Exergetic Life Cycle Assessment: A Review

The Role and Importance of Exergy in Technical Analysis

Exergy analysis provides a rational and meaningful framework for assessing and comparing complex processes and systems. While traditional energy analysis relies on the first law of thermodynamics—the conservation of energy—exergy analysis integrates the second law to account for energy quality. As a thermodynamic quantity, exergy describes the maximum possible useful work obtainable as a system is brought into a state of thermodynamic equilibrium with the common elements of its natural surroundings through reversible processes. This capacity for doing work serves as a definitive measure of energy quality, distinguishing between available energy and unavailable energy, the latter of which is often quantified as entropy.

Actual Performance Versus Ideal Potential

A primary utility of exergy analysis is its ability to provide a real evaluation of how actual performance tends toward or deviates from the ideal. This is achieved through exergy efficiency, a parameter that characterizes the performance of systems—such as power plants or buildings—by measuring the work potential of the energy involved. Unlike energy analysis, which may obscure the true sources of inefficiency, exergy identifies the types, causes, and locations of thermodynamic losses, also known as irreversibilities, more clearly. These losses represent exergy destruction within a system, which either reduces the useful results of a process or necessitates increased energy consumption to achieve the desired output.

The Critical Role of the Reference Environment

The calculation of exergy is intrinsically dependent on the specification of an idealized state, known as the reference environment. Because nature is in a state of thermodynamic disequilibrium, only common components such as the atmosphere, hydrosphere, or lithosphere are typically used as these idealized systems. The choice of this reference environment is critical for calculation accuracy, as the sensitivity of the results varies based on the operative conditions of the system under analysis.

By establishing a scientifically based indicator of resource utilization efficiency, exergy analysis allows researchers to quantify energy and material resources on a single scale. This enables a more accurate inventory of the manufacture, use, and disposal of goods, providing a comprehensive measure of sustainability that overcomes the limitations of subjective weighting used in other assessment tools.

Taxonomy of Exergy-Based Methods

Exergy-based methods encompass a variety of specialized techniques including conventional and advanced exergetic, exergoeconomic, and exergoenvironmental analyses. These methodologies are designed to describe the quantity and sources of inefficiencies, environmental impacts, and costs within a system. By utilizing exergy as a common denominator, these methods allow researchers to study the interconnections between thermodynamic performance and ecological sustainability. In the context of Life Cycle Assessment (LCA), several distinct methodologies have emerged to quantify resource consumption and environmental depletion.

Cumulative Exergy Demand (CExD)

Proposed by Szargut, Cumulative Exergy Demand (CExD) is defined as the sum of the exergy of all supplies required to provide a service or produce a product. While it is related to the traditional Cumulative Energy Demand (CED), CExD offers a significant advantage by accounting for the quality of energy inputs and including materials traditionally viewed as non-energetic. This method measures energy quality, exergy losses of materials, and emissions, providing a more in-depth indicator of resource use throughout a product's life cycle. However, CExD is limited to exergy losses of natural resources and excludes the broader ecological system losses.

Thermo-Ecological Cost (TEC)

The Thermo-Ecological Cost (TEC) method focuses strictly on the cumulative consumption of non-renewable primary exergy resources. Unlike methods that may incorporate monetary values, TEC is expressed solely in exergy units. The fundamental premise of this methodology is that sustainability assessments must prioritize the reduction and determination of the depletion of non-renewable natural materials. A noted limitation of TEC is its specific focus on non-renewables, which excludes renewable primary exergy resources from the final balance.

Cumulative Exergy Extraction from Natural Environment (CExENE)

The Cumulative Exergy Extraction from Natural Environment (CExENE) method acts as a boundary extension of CExD. While CExD accounts for energetic supplies and non-energetic materials like water, minerals, and metals, it ignores land use. CExENE incorporates land occupation into its framework, making it a more comprehensive resource indicator. Conceptually, CExENE evaluates the total exergy deprived from the natural system, regardless of whether that exergy is successfully transferred into the technological system. This distinguishes it from CExD, which only measures the exergy actually transferred into the technological process.

Extended Exergy Accounting (EExA)

Extended Exergy Accounting (EExA), proposed by Sciubba, moves beyond physical resource inputs to compute a commodity’s value based on its resource equivalent value rather than fiscal cost. This methodology relies on two primary assumptions:

• The cumulative exergy content is the sum of the product's constituents and a weighted sum of the exergies involved in the production process.
• Non-energetic costs, specifically labor, capital, and environmental emissions, can be reformulated in thermodynamic terms using global system balances.

The following table provides a comparative overview of these methodologies based on their scope and inherent limitations as identified in sustainability research:

Integration with Life Cycle Assessment (LCA) Frameworks

Exergy analysis serves as a vital thermodynamic indicator within the standard ISO 14040 Life Cycle Assessment (LCA) methodology by offering a rigorous physical basis for resource accounting. While traditional LCA often focuses on energy and material mass, exergy-based approaches provide a more appropriate measure for quantifying the depletion of natural resources and assessing resource utilization efficiency. By treating exergy as a measure of energy quality, researchers can evaluate how the actual performance of a system deviates from the ideal, providing a real evaluation of sustainability that transcends simple mass or energy balances.

Quantifying Resource Use via Cumulative Exergy Demand

The Cumulative Exergy Demand (CExD) method is the most widely applied exergy-based approach for evaluating environmental impacts. It functions by summing the exergy of all supplies required to provide a service or manufacture a product. Unlike Cumulative Energy Demand (CED), which is restricted to energy carriers, CExD accounts for the quality of energy inputs and includes non-energetic materials such as mineral ores, water, and metals. The calculation of CExD within an LCA framework is typically achieved through one of three specific techniques:

• Process Analysis: This involves the direct tracing and evaluation of exergy flows for each individual process step in a product’s manufacturing chain.
• Balance Equations: A system of equations is utilized to express the total CExD of a product as a summation of the exergy of intermediate products and the natural resources extracted directly from the environment.
• Extension from Cumulative Energy Consumption (CEC): CExD can be calculated by extending existing CEC data, which is often readily available in commercial LCA tools, through the application of gross calorific exergy-to-energy ratios.

Overcoming Subjectivity in Impact Assessment

A primary benefit of integrating exergy into the Life Cycle Impact Assessment (LCIA) phase is the ability to quantify various results of manufacture, use, and disposal on a single scale of exergy loss. Standard LCA methodologies often face characterization and valuation problems, particularly when attempting to weight different impact categories like global warming potential or acidification. These conventional factors depend on subjective or localized variables such as environmental fate, species exposure, and experimental toxicity effects.

Exergy-based methods bypass the need for these subjective weights by utilizing physical thermodynamic values. Because exergy measures the potential of dispersion of matter and energy in the environment, it provides a scientifically based, objective function. This allows practitioners to compare different types of resource consumption and environmental impacts using a unified unit, facilitating more robust decision-making and optimization in the design of sustainable systems.

Quantitative Analysis of Literature and Trends

Bibliometric Profile and Field Maturity

The bibliometric analysis of core literature reveals that exergetic Life Cycle Assessment (LCA) is a field still in its emerging stages. A systematic review of 25 core articles published between 1990 and 2018 underscores this status, as the volume of publications remains relatively low when compared to the broader timeline of environmental assessment research. Despite this emerging status, the research is anchored in a solid scientific foundation rooted in the second law of thermodynamics, providing a more objective physical parameter for sustainability evaluations than conventional energy-based methods.

Disciplinary and Geographic Concentration

Research efforts in this domain are heavily concentrated within specific technical and scientific disciplines. The literature primarily originates from the following areas:

• Environmental engineering and environmental sciences
• Thermodynamics
• Energy fuels
• Mechanics

Geographically, the concentration of reviewed publications is located primarily within European countries. This regional focus suggests that European academic and research institutions have been at the forefront of integrating exergy-based indicators into standard environmental accounting frameworks.

Terminology and Scope

A notable trend identified in the bibliometric data is the preference for specific nomenclature. The term 'Exergetic LCA' is significantly more prevalent in peer-reviewed literature than 'Exergy LCA' when describing the quantification of natural resource depletion through exergy loss over a product's life cycle. This terminology reflects the methodology's specific aim: using exergy to characterize resource quality and potential work capacity rather than just raw energy quantity.

Key Findings from Literature Analysis

The quantitative review of the selected 25 articles highlights several mechanisms and assumptions that define the current state of the art:

While the field is currently focused on resource accounting, the analysis identifies a limitation in current trends: the primary focus remains on resource extraction, whereas the exergy of life cycle emissions—which measures the disequilibrium between emitted substances and the environment—represents a significant opportunity for future methodological expansion.

Methodological Limitations and Characterization Challenges

A fundamental limitation in standard Life Cycle Assessment (LCA) is the "resource use characterization problem." In conventional frameworks, characterization factors for materials and processes are assigned based on environmental impact categories such as global warming potential. However, these factors are heavily dependent on localized fate, exposure, and effect models. Fate involves the duration and amount of substances in the environment, while exposure and effect models determine which species are impacted and the resulting toxicity. These variables are highly complex and geographically specific, making it difficult to establish a universal, objective baseline for impact assessment.

Gaps in Exergy-Based Accounting

While exergetic life cycle assessment (ELCA) offers a thermodynamic alternative to bypass these complex environmental models, current methodologies possess their own systemic gaps. Several exergy-based methods have been developed to address these issues, yet each has specific boundary limitations:

• Cumulative Exergy Demand (CExD): While this is the most widely applied method for measuring environmental impacts, it is primarily limited to exergy losses of natural resources and excludes the exergy losses occurring within ecological systems.
• Ecological Cumulative Exergy Demand (ECExD): This method addresses the gaps in CExD by reporting the exergy used up in ecological systems to produce natural wealth. However, it is currently limited to production processes and may suffer from aggregation errors due to a lack of detail regarding individual industrial processes.
• Cumulative Exergy Extraction from Natural Environment (CExENE): Although it extends the scope of CExD to include land use, it is not designed to track the exergy actually transferred into the technological system.

Supplemental Application and Reference State Sensitivity

Current ELCA practices are largely supplemental, utilized primarily as a tool for resource accounting—specifically quantifying the depletion of natural resources—rather than serving as a standalone, full impact assessment. Most studies use exergy as an additional indicator to complement traditional LCA categories like acidification or eutrophication potential, rather than integrating emissions into a unified exergetic framework.

Furthermore, the sensitivity of results to reference state definitions poses a significant challenge. The choice of the reference environment (the idealized state used for calculation) can cause results to vary significantly based on operative conditions. In high-temperature systems like power plants, the flows are less sensitive to the reference definition. However, in low-temperature applications such as HVAC systems for space heating and cooling, properties are close to base conditions. In these instances, minor changes in the defined reference state lead to great variations in the analytical results, complicating the standardization of ELCA in the built environment.

Future Directions: The Exe-LCA Proposal

The limitations of existing exergetic life cycle assessment methodologies, which primarily focus on resource accounting as a supplement to conventional Life Cycle Assessment (LCA), suggest a need for a more comprehensive framework. Authors propose an expanded approach termed "Exergy-based Life Cycle Assessment" (Exe-LCA). This unified framework aims to bridge the gap between resource use and emission measurements by utilizing exergy as a common denominator for all environmental interactions across a product's life cycle.

Quantifying Environmental Impact Potential

The fundamental mechanism of Exe-LCA relies on the thermodynamic principle that exergy measures the degree of disequilibrium between a substance and its environment. Consequently, the exergy of emissions serves as a physical measure of environmental impact potential. Unlike conventional LCAs that estimate impacts based on localized and complex factors such as fate, exposure, and effects, the exergetic approach treats chemical emissions as a loss of thermodynamic quality. This potential is quantified as a function of the emission mass, the standard chemical exergy of the substance, and its molar mass, relative to a reference environment typically defined at 298.15 K and 101.325 kPa.

Unification of Assessment Units

One of the primary benefits of the Exe-LCA proposal is its ability to solve the characterization and valuation problems inherent in standard LCA methodologies. In conventional assessments, environmental impacts are often reported in disparate units, such as carbon dioxide equivalence for global warming or sulfur dioxide equivalence for acidification. Exe-LCA allows for the following improvements in reporting:

• Cumulative Integration: The contributions of all identified chemical emissions can be summed as a cumulative value.
• Unit Consistency: Both life cycle resource use and life cycle emissions are expressed in the same unit of exergy, removing the need for subjective weighting.
• Absolute Quantification: The method provides absolute values rather than just relative indicators, which improves the robustness of comparative analysis.

Improving Decision-Making Robustness

By shifting from subjective characterization factors to standard thermodynamic properties, Exe-LCA provides a more objective and scientific base for sustainability evaluations. This approach allows for a single objective function in multi-objective optimizations, facilitating clearer decision-making in policy and engineering. The inclusion of life cycle emissions alongside resource use ensures that the evaluation captures the full extent of thermodynamic degradation caused by industrial and ecological processes, leading to more accurate benchmarking and process improvement.

Key findings

• Superiority of CExD over CED — Cumulative Exergy Demand (CExD) provides a more in-depth indicator than Cumulative Energy Demand (CED) by accounting for both energy carriers and the quality of non-energetic materials.
• Sustainability Objectivity — Using exergy as a basic physical parameter makes sustainability assessments more objective, as it relies on standard thermodynamic properties rather than subjective weights.
• Single Objective Function — Exergy indicators enable multi-objective LCA problems to be classified into a single objective function, facilitating design optimization.
• Unified Unit Measurement — Exergy allows for energy, material resources, and potentially emissions to be quantified simultaneously using the same unit, solving valuation problems in LCA methodology.

Method and assumptions

The methodology employed a systematic literature review and bibliometric analysis. The authors searched the Web of Science Core Collection (including SCI-Expanded, SSCI, A&HCI, and ESCI), Scopus, and Google Scholar using targeted keywords such as 'exergetic life cycle assessment' and 'exergy life cycle assessment' for the period from 1990 to December 2018. From an initial set of 43 articles, 25 relevant studies were selected for in-depth analysis. The analysis categorized these articles by aim, method, and relevant conclusions to determine the state-of-the-art. The study assumes standard thermodynamic reference environments (298.15 K, 101.325 kPa) for chemical exergy discussions and focuses on the integration of exergetic indicators within the ISO 14040 framework for Life Cycle Assessment.

Where it applies

• Green Building Design Optimization — Exergy-based models can be used to locate optimum insulation thicknesses and structural designs by minimizing cumulative exergy consumption.
• Industrial Process Improvement — Exergy analysis identifies specific locations and causes of thermodynamic losses in production chains, such as steel or cement manufacturing, to increase efficiency.
• Resource Recovery Assessment — ELCA provides a tool to evaluate the thermodynamic efficiency of recovering resources from waste streams, such as household waste or waste-to-energy plants.

Terms used

• Exergy — The maximum useful work obtainable as a system is brought to thermodynamic equilibrium with its environment.
• Cumulative Exergy Demand (CExD) — The sum of the exergy of all natural resources required to produce a product or provide a service.
• Irreversibility — Thermodynamic losses caused by real processes that deviate from the ideal, resulting in a reduction of work potential.
• Chemical Exergy — The work potential of a substance derived from its chemical composition relative to a reference state.
• Emergy — The total amount of available energy of one kind (usually solar) used directly and indirectly to make a product.
• Thermo-Ecological Cost — An exergy-based method focused specifically on the cumulative consumption of non-renewable primary resources.

Questions and answers

How does Exergetic LCA differ from conventional LCA?

Conventional LCA often uses subjective characterization factors and different units for different impact categories. Exergetic LCA uses thermodynamic exergy loss as a single, objective scale to quantify resource depletion and quality loss.

Can exergy be used to measure environmental emissions?

Yes. While currently emerging, the exergy of an emission measures its degree of disequilibrium with the environment, acting as a physical indicator of its environmental impact potential.

Why is the reference environment important in exergy analysis?

The reference environment (or 'dead state') provides the baseline for calculations. If the system state is close to the reference conditions, small variations in the reference definition can cause large variations in the resulting exergy flows.

What is the primary benefit of the proposed 'Exe-LCA' terminology?

It describes a comprehensive approach that unifies the exergy of life cycle resource use and the exergy of life cycle emissions into a single measurement framework, simplifying the interpretation of sustainability data.

How to cite

Nwodo, M.N., and Anumba, C.J. (2020). Exergetic Life Cycle Assessment: A Review. Energies, 13(11), 2684. doi:10.3390/en13112684.