|Fig. 1: Outline of LCA Stages|
Life Cycle Assessment (LCA) is defined as "the compilation and evaluation of the inputs, outputs and the potential environmental impacts of a product system throughout its life cycle."  LCA effectively allows for the establishment of the environmental profile of a given system. A system can be defined as a product and/or the process used to make that product.
The various product stages that a typical LCA spans include, but are not limited to: raw material acquisition, material processing, manufacture and assembly, use and service, retirement and recovery and disposal. Each of these stages can include material and energy inputs as well as waste (liquid, solid, gaseous) outputs.
The two primary types of LCA methodologies include process-based LCA and economic-input output LCA (EIO-LCA). Process-based LCA is centered on scientifically analyzing the actual process (i.e. mass/material balance, scientific characteristics, etc.). The ISO 14040 Standards, which will be further discussed, are primarily concerned with the process-based methodology. EIO-LCA uses the history of economic transactions to trace along a given supply chain where value is added or removed.  This quantified value can include resource requirements and can then be used to interpolate environmental impacts from corresponding datasets. Given the economic nature of EIO-LCA, the remainder of this paper will focus on process-based LCA.
Very specific standards from the International Organization for Standards (ISO) govern the exact requirements necessary to certify a process-based LCA. Fig. 1 shows each of the four stages and how they relate to one another.  ISO 14040 outlines the principles and framework of LCA. ISO 14041 governs the goal and scope definition and inventory analysis. The goal and scope definition is critical in properly drawing a boundary for an LCA.  The scope of the study is critical for a few reasons. First, the scope allows for defining a functional unit. For example, if one was to compare hand drying via paper towels or by an electric air dryer, a metric would need to be developed to make any accurate comparison. Pairs of hands dried could work as a metric in this case. Furthermore, system boundaries can also be drawn in order to exclude and include important and non-important items from the analysis. The inventory stage involves properly identifying mass, energy inputs and outputs as well as the environmental relevance of said inputs/outputs. The inventory analysis involves properly accounting for all material and energy inputs/outputs and waste outputs.
ISO 14042 details the necessary requirements for an impact assessment, which is the step where each of the inventory items are analyzed and converted to their according impacts.  The impact stage involves using the inventory analysis results and then assessing this data to come to relevant midpoint and endpoint indicators. Midpoint indicators might include acidification, radiation, climate, fossil fuel or ecotoxicity. Endpoint indicators can include respiratory diseases, seawater level or a reduced resource base.
ISO 14043 is concerned with the interpretation phase, which is a continuously occurring step that happens in sync with each of the three other stages previously mentioned.  The interpretation phase allows for a proper boundary definition, a rigorous inventory analysis, and a robust impact assessment. In addition, this phase most importantly involves formulating conclusions and improvements from inventory analysis and impact assessment results. Major environmental bottlenecks can be identified and alternate designs can be selected. It is also worth noting that ISO 14044 provides general requirements and guidelines of environmental management specifically related to LCA.  However, this document is not concerned with any one specific LCA step as are the other ISO standards mentioned previously.
|Table 1: Embodied Energies of Common Building Materials |
LCA serves as the fundamental analytical process by which the embodied energy of a product can be calculated. Table shows different embodied energies for various building materials depending on a number of factors. How these numbers are calculated as well as why there is significant variation within some of the numbers is worth understanding. Calculating embodied energy effectively involves performing an LCA on said product. In this case, steel, timber and concrete were analyzed. However, in order to come to the numbers present in the table above, a number of different intermediate impacts need to be calculated in order to understand what exactly is going on.
Equation 1 above details how embodied energy was calculated for the values found in Table 1.  M is the waste factor or scrap rate to allow for the actual impact to be calculated and not to omit materials that are thrown away and not included in the final product. M is a percentage value. C, S, A, W, R and P are the masses of the constituent materials, which include cement, sand, aggregate, water, replacements and plasticizers. The corresponding x factors for each material are the embodied energy (MJ/kg) for each of the constituent materials. O is the operational energy necessary to compile the final product in MJ. T represents the total transport energy of the final product in MJ. It is important to note the reference base used for this calculation. All of the quantity values (C, S, A, W, R, P, O, and T) should be using the same quantity of finished material as the denominator. In other words, if the analysis case was 100 kilograms of steel, then each of these numbers should be on a per 100 kilograms of steel basis. These impact factors are derived using the previously mentioned life cycle methodology. The final result then becomes a MJ number. This value really is on a per reference kilogram basis (functional unit as defined previously) even though the equation calculates a MJ number. The calculated values can vary widely due to the differences that exist across multiple databases. This is the main reason that the values in Table 1 vary across different sources listed on the left column. Furthermore, there are individual variations within the same dataset due to differences in boundaries used for the underlying LCA.
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