Based on some extremely back of napkin math (e.g. doing one of those activities where you cycle to power a light bulb) that seems pretty far off. Was the flow rate extremely low? Have a link?
250 litres x 7 metres is 1750 kilogram-force-metres.
Wolfram Alpha [0] says this is about 5 Watt-hours, and gives some other handy comparisons:
> ≈ 0.45 × metabolic energy of one gram of fat ( ≈ 38000 J )
> ≈ 0.63 × energy released by burning 1 gram of ethanol ( ≈ 27000 J )
> ≈ metabolic energy of one gram of sugar or protein ( ≈ 17000 J )
> ≈ (0.02 to 0.09) × typical kinetic energy of a car at highway speeds ( 200000 to 900000 J )
> ≈ 0.5 × typical battery energy content of an alkaline long-life C battery ( ≈ 9.6 W h )
> ≈ 0.55 × typical battery energy content of a nonrechargeable Lithium-Thionyl Chloride AA battery ( ≈ 8.6 W h )
> ≈ 0.92 × typical battery energy content of a carbon-zinc D battery ( ≈ 5.2 W h )
Gravitational potential energy is just really, really low energy density. Fortunately it's reasonable to have pumped-storage reservoirs that contain trillions of litres of water.
I think a bit more than that, the article says 250 million gallons which is more like 960 million litres. That squares with the dimensions given: 15m x 15m x pi x 75m x 20 tanks = 1 million cubic metres = 1 billion litres.
Oahu uses about 225MW on average in residential electricity [0], so this could store an hour or so of excess capacity. That's not nothing, but you might want to 5x or 10x it to really smooth out demand and supply peaks for the island.
Note that you also need a set of tanks or a lake at the bottom of the hill. The tanks are 75m tall, so how are you accounting for that in your 120m of vertical distance?
I subtracted 100 from their position on red hill at 220-ish meters to account for the fact that the tanks are buried in the ground (I don't know precisely where).
>Oahu uses about 225MW on average in residential electricity [0], so this could store an hour or so of excess capacity. That's not nothing, but you might want to 5x or 10x it to really smooth out demand and supply peaks for the island.
This solar plant & battery system was completed last month. The battery is able to store 144 MWh. This shows me that planners and people thinking about this are willing to invest & commit to power solutions at this scale. I don't know if you are aware or have read up on the red hill controversy. These tanks were used to store fuel by the Navy. They leaked earlier this year and poisoned the Red Hill aquifer and have caused quite a bit of local controversy.
I'm struggling with the idea of writing a letter to the editor outlining this concept, but also emphasizing how converting these tanks to a water storage/ energy storage facility would improve local resilience to drought, bring down water costs, improve environmental conditions etc.
I've seen some people talk about adding a heavy weight on top of the water in a pumped hydro system - something like a big concrete lid on top of a cylinder type reservoir.
They do say it is about equivalent to a single AA alkaline. It is not exactly an optimized efficient setup, but still, it gets the point across. Pumped hydro is a grid scale option, in places that have the ability to store a lot of water.
250L x 7m is 17kN, or about 5W/h, or about 2 AA 2450mAh cells from IKEA.
Pumped storage scales well though, as the stored volume scales squarely to the required container material (e.g. storing 25,000L will only take 10x the material while storing 100x the energy).
