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Comment by delinka | Hacker News Reader

Nit: Lake Mead is the battery. Hoover's generators are extracting power from that "battery" constantly.

The plan from TFA: Take water that's already passed through the generators at Hoover and, using solar and wind power, pump it back upstream into Mead to be processed again. This gives us the ability to supplement hydro with solar and wind. I guess if this is more efficient than other batteries, it's not a completely crazy idea. It does, however, have potential negative impact on downstream ecosystems.

Also: I assume that we don't extract 100% of the available potential from the dammed water: how much water flows past generators (instead of through them) and how does that change when upstream water supply changes? How much of a decrease in this "loss" can we get from this solar/wind plan?

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At Hoover Dam, 100% of the river flows through the turbines with no bypass. The dam was originally built for flood control. Power generation is driven by downstream water demands. [1]

[1] https://en.wikipedia.org/wiki/Hoover_Dam#Operation

"The entire flow of the Colorado River passes through the turbines."

But it isn’t 100% efficient in converting potential energy into electricity. Even if the generators were 100% efficient, resistance in the pipes leading to the turbines slow down the water, not all the water hits the turbine blades, the water that does doesn’t transfer all its energy (the water would have to have zero speed after the turbine to do that, and the turbines would have to be at the bottom water level)

Understood.

I took the OP's "how much water flows past generators" to address whether all the dam's outflow goes through the turbines or whether some is diverted around them.

The former is the case. The latter only happens when spillways are in use, which is rare.

From Wikipedia, https://en.wikipedia.org/wiki/Hoover_Dam#Operation:

> The amount of electricity generated by Hoover Dam has been decreasing along with the falling water level in Lake Mead due to the prolonged drought in the 2010s and high demand for the Colorado River's water. Lake Mead fell to a new record low elevation of 1,071.61 feet (326.63 m) on July 1, 2016 before beginning to rebound slowly.[95] Under its original design, the dam will no longer be able to generate power once the water level falls below 1,050 feet (320 m), which could occur as early as 2017. To lower the minimum power pool elevation from 1,050 to 950 feet (320 to 290 m), five wide-head turbines, designed to work efficiently with less flow, were installed.[96] Due to the low water levels, by 2014 it was providing power only during periods of peak demand.[97]

Which is really surprising.

FTA:

> The Los Angeles Department of Water and Power, the nation’s largest municipal utility, says its proposal would increase the productivity of the dam, which operates at just 20 percent of its potential, to avoid releasing too much water at once and flooding towns downstream.

>to avoid releasing too much water at once

Fixing that (digging the river deeper in downsteam towns) would probably be a cheaper way to allow Hoover to scale up and down to more variable demand from wind and solar.

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