Table of Contents
Introduction
Life cycle assessment, often shortened to LCA, is a way to look at the full environmental footprint of an energy system from beginning to end. Instead of focusing only on emissions during operation, LCA asks what happens from the moment materials are taken from the earth, through manufacturing and use, all the way to disposal or recycling. This perspective is essential for renewable energy, because many technologies have very low emissions during operation, yet still cause impacts when they are built, transported, and eventually dismantled.
The Life Cycle Perspective In Energy
An energy system such as a solar panel, a wind turbine, or a gas power plant does not appear out of nowhere and then suddenly vanish at the end of its use. It passes through a sequence of life stages. A life cycle perspective in energy looks at each of these stages systematically.
For most energy technologies, the life cycle includes raw material extraction, such as mining metals or drilling for fuels, processing and refining of these materials, manufacturing of components, construction and installation of the plant or device, operation and maintenance, and finally end of life which covers dismantling, recycling, or disposal. Each of these stages uses energy and materials, and can create emissions, waste, and other environmental pressures.
For fossil fuel systems, a large share of the environmental impact occurs during fuel extraction and combustion in the operational phase, because burning fuels releases significant greenhouse gases and air pollutants. For many renewable energy systems, the picture is different. Operational emissions can be very low, so the impacts linked to materials, manufacturing, and construction become relatively more important. Life cycle assessment helps reveal these differences clearly.
Basic Steps Of Life Cycle Assessment
Although detailed LCA studies can be complex, the overall process follows a structured set of steps. These steps are standardized in international guidelines and are applied to many products and services, including energy technologies.
The first step is goal and scope definition. In this step, the practitioner defines what questions the LCA is meant to answer, and for whom. They also define what is included and excluded from the study. For example, an LCA might aim to compare the climate impact of wind power with that of natural gas power, over 1 kilowatt-hour of delivered electricity. The scope must state which life cycle stages are covered, such as from cradle to grave, which means from raw material extraction to end of life, or from cradle to gate, which ends at the factory gate and does not include use or disposal.
Within this step, it is also necessary to define the functional unit. In energy LCAs, a common functional unit is 1 kilowatt-hour of electricity delivered to the grid, or 1 megajoule of useful heat. Choosing an appropriate functional unit is crucial for fair comparisons between different technologies or system designs.
The second step is life cycle inventory analysis, usually called LCI. Here, the practitioner collects data on all relevant inputs and outputs for each life cycle stage. Inputs include energy use, fuel consumption, raw materials, water, and land. Outputs include products, co-products, emissions to air such as carbon dioxide, methane, and nitrogen oxides, releases to water and soil, and solid waste. This step results in a large table or database of flows associated with producing the functional unit.
The third step is life cycle impact assessment, called LCIA. In this step, the inventory data are translated into indicators of environmental impact. For example, all greenhouse gas emissions are converted into carbon dioxide equivalents to calculate a global warming potential. Other impact categories may include acidification, eutrophication, human toxicity, or resource depletion. Each category uses scientific models to relate emissions and resource use to potential environmental effects.
The final step is interpretation. The practitioner examines the results, checks for consistency with the goal and scope, tests how sensitive the results are to key assumptions, and identifies which life cycle stages or processes contribute most to each impact category. Interpretation leads to conclusions and, when relevant, recommendations for design improvements or policy decisions.
Functional Unit And System Boundaries In Energy LCAs
Two basic choices, the functional unit and the system boundaries, shape the outcome of an energy LCA.
The functional unit is the quantified description of the service provided by the system. In energy systems, this is often an amount of energy, such as 1 kWh of electricity or 1 MWh of heat. Sometimes, the functional unit may be more specific, for example 1 kWh of electricity delivered at medium voltage and 99.9 percent reliability, or the energy service of transporting 1 passenger 1 kilometer.
System boundaries describe which processes are included in the assessment. For energy technologies, typical boundary types are cradle to grave, cradle to gate, and gate to gate. Cradle to grave includes all life stages from raw material extraction to final disposal. Cradle to gate ends at the factory gate and excludes the use phase and end of life. Gate to gate covers only one segment, such as the manufacturing process of solar cells.
A clear choice of system boundary is especially important when comparing technologies. For example, an LCA of a gas power plant that excludes methane leaks during fuel extraction and transport will underestimate its climate impact. Similarly, an LCA of a solar farm that excludes end of life waste treatment will miss potential impacts from landfilling or recycling.
For grid electricity, another complexity is whether to look at a single technology or at the whole mix of technologies on the grid. LCAs can be conducted for individual plants, or for average or marginal electricity mixes. These choices must match the practical question being asked.
Key Metrics In Life Cycle Assessment Of Energy
Several key metrics are used repeatedly when assessing energy systems by LCA. For beginners, the most important ones relate to greenhouse gas emissions and energy performance.
The global warming potential, GWP, of a system over a time horizon, commonly 100 years, is usually expressed in kilograms of carbon dioxide equivalent per functional unit, such as kg CO$_2$e per kWh. This metric sums up the effect of different greenhouse gases, such as CO$_2$, CH$_4$, and N$_2$O, using weighting factors based on their warming impact.
Another important metric is cumulative energy demand, often abbreviated CED. This measures the total amount of primary energy required over the life cycle to deliver the functional unit. It can be expressed in megajoules of primary energy per kWh of electricity. CED can be separated into renewable and nonrenewable energy inputs, which helps clarify how much fossil energy is still required to build and operate renewable technologies.
A related concept is the energy return on investment, or EROI, sometimes also called energy return on energy invested, EROEI. In simple terms, this is the ratio of the useful energy produced over the life cycle to the energy that had to be invested, for example in manufacturing and construction.
A simplified expression of energy return on investment is:
$$\text{EROI} = \frac{\text{Total useful energy output over life}}{\text{Total energy input over life}}$$
For electricity technologies, this can be approximated as:
$$\text{EROI} \approx \frac{E_{\text{lifetime output}}}{E_{\text{manufacturing}} + E_{\text{construction}} + E_{\text{O\&M}} + E_{\text{decommissioning}}}$$
Higher EROI values indicate that a technology delivers much more energy than it consumes over its life cycle.
LCA also uses impact indicators beyond climate change and energy use. Examples include land use per kWh, water consumption per kWh, and indicators for local pollutants affecting human health or ecosystems. These help reveal trade offs when one impact category improves but another worsens.
Applying LCA To Different Energy Technologies
When LCA is applied to different energy technologies, typical patterns emerge.
For fossil fuel power plants, such as coal, oil, and natural gas, the majority of life cycle greenhouse gas emissions arise from fuel combustion during operation. There are also significant emissions from fuel extraction, processing, and transport. Construction of the power plant usually contributes a smaller share. As a result, any changes in fuel efficiency, fuel type, or carbon capture have a strong effect on life cycle impacts.
For solar photovoltaic systems, emissions during operation are very low because no fuel is burned. Most life cycle impacts come from producing the solar cells and other components, such as glass, aluminum frames, and inverters. The carbon footprint per kWh is strongly influenced by the energy mix used in manufacturing, the efficiency of the panels, and their lifetime and performance in the field. A panel manufactured with renewable electricity and used in a sunny region over a long lifetime will show a much lower impact per kWh than the same panel produced using carbon intensive electricity and installed in a cloudy location.
Wind turbines follow a similar pattern. Most life cycle impacts arise from the production of steel, concrete, fiberglass, and other materials for the tower, nacelle, and blades, as well as from transport and installation. Over a typical lifetime, the energy produced is large compared with these inputs, so the carbon intensity per kWh is usually low. However, the assumed lifetime, capacity factor, and end of life treatment of components can significantly influence the LCA results.
Hydropower systems often show very low life cycle emissions from construction and operation, but can have additional greenhouse gas emissions from reservoirs, especially in tropical regions, where decomposing organic matter emits methane and carbon dioxide. These biogenic emissions, as well as land use changes and effects on ecosystems, need specific attention in the LCA.
Bioenergy systems are particularly complex in LCA, because their impacts depend heavily on how biomass is produced, what land uses change, how co products are handled, and how carbon flows between atmosphere, plants, soils, and products are modeled. Allocation methods for co products and assumptions about land use change can significantly alter the final results.
For nuclear, geothermal, marine, and other emerging technologies, LCA focuses on both material intensive infrastructure and specific operational emissions, such as fugitive gases or chemicals. Since these technologies are still evolving, LCAs are often updated as new designs and practices appear.
Methodological Choices And Uncertainties
LCA of energy systems involves many choices and assumptions. These do not make LCA useless, but they require transparency and careful interpretation.
One key issue is allocation. Many energy processes produce more than one product. For example, a combined heat and power plant generates both electricity and heat. A refinery produces fuels and other products. An LCA must decide how to share the environmental burdens between these outputs. Allocation can be based on energy content, economic value, physical properties, or other criteria. The choice can strongly affect the results.
Another issue is the background data used in the life cycle inventory. LCAs often rely on large databases that represent average conditions in particular regions or time periods. If a technology uses a unique supply chain or operates under very different conditions, these averages may not be fully accurate.
Temporal and geographic differences also matter. An LCA that assumes manufacturing is powered by a high carbon electricity mix will show a higher footprint than one that assumes a cleaner mix. As electricity grids decarbonize, the embodied emissions in future renewable technologies will likely fall, and LCA results need to reflect these changes to remain relevant.
Uncertainties arise from measurement limits, incomplete information, or simplified models in the impact assessment phase. Sensitivity analysis and scenario testing are common tools to explore how results change when key assumptions shift. Presenting ranges instead of single exact numbers can better reflect the true level of certainty.
Using LCA Results For Decision Making
Decision makers use LCA results to design better technologies, improve policies, and guide investment. For technology designers, LCA highlights which materials, processes, or life stages contribute most to impacts. This can lead to redesign, such as using less energy intensive materials, improving efficiency, or making components easier to recycle.
For policymakers, LCA helps avoid shifting environmental burdens from one stage or impact category to another. For example, a policy that favors an energy technology with low emissions during use, but ignores large upstream impacts on land or water, may not be sustainable overall. By relying on LCA, policies can be crafted to support options that truly reduce total impacts.
Actors in markets, including utilities and companies, can use LCA based labels or declarations to communicate the environmental performance of energy products and services. For instance, environmental product declarations for PV modules or batteries often rely on LCA results. In some regulatory schemes, such as public procurement requirements or low carbon fuel standards, life cycle metrics play a formal role in eligibility and ranking.
When interpreting and applying LCA results, it is important to compare like with like and to remain aware of uncertainties and assumptions. LCAs are powerful tools for understanding relative differences and identifying hotspots, but they should be combined with other types of analysis, such as economic, social, and technical assessments, to support well rounded decisions.
Conclusion
Life cycle assessment of energy systems provides a structured way to capture environmental impacts across all stages of an energy technology's life. By defining a clear goal and scope, selecting appropriate functional units and system boundaries, gathering inventory data, and assessing and interpreting impacts, LCA reveals where emissions and resource use really occur. In the context of renewable energy and sustainability, this helps ensure that technologies promoted as solutions to climate change and environmental degradation perform well not only during operation, but across their entire life cycle.