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Comment by dpflan | Hacker News Reader

How large is a typical system - how much land you need to excavate?

If we attach our installation to an existing reservoir, we'll take up nearly zero land above ground. If we build a new self-contained upper reservoir it will be about 0.5 miles on each side and 40 feet deep. It can be built with material excavated from the lower reservoir. This may seem large, but it's for a huge amount of storage 20GWh - enough to balance the load of a large city. And keep in mind that it's about the same size of the many large reservoirs that are scattered around a large city.

Again, the most promising option would be to simply attach our installation to an existing reservoir. We don't use any additional water, we just borrow it. For an ample sized reservoir, each cycle would just raise and lower the water level by an inch or so. Another promising option is that we can even use the ocean as an upper reservoir. Salt water can be accommodated -- See our notes about the Okinawa Yanbaru Station.

There are more details in our white paper posted on the website.

This would correspond to a height difference of about 1 km between upper and lower reservoir right?

yup! More fun facts about a 1 km head height... Off-the-shelf turbines are actually spec'd for a max head height of something closer to .5 km. So the design calls for a double-drop. This design approach is taken from the DOE research linked on our website.

so where the geology allows it, why not go even deeper with the lowest reservoir, and put multiple turbines in series, with perhaps small reservoirs each .5 km?

Then the total energy capacity is V * rho * g * h, so that energy store is proportional to height, while tunnel boring price is roughly constant as long as tunnel boring volume of the reservoirs is much larger than the volume of vertical shafts.

I realize its a bit oversimplified but if we consider 2 prices: p1 price per volume for boring horizontally (for reservoirs), and p2 price per volume for boring vertically, then increasing the reservoir size by a volume delta V, requires boring 2 * delta V (upper and lower reservoir), while boring vertically the difference in height depends on the diameter...

Why would a new upper reservoir need to be so wide and shallow, rather than having much less surface area and being much deeper?

Good question, it doesn't really need to be, those numbers are partly just to visualize it. But we do have some reasons to keep it with more surface area: - less digging - less reinforcing needed - it's more stable - In some cases we're interested in floating solar on top of the reservoir which wouldn't work well if the reservoir was too deep.

But it's certainly not out of the question to go deeper instead.

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