A Green Energy Closed-Loop System Based on Aluminum

This paper presents a focused review of a closed-loop system for sustainable hydrogen production utilizing the reaction between metallic aluminum powders and water, coupled with renewable energy-driven recycling of aluminum hydroxide (or alumina) byproducts back to metallic aluminum powders. A green energy closed-loop system based on aluminum could be achieved if the converting process is accomplished by a green Hall–Héroult process, where a cermet inert anode was used. Meanwhile, the byproduct alumina is converted back to the liquid form of aluminum at high temperature (up to 960 °C), producing pure oxygen. A high-pressure atomization process is then used to break the aluminum droplets into powder using argon gas. The technical feasibility, thermodynamic efficiency, economic viability, environmental sustainability, and comparison of this green aluminum cycle with existing hydrogen production and energy storage technologies are discussed. The aluminum–water reaction offers exceptional energy density (29.7 kJ/g of Al), ambient temperature operation, and zero direct carbon emissions. However, commercial implementation faces substantial challenges including overall round-trip energy efficiency (estimated 34.5–46.6%), technological maturity of the recycling process, passivation layer management, and economic competitiveness with conventional water electrolysis. Despite these challenges, the system demonstrates advantages for seasonal energy storage, off-grid applications, and integration with intermittent renewable energy sources. This analysis provides a framework for researchers, engineers, and policymakers to assess the potential role of aluminum-based energy cycles in the global energy transition toward carbon neutrality.