Fertilización

In the past few decades, scientists and professionals in the building industry have come to realize the importance of considering the whole life cycle when evaluating the impact of residential constructions. Of course, many of the costs associated with home ownership (like maintenance and utilities) are spread over the life of the house, so life cycle cost is a very commonly used metric in performance evaluation. However, some other impacts are traditionally taken into account only for one phase of the life cycle; that is the case for energy use, which is assumed to be significant only during the operation phase of the house (that is, for heating, cooling, ventilation, lighting and appliances). Despite this, many studies have proven that other impacts, such as energy consumption, are significant for the production and end-of- life phases, even when compared to the energy used to operate the house, and should be considered from a life cycle point of view (Itard, 2007, Dixit et al., 2010).

As buildings tend to have lower heating and cooling needs, the embodied energy represents a larger portion of the life cycle energy, sometimes as much as 30 to 60 percent (Gustavsson and Joelsson 2010, Dodoo et al. 2011). Embodied energy in low energy buildings also has a more significant contribution to total life cycle greenhouse gases emissions (Sharma et al. 2011). Furthermore, evaluating energy savings only for the operation phase of the building’s life can be deceiving, as the savings might not be as significant when put in a life cycle perspective (Blengini and Di Carlo, 2010). For those reasons, it is necessary to include life cycle analysis in the optimization process for high performance buildings. The following section looks at the methodology used in a life cycle analysis.

10 2.2.1. Life cycle analysis methodology

Life cycle energy use is expressed in terms of primary energy. Primary energy is defined as the total energy needed to provide a final product or service (Gustavsson and Joelsson, 2010). That energy includes all losses from extraction, transformation and distribution (Sartori and Hestnes, 2007). In the case of the LCE analysis of a house, LCE of all materials (refered to as embodied energy) is usually added to the operation energy required for some of the following tasks: space heating and cooling, domestic hot water, lighting and electric appliances. Which of the energy usage are included depends on each study: for example, it might not be relevant to include appliances energy use if the main focus is the insulation of the envelope, and the appliances do not vary from a design alternative to another. Differences in interpretation and other obstacles in collecting accurate life cycle primary energy use data for buildings are further discussed in Chapter 4.

Life cycle energy analysis is part of a greater process named life cycle assessment, while life cycle costing is a parallel process. Figure 2.1 shows the steps in the life cycle assessment procedure, as defined by the international standard ISO 14040:2006 (International Organization for Standardization, 2006).

11 In the literature, life cycle assessment (LCA) is defined as “a process whereby the material and energy flows of a system are quantified and evaluated” (Ramesh et al., 2010). Goal and scope definition is the setup of the problem, and includes, amongst other information, what metric are going to be studied, the limits of the system and the audience to which the results of the LCA are aimed. Life cycle inventory (LCI) is a critical part of LCA, as it is the step in which all the data concerning the resource, emissions and energy flows from and to the system are gathered. Life cycle assessment is the classification of LCI flows into impact categories and characterization into equivalent units (Trusty, 2010a). Impact measures can be of two natures: either mid-point or end-point. Mid-point measures involve an environmental loading, like acidification potential or green house gases emission. End-point measures aim at quantifying ultimate effects on human or ecosystem health; this proves to be more difficult and scientists do not, as of now, have a consensus on the methodology to follow in this case. Life cycle energy is not a measure of LCIA, but more of an addition of data from LCI (Trusty, 2010b), since in the LCI, phase the flows of materials and energy are determined. While LCE does not account for direct impacts on the environment, many studies have stated that energy is closely linked to environmental depletion (Dincer and Rosen, 1999, Scheuer et al., 2003, Dakwale et al., 2011), and therefore LCE is an interesting metric for life cycle optimization.

Specific primary energy use data is usually extracted from recognized life cycle inventory databases. Because the applicability of the data is highly location dependant (for example, the electricity mix used for the production of a good has a large impact on the efficiency of energy conversion, and therefore, on the primary energy use), it is preferable to use a database specific to the area studied. LCI databases are often included as part of a complete LCA tool. Bribían et al. (2009) have listed the main tools available both for general and building related LCA. In Canada, the most common tool is the Athena Impact Estimator (Athena Institute for Sustainable

12 Materials, 2008), which is used in practically all studies that analyse life cycle energy (Kassab, 2002, Yang and Zmeureanu, 2008, Frenette et al., 2010, Leckner and Zmeureanu, 2011, Van Ooteghem and Xu, 2012.) The most commonly used tool for the rest of North America is SimaPro, but it does include data adapted for Canada (PRé, N/A).