Life cycle analysis
Life-cycle assessment (LCA) has been developed in the last decades. A comprehensive framework has been established to enable the execution of the analysis. In this framework the concept of exergy or similar thermodynamic concepts have not yet been introduced. It is shown here that for sustainable development the depletion of exergy reservoirs and the exergetic emission to the environment have to be minimised. To accomplish this it is proposed to extend the LCA with an Exergetic Life-cycle Analysis (ELCA). The method of how to perform an ELCA is presented. In the paper the concept of a zero-exergy or a reduced exergy emission plant is introduced. It is shown that in the case of a zero-exergy emission plant the LCA can be replaced by an ELCA completely. An extension to the method of ELCA is presented in which emissions are translated in terms of exergy. As a case study the comparison between the disposable polystyrene cup and the porcelain mug has been made. It is shown that the extended ELCA is a valuable approach because it gives a good indication of the effort in terms of natural resources needed to prevent emissions.
In the last decades life-cycle assessment (LCA) for products or product systems has been developed to analyse environmental problems associated with the production, use and discard or recycling of these products. With the LCA an improvement analysis is connected to minimise the environmental burden of the analysed products. It will be shown that the tool LCA can gain in stength when the LCA is extended with an exergetic life-cycle analysis (ELCA).
The LCA uses the cradle to grave approach. The complete life cycle of the product is taken into account. Four components can be distinguished in the LCA. In the goal definition and scooping the subject of study is determined in relation to the application intended. The functional unit of the product, the spatial scale and time horizon have to be determined. In the inventory analysis the complete life-cycle of the product is analysed leading to the inventory table, a list of inputs from and outputs to the environment. The impact assessment is a step-wise process. In the first step, the classification, it is determined which environmental problems are considered and which extractions and emissions contribute to it. In the second step, the characterisation, the contribution of all extractions and emissions to the selected environmental problems is estimated. The final step is the valuation of the selected environmental problems. The last component of the LCA is the improvement analysis.
In the present LCA framework the depletion of abiotic resources is calculated by the material use per year divided by the material reserve or in formula form:
In the case of minerals the material reserve will always stay constant. Only the concentration and chemical structure of the minerals will change. Hence, with the use of sufficient energy it is always possible to obtain the desired concentration and chemical structure for the minerals. So, because there is no relation between reserve and material use the meaning of formula (1) as a criterion for depletion of minerals is less relevant. For the case of fossil fuels the material reserve will change by their use, because we are interested in their work potential, which the material looses (almost) completely after its use in the most common case of burning. Formula (1) is a way to describe the depletion of fossil fuels, however a criterion based on the work potential would be preferable. A new criterion for the depletion of abiotic resources is introduced, which displays the relation between the use of minerals and the depletion of fossil fuels and is based on the work potential of materials.
Exergy and sustainability
Up until now the exergy concept has not been used in the LCA. This can be seen as a lack in the LCA, because there is a direct relation between the exergy use of fossil fuels and minerals and the environmental problem of depletion of natural resources.
Exergy is based on the first and second law of thermodynamics and is defined as the maximal obtainable potential of work of a material or energy flow in relation to the environment. To minimise the use of natural resources an exergy analysis can be used. In an exergy analysis it is shown where the work potential of natural resources in relation to the surrounding environment is lost, i.e. where the irreversibility takes places. The maximal obtainable potential of work of a flow can only be obtained completely via reversible processes. For sustainable development the use of the exergy reservoirs of natural resources has to be minimised to a level where there is no irreversible damage to the environment and the supply of exergy to further generation is secured.
There is no need to distinguish between the different kinds of materials. For example, if the exergy reservoir of high concentration lead ore is depleted the exergy reservoir of coal or oil and other material needed to explore the lower concentration ore can be used to explore the lower concentration lead ore. One could argue that a distinction between abiotic and biotic resources should be made. However, in many situations in the present society abiotic resources are used to obtain biotic resources. For example, the use of fertilisers and green houses to produce food. So to make a distinction between these two types of resources becomes less meaningful. So exergy destruction should be adopted as criterion for the depletion of natural resources. However, depletion of biotic resources can still be a threat to some regions, for example extinction of species. So it is still meaningful on a regional level, but not as a general criterion on the global level.
Exergy can also be used as a more general criterion in a world, where more and more harmful emissions can be separated from each other and transformed to harmless waste or sometimes even useful products. To carry out these separation and transformation processes, which like all real processes are irreversible, an exergy destruction or irreversibility will be created. When all harmful emissions are handled by these processes the environmental burden of products can be compared by only the criterion of life-cycle irreversibility.
Zero-exergy emissions, i.e. emissions containing zero exergy, are harmless to the environment, because their chemical composition and physical properties are similar to the environment itself. So, in the case of a zero-exergy emission process the LCA can be completely replaced by an ELCA. However, a small exergy content of the emissions can already be harmful.
Exergetic Life-Cycle Analysis (ELCA)
The framework of the ELCA and LCA are quite similar. The goal definition and scooping are completely identical. The inventory analysis of the ELCA is more elaborate. A complete flowsheet of the mass and energy streams of the different production steps are needed. The material and energy balances have to be closed. Also the more simplified black box approach could be used of which only the inputs and outputs of the production processes are taking into account. The impact assessment is limited to calculation of the exergy of the flows and the determination of the exergy destruction in the different production processes. The cumulation of all exergy destruction in the life-cycle gives the life-cycle irreversibility of the product.
The exergy analysis pinpoints the places where the exergy destruction takes place and in the improvement analysis different possibilities can be presented to minimise this destruction. The improvement analysis can be extended by an exergo-economics analysis in which also the monetary costs are taken into account. When in the inventory analysis a black box approach has been used only different alternatives for the black boxes can be compared in the improvement analysis.
Zero-exergy emissions processes
In the special case of zero-exergy emission production processes the LCA can be completely replaced by the ELCA. A process is considered to be a sustainable process when the emission rate of the process is below the accepting level of the environment or below the allocated emission rates based on these levels. The analysed present emission rates were 50% to 90% above these levels. So instead of zero-exergy emissions the sustainable emission levels could be taken.
Zero-exergy emission ELCA (Zero-ELCA)
The ELCA can be extended with the abatement exergy of emissions. It can be assumed that the non zero-exergy emission processes are adjusted to zero-exergy emission processes by separation and transformation processes for the emissions, if possible. These processes will need exergy. So to the different environment distortion emissions an amount of exergy can be assigned, which we call the abatement exergy. When a comparison is made with other production processes it is just necessary to have equal emission per functional unit of the product. By this method the ELCA is extended to take into account all environmental problems and not only depletion of natural resources.
The present criterion for the depletion of natural resources is not very appropriate. As criterion for the depletion of natural resources exergy should be used. Because of this new criterion the present LCA has to be extended with an ELCA. In the case of a zero-exergy emission power plant the LCA can even be completely replaced by an ELCA. A method is proposed for the comparison of different emissions of processes on the basis of exergy. So the ELCA can extend to take into account all environmental problems. In the example of the disposable cup versus the porcelain mug it is shown that only a small increase in the use of natural recourses is needed to avoided harmful emissions
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