Replacing gas with low-cost, abundant long-duration pumped hydro in electricity systems

Fossil gas is often considered essential for balancing solar and wind beyond 2050, despite its status as a primary source of greenhouse gas emissions. We evaluate seventeen institutional long-term energy plans and find that they skew toward fossil gas because they use temporal aggregation methods that strongly constrain storage cycling, overlook off-river pumped hydro potential, and ignore near-optimal transition pathways. In a post-2050 Australian grid case study, we find near-optimal 100% renewable solutions using an off-the-shelf "segmentation" method. In addition, we develop a planning model that rapidly evaluates millions of near-optimal solutions. We show that batteries and long-duration pumped hydro (48–160 h) can replace gas without incurring a cost penalty, and we stress the importance of energy volume cost (