Development of a construction sub-sector embodied energy hybrid analysis
Document Type Working Paper
Background, Aims and Scope:
Embodied energy analysis is used to evaluate the total energy consumed by any product during all the stages leading up to its manufacture and delivery and can also be used to determine the energy-related environmental impacts such as CO2 emissions of buildings and other built infrastructure. In the wake of increase global awareness on climate change and the strong link between global warming and CO2 emissions, the role of new and improved analytical models to evaluate the energy embodied in products and its associated environmental impacts therefore takes an important role in environmental research studies. The development of a new hybrid embodied energy analysis model which methodologically improves the accuracy of the energy intensity of the construction sector is presented in this paper. This hybrid methodology is applied to four built infrastructure and their respective energy intensities determined. The four construction projects are; a bridge and three different building types. The building types are- a 3-bedroom terrace house, a 3-bedroom semi-detached house and a 4 bedroom detached house all in Dublin, Ireland. The bridge is for a railway line spanning Cork-to-Midleton in County Cork, Ireland. A variability and uncertainty analysis termed applicability error is carried out to determine the inaccuracy in using the sectoral or average construction sector energy intensity rather than the sub-sectoral energy intensities.
Material and Methods:
In this paper, a new hybrid embodied energy analysis which combines process analysis inventory and input-output analysis is proposed – the latter being undertaken at a disaggregated construction sub-sector level rather than at an aggregated sectoral level. The construction sector is divided into five different sub-sectors with each having different input-output energy intensities and accounting for different construction activities. When embodied energy analysis is carried out at the construction sub-sector level, there is a methodological improvement in the calculated values for the direct input-output as well as the total energy intensities over other traditional hybrid methods because of the use of disaggregated sub-sector construction data. Moreover, this hybrid methodology ensures that the input-output energy intensity applied to any built infrastructure is unique and specific to it rather than applying a construction sector input-output energy intensity which is the same for all construction products as other methodologies do. The paper proposes further improvements to hybrid methodologies which improve the accuracy of embodied energy results through the introduction and use of disaggregation constants in the input-output model. Disaggregation constants disaggregate an aggregated energy supply sector in the national input-output tables into different sub-sectors consisting of energy supply sector(s) and other commodity sectors. Finally, an error or uncertainty analysis termed applicability uncertainty is carried out to establish the error that will occur if the average energy intensity of construction is used rather than the energy intensity specific to the railway bridge or the different building types.
The input-output total energy intensity of Irish construction evaluated at the sectoral level was found to be 0.003647 GJ/€ while that of the four case studies; the bridge, the 3-bedroom terrace house, the 3-bedroom semi-detached house and the 4-bedroom detached house calculated at the construction sub-sector level were found to be 0.002686GJ/€, 0.002398GJ/€, 0.002408GJ/€ and 0.002397GJ/€ respectively. The applicability errors of the bridge, the 3-bedroom terrace house, the 3-bedroom semi-detached house and the 4-bedroom detached house was also subsequently found to be 29%, 44.5%, 43.9% and 44.6% respectively. It was also established that input-output analysis carried without the use of disaggregated constants can be over 3.5 times greater.
Each of the five sub-sectors of construction analysed using the hybrid methodology have different input-output energy intensities. This ensures that each activity or process undertaken during the construction of any structure is correctly allocated to a construction sub-sector and the accuracy of the calculated energy intensities enhanced. This is therefore a methodological improvement compared to other hybrid models in the determination of the input-output energy intensity of construction.
Uncertainties and errors in aggregated input-output construction data can be significant when compared to disaggregated sub-sectoral construction data and can greatly affect the results of embodied energy results. The use of disaggregated sub-sectoral data results in a decrease of between 29-44.6% in embodied energies in the case studies analysed. The inclusion of disaggregation constants to disaggregate the energy supply sectors significantly changes the input-output energy intensities. Although different building and infrastructure types belong to the same input-output sectors, this hybrid model ensures that each has unique total energy intensity.
Recommendation and Perspectives:
In order to improve the accuracy of hybrid embodied energy models, two levels of disaggregation, that is, the disaggregation of energy supply sectors and the disaggregation of the construction sector must be carried out during the analysis of input-output data