Life Cycle Inventory Analysis Basics

The Life Cycle Inventory (LCI) phase is where the theoretical framework of your LCA becomes grounded in real data. This is often the most time-intensive phase, but it's also where the assessment gains its empirical foundation.

What Is a Life Cycle Inventory?

A Life Cycle Inventory is a comprehensive accounting of all the inputs and outputs associated with your product system. Think of it as a detailed ledger that tracks everything flowing into and out of each process within your system boundary.

Inputs include:

• Raw materials and natural resources
• Energy (electricity, fuels, heat)
• Water
• Intermediate products from other processes

Outputs include:

• Products and co-products
• Emissions to air, water, and soil
• Solid waste
• Wastewater

The LCI Process Step by Step

Step 1: Create a Process Flow Diagram

Before collecting any data, map out all the processes within your system boundary. This visual representation helps you understand:

• Which processes are connected
• Where materials and energy flow
• What data you need to collect

A typical process flow diagram includes unit processes connected by material and energy flows, with system boundaries clearly marked.

Step 2: Define Data Requirements

For each unit process in your diagram, identify what data you need:

Step 3: Collect Data

Data collection is typically the most challenging and time-consuming part of LCI. You'll work with two main types of data:

Foreground data comes from the specific system you're studying. This includes:

• Direct measurements from production facilities
• Bills of materials from product design
• Energy bills and utility records
• Waste manifests and disposal records

Background data represents upstream and downstream processes not under your direct control:

• Raw material extraction
• Electricity grid composition
• Transportation networks
• Waste treatment processes

Step 4: Calculate Flows

Once you have raw data, you need to calculate all flows relative to your functional unit. This involves:

1. Scaling: Adjusting process data to match your functional unit
2. Unit conversion: Ensuring consistent units across all flows
3. Mass and energy balancing: Verifying that inputs and outputs balance

Example calculation:

If your functional unit is "1 kg of packaged product" and your manufacturing process produces 1,000 kg per batch:

Electricity per functional unit = (Batch electricity use) / (Batch output)
                                = 500 kWh / 1,000 kg
                                = 0.5 kWh/kg product

Step 5: Handle Allocation

Many processes produce multiple products. When this happens, you need to decide how to allocate environmental burdens among them. Common approaches include:

Physical allocation: Divide burdens based on physical properties like mass or energy content.

Economic allocation: Divide burdens based on the economic value of outputs.

System expansion: Avoid allocation by expanding the system to include the avoided production of co-products.

Data Quality Considerations

Not all data is created equal. Document the quality of your data using these criteria:

• Temporal representativeness: How recent is the data?
• Geographical representativeness: Does the data match your study's location?
• Technological representativeness: Does the data reflect the actual technology used?
• Completeness: Are all relevant flows included?
• Reliability: What is the source and how was it collected?

Many LCA practitioners use a pedigree matrix to systematically score data quality across these dimensions. For detailed guidance on conducting a formal Data Quality Assessment (DQA), see the Uncertainty Analysis module in Track 4.

Common LCI Challenges and Solutions

Challenge: Missing Data

Solution: Use proxy data from similar processes, literature values, or stoichiometric calculations. Document all assumptions and test their influence in sensitivity analysis.

Challenge: Confidential Business Information

Solution: Aggregate data across facilities, use ranges instead of exact values, or work with trade associations that can anonymize data.

Challenge: Cut-off Decisions

Solution: Establish clear cut-off criteria and document what has been excluded and why. Common thresholds are 1% by mass AND 1% by energy AND 1% of environmental relevance (all three criteria typically must be met for exclusion). EN 15804 for construction products specifies these combined thresholds, while other standards may vary. Total excluded flows should not exceed 5% of any indicator.

Building Your Inventory Table

The final output of LCI is an inventory table listing all elementary flows crossing the system boundary. A simplified example:

LCI Databases

Most LCA practitioners don't collect all data from scratch. They supplement primary data with established LCI databases:

• ecoinvent: The most comprehensive global database with thousands of processes
• GaBi/Sphera databases: Industry-specific datasets integrated with GaBi software
• US Life Cycle Inventory (USLCI): Free database with US-specific data
• Agri-footprint: Specialized database for food and agriculture
• ELCD: European reference data from the Joint Research Centre

Key Takeaways

1. LCI involves systematic collection and calculation of all inputs and outputs within your system boundary
2. Distinguish between foreground data (specific to your system) and background data (generic processes)
3. Scale all data to your functional unit and ensure mass/energy balance
4. Handle multi-output processes through allocation or system expansion
5. Document data quality and sources thoroughly
6. Use established databases to supplement primary data collection

Practice Exercise

Choose a simple product you use daily (a coffee cup, a notebook, a t-shirt). Create a basic process flow diagram showing the main life cycle stages. For each stage, list three to five inputs and outputs you would need to quantify for an LCI.

What's Next?

Now that you understand how to build a Life Cycle Inventory, the next lesson covers Impact Assessment—where we translate these inventory flows into potential environmental impacts that can inform decision-making.

---

Further Reading

• ISO 14044:2006 Section 4.3 (Life Cycle Inventory Analysis)
• Curran, M.A. (Ed.). (2012). Life Cycle Assessment Handbook. Wiley-Scrivener.
• Heijungs, R., & Suh, S. (2002). The Computational Structure of Life Cycle Assessment. Springer.