Embodied energy intensity extraction methodologies

The development of embodied energy intensity extraction methods has significantly advanced the understanding of the importance of embodied energy to the life cycle energy balance of a building and the potential for energy savings through materials selection. Questions remain, however, as to the reliability of embodied energy intensities due to significant variance in published data. These differences are largely the result of the various extraction methodologies and reporting conventions. This has contributed to the construction industry’s hesitancy in adopting and implementing wide-ranging energy use- reduction measures (Mackley, 1999).

There are three main methods of embodied energy intensity extraction: process-based, input–output-based and hybrid analysis. They vary significantly in terms of their system boundaries and reporting conventions, and this has led to significant confusion among researchers and within the industry. These methods and their relative advantages and disadvantages are briefly discussed below.

6.4.1 Process analysis

This can be described as embodied energy analysis that uses ‘any other source of information other than input–output tables’ (Treloar, 1997). It is generally accepted (Pullen, 1995; Alcorn, 1995; Treloar, 1996a) that its major advantage is the degree of accuracy possible for the precisely defined system to which it relates (e.g., the production of kiln-dried timber from a particular mill). Published figures derived by this method are, however, incomplete for a number of reasons:

䊉 it is difficult to account for direct energy inputs that occur more than two stages upstream of the process being analysed, and these are therefore not included in the calculations

Table 6.1 Ratio of life cycle operational energy to embodied energy*

Range Non-efficient Efficient

House Upper 5.8:1 3.6:1

Lower 10.8:1 5.1:1

Office Upper 10:1 1.5:1

Lower 8:1 2:1

䊉 indirect energy associated with the provision of capital goods required for the process both directly and upstream from the process are usually excluded

䊉 the energy lost in energy conversion processes (e.g., the energy lost as waste heat during electricity generation) is excluded from the calculation – if primary energy were to be reported, rather than delivered or end-use energy, then for many common building materials the energy intensity may be 30 to 40% greater (Pears, 1995).

6.4.2 Input–output analysis

This method uses a mathematical approach in an attempt to sum all of the upstream energy requirements, and so is a more complete method than process analysis. There are, however, problems related to the validity of the results as a range of assumptions must be made in regard to the homogeneity of some or all of the sectors of the economy relevant to the analysis, the energy tariffs paid by the various sectors, and the prices of the commodities that each sector produces. Most sectors of the economy are not homogeneous and consequently significant variations in embodied energy intensities may appear when the results obtained using this analysis are compared to those derived by applying process analysis to individual producers/manufacturers.

6.4.3 Hybrid analysis

As the name suggests this method is a combination of the previous two. It utilizes ‘the completeness of input–output analysis and the reliability of process analysis’ (Treloar, 1997). The significant energy pathways are identified and analysed using input–output analysis, then data for a specific process is used to calculate the energy intensity of a particular material. This avoids many of the problems that arise when either method is used in isolation. Importantly, the results are reported in terms of primary energy use and potential double counting of direct energy inputs is avoided. This method can produce results that are up to 90% complete (Treloar, 1998), although there are still unresolved problems related to energy tariffs, primary energy factors and sector homogeneity.

In spite of these remaining questions the major advantage of this method is the consistent quality of the data, and the energy intensities that are produced provide the most complete and constant indicators of the energy required to produce individual materials on a national basis.

Unfortunately these methods generally lack credibility in the construction industry, partly because few people have a clear understanding of the methodologies involved. Treloar (1996b) suggests that it is the ‘black box’ nature of the processes that makes the uninitiated suspicious of them. They are also questioned because they provide average material intensities but make no allowance for the relative efficiency of individual manufacturers. Ironically this is the greatest advantage of hybrid analysis.

It is evident that the validity of any reported embodied energy figures will depend on the method of analysis used, and on the system boundaries being clearly defined. This is particularly important if comparisons are to be made between embodied energy figures from a variety of sources. Table 6.2 demonstrates this point clearly: embodied energy intensities for rough processed timber, calculated by a number of different analysts are listed.

The differences in these figures highlight the problem. They vary by a factor of 5 from a low of 1.4 MJ/m3_{to 7.0 MJ/m}3_{. An average brick veneer home of 120 m}2_{, with timber}

frames to the inner half of external walls, all internal walls and roof/ceiling but excluding timber for trims and doors, contains an average of around 8 m3_{of timber framing. This}

would produce figures for total capital embodied energy for timber ranging from 11.2 GJ to 56 GJ. Pullen (1995) suggests that the average total capital embodied energy for typical residential buildings is 4–6 GJ/m2_{– based on this average the variance shown above for}

timber alone would produce an error of ± 0.37 GJ/m2_{. In the case of a 4 MJ/m}2_{house this}

would result in an error of ± 9.3%; for a 6 MJ/m2_{house the error would be ± 6.2%.}

The potential for further compounding differences is enormous given that there are a multitude of individual parts in each building, and similar variances could be expected for each part.