Definition Of Functional Unit Research Articles

Opening the black box of the use phase in circular economy life cycle assessments: Environmental performance of shell jacket reuse

AbstractLife cycle assessment (LCA) is highly needed and widely used to assess the environmental performance of circular economy (CE) measures such as reusing and sharing. However, the results of such LCAs are hampered by limited knowledge about the use phase of consumer products and oversimplification of important use phase aspects such as product functionality, user behavior, displacement, and rebound effects. This paper aims to validate the usefulness of a framework designed to assist practitioners in the generation and utilization of such knowledge in LCAs of circular measures. To validate the framework, a case study is used: reuse of shell jackets enabled by “premium secondhand” stores for outdoor equipment and clothing. The paper demonstrates that conclusions about the environmental performance of reuse can easily be altered depending on the functional unit definition, whether real user behavior data are used, and whether imperfect displacement and rebound effects are considered. For instance, shell jacket life cycles that include reuse and thus may be labeled “circular” have significantly higher environmental impact per use occasion than “linear” ones (used by one principal user the entire lifespan), since “circular” shell jackets are used less frequently, in particular during their first use span. Through facilitating the generation and utilization of environmentally relevant use phase data, which are otherwise often overlooked, the framework seems capable of supporting a better understanding of the environmental performance of CE measures.

Functional Unit definition in a circular economy perspective: implication for LCA normalisation for a footwear outsole

It is a significant challenge to address the complex issue of comparing multiple products with the same function but distinct characteristics in the circular economy framework and its implications for Life Cycle Assessment (LCA) in the footwear sector. Our strategy involved developing a specific methodology for determining the durability characteristic of physical products, which was accomplished by combining the results of various physical tests. In particular, a set of urban soles was examined for the case study. The environmental footprints of the footwear were then normalised using this durability metric, aligning them with the number of units required to reach practical quality levels. Using this method, we achieved substantial changes in the results of the LCA analysis, particularly in the context of products whose design prioritised particular qualities like durability. As a result, this research emphasises the necessity of redefining the functional unit, particularly from the circular economy perspective, and illustrates how it may positively impact company eco-design policies.

Life cycle assessment of stone consolidants for conservation of cultural heritage

The field of the conservation and repair of built cultural heritage shows a growing attention towards the environmental sustainability of materials and treatments used for conservation projects. As a standardized methodology that is widely adopted for the comparative evaluation of construction and building materials, life cycle assessment (LCA) can be usefully applied to conservation materials, if specific field-related issues on the definition of functional units and reference flows are properly addressed.In the present study, LCA is used to quantify the environmental footprint of four alternative consolidants for restoration of weathered carbonate substrates. An innovative treatment based on an aqueous solution of diammonium hydrogen phosphate (DAP) is compared against three commercial alternatives (lime nanoparticles, ethyl silicate, and acrylic resin). The issue of the definition of the reference flow (i.e., the amount of each consolidant required for the restoration of a given surface) is addressed through experimental tests on three model substrates (marble, limestone, and lime mortar), following two alternative criteria: i) application until apparent refusal, as recommended by product manufacturers, and ii) application to obtain a certain degree of consolidation, in terms of increase in compressive strength.The novel DAP consolidant is shown to have lower impacts than the alternatives on several environmental impact categories, mainly thanks to the absence of organic solvents in its formulation. The evaporation of volatiles after application (volatile organic compounds for the commercial consolidants, ammonia for the DAP consolidant) is highlighted as an environmental hotspot for all the consolidants.

The influence of the Functional Unit on the comparability of life cycle assessments in the construction sector: A Systematic Literature Review and attempt at unification for Reinforced Concrete

This article explores the potential for unifying Functional Units (FU) for all LCAs in any given product category in the construction sector. To this end, a Systematic Literature Review (SR) has been conducted. Due to the significant environmental impact of Reinforced Concrete (RC) and its ubiquitous global utilization, it is taken as an exemplary model for a unified FU.. The results of this SR show a significant heterogeneity in the choice and definition of FUs in the construction sector. FUs were extracted from the source material and categorized by the authors into five primary categories. The literature review found Reference Flows (RF) to be the most commonly named type of FU (75%), although FU and RF are not defined in the same way according to the LCA norm. The authors suggest a feasible standardized FU for RC structures. FUs for RC must include Reference Service Life (RSL), strength requirements, durability, and basic material specifications to achieve reasonable functional equivalence. Given the context of LCA as a support to decision-makers (DM), a standardized property FU for RC is most appropriate. This might be defined as 1 m3 of concrete normalized by the 28-day compressive strength and the RSL, including exposure class, specific concrete, and type and percentage of reinforcement. Lastly, the authors argue that a standardized FU is not enough to produce reliable and comparable LCA results. Data limitations, methodological issues, and a questionable practicality of implementation must also be addressed. Short-, mid-, and long-term measures to this end are suggested.

Life cycle assessment on construction and demolition waste recycling: a systematic review analyzing three important quality aspects

Abstract Purpose Life cycle assessment (LCA) is increasingly being applied to construction and demolition waste (CDW) recycling. But what is the current state of LCA studies on CDW recycling? In the context of circular economy, several aspects become important in LCA, such as avoided impacts and consideration of the quality of recycled materials. The aim of this study is to identify inconsistencies and best practices, and then provide recommendations for future LCA studies focusing on CDW recycling. Methods We conducted a systematic literature review on 76 journal articles. First, a general mapping of the selected studies was performed including the temporal and geographical distribution, and a bibliometric analysis to capture the linkages between the studies. Within the LCA content-based analysis, an in-depth assessment of three important quality aspects: (1) quality of the study based on the applied LCA methodology, (2) inclusion of material quality in LCA, and (3) data quality considering sensitivity and uncertainty analyses, was carried out. Major LCA components such as functional unit (FU), software, database, system approach (attributional or consequential), allocation method, life cycle impact assessment, and interpretation were evaluated. A special emphasis was placed on avoided impacts and the inclusion of recycled material quality in the LCA. Results and discussion In this review, it was found that many essential elements of LCA were missing or not implemented correctly. For example, in the definition of FU, some studies did not mention any FU, others defined an invalid FU, and most of the studies defined a uniform FU, which was most likely confused with the reference flow. The main problem observed is the lack of transparent reporting on the different elements of LCA. Regarding avoided impacts, for instance, only 13 studies reported the avoided materials and their substitution coefficients. Also, 6 studies used the term “virgin material” for avoided impacts without further information, which is a very broad term and difficult to interpret. Furthermore, only 12 studies included the quality of recycled material in the LCA. Conclusion To obtain reliable LCA results, the practitioners should follow the principal LCA methodology and peer-reviewers should ensure the proper implementation. In CDW recycling, the differentiation between downcycling and recycling is essential; therefore, the quality of recycled materials should be included in the LCA. Considering inconsistent implementation of avoided impacts, a standardized and well-defined avoided impact framework is suggested to be developed to improve the quality and reliability of future LCA studies.

Life cycle assessment (LCA) for flow batteries: A review of methodological decisions

A transition from fossil to renewable energy requires the development of sustainable electric energy storage systems capable to accommodate an increasing amount of energy, at larger power and for a longer time. Flow batteries are seen as one promising technology to face this challenge. As different innovations in this field of technology are still under development, reproducible, comparable and verifiable life cycle assessment studies are crucial to providing clear evidence on the sustainability of different flow battery systems. Based on a review of 20 relevant life cycle assessment studies for different flow battery systems, published between 1999 and 2021, this contribution explored relevant methodological choices regarding the sequence of phases defined in the ISO 14,040 series: goal and scope definition, inventory analysis, impact assessment and interpretation. Inspired by good practice examples, common gaps and weaknesses were identified and recommendations for comparative life cycle assessment studies were derived. This includes suggestions for an expanded functional unit definition, a provision of more detailed and transparent reporting of LCI data while using input/output tables. Outcomes of this study are also of relevance for the amendment of the Batteries Directive 2006/66/EC, where first drafts are under revision in the European Council, including the introduction of a battery passport, which should encourage battery producers to reduce their carbon footprint and avoid problematic materials.

A practice‐based framework for defining functional units in comparative life cycle assessments of materials

AbstractIn comparative life cycle assessment (LCA) studies of materials, there is a mismatch between the current practice and existing guidelines regarding functional unit definition. The purpose of this study is to develop a practice‐based framework for defining functional units in comparative LCAs of materials and provide guidance regarding in which situations different functional unit types are relevant. A literature review of comparative LCAs of materials identified three types of functional units: (i) the reference flow functional unit, (ii) the property functional unit, and (iii) the performance functional unit. These functional unit types, of which only the latter strictly complies with LCA guidelines, represent varying degrees of functional equivalence and technological maturity. The most relevant functional unit type depends on the goal of the study. We suggest that screening assessments of whether materials have comparable environmental impacts can apply reference flow functional units. Material comparisons for certain application areas with some important properties can apply property functional units. For comparisons of end products, performance functional units can be applied. However, even in such cases, complete functional equivalence can hardly be achieved due to more or less relevant product differences. The applicability of the framework is demonstrated for the case of comparing cemented carbide and polycrystalline diamond hard materials.

Functional unit choice in space conditioning life cycle assessment: Review and recommendations

Space cooling through air conditioners and electric fans, which already accounts for 10% of global energy consumption, is expected to increase as temperatures rise under climate change and as air conditioning becomes more accessible. The life cycle greenhouse gas (GHG) emissions of this energy-intensive activity are concerning in their potential to create a positive feedback loop for climate change. There is thus a growing need for life cycle assessments (LCAs) on space conditioning systems to be designed to facilitate comparison for climate change mitigation efforts, with similar standardization as has developed in the electricity LCA field, in which the vast majority of LCAs conform to a specific system boundary and functional unit. We present the first study that assesses the uniformity and identifies discrepancies in methodology between space conditioning LCAs. We analyzed the goals, system boundaries, and chosen functional units of 33 peer-reviewed LCAs on single-structure space conditioning systems published between 2000 and 2020. The findings indicate that the stated goals of these studies were mostly aligned, but there was a lack of consensus on functional unit definition, with multiple different types of units used across studies. This lack of consistency in functional units complicates and may even prevent the comparison of life cycle environmental impacts of technologies and methods of space conditioning, which is necessary to make decisions towards increased sustainability in space conditioning. Consequently, we recommend that a standard functional unit for space conditioning systems be defined as 1 MJ of heat added to or removed from conditioned space to ensure future studies are comparable.

Industrial solar collector vs alternative solar collector: Environmental impact comparison by LCA perspective / Colector solar industrial versus colector solar alternativo: Comparação do impacto ambiental por perspectiva LCA

Adequacy to sustainable development standards requires the use of methods and tools that enable the quantification and monitoring of environmental impacts related to production processes. As a subsidy to the potential reduction of impacts by solar collectors, this paper proposes an environmental evaluation, considering an alternative solar collector scenario to be compared with a commercial one, from the life cycle perspective. Using the Life Cycle Assessment (LCA) tool, the scenarios were evaluated using the SimaPro 8.5 software. The functional unit definition was defined with real system verification through the system preparation and operation in the laboratory, which is characterized as heating 26 L of water utilizing a thermosyphon system at a temperature greater than or equal to 38 ºC. Analyzing the LCA results, it was observed that the alternative system use offers environmental impacts reduction in all impact categories selected when compared to the commercial system. In addition, a sensitivity analysis was proposed considering a variation in polyester resin mass used in the alternative system. The simulation of changes in the resin resulted in even more decreases in the environmental impacts. Regarding thermal efficiency, the industrial system excelled in terms of absorption capacity and thermal reserve. Thus, the present paper using the analyzes proposed within the defined scope, allowed the comparison between the systems in such a way that it was possible to know whether the use of the alternative solar collector results in environmental advantages without losing thermal efficiency.

A methodological framework for comparing fractionated and non-fractionated products in life cycle assessments: The case of milk concentrates

Fractionation is increasingly applied in the process industries and aids in diversifying the use of biomaterials. However, comparing the environmental impacts of fractionated and non-fractionated products is challenging, since multi-product processes complicate the definition of functional units, and require by-product handling by means of allocation or system expansion. This paper proposes a generic scope-definition framework for multi-product processes in attributional life cycle assessment (ALCA) and in consequential life cycle assessment (CLCA). Considering the potentials and limitations of ALCA and CLCA, this framework addresses the choice of an appropriate functional unit and by-product handling method. Three distinct categories are defined: product systems with identical products, with identical functionalities, and with exclusively identical input materials. The framework is applied to new, fractionated dairy products – micellar-casein concentrate, whey-protein concentrate, and lactose – and their non-fractionated counterpart skim-milk concentrate. The case demonstrates how appropriate functional units can be identified that capture identical specific functionalities. The results for both ALCA and CLCA show that fractionation is environmentally viable. For fractionated products with non-identical functionalities, the methodological choice is reduced to an ALCA yielding a favorable eco-efficiency perspective for fractionation. The framework presented offers a systematic procedure for how to deal with multi-product processes in life cycle assessment that is useful for a wide range of applications.

Life Cycle Assessment of Innovative Materials for Thermal Energy Storage in Buildings

The politically endorsed reduction of greenhouse gas emissions entails the transformation of thermal energy systems towards renewable energies, especially in the building sector. This comes along with a demand in energy storage, as there is a time offset between energy availability and demand. As sensible heat storages induce major losses and have limited energy density, current water-based solutions are only partially sufficient to meet these demands. Within the project “Speicher-LCA” the environmental performance of a variety of innovative materials available for energy storage in buildings is assessed. The project provides the first extensive comparison of environmental profiles of various thermal energy storage materials, including phase change, thermochemical and sorption materials. The specific performances in the storage cycle are taken into account. All results will be publically accessible through a spreadsheet tool including a comprehensive set of materials, components as well as their integration into different building types.This paper discusses the methodological framework of the study and presents the environmental assessment results for selected materials. It highlights the main challenges in the assessment of innovative storage materials on different system levels which require specific definition of functional units accordingly. The first assessment results on material level for selected phase change (PCM) and thermo-chemical materials (TCM) allow an environmental characterization regarding their potential application in thermal storages. In addition, ranges of required numbers of storage cycles for amortization have been calculated for the non-renewable primary energy demand. For PCMs amortization cycles range between ∼20 to 150 cycles for salt hydrates and up to ∼280 cycles for paraffins. Regarding TCM, energetic amortization of silica gel and zeolite 13x is reached after ∼60 and ∼260 cycles respectively. Since the realization of storage components and systems which can actually be used in real applications will further increase the cycle number required for amortization, these storage materials may thus not be suitable for applications with a low number of cycles over lifetime, such as seasonal storage.

Territorial Life Cycle Assessment (LCA): What exactly is it about? A proposal towards using a common terminology and a research agenda

Evaluating holistically environmental impacts of land planning policies implies to take into account several aspects, intimately related both to territorial features and to production-consumption patterns, which have a specific local character and a potential impact at different scales. To address these challenges, life cycle thinking and assessment methods are crucial. Indeed, beyond the traditional application of Life Cycle Assessment as a product-oriented methodology, a new LCA-based approach called “territorial LCA” has gradually emerged to assess geographically or administratively defined systems. This paper aims to analyze how this new LCA-based approach differs from conventional LCA, highlighting main differences and added values. Territorial LCAs can be divided into two main approaches, i.e., i) type A, which focuses on the assessment of a specific activity or supply chain anchored in a given territory, and ii) type B, which attempts to assess all production and consumption activities located in a territory, including all environmental pressures embodied in trade flows with other territories. These two approaches are described and compared according to the four LCA phases to highlight differences and similarities with conventional LCA. This comparison is based on a detailed case study analysis for each territorial LCA type and it shows that most of the differences are in the goal and scope definition, especially for the territorial LCA of type B where the functional unit definition is no more the starting point of the assessment. Concerning territorial LCA of type A, there are no main divergences with conventional LCA as territorial contextualization already exists in some LCA applications, even if not systematically applied. Improvements in the application may entail a comprehensive contextualization of the four LCA phases, developing the synergies with the use of Geographic Information System (GIS) tools. Other specific challenges affecting both type A and B are related to i) territorial unique intrinsic multifunctionality determined by all human activities located within its boundaries, ii) specific territorial characteristics (i.e., spatial variability and organization), and iii) multiscale issues and the consideration of interactions between territories.

Functional unit and product functionality—addressing increase in consumption and demand for functionality in sustainability assessment with LCA

The static functional unit definition in the current LCA framework has limitations in addressing the changing product functionality and associated environmental impact of constantly evolving product technologies. As a result, it overlooks the changes in consumer behaviour of increased consumption of products in provided services as well as in growing volumes. This article aims to present a new framework in defining a dynamic functional unit of product technologies that caters for changes in consumer behaviour and growing market. A new approach to defining the functional unit is proposed that caters for changes in consumer behaviour and the use of technology from a technical performance perspective. A dynamic approach to definition of the functional unit is proposed that is based on Kano’s model of product function and satisfaction. The new approach is demonstrated on a case study in which the analysis of historical data for two TV product technologies—CRT and LCD—is used to show how the total environmental impact is increasing due to the increased functionality which triggers an increase in the volume of the market. Despite the efforts of improving product life cycle design, the society is still faced with increasing environmental impact from the product type overall. This article presents the challenges of using a static, single functional unit definition in an industrial culture with constant evolution of products that influences usage behaviour and demonstrates the vicious circle of improving product efficiency that leads to further consumption and environmental impact. To address this problem, a new framework of dynamic functional unit definition is put forward for performing comparative LCA to manage the development of product life cycle design that helps keep the total environmental impact of the company’s product portfolio within absolute boundaries.

Circular economy: To be or not to be in a closed product loop? A Life Cycle Assessment of aluminium cans with inclusion of alloying elements

Packaging, representing the second largest source of aluminium scrap at global level, deserves a key role in the transition towards the circular economy. Life Cycle Assessment (LCA) of aluminium products has been typically based on one life cycle considering pure aluminium flows and neglecting the presence of alloying elements and impurities. However, this simplification undermines the potentials of using LCA to quantify the environmental performances of products in multiple loops, as required in the circular economy. This study aims to investigate the effects of including the actual alloy composition in the LCA of aluminium can production and recycling, in order to understand whether a can-to-can (i.e. closed product loop) recycling should be promoted or not. Mass balance of the main alloying elements (Mn, Si, Cu, Fe) was carried out at increasing levels of recycling rate, corresponding to a temporal interval of five years. Different aluminium packaging scrap sources were considered: mixed packaging aluminium scrap and used beverage can scrap. The outcomes of the mass balance were used to quantify the amount of Mn and primary Al that needs to be reintegrated in each scenario according to the recycling rate and this information was further used to perform an LCA of 30 loops of aluminium can production and recycling, based on the actual alloy composition. The LCA revealed that the closed product loop option (considering used beverage can scrap) has lower climate change impacts over the other recycling scenario using mixed Al packaging scrap. The main recommendation from an LCA methodological point of view is to include the idea of multiple co-functions in the functional unit definition. To further improve the environmental performances of the aluminium beverage can sector towards circular economy implementation the key actions are: to reduce the weight of the lid, to develop methods to separate the body and lid at the point of collection, and to investigate the potentials of a closed supply chain loop for aluminium cans in terms of combined environmental and economic value creation.

Hybrid Life Cycle Assessment of Low, Mid and High-Rise Multi-Family Dwellings

We undertake Life Cycle Assessment (LCA) of the cumulative energy demand (CED) and global warming potential (GWP) for a portfolio of 10 multi-family residences in the U.S. We argue that prior LCA studies of buildings use an inconsistent boundary for processes to be included in the supply chain: The operational phase includes all energy use in a building, but supply chains for the production of appliances, equipment and consumables associated with activities done in the building are neglected. We correct this by starting the analysis with an explicit definition of a functional unit, providing climate controlled space, and including processes associated with this functional unit. Using a hybrid LCA approach, the CED for low, mid and high-rise multi-family residences is found to increase from 30, 34, to 39 GJ/m2, respectively. This increase is due to the need for energy-intensive structural materials such as concrete and steel in taller buildings. With our approach, the share of materials and construction of total life cycle energy doubles to 26%, compared with a 13% share that would be obtained with inconsistent system boundaries used in prior studies. We thus argue that explicit definition of functional unit leads to an increase in the contribution of supply chains to building energy life cycles.

Development of Specific Rules for the Application of Life Cycle Assessment to Carbon Capture and Storage

Carbon Capture and Storage (CCS) is a very innovative and promising solution for greenhouse gases (GHG) reduction, i.e., capturing carbon dioxide (CO2) at its source and storing it indefinitely to avoid its release to the atmosphere. This paper investigates a set of key issues in the development of specific rules for the application of Life Cycle Assessment (LCA) to CCS. The following LCA-based information are addressed in this work: definition of service type, definition of functional unit, definition of system boundaries, choice of allocation rules, choice of selected Life Cycle Inventory (LCI) results or other selected parameters for description of environmental performance. From a communication perspective, the specific rules defined in this study have been developed coherently with the requirements of a type III environment label scheme, the International EPD® System, according to the ISO 14025 standard.

Methodology for systematic analysis and improvement of manufacturing unit process life-cycle inventory (UPLCI)—CO2PE! initiative (cooperative effort on process emissions in manufacturing). Part 1: Methodology description

This report proposes a life-cycle analysis (LCA)-oriented methodology for systematic inventory analysis of the use phase of manufacturing unit processes providing unit process datasets to be used in life-cycle inventory (LCI) databases and libraries. The methodology has been developed in the framework of the CO2PE! collaborative research programme (CO2PE! 2011a) and comprises two approaches with different levels of detail, respectively referred to as the screening approach and the in-depth approach. The screening approach relies on representative, publicly available data and engineering calculations for energy use, material loss, and identification of variables for improvement, while the in-depth approach is subdivided into four modules, including a time study, a power consumption study, a consumables study and an emissions study, in which all relevant process in- and outputs are measured and analysed in detail. The screening approach provides the first insight in the unit process and results in a set of approximate LCI data, which also serve to guide the more detailed and complete in-depth approach leading to more accurate LCI data as well as the identification of potential for energy and resource efficiency improvements of the manufacturing unit process. To ensure optimal reproducibility and applicability, documentation guidelines for data and metadata are included in both approaches. Guidance on definition of functional unit and reference flow as well as on determination of system boundaries specifies the generic goal and scope definition requirements according to ISO 14040 (2006) and ISO 14044 (2006). The proposed methodology aims at ensuring solid foundations for the provision of high-quality LCI data for the use phase of manufacturing unit processes. Envisaged usage encompasses the provision of high-quality data for LCA studies of products using these unit process datasets for the manufacturing processes, as well as the in-depth analysis of individual manufacturing unit processes. In addition, the accruing availability of data for a range of similar machines (same process, different suppliers and machine capacities) will allow the establishment of parametric emission and resource use estimation models for a more streamlined LCA of products including reliable manufacturing process data. Both approaches have already provided useful results in some initial case studies (Kellens et al. 2009; Duflou et al. (Int J Sustain Manufacturing 2:80–98, 2010); Santos et al. (J Clean Prod 19:356–364, 2011); UPLCI 2011; Kellens et al. 2011a) and the use will be illustrated by two case studies in Part 2 of this paper (Kellens et al. 2011b).

A survey of unresolved problems in life cycle assessment

Life cycle assessment (LCA) stands as the pre-eminent tool for estimating environmental effects caused by products and processes from ‘cradle to grave’ or ‘cradle to cradle.’ It exists in multiple forms, claims a growing list of practitioners, and remains a focus of continuing research. Despite its popularity and codification by organizations such as the International Organization for Standards and the Society of Environmental Toxicology and Chemistry, life cycle assessment is a tool in need of improvement. Multiple authors have written about its individual problems, but a unified treatment of the subject is lacking. The following literature survey gathers and explains issues, problems and problematic decisions currently limiting LCA’s goal and scope definition and life cycle inventory phases. The review identifies 15 major problem areas and organizes them by the LCA phases in which each appears. This part of the review focuses on the first 7 of these problems occurring during the goal and scope definition and life cycle inventory phases. It is meant as a concise summary for practitioners interested in methodological limitations which might degrade the accuracy of their assessments. For new researchers, it provides an overview of pertinent problem areas toward which they might wish to direct their research efforts. Multiple problems occur in each of LCA’s four phases and reduce the accuracy of this tool. Considering problem severity and the adequacy of current solutions, six of the 15 discussed problems are of paramount importance. In LCA’s first two phases, functional unit definition, boundary selection, and allocation are critical problems requiring particular attention. Problems encountered during goal and scope definition arise from decisions about inclusion and exclusion while those in inventory analysis involve flows and transformations. Foundational decisions about the basis of comparison (functional unit), bounds of the study, and physical relationships between included processes largely dictate the representativeness and, therefore, the value of an LCA. It is for this reason that problems in functional unit definition, boundary selection, and allocation are the most critical examined in the first part of this review.

The Organization of Metabolic Reaction Networks: A Signal-Oriented Approach to Cellular Models

Complex metabolic networks are characterized by a great number of elements and many regulatory loops. The description of these networks with mathematical models requires the definition of functional units that group together several cellular processes. The approach presented here is based on the idea that cellular functional units may be assigned directly to mathematical modeling objects. Because the proposed modeling objects have defined inputs and outputs, they can be connected with other modeling objects until eventually the whole metabolism is covered. This modular approach guarantees a high transparency for biologists as well as for engineers. Three criteria are introduced to demarcate functional units. The criteria consider the physiological pathways, the organization of the corresponding genes, and the observation that cellular systems can be structured into units showing a hierarchy of signal transduction and processing. As an example, the carbon catabolic reactions in Escherichia coli are discussed as members of a functional unit catabolism.

Assessing the ecological and economic sustainability of energy crops

The production and use of biomass for energy has both positive and negative impacts on the environment. The environmental impacts of energy crops should be clarified before political choices concerning energy crops are made. An important aid to policy-making would be a systematic methodology to assess the environmental sustainability of energy crops. So far, most studies on the environmental aspects of energy crops deal mainly with the energy production of the crops and the possible consequences for CO 2 mitigation. The Dutch Centre for Agriculture and Environment (CLM) has developed a systematic methodology to assess the ecological and socio-economic sustainability of biomass crops. The method is best described as a multicriteria analysis of process chains and is very much related to Life Cycle Assessment (LCA). Characteristics of our methodology are the use of: • definition of functional units; • analysis of the entire lifecycle; • definition of yield levels and corresponding agricultural practices; • analysis of both ecological and economic criteria; • definition of reference systems; • definition of procedures for normalisation and weighting. CLM has applied the method to assess the sustainability of ten potentially interesting energy crops in four European regions. The results are used to outline the perspectives for large scale production of biomass crops with regard to the medium and long term land availability in Europe. For the crops considered, net energy budget ranges from 85 GJ net avoided fossil energy per ha for rape seed for fuel to 248 GJ net avoided fossil energy per ha for silage maize for electricity from gasification. The methodology of the tool and its results were discussed at the concerted action “Environmental aspects of biomass production and routes for European energy supply” (AIR3-94-2455), organised by CLM in 1996. Major conclusions of the research: • Multicriteria analysis of process lifecycles is at present the best available option to assess the ecological and economical sustainability of energy crops. However, it is recognised that further research and development activities in the field of LCA are necessary. • Use of crops for generation of electricity is preferred to use of crops for transport fuels since the latter score low on both ecological and socio-economical criteria. • Large scale use and production of energy crops requires (a) choices and goals from agricultural, environmental, and energy policies, and (b) policy instruments for financial incentives e.g. with payments or with tax rebates, preferably per kilogram avoided CO 2 emission.