The road up ends at a rim of loose red rock, and the ground beyond it simply stops.
Inside the bowl there is a black liner, welded in strips, and a flat sheet of water sitting where a volcanic vent used to be.
A pipe on the low side is running.
Nothing here looks like a power station, and yet the water in that bowl is the only reason the island below goes a whole day without diesel. Which leaves the question of how a hole in a mountain becomes a battery.
How a hole in a mountain stores electricity
You cannot store wind, so you move water instead.
When the turbines make more than the island is using, the surplus does not go to the grid. It goes to a bank of pumps that push water up the mountain into the crater.
That climb is the storage.
Every gallon sitting 2,300 feet up is potential energy waiting, and when the wind drops the valves open and the water comes back down through Pelton wheels. The round trip loses a large share of what went in, which is the price of storage and always has been.
The harder problem is not energy, it is frequency.
An island grid has no neighbor to lean on, so if generation and demand slip apart the whole thing collapses. The hydro machines carry speed regulators and heavy flywheels for that reason, giving the system enough mechanical inertia to ride out a gust a wind farm alone would not survive.
Five turbines, two tanks and two pipes
The wind farm is five machines.
Each is rated at 2.3 megawatts, for 11.5 across the ridge, a small farm by any mainland standard and an enormous one relative to this island.
The upper reservoir holds about 100 million gallons.
It sits in the crater, lined against leakage, feeding a lower tank near the coast that takes about 40 million gallons. Two steel pipes just under 2 miles long and 31 inches across connect them.
At the bottom sit four Pelton turbines.
Together they are rated at 11.3 megawatts, and facing them are the pumps that send the water back up, two of 1,500 kilowatts and six of 500, for 6 megawatts of lifting capacity. Everything else on the site exists to serve those two directions.
The record, and the line since
The island ran entirely on wind and water for 24 consecutive days, ending in August of 2019.
That figure comes from the national grid operator rather than from the plant, which matters, because the operator measures the island the way it measures everything else.
The trend since is the other half of the story.
Hours at full renewable supply came to 1,328 across 2021. The following year it was 1,008. The year after that it was 413 hours, which is about seventeen days of credit spread thinly over twelve months rather than seventeen days in a row.
Diesel saved tracks the same curve downward.
Roughly 6,600 tons of fuel were displaced in the first of those years and about 5,000 tons in the third. The plant has been in commercial operation since the middle of 2015, so this is not a startup problem.
What the island still burns
Across a full year, the system has never covered even half the island’s electricity.
The diesel station is not a museum piece. It runs often, and the wind and water plant is best understood as a very large fuel saver rather than a replacement.
Three desalination plants are part of why.
Fresh water on a volcanic rock comes out of a membrane, and membranes draw steady power whether or not the wind blows, which puts a hard floor under demand that a 100 million gallon crater cannot always meet.
The bill was substantial.
The plant cost between about 75 and 95 million dollars depending on whose accounting you take, several thousand dollars per installed kilowatt. The operator publishes the annual numbers, and the grid operator confirmed the 24 day run.
What the crater actually proves
About 11,000 people live here, on roughly 100 square miles of lava.
That is the scale at which this works, and the scale is not a detail. A grid this small can be carried by one crater and five machines in a way no city grid could, and it is also small enough that three water plants dominate the load.
The interesting result is not the record.
It is that a place with no interconnector, no gas and no nuclear can hold its own frequency on rotating machinery filled by wind, for weeks, without importing a single watt. Turbines keep turning up in awkward places, from painted blades to a flying machine over an old runway.
What the crater cannot do is get bigger.
More storage would raise the ceiling, and nobody has committed to building it. Until somebody does, this pumped hydro stays what it has been since the first day it carried the island, a genuine achievement and a partial one.
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