Advances in life cycle sustainability assessment of hydrogen energy systems

Abstract

The transition towards a sustainable energy sector, still dominated by fossil fuels, requires the integration of alternative solutions to fulfil the increasing global energy needs. Hydrogen is seen as a strategic energy carrier characterised by decisive strengths with respect to conventional fuels. For instance, it can be “extracted” from a wide range of feedstocks through several technologies by using energy in different forms (heat, light, electricity). However, techno-economic barriers need to be overcome for the deployment of hydrogen as a new energy actor. In this respect, economic, environmental and social implications shall be taken into account, and following a life-cycle perspective is a fundamental requirement to comprehensively check the overall performance. When evaluating options, sustainability criteria (that involve the pressure on the environment, economic feasibility, and social implications as a set of interrelated aspects) should be an important part of the strategies of governments and companies. However, general solutions to the complex problem of assessing sustainability do not exist, and the singularities of a given system usually require a tailor-made methodological framework. In light of this, this thesis focuses on developing a methodological framework for the sustainability assessment of hydrogen energy systems following a life-cycle perspective. The advances refer to straightening methodological consistency at the level of both sustainability dimensions (when jointly evaluating the environmental, economic and social life-cycle performance of a single case study) and case study (when comparing different case studies). The procedure to define a common Life Cycle Sustainability Assessment (LCSA) framework for hydrogen energy systems started with an extensive literature review on Life Cycle Assessment (LCA), Life Cycle Costing (LCC) and Social Life Cycle Assessment (SLCA) studies of hydrogen. The review pointes out the unbalanced situation in terms of the number of case studies addressing each sustainability dimension. A high range of case studies was found to address the environmental dimension, a significantly lower quantity was found to address the economic dimension, and a scarce number of case studies was found for the social dimension. This unbalanced situation, closely linked to the different level of methodological maturity in each pillar, has also influenced the scope of the methodological advances achieved in this thesis. Within the cloud of LCA studies of hydrogen, considerable differences in methodological assumptions were found between the studies, arising concerns about the actual comparability of the results. In this situation, in order to mitigate risks of misinterpretation and improve the comparability of LCA studies, harmonisation protocols for relevant life-cycle indicators of hydrogen systems (carbon, energy, and acidification footprints) were defined and applied within this thesis. From their application, libraries of harmonised life-cycle indicators were built and are currently available for LCA practitioners willing to robustly contextualise the environmental life-cycle performance of hydrogen. The use of harmonised impacts was proven to be valuable not only for case studies of hydrogen production but also when performing LCA studies with an extended scope (e.g., the use of hydrogen in fuel cell electric vehicles). Regarding the economic dimension, economic life-cycle indicators were first formulated, then calculated for relevant case studies of hydrogen (viz., hydrogen from biomass gasification and steam methane reforming), and finally implemented along with the environmental results in a joint two-dimensional interpretation through the standardised concept of eco-efficiency. In this phase of the thesis, the effect of the internalisation of socio-environmental externalities was also explored. Regarding the social dimension, a procedure to quantify social life-cycle indicators was detailed and consistently applied to the aforementioned case studies. The procedure allowed identifying social hotspots along the supply chain of both conventional and alternative hydrogen from biomass gasification. Finally, the specific procedure to consistently implement the social implications in the LCSA framework as a three-dimensional joint interpretation under sustainability criteria was detailed and applied. Overall, all the methodological developments in this doctoral thesis pave the way towards a sound life cycle sustainability assessment framework to support decision-making processes in the field of hydrogen energy systems. However, further efforts are still required to consolidate the framework, e.g. widening the number of sustainability life-cycle indicators and strengthening the applicability of the framework to systems beyond hydrogen production.