EMBODIED ENERGY

Section 2.2.3 outlined the resistance by certain industry sectors to thermal performance regulations, one of which was that only energy for heating/cooling was regulated, and not embodied energy. This section explains what embodied is, how it is measured, and its significance in the life-cycle energy of a house.

2.4.1 Embodied energy and how is it measured

The embodied energy of a material or product is the total energy used in its production including upstream activities such a raw material extraction, transport, manufacturing and assembly (Pullen 2007). It comprises direct and indirect energy. The direct energy is the energy used in the manufacturing process whereas indirect energy is the energy used to produce the goods, other materials, and services required for that manufacturing process to occur.

Numerous studies (Pullen 2007, Treloar et al 2001, Noller 2006, Crawford 2005, Baird et al 1997) have shown that differences in embodied energy figures for individual building products, (or m2 of building), are due largely to methodological differences in how embodied energy is calculated. However, they all show that process based figures are lower than those obtained using hybrid methods. While the method used is less important

when comparing which materials have a higher embodied energy, it becomes more important when calculating its contribution to life cycle energy.

There are a several methods used which vary in the comprehensiveness of the system boundaries selected for the analysis. Others (Crawford 2005, Lenzen 2002, Treloar 2000) have described in detail the difference between the methods. A summary of each method, and their advantages and disadvantages, is given below.

Process analysis

Process analysis usually only involves the calculation of the direct energy used in the manufacturing process. In that case, the energy system boundary of process analysis is limited to that which occurs within the boundary of the factory, or manufacturing plant. For some manufacturing processes, such as metals production the direct energy can amount to more than 50% of the total energy consumed (sum of direct and indirect energy), while for other products such as concrete and timber, the direct energy is a much lower proportion due to significant energy inputs in upstream raw materials manufacture (Crawford 2005). Further process analysis can be undertaken, for example of the material inputs into the main product, but this becomes increasingly time-consuming and is rarely undertaken. Process analysis is generally seen as accurate but only within the boundaries that energy is being measured. However, because of this finite boundary system, process analysis can be over 50% incomplete because significant energy contribution outside this boundary can be omitted (Treloar 2000).

Input-output analysis.

Input-output analysis is another method used to estimate embodied energy and uses national average economic data for each sector of the economy. Input-Output (I-O) tables, produced by the Australian Bureau of Statistics every few years, show the direct fiscal inputs for each industrial sector of the Australian economy. Researchers have combined these tables with national energy data from the Australian Bureau of Agricultural and Resource Economics (ABARE) to develop an energy-based I-O model of the economy. The I-O tables are divided into the sectors of the Australian economy, each having a

respective direct energy intensity and total energy intensity expressed in GJ/$ of product. This includes all the direct and indirect inputs of energy from every other sector. It is considered more comprehensive than process analysis because it has a systemically complete boundary system, accounting for both direct and indirect energy inputs (Treloar 1997, 1998: Lenzen, 2002, Crawford 2005). There are, however, disadvantages (Pullen 2007). The method uses national average costs for energy, but the price paid for materials by manufacturers is likely to vary, and in many cases be lower. Lower energy prices would have the effect of raising embodied energy coefficients. In addition, the method uses average energy intensities for a particular industrial sector of the economy. In reality, the energy intensity of sub-groups within that sector is likely to vary.

Hybrid analyses

Due to the limitations of both process analysis and input-output analysis, researchers have developed hybrid approaches. There are two types of hybrid analysis: process based analysis and I-O based analysis.

In process based hybrid analysis, for direct energy, the embodied energy of the individual material inputs are calculated from process analysis and I-O tables are used to calculate indirect energy. These are summed in proportion to the material quantities in the product (Pullen 2007). This type of analysis is more complete than the methods described above. However, as the supply chain is disaggregated to allow the integration of process data, there is the potential that downstream and sideways energy inputs are overlooked Extending the system boundary does not reduce the error to an acceptable level because of the complexity of the supply chain that would have to be investigated (Crawford 2005). The truncation of downstream and sideways energy inputs is depicted in figure 2.3 below.

Figure 2.3 – Truncation of downstream and sideways energy inputs

Source: Crawford, R (2010)

Input-output-based hybrid analysis was developed by Treloar et al (2001) and is currently the preferred method for embodied energy analysis of Australian buildings and building materials because it is more complete than other methods (Crawford 2005). This method addresses the problems of process-based analysis by starting with a disaggregated input- output model to which available process data is integrated. This avoids the possibility for sideways and downstream truncation errors discussed above, in addition to upstream truncation. The direct inputs to a specific product or process are calculated using process analysis. Upstream indirect processes can be accounted for by further process analysis or I- O analysis if it is considered too difficult or time consuming to collect relative to the significance of the process in question. The steps involved in I-O based hybrid analysis are summarized as follows:

(i) Extract the inputs from the relevant sector of the economy from which the product belongs.

(ii) Identify the inputs that have been calculated using process based analysis. (iii) The total energy intensity of each of the inputs represented in the process

analysis is subtracted for the total energy intensity of the sector.

(iv) The remainder of the unmodified inputs (the total energy intensity of the sector minus those inputs subtracted, in GJ/$1000) are then multiplied by the price of

the product ($) and divided by 1000 to give the additional embodied energy for the product, in GJ.

(v) The process based hybrid analysis figure is then added to this figure, minus the direct energy component (this is included in the remainder of the unmodified inputs) to give the I-O based hybrid analysis embodied energy total:

EEt=QM X W X EIM + (TEIn - TEIM)X $BP/1000 (ref)

where EEtis the total embodied energy through I-O based hybrid analysis, QMthe quantity

of materials in the basic product,Wthe wastage multiplier of the respective material, EIM

the material hybrid energy intensity, TEIn the total energy intensity of the I-O sector n, TEIM the total energy intensity of the I-O path representing the basic material, and $BP the

total price of the basic product.

2.4.2 The significance of embodied energy

Life cycle energy analysis (LCEA) of houses is a form of life cycle assessment, which focuses specifically on energy consumption and/or associated greenhouse gas emissions. Typically, it takes a cradle-to-grave approach whereby the energy associated with a house’s manufacture and use phase is assessed up to its end of life. The contribution of embodied energy to life cycle energy has generally been considered insignificant compared to contribution of operational energy. However, as more comprehensive methods of calculating embodied energy have been developed, the actual significance of embodied energy has been understood. Numerous housing LCEA studies have shown this is the case. Treloar et al (2001) found that embodied energy was approximately half of operational energy over a 30-year life. Mackley (1998) showed that savings in operational energy can be outstripped by embodied energy over a house’s life. Thormark (2006) notes that embodied energy can account for 40-60% of total energy use for low energy house over an assumed service life of 50 years.

If embodied energy can comprise half or even 80% of total operational energy over a house’s life, then given the breakdown of household energy use in figure 2.1, it follows that embodied energy could exceed the energy used for heating and cooling. Furthermore, rather than comparing their relative energy contributions, if embodied CO2 emissions and

the CO2 emissions associated with heating/cooling are compared, embodied emissions are

more likely to exceed heating/cooling emissions.