Fossil gas is sometimes presented as an enabler of variable solar and wind generation beyond 2050, despite being a primary source of greenhouse gas emissions from methane leakage and combustion. We find that balancing solar and wind generation with pumped hydro energy storage eliminates the need for fossil gas without incurring a cost penalty. However, many existing longterm electricity system plans are biased to rely on fossil gas due to using temporal aggregation methods that either heavily constrain storage cycling behaviour or lose track of the state-ofcharge, failing to consider the potential of low-cost long-duration off-river pumped hydro, and ignoring the broad suite of near-optimal energy transition pathways. We show that a temporal aggregation method based on ‘segmentation’ (fitted chronology) closely resembles the full-series optimisation, captures long-duration storage behaviour (48-and 160-hour durations), and finds a near-optimal 100% renewable electricity solution. We de
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Replacing Gas with Low-cost, Abundant Long-duration
Pumped Hydro in Electricity Systems
Timothy Weber1 (corresponding author: Timothy.Weber@anu.edu.au): Conceptualisation,
Writing – Original Draft, Software, Data Curation, Visualisation, Method, Investigation, Formal
Analysis
Cheng Cheng1: Software, Visualisation, Method, Writing – Review & Editing
Harry Thawley1: Software, Method, Writing – Review & Editing
Kylie Catchpole1: Writing – Review & Editing, Visualisation
Andrew Blakers1: Writing – Review & Editing, Data Curation
Bin Lu1: Writing – Review & Editing
Jennifer Zhao1: Writing – Review & Editing
Anna Nadolny1: Writing – Review & Editing
Summary
Fossil gas is sometimes presented as an enabler of variable solar and wind generation beyond
2050, despite being a primary source of greenhouse gas emissions from methane leakage and
combustion. We find that balancing solar and wind generation with pumped hydro energy storage
eliminates the need for fossil gas without incurring a cost penalty. However, many existing long-
term electricity system plans are biased to rely on fossil gas due to using temporal aggregation
methods that either heavily constrain storage cycling behaviour or lose track of the state-of-
charge, failing to consider the potential of low-cost long-duration off-river pumped hydro, and
ignoring the broad suite of near-optimal energy transition pathways. We show that a temporal
aggregation method based on ‘segmentation’ (fitted chronology) closely resembles the full-series
optimisation, captures long-duration storage behaviour (48- and 160-hour durations), and finds
a near-optimal 100% renewable electricity solution. We develop a new electricity system model
to rapidly evaluate millions of other near-optimal solutions, stressing the importance of
modelling pumped hydro sites with a low energy volume cost (
1Affiliation: The Australian National University, Canberra ACT 2601
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Context & scale
Biases in models used to develop long-term electricity system plans have fuelled the notion that
fossil gas ought to be expanded to support the renewable energy transition. Common methods
of simplifying multiple decades of high-resolution data to reduce computational complexity fail
to capture long-duration storage behaviour by either losing track of the state-of-charge or heavily
constraining the charging/discharging behaviour. Coupled with out-dated assumptions regarding
pumped hydro storage duration, costs, and availability, these electricity system models have no
alternative but to rely upon gas for balancing solar and wind generation. An alternative to gas
already exists in the form of pumped hydro, which remains the lowest-cost option for long-
duration energy storage (overnight storage and longer) and is abundantly available around the
world (over 800,000 potential sites). A combination of pumped hydro and batteries is capable of
providing the same frequency control, voltage control, black start, and inertia-related services as
gas generators.
We show that with suitable methods for simplifying the input data and updated pumped hydro
assumptions, long-term planning models can find reliable 100% renewable electricity systems
at a similar cost to gas-dependent systems. Using a new model with high computational speed,
we show that there is a wide variety of “near-optimal” clean energy development pathways. The
cost of these 100% renewable electricity pathways ought to be based on large-scale pumped
hydro with a long economic life, low energy volume cost, and a low real discount rate equivalent
to regulated natural monopolies such as transmission. Fossil gas is a primary cause of climate
change, both from methane leakage and from carbon dioxide due to combustion. With sufficient
policy and financial support from governments to develop large-scale long-duration pumped
hydro systems, fossil gas can be entirely replaced within the electricity systems of the future.
Costs are in 2025 US dollars.
Keywords: Energy planning; long-duration energy storage; temporal simplification; pumped
hydro; gas; 100% renewable energy3
Main
Long-term electricity system planning models are used to identify grid configurations that
achieve the objectives of the energy planner. About 89% of global net generation capacity added
in 2024 was either solar photovoltaics (PV) or wind, with PV capacity doubling roughly every three
years since 2015 [1, 2, 3]. Models of energy systems with a high penetration of solar PV and wind
must capture spatial [4] and temporal [5, 6, 7] variation in renewable generation. This means that
long-term planning model
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