Environ. Sci. Technol. 2007, 41, 8477–8483
Cumulative Exergy Extraction from the Natural Environment (CEENE): a comprehensive Life Cycle Impact Assessment method for resource accounting J . D E W U L F , * ,† M . E . B Ö S C H , ‡ B. DE MEESTER,† G. VAN DER VORST,† H. VAN LANGENHOVE,† S. HELLWEG,‡ AND M. A. J. HUIJBREGTS§ Research Group EnVOC, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium; Institute of Environmental Engineering, Ecological Systems Design, ETH Zurich, HIL G 35.2, CH-8093, Switzerland; Department of Environmental Science, Institute for Wetland and Water Research, Faculty of Science, Radboud University Nijmegen, P.O. Box 9010, NL-6500 GL Nijmegen, The Netherlands
Received May 15, 2007. Revised manuscript received October 01, 2007. Accepted October 02, 2007.
The objective of the paper is to establish a comprehensive resource-based life cycle impact assessment (LCIA) method which is scientifically sound and that enables to assess all kinds of resources that are deprived from the natural ecosystem, all quantified on one single scale, free of weighting factors. The method is based on the exergy concept. Consistent exergy data on fossils, nuclear and metal ores, minerals, air, water, land occupation, and renewable energy sources were elaborated, with well defined system boundaries. Based on these data, the method quantifies the exergy “taken away” from natural ecosystems, and is thus called the cumulative exergy extraction from the natural environment (CEENE). The acquired data set was coupled with a state-of-the art life cycle inventory database, ecoinvent. In this way, the method is able to quantitatively distinguish eight categories of resources withdrawn from the natural environment: renewable resources, fossil fuels, nuclear energy, metal ores, minerals, water resources, land resources, and atmospheric resources. Third, the CEENE method is illustrated for a number of products that are available in ecoinvent, and results are compared with common resource oriented LCIA methods. The application to the materials in the ecoinvent database showed that fossil resources and land use are of particular importance with regard to the total CEENE score, although the other resource categories may also be significant.
Introduction Second law analysis, i.e. exergy analysis, is a rapidly evolving approach with respect to process analysis and resource accounting (1–20). Exergy analysis provides a powerful tool for assessing the quality and quantity of a resource, as it * Corresponding author phone: ++32 9 264 59 49; Fax: ++32 9 264 62 43; e-mail:
[email protected]. † Ghent University. ‡ ETH Zurich. § Radboud University Nijmegen. 10.1021/es0711415 CCC: $37.00
Published on Web 11/08/2007
2007 American Chemical Society
represents the upper limit of the portion of the resource that can be converted into work, given the prevailing environmental conditions. In many cases, exergy analysis methods take on a life-cycle perspective, quantifying the cumulated exergy consumption of a product or process from “cradle to grave”. In this regard, it is similar to life cycle assessment (LCA). In fact, exergy analysis can be part of an LCA, representing a method for the life-cycle impact assessment (LCIA) of resource consumption. As an LCIA method for resource use, exergy analysis can benefit from the large effort that has been spent in the development of life-cycle inventory databases, such as ecoinvent (21), which contain resource use data for several thousands of technical processes. With regard to all these processes, a life cycle scope was adopted. For instance, for the generation of hydropower, the construction (and eventually disposal) of the dam and the turbines were considered as well as the direct inputs of land and energy/exergy. This idea, the coupling of exergy analysis and life-cycle inventory databases, has been put into practice by the authors: De Meester et al. (22) provided exergy data for minerals, based on up-to-date thermochemical data. Bösch et al. (5) put forward a comprehensive method for exergy-based analysis within LCA (cumulative exergy demand, CExD), accounting for all resource consumptions but land use. However, both approaches suffered from several weaknesses. For instance, some of the exergy factors of Bösch et al. (5) were not based on up-to-date thermochemical data, and neither of the two methods assessed exergy extraction from nature due to land use. In addition, the assessment of renewable resources was not entirely consistent. For instance, exergy intake of photovoltaic (PV) cells for electricity production was accounted for as the solar exergy insolating on the PV cell. By contrast, for biomass the exergy content of the biomass material was considered, although biomass growth also requires (a much larger amount of) solar exergy. Thus, technologies consuming solar exergy directly were put at a disadvantage toward technologies that consumed indirect solar exergy, via biomass or fossil fuels. The present paper aims at palliating these shortcomings by proposing a new method for exergy analysis. It was the goal to provide a scientifically sound method that is characterized by (1) full coverage of all natural resource categories including land use, (2) no need for weighting factors, by using one single unit that is physically interpretable and stable with time, and (3) avoidance of double counting. The exergy method proposed, the cumulative exergy extraction from the natural environment (CEENE), is coupled to a comprehensive state-of-the art life cycle inventory database, ecoinvent (21). Various product categories from ecoinvent are assessed with CEENE and the results are compared to previous exergy-based approaches and the energy based LCIA method cumulative energy demand.
Experimental Section Exergy Calculation Methods. Calculation and System Boundary Principle. The method to be developed aims at quantification of exergy that is taken away from the natural environment and that is the physical chemical “fuel and feedstock” for industrial production and consumption. Before calculating natural resources into exergy, special attention is needed to define exactly the resources that are transferred from the natural ecosystem to the industrial system: see Figure 1. Two kinds of exergy taken away from the natural environment have to be quantified. First, there are exergy stocks in the natural environment, e.g. water or fossil fuels. VOL. 41, NO. 24, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Natural resources taken away from the natural environment to establish industrial production and consumption: renewable resources (wind, solar energy, biomass, geothermal, hydropower), land use for infrastructure (built up, roads, . . .), fossil fuels, nuclear energy, metal ores, minerals, water resources, land resources, and atmospheric resources. One needs to define what particularly is going through the natural environment/industrial ecosystem boundary; e.g. in metal production, tailings are not considered to go into the industrial metabolism, although in some cases this assumption is not correct. Only the target mineral is taken into account in the present work. Next, there are flows of exergy (solar, wind, . . .) that have to be quantified in terms of exergy that powers the natural ecosystems, since the latter is being deprived by industrial use. The exergy quantity that is no longer available to sustain the natural processes within ecosystem due to withdrawal toward industrial use has to be taken into account. This means that, for solar energy, the fraction that is deprived from the first trophic level necessary for sustaining natural processes and cycles, is taken into account. In case of land use, it is considered that the natural ecosystem, and in particular again the first trophic level, can no longer make use of the solar exergy flow that is insolated onto this land. From a thermodynamic point of view, this solar influx is the driving force for the natural ecosystem to sustain itself. Reference Environment. For the calculation of feedstocks, the reference environment of Szargut et al. (23) was considered with its reference temperature T0 (298K), pressure P0 (1 atm) and composition, with an updated reference compound for aluminum (24). For the flows, not only the reference environment of (23) has to be considered, but at the same time the utilization rate of the natural environment. Exergy Calculation of Energy and Materials. The chemical exergy of any species can be calculated from the exergy values of the reference compounds, considering its reference reaction (24). Briefly, given the standard Gibbs free energy of the reference reaction ∆Gr0 (kJ/mol), the chemical exergy 0 of a compound i,bch,i (kJ/mol), is computed by 0 bch,i ) ∆Gr0 +
∑ν b
0 k ch,k
(1)
k
0 νkbch,k
where are the number of moles and the standard chemical exergy (kJ/mol) of the k th reference species, respectively. 0 denotes that reference temperature T0 and standard pressure P0. Alternatively, exergy values can be obtained from gross or net calorific values with calculation of exergy to heating 8478
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value ratios, β (23). These ratios depend on the elementary composition and can be calculated from equations reported in refs 23 and 25. Solar irradiations that are applied in land use were calculated into exergy by taking into account the geographical position of the different production sites and by making use of the solar energy engineering web site of the University of Massachusetts Lowell (26) and the exergy/energy factor of 0.9327 (23). Databases and Calculation of Exergy Data for Reference Flows. The considered resources extracted out of the ecosystem are the 184 reference flows as they are considered in the ecoinvent database version 1.2 (21) (see Supporting Information S1). Data on the composition that is necessary to do the exergy calculations, were retrieved from the ecoinvent reports (27–30), completed with data from (12, 31–38). The data were used to establish conversion factors, called X factors in this paper, which quantify the cumulative exergy extraction from the natural environment (CEENE) associated with (1) fossil fuels; (2) metal ores; (3) nuclear energy; (4) biomass; (5) land occupation; (6) renewable energy flows (wind, hydropower, solar); (7) minerals and mineral aggregates; (8) atmospheric resources; and (9) water resources. Volume occupation and land transformation are not considered because no exergy is deprived from the natural ecosystem. The factor X is defined as the exergy content of the considered reference flow (MJexergy or MJex) per unit of the reference flow as it is defined in ecoinvent, e.g., kg of oil or Nm3 of gas. Coupling with the Life Cycle Database of Ecoinvent. For each resource reference flow, the X factor allows the calculation of the cumulative exergy extracted from the natural environment for a product or service per functional unit:
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CEENEj )
∑ (Χ
i
× aij)
(2)
i)1
in which CEENEj is the cumulative exergy extracted from the natural environment for a product j (in MJex), calculated as the summation (over all resource reference flows) of the products of the Xi factor of the i th reference flow (Xi in MJex/ kg, MJex/MJ, MJex/Nm3, MJex/m2a) and the cumulative amount
aij from reference flow i (kg, MJ, Nm3, m2.a) necessary to obtain product j. Check for Double Counting. With respect to solar-based technologies and agricultural and forestry products, natural resources can be counted in two ways in life cycle inventory databases. On one hand they require land use, whereas at the same time the renewable products are considered. In fact, this double consideration can result in double counting: land occupation means solar exergy use whereas the renewable products represent part of this same solar exergy. This double counting is simply eliminated by setting correct boundaries: as soon as forestry and agriculture are intensive, they become part of the industrial metabolism. This means that they make use of land within the industrial system depriving the natural ecosystem from solar exergy. The same is valid for solar-based technologies. Their products are no longer purely natural and should be considered as any other products being marketed within the industrial society.
Results and Discussion Exergy Data. Fossils (Organic Nonrenewable Resources). Natural Gas. Based on the chemical compositions in ecoinvent report (29), β values of Dutch, German (sweet and sour), Norwegian and Russian natural gas were calculated. This is illustrated in Supporting Information S2. These exergy values are weighed by the shares of the different countries in the European gas consumption (S2). The resulting exergy factor X, i.e., the exergy content per unit in ecoinvent, is 38.28 MJex per Nm3 of gas for the European natural gas mix. Crude Oil. Based on the net calorific value (NCV) and gross calorific value (GCV) for crude oil as 43.2 MJ/kg and 45.8 MJ/kg respectively (30), and with β ) 1.07 (Supporting Information S3), an X value of 46.2 MJex/kg is obtained. A short sensitivity analysis shows that favoring and disfavoring of components that raise β has an impact of only (1% (S3). Formula accuracy is given to be (0.5%. The β value of 1.07 found is in good accordance with (23), in which the ratio of exergy over NCV is given as 1.07 for fuel oil and gasoline. Sulfur, In Ground. This resource reference flow is only created in ecoinvent to include the APME data (38). The resource ‘Sulfur, elemental’ used in the reports of Boustead, is transformed into the ecoinvent category “sulfur, in ground” (39). Although elemental sulfur can be extracted from nature (Frasch process), this process is totally replaced by recovering elemental sulfur from oil refining and metallurgical processes (40, 41). It is therefore assumed that the interpretation of elemental sulfur as byproduct of refining rather than as separate resource is more up-to-date. It can be assumed that this elemental sulfur, as well as the “bonded sulfur” in the APME reports, is won in oil refining. Therefore, its exergy is included in the oil category. The X factor of elemental sulfur is 18.94 MJex/kg (23). Peat. Peat, with a NCV of 8.8 MJ/kg and a GCV of 9.9 MJ/kg (28), is in fact the first step between biomass and coal. Its exergy/GCV factor therefore should be in the same order of that of biomass and lignite. The difference between both of them is only 1 percent. This can be considered to be small with respect to uncertainties in heating values or consumption in processes. Therefore, the biomass exergy/GCV factor is applied 1.031. The X factor is then 10.21 MJex/kg. Gas, Mine Off-Gas. Mine off-gas is an unconsumed exergy stream. The bulk of the methane is left untouched by the process and can thus not be seen as an exergetic resource utilization. Coal. The “coal, hard” reference flow is given the weighted average GCV of 19.1 MJex/kg, “For brown coal” a GCV of 9.9 MJex/kg. With β values of 1.03 and 1.04, the X factors are 19.7 and 10.3 MJex/kg (5). Metal Ores. Two types of metal ore reference flows can be distinguished. On the one hand, there are reference flows
that indicate the amount of a mineral, not of an element. It is in this form, that the mineral-and the metal included-is extracted. For this kind of reference flow, the calculation is straightforward and can be done from the data previously reported (24). Only two reference flows of this type are found among the metal ore minerals: cinnabar and rutile. Other metal ore reference flows’ names start with the target element, e.g., “aluminum, 24% in bauxite, 11% in crude ore, in ground”. In this case, the LCI results should be interpreted as the total amount of the element aluminum that is extracted. Although it is indicated that it is extracted in the form of bauxite, it is the weight of the aluminum that is calculated by the ecoinvent experts. In this case it is incorrect to take the specific exergy of aluminum (30 MJex/kg), as it is clearly not pure aluminum that is extracted. The representative ore minerals (up to three different species) were selected for each element based on the ecoinvent reports and literature data (37, 40, 42). Based on recent mineral exergy values of the ores (24) and the share of the specific ores in the supply of the metal, exergy values are calculated for 37 metal ores (see Supporting Information S4). One exception is garnierite for the reference flow Nickel, 1.98% in silicates, 1.04% in crude ore, in ground. The exergy of the mineral, which consists of Mg-rich phyllosilicates in which magnesium is to a large extent substituted by nickel, (Ni,Mg)6Si4O10(OH)8, had to be calculated with the method of Vieillard (43) (see Supporting Information S5). Metal production consists of ore extraction, beneficiation, and processing. Ore extraction provides the crude ore and generates overburden, which is deposited and not further processed. Beneficiation concentrates the crude ore to an ore concentrate and generates tailings. In some cases, these tailings may only be grinded, whereas in other cases they are chemically transformed. In contrast to extraction and beneficiation, processing of the concentrated ore always destroys the ore or mineral and generates wastes which are not earthen. Hence only exergy in the minerals which contain the target metals are accounted for, whereas the exergy in overburden and tailings are returned to the environment and potentially utilizable in the future. With respect to mixing exergy, a metal ore is modeled as a mixture of these minerals (see Supporting Information S6). From this modeling, it is can be seen that the exergy loss due to mixing is negligible. Nuclear Energy. In nuclear power generation, energy is extracted from the fission of both fissile U-235 and fertile U-238 isotopes. These isotopes have a nuclear exergy of 74.5 and 77.2 TJex/kg respectively (12). The isotope ratio as it is found in and extracted from nature, is 0.72% of U-235 and 99.28% U-238. Therefore, the nuclear exergy in the uranium ore extracted is 77.2 TJex/kg uranium. However, studying the processes and assumptions (27) more carefully shows that only less than one percent of the uranium atoms are “consumed” and the rest are brought to nuclear waste deposits or storage. The average percentage of atoms consumed in nuclear electricity production is calculated by following the steps modeled by the ecoinvent experts. The calculation scheme is presented in Supporting Information S7. The steps considered are mining, milling, conversion, enrichment, fuel element production, nuclear electricity production, burnt fuel reprocessing, and mixed oxide (MOX) fuel element production. Burnt fuel conditioning, waste storage and depleted U-238 storage are not included in the analysis, because they do not affect the amount of atoms that is actually consumed by fission. Some steps in the scheme require averaging over total production, e.g., enrichment and MOX content are not the same for every country. The standard value is an average over European production, approximated by the UCTE-zone (Union for the Co-ordination of Transmission of Electricity) production. The averages for enrichment, MOX content, burn up of fuel elements and plant VOL. 41, NO. 24, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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efficiency are calculated for the average UCTE production, national averages are weighed by national contribution to nuclear electricity production. The resulting X factor is 77.2 × 0.00608 TJex/kg ) 469 GJex/kg. It should be noticed that a similar approach is followed by the ecoinvent experts in the cumulative energy demand (CED) indicator calculation (44) and the approach of (5). This means that the CED value for natural uranium is dependent on the technology used to exploit the nuclear power, as is the case for the X factor calculated above. Biomass. Ecoinvent reference flows for biomass consist of data in m3 (biomass) and MJ (wood). To avoid double counting, exergy quantification is based on the energy content: details are given in Supporting Information S8 and S9. However, the resulting exergy values will not be implemented because of double counting, as explained in the paragraph “Check for Double Counting” above. Land Occupation. Out of the 184 resource reference flows, 22 are land occupation categories. It can be considered that all land surface occupation categories deprive the natural ecosystem from the solar exergy that is necessary to sustain its natural cycles, with the exception of “occupation, pasture and meadow, extensive”. Considering an average irradiation of 2.78 kWh/m2.day for Western European conditions (26) and the exergy/energy factor of 0.9327 (23), a solar exergy flow of 34071 GJex/ha.a can be calculated. A fraction of this exergy is used by the natural land ecological system (45) where for the marine systems this fraction can be considered to be negligible, being at least 2 orders of magnitude lower than for land based production (46). In case of the natural land ecosystem, only 45% is photosynthetically active radiation (PAR). Out of this PAR fraction, 5% is reflected and transmitted so that the natural ecosystem can make use of 40% of the overall solar exergy. With a maximum conversion efficiency of absorbed light to capture carbon dioxide of 27%, maximally 10.8% of the solar exergy is effectively metabolized within the natural vegetation. However, natural ecosystems merely attain 2.0% metabolization, where half of it is conserved and the other half is dissipated through respiration. This means that the exergetic value of land use during 1 year results in a withdrawal of 681.4 GJex/ha.a for sustaining the natural ecosystem. In conclusion, industrial (construction, traffic, . . .) and intensive agricultural land use means that the natural ecosystem is deprived of 681.4 GJex/ha.a, which corresponds to 14.8 ton oil equivalent (exergy value of 1 ton crude oil is 46.2 MJex). Renewable Exergy Flows: Wind Power, Hydropower, Geothermal Energy, Solar-Based Technologies. The CEENE method only considers the fraction of exergy useful for natural ecosystems, e.g., wind that cannot further transport pollen. The total kinetic exergy which is extracted from wind by wind mills, and the total potential exergy extracted by hydropower plants are accounted for. Geothermal energy stands for the low temperature heat in the ground, extracted by borehole heat exchangers and upgraded into high temperature heat to heat houses. This low temperature heat can be regarded as environmental heat. In its local environment it does not have any exergetic value. Therefore, we set the exergy value of “energy, geothermal” to zero. This may change in the future, when deep geothermal power plants come into play, as the temperatures are much higher. Ecoinvent reference flows for solar-based technologies take into account both land use and solar irradiation. However, considering both reference flows results in double counting, as explained in the paragraph “Check for Double Counting”. To avoid double counting, we only accounted for exergy extraction from land use. Minerals and Mineral Aggregates. Mineral aggregates, i.e., basalt, calcite, clays, granite, gravel, perlite, pumice, sand, 8480
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and shale, are collections of all sorts of ground, rock, and clay types. For these reference flows, average mineral compositions have been considered. However, these are averages and in some cases even estimates. Details of the calculations are in Supporting Information S10. The calculation results therefore have a rather high level of uncertainty. Due to the small exergy values of these resources, it is assumed that this will not jeopardize the validity. With respect to minerals, the X factors are in most of the cases the implementation of the specific exergy value of the mineral (Borax (tincal), diatomite (opal), anhydrite, Barite, chrysotile, colemanite, apatite (fluor and phosphor resource), dolomite, fluorspar, kaolinite, kieserite, magnesite, pyrite, spodumene, stibnite, sylvite, talc, ulexite, vermiculite), recently calculated (24). Only few reference flows require additional remarks. For feldspar, gypsum, and olivine, the composition of these flows have to be taken into account, next to the exergy value, the latter being available from (24). Orthoclase (KAlSi3O8), albite (NaAlSi3O8), and anorthite (CaAlSi3O8) represent the feldspar group. Orthoclase is the most important feldspar for industrial use (39), therefore it is assumed to account for half of the feldspar production. The remaining two minerals share the other half-equally. The gypsum reference flow is composed of about 65% natural gypsum (CaSO4.2H2O) and 34% natural anhydrite rock (CaSO4) (29). Olivine is not a mineral species, it is the name for a series of minerals between the iron-rich end member fayalite (Fe2SiO4) and the magnesium-rich end member forsterite (Mg2SiO4). Although intermediate members occur in nature, olivine is modeled as a mixture of forsterite (80%) and fayalite (20%) (47, 48). With these compositions, X data for feldspar, gypsum, and olivine of 0.103, 0.150, and 0.479 MJ/kg, respectively, are obtained. Finally for sodium chloride and sodium sulfate, data has been calculated as 0.248 and 0.127 MJex/kg respectively, in a similar way as in (24) based on the HSC database (49). Atmospheric and Water Resources. For air and water, the reference environment from ref 23 is valid. Application onto Products and Services That Are in the Ecoinvent Database and Comparison with Results Obtained with Common LCIA Methods. The CEENE method is applied to the product and infrastructure units in ecoinvent. The analysis of the energy production system exhibits that the major exergy source of energy production stems from the respective energy carriers utilized (Figure 2). The exergy extraction to build the facilities is, with the exception of solar energy production, small. In production of both solar electricity and solar heat, land occupation through which solar exergy is calculated, and the exergy for the infrastructure has a significant contribution to the total exergy score. In material production (Figure 3), exergy from fossils (for process heat, base material) as well as nuclear exergy (for electricity) exhibit a large share in the CEENE score, with an average of 42 and 6% for all products, respectively. However, exergy extraction for products made from biomass (agricultural products, paper, and cardboard) is dominated by exergy from land occupation to grow the biomass with an average of 84%. At a first glance, this looks different from the CED and CExD scores, where the renewable resources are much more prominent. However, the difference in scores can be explained by the fact that solar input in the CEENE method is accounted for through the land use and not biomass. In building materials, the exergy from minerals and mineral aggregates features an average contribution to the total score of 6%, whereas for metals, the average relative exergy extraction from metal ores amounts to 7%. Compared to resource indicators cumulative energy demand (CED) and cumulative exergy demand (CExD) (5), CEENE provides more detailed information on resource demand. By definition, CED only accounts for resources which may be used as energy
FIGURE 2. Composition of the CEENE score for energy production in ecoinvent. Note that biomass is not included as a resource because of double counting with land use.
FIGURE 3. Comparison of the composition of CEENE, CExD, and CED scores for material production in ecoinvent. For the CED and CExD methods, the categories for wind, solar, geothermal energy, and for biomass are aggregated into one category and displayed here in the CEENE category renewable resources. carriers and, hence, neglects resources traditionally considered nonenergetic like water, minerals, and metals. CExD includes these nonenergetic resources but does not evaluate land use. CEENE is the most comprehensive resource indicator which evaluates energy carriers, nonenergetic resources, and land occupation. In terms of fossils, nuclear, wind (kinetic energy), and hydropower (potential energy), all three indicators produce similar results, since the exergy to energy ratio of the respective energy carriers is close to unit. The difference between the resource evaluation in CExD and CEENE is due to incongruent system boundaries like the inclusion of land use in CEENE. CEENE differs from CExD in conceptual, quantitative, and qualitative aspects. CExD evaluates the exergy that is removed from nature and transferred into the technological system, whereas CEENE accounts for total exergy the natural system is deprived of. This conceptual disparity leads to a different evaluation of resources. CExD provides an exergy value for biomass that enters the technosphere (5), whereas CEENE accounts for the exergy deprived from the natural system due to land occupation during biomass growth. Quantitatively, the CEENE concept comprises an additional resource category, namely land occupation. The rationale of assigning an exergy value to land is that land occupation deprives the exergy of solar irradiation which could be used for photosynthesis in plant growth. This new approach perfectly reflects the inevitable land occupation
associated with solar based energy and materials: solar heat collectors, PV cells, and biomass. In this perspective, one must be aware that the category renewable resources does not comprise solar energy and biomass to avoid double counting. The exergy of metals in CExD is calculated from the whole metal ore that enters the technosphere (5), whereas CEENE only regards the metal-containing minerals of the ore, since the tailings from the beneficiation are often not chemically altered when deposited. The qualitative aspect is that CEENE and CExD base their exergy calculations for minerals on different literature sources, which lead to a different exergy value for aluminum. However, the impact on the final results is marginal. CEENE is seen as an improved method toward the previous CExD method because it is more consistent. For instance, the CExD factors of direct use of solar irradiation were very high in comparison to the exergy content of biomass or fossil resources, which ultimately were also formed by solar energy. Thus, using solar exergy directly, e.g., through photovoltaic panels, was put as a disadvantage toward indirect use, such as fuel incineration. This inconsistency no longer persists in the CEENE method as the new indicator quantifies the extraction of exergy that could have been useful for natural ecosystems. Apart from its consistency, the CEENE method has been developed to be compatible with existing life cycle databases VOL. 41, NO. 24, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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so that its applicability can be brought in practice. In this work, ecoinvent has been chosen as database to be coupled with; however, the methodology is, in principle, generic. Indeed, it starts from the reference flows as they are defined by the database under consideration, and assigns the appropriate exergy values to the respective flows. Finally, it is to be emphasized that CEENE enables to account for very different natural resource intakes, from renewable resources, nonrenewable resources, water and atmospheric resources, and land use. This approach will be helpful in substantiating the debate on sustainable use of the natural environment by mankind: it reflects the physical chemical price the natural environment pays for the withdrawal toward our industrial society. Through the eight different resource categories, and the integrated development with an up-to-date life cycle inventory, it shows the nature of the resources that are consumed, e.g., at the generation of so-called renewables-based products. Finally, through combination of CEENE with a sound emission-oriented LCIA, decision-makers are provided with comprehensive LCIA methods taking into consideration the impact of resource input as well as emission output.
Supporting Information Available Details on the obtained X factors and the calculations are included in sections S1-S10. This information is available free of charge via the Internet at http://pubs.acs.org.
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