comparing the environmental impacts of a jet engine and an electric propulsion system based on the same thrust output or transport service

Data Collection and Impact Assessment

• Data collection and inventory analysis should cover the inputs and outputs associated with each life cycle stage, such as fuel consumption (jet fuel, electricity), emissions (CO2, NOx), material usage (metals, composites), and waste generation (manufacturing scrap, end-of-life components)
• The environmental impact categories considered in the LCIA should be relevant to propulsion systems, such as global warming potential (contribution to climate change), acidification potential (impact on ecosystems), and resource depletion (consumption of finite resources)
• Sensitivity analysis can be performed to assess the influence of key parameters (fuel efficiency, component lifespan) and assumptions (recycling rates, electricity mix) on the LCA results

Interpreting LCA Results

Identifying Environmental Hotspots

• Interpreting LCA results involves analyzing the contribution of different life cycle stages and processes to the overall environmental impacts of the propulsion system
• Environmental hotspots are the life cycle stages or processes that have the highest contribution to specific environmental impact categories
  • For example, the operation stage may be identified as a hotspot for global warming potential due to the fuel combustion emissions (CO2, H2O)
  • Manufacturing stage may be a hotspot for resource depletion due to the use of critical materials (rare earth elements in electric motors)

Improvement Opportunities and Trade-offs

• Improvement opportunities can be identified by targeting the environmental hotspots and exploring alternative technologies (advanced materials, alternative fuels), materials (recycled content, bio-based), or operational strategies (optimized flight routes, maintenance schedules)
• Trade-offs between different environmental impact categories should be considered when interpreting LCA results and proposing improvements
  • For instance, using lightweight materials (composites) may reduce fuel consumption and global warming potential but increase resource depletion and end-of-life challenges
• Benchmarking the LCA results against industry standards (ICAO goals, IATA targets) or best practices can provide insights into the relative environmental performance of the propulsion system

Limitations of LCA

Data Quality and Methodological Choices

• LCA of propulsion systems is subject to various limitations and uncertainties that should be acknowledged and critically assessed
• Data quality and availability can be a major limitation, particularly for emerging technologies (electric propulsion, hydrogen fuel cells) or complex supply chains (rare earth elements, biofuels)
  • Assumptions and proxy data may be required to fill data gaps, introducing uncertainties into the LCA results
• Methodological choices, such as the selection of impact assessment methods (ReCiPe, CML) or allocation procedures (mass-based, economic), can influence the LCA results and should be transparently reported

Uncertainty and Dynamic Nature of Propulsion Systems

• The dynamic nature of propulsion systems, with evolving technologies and operational conditions, poses challenges for LCA and may require regular updates and refinements
  • For example, advancements in battery technology or changes in the electricity mix can significantly impact the LCA results of electric propulsion systems over time
• Uncertainty analysis techniques, such as Monte Carlo simulation (probabilistic approach) or scenario analysis (exploring alternative future developments), can be applied to quantify and communicate the uncertainties associated with LCA results
• The limitations and uncertainties should be considered when interpreting and communicating LCA results to stakeholders (policymakers, industry partners) and decision-makers (technology selection, investment decisions)