Comparative life cycle assessment of hydrogen and battery storage pathways

Abstract

As the global energy system transitions toward decarbonization, the integration of variable (VRES) renewable energy sources necessitates effective long-term energy storage solutions. One such concept is the Power-to-Power (P2P), which involves converting surplus electricity from renewables into another energy carrier (such as hydrogen) for storage, and subsequently reconverting it into electricity when needed. While battery storage systems are widely adopted due to their high efficiency and technological maturity, they are limited in their capacity for long-duration energy storage.

This has brought hydrogen into the spotlight as a key energy carrier for the future energy system, to complement the fluctuation of VRES. However, the conversion processes of electricity to hydrogen via electrolysis and hydrogen back to electricity via fuel cells are associated with significant energy losses due to the relatively low efficiencies of current electrolyzer and fuel cell technologies.

Despite these inefficiencies, hydrogen is often promoted as a clean and emission-free energy solution, while little attention is paid to the upstream emissions and environmental impacts associated with the manufacturing and operation of hydrogen infrastructure. Therefore, a comprehensive assessment is crucial to understand the true environmental implications of hydrogen-based energy systems.

This thesis investigates the environmental sustainability of three P2P pathways through a comparative life cycle assessment (LCA), adopting a cradle-to-use perspective. The configurations assessed include: (P2P 1) PEM electrolyser → pressurised hydrogen storage → PEM fuel cell, (P2P 2) PEM electrolyser → metal hydride storage → PEM fuel cell, and (P2P 3) lithium-ion battery + inverter. Each pathway was evaluated based on key midpoint categories (global warming potential, abiotic resource depletion, and marine aquatic ecotoxicity) and endpoint damage assessment indicators.

Life cycle modelling and impact assessments were carried out using SimaPro software, based on a functional unit of 1 MWh of electricity delivered. Among the three P2P pathways evaluated, the battery-based system (P2P 3) showed the lowest global warming potential due to higher round-trip efficiency and reduced electricity use. However, it also exhibited the highest abiotic depletion and marine ecotoxicity impacts due to resource-intensive battery materials. Hydrogen-based systems (P2P 1 and P2P 2) had lower efficiencies and higher use phase impacts. Among the hydrogen-based pathways, the P2P 1 system exhibited lower impacts compared to P2P 2, largely due to the material intensity and processing requirements associated with metal hydride tanks. The obtained results are compared to previously published studies.

This work contributes to the ongoing discourse on hydrogen as an energy storage medium by providing a detailed LCA-based comparison with the battery storage systems. It also offers recommendations for future research and technological development, particularly in refining the LCA scope, through more accurate assumptions, verified data and inclusion of end-of-life processes, in hydrogen and battery-based systems.