Life-cycle assessment of diesel, natural gas and hydrogen fuel cell bus transportation systems
Introduction
The Sustainable Transport Energy Programme (STEP) is an initiative to examine alternative transport fuels for Western Australia (WA). The project includes three buses manufactured by DaimlerChrysler, which operate with fuel cell engines from Ballard Power Systems, and a raw hydrogen supply provided by the BP Kwinana oil refinery. The STEP trial is in partnership with the Clean Urban Transport for Europe (CUTE) trial, the Ecological City Transport System (ECTOS) trial in Iceland, and the Global Hydrogen Bus Platform (HyFLEET:CUTE) [1].
Recent work has evaluated the potential for the establishment of a hydrogen economy in Australia [2] and current activities in the field [3]. These studies have presented a qualitative overview. A policy framework for hydrogen has yet to be established. There is a recognized need for detailed quantitative analysis and testing [4].
The Government of Western Australia, through the Department for Planning and Infrastructure (DPI), has commissioned several research projects to develop academic knowledge and expertise from the fuel cell bus trial. The life-cycle assessment (LCA) is one such project, that is aimed at evaluating the hydrogen infrastructure and fuel cell buses in relation to the existing diesel and natural gas transportation systems. Based at Path Transit's Morley Bus Depot in the Malaga suburb of Perth, the fuel cell buses are operating in regular service alongside the conventional Transperth natural gas and diesel bus fleets.
The life-cycle models are designed to be flexible and thereby allow for future scenario analysis that examines different primary energy sources, fuel production processes, and expected improvements in technology. Concepts for sustainable bus transportation can be incorporated using the methodologies and boundary conditions defined during the project. Continued efforts to develop and refine these models can identify industry opportunities, as the entire product life-cycle moves towards optimization, and important problems are resolved in the early stages of the emerging hydrogen economy. The knowledge gained from this research may be used to define the direction of future programmes and policies.
The application of LCA and similar well-to-wheels (WTW) methods to hydrogen fuel cell vehicles has become an active field of research. A precursor and important basis for this study is the research conducted by Faltenbacher et al. [5], [6] as part of the CUTE trial evaluation. The preliminary results from the CUTE trial, reported in [5], [6], provided a base of methodology for the subsequent LCA work on the CUTE, ECTOS and STEP trials. In a recent literature review [7], several common deficiencies in the hydrogen futures literature were raised. Many studies lack participation from stakeholders and use a top-down theoretical approach with little discussion of the issues experienced by technology on the ground. These issues are categorically addressed by this study through the use of data provided by the participating companies and collected from the field results of the STEP fuel cell bus trial.
Research on the capabilities of hydrogen fuel cell technology in relation to conventional and other alternative transport solutions has been undertaken in the LCA context using a variety of methods. The Comparison of Transport Fuels conducted by Beer et al. [8] referenced the GREET model and examined a very broad range of transport fuel alternatives. The only hydrogen pathway examined was production from steam reforming of natural gas, which is just one of the many possible pathways. Colella et al. [9] evaluated the change in emissions and energy use from an instantaneous change to a hydrogen fuel cell vehicle fleet. Granovskii et al. [10] conducted an LCA of hydrogen fuel cell and gasoline vehicles using a first-principal methodology, which was based on theoretical calculations of the required economic and energetic data. Zamel and Li [11] performed an LCA of fuel cell and internal combustion engine vehicles in Canada, with fuel-cycle calculations carried out using GREET [12] and vehicle cycle data derived from published literature. General Motors conducted two WTW studies [13], [14], one based in North America and the other in Europe, of which the latter examined a total of 88 fuel supply pathways including 14 hydrogen-based pathways. The GM studies did not include hydrogen sourced as a by-product of petroleum refining, which is the case in the STEP project. Pehnt [15] made an LCA of fuel cell stacks in accordance with ISO methodology and included discussion of allocation rules regarding Platinum Group Metals (PGM's) and recycling concerns. Ahluwalia et al. [16] and Schäfer et al. [17] have reported performance expectations for future fuel cell vehicles, together with a range of results due to the large uncertainty associated with both this developing technology and the specific boundary conditions chosen for each study.
There is a need for present-day LCA results, which adhere to internationally accepted methodology standards, to indicate the current state of the technology and highlight the issues from an operational trial. The LCA research conducted in Perth aims to address this need and to develop a set of validated models that can be used for scenario and sensitivity analysis.
Section snippets
Methodology
The premise for LCA studies is the comprehensive evaluation of all energy and material flows through a product system over its entire life-cycle. A system boundary is defined which encompasses the important processes of the product system and specifies the scope of the study, as shown in Fig. 1. Energy and material flows across the system boundary are accounted for in the LCA, and the processes contained within the LCA are studied in detail. The conservation of mass and energy across the system
Goal and scope definition
The objectives for this research are
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evaluation of the environmental impacts and energy demands of the hydrogen fuel cell bus transportation system life-cycle;
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parallel comparative evaluation of the established diesel and natural gas bus transportation systems;
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scenario analysis examining different technologies and the impact of future technological improvements.
These objectives are conducted with an aim to provide input to the strategic decision-making process for future transport energy policy,
Life-cycle inventory
The collection of data that describes the systems to be examined is termed the life-cycle inventory (LCI). In compiling the LCI, each material and energy flow through each process and across the system boundaries must be carefully enumerated. For complex product systems, this can be an enormous task. PE Europe GmbH has provided the GaBi 4 software system and datasets on material and energy flows, which greatly reduces the data-collection workload for common industrial processes [22].
Life-cycle impact assessment (LCIA)
A main objective of the LCA is to determine the outputs to the environment by calculation of the material and energy flows. Outputs with similar environmental impacts can be grouped and aggregated into a single parameter, known as an ‘impact category’. As stated in ISO 14042 [20], if comparative assertions from LCIA are disclosed to the public they should be internationally accepted impact categories, and be environmentally relevant to the spatial and temporal context.
The impact categories
Interpretation
The life-cycle of each bus transportation system was modelled individually, and the results compared with respect to a functional unit of vehicle kilometres. The average bus in Perth travels 55,000 km annually, with a lifetime of 16 years [33].
The life-cycle impacts for each of the selected impact categories, as well as the overall energy demand, are shown in Fig. 3. As expected, tailpipe emissions generally dominate the diesel and CNG profiles, while fuel production dominates the hydrogen
Key parameters for an improved life-cycle profile
This project has established a benchmark LCA model, which can be applied to a wide range of scenarios and advanced modelling applications. The assessment clearly shows the relative magnitude that each process has on the overall environmental profile and thus provides feedback to identify the critical processes that need to be addressed.
It has been noted in several publications that renewable energy would achieve a greater reduction in GWP by displacing existing fossil-fuel generation systems,
Conclusions
The hydrogen infrastructure implemented in Perth provides a measure of the current state of technology, and a benchmark that can be used to measure future progress. The LCA results highlight the key areas for future research, and a realistic scenario analysis has shown how technological developments can affect the overall life-cycle profile of the transportation system.
This research can be used for strategic decision-making on the future of transport energy policy, and can also be developed
Acknowledgements
The authors acknowledge the support of the Department for Planning and Infrastructure of the Government of Western Australia, Ballard Power Systems, BOC Gases, Linde, BP and Path Transit. Valuable contributions were also made by Marc Allen, Alan Bray, Paul Burke, Colin Cockroft, Brendan Davis, Michael Faltenbacher, Geoff Grenda, Glen Head, Christine Howland, Robert Ilg, Ian Kerr, John Lambert, Brian Marsden, Manfred Schuckert, Emma Vanderstaay, Simon Whitehouse, and Tim Woolerson.
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