Energy everywhere · banking it

Storing power for the dark and the calm

The sun sets and the sea sometimes goes flat. None of the harvesting on this ladder matters without somewhere to put the surplus for later, and there are many honest ways to do it: water lifted uphill, sand heated in a silo, iron left to rust, air chilled to a liquid, steel spun in a vacuum. The honesty gets banked too: every option shows its real round-trip loss, not just its capacity, because a battery that quietly throws away most of what you put in isn't free, it's just differently expensive.

Start with water uphill Compare them all

The biggest battery of all

Water uphill.

Known Speculative

Pump water up to a higher pond when power is spare, and let it fall back down through a generator when it's needed. It sounds almost too simple, and yet pumped hydro is by far the largest and longest-proven grid storage on Earth, holding more energy worldwide than every chemical battery combined. The turbine sits sealed inside a steel pipe and a concrete powerhouse, never in open water where anything swims, so it passes this ladder's one rule: no blade a living thing can reach.

Reference system	What it is	What it teaches
Snowy 2.0	A giant pumped-hydro build linking two existing dams through tunnels	What long-duration storage really costs in tunnels, patience and public trust
Wivenhoe	Queensland's own pumped-hydro station beside Brisbane's water supply	That stored height can answer the evening peak close to home
Kidston	A new store built inside the pits of an old, abandoned gold mine	The lesson this island cares about most: a dug-out void becomes the reservoir
Borumba	A major new Queensland pumped-hydro project in development	The depth of modelling, consultation and care a serious idea gathers before a shovel moves
ANU RE100 atlas	A global map screening the whole planet for pumped-hydro sites	How to weigh height, water, distance and duration for any place, a sand island included

Straddie as Straddie

Three ways the island asks it differently.

The island is not the Snowy Mountains. It has no rock gorges and no snowmelt. But it has its own map, and three honest answers to the same question.

Dune ridges and perched lakes

the island's own height

The high dunes give real height to work with, and the perched lakes already sit above the water table. A pair of small ponds, one up, one down, is the gentlest version, small enough to model on a school rig before anything bigger is even sketched. Sacred and protected waters are ruled out first, not last.

Tank pairs and dune height → small, modellable pumped storage
SpeculativeThe physics is textbook; the island-specific siting, and the absolute protection of the natural lakes, is the whole design problem.

The ocean as the lower pond

seawater pumped hydro

If the sea is your lower reservoir, it never runs dry and never needs filling. Pump seawater up to a coastal-dune pond, let it fall back to the ocean through a sealed turbine. A plant ran exactly this way in Japan for years, and more are being planned in Europe.

Coastal pond + the sea below → a bottomless lower reservoir
ModelledProven at one site overseas; salt corrosion and any seepage toward the lens are the honest engineering questions, settled in the twin before anything is dug.

The reservoir the tunnels leave behind

underground pumped hydro

Here is the elegant one. The tunnelling work digs voids on its way to making the city; the same voids, sealed and deep, can be the lower reservoir, with a pond up on the surface as the upper one. The machine that builds the city also builds the battery, and Kidston already proves a dug-out mine can hold the water.

Surface pond + a sealed void below → a battery made of the tunnels
SpeculativeThe deepest tie between parts of the whole ladder: read it alongside the tunnels page. Depends entirely on the aquifer model coming first.

The honest comparison

Every store, and what you actually get back.

Known

Round trip means: put in 100 units, get this many back. Nobody selling storage leads with this number, so here it leads.

Store	What it holds	Round trip	Precedent
Sand thermal battery	Heat, around 500-600°C, in an insulated silo of the island's own sand	Excellent as heat; poor back to electricity (~25-30%), and that conversion step isn't demonstrated at this scale	Polar Night Energy, Finland: heating a town since 2022, on air, delivering heat rather than electricity
Molten salt	Heat, around 290-565°C, in a tank of nitrate salts	Good as heat; similar losses to sand if converted back	Concentrated solar plants worldwide, incl. Kogan Creek, QLD
Vanadium flow battery	Electricity, directly, in tanks of liquid electrolyte	Good, roughly 70-80%	Grid-scale installations expanding across Australia
Lithium battery	Electricity, directly, in a sealed cell	Very good, roughly 90%+	Household and grid batteries everywhere today
Compressed air (in a sealed cavity)	Air pressure, pumped into a sealed underground space	Moderate, roughly 50-70% depending on heat recovery	Operating compressed-air plants exist overseas; local caverns need engineering review
Hydraulic accumulator	Pressurised fluid, charged by storm surge on the coast	Good over short bursts; not a long-duration store	Standard industrial hydraulics; the storm-lock mode on the ocean energy page
Pumped hydro	Water lifted to a higher pond, released down through a sealed turbine	Good, roughly 70-80%; the world's biggest and longest-proven store	Snowy 2.0, Wivenhoe and Kidston, Australia (see above)
Gravity (lifted mass)	Heavy blocks or sand hoisted up a shaft, dropped to generate	Very good, roughly 80-85%; never degrades with age	Commercial towers running overseas; a natural fit for a shaft in the dunes
Iron-air battery	Electricity, stored by rusting and un-rusting iron, for days at a time	Lower, roughly 50%, but dirt-cheap and made of the commonest metals	First grid-scale plants being built in the United States now
Sodium-ion battery	Electricity, like lithium but built from sodium out of common salt	Very good, roughly 90%; cheaper and safer than lithium	Reaching commercial production; salt is something the sea gives freely
Green hydrogen	Water split into hydrogen when power is spare, burned or fuel-celled back	Low as electricity (~30-40%), but stores for months and doubles as fuel and feedstock	Pilot plants worldwide; ties to the alchemy page's chemistry
Liquid air (cryogenic)	Air chilled to a liquid when power is spare, expanded back through a generator	Moderate, roughly 50-60%; shares its kit with the cold chain	Grid plant operating in the United Kingdom; see the breath of the city
Flywheel	A heavy rotor spun up inside a sealed vacuum, for seconds to minutes	Excellent, roughly 85-90%, but empties fast: it steadies the grid, it doesn't run the night	Standard grid-stabilising kit; sealed, nothing exposed

The honest reading of this table: use heat stores for heat needs (hot water, process heat, warming a room), and electricity stores for electricity needs. Trying to force one to do the other's job is where the real losses hide. The sand battery isn't a bad electricity battery by accident; it's an excellent heat battery being asked the wrong question.

Where storage meets alchemy

A battery is just a paused chemical reaction.

Follow the thermal loop on the alchemy page and storage stops being a separate topic: every furnace and reactor on this ladder rejects heat, and the sand silo is where that heat waits instead of leaking into the sky. Storage isn't a bolt-on. It's the alchemy page's patience, given a physical shape.