Life cycle assessment of the transmission network in Great Britain
Introduction
Carbon dioxide (CO2) emissions, along with cost and reliable energy supplies are the main concerns for electricity utilities. All major generators and network companies in the UK are part of the European Union Emissions Trading Scheme, which aims to lower emissions under a cap-and-trade arrangement. This offers risks for companies who emit more than their allowance while offering opportunities for those able to reduce emissions. There is accordingly interest in the emissions associated with operation of power plants and network infrastructure. At the same time transmission network companies are under continued regulatory pressure to keep costs as low as possible, often focussing on minimising capital expenditure. Decisions over network infrastructure often have significant implications for life time costs, energy use and carbon emissions. As such, there is a need to see operational emissions in the context of the whole network life cycle.
Transmission network infrastructure comprises substantial volumes of materials that consumed energy and emitted CO2 during their manufacture and installation. Transmission losses throughout their operational lifetimes mean that networks also consume energy, resulting in additional CO2 emissions from power plants. They also require routine maintenance and refurbishments that also consume energy and result in CO2 emissions. Finally, there are energy and carbon implications when decommissioning infrastructure at the end of its useful life. Such aspects can be captured as part of a life cycle assessment (LCA).
Previous LCA work in the electricity industry has predominantly focussed on electricity generation technologies, comparing the energy generated and carbon emitted over the operational lifetime relative to the energy and carbon required to procure the fuel, build, operate, and decommission the power plants. Extensive studies show that renewable energy technologies including wind, wave and tidal stream, have significantly lower life cycle CO2 emissions than fossil fuelled electricity generation (see, e.g., Douglas et al., 2008; Fthenakis and Kim, 2007; Kim and Dale, 2005; Meier et al., 2005; Parker et al., 2007; Vestas, 2005; Weisser, 2007).
In contrast, there is only a modest amount of work reported on electricity networks. Vattenfall (1999) summarises an analysis of the Swedish system across multiple voltage levels while others concentrate on specific assets or asset classes. Cigre Working Group B2.15 (2004) reports assessments of overhead transmission lines (OHL) while Jones and McManus (2008) provide a detailed comparison of embodied energy and carbon for a hypothetical one kilometre-long section of 11 kV rural distribution circuit with alternative OHL and cable constructions. Comparisons have also been published for oil-filled and cross-linked polyethylene (XLPE) cables (Drugge et al., 1996). Several manufacturers publish LCAs in the form of environmental product declarations (EPD) for specific assets, e.g. ABB (2003). Many LCAs focus on sulphur hexafluoride (SF6) gas in circuit breakers (Bessede and Krondorfer, 2000; Neumann et al., 2004; Preisegger et al., 2001). Due to excellent electric arc-quenching and insulating properties, SF6-based circuit breakers are generally replacing older technologies (Preisegger et al., 2001). The interest arises as SF6 has a global warming potential 22,800 times that of CO2 (over a 100-year time horizon; Forster et al., 2007) and that significant amounts of gas leak each year, particularly from older types of circuit breakers. Neumann et al. (2004) examined the relative merits of air insulated (AIS) and SF6 gas insulated switchgear (GIS) within representative 10–110 kV German distribution networks. It found that despite the release of SF6, GIS-based systems performed better, primarily due to the ability to site compact, indoor, GIS substations close to urban load centres. This allowed very different network topologies with fewer substations and shorter higher voltage circuits that reduced overall material use but, more importantly, significantly reduced power losses. Other studies concur that losses tend to dominate both energy use and CO2 emissions.
This paper presents a detailed account of the energy and CO2 impacts of the entire high voltage electricity transmission network in Great Britain (GB). Although it deliberately applies some simplifying assumptions to achieve broad yet representative coverage, it provides a credible first approximation and identifies future research and regulatory challenges.
Section snippets
Life cycle assessment
Life cycle assessment (LCA) is an analysis of all the environmental impacts of a product, process or service from the extraction of raw materials, through production and use to eventual disposal. LCA studies should comply with the international standard ISO 14040 series (ISO, 2006), which specifies the general framework, principles and requirements for conducting and reporting such assessments. LCA can be used as a design improvement tool to identify the environmental factors attributed to
Procedure
The raw materials used in the network infrastructure have been analysed directly in terms of their materials and mass. Where available, use was made of existing LCA studies of specific assets, by re-engineering them to suit UK embodied values based on the material content and energy use in manufacturing. Power losses during asset use are discounted as these are dealt with for the network as a whole.
The infrastructure is rather eclectic and represents the changes in transmission technology over
Energy and carbon intensity
The energy and carbon intensity offer a convenient means of comparison between networks and other energy delivery mechanisms. These are calculated by dividing the overall embodied energy and CO2 emissions by the electricity transmitted over the 40 year life cycle. The annual volume of electricity transmitted in 2006/07 was 350 TWh (1260 PJ; National Grid 2008). Assuming that, this losses and generation mix remain constant, the energy intensity of the transmission network is 190 kJ/kWh and the
Conclusions
This paper presented a detailed account of the energy and CO2 impacts of the entire high voltage electricity transmission network in Great Britain. It showed that the multi-millions of tonnes of materials used in the infrastructure had a substantial amount of embodied energy and CO2, but that network losses had an impact that was two orders of magnitude more significant. Overall, the network was seen to transmit around nineteen times more energy than is embodied and its carbon footprint is of
Acknowledgements
This work was part-funded through the EPSRC Supergen V, UK Energy Infrastructure (AMPerES) grant in collaboration with UK electricity network operators working under Ofgem’s Innovation Funding Incentive. The authors would like to thank Dr Mark Osborne at National Grid plc for extensive assistance as well as Gordon Kelly of Scottish Power and Dr Keith Bell of the University of Strathclyde for help in defining typical infrastructure. The assistance of staff at ABB, AMEC, Elexon, Lovat and Murphy
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