Life Cycle Analysis Methods in Buildings
Introduction
Life cycle analysis (LCA) is an important methodology in assessing the sustainability of new buildings and of maintaining, refurbishing and replacing existing buildings. Embodied energy and carbon is only one part of a building's life cycle, but is of increasing significance.
The embodied energy and carbon, associated with building construction, is the energy required and carbon emitted to construct a building (including extraction of raw materials, manufacture of building products and construction of the building.) The other stages of a building's full life cycle are operational energy and carbon (relating to the use of a building) and end?of?life energy and carbon (relating to its demolition and disposal).
Carbon dioxide (CO2) is a significant greenhouse gas and, consequently, these emission reduction targets apply to CO2 emission, often simply referred to as carbon emissions. These emissions are inextricably linked to energy consumption, when energy is produced through the combustion of fuels. Buildings require large quantities of energy for operation. The construction industry is also, globally, the highest consumer of materials, using around 6 tonnes of material per person per year.
The construction industry has become proficient at designing and erecting low?energy buildings within a relatively short time period (mid 1990s to date). Coupled with this is a growing understanding by the general public of the need to conserve energy both for security of future supply and to protect the environment. In times of critical financial uncertainty, the general public is acutely aware of energy costs and is particularly eager to cut fuel bills. The 2010 BP Statistical Review of World Energy (BP plc, 2010) revealed a decline in the amount of energy used throughout the recent recession; the first decline since 1982.
There are a wide number of technologies, software programs, tools and initiatives now in use to help reduce operational energy and carbon, but the practical application of methods to assess and reduce the embodied energy and carbon of building construction is still in relative infancy. Energy and carbon assessment in the construction industry has also focused much on the construction of new buildings.
Assessing the energy and carbon related to existing buildings was all too often limited to only operational energy. The energy and carbon associated with the maintenance and repair, upgrade and refurbishment, and demolition and replacement of existing buildings is not that often assessed properly, if at all.
Embodied Energy and Carbon
The embodied energy and carbon of a building are one part of the overall energy and carbon of the building, measured over its complete life (life cycle). So it is important, firstly, to introduce the terms energy and carbon, and then describe the concept of life cycles and their assessment, before discussing in more detail embodied energy and carbon, and, finally, what impacts on them.
Energy is needed to construct and operate a building. This energy is often produced through the combustion of fossil fuels, such as coal, gas and oil. The combustion process generates the energy used, for example, in the construction and operations of a building. However, this process also produces, as by?products, CO2 and other gases, emitted into the atmosphere. It is these CO2 emissions which are, generally, referred to in shortened form as 'carbon'. In other words, energy, generally measured in joules (J) or mega joules (MJ), is converted into carbon, measured in kilograms (kg) or tonnes (t). Energy 'consumption' is, therefore, inextricably linked to carbon emissions, when (fossil) fuels are used. However, other forms of energy exist, for example, solar energy (a form of radiant energy) or wind energy (a form of kinetic energy), which do not generate carbon emissions when converted.
Physically, energy is neither consumed nor produced, but converted from one form of energy into another. Energy can be in the form of, by example, thermal energy (heat), electrical energy (electricity) and kinetic energy (movement).
CO2 is only one of several greenhouse gases impacting on climate change through global warming. Instead of using CO2 only, when assessing global warming, equivalent carbon dioxide (CO2e) is frequently used as a measure. This is a way of describing how much global warming a given type and amount of greenhouse gas may cause, using the functionally equivalent amount, or concentration, of CO2. Put simply, if CO2 has a global warming potential (GWP) of 1, then methane has a GWP of 25 and nitrous oxide a GWP of 298.
It is important not to confuse the term 'carbon', as generally used in this report, with the chemical element of the same name. Carbon, in the context of this report, refers to CO2 emissions. However, the word 'carbon' can also refer to the chemical element 'carbon' (chemical symbol C), which occurs as element in form of graphite, diamond or amorphous carbon (the element's free, reactive form), or as a compound in, by example, CO2, carbonate rock (e.g. limestone, dolomite and marble) and hydrocarbons (e.g. coal, petroleum and natural gas). Plants can absorb CO2 from the air, storing the carbon element in form of carbohydrates (sugar). This storing of carbon is referred to as carbon sequestration or carbon capture. That means that chemical carbon can be contained, as a chemical compound, in materials which are used in building construction, e.g. building stones and timber. However, this storage of carbon in a (raw) material does not relate to the CO2 emissions released in the process of making a building product out of the (raw) material. So, when in this report the term carbon is used, it refers to CO2 emissions and not to the chemical element.
Three Main Stages
Generally speaking, a building product (i.e. construction material, product or component) has three main stages to its Cradle?to?Cradle energy life cycle: embodied energy, operational energy and end?of?life energy.
Embodied energy is the energy related to the construction of a building; operational energy is associated with the use of a building; and end?of?life energy is related to the disposal or recycling of a building. The sum of embodied, operational and end?of?life energy is referred to as overall energy or total energy footprint.
Carbon is a conversion of energy from mega joules to kilograms of CO2. Consequently, embodied carbon relates to the construction of a building, operational carbon to its use, and end?of?life carbon to its disposal or recycles. The sum of all three is the overall carbon or total carbon footprint.
In other words, embodied carbon can also be defined in relation to life cycle variants as a Cradle?to?Gate or Cradle?to?Site analysis (see Table 1), based on energy inputs only (i.e. those energy inputs relating to raw material extraction, transportation, processing, manufacturing and packaging).
Table 1: Life cycle variants commonly used in LCA
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Cradle to Gate
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Describes the impacts associated with products, materials or processes up to the point at which they are packaged and ready for delivery to site.
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Cradle to Site
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Describes the impacts associated with suppliers (raw materials), transportation to manufacturing centre, manufacturing, packaging and transportation to site. In the case of construction impacts, this would also include any processing required on site to make use of the product or component.
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Cradle to Grave
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Describes all the processes which a product or component goes through from raw material extraction to obsolescence and final disposal. It assumes no end?of?life residual value.
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Cradle to Cradle
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Is similar to Cradle?to?Grave', but assumes that an obsolete building, product or component has a residual value at the end of its first life. It assumes that construction waste can be recycled and used to provide raw materials for the re?manufacture of the same product or the manufacture of new and different products.
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Product manufacturers may give embodied energy figures for their products which take into account only some of these stages. The definition of the boundaries of a LCA is critical in drawing useful conclusions on the embodied energy of a product. In general, the more manufacturing processes a product undergoes the higher its embodied energy will be.
Life Cycle Analysis
To create a building, energy is needed for the extraction of raw materials, the manufacture of building products and the construction of the building on site. Energy is also needed to operate a building in use, e.g. for heating, lighting, cooking and working. And energy is needed at the end of the life of a building for its demolition and disposal. Figure 1 illustrates the various life cycle stages of a building.
If a boundary is drawn around this life cycle of a building, an assessment of the inputs and outputs which cross this boundary can be made. Such an assessment is called life cycle assessment or life cycle analysis (LCA). It includes the entire life cycle of a product, process or a system: encompassing the extraction and processing of raw materials; manufacturing, transportation and distribution; and use, reuse, maintenance, recycling and final disposal (Consoli et al., 1993).
The execution of a LCA relies heavily on data generally provided in life cycle inventory (LCI) databases.
LCA can be used as an accounting methodology to quantify energy use and carbon emissions for the whole life cycle of a building. In this context, LCA often utilises accountancy?speak: energy and carbon can, for example, be expended, invested or spent.
LCA is a much?explored concept and has been used as an environmental management tool worldwide since the late 1960s. It is an internationally recognised tool for assessing the environmental impact of products, processes and activities, using environmental impact indicators. LCA based, for example, on the BRE's Environmental Profiles methodology (BRE, 2011) identifies 13 environmental impact indicators, as listed in Table 2. It is worth noting that 'climate change' (or global warming) is only one of them and accounts for just less than 25% of the impacts.
Table 2 LCA environmental impacts (Anderson et al., 2009)
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Climate Change
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Water extraction
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Mineral resource extraction
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Stratospheric ozone depletion
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Human toxicity
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Ecotoxicity to fresh water
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Nuclear waste
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Ecotoxicity to land
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Waste disposal
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Fossil fuel depletion
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Eutrophication
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Photochemical ozone creation
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Acidification
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Life cycle energy analysis (LCEA) emerged in the late 1970s and focuses on energy as the only measure of environmental impact of buildings or products. The purpose of LCEA is to present a more detailed analysis of energy attributable to products, systems or buildings. It is not developed to replace LCA, but to compare and evaluate the initial (capital, embodied) and recurrent (operational) energy in materials and components. End of life (EoL) issues should also be considered, i.e. the energy required to recycle and reprocess materials, or to dispose of them safely. LCEA is often used to estimate the energy use and savings over a product or building life, and to compare energy payback periods.
LCA Methodologies
There are a number of recognised LCA approaches / LCA methodologies. These include process analysis, input?output analysis and hybrid analyses. There are also simplistic / alternative approaches. These LCA approaches are briefly summarised in Figure 4 and described in more detail below.
Process analysis
Process life cycle analysis (P?LCA) is the oldest and still most commonly used method, involving the evaluation of direct and indirect energy inputs to each product stage. It usually begins with the final product and works backwards to the point of raw material extraction. The main disadvantages centre on the difficulties in obtaining data, not understanding the full process thoroughly, and extreme time and labour intensity. These result in compromises to system boundary selections (which are generally drawn around the inputs where data is available). Furthermore, it is likely to ignore some of the processes, such as services (banking and insurance, finance), inputs of small items, and ancillary activities (administration, storage). The magnitude of the incompleteness varies with the type of product or process and the depth of the study, but can be 50% or more (Lenzen et al., 2002). For these reasons, results are found to be consistently lower than the findings of other methodologies. P?LCA is better used to assess or compare specific options within one particular sector.
Input?output analysis
Originally developed as a technique to represent financial interactions between the industries of a nation, the input?output life cycle analysis (I/O?LCA) can be used in inventory analysis to overcome the limitations of process analysis. This method is based on tables which represent monetary flows between sectors and can be transformed to physical flows to capture environmental fluxes between economic sectors. The number of sectors and their definition vary within each country. The great advantage of this method is data completeness of system boundaries: the entire economic activities of a nation are represented. However, despite the comprehensive framework and complete data analysis, I/O?LCA is subject to many uncertainties, due mainly to the high level of aggregation of products. Many dissimilar commodities, or sectors containing much dissimilarity, are put into the same category and assumed identical; assumptions are based on proportionality between monetary and physical flows. In some countries I/O tables are not updated frequently, resulting in temporal differences with irrelevant or unrepresentative data. Unsurprisingly, P?LCA and I/O?LCA yield considerably different results. I/O?LCA is suitable for strategic policy making decisions (comparing sectors) as well as providing complementary data on sectors not easily covered by P?LCA. To assess life cycles of older buildings, I/O?LCA would be impractical, as the economic input and output data for the time of construction is not, or at least not easily, available.
Hybrid analyses
The disadvantages of the previous methods can be reduced if a hybrid LCA method, combining both P?LCA and I/O?LCA methodologies, is employed. In this model, some of the requirements are assessed by P?LCA, while the remaining requirements are covered by I/O?LCA. The main disadvantage of these techniques is the risk of double counting.
There are, generally speaking, four types of hybrid analysis:
- Process?based hybrid analysis
This method uses process analysis as the starting point and escapes the excessive amount of time needed to acquire the last few percent of the total, but is criticized for lacking transparency.
- Input/output?based hybrid approach
This method uses conventional I/O analysis data as the starting point. Single or groups of data in the I/O matrices are substituted with process analysis data.
Process analysis is employed for use and disposal phases as well as for several important upstream processes; the remaining input data is taken from an I/O?based LCI. Aside from the risk of double?counting, the interaction between the two methodologies can be difficult to assess in a systematic way.
- Integrated hybrid analysis
Process analysis and I/O analysis are developed independently and then systematically merged into one system to form a computational structure and a consistent mathematical framework.
Simplistic / alternative approaches
Due to the complexities of LCA and the complications in LCI studies, a number of simplistic methods were developed for industrial use, mainly aiming to develop quick decision making tools. Researchers have introduced a 'hotspot' approach, which selects essential issues in the inventory and applies generic data to quickly analyse products. This has been developed into a method of 'screening' and 'streamlining' to restrict LCA scope: data is obtained from a number of sources to identify environmental hotspots. These hotspots are then subject to further and fuller analysis.
Which LCA methodology should be use?
The most commonly used methodologies for building components and whole new buildings are P?LCA, I/O?LCA and hybrid approaches. In practice it is usual to find a truncated P?LCA approach, whereby the most energy intensive components and processes are captured (e.g. structure, envelope, windows, fit?out) while smaller, less energy intensive components are ignored (e.g. certain fittings, signage and ironmongery). I/O?LCA approaches will, by nature of the methodology, be more inclusive, but run the risk of double counting and data quality issues. Hybrid approaches, which capture much of the building through P?LCA and use I/O?LCA to fill in the 'gaps', have also been used (Treloar et al., 2000b). It is not possible to use I/O?LCA to assess the embodied energy of existing buildings, as I/O data tables are not available for the period in which these buildings were constructed. Upgrades to existing buildings are, normally, most appropriately assessed using P?LCA methodologies.